Influences of welding parameters on mechanical properties of AZ31 friction stir spot welds

Abstract

In this study, the influence of welding parameters, including tool rotational speed, plunge rate and dwell time, on the overlap tensile shear properties of AZ31 friction stir spot welds was investigated. The microstructures in stir zones and fracture surfaces were observed using optical microscope and scanning electron microscope respectively. The bonded width and h value (the distance from the tip of the partially bonded region to the top of the weld surface) were measured. The results indicated that larger bonded width and higher h value of the AZ31 weld result in better mechanical property. It is proposed that high tensile shear loads are produced when the tool rotational speed range of 1500–2250 rev min^?1 and 1 s dwell time are applied during the friction stir spot welding of AZ31. The plunge rate range from 2·5 to 10 mm s^?1 has insignificant influence on the tensile shear load of AZ31 joints under the present conditions. The failure mode changes from interfacial to pullout when the tool rotational speed is >2250 rev min^?1. The fracture feature of AZ31 welds is brittle fracture.     

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.     

Development of novel heating tool friction stir spot welding (HT-FSSW) for AZ31 magnesium alloy

In this work, a new heating tool friction stir spot welding (HT-FSSW) process was developed, and its impacts on the microstructure and mechanical properties of the welded AZ31 magnesium alloy joints were investigated by microstructure observation, tensile tests and microhardness tests. An increase in the heating tool temperature resulted in a decrease in the grain size of the stir zone (SZ) and an increase in the grain size of the thermomechanically affected zone (TMAZ). The rising heating tool temperature also aggrandised the bonded zone width and enhanced the tensile shear load strength per unit area of the HT-FSSW welded joints. With an increase in the heating tool temperature, the microhardness of SZ increased while that of the TMAZ decreased. Moreover, the slope of the Hall–Petch relationship between microhardness and grain size of the TMAZ is larger than that of the SZ.     

Hook formation and mechanical properties in AZ31 friction stir spot welds

Hook formation and the failure load properties of AZ31 friction stir spot welds are investigated in friction stir spot welds made between wrought and heat-treated AZ31 sheets having widely different average grain sizes.     

AZ31镁合金搅拌摩擦点钎焊接头微观组织和力学性能

研究了几种钎料及硬质相(锌钎料、锡钎料及SiC硬质相颗粒)对镁合金搅拌摩擦点钎焊接头微观组织、剪切力及硬度分布的影响。结果表明,加入锌钎料时,钎料能充分与母材发生冶金反应,生成MgZn2硬质相,焊核区的硬质相阻碍了金属的塑性流动,且在钎焊区会形成较大的钎焊界面,焊接接头剪切力从3．5 kN提升到了5．8 kN,且焊接接头具有较高的硬度。     

Microstructures and mechanical properties of friction stir welded joints of Zr–Ti alloy

Zirconium–titanium alloy joints were successfully produced by friction stir welding. Unlike the (α+β) dual phase microstructure in base metal, only the β phase existed in the region in which temperature exceeded the β transient point for the as welded joint. Accordingly, the hardness in these regions exhibited integral decrement and uniform distribution features. The thermal simulation further showed that hardness variation was mainly determined by phase composition. Microstructure development in the nugget zone was mainly governed by continuous dynamic recrystallisation. Satisfactory ultimate tensile strength and elongation equal to the base metal were achieved in the as welded joint. Tensile fracture occurred at the heat affected zone near the retreating side of the joint. The fracture surface of the joint exhibited a mixing feature with quasi-cleavage facets and small dimples.     

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     

厚板AZ31镁合金搅拌摩擦焊焊接接头的组织与性能

对10mm厚板A231镁合金成功进行了搅拌摩擦焊接,获得成形良好、表面光滑、无裂纹、无气孔的焊接接头.研究该搅拌摩擦焊接头不同区域的显微组织特征,并通过拉伸、冲击和硬度试验分析了焊接接头的力学性能.结果表明,焊缝中心区是均匀细小的等轴晶粒,热力影响区晶粒大小不均匀,存在较明显的塑性流变带结构;焊接接头的抗拉强度达到母材的80%以上,焊接接头的冲击韧性比母材高,焊接接头的显微硬度比母材稍有降低,焊接接头具有较好的力学性能,说明搅拌摩擦焊是焊接厚板镁合金的一种有效方法.     

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.     

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.     

