Proceedings of the 14th International Conference on the Technology of Plasticity - Current Trends in the Technology of Plasticity

CubeSats are kind of small satellites that follow specific standards, and because of that, they are less costly to develop and launch compared to conventional satellites. The careful design of a CubeSat’s structure allows for weight savings and increases the chances of a successful mission. In addition to being light in weight, the CubeSat structure must provide enough stiffness and strength to support the other subsystems. Superplastic forming (SPF) is a manufacturing process used to produce complex and lightweight components. It is known to significantly save manufacturing cost and time in certain niche applications. Up to our knowledge, this process has never been implemented in the CubeSat industry. This study considers the use of the superplastic forming process to produce a CubeSat enclosure that consists of six SPF panels. The Aluminum alloy AA5083 was used in the study, as it already has applications in the space industry. Several finite element simulations were carried out to ensure that the proposed structure passes the requirements of the CubeSat design specifications. At first, the SPF process was simulated to make sure that the panels’ thickness profiles meet the industry requirements and that no excessive thinning is observed. The generated panels were integrated in a 1U CubeSat and then subjected to both quasi-static and modal analyses. The results show that the proposed structure has a mass that is 226g less than that of a certain commercially available off-the-shelf CubeSat structure. This is considered significant when compared with the typical mass of a complete 1U CubeSat, which is 2.0 kg. The results also demonstrate that the proposed structure satisfies all the CubeSat dimensional, stiffness, and strength requirements.

In metal forming processes, contact mechanics in the tool-workpiece interfaces determine the resulting friction and thus influence the quality and stability of the process. In models representing dry, thin film and boundary lubrication conditions in metal forming, friction may be assumed proportional to the ratio of the real contact area to the apparent contact area. It is therefore important to obtain a fundamental understanding of as many of factors influencing asperity flattening as possible. This paper presents asperity flattening of pyramidal model asperities loaded normally by a plane tool. The experiments are performed in a recently developed test setup, which allows for different in-plane strain ratios in combination with the normal load. The in-plane deformation is known, through the yield criterion, to influence the necessary yield pressure to flatten the asperities and hence the resulting real contact area. In-plane balanced biaxial stretching is shown to result in further asperity flattening than plane strain. When comparing to previous results, it is shown that in-plane strains reduce the effect of the asperity shape on the resulting real contact area ratio.

Auxetic materials and cellular structures offer unique mechanical properties with respect to negative Poisson ratios, which essentially leads to elevated shear moduli and delivers superior mechanical resilience under specific mechanic and dynamic loading conditions. In the present study, we provide a literature review about auxetics, their specific properties and the resulting mechanical behavior. Systematically, we present concepts to apply these properties to the elementary functions of adjustable tools in groups of passive and active implementations of auxetic structures into forming tools. Conceptual application clusters are derived with respect to three scales of flexible forming tools: i) smallest adjustments of a surface layer to influence and control the material flow by pressure distribution, ii) intermediate adjustments to comply with manufacturing tolerances (e.g. elastic springback of the product) and iii) large tool surface adjustments by actuated substructures to produce variants. We present protype concepts of an elastically deformable cellular auxetic structure and a non-assembly mechanism with auxetic properties and quantify the benefits and limitations of both concepts. In future, we aim to apply these concepts to demonstrators used in sheet metal forming processes.

Increasing ecological and economic demands are setting new requirements for industrial manufacturing processes and components. The use of functional integration as a strategy of lightweight design offers the possibility to meet these requirements. However, a high functional integration implies an increasing component complexity, which challenges conventional forming processes to their limits regarding formability and efficiency. The novel process class of sheet-bulk metal forming (SBMF) offers the opportunity to combine the advantages of sheet metal and bulk forming by applying bulk forming operations to sheet metal semi-finished products. SBMF from coil offers further economic and ecological benefits through high output rates, short process times by eliminating costly handling tasks and a greater material efficiency. However, forming from coil also poses coil-specific challenges such as anisotropic material flow and reduced geometric part accuracy. Against this background, in this study, a backward extrusion process from coil for forming geared functional components is investigated on a high-speed press. The objective of this study is to analyze measures for a targeted material flow control in order to avoid unequal material flow and to improve part accuracy. Therefore, the mild deep drawing steel DC04 with an initial sheet thickness of 2.0 mm is analyzed in a combined numerical-experimental approach. Within these investigations, the influences of coil width, friction and feed width are analyzed and evaluated based on component and process parameters. Finally, measures for a purposive material flow are derived.

5A06 aluminum alloy exhibits low density, high strength, and excellent corrosion resistance, which is widely used in areas of the aerospace, aviation, missile, ship and other fields. However, the defects, such as springback and thinning will occur during the process of stamping. In this research, the formability of 5A06 aluminum alloy thin-walled elbow is studied by experiments and FE simulation. Thinning and springback of 5A06 aluminum alloy thin-walled elbow is analyzed when the BHF is 10 kN, 15 kN and 20 kN, and the friction coefficient is 0.15 and 0.3, respectively. The result shows that the maximum thinning rate increases with increasing of BHF and friction coefficient. Thinning occurs on the short edge and reaches the maximum value around the curved edge. When BHF and friction coefficient are 10 kN and 0.15, respectively, the maximum thinning rate is 5.22%. A large BHF contributes to control springback. When BHF is 20 kN, the maximum springback is 3.01 mm. The simulation results exhibit good consistency with the experiment results.

6005A aluminum alloy is a medium-strength alloy widely used in the aerospace industry and throughout railway applications. The forming limit curve of 6005A aluminum alloy at 350–450 ℃ is obtained by forming limit test and the microstructure was analyzed. Particle swarm optimization (PSO)-BP neural network combined with genetic algorithm optimization method is used to obtain the best hot stamping process parameters for small curved beam parts of high-speed trains, and used to optimize the hot stamping forming parameters of the 6005A aluminum alloy track train small curved beam parts. The optimization results from PSO-BP neural network and genetic algorithm are verified by finite element simulation analysis and forming limit diagram. The results show that the deformation temperature has a great influence on the forming limit of aluminum alloy. The optimal process parameters for hot stamping of 6005A aluminum alloy small curved beam parts were determined: blank holder force is 4.051KN, stamping speed is 141 mm/s, friction coefficient is 0.05, and maximum thinning rate is 21.2%.

Microchannels on metallic substrates are essential parts of microreactors, micro heat exchangers, micro heat sinks, and fuel cell bipolar plates. Because of superior properties, such as good corrosion resistance, excellent thermal conductivity, high strength, excellent ductility and structural stability, composite metal foils are gaining popularity as substrates for microchannels. With the increasing demands of low cost, high efficiency and accuracy, novel micro forming technologies must be capable of creating intricate and precise micro features of microchannels at a low cost. An ultra-thin metal foil rolling for the fabrication of micro composite channels is proposed in this study, which can improve product function and reduce operational costs. Copper/ stainless steel 304L (SS304L) composite foils with a thickness of 0.41 mm after annealing treatment at 800 ℃ and holding for 0.5 h, 1.0 h and 2.0 h are used to fabricate microchannels under the rolling reduction of 60.9%. The hardness of composite foils has been characterised before and after different annealing processes, in which the hardness decreases due to the synergistic effects of decreased dislocation density, grain coarsening and phase transformation. Further, the formability of composite foils after annealing is discussed. In addition, the processing characteristics, material deformation behaviours and mechanisms during micro rolling are also investigated.

Folding-shearing, is a novel process, which in previous work has been used to create isolated shrink corners. The top surface of a blank was clamped, and the part formed by manual folding followed by a shearing process. For the first time, this paper demonstrates folding-shearing in a press-tool. A single set of tooling is used to crash form the upper surface of the part, followed by folding and in-plane shearing with all tools moving vertically, avoiding any requirements for cams or other mechanisms. The results of the paper demonstrate that folding-shearing can be implemented in a conventional press-tool. A process window is defined by process parameters and is limited by springback, thinning or thickening. These limits are influenced by process parameters including part radius, feed radius, fold geometry, part height and material parameters. An analytical description of the mechanics of the process is proposed and is validated though finite-element simulations and experimental trials.

