Nugget formation and growth during resistance spot welding of aluminium alloy 5182

Abstract

Surface interaction at the worksheet/worksheet interface during resistance spot welding of aluminium alloy 5182 with spherical tip electrodes was investigated. Oxide layer cracking and nugget formation were focused. Both experimental work and finite element analysis were employed to explain the contact behaviour at this interface. It was found that sheet separation and thus bending occurred during the squeezing phase of the resistance spot welding process and suggested a profound influence on nugget formation. The sheet separation caused enlarged and aligned cracks in the surface oxide layers which led to a good metal‐to‐metal contact near the periphery of the faying surface. High current densities which occurred at the beginning of the current phase caused significant heat generation in this zone. Consequently, the melting at the faying surface started near the periphery and moved in towards the central zone of the contact region to produce a ‘doughnut shaped’ nugget with a filled‐in but thin central region.

On a étudié l'interaction de la surface à l'interface feuille de travail‐feuille de travail lors du soudage par points par résistance de l'alliage d'aluminium 5182 avec des électrodes à bout sphérique. On s'est concentré sur la fracture de la couche d'oxyde et sur la formation du noyau. On a utilisé tant le travail expérimental que l'analyse par éléments finis pour expliquer le comportement de contact à cette interface. On a trouvé que la séparation de la feuille, et donc le pliage, se produisait lors de la phase de compression du procédé de soudage par points par résistance, suggérant une influence profonde sur la formation du noyau. La séparation de la feuille résultait en fissures agrandies et alignées dans les couches d'oxyde de la surface, ce qui amenait un bon contact de métal à métal près de la périphérie de l'aire de contact. Des densités élevées de courant, qui se produisaient au début de la phase de courant, résultaient en un dégagement important de chaleur dans cette zone. Conséquemment, la fonte de l'aire de contact commençait près de la périphérie et se déplaçait vers la zone centrale de la région de contact, produisant un noyau en forme d'anneau avec une région centrale remplie, mais mince.     

Weld metal composition change during conduction mode laser welding of aluminum alloy 5182

Selective vaporization of volatile elements during laser welding of automotive aluminum alloys affects weld metal composition and properties. An experimental and theoretical study was carried out to seek a quantitative understanding of the influences of various welding variables on vaporization and composition change during conduction mode laser welding of aluminum alloy 5182. A comprehensive model for the calculation of vaporization rate and weld metal composition change was developed based on the principles of transport phenomena, kinetics, and thermodynamics. The calculations showed that the vaporization was concentrated in a small high-temperature region under the laser beam where the local vapor pressure exceeded the ambient pressure. The convective vapor flux driven by the pressure gradient was much higher than the diffusive vapor flux driven by the concentration gradient. The computed weld pool geometry, vaporization rates, and composition changes for different welding conditions agreed well with the corresponding experimental data. The good agreement demonstrates that the comprehensive model can serve as a basis for the quantitative understanding of the influences of various welding variables on the heat transfer, fluid flow, and vaporization occurring during conduction mode laser welding of automotive aluminum alloys.     

微量Sc、Zr和Er对5182铝合金焊缝组织与性能的影响

在ER5356铝合金焊丝中添加了微量的Sc、Zr和Er,制备出微合金化ER5356铝合金焊丝.使用微合金化焊丝焊接4 mm厚的5182铝合金板材,焊接采用手工钨极惰性气体保护焊.利用金相显微镜,X射线衍射以及透射电镜研究了微量的Sc、Zr和Er对5182铝合金焊缝处显微组织的影响.结果表明,在ER5356铝合金焊丝中添加的Sc、Zr和Er可以有效改善5182铝合金板材焊后焊缝处的显微组织,显著细化组织晶粒.其中,ER5356铝合金焊丝中单独添加0.3%Sc焊后效果最佳.     

Effect of bake hardening treatment on the mechanical behaviors of aluminum alloy spot welding joints

In this paper, the mechanical behaviors of the spot welding aluminum alloy in both baked and unbaked conditions are investigated. The spot welded specimens with two different base aluminum alloy sheets are studied by tensile and fatigue tests. The test results of unbaked specimens are compared with the baked which are baked at 185°C for 20 min. The stress distribution of the specimens and the fatigue life under different stress is simulated by finite element method (FEM). The paint bake treatment can increase the ductility of both base materials and spot joints. The fatigue life is increased with high loading stress due to the pinning effect; while with low fatigue load, it has the opposite rule because the compressive residual stress is released during the bake hardening process, which can restrain the fatigue crack growth rate in low stress. The simulated results of FEM model can well verify the experiment data.     

