Microstructure and Mechanical Properties of Ti-6Al-4V Manufactured by Selective Laser Melting after Stress Relieving, Hot Isostatic Pressing Treatment, and Post-Heat Treatment

Abstract

Additive manufacturing (AM) is defined as a technology performed for tooling applications. It is used for manufacturing tools that have complex shapes and figures. In this study, an extensively applied Ti-6Al-4V alloy was made using the selective laser melting method. Post-production heat treatments were applied to decrease thermal stresses and to enhance the mechanical properties and the microstructure. The study investigates the fatigue mechanical properties, microstructure, hardness, and porosity of the AM Ti-6Al-4V after stress relieving (SR) and after SR followed by hot isostatic pressing (HIP). The samples’ upper and lower parts were independently examined to determine the effects of thermal conditions and the heat treatment of the microstructure. The microstructures were examined through optical microscopy, scanning electron microscopy and x-ray diffraction methods. The mechanical properties were investigated through microhardness testing, alongside assessment by fatigue testing at room temperature. The findings demonstrated that the microstructure after SR at 704 °C for 2 h is 100% fine martensitic α'-Ti, with a microhardness value of 408 HV. Air and furnace cooled samples have a more homogenous structure and are characterised by mixture (α + β) with microhardness values of 382 and 356 HV, respectively. After HIP at 920 °C and 100 MPa for 2 h was applied, the martensite was converted into a lamellar (α + β) microstructure, whereby the α phase is presented as fine needles situated among the β ridges in the microstructure, with the existence of the prior β grain boundary.

Investigation of Microstructure and Mechanical Properties for Ti-6Al-4V Alloy Parts Produced Using Non-Spherical Precursor Powder by Laser Powder Bed Fusion

“…The corresponding hardness characteristic of the HIP microstructures shown in Figures 9 and 10c,d averaged HRC 40.7; representing a reduction of~7% from the annealed components noted above. These results are similar to components fabricated from commercial, spherical Ti64 powder by LPBF where the HIPed (at 920 • C) microhardness dropped by~13% in contrast to as-built and stress-relief annealed (at 704 • C) components [18].…”

Powder Reuse in Laser-Based Powder Bed Fusion of Ti6Al4V—Changes in Mechanical Properties during a Powder Top-Up Regime

“…The reported Vickers hardness of annealed or additively manufactured Ti6Al4V after HIP heat treatment ranges between 320 HV and 380 HV [ 10 , 14 , 19 ]. However, the highest hardness found in the present work approached 400 HV, which was higher than this range.…”

“…The mechanical properties of titanium alloy components largely depend on their microstructure, defect levels, and chemical composition [ 9 ]. Ti6Al4V is an α + β titanium alloy, and due to the rapid cooling rate (over 410 °C/s) that occurs during the L-PBF process, its as-built components typically exhibit an acicular α’ martensitic microstructure [ 10 , 11 ]. This form of microstructure has been shown to be associated with high strength but low ductility [ 12 ].…”

Development of VUMAT and VUHARD Subroutines for Simulating the Dynamic Mechanical Properties of Additively Manufactured Parts

“…Thus, the DMLS process leads to the formation of meta-stable microstructures with non-equilibrium composition of resulting phases such as the acicular (α’) martensitic microstructure in the case of Ti6Al4V (ELI). This type of microstructure is characterised by high strength and low ductility [ 13 ]. However, improved ductility with reduction in strength is realised for DMLS Ti6Al4V (ELI) following successive heat treatments involving the transformation of the non-equilibrium α’-phase into an equilibrium mixture of (α + β) phases.…”

“…Numerous studies have applied one stage and two-stage heat treatment cycles at different temperatures on the as-built parts with the aim of optimising the mechanical properties of AM alloys to satisfy industrial requirements [ 10 , 11 , 13 , 14 , 15 ]. Different heat treatment cycles yield different mechanical properties of strength, ductility, and toughness, based on the resulting microstructure.…”