The investigation of abnormal particle-coarsening phenomena in friction stir repair weld of 2219-T6 aluminum alloy

The single-pass friction stir weld of aluminum 2219-T6 with weld-defects was repaired by overlapping friction stir welding technique. However, without any post weld heat treatment process, it was found that the phenomena of abnormal particle-coarsening of Al₂Cu had occurred in the overlapping friction stir repair welds. The detecting results of non-destructive X-ray inspection proved that not only one group of repair FSW process parameters could lead to occurrence of the abnormal phenomena. And the abnormally coarsened particles always appeared on the advancing side of repair welds rather than the retreating side where the fracture behaviors occurred after mechanical tensile testing. The size of the biggest particle lying in the dark bands of ‘Onion-rings’ was more than 150 μm. After the related investigation by scanning electron microscope and X-ray energy spectrometer, three types of formation mechanisms were proposed for reasonably explaining the abnormal phenomenon: Aggregation Mechanism, Diffusion Mechanisms I and II. Aggregation Mechanism was according to the motion-laws of stir-pin. Diffusion Mechanisms were based on the classical theories of precipitate growth in metallic systems. The combined action of the three detailed mechanisms contributed to the abnormal coarsening behavior of Al₂Cu particles in the friction stir repair weld.     

Analysis of residual stresses in thick aluminum friction stir welded butt joints

Plates of aluminum alloys 2219-T62 were joined in a butt joint by friction stir welding. The residual stresses on the top and bottom surfaces were measured using the hole-drilling strain-gauge method. In the test specimen, it was found that the residual stresses on the top surface peaked at about 171 MPa, while the value reached 243 MPa for the weld with tunnel defect and had the conventional “M” profile with tensile stress peaks in the heat-affected zone. Those attached on the bottom surface had the inverted “V” profile with tensile stress peaks in the weld centre and the corresponding value was 99.4 MPa. Meanwhile, with the increase of rotary speed, the longitudinal residual stress decreased on the top surface, but increased on the bottom surface.     

Process optimization in the self-reacting friction stir welding of aluminum 6061-T6

Self-reacting friction stir welding (SR-FSW), also called bobbin-tool friction stir welding (BT-FSW), is a solid state welding process similar to friction stir welding (FSW) except that the tool has two opposing shoulders instead of the shoulder and a backing plate found in FSW. The tool configuration results in greater heat input and a symmetrical weld macrostructure. A significant amount of information has been published in the literature concerning traditional FSW while little has been published about SR-FSW. An optimization experiment was performed using a factorial design to evaluate the effect of process parameters on the weld temperature, surface and internal quality, and mechanical properties of self-reacting friction stir welded aluminum alloy 6061-T6 butt joints. The parameters evaluated were tool rotational speed, traverse speed, and tool plunge force. A correlation between weld temperature, defect formation (specifically galling and void formation), and mechanical properties was found. Optimum parameters were determined for the welding of 8-mm-thick 6061-T6 plate.     

Friction stir welding characteristics of different heat-treated-state 2219 aluminum alloy plates

Friction stir welding (FSW) of 2219-O and 2219-T6 aluminum alloys was performed to investigate the effects of the base material conditions on the FSW characteristics. The experimental results indicated that the base material condition has a significant effect on weld morphologies, weld defects, and mechanical properties of joints. In the 2219-O welds, no discernible interface exists between the stir zone (SZ) and the thermal-mechanically affected zone (TMAZ), and weld defects are liable to form in the lower part of the weld. In the 2219-T6 welds, there is visible interface between the SZ and the TMAZ, and a weld nugget with an “onion ring”-like morphology clearly exists. The defects are liable to form in the upper part of the weld. The strength efficiency of 2219-O joints is 100%, while that of 2219-T6 joints is only up to 82%. In addition, the two types of joints have different fracture location characteristics.     

Development of a process envelope for friction stir welding of DH36 steel – A step change

Friction stir welding of steel presents an array of advantages across many industrial sectors compared to conventional fusion welding techniques. However, the fundamental knowledge of the friction stir welding process in relation to steel remains relatively limited. A microstructure and property evaluation of friction stir welded low alloy steel grade DH36 plate, commonly used in ship and marine applications has been undertaken. In this comprehensive study, plates of 2000 × 200 × 6 mm were butt welded together at varying rotational and traverse speeds. Samples were examined microscopically and by transverse tensile tests. In addition, the work was complemented by Charpy impact testing and micro-hardness testing in various regions of the weld. The study examined a wide range of process parameters; from this, a preliminary process parameter envelope has been developed and initial process parameter sets established that produce commercially attractive excellent quality welds through a substantial increase in the conventionally recognised weld traverse speed.     

