Effect Of Process Conditions Solidification Microstructure Revolution And Properties Of AZ91 Alloy

Magnesium alloys,with a number of desirable features including low density,the highest strength-to-weight ratio,the highest specific toughness,better damping characteristics and shielding properties,widely used in aeronautical industries,automotive,3C products and other fields.However,the precipitation and distribution of second phase were variability in magnesium alloys and with wide temperature result in solidification defects,greatly affected the performance stability and service quality in the process of reliability.Based on AZ91 alloy,the evolving law of solidified structure and change of mechanical properties of materials has been studied through OM,SEM,XRD and Universal compression testing machine and other devices.The result shown that:Change pressure and cooling rate in the process of solidification,has important influence on grain size,precipitation time and species distribution of ?-Mg₁7A1₁2 phase.With relative centrifugal force increased from 44(g)to 263(g),under heat preservation pressure condition,alloy grain size by 162 ?m decreases to 82 ?m,precipitation of second phase ?-Mg₁7Al₁2 decreases from 9.2% to 4.3%,Its shape from uneven distribution of rough granular structure to fine dispersion shift.While under no heat preservation pressure condition,alloy grain size by 68 ?m decreases to 50?m,Its refined degree was less than heat preservation sample,the?phase increase from 26.3% to 33.1%,and the morphology of ?from tiny discontinuous distribution structure changes to the continuous bulky mesh structure.Centrifugal stress and cooling rate have a certain degree of influence on solute elements content of AZ91 alloy.High density elements(Al,Zn)under the action of centrifugal force,move to the large centrifugal radius side.With the increase of centrifugal solidification pressure,more solute elements solid soluble in matrix phase(?-Mg),and the content of Al and Zn difference greater at different radius with cooling rate become lower in matrix phase.The number of shrinkage porosity and cavity decreased significantly in sample with increase in pressure of solidification,the metal fluidity become bad and pressure is not fully applied to the feeding in same solidification pressure with cooling rate become large.With the increase of solidification pressure,based on change of second phase precipitation and distribution,decrese of solidification defect and grain size,and improve of other solidification microstrcture,the comprehensive mechanical properties of AZ91 magnesium alloy were improved,under heat preservation,sample compressive strength increase from 216 MPa to 303 MPa and the maximum degree of plastic deformation increase from 18% to 28%.while under no heat preservation pressure condition,sample compressive strength increase from 272 MPa to 342 MPa and the maximum degree of plastic deformation increase from 16.3 % to 22.3% finaly reduce to 20.1%.The plastic of heat preservation sample was better than no heat preservation sample but its strength is less than no heat preservation sample in same solidification pressure.