Strengthening of spot-welded lap joints by new hardened zone

This paper deals with strengthening of spot-welded lap joints by a new hardened zone. Based on the equation to predict the strength proposed by past researchers, the factors necessary for strengthening were extracted. And, based on these factors, the strengthening of spot-welded lap joints was examined. First of all, in order to increase the hardened zone, the vicinity of the nugget edge was heated with a laser. Next, tensile shear tests were conducted by using spot-welded lap joints, which were heated by laser at various distances from the nugget. Finally, the relationship between tensile shear strength and the distance from the nugget centre to the crack was clarified. As a result, by increasing the hardened zone, we found that the fracture position was far away from the nugget edge. And, we found that the tensile shear strength could be improved by 15%.     

铝锂合金填充式摩擦点焊工艺

以5A90铝锂合金搭接接头为研究对象,进行了不同工艺参数下的填充式摩擦点焊试验,并对点焊接头进行了剪切试验、十字拉伸试验,在此基础上对典型接头微观组织结构进行了金相分析和横截面显微硬度测试.结果表明,采用填充式摩擦点焊技术焊接1.5 mm壁厚铝锂合金搭接接头在旋转频率1800 r/min、焊接时间1.5 s左右可获得良好的抗剪能力;十字拉伸强度对于焊接工具旋转频率、下压量不敏感,但随焊接时间的增加而增加;对接头横截面进行金相观察发现,焊点与母材的竖直连接界面为连接薄弱区域,接头横截面显微硬度分布表明,在焊点中心附近水平连接面具有较低的硬度分布.     

板厚对不锈钢车体激光叠焊接头抗剪强度和疲劳强度的影响

为了研究板厚对不锈钢激光叠焊接头抗剪强度和疲劳强度的影响,该文针对0.8 mm+2 mm和2 mm+2 mm²种不同板厚搭配的不锈钢激光叠焊接头分别进行了拉伸试验和疲劳试验。结果表明,2 mm+2 mm接头的抗剪强度和疲劳强度均高于0.8 mm+2 mm接头。失效分析发现,2种接头的拉伸破坏由焊肉部位剪断引起的;2种接头的疲劳裂纹均萌生于2 mm未焊透板,裂纹起始位置在2个焊板之间靠近焊核附近未焊透板的热影响区,裂纹沿着焊核边缘向未焊透板外表面方向扩展,直至穿透未焊透板。对焊接接头部位的有限元受力分析可知,2种接头的应力集中程度的不同是造成它们抗剪强度和疲劳强度差异的主要原因。     

超高强钢点焊结构拉剪试验及数值仿真

以超高强淬火钢22MnB5搭接型点焊结构为研究对象,通过拉剪试验、光学试验和维氏硬度测试,获得了点焊结构的部分力学参数.在ANSYS中进行强度仿真,并与试验结果进行对比.结果表明,将点焊模型分为焊核、塑性环、母材三部分的有限元模型是合理的.同时仿真结果还表明点焊结构交界面的塑性环边缘处是最薄弱的位置,结构失效主要由切应力所引起,并分析了部分尺寸参数对结构强度的影响,当焊核半径、两板重叠长度与板厚越大,结构的最大Von Mises应力值就越低,但增大到一定程度后,其影响降低,为焊接工艺参数的优化提供参考.     

Fracture behaviour and numerical study of resistance spot welded joints in high-strength steel sheet

Cross tension tests of resistance spot welded joints with varying nugget diameter were carried out using 980 MPa high strength steel sheet of 1.6 mm thickness. In proportion, as nugget diameter increased from 3√t to 5√t (where t is thickness), cross tension strength (CTS) increased while fracture morphology simultaneously transferred from interface fracture to full plug fracture. In cases of interface fracture, circumferential crack initiation due to separation of the corona bond arose at an early stage of loading. The crack opening process without propagation was recognized until just before fracture and then the crack propagated to the nugget immediately in a brittle manner around CTS. In full plug fracture, main ductile crack initiation from the notch-like part at the end of sheet separation occurred with the sub-crack initiated at an early stage. The ductile crack propagated toward the HAZ and base material to form full plug fracture. The mode I stress intensity factor was considered as a suitable fracture parameter because the circumferential crack behaved pre-crack for brittle fracture in the nugget region at the final stage. Based on the FE analysis, the mode I stress intensity factor was calculated as 116 MPa √m at CTS as fracture toughness for the nugget. With respect to full plug fracture, ductile crack initiation behaviour from the notch-like part was expressed by concentration of equivalent plastic strain. On the assumption that the ductile crack arose in critical value of equivalent plastic strain, the value was calculated as 0.34 by FE analysis. Reasonable interpretation for interface fracture and full plug fracture in the resistance spot welded joint was proposed due to first crack initiation by stress concentration, brittle fracture by using mode I stress intensity factor, and ductile crack initiation by using equivalent plastic strain.     

