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.

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.

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。

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.

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.

基于机器视觉的铆接孔圆度误差评定方法

针对飞机铆接孔圆度误差评定的问题,提出了一种基于机器视觉的铆接孔圆度误差评定方法。该方法首先使用CCD工业相机获得铆接孔图像信息,其次对图像进行高斯滤波去噪处理,并采用拉普拉斯算子提取图像边缘信息,最后使用最小包容区域法对边缘图像进行圆度拟合以及误差评定。结果表明,该方法测得铆接孔圆度误差平均值为0.095,均方误差为0.09,圆度相对误差率为0.0026,为铆接孔圆度误差评定提供了一种可行的方法。

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.

滚筒洗衣机框架高强度铆接技术研究与应用

本文通过对滚筒洗衣机框架铆接工艺和设备方案进行调研分析、设计与实施,着重研究了铆接可靠性、检测方法和防错防呆的解决方案,实现滚筒洗衣机框架部件连续自动化生产,提高铆接质量,降低生产成本,具有较大的参考价值。

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.

基于机器学习的铆钉缺陷检测技术

针对传统铆钉缺陷检测中由于人工操作使得检测速度慢,并且结果会引入人为误差的问题,设计出一套基于Faster-RCNN为目标检测算法,以TensorFlow为深度学习框架的铆钉缺陷检测系统。利用LabelImage标注工具进行标注,将数据集转换为Pascial VOC 2012格式。迭代4万次的VGG-16训练模型在1008幅测试集进行测试,实验结果表明:平均准确率为97.37%,平均精确率为93.9%,平均召回率86.48%。

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.

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.

基于机器视觉的自冲铆接接头剖面尺寸检测研究

针对实际工业检测中自冲铆接工艺接头的剖面尺寸人工检测存在精度低、效率低、测量不一致等问题,开发了一个基于机器视觉的自冲铆接工艺接头剖面尺寸检测系统,提出了一种基于边缘提取和角点检测的尺寸缺陷检测算法。对获取的图像进行图像灰度化、图像双边滤波和图像增强,通过Canny边缘检测进行图像分割实现测量区域的边缘提取,采用亚像素级角点检测提取剖面尺寸的测量点,采用最小二乘法拟合直线边缘。通过系统测试对尺寸缺陷检测算法进行了验证和分析,验证结果表明,算法的精确率达到98.67%,测量精度达到0.05 mm,平均检测节拍为0.31 s。该系统检测精度高、稳定性好、检测效率高,可以有效替代传统的人工测量。

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.

轿车轮毂轴承单元新型铆合装配机床的研究

提出了新型的具有铆头倾角可调、铆装压力和工作台位移实时测量、自动精密调整铆装终止位置等功能的精密铆合装配专用机床,理论分析了机床的铆头倾角、进给量和铆装力等参数,试制了机床样机.试验结果表明,该机床最大轴向铆装力达30T,工作台的定位精度小于2μm,生产效率可达3 000件/班,铆装出的轮毂轴承单元可靠性理论上可达到30万公里以上,满足了轿车轮毂轴承单元的装配要求.

铝电解电容器盖板铆接机控制系统设计

根据目前铝电解电容器盖板铆接机存在自动化程度低、劳动强度大、生产工序不连贯等不足，设计了铝电解电容器盖板铆接机控制系统。该系统以三菱FGA系列PLC为核心，结合步进电机、气缸、磁性接近传感器和位置传感器等元件的检测定位，实现芯包移动、盖板铆接、自动脱模、芯包翻转等工序的自动连续运行：采用触摸屏作为上位机，实现参数设定和系统运行状态监测。现场调试运行表明：该系统运行可靠平稳、操作简单、劳动强度低。能够满足生产线高效率、高质量的自动化加工要求。

LMB50型智能旋铆机的设计及研究

针对轮毂轴承铆接加工过程中存在的微调不方便、加工精度不高等问题,基于闭环控制原理,设计研究了一种可编程序控制器和伺服电机控制进给位移的新型旋铆机。该旋铆机的升降装置采用了大功率伺服电机、行星减速机和重型滚珠丝杠驱动,外加光栅尺构成了闭环控制;检测装置采用了精密级高度计作为检测传感器检测实际毛坯高度和加工后成品的高度;通过在工件夹具下方装设轮辐称重传感器以实时反馈旋铆加工时的工作压力。实践结果表明,该旋铆机实现了加工坐标值自动调整和毛坯材料性能的自动检测,从而实现了轮毂轴承零游隙和负游隙的控制,同时降低了轮毂轴承的各零件制造精度要求。

基于机器视觉的多铆钉自动检测算法研究

为了克服传统人工铆钉检测工效低、精度不易控制等弊端,提出了一种基于机器视觉的多铆钉非接触式自动检测算法。采用改进的OTSU算法对采集到的铆钉图像进行分割,有效地减少了污点区域的误划分。为解决铆钉方位的随机性对检测过程造成的影响,采用最小外接矩形法定位铆钉轮廓主轴,克服了复杂工业环境中的不确定因素。通过计算铆钉轮廓各点在两侧支撑区间内曲率的方法识别轮廓特征点,有效减少了噪声在曲率计算中的权重,并提高了识别精度。实验表明:该系统检测精度高,且误检率低,能满足铆钉生产在线检测的要求。