在45#钢基体上以TH-2激光合金化专用粉进行激光合金化处理，通过扫描电镜动态原位观察裂纹从基体向激光合金化区(LAZ)扩展的过程，研究了疲劳裂纹扩展速率与应力强度因子幅(SIFR)的关系，分析了疲劳断口形貌和表面裂纹扩展路径的组织形态。经历一定循环次数之后，激光合金化区内部也出现裂纹，但是合金化区的边界能强烈阻滞裂纹的扩展，一旦基体中的裂纹与合金化区中的裂纹汇合，裂纹失稳扩展。裂纹扩展速率与应力强度因子幅之间的关系不满足传统的Paris公式，裂纹在基体中扩展的断口形貌有典型的疲劳辉纹，而合金化区的断口呈脆性断裂的形貌。从抗疲劳裂纹扩展的要求来看，有必要针对不同的基体材料，设计不同的合金化粉末，通过适当增加合金化区的韧性相，进一步提高激光合金化区抗疲劳裂纹扩展的能力。

In this paper No.45 steel (equivalent to AISI 1045) was laser alloyed with a special laser alloying powder TH-2. With SEM in situ observation of crack growth from base to alloying zone, the relationship between FCGR and stress intensity factor range (SIFR) was investigated, and the structure morphology of surface crack path and the fatigue fracture was analyzed. It is found that crack would appear on laser alloying zone (LAZ) after a certain cycles, but the boundary of LAZ would retard crack growth. Unstably propagation would occur when the crack from base meet the crack on LAZ. The relationship between FCGR and SIFR was not agreed with Paris formula. Typical fatigue strips was found on fatigue fracture of base and the morphology of LAZ was brittle fracture. It was necessary to design special alloying powder for different base material. To increase ductile phase in LAZ would improve resistance to fatigue crack growth of LAZ.