Embossings on sheet metal surfaces allow a targeted enhancement of local part strength and affect thinning in deep drawing. However, conventional manufacturing of tailor embossed blanks on press machines with simultaneous intrusion of embossing punches results in high force demands and tool wear. Incremental forming of each indentation reduces the embossing forces required but increases cycle time. Embossing by rolling provides limited flexibility in embossing patterns. Therefore, presented paper deals with the development of a new approach for an efficient and flexible embossing of sheet metals – the Plate Roll Embossing Process (PRE Process). Here, a modular design of an upper and lower tool is used, into which various embossing, counter punch or blank positioning inserts can be integrated. The sheet metal is placed between the tools and the setup is moved through a defined roll gap of a mill stand. The line contact between rolls and tool defines a relatively small forming zone, which is moved translationally through the workpiece by the rotation of the rolls. Numerical and experimental investigations were performed with aluminium AA6014 and steel DC01, showing homogenous embossing results with reduced loads compared to conventional embossing. Furthermore, the paper demonstrates that the high flexibility of the tool and process design enables embossing of flat workpieces as well as the simultaneous embossing and forming of curved parts. Finally, tensile tests performed with embossed specimens show that the PRE Process represents a fast and efficient sheet metal forming method for the targeted modification of component properties.

Multi-stage press hardening enables a stepwise control of the thermo-mechanical history during press hardening, thereby facilitating a flexible hardness tailoring. Further degrees of freedom arise from utilizing rapid austenitization by induction heating within the first process stage. To assess the potential of rapid austenitization and a subsequent multi-stage thermo-mechanical history for tailoring the hardness, near-process characterizations are carried out with 22MnB5 steel. By a variation of the austenitizing temperature and dwell time, when cooling at a rate of 30 K/s, a hardness between 310 and 448 HV was set. By inducing a strain of 0.1 after rapid austenitization, the hardness was reduced by 15% due to an accelerated ferrite and bainite transformation. However, it was observed that a second, subsequent induced strain can potentially increase the hardness by work hardening. Using the example of multi-stage press hardening with rapid austenitization implemented in a progressive die, the hardness of 22MnB5 sheet material is tailored. A hardness between 350 and 445 HV was set by varying the austenitization parameters and the strain induced with a controllable blank holder.

Tools for high-volume sheet metal forming processes generally sacrifice flexibility for productivity. For a tribologically severe process like ironing, this lack of flexibility makes mitigating galling during internal ironing challenging. In a novel concept of an adjustable ironing punch, the diameter of the punch can be rapidly and accurately adjusted to control the part height and decrease contact pressure during punch retraction, thereby reducing tool wear and lubrication requirements, while also making the process more flexible. The present study shows numerical analysis of the process of ironing the interior of a deep drawn cup made from steel grade 1.4301 and puts forward a method for estimating the required punch contraction to substantially reduce wear during retraction. Different punch diameters, cup wall thicknesses and reduction ratios are considered in a simplified process setup, with the effect of simplifications being explained based on selected examples. Results show that the required absolute punch contraction is positively correlated, and the strain required to reach that punch contraction is negatively correlated to the punch diameter. Both response variables were found to be positively correlated to the thickness change performed in the ironing process. Consequently, the design requirements of an adjustable ironing punch are the loosest for large diameter cups and low thickness changes.

Springback notoriously depends on the strain-path history of the sheet material during forming. The bending-under-tension (BUT) test is used in this work for a discriminant characterization of springback in various conditions. In order to generate non-linear strain-paths, tensile prestrain is usually applied in the literature. In this work, an original biaxial prestrain device was designed and build. It allowed for the uniform prestrain of large samples of 500 mm diameter in DP600, up to 9.5% of strain. Subsequent samples for bending-under-tension were cut and tested at different combinations of tool radius and backforce. Classical models were used to describe the material’s behavior within finite element simulations of the BUT test. These experiments allowed for a discriminant comparison between the springback predictions of competing constitutive models.

Studies on crash performance show that hot stamped components with homogeneous mechanical properties can reach their limits in highly stressed car body areas. To face this challenge, hot stamped components can be optimized by integrating ductile areas. A suitable technology for adjusting tailored properties is based on a special furnace chamber within a multi-chamber furnace in which a cooled aluminum mask protects the blank locally from incident heat radiation. Simultaneously, the mask absorbs the blank’s own radiation. In addition to the temperature and radiation properties, this heat exchange depends in particular on the orientation and position of the radiating surfaces in the furnace chamber. For this reason, the mask distance to the blank can influence the width and characteristics of the transition zone after partial hot stamping. In order to predict the correlation between hard and ductile area, the process of partial hot stamping with varying mask distances is modelled in LS-DYNA® and validated experimentally. The results indicate that it is feasible to simulate the influence of the mask distance on the resulting hard, soft and transition zone after partial hot stamping. Furthermore, the results demonstrate that transition zones on a partially hot stamping part can be designed more flexibly, thus extending the limits in terms of part complexity.

Multi-materials, such as fiber metal laminates, offer several advantages over monolithic materials, i.e. the possibility of tailoring the material properties. The in-situ hybridization process combines deep drawing and thermoplastic resin transfer moulding (T-RTM) to manufacture fiber metal laminates with complex geometries in a single process step. However, due to strong fluid-structure interactions between fabric, metal and the low viscous matrix during forming and infiltration, other parameters besides those in pure metal deep drawing affect the process and part quality. Therefore, the influence of blank holder force, injection time, punch velocity and matrix viscosity on the process is experimentally investigated on part scale. The manufactured parts are subsequently assessed by measuring the metal blank surface strains and the thickness of the glass fiber reinforced layer to evaluate the forming behavior and matrix flow. Based on these findings, the most influential parameters and parameter interactions during the deep drawing of fiber metal laminates with a low viscous matrix are identified, and their influence on the process is discussed.

Residual stresses in metal components induced by forming operations significantly influence subsequent manufacturing steps. In particular, two aspects for the final processing of formed thick sheets by cutting and machining operations result: First, the achievable geometric accuracy is limited by distortion due to the modification of residual stresses across the process step sequence. In addition, the residual stresses present in the final component are determined by the process parameter settings of the individual substeps. In this work, the influence of the forming-induced residual stresses on a subsequent cutting operation was quantitatively investigated. For this purpose, the step sequence of forming and wet abrasive cutting for aluminum AA7075 thick sheets was implemented both numerically and experimentally. This allows the residual stress redistribution to be taken into account in future virtual setups of the production chain by specifically adjusting the process parameters.

Pressure fields in electrohydraulic forming often have a very complex character due to the combination of the many physical phenomena involved. Among several energy-force factors generated by underwater discharge, direct shock waves play a leading role in the deformation of the sheet blank. The measuring method based on a multipoint membrane pressure gauge makes it possible to obtain detailed information about shock-wave pressure distribution over a large area (field). The analysis of pressure fields shows three types of loaded segments: zones of direct shock wave loading, zones of shock waves’ interaction, and zones of concentration along lateral rigid walls. In their profile, the pressure distributions in the above-mentioned zones are very similar to the Gaussian distribution curve. The Laplace exponential normalised function was thus selected as a basis for the test approximation dependencies. In the proposed approximation method, the loaded zones are assumed to be independent pressure sources and the principle of superposition is valid. The resulting pressure value at each point is determined as the sum of pressure values from all the loaded zones. The approximation results correlate well with the test data for two simultaneous discharges in a rectangular chamber. The approximations of pressure fields give more exact data on the configuration of each loading zone and its share in the total field formation and can serve as a technological characteristic of a discharge chamber.

Medium-Mn (MMn) steels with low austenitisation temperatures have recently attracted much research attention because of their suitability for hot stamping to save cost and improve productivity. However, research on their formability under conditions that mimic industrial hot stamping is still lacking. In this study, an innovative method for biaxial testing, which includes a cruciform specimen design, is used on a Gleeble 3800 simulator to obtain the forming limit curves (FLCs) and fracture forming limit curves (FFLCs) of a representative MMn (8Mn) steel. The localised necking strain and fracture strain are measured using a newly developed spatio-temporal method. The FLCs and FFLCs are constructed using strain limits in different loading states, e.g. uniaxial, plane strain and equi-biaxial stretching. The results show that the loading state conditions are acceptably simulated using the biaxial test system, and the FLCs and FFLCs are well-constructed, displaying a V shape. The data will contribute to the design of forming process. In addition, the forming strain limits of the MMn steel are much higher than those of a conventional boron steel (22MnB5) under the same hot stamping conditions, suggesting that the MMn steel has great potential and benefit as an alternative in hot stamping.

