Study Of Directional Solidification On Mechanical Properties And Thermal Conductivity Of Mg-xZn-Y Alloy

At present,the magnesium alloys which are obtained by ordinary solidification,its room temperature and high temperature mechanical properties,creep resistance,thermal conductivity,conductive conductivity and other properties are often difficult to meet the performance requirements.And directional solidification as a new solidification technology,by controlling the heat dissipation to gain columnar crystal structure,it improves the performance of magnesium alloy.In this paper,the high thermal conductivity,high specific strength of Mg-xZn-Y(x=1,3,5 wt.%)alloys were designed by directional solidification device,optical microscope,scanning electron microscope,XRD diffraction,material testing machine and hardness tester to study how the ordinary solidification and temperature gradients(70 K/cm,100 K/cm)of directional solidification and growth rates(5 μm/s,10 μm/s,25 μm/s,50 μm/s,100 μm/s)effect the mechanical properties and thermal conductivity of Mg-Zn-Y alloy.And draw the following conclusions:(1)The microstructure of Mg-Zn-Y alloy is mainly composed of α-Mg matrix,W-phase and I-phase.The matrix phase is equiaxed grain and the second phases like white points distribute in matrix or grain boundaries.Directional solidification can change the microstructure of the as-cast alloy from equiaxed grain to columnar crystal,and the number of dispersed second phase is increased compared with the as-cast structure.(2)Zn can refine the microstructure of the alloy in the condition of ordinary solidification.With the increase of Zn content,the grain size was refined,the number of the dispersed second phase increased,and the distribution was more uniform.In the condition of directional solidification condition,if the growth rate is constant,with the increase of the temperature gradient,columnar crystal was coarser,more uniform,and better directivity;if the temperature gradient is constant,with the increase of the growth rate gradually,the average width of the columnar crystal gradually narrowed down,the uniformity of the distribution ofgrain gradually decreased,the growth direction and continuity of grain gradually deteriorated.(3)The room temperature mechanical properties of as-cast alloy increases with the increase of Zn content,and at the content of Zn is 5%,the maximum tensile strength is 198.9 MPa,the elongation is about 10%,and the hardness is71.1 HV.If directional solidification growth rate constant,with the increase of the temperature gradient,tensile strength,elongation and hardness all have an increasing trend.By contrast,it found that in the high temperature gradient condition of tensile strength than the tensile strength of in the low temperature gradient condition increased by about 9.3% and elongation increased by 15.0%,alloy hardness increased by 11.3%;if temperature gradient constant,with the increase of the growth rate,tensile strength,elongation and hardness firstly increased and then decreased,when the temperature gradient reached 100 K/cm,the growth rate reached 50 um/s,the highest tensile strength reached 245.0 MPa,the elongation reached 13.3%,the hardness of the alloy reached 71.7 HV,compared to the as-cast alloys,respectively increased by 35.7%,49.4%,26.0%.(4)The thermal conductivity of as-cast alloy decreases linearly with the increase of Zn content(1~5 wt.%),so the influence of Zn content on the thermal conductivity is very obvious.In the directional solidification condition,thermal conductivity of the alloy with the increase of the temperature gradient showed a slight increasing trend,with the increase of the growth rate showed a declining trend.But compared to the as-cast alloy,thermal conductivity of directionally solidified alloy improved.For an example of Mg-3Zn-Y alloy,when the temperature gradient is 100 K/cm and the growth rate is 5 μm/s,thermal conductivity of the alloy reached a maximum of 149.73 W/(m ? K),increased by 36.0% than as-cast alloy;when the temperature gradient is 100K/cm,the growth rate is 100 μm/s,thermal conductivity of the alloy reached the lowest 128.64 W/(m?K),increased by 13.1% than as-cast alloy.