Microstructure and failure behavior of dissimilar resistance spot welds between low carbon galvanized and austenitic stainless steels

Resistance spot welding was used to join austenitic stainless steel and galvanized low carbon steel. The relationship between failure mode and weld fusion zone characteristics (size and microstructure) was studied. It was found that spot weld strength in the pullout failure mode is controlled by the strength and fusion zone size of the galvanized steel side. The hardness of the fusion zone which is governed by the dilution between two base metals, and fusion zone size of galvanized carbon steel side are dominant factors in determining the failure mode.     

Microstructure development in resistance spot welded galvannealed IF steel sheet

Abstract

Interstitial free (IF) steels having excellent drawing and forming characteristics find extensive use in autobody panels. Although, resistance spot welded joints are widely used in the automobile industry, little is known about the metallurgical changes which occur during the spot welding process. The investigation of the metallurgical changes is very important for the safety strength of the welded joints. In the present research work, microstructures of the different zones of spot welded interstitial free steels have been characterised by optical, scanning electron and transmission electron microscopes. Microstructural changes at weld and heat affected zone have also been correlated with welding heat input and microhardness values.     

Resistance spot welding of AISI 430 ferritic stainless steel: Phase transformations and mechanical properties

The paper aims at investigating the process–microstructure–performance relationship in resistance spot welding of AISI 430 ferritic stainless steel. The phase transformations which occur during weld thermal cycle were analyzed in details, based on the physical metallurgy of welding of the ferritic stainless steels. It was found that the microstructure of the fusion zone and the heat affected zone is influenced by different phenomena including grain growth, martensite formation and carbide precipitation. The effects of welding cycle on the mechanical properties of the spot welds in terms of peak load, energy absorption and failure mode are discussed.     

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.     

Fatigue life estimation of ultrasonic spot welded Mg alloy joints

Lightweight magnesium alloys are increasingly used in automotive and other transportation industries for weight reduction and fuel efficiency improvement. The structural application of magnesium components requires proper welding and fatigue resistance to guarantee their durability and safety. The objective of this investigation was to identify failure mode and estimate fatigue life of ultrasonic spot welded (USWed) lap joints of an AZ31B-H24 magnesium alloy. It was observed that the solid-state USWed joints exhibited a superior fatigue life compared with other welding processes. Fatigue failure mode changed from interfacial failure to transverse-through-thickness crack growth with decreasing cyclic load level, depending on the welding energy. Fatigue crack initiation and propagation occurred from both the notch tip inside the faying surface and the edge of sonotrode indentation-footprints due to the presence of stress concentration. A life prediction model for the spot welded lap joints developed by Newman and Dowling was adopted to estimate the fatigue lives of the USWed magnesium alloy joints. The fatigue life estimation, based on the fatigue crack growth model with the global and local stress intensity factors as a function of kink length and the experimentally determined kink angle, agreed fairly well with the obtained experimental results.     

Combined effect of strength & sheet thickness on fatigue behaviour of resistance spot welded joint

Fatigue performance of spot welded lap shear joint is primarily dependent on weld nugget size, sheet thickness and corresponding joint stiffness. Two automotive steel sheets having higher strength lower thickness and lower strength higher thickness are resistance spot welded with established optimum welding condition. The tensile‐shear strength and fatigue strength of lap shear joint of the two automotive steel sheets are determined and compared. Experimental fatigue life of spot welded lap shear joint of each steel are compared with predicted fatigue lives using different stress intensity factor solutions for kinked crack and spot weld available in literature. Micrographs of fatigue fractured surfaces are examined to understand fracture micro‐mechanisms.     

Revealing microstructural evolution and mechanical properties of resistance spot welded NiTi-stainless steel with Ni or Nb interlayer

Dissimilar welding of NiTi and stainless steel(SS)for multifunctional device fabrication is challenging due to the brittle nature of intermetallic compounds(IMCs)that are formed in the weld zone.In this work,Ni and Nb interlayers were applied for the resistance spot welding(RSW)of NiTi and SS to replace the harmful Fe2Ti phase and to restrict the mixing of dissimilar molten metals,respectively.Microstructural evolution and mechanical properties of the joints were investigated.It was shown that a conventional weld nugget was created in the absence of any interlayer in the welded joint suffering from traversed cracks due to the formation of brittle IMCs network in the fusion zone(FZ).By the addition of Ni from the interlayer,Fe2Ti dominated weld nugget was efficaciously replaced by Ni3Ti phase;however,the presence of the large pore and cracks reduced the effective joining area.The use of a Nb interlayer resulted in a fundamentally different joint,in which FZs at NiTi and SS sides separated by the unmolten Nb would suppress the mixing of dissimilar molten metals.Nb-containing eutectic structures with low brittleness formed at the interfaces,contributing to the enhancement of joint strength(increased by 38％on fracture load and 460％on energy absorption).A high-melting-point interlayer showed great potential to realize a reliable and high-performing RSWed NiTi-SS joint.     

