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. can be calculated by Eq. (1) [4]:
\(E=\frac{p}{v}\) \(\left(1\right)\)
Where 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.