Study on quality of resistance spot welded aluminum alloys under various electrode pressures

The electrode force is one of the main parameters in resistance spot welding (RSW). It is very important to guarantee the quality of aluminum alloys and determine whether the electrode pressure is stable or adjustable in the welding process. With the drive set of a servo-motor, we conduct the RSW tests and tensile shear tests on the 5052 aluminum alloy sheets. Results of these tests show that all variable pressure curves are suitable for spot welding, and all have their own rules in affecting the tensile strength of the spot welded joints.     

Effect of adhesive on fatigue property of Aural2 to AA5754 dissimilar aluminum alloy resistance spot welds

General Motors (GM) has developed a proprietary resistance spot welding (RSW) process using a multi-ring, domed electrode geometry that has been used successfully in automotive aluminum welding operations. To enhance structural performance, one-part epoxy adhesives are frequently applied prior to RSW to create weld-bonded joints. The addition of adhesive can result in additional porosity created within the weld nugget. Therefore, the adhesive's impact on mechanical properties, especially fatigue properties requires further investigation.Load-controlled fatigue testing was conducted on dissimilar aluminum alloy spot welds made of AA5754 wrought sheet and Aural2 die casting sheet with and without the addition of adhesive prior to welding. The same GM RSW electrode and weld schedule was used for both conditions. The results show that the addition of adhesive results in a larger nugget size, but similar maximum load in tension-shear testing. X-ray computed tomography during interrupted fatigue testing of the spot welds shows that the main fatigue crack initiates at the edge of the nugget in the plane of the faying interface and penetrates through the Aural2 die cast sheet in the thickness direction. Using the structural stress concept, it was also found that the structural stress range–fatigue life curve for these spot welds, both with and without adhesive, falls onto a single master curve indicating that the nugget size which corresponds to the tensile and bending strength dominates the fatigue life and that adhesive-induced porosity within the weld nugget does not harm fatigue performance.     

7075铝合金高速搅拌摩擦焊材料流动行为及性能研究

目的 针对7075–O铝合金高焊速、高转速搅拌摩擦焊接缺陷多、质量差等问题,研究焊接接头材料流动对焊缝性能的影响。方法 选用焊接速度1 000 mm/min,搅拌转速分别为1 000、1 200、1 600、1 700 r/min的条件对7075–O铝合金板进行搅拌摩擦焊接,分析不同焊接工艺参数下焊接接头的显微组织及力学性能。同时,利用Fluent软件模拟7075–O铝合金搅拌摩擦焊接过程中的材料流动场分布,分析焊接材料流动与缺陷形成的关系。结果 利用7075–O铝合金三维流动模型,预测出高焊速条件下焊缝前进侧形成一个低压区,孔洞等缺陷易出现在此区域,数值模拟预测与试验结果吻合。在高焊接速度1 000 mm/min、焊接转速1 200 r/min时,焊缝表面光滑平整,焊核区域的硬度分布更加均匀。结论 随着搅拌转速从1 000 r/min增大到1 700 r/min,热输入量逐渐增大,孔洞缺陷由隧道型孔洞转变为不连续的小孔。同时,随着搅拌转速的增大,焊缝高硬度区域的宽度先增大而后降低。当搅拌转速为1 200 r/min时得到了优质的焊接接头,焊缝焊核区硬度分布均匀,硬度值最高为176HV。     

Effects of various tool plunge depths on microstructure evolution,mechanical properties and dome structure features of friction stir spot welded AA5052-H32 similar joints

The solid-state nature of friction stir spot welding process provides outstanding advantages for the sound joining of aluminum alloys. Within this study, 3 mm-thick AA5052-H32 sheets are successfully joined by friction stir spot welding using 2344 hot-worked steel pin to investigate the effects of various tool plunge depths on the microstructure, mechanical and metallurgical properties of similar welds. Therefore, the experiments are performed at different plunge depths in the range of 3 mm–4 mm. Accordingly, the relationships between the process parameter (tool plunge depth) and the responses (microstructure, dome structure, microhardness and lap shear tensile load) are established. Microstructure analyses demonstrate that the increase in the plunge depth leads to more grain refinement within the stir zone, which significantly affects the mechanical performance of the similar joints. This study also indicates that the tool plunge depth in friction stir spot welding process has a noteworthy influence on the characteristic features of the 5052 aluminum alloy joints, such as the dome structure. Moreover, an explicit increase in the microhardness towards the weld stir zone is observed in all specimens. It is found that the average maximum tensile-shear force enhances with the increment in the tool plunge depth from 3 mm to 4 mm.     

