Geometry Effect on Microstructure and Mechanical Properties in Laser Powder Bed Fusion of Ti-6Al-4V

Abstract

Laser Powder Bed Fusion (LPBF) of Ti-6Al-4V enables the manufacturing of complex parts for lightweight applications. The emerging microstructure in the LPBF process and thus the mechanical properties are defined by the thermal cycles, which are locally variable for complex geometries. Predictions of local mechanical properties by simulation would reduce the development time of new applications drastically but are today not possible on part scale, so new part applications must be qualified experimentally at great effort. In this study, representative geometry sections were transferred into a simplified sample shape to mechanically characterize different geometry-dependent microstructures. In areas exposed to comparatively increased heat input over time, a lamellar α + β microstructure with β fraction up to 20% was measured in contrast to the common martensitic α′ microstructure of LPBF-manufactured Ti-6Al-4V, resulting in reduced tensile strength and fatigue life. For the first time, a correlation was successfully established between ultimate tensile strength of multiple geometries and the corresponding temperature–time cycles. With reduced computational effort by use of simplifying assumptions in the simulation, this correlation model can theoretically be applied to the part level. This work has laid the foundation for the simulation-based prediction of mechanical properties for entire parts manufactured with LPBF.

Geometrical Influence on Material Properties for Ti6Al4V Parts in Powder Bed Fusion

“…Additionally, Mohr et al showed an influence of the part geometry on heat flux leading to an in situ heat accumulation [16]. This effect was further proven by Munk et al [17], who linked the effect of different part geometries on the microstructure to the tensile strength. For a prediction of part defects dependent on geometrical features, Paulson et al [18] established machine-learning models that correlate thermal histories of single-track deposits to subsurface porosity formation.…”

“…Nonuniform geometries can result in varying heat flow through the part, thus leading to different thermal properties affecting the microstructure and part quality. Therefore, understanding the correlation between thermal history and material properties are the bases of recent investigations [15][16][17][18][19]. Both Williams et al [15] and Mohr et al [16] studied the effects of varying inter-layer cooling times during PBF-LB.…”

Selective Laser Melted Rare Earth Magnesium Alloy with High Corrosion Resistance

“…The main processing parameters were as follows: Laser scanning speed 600 mm/s, laser power 135 W, layer thickness 50 μm, and hatch spacing 50 μm. The previous study reported that the geometry significantly affects the part properties[ 32 - 34 ]. To eliminate the influence of shape on properties, all samples in this work were cubic shape.…”

Martensite Decomposition and Ultrafine Grain Formation during Small Punch Creep Testing of Additively Manufactured Ti64

“…Zones with less heat transfer showed higher β phase fraction. This amount varies from 0% to 20% within only 5 mm [5]. For these reasons, it seemed important to use methods to probe the mechanical properties of the material locally.…”