Optimising parameters for meaningful measurement of residual strain by neutron diffraction in self-pierce riveted joints

Abstract

The aim of this work is to examine the uncertainties involved in measuring residual strain and their dependence on both the gauge volume of the neutron beam and the acquisition time in self-pierce riveted (SPR) joints while using the neutron diffraction technique. The main challenge involved dealing with the very small dimensions of SPR joints and developing optimum instrument configuration that allowed faster and/or more accurate stress measurement in SPR while maintaining the same time resolution required for the millimetre scale of the problem. Two different gauge volumes were used (0·125 and 1·0 mm³), and two different measuring directions were chosen in order to examine the rotational accuracy of the sample table. All measurements were conducted in steel‐steel joints. Even though small sizes were involved, meaningful results were obtained and measurement errors were reduced by optimising the instrument parameters.     

铝合金自冲铆接头疲劳性能及失效机理

自冲铆是轻量化材料的有效连接技术,为促进该技术的广泛应用,文中基于两组铝合金自冲铆接头,采用疲劳测试、统计方法、断口分析和X-射线能谱仪元素分析,获得接头疲劳特性和断口典型部位微观组织特征,从而对铝合金自冲铆接头疲劳性能及失效机理进行研究。结果表明,随着疲劳载荷降低,接头疲劳寿命稳定性和相对滑移量下降。由于多铆钉接头有效减小了应力集中,其疲劳强度比单铆钉接头提高了31.36%~23.14%,且多铆钉接头的疲劳寿命稳定性较高。多铆钉接头中存在首要和次要承载顺序,疲劳断裂表面为首要承载顺序所在位置。接头疲劳宏观失效模式均为下板断裂,疲劳裂纹主要萌生于铆钉管腿与下板接触部位,微振磨损区域自铆钉管腿底部向铆钉头方向生长。减缓该部位的摩擦作用,可有效延迟疲劳裂纹萌生和减缓裂纹扩展,从而提高接头疲劳寿命。     

电磁铆接试样质量分析

采用试验方法,通过电磁铆接和气动铆接方式,对铆接试样从铆钉宏观变形和微观连接方面进行质量对比分析。结果表明,与气动铆接相比,电磁铆接能一次成形,铆接试样钉杆变形均匀,波动度小,铆接质量稳定,不易出现钉头被铆歪、开裂等现象。在铆钉成形钉头与铝合金板连接处,电磁铆接铆钉变形较气动铆接剧烈。在铆钉钉杆与铝合金板连接处,电磁铆接试样铆钉与铝合金板连接的紧密程度要好于气动铆接。在铆钉钉杆与复合材料连接处,电磁铆接铆钉对复合材料的挤压程度要小于气动铆接,未出现复合材料分层和开裂。     

冲击式自穿孔铆接方法及其接头性能

采用一种新型的应用火药作为驱动力的铆接方法和试验设备.对影响该方法的主要因素如模具尺寸、铆钉硬度进行分析.用该设备铆接铝合金板形成的接头的拉剪强度、十字抗拉强度以及疲劳强度与点焊形成的接头进行比较.试验证明这种新型的自穿孔铆接接头比点焊接头有更好的力学性能.     

Major factor causing improvement in fatigue strength of butt welded steel joints after laser peening without coating

Abstract

Laser peening without coating (LPwC) can introduce compressive residual stress to the surface and, therefore, is effective in enhancing the fatigue strength. This study used butt welded structural steel joints to investigate changes in the residual stress and the hardness near the toe of the welded zone and to examine the major factor causing the improved fatigue strength due to LPwC. It is concluded that the generation of compressive residual stress by LPwC is the major factor improving the fatigue strength, because the reduction in compressive residual stress due to stress relief annealing decreased the fatigue strength to the same level as that of butt welded joints without LPwC.     

