Study On Microstructure And Properties Of Magnesium Matrix Composites Fabricated By Repeated Compression Severe Plastic Deformation

In this dissertation, magnesium matrix composites were fabricated by repeatedcompression severe plastic deformation (SPD) technologies. Microstructure and mechanicalproperties of the composites were investigated. AZ31–Si in-situ magnesium matrix compositebillets were fabricated by adding Si into AZ31magnesium alloy melt. Repeated compressionSPD technologies, cyclic closed-die forging (CCDF) and repeated upsetting (RU), wereadopted to refine and uniform the microstructure, and in situ Mg2Si reinforced magnesiummatrix composites with homogeneous microstructure were fabricated. Effects of Si content onmicrostructure, mechanical properties and wear resistance properties of as-cast compositebillets were examined. Effects of technological parameters (deformation pass, processingtemperature) on size, morphology, distribution of matrix structure and reinforcements,mechanical properties were studied. Influence of repeated compression on fracture behaviorduring tensile test at room temperature was analyzed. Breaking mechanism of Mg2Si phasesduring repeated compression of AZ31–Si was discussed. The temperature field, flow field,stress field, and strain field of composite were simulated with finite element method. The dieand technology were optimized. SiC nanoparticles were added into Mg melt by high intensityultrasonic method and Mg–1wt.%SiC nanocomposite billets were fabricated. Cyclicextrusion compression (CEC) severe plastic deformation technology was used to refine anduniform the microstructure, disperse the SiC nanoparticles, and magnesium matrixnanocomposite with homogeneous distribution of SiC nanoparticles was fabricated.Influences of CEC passes number on microstructure and properties of Mg–1wt.%SiCnanocomposite billets were studied.Effects of Si content on microstructure and room temperature properties of AZ31–Si in-situ composite billets were investigated, and the results show that as-cast AZ31–Si in-situcomposites are composed of α-Mg matrix, Mg17Al12phase, dendritic primary Mg2Si phaseand Chinese script type eutectic Mg2Si phase. With Si contents increasing from0to5%(wt.%), size and volume fraction of Mg2Si phase gradually increase. Hardness, yield strength(YS) and wear resistance also gradually increase. Both elongation and ultimate tensilestrength (UTS) decrease due to stress concentrations occur easily in the matrix near the sharptips of Mg2Si particles. The tensile fracture mode at room temperature transforms from ductileand brittle mixed transgranular fracture to cleavage brittle fracture.The mechanical properties and wear resistance properties of as-cast AZ31–Si compositebillets at elevated temperature were examined, it is found that with Si contents increasingfrom0to5%(wt.%), both UTS and elongation of as-cast AZ31–Si composites graduallydecrease. As tensile temperature increases from100°C to200°C, UTS of AZ31–2wt.%Sicomposite gradually decrease and the elongation increase. Wear loss of composite graduallyincrease with temperature rise from30°C to190°C, due to decrease of strength and hardness,interaction of thermal stress and contact stress at high temperature.Influence of repeated compression pass on microstructure of AZ31–2wt.%Si billets wasstudied, and it is found that as CCDF and RU passes number increasing from0to5, averagegrain size of AZ31–Si composites decreases and homogeneity of size distribution improvesfor dynamic recrystallization. Both dendritic and Chinese script type Mg2Si are broken up intosmaller polygonal pieces due to the shear stress imposed by the matrix. The broken Mg2Siparticles repeatedly flow and redistribute during multi-pass processing, and their homogeneityis gradually enhanced. Fine and dispersive distribution of Mg2Si is obtained after5passes.Finite element simulation of the billet during repeated compression shows that shear stressexists in strain field. Accumulated strain and strain homogeneity in strain field graduallyimprove with increasing passes number. These factors result in homogeneity of Mg2Siparticles gradually improves with increasing passes number.Effect of repeated compression pass on properties of AZ31–2wt.%Si billets wasinvestigated. It shows that with CCDF and RU passes number increasing from0to5, YS,UTS, elongation and wear resistance of AZ31–Si increase due to gradual refinement of matrixgrain and dispersion of Mg2Si phases. The tensile fracture mode transforms from cleavagebrittle fracture to ductile and brittle mixed fracture.Research on microstructure and properties of AZ31and AZ31–2wt.%Si billets processed by repeated compression at different temperature show that as CCDF processingtemperature increases from350°C to450°C, basal texture weakens, the average grain sizeand elongation of AZ31alloy increase, while the YS and UTS decrease. For AZ31–2wt.%Sicomposite, the finest grain size and Mg2Si particles along with the highest strength andelongation are achieved at processing temperature of400°C.Influences of RU die structure and technological parameters on material flow wereanalyzed by finite element method. It is found that reduction of cavity width could increasethe equivalent strain for each pass, however, it deceases homogeneity of strain distributionand retentivity of billet shape and size. Transitional angle can enhance the forming quality ofbillet. With increasing transitional radius, homogeneity of strain distribution is slightlyimproved. Most of the billet suffers compressive stress in three direction and sheardeformation always exists due to inhomogeneous flowing velocity and different flowingdirection during RU process. Both accumulated strain and strain homogeneity improve as RUpasses number increase. More homogeneous strain is obtained in the billet processed withroute B (rotate90°around Z axis after former processing pass) than route A (without rotationafter former pass). With increasing RU temperature, the maximum upsetting load decreaseswhile homogeneity of strain and stress distribution in the billet increase.Microstructure and properties of Mg alloy processed by RU with route A and B wereinvestigated, and it shows that more intense grain refinement and homogenization of AZ31alloy is obtained when processed with route B at350°C for5passes. Thus, more significantimprovement of strength and ductility of AZ31alloy processed with route B is obtained.Influences of CEC passes number on microstructure and properties of Mg–1wt.%SiCnanocomposite billets were studied, and it is found that with passes number increasing from0to8, a finer average grain size and more uniform nanoparticle distribution are obtained alongwith significant improvement in hardness and wear resistance. Nanoparticle declusteringoccurs due to intense shear deformation of Mg matrix during CEC and the SiC nanoparticlesdisperse homogeneously. The property improvement is mainly attributed to dispersionstrengthening and fine grain strengthening.