差强差厚三层板电阻点焊熔核尺寸的控制方法

针对差强差厚三层板电阻点焊过程熔核形成规律展开研究,提出采用不对称电极帽匹配方法改善点焊熔核尺寸及可焊性窗口宽度.基于建立的有限元模型,分析了两种电极帽匹配下薄板侧熔核形成过程中的电流密度、温度场及熔核尺寸的变化规律.通过试验对比分析了不同焊接时刻的薄板侧熔核尺寸,并建立了两种电极帽匹配下的差强差厚三层板点焊可焊性工艺窗口.结果表明,该方法能有效解决差强差厚三层板点焊过程中的熔核偏移及薄板侧熔核尺寸偏小的问题,提高电阻点焊过程的鲁棒性.     

含镀锌板的三层板点焊熔核形成机理研究

基于车身制造需求,对含镀锌板的低碳钢三层板(0.65 mm厚的镀锌板、0.65及0.70 mm厚的裸板)按照不同焊接参数进行点焊工艺试验,并测试了其力学性能,观察了点焊熔核金相组织,从而分析了其熔核形成机理.研究表明:对于含镀锌板的三层板点焊,熔核最初是在裸板与裸板的接触面上形成的,经历横向和纵向扩展,直至形成完整的熔核.     

外加磁场对非等厚铝合金电阻点焊熔核偏移的影响

针对两层板及三层板组合下的5052铝合金进行了外加磁场与常规情况下的电阻点焊,研究了外加磁场对铝合金点焊熔核偏移影响. 结果表明,外加磁场改善了两层板及三层板非等厚度铝合金的熔核偏移情况,尤其是针对熔核偏移较严重的板材厚度组合下,外加磁场改善熔核偏移的作用更加明显. 之后针对外加磁场和常规点焊情况的不同两层及三层板铝合金板厚组合式样进行了剪切拉伸测试. 结果表明,外加磁场在改善点焊熔核偏移的同时,提高了点焊接头的强度. 因此,外加磁场是一种改善非等厚度铝合金点焊熔核偏移及提高接头剪切拉伸性能的有效方法.     

镀锌钢板电阻点焊的多元非线性回归模型

为了研究用于家用轿车车身制造的镀锌钢板电阻点焊工艺,采用多元非线性回归正交组合的方法设计试验.试验将电阻点焊熔核形状参数和焊接接头抗剪强度作为考察指标,将焊接电流、电极压力、通电时间、预热电流四个参数,以及各参数之间的交互作用作为影响指标的考察因素,得到可预测熔核形状和焊接接头力学性能的四元二次回归数学模型,并通过方差分析对模型进行优化.结果表明,优化的回归数学模型可实现焊接接头熔核成形及力学性能较为准确的预测.在模型的基础上研究各参数及各交互作用对焊点质量的影响规律,从而可实现电阻点焊工艺参数的优化设计.     

铝合金和镀锌钢异种金属CMT焊接性分析

针对汽车工业中铝合金和镀锌钢板的轻量化结构制造,利用正交试验法对铝合金与镀锌钢薄板搭接件进行了异种材料冷金属过渡(CMT)熔钎焊连接,研究了工艺参数、焊丝成分以及镀锌层厚度对焊缝表面成形和接头力学性能的影响.铝/镀锌钢板CMT熔钎焊的接头为典型的搭接接头,在整个焊接过程中,硅和锌很好的起到了促进熔化的铝在钢板表面润湿和铺展的作用.通过试验得出理想的工艺参数,可以得到良好的焊接接头.     

管板单边电阻点焊形核过程有限元模拟

根据管板单边电阻点焊结构特点,建立了轴对称有限元模型.通过结构、热电场的全耦合,分析了焊接过程中接触面压力变化规律,单边焊熔核形成过程以及形核特点等.结果表明,单边点焊和传统点焊焊接过程有很大的不同.单边点焊焊接过程中工件变形严重,电极和板以及板和管子间接触状态变化复杂,熔核形成需要电流大、时间长,且最终形成环状熔核.与金相试验比较,管板单边焊熔核特征的计算结果与试验结果相符合,证实了所建模型的正确性和适用性.为研究单边点焊过程中焊接参数对熔核形成过程的影响规律及确定合理的管板焊接参数奠定了基础.     