Brazing alloys based on the Ni–Cr–Zr system for brazing creep-resisting nickel alloys

Summary

Offering the advantages of high welding speed and low heat distortion, laser welding is an attractive process for joining thin steel sheet. This paper describes an investigation of the static and fatigue strength of laser-welded lap joints in thin steel sheet with different sheet thicknesses and tensile strengths and compares the results with those obtained for spot-welded joints. To evaluate the static strength of the joints, a method for estimation of the joint strength and fracture mode is established. To evaluate the fatigue strength of the joints, the mixed-mode fracture-mechanics criterion of Erdogan and Sih is used, giving good characterisation of the fatigue strength, including that of the spot-welded joints.     

Study of top sheet thinning during friction stir lap welding of AZ31 magnesium alloy

Abstract

Friction stir lap welding was applied to AZ31 magnesium alloy sheets to investigate the effect of tool designs and welding variables on top sheet thinning. Three tools with different shoulder designs were used. Sheet thinning aroused by the hooking or cold lap feature was quantified, and the lap shear fracture load of the joints was evaluated and correlated with the effective top sheet thickness. The tool geometry has a significant effect on the morphology and extent of both hooking and cold lap features, as well as welding variables such as welding pitch and plunge depth. The morphology and extent of these features influenced the effective top sheet thickness, which exhibited a linear relationship with the unit width fracture load via the welded material strength. High fracture loads can be achieved by suppressing the hooking and cold lap features as well as by strengthening the material in the nugget.     

Effect of interlayer on microstructure and mechanical properties of Al–Ti ultrasonic welds

Joints of Al6061 and Ti6Al4?V alloys with pure Al-particle interlayers were conducted using ultrasonic spot welding. The microstructure, hardness, lap shear strength and fracture energy were measured for different welding energies. With increasing welding energy delivered through the sonotrode, the lap shear strength of the joints increased, reaching about 106?MPa at a welding energy of 1100?J, at which failure occurred in the pull-out mode. In the weld region, the hardness of Al6061 alloy increased with increasing weld energy, whereas the hardness of Ti6Al4?V did not change discernibly. No brittle intermetallic compounds were observed in the joints. Moreover, two simple mechanisms were described for the formation of ultrasonic spot-welded Al–Ti joints with and without the pure Al interlayer.     

Effects of HAZ softening on the strength and elongation of resistance spot-welded joints in high-strength steel sheet in an in-plane tensile test

ABSTRACT

In this study, we investigated the effects of heat-affected zone (HAZ) softening on the strength and elongation of resistance spot-welded joints in high-strength steel sheet in an in-plane tensile test. The fracture in the softened HAZ had a little effect on the maximum stress of the resistance spot-welded specimen; however, the fracture elongation decreased. The nugget diameter and HAZ softened width had little effect on the fracture elongation of the resistance spot-welded specimen. Also, the fracture elongation decreased slightly with the decrease in the sheet thickness. The major factor affecting the fracture elongation was the HAZ hardness ratio (= Softened HAZ/Base metal × 100%). For the resistance spot-welded specimen with a thickness of 1.6 mm, when the HAZ hardness ratio decreased to less than 80%, the fracture position changed from the base metal to the softened HAZ and the fracture elongation decreased sharply. In addition, with a decrease in the hardness ratio, the fracture elongation decreased.     

Static strength of hot-stamped spot welded joints: study on spot welding tailored blank technology

Hot stamping spot welding tailored blank technology is a process to produce spot welded automotive body parts by the following process: spot welding steel sheets in lap configuration → hot stamping (heating to about 900°C) → quenching and forming in water-cooled die → shot blasting to remove scale. This process has the advantage of producing high strength lap welded automotive body parts without increasing the number of forming dies. In this study, the mechanical properties of the hot stamped spot weld (spot welding → hot stamping) and conventional spot weld (hot stamping → spot welding) of the 1500 MPa class uncoated boron steel sheets were compared. The obtained results are as follows. The tensile shear strength (TSS) of the hot-stamped spot weld and conventional spot weld were comparable and the fracture modes were the same. On the other hand, the cross tension strength (CTS) of hot-stamped spot weld was significantly higher than that of the conventional spot weld. The fracture position of the hot-stamped spot weld was outside the nugget and conventional spot weld was inside the nugget. The high CTS of the hot-stamped spot welds might be caused by the improvement of the fracture toughness of the nugget, which was caused by reduction of the solidification segregation of the phosphorus. It is assumed that the heating process after spot welding leads to the reduction of the solidification segregation. For the tension test because there was no HAZ softening in the hot-stamped spot weld, no fracture was observed in HAZ and a higher elongation was obtained.     

