Effect Of Solidification Microstructure On The Deformation Behavior Of AZ31 Magnesium Alloy

AZ31 magnesium alloy is the lightest metal structure material,which plays an important role in energy saving and emission reduction.But it has a bad effect on the plastic forming,which seriously restricts its development and application.Firstly of AZ31 magnesium alloy is remelted in rotating device for insulation centrifugal solidification under pressure and holding centrifugal solidification under pressure two obtained samples with different microstructures.Through continuous variable cross-section recycled extrusion(CVCE)deformation,the deformation behavior of the solidified samples under different cooling rate and different centrifugal pressure is analyzed,the grain refinement process and deformation mechanism are discussed.Further study the relationship between the solidification mode,the solidification structure,the deformation behavior and the microstructure and properties of different conditions.The experimental results are as follows:In the process of CVCE deformation,the specimen was first cracked at the edge of the extrusion rod and the contact point of the specimen,and along the direction of 45 degrees,the shear stress of the specimen was the largest.In the original sample,the cracks were formed at the loose shrinkage cavity.After the solidification of the centrifugal pressure,the shrinkage cavity was eliminated and the density was increased.Finally Crack produced on the second phase of intermittent distribution;the end of the conical crack is the combine of intergranular fracture and transgranular fracture.Side of crack is single transgranular fracture and there is a big difference between the two sides of the crack.The CVCE deformation process is not uniform,with the increase in the number of deformation,the grain has been refined.The uneven deformation of initial extrusion is eliminated,the second phase and impurity are evenly distributed in the grain boundary;By 20 times heating deformation,recovery and dynamic recrystallization are increased.When the velocity of grain growth is greater than the deformation thinning,at this time,the grain size is no longer reduced and the growth is accompanied by the initiation of fine crack;The CVCE deformation process of as cast AZ31 magnesium alloy is mainly twins,dynamic recrystallization is occurred during deformation and the original coarse grains can keep the heredity of the original crystal position while forming fine equiaxed grains.By centrifugal pressure solidification,the centrifugal force is in the middle position of the heat preservation,where the impurities and the second phase content are the least,the largest number of deformation and the best plastic.After 8 passes,the second phase at the grain boundary is melted and dissolved in the matrix.The grain size is decreased from 80μm to 12μm,which forms equal axial grain microstructure.The microhardness of the sample is increased after 8 passes of CVCE,but the distribution of microhardness on the whole longitudinal section is not uniform.After 20 passes,the area of low hardness value is greatly reduced,and the distribution of microhardness is uniform.With the increase of deformation number,the compressive strength appears abnormal,it is not improved but decreased.The rheological softening phenomenon has been occurred,but the compression ratio has been improved.This is mainly through the dynamic recrystallization after many times of deformation and softening phenomenon occurs in the crystal and relate to the morphology and distribution of the second phase in the deformation process.After CVCE compressiondeformation of the specimens of thermal insulation and centrifugal compression solidification,microscopic fracture morphology of the river pattern was significantly reduced,the cleavage steps increase and size decrease.After 20 times deformation to the Intermediate specimen of heat preservation solidification,there are many micropores in the fracture surface.Under the scanning electron microscope,we can see the dimple fracture.The original sample after 2 passes of deformation,the grain was elongated along the extrusion direction and produced a large number of(0002)basal fiber texture.After 8 passes of deformation,the grain was refined and the basal texture was weakened.For heat preservation solidification of intermediate samples,in the process of CVCE deformation,the slippages of AZ31 magnesium alloy cylinder(1 0?(?) 0)(1 1?(?) 0)and cone(1 0?(?) 1)are excited,the relative strength is higher than that of the original sample.