Study On Microstructures And Properties Of Spray Formed Ultra High Strength Aluminum Alloys Joint By Friction Stir Welding

The microstructure is finer and no obvious macro segregation gotten when aluminum alloy is produced by spray forming. Softening phenomenon is serious and the property will be reduced greatly when spray formed aluminum is welded by traditional fusion welding. As a kind of solid-state welding method friction stir welding is an effective welding method for spray formed aluminum.Spray formed 7055 was welded under T76 state and the temperature was measured in the welding process to study the effects of welding parameters on the temperature field and organization. Studied the effects of welding parameter on mechanical properties by tensile and hardness. Analyzed the microscopic by OM and SEM and researched the relationship between welding parameter, microstructure and properties. Welded 7055 under squeezed state and solid solution state and heat treatment was carried out on the joints to study the effects of original state of base material and heat treatment on microstructures and properties.Temperature measurement results show that each point in the material rise first and then decreased. The maximum temperature in the end stage is higher than that in the initial and stability stages. The temperature of each point increased with the increase of rotation speed and the reduction of advancing speed. Temperature in the advancing side is higher than that in the retreating side and the father the distance on the weld surface the maximum temperature is lower.It can be seen on the macroscopic of the joint that flash mainly appeared in the back side. When the press was fixed flash was more serious with reduced advancing speed and increased rotation speed. The width of the central of nugget zone became wider with the decrease pf welding speed or the increase of rotation speed. Microscopic analysis showed that with the decrease of welding speed and the increase of rotation speed recrystallization occurred in nugget zone and part of the thermal mechanical affected zone and mechanical properties in these zones were fine. Grain grown in the thermal affected zone and this zone is the weak area of the joint.The tensile strength and extension rate of the joint reduced first and then increased with the increase of rotation speed and advancing speed. When rotation speed is 1350 rpm and advancing speed is 75mm/min, tensile strength of the joint is 467.12 MPa, up to 86.0% of base material.With the improvement of rotation speed, the width of heat affected zone is bigger and the minimum hardness of the heat affected zone is smaller. The change of the width of heat affected zone under different advancing speeds was not big and the width was about 3mm. But the minimum hardness of the heat affected zone reduced first and then improved.No obvious macroscopic defect was found in the joints of base material under squeezed state, solid solution state when rotation speed was 1350 rpm and advancing speed was 75mm/min. Also, no obvious defect was found when heat treatment was carried out on the joints. Grains in the nugget zone grew up unusually when T76 heat treatment was carried out on the squeezed state joint but the tensile strength is 496.1MPa, up to 91.4% of base material. Hardness in each zone was about 195 HV, which was similar to that of base material.