Refinement Of Primary Silicon Grains And Properties Of Hypereutectic Al-Si Alloy Processed By A Copper Serpentine Channel Method

As the study object of this paper,the hypereutectic Al-Si alloys with the silicon content of 30%and 35%were prepared by using the mechanical vibration water-cooled copper serpentine channel pouring processing and the composite processing of metamorphism and mechanical vibration serpentine channel pouring.Besides,it systematically studied the effects of the serpentine channel pouring processing parameters,Si content and phosphorus refining processing parameters on the primary Si grains within the hypereutectic Al-Si alloy.It also discussed the nucleation and growth process of primary Si crystals within hypereutectic Al-Si alloy and the refinement mechanism and evolution process of the primary Si grains during the preparation process in this experiment.On this basis,the effects of rheo-diecasting process parameters on the mechanical properties of Al-30%Si alloy samples were further investigated.Moreover,it also studied the effects of the T6 heat treatment on the alloy properties.The main achievements are as follows:(1)The primary Si within the microstructure of the Al-30%Si alloy can be fully refined by using the eight-curve and twelve-curve mechanical vibration serpentine channel pouring processing.The optimum processing parameters are as follows:the pouring temperature is 850℃,the vibration frequency is 80Hz and the curve number of the copper serpentine channel is eight.With the optimum processing,the average equivalent diameter of the primary Si grains is refined from 87.6μm to 29.8μm,and the average shape factor is 0.726.(2)The primary Si grains in the Al-30%Si alloy can be further refined by using composite processing of metamorphism and mechanical vibration serpentine channel pouring.Ancd the optimum processing parameters are as follows:the pouring temperature is 860℃,the addition of the phosphor-copper master alloy is 1.4%,the processing temperature is 900℃ and the proeessing time is 30min.With the optimum processing,the equivalent grain diameter and the average shape coefficient of the primary Si grains are 22.4 μm and 0.617.respectively.(3)The primary Si within the microstructure of the Al-35%Si alloy cannot be fully refined by using the mechanical vibration water-cooled copper serpentine channel pouring or composite processing of metamorphism and mechanical vibration serpentine channel pouring.Through this preparation,the refinement of the primary Si grains is uneven,and there are always some thick lath-like primary Si grains in the alloy microstructure.(4)Due to the intense chilling effect of the copper serpentine channel,the channel inner wall can be used as a base of the primary Si’s heterogeneous nucleation when the hypereutectic Al-Si alloy melt through the mechanical vibration copper serpentine channel,thus the Si atoms group preexisting in the alloy melt can be generated to primary Si crystal nucleus in the channel inner wall.Meanwhile,the vibration of the serpentine channel can accelerate the exfoliation of primary Si grains,it can also improve effective nucleation rate of the primary Si crystals in the alloy melt.In addition,the effect of the mechanical vibration can enhance the turbulence "stir" role of the melt when the alloy melt through serpentine channel,thus the relative movement between the alloy melt and the primary Si grains is enhanced,and the growth of the primary Si grains is restricted.Finally,the primary Si grains in the alloy microstructure are refined successfully.(5)With the decrease of the pouring temperature,and the increase of the vibration frequency of channel,the addition of phosphorus refiner and the injection pressure,the mechanical properties index of the Al-30%Si alloy rheo-diecasting samples increase first and then decrease.The optimum forming processing parameters are as follows:the pouring temperature is 850 C,the vibration frequency is 80Hz,the injection pressure is 60MPa and the temperature of the mould is 200～250℃.Under this conditions,the tensile strength and the elongation of the Al-30%Si alloy rheo-diecasting samples can reach the maximum value.The maximum tensile strength of the samples is 212.1 MPa,and the maximum elongation is 0.33%.(6)For the Al-30%Si alloy rheo-diecasting samples,the reasonable T6 heat treatment process is solution treatment for 6h under 480℃ and artificial aging treatment for 9h under 170℃.After T6 heat treatment,the maximum tensile strength of the Al-30%Si alloy rheo-diecasting samples is 251.8MPa,and the corresponding elongation is 0.27%.(7)Compared with the as-cast tensile samples,the tensile strength of the T6 heat treatment samples is increased by 21.6%.The wear resistance and the thermal expansibility of the Al-30%Si alloy rheo-diecasting samples can satisfy the property requirements of the piston alloy.However,the room temperature elongation of Al-30%Si alloy rheo-diecasting samples are less than 0.5%.