Effects of surface conditions on resistance spot welding of Mg alloy AZ31

Abstract

In this work, resistance spot welding of Mg alloy AZ31 sheets was investigated in as received and acid cleaned surface conditions. As received sheets had higher contact resistance which required lower current thresholds for weld initiation and for four root t nugget size (where t is sheet thickness). However, it also led to both serious expulsion and internal defects. The fracture mode of welds in as received sheets was interfacial failure while that of the acid cleaned specimens shifted from interfacial to nugget pullout and exhibited better strength. The acid cleaned sheets also produced less damage on electrode tip faces.     

Similar and dissimilar resistance spot weldability of galvanised DP1000 and TWIP980 steels

Similar and dissimilar combinations of a 1000?MPa galvanised dual phase (DP) steel and a 980?MPa twining-induced plasticity (TWIP) steel were resistance spot welded under different welding parameters. The microstructure, expulsion situation, nugget size and mechanical properties of spot welds were evaluated systematically. The results showed that the differences of microstructure and chemical compositions caused that the weld nugget hardness increases in the order of TWIP/TWIP, DP/TWIP and DP/DP. The lower melting point and heat conductivity of the TWIP steel and the lower electric resistance of the zinc coat on the DP steel caused that the expulsion occurring current increased in the order of TWIP/TWIP, DP/TWIP and DP/DP and under the same welding condition the nugget diameter increased in the order of DP/DP, DP/TWIP and TWIP/TWIP. Furthermore, the tensile shear failure mode and location depends on the nugget size, microstructure and hardness distribution of spot weld.     

Static and fatigue performance of weld bonded dual phase steel sheets

Abstract

Lap joints of dual phase steel sheets of 1·0 mm were prepared by adhesive bonding, spot welding and weld bonding processes using a one component epoxy base structural adhesive. Mechanical properties of the joints were evaluated by tensile shear and fatigue tests. The size of the weld nugget for both spot weld and weld bond was measured for different welding parameters (current, time) and compared. For identical welding parameters, weld bonded nuggets exhibit higher nugget diameter. Tensile shear strength of weld bonded joints is 40 and 58% higher than spot welded joints and 15 and 39% higher than adhesive bonded joints and for DP590 and DP780 steels respectively. Considering 10⁶ cycles, the endurance limit of weld bonded joint is much higher than spot welded joint but smaller than adhesive bonded joints. Overall the performance of weld bonded joints is superior to those of resistance spot welding.     

Failure mode transition and mechanical properties of similar and dissimilar resistance spot welds of DP600 and low carbon steels

Abstract

The present work addresses the microstructure and mechanical properties of similar and dissimilar resistance spot welds of low carbon steel (LCS) and dual phase steel (DP600). Correlations between the critical fusion zone size required to ensure pullout failure mode, the weld microstructure and the weld hardness characteristics were developed. Dissimilar DP600/LCS spot welds exhibit the lowest tendency to fail in interfacial failure mode. Effects of weld physical attributes and weld microstructure on the peak load and energy absorption of similar and dissimilar DP600/LCS resistance spot welds are analysed.     

Effect of dynamic contact resistances on advanced high strength steels under medium frequency DC spot welding conditions

Abstract

The effect of dynamic contact resistance (DCR) during MFDC spot welding of dual phase and martensitic steels was evaluated. A comparative analysis of DP590 to DP590 with DP780 to DP780 steel welds, and DP780 to DP780 with M1200 to M1200 steels welds was carried out. The DCR of DP780 steel is higher than DP590 steel during the initial stages of weld time, but is reversed later. The bulk resistance component, which is higher in DP780 steel, is dominant and generates more energy early in the process and controls melting. Although the total energy input is almost same, the higher β-peak and its early occurrence ensures better heat utilisation resulting in larger nugget size. Contrarily, in martensitic steel the interface resistance component remains high throughout the entire welding process and compensates for the lower bulk resistance effect. Even with relatively lower energy input the nuggets produced in M1200 steel are comparable to DP780 steel.     

