Influence of the preheating on the microstructure evolution within thick deposits of AISI M4 high speed steel processed by Laser-DirectedEnergy Deposition, based on thermal modelling, hardness characterization, and dilatometry tests

The AISI HSS M4 alloy that contains many strong carbide forming elements is a medium carbon tool steel which is among the most ones that have been proceed under additive manufacturing. It has been well established that preheating is required to obtain sound and crack-free parts when both L-PBF and L-DED processes are considered. However, little is known about the evolution of the microstructure during the manufacturing process.
This work is based on the comparison of HHS M4 L-DED thick deposits cladded on a AISI 4140 steel substrate, which has been preheated following three different routes: a preliminary preheating within a furnace, a continuous induction preheating, and a preheating using laser beam. The microstructure of the deposits is characterized via macro and nano hardness profiles. Then a validated thermal model is used to reconstruct the thermal histories within the deposit during laser processing, and the final structure obtained in each deposit is linked to the temperature evolution. Based on adequate dilatometry tests, a discussion on the influence of step thermal gradients on solid phase transformations is carried out.