Introduction
Manufacturing technologies are an
important aspect of industrial framework because through them,
fabrication of components for service industries such as; aerospace,
energy, nuclear and automobile is accomplished [1, 2]. These
technologies can be categorized into traditional subtractive and
formative, with newer technologies such as additive manufacturing [3,
4]. Additive manufacturing (AM) is a process of joining materials to
make parts from 3D model data, using layer upon layer [5] . This
process is currently a preferred alternative to conventional
manufacturing techniques owing to its many advantages such as, its
ability to manufacture complex parts using hard to machine materials
such as titanium alloys [6-8]. There are several AM techniques
classified by their energy source (arc, electron and laser beam), method
of feed material (powder bed, blown powder) and the feedstock (sheet,
wire or metal powder) [9, 10]. Laser additive manufacturing
facilitates the fabrication of three dimensional products with less
defects through the optimization of its process parameters such as laser
power, scanning speed, etc. [9, 10]. The process is flexible,
versatile and customized, allowing for the manufacturing of more complex
and intricate structures [2, 11]. However, this technique can be
susceptible to defects creating the need to optimize process parameters
and improve the overall quality and properties of AM products
[12-14].
Laser energy source of additive manufacturing has become an effective
direct energy deposition method used in producing geometries with small
heat affected zones [15, 16]. The direct energy deposition or powder
blown method is used for creating coatings or cladding on the surface of
a chosen substrate as well as build up components [17-19] . The
first set of materials used for systematic studies of laser additive
manufacturing (LAM) ranged from stainless steels, aluminum to nickel and
titanium alloys [20, 21], however, due to the shortcomings of these
materials; low strength at elevated temperatures, little or no corrosion
and wear resistance to name a few, there has been a need to use other
materials such as high entropy alloys. High entropy alloys (HEAs) are
materials that are composed of at least five metallic components with a
molar atomic concentration within a range of 5 to 35 at.% [22-24].
Recently, high entropy alloys are used and reported to be more durable,
ductile, and flexible than some titanium and steel alloys [23-25].
Their excellent strength at elevated temperatures makes them potential
materials for coatings in high temperature and corrosive environments,
such as refractory materials for aero engine parts [26-28].
Tong, Chen [29], synthesized AlxCoCrCuFeNi HEAs with
molar ratio (x=0-3.0) using casting and arc-melting method and the
results indicated that the alloy’s wear resistance was as high as that
of SKD 61 steel. This was attributed to the excellent work hardening
ability of the alloy. Kao, Chen [30] prepared the most studied
AlxCoCrFeNi HEA using vacuum arc melting with the aim of investigating
the magnetic, electrical and thermal properties of the amalgam. The
alloy showed a single FCC and a single BCC phase structures from X-ray
diffraction studies, and there were no stress-induced phase
transformations during deformation. The authors stated that the alloy’s
main strengthening mechanism is also work hardening and that the
hardening ability of the alloy was twice in the FCC than in the BCC
phase. Although , these alloys are usually cast, there have been reports
of oxides formation hindering the improvement of its mechanical
properties caused by the manufacturing route adopted [31].
In this case, alternative methods are required to fully take advantage
of the benefits of HEAs. For instance, Jiang, Han [32] investigated
the fabrication and characterization of AlCoCrFeNiNbx HEA coatings
produced by laser cladding. The study revealed that the microstructures
of the alloy transitioned from equiaxed grain to hypoeutectic, from
hypoeutectic to full eutectic and then to hypereutectic as the
composition of Nb varied from 0-0.75 at. %, attributed to the cocktail
effect of the alloys. Chao, Guo [33] produced AlxCoCrFeNi HEA by
laser coating and stated that clads were defect- free with isothermal
treatments at 1000 °C at optimized process parameters with minimal
dilution and an increase in the hardness values of the composition.
According to Hofman [34, 35] the three most important processing
parameters to ensure the quality of the alloys are the laser power, the
scanning speed and the powder feed-rate.
Linear energy has a significant impact on the microstructure and
properties of materials reported by Deng et al. [36].The
linear energy density input (\(E\)) in J/mm was calculated as an
alternative to determine the cooling rates and thermal history of the
whole process in a single value. E can be calculated by Eq.
(1) [4]:
\(E=\frac{p}{v}\) \(\left(1\right)\)
Where p the Laser power is in\(\ J/s\),\(v\) is the laser
scanning speed in\(\ mm/s\).
The energy density was used as a measure of the energy input during the
laser processing, which takes into account the relationship between the
scanning speed and laser power [37].
These process parameters have the greatest influence on the overall
quality and properties of the coatings. On the other hand,
serviceability of alloys manufactured using LAM hinge on their
microstructure and mechanical properties [6, 38, 39]. Therefore, the
optimization of the process parameters was essential to achieving the
desired properties for aerospace applications. This study investigates
the effect of the variation of laser power, scan speed and the concept
of energy density on the microstructure, melt pool geometry and hardness
properties of AlCoCrFeNiCu and AlTiCrFeCoNi HEAs created by LAM for
aerospace applications.