Study of resistance spot weldability of a new stainless steel

ABSTRACT

The weldability of a newly developed stainless steel was investigated by comparing with hot-dipped galvanised dual phase and low carbon steels in a two-sheet stack-up of similar metal. The weld lobes, mechanical properties, weld morphology, microstructures and microhardness profiles of the welds made with the three materials were comparatively discussed. Results showed that significant softening was observed in the stainless steel weld nugget. Compared to the dual phase steel and low carbon steel, the stainless steel has a very narrow weld lobe but requires lower welding current and shorter welding time, and the stainless steel weld performed better mechanical properties in the case of welds with same nugget size, but it tended to exhibit interfacial fracture in tensile shear tests.     

Critical sheet thickness for weld nugget growth during resistance spot welding of three‐steel sheets

Abstract

One of the most important issues in resistance spot welding of three‐sheet stack joints is the insufficient growth of the weld nugget, which may cause problems in places needing larger weld nuggets (i.e. sheet/sheet interface). In this paper, the effect of sheet thickness on the pattern of weld nugget development during resistance spot welding of three‐steel sheets of equal thickness is studied. Results showed that there is a critical sheet thickness of 1·5?mm at which the fusion zone size at the sheet/sheet interface is nearly equal to the fusion zone size at the geometrical centre of the joint. Increasing the sheet thickness beyond the critical size causes a shift in the location of weld nugget formation from the geometrical centre to the sheet/sheet interfaces. Below the critical sheet thickness, the weld nugget growth in the geometrical centre of the joint is higher than that in the sheet/sheet interface.     

Influence of heating source position and dilution rate in achieving overmatched dissimilar welded joints

Abstract

The investigation addresses the overall performance of black and white joints (BWJ) of low carbon steel (LCS) and stainless steel thin sheets achieved by laser hybrid welding. Assuming that the structural integrity is directly influenced by the processing temperature, a thermal simulation of BWJ of thin sheets was developed. Afterwards, the base metals apportionment at joint formation, namely their distinct dilution rate, was originally estimated from the top surface temperature variation. Defect-free laser hybrid dissimilar welds were experimentally obtained using the selected filler metal and the energetic input from the process simulation, even for a critical analysed case of heat source displacement from the weld gap centreline towards LCS. Detailed macro and microstructural examination of the BWJ and related microhardness analysis results are presented. The tensile tests results indicate that in the case of transversally loaded BWJ, the positive difference in yield between the weld metal and the base materials protects the weld metal from being plastically deformed; the flat transverse tensile specimens loading up to failure reveals large strains in LCS, far away from the weld.     

Effects of welding parameters and surface pretreatments on resistance spot welding of AZ31B Mg alloy

The resistance spot weldability of AZ31B magnesium alloy using AC (alternating current) was investigated in this paper. The surfaces of sheet coupons were prepared by acetone cleaning (as-received) and some coupons were also cleaned with chromic acid (H₂Cr₂O₇) after acetone cleaning (acid-cleaned). The surfaces of the as-received AZ31B sheets consisted of MgO, Mg(OH)₂, and MgCO₃. The extent and thickness of the surface oxide/reaction layer of the sheets varied according to the fabrication process of the sheets, and resulted in different contact resistance. Effects of welding current and weld time were investigated while the electrode force was kept constant. The results indicated that the as-received sheets produced expulsion both at the faying surface and at the work piece/electrode interface due to high contact resistance. Diluted chromic acid cleaning reduced contact resistance and produced good weld quality. Increasing the welding current and weld time enhanced not only the nugget size and joint strength, but also increased indentation on the weld surfaces. A weld lobe was established considering the minimum joint strength, weld expulsion, and surface indentation based on a relevant American Welding Society Standard.     

On formability of tailor laser welded blanks of DP/TRIP steel sheets

Abstract

This paper aims to evaluate the formability of tailor welded blanks of dual phase (DP600)/transformation induced plasticity (TRIP700) steel sheets. In this work, bead on plate butt joints of 2·5 mm DP600 and 1·2 mm TRIP700 steel sheets were performed using CO₂ laser beam welding. Microhardness measurements and transverse tensile testing were carried out to characterise the welds. The formability of base metals and welds were investigated by standard Erichsen test. In a perpendicular tensile test to the weld line, all specimens were fractured at the TRIP base metal, and the strengths were somewhat higher than those of base metal. There was a significant reduction in formability caused by welding of the DP600/TRIP700 steel sheets, and the formability increased with increasing welding speed.     

