Page 35 - MSAM-2-2

Materials Science in Additive Manufacturing Tensile and fatigue properties of Ti6Al4V SLM parts

1. Introduction (BJ), and directed energy deposition (DED). SL method
can be brazing for metal materials. It is unnecessary
1.1. Titanium and Ti-6Al-4V (Ti6Al4V) regions are cutoff layer by layer and removed after the SL
Ti6Al4V is the most frequently used titanium alloy in process is completed. BJ builds up the parts layer by layer
aviation and medical industries for its high strength, by selectively applying liquid bonding agents onto thin
resistance to corrosion, and low density . However, due layers of powdered materials. Metal parts are typically
[1]
to its thermal conductivity, Ti6Al4V is difficult to machine heat-treated in a furnace after the printing is done. Direct
using traditional methods as high cutting temperatures energy deposition uses a laser or electron beam as energy
[2]
lead to thermal softening and tool wear . source to melt powder or wire metal materials onto the
surfaces of the part. This is similar to automated build-up
Titanium alloy is popular because it has superior [13]
physical properties. It has a high strength-to-weight ratio, welding . SLM, which belongs to the category of PBF,
melts successive layers of metal powder with a laser beam
ideal for applications requiring light weight, and high of sufficient power to form a molten pool, which cools
strength, such as the aerospace industry. Commercially down quickly and solidifies for form the shape. After
available pure titanium has an ultimate tensile strength each layer is scanned and built, the building plate is
(UTS) of 434 MPa, which is comparable to low-grade steel recoated with a fresh layer of powder after being lowered
alloys and 45% lighter than steel . Hence, using titanium by a pre-set layer thickness. This process repeats, and the
[3]
alloys enhances components performance and fuel consolidated material forms the final metal product [14,15] .
efficiency of aircrafts . Therefore, a combination of the SLM technology is compatible with many materials, highly
[4]
above properties makes titanium alloy an ideal material efficient, and capable of producing complex end-products
for use in load-bearing orthopedic implants . Titanium of high precision [16,17] . Companies such as General Electric,
[5]
is highly biocompatible due to its surface texture and Airbus, Boeing, and Rolls-Royce are all at the forefront of
wettability. Examples of titanium alloy-based biomedical using more metal AM in their production line [18-20] .
implants include replacements for hip and knee joint,
surgical screws, cardiac valve prostheses, and artificial Despite the advantages of SLM, research has shown
hearts [6,7] . Titanium alloy is also used for aircraft and that surface quality and fatigue properties of SLM-
missile applications and biomedical applications because printed products are not comparable to those fabricated
[21]
they have low density yet can resist large variations of by traditional manufacturing methods . Process-related
temperature . However, conventional manufacturing defects, such as porosity and unmelted powders, lead to
[8]
[22]
processes of titanium are very costly and difficult in terms stress concentrations and mechanical inferiority . The
of machining. fatigue property of metal AM components, depending
on the material used, is typically lower than parts made
Ti6Al4V is the most popular titanium alloy, and it by conventional methods. For example, the high cycle
accounts for more than 50% of the titanium usage. Being fatigue (HCF) life of Ti6Al4V parts built by PBF is at 2
an alpha-beta titanium alloy, Ti6Al4V can be solution- million cycles . Surface roughness of parts built by PBF
[23]
treated and aged, which results in a significant increase in has impact on the HCF life. Meanwhile, heat treatment
strength. This treatment also optimizes properties, such as such as annealing also has major effect on the fracture
fracture toughness, fatigue strength, and high-temperature toughness and the threshold for fatigue crack initiation .
[24]
creep resistance. Ti6Al4V comprises 6% aluminum and However, there are only a limited number of studies done
4% vanadium by weight. Aluminum reduces density and on the fatigue strength of SLM. From their research, the
strengthens and stabilizes the alpha phase. Vanadium gives reported strengths of SLM-printed Ti6Al4V parts are
[9]
the material more ductile beta-phase for hot-working . significantly lower than the conventional buildings .
[25]
However, Ti6Al4V has some disadvantages. It exhibits This is an extremely glaring problem for SLM-printed
high coefficient of friction, experiences significant damage aerospace parts as the engine produces large amounts of
from adhesion and has a strong tendency to seizing and vibrations that cause metal fatigue. The four main printing
low tolerance for abrasion . process parameters of SLM are scanning speed, hatch
[10]
distance, laser power, and layer thickness . Two other
[26]
1.2. Selective laser melting (SLM) parameters to be considered are scanning strategy and
[26]
Additive manufacturing (AM) is an efficient process as it build orientation .
uses near net-shape fabrication, which reduces additional This study aims to investigate the relative density,
machining and conserves raw materials [11,12] . The four tensile, and fatigue properties of Ti6Al4V fabricated by
broad categories of metal AM technologies are powder SLM and to optimize the printing process parameters to
bed fusion (PBF), sheet lamination (SL), binder jetting reduce process-related defects.

Volume 2 Issue 2 (2023) 2 https://doi.org/10.36922/msam.0912