镀锌钢板点焊时熔核结晶形态分析

该文对普通冷轧钢板，电镀锌钢板和两种不同锌层厚度的热镀锌钢板在几种焊接参数下进行了点焊试验，并对其焊点熔核结晶形态作了详细的分析和比较，说明了镀锌层及点焊参数对焊点熔核结晶形态的影响，从而对镀锌钢板点焊参数选择了一些建议。     

三层板铝合金电阻点焊搭接接头的弹塑性模拟

建立了1.5 mm等厚5052铝合金三层板电阻点焊接头在拉剪载荷作用下的弹塑性有限元模型.有限元模型的计算结果与试验结果吻合良好.结果表明,不同的接头设计形式导致不同的峰值载荷和断裂模式.熔核旋转对控制峰值载荷及断模式起重要作用,熔核旋转角度增加导致峰值载荷降低.断裂模式不仅与熔核旋转有关,也受接头应力分布影响.当点焊接头在拉伸过程中发生旋转时,熔核周围受到的剪切应力增大,断口形貌呈现拉长的椭圆形韧窝.当点焊接头在拉伸过程中不发生旋转时,熔核周围受到的较大拉应力,断口形貌呈等轴韧窝.     

镀锌钢板单脉冲与双脉冲点焊工艺性能比较

通过试验,比较了在相同焊接规范下镀锌钢板单脉冲和双脉冲点焊工艺性能的差异.试验结果表明,双脉冲焊接参数对镀锌钢板镀锌层有良好的挤出效果;采用双脉冲参数进行焊接时,焊点接头熔核直径更大,焊点接头抗拉剪载荷也更大.     

点蚀对镀锌高强钢点焊熔核形成的影响

镀锌高强钢具有高强度特性以及良好的抗腐蚀性,能够满足汽车轻量化与安全性需要,在车身制造中逐渐得到广泛应用。但镀锌高强钢点焊的点蚀磨损特征变化明显,对熔核形成产生较大影响。建立点焊过程的点蚀有限元模型,采用数值分析和试验方法研究点蚀对熔核形成影响。结果表明,当电极中心部位点蚀面积增加时,工件与工件界面上实际接触面积增大,电流密度减小,熔核直径减小。点蚀的产生改变了电流密度的分布,更加不利于熔核的形成。随着点蚀面积的增大,形成环状熔核几率增加。     

Contact conditions on nugget development during resistance spot welding of Zn coated steel sheets using rounded tip electrodes

This paper investigates the influence of coating on the nugget development during resistance spot welding of Zn coated steel sheets using rounded tip electrodes. The study relies on an experimental observation of zinc behaviour coupled with a numerical Electro-Thermo-Metallurgical–Mechanical analysis. The electro-thermal contact surface enlargement due to zinc ejection at sheet/sheet and electrode/sheet interfaces is found to have a negligible effect on the nugget development. The numerical analysis shows that the dispersion on the efficient weld size, attributable to the zinc effect, is below 10%.     

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.     

Effects of low temperature thermal treatment on zinc and/or tin plated coatings of AZ91D magnesium alloy

A new method was investigated to obtain composite coatings on the AZ91D magnesium alloy by electrodeposition and low temperature thermal treatment. Zinc and tin were introduced to AZ91D Mg alloy surface by electroplating firstly. And a succedent thermal treatment was carried out at 190 ± 10 °C for 12 h. The surface and cross-section morphologies of the plated coatings with and without thermal treatment were studied by scanning electron microscopy (SEM). And the microstructure was determined by X-ray diffraction (XRD). The results reveal that it was difficult to obtain good adhesion plated Sn coating but easy to get well-adherent plated Zn coating. And the thermal treatment promoted the formation of Mg₂Sn in the plated Sn coating and the recrystallization in the plated Zn coating. The plated double Zn-Sn coating owned good adhesion and uniform surface. Furthermore, when the plated double Zn-Sn coating was treated at 190 ± 10 °C for 12 h, a three-layer structure coating was formed due to the diffusion of tin. The results of the anodic polarization behaviors in 5 wt.% NaCl solution show that the three-layer structure coating could provide better protection for AZ91D substrate than the plated Zn-Sn coating.     

