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.     

永磁体磁场对铝合金电阻点焊力学性能及微观组织的影响

文中采用永磁体作为磁场源,研究了外加稳定磁场对铝合金电阻点焊的熔核尺寸、焊点力学性能、断裂模式以及微观组织的影响.文中还对熔核直径与峰值载荷之间的关系进行了研究.结果表明,永磁铁的工作距离对熔核直径大小具有重要影响.距离越近,外磁场促进熔核直径增加的效果越明显.外加磁场能够有效增加焊点熔核直径、提高点焊接头的剪切拉伸强度、促进断裂模式由界面断裂向纽扣断裂转变.在不同工艺参数下,熔核直径增长在3.5%~14.1%,剪切拉伸力可提高4%~25%.外加磁场可促进点焊熔核内等轴晶的形成和细化.峰值载荷与熔核尺寸、工件厚度、母材极限抗拉强度的乘积具有良好的线性关系.     

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.     

On the resistance spot welding of four-sheet stack of unequal sheet thickness

There is a lack of sufficient understanding regarding resistance spot welding behaviour of multi-layer structure. This paper investigates the weld nugget development and failure characteristics of four-sheet joint of dissimilar sheet thickness (0.7/1.2/1.2/0.9?mm) made on low carbon steel. The heat dissipation via water-cooled electrode hinders the weld nugget penetration into the thin/thick sheet interface. It was shown that increasing heat input led to bonding mechanism transition from solid-state welding to fusion welding at thin/thick sheet interface. Increasing welding current beyond a critical value changed the failure mode from interfacial to pullout leading to improvement of energy absorption of the joint. Fusion zone size along middle sheet/sheet interface proved to be the most important controlling factor for mechanical properties of four-sheet resistance spot welds.     

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.     

Influence of microstructure and weld size on the mechanical behaviour of dissimilar AHSS resistance spot welds

Abstract

Resistance spot welds were produced in dissimilar combinations of advanced high strength steels. A 600 MPa dual phase (DP) steel was welded to a high strength low alloy, a 780 MPa DP, and a 780 MPa transformation induced plasticity steel. The microstructure and mechanical properties were characterised using metallurgical techniques and lap shear and cross-tension testing. The results show that a pullout failure mode with improved mechanical properties is obtained when DP600 is paired with other advanced high strength steels, compared to the DP600 welded to itself, which is prone to interfacial failure and poor mechanical properties, given the same weld size. An in depth comparison of the interfacial to pullout failure transition in similar DP600 and DP780 and dissimilar DP600–DP780 welds was performed. The results show that the interfacial to pullout transition for the DP600–DP780 welds is significantly lower than with DP600 welded to itself. Increased fusion zone strength through dilution with the DP780 promotes button pullout at smaller weld sizes. Furthermore, it was observed that softening in the heat affected zone of DP780 promoted a pullout failure mode in that material.     

Studies on the micro-laser spot welding of an NdFeB permanent magnet with a low carbon steel

In search of high speed and miniaturization, the welding of NdFeB permanent magnet material is currently of increasing interest. Previous studies have shown that dissimilar material joining between magnet and steel sheets can be realized by laser irradiation, but it is still not clear how the welding parameters affect the weld quality. In this paper, the results are reported of experiments studying effects of laser pulse power, pulse duration, and defocusing distance on joint dimensions and mechanical behavior in laser spot welding of an NdFeB magnet (N48) with a low carbon steel (AISI 1006). Both conduction mode and keyhole mode welding were performed in the present study. The welding modes can be altered by changing peak power or defocusing distance, but not by changing pulse duration. Three fracture modes were found in shear tests, i.e., ‘nugget pullout’, ‘through nugget’ and ‘magnet crush’, while in peel tests, only ‘nugget pullout’ fracture mode was observed. The different fracture modes under different loading conditions were attributed to the mechanical locking effects present under shear testing but not under peel testing. For ‘nugget pullout’ mode fracture, the metallurgical quality of joints is the controlling factor of fracture forces; for ‘nugget through’ and ‘magnet crush’ mode fractures, the controlling factors are the size and strength of the nugget and base magnet. Certain nugget penetration should be achieved to avoid ‘nugget pullout’ fracture, while excessive growth of nugget should be limited to avoid ‘magnet crush’ fracture by appropriately adjusting process parameters.     

