Control of solid-state phase transformations during the wire arc additive manufacturing process to reduce residual stress and deformation in deposits

The heating and cooling conditions during the wire arc additive manufacturing process (WAAM) generate complex thermal, metallurgical and mechanical phenomena that induce stresses and strains within the part. They are mainly due to incompatibilities between local deformations induced by thermal expansion gradients. In steels, certain phase transformations, between ferrite, austenite and martensite, can also interfere with the state of stress and strain. To reduce residual stresses and strains, we propose to control solid-state phase transformations, through local chemical composition, to drive metallurgical deformations that counter thermal distorsions. To that end, we study the various phase transformations arising from the martensitic, austenitic and ferritic stainless steels combinations. The microstructure of the 12 alloys produced in situ are analysed and related to the deformation and residual stress state, measured by neutron diffraction, to reveal their correlations. Phase evolution has been shown to have a significant influence on the stress establishment in the part, with up to 66% less strain depending on the transformations encountered. These results enable us to identify the chemical compositions for which we obtain the least deformation and stress, and therefore the microstructures to be favoured. Finally, this study paves the way for the development of functionally graded materials produced by WAAM for improved final properties.