Friction self-piercing riveting(F-SPR)of aluminum alloy to magnesium alloy using a flat die

Friction self-piercing riveting(F-SPR)process based on a pip die has been invented to solve the cracking problems in riveting high-strength and low-ductility light metals,such as magnesium alloys,cast aluminum,and 7 series aluminum alloys.In this paper,in order to solve quality issues caused by the misalignment between rivet and pip-die in F-SPR,a flat-die based F-SPR process was proposed and employed to join 1.27 mm-thick AA6061-T6 to 3 mm-thick AZ31B.The results indicate that a 1.0 mm die distance is effective to avoid rivet upset and insufficient flaring.As the feed rate increases,the heat input in the whole process decreases,resulting in a larger riveting force,which in turn increases both the bottom thickness and interlock amount.Besides,solid-state bonding,including Al-Mg intermetallic compounds(IMCs),Al-Mg mechanical mixture,and Al-Fe atom interdiffusion was observed at the joint interfaces.The upper Al layer was softened,but the lower Mg layer was hardened,and both sheets exhibited a narrowed affected region with the increase of feed rate,while the rivet hardness shows no obvious change.Three fracture modes appeared accompanying the variations in lap-shear strength and energy absorption as the feed rate increased from 2 mm/s to 8 mm/s.Finally,the F-SPR process using a flat die was compared to those using a pip die and a flat bottom die to show the advantage of flat die on coping with the misalignment problem.

An Overview of Self-piercing Riveting Process with Focus on Joint Failures, Corrosion Issues and Optimisation Techniques

Self-piercing riveting(SPR)is a cold forming technique used to fasten together two or more sheets of materials with a rivet without the need to predrill a hole.The application of SPR in the automotive sector has become increasingly popular mainly due to the growing use of lightweight materials in transportation applications.However,SPR joining of these advanced light materials remains a challenge as these materials often lack a good combination of high strength and ductility to resist the large plastic deformation induced by the SPR process.In this paper,SPR joints of advanced materials and their corresponding failure mechanisms are discussed,aiming to provide the foundation for future improvement of SPR joint quality.This paper is divided into three major sections:1)joint failures focusing on joint defects originated from the SPR process and joint failure modes under different mechanical loading conditions,2)joint corrosion issues,and 3)joint optimisation via process parameters and advanced techniques.

A Comparative Study of Friction Self-Piercing Riveting and Self-Piercing Riveting of Aluminum Alloy AA5182-O

In this paper,self-piercing riveting(SPR)and friction self-piercing riveting(F-SPR)processes were employed to join aluminum alloy AA5182-O sheets.Parallel studies were carried out to compare the two processes in terms of joint macrogeometry,tooling force,microhardness,quasi-static mechanical performance,and fatigue behavior.The results indicate that the F-SPR process formed both rivet–sheet interlocking and sheet–sheet solid-state bonding,whereas the SPR process only contained rivet–sheet interlocking.For the same rivet flaring,the F-SPR process required 63%less tooling force than the SPR process because of the softening effect of frictional heat and the lower rivet hardness of F-SPR.The decrease in the switch depth of the F-SPR resulted in more hardening of the aluminum alloy surrounding the rivet.The higher hardness of aluminum and formation of solid-state bonding enhanced the F-SPR joint stiffness under lap-shear loading,which contributed to the higher quasi-static lap-shear strength and longer fatigue life compared to those of the SPR joints.

