Method for the evaluation of the low-cycle fatigue strength of welded joints using the J-integral energy criterion

Summary

The purpose of this study was to investigate effects of aluminium in the Zn coating on electrode life in welding galvanized steel sheet. Three hot‐dip galvanized and one electro‐galvanized steel sheet types were prepared for this study. Aluminium content in the coatings varied within the range 0.26–0.87 mass% for the three types of hot‐dip galvanized sheet.

The approach used here included EDX, AEX analysis of the coating layers, electrode life tests and EPMA analysis of electrodes after 900 spots were welded.

The results were as follows:

1. The electrode lives of HDG materials were changed at approximately 0.3–0.4 mass% Al content in Zn coating. Materials with low‐Al coating content showed over three times longer electrode lives than materials with high‐Al coating content.

2. Although the thickness of Al oxide layers was in proportion to the Al content in Zn coatings, the obvious correlation between electrode life and thickness of Al oxide layers was not observed.

3. In the case of low‐Al coating content, it was observed that Fe‐Zn alloy grew from the steel‐coating interface to the Zn coating. It was considered that, in the initial stage of welding, the content of Fe in Zn coating increased immediately.

4. In the case of high‐Al coating content, Fe‐Al alloy was observed at coating‐steel interfaces instead of Fe‐Zn alloy. It was known that Fe‐Al alloy suppresses the Fe‐Zn alloying reaction. Zn coating was not alloyed with Fe in initial stages of welding.

5. From these results, it was concluded that aluminium in coatings affected electrode life by changing the melting point of coating layers between the electrode and the steel. The melting point of low‐Al content coating layers rose because of the diffusion of Fe into the Zn coating. This phenomenon decreased electrode wear and electrode life was long. In contrast, the melting point of high‐Al content coating layers remained low. This phenomenon caused electrode alloying easily and also increased electrode wear. As a result, electrode life became shorter.

     

Friction anchor welding between Al alloy and Zn-coated steel using insert steel difference in the mechanism of friction anchor welding between the types of Zn coatings on Zn-coated steel

The lap joints of upper Al alloy sheets (1.0-mm-thick A5052) and lower Zn-coated steel sheets (1.2-mm-thick GI steel or GA steel) were welded using insert steel sheets (0.6-mm-thick SPCC) by a spot welding process with a tool having a spherical ceramic tip, i.e. ‘Friction Anchor Welding.’ As a result, straight (not-rugged) steel projections were formed in the Al alloy sheets for both the GI and GA, while steel projections were not formed for the GI, rugged steel projections were formed for the GA without the insert steel sheets. In addition, the tensile shear strength for the GI was greater than that for the GA. In other words, the tensile shear strengths reached about 3.9 kN/point for the GI and about 3.2 kN/point for the GA, which were greater than those of the welds without the insert steel sheets. On the other hand, the cross tensile strengths for the GI and GA were almost the same, which reached about 2.6 kN/point. Additionally, for the GI, the Zn layer on the GI steel sheet melted and was totally removed due to the pressure and heat caused by the rotating tool, which facilitated the welding between the SPCC and GI steel sheets. For the GA, however, the Zn-Fe layer on the GA steel sheet changed to a solid-liquid mixture and was not completely removed, which prevented the welding between the SPCC and GA steel sheets. Therefore, the thickness of the steel-steel welded region (i.e. the SPCC-GI or the SPCC-GA welded region) for the GI was greater than that for the GA. We estimated that the difference in this thickness is significantly related to the fracture mechanism during the tensile shear test and the cross tensile test.     

Surface modified long-life electrode for resistance spot welding of Zn-coated steel

In order to increase the life of conventional copper electrodes in resistance spot welding of Zn-coated sheet steel, a multi-layer Ni/(TiC_P/Ni)/Ni composite coating was deposited onto the copper electrode top surface by electro-spark deposition. Scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction and micro-hardness tests were employed to characterize the microstructure and property of the coating. The results showed that the copper electrodes with a dense Ni/(TiC_P/Ni)/Ni coating slightly increased the resistance of the weld system and hence the welding current could be reduced to produce a weld with the same button size as that made by uncoated electrodes at a high welding current. The coating was gradually cracked during welding under the action of welding force, forming Ni/(TiC_P/Ni)/Ni composite islands which were strongly adherent to and further, punched into copper substrate. The coating could significantly reduce the alloying between copper electrode substrate and molten Zn. As a result, coated electrode showed a much longer life than an uncoated electrode even though the welding current for coated electrode was lower than that for uncoated one.     

