Effects Of Si And Si-Sb Added On The Microstructure And Mechanical Properties Of AZ91 Magnesium Alloy

Magnesium alloys have the advantages of light-weight, high strength/weight ratio, high stiffness/weight ratio, high damping property and electromagnetism shielding, so it will act an important role in automobile, computer and communication as a kind of weight reducing material. AZ91 alloy which is widely used at the present time has many advantages, such as high ambient temperature strength, good casting property and low cost. But it is unsuitable for using at temperatures above 120 degree due to its poor mechanical properties at elevated temperature. Although AS and AE alloy series were developed for application at elevated temperature, their limited castability or higher cost has restricted their productions and applications. Despite of the growing interest in magnesium alloys, very little data exist on their friction and wear behavior, wear is also one of important factors which limit the applications of magnesium alloys. Therefore, it is important to develop some new magnesium alloys with low cost, high mechanical properties at room and elevated temperature, high toughness and wear resistance on the basis of AZ91 alloy.In this paper, we try to strengthen AZ91 based alloy by adding various amount of Si and Si-Sb mixed, conducing solid solution treatment processe. The influences of different Si, Sb additions and different hours of solid solution treatment on microstructures and properties of AZ91 based alloy are systemically studied by measurements of Optical Microscopy(OM), X-ray Diffraction(XRD), Scan Electric Microscopy(SEM), Energy Dispersive Spectro- meter(EDS), Friction and Wear test, Hardness and Tensile test at room temperature.The results of alloy elements added show that: 1) With Si added to alloy, the microstructures of as-cast AZ91 were refined and a new phase Mg2Si appeared. Morphology of Mg₂Si phases changed from short staff shape to coarse Chinese script shape when contents of Si increased. Under stress the microcrack prefer to occurring in the interface between the phase Mg2Si and matrix which resulted the decrease of mechanical properties at room temperature. 2) Addition of Si-Sb mixed to AZ91 alloy resulted in refinement of microstructure, and the effect of refinement is better than that of refinement added with Si only. Morphology of the Mg2Si phases changed from coarse Chinese script shape to the fine polygonal shape. The modification mechanism of Sb is polygonal type Mg₂Si particles nucleating from Mg3Sb2 particles respectively. Such improved microstructure of modified alloys results in improvement in tensile properties and toughness as compared to the base alloy. 3) Because of high hardness of Mg₂Si phase, the hardness of AZ91 alloy was greatly increased by addition of Si.In contrast with that of the AZ91 magnesium alloy, the friction and wear performance of AZ91 alloys containing Si and Si-Sb mixed were studied. Under the same experiment, the wear mechanism of alloys all changes with the increasing test loads from minor wear (mainly oxidation wear, abrasive wear and minor delamination wear) to serious wear (mainly serious delamination wear and fusion wear).Addition Si and Si-Sb prolonged solid solution time. The wholeβ-Mg₁₇Al₁₂ phase dissolves into theα-Mg after longer solution time. The Mg₂Si phases didn't dissolve and their morphology and size didn't change either. Ultimate tensile and elongation of AZ91 with or without Si and Si-Sb are improved after solution treatment, but the yield strength decreases because the crystals are coarser and coarser. The hardness of the alloy decreases due to formation of solid solution. Theβ-phase dissolved into theα-Mg matrix to form solid solution being low micro-hardness.