A Review on Machinability Aspects of Ti-6Al-4V: A Titanium Grade 5 Alloy

Ayanesh Y. Joshi and Anand Y. Joshi

#### Abstract

Ti-6Al-4V (Grade 5) is a very popular titanium alloy used in the automobile, biomedical, chemical and aerospace industries as it possesses properties like high strength to low weight ratio, capability to retain strength even at elevated temperatures, corrosion resistance and very good biocompatibility. However, Ti-6Al-4V possesses low machinability owing to its properties of low thermal conductivity, high hardness and high chemical reactivity at elevated temperatures, and low elasticity. This paper reviews the machining difficulties conferred by various researchers and their apparent solution, which can assist in reducing tool wear and achievement of a high surface finish. The impact of titanium properties on the machinability is also discussed in the paper.

Keywords: titanium alloy, Ti-6Al-4V, machinability, tool-wear, lubrication effect, chip formation

#### 1. Introduction

Titanium and titanium alloys find their extensive application in the field of aerospace as they have excellent specific strength (ratio of strength to weight) even at raised temperatures, in addition to their exceptional corrosion resistance and their fracture resistant characteristics. These characteristics have resulted in rapid growth of titanium industries in the last few decades [1]. Titanium is gaining popularity in commercial and industrial applications, such as chemical processing, surgical implantation, petroleum refining, nuclear waste storage and pollution control, food processing, electro-chemical and marine applications [2, 36]. However, titanium and titanium alloys are expensive because of the difficulty of melting, intricacy of the extraction-process, fabrication and machining complications [2, 3]. Basic formation methods like forging (isothermal), casting and powder metallurgy are followed to reduce the costs of titanium components [4, 5]. Longer service life and excellent properties counterbalance the high cost of production.

The machinability of titanium and titanium alloys is poor because of several properties of the materials. Siekmann pointed out that "Irrespective of the techniques that are employed to transform titanium into chip, it's machining would always be a problem." [6]. Chemically, titanium is very reactive and, hence, during machining it might get welded to the cutting tool and lead to chipping and early

A Review on Machinability Aspects of Ti-6Al-4V: A Titanium Grade 5 Alloy DOI: http://dx.doi.org/10.5772/intechopen.81083

failure of the tool. The tool/workpiece interface temperature will be high as it has low thermal conductivity, which affects the tool life adversely. Because of the high strength maintained at elevated temperatures and low elasticity, its machinability is further impaired [7]. Many large companies like Rolls-Royce and General Electrics have invested large sums of money in developing techniques to reduce machining cost because of poor machinability of these alloys.

Researchers have attempted various methods to improve machining of titanium alloy but cost is still a challenge to the titanium alloy part manufacturer and hence, research has to focus on new technologies and methods. This article emphasizes metallurgical aspects, chip formation, tool wear, lubrication during machining and surface integrity of Ti-6Al-4V. These factors are responsible for manufacturing challenges during working with titanium alloy. At the end, observations are summarized along with some recent techniques and future research scope.

#### 2. Metallurgical aspects of Ti-6Al-4V, titanium alloy–grade 5

Titanium is an element (atomic number 22; atomic weight 47.9 and symbol Ti) [8]. Pure titanium experiences a transformation (allotropic) at 882°C, changing from the α - phase (low-temperature CPH structure) to the β - phase (higher-temperature BCC structure). Alloying elements, such as oxygen (O) and aluminum (Al) cause an increase in the transformation temperature, whereas tin (Sn) dissolved in titanium, does not have such an effect, such elements are called α - stabilizers. Elements that decrease the phase-transformation temperature are called β - stabilizers. Significant β alloying additions are vanadium (V), molybdenum (Mo) and niobium (Nb). They are generally transition metals [9]. Elements are alloyed in titanium to stabilize the α phase or β - phase that modifies the transformation temperature and changes the shape as well as the extent of α + β field [10, 11]. Aluminum (Al) is a strengthening element at ambient and elevated temperatures up to 550°C. The density (low) of Al is a vital advantage. O, N and C are observed as impurities in commercial alloys. For strength and fabricability, O is used as a strengthening agent to provide various grades of commercially pure titanium.