铝合金焊接性能及焊接接头性能的研究

在高铁、地铁列车的制造中,铝合金材料更是列车车体的主要材料之一,然而由于铝合金材料在焊接性能、焊接接头性能方面仍存在一定的不足,经常会出现气孔、裂纹等缺陷,因此高铁、地铁列车铝合金车体的焊接施工质量仍然很难保证。本文对铝合金的焊接性能以及焊接接头性能进行了分析。     

汽车钢板点焊工艺的发展

通过文献调研分析了冷轧、镀锌及高强汽车板的焊接特点。采用普通点焊工艺,铁锌合金镀锌板比纯镀锌板点焊工艺窗口宽约30%,但预热后纯镀锌板焊接性能较好;FEA模拟Mn-Si系TRIP800钢的焊点十字拉伸强度明显高于Mn系DP450钢,但实际试验结果却明显较低;Mn系1.5mm规格的DP800钢在点焊电流11 000A及时间0.6s时,焊点拉剪试验以韧性特征为主,焊点拉剪力高于20kN。1 300MPa热成型钢点焊接头有一定软化倾向。不同种类高强钢的合金元素含量和生产工艺的不同,其焊接性能体现出较大差异。     

6061-T6铝合金薄板的搅拌摩擦焊接

采用搅拌摩擦焊(FSW)技术对1 mm厚6061-T6铝合金薄板进行了对接.研究了焊接工艺参数的范围,实验测试了焊接接头的强度、硬度和延伸率,利用金相显微镜、扫描电镜和透射电镜分析了接头的微观组织.结果表明:对于1 mm厚度6061-T6铝合金,FSW的最优工艺参数为旋转速度1 800 r·min^-1,焊接速度1000 mm·min^-1;在此参数下,接头的硬度值达到母材的80%左右,抗拉强度达到母材的103%,延伸率达到母材的54%;接头的力学性能与微观结构相符.     

Through-thickness texture variation in cold-rolled AA 5182 aluminum alloy with an initial {001}〈110〉 texture

Through-thickness texture variation in cold-rolled AA 5182 aluminum alloy with an initial {001}〈110〉 texture was investigated by X-ray diffraction. A detailed quantitative analysis of texture volume fractions in several layers was performed. The effect of initial through-thickness texture gradient and roll-gap geometry on the texture evolution during multipass rolling was determined. A pronounced texture variation through the thickness was found in the initial material. The shear deformation produced by the friction between the roll and the sheet contact surface may be neglected in the present experiment with oil as a lubricant. At low strains with the l/h ratio from 0.9 to 2.0, the shear deformation caused by the roll-gap geometry slightly decreased the lattice rotation from the {001}〈110〉 orientation to the C orientation at the intermediate layer, while the roll-gap geometry did not produce a shear strain at large strains with higher values of the l/h ratio. The strong initial r-cube texture at the center layer stimulated the transformation from the r-cube orientation to the C orientation and the formation of the β fiber during rolling.     

5182铝合金热变形行为研究

采用了MMS-100热力模拟试验机对5182铝合金进行单道次压缩实验,对其热变形行为展开研究,构建了流变应力模型和加工图.结果发现:5182铝合金的流变应力随温度的升高、应变速率的降低而逐渐减小;高温条件会促使动态再结晶的发生,而应变速率的影响可以忽略;合金的真应力-真应变曲线在高应变速率时会出现锯齿状波动;合金在加热温度420~500 ℃、真应变ε= 0.4、应变速率的热变形条件下会有一个高功率耗散因子区域;合金在450 ℃附近存在较大安全加工区域.      

Effects of au plating on small-scale resistance spot welding of thin-sheet nickel

The effects of Au plating on joint formation and joint strength in small-scale resistance spot welding (SSRSW) of Ni sheets have been investigated using tensile-shear testing, optical microscopy, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). The results show that the mechanisms of the joint formation during the welding of Au-plated Ni sheets involve solid-state bonding, brazing, and fusion welding. The comparison between SSRSW of Au-plated Ni and bare Ni sheets and large-scale resistance spot welding (LSRSW) of Zn-coated steels is also discussed.     

