Research On Rheological Squeeze Casting Process Of High Silicon Aluminum Alloy Cylinder Liner

In the high-silicon aluminum alloys,there are a large number of primary phases such as primary silicon and Fe-rich alloys.Therefore,the materials have good wear resistance,heat resistance,and low coefficient of thermal expansion.They can be replaced the traditional cast iron,cast steel cylinder liner.However,due to the large size of primary silicon and Fe-rich phases in the conventional casting,the splitting effect on the matrix is significant,which reduces the mechanical properties of the material and causes the industrial application of high silicon aluminum alloy cylinder liners to be extremely limited.Therefore,in order to improve the practical application worth of high silicon aluminum alloy cylinder liners,semi-solid technology is used to refine the hard phases in high-silicon aluminum alloys.However,due to the presence of solid-liquid two phases in the semi-solid slurry,solid-liquid separation may occur during the forming process,affecting the properties of the casting.In this paper,based on the previous study of the research group,a set of rheological extrusion casting molds for high silicon aluminum alloy cylinder liners was designed.The influence of forming process parameters on the microstructure and properties of semi-solid high silicon aluminum alloy rheological forming cylinder liners was studied.Based on this,the distribution rule of the hard phase in semi-solid high silicon aluminum alloy cylinder liner in radial direction was investigated.It was found that at a certain extrusion rate,as the forming pressure increased,the average equivalent diameter of primary Si particles and Fe-rich phase gradually decreased,the shape factor gradually increased,and the primary Si particles and Fe-rich phase were more rounded,but it has little effect on the distribution of hard phases;with the increase of extrusion rate under the same forming pressure,the average equivalent diameter of the primary Si particle and the Fe-rich phase reduces firstly and then increases,and the shape factor increases firstly and then decreases,and the increase in extrusion rate can make the hard phase more evenly distributed in the axial direction,but when the extrusion rate is too high,the hard phase will be segregated in the radial direction of the tissue,and the larger the extrusion rate,the more unstable the flow of the filling front end.There may be turbulence and increased microstructure defects.When the forming pressure is 160 MPa and the extrusion rate is 16mm/s,the hard phase of rheological squeezing casting high silicon aluminum alloy cylinder liner is finest and round,and the average equivalent diameter of primary Si particles is 28.2um.The shape factor is 0.76,the averageequivalent diameter of the Fe-rich phase is 34.4um,the shape factor is 0.57,and its distribution is more uniform.The proper semi-solid extrusion parameters can make the properties of the rheological forming cylinder best.At the same extrusion rate,the mechanical properties of high silicon aluminum alloy cylinder liner will also increase with the increase of forming pressure.At the same forming pressure,the mechanical properties of cylinder liner will firstly increase and then decrease with the increase of extrusion rate.Under forming pressure is 160 MPa,the extrusion rate is 16mm/s,the mechanical properties of the high-silicon-aluminum alloy are the best.The tensile strength was 121 MPa and the elongation was 4.5%.Compared with the liquid extrusion liner under the same process,the tensile strength and elongation increased by 32.9% and 125%,respectively.Through the stress-strain curve and fracture morphology analysis of the rheological extrusion and conventional liquid extrusion of high silicon aluminum alloy cylinder liners,it is found that the fracture modes of the two are the same,all of them are brittle fractures,but the crushing plane distribution of liquid extrusion is more concentrated,larger in size and there is a small sharp corner.Through the analysis of the radial cross section microstructure of the cylinder liner under inner gate extending out of 5mm and 9mm,it was found that the volume fraction and the average equivalent diameter of the hard phase under the curved gate extending 5mm from the inner side of the cylinder liner the outside gradually decreases.The reason for the gradient is that the limiting effect of the inner gate makes the stress field and flow field of the high silicon aluminum alloy semi-solid slurry different during the filling process,resulting in the velocity of the liquid phase and the hard phase inside and outside of the cylinder liner or in the direction of deviation,so as to form the unidirectional gradient distribution of the hard phase.The Arc inner gate of the9 mm is too narrow,it will make the filling speed is too large,so that turbulent flow front of the filling,not to advance in laminar flow,leading to hard phase segregation too serious,its distribution is not effectively controlled.