Semi-solid Processing Behavior Of Hypereutectic Al-Si-Fe Alloy

The hypereutectic Al-Si-Fe alloy is an important casting Al-Si alloy, which is a lightwear-resisting material with hard spots distribute on soft substrates.The hypereutectic Al-Si-Fealloy is especially suitable for the manufacture of lightweight, wear-resistant parts, and is anideal alternative to traditional wear-resisting material. However, the application ofhypereutectic Al-Si-Fe alloy is limited by the bulky phase which seriously affect itsorganization integrity and mechanical properties.In this paper, the nonlinear mathematicalexpression of liquid mass fraction in solid-liquid phase transition is derived by theexperimental and theoretic study on semi-solid thixotropy extrusion of the Al-Si-Fe alloy. Theexpression could reveal the evolution rule of liquid mass fraction of semi-solid alloy in theprocess of reheating.The thixoforming constitutive equations of hypereutecticAl-20Si-3Fe-1Mn-4Cu-1Mg alloy are established and the semi-solid thixotropic extrusionforming process parameters are optimized by numerical simulation, more ever, the influenceof semi-solid thixotropic extrusion forming process on the properties and organization of thealloy is studied.The results show that electromagnetism stirring only has a obvious effect on therefinement of the matrix phase ?-Al in hypereutectic Al-20Si-3Fe-1Mn-4Cu-1Mg alloy, whenthe electric voltage is at120V~150V, fine and spherical non dendrite ?-Al matrix can beobtained. The solidification temperature and completely melting temperature ofAl-20Si-3Fe-1Mn-4Cu-1Mg alloy are507?and627?this alloy has a wide range ofsolidification temperature and is suitable for semi-solid processing. At the range of57?0~580?, the alloy has smaller solid fraction temperature sensitivity and suitable for reheating.The microstructures of eutectic Si,primary Si and Fe-riched phase of the alloy are morefiner and spherical,with the reheating temperature increasing, the ?-Al is more spherical andlarger, and this effect is obvious when the temperature is higher. The grain growth is verysensitive to the heating time.The curve of liquid mass fraction and heating temperature of thealloy is obtained by DSC, the liquid mass fraction is not simple linear increased with heatingtemperature as Scheil expression described, but nonlinear increased.The growth rate of liquid mass fraction was not entirely consistent in different temperature range, which formedS-shaped curve.The nonlinear mathematical expression of liquid mass fraction in solid-liquidphase transition of semi-solid alloy is derived from thermodynamic equations.The research ofsolid-liquid phase transition in the alloy show that the nonlinear mathematical expression has asmaller discrete degree compared with the Scheil linear regression expression.The semi-solid thixoforming behavior of hypereutectic Al-20Si-3Fe-1Mn-4Cu-1Mg alloyis studied by single-pass compression experiment,the results show that the flow stress anddeformation degree decrease with the increase of temperature,the mechanical properties of thealloy are hardly improved when temperature is too high to break the thick phase of thealloy.The alloy is a positive strain rate sensitive material at the same temperature, the flowstress is increased with strain rate rising. At the same strain rate, the flow stress is decreasedwith temperature increasing and the strain rate is too low to break the thick phase of thealloy.The process parameter of semi-solid thixotropic extrusion forming is: temperature at570?~580?,strain rate in0.1~0.01s-1.A new method is discussed that semi-solidthixoforming process can be separated into elastic deformation, strain hardening andrheology-viscoplastic deformation stage in different true strain range,and the thixoformingconstitutive equations are derived by multiple regression in different deformationstage,according to the characteristic of semi-solid thixotropic compression stress-straincurve.The semi-solid thixoforming die can meet the requirements of forming process after theanalysis of numerical simulation. The optimization of semi-solid thixotropic extrusion processparameters obtained via numerical simulation is: reheating temperature is580?,moldtemperature is55?, thespeedofextrusionis2mm/s.The microstructure of the alloy is finer cause thick phase of the alloy is broken and splitinto small pieces by semi-solid thixotropy extrusion.The anisotropy textural microstructure ofalloy which is constitute of the directional chain distribution of Si phase and continuousdistribution of Al matrix organization improve mechanical properties of the alloy. The tensilestrength reaches227MPa,192%higher than as-cast, hardness increase to121HB, elongationis1.8%, increased by100%. The properties of the heat treatment for thixoforming alloyis:tensile strength is264MPa,123.3%higher than that of as-cast,hardness is increased to138HB,plastic maintained a higher level.The high temperature wear experiments show that thewearability of semi-solid alloy by thixotropic extrusion was significantly better than A390 alloy, the T1, T6heat treatment of the alloy also have high wear resistance.