根部未焊合对AZ31B镁合金搅拌摩擦焊接头性能的影响

针对厚4.3 mm的AZ31B镁合金短针设计搅拌摩擦焊接,研究工艺参数对根部未焊合率的影响,并通过接头组织与拉伸断口形貌分析,分析了根部未焊合对接头抗拉强度的影响。结果表明:焊接速度一定时,当搅拌头旋转速度超过临界值950 r/min时,随着转速的增加,接头根部未焊合率降低。当旋转速度一定时,焊接速度对接头未焊合率几乎无影响。所有拉伸试样均在根部未焊合界面处起裂并发生断裂,而焊合区断口呈现典型的韧脆混合断裂特征。此外,接头抗拉强度随着根部未焊合率的降低而升高,当旋转速度为1 180 r/min、焊接速度120 mm/min时,接头的抗拉强度达到最大值188 MPa,为母材强度的76.4%。     

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.     

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.     

Study of top sheet thinning during friction stir lap welding of AZ31 magnesium alloy

Abstract

Friction stir lap welding was applied to AZ31 magnesium alloy sheets to investigate the effect of tool designs and welding variables on top sheet thinning. Three tools with different shoulder designs were used. Sheet thinning aroused by the hooking or cold lap feature was quantified, and the lap shear fracture load of the joints was evaluated and correlated with the effective top sheet thickness. The tool geometry has a significant effect on the morphology and extent of both hooking and cold lap features, as well as welding variables such as welding pitch and plunge depth. The morphology and extent of these features influenced the effective top sheet thickness, which exhibited a linear relationship with the unit width fracture load via the welded material strength. High fracture loads can be achieved by suppressing the hooking and cold lap features as well as by strengthening the material in the nugget.     

Microstructure and mechanical behaviour of pinless friction stir spot welded AA2198 joints

In this study, pinless friction stir spot welding of 1.8 mm thick 2198-T8 aluminium–lithium alloy plates was carried out. The change of the angle between the nugget edge and the surface, and the relationship between this angle and joint mechanical property were analysed. The results show that the angle increases rapidly initially and then approaches 45°, which is due to the extrusion of nugget material and its flow along the surrounding ‘cold’ metal during welding. The tensile strength is determined by the nugget edge angle and hook defect. Tensile loads reach a higher value when the nugget edge angle approaches 45° but have a slight decrease with the hook angle changing from obtuse to acute. The maximum tensile/shear strength could be 8.57 kN at the rotation speed of 1500 rev min^??1 and the dwell time of 12 s.     

镁/铝合金回填式搅拌摩擦点焊的组织与性能

选用5083铝合金和AZ31B镁合金为研究对象,研究焊接时间对异种合金回填式搅拌摩擦点焊接头的金属间化合物层和拉剪性能的影响.结果表明,当焊接时间为1 s时,Mg合金的流动性较差,接头中出现明显的孔洞缺陷;随着焊接时间的变长,孔洞缺陷消失.由于铝镁表面的氧化膜在焊接过程中被打碎且焊接温度高于铝镁的共晶温度,接头中心会形成一层液相层,焊后液相层凝固形成金属间化合物.接头的抗拉载荷随着焊接时间的延长先升高后降低,最优载荷在焊接时间为2 s时取得,为3.1 kN.     

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.     

Effects of mix activated fluxes coating on microstructures and mechanical properties of tungsten inert gas welded AZ31 magnesium alloy joints

Abstract

The effects of mix activated fluxes on the penetrations, microstructures and mechanical properties of tungsten inert gas (TIG) welded AZ31 magnesium alloy joints were studied. The results showed that an increase in the amount of TiO₂ coating improved the weld penetration and depth/width ratio of the TIG welded AZ31 magnesium alloy seams. The α-Mg grains of the fusion zone (FZ) became fine gradually with the addition of CaF₂ coating but coarsened sharply when the amount of TiO₂ coating was over 70%. In addition, the porosities and total length of solidification cracks in the FZ were reduced by the CaF₂ and TiO₂ coatings. The ultimate tensile strength value of the welded joints increased with the addition of TiO₂ coating but decreased sharply when more TiO₂ coating was adopted.     

AZ31镁合金搅拌摩擦焊接头力学性能

成功实现了2．5mn厚AZ31镁合金的搅拌摩擦焊接。焊接参数的选择决定接头的力学性能，通过拉伸和弯曲试验，分析了焊接参数的选择对接头性能的影响。试验结果表明，当焊接速度为75～150mm／min，旋转速度为900～l500r／min之间时，可以得到性能好的接头。为简化焊接参数的选择，同时考虑焊接速度和旋转速度的相互影响。提出一个新的评定参数R/ε，即单位长度上的旋转次数，并分析R/v与接头性能之间的关系。结果表明，iv R/r=8～15时，接头性能良好，可知，当焊接速度和旋转速度的选择在一个比较合理的范围时，R／v可以作为搅拌摩擦焊接参数选择的依据。