Numerical and experimental study of nugget size growth in resistance spot welding of austenitic stainless steels

A 2D axisymmetric electro-thermo-mechanical finite element (FE) model is developed to study the effect of welding time and current intensity on nugget size in resistance spot welding process of AISI type 304L austenitic stainless steel sheets using ANSYS commercial software package. In order to improve the accuracy, temperature-dependent properties of materials are taken into account during the simulation. The diameter and thickness of computed weld nuggets are compared with experimental results. The FE predicted weld nugget growth and nugget size agree well with experimental results. The effects of welding time and current intensity on nugget growth are also studied. Generally, increasing welding time and current is accompanied by an increase in the fusion zone size with a decreasing slope. However if expulsion occurs, nugget size reduces due to melt spattering. Except for an initial nugget formation stage, welding time has minor effect on nugget size in comparison with welding current.     

Expulsion monitoring in spot welded advanced high strength automotive steels

Abstract

Although there have been a number of investigations on monitoring and controlling the resistance spot welding (RSW) of low carbon galvanised steels, those of advanced high strength steels (AHSS) are limited. A data acquisition system was designed for monitoring weld expulsion via the measurement of voltage, current, electrode force and displacement and the calculation of resistance. The dynamic resistance, electrode force and tip displacement were characterised and correlated with the phenomenon of expulsion during RSW of dual phase (DP) steel using an ac welder. Two control strategies for DP600 spot welding were proposed on the basis of the rate of change in the dynamic resistance and the electrode force.     

Process–structure–property relationships for nugget and heat affected zone regions of AA2524–T351 friction stir welds

Abstract

The effects of weld tool rotational speed ω, welding speed v and z-axis force F_Z during friction stir welding of the aluminium alloy 2524-T351 on the resulting process response variables, nugget microstructure, nugget tensile properties and heat affected zone hardness variations were investigated. For the range of conditions examined, the results indicate that ω has the dominant effect on nugget properties and structure, that optimum nugget tensile properties can be obtained by increasing ω to obtain a peak temperature that is just below the incipient local melting temperature, and that excessive values of ω result in low nugget ductility because of localised embrittlement near the weld crown. The study has also shown that the peak weld temperature is inversely related to the measured torque T ₀. The T ₀–ω relationship appears to be a useful guide for weld modification, as it is indicative of conditions leading to overheating in the nugget region.     

Weld growth mechanisms during resistance spot welding of two and three thickness lap joints

Abstract

Understanding the mechanisms of weld growth during resistance spot welding is a prerequisite for the development of optimum welding conditions which ensure high levels of joint quality in autobody manufacture. This study compared the heat development/weld growth of double and triple sheet lap joints as a means of understanding the role of welding conditions, sheet thickness configuration, and physical material properties on joint microstructure and performance. High temperature heat patterns have been determined using metallographic and thermographic techniques for double and triple sheet lap joints in uncoated low carbon steel. In the case of double sheet joints, initial heat generation and weld nugget formation was observed to occur where the resistance to the flow of current was the greatest, i.e. at the sheet/sheet interface. This was also the case for heat generation in triple sheet joints, but in this more complex case weld nugget formation was dependent on sheet thickness and joint configuration. Furthermore, the incubation period to weld nugget formation was dependent on total joint thickness with a longer weld time required for joints of greater thickness. For non-uniform triple sheet joints, heating was observed to be concentrated around the thinnest sheet as a result of the way heat was dissipated by the steel substrate.     

