先进高强钢电阻点焊接头断裂模式的研究进展

随着汽车轻量化的发展趋势,先进高强钢(Advanced high strength steels,AHSS)在汽车结构中的应用逐渐增加,因此研究AHSS电阻点焊接头具有重要意义。重点关注AHSS点焊接头的断裂模式问题,综述近年来国内外关于同种材料两层板断裂模式、异种材料两层板断裂模式、三层板断裂模式、断裂模式的数值模拟以及界面断裂抑制方法等方面的研究进展,提出AHSS点焊接头的断裂模式及机制研究中有待解决的问题,展望未来关于AHSS点焊接头的断裂模式的研究热点和方向。     

镀锌Q & P980钢电阻点焊接头液态金属脆裂纹的影响因素分析

新一代超高强Q&P淬火延性钢在具有高强度的同时具有较好的断后伸长率,在车身制造中具有广阔的应用前景.然而在对镀锌Q&P980钢进行电阻点焊试验后,在焊接接头中发现了表面裂纹,前期研究表明该表面裂纹是由液态金属脆化机制所引起的,即钢板在液态锌和应力的共同作用下发生的沿晶界开裂.结果表明,点焊中的工艺参数,包括焊接电流、焊接时间及电极压力、以及加工条件,包括电极端面形状、加压模式、电极对中度均会对接头中的液态金属脆裂纹产生不同程度的影响,因此可以通过调整工艺参数及加工条件来降低裂纹的敏感性.     

B1500HS热成形钢与DP钢的点焊接头拉剪性能

文中以3种强度级别的双相钢(DP780,DP980,DP1180)和B1500HS热成形钢的电阻点焊接头为研究对象,研究了DP钢强度对点焊接头拉剪性能的影响,分别观察和分析了接头宏观形貌和微观组织,测试和分析了接头的硬度分布及接头的断裂模式。结果表明,DP钢强度对接头的拉剪强度影响很小,但会影响其断裂模式。B1500HS/DP780的断裂是焊核从DP钢侧拔出,另2种的则是焊核从B1500HS侧拔出,但它们的初始起裂位置均位于亚临界热影响区。B1500HS侧亚临界热影响区软化严重,较基体硬度下降约29%～36%,而DP780无明显软化现象,DP980和DP1180侧的亚临界热影响区软化率分别为17%和25%。说明在异种材料电阻点焊过程中其热影响区的软化程度会影响点焊接头的断裂模式。创新点： 对B1500HS分别与DP780,DP980,DP1180组成的RSW接头展开对比研究。     

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.     

Parameter optimization of dissimilar resistance spot welding on ultra-high strength hot-stamped steel and mild steel by numerical simulation

In this study,a coupled axisymmetric finite element model (FEM) was built to simulate the resistance spot welding (RSW) process between ultra-high strength hotstamped (UHSS) and mild steel by SORPAS software.Via simulating this process,the temperature distribution and dynamic temperature curves of the welding area were studied,and welding spatter phenomena were predicted and validated by comparing them with experimental results.By adjusting the welding parameters in numerical simulation,appropriate welding parameters were achieved.Moreover,the mechanical properties of the welding joints under the optimized conditions were also compared with those of not optimized.The study results have already been applied in a manufacturing production.It can also provide guidance for the RSW on UHSS and mild steel.     

Effect of pulse scheme on the microstructural evolution,residual stress state and mechanical performance of resistance spot welded DP1000-GI steel

In the present study, the effect of resistance spot welding scheme (i.e. single and double pulse welding) on the mechanical behaviour of resistance spot-welded DP1000-GI steel is investigated. It is shown that double pulse welding at low welding current decreases the maximum cross-tension strength and mechanical energy absorption capability of the welds. The factors that lead to the lower mechanical performance of double pulse welds are scrutinised. Local residual stress mapping reveals that the compressive residual stress perpendicular to the plane of the pre-crack either decreases or is fully released at the weld edge of double pulse welds. Orientation imaging microscopy analyses show that the martensite formed in front of the pre-crack of double pulse weld has a lower fraction of high-angle grain boundaries and a coarser structure of Bain groups as opposed to the corresponding area of single pulse weld. Lower mechanical performance of double pulse welds produced at lower welding current is ascribed to the lower compressive residual stress normal to the plane of crack and the formation of martensitic structure in front of the pre-crack with a lower fraction of high-angle grain boundaries and coarser Bain groups.     

Fatigue behavior of friction stir spot welded AZ31 Mg alloy sheet joints

The fatigue behavior of friction stir spot welded (FSSW) AZ31 magnesium alloy sheet joints was investigated by tension–compression of fatigue test. The results suggest that all the fatigue failures occur at the stir zone of the FSSW AZ31 sheet joints, and all cracks initiate at the stir zone outer edge between the upper and lower sheet. When the cycle force equals 1 kN, the crack propagates along the interface of heat-affected zone and thermo-mechanical zone, simultaneously across the direction of force; while the cycle force equals 3 kN, the crack propagates along the diameter of stir zone and shear failure occurs finally. Moreover, the transverse microsections indicate that there is a tongue-like region at the outer edge of stir zone between the two AZ31 sheets, and the direction of tongue-like region is toward outside of the stirred zone and all fatigue cracks initiate at the tongue-like region.     