单面双点和单面点焊接形式对电阻点焊焊点强度的影响

采用外界铜板作为导电体的点焊,有单面双点和单面单点两种形式.用一台并列双电极式加辅助电极形式的焊接设备焊接三种搭接接头形式的试件,试件搭接方式分别是:薄板在铬锆铜乎台接触,厚板在电极侧,完成单面双点和单面单点的焊接;厚板在铬锆铜平台侧,薄板在电极侧,用单面双点方式进行焊接焊接电流、压力、时间相同的条件下,对三种形式的焊接试样分别进行拉伸试验和金相试验,分析拉伸力、熔核中的气孔尺寸和熔核尺寸,得出在表面不涂装的不锈钢车辆的侧墙、端墙生产中的焊接影响因素.     

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%.     

TiB2-ZrB2涂层电极与TiB-ZrB2/Ni涂层电极失效对比分析

采用扫描电镜、能谱分析、X射线衍射仪以及显微硬度等表征手段,分析TiB2-ZrB2涂层与TiB2-ZrB2/Ni涂层电极点焊镀锌钢板时的失效过程.研究发现,无论是ZrB2-TiB2涂层还是ZrB2-TiB2/Ni涂层,在一定程度上均具有阻碍钢板镀层与点焊电极基体产生合金化反应的能力.ZrB2-TiB2、ZrB2-TiB2/Ni涂层电极失效过程存在些许不同,前者由于涂层与基体间结合力差,且涂层内塑性相相对较少,导致在点焊热和力的作用下,涂层逐渐脱落,点焊电极最终在合金化及塑性变形作用下发生失效.后者由于具有一定塑性,且涂层与基体结合力较好,点焊过程中涂层不会出现完全脱落现象.涂层作用一直持续至电极失效,电极失效形式主要表现为塑性变形.     

深冷处理提高镀锌钢板点焊电极寿命的机理

为了提高镀锌钢板点焊电极寿命，作者提出了镀锌钢板点焊电极的深冷处理方法。采用不同深冷处理工艺参数处理了点焊镀锌钢板的Cr—Zr—Cu合金电极，用这些电极进行了镀锌钢板点焊电极寿命试验并与未深冷处理的电极寿命进行了比较。用扫描电子显微镜对深冷处理电极进行了背散射及面扫描分析，用X射线衍射法观测了深冷处理前后的电极晶粒度，测试了电极在深冷处理前后的电阻率。通过对深冷电极微观结构的观测及电极性能的测试，探讨了深冷处理提高镀锌钢板点焊电极寿命的机理。研究结果表明，深冷处理提高了镀锌钢板点焊电极基体的致密性，改变了合金元素的分布，细化了电极材料的晶粒，提高了镀锌钢板点焊电极的导电、导热能力及电极抗压渍变形的能力，从而提高了镀锌钢板点焊电极的寿命。     

Surface modification of resistance welding electrodes by electro-spark deposited composite coatings: Part II. Metallurgical behavior during welding