Effect of post-weld natural aging on mechanical and microstructural properties of friction stir welded 6063-T4 aluminium alloy

Influence of natural aging on mechanical and microstructural properties of friction stir welded 6063-T4 aluminium alloy plates was investigated through mechanical testing, X-ray diffraction studies, and transmission electron microscopy, for aging times up to 8640 h. Mg–Si co-clusters formed during the natural aging process resulted in an increase in strength, decrease in ductility, and occurrence of serrated plastic flow. Hardness increase from aging was fastest in welds obtained at higher tool rotational speeds due to greater amount of “quenched-in” vacancies from higher peak stir zone temperatures. Peak broadening analyses and classical Williamson–Hall plots were used to investigate the effect of friction stir welding and post weld natural aging on microstrain in different weld regions. Higher microstrain was found in stir zone as well as heat affected zone as compared to that for base metal, albeit for different reasons.     

Defect tolerance of friction stir welds in DH36 steel

Friction stir welding of steel is in the early stages of development. The aim to commercialise this process creates a trade-off between welding time, cost and quality of the joint produced. Therefore, it becomes critical to analyse the lower quality bound of steel friction stir welds in conventional square edge butt welding configuration. Work has been undertaken to evaluate the microstructure and fatigue performance of 6 mm thick DH36 steel plates friction stir welded with sub-optimal process conditions, resulting in the development of embedded and surface breaking flaws. The defective weldments were characterised to understand the nature of the flaws and a programme of mechanical testing was undertaken (including fatigue assessment) to determine the relationship between the flaw geometry, location and weld quality. A number of characteristic flaws were identified and seen to interact with the samples' fatigue fracture mechanisms. Samples with wormholes at the weld root produced the lowest fatigue performance. Fracture from incomplete fusion paths at the retreating side of the welds' top surface was seen to correspond to the highest recorded fatigue lives. The work provides an insight into the complex nature of characteristic flaws in steel friction stir welds and their interaction with fatigue behaviour.     

Effect of friction stir processing on corrosion resistance of aluminum–copper alloy gas tungsten arc welds

Gas tungsten arc welds in aluminum–copper alloy AA2219-T6 were friction stir processed (to a depth of about 2 mm from the weld top surface) for improving their corrosion resistance. Unprocessed and friction stir processed welds were comparatively evaluated for their microstructural characteristics and corrosion resistance. Friction stir processing was found to result in substantial microstructural refinement with fine, uniformly distributed CuAl₂ intermetallic particles. Friction stir processing was also found to result in a more uniform copper distribution in the weld metal, leading to significant increase in weld corrosion resistance. This work demonstrates that friction stir processing is an effective strategy for overcoming corrosion problems in aluminum–copper alloy fusion welds.     

Precipitate evolution in friction stir welding of 2219-T6 aluminum alloys

Precipitate evolution in friction stir welding of 2219-T6 aluminum alloys was characterized by transmission electron microscopy. In the weld nugget zone and the thermo-mechanically affected zone some metastable precipitates overaged to equilibrium phase while others solutionized into the aluminum solid solution. In the heat-affected zone the precipitates coarsened.     

铝合金超声辅助搅拌摩擦焊接接头组织性能研究

目的探究超声振动对铝合金搅拌摩擦焊的作用效果。方法分别采用普通搅拌摩擦焊和超声辅助搅拌摩擦焊方法,对7075铝合金进行焊接试验,并对焊接接头的微观组织、力学性能、断口形貌进行分析。结果普通搅拌摩擦焊焊缝中生成了隧道型缺陷,施加超声振动后,缺陷消失,形成了无缺陷的良好接头,且与普通搅拌摩擦焊相比,超声辅助搅拌摩擦焊焊缝热影响区晶粒长大程度较小,焊核晶粒细化。接头强度明显提高,达到铝合金母材强度的71.5%,接头断裂模式为韧窝和准解理的混合断裂形式。结论超声振动促进了塑性金属的流动,能有效抑制孔洞、隧道型缺陷等的形成,同时超声振动能在提升金属塑性的同时,降低焊缝的热输入。     

Joint properties of dissimilar Al6061-T6 aluminum alloy/Ti–6%Al–4%V titanium alloy by gas tungsten arc welding assisted hybrid friction stir welding

Hybrid friction stir butt welding of Al6061-T6 aluminum alloy plate to Ti–6%Al–4%V titanium alloy plate with satisfactory acceptable joint strength was successfully achieved using preceding gas tungsten arc welding (GTAW) preheating heat source of the Ti alloy plate surface. Hybrid friction stir welding (HFSW) joints were welded completely without any unwelded zone resulting from smooth material flow by equally distributed temperature both in Al alloy side and Ti alloy side using GTAW assistance for preheating the Ti alloy plate unlike friction stir welding (FSW) joints. The ultimate tensile strength was approximately 91% in HFSW welds by that of the Al alloy base metal, which was 24% higher than that of FSW welds without GTAW under same welding condition. Notably, it was found that elongation in HFSW welds increased significantly compared with that of FSW welds, which resulted in improved joint strength. The ductile fracture was the main fracture mode in tensile test of HFSW welds.     

