铝/镀锌钢搅拌摩擦铆焊接头组织与力学性能

为实现铝钢之间的优质连接,采用搅拌摩擦铆焊新方法对6061铝合金和DP600镀锌钢进行搭接点焊,利用扫描电子显微镜、能谱仪及拉伸试验对接头的微观组织及力学性能进行了研究.结果表明:接头成形平整美观,中心没有匙孔;接头包含铆接区和扩散区,其中在铆接区铝合金以铝柱的形式嵌入到钢板的圆孔中,形成了一个"铝铆钉",底部有富铝的α固溶体偏聚,圆孔四周形成扩散区,铝和钢形成了冶金结合,依靠金属间化合物Fe Al3连接在一起;接头有3种断裂形式,在最佳工艺参数下接头的抗剪力达到8.2 k N;铝柱上断口的微观形貌是被拉长的韧窝,扩散区的断口由灰色基体和白色颗粒组成.     

汽车车身 6061 铝合金电阻点焊工艺优化

目的优化6061铝合金电阻点焊工艺参数。方法采用正交实验对2 mm厚6061铝合金薄板进行对接,并进行了方差分析和性能测试。结果当选取方差最优方案,即电流22 k A、电极压力0.15 MPa、焊接时间15个周波这组焊接参数时,接头抗拉力为5.444 k N,较正交实验最大值提高了25.5%。而选取电流22 k A、电极压力0.20 MPa、焊接时间11个周波的焊接参数时,接头抗拉力最大,为6.262 k N,较正交实验最大值提高了44.35%。结论 6061铝合金最佳焊接参数为:电流22 k A、电极压力0.20 MPa、焊接时间11个周波。     

Failure mechanisms of friction stir spot welds of AA6061‐T6/DP590 steel during tensile‐shear testing

The tensile‐shear test was conducted for the evaluation of shear load and failure mechanisms of dissimilar friction stir spot weldments of AA6061‐T6/DP590 dual‐phase steel sheets. The joints were fabricated using a penetrated pin into the steel sheet (lower member). Such design resulted in the formation of a hook at the joint interface and an intermetallic compound layer (IMC) between the upper part of the hook and Al‐side. A maximum tensile‐shear load of ~2950 N was measured for the joint fabricated using a heat input of ~10 kJ; a lower strength was noted by varying the heat input. Partial plug was the failure mechanism in all joints. However, three different types of crack propagation paths were found depending on the heat input: along the interface between 6061 Al and IMC layer, thru 6061 Al near the joint interface, or within the IMC layer using relatively low, moderate, and high heat inputs.     

Ti/Al异种金属微电阻点焊接头力学性能及显微组织

目的 对0.2 mm厚的TC4和2A12薄板进行微电阻点焊研究,并确定最佳工艺参数。方法 通过测量接头剪切强度以及观察接头横截面形貌,研究工艺参数对接头拉剪力的影响,采用扫描电子显微镜(SEM)对点焊接头的连接特征及断裂行为进行深入研究,借助金相显微镜对接头微观组织进行分析。结果 焊接时间对接头的抗拉剪力没有显著影响,当焊接电流为4.2 kA,电极压力为110 N,焊接时间为10 ms时,取得最大剪切强度125.82 N。接头有两种断裂方式,分别为沿熔核中心断裂和纽扣状断裂。沿熔核中心断裂的断口呈现脆性断裂的特征,钮扣状断裂的断口熔核中心处呈韧性断裂特征,其热影响区呈脆性断裂特征。结论 实现了Ti/Al薄板的微电阻点焊,并通过改变工艺参数获得良好的剪切性能。观察焊缝的显微组织发现,远离熔核中心、靠近铝母材侧的区域,由于铝侧母材散热较好,组织为等轴晶以及细小的柱状晶,晶粒较小。靠近熔核中心的区域为组织较为粗大的柱状枝晶。     

Microstructure and mechanical properties of dissimilar Al alloy/steel joints prepared by a flat spot friction stir welding technique

