Intrinsic Heat Treatment vs. Reduced Cooling Rate - Factors for Microstructure Formation in Laser Powder Bed Fusion of Ti-6Al-4V

Abstract

Laser Powder Bed Fusion (LPBF) of Ti-6Al-4V can be used to manufacture lightweight structures that are characterized by a load-specific material distribution based on bionic design principles. It is essential to know the maximum tolerable mechanical load that the material can withstand to prevent failure during operation. For this, the local microstructure and thus the associated mechanical properties in the additively manufactured component need to be known, but the microstructure in LPBF is strongly influenced by the thermal history that represents the thermal cycles of repeated heating and cooling in the layer-by-layer build up process. Since the thermal history largely depends on the component geometry, components with complex shapes can lead to inhomogeneous microstructures and therefore to inhomogeneous mechanical properties [1]. In non-overheating process conditions, LPBF of Ti-6Al-4V normally results in a microstructure dominated by the martensitic α’-phase [2]. Overheating or intrinsic heat treatment due to poor heat dissipation, e.g. due to geometric bottlenecks, can lead to a deviation from the standard LPBF as built Ti-6Al-4V microstructure, i.e. α + β microstructures form with a β phase fraction depending on the related thermal history [1,3]. This contribution will present results that contribute to the understanding of the formation of the β-phase as a function of the thermal history in LPBF of Ti-6Al-4V. Two driving forces for the local formation of the β phase were identified: First, reduced cooling rate is present when an overheated process below a point of interest exists, i.e. the cooling time between the fabrication of two layers are not sufficient to dissipate the heat. Secondly, intrinsic heat treatment occurs when a comparatively large amount of material is built within a short period of time above a point of interest, e.g. in terms of geometry, if there are rapidly increasing exposure areas in the build-up direction. Characteristic threshold values will be presented on the basis of thermal finite element simulations with the super layer approach, which allow an estimation of the β phase fraction depending on geometry. References [1] P. Barriobero-Vila Scripta Materialia, 2020, 182, 48-52. [2] L. Thijs Acta Materialia, 2019, 9, 3303-3312. [3] J. Munk Metals, 2022, 12, 482.