核用ODS钢电阻点焊性能

为评价氧化物弥散强化合金(oxide dispersion-strengthened alloy, ODS)钢的电阻点焊性能,使用金相显微镜及扫描电镜观察了9CrYWT-ODS钢焊点不同区域的组织,测试了焊点的拉伸剪切性能,确定了合适的焊接电流范围. 结果表明,焊点不同区域内氧化相尺寸存在明显差异,热影响区回火区温度较低,氧化相细小,热影响区相变区温度较高,氧化相有所长大,熔核区温度很高,氧化相明显粗化. 随着焊接电流增大,熔核尺寸增大导致焊点最大拉伸剪切力升高的同时,失效方式由界面失效逐渐转变为部分界面?部分焊点拔出失效及完全焊点拔出失效,继续增大焊接电流到飞溅产生时,熔核尺寸减小引起最大拉伸剪切力降低,失效方式再次转变为部分界面?部分焊点拔出失效和界面失效. 根据拉伸剪切试验结果确定合适的焊接电流范围为6.6 ~ 7.0 kA.     

热镀锌双相钢点焊结构的临界试样厚度

针对热镀锌双相钢DP600电阻点焊结构的分界面断裂失效,利用统计分析方法,研究了不同熔核长度(5～8 mm)下,板厚分别为0.8 mm和1.5 mm的DP600点焊结构拉剪测试失效模式,建立DP600点焊结构发生分界面断裂失效的概率与熔核长度和板厚的关系模型,从而确定其临界试样厚度,以及熔核长度对临界试样厚度的影响.结果表明,熔核长度和板厚对DP600点焊结构分界面断裂失效的影响较大;随着熔核长度的递增,临界试样厚度呈线性增加.研究结果可为车身用高强钢电阻点焊结构的优化提供指导.     

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.     

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.     

AZ31B镁合金电阻点焊接头断裂形式分析

以AZ31B镁合金板材为研究对象,采用基于断裂力学理论分析电阻点焊接头 拉剪力试验后的两种断裂模式,即纽扣断裂和结合面断裂.结果表明,AZ3lB镁合金焊点断裂形式受熔核边界的最大拉应力影响,当熔核边界的最大拉应力小于母材抗拉强度时,焊点发生纽扣断裂,反之则发生结合面断裂;同时,AZ31B镁合金的焊点从纽扣断裂过渡到结合...     

Failure Behavior of Three-Steel Sheets Resistance Spot Welds: Effect of Joint Design

There is a lack of comprehensive understanding concerning failure characteristics of three-steel sheet resistance spot welds. In this article, macro/microstructural characteristics and failure behavior of 1.25/1.25/1.25?mm three-sheet low carbon steel resistance spot welds are investigated. To evaluate the mechanical properties of the joint, the tensile-shear test was performed in three different joint designs. Mechanical performance of the joint was described in terms of peak load, energy absorption, and failure mode. The critical weld nugget size required to insure pullout failure mode was obtained for each joint design. It was found that the joint design significantly affects the mechanical properties and the tendency to fail in the interfacial failure mode. It was also observed that stiffer joint types exhibit higher critical weld size. Fusion zone size along sheet/sheet interface proved to be the most important controlling factor of spot weld peak load and energy absorption.     

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.     

