Development, metallurgical and tribological characterization of complex alloys and metal matrix composites obtained from manufacturing processes under non-equilibrium conditions

Additive manufacturing (AM) has experienced significant growth over the past 25 years in several areas, ranging from the conception of new machines to the number and complexity of parts produced.
Currently, a particularly interesting research field is the development of alloys and composite materials which are only accessible due to the particularities of AM processing. The improvement or modification of existing alloys by AM is a further step to open new possibilities for the industries. Such modification of the existing alloys can be achieved by combining available raw materials to adapt their compositions or create metal matrix composites. The fabrication of these materials can be carried out via powder-bed fusion processes (mostly known as selective laser melting) with prior mixing of the powders or via powder feeding techniques as laser cladding (LC) with powder mixing during the process. This is the context of the present PhD research, focused on understanding the underlying metallurgy of materials composed of 316L stainless steel and hard carbides fabricated by LC.
By considering three different reinforcements, a deeper understanding is gained of the interactions between ceramic powders and molten metal during LC. Indeed, depending on the nature of the reinforcement, partial or total dissolution of the particles occurs and the dissolved elements enrich the composition of the parent 316L matrix. This modification of the composition and the variation of the process conditions during deposition were considered in order to understand the resulting microstructures and mechanical properties.
In order to elucidate the new solidification routes of these complex materials, a novel methodology was first implemented on a complex graphitic high speed steel fabricated by centrifugal casting, and then its application was extended towards microstructures obtained after AM. This so-called “reverse analysis” is based on the backwards consideration of the differential thermal analysis heating curve and the features of the phases in the microstructures.
Macro-hardness, nano-hardness and wear resistance of the materials were evaluated to correlate the microstructures to the mechanical properties. Results highlight a remarkable improvement in all the considered mechanical properties compared to the parent 316L. Interestingly, a variation of these properties was observed, depending on the thickness of the coating, the volume percentage and the nature of the reinforcements. Pin-on-disc tests gave a good estimation of the improvement of the wear resistance and different wear behaviours. In particular, the novel approach of the interrupted tests is found efficient in order to elucidate both the wear mechanisms and the wear sequence.
In conclusion, this work highlights the possibility to tailor the microstructure of complex AM materials depending on the needs of the application.