Mechanical and microstructural behaviour of 2024–7075 aluminium alloy sheets joined by friction stir welding

The aim of the present work is to investigate on the mechanical and microstructural properties of dissimilar 2024 and 7075 aluminium sheets joined by friction stir welding (FSW). The two sheets, aligned with perpendicular rolling directions, have been successfully welded; successively, the welded sheets have been tested under tension at room temperature in order to analyse the mechanical response with respect to the parent materials. The fatigue endurance (S–N) curves of the welded joints have been achieved, since the fatigue behaviour of light welded sheets is the best performance indicator for a large part of industrial applications; a resonant electro-mechanical testing machine load and a constant load ratio R=σ_min/σ_max =0.1 have been used at a load frequency of about 75 Hz. The resulted microstructure due to the FSW process has been studied by employing optical and scanning electron microscopy either on ‘as welded’ specimens and on tested specimen after rupture occurred.     

Formation of interfacial microstructure in a friction stir welded lap joint between aluminium alloy and stainless steel

The interfacial microstructure produced through tool transit of a friction stir welded lap joint between an aluminium alloy and stainless steel was studied by transmission electron microscopy in order to clarify its early stages of formation. Transmission electron microscopy studies of the bottom surface of the exit hole revealed the presence of several mixed layers of an ultrafine intermetallic compound (IMC) and stainless steel. The joining between dissimilar materials was achieved through a continuous flow of the stirred aluminium alloy into the mixed layers and the resultant growth of the ultrafine IMCs due to the heat induced by the friction between the tool and the specimen. The continuous thin reaction layer finally produced at the interface was found to be stronger than the base aluminium alloy.     

Microstructures and mechanical properties of friction spot welded Alclad 7B04-T74 aluminium alloy

Two millimetre thick Alclad 7B04-T74 aluminium alloy was friction spot welded at different tool rotation speeds. The weld formation, Alclad redistribution, microstructures and mechanical properties of the joints were investigated. The results indicate that inappropriate tool rotation speeds can give rise to weld defects, such as annular groove, void and surface concavity. After welding, the original surface Alclad is redistributed as a U shaped Alclad layer in the weld. When the tool rotation speed is relatively high, eutectic films can be observed in the stir zone, and the Alclad layer in the weld is a preferred crack propagation path during tensile shear testing. The optimised joint with a tensile shear failure load of 11?921 N can be obtained at a tool rotation speed of 1500 rev min^?1.     

搅拌摩擦焊AA7075-T651铝合金接头的疲劳裂纹扩展(英文)

研究12 mm厚AA7075-T651铝合金板搅拌摩擦焊接头的疲劳裂纹扩展行为。从搅拌摩擦焊接头以及母材中截取试样,对试样进行疲劳裂纹扩展实验。对搅拌摩擦焊接头以及母材的横向拉伸性能进行评估。用光学显微镜和透射电镜分析焊接接头的显微组织。用扫描电镜观察试样的断裂表面。与母材相比,焊接接头的ΔKcr降低了10×10-3 MPa·m1/2。搅拌摩擦焊AA7075-T651接头的疲劳寿命明显低于母材的,其原因可归结于焊缝区的析出相在搅拌摩擦焊接过程中的溶解。     

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.     

Microstructural characterization and corrosion behavior of multipass friction stir processed AA2219 aluminium alloy

Multipass friction stir processing of AA 2219-T87 aluminium alloy to a depth of 2 mm in a 5 mm plate resulted in fine α-Al grains, reduction and dissolution of both eutectic phase (CuAl₂) and the strengthening precipitates (CuAl₂). Anodic polarization and electrochemical impedance tests in 3.5% NaCl showed an improved corrosion resistance of the processed alloy, which increased with the number of passes. Salt spray and immersion tests also showed improved resistance to corrosion. The increased resistance to corrosion is attributed to the dissolution of CuAl₂ particles, which was established by XRD and DSC studies.     

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.     