The 6061-T6 Al alloy and mild steel plate with a thickness of 1 mm were successfully welded by the flat spot friction stir welding technique, which contains two steps during the entire welding process. The rotating tools with different probe lengths of 1.0, 1.3 and 1.5 mm were used in the first step, during which a conventional spot FSW was conducted above a round dent previously made on the back plate. However, sound Al/Fe welds with similar microstructure and mechanical properties can still be obtained after the second step, during which a probe-less rotating tool was used to flatten the weld surface. The sound welds have smooth surface without keyholes and other internal welding defects. No intermetallic compound layer but some areas with amorphous atomic configuration was formed along the Al/Fe joint interface due to the lower heat input. The shear tensile failure load can reach a maximum value of 3607 N and fracture through plug mode. The probe length has little effect on the weld properties, which indicates that the tool life can be significantly extended by this new spot welding technique.     

马氏体含量对合金化热镀锌双相钢电阻点焊接头组织与性能的影响

目的 研究在双相钢电阻电焊过程中马氏体含量对点焊接头组织、性能的影响规律。方法 使用电阻点焊机对DP780、DP980、DP1180 3种马氏体含量不同的锌铁合金化热镀锌双相钢进行焊接,利用欧姆表、光学显微镜、扫描电镜、拉伸机和显微硬度计等设备,对基板的电阻率、工艺窗口、接头力学性能、焊点断裂模式、金相组织进行表征。结果 在AWS D8.9M-2012焊接标准体系下,DP780、DP980、DP1180焊接电流窗口依次减小,DP780、DP980、DP1180 3种材料在最大焊接电流下的焊核直径基本一致;熔核区硬度呈增大趋势,DP780点焊接头软化不明显,DP980和DP1180的热影响区出现明显的软化现象,这主要是由母材热影响区中的马氏体回火造成的。DP780、DP980、DP1180的最大剪切力分别为23 062、27 317、28 183 N。DP780为拔核断裂模式,DP980和DP1180为部分拔核断裂模式。结论 双相钢中马氏体含量的增加会使焊接电流窗口降低,整体向焊接电流减小的方向偏移,但是会提高上限电流的焊点承载强度。     

Impact performance of friction welded butt joints between 6061-T6 aluminium alloy and type 304 stainless steel

Abstract

The tensile strength and energy absorption for dissimilar metal friction welds between 6061-T6 Al alloy and type 304 stainless steel at high rates of loading were determined using the split Hopkinson bar. Cylindrical tensile specimens machined from as welded butt joints of 13 mm in diameter were used in both static and impact tests. Friction welding was conducted using a brake type friction welding machine under two different welding conditions. The effects of welding conditions and loading rate on the joint tensile properties were examined. Results show that the joint tensile properties were greatly affected by the welding parameters, and were slightly enhanced with increased loading rate. Scanning electron microscope observations revealed that the tensile fracture modes in the butt joint specimens varied with loading rate and depend on welding conditions. Microhardness profiles across the weld interface were measured to investigate the extent of the heat affected zone. The slight enhancement of the joint tensile properties with increasing loading rate is primarily attributed to the strain rate dependence of the thermally softened 6061-T6 Al alloy base material.     

铝/钢搅拌摩擦辅助铆接接头界面特性研究

目的探究搅拌摩擦辅助铆接铝/钢接头界面的结合特征,解决传统铆接力学性能较低的问题。方法采用搅拌摩擦焊技术实现了对3mm厚不锈钢板和4mm厚的铝合金板的搭接点焊,采用OM及SEM对铝/钢接头界面结合情况进行分析,并利用EDS对界面处元素分布进行分析。结果搅拌头转速1180 r/min、焊接时间120 s、下压量0.2 mm时,铝/钢接头界面结合较好,平均拉剪力达到6519 N,且在铝/钢界面处产生FeAl金属间化合物。受摩擦热作用的影响,位于下板的铝母材晶粒发生长大变粗,铝铆钉与铝板结合紧密,铝铆钉与铝板的结合情况受搅拌头压力的影响更为显著。结论搅拌摩擦辅助铆接铝/钢异种合金,实现了铆钉与铝板和钢板的有效冶金结合,在铝钢结合界面处存在原子的互扩散现象,且有相应的金属间化合物生成。     