A comparison of the mechanical behaviour of self-piercing riveted and resistance spot welded aluminium sheets for the automotive industry

The increased application of lightweight materials, such as aluminium has initiated many investigations into new joining techniques for aluminium alloys. The resistance spot welding (RSW) concept for aluminium has always attracted many researchers from different organizations. Self-piercing riveting (SPR) is the major production process used to join aluminium sheet body structures for the automotive industry. The research team at the University of Warwick has investigated these two major joining technologies for aluminium assembly. The paper reported here gives an in depth comparison of the mechanical behaviour for each joint type under different loading conditions. It covers symmetrical and asymmetrical assembly from thin gauge of 1.0 mm to thick gauge of 3.0 mm. The results suggest that generally RSW can provide similar strength performance to SPR with the exception of T-peel; the energy to maximum load needs be considered ‘case to case’ and is dependent largely on loading conditions and the failure mode particularly with respect to SPR. The spread of results for SPR is generally smaller than for RSW, and the performance of SPR joints improves as the thickness increases.     

Interfacial microstructure and mechanical property of resistance spot welded joint of high strength steel and aluminium alloy with 4047 AlSi12 interlayer

Dissimilar materials of H220YD galvanised high strength steel and 6008-T66 aluminium alloy were welded by means of median frequency direct current resistance spot welding with employment of 4047 AlSi12 interlayer. Effects of interlayer thickness on microstructure and mechanical property of the welded joints were studied. The welded joint with interlayer employed could be recognised as a brazed joint. The nugget diameter had a decreased tendency with increasing thickness of interlayer under optimised welding parameters. An intermetallic compound layer composed of Fe₂(Al,Si)₅ and Fe₄(Al,Si)₁₃ was formed at the interfacial zone in the welded joint, the thickness and morphology of which varying with the increase of interlayer thickness. Reaction diffusion at the steel/aluminium interface was inhibited by introduction of silicon atoms, which restricted growth of Fe₂(Al,Si)₅. Tensile shear load of welded joints experienced an increased tendency with increasing interlayer thickness from 100 to 300 μm, and the maximum tensile shear load of 6.2 kN was obtained with interlayer thickness of 300 μm, the fractured welded joint of which exhibiting a nugget pullout failure mode.     

Fatigue fracture behaviour of spot welded B1500HS steel under tensile‐shear load

The microstructural characterizations, micro‐hardness measurements and fatigue tests of B1500HS steel spot welded tensile‐shear specimens were performed. The high hardness values of base material (470 HV) and nugget (515 HV) are closely related to the dominant formation of martensitic microstructures, while the occurrence of soft zone is the result of the formation of ferrite phases in inter‐critical heat‐affected zone (HAZ), as well as martensite tempering in sub‐critical HAZ. The fatigue failure modes involve the fracture along the circumference or along the direction of width. The fatigue property of spot welded B1500HS is found to be better than that of spot welded M190 because of the thicker sheet and suitable nugget size, which follows the standard rule of 5t^0.5, where t is the sheet thickness.     

Resistance spot welding macro characteristics of the dissimilar thickness dual phase steels

In the present work, macro characteristics of the dissimilar thickness dual phase steel resistance spot welding joints were described in terms of melting rate, indentation rate, nugget diameter and indentation diameter. The results revealed that the melting rate of the DP600 side was higher than that of the DP780 side and the indentation rate of the DP600 side was lower than that of the DP780 side of the welded joints. The base metal lap order had the important effect on nugget diameter, and the DP780/DP600 spot welded joints tended to get the larger nugget diameter than DP600/DP780 spot welded joints with the same process parameters. The indentation diameters of DP600 and DP780 sides depended on the electrode geometry and force.     

Effects of weld geometry and sheet thickness on stress intensity factor solutions for spot and spot friction welds in lap-shear specimens of similar and dissimilar materials

In this paper, three-dimensional finite element analyses for spot welds with ideal geometry in lap-shear specimens of different materials and thicknesses were first conducted. The computational results indicate that the stress intensity factor and J integral solutions based on the finite element analyses agree well with the analytical solutions and that the analytical solutions can be used with a reasonable accuracy. Three-dimensional finite element analyses based on the micrographs of an aluminum 6111 resistance spot weld, an aluminum 5754 spot friction weld, and a dissimilar Al/Fe spot friction weld were also conducted. The computational results indicate that the stress intensity factor and J integral solutions based on the finite element analyses for the aluminum 6111 resistance spot weld and aluminum 5754 spot friction weld with complex geometry are in good agreement with the analytical solutions for the equivalent spot welds with ideal geometry. However, the stress intensity factor and J integral solutions based on the finite element analysis for the Al/Fe spot friction weld with complex geometry are completely different from the analytical solutions for the equivalent spot weld with ideal geometry. Different three-dimensional finite element analyses based on the meshes that represent different features of the complex geometry of the Al/Fe spot friction weld were then conducted. The computational results indicate that the stress intensity factor and J integral solutions for the Al/Fe spot friction weld based on the finite element analysis agree reasonably well with the analytical solutions for the equivalent spot weld with consideration of gap and bend. The computational and analytical results suggest that the stress intensity factor and J integral solutions based on the finite element analysis and the analytical solutions with consideration of gap and bend may be used to correlate with the fatigue crack growth patterns of Al/Fe spot friction welds observed in experiments.     