Dissimilar metal joining of aluminium to zinc-coated steel by ultrasonic plus resistance spot welding – microstructure and mechanical properties

Vehicle body structures are increasingly utilising multi-materials designs with advanced high strength steels (AHSS) and aluminium alloys. A robust process for joining aluminium alloys to AHSS based on resistance spot welding (RSW) is essential to widespread application of such bi-metallic structures in fuel-efficient vehicles. In this study, ultrasonic plus RSW was applied to join AA6022 to Zn-coated dual-phase steel DP980. During solid-state ultrasonic spot welding, an interface structure comprising multilayer, Al–Zn and Zn–Fe intermetallics formed due to alloying of aluminium with steel coating. Such structure was subsequently melted into the aluminium nugget, and new Al–Fe intermetallics formed during RSW. Ultrasonic plus resistance spot-welded joints had superior fracture energy than direct resistance spot-welded joints.     

Effect of spot weld distribution characteristics on the fatigue strength of resistance welded joints

Abstract

Resistance spot-welded joints are often the Achilles heel of structures subject to fatigue stresses. Although many papers, applying both numerical and experimental techniques, have been published in the last few years, the problem of correlation between joint design and fatigue strength has not yet been satisfactorily resolved.

The aim of this article is to identify such a correlation for simple lap joints, to provide designers with a useful tool allowing them to evaluate easily and quickly the improved fatigue strength obtainable by changing the number of weld spots.     

铝合金薄板搅拌摩擦焊搭接界面缺陷与接头性能

针对2A12–T4铝合金薄板进行了搅拌摩擦焊搭接试验,研究了焊接参数对缺陷形态与接头性能的影响规律. 结果表明,勾状缺陷具有更大的高度和弯曲角度,最大缺陷高度为上板厚度的12.7%. 随焊接速度增大,缺陷高度减小.随转速提高,勾状缺陷高度先增加后减小,冷搭接缺陷高度呈“V”形变化. 在950 r/min,200 mm/min下接头强度最高,接头系数可达84%. 维氏显微硬度分布呈“W”形,上板出现接头软化,焊核区下部硬度高于上部硬度. 冷搭接缺陷是影响接头性能的主要因素,由于有效搭接宽度较小,接头断裂方式为沿搭接面的剪切断裂.     

Microstructure,static and fatigue properties of refill friction stir spot welded 7075-T6 aluminium alloy using a modified tool

ABSTRACT

Defect-free joints were produced in 2.0?mm thick 7075-T6 Al alloy by refill friction stir spot welding using a modified tool. Weld performance was evaluated in terms of microstructure, interfacial bonding, hardness, static and fatigue strength based on the experimental observations. The results indicated that grain size, interfacial bonding quality and lap shear strength significantly depend on sleeve penetration depth (SPD), with sufficient interfacial bonding and stable lap shear strength achieved when SPD?≥?1.8?mm. The SPD and applied load played a dominant role in determining the static lap shear and fatigue failure mechanisms, respectively. More precisely, the lap shear failure mode was an interfacial failure when SPD?≤?1.8?mm, and nugget pullout when SPD?≥?2.0?mm. The fatigue fracture mode involved nugget pullout under high applied load, while fracture through the parent sheet occurs under medium and lower applied loads.     

Influence of polarity on mechanical properties of dissimilar resistance spot welds of DP 600/AISI 304 steels

The influence of polarity during resistance spot weld (RSW) of dissimilar lap joints on mechanical properties and failure mode is assessed in this work. A dissimilar lap joint was set using DP 600 dual phase steel with AISI 304 stainless steel. The experiments were performed on a medium-frequency direct current spot welding machine with proper data acquisition of voltage and welding current. Temperature evolution was also acquired by using IR camera. The mechanical properties of spot welds were evaluated by using a coach peel testing. The effect of welding current and welding time on mechanical properties was also evaluated. Correlations between polarity with the welding nugget size, failure mode, tensile strength, temperature evolution and dynamic resistance were analysed. The analysis confirmed that the polarity on dissimilar lap joints affects the behaviour of dissimilar RSWs.     

复合加载方式作用下熔合线上含裂纹焊接接头的裂端场与断裂参量

针对复合载荷作用下熔合线含裂纹的焊接接头,应用弹塑性有限元方法分析了其裂端应力场的分布规律.并对不同组配焊接接头的COD断裂参量及其复合角进行了数值计算,讨论了加载角度和接头强度组配对焊接接头断裂行为及断裂参量的影响机制.研究结果表明,对于复合载荷作用下的熔合线含裂纹焊接接头,其裂端应力场同时受到加载角度以及材料非均匀性的影响.当加载角度较小时,复合载荷对接头性能的影响相对于接头组配的影响更为显著.而当加载角度达到并超过30°时,情况与之相反.     