铝/钢异种金属的超声振动强化搅拌摩擦焊接工艺

采用新型超声振动强化搅拌摩擦焊接工艺实现了6061-T6铝合金以及QP980高强钢的搭接焊, 对比分析了有无超声作用下, 接头的宏观形貌、微观组织和拉伸剪切性能, 同时研究了超声振动对焊接载荷的影响。结果表明: 焊接前对母材施加超声振动, 可以起到软化母材的作用, 促进了材料的塑性流动, 扩大了铝/钢界面区和焊核区, 使更多的钢颗粒随搅拌针旋转进入铝合金侧, 在界面区边缘形成钩状结构, 进而提高了接头的失效载荷; 超声改变了FSW接头断裂位置和断口形貌, 提高了接头力学性能, 在本实验工艺参数范围内, 接头最大的平均失效载荷为4.99 kN; 当焊接速度为90 mm/min, 下压量为0.1 mm时, 施加超声振动使接头的平均失效载荷提高了0.98 kN, 拉剪性能提升28.24%;施加超声振动后轴向力F_z、搅拌头扭矩M_t和主轴输出功率分别下降2.46%, 6.44%和4.59%。     

Fatigue behaviour of dissimilar friction stir spot welds between A6061‐T6 and low carbon steel sheets welded by a scroll grooved tool without probe

A6061 and low carbon steel sheets, whose thicknesses were 2 mm, were welded by a friction stir spot welding (FSSW) technique using a scroll grooved tool without probe (scroll tool). Tensile‐shear fatigue tests were performed using lap‐shear specimens at a stress ratio R = 0.1, and the fatigue behaviour of dissimilar welds was discussed. Tensile‐shear force of the dissimilar welds was higher than that of the A6061 similar ones. Furthermore, the dissimilar welds exhibited nearly the same fatigue strengths as the A6061 similar ones, indicating FSSW by a scroll tool was effective technique for joining aluminium to steel sheet. Fatigue fracture modes of the dissimilar welds were dependent on load levels, where shear fracture through the interface between A6061 and steel occurred at high load levels, while crack grew through A6061 sheet at low load level.     

A parametric study on fatigue strength of spot‐weld joints

Fastening elements usually lead to high stress concentrations; fatigue failure thus becomes the most critical failure mode for a fastening element itself or the region around it under fluctuating stresses. A designer should seek the ways of increasing fatigue strength of a joint to ensure the safety of the whole structure. Resistance spot welding is the most preferred method to join metal sheets. The design variables for spot‐weld joints affecting their strengths are basically sheet thickness, spot‐weld nugget diameter, number of spot welds and the joint type as exemplified in tensile shear (TS), modified tensile shear (MTS), coach peel (CP) and modified coach peel (MCP) specimens. In this study, the effects of these parameters on the fatigue life of spot‐weld joints have been investigated. For this purpose, one of the most reliable fatigue assessment models, Coffin–Manson approach, was used. In order to accurately determine the stress and strain states, a nonlinear finite element analysis was carried out taking into account plastic deformations, residual stresses developed after unloading and contacting surfaces. The results provide designers with some guidelines to foresee the impact of design changes on fatigue strength of spot‐weld joints.     

Optimization of welding parameters for resistance spot welding of TRIP steel with response surface methodology

Many automotive companies are endeavouring to reduce the weight of the car body in response to various environmental issues. One initiative is the development of TRIP (Transformation Induced Plasticity) steels with a high strength and ductility. Resistance spot welding is a complex process, which requires specific optimal welding conditions based on experimental data. However, the trial-and-error method to determine the optimal conditions requires a large number of experiments, and so response surface methodology has been employed to overcome this problem. The second-order model was used here. This has been used in the resistance spot welding process of the TRIP steel and galvanized TRIP steel with a zinc-coated layer to optimize the welding parameters. The welding current, welding time, and welding force were selected as input variables, and the shear strength and indentation were selected as output variables.     