Electrodes with monolithic TiC_P/Ni coatings and multi-layer Ni/(TiC_P/Ni)/Ni coatings were used to resistance spot weld Zn-coated sheet steel to investigate metallurgical behaviour of the coatings during welding. Scanning electron microscopy, energy-dispersive X-ray analysis and X-ray diffraction were employed to characterize the microstructure of coatings, reactions of electrodes with Zn-coating and alloy layer formation. The results showed that molten Zn penetrated TiC_P/Ni coatings via the cracks that were present within as-coated TiC_P/Ni coating, starting from the first weld. Additional cracks continually formed in the coating during welding due to action of the welding force on the low toughness coating, resulting in formation of a granular loose overlay at the outer surface which were easily detached and stuck onto the work sheet. On the contrary, cracks could be rarely found within Ni/(TiC_P/Ni)/Ni coating until 100 welds or more were made, and much fewer cracks formed up to 400 welds, compared to the TiC_P/Ni coating. With Ni/(TiC_P/Ni)/Ni coating on the electrode surface, alloying between copper alloy and molten Zn as well as pitting (erosion) of the electrode tips were remarkably reduced, and hence, a slower growth rate of tip diameter was observed.     

Inspection of the physical contact between two steel sheets by means of a potential drop technique

In this paper, we propose a potential drop technique for inspecting the physical contact between two steel sheets. One of the important targets in this study is the examination of spot welds in the steel sheets and we evaluated this using the electrical circuit in a welding machine. First, we used numerical analysis to analyze the changes in the potential drop due to the size of the physical contact between the steel sheets. In the conventional setup for spot welding, two steel sheets are inserted between two Cu electrodes. In the analysis, the current is supplied through two terminals, one on the surface of each electrode, and the potential drop is measured between two other terminals, again, one on the surface of each electrode. The validity of the analysis was confirmed experimentally where the physical contact between the steel sheets was modified by inserting shims between the sheets.     

热镀锌钢板连续点焊时电极损耗对焊接质量的影响

镀锌钢板电阻点焊时由于锌层的影响电极损耗非常严重。以厚0.8mm家电用热镀纯锌钢板DX51D+Z为研究对象,探究在正交试验优化焊接参数后,热镀锌钢连续点焊时焊接接头质量变化的规律和电极损耗的特点。结果表明,当采用I=11 kA、P=2 500 N、T=13 cyc的点焊工艺参数组合可获得力学性能最佳的点焊接头;连续点焊试验初期(150点前)焊点力学强度稳定且焊点成形较好,焊接后期(150～300点)接头力学强度波动大,焊接状况不稳定;电极损耗至失效是接头质量急剧下降的主要原因,电极失效形式为头部塑性变形、端面坑蚀和Cu-Zn合金化,在本试验条件下,Cr-Zr-Cu电极焊至150点后显现不稳定现象,电极寿命约280点;失效电极端面的Cu-Zn合金层约65μm。     

Seal spot welding of steel and aluminium alloy by resistance spot welding: dissimilar metal joining of steel and aluminium alloy by Zn insertion

This research concerns a dissimilar metal joining of steel and aluminium (Al) alloys by means of zinc (Zn) insertion. The authors propose a joining concept for achieving strong bonded joints between Zn-coated steel and Al alloys. A eutectic reaction between Zn in the Zn coating and uniform Al–Fe intermetallic compound (IMC) layer at the joint interface, leading to a strong bonded joint. The ultimate aim of this research was to apply this joining concept in the resistance spot welding process for manufacturing vehicle bodies. As a practical issue characteristic to joints of dissimilar metals, anticorrosion measures against electrochemical corrosion must be undertaken. If there is moisture near a joint interface of dissimilar metals, electrochemical erosion will progress. Therefore, a sealing function that could prevent moisture intrusion is required. By applying the above-mentioned welding process to a set of metals with thermosetting resin spread in between, we realized seal spot welding, which not only prevented moisture intrusion but also retained high tensile strength. In this research, first, a cyclic corrosion test was performed on the seal spot-welded joint of galvanized (GI) steel, a steel grade widely distributed in Japan, and Al alloy was bonded by seal spot welding, and the following topics are discussed. Complete removal of sealant from the joint interface is the key to realizing the high tensile stress joint, because remaining sealant will lead to reduction in tensile strength. Therefore, heat generation at the interface was monitored by measuring electrical current and potential difference between the two electrodes, and a precise temperature control was performed. Moreover, the bonding process was clarified by stepwise analysis of the joint interface using optical microscopy, and a guideline for producing strong joints was proposed. And finally, a TEM observation also confirmed that the interface structure of the seal spot-welded joint was the same as joints without the resin; a thin and uniform Al–Fe IMC layer was formed and a strong metallurgical bonding was achieved.     

