| Considering the effect and cost,the aluminum/steel mixed car body is currently one of the main trends for vehicle lightweighting.However,the application of advanced high-strength steel(AHSS)and ultra-high strength steel(UHSS)brings a great challenge to some typical technologies used in multi-material vehicle car body assembly,such as self-piercing riveting(SPR)and friction stir spot welding(FSSW).In this paper,an innovative process named flat die based friction self-piercing riveting(F-SPR),which integrates advantages of SPR and FSSW,is developed to effectively join aluminum to AHSS/UHSS.Its process includes two stages: driving the high-speed rotating rivet to penetrate the upper aluminum sheet and then forming a solid-state joint between rivet leg and lower steel sheet with the help of axial compressive force and circumferential friction torque.Finally,frictional heat.Then,the upper sheet is "locked" between rivet head and lower sheet,so that mechanical-metallurgical joint is obtained.As a new technology with both features of SPR and FSSW,the process of F-SPR is complicated and the forming mechanism of different bonds among rivet and sheets is still not clear.Therefore,to realize a sound joining of dissimilar materials using flat die based F-SPR,the following research work was carried out:Study on interaction regularity of rivet and steel sheet.Based on the self-developed F-SPR equipment,the mechanism of the solid-state welding between steel rivet and DP780 without aluminum sheet was studied.The influence of rivet hardness and process parameters on the forming process and joint quality was analyzed.Results show that rivet with higher hardness has a wider range of rivetability and is more suitable to be a benchmark rivet.Among three process parameters,the rotation speed and feed speed mainly influence heat input while the feed depth is the major factor of solid-state welding.The joint strength is positively correlated with the average hardness of the nugget at rivet/steel interface.Study on joining mechanism of F-SPR for aluminum and steel.Considering the flow characteristics and volume distribution of aluminum,the geometry of rivet was redesigned.Afterwards,the forming process of F-SPR joint was analyzed,and the influences of rivet hardness and process parameters on forming characteristics and tensile strength of the joints were studied systematically.Results show that forming process of F-SPR could be divided into two stages: rotating penetration and friction welding.The joint quality could be evaluated by three indicators,which are state of welding zone,average hardness of the nugget and range of hardening zone.Multi-stage feed process could not significantly enhance the joint strength.The way to increase joint strength is to reduce heat input during the rotating penetration stage,while making the feed depth and heat input in a proper range in friction welding stage.Optimization of F-SPR joint strength.Since SPR cannot joint AA6061-T6 to QP980 effectively,the research on characterization of FSPR joints for AA6061-T6/QP980 was carried out.The influence of process parameters on the characteristics of joints was analyzed,and the process parameters were optimized by orthogonal experimental design.Results show that changes of materials and thickness of steel sheet will make a largely difference to the applicable range of process parameters for optimal heat input.The fast feed with slow rotation are the optimal process parameters for this combination,and its average tensile shear strength of the joint is 4.6 k N,which is 33% higher than that of SPR joint.In conclusion,the F-SPR process shows a very significant technological advantages over SPR process. |