The spatiotemporal coupling relationship of strain in different local deformation regions was characterized by digital image correlation (DIC). The Lüders band nucleates at the deviation point of the elastic stress-strain line and begins to propagate at the lower yield point. Propagation of the Portevin-Le-Chatelier (PLC) band leads to the formation of the Band Propagation Region and No-band Region in the entire deformation region. The occurrence of stress serrations is attributed to the periodic strain transferring between the Band Propagation Region and the No-band Region at the mesoscopic scale.

Additive manufacturing of hot forming tools provides increased flexibility regarding geometry and proximity of the cooling channels to the tools’ surface. In this field, the application of Directed Energy Deposition has not been investigated extensively. The conventional manufacturing route of cooling channels in hot stamping tools comprises the segmentation of the tool and hole drilling. This leads to the disadvantages of possible leakages and low flexibility regarding the channels’ size and location. Thus, a novel combination of Directed Energy Deposition and ball burnishing for leveling the additively manufactured surfaces is established to manufacture hot stamping tools for the forming of hat profiles. In the additively manufactured punch, near-surface cooling channels are integrated after a load-adapted definition of channel geometry and path, both based on numerical and experimental analysis. On the blank holder side, an additively manufactured texture, targeting the control of the heat balance and material flow, is applied. With these tools, hot stamping tests with a hat profile (22MnB5) are conducted and compared with conventionally manufactured tools with drilled cooling channels. It is demonstrated that the near-surface cooling channels are able to decrease the punch temperature in the hot stamping process by up to 50% in the investigated configuration. The textured blank holder creates a more homogeneous temperature distribution over the hat profile during forming. As a result, part properties can be enhanced by a lowered sheet thickness reduction and a higher and evenly distributed hardness. Numerical simulations can be used to depict those effects during process planning.

Forming limit diagrams and forming limit curves are used to determine the failure limits of sheet metal materials by conducting a forming limit analysis. The aim is to enable the selection of suitable materials, geometries and process parameters at an early stage in the component development. In this way, engineering time and costs can be reduced. In addition to experimental forming limit analyses, different models and model approaches exist to numerically predict damage and failure. All have in common that calibration is necessary based on experimental investigations. The correspondingly determined parameters specify when ductile damage initiation begins, how it evolves, and how the material finally fails. Thus, the calibration and the parameters determined have a significant impact on the models’ accuracy. The present work focuses on the numerical prediction of DP 800 dual phase steel in a Nakajima test using a modified Bai-Wierzbicki model. Besides the calibration influence of the damage model coming from a non-unique parameter selection is investigated. A sensitivity analysis of the experimentally determined parameters of the damage model is carried out with the aim of predicting the crack initiation with high accuracy and to determine the influence of the calibrated parameters, especially the range between the damage initiation locus, the failure locus and the damage evolution rate. The objective is to determine which damage model parameters possess a high influence on the failure prediction, in order to be able to make a risk assessment with regard to the prediction accuracy for later applications.

The newly developed hybrid forming process allows producing steel-long fiber reinforced plastic hybrid components in a single process step. The hybrid components produced in this way exhibit good failsafe behavior and enormous energy absorption potential combined with high structural integrity. They are also cost-efficient and environmentally friendly. To explain the field of application, this process was further investigated to produce hybrid components with aluminum-LFT and –PP-GMT (glass mat thermoplastic). Therefore, it is suitable for developing automotive structural components, e.g., crash management systems (CMS). To develop a lightweight hybrid-CMS with approx. 15% weight reduction compared to a full aluminum (Al) reference, different requirements must be fulfilled by Al and GMT. To achieve this, firstly, tool concept for axial crash and 3-point bending components were developed. Corresponding components were produced and tested for their performance.

Material heterogeneity has a considerable effect on spring back. Integrating heterogeneity in a simulation to obtain a more accurate spring back prediction is complex and often requires a significant number of tests and parameters. Whilst in the industry, the necessity of predicting forming process outcomes, e.g. the spring back of the material, is becoming essential, it does inevitably require a performant and easy to calibrate model. At academic level, a compartmentalized model [1] considers the heterogeneity of the material as one of the parameters driving its plastic behaviour. Its ability to describe load-unload cycles with a small number of adjustable parameters has demonstrated [2]. In this paper, we propose to optimize the identification process on an industrial material.The chosen material, a DP 600 steel, already presents high heterogeneity level due to its initial structure. The main objective is to calibrate the compartmentalized model by using only a monotonous tensile test. The identified model is integrated in a finite element code and simulations of a loading/unloading path are then carried out to evaluate the prediction accuracy of the material behaviour at small strain.. This possible new method could help reduce the number of tests required to obtain the required prediction accuracy, especially when a large spring back effect is present. This would allow to achieve more accurate optimization of the process parameters in the bending operation.

The low ductility of Ti–6Al–4V alloy at forming temperatures below 780 ℃ is still a problem. Ti–6Al–4V is usually hot press formed at 780 to 900 ℃. Hence, it is necessary to develop a new press forming for Ti–6Al–4V alloy sheets at forming temperatures below 780 ℃ to improve productivity. We developed a method to achieve of warm and cold press forming of Ti–6Al–4V alloy sheets by combining press motion control and clearance between a die and a blank. In this study, we tried tube forming by developed press forming method. In this method, the following two processes were separated: punch motion process without BHF and BHF motion process without punch motion. These two processes were carried out alternately during deep drawing or drawing and ironing. Three forming parameters, namely, the punch speed Spi, punch stroke Psi, and blank holding force BHFi, were controlled separately and the effect of forming conditions on deformation behavior was investigated by using experiment and finite element analysis (FEA). As a result, warm and cold tube forming of Ti–6Al–4V alloy was achieved without any undesirable deformation by applying the developed press forming method.

A die-embedded sensing system by embedding a 3-axis piezo sensor in the die was developed to measure the dynamic characteristics of sliding friction between materials and dies during stamping process. A hat-bending of steel sheets of high-strength steels was performed using the developed monitoring system. The influence of material strength, slide speed, cushion pressure, etc. on the dynamic characteristics of friction was evaluated. It is found that the friction coefficient changes dynamically depending on the processing conditions and material strength and the mixed lubricating state lasted for a certain duration time, and the duration time was theoretically calculated based on the squeeze effect theory. A pulse motion was adapted to the hat-bending process based on the calculation and the effect of the friction reduction was verified.

In this paper, the process model for cryogenic forming with macro-structured tools is analyzed regarding the continuously changing thermal conditions during the deep drawing process. For this purpose, the transient contact conditions within the macro-structure are investigated for different tool designs of the macro-structure as a function of the immersion depth and resulting contact pressure in a numerical analysis. The determination of the heat transfer coefficient between the aluminum sheet metal and the tools as a function of the contact pressure is carried out experimentally. Numerical investigations are used to determine an improved macro-structure in terms of low-heat flow and reliable suppression of wrinkling. The influence of the tool design on the temperature distribution is presented and compared with the conventional deep drawing process.

The use of aluminium alloys with high mechanical properties is very interesting to reduce the weight of parts, especially in the automotive industry. However, forming of these alloys at room temperature (RT) is difficult, especially for complex parts where fracture often occurs before the end of the forming process. The warm forming process, which consists of heating aluminum alloys to a temperature between 150 and 250 $$^{\circ }$$ ∘ C, increases formability and thus allows complex shapes to be formed. In this study, forming of a cylindrical cup using a specific two-step drawing device by two reductions of the blank diameter is performed at RT and 200 $$^{\circ }$$ ∘ C for a 7075-T6 alloy. These two forming steps are also numerically simulated using Abaqus. Forming at RT leads to the failure of the cup in the first stage, which is also obtained by numerical simulation with plastic deformations higher than what the material can support. At 200 $$^{\circ }$$ ∘ C, the cylindrical cup can be formed in two steps, which results experimentally and numerically in a significant reduction of the forming forces.

Skin sheets of metal aircrafts have complex thickness change patterns. Thus, their forming with press brake is quite difficult.To automate and ensure high quality, a cutting-edge press brake equipment with synchronized material handling system, position variable punches, die, curvature measurement devices, and bending mechanism with curvature feedback/feedforward technology has been developed. In order to determine specifications of many innovative devices such as material handling systems, variable punches, die, etc., and to realize new bending process, a huge number of studies and development have been conducted.Here in this paper, a bending characteristics of Aluminum Alloy 2524-T351 clad and an optimization of each variable punch strokes are described. The bending characteristics are affected by the phase in which only clad aluminum plasticity, the phase in which the clad layer re-yields upon unloading, and the phase in which the base material plasticizes, resulting in a complex curve. It depends on the thickness, die width and previous bending in sequential bending.

