Research On Forming Mechanism And Joint Properties Of Al5052 Sheet Metal By Double-Sided Self-Pierce Riveting

As a new cold-formed joining process,the double-sided self-pierce riveting process possesses the characteristics of high joint strength,hidden interlock,no need for prefabricated holes,and smooth sheet surface,showing good application potential in automobile,aerospace,and other fields.However,there are still some problems in the application of the double-sided self-pierce riveting process,such as limited joint profile controlling,unclear joining mechanism,and difficulties in multi-rivets connection.Based on these,the research on the new parameter design of tubular rivet structure,combined rivet structure,and multirivets joining methods were carried out,aiming to break through the limitations of the double-sided self-pierce riveting process and promote its engineering application.The main research contents and conclusions are as follows:First of all,aiming at the limited control of the existing design parameters of tubular rivets on the joint profile,a new design parameter of tubular rivets—chamfered angle position l was proposed.The deformation behavior,mechanical properties,and failure modes of the joints at five chamfered angle positions were studied through the process tests.The results showed that with the increase of chamfered angle position l,the bending direction of the rivet tip gradually changed from outside to inside.When l was 0.25,the joint profile was better.And when l was 1,the rivet was bent and deformed seriously,forming an invalid joint.When the chamfer angle was 30° and 45°,and the l was 0.25,the tensile shearing strength of the joint was 82.53%and 51.21%higher than that of the traditional ones,respectively.When the chamfer angle was 45° and 60°,the tensile shearing strength of the joint was larger than 6 kN;Three failure modes were obtained by analyzing the tensile shearing failure characteristics of the joints,namely,invalid joint failure,sheet tearing failure,and rivet pull-out failure.Secondly,DEFORM software was used to carry out numerical simulations on the double-sided self-pierce riveting process,and the influence of chamfered angle position on joint profile,the force state,the distribution of effective stress,strain,and the material flow during the joining process were analyzed.The simulation results showed that the effective stress and strain in the rivet middle region increased with the increase of the stroke and gradually extended from the middle to both ends,while the rivet sharp region first increased and then remained stable.The stress and strain of the sheet decreased first and then increased with the rivet bending direction changing from outside to inside,and the stress concentration changed gradually from inside to outside;When l was 0.25,the stress concentration in the rivet middle region could be decreased,a larger joint profile height and more materials in the interlocks would be obtained,and the gap between the sheets was reduced,thus enhancing the mechanical properties of the joint.It was found that the counter force Fi determined the rivet bending direction,and F2 and F3 determined the bending degree by analyzing the force state of the rivet.Thirdly,in order to break through the strength limitation of a single rivet joint,a combined tubular rivet structure was proposed,and the riveting tests of Al5052 aluminum alloy sheets were carried out.The results showed that the combined tubular rivet structure was feasible,and the reliable joining parts of the Al5052 aluminum alloy sheet were realized,and the mechanical properties of the joint were effectively improved.The shearing strength of the combined riveted joint decreased with the increase of the chamfered angle position,which were 10.5 kN,10.4 kN,and 9.8 kN,respectively.The energy absorption value of l=1 was 9.6 times that of the external riveted joint,and the failure mode was the same as that of the single riveted joint;By comparing the mechanical properties of the outer rivet joint with that of the inner rivet joint without flange,it was found that the mechanical properties of the riveted joint with larger diameter was poor.The results showed that the combined tubular rivet structure could effectively improve the mechanical properties of the joint.Finally,based on the problem that the double-sided self-pierce riveting process was difficult to realize multi-rivets connection joint,two multi-rivets joining methods(intermittent successive riveting method and two-steps riveting method)were proposed,and the influence of rivet distance and rivet number on joint interference was studied.The warping angle of riveted parts was used to evaluate the joint interference.The results showed that the warping angle decreased with the increase in distance and the decrease of the rivet number.Based on the intermittent successive riveting method,when the rivet number was 2 and the distance are 15 mm and 25 mm,there was significant interference in the riveted joint,with warping angles of 6.27° and 5.98°,respectively.When the rivet number was two and the distance was more than 25 mm,the two-step riveting method could be used to achieve a flat surface and a negligible warping angle(<1°)of the multi-joints parts.The results showed that the two-steps riveting method was feasible to realize multi-rivets joining parts.