6 - Additive manufacturing: process and microstructure

Abstract

Fusion-based additive manufacturing (AM) technologies use layer-by-layer melting and solidification processes to fabricate the 3D metal and/or alloy components. This approach uses high-energy beams of either laser or electron to melt the raw materials in the form of either powders or thin wires, followed by the rapid solidification process. Although the AM has a significant advantage over the conventional manufacturing techniques, the reliability of AM components is a major challenge for applications. Much research has been focused on increasing the reliability of the AM process by developing a fundamental consolidation approach and the subsequent quality and microstructure evolution. The AM processing parameters largely control the quality and microstructure of the AM components. The principal processing parameters are energy density, scan speed, hatch distance, and layer thickness. This chapter highlights the fundamental understanding of AM and the principal parameters involved in AM. A discussion on how the principal parameter affects the porosity development and surface roughness in the AM component is presented and a potential approach to achieve a good quality in terms of porosity and surface roughness components. The chapter also discusses the microstructure evolution as a function of principal parameters and the underlying mechanisms responsible for microstructural evolution during fusion-based AM. An example of binary alloy, Ti–45Al (at.%) alloy is investigated through a computational effort. A phase-field method is presented to simulate the solidification microstructure at different locations within the melt pool during laser-based AM. The investigation is accomplished using a binary approximation, with a focus on site-specific microstructure evolution in the melt pool. It is found that site-specific microstructure variations along the build direction are more significant than that along the transverse direction since thermal variations are more pronounced at the build direction due to enhanced constitutional undercooling effects resulting from microsegregation. This chapter envisioned to provide a basic framework on AM process and the approach to achieve the desired quality and microstructure.