Mechanical performance of three thickness resistance spot welded low carbon steel

Abstract

Resistance spot welding is the dominant process for joining sheet metals in automotive industry. Despite the application of three thickness resistance spot welds in this industry, present guidelines and recommendations are limited to two thickness spot welds. Study towards better understanding of weld nugget growth and mechanical properties is the first step to understanding the welding behaviour and developing proper guidelines for the three thickness resistance spot welding. In this paper, weld nugget growth, mechanical performance and failure behaviour of three thickness low carbon steel resistance spot welds are investigated. Macrostrcutural and microstructural investigations, microhardness tests and quasi-static tensile–shear tests were conducted. Mechanical performance of the joint was described in terms of peak load, energy absorption and failure mode. In order to understand the failure mechanism, micrographs of the cross-sections of the spot welded joints during and after tensile–shear are examined by optical microscopy. Unlike two thickness resistance spot welded joint, weld nugget was formed in the geometrical centre of the joint (i.e. centre of the middle sheet). Weld nugget size along sheet/sheet interface was greater than that of along geometrical centre of the joint. Increasing welding time leads to increases in peak load and energy absorption of the joint and transition of interfacial failure mode to pullout failure mode, primarily due to the enlargement of weld nugget size along sheet/sheet interface.     

GH4145激光点焊工艺及接头性能研究

目的研究不同工艺参数下高温合金的微激光点焊接头性能。方法利用Nd:YAG激光器对厚度为0.23 mm和0.13 mm的GH4145进行搭接点焊,利用微型拉伸机、金相显微镜、硬度计对接头的物理性能进行测试,并观察组织结构。结果脉宽长度为6.0 ms,输出功率百分比为20%(输出功率16 W)时,最大拉伸剪切力为178.59 N,靠近焊点中心位置两边测试点硬度分别达到HV517和HV506。结论接头表面熔化尺寸、焊接接头的拉伸剪切力和硬度随着激光功率的增大而增大,增大到一定值时停止变化,不同参数下达到不同的最大值,组织无明显规律变化。近表面处出现等轴晶,表面以下至熔合线的显微组织为树状晶。通过激光搭接点焊将两片GH4145连接,接头处拉伸剪切力、硬度均低于母材,功率增大有助于提高接头的抗拉强度。     

Overload failure behaviour of dissimilar thickness resistance spot welds during tensile shear test

Abstract

Resistance spot welding is the dominant process for joining sheet metals in automotive industry. Even thickness combinations are rarely used in practice; therefore, there is clearly a practical need for failure behaviour investigation of uneven thickness resistance spot welds. The aim of the present paper is to investigate the failure mode and failure mechanism of dissimilar thickness low carbon steel resistance spot welds during tensile shear overload test. Microstructural investigations, microhardness tests and tensile shear tests were conducted. Mechanical properties of the joints were described in terms of peak load, energy absorption and failure mode. In order to understand the failure mechanism, micrographs of the cross-sections of the spot welded joints during and after tensile shear are examined by optical microscopy. It was found that for well established weld nuggets, the final solidification line is located in the geometrical centre of the joint. In pull-out failure mode, failure is initiated by necking of the base metal at the thinner thickness sheet. Finally, it was concluded that weld nugget size, weld penetration and the strength of the thinner sheet are the main controlling factors of the peak load and energy absorption of dissimilar thickness spot welds.     

Analysis of process parameters effects on friction stir welding of dissimilar aluminum alloy to advanced high strength steel

Thin sheets of aluminum alloy 6061-T6 and one type of Advanced high strength steel, transformation induced plasticity (TRIP) steel have been successfully butt joined using friction stir welding (FSW) technique. The maximum ultimate tensile strength can reach 85% of the base aluminum alloy. Intermetallic compound (IMC) layer of FeAl or Fe₃Al with thickness of less than 1 μm was formed at the Al–Fe interface in the advancing side, which can actually contribute to the joint strength. Tensile tests and scanning electron microscopy (SEM) results indicate that the weld nugget can be considered as aluminum matrix composite, which is enhanced by dispersed sheared-off steel fragments encompassed by a thin intermetallic layer or simply intermetallic particles. Effects of process parameters on the joint microstructure evolution were analyzed based on mechanical welding force and temperature that have been measured during the welding process.     