22MnB5热成形钢点焊接头组织演变与性能分析

采用不同工艺参数对22MnB5热成形钢进行点焊试验,分析工艺参数对焊点性能的影响,并研究22MnB5热成形钢点焊接头组织演变及组织—性能关系. 结果表明,焊点熔核直径与拉剪力两者表现出正相关关系. 与电极压力相比,焊接电流对焊点力学性能具有更大的影响. 焊点各区域的组织演变导致了明显的硬度差异. 熔核区、过临界热影响区、亚临界热影响区及母材区均为马氏体组织. 临界热影响区为铁素体 + 马氏体双相组织,导致硬度显著降低. 该软化区增加了焊点失效时的承载能力及能量吸收能力,促使接头失效以“熔核拔出”方式发生.     

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.     

Welding behaviour,microstructure and mechanical properties of dissimilar resistance spot welds between galvannealed HSLA350 and DP600 steels

Abstract

Resistance spot welding experiments were conducted on dissimilar material combination of HSLA350/DP600 steels. The welds were characterised using optical and scanning electron microscopy. The fusion zone of the dissimilar material spot weld was predominantly martensitic with some bainite. Mechanical properties were also determined by tensile shear, cross tension and fatigue tests. The performance of dissimilar material spot weld was different from that of the similar ones in each of the HSLA350 and DP600 steels and exhibited different heat affected zone hardness. The DP600 weld properties played a dominating role in the microstructure and tensile properties of the dissimilar material spot welds. However, the fatigue performance of the dissimilar welds was similar to that of the HSLA350 welds. Fatigue tests on the dissimilar material spot welds showed that the 5·5 mm diameter nugget exhibited higher fatigue strength than the 7·5 mm diameter nugget.     

先进高强钢电阻点焊接头的断裂模式分析与预测

研究了先进高强钢(advanced high strength steel, AHSS)两层板电阻点焊接头的断裂模式,不同的断裂模式会影响点焊接头断裂时的机理、力学性能及断裂位置,基于不同组合下的临界熔核尺寸、最大载荷、断口宏观形貌、初始断裂位置、宏观金相组织以及微观硬度曲线等试验结果,阐明了板材厚度和板材强度两类因素对于断裂模式的影响规律. 结果表明,板材强度因素会直接影响断裂模式、初始断裂位置以及最大载荷;板材厚度因素影响断裂模式但不改变初始断裂位置及最大载荷. 临界熔核尺寸的影响因素有板材厚度、板材强度、熔核中缺陷以及拔出断裂位置距熔合线的距离. 在此基础上,文中提出了临界熔核尺寸(D_CR)的预测模型及预测方法,该方法与试验值符合较好,为实际工业应用中的临界熔核尺寸判定提供了理论依据.     

Dependence of overload performance on weld attributes for resistance spot welded galvanized low carbon steel

Microstructure and failure behavior of galvanized low carbon steel resistance spot welds were investigated. Failure mode, peak load and energy absorption obtained in tensile-shear test were used to describe spot welds performance. It was found that weld fusion zone size, electrode indentation and expulsion can significantly affect the mechanical performance of spot welds. Failure mechanism of spot weld which fail via pullout mode during tensile-shear test was “through thickness” localized necking in the base metal. However, those spot welds which have experienced severe expulsion during welding, failed at the fusion zone/HAZ interface. This can contribute to the reduction in energy absorption capability of spot welds due to the harder microstructure of the fusion zone/HAZ compared to the soft ferritic base metal.     

Microstructural characterisation of double pulse resistance spot welded advanced high strength steel

The effects of single and double pulse resistance spot welding on the microstructures of an advanced high strength automotive steel are presented in this work. The double pulse welding schemes partially remelt the primary weld nugget and anneal the area at the fusion boundary of the nugget. The effects of the annealing treatment on the segregation and the microstructure have been studied by electron probe microanalysis (EPMA) in combination with electron backscatter diffraction (EBSD). Results show that phosphorus has been redistributed at the primary weld nugget edge of the double pulse welds, while the mean block width and ellipticity of the prior austenite grains were smaller in welds subjected to double pulsing compared with single pulse weld. A favourable failure mode was obtained for the double pulse welds although behaviour did not correlate with the measured grain size.