Computational Assessment of Thermokinetics and Associated Microstructural Evolution in Laser Powder Bed Fusion Manufacturing of Ti6Al4V Alloy

Abstract

Although most of the near non-equilibrium microstructures of alloys produced by laser powder bed fusion (LPBF) additive manufacturing (AM) are being reported at a rapid rate, the accountable thermokinetics of the entire process have rarely been studied. In order to exploit the versatility of this AM process for the desired properties of built material, it is crucial to understand the thermokinetics associated with the process. In light of this, a three-dimensional thermokinetic model based on the finite element method was developed to correlate with the microstructure evolved in additively manufactured Ti6Al4V alloy. The computational model yielded the thermal patterns experienced at given location while building a single layer through multiple laser scans and a whole part through multiple layers above it. X-ray analysis of the resultant microstructure confirmed the presence of acicular martensitic (α′) phase of (002) texture within the build-plane. Computationally predicted magnitude of the thermal gradients within the additively manufactured Ti6Al4V alloy in different directions (X, Y, and Z) facilitated the understanding about the evolution of grain morphology and orientation of acicular martensite in prior β grains. The scanning electron microscopy observations of the alloy revealed the distinct morphology of phase precipitated within the martensitic phase, whose existence was, in turn, understood through predicted thermal history.

Crystallographic texture dependent bulk anisotropic elastic response of additively manufactured Ti6Al4V

“…The preliminary results on phase evolution in LPBF-AM Ti6Al4V examined using X-ray diffraction, transmission electron microscopy, and atom probe tomography have been reported in previous publications by the authors 3,4 . These findings indicated the predominant presence of hierarchical hcp α ′ martensite laths under rapid cooling rates (2.56 × 10 7 -4.82 × 10 4 K/s).…”

“…The center to center distance (hatch spacing) between the adjacent laser tracks was 120 µ m. Combination of these laser processing parameters generated laser energy density of 52.08 J/mm 3 on the sample surface. Further details about the LPBF-AM process can be located in previous publication by the authors 3,4 . In order to conduct post LPBF-AM process, NDT and microstructural characterization/analysis, the AM printed Ti6Al4V cubical block was separated from the circular build plate using electrode discharge machining.…”

“…To get insights into microstructure and crystallographic texture and interpret the data from ultrasonic measurements, the LPBF-AM samples were sectioned in XZ plane using an oil cooled slow speed diamond laced wheel saw. The sectioned samples were placed in epoxy mounts and prepared using protocol described in previous paper by the authors 3 . The microstructure was observed in FEI Nova NanoSEM 230 equipped with a Hikari super electron backscatter diffraction (EBSD) detector operated at 20 kV and spot size of 6.…”

“…Although various additive manufacturing (AM) techniques associated with distinct physical phenomena have emerged, the laser powder bed fusion (LPBF) based AM is widely explored as it offers significant control over the geometric tolerance of the printed component [1][2][3][4] . Additionally, in LPBF-AM, a broad spectrum of process parameters allows monitoring the set of properties of the printed part [5][6][7] .…”

“…Additionally, in LPBF-AM, a broad spectrum of process parameters allows monitoring the set of properties of the printed part [5][6][7] . LPBF-AM, by its principle nature, yields smaller melt pool dimensions ( ∼ 30-100 µ m in depth and ∼ 80-400 µ m in width) in a significantly large volume of powder, which in turn leads to a rapid cooling rates (10 5 -10 7 K/s) and steeper thermal gradients (10 5 -10 7 K/m) 3,7 . Such rapid thermokinetics often result in microstructural anisotropy (texture) and heterogeneity, impacting the macroscopic physical properties 8 .…”