Study of galvanic corrosion and mechanical joint properties of AZ31B and carbon-fiber–reinforced polymer joined by friction self-piercing riveting

A new testing methodology was developed to quantitively study galvanic corrosion of AZ31B and thermoset carbon-fiber–reinforced polymer spot-joined by a friction self-piercing riveting process.Pre-defined areas of AZ31B in the joint were exposed in 0.1 M NaCl solution over time.Massive galvanic corrosion of AZ31B was observed as exposure time increased.The measured volume loss was converted into corrosion current that was at least 48 times greater than the corrosion current of AZ31B without galvanic coupling.Ninety percent of the mechanical joint integrity was retained for corroded F-SPR joints to 200 h and then decreased because of the massive volume loss of AZ31B。

Prediction of Cross-Tension Strength of Self-Piercing Riveted Joints Using Finite Element Simulation and XGBoost Algorithm

Self-piercing riveting(SPR)has been widely used in automobile industry,and the strength prediction of SPR joints always attracts the attention of researchers.In this work,a prediction method of the cross-tension strength of SPR joints was proposed on the basis of finite element(FE)simulation and extreme gradient boosting decision tree(XGBoost)algorithm.An FE model of SPR process was established to simulate the plastic deformations of rivet and substrate materials and verified in terms of cross-sectional dimensions of SPR joints.The residual mechanical field from SPR process simulation was imported into a 2D FE model for the cross-tension testing simulation of SPR joints,and cross-tension strengths from FE simulation show a good consistence with the experiment result.Based on the verified FE model,the mechanical properties and thickness of substrate materials were varied and then used for FE simulation to obtain cross-tension strengths of a number of SPR joints,which were used to train the regression model based on the XGBoost algorithm in order to achieve prediction for cross-tension strength of SPR joints.Results show that the cross-tension strengths of SPR steel/aluminum joints could be successfully predicted by the XGBoost regression model with a respective error less than 7.6%compared to experimental values.

Fatigue strength evaluation of self-piercing riveted joints of AZ31 Mg alloy and cold-rolled steel sheets

The application of magnesium alloys to automobiles is increasing due to their superior specific strength and specific stiffness.In this study,an upper sheet of AZ31 magnesium alloy and a lower sheet of cold-rolled steel were joined by self-piercing riveting(SPR),a method commonly used to join automotive panels.A cross-shaped specimen was fabricated with a punching force of 35 kN,which exhibited the best joint strength for the SPR joint specimen geometry.Monotonic and fatigue strengths were evaluated using cross-shaped specimens at loading angles of 0°,45°,and 90°.The load amplitude corresponding to the fatigue endurance limit was assumed to be at 106 cycles,and the fatigue ratios(=fatigue endurance limit/static strength)at the loading angles of 0°,45°,and 90°are 22%,13%,and 9%,respectively.For all three loading angle specimens,fatigue cracks initiated at the triple point where the rivet shank,the upper sheet and the lower sheet are in contact with each other,with the cracks propagating through the thickness of the upper sheet and ultimately leading to fracture.The fatigue lifetimes were evaluated through the von-Mises stress,maximum principal stress,and equivalent stress intensity factor.It was found that the fatigue lifetimes could be evaluated most appropriately through the maximum principal stress.

Mechanical joint performances of friction self-piercing riveted carbon fiber reinforced polymer and AZ31B Mg alloy

Carbon fiber reinforced polymer(CFRP) and AZ31B Mg alloy were joined by the friction self-piercing riveting(F-SPR) with different steel rivet shank sizes. With the increase of rivet shank size, lap shear fracture load and mechanical interlock distance increased. Ultrafine grains were formed at the joint in AZ31B as a result of dynamic recrystallization, which contributed to the higher hardness. Fatigue life of the CFRP-AZ31B joint was studied at various peak loads of 0.5, 1, 2, and 3 kN and compared with the resistance spot welded AZ31B-AZ31B from the open literature. The fatigue performance was better at higher peak load(>2 kN) and comparable to that of resistance spot welding of AZ31B to AZ31B at lower peak loads(<1 kN). From fractography, the crack initiation for lower peak load(<1 kN) case was observed at the fretting positions on the top and bottom surfaces of AZ31B sheet. When peak load was increased, fretting between the rivet and the top of AZ31B became more dominant to initiate a crack during fatigue testing.