Microstructure and tensile properties of friction welded aluminum alloy AA7075-T6

Solid-state welding processes like friction welding and friction stir welding are now being actively considered for welding aluminum alloy AA7075. In this work, friction welding of AA7075-T6 rods of 13 mm diameter was investigated with an aim to understand the effects of process parameters on weld microstructure and tensile properties. Welds made with various process parameter combinations (incorporating Taguchi methods) were subjected to tensile tests. Microstructural studies and hardness tests were also conducted. The results show that sound joints in AA7075-T6 can be achieved using friction welding, with a joint efficiency of 89% in as-welded condition with careful selection of process parameters. The effects of process parameters are discussed in detail based on microstructural observations.     

Effect of process parameters on the strength of resistance spot welds in 6082-T6 aluminium alloy

In this study the microstructural and mechanical behaviour of resistance spot welds (RSW) done on aluminium alloy 6082-T6 sheets, welded at different welding parameters, is examined. Microstructural examinations and hardness evaluations were carried out in order to determine the influence of welding parameters on the quality of the welds. The welded joints were subjected to static tensile-shear tests in order to determine their strength and failure mode. The increase in weld current and duration increased the nugget size and the weld strength. Beyond a critical nugget diameter the failure mode changed from interfacial to pullout. Taking into consideration the sheet thickness and the mechanical properties of the weld, a simple model is proposed to predict the critical nugget diameter required to produce pull-out failure mode in undermatched welds in heat-treatable aluminium alloys.     

An experimental investigation of fatigue behavior and deformation of double spot friction welded joints

Fatigue behavior of double spot friction welded joints in aluminum alloy 7075-T6 plates is investigated by conducting monotonic tensile and fatigue tests. The spot friction welding procedures are carried out by a milling machine with a designed fixture at the best preliminary welding parameter set. The fatigue tests are performed in a constant amplitude load control servo-hydraulic fatigue testing machine with a load ratio of (R = P_min/P_max) 0.1 at room temperature. It is observed that the failure mode in cyclic loading (low-cycle and high-cycle) resembles that of the quasi-static loading conditions i.e. pure shearing. Primary fatigue crack is initiated in the vicinity of the original notch tip and then propagated along the circumference of the weld’s nugget.     

Single-sided laser beam welding of a dissimilar AA2024–AA7050 T-joint

In the aircraft industry double-sided laser beam welding of skin–stringer joints is an approved method for producing defect-free welds. But due to limited accessibility – as for the welding of skin–clip joints – the applicability of this method is limited. Therefore single-sided laser beam welding of T-joints becomes necessary. This also implies a reduction of the manufacturing effort. However, the main obstacle for the use of single-sided welding of T-joints is the occurrence of weld defects. An additional complexity represents the combination of dissimilar and hard-to-weld aluminium alloys – like Al–Cu and Al–Zn alloys. These alloys offer a high strength-to-density ratio, but are also associated with distinct weldability problems especially for fusion welding techniques like laser beam welding. The present study demonstrates how to overcome the weldability problems during single-sided laser beam welding of a dissimilar T-joint made of AA2024 and AA7050. For this purpose a high-power fibre laser with a large beam diameter is used. Important welding parameters are identified and adjusted for achieving defect-free welds. The obtained joints are compared to double-sided welded joints made of typical aircraft aluminium alloys. In this regard single-sided welded joints showed the expected differing weld seam appearance, but comparable mechanical properties.     

Microstructure and failure mechanisms of refill friction stir spot welded 7075-T6 aluminum alloy joints

In this paper, the microstructure and mechanical properties of 7075-T6 aluminum alloy joints joined by refill friction stir spot welding (RFSSW) were investigated. The keyhole was refilled successfully, and the microstructure of the weld exhibited variations in the grain sizes in the width and the thickness directions. There existed defects (hook, voids, bonding ligament, etc.) associated to the material flow in the weld. Mechanical properties of the joint have been investigated in terms of hardness and tensile/shear and cross-tension test, and the fracture mechanisms were observed by SEM (scanning electron microscope). The hardness profile of the weld exhibited a W-shaped appearance in the macroscopic level, which reached the minimum at the boundary of the sleeve and the clamping ring. The variation laws between tensile/shear and cross-tension strength and processing parameters were rather complicated. The void in the weld played an important role in determining the strength of the joint. On the whole, the preferable strength can be obtained at lower rotational speed. Shear fracture mode was observed under tensile–shear loadings, and nugget debonding, plug type fracture (on the upper sheet) and plug type fracture (on the lower sheet) modes were observed under cross-tension loadings. It was also observed that the main feature affecting the mechanical properties of the joint is the alclad between the upper and lower sheets and the connecting qualities between the stir zone and thermo-mechanically affected zone.     

