半固态挤压铸造高铁高硅铝合金中硬质相梯度分布控制及机理研究

Because of its good wear resistance and thermal stability, the high Si alloy with high Fe content become the ideal material for cylinder liner used in the Al engine. The semi-solid squeeze casting research on this alloy showed that the volume fraction of the hard phases in the liner-like tube distributed with one-way gradient along the radial, and reduced gradually from the inner wall to the outer wall. In the light of the characteristics of complex semi-solid slurry system with kinds of hard phases such as primary Si and Fe-rich phases, the shear rheological behavior of the slurry will be firstly investigated under the conditions of isothermal and continuous cooling in this project. And combined with the addition of Fe to the Al melt and the formation of Fe-rich phases, the controllable equation of slurry apparent viscosity will be established. Secondly, the simulation of the effects of the inner gate shape and slurry filling velocity on the filling process and free surface morphology of the slurry front will be executed by means of the numerical analysis software. According to the simulation results, the change of the temperature field, stress field and flow field in the slurry during the filling process will also be analyzed. Thirdly, the formation mechanism of the microstructure with one-way gradient distribution will be revealed according to the simulation results and microstructure characteristics in the studied Al alloy tube obtained by the semi-solid squeeze casting. The objective to control the gradient magnitude of microstructure distribution will be reached finally. The project’s research can promote the engineering application of high Si Al alloys with high Fe, and also can enrich the rheology of semi-solid metal. Its theoretical value for obtaining microstructure with gradient distribution in other alloy and composite material will be highlighted fully.

高铁高硅铝合金因其良好的耐磨性、热稳定性而成为全铝发动机中缸套的理想材质。半固态挤压铸造研究发现，此合金类缸套管件壁中硬质相由管内壁沿径向至外壁呈逐渐减少的单向梯度分布。项目针对其半固态浆料为含初生硅、富铁相等多种固相的复杂体系特点，结合铁元素添加、富铁相的形成等因素，研究等温和变温条件下高铁高硅铝合金浆料的剪切流变行为，建立表观粘度控制方程；借助数值分析软件模拟内浇口形状和充型速度对半固态浆料充型过程及其前端自由曲面形态的影响，分析浆料流动过程中温度场、应力场和流场的变化；综合模拟结果和半固态挤压铸造高铁高硅铝合金组织特点，揭示硬质相单向梯度分布的形成机理，达到控制单向梯度分布幅值的目的。项目研究能促进高铁高硅铝合金的工程应用，同时能丰富半固态金属流变学，对获取组织梯度分布的其它合金和复合材料具有理论指导意义。

含Fe高硅铝合金(20~25%Si)具有良好的耐磨性、耐热性和较低的热膨胀系数，是替代汽车行业发动机中铸铁、铸钢质活塞、缸套的理想材料。但高硅铝合金凝固组织中由于Fe的加入而出现长针状富Fe相，同时初生Si也呈粗大块状或多边形形状，这两种硬质相对基体具有显著的割裂作用。.本项目采用添加Mn元素的方法，将含2%Fe的高硅铝合金(22~25%Si)中针状富Fe相转变成块状，结合转棒诱导形核半固态熔体处理技术细化合金中硬质相，同时转棒对熔体的整体过冷促进了硬质相分布的均匀化。项目通过SEM、XRD、FIB以及TEM等表征技术获得了Al-25Si-2Fe合金，Al-25Si-2Fe-1.4Mn合金中主要富铁相的晶体结构与晶格常数，丰富、明确了此类化合物的晶体信息；结合Mn/Fe比为0.7的含2%Fe高硅铝合金铸态与半固态组织分析、DSC曲线及热力学计算结果，获得了其中富Fe相析出温度及物相间反应。.为深入研究高铁高硅铝合金半固态浆料流变行为，设计并制造了高精度的高温旋转粘度计用以测量测量合金熔体在不同状态下的表观粘度，构建稳态条件下合金半固态浆料的表观粘度数学模型；采用多相流耦合计算方法模拟合金半固态浆料固液两相混合流动行为，结果表明，直浇口条件下，充型速率越大，管壁轴向各位置体积分数越接近；模拟了不同浇口形状下，合金半固态浆料充型前沿自由液面以及浆料流动形式。.根据多相流数值模拟结果，设计合金半固态挤压铸造模具内浇口形状与尺寸，通过试验验证合金半固态浆料充型数值模拟结果，控制浆料中硬质相的分布，获得径向硬质相具有线性梯度分布管材零件；结合数值模拟与试验验证结果分析了合金半固态浆料自由液面、充型模式与硬质相分布之间的内在关联机制。.明确硬质相形貌对合金半固态成形零件耐磨性能及抗拉强度的影响，结果表明，针状富铁相对合金耐磨性及抗拉性能影响恶略，细化后的块状富铁相有利于合金耐磨性能提升，同时改善了合金的延伸率等性能。 .通过本项目的实施，促进了高铁高硅铝合金的工程应用，丰富了半固态金属流变学，对获取组织梯度分布的其它合金和复合材料具有理论指导意义。