Friction stir welding of thick section Al‐Zn‐Mg‐Cu aluminum alloy

Friction stir butt welding of 25 mm thick AA7075–T651 plates has been investigated. Careful process parameter selection resulted in single pass, full‐penetration defect free welds. The weld nugget exhibits a significant grain refinement while facing the dissolution of strengthening precipitates. Microhardness survey gives a W‐shaped profile with lower hardness values recorded in the thermo‐mechanically‐affected zone. Tensile fractures occur, again, in the thermo‐mechanically‐affected zone, where minimum hardness occurred. The friction stir welds demonstrate an excellent root bend performance while falling behind base material in face bend test. The welds also displayed outstanding impact toughness compared to that of parent material. It is concluded that defect free single pass friction stir welds can be successfully made on 25 mm thick AA7075–T651 plates.     

Mechanical performance optimization of similar thin AA 7075-T6 sheets produced by refill friction stir spot welding

Refill friction stir spot welding was applied to weld similar thin AA 7075-T6 aluminum alloy sheets in a spot-like joint configuration without a keyhole. The welds were produced using a small tool consisting of sleeve and probe with diameters of 6 mm and 4 mm, respectively. Design of experiment was employed to optimize the welding parameters in terms of the cross tensile strength by using Box Behnken Design. Based on analysis of variance, it can be concluded that plunge depth strongly affects the mechanical performance of the weld. Optimal welding parameters in terms of rotational speed, plunge depth and speed are identified to reach a cross tensile strength of up to 660 N.     

铝/钢搅拌摩擦焊对接焊缝组织及机理研究

为了提高Q235钢板和6082-T6铝合金对接的连接强度,采用搅拌摩擦焊进行对接焊接.研究了不同尺寸和形状的搅拌头、转速、焊接速度和偏移量等对铝钢对接焊缝组织的影响,进而优化了搅拌摩擦焊工艺.实验结果表明:不同形状的搅拌头影响接头"钉子"形状,接头的不同位置处由于受到不同热循环和搅拌导致晶粒尺寸不同,从而影响接头的力学性能.当搅拌针旋转速度260 r/min,焊接速度16 mm/min,针头偏向铝侧0.2 mm时,所得焊缝的拉伸强度为141.204 MPa,为最佳工艺参数.在此最优参数下获得过渡层的厚度约为8μm,界面的主要成分是Fe Al3.     

Influence of tool geometries and process variables on friction stir butt welding of Al–4.5%Cu/TiC in situ metal matrix composites

Present work describes friction stir welding of in-house produced and hot rolled Al–4.5%Cu/TiC in situ metal matrix composites by using hardened bimetallic tool with varying shoulder surface geometries and other process variables. Joining of the said composite using friction stir welding process has been seen to provide beneficial effects such as grain refinement of the matrix and subsequent redistribution and refinement of reinforcements. A predictive model has also been developed to estimate the weld properties such as tensile strength and ductility with respect to the tool geometry used and input process variables. The X-ray diffraction analysis results of Al–4.5%Cu/TiC butt welds indicated formation of CuAl₂O₄ and CuAl₂ to some extent in the stir zone. Fractography of the weld samples revealed dimpled ductile nature of fracture. Through multi response optimization of the welding parameters and tool geometry, weld strength of 89% that of the base material was achieved.     

旋转速度与焊接速度对铝铜异种材料搅拌摩擦焊接头成形的影响

目的为了获得较好的铝铜异种材料搅拌摩擦焊焊缝,研究旋转速度与焊接速度对焊缝成形的影响规律。方法采用对接的方式对4mm厚的纯铝和紫铜进行搅拌摩擦焊,对比了不同旋转速度和焊接速度对铝铜异种金属焊缝表面的影响,并对焊缝内部成形变化进行了分析。结果旋转速度和焊接速度对焊缝的成形影响较大,当旋转速度为1200 r/min,焊接速度为10 mm/min时,焊缝表面成形美观,焊缝表面相对较为光滑,焊缝内部存在相互交叠的片层结构和漩涡状结构,焊缝内部未见明显缺陷;与旋转速度1200 r/min,焊接速度10mm/min相比,在焊接旋转速度减小或者焊接速度变大时,由于焊接热输入减小,造成焊缝表面出现沟槽、孔洞等焊接缺陷,同时搅拌针的旋转搅拌作用影响焊缝成形和焊缝内部缺陷的产生。结论选择合适的旋转速度和焊接速度能够获得宏观成形较好和内部缺陷较少的焊缝。     

