Microstructure and mechanical properties as a function of process parameters for Ti6Al4V produced by high power laser powder bed fusion

Abstract

Laser-powder bed fusion (L-PBF) processing finds its application in various metal forming industries such as aerospace, automotive and medical industries. Ti6Al4V alloy is widely used in aerospace applications. The main interest of studies on additive manufacturing of Ti6Al4V is to investigate the material properties (strength, toughness and corrosion resistance) with regards to applications in the aerospace industry. The L-PBF process allows great flexibility with regards to process control and process design, and therefore control over microstructure and properties. The aim of the project was to study the effect of process parameters (laser power, scanning speed, hatch spacing, spot size and energy density) on Ti6Al4V microstructure and hardness of samples produced by L-PBF processing. The main objective was to analyse, and statistically predict part properties based on selected process parameters in order to enhance process understanding. The equipment that was used to manufacture the samples is a prototype powder bed fusion setup, with an Ytterbium laser system housed in a LENS (laser engineering net shaping) chamber. Experiments were carried out using a laser power of 1 to 3 kW, 2 to 4 m/s scanning speed, 0.10 to 0.24 mm hatch spacing, 250 to 450 μm spot size, and laser energy density of 33 to 200 J/mm3. Porosity analysis was conducted using the OHAUS Explore® balance precision weighing equipment. Optical microscope (OM) and EBSD analysis scanning electron microscope (SEM) was used to analyse microstructures of the samples. Porosity was found to be a function of laser power, scanning speed, hatch spacing and energy density. Linear regression relationships were developed to predict porosity of Ti6Al4V under the set of parameters used in the study. The lowest level of fraction porosity obtained from the built parts was 0.6% (2 kW laser power, 2 m/s scanning speed, 0.24 μm hatch spacing and 450 μm spot size). The amount of porosity varied with laser power. A higher laser power resulted in increased micro round porosity. A microstructure of acicular  martensite within columnar prior  grains was obtained for all energy density values used. Changes in process parameters used in the project scope were found to have a significant effect on the microstructure and not so much on the hardness range. However, through electron backscatter diffraction analysis a change in β content of (0.2 to 5.5%) was found with increasing energy densities, whilst content decreased with increasing energy densities. The hardness was between 326 and 418 HV (300 g).