宽带激光熔覆高硬度火焰喷涂层组织和裂纹行为

对镍基合金火焰喷涂层进行宽带激光熔覆试验，分析熔覆层组织、物相组成和硬度分布规律以及工艺参数对组织、结合强度和硬度的影响。激光熔覆过程骤冷骤热的特点使得熔覆层内存在较高的拉应力，同时，由于激光熔覆镍基合金火焰喷涂层主要由g (Fe, Ni) 固溶体和弥散分布的高硬度脆性碳化物相(Ni, B) 和(Cr, Si) 相组成，熔覆层强度和硬度显著提高而延性降低，在凝固过程残余内应力的作用下极易开裂，熔覆层裂纹为低延性穿晶脆性冷裂纹。基体的预热和熔覆后保温缓冷可有效降低温度梯度，释放残余内应力，当预热温度为300 ℃时，熔覆层裂纹消失。

MICROSTRUCTURE AND CRACKING BEHAVIOR OF WIDE-BAND LASER CLADDING FLAME SPRAYED HIGH HARDNESS COATINGS

Wide-band laser cladding experiments of flame sprayed nickel based alloy coatings are carried out. Microstructures, phase compositions and hardness distribution of the cladding layers are characterized. Effects of process parameters on microstructures, bonding strength and layer hardness are ana-lyzed. Due to the emergency cooling and heating characteristics of laser cladding process, higher tensile strength often exists in the cladding layers. Simultaneously, because the laser cladding coatings of flame sprayed nickel based alloys are mainly com-posed of g(Fe, Ni) solutions and dispersedly distributed high- hardness brittle carbide phase, (Ni, B) and (Cr, Si) phases, the strength and hardness of the cladding layers are enhanced while the ductility decreases. Under the actions of internal residual stresses during the solidification process, the cladding layers are liable to crack. The cracks are transgranularly distributed, and are induced by the brittle and low ductility properties of the cladding layers. Through preheating of the substrate, holding and slow cooling after the process, temperature gradient can be effectively reduced and residual stresses can be released. With a preheating temperature of 300 ℃, cracks can be completely eliminated.