钛-铝异种材料的点焊技术研究新进展

钛合金和铝合金的应用在航空航天及汽车制造领域都在逐步增加,而Ti/Al异种金属复合结构中搭接连接无疑是不可避免的连接形式,因此对于钛/铝异种金属点焊技术的研究是非常必要的。由于Ti/Al两种材料的物理和化学性质存在巨大差异,导致两者的焊接尤为困难,目前能实现钛铝异种材料的高质量焊接的方法研究较少。先从钛和铝材料的本身特性入手,阐述目前3种常用的点焊技术:电阻点焊、搅拌摩擦点焊和超声波点焊的研究现状。主要涉及点焊接头成形、焊接参数、接头的组织特征以及力学性能,探究了各个方法的优缺点,并指出Ti/Al异种金属焊接质量的关键主要在于对金属间化合物的有效控制。为Ti/Al异种结构的连接提供合适的设计思路,并为新的连接方法提供借鉴思路。     

环境温湿度对铝合金焊缝气孔和力学性能的影响

对于高速轨道客车铝车身的生产制造,气孔是焊接中最常见的缺陷。采用X射线法研究了不同温度和湿度下铝合金6082和5083熔化极氩弧焊(MIG)焊缝的气孔情况。结果表明,在焊接过程中环境的绝对湿度(是温度和湿度的综合体现)对焊缝的气孔率有重要影响,铝合金6082焊缝的气孔敏感性要比铝合金5083高。在拉伸试验中铝合金6082接头的断裂位置一般在焊接热影响区(HAZ),随着绝对湿度的增加,接头的抗拉强度和断后伸长率几乎保持不变,但接头的正弯和背弯角度分别减小了74.4%和64.4%。铝合金5083接头的断裂位置一般出现在熔合区,随着绝对湿度的增加,接头的抗拉强度和断后伸长率分别减小了4.0%和15.7%,但是弯曲性能变化不大。     

5052铝合金金字塔型点阵夹芯板的成型及其组织性能研究

采用冲压成型的5052铝合金板作为金字塔点阵芯层,2024铝合金作为面板,以Zn-Cd-Ag-Cu为钎料,分别在430℃、460℃进行炉中钎焊。观察焊后钎料及夹芯板的焊接接头组织,并进行显微硬度测试和压缩试验。结果表明,5052金字塔型点阵夹芯板在430℃焊接具有良好的组织和性能。     

Resistance Element Welding of 6061 Aluminum Alloy to Uncoated 22MnMoB Boron Steel

A novel resistance element welding technology was applied to join 6061 Al alloy and uncoated 22MnMoB boron steel. To conduct the resistance element welding process, a technological hole was drilled in the Al sheet into which a Q235 steel rivet was inserted. Resistance spot welding was carried out at the rivet. The mechanical properties, fracture morphology, nugget formation process, dynamic resistance, microstructure, and hardness distribution of the resistance element welding were investigated. Traditional resistance spot weld joints were also prepared for comparison. Resistance spot welding could barely join Al 6061 and boron steel, and had a maximum tensile shear force of less than 1000 N. Novel resistance element welding could join the metals reliably with a maximum tensile shear force of over 7000 N and a relatively high toughness. Nugget formed at the interface of rivet and steel acted as loading position, and IMC interlayer connected rivet and aluminum.     

5083厚板TIG焊焊接工艺研究与应用

介绍了铝合金厚板(δ=28mm)TIG焊对接焊缝的工艺研究及应用。分析了大厚度5083材料的焊接工艺特点,采用合理的坡口形式,经过焊接工艺评定取得了较合理的工艺参数,最终焊制出了满意的焊接接头。     

Effect of surface treatment on the spot weldability of Fe-Mn-Al-Cr alloys

An Fe-26.3Mn 6,8Al-5.5Cr-0.9Mo-1.0C alloy was employed to study spot weldability. A series of Lobe curves was obtained under various welding conditions including electrode force, weld current, weld time and hold time. The acceptable weld currents ranged from 3.5–6kA, which is narrower than those of HSLA and plain carbon steel. The effect of surface treatment on the weldability of this alloy system was investigated. For an untreated specimen with -Al₂O₃ layer on the surface, the Lobe curve exhibits a narrower range than those of surface acid-pickled specimens. In addition, for the surface untreated specimen, the plug size was linearly proportional to the weld time. However, for the surface acid-pickled specimen, the degree of dependence of plug size on the weld time decreased as the electrode force increased. It was also observed that the indentation was proportional to the weld current for this alloy system. The dependence of indentation on the weld time was influenced by both the surface condition and the electrode force.     