不锈钢焊条熔滴过渡及其影响因素研究

采用水中收集熔滴、平板堆焊、高速摄影等试验方法,研究了典型不锈钢要熔滴过渡特性及其影响因素。结果表明,熔滴过渡指数为优化值时,焊条熔滴呈渣壁过渡形态,焊条的综合工艺性最佳;药皮中的ＳｉＯ２细化熔滴作用最强,增大药皮厚度和焊接电流均使熔滴细化,但受焊条工艺性制约;熔滴反应区的渗Ｓｉ反应亦有利于细化熔滴,但受焊缝最高含Ｓｉ量的限制;焊接熔渣的碱度、组成物及含量对熔滴过渡特性亦有一定的影响,提出了“焊条     

药皮类型对E308-XX型不锈钢焊条工艺质量的影响

介绍了E308-XX型不锈钢焊条药皮类型、熔滴过渡形态和冶金学过程的特点,探讨药皮类型对焊条工艺质量的影响。结果表明,E308-XX型焊条药皮类型有五种,其所用电源类型、极性以及适用的焊接位置各异。药皮类型15和25焊条具有短路过渡形态,药皮类型16焊条具有短路+渣壁混合过渡形态,药皮类型17和26焊条具有渣壁过渡形态。药皮类型16、17和26焊条,在熔滴过渡区的渗Si反应使熔滴增氧、细化,有利形成渣壁过渡。药皮类型25和26焊条分别克服了15和16焊条的工艺质量问题,但前者不能完全取代后者;药皮类型17焊条的工艺性最好,但对气孔敏感;药皮类型15、25焊条焊缝的综合性能比16、17和26更好。     

Correlation between electrochemical impedance and current distribution of carbon steel under organic coating

The current distribution of carbon steel beneath intact organic coating exposed to 3.5% NaCl solution was investigated by the wire beam electrode (WBE) method. The electrochemical impedance spectroscopy (EIS) of individual electrodes with high current was characterized to analyze the behavior of the coating degradation and metal corrosion at coating/metal interface. The polarity switch of the electrode under the coating was observed. The mechanism of coating degradation and carbon steel substrate corrosion was discussed from a point of electrochemical distribution.     

无机固体润滑处理热镀锌板在汽车中的应用

目的降低汽车生产成本,提高汽车生产效率。方法采用平板拉延法测定无机固体润滑处理热镀锌板与普通热镀锌板等在拉延过程中摩擦系数,分析其随时间变化的趋势;采用SEM观察拉延后各钢板的表面形貌差异;采用拉深试验对比无机固体润滑处理热镀锌板与普通热镀锌板的最大拉深力和最大杯高;采用模拟焊接法对比两者的可焊电流区间、可焊电流宽度、最大剪切力和纽扣直径;采用随车挂片的方法对比两者的磷化、电泳性能。即分别评价无机固体润滑处理热镀锌板和普通热镀锌板在冲压、焊接、涂装性能的差异性。结果冲压性能方面,无机固体润滑处理热镀锌板的摩擦系数较普通热镀锌板降低23%,且微区表面与其他工艺表面的钢板相比几乎无磨损、划伤存在。拉深试验中最大拉深力下降4.5%,最大杯高增加9.5%,无机固体润滑处理热镀锌板具有更优的冲压性能。焊接性能方面,前者较后者的可焊电流区间下降0.6 k A,可焊电流宽度都为1.2 k A,前者的最大剪切力为2.75 k N,后者剪切力为2.6 k N,前者的纽扣直径为5.8 mm,后者的纽扣直径为5.2 mm,焊接性能相差不大。涂装性能方面,前者磷化膜重为3.12 g/mm~2,后者磷化膜重为3.09 g/mm~2,磷化性能相当,且两者的电泳膜性能试验全部通过。经试冲500次发现,无机固体润滑处理热镀锌板可有效防止模具粘锌。结论无机固体润滑处理热镀锌板和普通热镀锌板相比,有更优的冲压性能,焊接、涂装性能相当,且能有效改善锌层脱落。而且,无机固体润滑处理热镀锌板替换普通热镀锌板,可降低整车生产成本,提高生产效率。     