Flexible servo riveting system control strategy based on the RBF network and self-pierce riveting process

As more and more composite materials are used in lightweight vehicle white bodies,self-pierce riveting(SPR)technology has attracted great attention.However,the existing riveting tools still have the disadvantages of low efficiency and flexibility.To improve these disadvantages and the riveting qualification rate,this paper improves the control scheme of the existing riveting tools,and proposes a novel controller design approach of the flexible servo riveting system based on the RBF network and SPR process.Firstly,this paper briefly introduces the working principle and SPR procedure of the servo riveting tool.Then a moving component force analysis is performed,which lays the foundation for the motion control.Secondly,the riveting quality inspection rules of traditional riveting tools are used for reference to plan the force-displacement curve autonomously.To control this process,the riveting force is fed back into the closed-loop control of the riveting tool and the riveting speed is computed based on the admittance control algorithm.Then,this paper adopts the permanent magnet synchronous motor(PMSM)as the power of riveting tool,and proposes an integral sliding mode control approach based on the improved reaching law and the radial basis function(RBF)network friction compensation for the PMSM speed control.Finally,the proposed control approach is simulated by Matlab,and is applied to the servo riveting system designed by our laboratory.The simulation and riveting results show the feasibility of the designed controller.

芯杆齐平度偏差对抽芯铆钉力学性能影响及机理分析

抽芯铆钉广泛应用于航空制造与装配领域,其芯杆断面相对钉套头表面凸出量(芯杆齐平度)有定量规定标准,但芯杆齐平度对于连接结构力学性能的影响并不明确。针对这一问题,开展了不同芯杆齐平度下装配结构的静力学性能及疲劳性能试验,并对结构的装配以及静力学性能进行了仿真验证。通过试验探究发现,芯杆齐平度合格样件剪切力破坏强度平均值为9570.535 N,标准差为107.001 N,芯杆齐平度不合格样件剪切力破坏强度平均值为9532.14 N,标准差为327.261 N,两者标准差差异显著,可见相较于超差试样,合格试样结构稳定性更强;疲劳试验中芯杆齐平度合格样件疲劳循环次数平均值为456770次,芯杆齐平度不合格样件疲劳循环次数平均值为456022.7次,疲劳强度受该方面的影响较小。对抽铆的仿真发现,两种情况下芯杆的绝对干涉量几乎没有区别,该结论解释了为何两组试验的剪切力没有太大区别,并且在仿真过程中发现两者的力–位移曲线几乎没有区别,进一步验证了试验结论。

基于铆接结构的模块化平车车体结构探讨

文章提出了一种基于铆接和模块化的铁路平车设计理念,阐述了铆接结构和模块化的优势,详细介绍了该新型平车的结构特点和设计思路,为铁路货车设计提供了新的方向。

Influence of die geometry on self-piercing riveting of aluminum alloy AA6061-T6 to mild steel SPFC340 sheets

The self-piercing riveting (SPR) process was used to join 2.0-mm-thick aluminum alloy 6061-T6 and 1.2-mm-thick mild steel SPFC340 sheets. SPR joints produced with a conventional flat-bottom die and conicalsection dies were investigated both experimentally and numerically. Lap shear tests were conducted under quasistatic conditions to evaluate the load-carrying capability of these SPR joints. The effect of variation in die geometry (such as variation in the die groove shape, cone height, and die radius) on the main mechanical response of the joints, namely the peak load and energy absorption, was discussed. The results showed that SPR joints produced with the conical-section dies exhibited a failure mode similar to those produced with a conventional die. All the joints failed by tearing of the top steel sheet. Cracks that occurred in the bottom aluminum alloy 6061-T6 sheet around the rivet leg were a result of tangential tensile stress. The cone height of a conical-section die is the most important parameter affecting the surface quality of Al/steel SPR joints. Conical-section dies with a moderate convex can ensure a good surface quality during the SPR process. In addition, SPR joints with single conical-section die allow higher tensile strength and energy absorption compared to those with double conical-section die.

