粉末冶金生坯边位印压的裂纹扩展行为

Crack Propagation Behaviors of Green Powder Metallurgy Compacts Subjected to Edge-Indentation

生坯加工是粉末冶金零件制造的一种重要方式,其加工过程中极易产生裂纹和边崩等缺陷.为了获得高完整性的已加工表面,文中研究了粉末冶金生坯边位印压时裂纹的产生和扩展行为.结果表明:当印压载荷达到某一临界值时,裂纹萌生,并沿印压速度方向近似呈直线缓慢扩展,然后转向试件外表面扩展,直至被裂纹包围的块状材料脱落;裂纹微观上呈锯齿形,与印压速度方向存在一小的夹角,锯齿形裂纹未深入到已加工表面;裂纹扩展角随压头楔角的增加而降低,随印压厚度的增加基本不变;裂纹长度随印压厚度的增加而显著增加;当压头楔角小于70°时,裂纹长度随压头楔角的增加而增加,当压头楔角大于70°时,裂纹长度基本保持不变.

The green machining is an important approach for parts manufacturing of powder metallurgy,during which cracks and breakouts occur easily at the same time. The crack initiation and propagation behaviors of the green PM compacts subjected to edge-indentation are investigated to obtain the machined surface with high integrity.The results show that a crack initiates and spreads slowly along the indentation direction in a rectilinear form when the indentation load reaches a critical value. After that,the crack extends toward the outer surface of the specimen and eventually leading to the shedding of block materials encompassed by the crack. The crack is serrated in microlevel and forms a small angle with the indentation direction. Besides,the serrated crack does not penetrate into the machined surface. The crack propagation angle decreases with the increase of the interior angle of the indenter while remaining unchanged with the increase of the thickness of an indenter. The crack length increases significantly with the growth of the indentation thickness and increases as the growing of the interior angle of indenter when the interior angle is less than 70°,and then it barely changes with the angles that are larger than 70°.