Research On Forming Mechanism And Controlling Of Defects Of Al-Si Alloy Semi-Solid Die Casting Products

There are increasing requirements on ensuring production quality of aluminum alloy components to satisfy the widely applications of aluminum alloy,such as in automobile industry.Semi-solid die casting is a novel technology which can produce high quality aluminum alloy components with low cost.The application target is the high-end markets which ask good quality for aluminum alloy components.Therefore,defects control is very important for semi-solid die casting.However,systematic researches and controlling methods of the defects in semi-solid die casting products are absent and engineers are still beset by defects in industry practice.The purpose of this project is to uncover the formation mechanism and influence factors of defects in semi-solid die casting products,and to obtain the methods for controlling defects.Experimental and computing simulation methods are used synthetically to systematically investigate major defects.The defects of gas hole,blister and shrinkage porosity have been identified to be the major defects in semi-solid die castings.Achievements on the forming mechanism and controlling methods of these types of defects are given below:(1)Gas entrapment is the main cause of gas hole in semi-solid die casting.Ingate velocity and initial solid fraction are the main factors of forming the gas entrapment due to front turbulence of metal flow.Decreasing ingate velocity is helpful to reduce the inner gas hole and subsurface gas hole.Increasing initial gas hole is good to reduce amount of the inner gas hole.And the formation probability of subsurface gas hole is minimum under the condition of 0.45 of initial solid fraction.For the gas entrapment by the patching of casting surface,the effect of mold temperature is more obvious than initial solid fraction.Increasing mold temperature or decreasing initial solid fraction is beneficial to decrease subsurface gas hole.The entrapped gas in semi-solid metal is unable to escape by the work of buoyancy.(2)A critical temperature exists for blister formation.When heat treatment temperature is below the critical value,blister cannot form.With intensification pressure increasing,the formation temperature for blister decreases and the average diameter of blisters increases.Decreasing the initial pressure in gas hole,the long-short axial ratio of gas hole and the volume of gas hole,as well as increasing the depth of gas hole and the strength of alloy are helpful to increase the critical temperature of forming blister.Among the factors,the volume,shape and depth of gas hole are of relatively major influence,and others have weak influence.(3)Shrinkage porosity is always in the center of semi-solid die casting.With increase of intensification pressure or initial solid fraction,the volume fraction of shrinkage porosity in casting center decreases.With increase of intensification pressure or decrease of initial solid fraction,the distance from the position of forming shrinkage to the casting surface increases.Increasing intensification pressure is the most effective way to remove shrinkage porosity in semi-solid die casting process.The criterion of intensification pressure is:(?),when temperature in the center of sample reaches the solidus of alloy.(4)Six controlling methods of defects in semi-solid die casting produces are identified as:(a)to carefully choose the right alloy,(b)to make a clean melt,(c)to prepare a good semi-solid slurry,(d)to decrease ingate velocity and to increase mold temperature,(e)to ensure enough vent and to avoid trapped gas,(f)to avoid hot spots during solidification by optimizing the die design.These controlling methods have been successfully applied in industry practice,in assistance mold design and process development,to produce several semi-solid die casting products with defects free.The innovation of this project includes:(1)obtaining the effect of initial solid fraction of semi-solid metal on the forming of gas hole in Al-Si alloy semi-solid die casting process.(2)systematically studying the formation mechanism and factors of blister defect,and determining the main factors in the heat treatment process of Al-Si alloy semi-solid die castings.(3)obtaining the effect of initial solid fraction of semi-solid metal on the forming of shrinkage porosity in Al-Si alloy semi-solid die casting process.(4)obtaining the controlling methods of defects in Al-Si semi-solid die casting products.The research achievements in this project will be helpful for engineers to understand the formation mechanism of defects in semi-solid die casting products,and provide a guideline for them to predict and deal with defects in casting practice.