铍铝合金挤压铸造凝固行为及其微观组织形成机理研究

Due to large freezing range, high latent heat of fusion, and low capability of heat diffusion, Be-Al alloy exhibits coarse columnar dendritic structure and severe tendency towards thermal cracking. Squeeze casting offers the advantages of solidifying under an external pressure introduces plastic deformation that compensates for volume shrinkage and increases heat rejection capability, which has a great potential for Be-Al alloy to get more compact and uniform structure, and excellent properties. Squeeze casting could be the method to prepared high quality Be-Al billet. At the present time, limit researches have been done about squeeze-cast Be-Al alloy, and the understanding of solidification behavior during squeeze casting process of Be-Al alloy and its effect on microstructure is still limited. In this project, several scientific problems of squeeze-cast Be-Al alloy will be studied. Both the start and end temperatures of primary Be formation and eutectic reaction under the applied pressure will be investigated using computer-aided cooling curve analysis (CA-CCA). The evolution rule of solid fraction during solification will be ravelled, the influence of applied pressure on grain size, SDAS, and eutectic structure fraction and its mechanism will be indicated, the internal relations between morphology of the solid solubility and applied pressure will be definituded. The results of the project will complement solidification theory of Be-Al alloy, and provides the theory basis and technical sustain for developing jumbo size Be-Al alloy parts used in aerospace equipment.

铍铝合金具有宽的凝固区间，熔化潜热高、热扩散能力低，凝固组织为粗大的柱状晶，热裂倾向严重。通过对液态金属施加机械压力能够补偿凝固收缩，提高热散失能力，并使铸件产生少量塑性变形，有望使铍铝合金获得致密、均匀的组织和优异的性能，解决高质量铍铝合金生产的难题。然而，目前对铍铝合金在压力作用下的凝固过程缺乏认识，其组织特性与形成机理还不清楚。本项目关注铍铝合金挤压铸造过程中的基础科学问题，拟利用冷却曲线计算机辅助分析（CA-CCA）研究压力对初生Be相以及共晶组织形成与终了温度的影响规律，阐明凝固过程中固相的生长规律及机理，弄清晶粒尺寸、二次枝晶臂间距、共晶组织体积分数在压力作用下的变化规律，揭示凝固组织的形成机理，明确铝的固溶度与压力的关系，丰富铍铝合金的凝固理论，为航天用铍铝合金大尺寸部件的研制提供理论基础与技术支撑。

铍铝合金具有宽的凝固区间，熔化潜热高、热扩散能力低，凝固组织为粗大的柱状晶，热裂倾向严重。本项目探索采用挤压铸造方法制备铍铝合金，力求获得致密、均匀的组织和优异的性能，解决高质量铍铝合金生产的难题。获得了Be-38wt.%Al二元合金在80t（31MPa）压力下的冷却曲线，利用冷却曲线计算机辅助分析（CA-CCA）方法研究了合金的凝固开始点和结束点，发现铍铝合金由于是双相假合金，Be相和Al相凝固相对比较独立，需要在凝固曲线上拟合两条零位曲线，从而获得了Be相的凝固开始点与结束点和Al相的凝固开始点与结束点，说明Be-38Al合金存在两个明显的凝固过程；在压力状态下Be-38Al合金组织发生显著变化，铸态合金组织主要由粗大的柱状树枝晶组成，一次枝晶臂长度可达2~3mm，平均二次枝晶臂间距约为30um，压力下获得的Be-38Al合金晶粒转变为等轴晶组织，平均晶粒尺寸约为250um；铸态Be-38Al合金组织中存在大量疏松缺陷，这是合金凝固过程中Be相凝固后堵塞了补缩通道导致Al相无法补缩造成的，在压力作用下保证了Al相的补缩使得合金组织非常致密，通过水浸超声法测试未见疏松等缺陷；晶粒组织的细化以及疏松缺陷的消除是Be-38Al合金力学性能较铸态显著增加，抗拉强度由铸态时的69MPa，提高到214MPa；在压力作用下铍铝合金Be相中Al元素的固溶度没有显著变化；铍铝合金锭熔化后在1120℃、40MPa压力下进行保压10min，断电随炉冷却，在600℃卸压即可获得的组织致密的Be-38Al合金；基本实现了组织可控的铍铝合金挤压铸造成形，制备的Be-38Al铸锭用于了航天光学系统支架的加工，取得了良好的效果。