2, IWT Stiftung Institut für Werkstofftechnik, Bremen, , Germany
Additive manufacturing (AM) opens new horizons for engineering by allowing for unprecedented geometries of parts, at simultaneously virtually no limitations regarding the scale of their production. Consequently the development of AM technology has evolved from providing relatively simple geometric prototypes out of polymers towards the production of functional, load bearing components made by Laser/powder based metallic printing. However, their mechanical properties are often not fully competitive with established production routes. One the one hand, this is caused by microstructural artefacts typical for powder metallurgical processes, especially porosity. More important though are phenomena linked to the rapidly solidified, as-cast microstructure of printed metallic parts, such as chemical inhomogeneity as well as undesirable phases and crystallographic texture. This is caused by the fact that the most commonly used metallic alloys – such as stainless steels, Ti or Ni based alloys – were originally designed and developed for a completely different production route and thermomechanical history, namely bulk casting followed by forging or rolling. This represents a severe limitation to the further proliferation of AM.
In this talk we present how these limitations can be overcome by alloy design strategies based on a systematic analysis of the phenomena occurring in AM. Two examples showcase how they allow for opening up pathways towards metallic materials with unprecedented property profiles. The first demonstrates how a new class of nano-structured steel – TiB2 composites can overcome the trade-off between the otherwise mutually exclusive properties strength, stiffness, ductility and density. Owing to the nano-sized dispersion of the TiB2 particles of extreme stiffness and low density – obtained by the in-situ formation with rapid solidification kinetics – the new material has the mechanical performance of advanced high strength steels, and a 25 % higher stiffness / density ratio than any of the currently used high strength steels, aluminium, magnesium and titanium alloys, meeting all key requirements for high performance and cost effective lightweight design. The second example shows how the typically unwanted reactions with oxygen and / or nitrogen during additive manufacturing can be exploited for obtaining novel generations of cost effective and lean high strength materials, especially for high temperature applications. Exemplified on stainless steel alloys, even without substantial optimization of process and materials, more than 2 vol.% of hard and stable chromium nitride particles with sizes down to 80 nm could be evenly dispersed, resulting in pronounced strengthening at both room temperature and 700 °C without significant loss in ductility. Future possibilities for broadening these innovative metallurgical design strategies to develop AM to its full potential are outlined and discussed.