镀锌钢的液态金属脆现象及其在电阻点焊过程中的表现

液态金属脆是指通常具有韧性的固体金属或者合金与液态金属直接接触且受到拉伸应力时,其塑性降低并发生脆性断裂的现象。钢在液态锌中会发生液态金属脆现象,这在镀锌钢的热拉伸实验中得到了证实。此外,研究人员发现在镀锌高强钢的电阻点焊过程中也会出现液态金属脆现象,表现为在焊点表面出现大量裂纹,这些裂纹对焊点性能存在潜在危害。本文回顾了镀锌钢液态金属脆现象的热拉伸实验研究,阐明了影响脆化现象的实验因素;综述了镀锌钢在电阻点焊过程中发生液态金属脆现象的研究进展,分析了产生裂纹的位置及其影响因素,并总结了可能的解决方案。     

Flat friction stir spot welding of low carbon steel by double side adjustable tools

2 mm low carbon steel plates were successfully welded by the flat friction stir spot welding(FSSW) using double side adjustable tools, by which the keyhole formed in the conventional FSSW was eliminated and a flat surface on both the top and bottom sides of the welded joints was obtained. In addition, the hook shape usually generated in the conventional FSSW was eliminated by this technique, and the unbonded interface was parallel to the surface of the sheets. Owing to the enlarged bonded interface width by eliminating the keyhole and the intermixed interface by the adjustable probe, the plug fracture occurred under all the welding conditions in the present study. Due to the suppression of the thickness thinning and elimination of the hook shape, the joint performance was improved in the plug fracture mode. The shear tensile performance was considered to strongly depend on the microstructure in the tip area of the unbonded interface and the maximum shear fracture load of 23.0 kN was achieved in this study.     

Fatigue crack propagation in tensile shear stainless steel spot welded specimens

Fatigue tests were carried out on 4 mm thick spot welded joints; the material was stainless steel AISI 301, quarter hard. Some specimens were instrumented with a strain gauge bonded in correspondence with one of the edges of the spot weld. Strain gauge output was demonstrated to be a reliable instrument to monitor the nucleation and propagation of fatigue cracks. A good correlation was found between strain gauge output and spent fatigue life. Some fatigue tests were suspended when the strain gauge output was equal to pre-fixed values, corresponding to fatigue life in the range from 15 to 85%. Subsequently, the specimens were dissected to observe fatigue cracks. The same correlation existed between crack depth and fatigue life. Small cracks were observed in specimens fatigue tested up to 15% of the mean fatigue life; fatigue cracks in the joints under examination would be nucleated between 5 and 10% of fatigue life.
Finite Element calculations were carried out, introducing in the models cracks similar to those observed in the fatigue tests. Calculated strain at the external surface compared well with the measured strain as a function of crack depth. Calculations demonstrated that small errors in strain gauge position can be tolerated without appreciable deterioration in crack dimension prediction.     

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.     

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.     

Mechanical characterization of spot friction stir welded joints in aluminum alloys by combined experimental/numerical approaches: Part II: Macromechanical studies

This is part II of a two part paper summarizing material characteristics of Spot Friction Stir Welded (SFW) lap joints in aluminum alloy 6111 at the macromechanical and micromechanical levels. In this paper, modal vibration testing and static flexure testing at the macromechanical level combined with numerical finite element models have been used to indirectly determine the elastic moduli of the base metal and weld zone. It was observed that the modal frequencies (and the corresponding apparent stiffness) of the joint oscillate at low amplitudes with increasing processing time. For each vibration mode, the amplitude of the oscillation in the frequency vs. processing time is only a few percent of the mean frequency, while the corresponding lap shear strength increases monotonically by a factor of about 8 as the processing time increases. Comparison of predicted modal frequencies and static load–displacement response of SFW joints with the corresponding measured responses seem to indicate that the weld zone is not as stiff as the base metal. Parametric studies to determine the effect of weld zone measurements on the modal frequencies have been carried out using finite element models.     

Process analysis and optimization for failure energy of spot welded titanium alloy

Titanium and its alloys have been applied in many industrial fields because of their high specific strength, good corrosion resistance and high thermal stability. Whereas, there is limited valuable references for recommendations of welding parameter selection and specific standards of the small scale resistance spot welding (SSRSW) of titanium alloy though it has been applied in many industrial production fields. Seventeen tests were designed according to the three-level three-factor Box–Behnken experimental design and the mathematical model correlating the process parameters and the failure energy was established on the basis of response surface methodology (RSM) technique. And then this model was used to analyze the effects/interactions of the welding parameters on the failure energy. The verification test results which were conducted with completely new welding parameters verified that the model presented in this paper was effective and robust. Sensitivity analysis was also carried out to explore the impact of each process parameter on the quality of welding joint. The optimal combination of process parameters for maximum failure energy of the welded joint was obtained using the model based on artificial fish swarm algorithm (AFSA).