Microstructure And Properties Of Spray Formed 7055 Aluminum Alloy FSW Joint Under Different Cooling Conditions

Spray formed 7055 Al alloy?Al-Zn-Mg-Cu alloy?is widely used in aerospace,high speed trains and other fields due to its high strength and toughness.Due to the unique strengthening method,it is very difficult to weld spray formed Al alloy by traditional fusion welding.In this paper,the 4mm spray formed 7055 aluminum alloy was welded by friction stir welding.The microstructure and properties of friction stir welded joint under air cooling condition and water mist cooling condition were studied respectively.The mechanism of water mist in friction stir welding under the condition of water mist cooling was analyzed.In air cooling condition,when the welding speed is 175mm/min and the rotational speed is 900rpm,the transgranular fracture occurs in the heat affected zone,when the rotational speed is 1500rpm,the mixed fracture of transgranular and intergranular occurs in the weld nugget area.The analysis of EDS and DSC showed that the fracture of the two joints is mainly caused by the precipitation of Al-Cu phase at the grain boundary.When the rotation speed is 1300rpm and the welding speed is 75mm/min,the maximum strength?488MPa?of the friction stir welding joint under water mist reaches 77%of the strength of the base material?632MPa?,and the properties were better than the joint performance under the air condition at corresponding parameters.The effect of water mist cooling changed the vickers-hardness distribution of joints.The hardness of the heat affected zone was obviously improved,and the softening zone of the joint appeared in the nugget area.The microstructure of the joint under different cooling condition at the same parameters were compared with the methods of measuring the thermal cycle of the joint and combining the methods such as SEM&EDS,XRD,DSC and TEM analysis.The results showed that the effect of water mist cooling can effectively reduce the peak temperature of the test point in the welding process.At the end of the welding process,the temperature of the test point under the water mist condition is close to the room temperature,and the temperature of the test point under the air condition is still around 100?.SEM&EDS analysis showed that the grain size in the nugget area is very small under the water mist condition.The Cu content in the heat affected zone of the water mist cooling joint decreases compared with that under the air cooling condition.Under the air cooling condition,MgZn₂mainly precipitates in the grain of the nugget zone.Under the water mist cooling condition,MgZn₂ mainly precipitates in the grain boundary of the nugget zone.The analysis of DSC and XRD showed that the strengthening phases in the joint were precipitated from the matrix,whether under air cooling condition or water mist cooling condition.The TEM analysis showed that the size of the precipitated phase in the heat affected zone of the joint is much larger than that under the water mist cooling condition under air cooling condition.The high resolution of TEM indicated that the phase relationship between Al₂Cu,Al₂MgCu and MgZn₂ precipitated in the heat affected zone under air cooling condition is coherent or incoherent.The precipitation phase Al₂Cu and Al₂MgCu in the nugget zone of joints are incoherent with the matrix phase under the air cooling condition.Under water mist cooling condition,the joints in heat affected zone and nugget area retained more?and S phases and formed a good semi-coherent and incoherent relationship with the matrix.Therefore,water mist cooling can enhance the strengthening effect of the second phase of the joint,and the joint performance is improved by the incoherent strengthening of the precipitated phase.