Research on microstructural and mechanical properties of rotational friction welded joints of HT700 superalloy
The HT700 superalloy was joined by rotational friction welding(RFW)method,and the welded specimens were subjected to post weld heat treatment(PWTH).The microstructural evolution and mechanical properties of the joints were systematically investigated by optical microscopy,scanning electron microscopy,transmission electron microscope,micro-hardness,and tensile tests at both room temperature and 750 ℃.The results show that,the as-welded joint shows three typical zones across the weldline:weld center zone(WCZ),thermomechanically affected zone(TMAZ),and heat affected zone(HAZ),in which the microstructure gradually changes from equiaxed fine grains(WCZ)and deformed coarse grains(TMAZ)to equiaxed grains(HAZ)that are similar to the base material.The dynamic recrystallization and dissolution of strengthening phases have occurred in the WCZ during RFW,in which y'strengthening phase dissolves to a larger extent than M23C6 or MC carbides.The microstructure of the as-welded joint including the grain size,shape,and the distribution of precipitates gradually changes from the weldline to the parent alloy.Consequently,the as-welded joints exhibit relatively poor mechanical properties due to the dissolution of γ'which becomes even worse at 750 ℃ because of the grain-boundary sliding.After PWHT,the as-welded microstructure can be homogenized by grain growth and the re-precipitation of strengthening phases,which is responsible for the remarkable improvement in tensile strength at both room and high temperature after PWHT.And the high-temperature ductility of PWTH joints has been improved to a certain extent.This study gives new insights into the high-quality welding of the HT700 superalloy.
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