Fatigue strength improvement of stainless steel using weld toes dressing with low transformation temperature welding wire

In the present study, low transformation temperature welding wire (LTTW), which can induce residual compressive stress around 304L welded joints, has been developed to improve the fatigue performance of welded joints. Procedure of design and chemical composition of weld metal and deposited metal of LTTW are introduced. The microstructure of weld metal of LTTW is composed of low carbon martensite and residual austenite. The residual stress distribution of a single welding bead of LTTW was measured and the result shows that compressive residual stress is generated. In addition, fatigue test was also carried out on no-load cruciform welded joints of 304L stainless steel under three conditions: as welded, 308L dressing and LTTW dressing. The result shows that the fatigue life of LTTW dressing joints (2×10⁶ cycles) is improved by 14–23 times and 3–6 times compared with that of as welded joints and 308L dressing joints.     

钉脚张开度对自冲铆构件机械内锁刚度的影响

利用数字图像分析技术研究了铆钉张开度对自冲铆接头机械内锁结构刚度的影响.借助材料性能试验机对具有不同铆钉张开度的5052铝合金板自冲铆接头进行单向静拉伸试验,加载过程中利用CCD获取自冲铆机械内锁结构形变的图像序列,利用图像相关分析计算不同载荷水平下铆钉脚与底板的滑移错动量,建立滑移-载荷曲线,并据此评估结构件刚度大小;建立自冲铆接头有限元模型,对比分析不同钉脚张开度的自冲铆机械内锁的失效过程.结果表明,在受载状态下,自冲铆接头存在一个临界滑移启动载荷值,当外载荷小于此临界值时,机械内锁结构保持相对完整,处于弹性变形阶段;大于此临界值时,滑移错动量明显增大,结构发生塑性大变形,并最终呈现脱离失效模式.其次,钉脚张开度对自冲铆构件的强度和刚度影响较大,提高钉脚张开度有助于增大构件整体刚度.     

Resistance spot welding of steel and aluminium sheet using insert metal sheet

Abstract

The present paper reports the resistance spot welding of steel and aluminium sheets using aluminium clad steel sheets as insert metals. Intermetallic compound layers were formed in the weld zones in direct spot welding of steel sheets to aluminium sheets. Thus, the strength of these joints was lower than that of aluminium to aluminium joints. Intermetallic compound layers were also formed at the steel/aluminium interfaces of the insert metal in welding of steel to aluminium using an insert metal sheet. However, the strength of these joints was of the same order as that of the aluminium joints. The fracture mode of these joints varied with the welding current. The suitable welding current for steel to aluminium joints varied between the values suitable for steel to steel and aluminium to aluminium joints. The fatigue strength of joints using insert metals was somewhat lower than that of the aluminium joints.     

Non-destructive measurement of residual stresses in carbon steel weld joints

Abstract

Residual stress measurements have been carried out on 8 and 12 mm thickness carbon steel single V weld joints by X -ray diffraction and ultrasonic techniques. The maximum tensile and maximum compressive stresses on the surface of the 12 mm carbon steel weld joint are higher than those on the surface of the 8 mm carbon steel weld joint. The results also indicate that the variation in the surface residual stress across the weld is different from that for the through thickness residual stresses. The effect of constraint, which depends on the thickness of the weld joint, is found to influence the surface and through thickness residual stresses. The effect of stress relief annealing heat treatment on residual stresses was also studied.     

Self-piercing riveting of high tensile strength steel and aluminium alloy sheets using conventional rivet and die

High tensile strength steel sheets having different strengths were joined with an aluminium alloy sheet by a self-piercing rivet. In the joining, a conventional rivet and die used for aluminium alloy sheets were employed in order to have the versatility for various steel sheets. The effects of the flow stress of the high strength steel sheets and the combination of the sheets on the joinability of the sheets were investigated by finite element simulation and an experiment. As the tensile strength of the high strength steel sheet increases, the interlock for the upper high strength steel sheet increases due to the increase in flaring during the driving through the upper sheet, whereas that for the lower high strength steel sheet decreases. The joint strength for the lower high strength steel is comparatively smaller than that for the upper high strength steel sheet. It was found that the high tensile strength steel sheets below 590 MPa were fully joined with the aluminium alloy sheet even with the conventional self-piercing rivet and die.     