Effects of Au plating on dynamic resistance during small-scale resistance spot welding of thin Ni sheets

The effects of Au plating on dynamic resistance during small-scale resistance spot welding of thin Ni sheets have been investigated. Compared to the small-scale resistance spot welding of bare Ni, the Au-plated material showed much lower initial static resistance, and the contribution of constriction resistance to overall dynamic resistance reduced to a negligible value very early in the welding sequence because of the low softening temperature and its lack of surface oxide film. For the same reason, the asperity heating and surface breakdown stages during resistance spot welding of bare Ni were not observed during resistance spot welding of Au-plated Ni. Furthermore, the partial surface melting stage was replaced by the solid-state bonding and brazing stages. Step increases in the sheet-to-sheet dynamic resistance curves of Au-plated Ni were shown to be due to the peripheral unzipping of a prior solid-state bond caused by uneven local thermal expansion. The electrode-to-electrode dynamic resistance curve when welding Au-plated material is not useful for monitoring the state of nugget formation.     

应变测量在点焊接头失效分析中的应用

电阻点焊接头的失效分析是整车结构强度和碰撞分析中的难点。采用应变测量系统记录和分析了剪切拉伸试验中焊点的破坏过程,分析结果显示:在焊点拔出的失效方式下,破裂点的应变路径为平面应变类型;低速拉伸下的试验速度对应变路径无明显影响,破裂点的断裂应变接近材料的极限应变,应变速率在剪切拉伸过程中呈加速增长趋势。测试分析表明,采用应变分析方法进行点焊接头失效分析是可行的。     

TRIP600 高强钢的点焊工艺研究

对1．8mm规格的TRIP600钢进行了点焊试验、拉剪试验和接头的金相分析,研究了焊接电流、焊接时间等点焊工艺参数对接头力学性能的影响。结果表明,焊接时间为0．20、0．24、0．30S,TRIP600钢的可焊电流范围分别为11-12、10．5～12、10-11．5kA,拉剪试验失效模式为扣式或母材撕裂、且无飞溅的焊点,拉剪力达到27-30kN。     

Parameters controlling microstructure and hardness during friction-stir welding of precipitation-hardenable aluminum alloy 6063

The aluminum (Al) alloys 6063-T5 and T4 were friction-stir welded at different tool rotation speeds (R), and then distributions of the microstructure and hardness were examined in these welds. The maximum temperature of the welding thermal cycle rose with increasing R values. The recrystallized grain size of the weld increased exponentially with increasing maximum temperature. The relationship between the grain size and the maximum temperature satisfied the static grain-growth equation. In the as-welded condition, 6063-T5 Al was softened around the weld center, whereas 6063-T4 Al showed homogeneous hardness profiles. Different R values did not result in significant differences in the hardness profile in these welds, except for the width of the softened region in the weld of 6063-T5 Al. Postweld aging raised the hardness in most parts of the welds, but the increase in hardness was small in the stir zone produced at the lower R values. Transmission electron microscope (TEM) observations detected a similar distribution of the strengthening precipitates in the grain interiors and the presence of a precipitation-free zone (PFZ) adjacent to the grain boundaries in all the welds. Microstructural analyses suggested that the small increase in hardness in the stir zone produced at the lower R values was caused by an increase in the volume fraction of PFZs.     

CO2 laser beam welding of 6061-T6 aluminum alloy thin plate

Laser beam welding is an attractive welding process for age-hardened aluminum alloys, because its low heat input minimizes the width of weld fusion and heat-affected zones (HAZs). In the present work, 1-mm-thick age-hardened Al-Mg-Si alloy, 6061-T6, plates were welded with full penetration using a 2.5-kW CO₂ laser. Fractions of porosity in the fusion zones were less than 0.05 pct in bead-on-plate welding and less than 0.2 pct in butt welding with polishing the groove surface before welding. The width of a softened region in the-laser beam welds was less than 1/4 times that of a tungsten inert gas (TIG) weld. The softened region is caused by reversion of strengthening β″ (Mg₂Si) precipitates due to weld heat input. The hardness values of the softened region in the laser beam welds were almost fully recovered to that of the base metal after an artificial aging treatment at 448 K for 28.8 ks without solution annealing, whereas those in the TIG weld were not recovered in a partly reverted region. Both the bead-on-plate weld and the butt weld after the postweld artificial aging treatment had almost equivalent tensile strengths to that of the base plate.     

