【作者】 王耘涛；

【导师】 袁晓光； 于宝义；

【作者基本信息】 沈阳工业大学 ， 材料加工工程， 2014， 博士

【摘要】 过共晶Al-Si-Fe合金是一种重要的铸造Al-Si系合金，该合金是一种软基体上分布着硬质点的理想轻质耐磨结构材料，特别适用于制造轻质、耐磨零件，是传统耐磨材料的理想替代材料。但过共晶Al-Si-Fe合金因其组织中的粗大相严重影响了其组织完整性和力学性能的发挥，限制了过共晶Al-Si-Fe合金的应用。本文以过共晶Al-20Si-3Fe-1Mn-4Cu-1Mg合金为研究对象，通过半固态触变挤压成形工艺及理论研究，建立反映半固态合金固液相变行为的非线性数学模型，揭示半固态合金在二次加热过程中液相率的变化规律，通过对合金变形行为的研究，构建该合金在半固态触变挤压成形过程的本构关系，经数值模拟对半固态触变挤压成形工艺参数进行了优化，并对半固态触变挤压成形工艺对该合金的组织、性能的影响进行了研究。研究表明，电磁搅拌对过共晶Al-20Si-3Fe-1Mn-4Cu-1Mg合金中的α-Al基体相细化作用明显，而对其它相效果较差。电磁搅拌电压在120V~150V为宜，能够得到基本细小，圆整化的非枝晶α-Al基体。通过DSC实验，获得该合金的开始凝固温度为507℃，完全熔化温度为627℃，凝固温度区间较宽，适于半固态加工。在570℃~580℃的温度区间，该合金具有较小的固相分数温度敏感性，适合进行二次加热。合金组织中共晶Si、初生Si和富Fe相随二次加热温度的升高而细化、圆整化，α-Al基体则球化、粗化，温度越高这种作用越明显。随着保温时间的延长，合金组织具有长大趋势，晶粒长大对加热时间更敏感。采用热涵法获得了该合金液相率与加热温度曲线，二者的关系不是Scheil公式描述的简单线性增加，而是随温度升高液相率非线性增加；在不同的温度区间，液相率的增长速率并不完全一致，导致曲线呈现S形，依此建立了半固态合金固液相变液相率的非线性数学模型；采用过共晶Al-20Si-3Fe-1Mn-4Cu-1Mg合金验证了模型的正确性，并表现出较Scheil线性回归模型离散度更小、准确性更高的特点。Al-20Si-3Fe-1Mn-4Cu-1Mg合金的半固态单向压缩实验表明，流变应力及变形程度随温度的升高而减小，过高的温度不易破碎组织中粗大相，不利于提高合金的力学性能。变形抗力在变形温度相同的情况下随应变速率的增加而增大，说明该合金是正应变速率敏感材料。变形抗力在相同应变速率条件下随变形温度的升高而降低，应变速率过低不利于破碎粗大相。半固态触变挤压成形工艺参数选择范围为：变形温度在570℃~580℃区间，应变速率在0.1s-1~0.01s-1。根据半固态触变压缩真应力-真应变曲线的特征，提出在不同的真应变范围分为3个不同的变形阶段，即类弹性变形、加工硬化和流变-粘塑性变形阶段，并分阶段建立了半固态触变本构方程，采用多元回归进行了求解。通过对半固态触变挤压成形过程进行的数值模拟表明，模具结构设计能够满足制备合格制件的要求，经模拟优化得到了半固态触变挤压工艺参数：坯料二次加热温度为580℃，模具温度为550℃，挤压速度为2mm/s。经半固态触变挤压成形后，Al-20Si-3Fe-1Mn-4Cu-1Mg合金组织中粗大相显著破碎并分裂为小块，组织明显细化。在变形较大的部位，呈方向性链状分布的Si相与连续分布的Al基体使合金组织具有一定的方向性，有助于提高合金的力学性能。Al-20Si-3Fe-1Mn-4Cu-1Mg合金经半固态触变挤压成形之后的抗拉强度达到了227MPa，为铸态抗拉强度的1.92倍，硬度也增加为121HB，伸长率则达到了1.8%，提高了1倍。而经热处理后的触变成形合金，其抗拉强度达到了264MPa，比铸态提高了123.3%，硬度则提高到138HB，塑性也保持了较高的水平。高温干摩擦实验表明，经半固态触变挤压的合金的耐磨性显著好于A390合金，经T1、T6热处理的合金的耐磨性进一步提高。更多还原

【Abstract】 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 α-Al in hypereutectic Al-20Si-3Fe-1Mn-4Cu-1Mg alloy, whenthe electric voltage is at120V~150V, fine and spherical non dendrite α-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 α-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.更多还原