Selected Topics in Manufacturing

This paper deals with micro mechanical machining process of 17-4 PH stainless steel samples fabricated by selective laser melting. An analysis of the material removal behaviour during micro-milling operations for the selection of the optimal feed rate value was performed on 17-4 PH additive manufactured samples studying the variation of the specific cutting force as a function of the feed per tooth. The transition from shearing to ploughing regime was analysed by considering the variation of the specific cutting forces. The minimum uncut chip thickness was calculated to identify the transition between the cutting regimes (shearing, ploughing or their combination) that affects the final product quality in terms of surface integrity and dimensional accuracy. Moreover, the surface roughness and the burr extension were analysed as a function of the feed rate.

In scheduling problems with fixed routing, usually the transportation of jobs among the machines is not considered (i.e., the transportation time between two stages is negligible, and the number of transportation resources is unlimited). However, in real contexts, this assumption can be unrealistic, especially when human supervision is needed for transportation, and hence not considering transportation can lead to low quality scheduling solutions. This paper considers a job shop in which transportation resources are limited and free to move among all the machines (no fixed routes). The aim is the integration of machine scheduling and transportation resource allocation, i.e., to decide for each machine the job sequence, and for each free transportation resource the routing. Due to the complexity of the problem, a Discrete Event Simulation approach is used to compare different scheduling and transportation resource allocation policies through scenario analysis.

Milling of thin-wall components often entails significant workpiece static deflections, which make manufacturers use conservative cutting parameters along the toolpath to meet the tolerance required. This paper presents a technique to define the 3-axis toolpath that maximizes cutting parameters, without compromising the accuracy of the component. This goal is achieved by coupling a FE model of the workpiece, updated to include material removal mechanism, to a mechanistic model of the cutting forces. The algorithm follows the milling cycle in the reverse order: starts from the finished part, computes the maximum allowable radial depth of cut, and adding material accordingly, generates the toolpath until the stock is build. The proposed technique has been experimentally validated through comparisons between milling tests and numerical results, both traditional and optimized toolpaths have been tested to assess accuracy, benefits and limitations of the method.

Energy efficient control policies that switch off/on machine tools aim to reduce the energy consumed while not producing parts. Commonly, a transitory is required before resuming the service; thus, to switch off/on the machine might be not advantageous. This paper analyzes a time-based threshold policy that disables/enables machine components with separated control commands instead of controlling the whole machine simultaneously. According to the subset of controlled components, multiple sleeping states and transitory duration can be defined. Machine idle times are assumed stochastic and the expected value of the energy consumed per produced part is reduced while assuring a certain target of machine utilization. A simulation optimization algorithm is used to search for the optimum. The analysis is based on two real CNC machining centers. Potential benefits are compared to state-of-the-art policies and discussed for a set of realistic numerical cases representing several production environments.

The effects of different reinforcement shapes on stability and repeatability of micro electrical discharge machining were experimentally investigated for Ultra-High Temperature Ceramics based on zirconium diboride (ZrB₂) doped by SiC. Two reinforcement shapes, namely SiC short fibres and SiC whiskers were selected in accordance with their potential effects on mechanical properties and oxidation performances. Specific sets of process parameters were defined minimizing the short circuits in order to identify the best combination for different pulse types. The obtained results were then correlated with the energy per single discharge and the discharges occurred for all the combinations of material and pulse type. The pulse characterization was performed by recording pulses data by means of an oscilloscope, while the surface characteristics were defined by a 3D reconstruction. The results indicated how reinforcement shapes affect the energy efficiency of the process and change the surface aspect.

WAAM (Wire Arc Additive Manufacturing) is a metal additive manufacturing process based on gas metal arc welding which enables to create large parts with a high deposition rate. WAAM is prone to the heat accumulation issue, i.e. a progressive increase of the workpiece internal energy due to the high heat input of the welding process, which can cause defects such as part structural collapse, uneven layers geometry or non-homogenous microstructure. A promising technique to mitigate such issue is to use an air jet impinging on the deposited material to increase the convective heat transfer. This paper presents an analysis of air jet impingement performances by means of a hybrid numerical-experimental approach. Different samples of a test case are manufactured using free convection cooling, air jet impingement and different interlayer idle times. Substrate temperatures are measured and compared with the results of a finite element simulation to assess its accuracy. The performances of air jet impingement are analyzed in terms of measured substrate temperatures and of simulated interlayer temperature, evaluated at the top of each layer. The results highlight that air jet impingement has a significant impact on the process, limiting the progressive increase of interlayer temperature compared with free convection cooling.

