Study On Microstructure And Properties Of Cold Metal Transition (CMT) Brazing Welding Of 7075 High Strength Aluminum Alloy/Galvanized Steel Sheet

As the energy and environmental crises intensify,lightweight automobile has become one of the major trends in the development in the automotive industry.High-strength aluminum alloy has become one of the ways to reduce the weight of automotive vehicles.However,iron and aluminum have the great differences in the physical properties,As well as the Intermetallic compounds?IMC?will be formed,affected the connection of aluminum and steel.In this paper,AlSi5 wire were used as filler metal,the wettability of the wire in the galvanized steel was studied,and the 7075 aluminum alloy and galvanized steel were melting brazing connection by CMT technique.and in order to improve the performance of joints,to take preheating before welding,studied the microstructure and properties of joints.AlSi5 wire is more wettable on galvanized steel than on non-galvanized steel,wetting angle smaller,spreading longer.Fe₂Al₅and Fe Al₃compounds were formed on the wetting interface,and the IMC thickness of AlSi5 wire on galvanized steel was much smaller than that of non-galvanized steel.As the heat input increases,the wetting angle of the AlSi5 wire on the galvanized steel decreases,the wetting length and the IMC thickness increases.CMT melting brazing test of 7075 aluminum alloy and galvanized steel found that the weld surface forming a good.Welding joints can be divided into zinc-rich area,weld zone,heat affected zone,the IMC area.The zinc-rich zone is Al-Zn solid solution;the weld zone consists of?-Al solid solution and Al-Si eutectic;the IMC zone consists of Fe₂Al₅close to the steel side and Fe Al₃phase close to the weld;The microstructure of a heat-affected zone also have some changes due to the influence of heat input.The mechanical properties of the joints were tested and found that when the heat input is small,the IMC thickness is thinner and the wetting length is smaller,the fracture occurs on the steel side IMC layer.With the increase of heat input,the IMC thickness increases to 4-5um and the wetting length also increases,and the fracture occurs in the fusion zone.Heat input continues to increase,the IMC thickness increased to about8-10um,fracture occurred in the interface area.The fracture occurred in the fusion zone belonging to the dimple fracture,while the fracture in the interface zone was the dissociation fracture.When the wire feed speed is 5m/min,the welding speed is 0.5m/min,the maximum load of the joint is 8582N.Hardness analysis of the joint found that the weld center hardness of about 60-90HV,heat-affected zone hardness of about 140-160HV,heat-affected zone softening zone distance from the weld line increases as the heat input increases.Preheating before welding method found that preheating can improve the AlSi5 wettability on galvanized steel,wetting angle decreases,the wetting length and the IMC thickness increases.The effect of preheating on melting brazing joint can promote the increase of wetting length and the decrease of wetting angle,and the change of IMC layer is not obvious.The temperature field of the welding process is simulated by finite element method.It is found that the temperature gradient near the aluminum plate side is larger,and the temperature gradient near the steel plate side is smaller.The maximum temperature during welding is 1110?.The temperature at the front of the zinc-rich zone is 750?,causing the zinc in the zinc-rich zone to melt and not completely evaporate,so the wetting of the liquid solder is on the liquid zinc surface and partly on the steel interface.The preheating of the temperature field simulation found that preheating 100?,the same location of zinc-rich zone temperature increased to about 900?,So that the amount of molten zinc increased,promoting the front of the solder wetting spread further.and the IMC layer of the temperature increase not obvious,so the thickness of the IMC thickness is not obvious.