Microstructure,hardness and residual stress distribution in maraging steel gas tungsten arc weldments

Abstract

The distribution of residual stresses due to welding has been studied in maraging steel welds. Gas tungsten arc welding process was used and the effect of filler metal composition on the nature of residual stress distribution has been investigated using X-ray diffraction technique with Cr K_α radiation. Three types of filler materials were used, they include: maraging filler, austenitic stainless steel and medium alloy medium carbon steel filler metal. In the case of maraging steel weld, medium alloy medium carbon filler, the residual stress at the centre of the weld zone was more compressive while, less compressive stresses have been identified in the heat affected zone of the parent metal adjacent to the weld metal. But, in the case of austenitic stainless steel filler the residual stresses at the centre of the weld and heat affected zone were tensile. Post-weld aging treatment reduced the magnitude of stresses. The observed residual stress distribution across the weldments has been correlated with microstructure and hardness distribution across the weld.     

Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590

The hybrid use of aluminum alloy and advanced high strength steel (AHSS) has become an inevitable trend for fabricating a lightweight auto-body. Self-piercing riveting (SPR) as a preferred cold-forming fastening method is facing problem like weak interlocking when joining dissimilar combinations with considerably unequal thickness. In this study, a hybrid joining method, named rivet-welding (RW) was proposed to improve the robustness and strength of the SPR joint, by applying an electric current to it. For better evaluating the new process, the effects of heating time and electrode design on the microstructure, micro-hardness distribution, and mechanical performance of the RW joints were studied and compared systematically with the traditional SPR ones. The results showed that the electric current could improve the microstructure of the steel rivet and bottom DP590, and under long heating time, the inter-metallic compounds (IMC) could be formed at the interface of trapped AA6061-T6 and bottom DP590. Meanwhile, the electric current could increase the micro-hardness of the rivet and bottom DP590, and soften the AA6061-T6 around the rivet leg. In addition, the RW process using lower annular electrode A (LAE_A) could obtain 12.1% higher tensile-shear strength compared with the traditional SPR process.     

Suggestion of welding procedure for compressive residual stress induction by using low transformation temperature welding materials

ABSTRACT

To understand the cause of compressive residual stress in welded joints, we analysed by numerical analysis the effect of welding pass sequence using low transformation temperature (LTT) welding materials on residual stress around the weld toe of boxing fillet welded joints. It was determined by numerical analysis that the produced compressive residual stress and the influence of the stiffeners are reduced in the equivalent position of the weld toe in a fillet welded joint because of the influence on the behaviour of the stiffener in the weld being due to residual stress distribution around the weld toe. The residual stress reduction method of extending the length of the welded bead and releasing the weld toe from the stiffener, similar to the concept of discarding a bead to reduce tensile residual stress, was effective in fillet welded joints. Numerical analysis of the relationship between residual stress around the weld toe and width of the weld bead in the bead-on-plate welding model clarified that compressive residual stress can be introduced around the weld toe by having a wide width weld bead. In addition, a fully penetrated welded joint was very effective for causing compressive residual stress around the weld toe.     

Investigation on friction stir lap welding of aluminium to aluminium clad steel sheets

Abstract

In this work, the feasibility of friction stir lap welding an aluminium alloy (Al-5083) to the aluminium clad steel sheet was studied. The welded joints were characterised by various methods including shear–tensile test and optical microscopy and scanning electron microscopy. The results indicated that sound and defect free joints were obtained. The fracture loads of the samples reached up to 94% compared with that of the steel base metal. Moreover, the joint strength linearly increased with the decrease in thickness of the aluminium layer roll bonded on the steel sheet. It was also shown that welding rotation and travel speed variation did not considerably impact the fracture loads in the studied range.     

Compression tests of carbon steel ball mould for casting

Abstract

In this study, the carbon steel boll mould that has merit of the metal mould and the sand mould was produced. Steel balls were heated and joined to produce the steel ball mould. The compressive strength was measured for the strength of the mould. The compression specimen was manufactured after putting steel balls into the carbon vessel and heating to join. The compression specimen was 10 mm in diameter and about 10 mm in height. The influence of the diameter of the steel ball and temperature of heating on the strength of the steel ball mould was examined. The compressive strength decreased as the ball diameter became large. Steel balls were oxidised and joined by heating. Melted aluminium was cast into the carbon steel ball mould. The macrostructure of the solidified specimen was observed.     