Microstructure and mechanical properties of friction spot welded 6061-T4 aluminum alloy

Friction spot welding (FSpW) is a relatively new solid state joining technology developed by GKSS. In the present study, FSpW was applied to join the 6061-T4 aluminum alloy sheet with 2 mm thickness. The microstructure of the weld can be classified into four regions, which are stir zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and the base material (BM), respectively. Meanwhile, defects such as bonding ligament, hook and voids are found in the weld, which are associated to the material flow. The hardness profile of the weld exhibits a W-shaped appearance and the minimum hardness is measured at the boundary of TMAZ and SZ. Both the tensile/shear strength and cross-tension strength reach the maximum of 7117.0 N and 4555.4 N at the welding condition of the rotational speed of 1500 rpm and duration time of 4 s. Compared to cross-tension strength, the tensile/shear strength were stable with the variation of processing parameters. Three different fracture modes are observed under tensile/shear loading, which are plug type fracture, shear fracture and plug-shear fracture. There are also there different fracture modes under cross-tension loading, which are plug type fracture (on the upper sheet), nugget debonding and plug type fracture (on the lower sheet).     

镀锌钢板与6061铝合金搭接搅拌摩擦钎焊

目的 分析Q235镀锌钢与6061铝合金搅拌摩擦钎焊接头在不同旋转速度下的组织性能。方法 使用0.3 mm的Zn作为中间层,通过搅拌摩擦钎焊,焊接6061铝合金与Q235镀锌钢,观察测试其接头组织和力学性能。结果 转速从660 r/min增加到1750 r/min时,随着进入到6061铝合金近缝区Zn元素的增加,铝合金搅拌区孔洞变小。界面Zn过渡层变薄。在适中的转速下,界面结合良好。接头最大平均拉剪力先增加后降低,界面显微硬度升高,硬度梯度增加。结论 搅拌头在1320 r/min转速下,测得搅拌摩擦钎焊接头平均拉剪力为2.33 kN。     

Resistance spot weldability and high cycle fatigue behaviour of martensitic (M190) steel sheet

Resistance spot welding characteristics of martensitic sheet steel (M190) was investigated using a peel test, microhardness test, tensile shear test and fatigue test. Tensile shear test provides better spot weld quality than conventional peel test and hardness is not a good indicator of the susceptibility to interfacial fracture. Unlike DP 600 steel, the maximum load carrying capability is affected by the mode of fracture. At high load low cycle range, weld parameters have a significant difference in the S–N curves. But, almost similar fatigue behaviour of the spot welds is noted at low load high cycle range. However, when applied load was converted to stress intensity factor, the difference in fatigue behaviour between welds and even DP 780 steel diminished. Furthermore, a transition in fracture mode, that is, interfacial and plug and hole type at about 50% of yield load were observed.
[* Note: Correction made on 16 Aug 2010 after first publication online on 28 June 2010. The authors' affiliations were corrected. Under Results and Discussion, in reference to the HAZ hazardness in the ‘Micro hardness profile’ section, Figure 2 was changed to Figure 3. In reference to the welding parameters under ‘Tensile properties’ section, note that Figure 4 represents 7/200 and Figure 5 represents 5/300. In reference to the low cycles behaviour of S‐N curves in the ‘Fatigue’ section, Figure 5 was changed to Figure 6.]     

Microstructure and Property Relationships in Resistance Spot Weld between 7114 Interstitial Free Steel and 304 Austenitic Stainless Steel

Due to the differences in physical, chemical and mechanical properties of the base metals, the resistance spot welding of dissimilar materials is generally more challenging than that of similar materials. The influence of the primary welding parameters affecting the heat input such as peak current on the morphology, microhardness, and tensile shear load bearing capacity of dissimilar welds between 304 grades austenitic and 7114 grade interstitial free steel has been investigated in this study. The optimum welding parameters producing maximum joint strength were established at a peak current of 9 kA, where the electrode force is kept 6×10-5 Pa and weld time is kept constant 17 cycles, respectively. The primary cause of weakening the weldment is identified as the excessive grain growing region of heat affected zone (HAZ) in case of 7114 grade interstitial free steel.