Development of master alloys for improving the sintering response and mechanical properties of high performance steel components produced by powder metallurgy and additive manufacturing

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Final density of components produced with metal powders as base material is key to achieve mechanical properties approaching those of wrought alloys. Should higher densities become reachable in an industrial context, low-cost and more sustainable production processes such as powder metallurgy or binder-jetting additive manufacturing could capture a larger volume of high-performance metallic components. The main objective of this research is to develop master alloys to promote densification of green steel compacts and optimum distribution of alloying elements through an approach based on liquid phase sintering. Interestingly, the challenges related to this objective are common to conventional powder metallurgy (PM) and binder-jetting additive manufacturing processes. Our project aims at developing master alloys with a melting temperature lower than the typical sintering temperature used for PM steels. The liquid phase that will form during sintering will serve several purposes: 1) Reduce the chemical activity of elements with high affinity for oxygen making them less sensitive to oxidation; 2) Form a liquid phase (5-8vol-%) that will wet the iron particles favouring their rearrangement in a denser stacking; 3) Show significant solubility for Fe to improve mass transport and favour neck growth through a process of dissolution-reprecipitation; 4) Accelerate spatial distribution of alloying elements within the steel matrix through liquid phase sintering; 5) Round the residual pores making them less susceptible to act as stress concentrators, crack initiation/propagation sites, etc. The master alloys will be atomized using the water atomizer available at the Powder Metallurgy Laboratory of Université Laval (LAMPOUL). Conventional PM parts as well as AM binder-jetted specimens will be pressed/printed. Studies will be carried out to determine the influence of particle size distribution, sintering profile as well as sintering atmosphere in terms of composition and dew point. Work will also be performed to model and control dimensional change as a function of the variables identified above. Finally, tensile properties as well as fatigue resistance of the most promising master alloys will be quantified. The research summarized here is directly related to 2 of the 4 main topics identified by the Metal Powder Industries Federation in its 2017 PM Industry Roadmap, namely: 1) High-density PM components and 2) Metal AM. The work briefly summarized here will foster the development of cost-effective high-density components manufactured from materials with mechanical properties comparable to wrought. Moreover, these gains will be achieved using sustainable manufacturing processes that are recognized for reducing energy consumption and production costs.