Structuring Difference: The Additive Manufacture of Spatially & Functionally Differentiated Microstructures

Additive manufacturing is an enabling technology for achieving spatially-differentiated functionally-graded materials at a resolution that could not be realised by conventional methods. This work covers the development of an in-situ magnetically and microstructurally graded material, built by selective laser melting from a single composition of 17-4PH stainless steel. The high solidification rates of additive manufacture are exploited to drive very fine austenite grain and cell sizes, suppressing the thermally-driven martensitic transformation and stabilising austenite. But additively manufactured parts are known to suffer from thermal strain, and the low stacking fault energy of metastable retained austenite makes it susceptible to deformation-driven transformation. Regions engineered to have low thermal strain remain fully austenitic and demonstrate paramagnetic behaviour, while other regions, tailored to have higher thermal strain, partially transform to ferromagnetic martensite. The magnetic properties are proportional to the phase composition, and therefore to thermal strain, and are shown to be controllable through the build parameters and geometry. The applications for this type of magnetically graded material include electrical machine architectures, and a SLM-built in-situ magnetically graded rotor is demonstrated in a synchronous motor.