粉床型电子束3D打印纯钨及W-Nb合金的显微组织及开裂机制

Microstructure and cracking mechanism of pure tungsten and W-Nb alloys fabricated by selective electron beam melting

摘要:

采用粉床型电子束3D打印技术制备了纯钨及固溶强化型W-Nb合金,对两种钨基材料的显微组织和裂纹缺陷进行了分析。研究发现：二者均沿逐层累积的方向形成柱状晶,Nb元素的加入使柱状晶的平均尺寸由109.78μm降至25.10μm。纯钨并未发生明显的开裂现象,但是W-Nb合金内部存在少量沿晶界分布的微裂纹。从热应力累积和凝固过程两个方面分析了钨基材料开裂的原因,粉床型电子束3D打印过程包含了熔池的快速凝固-快速冷却-高温保温几个阶段,过程中纯钨及W-Nb合金均发生了回复与再结晶,使成形过程中累积的热应力得到一定的释放,缓解了热应力累积引起的开裂现象。W-Nb合金的开裂主要是由于凝固过程中液态金属无法短时间内在枝晶间进行充分流动补缩,形成大量沿晶界分布的纳米微缩孔,在微小的应力作用下就会造成合金沿晶界开裂。

Abstract:

Pure tungsten and solid solution strengthened W-Nb alloy were fabricated by selective electron beam melting (SEBM). The microstructure and crack were analyzed. Results showed that columnar crystal structure along the building direction of SEBM formed both in the pure tungsten and W-Nb alloy. The addition of Nb reduced the average size of columnar crystals from 109.78μm to 25.10μm. No significant cracks formed in the pure tungsten, but microcracks along the grain boundaries were found in W-Nb alloys. The rapid solidification, rapid cooling and holding at elevated temperature process in SEBM caused recovery and recrystallization of W and W-Nb alloys, so that the accumulated stress during the forming process was released. The cracking of W-Nb alloy is mainly formed during solidification, that is, because the liquid metal cannot feed the dendrite in a very short time during the solidification process, nanopores formed and gathered at the grain boundaries, and caused cracks along the grain boundary under little stress.