铝合金电阻点焊电极寿命及其表面特征分析

为了研究采用不同焊接电源时,铝合金电阻点焊的电极寿命、电极头磨损行为及其对焊接质量的影响,运用图形分析方法对电极头磨损行为进行了定量分析.基于对电极压印的分析,定义了3种电极头表面特征参数:相对半径(Rr)、边缘聚集度(EC)和偏心度(ECC).研究表明:铝合金电阻点焊采用逆变直流电源与交流电源比较,前者电极寿命约为后者的1/4;焊点的剪切强度随着Rr和EC的增加而下降;电极头中部残存接触区的存在有助于维持较好的焊接质量.     

Magnetism-current inverse image reconstruction of the nugget in aluminum alloy spot welding based on ART algorithm

In this work, the computer tomography (CT) theory and its reconstruction algorithm were used to deal with the magnetism-current inverse problem in the resistance spot welding (RSW). At first, the magnetic fields around the nugget were detected. Then, the current distribution of the nugget section was calculated by reconstruction algorithm. At last, we changed the current distribution data into a graph using Matlab. The inversed graph of the nugget-section current distribution in the inverted RSW can be achieved, and by this graph the details of the nugget can be observed directly, which can help evaluate the joint quality.     

Effects of sleeve plunge depth on microstructures and mechanical properties of friction spot welded alclad 7B04-T74 aluminum alloy

Friction spot welding (FSpW) was applied to join the 7B04-T74 aluminum alloy successfully, and effects of sleeve plunge depth on weld appearance, microstructures and mechanical properties were investigated in detail. When the sleeve plunge depth was larger than 2 mm, a surface indentation with a depth of 0.2 mm should be applied in order to eliminate the defect of annular groove. The tensile shear properties of the joints were dependent on hook geometry, location of alclad layer, and hardness of stir zone (SZ). With increasing the sleeve plunge depth from 2 to 3.5 mm, the hook height increased, the alclad layer downward migrated further and the hardness of SZ decreased. The optimized FSpW joint was obtained when the sleeve plunge depth was 3 mm, and the corresponding tensile shear failure load was 11921 N. Two different failure modes, i.e. shear fracture mode and tensile-shear mixed fracture mode, were observed in the tensile shear tests.     

电阻点焊质量检测技术研究现状

电阻点焊过程由于受到各种因素的影响,熔核区域容易出现裂纹、缩孔、未熔合等缺陷,焊点质量直接影响焊接部件的使用寿命,因此对焊点的缺陷检测与质量评定非常重要.对电阻点焊原理进行概述,总结了电阻点焊质量检测技术最新研究成果及应用,分析了焊接过程参数监控方法、焊后无损检测方法的检测机理、质量评定方法及其在实际应用中的优缺点,对...     

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.     

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.     

铝及铝合金表面处理研究进展

铝合金耐磨性差、特殊条件下耐蚀性差的缺点限制了它的进一步利用,对铝合金进行表面处理长期以来一直是扩大铝合金使用范围地行之有效的方法.文章综述了铝合金的各种表面处理方法,比较了它们的优缺点,指出表面氧化是铝合金表面处理的主流,复合处理、纳米化处理将是今后铝合金表面处理的主要研究方向.     

Improving mechanical properties of pinless friction stir spot welded joints by eliminating hook defect

Hook defect (HD) seriously decreases the mechanical properties of friction stir spot welded (FSSW) joints. In this study, two methods were therefore used to eliminate the HD in pinless FSSW joints. The one is changing welding parameters such as rotating speed and dwell time. The other one is FSSW plus subsequent friction stir welding (FSSW-FSW), which is an innovative method proposed in this study. Experimental results showed that the HD in pinless FSSWed AA2024 joints was successfully eliminated by using FSSW-FSW, not by changing process parameters. The joints without HD exhibited a tensile–shear load of as much as 12 kN, which was higher than that of 6.9 kN in the joints with HD. Furthermore, it was proved that the tensile–shear load is not greatly improved only by increasing the nugget zone when HD still existed in the FSSW joints. In addition, the fracture morphology analysis demonstrated that the shear fracture of the FSSW-FSW joints took place along the boundary between the upper and lower sheets through the weld nugget, and the faying surface between the two sheets was completely sheared off.     

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).     

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.     

Fatigue behavior of dissimilar friction stir welds between cast and wrought aluminum alloys

Cast aluminum alloy, AC4CH-T6, and wrought aluminum alloy, A6061-T6, were joined by means of friction stir welding (FSW) technique. The effect of microstructure and post heat treatment on fatigue behavior of the dissimilar joints was investigated. Near the weld centre, Vickers hardness was lower than in the parent metals and the hardness minima were observed along the trace route of FSW tool’s shoulder edge. Tensile fracture took place on A6061 side where the hardness was minimal, resulting in the lower static strength of the dissimilar joints than AC4CH or A6061. Fatigue fracture occurred on AC4CH side due to casting defects and the fatigue strength of the dissimilar joints was similar to that of AC4CH, but lower than that of A6061. Friction stir process (FSP) and post heat treatment successfully improved the fatigue strength of the dissimilar joints up to that of the parent metal, A6061. __________ Translated from Problemy Prochnosti, No. 1, pp. 150–154, January–February, 2008.     