轴肩下压量对搅拌摩擦焊搭接接头力学性能的影响

采用表面为右螺纹圆柱型搅拌针的搅拌头对3mm厚LY12铝合金进行了搅拌摩擦焊搭接试验,研究了轴肩下压量对接头界面迁移和搭接接头力学性能的影响.结果表明,随轴肩下压量增大,焊缝两侧的搭接界面均向下迁移,其迁移量随下压量的增加而增加,焊缝返回侧的界面迁移量大于前进侧.控制轴肩下压量,当轴肩下压量合适时,焊缝两侧的界面仅发生较小的迁移.当焊缝两侧的界面仅发生较小的迁移量时,搭接接头有较高的抗剪强度,且断裂都发生在上板,呈正断.焊缝两侧的搭接界面向下迁移时,断裂都发生在下板,接头抗剪强度随界面迁移高度的增加而减小,且受拉侧在下板前进侧时,接头的抗剪强度较受拉侧在下板返回侧时的高.     

通电时间对热冲压高强钢点焊接头性能及失效方式的影响

热冲压硼钢电阻率大、焊接热循环升温速度快、峰值温度高、局部冷却速率的不均匀,决定了点焊接头性能差别极大.点焊接头在剪切载荷作用下有界面断裂、焊核拔出和部分焊核拔出三种失效方式.以1.6 mm厚的热冲压硼钢为对象,研究不同通电时间对接头显微硬度、承载性能、失效方式的影响规律,分析失效的形成原因和失效机理.结果表明,上述三种失效方式承载能力和破坏吸能力相差不大,焊核拔出并不能作为评判高强钢点焊接头质量好坏的标准.焊接缺陷、承载时的高度应力集中以及热影响区局部韧性恶化是影响热冲压高强钢点焊接头失效方式的三大主要因素.     

Microstructure and mechanical behaviour of pinless friction stir spot welded AA2198 joints

In this study, pinless friction stir spot welding of 1.8 mm thick 2198-T8 aluminium–lithium alloy plates was carried out. The change of the angle between the nugget edge and the surface, and the relationship between this angle and joint mechanical property were analysed. The results show that the angle increases rapidly initially and then approaches 45°, which is due to the extrusion of nugget material and its flow along the surrounding ‘cold’ metal during welding. The tensile strength is determined by the nugget edge angle and hook defect. Tensile loads reach a higher value when the nugget edge angle approaches 45° but have a slight decrease with the hook angle changing from obtuse to acute. The maximum tensile/shear strength could be 8.57 kN at the rotation speed of 1500 rev min^??1 and the dwell time of 12 s.     

铝胶焊双搭接接头上应力分布的数值分析

用弹塑性有限元法研究了胶粘剂弹性模量和焊点相对位置对铝胶焊双搭接接头胶层中应力分布的影响.结果表明,胶粘剂弹性模量对铝胶焊双搭接接头胶层中部和界面附近的应力分布的影响较为显著,采用低弹性模量胶粘剂使得焊点的承载作用突出;采用高弹性模量胶粘剂时,胶层所承担载荷增加.焊点中心位置对铝胶焊双搭接接头胶层中部的应力分布有一定影响,随焊点中心向搭接区左侧移动,铝胶焊双搭接接头胶层中部的应力峰值逐渐增大,等效应力由焊点中心与搭接区中心重合时的55.2MPa增大到焊点中心距搭接区中心7mm时的77.4MPa,增幅达40.2%.因此采用低弹性模量胶粘剂和使焊点中心与搭接区中心重合可以有效提高接头的承载能力.

Abstract：

The influence of the elastic modulns of adhesives and nuggets position on the stress distribution in adhesive-welded double lap joints of aluminum was investigated by elastic-plastic finite element method (FEM). The results obtained show that the influence of the elastic modulus of adhesive on the stress distribution in adhesive-welded double lap joints of aluminum is significant. The load subjected by the nuggets is greater when the elastic modulus of adhesive is lower and the load subjected by the adhesive layer increases when the elastic modulus of adhesive is higher. The effect of nuggets location is also significant when the center of the nugget is moved to the left end of the overlap zone. The peak stress along the mid-bonding line of the aluminum double lap joint increases when the center of the nuggets are moved to near the left end of the overlap zone. The peak value of the von mises equivalent stress increases from 55.2 MPa to 77.4 MPa when the nugget center is shifted from the point at 12.5 mm to 5.5 mm. Therefore, the load bearing ability of the adhesive-welded double lap joints of aluminum can be improved by lower elastic modulus adhesive and the center ovedapping of the nugget and the lap zone.