| This article used Al-20Si alloy as the research object,adopt two methods of metamorphism and different cooling rates to refine the microstructure of Al-20Si alloy.Through high-precision thermometers,OM,SEM,EDS,XRD and Friction and Wear tests,the effects of two refining methods,modification and cooling rate,on the solidification microstructure and properties of Al-20Si alloy were investigated.The mechanism of collaborative refinement of primary Si and eutectic Si in Al-20Si alloy was also investigated,provided the theoretical foundation for the preparation of pistons,cylinder liners and radiators for hypereutectic Al-Si alloy materials.(1).The Al-20Si alloy was modified with P and RE modifiers.The results showed that the Al-P complexes formed by the P and Al as the foreign heterogeneous nucleus of the primary Si phase,It can effectively refine the primary Si in the alloy.The RE element were adsorbed and dissolved on the growth step of the Si phase,which caused the lattice constant of Si phase in the Al-20Si alloy was increased and the X-ray diffraction pattern is shifted by a small angle.With the change of metamorphic time,the lath-shaped primary Si in the alloy becomes Fine and rounded,the average grain size decreased from 85.6μm to 13.2μm.The eutectic Si was changed from a long needle to a short rod,diffusely distributed in the matrix,the hardness was increased from 68.5HB to 80.8HB,and the tensile strength increased from 132MPa to 185MPa,the elongation increased from 1.29%to 2.68%.(2).The Al-20Si alloy was refined with different cooling rates.The results showed that the average grain size of primary Si(D)in Al-20Si alloy had a power function relationship with the cooling rate(v):D=260.6v-3/4,and the linear relationship with the Faihui temperature(Tm)was D=0.25Tm-143.12;Reducing the Faihui temperature of primary Si was the key to control the grain growth,the high thermal storage coefficient of Copper mold can continue to reduce the nucleation temperature and Faihui temperature of primary Si,so that the primary Si grain size was small;The critical supercooling degree of primary Si transformed from facet growth to rough interface growth is 70K,which was consistent with the theoretical calculation(74K).With the addition of modificator,the increase of cooling rate,the increase of undercooling and the decrease of Faihui temperature,the solidified microstructure of Al-20Si alloy was refined remarkably.The tensile strength of the alloy increased from 119MPa to 210MPa and the elongation increased from 1.01%to 3.89%.(3).The wear behavior of Al-20Si alloy is dominated by exfoliation wear,adhesive wear and abrasive wear.The exfoliation wear of the sample was exacerbated by the increase of primary Si grain size in the alloy.After refinement in Al-20Si alloy,the grain size of Si phase in the microstructure was decreased,but the hardness was increased significantly,the amount of wear was significantly reduced. |