Microstructures and properties of friction stir spot welded DP590 dual phase steel sheets

DP590 steel sheets were joined by friction stir spot welding using polycrystalline cubic boron nitride tool with an objective to produce bond diameters similar to conventional spot welding nuggets. A range of spindle rotation (400–2400 rev min^?1) and plunge speeds (0·03–3·8 mm s^?1) were exercised to attain defect free welds in 1·6 mm thick sheets. A bond diameter of 4t^1/2, alike minimum nugget diameter criteria for resistance spot welds, resulted in superior mechanical properties than conventional spot welds. The heat inputs corresponding to different welding parameters influenced the weld microstructure, including grain size, phases and their morphology. The bond diameter was higher for higher heat inputs. However, low heat input welds with weld time cycles ～4 s produced more refined microstructure and exhibited similar strengths even with reduced bond size. Plug type failure was associated with larger bond diameters (～7·1 mm), while interfacial failure was observed with smaller welds (～5·4 mm).     

An experimental study on effects of post-heating parameters on resistance spot welding of SAPH440 steel

Abstract

The fracture mode of spot welded joints, made of SAPH440 steel sheets, is investigated. It was found that the weldment failure in the peel test of the joints occurred through the weld nugget. This is called an interfacial failure and is not acceptable because it is a sign of insufficient mechanical strength. Investigation showed that this kind of fracture is attributed to the brittleness of the nugget zone, caused by its martensitic microstructure due to the high cooling rate in the welding. For eliminating this defect, resistance spot welding procedures were augmented with post-heating stage. This approach is intended to reduce the cooling rate after welding and also to temper the weld nugget, generating a more ductile microstructure in the weld zone. The results of this research can be used for planning spot welding process and provides a guideline for analysing the results of hardness and peel test.     

双相钢搭接点焊接头疲劳寿命分析

研究了双相钢焊点特征,对不同匹配双相钢搭接焊点进行了疲劳试验,获得了焊点的载荷寿命曲线.研究了双相钢焊点的疲劳裂纹扩展及失效形式,分析和解释了疲劳过程中的现象,并根据裂纹的实际扩展路径,提出了局部等效张开应力强度因子keq,从断裂力学的角度对双相钢焊点的疲劳失效进行了分析.结果表明,keq能够有效地关联具有不同厚度,不同熔核直径的搭接焊点试样的疲劳寿命,是反映双相钢焊点疲劳强度的有效参量,能够用来预测焊点疲劳寿命.     

Effect of weld nugget size on overload failure mode of resistance spot welds

Abstract

In the present paper, effects of welding current, welding time, electrode pressure and holding time on the weld nugget size were studied. A failure mechanism was proposed to describe both interfacial and pullout failure modes. This mechanism was confirmed by SEM investigations. In the light of this mechanism, the effect of welding parameters on static weld strength and failure mode was studied. Then, an analytical model was proposed to predict failure mode and to estimate minimum nugget diameter (critical diameter) to ensure pullout failure mode in shear tensile test. On the contrary to existing industrial standards, in this model, critical nugget diameter is attributed to metallurgical characterisation of material (weld nugget hardness to failure location hardness ratio), in addition to sheet thickness. For a given sheet thickness, decreasing H_WN/H_FL increases interfacial failure mode tendency. The results of this model were compared with experimental data and also with the literature.     

铝和镀锌钢板的搅拌摩擦焊搭接分析

对异种材料DP600镀锌双相钢板和1060工业纯铝进行了搅拌摩擦焊搭接试验（铝板在上,钢板在下）．从接头横截面的金相形貌可以发现,在前进侧和返回侧,钢像两个钉子一样进入铝中．在微观上,钢和铝以河流状的形式充分地混合在一起．在焊核区和热力影响区,钢和铝之间形成了一层有一定宽度的由两种材料呈带状交错分布组成的过渡层．力学性...     

Prediction of the failure mode of automotive steels resistance spot welds

ABSTRACT

In this study, the critical nugget size, at which the failure state in tensile shear test changed from the interfacial failure mode to the pull-out failure one, was estimated as a function of nugget and base metal hardness. The proposed approach could address the effect of various parameters involved in resistance spot welding process, such as sheet thickness, base metal chemical composition and physical properties of electrodes and sheets. The reliability of the present model was evaluated using independent experimental results. Based on the obtained results, the effect of steel composition on critical nugget diameter was found to be more important, especially for the sheets thicker than about 1?mm, whereas predicted nugget sizes by previous models could not guarantee the pull-out failure mode.