Abstract
Spatially resolved functional grading is a key differentiator for additive manufacturing, achieving a level of control that could not be realised by conventional methods. Here we use the rapid solidification and thermal strain associated with selective laser melting to create an in-situ microstructurally and magnetically graded single-composition material, exploiting the solid-state austenite-martensite phase transformation. The fine grain sizes resulting from high cooling rates suppress the thermal martensite start temperature, increasing the proportion of retained austenite. Then the thermal strain accrued during the build causes in-situ deformation-driven martensitic transformation. By controlling the thermal strain, through appropriate selection of build parameters and geometry, we have been able to control the final ratio of austenite to martensite. Fully austenitic regions are paramagnetic, while dual-phase regions show increasingly ferromagnetic behaviour with an increasing proportion of martensite. We exploit this to build a magnetically graded rotor which we run successfully in a synchronous motor.
Highlights
Thermal strain is normally regarded as an undesirable aspect of additively manufactured components, but, if it can be controlled through selection of build parameters, it may be possible to use this to control the extent of deformation-driven martensitic transformation and produce a microstructurally and magnetically graded single-composition material
Steels built by selective laser melting (SLM) are characterised by an austenite grain size of 10–100 μm, where the grains contain a forest of elongated solidification cells, 0.5–2 μm in diameter [7,28,29,30]
We found that the cuboidal sample manufactured with graduated 4 mm thick slices of the five conditions (Fig. 1a) successfully demonstrated an in-situ magnetically graded response, from magnetic at the high energy density (S1) end to non-magnetic at the low energy density (S5) end
Summary
This gives scope for building an in-situ magnetically graded material from a single alloy composition by selectively allowing or suppressing martensitic transformation. Thermal strain is a common issue with additively manufactured (AM) parts, due to the extreme thermal gradients caused by rapid melting and solidification [17] These can cause distortion of the final component [18], but could drive phase transformation in a susceptible alloy. Thermal strain is normally regarded as an undesirable aspect of additively manufactured components, but, if it can be controlled through selection of build parameters, it may be possible to use this to control the extent of deformation-driven martensitic transformation and produce a microstructurally and magnetically graded single-composition material. Using AM to functionally grade during build gives the opportunity to achieve magnetic variation from a single alloy composition with a high level of spatial resolution
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