5A06铝合金搅拌摩擦焊接接头的金属流动规律,显微组织以及腐蚀行为研究

对5A06铝合金在搅拌摩擦焊接过程中的金属流动规律以及焊后显微组织演变和腐蚀行为进行研究。结果表明:由于受到扭转和漩涡运动的综合作用,前进侧金属被直接卷入搅拌针的螺纹内部,然后被向下挤压进入焊核区。然而,后退侧的金属展现出无序的金属流动模式。通过扫描电子显微镜电子背散射分析发现,搅拌区的晶粒在动态再结晶的作用下发生了明显的细化,再结晶后平均晶粒尺寸约为6μm。对搅拌区的扫描电子显微分析结果表明,β相(Al_3Mg_2)和Al6(Mn,Fe)呈弥散分布,且和母材相比Al6(Mn,Fe)颗粒明显细化。同时,焊缝截面的硬度分布表明焊后5A06铝合金具有软化趋势,最小硬度值(HV)为720 MPa,位于搅拌区和热机影响区的界面处。通过电化学分析和浸泡后的组织观察发现搅拌区的耐腐性明显优于母材,母材的腐蚀电位为–0.725 V,而搅拌区域腐蚀电位升高35 mV。     

搅拌摩擦加工AZ31镁合金低周疲劳性能研究

作为新型大塑性变形技术,搅拌摩擦焊接技术（FSW）已成功应用于超细晶材料的制备。本文采用FSW对热轧态AZ31镁合金进行连接,对母材和焊接接头的低周疲劳性能进行了研究。结果表明：在疲劳测试和单轴拉伸测试过程中,焊合区和前进侧热机械影响区交界处为性能薄弱区域。FSW AZ31接头的低周疲劳寿命,屈服强度,抗力强度以及断后延伸率均低于母材。AZ31镁合金疲劳测试过程种的主要变形机制为位错滑移,疲劳断口表面呈现出明显的疲劳纹。最终,我们发现母材和FSW接头低周疲劳行为符合Coffin-Manson和 Basquin关系     

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.     

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.     

Effects of microstructural asymmetries across friction stir welded AA2024 joints on mechanical properties

The effect of offset of the centre of a tensile specimen to the weldline on global tensile properties of friction stir welded AA2024 joints was investigated using experiments and numerical analysis. The size and geometry of these discrete zones, such as the central low hardness zone, the low hardness zone-I near the thermo-mechanically affected zone, the low hardness zone-II near the base metal and the base metal, were determined from a cross-sectional hardness map. Results show that tensile specimens with different area of the joint placed in the centre of the specimen do not affect the tensile strength and fracture path of a joint, strain is affected. Predictions based on local tensile properties follow well the measured global tensile behaviour.     

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.     

Influence of tool thread on mechanical properties of dissimilar Al alloy friction stir spot welds

Abstract

The influence of threaded and wear simulated (half thread) tools on the mechanical properties of dissimilar Al alloy friction stir spot welds is investigated. With lower tool rotational speed settings, the failure loads of Al 5754/Al 6111 lap joints made using a threaded tool were clearly higher than that of a half thread tool. However, the failure load of the joints made using a half thread tool increased when the tool rotational speed increased, and finally, as the rotational speed was further increased, the failure load became almost the same as the failure load of joints made using a threaded tool. In Al 5052/Al 6061 butt joints made using the threaded and half thread tools, the area of the stir zone on the bonded cross-section corresponded with the actual bonded region on the fracture surface. Therefore, the thread on the rotating pin has limited influence on the mechanical properties of the friction stir spot lap joints.     

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.     

