Microscale evaluation of mechanical properties of friction stir welded A6061 aluminium alloy/304 stainless steel dissimilar lap joint

Abstract

Microscale evaluation of the mechanical properties of a friction stir welded A6061/SUS 304 grooved lap joint was performed using a microtensile test and transmission electron microscopy. The microtensile test revealed that ～62% of the area along which the rotating tool passed the specimen was regarded as the bonded region and that the joint was fractured at the A6061 matrix owing to the formation of very thin interfacial reaction layers. Equiaxed aluminium grains were observed at the interface of the specimen after it was fractured, indicating that the interface deformed only slightly during the microtensile test. It should be noted that although the maximum tensile strength of the joint was approximately the same as that of the base alloy, the proof stress of the joint decreased with the dissolution of the β″ phase in the A6061 aluminium alloy.     

Numerical and experimental investigation on friction stir lap welding of aluminium to steel

Use of multimaterial fabrication such as aluminium to steel to reduce overall vehicular body weight has gained significant attention in the automotive industries. Since fusion welding of aluminium to steel is difficult, friction stir welding of the same is considered as an effective recourse. Quantitative studies on friction stir welding of aluminium to steel are thus important but scarce in the literature. We present here a numerical and experimental study on friction stir lap welding of AA6061 to high strength interstitial free coated steel sheets under different combinations of tool rotational speed and welding speed. The computed values of thermal cycle, torque and traverse force are found to be in good agreement with the corresponding experimentally measured values. The computed thermal cycles along the AA6061 to steel interfaces are related qualitatively with the experimentally measured trend and distribution in Fe–Al intermetallics along the weld joint interface.     

High strength lap joint of aluminium and stainless steels fabricated by friction stir welding with cutting pin

Abstract

Dissimilar lap joints of aluminium and stainless steel were first friction stir welded by the tool with a cutting pin. The results showed that sound joints could be obtained by this method. When the pin was inserted into the lower steel sheet, macrointerlocks were formed by the steel flashes plugging into the upper aluminium at both sides of the nugget bottom. At the aluminium/steel interface, a thin intermetallic compound (IMC) layer and the mechanical bonding of microinterlocks were formed. In addition, the aluminium near the interface was also strengthened by grain refinement and IMC particles. Therefore, the beneficial effect of the macrointerlocks provided by the steel flashes was removed, the shear strength of the joint reached 89·7 MPa, which was even higher than that of the base metal of aluminium.     

Microstructural and mechanical properties of friction stir welded aluminum/copper lap joints

This paper focuses on the microstructural and mechanical properties of the friction stir welding (FSW) of 1060 aluminum alloy to a commercially pure copper. A number of FSW experiments were carried out to obtain the optimum mechanical properties by adjusting the rotational speed and welding speed in the range of 750–1500 rpm and 30–375 mm/min, respectively. Various microstructures with different morphologies and properties were observed in the stir zone. The results indicated that Al₄Cu₉, AlCu and Al₂Cu are the main intermetallic compounds formed in the interfacial region. The effect of formation of hard and brittle intermetallic phase at the interface of the joints on the shear strength of the joint is discussed.     

Achieving high property friction stir welded aluminium/copper lap joint at low heat input

Abstract

Aluminium and copper plates with 3 mm thickness were successfully friction stir lap welded at a lower rotation rate of 600 rev min^?1 using a larger pin 8 mm in diameter. Good metallurgical bonding on the Al/Cuinterface was achieved due to the formation of a thin, continuous and uniform Al–Cu intermetallic compound layer. Furthermore, many Cu particles consisting of pure Cu and intermetallic compound layers were generated at the lower part of the nugget zone, forming a composite structure with increased hardness. A lower rotation rate resulted in a decrease in annealing softening in the heat affected zone (HAZ), and a larger diameter pin increased the Al–Cu bonding area. These factors resulted in that the friction stir welded lap joint exhibited a high failure load of 2680 N with failure in the HAZ on the aluminium side.     

Towards attaining dissimilar lap joint of CuCrZr alloy and 316L stainless steel using friction stir welding

CuCrZr alloy (Cu-0.8wt-%Cr-0.1wt-%Zr) and 316L stainless steel (Fe-0.03wt-%C-16wt-%Cr-10wt-%Ni) plates were successfully friction stir lap welded resulting in significant mechanical mixing of the two matrix elements, Cu and Fe, in the stir zone. The severe mixing not only led to improved load bearing response but also leads to form Cu-rich and Fe-rich regions in the weld nugget. The formation of these phases governs the failure mechanism of the joint. Tensile properties of the weld showed promising response when compared with joints made for the similar alloy pair by other welding techniques. This suggests strong feasibility of applying FSW for joining Cu and steel for nuclear applications.     

