Friction Stir Welding and Processing X

Dissimilar aluminum alloys 2024-T351 and 7075-T651 were friction stir welded utilizing a conventional tool and a bobbin-style tool. The welds produced with the conventional tool yielded higher mechanical properties than those produced with the bobbin tool, and fracture of all tensile specimens occurred on the 2024 side of the weld regardless of the tool or weld configuration. Temperature data and modeling demonstrated that the temperature distribution from either tool skews toward the advancing side. Ultimately, the mechanical properties and hardness profiles across the welds correlated with the temperature distribution and the associated precipitation behavior of the alloys. Optical microscopy revealed distinct layers of the alloys interwoven within the stir zone and identical grain sizes in both alloys. Grain boundary orientations from the stir zone followed the Mackenzie plot, suggesting complete recrystallization and a lack of texture within this zone.

Strong dissimilar material welds (DMW) of PA66 and 6061 Al was produced by friction lap welding (FLW) at welding speeds as high as 5 m/min. The temperature difference at various locations of the welds did not affect the local joining strength. In an attempt of explaining the joint strengths, special samples were made by evaporation of aluminum oxide onto a polyamide 66 (PA66) substrate to form metal/polymer interface. X-ray photoelectron spectroscopy (XPS) analysis showed that the key chemical bond developed across the PA66/alumina interface is of Al–O–C type which would have very likely contributed to good joint strengths in such metal/polymer joints. Elevated temperatures are not essential for formation such chemical bonds as long as a fully intimate atomic contact between the PA66 and Al plates can be achieved.

Ultrasound Enhanced Friction Stir Welding (USE-FSW) is an innovative hybrid method for solid-state joining. This process resulted in remarkably positive findings in the successful realization of hybrid aluminum/magnesium joints as well as in first investigations on the microstructure of aluminum/steel joints due to the parallel and synchronous transmission of power ultrasound into one of the joining partners. The present work investigates the impact of additional power ultrasound on the mechanical properties of AA6061/SAE1006-joints by comparing FSW and USE-FSW. Therefore, light microscopy, as well as scanning electron microscopy, was carried out for examining the microstructure of the joints. Furthermore, mechanical tests on the microhardness of the weld zone of the joints as well as tensile and first fatigue tests were examined. The investigations proved an influence of the power ultrasound by a change in the morphology of the nugget. It showed to be more cleared up and also contains a thinner intermetallic phase of FeAl₃ at the interface aluminum to steel. Furthermore, an increase in the tensile strength of the joints of about 15% could be observed. First stepwise load increase tests resulted in slightly different stress levels for the estimated fatigue limit.

Dissimilar metal joints tend to fracture along the welded interface during tensile testing, particularly in butt joint configuration. A common explanation relates formation of brittle intermetallic compound layer at dissimilar weld interface to crack initiation and propagation. This typically leads to lower strength and ductility of the dissimilar material joint. However, another critical aspect determining strength and fracture behavior of dissimilar material joints is the existence of stress concentration at the welded interface during mechanical loading. Mismatch of elastic modulus of dissimilar materials creates stress concentration at the initial stage of mechanical loading, which facilitated crack initiation at the welded interface. In this overview, factors leading to stress concentration and their impact on dissimilar joint strength and fracture behavior have been highlighted.

Composite materials consisting of a titanium alloy reinforced with continuous silicon carbide fibres, called TiSiC, are currently being investigated to enhance performance for applications where titanium alloys are used. Conventional fusion welding techniques create difficulties due to the detrimental impact of high temperature on the fibre/metal interfaces. This study describes the application of stationary shoulder friction stir welding (SS-FSW) technique to join TISIC components to monolithic titanium. Microscopic investigations had shown flaw-free welding, until the SSFSW tool started to mechanically interact with the SiC fibres. When subjected to tensile testing, the weld properties were superior to the parent monolithic titanium. The comprehensive investigation of fracture toughness, residual stress and fatigue properties of the weld components are presented and potential advantages discussed.

Wear affects shape and size of the friction stir welding (FSW) tool, and leads to unexpected weld properties and shorter tool life. Understanding wear mechanisms during FSW is important to prevent or reduce tool wear and ensure longer tool life for joining of high melting point metallic (HMPM) materials. Severe tool wear is a consequence of extreme thermo-mechanical environment around the tool during welding. The macroscopic and microscopic investigations of wear mechanism are conducted by performing 3D profilometer and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). Analysis of the scratch formation on the tool surface is used to ascertain the presence of abrasive wear mechanism. Adhesive wear is confirmed by investigating the tool–workpiece interface layer, which features diffusion of copper. This understanding of tool wear mechanism during FSW of high melting point metallic materials will enable selection of better tool materials and improved weld properties.