Due to the demographic developments of an increasingly aging society, the number of employees retiring is also rising. Likewise, the fluctuation within the individual companies has increased drastically within the last few years. As a result, experience gained over many years is being lost. On the other hand, there is a strong trend towards an ever-increasing number of very complex manufacturing processes with a large number of process influencing variables, where the human monitoring capabilities are limited. In this work we study the complex process of hot stamping of metal blanks for car body parts with a demonstrator plant. The demonstrator plant consists of five main components, comprising a magazine for material feed, an industrial robot for transferring the raw material and the finished part, an annealing furnace, and a hydraulic press with the temperature controlled hot forming tool inside. All machines have open communication interfaces with which all sensor data can be accessed. The demonstrator plant is additionally outfitted with different sensors measuring crucial process parameters. We use temperature sensors in the punch and die and make use of capturing the structure-borne sound and mechanical vibrations during the process to closely monitor the process. This sensor data is analyzed and methods of machine learning are used to detect anomalies in the process data. The goal of this work is to develop a methodology for the systematic detection of anomalies in the hot forming process using machine learning methods within a demonstrator plant that are suitable for real time anomaly detection for a high level control system. We use unsupervised machine learning techniques and neural networks to detect anomalies with great accuracy on our test data.

As trade patterns have shifted to high variety and small batches, metalworking techniques used in metalcraft have been reconsidered. Therefore, the purpose of this article is to study one of these techniques, the English Wheel, which can produce a curved sheet metal part by pulling and pushing the sheet, clamped under a pressing force, back and forth along the working path between the roller and the anvil wheel inside a giant C-clamp. In this study, SPCC sheets were used to form parts with a single curvature. As a result, the curvature of the parts formed by the English Wheel was measured and determined by a 3D optical digitizer and approximated by its circumscribed circular cylinder. The curvature of the part formed by a sawtooth path is the same as the superimposed curvature of each part formed by individual parallel paths decomposed from the sawtooth path. Based on these findings, this study then provides a processing scheme for forming with English Wheel, which is validated by the finite element method on an example with a given curvature.

Knowing exactly when a tool needs to be replaced helps to avoid unscheduled downtime due to failure. In roll forming, as in many other forming processes, especially abrasive wear is critical for tool life. Since numerical modeling of wear phenomena is complex and time-consuming due to required microscopic accuracy, a practical data-based approach is investigated to predict wear states. In roll forming, driven tool rolls are responsible for both the incremental profile bending at each forming step and the overall profile transport induced by all forming steps. The driving diameter of a tool roll is defined as being located at the position with rolling friction, i.e., without relative sliding velocity between the tool and the workpiece. The effective direction of the tool torque corresponds to the transport direction of the frictional force between tool and workpiece. Sensor data shows, that the torque of a driven tool roll is sensitive to changes in the tool gap between the rolls, to changes in transport velocity and to changes in geometrical bending parameters. Therefore, it is considered to be an important inline measured process variable. In this approach, idealized wear states in terms of enlarged tool radii are experimentally investigated using four discrete levels. As a result, the average torque level show correlations to the wear states for all the four investigated forming steps individually and simultaneously allowing to determine data trends. The trends are explained by a geometric displacement of the driving diameter at larger tool radii. Furthermore, stationary and non-stationary forming conditions are identified from data, allowing the forming process to be analyzed over time. Using the presented approach, four discrete wear states are classified for one of the forming stages by an artificial neural network. Using these results, torque monitoring can be applied to predict wear and estimate the end of the tool life.

This study investigates the relationship between stress relaxation behavior and kinematic hardening of sheet metals. Stress relaxation tests and tension-compression tests for a high-strength steel sheet and an aluminum sheet were carried out to discuss whether it is possible to estimate the back stress as the amount of kinematic hardening based on the results of the stress-relaxation tests. In the stress relaxation after uniaxial tension, stress decreases with time. On the other hand, in the stress relaxation after uniaxial tension followed by full unloading, stress increases. It was found that in the stress relaxation after uniaxial tension followed by unloading to a certain stress level, stress does not change during the relaxation test. As for the high-strength steel sheet, the unloading stress level where stress does not change in the relaxation test was found to be relatively high and its Bauschinger effect is significant. On the contrary, as for the aluminum sheet, the unloading stress level is found to be relatively low and its Bauschinger effect is weak. These results suggest that the externally applied stress and the internal back stress is balanced at a certain unloading stress level where stress does not change in the relaxation test, and the unloading stress level is correlated with the amount of kinematic hardening.

Superplastic bulging forming is a hot manufacturing process widely used in the aerospace industry for its capability to produce complex shapes and decrease the joining operations but it has some drawbacks that make its monitoring quite difficult in terms of pressure regulation and strain rate control. A finite elements model of superplastic rectangular bulging of Al-Li alloy AA8090 was built and developed using Abaqus commercial code. The FE model was validated based on the thickness ratio distribution along the length and the width of the rectangular cavity of the die, besides the ratio of max creep strain rate over the target creep strain rate profile. The development of the model was proposed to enhance the accuracy of thickness distribution results by changing the friction coefficient of the contact between the die and the sheet. The obtained results were analyzed and explained and the new model gave promising results with good accuracy with respect to the experimental findings.

Thin-walled shells with curved surfaces are widely used in the aerospace field. As the thickness to diameter ratio decreases, it becomes increasingly challenging to control inner wrinkling during the conventional deep drawing process. In this paper, a novel deep drawing process with positive local bulging was proposed to suppress inner wrinkles in the deep drawing of a semi-ellipsoidal part. The effects of loading paths, rigid ring numbers, and thickness to diameter ratio of blank on forming defects were analyzed by experiments. The results indicated that the defects-free parts with a small ratio of thickness to diameter were formed by the novel process, which cannot be formed by the conventional deep drawing process. Both inner wrinkling and splitting were suppressed by the optimized loading path of the reverse bulging force. Although the local thickness of the curved surface part was decreased by the reverse bulging tools, the minimum thickness was improved by the increased rigid rings. In addition, a larger reverse bulging force was required to prevent the inner wrinkles for a smaller thickness to diameter ratio. This research provides valuable experimental guidance for the development of a novel deep drawing process for curved surface parts.

Aluminum alloy thin components are widely applied as structures in aerospace, aircraft, and electric vehicle. It is very challengeable to overcome the coexistence of wrinkling, splitting, and destroyed microstructure by the cold forming or hot forming processes when the components’ size is larger and thickness is thinner. A phenomenon was found by authors that the ductility and hardening ability can be simultaneously enhanced at cryogenic temperatures, which can be used to develop a novel cryogenic forming process for overcoming those problems. In this paper, the bulging and drawing abilities were studied at cryogenic temperatures, which demonstrates that aluminum alloys also have excellent cryogenic formability at complex stress states. Thus, a novel forming process at ultra-low temperature gradient has been developed, and the related development and prospect were introduced. The world’s first cryogenic forming device was developed with a drawing force of 22 MN and a platform size of 3 m. An integral 2219 aluminum alloy rocket tank dome with a diameter of 2.25 m was directly formed from a blank by the novel process at an ultra-low temperature gradient. The cryogenic forming process has considerable potential for large-sized and thin-walled components made of high-strength aluminum alloys, such as 2219 and 2195 aluminum alloys.

The simple shear test for sheet metals has become a topic of growing interest in recent years, whose significance could be reflected in two aspects: obtaining the mechanical behaviors under large strain and investigating the ductile fracture under low-stress triaxiality. In the current stage, the community lacks a widely-accepted and standardized procedure to conduct simple shear tests, and it makes the interpretation and comparison of experimental results to be indirect and difficult. Shear strain is commonly used to characterize the degree of deformation, but the method to estimate it in experiments is often different from one to another, and uncertainties will appear due to the inconsistency. In this work, some representative methods to obtain shear strain will be evaluated analytically and experimentally. The influence of the perturbation phenomenon (that is, the emergence of axial deformation and the tilt of shear direction with respect to the coordinate axis of strain measurement during simple shear) will be investigated in detail, and it seemed that there were no universal methods to estimate shear strain in experiments (to the best of the authors’ knowledge). Some recommendations on this topic would be provided and the results were expected to enhance the understanding of the experimental mechanics of simple shear tests.

