Evolution Of TiP And Coupling Nucleation Of Primary Si In High-Si Aluminium Alloy

In this paper,two kinds of methods were proposed to enhance the efficiency on the heterogeneous nucleation of Si,which is helpful to improve the effect on refining high-Si Al-Si alloys.The one method is to preparea kind of Si-Ti-P master alloy.The phase constitution,morphology of Si-Ti-P master alloy and its evolution process in Al-Si melt has been studied.Base on that,the refining process of high-Si Al-Si alloy was optimized.Moreover,the Si-Ti-P master alloy showed better refining effect on Al-₂7Si alloy comparing with Al-3P and Cu-14P master alloys.And the effect from P refinement on the coefficient of thermal expansion and the tensile properties of Al-₂7Si alloy were studied.The other method is to simultaneously introduce AlP and TiB₂ particles.It was verified that TiB₂@AlP particles can be a high-efficiency nucleating agent for primary Si.The formation of TiB₂@AlP conforms to the adsorption model rather than the traditional spherical cap model.The major research results are as follows:?1?The evolution behavior of Si-Ti-P master alloy in Al-Si meltIt was found that phosphide would change after adding transition metal elements?Ti,Fe,Cu?into Si-Mn-P alloy.The effect on retaining P element is different from transition metal elements.In Si-Ti-P master alloy,TiP will evolve into AlP and?Al,Si,Ti?ternary compound when baring in Al-Si melt.The evolution of TiP in Al-Si melt is a liquid-solid diffusion process,driven by chemical potential difference.After adding Si-Ti-P master alloy into Al-Si melt,TiP would gradually expose in the melt and contact with A1 as Si matrix melting.Driven by chemical potential difference between A1 and TiP,TiP would evole into AlP.Due to the reaction rate is controlled by diffusion and it needs time for products to spread evenly,sufficient holding time is necessary for Si-Ti-P master alloy to achieve better refining effect.For shortening the holding time,it is suggested to increase the holding temperature or convection in melt.?2?Refining effect of hypereutectic Al-Si alloy by Si-Ti-P master alloyComparing with Cu-14P and Al-3P,Si-Ti-P performed more excellent refining effect on primary Si with smaller size,more regular morphology and more uniform distribution.After addition of 350ppm P via Si-Ti-P master alloy,primary Si in Al-₂7Si alloy was refined from coarse plate?above 488?m?to regular block?14.7±1.3pm?.Moreover,it was found that P refinement is beneficial to reduction of the thermal expansion coefficient and enhancement of tensile properties of Al-27Si alloy.For refined Al-27Si alloy by Si-Ti-P,a thermal expansion coefficient of 16.25×10-6/K,an increased ultimate tensile strength and elongation by 106%,235%than those of unrefined alloy were obtained respectively.?3?Refinement of high-Si Al-Si alloy by coupling AlP particlesThe formation mechanism of the TiB₂@AlP particles has been proposed and the refining effect on Al-45Si alloy by the TiB₂@AlP particles was verified.In Al-Si melt,P atoms tend to be adsorbed on the interface of TiB₂ particles,forming an adsorbed layer.As temperature decreasing,the adsorbed layer evolves to AlP crystal driven by undercooling and then the TiB₂@AlP coupling particles present.As the results of density functional theory calculations,the adsorbed P atoms trend on the Ti-terminated interface of?0001?TiB₂ and prefer to be close to Ti atoms.The refining effect of Al-45Si alloy by introducing TiB₂@AlP particles is much better than that by only introducing AlP.After holding 5 min on 1300?,the size of primary Si in Al-45Si alloy could be reduced to 30.1±1.9?m.