Mg-Zn-(Ce,Gd)合金凝固过程热裂纹萌生、扩展行为及其机理的研究

The research studies about magnesium alloys steering wheel,gear box and other automotive castings focused on elements alloying and mechanics performances. However, magnesium alloys show a high susceptibility to hot tearing during the solidification, especially if casting in a permanent mold. In this project, Mg-Zn-(Ce,Gd)Al alloys are studied which are sensitive to hot tearing. The present work evaluates the hot tearing susceptibility (HTS) of Mg-Zn-(Ce,Gd) ternary system using thermodynamic calculations based on Clyne-Davies’ mode using qualitative and quantitative methods for validation. Hot crack initiation and propagation are studied using XRD, OM, SEM and X-ray 3D computed-tomography characterization methods. The relationships among the alloy compositions(light and weight RE), mold temperature, solidification contraction force recorded and hot crack initiations and propagations are investigated. It reveals the microscopic mechanism of hot tearing behaviors of hot tearing according to the rules of hot crack initiations and propagations. This study is very important to theoretical and practical significance for widening magnesium applications. At the same time, it is a new way to characterize rules of hot crack initiations and propagations using X-ray 3D computed-tomography characterization method. And a simple and efficient observation method is provided to analysis magnesium alloys hot tearing behaviors.

目前有关于镁合金方向盘、变速箱壳体等汽车用轻量化铸件的研究多集中于合金化及性能等方面，而铸件在铸造过程中容易出现热裂现象使得材料的性能有所降低常被忽视。本项目中，以对热裂较为敏感的Mg-Zn-(Ce,Gd)系合金为研究对象。利用Clyne-Davies 模型的热力学计算预测Mg-Zn-(Ce,Gd)系合金较为敏感的合金成分区域并用定性定量的分析方法进行验证。通过XRD、OM、SEM、TEM 及特有的计算机断层扫描系统等分析手段，探究热裂纹的萌生、扩展的方式以及合金成分（轻重稀土元素）、模具温度、凝固收缩应力变化率等对其的影响，研究热裂纹萌生条件及扩展规律，揭示镁合金热裂行为的微观机制。本项目的开展将对汽车用镁合金、扩展镁合金应用领域提供理论依据和实验支持，同时采用计算机断层扫描系统表征合金热裂纹萌生扩展的演变规律，是一种新方法的尝试，有望对合金热裂行为的分析提供一种简单高效的观察方法。

本项目中，以对热裂较为敏感的Mg-Zn-Gd（Ce）系合金为研究对象。利用Clyne-Davies模型的热力学计算预测Mg-Zn-Gd（Ce）系合金的热裂敏感性，二元Mg-Gd(Ce)的热裂敏感性趋势符合“λ”型。通过XRD、OM、SEM、TEM及特有的计算机断层扫描系统等分析手段，探究热裂纹的萌生、扩展的方式以及合金成分、模具温度、凝固收缩应力等对其的影响，得到了凝固收缩应力、模具温度与裂纹体积成反比；研究热裂纹萌生条件及扩展规律，揭示镁合金热裂行为的微观机制为液膜理论凝固收缩补缩理论与晶间搭桥理论相互作用。本项目的研究结果将对汽车用镁合金以及扩展镁合金应用领域提供理论依据和实验支持。