触变注射成形AZ91D合金组织和拉伸断裂特性研究

通过不同工艺制备的合金的组织特点和性能变化,研究了触变注射成形AZ91D合金的拉伸断裂行为。结果表明,典型触变注射成形AZ91D合金的室温组织主要由未熔固相α-Mg和液相急冷组织构成。未熔固相有类球状、不规则状、包裹液相和内部小液池等4种形态。液相急冷组织包含初生α-Mg固相和共晶相,初生固相较规则,尺寸较小,主要受液相的过冷度影响。当组织变化不大时,孔隙率对合金性能的影响最大,而当孔隙率变化不大时,组织的变化对性能的影响较大,固相率较低和初生固相较细小的试样的力学性能较好。试样拉伸时裂纹主要沿着未熔固相与液相的界面或初生固相与共晶的界面扩展。当固相率较高时,裂纹大部分沿着未熔固相与液相的界面扩展,试样的性能较低;而当固相率较低时,裂纹大部分沿着初生固相与共晶相的界面扩展,并通过β相的桥接进行拓展,性能较好。

Microstructure and Fracture Behavior of Thixo-molded AZ91D Alloy

By understanding microstructure and properties of thixo-molded AZ91D fabricated at different processing parameters, the fracture behavior was investigated. The results show that ambient microstructure of typical thixo-molded AZ91D alloy is mainly composed of un-melted α-Mg and solidified phase. The un-melted solid phase can be divided into four types, such as spherical solid particles, irregular solid particle, solid particle with liquid pool inside and solid particle with liquid alloy inside. Solidified phase includes primary α-Mg phase and eutectic phase, in which primary phase exhibits regular structure with fine grain, controlling by under-cooling rate of liquid alloy. Porosity rate is the main factor influencing the mechanical properties of the alloy with similar microstructure, while effects of microstructure on the mechanical properties of the alloy with similar porosity rate is dominant. The samples with a lower solid fraction and a finer primary solid phase exhibits acceptable properties. Crack in the sample during tensile testing is propagated along interface of un-melted solid phase and liquid phase or along primary solid phase and eu-tectic phase. With a higher solid fraction, crack is mainly propagated along interface of un-melted solid phase and liquid phase, resulting in the deterioration of mechanical properties, while with a lower solid fraction, crack is mainly propagated along interface of primary solid phase and eutectic phase, leading to the improvement of mechanical properties as a result of bridging effects though β phase.