Study On Microstructures And Properties Of 7B04 Aluminum Alloy By Friction Stir Welding

7B04 is a high strength Al-Zn-Mg-Cu aluminum alloy;which is based on the development of the 7A04 aluminum alloy.7B04 aluminum alloy,which has a high hardness and strength,good thermal formability and welding performance,good resistance to corrosion and toughness,is a kind of very important material in the aerospace industry.At present,it has been widely used in all the fuselage of the aircraft,cabin siding and rocket structure parts of manufacturing.Friction stir welding(FSW)is a solid state autogeneous method for joining two or more workpiece where a tool,moving in a cyclic manner relative to the workpiece,enters the joint region,local plasticizes and moves material along the interface thus developing a solid joint between the workpiece.In the present study,FSW was carried out on A1-7B04 and the welding parameters were optimized.The sound FSW joints with excellent mechanical properties were obtained.Further,the post-weld heat treatment was also conducted in order to provide some advises for the industry.From the research,the results are as followed:(1)The optimum welding parameter for FSW A1-7B04 was obtained which was 1200rpm(rotation speed)with 400mm/min(welding speed),and the sound welding joints were produced.The ultimate tensile strength,yield strength,elongation,joint efficiency of the weld joints were 480MPa,220MPa,10%and 81%,respectively.(2)The shape of hardness distribution under natural aging state was W feature,where the base metal showed the highest hardness and the lowest one was between thermo-mechanically affected zone and heat affected zone.The hardness distribution under post-weld heat treatment states was different form that of NA state,the hardness was uniformed for solution treatment(ST)and solution treatment and artificial aging(STA),further,the hardness is much higher for STA.It is still W shape for natural aging(NA),artificial aging(AA)and step-aging(SA)states.(3)Different degrees of grain abnormal growth(AGG)occurred in the NZ after solid solution treatment,and AGG in NZ grew obviously with the increase of rotational speed.Meanwhile,the decrease of the welding speed is benefitted to limit the degree of AGG.(4)The distribution of the precipitates was non-uniform in the NZ.After AA,both fine and coarse precipitates were observed where the distribution of the fine precipitates is more uniform.The precipitate free zone(PFZ)was found and the precipitates in the thermo-mechanically affected zone got coarsened.The recipitates dissolved during ST and re-precipitated subsequently,however,the amount of them was small.The precipitates got more uniform and finer during STA.