Fatigue properties of friction stir welded joint of ultrafine-grained 2024 aluminium alloy

Ultrafine-grained (UFG) 2024 aluminium alloy prepared by the equal channel angular pressing was friction stir welded (FSW). The high cycle fatigue and crack growth behaviour of the FSW joint were investigated in air and NaCl solution, respectively. This study demonstrated that FSW was a viable technique for joining UFG materials. The UFG microstructure was retained in the nugget zone (NZ). Compared with the UFG base metal (BM), FSW joint exhibited lower ultimate tensile strength and hardness, and the minimum hardness value was located in the heat affected zone (HAZ). NaCl solution significantly reduced the fatigue strength of FSW joint. Fatigue crack propagation rates in the NZ and HAZ were slower than that in the BM in the whole fatigue life.     

Filling exit holes of friction stir welding lap joints using consumable pin tools

Abstract

In this study, filling friction stir welding was used to remove the exit hole of friction stir welding lap joints made from AA5456 sheets. For this purpose, the exit holes were filled by consumable pins with various geometries and different pin applying methods. Then, the structures and mechanical properties of the resulting joints were investigated. Results showed that the strength of 7% higher than the strength of the joint with the non-filled exit hole, ～91% of the corresponding defect free joints, is obtainable with this technique. The best results were found by a pin with 11° cone angle, 8?mm diameter and 7?mm length, and with a 6?mm plunge without rotation.     

异种铝合金单层板与双层板对搭接搅拌摩擦焊

将一块厚4 mm的LF5铝板与两块厚2 mm的6063铝板组成并种铝合金对搭接复合接头,进行搅拌摩擦焊工艺试验并优化焊接参数,获得优质焊缝.重点研究搅拌头转速、搅拌针偏移量对复合接头抗拉强度的影响,并对焊缝表面及横截面宏观形貌、焊缝“洋葱环”组织形貌、焊缝缺陷等进行观察.分析在搅拌针选取不同偏移量的条件下,双层板一侧的...     

Interface shape and microstructure controlled dissimilar friction stir lap welded steels

Abstract

When fusion welding is conducted on the dissimilar materials between a reduced activation ferritic/martensitic steel F82H steel and an austenite stainless steel SUS 316 steel, δ ferrite is generally formed and inevitably deteriorates the weld properties. In this study, dissimilar welding of F82H to SUS 316 steel was successfully achieved by friction stir lap welding technique. It revealed that the shape and microstructure of the joint interface can be controlled by controlling the welding temperature, in another word, by changing the applied load. By controlling the welding temperature at ～710°C, a sound dissimilar joint can be obtained with a smooth joint interface and no mixed microstructure, despite the relative overlapping position of the steel plates. All the dissimilar joints showed high shear tensile strength and fracture in the base metal of F82H steel plate, which has lower strength than the SUS 316 steel plate at room temperature.     

铝合金与钢的搅拌摩擦焊焊缝成形及接头性能

用搅拌摩擦焊方法焊接了异种材料铝合金与低碳钢,分析了接头的截面形貌及力学性能.结果表明,当焊接工艺参数合适时,可以获得表面成形良好、无变形的铝合金与钢的对接接头和搭接接头.对于对接接头,金相分析表明,在焊缝横截面,低碳钢与铝合金呈较好的混合状态,在平行焊缝表面的平面内,两种材料呈交叠分布.力学性能试验表明,焊核内局部区域具有较高的显微硬度,可能是形成了钢与铝的金属间化合物,拉伸试样断裂在焊核区边缘偏低碳钢一侧.对于搭接接头,接合面处的钢呈形似"钳子"或弯钩状的分布,钢铆进了铝合金焊缝中,两种材料能有效地达到塑性结合,接头的抗剪切性能较好.     

Characterization of aluminum/steel lap joint by friction stir welding

The welding of a lap joint of a commercially pure aluminum plate to a low carbon steel plate (i.e., Al plate top, and steel plate bottom) was produced by friction stir welding using various rotations and traveling speeds of the tool to investigate the effects of the welding parameters on the joint strength. The joint strength depended strongly on the depth of the pin tip relative to the steel surface; when the pin depth did not reach the steel surface, the joint failed under low applied loads. Meanwhile, slight penetration of the pin tip to the steel surface significantly increased the joint strength. The joint strength tended to increase with rotationspeed and slightly decrease with the increase in the traveling speed, although the results were quite scattered. The effects of the welding parameters were discussed metallographically based on observations with optical and scanning electron microscopes.     

Friction stir lap joining of aluminium alloy to polypropylene sheets

Joining feasibility of aluminium alloy to polypropylene sheets via friction stir lap joining was examined. Effects of heat-input on microstructure and mechanical behaviour of the joints were investigated. A covering plate was used to confine flow of molten polymer. The results showed a distinctive interaction layer at polymer/aluminium interface, consisted mainly of C, O and Al. Shear strength of the joints decreased by enhancement of the heat-input due to increase in the thickness of the interaction layer as well as the gap width between this layer and both aluminium and polymer matrices. Maximum shear-tensile strength of 5.1?MPa (～20% of polymer shear strength) was obtained, which was higher than or comparable to that of the joints produced by other processes.     