Friction-stir welding effects on microstructure and fatigue of aluminum alloy 7050-T7451

Aluminum alloy 7050 was friction-stir welded (FSW) in a T7451 temper to investigate the effects on the microstructure and mechanical properties. Results are discussed for the as-welded condition (as-FSW) and for a postweld heat-treated condition consisting of 121 °C for 24 hours (as-FSW + T6). Optical microscopy and transmission electron microscopy (TEM) examination of the weld-nugget region show that the FS welding process transforms the initial millimeter-sized pancake-shaped grains in the parent material to fine 1 to 5 μm dynamically recrystallized grains; also, the FS welding process redissolves the strengthening precipitates in the weld-nugget region. In the heat-affected zone (HAZ), the initial grain size is retained, while the size of the strengthening precipitates and of the precipitatefree zone (PFZ) is coarsened by a factor of 5. Tensile specimens tested transverse to the weld show that there is a 25 to 30 pct reduction in the strength level, a 60 pct reduction in the elongation in the as-FSW condition, and that the fracture path is in the HAZ. The postweld heat treatment of 121 °C for 24 hours did not result in an improvement either in the strength or the ductility of the welded material. Comparison of fatigue-crack growth rates (FCGRs) between the parent T7451 material and the as-FSW + T6 condition, at a stress ratio of R = 0.33, shows that the FCG resistance of the weldnugget region is decreased, while the FCG resistance of the HAZ is increased. Differences in FCGRs, however, are substantially reduced at a stress ratio of R = 0.70. Analysis of residual stresses, fatigue-crack closure, and fatigue fracture surfaces suggests that decrease in fatigue crack growth resistance in the weld-nugget region is due to an intergranular failure mechanism; in the HAZ region, residual stresses are more dominant than the microstructure improving the fatigue crack growth resistance.     

多向锻造道次及终锻温度对5182铝合金组织性能的影响

采用多向锻造实验的方法研究了锻造道次及终锻温度对5182铝合金组织性能的影响.研究结果表明:终锻温度为340℃时,随锻造道次由3增至6,试样锤头附近的组织破碎变细,但未发生明显再结晶;试样心部的变形组织在经3道次锻造后开始发生部分再结晶;经6道次锻造后试样心部发生完全再结晶.340℃以上进行变形时,变形程度对5182铝合金的硬度影响不大.终锻温度为230℃时,随锻造道次由3增至6,试样锤头附近及心部的组织均破碎变细,但未发生明显再结晶,经6道次锻造后试样心部为纤维状变形组织.终锻温度为230℃时,变形程度大的试样心部区域的硬度值较高.     

淬火介质对2519铝合金抗晶间腐蚀性能的影响

2519铝合金热轧板在530℃下保温2 h后,分别于室温(20℃)水、机油和空气中淬火,经10%冷轧变形后,于180℃进行人工时效。通过显微硬度测试、金相显微镜和透射电镜观察以及动电位极化曲线的测试,研究淬火介质对2519铝合金抗晶间腐蚀性能的影响。结果表明:经空气淬火的合金晶界析出第二相,晶界析出相粒子粗大,呈不连续分布,无沉淀析出带宽度为130 nm,晶间腐蚀深度为300μm;经室温水淬和机油淬火并时效后,晶界上的析出相尺寸较小,呈链状分布,无沉淀析出带宽度分别为20 nm和30 nm。经室温(20℃)水淬火的合金未发生晶间腐蚀,经机油淬火的合金只发生局部点蚀,说明经室温水淬的合金比经空气和机油淬火的合金时效后具有更好的抗晶间腐蚀性能。     

Improved microstructure and properties of 6061 aluminum alloy weldments using a double-sided arc welding process

Due to its popularity and high crack sensitivity, 6061 aluminum alloy was selected as a test material for the newly developed double-sided arc welding (DSAW) process. The microstructure, crack sensitivity, and porosity of DSAW weldments, were studied systematically. The percentage of fine equiaxed grains in the fully penetrated welds is greatly increased. Residual stresses are reduced. Porosity in the welds is reduced and individual pores are smaller. It was also found that the shape and size of porosity is related to solidification substructure. In particular, a weld metal zone with equiaxed grains tends to form small and dispersed porosity, whereas elongated porosity tends to occur in columnar grains.