2-15. Study on the microstructural evolution of a beta titanium alloy during high throughput laser melting under a pulsed laser mode

Chunlei Qiu*, Qi Liu
School of Materials Science and Engineering, Beihang University, Beijing, 100191, China

Abstract:Owing to high energy density, high flexibility and good controllability, laser provides a new high throughput manufacturing approach for rapid melting and synthesis of complex alloys with multiple components that can have significant difference in physical and chemical properties. Meanwhile, due to the high controllability on the laser processing parameters, it offers a new route to control key solidification and cooling conditions and thus the final microstructure during metallic laser melting by controlling key processing parameters. In this study, a  titanium alloy, Ti-10V-2Fe-3Al, was laser melted under a modulated pulsed laser mode with different processing conditions. The as-fabricated samples were examined using a range of characterization techniques and properties evaluated through tensile testing. It is shown that with a small powder layer thickness (30 m), a low laser power and a short exposure time (i.e., low energy density) led to development of fine  columnar grains and widespread cell structures whereas increased laser power and exposure time (i.e., high energy density) resulted in pronounced grain growth, increased texture and significantly decreased cell structures. Increasing powder layer thickness effectively promoted the columnar-to-equiaxed grain transition (CET), leading to a greatly reduced texture and a hybrid microstructure which consists of small and chunky equiaxed grains together with a small number of large columnar grains. Athermal  precipitates were observed in all the as-fabricated samples. In the samples made with high energy densities,  laths which constitute a grid-like structure were observed. The  precipitation was found to have a strong variant selection trend, leading to the formation of strong  microtexture. The -free samples with the finest columnar grains show both high strengths and good ductility thanks to full plastic deformation through both slipping and twinning. The samples with the hybrid grain structure, however, exhibit a highly limited or no ductility due to intergranular fracturing. The -containing samples which also have coarse grains all failed in a cleavage fracture mode and exhibited almost no ductility. Transmission electron microscopy study reveals that the -demarcated grid structure tended to confine plastic deformation within the  matrix and suppress the macroscopic plastic deformation throughout the samples.

Keyword:Laser melting; Titanium alloy; Microstructure; Columnar-to-equiaxed grain transition; Tensile behaviour

脉冲激光模式下激光冶金高通量制备一种钛合金的微观组织演变研究 

邱春雷, 刘琦

北京航空航天大学材料科学与工程学院 

摘要:激光具有高能量密度、高效灵活、高适用性及精确可控等特性，为具有多组元且组元性质差异大的复杂合金的快速熔化、合金化及合成提供了全新的高通量制备方法。同时，由于激 光冶金工艺参数具有灵活可控等特点，为通过控制激光参数实现对熔池凝固及冷却等关键条件 的控制，从而突破高性能复杂合金激光冶金高通量制备凝固过程及组织的控制奠定了技术基础。 本研究在脉冲激光模式下，通过改变激光冶金工艺参数如激光功率、曝光时间及粉末层厚度等 对一种钛合金的微观组织演变进行了深入的研究。结果表明，当激光功率低曝光时间短及粉末 层厚度小(即低能量密度)的时候，钛合金获得了非常细小的纤维状柱状晶组织。随着激光功率或 曝光时间的增加(能量密度随之增加)，晶粒显著变粗大，沿〈001〉的织构也随之增强。增加粉 末层厚度有效促进了柱状晶向等轴晶的转变，极大降低了织构程度，突破了激光冶金过程易形 成柱状晶的局面，形成了含有细小和粗大等轴晶以及少量柱状晶的混合晶粒组织。在低能量密 度下制备的晶粒含有大量的胞状结构,而高能量密度则导致胞状结构合并粗化，极大减少了胞状 结构的存在。在所有的样品中均观察到相的析出，在高能量密度条件下制备的样品还有纳米 相的析出且随能量密度增加相的体积分数持续增加。这种细小的相倾向于以网格状的形式存 在,具有明显的取向选择性及微观织构。由于微观结构的差异，这些样品显示出了不同的拉伸行 为。具有微细柱状晶的样品不仅具有高的强度,还有很好的塑性(延伸率13%),且以位错滑移和孪 晶形变方式协同促进塑性变形。存在网格状相且以粗大的柱状晶组织为主的样品则具有高的抗 拉强度,但没有屈服强度和塑性，且以解理断裂的方式失效。进一步的研究表明该类样品主要以 局域化的塑性变形为主要形变机制,即塑性变形被限制在相网格围成的基体内,大范围的塑性 形变被抑制。含有大量等轴晶的混杂微观组织具有高的强度，但塑性极其有限(延伸率1%),且 以延晶断裂模式失效。

关键词:激光冶金;钛合金;微观结构;柱晶-等轴晶转变;拉伸行为