Inoculation of Aluminium Powders for Additive Manufacturing guided by Differential Fast Scanning Calorimetry

我们项目的主要目标是开发用于无热裂纹的高强度铝合金PBF-LB/M的新粉末。这一目标应通过NP接种粉末的设计来实现。在快速凝固过程中，NP应起到晶核的作用，从而在最终凝固过程中形成具有细小分布的剩余熔体的细晶粒显微组织。除表面孕育外，粉末颗粒中的体积孕育更有效。我们假设，NP在粉末表面上的低成核效率与最初分离铝合金和NP的铝合金粉末颗粒上的氧化物层有关。相反，粉末颗粒体积内的NP固有地与熔体直接接触，因此将更有效地改善凝固成核。这一点已经在自己的初步工作中得到了证明，在7075粉末中使用了TiC NP和AlN NP。NP接种粉末的设计应通过差示快速扫描量热法（DFSC）进行指导。原位DFSC允许分析在铝合金的PBF-LB/M期间出现的冷却速率为10^3至10^5 K/s的单个粉末颗粒的快速凝固行为。在第一个资助期内，我们成功地将DFSC的凝固起始温度与高强度铝合金的无裂纹PBF-LB/M组件相关联。有效的形核与较低的过冷度、细小的晶粒组织、最终凝固时细小的残余熔体相联系，从而避免热裂纹的产生。在第一个供资期，这种相关性已经实现了凝固开始。由于热裂纹主要发生在凝固完成期间，因此取决于材料和冷却速率的该特征温度也是非常感兴趣的。因此，我们将采用一种新的等温DFSC方法，也分析凝固完成。我们将进一步完善我们提出的快速凝固行为与PBF-LB/M之间的相关性，与几个合作伙伴合作，建立不同的铝合金和不同的NP核的相关性。最后，我们将重点关注NP在PBF-LB/M组分中的重要影响，这是迄今为止被忽视的。尽管它们在凝固过程中的作用是预期的并且至少部分地被理解，但NP也存在于凝固的微观结构中，并且在铝合金的进一步冷却和随后的热处理过程中对固/固相转变产生影响。在时效硬化铝合金中，NP可促进冷却过程中粗沉淀物的过早形成，这显著降低了构件在构建条件下的强度以及进一步老化的可能性。应理解这一机制，并应选择NP，这对高强度铝合金的凝固有利，但对时效硬化无害。

The main objective of our project is to develop new powders for PBF-LB/M of high strength aluminium alloys without hot cracks. This objective shall be achieved by design of NP inoculated powders. NP shall act as nuclei during rapid solidification, resulting in a fine-grained microstructure with fine distributed remaining melt during final solidification. Besides surface inoculation, volume inoculation in powder particles is proposed to be more effective. We assume, that the low nucleation efficiency of NP on powder surfaces is associated with oxide layers on aluminium alloy powders particles which initially separate aluminium alloy and NP. Instead, NP inside the powder particle volume are inherently in direct contact with the melt and shall thus be more effective to improve the solidification nucleation. This has already been proven in preliminary own work with TiC NP and AlN NP in 7075 powder.Design of NP inoculated powders shall be guided by Differential Fast Scanning Calorimetry (DFSC). In-situ DFSC allows to analyse the rapid solidification behaviour of single powder particles with cooling rates of 10^3 to 10^5 K/s, which appear during PBF-LB/M of aluminium alloys. In the 1st funding period, we have successfully correlated solidification onset temperatures from DFSC with crack-free PBF-LB/M components for high strength aluminium alloys. Effective nuclei could be linked with lower undercooling, fine-grained microstructures, fine distributed remaining melt during final solidification and thus avoid hot cracks. In the 1st funding period, this correlation has been achieved for solidification onset. As hot cracking mainly occurs during solidification finish, this characteristic temperature, depending on material and cooling rate is of great interest too. Therefore, we will apply a new isothermal DFSC method, to analyse also solidification finish. We will further improve our proposed correlation between rapid solidification behaviour and PBF-LB/M in cooperation with several partners, to establish the correlation for different aluminium alloys and different NP nuclei.Finally, we will focus on a significant influence of NP in PBF-LB/M components, which has been neglected up to now. Whereas their role during solidification is intended and at least partially understood, NP are also present in the solidified microstructures and influence on solid/solid phase transformations during further cooling and subsequent heat treatment of aluminium alloys. In age hardening aluminium alloys, NP can promote premature formation of coarse precipitates during cooling, which significantly lowers strength in as-build condition as well as further ageing potential of components. This mechanism shall be understood and NP shall be selected, which are beneficial for solidification but not detrimental for age hardening of high-strength aluminium alloys.