Investigation On Fabrication, Microstructure And Mechanical Properties Of Friction Stir Processed Fine-grained Magnesium Alloys

Friction stir processing (FSP) is a novel severe plastic deformation technology, whichwas developed on the basis of friction stir welding (FSW). Due to its high strain rate and largeplastic deformation, FSP has great potential in the preparation of fine-grained metallicmaterials. In this article, FSP was used to process as-cast AZ91magnesium alloy andhot-rolled MB8magnesium alloy. The processes of fabricating fine-grained magnesium alloyand microstructure transformation mechanism were studied. The effect of processingparamiters on grain size and mechanical properties were investigated. The superplasticbehavior of the fine-grained magnesium alloys was analyzed and the superplastic deformationmechanism and fracture mechanism was discussed.(1) The results showed that defect-free materails could be gained under rotation speed of400-1300r/min and travel speed of60-200mm/min in as-cast AZ91magnesium alloy by FSP.The microstructure observation indicated that the coarse dendrites of BM turned into fineequi-axed crystal grains after FSP. The average grain size of is about1.3m at the rotationspeed700r/min and processing speed of60mm/min. The network structure of phase wasbroken into fine particles distributing on the grain boundary. The tensile test results at roomtemperature showed that the hardness and tensile strength of AZ91magnesium alloy weresignificantly improved after FSP, but the elongations were reduced at some processingparameters. The analysis of fracture demonstrated that as-cast alloy failed in brittle fractureunder room temperature, while the fractured surface of FSPed alloy exist many dimples whichis belonging to ductile fracture.(2) Tensile properties at elevated temperature of as-cast and FSP AZ91alloy wereexamined. The results showed that as-cast AZ91magnesium alloys had no superplasticdeformation, while AZ91magnesium alloy after FSP exhibited excellent superplasticity. Themaximum elongation of1604%was obtained at573K and1×10-4s-1. It also showedsuperplasticity with large elongation (>200%) at low temperature (<300C) and high strainrate (>3×10-3s-1). Grain growth was considered to be the dominating mechanism for themicrostructure evolution during superplastic deformation at high temperature. The main failure mechanism during superplastic deformation was cavity coalescence. Cavitiesnucleated around the second phase particles and connected to each other due to the grainboundary sliding (GBS).(3) The effect of processing parameters on the microstructure and room temperaturemechanics properties of MB8magnesium alloys processed by FSP was studied. The resultsshowed that the original course and nonuniform microstructure turned into equiaxial grainsdue to dynamic recrystallization. The average grain size of FSP MB8magnesium alloy wasrefined from16.5to6.0μm at the rotation speed of800r/min. The tensile experimental resultsshowed that the tensile strength of FSP MB8decreased greatly, but the elongation increasedsignificantly. A maximum elongation of57%was gained at the rotation speed of1200r/min.(4) The high temperature tensile tests of MB8magnesium alloy before and after FSPwere carried out. The results indicated that, MB8magnesium alloy before FSP exheibitedgood superability and the maximum elongation of441.6%was obtained at673K and4×10-4s-1.However, FSP MB8magnesium alloy had poor superplasticity due to its grain coarseningunder high temperature and the maximum elongation was only231.2%at673K and4×10-4s-1.