## 3 Machinability of AM Ti6 Al4V Alloy
### Titanium Alloy  
Titanium alloy, also known as Ti64, is extensively used for structural and engine components in the aerospace industry and for onshore and offshore equipment owing to its excellent corrosion resistance, high strength, and low density. Ti6 Al4V alloy is the most widely used alloy, accounting for over half of all titanium goods used worldwide. In the 1950s, Ti6 Al4V alloy was first fabricated for structural uses in the aircraft industry [12]. In addition to its widespread use in the aerospace industry, titanium and its alloys are utilized in various other applications, including surgical implants, heat exchangers, bioengineering, valve bodies, marine hardware, missile fuel tanks, compressors, and lightweight springs [42]. Titanium alloys can be classified into three classes based on their crystal structure, as follows [7; 18; 41; 43]:  
* Pure titanium, such as commercially pure Ti, has excellent corrosion resistance but low mechanical strength properties and good weldability. The addition of small amounts of Fe and O could increase its strength. * Alpha and near-alpha class (\(\alpha\)-phase), such as Ti3 Al2.5V, contain alpha stabilizers and possess excellent creep resistance. * Alpha-beta class (\(\alpha\)-\(\beta\)-phase), such as Ti6 Al4V and Ti5 Al2.5 Fe; this class at room temperature presents both 'alpha' and 'beta' phases. It is mainly used for mechanical applications requiring high strength and low density at raised temperatures. * Beta class (\(\beta\)-phase), such as Ti5 Al5 Mo5V3 Cr, contains a significant amount of beta stabilizers and is characterized by high density and hardenability. Moreover, \(\beta\)-phase has been known for its excellent biomedical properties and is used as an orthopedic (bone) implant. At room temperature, titanium alloy exists as a stable closed packed hexagonal (HCP) structure called the alpha phase; it changes to a stable body-centered cubic (BCC) structure called the beta phase at higher temperatures (880 \({}^{\circ}\)C). The \(\alpha\)-phase of titanium has an HCP structure with 12 primary slip systems. The \(\alpha\)-phase exhibits higher strength, hardness, and toughness than the \(\beta\)-phase, which contains many \(\beta\)-phase additions and is a BCC structure containing standard 48 slip systems. Vanadium, niobium, and tantalum are \(\beta\)-phase stabilizers, while aluminum and tin are \(\alpha\)-phase stabilizers [16; 44; 45; 46]. The cited papers overlook important aspects, such as the focus on emerging technologies like AM, which are increasingly significant for titanium-based materials. Additionally, they do not address the cost and sustainability of titanium materials, factors that are critical for assessing their economic feasibility and industrial adoption. Aluminum, vanadium, oxygen, hydrogen, zirconium, and palladium are the alloying elements of titanium. Aluminum and vanadium are the principal alloying elements. Aluminum offers corrosion resistance and strengthens solid solutions, while vanadium is a beta stabilizer that widens the temperature range to retain the beta properties. The corrosion resistance of the one-way scanning method was found to be superior to that of cross scanning. However, both methods demonstrated slightly lower corrosion resistance compared to wrought materials. This difference is likely due to variations in phase and element distribution, grain size, and crystal texture [47]. Table 3 shows the mechanical properties of different materials, with Ti6 Al4V alloy having low density coupled with high strength and good corrosion resistance. Reports indicate that the Ti6 Al4V alloy exhibits a corrosion rate of 5 \(\upmu\)m per year, which is considered negligible compared to that of aluminum, of 30 \(\upmu\)m per year [48; 49]. In machining operations involving challenging materials such as Ti6 Al4V alloy, substantial mechanical and thermal loads are generated, which may be detrimental to the performance and life of a cutting tool during the chip formation process [50].