电阻点焊工艺对先进高强钢焊点裂纹缺陷的影响

研究不同的焊接工艺对高强度双相钢电阻点焊焊接质量和焊点裂纹缺陷的影响。结果表明,增加焊后回火脉冲,能够改善焊点表面边部裂纹;增加预热脉冲或回火脉冲,能够明显提升焊点的抗剪性能;同时增加预热脉冲和回火脉冲,能够改善或消除焊缝区的软化点。     

TRIP980高强钢电阻点焊接头的组织及力学性能

采用电阻点焊对TRIP980高强钢进行焊接. 通过单因素法和焊后回火优化了焊接参数和工艺,研究了较优焊接参数和工艺时的接头熔核显微组织及力学性能,结果表明,优化参数为9.5 kA,22 cycle,3 kN,接头熔核为粗大的马氏体组织,接头硬度为617.1 HV,最大拉剪载荷为17.8 kN;在此基础上增加焊后回火,回火电流6.3 kA、回火时间13 cycle,接头组织显著细化,接头硬度降低至574 .0 HV,接头最大拉剪载荷提高到19.5 kN,增幅为9.6%,断口形式由原先的界面断裂转变为纽扣断裂.     

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.     

镀锌钢板点焊电极寿命预测

采用连续点焊试验与数值模拟相结合的方法预测电极寿命,即利用镀锌钢板连续点焊试验得出电极端面直径随焊接点数增加的规律,并用ANSYS软件模拟不同电极端面直径下点焊区域温度场,得出对应的熔核直径.综合试验及模拟结果得出焊接点数与熔核直径的关系,在定义了保证焊点质量的临界熔核直径情况下,即可得出相应的临界电极端面直径和连续点焊可焊点数,从而预测电极寿命.结果表明,两种方法得出的熔核直径的误差不超过15%.此方法对实际生产过程中的电极修整及更换周期制定具有一定的指导作用.     

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.     

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.     

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.     

Effect of joint design on the failure behaviour of three-stack-up austenitic stainless steel resistance spot welds

Tensile shear tests were carried out on three-stack-up austenitic stainless steel resistance spot welds having four types of joint design. Mechanics-based criteria were applied to reveal the difference in the stress state at the microstructural level. Optical microscopy and scanning electron microscopy showed evidence of the different stress states for different joint designs. Nugget rotation generated combined tensile/shear stress at the microstructural level. The peak load and the energy absorption of the joints reduced with the growth of the normal component of the global loads. Joints for which both interfaces bear the load were more prone to failure in pull-out mode.     

纯铝中间层厚度对铝/钢点焊接头性能的影响

采用纯铝作为中间层对铝合金与低碳钢进行了电阻点焊,分析中间夹层厚度对界面反应层厚度和接头抗拉强度的影响。在钢/中间夹层界面观察到有界面反应层生成,其主要由靠近钢侧的Fe2Al5和靠近中间夹层铝侧的FeAl3两种金属间化合物组成。与不加中间夹层相比,利用纯铝作为中间夹层点焊的铝合金与低碳钢的接头具有较薄的界面反应层和较高的接合强度。随着中间夹层厚度的增加,界面反应层厚度逐渐减小,而接头抗拉强度则呈增大趋势。结果表明,采用纯铝作为中间夹层点焊铝合金与钢具有一定的有效性。     

超高强度钢与镀锌双相钢电阻点焊接头强度试验

试验研究了超高强度硼钢板/镀锌双相钢板的电阻点焊接头质量缺陷及其产生原因,通过正交试验设计,重点讨论了焊接电流、通电时间和电极压力对点焊接头强度的影响.结果表明:超高强度硼钢板/镀锌双相钢点焊中超高强度钢板侧更易出现飞溅和烧穿问题,通电时间和焊接电流强度时点焊接头拉剪强度影响显著,这类钢板组合的焊接应优先采用大电流、短...     

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.     

Mechanical clinching of ultra-high strength steel sheets and strength of joints

Ultra-high strength steel sheets having low ductility were joined by mechanical clinching with dies for control of metal flow. The diameter and depth of the die were modified to relieve concentration of deformation of the sheets for avoidance of the occurrence of sheet fracture. As the tensile strength of the steel sheets increased, the interlock decreased due to small metal flow. Two kinds of the ultra-high strength steel sheets having different ductility were used. The ultra-high strength steel sheets having large ductility were successfully joined using die having modified shape, whereas the sheets having small ductility were not joined. The static and fatigue strengths of the mechanically clinched joint were compared with those of the resistance spot welded joint. Although the static load of the mechanically clinched joint was smaller than that of the resistance spot welded joint in both tension-shearing and cross-tension tests, the fatigue load of the clinched joint was larger in the large number of cycles. It was found that mechanical clinching has superior fatigue strength due to the large yield stress of the sheets and relaxation of the stress concentration.     

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.