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

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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.