Microcavities accompanying a zigzag line in friction stir welded A6082-T6 alloy joint

Abstract

An Al–Mg–Si alloy was friction stir welded (FSW), and the microstructure of the zigzag line in the welds was investigated using optical microscopy, energy dispersive X-ray spectroscopy and scanning electron microscopy. The effect of the zigzag line on the mechanical properties of the as welded joints was also examined. It was found that in the welds with high heat input, small discontinuous cavities or microcracks of several micrometres in size exist along the zigzag line, and the microcracked zigzag line was found to significantly affect the face bend properties of the FSW joint, but had limited influence on the tensile properties of the butt welds. In joints with low heat input, the zigzag line was only composed of oxide particles, no cavities or microcracks were detected at the zigzag line.     

Mechanical properties and microstructure analysis of refilling friction stir welding on 2219 aluminum alloy

The 2219 aluminum alloy under refilling friction stir welding (RF-FSW) was investigated. The micrographs showed that the bead could be divided into six zones, and the grain size and shape were greatly different in these zones. According to the microstructure analysis, the weld nugget zone and the shoulder stirring zone consisted of equiaxed grains, while the grains in the heat affected zone were seriously coarsened. It was obvious that bending deformation occurred in the thermo-mechanically affected zone. According to the microhardness analysis, the lowest hardness of the weld was at the thermo-mechanically affected zone, and the microhardness increased with the retraction of the stir-pin. The tensile strength and elongation of the bead were 70% and 80% of the base metal, respectively. The tensile strength was slightly different for the stable stage and the retraction stage, while the elongation decreased in the retraction stage. The mechanical properties and microstructure responded to different retraction speed were analyzed, and it showed that the elongation decreased with increasing retraction speed.     

Numerical modelling of gas metal arc joining of aluminium alloy and galvanised steels in lap joint configuration

Advanced pulsed current gas metal arc based processes are increasingly attempted for the joining of aluminium alloys and galvanised steel sheets. The bead profile and the thickness of the interfacial Fe–Al intermetallic (IMC) layer significantly influence the failure strength of these joints. Although several experimental studies have examined the nature and extent of the IMC phases and consequent joint strength, quantitative efforts to estimate bead profile and the IMC layer thickness as function of process conditions and resulting heat input are scarce. We present here for the first time a coupled theoretical and experimental study to estimate the bead profile and Fe–Al IMC layer thickness for joining of galvannealed steel and aluminium alloy sheets in a typical lap joint configuration. The computed values of bead profile and IMC layer thicknesses are validated with the corresponding experimental results.     

35mm厚板铝合金搅拌摩擦焊接头组织和性能

采用搅拌摩擦焊双面焊工艺,对35 mm厚板6005A-T6铝合金型材进行了搅拌摩擦焊接,获得成形良好、表面光滑、无隧道孔和沟槽缺陷的焊接接头.应用光学显微镜、扫描电镜、显微硬度仪及电子拉伸试验机等对搅拌摩擦焊接头组织与性能进行研究.结果表明,接头焊核区组织为细小等轴晶;前进侧出现明显的螺旋纹及清晰的结合线,热力影响区晶粒被明显拉长呈条状组织,热影响区受热晶粒粗大;后退侧未见螺旋纹,晶粒比前进侧细小,过渡区较前进侧宽.在搅拌头旋转频率为650 r/min,焊接速度为200 mm/min工艺条件下接头抗拉强度为213 MPa,达到母材强度的84.8%,断裂起始于焊缝前进侧的热影响区,扩展至双面焊接重合区时,沿着焊缝后退侧热影响区直至断裂;接头显微硬度最低值出现在前进侧热影响区,最低值为50 HV.     

镁和钢搅拌摩擦焊接头组织分析

采用搅拌摩擦焊对镁合金(AZ31B)和钢(Q235)异种材料进行焊接,通过优化工艺参数获得最佳成形接头,并采用光学显微镜对接头显微组织进行观察,通过SEM沿板厚方向分析焊核与钢侧界面不同位置的微观形态.结果表明,镁/钢连接紧密,焊核勺子状区与镁侧分界面明显,晶粒较母材晶粒明显长大;钢侧热力影响区受机械和热的复合作用,组织不均匀,既有等轴晶组织也有条状组织,镁侧热力影响区不明显;镁热影响区晶粒粗化较钢侧严重.接头横截面钢侧显微硬度距离焊核越近硬度值越高,焊核硬度分布不均,局部区域硬度很高,最高为324.7 MPa,镁侧硬度值较均匀.     

Asymmetric microstructure and fracture behaviour of friction stir welded joints of 2024 aluminium alloy under cyclical load

In this study, the effect of asymmetric microstructure on fracture behaviour of friction stir welded joints of 2024-O aluminium alloy under cyclical load was investigated via optical microscopes, electron backscattering diffraction, microhardness and infrared thermography, and digital imaging correlation (DIC). The results demonstrate that the thermo-mechanical affected zone (TMAZ) at retreating side possesses a relatively higher fraction of low-angle grain boundaries and coincidence site lattice boundaries in comparison to that at the advancing side (AS), which results in the strain localisation and fracture path along the region between nugget zone and TMAZ at the AS under cyclical load. The profile of microhardness presents the asymmetric characteristic as well. The temperature evolution of the joint obtained by infrared camera is in good agreement with strain distribution measured by DIC. The thermography proves to be generally sufficient to predict the fracture path before failure under cyclical load.