Preparation Of Semi-solid Al-25Si Billet And Research On Its Plastic Extrusion Formin

High silicon aluminum alloy has the advantages of low coefficient of thermal expansion,low density,high strength,good wear resistance and good casting performance,so it is suitable for parts such as cylinders and pistons which need to serve for a long time at high temperature.at the same time,it is also widely used in aviation,automobile and other industries.However,high silicon aluminum alloy contains a large amount of primary silicon,too high Si content can easily lead to too much Si phase in the alloy,and there are coarse primary silicon and eutectic silicon in the as-cast alloy,which seriously splits the matrix,which is easy to produce microcracks in the aluminum matrix at the tip of primary silicon during deformation,which reduces its plasticity and toughness,which restricts the application and development of high silicon aluminum alloy.Therefore,how to use advanced preparation technology to refine the microstructure of high silicon aluminum alloy,improve the morphology,size and distribution of silicon phase,improve its comprehensive properties,and finally prepare high silicon aluminum alloy materials with excellent comprehensive properties,it has become the focus of research at home and abroad in recent years.In this paper,the rotating rod induced nucleation device is used to prepare semi-solid slurry to refine the primary silicon grains,and the semi-solid forming technology is used to improve the properties of high silicon aluminum alloy.On the basis of previous research,the effect of rotating rod pulping process on the microstructure of semi-solid slurry was further studied,and conventional as-cast Al-25%Si blank extrusion forming and semi-solid Al-25%Si blank extrusion forming were carried out under the same forming process parameters.The differences of microstructure and mechanical properties were analyzed,and the effects of forming process parameters on the microstructure and properties of semi-solid Al-25%Si billets extruded parts were studied.It is found that the primary silicon grains can be effectively refined by changing the rotating speed of the rotating rod,the water flow rate into the cooling water and the temperature of the slurry before water quenching in the treatment of Al-25%Si alloy melt by rotating rod induced nucleation method.Compared with the microstructure,it is found that when the rotating speed of the rotating rod is 600r/min,the water flow rate into the cooling water is 10L/min,and the temperature of the prepared semi-solid slurry before water quenching is 585C,the refinement effect of primary silicon is the best.The grain size of primary silicon can be refined to about28μm,the shape factor can reach 0.7,and the number of nucleation per unit area is about 8000/cm^-2.Comparing the microstructure and properties of common casting Al-25%Si billet extruded parts and semi-solid Al-25%Si billet extruded parts,under the condition of common casting billet,the primary silicon in the extruded part is coarse and unevenly distributed,the tensile strength is 103.5MPa,the elongation is 1.83%,and the extrusion forming under the semi-solid billet condition,the microstructure of the primary silicon grain is fine and uniformly distributed,and the tensile strength is127.2MPa.The elongation is 2.87%.Compared with the extruded parts under the condition of ordinary casting billet,the tensile strength is increased by 22.8%,and the elongation is increased by 56%.The effects of heating temperature and extrusion speed on the microstructure and properties of semi-solid Al-25%Si billets were studied.The effects of extrusion temperatures of 380℃,420℃,460℃,500℃,540℃and 580℃on the microstructure and properties of extruded parts were analyzed.It is found that with the increase of extrusion temperature,the grain size of primary silicon in the extruded part is refined at first and then coarsening,and the mechanical properties of extruded parts increase at first and then decrease with the increase of extrusion temperature.When the extrusion temperature is 420℃and 460℃,the microstructure of the extruded part is finer,and when the extrusion temperature is higher than 460℃,the microstructure of the extruded part coarsens continuously.The effects of extrusion speeds of 2mm/s,3mm/s,4mm/s,5mm/s,6mm/s,7mm/s and8mm/s on the microstructure and properties of extruded parts were analyzed.When the extrusion speed is too low,the surface roughness of the extruded parts is larger,and when the extrusion speed is too low,the production efficiency will be reduced.When the extrusion speed is too high,the tensile strength and elongation of the extruded parts are relatively low.When the extrusion speed is 6mm/s,the microstructure and tensile strength of the extruded parts are better.That is,when the extrusion temperature is 420C and the extrusion speed is 6mm/s,the tensile strength is 135.7MPa and the elongation is 2.95%.When the extrusion temperature is 460C and the extrusion speed is 6mm/s,the tensile strength is 132.3MPa and the elongation is 3.2%.