The alloys are categorized into four main groups; (1) Unalloyed Titanium: Alloys have excellent corrosion resistance but low strength properties which can be improved by adding O and Fe. (2) α and near-α Alloys: These alloys contain α stabilizers and have an exceptional creep resistance property. These alloys have very minimum quantity β stabilizers, but behave more like conventional α alloys. (3) α + β Alloys: At room temperatures, these alloys offer a mixture of α and β stabilizers. They are largely used in aerospace industries and an example is Ti-6Al-4V. (4) β Alloys: Alloys have significant quantity of β stabilizers and a high density and high hardenability [39].


#### Table 1.

Chemical composition of Ti-6Al-4V.

In the late 1940s, the Ti-6Al-4V alloy was developed. Ti-6Al-4V is sometimes called TC4 and is a two phase α + β titanium alloy, with aluminum and vanadium as alpha and beta stabilizer respectively. The Ti-6Al-4V alloy is the most commonly and commercially used titanium alloy. Table 1 shows the chemical composition and Table 2 shows the physical properties of the alloy.

It is noted from the property table that the alloy has good Mechanical properties with respect to steel but has poor thermal properties. Ti alloys specific heat is higher than steels but its volumetric specific heat is less as the density is much lower. The strength is because of the alloying content but an increase in β content results in poor machinability.


#### Table 2.

Physical properties - Ti-6Al-4V.

#### 3. Machining of titanium alloy

Machinability of any material refers to the ease with which a material/metal can be cut (machined) permitting removal of material with satisfactory finish efficiently. The machinability index of a material is usually determined based on measures such as cutting force, chip formation, tool wear, cutting temperature, tool life, surface integrity and chip size.

Ti-6Al-4V is categorized as a difficult to machine material by traditional methods because of its physical, chemical and mechanical properties. Ti-6Al-4V has high hardness, low elastic modulus, low thermal conductivity, strength at elevated temperature and metallurgical characteristics that make it slightly more difficult to machine than equivalent hardness steels. The influence of titanium alloys properties on its machinability is summarized in Table 3 [13–16, 19].

Metallurgical properties of the material challenge the workpiece material in the region near the cutting edge and lead to a severe strain and numerous physical and metallurgical alterations [17]. Poor thermal conductivity of the alloys results in accretion of heat in the primary shearzone that results in localized shear and chip segmentation, leads to temperature increases and α - β phase transformation in the secondary deformation zone [18].

A Review on Machinability Aspects of Ti-6Al-4V: A Titanium Grade 5 Alloy DOI: http://dx.doi.org/10.5772/intechopen.81083


Table 3. Titanium properties and its effect on machinability.

#### 3.1 Chip formation

To minimize the negative effects of the chip produced while machining on cutting tool and surface of the workpiece, it's important to comprehend the cutting conditions as the chips produced during machining affects temperature of the cutting zone, machining forces, workpiece's surface integrity and tool life. The analyses of chips developed during machining of titanium alloys indicated that adiabatic shear (thermo-plastic instability) formation bands is the most studied feature [20, 21]. In titanium alloys, when the degree of thermal softening goes above the degree of strain hardening, adiabatic shear occurs [22]. While operating at low cutting speeds, the initiation and propagation the crack starts from the tip of the tool and spreads to the workpiece free surface and/or vice versa [23]. There will be noteworthy periodic deviation of machining forces because of localization of the shear, subsequently periodic variation of machining forces enforces fatigue to the tool or might result in chipping or breakage of the cutting tool and hence is not desired [24]. Experiments were carried out to form chips of Ti-6Al-4V at a high cutting speed, ranging from 30 to 6000 m/min and the results indicated that the segmentation structure changes at a cutting speed beyond 2000 m/min and there was no variation in specific cutting energy [25, 26].

In the case of Ti-6Al-4V alloys, segmented chips form at all speeds, but at high speeds it becomes continuous macroscopically and hence can be concluded that the microstructural state of the material strongly influences the chip formation mechanism. This is a major concern during machining of these alloys.