The “reduced texture methodology” enables the numerical modeling of laboratory specimens with mean field polycrystalline plasticity at an affordable cost. It makes it possible to model not only plastic anisotropy, anisotropic hardening and non-proportional loadings but also the various ductile fracture mechanisms observed in metallic alloys: initiation, growth, rotation, flattening and coalescence of microscopic voids, transgranular fracture, secondary voids nucleation at the grain scale and coalescence at the slip system scale. The extension to numerical modeling of processes and structural behavior can be achieved in the longer term.

The non-quadratic yield locus YLD2000-2D is widely used in sheet metal forming to describe anisotropy. In this yield function, the exponent, often denoted by $$m$$ m , is a material parameter that determines the yield locus shape and is typically defined by the grain structure ( $$m=6$$ m = 6 for body-centered cubic (bcc) and $$m=8$$ m = 8 for face-centered cubic (fcc)). However, recent studies have shown that this assumption is not always the best choice for reproducing the strain distribution of standard experiments, such as the Nakazima tests. In the present work, an inverse time-dependent optimization is proposed to determine the parameter $$m$$ m for the metastable austenitic stainless steel 1.4301. The material is subject to the Transformation Induced Plasticity (TRIP) effect, and the microstructure transforms from austenite (fcc) to martensite (bcc) during cold forming. The strain distributions along cross-sections of different Nakazima experiments are evaluated at different drawing depths for the inverse optimization. The FE simulation results are compared with the experimental cross-section data in order to determine the optimal values of $$m$$ m . The outcome is analyzed to quantify the influence of the Nakazima specimen and the drawing depth. Despite the increase in martensite content in the material, the overall optimal value of $$m$$ m for the stainless steel 1.4301 was determined to be $$m=8$$ m = 8 .

This study presents the experimental results of layer-compression tests in plane strain state as well as the role of the obtained parameters on the simulated thinning tendency of a cylindrical deep drawn part. The work hardening law has a special importance in numerical simulations, thus it is mostly expected to be known over a large range of deformation. Tensile test, which counts as a fundamental method for material characterization, is therefore often complemented by other experiments, such as hydraulic bulge test, or plane strain compression test in sheet metal forming. The main drawbacks of the plane strain compression test are the lateral tool misalignment, the stress concentration at the tool edges and the undesirable effect of the friction. The thinner the applied sheets, the first two things are more pronounced due to the geometrical constraints that ensure the plane strain state. To overcome these disadvantages, layered specimens were investigated with rounded-edge tools in the present research. Our results show that the shear contribution in the compression zone can be effectively reduced by two- or three-layered sheets, with a minimum of two times higher deformation than it occurs in tension. Besides, a better lubricant condition is also easier to achieve in the multi-layered arrangement. The effect of the obtained work hardening parameters is also briefly outlined using numerical simulations and the Taguchi method. It was initially observed that none of the hardening parameters have more significance on thinning than others.

The objective of this study is to investigate whether the material model identified using linear stress path (LSP) experiment can predict the deformation behavior of the material subjected to nonlinear stress paths (NLSPs). First, many LSPs are applied to a test sample (6000-series aluminum alloy sheet A6116-T4) with a nominal thickness of 1.1 mm to measure the contours of plastic work and the directions of the plastic strain rates, $${\mathbf{D}}^{\mathrm{p}}$$ D p . Second, proper yield functions that reproduce the stress points forming counters of plastic work and the directions of $${\mathbf{D}}^{\mathrm{p}}$$ D p are determined. Third, a NLSP is applied to the test sample; the NLSP consists of several linear stress paths without intermediate elastic unloading. Forth, the elasto-plastic deformation behavior of the test sample subjected to the NLSP is compared with that predicted using the yield function determined from the LSP experiment. It is concluded that although the measured $${\mathbf{D}}^{\mathrm{p}}$$ D p slightly deviated from those predicted from the normality flow rule, the deformation behavior of the test sample predicted for the NLSP using the model determined from the LSP experiment is generally consistent with the measurement within the experimental conditions of the present study.

There are several methods of revealing temperature effect in hot cylinder compression test on actual flow curves. However, few methods were verified by the direct comparison between the experimental and predicted compression load-stroke curves. In this study, the systematic and scientific method of obtaining the flow curves formulated by the generalized C–m model considering the actual test environments is applied to the temperature-sensitive Ti6Al4V alloy in the temperature range of warm forming. Finite element analyses of the cylinder compression tests are conducted at various sample temperatures and strain rates using the fitted flow function, revealing that the predicted compression load-stroke curves are consistent with the experimental results.

During forging, temperature effect is relatively weak in the case of an A6082 alloy to magnesium and titanium alloys and both friction and temperature compensations are thus necessary to accurately characterize it. This point is quite different from other materials for which friction-compensation can be neglected. The characterized flow information of the A6082 alloy is verified by comparing the predicted and experimental stroke-compression load curves. The improved flow information is utilized to simulate a hot forging process of an A6082 alloy automotive part with high accuracy. The predictions are compared with the experiments, showing a good agreement with each other. The effect of friction on the flow pattern of the aluminum forging process is also investigated to practically find the ac-acceptable frictional condition.

Tension-Compression Asymmetry (TCA) of a 1.5-mm-thick 5000-series aluminum alloy sheet, A5083-O, is measured using a uniaxial tensile test and in-plane compression test. In particular, the TCA observed for annealed materials is referred to as the Strength Differential Effect (SDE). The in-plane compressive flow stress is 2–6% higher than the uniaxial tensile flow stress; therefore, the material exhibits the SDE. Moreover, a stacked compression test in the thickness direction of the test sample is also performed to measure the hydrostatic stress dependency of the yield stress. The uniaxial compressive flow stress in the thickness direction is almost identical to the equibiaxial tensile flow stress measured using a cruciform equibiaxial tension test (ISO 16842) and a hydraulic bulge test; therefore, the hydrostatic stress dependency of the yield stress has not been confirmed. Hence, it is concluded that the SDE observed in the A5083-O is not caused by the hydrostatic stress dependency of the yield stress.

The objective of present paper is to examine the plastic anisotropy behaviour of steel sheet, employing the Barlat´s Yld 2000-2d yield stress criterion and the corresponding non-associated plastic flow rule. New Barlat´s coefficients of anisotropy were defined and calibrated from material experimental data of simple uniaxial tension and equal biaxial stress tests. The new set of coefficients calculated from the experimental Lankford anisotropy coefficients (r-values), normalized yield stress (s-values), equal biaxial stress parameters (rb and σb) were numerically obtained using the Newton-Raphson method. The investigated metal was the highly anisotropic AISI 439 steel sheets found in the literature. In the results analysis and discussion, the new coefficients of anisotropy of the Barlat´s non-associated plastic flow rule were calculated and validated by plotting on the same graph the predicted r-value and s-value curves and the experimental data for the anisotropic steel sheets. The correlations have revealed that the Barlat´s yield criterion and the plastic flow stress potential were not coincident. Furthermore, the predicted limit strain curve of 439 steel correlated better with the experimental FLCTD transverse curve when using the shear stress fracture criterion and the non-associated plastic potential than the associated flow rule. Therefore, the Barlat´s Yld 2000-2d non-associated plastic flow rule provides a better fit with the experimental Lankford and equal biaxial coefficients of anisotropy and the FLCTD curve results of AISI 439 steel sheets.

The use of hollow aluminum extrusions fabricated using porthole dies in automotive applications is increasing. In a porthole die, the aluminum feed splits into multiple paths to pass around the bridges which support the internal mandrel and then recombines in the weld chamber of the die. A challenge for these products is the spatial variation of microstructure near the weld seam, in particular crystallographic texture. In the current study, an Al-Mg-Si alloy with a high density of Mn/Cr containing dispersoids was extruded through a simple die with two portholes and a single bridge into a rectangular strip. The high density of dispersoids minimized recrystallization during and after the extrusion process. The microstructure in the profile was analyzed using electron backscatter diffraction (EBSD) maps. It was observed that the microstructure could be segmented into a number of regions with different crystallographic textures. Using the information on local texture, the visco-plastic self-consistent (VPSC) polycrystal plasticity code was used to predict the local anisotropic plastic response in each region. The VPSC results were used to fit a Barlat YLD2004-18p flow rule which was employed in an LS-Dyna finite element method (FEM) model to predict the spatial variation of plastic strain near the weld seam. The FEM simulations were validated with tensile testing where the local strain was characterized using digital image correlation (DIC). Good agreement was found between the DIC and FEM results.

