Study On Laser-MIG Hybrid Welding Of Dissimilar Metals Of SYG960E Ultra-high Strength Steel And 6061 Aluminum Alloy

With the excessive energy consumption and increasing atmospheric pollution,the lightweight of automobiles has become an important measure to reduce energy consumptions and emissions.In addition,the application of ultra-high strength steel,aluminum alloy and other light materials is an important way to realize the lightweight of automobiles.But there are significant differences between steel and aluminum in both physical and chemical properties and the formation of brittle Fe-Al intermetallic compounds(IMCs)during welding process deteriorates the mechanical properties of steel/Al joints seriously,which restrict the development of lightweight technology.Therefore,it is of great theoretical significance and practical value to carry out the research on laser-MIG hybrid welding of ultra-high strength steel and aluminum alloy.In this paper,the microstructures and mechanical properties of laser-MIG hybrid welded joints of SYG960 E ultra-high strength steel and 6061 aluminum alloy were studied.And the effects of alloying elements and welding parameters on microstructures and mechanical properties of laser-MIG hybrid welded steel/Al joints were investigated.The results showed that the steel/Al joint of laser-MIG hybrid welding had dual characteristics of fusion welding and brazing and it was divided into interfacial zone(IZ),weld zone(WZ),heat affected zone at steel side(HAZ1)and heat affected zone at aluminum side(HAZ2).The IZ was a double-layer structure consisting of tongue Fe2Al5 and needle Fe4Al13 and it had high crack sensitivity.The WZ consisted of near equiaxed ?-Al solid solution.The HAZ1 consisted of bainite and pearlite and the ?-Al grains coarsened in HAZ2.Due to the brittleness and hardness of Fe-Al IMCs,the micro-hardness of interfacial zone was the highest(392HV)and the joints fractured at the interface between steel and aluminum during the tensile test.Thus,the interfacial zone was the weakest link of laser-MIG hybrid welded joints of ultra-high strength steel and aluminum alloy.Alloying elements had great effects on microstructures and mechanical properties of laser-MIG hybrid welded joints of ultra-high strength steel and aluminum alloy.The results showed that adding Cu was beneficial for inhibiting the growth of Fe-Al IMCs and reducing the thickness of interfacial layer.In addition,Cu atoms replaced some of Fe atoms of Fe-Al IMCs to form(Fe,Cu)2Al5 and(Fe,Cu)4Al13,which improved the brittleness of IMCs,reduced the hardness of interfacial zone and improved the crack sensitivity of interfacial layer.Based on the two aspects above,the maximum tensile strength of steel/Al joint was increased by 110.7% with Cu addition,compared with that without Cu addition.Adding Ni was also beneficial for inhibiting the growth of Fe-Al IMCs and reducing the thickness of interfacial layer.But the formation of(Fe,Ni)2Al5 and(Fe,Ni)4Al13 had limited effect on improving the brittleness of IMCs and the interfacial layer still had high crack sensitivity.The maximum tensile strength of steel/Al joint was increased by 15.4% with Ni addition.Adding Zn improved the wettability of liquid aluminum on solid steel surface significantly.But the evaporation of Zn affected the atomic diffusion and interfacial reaction seriously,inhibited the formation of Fe-Al IMCs and hindered the effective joining between steel and aluminum.Therefore,adding Zn was not conducive to improving the welding quality.The welding parameters had great effects on weld appearance,the thickness of interfacial layer and the mechanical properties of laser-MIG hybrid weled joints of ultra-high strength steel and aluminum alloy.With the increase of laser power,the weld width decreased firstly and then remained unchanged,the weld penetration increased,the reinforcement decreased,the thickness of interfacial layer increased but without any cracks throughout and the tensile strength of joints increased firstly and then decreased.With the increase of welding current and the decrease of welding speed,the weld width and reinforcement increased,the thickness of interfacial layer increased and the cracks occurred,the weld penetration remained unchanged,the tensile strength of joints increased firstly and then decreased.Under the condition of this experiment,the laser power 1000 w,welding current 150 A and welding speed 60cm/min was favorable for improving the welding quality and the mechanical properties of laser-MIG hybrid welded joints of ultra-high strength steel and aluminum alloy.