*3.4.1 Helix vector scan strategy*

Helix vector scan strategy is most suitable for producing complex parts and it reduces deformation caused by steep thermal gradients in the parts produced. A Voronoi diagram is used to build each layer and a tool path algorithm applies to the diagram and generates the recursive helix scan path for every layer (**Figure 1**).

### *3.4.2 Island scan strategy*

This is a strategy that tries to remove thermal residual stresses and this is achieved by putting separating exposed areas in a track into smaller sections called islands and this is usually 5 × 5 mm by default. The islands are then scanned in a random sequence with short scan tracks eliminating localized heating of the larger sections and subsequently reducing the thermal gradients and residual stresses (**Figure 2**).

### *3.4.3 Layer scan strategy*

Layer scan strategies comprise an orthogonal scan strategy and inter-layer stagger strategy. An orthogonal scan strategy is used to reduce porosity and stresses building up along the scan track by changing the direction of the scan after each layer is built. This is achieved when consecutive layers are scanned orthogonally to each other. The inter-layer or knitting strategy is used to repair defects observed in previously scanned layers through overlapping. The defects are corrected by melting all the powder in the overlapping zone causing a strong bond between the layers (**Figure 3**).

## *3.4.4 Vector scan strategies*

The vector scan strategy consists of the progressive and 'raster scan strategies. The 'raster scan strategy alternates the vector track after every scan. The laser scans

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**Figure 4.**

**Figure 2.**

**Figure 3.**

*Island strategy; adapted from [37].*

*Layer scan strategies; adapted from [38].*

*Vector scan strategies; adapted from [39, 40].*

from the beginning to the end of a vector before moving to the next vector beginning with the next vector at close range to the end of the previous vector. While the progressive scan strategy as the word progressive states is a scan strategy that does

not stop but continues from one vector to another (**Figure 4**).

*High Entropy Alloys for Aerospace Applications DOI: http://dx.doi.org/10.5772/intechopen.84982*

**Figure 1.** *Helix scan strategy; adapted from [36].*

*High Entropy Alloys for Aerospace Applications DOI: http://dx.doi.org/10.5772/intechopen.84982*

#### **Figure 2.** *Island strategy; adapted from [37].*

**Figure 3.** *Layer scan strategies; adapted from [38].*

### **Figure 4.**

*Vector scan strategies; adapted from [39, 40].*

from the beginning to the end of a vector before moving to the next vector beginning with the next vector at close range to the end of the previous vector. While the progressive scan strategy as the word progressive states is a scan strategy that does not stop but continues from one vector to another (**Figure 4**).

### **3.5 Benefits and limitations of laser additive high entropy alloys**

Aero engines comprise different parts and those parts are composed of several materials; aluminum alloys, steels, titanium alloys, nickel superalloys, ceramics, composites and intermetallics to name a few, however, most of these materials have limiting properties. High entropy alloys fabricated using laser additive manufacturing through research and development show promising properties; elevated temperature strength, oxidation resistance, favorable compressive yield strength, advantages over other materials used in the jet engines despite its challenges [41].

The high entropy alloy system Al*x*-Co-Cr-Cu-Fe-Ni fabricated at *x* = 0.5 exhibits high strength at elevated temperatures. The aluminum content in the high entropy system influences the crystal structure of the alloy. At reduced aluminum content, a ductile FCC phase will be formed which is resistant to changes at high temperatures and the strengthening is as a result of the solid solution phases. The wear resistance of the alloy acts independently with respect to its hardness [42]. As opposed to some alloys that the resistance to wear is directly proportioned to its hardness, this alloy system has been reported to having a high wear resistance despite its reduced hardness and this is attributed to the surface hardening of the ductile FCC phase. At low aluminum contents, delamination wear is observed while at high aluminum content, oxidation wear is observed.

Although this high entropy system shows variation in its corrosion properties from favorable to not favorable in both NaCl and H2SO4 solutions it has been reported to be susceptible to pitting corrosion in chloride environments, which is increased by anodizing in H2SO4.

The aluminum and chromium content in the high entropy alloy system has shown to improve the oxidation properties of the alloy. Aluminum achieves this by creating a protective aluminum oxide (Al2O3) layer on the surface while chromium also creates a protective chromium oxide (Cr2O3) layer on the surface [43].

The fatigue resistance of the alloy has been reported to be favorable between 540 and 950 MPa. However, there is a need to improve the fatigue resistance of the alloy as recent studies have shown that Al0.5CoCrCuFeNi high entropy alloy is sensitive to defects, such as micro-cracks, introduced using the conventional manufacturing techniques [44]. These manufacturing defects arise and contribute to a reduced fatigue life of the material and an increase in the cost of reproduction, therefore, the removal of these defects and an increase in the fatigue resistance of the material will cause improvements of the technology of production. SLM and LENS melting technique are versatile and achieve accuracy in geometry. SLM uses a powder bed and LENS uses a blown powder method by the laser beam. Formation of fine grains, non-equilibrium phases and new chemical compounds result in improved mechanical properties
