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).