#### 3.2 Surface integrity

It is the nature of the surface of a workpiece after being modified by a manufacturing process that has a noteworthy impact on the product reliability, performance and durability. These modifications include metallurgical, chemical, mechanical and other changes. Though the changes are restricted to a thinner surface layer, it might have a limit on the component quality or render the surface unacceptable. So it's important to improve the product's quality. Surface integrity is a prime requirement as titanium is used for parts requiring the highest reliability. During machining, the surface of these alloys is easily impaired due to its poor machinability. Therefore, it is required to optimize process parameters for improved surface finish and tool life. The damage appears in the form of work hardening, formation of heat-affected zones, micro cracks, built-up edge, and tensile residual stresses [12, 37].

#### 3.3 Cutting fluids

Cutting fluids are used to reduce high temperatures generated in the machining zone during machining. Use of cutting fluid increases the tool life, improves surface conditions and increases process efficiency. M. Venkata Ramanaa et al. performed an experiment at low cutting speeds at different machining conditions such as dry machining, using servocut oil mixed with water and synthetic oil conditions to find out its effect on surface roughness. Experimental results indicated that for dry machining, higher cutting speeds resulted in higher surface roughness compared to servocut oil mixed with water and synthetic oil conditions. Results indicated that, compared to other conditions, with synthetic oil, the surface roughness is less [27, 28]. Researcher Ibrahim Deiab et al. explored the effect of various lubrication parameters on the surface roughness (Ra) [29].

At a higher feed rate, surface roughness is higher. For synthetic oil conditions, the surface roughness is low at lower feed rates, but at higher feed rates, surface roughness is high for servocut oil added with water conditions compared to dry and synthetic oil conditions.

Surface roughness is low for lower depth of cut for synthetic oil conditions compared to dry and servocut oil with added water conditions. Results also indicate that the value of surface roughness is high for a higher depth of cut for synthetic oil conditions compared to dry and servocut oil mixed with water conditions [28]. It is understood that at low feed rates, the surface roughness is less and it increases as the speed increases for the same feed rate. Conventional flood cooling mechanism is preferable when operating at low feed rates and cryogenic machining is to be used when operating at higher feed rates for better results.

Moaz H. Ali et al. modeled a finite element model to forecast the effect of varied feed rates on surface roughness for dry milling conditions. FEM can help us to reduce machining time and manufacturing costs at the same time. Accuracy of the experimented and predicted cutting force values was about 97%. Results also indicated that the cutting forces had an insignificant effect on surface roughness [29].

#### 3.4 Tool wear

The tool wear depends on tool and workpiece material, tool geometry, machining parameters, machine-tool characteristics and application of cutting fluids. The wear land is the cutting tool area near the cutting edge, which gets worn while machining [31]. The basic types of tool wear are flank wear and crater wear. Tool wear characteristics are signified as a relation between material wear and sliding

A Review on Machinability Aspects of Ti-6Al-4V: A Titanium Grade 5 Alloy DOI: http://dx.doi.org/10.5772/intechopen.81083

distance. Tool wear occurs in three different regions. Region 1 - Initial wear region: where wear rate is relatively high as it is depending on accelerated wear due to damage of tool layer while manufacturing, Region 2 - Steady-state region: where normal operation for the cutting tool occurs, Region 3 - Severe wear region: which ends with failure. In this region the cutting forces and temperatures are high accompanied by severe tool vibrations.

Crater wear occurs on the rake face of the tool and forms a crater. Severe pressure and temperature loads acting on the rake face causes diffusive wear of the chip material in cutting tool material on the rake face. Crater wear is temperature sensitive and depends on the tool material's solubility in the chip material. Flank wear is wear formed on the cutting tool's flanks. Abrasion of the clearance face against workpiece material causes flank wear. Li, Zhang and Wang investigated tool life and cutting forces in end milling operation of Inconel 718 using dry and MQL cutting conditions [30].

## 4. Techniques for enhancement of machining of Ti-6Al-4V

Various techniques to enhance machining efficiency of Ti-6Al-4V are summarized here. Most of the techniques make use of especial cooling or lubrication methods for temperature control and friction at the tool-workpiece interface.