In accordance with the emergence of new automotive steel types, such as 3rd Generation advanced high strength steel, the reasonable estimation of their local ductility (tight-radius bendability, edge stretchability, etc.) has become of great importance to the automobile industry. Regarding this, the local/global formability map, expressed by true fracture (or thickness) strain and uniform strain, has recently been introduced to present intrinsic local and global formability of steels simultaneously. In this study, an improved form of the local/global formability map is proposed in combination with limit strain models. Isolines of tight-radius bending limit and hole expansion ratio at arbitrary intervals are superimposed on the existing map, thereby enabling designers of stamping parts to easily confirm whether their material selections are appropriate for concerned part geometries, or vice versa.

In order to improve and enhance the solution accuracy and practicability in prediction of dynamic recrystallization kinetics, we present a direct method to characterize the flow-related DRX parameters including two essential parameters when predicting DRX microstructural evolution which are the peak strain and the strain at 50% recrystallization. In this method, flow curves are described accurately using the general and improved C-m models in which C and m are defined as functions of strain rate at varying strains and temperatures. It eliminates the need for mathematical modeling of these parameters in order to predict the volume fraction of DRX grains and increases the practical utility of microstructural prediction by a great deal. The DRX behavior of extruded 42CrMo high-strength steel is investigated with a finite element approach using the direct method and the Avrami kinetic model. Our present method predicts the DRX kinetics volume fraction and grain size during microstructural evolution with remarkable accuracy when compared to other approaches and experiments. DRX Microstructural predictions are greatly enhanced by this method since it eschews mathematical modeling of the peak strain and the strain at 50% recrystallization in Avrami kinetic model.

Formability evaluation of the laminated polymer-metal films is important for manufacturing the pouch type Li-ion battery cell. In this work, the Marciniak-Kuczynski (M-K) model was enhanced to predict the forming limit of the multi-layered films used for the battery pouch. A rate-dependent Swift hardening law and anisotropic Hill48 yield criterion under the non-associated flow rule were adopted to accurately describe the plastic deformation of each layer. By assuming the iso-strain condition and perfect bonding between layers for the simplicity of modeling, the M-K model was re-formulated with the unknown variables such as strain increment and strain ratios in the groove zone based upon the compatibility condition and force equilibrium. In order to experimentally verify the predictive accuracy of the developed model, the calculated forming limit curve (FLC) was compared with the experimental FLC of a three-layer AA5182-O/Polypropylene/AA5182-O (AA/PP/AA) sandwich sheet. The results demonstrate the high accuracy of the proposed enhanced M-K method. In addition, based on the established modeling framework, the effect of each layer and its thickness on the formability of the laminated film were investigated for a nylon/AA/PP sandwich film for the battery pouch.

A methodology is described for obtaining optimum biaxial cruciform specimen designs by combining equibiaxial tension test data with finite element analysis (FEA) in conjunction with the application of an appropriate optimization strategy. This approach is applied for developing optimum specimen designs for an ASTM A1008 steel, a SS 316L, and an AA 6xxx-T4 alloy. This methodology requires in sequence the (a) selection of an appropriate initial biaxial specimen design and development of an equivalent, verified FEA model, b) optimization of the verified FEA models, (c) fabrication of cruciform specimens according to the optimized dimensions and testing under appropriate loading conditions, and (d) simulating the validation test with an equivalent FEA model with boundary conditions obtained from the tests. This study shows that such a combined “test-FEA-optimization” approach can be successfully applied to develop optimum cruciform specimen designs for advanced lightweighting materials.

Uniaxial tension is a universal material characterization experiment. However, studies have shown that increased formability can be achieved with simultaneous bending and unbending of the material. This so-called continuous bending under tension process is an example of bending stress superposition to a uniaxial tension process. In this research, experiments are conducted on stainless steel 304 to investigate the effects of bending stress superposition on the austenite to martensite phase transformation. Two vortex tubes are mounted to the carriage of the machine and used to decrease the temperature in a localized region of the specimen to evaluate two temperature conditions. The in-situ strain and temperature fields are captured using 3D digital image correlation and infrared cameras. The deformation induced $$\upalpha^{\prime}$$ α ′ -martensite volume fraction is measured at regular intervals along the deformed gauge length using a Feritscope. The number of cycles that the rollers traverse the gauge length, corresponding to the strain level, is also varied to create five conditions. The deformed specimens revealed heterogeneous martensite transformation along the gauge length due to the non-uniform temperature fields observed for each test condition. Decreasing the temperature and increasing the number of cycles led to the highest amount of phase transformation for this bending-tension superposed process. These results provide insight on how stress superposition can be applied to vary the phase transformation in more complex manufacturing processes, such as incremental forming, which combines bending, tension, and shear deformation.

For some modern steels with a body-centered cubic (bcc) crystal structure, it is observed that both tensile strength and ductility are significantly improved with decreasing temperature, which motivates the exploration of the cryogenic formability and fracture properties of these materials. The temperature-dependent plasticity and fracture phenomena of a modern bainitic steels with the bcc structure have been investigated by performing a comprehensive experimental program and finite element simulations, covering a broad range of loading conditions. Uniaxial tensile tests have been performed at different temperatures along three loading directions. Tensile tests using flat specimens with various geometries, including shear, central hole and notched dog bone, have been performed along the rolling direction at room temperature and –196 ℃. An advanced non-associated constitutive plasticity model is used to describe the temperature-dependent strength and hardening properties of the material. The local critical stress and strain variables extracted from finite element simulations of different fracture tests have been used to calibrate a unified fracture criterion, which considers the stress state dependence. The effects of temperature on the plasticity and stress state dependent fracture behavior of the modern bcc steels have been quantitatively determined.

Due to the limited formability of ultra-thin titanium sheet at room temperature, it is difficult to form fine flow channels with increasing geometrical complexity of titanium bipolar plate. Premature occurrence of cracks is commonly observed in conventional room-temperature stamping process, restricting further improvement of the performance of PEMFC. In this work, hot stamping using an on-site resistance heating system is applied to ultra-thin titanium sheet to improve formability of titanium bipolar plate. It is demonstrated that fracture limits of ultra-thin titanium sheet at 700 ℃ are obviously higher than those at room temperature. Finally, a crack-free titanium bipolar plate with fine flow channels is exemplarily formed via hot stamping process.

Recently, Cazacu [1] derived the expression in terms of stress invariants of the Yld91 yield function [2]. This enables the determination of the anisotropy coefficients analytically and makes possible the investigation of the influence of the data subset used for identification on the predictions. In this paper, we conduct such a study for AA 7079 extrusion. It is shown that if Yld91 and respectively Hill’s parameters are obtained analytically using the same data subset, similar predictions of the anisotropy in uniaxial tension are obtained. However, there are distinct differences in terms of the predicted yield surface evolution with the level of shear.

With a view of sustainability and the rising energy costs currently, manufacturing processes of metals are becoming increasingly focused on optimizing process parameters such as energy and time consumption. A conventional hot-forming process route currently involves casting an ingot, letting it cool down, and heating it up again for the hot-forming process (see Fig. 1a). In order to implement the combination of casting and forging, avoiding the reheating cycle and using less energy, by utilizing the casting heat (see Fig. 1b), a methodology was developed within the present work to quantify the influence of the resulting microstructure as a function of the cooling rate on the forming and recrystallization behavior (see Fig. 1c).For this purpose, AISI 301 austenitic stainless-steel cast samples with different cast cooling rates were generated. An in-situ high-temperature microscope is used to determine the holding time and the heating rate. Dilatometer tests are performed to characterize the interaction between initial microstructure and the flow curves to verify the determination method (see Fig. 1d). The aim was to demonstrate whether the microstructure evolution and mechanical behavior is affected by the initial microstructure. The flow curves and the post-forming microstructure show a higher degree of recrystallization in fast-cooled microstructure than slow-cooled microstructure. Hence, it was found that the initial microstructure and the associated temperature history does have an impact on the mechanical properties.

