Mechanical Properties And Microstructure Of High Strength Aluminum Alloy FSW Joints

Friction stir welding is a newly invented welding technology especially efficient in joining aluminum alloys and other materials that traditionally considered difficult or impossible to weld.2195 Al-Li alloy,with outstanding mechanical properties,is one of the most promising 3^rd generation Al-Li alloys.And given to the cost,it is convenient to manufacture spacecraft with both 2195 and 2219,one of the most popular materials in the space industry.However,the mechanism and microstructure evolution of either 2195 friction stir welding or 2195-2219 dissimilar friction stir welding remain unclear.And the cryogenic mechanical behavior of the joints also need more research.2219-T6 and 2195-T8 sheets are employed in this research.The paper focus on the mechanical properties and microstructure evolution of 2219-2195 dissimilar friction stir welding and 2195 friction stir welding joints.The macrostructure of both joints are typical for FSW joints.There is a clear boundary in the middle of WNZ?welding nugget zone?comes from the original surface of the welded sheets.The boundary between WNZ and TMAZ?thermo-mechanically affected zone?on AS?advancing side?is clear dividing recrystallized grains and twisted grains in two different regions.While on the RS?retreating side?some of the twisted grains can be found in the WNZ blurring the boundary.This phenomenon revealed the fact that materials on RS experienced less thermal and mechanical affection.The mechanical properties of dissimilar joints increase with the travel speed of FSW.And joints with 2219 on RS has better mechanical properties than those with 2219 on RS.It turns out that strengthening precipitates in 2219 is vulnerable to heat and either higher travelling speed or RS location can protect the precipitates from excessive temperature.While the longitudinal tensile specimens has poorer strength than expected.Non-uniformity of 2219 and 2195 in WNZ may be one of the reasons.But it is also possible that the strength between different layers stirred by the welding tool is much weaker than that of the homogeneous fine grains.Increased travelling speed seems to be of no benefit for the 2195 FSW joints.On the contrary,insufficient heat for the joining of the faying surface leads to an obvious drop in mechanical properties.Moreover,by comparing FSW joints and FSP specimens without faying surface,it is revealed that travelling speed has little impact on the strength of 2195 joints.And at cryogenic temperature the2195 FSW joints have overwhelming increase in strength over the dissimilar ones.It is the difference in precipitates that causes the significant difference between 2195 and 2219.