The Microstructure, Property And Strengthening Mechanism Of Mg-Al-Si Alloy By Reciprocating Extrusion

In this paper Mg-4Al-2Si (AS42), Mg-4Al-4Si (AS44) and Mg-6Al-6Si (AS66) alloys were prepared by traditional solidification. The effects of solidification, reciprocating extrusion (RE), heat treatment and alloying with Sb on microstructures and mechanical properties were investigated. The grain and strengthening phase refinement and strengthening mechanisms were analyzed.The results show that AS42, AS44 and AS66 alloys can be prepared by adding Al-Si intermediate alloy into magnesium-aluminum melt system. The microstructures of these alloys are composed ofα-Mg matrix,β-Mg₁₇Al₁₂ and Mg₂Si which in Chinese script or/and massive shapes.RE is an effective method to be as grain and strengthening particles refining to AS42 alloy. The grain refinement of AS42 is mostly due to dynamic recrystallization RE. The dimension of theα-Mg matrix and Mg₂Si particle decreases with increasing passes of the RE. As fine as 3μm and 1.4μm of equaxedα-Mg and Mg₂Si particle, respectively, can be achieved after REed 6 passes. And 8 pass of RE also brought about finer matrix grain and Mg₂Si particle and maximum ultimate tensile strength and yield strength of 283.5MPa and 269.1Mpa, respectively. The results also showed that 6-8 pass of RE resulted in limitaion of finememt of grain The strain-hardening and dynamic recrystallization are equilibrium, which means that the rate of multiplication and annihilation of dislocations are almost the same. The grain size and particle size begin to coarse due to high temperature from 11 passes RE.RE can refine microstructures of AS44 and AS66 alloys apparently. The dynamic recrystallization of AS44 and AS66 matrix has happened; the 'Chinese' script Mg₂Si particle has been refined; and the large skeleton Mg₂Si has broken into large block in matrix by extrusion.RE can improve mechanical properties of as-cast AS42 alloy. The tensile properties at room temperature, such as yielding strength, ultimate strength and hardness of the alloys increase with increasing the pass of extrusion.The results of cracking Mg₂Si particles of AS42 alloy showed that Mg₂Si particles size distribution fits 'Weibull' rule. The 'Chinese' script Mg₂Si particles could break into massive or strip particles by the bending mechanisms, the plate particles could crack by the short-fiber loading mechanisms, the massive particles, crack by the shear mechanism during RE process. The crack probability of Mg₂Si particles are proportional to relative density of Mg₂Si particles, and also proportional to square of matrix grain size and 5 power of applied true stress.AS42 alloy were heat treated after extrusion. The results show that the morphology and size of Mg₂Si particles were changed slightly by T4 and T6 treatment. It indicated that Mg₂Si phase has a high thermal stability. However, the matrix grain coursed obviously and resulted in decreased mechanical properties.The tensile results of AS42 alloy after RE at 150℃showed that: RE pass have little effects on the ultimate tensile strength when it is less than 8 passes; and the ultimate tensile strength improved rapidly when the pass is more than 8 passes. RE-11 passes resulted in the maximum ultimate tensile strength, 256Mpa. The ultimate tensile strength increased by 163.9% compared with as-cast AS42 at 150℃. Moreover, the ultimate tensile strength of RE AS42 was higher than that of RE AS44 and RE AS66 at 150℃.The morphology of Mg₂Si particle in as-cast AS44 and AS66 changed from coarse dendritic-like to fine 'Chinese' script-like by alloying element Sb. The ultimate tensile strength and yield strength are also improved by refine microstructures. It is considered that the high melting-point Mg₃Sb₂ phase is dispersed in the matrix due to addition of Sb. Mg₃Sb₂ phase is conherent to Mg₂Si phase and become the inhomgenious nuclei of Mg₂Si and Mg₃Sb₂ phase gathered in the front of primary Mg₂Si phase and hinder its growth.