HT700高温合金旋转摩擦焊接接头组织和力学性能的研究

Research on microstructural and mechanical properties of rotational friction welded joints of HT700 superalloy

用旋转摩擦焊接(RFW)方法对HT700高温合金进行焊接,并对焊接试样进行焊后热处理(PWTH)。通过光学显微镜、扫描电子显微镜、透射电子显微镜、显微硬度计、室温和750℃拉伸实验,对接头微观组织和力学性能进行系统研究。结果表明:焊态接头分为焊缝中心区(WCZ)、热力影响区(TMAZ)和热影响区(HAZ)3个典型区域,其中显微组织从等轴细晶粒(WCZ)、变形粗晶粒(TMAZ)逐渐转变为与母材类似的等轴晶粒(HAZ);RFW过程中WCZ发生了动态再结晶及强化相的溶解,其中γ′强化相的溶解程度大于M23C6或MC碳化物;从焊接界面到母材,焊态接头的晶粒尺寸、形状和强化相分布等微观结构逐渐发生变化,γ′相的溶解导致焊态接头力学性能较差,且在750℃时,晶界滑动使其力学性能进一步下降;焊后热处理后,晶粒生长、强化相的再析出和焊后微观组织均匀化使得接头室温和高温拉伸强度显著提高,且接头高温延展性得到一定程度的改善。本研究可为HT700高温合金的高质量焊接提供新思路。

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 γ′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.