熔体阶段控制冷却铸造铜锡合金的偏析抑制及其与性能之间的关联机制

Cu-Sn alloys have low friction coefficient, good abrasion resistance and corrosion resistance, etc. But, poor pressure resistance and dynamic brittleness are caused by low melting segregation during the alloy solidification process. In this project, for avoiding the segregation of casting Cu-Sn alloys, the method of controlling cooling condition of alloy melt will be applied. At first, the melt flow through the slit channel moments caused the undercooling, the primary phases rapid nucleation and suppress Tin atoms diffusion into the liquid phase. Subsequent crystallization latent heat of primary phases will form the isothermal process in the collected alloy slurry. Then, Tin atoms are further promoted to diffuse into the fine solid phases, and the concentration of Tin element in liquid phase is reduced. At last, low segregation semi-solid slurry could be produced for squeeze casting. The microstructure evolution rules will be researched in the project, such as the nucleation and growth of solid phase grains when melt instantaneous supercooling or during approximate isothermal process，the solid-liquid interface morphology and the solute Sn element distribution. The temperature field and flow field of alloy melt in the runner will be calculate by numerical simulation. The mechanism of restraining segregation by controlled cooling process of melt will be investigated by combining the thermodynamics conditions and dynamics conditions of diffusion，such as the solute spreading depth in solid phase，diffusion direction and the number of diffusion couples. According to the concentration change of Tin atom in liquid phase and solid phase, the evaluation index of segregation degree will be confirmed.

铜锡合金具有摩擦系数低、耐磨耐蚀性好等优点，但其凝固过程易形成低熔点偏析导致耐压性差而出现动态脆性。项目针对铸造铜锡合金偏析的抑制，采用阶段控制冷却方法处理熔体，首先熔体流经缝隙式通道产生瞬间过冷，初生相快速形核并抑制锡原子向液相内扩散；后续利用初生相形核的结晶潜热，在浆料收集时形成类等温过程，进一步促进锡原子向细小固相内扩散，以降低液相中锡元素的浓度，为挤压铸造提供极低偏析的半固态浆料。项目研究熔体瞬间过冷、类等温过程中固相形核长大、固液界面形态、溶质锡原子的分布等组织演变规律；采用数值模拟计算通道内熔体温度场、流动场；结合溶质在固相内部分布厚度、扩散方向、扩散偶数量等扩散热力学和动力学条件，揭示熔体阶段控制冷却抑制偏析的机理；以锡原子在液相和固相中的浓度变化，编制偏析程度等级评价指标，建立偏析程度与性能之间的关联机制。项目既能丰富金属凝固理论，又促进高品质铸造锡青铜铸件开发和工程应用。

CuSn10P1合金铸件具有高强、耐磨、耐蚀等特性，满足重载（20 MPa以下）、高滑动速度（8 m/s）和高温并受强烈摩擦的工况要求，广泛用于制造高铁、船舶、航空等行业的轴套、连杆、轴瓦、涡轮等零部件。但CuSn10P1合金铸件中低熔点锡元素容易向晶间迁移，形成晶间偏析和逆偏析，显著降低铸件的强度和塑性。.本项目采用层流紊流转变理论计算结合ProCAST温度场模拟仿真，设计基于爆发性形核原理的熔体约束流动诱导形核半固态浆料制备设备。采用该设备制备CuSn10P1合金半固态浆料并分析组织细化和锡元素偏析改善机理。结果表明，熔体在约束通道内流动过程中受到通道壁的快速激冷而大量形核，晶核数量增加有利于晶粒的细化；另外，快速凝固使部分锡元素来不及扩散而固溶在初生相内，从而改善锡元素偏析现象。.类等温过程中局部微热对流为初生相旋转和游离提供动力学条件，界面能最小化原理推动初生相发生合并与熟化，同时，浓度梯度驱动Sn元素从液相向初生相内扩散，提高初生相内部Sn元素的整体含量从而改善偏析现象。.明确初生相尺寸、分布及锡元素偏析程度对零件抗拉强度、塑性及耐磨性的影响，结果表明，施加类等温后进行流变挤压成形，铸件的显微组织均匀性得到提高和锡元素偏析现象得到改善，同时改善了合金的抗拉强度和延伸率等性能。对铸件进行固溶处理后，零件的晶间偏析基本消除和延伸率得到显著提升。.通过本项目的实施，促进了高品质铸造锡青铜铸件的开发和工程应用，丰富了半固态金属流变学理论，对需改善偏析现象的其它合金具有理论指导意义。