Fatigue properties of friction stir welded butt joint and base metal of MB8 magnesium alloy

The fatigue properties of friction stir welded(FSW) butt joint and base metal of MB8 magnesium alloy were investigated.The comparative fatigue tests were carried out using EHF-EM200K2-070-1A fatigue testing machine for both FSW butt joint and base metal specimens.The fatigue fractures were observed and analyzed using a scanning electron microscope of JSM-6063 LA type.The experimental results show that the fatigue performance of the FSW butt joint of MB8 magnesium alloy is sharply decreased.The conditional fatigue limit(2 × 10~6) of base metal and welded butt joint is about77.44 MPa and 49.91 MPa,respectively.The conditional fatigue limit(2 × 10~6) of the welded butt joint is 64.45%of that of base metal.The main reasons are that the welding can lead to stress concentration in the flash area,tensile welding residual stress in the welded joint(The residual stress value was 30.5 MPa),as well as the grain size is not uniform in the heat-affected zone.The cleavage steps or quasi-cleavage patterns present on the fatigue fracture surface,the fracture type of the FSW butt joint belongs to a brittle fracture.     

2219铝合金搅拌摩擦焊接头晶间腐蚀分析

采用晶间腐蚀试验研究了2219铝合金搅拌摩擦焊接头的晶间腐蚀行为,结合接头显微组织、微观硬度、腐蚀形貌及腐蚀深度,分析母材与焊核区的差异,并对接头晶间腐蚀机理进行了初步的探讨.结果表明,焊核区为细小的等轴晶组织,且接头上表面焊核区的晶粒要大于下表面焊核区的晶粒;母材区硬度最高,下表面焊核区硬度最低;焊核区的耐蚀性优于母材,且上表面焊核区耐蚀性优于下表面焊核区,母材最大腐蚀深度为145.9 μm,上表面焊核区及下表面焊核区最大腐蚀深度为46.3 μm和84.1 μm.     

Fatigue behavior of friction stir spot welded AZ31 Mg alloy sheet joints

The fatigue behavior of friction stir spot welded (FSSW) AZ31 magnesium alloy sheet joints was investigated by tension–compression of fatigue test. The results suggest that all the fatigue failures occur at the stir zone of the FSSW AZ31 sheet joints, and all cracks initiate at the stir zone outer edge between the upper and lower sheet. When the cycle force equals 1 kN, the crack propagates along the interface of heat-affected zone and thermo-mechanical zone, simultaneously across the direction of force; while the cycle force equals 3 kN, the crack propagates along the diameter of stir zone and shear failure occurs finally. Moreover, the transverse microsections indicate that there is a tongue-like region at the outer edge of stir zone between the two AZ31 sheets, and the direction of tongue-like region is toward outside of the stirred zone and all fatigue cracks initiate at the tongue-like region.     

压铸态镁/铝异种金属搅拌摩擦焊接接头组织及性能

对3 mm厚压铸态AZ91D镁合金和压铸态ADC12铝合金进行了结果表明搅拌摩擦焊,采用金相显微镜、扫描电镜、X射线衍射、万能试验机等分析了接头的微观组织和力学性能。结果表明:铝合金置于前进侧,前进速度为40 mm/min,转速为900～1300 r/min时,可获得成形良好的焊接接头;接头焊核区域镁铝合金相互混合,发生机械互锁,组织呈现条带状特征;母材中粗大铸态枝晶明显细化,非镁/铝基体相主要有Si、Mg_2Al_3、Mg_(17)Al_(12)和Mg_2Si,颗粒状尺寸随转速的增加呈先降后增趋势;接头平均抗拉强度随转速增加呈现先增大后减小的趋势;断裂均发生在前进侧热机影响区和焊核的交界处。关键词:搅拌摩擦焊;微观组织;力学性能;压铸镁合金;压铸铝合金     

Microstructural investigation and mechanical properties of dissimilar friction stir welded magnesium alloys

Abstract

Dissimilar joints of magnesium alloys were obtained by friction stir welding. Detailed microstructure and texture examinations were performed on the joints. Significant difference in microtexture distribution and microstructural features is observed between crown and stir zones in the dissimilar joints. However, an overall effect of these factors on mechanical properties of layered joints is not obvious as both the up and middle samples present quite similar yield strength and strain hardening behaviour. Moreover, both samples fracture in the Mg–3Al–1Zn alloy side. The fracture of the middle sample starts at the boundary of transition and stir zones, while the up sample starts in the crown zone side.