Friction stir lap welding (FSLW) with linear speed of 1 m/min or higher can open up opportunities for industrial implementation. This paper will provide an overview of several challenges and mitigation strategies to produce effective joints between several Al alloys. FSLW of Al alloys at high speed presents challenges including interface hook features and defects at the interface. 5xxx, 6xxx, and 7xxx Al alloys were joined and characterized. Load-bearing capacity in excess of 80% of base material strength has been demonstrated at the welding speed of 2 m/min for 6xxx and 5xxx similar lap joints. The effects of parameters like welding speed, RPM, tool geometry in material flow features, void volume, and hook geometry is investigated and this knowledge base is applied to improve the joint strength of 7xxx lap welds.

The aeronautic industry is continuously looking for new structural concepts with the aim of reducing dangerous gas emissions as well as reducing manufacturing costs and times. The development of advanced lightweight structures is an effective alternative to achieve the mentioned goals. Reinforced panels produced by the third generation aluminum–lithium alloys and Friction Stir Welding (FSW) can bring new solutions for more efficient aircrafts. This work presents the results obtained in the development and characterization of FSW joints directed to reinforced panel manufacturing. FSW lap joints were produced using aluminum–lithium alloys AA2099-T83 extrusions and AA2060-T8E30 sheets. Several welding parameter combinations and FSW tool designs were used to produce the joints. Joint properties were investigated by metallographic examination, microhardness tests as well as mechanical strength testing. The appropriate FSW conditions to optimize joint properties were established.

The effect of process parameters on temperature distribution in stationary shoulder friction stir welding (SSFSW) and conventional FSW of AA7010-T6 alloy are studied using a three-dimensional heat conduction analysis. The computed results are validated from experimentally measured results reported in independent literature. The tool torque, traverse force and the mechanical stresses on the FSW tool were evaluated analytically using mechanics based principles. The estimated results showed that the tools used in the SSFSW process were more likely to early failure.

The material bonding defects such as root flaws have been an important kind of welding defects in friction stir welds. In recent years, the growing application of friction stir welding (FSW) in fabricating many critical components, e.g., rocket fuel tank, renewed the need for in-depth understanding for the formation of material bonding defects. This study provides a quantitative investigation to access the material bonding behavior via numerical simulation. It is found that the rapid growth of the bonded fraction is developed owing to the localized thermal-mechanical processing during FSW. The thermal-mechanical condition at the root region is deteriorated greatly as the tool rotation rate decreases. The validity of the simulation results at different welding parameters is confirmed by our microstructural observation. The concepts that we develop open up quantitative prospects for the elimination of defects due to insufficient solid state bonding in FSW and similar material processing approaches.

The objective of this study is to predict the thickness of intermetallic compound at the weld interface of dissimilar friction stir weld of Al 1050 and copper. The mechanical properties of the dissimilar friction stir weld are significantly affected by the intermetallic compounds formed during the process. The formation of intermetallic depends on the concentrations of the dissimilar materials, which are determined by their diffusion across the weld interface. A numerical model is developed which consists of Fick’s second law based diffusion model in conjunction with a thermo-mechanical model. The numerical model captures the movement of the interfaces between intermetallic species due to the diffusion of the Al. A representative friction stir butt weld is performed with Al 1050 alloy and pure copper. The thickness of the intermetallic layer at the weld interface is determined by scanning electron microscopy and energy dispersive spectroscopy mapping of the weld cross sections. Predicted intermetallic compound thickness is compared well against the experimental observation.

Key advancements in friction stir welding and processing (FSW&P) have been chronicled in these biennial symposia. Insights gained through fundamental and applied research published in symposia proceedings hold significant value in maturing and furthering the development and deployment of FSW&P. However, not all of this research can be replicated scientifically to enable this purpose due to insufficient information provided in the published articles. Providing more complete process information in symposium papers will serve to advance broader acceptance of FSW&P by industry and regulatory agencies. This objective can be facilitated by including appropriate detail required by national and international standards in published research. Without such detail, the maturity of FSW&P will remain in question throughout the different industry sectors due to a lack of uniformity and consistency in published results. A study carried out in coordination with the Metallic Materials Properties Development and Standardization (MMPDS) handbook steering committee illustrates the value of such discipline. The study was undertaken to investigate the potential for developing design data for FSW. Also, because successful implementation of FSW&P is reliant upon understanding and controlling the local metalworking conditions around the weld tool (both thermally and mechanically), utilizing process feedback signals is needed to confirm the consistency and effectiveness, and thus the maturity, of these technologies to organizations and agencies charged with quality assurance.