Local melting and tool slippage during friction stir spot welding of Al-alloys

Local melting and tool slippage during friction stir spot welding of different Al-alloy base materials is examined using a combination of detailed microscopy and temperature measurement. The stir zone peak temperature during welding is limited by either the solidus of the alloy in question or by spontaneous melting of intermetallic particles contained in the as-received base material. When spontaneous melting occurs this facilitates tool slippage at the contact interface. Accurate stir zone temperature and grain size measurements are essential elements when estimating the strain rate using the Zener–Hollomon relation. In Al 2024 and Al 7075 spot welds spontaneous melting of second-phase particles produces a drastic reduction in strain rate values. In Al 5754 and Al 6061 spot welds there is a strong correlation between tool rotational speed and estimated strain values. Local melted films dissolve rapidly in the high temperature stir zone and when the spot weld cools to room temperature following welding. Evidence of local melting is observed in Al 7075 friction stir spot welded joints made using a combination of rapid quenching, high plunge rates, and extremely short dwell time settings.     

Welding failure of as-fabricated component of aluminum alloy 5052

This paper describes the failure analysis of the “tray section” made up of aluminum alloy 5052 which is used as a specimen holder in a research reactor. Fracture was observed in the central rod of alloy 5052 before it was taken for service. The fracture had occurred in a brittle mode without any gross plastic deformation at a location where the rod was welded to the stopper plate. Detailed microstructural examination was done using both optical and scanning electron microscopy. The weld fusion zone showed presence of high porosity and eutectic phases mainly along the inter-dendritic regions. These low melting temperature eutectics were rich in Si and Fe and led to weld cracking along the dendritic grains during solidification of the welds. Solidification cracking of alloy 5052 was related to pure aluminum filler wire used for welding that shifted the composition of the welds towards peak cracking sensitivity of 1.5 wt% Mg. The failure of the tray section was concluded to be due to welding defects, e.g. high porosity and solidification cracks. Recommendations to avoid this type of failure are also proposed.     

7075铝合金脉冲变极性等离子弧焊接头的双级时效行为EI北大核心CSCD

通过力学性能、电导率测试和TEM分析,对10 mm厚7075铝合金脉冲变极性等离子弧焊(pulse variable polarity plasma arc welding,PVPPAW)接头的双级时效行为特征进行研究,并确定了较为合理的双级时效工艺。结果表明:7075铝合金PVPPAW接头的抗拉强度随着终时效温度的升高和终时效时间的延长先增大后减小。经160℃,12 h终时效处理后的接头抗拉强度为545.1 MPa,比焊态时提高了37%。此时接头的电导率为27.9%IACS,抗应力腐蚀性能有所提高。焊缝中心主要强化相为η′相和η相,随着终时效温度的升高和终时效时间的延长,晶内与晶界的析出相逐渐长大和粗化,晶界的无沉淀析出带变宽。     

Resistance spot welding joints of AISI 316L austenitic stainless steel sheets: Phase transformations,mechanical properties and microstructure characterizations

In this paper, we aim to optimize welding parameters namely welding current and time in resistance spot welding (RSW) of the austenitic stainless steel sheets grade AISI 316L. Afterward, effect of optimum welding parameters on the resistance spot welding properties and microstructure of AISI 316L austenitic stainless steel sheets has been investigated. Effect of welding current at constant welding time was considered on the weld properties such as weld nugget size, tensile–shear load bearing capacity of welded materials, failure modes, failure energy, ductility, and microstructure of weld nuggets as well. Phase transformations that took place during weld thermal cycle were analyzed in more details including metallographic studies of welding of the austenitic stainless steels. Metallographic images, mechanical properties, electron microscopy photographs and micro-hardness measurements showed that the region between interfacial to pullout mode transition and expulsion limit is defined as the optimum welding condition. Backscattered electron scanning microscopic images (BE-SEM) showed various types of delta ferrite in weld nuggets. Three delta ferrite morphologies consist of skeletal, acicular and lathy delta ferrite morphologies formed in resistance spot welded regions as a result of non-equilibrium phases which can be attributed to the fast cooling rate in RSW process and consequently, prediction and explanation of the obtained morphologies based on Schaeffler, WRC-1992 and Pseudo-binary phase diagrams would be a difficult task.     

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.