平点自冲铆接过程的数值模拟分析

为研究平点自冲铆接过程,以两层厚度均为1mm的铝板AL-1100以及材质结构为合金结构钢DIN 14NiCr14的半空心铆钉为研究对象,借助于有限元分析软件Deform-2D进行数值模拟并得到强度合适的接头以及冲头载荷随位移变化的曲线;选取接头自锁量和底部最小厚度作为评价指标,研究了不同铆钉高度对接头的影响。结果表明:平点自冲铆接技术是可行的;铆钉腿部的变形是引起冲头载荷迅速增加的主要因素;随着铆钉高度的增加,两个评价指标的变化曲线几乎呈线性关系。

电磁成形技术的研究与应用

电磁成形技术作为高能、高效率技术有着传统成形方法不能比拟的优越性,在工业生产中应用十分广泛。主要介绍了电磁成形在管材成形、平板件成形、冲裁、铆接、焊接、电磁粉末压制等方面的应用。

304不锈钢扁平头半空心厚壁铆钉翻铆缺陷分析

为了揭示304不锈钢扁平头半空心厚壁铆钉在翻铆过程中易出现压不到位、空心壁开裂和腰部镦粗等缺陷产生的原因,采用有限元、机理分析和实验的方法分析了半空心厚壁铆钉的制作及翻铆过程,获得了铆钉制作及翻铆时铆钉的应力场、硬度分布和显微组织。结果表明:翻铆时翻卷半径过小,材料在弯角内侧堆积,导致压不到位。铆钉空心壁在铆钉制作和翻铆过程中变形量最大,加工硬化最严重,且有部分马氏体相变,导致铆钉空心壁部分的伸长率大幅下降;翻铆时铆钉空心壁外翻直径过大,超过拉伸极限,则铆钉在硬度最高的空心壁处开裂。铆钉制作和翻铆时腰部变形量最小,硬度最低,当翻铆力大于其抗压缩失稳的能力时,则会在腰部发生镦粗。

基于双目多线结构光的铆钉齐平度测量方法

对于铆钉齐平度的测量,使用传统检具或非接触式点云扫描的测量方法分别存在局限较大、准备周期长、数据难处理等缺点。针对上述问题,提出一种基于双目多线结构光的铆钉齐平度测量方法,首先对双目系统采集到的结构光图像进行双目匹配与重建,提取、拟合出在测量坐标系下铆钉头圆轮廓内离散像素点的坐标与局部蒙皮平面的表达式,随后引入离散点筛选比例系数进一步优化局部蒙皮平面的拟合精度,将其代入铆钉齐平度测量模型中进行计算获得结果。相比于传统接触式测量与点云扫描,此研究方法节省了处理点云数据所需的大量时间,提高了测量效率,通过机械臂搭载小型测量末端能够完成对飞机进气道等狭窄内腔的检测任务,降低了局限性。试验结果表明,该方法对于铆钉头凹凸两种特征均适用,且齐平度的最大测量误差小于0.03mm,满足视觉测量要求。

低压交流接触器铁心底面平面度超差的原因分析及解决措施

低压交流接触器铁心通过硅钢片冲压、叠铆、极面磨削和喷油等加工工艺制造完成。铁心冲片后铆压是铁心加工的一个重要工序,而铆压工艺质量控制的好坏是造成铁心底面平面度超差的主要原因,也是影响后道工序和整个产品性能质量的重要标志。介绍了低压交流接触器铁心底面平面度超差产生的主要原因及基本的解决措施,为低压交流接触器铁心的制造提供了一定的技术参考。

Impact of Stack Orientation on Self-Piercing Riveted and Friction Self-Piercing Riveted Aluminum Alloy and Magnesium Alloy Joints