Precise temperature control of FSW benefits from control and manipulated signals that are responsive and smooth. Accurate measurement of tool temperature and spindle speed feedback are important to temperature control, but often noise in these signals prevents optimal control. Two different methods are developed in this paper to improve signal quality. A series of Bezier curves are used to compensate signals which exhibit a periodic but arbitrarily-shaped offset. Least-squares fitting is used to obtain quality derivatives from discrete or noisy signals. The Bezier method is used to decrease the inaccurate temperature fluctuation measurements reported by telemetry collar error and adds no time delay or phase shift. The least-squares approach is used to estimate spindle speed and temperature derivatives and adds only minimal time delay while substantially reducing noise.

The cost limitations of post-weld inspection have driven the need for in situ process monitoring of subsurface defects. Subsurface defects are believed to be formed due to a breakdown in the intermittent flow of material around the friction stir tool once per revolution. This work examines the intermittent flow of material and its relation to defect formation. In addition, advances have been made in a force-based defect detection model that links changes in process forces to the formation and size of defects. A range of aluminum alloys has been examined, showing that softer aluminum alloys produce less distinct changes in process forces during defect formation and harder aluminum alloys produce more distinct changes when using the same tool geometry.

Copper-graphite composites wires are manufactured by a novel friction stir processing named Shear-Assisted Processing and Extrusion (ShAPE). Two types of precursors have been prepared respectively: a blend of copper and graphite powder; solid copper cylinders having pre-drill holes filled with graphite powder. The precursor material was consolidated and extruded in one step by ShAPE. Up to 800 mm long defect-free wires were produced. The metallographic inspection on both transverse cross-section and longitudinal cross-section confirms the good integrity of the ShAPE Cu-graphite wires. Energy dispersive spectroscopy and electron backscatter diffraction indicate the graphite particles were reduced to sub-micro size and uniformly dispersed in the copper matrix. The ultrafine graphite particle inhibits the grain growth thus improving the hardness. The processing temperature is below 550 °C which is much lower compared to conventional manufacturing methods.

Friction stir dovetailing (FSD) was used to join 0.5 in. (12.7 mm) AA7099 to 0.5 in. (12.7 mm) Ni-Cr-Mo steel in a lap configuration. Two new FSD approaches are reported that significantly reduce zinc embrittlement of Fe–Al intermetallic compounds (IMCs) which form during conventional friction stir welding (FSW). The first method uses the general FSD approach where a custom designed tool is employed to extrude the AA7099 into the pre-machined dovetail groove of underlying steel by forming mechanical interlocking and metallurgical bonding simultaneously. The second method uses a two-step approach where FSD of AA6061 is first used to form a silicon-rich Fe-Al IMC within the dovetail groove. AA7099 plate is then joined to the AA6061 within the dovetail using conventional FSW. A discussion of the new FSD technique, joint configurations, and process parameters are provided along with joint microstructural analyses and mechanical performance.

During corner AdStir fillet stationary shoulder friction stir welding (FSW), filler material with surface oxide is fed into the stir zone. In this study, the material flow of the filler material and surface oxide layer during the process was investigated by microstructure observation. To visualize the material flow, a marker insert technique was employed. Similar to conventional FSW, fine and equiaxed grain structure was observed in the stir zone. EBSD investigation revealed that the material flow was governed by the simple shear deformation induced by the rotating probe. The filler material was widely distributed in the stir zone, suggesting that some amount of the filler material moved downward due to the vertical material flow. The initial surface layer on the filler material was finely broken up by the material flow, achieving metallic bonding between the filler wire and the plates. Any harmful effects by adding the filler material were not found in the mechanical tests in this study.

In this work, we employed a unique solid-state joining process, friction self-piercing riveting (F-SPR), to join carbon fiber composites to the low-ductility magnesium alloy AZ31B. The localized frictional heat generated between the rotating rivet and the underside of the magnesium sheet softened and prevented crack generation in AZ31B. A consumable joining rivet was designed to join the selected material stacks by F-SPR. Lap shear tensile testing was used to assess the joint quality of specimens produced by F-SPR. The joint interface from the cross-sectioned F-SPR specimen was evaluated by optical microscopy.

Cu-based binary and ternary immiscible alloys were synthesized from elemental powders via friction stir processing (FSP) as a pathway to obtain thermally stable bulk nanostructured alloys with forced miscibility. The processed alloys were characterized using scanning electron microscopy (SEM). High magnification SEM confirmed the formation of forced mixing in the friction stir processed layer. Forced miscibility in immiscible alloys systems was possible due to high temperature intense severe plastic deformation during FSP. Mixing characteristics in Cu–Ag–Nb and Cu–Fe immiscible alloys were carried out and a mixing mechanism was proposed. As-processed alloys exhibited hardness in the range of 215–320 HV0.3.

