| Magnesium alloy is considered to be the most promising structural material with broad development prospects.Liquid die-casting is a common method for preparing magnesium alloy castings.However,due to the active chemical properties of magnesium alloys and easy oxidation and combustion at high temperatures,the development of magnesium alloys is restricted to a certain extent,and the semi-solid forming technology provides new ideas for solving this problem.There are two main methods to improve the properties of magnesium alloys:heat treatment and microalloying.However,the current research on semi-solid die casting of magnesium alloys is mostly based on AZ alloys.The disadvantages of weak heat treatment of these alloys make the advantages of heat treatment of semi-solid die castings impossible.fully use.Mg-Zn-Cu series magnesium alloy has excellent aging strengthening effect.In this paper,Cr element is added on the basis of Mg-7Zn-1Cu-0.1Mn alloy for microalloying.It is expected to develop a suitable semi-solid forming technology for die casting and its Semi-solid die castings have a strong aging strengthening of high strength and toughness magnesium alloy.This article uses OM,XRD,SEM,TEM and microhardness testing methods to obtain:After the Mg-7Zn-1Cu-0.1Mn alloy is microalloyed with Cr,the as-cast alloy's surrounding structure is significantly improved,the crystal grains are refined,and the dendrite size is reduced.The segregation of alloy components is weakened,and the microstructure of the as-cast alloy is improved.hardness.When the addition amount of Cr element is 0.1wt.%,the optimization effect of the alloy structure is the best,and the alloy microhardness page reaches 56.2 HV.The as-cast Mg-7Zn-1Cu-0.1Mn-xCr alloy is mainly composed of?-Mg matrix and Mg Zn2 and Cu Mg Zn phases,and no Cr-containing phase is produced.The Mg-7Zn-1Cu-0.1Mn-xCr alloy semi-solid ingot was prepared by isothermal heat treatment.It is found that when the content of Cr element is 0.1wt.%,the size of the non-dendritic structure of the alloy is the smallest and uniform.The Mg-7Zn-1Cu-0.1Mn-0.1Cr alloy obtains the best non-dendritic structure after isothermal heat treatment at 580?for 30min.The average solid particle size,shape factor and solid phase ratio are all at least 45?m,1.37 and 65%.The obtained semi-solid non-dendritic structure is mainly composed of two phases of solid and liquid.The solid phase includes primary?1-Mg and secondary?2-Mg.The liquid phase has two types of eutectic molten pools distributed around the solid particles and"liquid pools"contained in the solid particles.When the alloy undergoes isothermal heat treatment,it mainly undergoes four stages:rapid dendrite coarsening and merging,structure separation,spheroidization,and crystal grain merge and coarsening.T4 and T6 heat treatments were performed on the alloy,and it was found that the best solution process for the as-cast experimental alloy and the semi-solid experimental alloy was kept at 410?for 24h,and the as-cast alloy reached peak aging at 160?for32h,and the microhardness was 86.4 HV,The semi-solid alloy reaches its peak aging at 160?for 28h.One type of the as-cast alloy peak aging precipitation phase is a rod-shaped?1'perpendicular to the{0001}Mg basal plane,and the other is a plate-shaped or disc-shaped?2'parallel to the{0001}Mgbasal plane.During semi-solid peak aging,a new lath?2'precipitated perpendicular to the{0001}Mg basal plane was observed.The semi-solid peak aging precipitation phase density is higher than the peak aging precipitation phase density of the as-cast state,so the semi-solid peak aging hardening effect is better.The precipitation sequence of as-cast experimental alloy aging and semi-solid experimental alloy aging is:SSSS?G.P.zone???phase??phase.Among them,?1',which is perpendicular to the{0001}Mgbasal plane,has a major role in strengthening the aging of the alloy. |
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