Small-scale resistance spot welding of austenitic stainless steels

Small-scale resistance spot welding (SSRSW) was carried out for austenitic stainless steels. A weld lobe that shows the process window for making sound joints was obtained for type 304 stainless steel thin sheets, and the effects of welding current, force and weld time on joint strength and nugget size were investigated. The cooling rate that was estimated from the solidification cell size was approximately 2.4 × 10⁵ K/s which is almost similar to that produced by laser beam welding. The microstructures of weld zones were almost fully austenitic due to the rapid solidification rate. Despite the fully austenitic microstructure, no hot cracking was found in types 302, 304, 316L, 310S and 347 austenitic stainless steels by SSRSW. Rapid cooling rate in SSRSW made it difficult to predict the microstructures from the conventional Schaeffler diagram.     

固溶后时效成形对7075铝合金FSW构件性能的影响

为评价搅拌摩擦焊(FSW)构件的可时效成形性,对7075铝合金FSW构件固溶处理后进行时效成形工艺,试验研究了固溶处理影响下FSW构件时效成形后构件性能的变化规律.试验结果表明,固溶处理后时效成形工艺有助于提高FSW构件的抗拉强度、延伸率和电导率.固溶处理后180℃时效成形时,FSW试样抗拉强度峰值显著提高,达到449.4 MPa,是仅时效成形工件抗拉强度的139.5%,为原始焊接试样抗拉强度的1.4倍,此时焊件延伸率达到峰值4.2%,固溶处理对试样延伸率有一定的提升作用.微观结构观察结果表明,固溶处理的引入对FSW构件时效成形过程焊缝区特征组织产生显著影响,有助于消除焊缝的不均匀组织特征的影响,从而有利于成形试样的性能均匀化和提高.     

SiCp/Al复合材料电阻点焊剪切断口形貌

为了优化SiC p/Al复合材料电阻点焊工艺参数,采用不同焊接电流和焊接时间对SiCp/Al复合材料进行了电阻点焊连接,对接头进行了剪切强度试验,用扫描电镜对不同的点焊剪切断口进行微观形貌分析.结果表明:最优的焊接电流和时间匹配值为焊接电流I=14.6 kA,焊接时间t=0.2 s,配合电极压力F=2 500 N点焊,熔核直径适中,接头拉剪力可达1 693 N;撕开后的焊点断口两侧分别呈规则的圆凸台和圆孔状,呈纽扣型断裂,接头成型良好.当焊接电流和时间的匹配值小于最优参数时,点焊接头只有少量的点形成冶金结合,呈结合面断裂,焊接强度较低;当焊接电流和时间的匹配值大于最优参数时,点焊接头易过热,断口上出现气孔、裂纹、电极粘附烧蚀缺陷,接头强度降低.     

铝合金板材热塑性变形行为研究

为了研究铝合金板材的热塑性变形行为,进行了热态胀形试验,获取了不同温度及压力率下的胀形压力-高度曲线,分析了压力率对胀形高度的影响规律.基于同一压力率、不同温度下胀形压力与等效应变之间的关系,提出胀形压力关于等效应变及压力率的拟合方程,同时获得压力率和应变率之间的函数关系.试验结果表明,板材充液热成形工艺过程中,压力率对金属材料的变形行为影响显著,同时压力率能够表征材料成形过程中的变形快慢.     

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.     

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。     

TiNi形状记忆合金精密脉冲电阻对焊接头相变温度变化规律的研究

通过示差热分析（DSC）和X射线衍射等分析手段，研究了焊接热量、焊接压力、随机后热处理等焊接参数以及冷热循环对焊接接头相变温度的影响，并总结了其影响规律。研究表明，焊接热量和焊接压力对焊接接头相变温度的影响较小，相变温度区间基本保持不变，焊接接头相变类型为单一的马氏体→奥氏体的转变。焊后随机后热处理对焊接接头相变温度具有一定的影响，马氏体向奥氏体转变的起始温度As和终了温度At均随后热处理时间的增加而提高。在热处理时间为99cyc时，在加热过程中首先发生马氏体→R相→奥氏体的转变，由于发生了R相变，延迟了奥氏体相变的温度，使TiNi形状记忆合金的温度滞后现象加剧，同时使奥氏体的相变区间缩小。冷热循环对焊接接头的相变过程和相变温度具有重要的影响，导致了R相变的出现。R相变进行的程度随着冷循环次数的增加而提高，并逐渐趋于稳定。     

Effects of rotation speed and dwell time on the mechanical properties and microstructure of dissimilar aluminum-titanium alloys by friction stir spot welding (FSSW)

In this study, the effects of rotation speed and dwell time on the mechanical properties and microstructure of friction stir spot welded joints of dissimilar aluminum and titanium alloys were investigated. Aluminum AA6061 and titanium Ti-6Al-4 V alloys were selected as the work piece. The joint quality, mechanical behavior, and microstructural evolution in the welded regions were considerably affected by the welding parameters. The results of scanning electron microscopy showed the formation of Ti₃Al intermetallic compounds near the thermomechanical affected zone, which significantly affected the properties of the welding joint. Maximum tensile shear load was produced at 1000 min⁻¹ and 10 s dwell time. Moreover, the welding joint microhardness was improved with increasing the rotation speed.     

铝制板翅式换热器焊接焊材的选用及预处理案例分析

本文从焊接防腐、焊接强度等方面介绍了铝制板翅式换热器焊接的特点及焊料焊材的选用。从实际应用和技术实践方面,提出和规范了笔者所在企业的焊接工艺流程和企业标准,明确焊料焊材选用和技术处理应用的适用性和重要性,以期为铝制板翅式换热器产业的生产和质量控制,提供实际的技术指导。