Stationary shoulder friction stir processing (SSFSP) as a low heat input grain refinement technique is projected in this study. SSFSP can be considered as a variant of friction stir processing (FSP) with modified tooling system. It uses stationary shoulder tool and rotating probe, which helps to reduce heat input in great manner during process. Present work aims to refine grain size in thick AZ31B magnesium alloy using SSFSP without using external cooling at different tool rotational speeds (700–1300 rpm). The smooth surface with little flash without any defect was obtained in all the samples, which had confirmed the wide processing range of SSFSP. Probe-dominated stir zone (SZ) achieved for all rotational speeds, which confirmed smaller temperature gradient throughout the SZ thickness. SZ produced at the lowest rotational speed (700 rpm) exhibited reduction in grain size and subsequently enhancement in mechanical properties (hardness and tensile).

Friction Stir Processing (FSP) is a novel solid-state processing technique for fabrication of high strength surface composites. In present study, FSP was used to compare the cold formability of individually reinforced, hybrid and reference FSP samples of aluminum alloy Al5083. A plate of alloy containing MultiWall Carbon NanoTubes (MWCNTs) and boron carbide particles (B₄C) was processed by FSP and characterized. FSP composite containing MWCNTs was found to fracture during the bend-ductility test, while boron carbide particles reinforced FSP composites had superior cold bending formability along with the reference FSP sample. Cracking was also observed in hybrid FSP composite samples in lesser extent as compared to individually reinforced MWCNTs FSP composite. Possible cause of failure was identified as clustering of MWCNTs and weak interfacial bonding with the aluminum alloy matrix. Detailed metallographic and mechanical testing investigations revealed that the distribution of reinforcement at nanoscale and single pass processing played a vital role in generating defects and sinking of reinforcement particles in Al5083 matrix.

The service life of automotive components often depends on their surface properties. Consequently, improved surface properties with the retainment of bulk characteristics are necessary for such components to guarantee enhanced mechanical and tribological properties. In this research, friction stir processing (FSP) is used to produce surface composites characterized by extruded AlSi12CuNiMg matrix and micro and nano-sized Al₂O₃ particles as reinforcing phase. Multiple passes of FSP using two different strategies were applied to distribute the Al₂O₃ particles. The effect of the different FSP parameters and sequence of rotation direction for the applied passes was investigated. The processed surface layers were analyzed through optical and scanning electron microscopy, hardness, and wear testing. The properties of the processed composite surface showed to be affected by both the size of reinforcing particles and the processing direction sequence. A comparison between properties of the produced surface composites and the base metal was also carried out. Bench-type test developed to measure the weight loss of samples under sand erosion conditions.

In order to restrain global warming, automotive producers have adopted electric vehicle technology as one of the solutions to produce zero-emission cars as replacement for fuel combustion car. One of the challenges in production of the battery for electric vehicle is to weld thin electrode materials that have good electric conductivity, in multilayers configuration, which is challenging for conventional technologies. Refill friction stir spot welding is a spot-like joining process used as a nonconsumable tool to generate frictional heat during the process. Refill friction stir spot welding is able to weld various material combinations with good mechanical properties and surface quality. In this presentation, the capability of refill friction stir spot welding to join multilayer materials up to 80 layers of similar aluminum alloys will be presented. The presentation consists of the mechanical properties of the welds, electric conductivity/resistance of the welds and the temperature measurement data during welding process.

Joining between metal and polymer has attracted significant attention recently due to its advantage of great weight reduction and excellent integrated physical/chemical properties. In this study, specially designed biomedical additive manufactured porous TC4 titanium alloy plate was successfully joined to ultra-high molecular weight polyethylene (UHMWPE) plate by friction spot welding (FSpW). The z-axial load (F_z) evolution has been measured with load cell, and welding temperature (T_w) near TC4/UHMWPE interface has been measured with thermocouple. High tensile shear strength (~3000 N) has been realized through strong mechanical interlocking. Good macro-penetration of UHMWPE into TC4 porous structure (up to 80% filling rate) and sound micro-interlocking between metal and polymer were obtained. Relationship between T_w/F_z and joint quality has been unveiled for the fabrication of defect-less joints.

This study examines the connections between friction stir welding (FSW) parameters, the simple pseudo-heat index (PHI) metric, and the prediction of resultant residual stresses on AA5052-H32 plates. A range of weldments was produced with different tool rotational speeds (283–1732 RPM) and traverse speeds (200–800 mm/min) to produce the same values of PHI with distinctly different FSW conditions. Residual stresses were measured on the surfaces of the welded plates using x-ray diffraction. All of the friction stir welds produced the typical, M-shaped longitudinal residual stress profiles across the weld. The largest tensile stresses were produced for low PHI conditions, which did not fully consolidate the material. For sound welds, increasing traverse speed with fixed rotational speed did systematically increase the residual stresses inside the stir zone. However, the simple metric of PHI was not a good predictor of the stir zone residual stress.