Study On Single-sided Joining Of Aluminum Alloys Using Friction Self-piercing Riveting Process

Aluminum sheets,aluminum profiles and aluminum castings are used to achieve the purpose of reducing the weight without sacrificing strength or structural stiffness of vehicle bodies.When it comes to aluminum alloy profiles,castings,closed / semi-closed structures,two-sided joining processes are difficult to achieve due to space constraints,and single-sided joining processes are required.There are few types of existing single-sided joining processes.Only the flow drill screw process is widely used in vehicle body manufacturing,but the screw has problems such as heavy weight and high cost.This paper proposes a single-sided friction selfpiercing riveting(F-SPR)process for single-sided joining of aluminum alloy.The single-sided F-SPR process starts by rotating a semi-tubular rivet at a high speed and meanwhile feeding it towards the sheet stackup.The rotational motion generates frictional heat in piercing into the stackup and thus could improve the rivetability of low ductility materials.The joint formation process is complicated due to the thermo-mechanical coupling,and the influence of the process parameters on joint formation is still unclear.Therefore,the following research is carried out through experiments for the application of single-sided F-SPR process:(1)Research on the joint formation mechanism of single-sided F-SPR process.The macroscopic morphology,microstructure and hardness distribution of joints under different process parameters are studied according to the riveting force-displacement curves and torquedisplacement curves.Then a two-stage process method is proposed to avoid joint defects and improve joint quality.Results show that the higher rotation speed and lower feed rate could avoid rivet shank fracture but lead to insufficient undercut.The material near the interface between sheets and rivet and the material in the rivet cavity would recrystallize and form fine grain regions under the agitation of rivet.The hardness of fine grain regions is significantly reduced due to dissolution of precipitate phase and elimination of work hardening.Under some parameters,metallurgical joining is formed between two sheets.(2)Research on mechanical properties and failure modes of joints.The tensile-shear and cross-tension properties of joints under one-stage process parameters are studied,and the failure modes are also summarized.Results show that under one-stage process parameters,the tensile-shear property is mainly determined by riveting depth.Therefore,the increase of the feed rate would reduce the tensile-shear strength.The cross-tension property is sensitive to two factors: the undercut and the tightness of bond between rivet tips and surrounding material.After using the two-stage process parameters,the tensile-shear strength is significantly improved,but the cross-tension strength is slightly reduced.(3)Optimization of two-stage process parameters by orthogonal experimental design.The joint geometrical indexes and mechanical properties are used as test indicators in orthogonal experimental design.The optimal two-stage process parameter is selected by analyzing the trend of test indicators.Results show that the maximum tensile-shear strength and cross-tension strength are 5231.0 N and 2081.9 N,respectively.