Developments of mathematical models for prediction of tensile properties of dissimilar AA6061-T6 to Cu welds prepared by friction stir welding process using Zn interlayer

Amount of intermetallics formed at the weld interface in dissimilar friction stir welding may be reduced by use of suitable interlayer materials such as Zn. In the present investigation, mathematical models have been developed for prediction of tensile properties of dissimilar AA6061-T6 to pure Cu welds prepared by friction stir welding process with Zn interlayer. Experiments were planned as per Box–Behnken design of response surface methodology. Three-factor, three-level Box–Behnken design with 15 runs was employed. Three process parameters: tool rotation speed (710, 1000 and 1400 rpm), tool travel speed (28, 56 and 80 mm/min) and tool pin offset (+0.5, +1.0 and +1.5 mm towards AA6061-T6 sheet) were considered. Lacks of fit for the developed models were assessed using analysis of variance (ANOVA). Validities of the developed models were checked by conducting confirmation runs. Predicted and experimental results of confirmation runs were found in reasonable agreements. Microstructural characterization revealed typical microstructure composed of intercalation of base metals. It was observed by X-ray diffraction analysis that use of Zn interlayer coupled with tool offset of +1.0 and +1.5 resulted in elimination of intermetallics of Al–Cu system at the weld interface, improving dissimilar weld quality.     

The time–temperature–corrosion susceptibility in a 7050-T7451 friction stir weld

The temperature gradient occurring for a short time during friction stir welding greatly affects the localized corrosion properties of welded 7050-T7451 plates. An immersion experiment in a salty solution was carried out in order to verify the influence of short-term post-weld heat treatments at temperatures similar to those taking place during friction stir welding on the corrosion behaviour of friction stir welded 7050-T7451. The experiment consisted of inserting thermocouples at different weld regions for the registration of the temperature development with time, and partially immersing the welded plate in a salty solution at 480 °C. In this manner, the weld experienced different temperature expositions at different locations. It was found that a temperature exposition above 180 °C for 20 min significantly increases the general corrosion resistance of friction stir welded 7050-T7451. The re-exposition of 7050-T7451 friction stir weld to a time–temperature combination similar to that occurring during the welding process on the thermomechanically heat affected zones of the weld, significantly improves the corrosion properties and the environmental cracking resistance. After the short-term temperature exposition, the fracture location of samples tested in a 3.5 wt.% NaCl solution moved from the corrosion susceptible thermomechanically affected zone to the heat affected zone.     

Mikrostrukturelle,mechanische und korrosive Eigenschaften reibrührgeschweißter Stumpfnähte in Aluminiumlegierungen

Microstructural, mechanical and corrosive properties of friction stir welded aluminium joints Friction stir welding (FSW) is a novel solid state welding process. It allows joining of high strength aluminum alloys, generally considered as difficult-to-weld with conventional technologies, without loss in joint strength. Results of investigations on selfmade FSW butt joints of the aluminum alloys 2024-T3 and 6013-T4 are presented. First, the microstructure of the weld seam and heat affected zone is characterised metallographically and by hardness measurements. By tensile, fatigue endurance (SN) and fatigue crack propagation tests it is demonstrated, that especially the FSW-joints of 2024–T3 sustain high mechanical loadings. Investigations on the corrosion properties reveal a certain sensitivity of the 2024-T3 joints to intergranular and exfoliation corrosion.     

Influence of friction stir welding process and tool parameters on strength properties of AA7075-T6 aluminium alloy joints

The aircraft aluminium alloys generally present low weldability by traditional fusion welding process. The development of the friction stir welding has provided an alternative improved way of satisfactorily producing aluminium joints, in a faster and reliable manner. In this present work, the influence of process and tool parameters on tensile strength properties of AA7075-T₆ joints produced by friction stir welding was analysed. Square butt joints were fabricated by varying process parameters and tool parameters. Strength properties of the joints were evaluated and correlated with the microstructure, microhardness of weld nugget. From this investigation it is found that the joint fabricated at a tool rotational speed of 1400 rpm, welding speed of 60 mm/min, axial force of 8 kN, using the tool with 15 mm shoulder diameter, 5 mm pin diameter, 45 HRc tool hardness yielded higher strength properties compared to other joints.