Study On Microstructure And Mechanical Properties Of Aluminum/Steel Riveting-welding Hybrid Joining

Automotive lightweight is conducive to reducing energy consumption and improving energy utilization,and has been the goal of the automotive industry’s manufacturing development.By applying lightweight materials in the body is the most effective means of reducing body weight,and high-strength steel and aluminum alloy materials are widely employed in the field of automotive manufacturing.However,the great difference in physical and chemical properties such as melting point,electrical conductivity and mutual solubility between aluminum alloys and high-strength steels makes them prone to form brittle intermetallic compounds（IMC）when they are welded,reducing the quality of the welded joints.Therefore,many researchers sought or developed multiple joining processes to solve the jointing problem between aluminum alloys and high-strength steels.In this paper,QP980 high-strength steel and 6061 aluminum alloy connection was studied using the riveting-welding hybrid joining process.Pulsed laser,laser-arc hybrid and continuous laser were employed as welding heat sources to obtain the influence law of different welding heat sources on the forming quality of the joint,mechanical properties and microstructure of the weld,revealing the mechanism and influencing factors of welding heat sources on the distribution of weld elements and the IMC.The microstructure and properties of aluminum（Al）-steel dissimilar metal riveting-welding joints obtained by the pulse laser and laser-arc hybrid welding methods were investigated.For simple laser beam riveting-welding Al and steel joint,the maximum tensile shear load of the joints was 6.10 k N,as the average laser beam power was about 438 W.The microstructure of the fusion zone（FZ）of the joint was lower bainite（B_L）,and there were small cracks in the weld.The Al elements were uniformly distributed in a vortex-shaped in the mixed region consisting of QP980 steel,6061 Al alloy,and Q460 rivet,and the intermetallic compounds（IMCs）at the Al-steel interface had Fe Al,Fe₂Al₅,and Fe Al₃.For the laser-arc hybrid riveting-welding joint,the maximum tensile shear load of the joint with an average laser power of 160 W and a tungsten inert gas（TIG）current 100 A was 7.30 k N.Lath martensite（LM）,ferrite（F）,and retained austenite（RA）were generated in the FZ of the joint,and the Al element aggregates in bulk and sporadically distributed in the mixed zone consisting of the steel plate,Al plate,and rivets.Furthermore,the differentiated distribution mechanism of Al element in riveting-welding hybrid joints was discussed in this paper.Subsequently,a continuous laser was employed as a welding heat source to achieve riveting-welding hybrid joint between aluminum alloy and high-strength steel,and it was found that the microstructure of the FZ differed for different laser powers.The microstructure of the joint FZ with laser power of 1.62 k W were martensitic.When the laser power was increased to1.70 k W,the top area of the joint FZ was martensitic and ferrite,and the middle area and the bottom were martensitic.The Al content in the welds was low and uniformly distributed,and the Al elements were heavily aggregated at the Al/steel interface,and the generated IMCs were in the form of continuous layers,needles,and free islands,consisting ofα-Fe,Fe Al,Fe₂Al₅,Fe Al₃.When the laser power was 1.78 k W,the top of the joint weld was columnar crystal,the middle of the weld was martensite,and the bottom was island-like IMC and martensite.The average maximum tensile shear load of the laser-welded rivet head was 8.31 k N,which was13.7%higher than that of the laser-arc riveting-welding hybrid joint,and the fracture of the rivet occurred in this joint.The non-uniform hardness distribution of the joint was related to its microstructure.