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.     

Influence of initial gap on weld expulsion in resistance spot welding of dual phase steel

Abstract

The weld expulsion is prone to occur and severely affects the nugget quality when the initial gap between dual phase (DP) steel sheets exist in resistance spot welding (RSW). To investigate the effect of initial gap on weld expulsion, a finite element model was developed to analyse the weld nugget formation process with different initial gaps for DP steels. An estimation method of expulsion occurrence based on the ratio of the nugget radius R_n and the contact radius R_c between sheets was proposed to get the critical initial gap without expulsion. The simulation and experimental results showed that the weld expulsion would not happen until the gap spacing reaches the critical value. The critical initial gap of DP steel is much smaller than that of low carbon steel. For both DP steel and low carbon steel, the critical initial gap would increase with the thickening of the steel sheet.     

Numerical and experimental study on nugget formation in resistance spot welding for three pieces of high strength steel sheets

A resistance spot welding for three pieces of high strength steel sheets was selected as a research subject and the nugget formation processes at the various welding conditions were investigated by both experimental measurement and FEM simulation. The FEM program developed by the authors considered the coupling of the electrical field, thermal field and mechanical field. The interface elements were used to model the transient contact states between two worksheets or between a worksheet and an electrode. The electrical resistance and thermal–mechanical properties of the interface elements change with the contact state, and can have different values from the worksheets and electrodes. The nugget sizes and its formation process predicted by FEM agreed very well with experimental results. The welding conditions (current, cycles and force) to produce sound nuggets for both two pieces and three pieces of high strength steel sheets were accurately estimated by the simulation.     

异种钢电阻塞焊工艺优化及接头性能分析

采用一种新型复合焊接工艺——电阻塞焊对1.5 mm等厚TRIP980高强钢/SPCC低碳钢板进行焊接,利用正交试验优化其焊接参数,随后增加焊前预热,分析对比无预热和焊前预热两种条件下较优焊接参数时接头的力学性能及组织、硬度特点.结果表明,各焊接参数对接头的拉剪载荷影响程度由大到小依次为焊接电流、填充物直径、焊接时间及电极力;在电极头端面直径为8 mm的条件下,填充物直径为5.5 mm时的接头力学性能优于其它直径;相同焊接参数时,焊前预热塞焊接头的拉剪载荷比无预热提高7%以上;两种条件下较优焊接参数时,焊前预热塞焊接头的熔核偏移量小于无预热,熔核和熔合区的硬度比无预热时有所下降,熔合区脆硬马氏体组织比无预热少;焊前预热塞焊接头断口为韧性断裂,无预热塞焊接头断口为脆性断裂;其主要机理是,预热减小了熔核区金属过热倾向,使熔核扩展均匀,熔合区脆硬组织减少,有利于接头强度的提高.     

Heat distribution in welds by short-time high-current post-heating and its improving effect on cross tension strength: development of resistance spot welding with pulsed current pattern for ultrahigh-strength steel sheets

In order to achieve high joint strength in resistance spot welding of ultrahigh-strength steel, the effect of adding a ‘pulsed current pattern’ consisting of a combination of short cool time and short-time high-current post-heating was investigated. Finite element analysis (FEA) for post-heating patterns and experimental evaluation for joint strength were conducted using 980 N mm^?2 grade steel sheets. FEA shows that the short-time high-current post-heating leads to rapid heating in the nugget and heat-affected zone (HAZ) compared to a conventional temper pattern sheet interface higher than the centre of the nugget. The pulsed current pattern utilizes the effect of this high-current post-heating to properly reheat the nugget and HAZ, which prevents brittle fracture through the nugget without remelting it, even in a short cool time of eight cycles. The experimental results show that the pulsed current pattern improves the failure mode from partial plug failure to plug failure and increases cross tension strength (CTS). The pulsed current pattern does not decrease the hardness of the nugget and results in retention of sufficient tensile shear strength (TSS), while the softened nugget by the conventional temper pattern causes lowering of TSS. A wider proper current range with high CTS over 10 kN and plug failure can be obtained in pulsed current pattern than in the conventional temper pattern.