Self-piercing riveting(SPR)is a mature method to join dissimilar materials in vehicle body assembling.Friction self-piercing riveting(F-SPR)is a newly developed technology for joining low-ductility materials by combining SPR and friction stir spot welding processes.In this paper,the SPR and F-SPR were employed to join AA6061-T6 aluminum alloy and AZ31B magnesium alloy.The two processes were studied in parallel to investigate the effects of stack orientation on riveting force,macro-geomet-rical features,hardness distributions,and mechanical performance of the joints.The results indicate that both processes exhibit a better overall joint quality by riveting from AZ31B to AA6061-T6.Major cracking in the Mg sheet is produced when riveting from AA6061-T6 to AZ31B in the case of SPR,and the cracking is inhibited with the thermal softening effect by friction heat in the case of F-SPR.The F-SPR process requires approximately one-third of the riveting forces of the SPR process but exhibits a maximum of 45.4%and 59.1%higher tensile-shear strength for the stack orientation with AZ31B on top of AA6061-T6 and the opposite direction,respectively,than those of the SPR joints.The stack orientation of riveting from AZ31B to AA6061-T6 renders better cross-section quality and higher tensile-shear strength and is recommended for both processes.

Microstructural evolution in friction self-piercing riveted aluminum alloy AA7075-T6 joints

Friction self-piercing riveting(F-SPR)is an emerging technique for low ductility materials joining,which creates a mechanical and solid-state hybrid joint with a semi-hollow rivet.The severe plastic deformation of work materials and localized elevated temperatures during the F-SPR process yield complex and heterogeneous microstructures.The cut-off action of the work materials by the rivet further complicates the material flow during joint formation.This study employed the F-SPR process to join AA7075-T6 aluminum alloy sheets and systematically investigated the microstructural evolutions using electron backscatter diffraction(EBSD)techniques.The results suggested that as the base material approached the rivet,grains were deformed and recrystallized,forming two distinct fine grain zones(FGZs)surrounding the rivet and in the rivet cavity,re s pectively.Solid-state bonding of aluminum sheets occurred in the FGZs.The formation of FGZ outside the rivet is due to dynamic recrystallization(DRX)triggered by the sliding-to-sticking transition at the rivet/sheet interface.The FGZ in the rivet cavity was caused by the rotation of the trapped aluminum,which created a sticking affected zone at the trapped aluminum/lower sheet interface and led to DRX.Strain rate gradient in the trapped aluminum drove the further expansion of the sticking affected zone and resulted in grain refinement in a larger span.

Combined strengthening mechanism of solid-state bonding and mechanical interlocking in friction self-piercing riveted AA7075-T6 aluminum alloy joints

A recently developed friction self-piercing riveting(F-SPR)technique based on the combination of fric-tion stir processing and riveting has been reported to possess both solid-state bonding and mechanical fastening characteristics.However,there is still a lack of quantitative understanding of the hybrid en-hancement mechanism,hindering its engineering application.To fill in this gap,the current research investigated the microstructure evolution,microhardness distribution,and miniature-tensile performance of the aluminum alloy AA7075-T6 F-SPR joints by experiments.An accurate numerical simulation model was established to quantitatively evaluate the individual contributions of microstructure,local bonding strength,and macro interlocking to the performance of the joint,which could well explain the experi-mental results.It was found that due to the friction stirring of the rivet,solid-state bonding driven by dynamic recrystallization is realized between the trapped aluminum in the rivet cavity and the bottom aluminum sheet.The solid-state bonding zone has 75%yield strength,81%ultimate tensile strength,and 106%elongation compared to the base material.This solid-state bonding enables the internal interlock-ing between the trapped aluminum and the rivet to withstand the additional load,which forms a novel dual-interlock fastening mechanism and increases the peak cross-tension force by 14.3%compared to the single-interlock joint.

连接板厚度对沉头铆钉连接强度的影响分析

采用考虑材料弹塑性、接触及几何大变形等因素的非线性有限元方法对沉头铆钉连接结构进行模拟。主要研究连接板的厚度对沉头铆钉连接强度的影响,并与平锥头铆钉进行对比。分析结果表明铆钉在承受剪力的同时会承受一部分拉力,致使铆钉的连接强度有所降低。特别是对在较薄平板上安装的沉头铆钉,因锪窝对铆钉孔削弱较大,铆钉孔变形较大。较大的变形使铆钉轴线发生倾斜,导致其受拉变严重,从而对其连接强度产生了显著的影响。而连接板厚度对平锥头铆钉的连接强度影响不明显。