ABSTRACT

Additive manufacturing (AM) of titanium alloy entails severe microstructural heterogeneity and layer bands due to diverse thermal histories. While the thermal-microstructure relationship in AM has been reported, the details on how complex thermal histories influence the microstructural evolution have not been so addressed, and the formation of layer bands in multi-layer multi-pass builds is still unclear. To undertake such investigation, a thermal model is firstly calibrated using two part-scale blocks fabricated on differently sized substrates, and then used to study the relationship between key microstructural characteristics and the thermal cycling involved. Results show that the different evolutions of the temperature ranges just underneath the β-transus temperature (T_β) controlled by the printing path are responsible for the different band distributions at the centre and corner of the blocks. Also, the α sizes in the normal region are closely linked to the integral area obtained from the thermal curve as temperature fluctuates between T_β and α dissolution temperature, which helps linking AM variables to metallurgy. This further demonstrates that the α coarsening during thermal cycles is primarily driven by multi dissolution and precipation transformations instead of Ostwald ripening. Finally, the quantitative thermal-microstructure-microhardness relationship is established, helpful for the microstructural design.

摘要

由于不同的热历史，钛合金的增材制造 (AM) 需要严重的微观结构异质性和层带。虽然已经报道了 AM 中的热-显微结构关系，但关于复杂的热历史如何影响显微结构演变的细节尚未得到解决，多层多通道构建中层带的形成仍不清楚。为了进行这样的研究，首先使用在不同尺寸的基板上制造的两个零件比例块校准热模型，然后用于研究关键微观结构特征与所涉及的热循环之间的关系。结果表明，温度的不同演变范围刚好低于β- transus 温度 ( T _β) 由打印路径控制，负责块中心和角落的不同带分布。此外，当温度在T _β和α溶解温度之间波动时，正常区域中的 α 尺寸与从热曲线获得的积分面积密切相关，这有助于将 AM 变量与冶金联系起来。这进一步表明热循环过程中的α粗化主要是由多重溶解和沉淀转化而不是奥斯特瓦尔德熟化驱动的。最后，建立定量的热-显微组织-显微硬度关系，有助于显微组织设计。