Thermo-mechanical uniaxial tensile tests on sheet metals have been widely carried out to characterize the mechanical behavior of the materials at high temperatures. Because of nonuniform temperature along gauge length, however, nonuniform deformation occurs from the beginning of deformation, leading to challenges to determine stress–strain curves. In the present study, a robust method for determining true stress–true strain curves has been proposed; in this method, true strains are obtained within a local area in which fracture occurs, and true stresses are computed based on the engineering strains within this local area. Uniaxial tensile tests on aluminum alloy AA6082 sheets under hot stamping conditions have been carried out, with the strain measurement using the digital image correlation (DIC) technique. The onset of necking in those tests has been determined using the spatio-temporal method. Then the data before the necking have been analyzed and the robust method has been applied to determine the associated true stress–true strain curves. The true stresses computed using the robust method have also been benchmarked against those computed using the standard method. It concludes that this robust method is able to accurately determine the true stress–true strain curves at high temperatures.

In the present work, an analytical framework for the computation of the final height of cylindrical cups produced by multi-stage deep drawing operations is proposed. Its capabilities of making accurate predictions are demonstrated by means of an application example.

In this paper a novel model for 3D finite element sheet metal rolling calculations is presented. A global model which represents the behavior and stress state of the strip outside the roll bite is coupled to a local model which represents in detail the mechanics of deformation within the roll bite. Shell finite elements are used for the global model of the rolled sheet, while 2D plane strain elements are used for the local model of the strip and the roll. The coupling is made via an equivalent roll bite model, incorporated into the shell model to represent the physics of the roll bite. The rolling velocity, the zero out-of-plane position and the thickness strain are enforced at the roll line via a set of constraint equations. The amount of prescribed thickness reduction is determined based on the local tensions and friction coefficient. A metamodel which provides the relation between these conditions and the local thinning is obtained from 2D off-line rolling calculations. The proposed model can be used instead of a full 3D rolling model, as it is computationally less expensive, especially for thin strip rolling simulations. It is shown how the developed model can be applied to analyze instability phenomena in cold rolling processes, specifically strip buckling due to disruptions in the process conditions.

The in-plane torsion test is more frequently used in various research facilities to obtain flow curves and the Bauschinger-coefficients of sheet materials. The execution and the experimental setup of the test currently depend on the experience, available machines as well as tools at the respective testing laboratories. For this reason, the author’s aim is to standardize the in-plane torsion test in order to create a common basis for carrying out the tests and to enable the characterization of comparable material parameters. The shape and structure of the inner clamping are often selected based on experience without knowing the exact effects on the process. Although the clamps appear less relevant, they are the only component in contact with the specimen and have a major influence on the process limits of the in-plane torsion test, such as wrinkling and slippage. To provide such a base, the shape and the surface structure of the clamping on the process limit of slipping are investigated. Results show that a ring-shaped clamping surface can transmit up to 50% more torque at the same clamping force compared to a full-circle clamping surface. It is derived analytically that torque transmission through plain clamping surfaces is not possible for a part of thin sheets (t = 0.5 and 3.0 mm). For such cases, structured clamping surfaces were used to enable successful torque transmission.

A fully-explicit stress integration method is comprehensively discussed via finite element analyses using three different classes of continuum plasticity models, i.e., anisotropic yield function, anisotropic hardening, and continuum damage model. A unified but straightforward formulation covers a wide range of rate-independent plasticity models. High accuracy, numerical robustness, and great efficiency are achieved through the use of generalized effective plastic strain and stress projection equations. In order to estimate the stress integration quality, an associated variable mapping technique, namely, the precision map, is introduced. Based on the precision map study, the current fully-explicit integration exhibits excellent accuracy. Furthermore, this non-iterative stress calculation reduces the computation cost by about 50% compared to conventional iterative stress update algorithms.

Formability or plasticity of light metals, such as aluminum alloys and titanium alloys, are quite strain rate sensitive. For most normal plastic forming processes like forging, stamping, rolling and extrusion, their strain rates are usually at medium strain rates between 10–2 s−1 and 102 s−1, the formability becomes lower with increasing of strain rates. While at very low strain rates, the formability is often much higher, especially between 10–5 ~ 10–3 s−1, at which super-plasticity is found for many metals and alloys. Spring-back is quite normal for sheet and tube forming, it is very small when the strain rate is lower than 10–3 s−1, while it is usually very high when the strain rate is higher for medium strain rates (10–2 s−1 and 102 s−1), especially for titanium alloys. It has been found that there exists a range of strain rates around 1 × 103~5 × 103s−1 for many light metal alloys that the formability is increased remarkably, and the spring-back is also quite low. In this paper, variations of the formability and the spring-back with varied strain rates have been investigated, The research aims to find a suitable strain rate range with remarkable increased formability and lower spring-back, so that many sheet and tube parts of light metals can be formed with sufficient formability and very small spring-back at room temperature.

Sheet metal forming is an important manufacturing process widely used to produce complex stamped parts from flat sheet stock in industries such as automotive and packaging. Due to the global economic climate, these industries need to be highly competitive by reducing production costs and increasing process efficiency. Numerical simulation combined with sheet metal forming expertise is one of the technological innovations adopted to meet these requirements by reducing the traditional time-consuming and costly testing steps. With the progress of finite element simulation, questions about the accuracy or limitations of the type of material description adopted have become particularly important. The influence of the plasticity model is examined in this work by a numerical study using the hole expansion test. This work first presents the yield locus criterion adopted and developed by Tata Steel, which is hereafter referred to as the Tata Steel material model. Hole expansion tests are performed at different hole diameters and the results are compared with FE simulation. The simulations are performed with the finite element software Autoform R11 in which the yield criterion proposed by Abspoel & Scholting [1] has been implemented. The discussion therefore focuses on the influence of the material model on the numerical predictions and its accuracy based on the optimization of the different material parameters measured.

Finite element simulations of sheet metal forming processes achieve high precision of the process model. Previous works [1] on the impact of the selection of the material law on the prediction accuracy of the formed results have shown that more sophisticated yield functions allow better prediction compared to von Mises or Hill 48 models. However, this choice increases the cost of calibration, as it requires extensive material testing, and in some cases, crystal plasticity modeling predictions. In this work, the selection of the material model is investigated with respect to the accuracy of the force prediction in single point incremental sheet forming (SPIF) simulations. It is shown that the calibration of the material model using shear tests instead of tensile tests can satisfy the required accuracy of SPIF force prediction. This study investigates the accuracy and computation cost of using only von Mises yield surface and an isotropic hardening model versus Hill 48 and Barlat Yld2004-18p yield criteria. AA7075-O cone parts are used to compare the force prediction results.

In this study, a new multi-axial vibration assisted incremental sheet forming (MV-ISF) process is introduced by developing a vibrated toolpath and an ellipsoidal headed tool design. The process has been shown to improve the formability of tested materials due to the introduction of a low-frequency and high-amplitude vibration field. To understand the forming characteristics of the MV-ISF process, the geometrical accuracy, surface quality and formability in forming a conic geometry are investigated through experiments and finite element analysis using two aluminium alloys and compared with that formed using the conventional ISF tool path design. An analytical model of the material deformation under the vibration condition is also proposed to assess the enhanced formability of the MV-ISF process. The new vibrated tool path design combined with the use of the ellipsoidal headed tool provides a simple and potentially effective solution for improving the forming accuracy, surface quality and formability of ISF manufactured parts.

This research investigates the anisotropic-asymmetric hardening behaviors of different types of metals by experiments and analytical modeling. Experiments are conducted at shear, uniaxial tension, plane strain tension and equibiaxial tension states to characterize the anisotropic hardening behaviors of aluminum alloy 7075-T6 and magnesium alloy AZ31. The strain hardening behaviors are compared between different loading directions and stress states. An obvious anisotropic-differential hardening evolution is observed for AZ31. The anisotropic-asymmetric hardening behaviors are characterized by a newly proposed stress invariants-based function with isotropic and anisotropic forms. A newly linear transformation tensor is introduced based on that of Barlat et al. (1991) to extend the isotropic stress invariants-based function to an anisotropic form. The comparison shows that the anisotropic-asymmetric hardening behaviors are precisely modeled by the proposed yield function under different stress states of shear, uniaxial tension, plane strain tension and equibiaxial tension, and different loading directions. The extended anisotropic form is recommended for strongly anisotropic materials such as AZ31 due to its less computationally cost while maintaining high accuracy.

