稀土镁合金废屑放电等离子烧结/旋转挤压固相再生强化机理

Large amounts of magnesium alloy scrap will be produced in the forming and machining process. Rare earth magnesium alloy is hard to be recycled by remelting methods due to the addition of rare earth elements, this leads to a serious waste of resources.The density of prepared billets and properities of the formed product are lower by traditional solid state recycling of magnesium alloy. In order to improve the properities of the recycled products, solid state recycling method by spark plasma sintering (SPS) and rotating extrusion is proposed. In this method, SPS is used to prepare the billets. Defects of voids and cracks can be eliminated, and metallurgical bonding between chips can be obtained by SPS. Rotating extrusion is used to improve the properities of the recycled products, in which shear strain is imposed into the extruded parts and severe plastic deformation can be realized in single pass. Solid state recycled billets with high relative density and formabilty can be obtianed through this investigation, and rules for SPS and heat treatment will be determined. Effects of shear on deformation will be revealed. Phase compositions and microstructure evoluton during SPS and rotating extrusion will be studied. Strengthening mechanisms of mechanical properties will be clarified. The investigation will provide a method for solid state recycling of rare earth magnesium alloy and other light alloys, and lays a theoretical foundation for producing the recycled parts with high properities.

镁合金在成形和加工过程中产生大量废料，特别是稀土镁合金，由于添加了多种稀土元素，难于通过回炉重熔法实现回收，严重造成资源浪费。传统的镁合金固相再生工艺，存在预制坯致密度不高及再生制件性能不理想等问题。为了提高固相再生制件性能，本项目提出基于放电等离子烧结制坯和旋转挤压成形的固相再生新方法，该方法采用放电等离子烧结来进行预制坯制备，消除传统制坯时存在的空洞、裂缝等缺陷，实现切屑之间的冶金结合；采用引入剪切变形的旋转挤压进行后续成形，在单道次成形中实现大塑性变形、以强化组织性能。通过本项目研究，确定放电等离子烧结的合理工艺参数和热处理制度，制备出具有较高致密度和成形性能的预制坯，揭示旋转挤压过程中剪切变形对金属流动的影响规律，明晰烧结及旋转挤压过程中的相组成及组织演变规律，阐明性能强化机制，为稀土镁合金及其它轻合金废料的固相再生利用、制备具有较高使用性能的再生制件提供新的工艺途径和理论基础。

稀土镁合金中由于添加了多种稀土元素导致其成本昂贵，稀土镁合金构件机械加工过程中产生大量的废屑，造成极大的稀土资源浪费。传统的液相再生和固相再生方法制备的坯料均存在力学性能较差等缺陷。本项目针对稀土镁合金固相再生制备的瓶颈问题，提出放电等离子烧结（SPS）制备预制坯和旋转挤压成形强韧化的固相再生工艺。本项目首先研究了稀土镁合金预制坯的放电等离子烧结制备，对于Mg-7Gd-4Y-2Zn-0.6Zr合金而言，最佳的放电等离子烧结参数为烧结温度500℃，烧结时间为10min。对于固相再生坯料的后续热处理，热处理时间对固相再生坯料的室温压缩力学性能影响较小，而冷却方式显著影响固相再生坯料的力学性能，固相再生坯料空冷后的力学性能要显著优于炉冷后的力学性能。其次，研究了放电等离子预制坯旋转挤压过程中组织演变和强化机制，建立了考虑剪应力影响的高温本构方程，阐明了放电等离子烧结固相再生预制坯烧结界面在旋转挤压剪切作用下发生了变形乃至破碎，揭示了纤维强化以及固溶强化是稀土镁合金固相再生坯料的主要强化机制。再次，研究了SPS预制坯制备及旋转挤压过程中的氧化相及其对性能的影响，烧结界面上的稀土氧化相Gd2O3，Y2O3导致固相再生坯料烧结界面处存在大量的稀土元素聚集。旋转挤压制件的抗拉强度提升一方面来源于旋转挤压施加剧烈剪切变形而造成的细晶强化，另一方面则来源于破碎的烧结结合界面氧化相的颗粒强化作用。最后，研究了SPS坯料旋转挤压制件的使用性能，旋转挤压制件底部的强度略高于侧壁的强度，旋转挤压制件热处理之后的室温力学性能趋于稳定，旋转挤压制件腐蚀过程中产生的腐蚀产物会阻碍腐蚀向挤压制件内部进行，有效提高制件的耐腐蚀性能。本项目的研究结果可以为Mg-Gd-Y-Zn-Zr合金废屑的回收提供理论指导。