Study On The Mechanism Of Microstructure Evolution Of Directionally Solidified Hypereutectic Aluminum Alloys And Fabrication Of Functionally Graded Materials Under Static Magnetic Field

The solidified microstructure of the material has a decisive influence on its properties.The applied static magnetic field during solidification can effectively control the solidification microstructure of the alloy and thus will be useful to fabricate the high performance material.Hypereutectic aluminum alloy is one of the most widely used nonferrous metal structural materials in industry.It has the characteristics of wear resistance,heat resistance,corrosion resistance and low coefficient of thermal expansion,attracting much attention from researchers.This paper adopts directional solidification technique under a static magnetic field and correspongding the numerical simulations to systematically investigate thermoelectric magnetic convection?TEMC?in the liquid,thermoelectric magnetic force?TEMF?on solid phase,solute migration and solid migration near the mushy zone and microstructure of hypereutectic aluminum alloy Al-Cu,Al-Ni,Al-Si and Al-Zn.The main results are as follows:In this paper,the finite element software COMSOL is used to simulate the TEMC and its distribution near the mushy zone during directionally solidified Al-Cu alloy under various magnetic fields.The applied transverse magnetic field will cause the formation of unidirectional TEMC in the liquid near the mushy zone in longitudinal section perpendicular to the magnetic field direction.The TEMC increases first and then decreases with the increase of the magnetic field.In addition,the applied axial magnetic field will cause the formation of unidirectional TEMC in the liquid near the mushy zone in transverse section of the sample.The TEMC increases first and then decreases with the increase of the magnetic field.At the same time,by simulate thermoelectric effect under different conditions,we can find that the temperature gradient,length of the mushy zone and sample diameter can enhance the TEMF in the melt and promote the TEMC.The applied transverse magnetic field can lead to the change of the solid/liquid interface morphology.For Al-40 wt.%Cu and Al-12 wt.%Ni,the magnetic field will cause decrease of the area fraction of the primary Al₂Cu and Al₃Ni phase on one side of samples and form the channel segregation along the longitude direction.The magnetic field results in the increase of the area fraction of the primary Al₂Cu and Al₃Ni phase on the other side of the samples.The above results show that the transverse magnetic field promotes growth of the primary Al₂Cu and Al₃Ni phase on the right side the samples,while inhibiting the growth of primary phases on the left of the samples.Thus,the radial gradient distribution of primary phase forms.This phenomenon may be attributed to the TEMC under transverse magnetic field,which promotes the Al elements to be enriched on the left of the sample.For Al-21 wt.%Si alloy,the primary Si phase distributed on Al-Si eutectic matrix is changed by the magnetic field,which changes distributed uniformly primary Si phase in the longitudinal section of the sample into an inclined strip distributed primary Si.The radial gradient distribution of the primary Si phase is obtained.The above results are attributed to the combined effect of the TEMF acting on the primary Si phase and the TEMC in the melt.The effect of axial magnetic field on the directional solidification microstructure of Al-40 wt.%Cu,Al-12 wt.%Ni,Al-21 wt.%Si and Al-96 wt.%Zn alloy is investigated.It is found that the distribution of primary phase in the whole sample can be changed by axial magnetic field.The area fraction of primary phase decreases with increasing growth distance and the longitudinal macro segregation of the primary phase forms.Moreover,the magnetic field can enhance the longitudinal macro segregation of primary phase.The application of magnetic field can also shorten the length of mushy zone and with the increase of magnetic field the length of the mushy zone become shorter and even disappears,which is consistent with the change of the longitudinal macro segregation of primary phase.In addition,the increase of temperature gradient and the decrease of growth rate can enhance the effect of the magnetic field and promote the longitudinal macro segregation of the primary phase.The above results are attributed to the effect of TEMC under axial magnetic field,the temperature field,the gravity on solute migration and growth of the primary phase during directional solidification.For the Al-21 wt.%Si alloy with granular primary Si phase,the application of magnetic field will lead to the change of primary phase interface in the mushy zone,quasi flat interface?U-shaped interface?flat interface.with the formation of the U-shaped interface from the quasi flat interface,the radial macro segregation of the primary Si phase occurs.The above results are attributed to the effect of TEMC under axial magnetic field and the temperature field on solute migration and growth of the primary phase during directional solidification.