In this paper, the virtual field method (VFM) is used to identify the dynamic anisotropic plasticity of the sheet metal subjected to highspeed impact. First, the selected dynamic anisotropic plasticity constitutive models and the principle of the VFM-based characterization method are introduced. Then, a heterogeneous highspeed impact testing prototype using the double-notched specimen configuration is proposed and the virtual inertial impact test carried out. The simulated strain and acceleration field data are extracted and analyzed, from which the target dynamic anisotropic plasticity constitutive parameters are retrieved. The results show that using the current identification scheme the anisotropic yielding and the rate-dependent hardening parameters can be simultaneously identified from the heterogeneous inertial kinetic fields with a single impact test. The proposed identification method is exempt from the one-dimensional stress wave propagation and homogeneous deformation state preconditions and can significantly simplify the identification process of the dynamic anisotropic plasticity of sheet metals.

Focusing on non-proportional loading condition during forming process of HCP metals, the equivalent plastic work contours of AZ31 Mg alloy are investigated experimentally under combined biaxial test. Strong anisotropy including initial strength differential effect and subsequent distortional hardening are detected. The underlying deformation mechanisms are discussed based on microstructure observation. It is found that the specimen undergoes the main plastic deformation and high stress state along the direction with larger strain ratio. Under the combined load of tension and compression, the c-axis change of the grains in the sample is mainly the deflection along the normal direction, which results in the deformation dominated mainly by dislocation slip. The difference in deformation mechanism leads to different evolution of equivalent plastic work contour in four quadrants.

In the last few years, there has been a growing interest in the sheet metal forming community for analyzing the enhancement of formability attained in non-proportional forming processes such as incremental sheet forming (ISF), and especially in its dieless variant single point incremental forming (SPIF). To this regard, this analysis using classical forming limit diagrams (FLD) do not provide enough information for understanding the conditions upon which this postponed failure is attained. As an alternative, some recent studies made use of equivalent strain versus stress triaxiality diagrams, which proved to be more suitable for evaluating the non-proportional strain paths that lead to failure in SPIF.On the other hand, flanged parts are commonly used in the aircraft industry to provide stiffness to the manufactured components as well as a target area for the assembly to other metallic parts. To this regard, the authors have recently carried out research works aiming investigating the formability and failure modes of AA2024-T3 sheet 1.2 mm thickness in stretch and shrink flanging by SPIF. These studies were carried out combining experimentation and numerical simulation using Finite Elements (FE), with the aim of evaluating and assessing the different modes of failure that occurred.In this context, the present contribution consists on a numerical evaluation of failure by using a FE modeling of the stretch flanging process in LS-DYNA. To this purpose, Barlat’s anisotropy is considered for the assessment of the forming limit at fracture within the material equivalent strain versus stress triaxiality diagram. The resulting numerical model, calibrated using principal strains experimental results, allows predicting the sheet material fracture and the mode of failure attained, which can be either failure by fracture at the flange edge or at the flange corner depending on the set of process parameters.

A 5-layer laminate composite was produced by roll-bonding at room temperature of a stacking of ARMCO steel and commercial purity aluminum alloy (AA1050). The large rolling draught led to heterogeneous deformation across the thickness of the layered aggregate, which was investigated using finite element modeling. The evolution of microstructure and crystallographic texture was probed using EBSD. Intense localized shear inside the AA1050 layers gave rise to continuous recrystallization, whereas a rolling texture developed in the adjacent iron layer. Both were reproduced using crystal plasticity modeling.

Uranium aggregates accumulate large plastic deformation during thermal cycling (ratcheting). This process is the result of highly anisotropic single crystal properties, texture, and a complex coupling between thermal, elastic and plastic mechanisms of deformation. Here we present a thermo-elastic-viscous crystal plasticity model and apply it to the prediction of thermal ratcheting in clock-rolled uranium. The model captures the plastic relaxation of intergranular stress induced by thermal cycling and leads to an understanding of the coupling of the deformation mechanisms in uranium. Results are compared with ratcheting measurements.

A dual-phase 980 steel sample consisting of hard martensitic and ductile ferritic phases was investigated in terms of its nonlinear unloading behavior and the Bauschinger effect. The microstructural characters obtained from a set of electron back-scattered diffraction scans were used to analyze the crystallographic texture for each phase. The incremental elasto-visco-plastic self-consistent model was applied with an empirical Voce hardening law. Neither backstress nor the effect of geometrically necessary dislocation was accounted for. Despite the simplicity of this baseline $$\Delta $$ Δ EVPSC model, the nonlinear unloading and the Bauschinger effect were captured. It seems that the contrast in the critical resolved shear stress of the two phases suffices to induce earlier and selective re-yielding of ductile ferrite.

This paper explores springback levels in AA6016-T4 sheets that were first pre-strained to various levels in uniaxial, plane-strain, and biaxial tension, then subjected to a pure bending operation and released. Finite element modeling of the pre-strain and bending steps was performed using both isotropic and elasto-plastic self-consistent (EPSC) crystal plasticity approaches. Because the EPSC model incorporates backstresses informed by GND content, the springback predictions were far more accurate than those of the isotropic model. In the unstrained base material, the EPSC predictions had a maximum error of 7% versus experiment, compared to 49% for the isotropic approach. The EPSC model accurately predicted the transition from springforward to springback as a function of applied pre-strain, but only when the backtress term was included in the hardening law. The backstress contribution was also found to be increasing influential in predicting springback as pre-strain levels increased, especially in the case of a uniaxial tension pre-strain.

Professor Lihui Lang is an internationally renowned scholar in the field of metal forming technology. He was deputy director of Youth Work Committee of China Society for Technology of Plasticity, editorial board members of many scientific journals and chair of many international conferences. During his more than 20 years’ research in theories, technologies and equipment in hydroforming and powder hot isostatic pressing field, he has published about 10 textbooks and monographs, more than 300 academic papers, obtained more than 40 authorized invention patents, and cultivated nearly 100 doctoral and master students. Based on these contributions, he won international and national awards 3 times, including the Thomas Stephen Group Prize and second prize of Chinese National Award for Technological Inventions.

A flexible medium hydroforming and curing (FMHC) process for fiber metal laminates (FMLs) was proposed and utilized to manufacture FMLs parts with complex structures. The formation mechanisms of wrinkling defects in the FMHC process of complex FMLs structure were studied through both numerical simulation and experimental methods. The results show that the compressive stress will occur at the T-shaped cavity when forming complex FMLs parts by FMHC process, which will cause wrinkling defects. To further eliminate the wrinkling, the steel-plate assisted flexible medium hydroforming and curing (SPAFMHC) process was proposed and validated. Industrial CT scanning technology was used to further analyze the distribution of internal defects in the FMLs parts formed by SPAFMHC. The results show that the defects are mainly concentrated in the corner of the ∞-shaped cavity, and the internal defects of the FMLs parts account for 4.7% of the total area of the part; The defects position was cut and observed. It is found that internal porosity and delamination are the main forms of internal defects.

The diffusion bonding process can take full advantage of 2198 Al-Li alloy to produce high-efficiency integral structures. However, the dense and stable oxide layer on the surface to be bonded seriously hinders the diffusion bonding process. The suitable surface treatment method was selected to remove the oxide layer. Furthermore, the effects of bonding temperature, time and pressure on interface performance were studied. Optical microscopy (OM) and scanning electron microscope (SEM) are used to observe the bonded interface and fracture surface after the shear test. The maximum shear strength and strength ratio reach up to 131 MPa and 96%, respectively. In general, shear fracture surfaces account for the majority of bonded interfaces, which show ductile areas with dimples and brittle regions with quasi-cleavage behavior. This study indicates 2198 Al-Li alloy can be well bonded by suitable surface treatment and process parameters.

In this paper, Fast Gas Forming with in-die Quenching (FGFQ) of TC31 thin-walled components was developed to integrate gas forming and heat treatment together. This forming process consists of rapid resistance heating, fast gas forming, and in-die quenching. The heating rate and pressurization rate can reach up to 30 ℃/s and 20 MPa/s respectively. The whole forming cycle takes less than one minute, which can significantly improve the forming efficiency. The component possesses the best mechanical properties at the forming temperature of 1040 ℃ and the pressure of 15 MPa. The ultimate tensile strength of the component could reach up to 1292 and 837 MPa respectively at room temperature and 650 ℃, which are improved by 18.2% and 29.8% compared with the initial state. Moreover, the elongation of 11.2% and 16.5% could also remain. The rapid resistance heating and fast gas forming can effectively prevent prior β grains from coarsening during the forming process, which results in the formation of high-density fine martensite and therefore enhance the post-form strength. Also, the refinement of the martensite contributes to the great ductility.