激光增材修复单晶高温合金的热裂纹形成机制

Formation Mechanisms of Hot Cracks in Laser Additive Repairing Single Crystal Superalloys

以第二代单晶高温合金DD432为研究对象开展激光增材修复实验,利用实验与理论计算相结合的手段,分析和研究了单晶高温合金在激光增材修复过程中的热裂纹形成机制。结果表明,单晶修复区内热裂纹在大角度晶界处形成,裂纹两侧呈现显著应力集中,裂纹源区域分布大量MC型碳化物。热裂纹的形成取决于液膜的稳定性、应力集中及碳化物析出相的共同作用。液膜稳定性取决于枝晶凝并过冷度,并与相邻晶粒间的晶界角度密切相关。基于Rappaz枝晶凝固过冷理论,计算获得DD432单晶合金形成稳定液膜的最小晶界角为2.9°,即该合金热裂纹形成的临界角;大角度晶界处的枝晶凝并过冷度为395 K,远高于晶粒内部枝晶间液膜的过冷度(29.58 K)以及小角度晶界(3.6°)处的枝晶凝并过冷度(56 K),大角度晶界为开裂提供了稳定液膜;沉积区内部的高水平应力集中驱动了热裂纹的萌生与扩展;MC型碳化物析出相通过“钉扎作用”抑制液相补缩及弱化与基体之间结合强度等作用进一步促进了热裂纹形成。

Hot cracking is a prevalent defect in metallurgy that often occurs during the laser additive repair of single crystal superalloys.The understanding of the cracking mechanism is vital for defect prevention.Consequently,this study entails combining experimental analysis and theoretical calculations to investigate the hot cracking mechanism in a second-generation single crystal superalloy,DD432,during laser additive repairing.The incident of hot cracking was observed predominantly at high-angle grain boundaries(HAGBs).High-magnitude stress concentrations were identified on both sides of the crack,accompanied by an extensive distribution of MC-type carbides in the crack initiation region.Hot cracking depended on factors such as liquid film stability,stress concentration,and MC-type carbide precipitates.The stability of the liquid film depended on dendrite coalescence undercooling,which in turn was related to the angle of grain boundaries.According to Rappaz's theory of dendrite coalescence undercooling,the calculated dendrite coalescence undercooling at HAGBs was 395 K.This figure was substantially higher than the 38 K liquid film undercooling found within a single dendrite,and far exceeded the undercooling at a low-angle grain boundary(3.6°) with a value of 56 K.The elevated level of stress concentration served as a driving force for crack initiation and propagation.MC-type carbide precipitates promoted crack initiation through a pinning effect on the liquid feed,thereby weakening the interface bonding strength with the substrate.