TiB_2-ZrB_2/Ni涂层电极点焊镀锌钢板失效过程分析

目的了解TiB_2-ZrB_2/Ni涂层电极点焊镀锌钢板时的失效机理。方法通过扫描电镜(SEM)、X射线衍射仪(XRD)、显微硬度测试等表征方法,研究TiB_2-ZrB_2/Ni涂层点焊电极点焊镀锌钢板时表面结构、物相及性能的变化。结果 TiB_2-ZrB_2/Ni涂层电极对提高点焊电极寿命有很大帮助,点焊电极寿命可提高5倍左右。涂层使点焊电极表面的硬度得以明显提高,减缓了点焊电极端部塑性变形的进程。ZrB_2-TiB_2/Ni涂层在一定程度上减缓了钢板镀层与点焊电极产生合金化反应的进程。结论 ZrB_2-TiB_2/Ni涂层电极由于具有一定的塑性,点焊过程中涂层不会出现完全脱落现象。涂层作用一直持续至电极失效,电极失效的形式主要表现为塑性变形。     

Metal transfer and arc behaviour of novel consumable and non-consumable electrode indirect arc droplet welding

Abstract

The consumable and non-consumable electrode indirect arc droplet welding is a novel method that applies an indirect arc generated between a consumable electrode and a non-consumable electrode to welding. It was successfully used to overlap join the thin zinc coated steel plate. In the point to point (consumable electrode and non-consumable electrode) configuration, the stable metal transfer was obtained. The globular, projected spray and streaming spray transfer modes were promoted. With the increase in welding power, the metal transfer became faster and more stable. At the same time, the indirect arc became brighter, smaller and more stable. The arc self-regulation guaranteed the stable burning of the indirect arc. The desire stability was observed from the fluctuation of the welding current and the arc voltage waveforms. The side concentrated melting on the root of the droplet produced high pinch effect, which facilitated the detachment of the droplet.     

用电流波形控制法提高镀锌钢板点焊电极的寿命

作者通过分析镀锌钢板点焊电极寿命降低的原因，提出了提高镀锌钢板点焊电极寿命的电流波形控制法。采用该方法进行了点焊电极寿命试验，研究了工件表面飞溅率和电极寿命随焊前电流脉冲的幅值、焊前电流通电时间的变化规律。试验结果表明：采用电流滤形控制法可将镀锌钢板点焊电极的寿命由原来的400点以980点；工件表面的飞溅率及电极寿命与焊前电泳脉冲幅值、焊前电流通电时间有关。     

用普通缝焊机焊接镀Zn油箱

分析了镀Ｚｎ钢板缝焊的焊接性，进而研究了使用普通缝焊机替代专用设备焊接镀Ｚｎ油箱的新工艺，提出了避免滚轮电极粘附Ｚｎ及Ｃｕ－Ｚｎ合金对焊接不良影响的措施，并在生产实践得到了应用，得取了良好的经济效益。     

Dissimilar spot welding of aluminium alloy and galvannealed steel by metal flow-application of friction anchor welding to aluminium alloy and Zn-coated steel

ABSTRACT

Lap joints of an upper Al alloy (1.0-mm-thick A5052) and lower hot dip galvanized steel (1.2-mm-thick GI steel) were welded by a novel spot welding process for dissimilar metal lap joints using a new tool with the tip made of spherical ceramics, i.e. ‘Friction Anchor Welding.’ As a result, the rotating tool was plunged only to 1.3–1.4 mm from the Al alloy surface, and accordingly, a steel projection was not formed in the Al alloy sheet. Further, the Al alloy near the rotating tool was removed. However, near this removed area, the Zn layer on the GI steel melted and was removed by friction heat, and consequently, the GI steel and the Al alloy were welded. Thus, the tensile shear strength reached about 2.6 kN/point.