Process modelling & heat treatment simulations of Scalmalloy® fabricated by laser powder bed fusion (L-PBF)

Standard-Scalmalloy® (Al-4.5Mg-0.75Sc-0.35Zr-0.45Mn) produced via laser powder bed fusion (L-PBF) has proved to be a game changer due to its bimodal microstructure combined with nano Al3(Sc,Zr) precipitates giving high mechanical properties along with good ductility [1]. In addition to these exceptional static strength properties, D. Schimbäck et al [2] fabricated Scalmalloy® with high fatigue properties by controlling melt puddle dynamics. Despite these advantages (static & fatigue), the implementation of 3D-printed Scalmalloy® for aerospace application is challenged due to the time / cost associated with the laser powder bed fusion and post processing, for example : As-built : low built rates ~30cm3/hr, porosity due to keyhole melting, lack-of-fusion microstructure imperfections, evaporation of volatile elements like Magnesium. Post built - heat treatment (HT) at 325°C for 4hr or alternatively Argon medium based hot isostatic pressing (HIP) heat treatment (at 325°C for 4hr [3]) This lack of in-depth understanding of the root causes of low built rates, keyhole porosity formation is an impediment for designing Scalmalloy® engineering parts for safety-critical applications in aerospace, mainly fatigue driven. The approach proposed in this study aims to identify the high built rate conditions for Scalmalloy® by understanding the underlying melt pool dynamics using multiphysics simulations coupled with thermodynamics. First, the solidification path of the Scalmalloy® is calculated relying on the Scheil-Gulliver assumption taking into account the solute trapping that is representative of high cooling rates of L-PBF. Second, the intrinsic material properties like thermal conductivity, dynamic viscosity, surface tensions, evaporation enthalpy are computed using Thermocalc® as a function of temperature. Finally, a multi phase Volume of fluid (VoF) framework is proposed, taking inputs from thermodynamics, for computationally deriving correlations between process-induced melt pool physics and LPBF process parameters. The model links process parameters to temperature-time profile, solidification conditions and identifies conditions that cause lack-of-fusion defects, melt- and solidification-induced voids, as well keyhole porosity formation. Similarly to tackle the expensive and time consuming post processing, an innovative accelerated liquid-medium heat treatment (LHT) is also proposed. This is done via post process precipitation simulation that deals with identifying the HT parameters that try to predict how to maintain the Al3(Sc, Zr) precipitates density and size distribution while shortening the HT sequence tremendously by a factor > 10 (i.e. 15 min. instead of 240 min). This involves use of thermo-metallurgical precipitation model and strengthening model to estimate the corresponding increase in static mechanical properties post LHT. The outcomes of the LHT simulations can in principle be transferred (further developed) to other interesting Aluminum alloys fabricated by LPBF, thereby reducing the post fabrication efforts. References : A.B. Spierings, K. Dawson, T. Heeling, P.J. Uggowitzer, R. Schäublin, F. Palm, K. Wegener, Microstructural features of Sc- and Zr-modified Al-Mg alloys processed by selective laser melting, Mater. Des. 115 (2017) 52–63, D. Schimbäck, P. Mair, L. Kaserer, An improved process scan strategy to obtain high-performance fatigue properties for Scalmalloy, Materials & Design 224 (2022) 111410 Rosso, M. et al., Liquid hot isostatic pressing process to improve properties of thixoformed parts. Materials Science and Technology, ISSN 0267-0836, vol. 18(2), (2000), p. 16-20