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

本项目的主要目标是为无热裂纹的高强铝合金PBF-LB/M开发新的粉末。这一目标应通过设计NP接种粉末来实现。Np在快速凝固过程中起核心作用，在最终凝固过程中形成细晶组织和细小分布的残余熔体。除表面孕育外，粉末颗粒内的体积孕育更有效。我们认为，Np在粉末表面的低形核效率与铝合金粉末颗粒上最初分离出铝合金和Np的氧化层有关。相反，粉末颗粒体积内的Np本质上与熔体直接接触，因此将更有效地改善凝固形核。这一点已经在7075粉末中的TiC NP和AlN NP的初步工作中得到了证实。NP接种粉末的设计应以差示快速扫描量热法(DFSC)为指导。原位DFSC可以分析冷却速度在10^3~10^5 K/S范围内的单一粉末颗粒在铝合金PBF-LB/M过程中的快速凝固行为。在第一个资助期，我们成功地将DFSC的凝固起始温度与用于高强度铝合金的无裂纹PBF-LB/M组件进行了关联。有效形核可以与较低的过冷度、细小的显微组织、细小分布的最终凝固残余熔体相联系，从而避免热裂纹的产生。在第一个资助期，已经实现了固化开始的这种相关性。由于热裂纹主要发生在凝固过程中，这一取决于材料和冷却速度的特征温度也很有意义。因此，我们将应用一种新的等温DFSC方法，来分析凝固过程。我们将与几个合作伙伴合作，进一步完善我们提出的快速凝固行为与PBF-Lb/M之间的关联，以建立不同铝合金和不同Np核之间的关联。最后，我们将关注Np在PBF-Lb/M组件中的显著影响，这一点到目前为止被忽略了。虽然它们在凝固过程中的作用是有意的，至少是部分了解的，但NP也存在于凝固组织中，并对铝合金进一步冷却和随后的热处理过程中的固/固相变产生影响。在时效硬化铝合金中，NP会促进冷却过程中粗大析出物的过早形成，这会显著降低铸态强度，并进一步降低部件的时效潜力。应了解这一机制，并选择有利于凝固但不利于高强度铝合金时效硬化的NP。

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.