As the use of short fiber and textile thermoplastic composites is expanding in many industrial fields, particularly the automotive, it is necessary for each manufacturer in every sector to assess the mechanical characteristics and behavior of these materials in various loading and environmental conditions. Among the most difficult mechanical tests are those for calculating the shear properties and behavior of these materials. As a result, a variety of standards have been developed throughout the years. Among these, the most promising one may be considered the V-notched Rail Shear test as it incorporates the unique features of two different mechanical tests namely the Iosipescu and rail shear test. In the present work, this state-of-the-art mechanical test, originally designed for unidirectional composites, was implemented in different material architectures and its apparatus was modified and used for mainly two purposes; i.e. the investigation of the effect of the infusion direction on the mechanical properties of short and the warp-weft direction of the woven textile thermoplastic composites, as well as producing reliable data regarding the mechanical characteristics and the behavior of these materials for simulating the manufacturing process. The testing device was developed in a way to produce robust and accurate results, ensuring the alignment of its components as well as the stress uniformity in the section gauge. The results revealed a dependency of the infusion direction of the short fiber thermoplastics and the textile direction of the woven composites. To this end, this mechanical testing technique may be considered as a benchmark on the material characterization in shear deformation.

The manufacturing sector has to be able to manage high-variety and low-volumes per product, causing the adoption of a dedicated production system/cell to be unfeasible. In this context, reconfigurable pallets and flexible fixtures are enablers to manage product variety and volume variability. Namely, as a pallet is reconfigured, the associated part program needs to be verified to check for possible collisions between the tools and the new machining environment. An approach is proposed to verify the machinability of a pallet configuration given an existing part program. The approach grounds on an approximated collision check method exploiting a 3D representation of the machining environment (fixtures and parts). The approach is validated through an application to a realistic use case and the comparison with the results of a traditional collision check approach.

Selective Laser Melting of AlSi10Mg parts has a lot of applications in different fields such as aerospace and automotive for its abilities to fabricate components characterized by complex shapes, good mechanical properties and low porosity. One of the main drawbacks for its application is the obtainable surface roughness widely not suitable for functional requirements. Typically, the improvement is handled by secondary operations thus markedly increasing the production time and costs. In this paper the possibility to improve the surface roughness by tuning Selective Laser Melting process parameters is investigated. A design of experiments is carried out considering not only the common laser process parameters but also the changing of the contour, upskin and downskin strategy definitions. This way the attained results show a marked decreasing of the roughness for vertical and horizontal surfaces.

The paper presents a deep analysis of surface modifications induced by roller burnishing process of Ti6Al4V titanium alloy. The extensive experimental campaign has been performed based on a Design of Experiments at varying lubrication/cooling strategies (dry, cryogenic and MQL), roller radius, burnishing speed and burnishing depth. The resulting surface integrity has been analyzed in terms of surface roughness, micro hard-ness, microstructural changes and tribological performance. In particular, the wear rate of the burnished sample has been evaluated as a quality indicator of the process. The overall results show the influence of burnishing process parameters on surface quality and wear resistance of Ti6Al4V highlighting the capability of the process to significantly improve the above performance especially when cryogenic cooling is applied. Finally, the extensive experimental activity allowed to find a combination of processing parameters and lubrication conditions able to significantly improve the surface quality of the final component.

Products whose quality features deteriorate with the time they spend in the manufacturing system are common in industry. Usually, they are perishable products that loose their performance or value whenever their sojourn time in the system exceeds a certain threshold. A typical example is the food industry where such products need to be identified and scrapped. In other cases, perishable products can be reworked for restoring their functions when their sojourn exceeds a threshold. For instance, automotive painting processes suffer of dust settling on parts before the drying process and semiconductor wafers risk contamination between the cleaning and heating processes. This work develops an analytical method to evaluate the performance of systems with rework operations triggered by an extended residence time. The approach is based on the exact computation of the lead time distribution in production lines featuring general markovian machines. By exploiting the developed method, specific production policies aiming at minimizing the rework rate and maximizing the effective throughput in such systems are investigated. The method is validated within a real case showing high benefits through industrial applications.