Diode laser brazing of aluminium alloy to steels with aluminium filler metal

Abstract

Diode laser brazing of aluminium alloy (A5052) to interstitial free steel (IF steel) or type 304 stainless steel (SUS304) was conducted using aluminium filler metal (BA4047) with Nocolock flux. The processing parameters of laser power, wire feed rate and travel speed were varied. The strength of lap joints of A5052 on steels was evaluated by tensile shear test. The joint strength of A5052/steels was increased with increasing laser power and reached the maximum strength, more than approximately 80% of the A5052 base metal strength, at a laser power of 1300 W. Voids and incomplete penetration of filler metal were observed at the A5052/braze layer interface when the laser power was below 1100 W. The Fe–Al intermetallic compounds were formed at the steel/braze layer interfaces and grew drastically when the laser power exceeded 1300 W. Superior brazability of A5052/steels was found at brazing conditions corresponding to a temperature of filler metal droplet of 1050–1250 K.     

Characterisation of force–displacement curve in self-pierce riveting

Abstract

A systematic study of the force–displacement curve of the self-piercing riveting (SPR) process and associated sample characterisation have been carried out in this work. Carbon steel sheet with three different thicknesses and steel rivets with three different hardness levels have been used to examine their impact on the force–displacement characteristic curve. The force–displacement curve is segmented into four different stages, which give the signature of the quality of the joint depending on different process variables. This study shows that the force–displacement curve can be used to monitor the quality of the joint within the same process parameters (rivet length and hardness, total stack thickness and die geometry).     

High strength lap joint of aluminium and stainless steels fabricated by friction stir welding with cutting pin

Abstract

Dissimilar lap joints of aluminium and stainless steel were first friction stir welded by the tool with a cutting pin. The results showed that sound joints could be obtained by this method. When the pin was inserted into the lower steel sheet, macrointerlocks were formed by the steel flashes plugging into the upper aluminium at both sides of the nugget bottom. At the aluminium/steel interface, a thin intermetallic compound (IMC) layer and the mechanical bonding of microinterlocks were formed. In addition, the aluminium near the interface was also strengthened by grain refinement and IMC particles. Therefore, the beneficial effect of the macrointerlocks provided by the steel flashes was removed, the shear strength of the joint reached 89·7 MPa, which was even higher than that of the base metal of aluminium.     

铝锂合金压铆壁板残余应力与疲劳性能分析

自动压铆是航空制造工业中的重要装配技术,压铆过程结束后铆孔周围产生的残余应力的分布形式与压铆结构的疲劳性能息息相关。本文使用ABAQUS软件建立了2060-T8铝锂合金壁板压铆过程的有限元模型,通过有限元分析发现了压铆后铆孔壁面上的残余应力由靠近镦头处到靠近钉头处逐渐降低的分布规律。随着压铆力由28.5kN增大至46kN,铆钉材料为2117-T4的压铆壁板孔壁平均残余应力提高33%,残余应力沿壁板厚度上分布的均匀度提升180%;铆钉材料为7050-T73的压铆壁板孔壁平均残余应力提高58%,残余应力沿壁板厚度上分布的均匀度提升184%。疲劳裂纹萌生于铆接下板孔壁附近,随着压铆力由32.5kN增大至42kN,铆钉材料为2117-T4的压铆壁板疲劳寿命提升了31%~80%,铆钉材料为7050-T73的压铆壁板疲劳寿命提升6%~161%。相比于铆钉材料为7050-T73的压铆壁板,相同工艺条件下铆钉材料为2117-T4的压铆壁板疲劳寿命提升12%~44%。     

Delayed fracture in cold blanking of ultra-high strength steel sheets

Hydrogen-induced delayed fracture at cold-blanked edges of 1–1.5 GPa ultra-high strength steel sheets was investigated. The blanked edges undergo large shear deformation and tensile residual stress, and thus the risk of delayed fracture is high, especially for the 1.5 GPa sheet. The effects of residual stress, surface quality and hardness of the sheared edge on the occurrence of delayed cracking were examined. Delayed cracking was caused by press blanking, whereas no cracking occurred for laser blanking because of compressive residual stress. For the 1.5 GPa sheet, delayed cracking was prevented by heating above 250 °C and a stain above 0.005.