An experimental and numerical study on the face milling of Ti–6Al–4V alloy: Tool performance and surface integrity

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Abstract

This paper is concerned with the experimental and numerical study of face milling of Ti–6Al–4 V titanium alloy. Machining is carried out by uncoated carbide cutters in the presence of an abundant supply of coolant. Experimental analysis is conducted by focusing on the measurement of specific cutting energy, surface integrity and tool performance. The experimental analysis is supplemented by simulations from a 3D finite element model (FEM) of face milling simulation where needed. A tool wear model parameterized from FEM predictions of the tool–chip interface temperature, contact stress and chip velocity is presented. Tool wear patterns are described in terms of various cutting conditions and the influence of tool wear on surface integrity is investigated. Tool wear predictions based on the 3D FEM simulation show good agreement with experimental tool wear measurements. The highest cutting speed realized for the cutting tool material is 182.9 m/min (600 sfpm). Good surface integrity in terms of favorable residual stress and surface finish is achieved under the machining conditions used with limited tool wear. Residual stresses imparted to the machined surface are shown to be compressive.

Introduction

Among the different alloys of titanium, Ti–6Al–4V is by far the most popular one with its widespread use in the chemical, surgical, ship building and aerospace industry. The primary reason for wide applications of this α–β titanium alloy is its high strength-to-weight ratio that can be maintained at elevated temperatures and excellent corrosion and fracture resistance. However, Ti–6Al–4V is notorious for poor machinability due to its low thermal conductivity that causes high temperature on the tool face and strong chemical affinity with most tool materials, thereby leading to premature tool failure. Furthermore its inhomogeneous deformation by catastrophic shear makes the cutting force fluctuate and aggravates tool-wear and chatter. This poor machinability has limited cutting speed to less than 60 m/min in industrial practice as described by Komanduri and Von-Turkovich (1981) and Chandler (1989).

Therefore, over the years numerous research efforts have been made to improve the machinability of Ti–6Al–4V by investigating tool-wear and related issues to assist in choosing suitable machining conditions. Komanduri and Von-Turkovich (1981) studied the chip formation mechanism when machining Ti–6Al–4V and reached a conclusion that prolonged contact between tool face and chip underside during the upsetting stage under high temperature conditions caused rapid tool wear due to Ti–6Al–4V's chemical affinity with most tool materials. This observation was later successfully utilized by Komanduri and Reed (1983) in designing a new cutting tool holder that yields high clearance and negative rake angles, which resulted in an improvement in tool-life. They also discovered the non-suitability of ceramics for machining Ti–6Al–4V. In contrast, Narutaki and Murakoshi (1983) discussed the feasibility of using natural diamond as a cutting tool material for machining Ti–6Al–4V with an abundant coolant supply, and also described the associated surface integrity. The effectiveness of using diamond and carbide cutters has also been described by Hartung and Kramer (1982), who demonstrated the formation of a wear resistant titanium carbide reaction layer on these tool materials during machining of Ti–6Al–4V. Later, Kramer and Chin (1993) used the observations made in Hartung and Kramer (1982) to evaluate the potential of various rare-earth metal compounds as tool materials for machining titanium alloys.

All the work mentioned in the previous paragraph used low to moderate cutting speeds with most of the tests performed under orthogonal conditions. Industrial applications are, however, mostly three-dimensional and require higher speeds whenever possible to improve productivity. Due to the low thermal conductivity of titanium alloys, controlling the heat during machining is vital in successful machining of titanium. The interrupted nature of the milling process does not allow build-up of heat energy generated during cutting, which can be beneficial from a tool-wear point of view. Several researchers have worked along this line of thought. Kitagawa et al. (1997) used a high cutting speed of 628 m/min in end milling of Ti–6Al–4V with and without a coolant. This work stressed the benefits of improving tool-life by using intermittent cutting along with a coolant. Kuljanic et al. (1998) studied the possibility of end milling Ti–6Al–4V compressor blades with a polycrystalline diamond (PCD) cutter in the presence of coolant. The authors arrived at an economical cutting speed of 110 m/min with a tool life of 381 min. Recently, Barnett-Ritcey et al. (2001) investigated the use of a special coolant delivery system in high speed milling of Ti–6Al–4V with polycrystalline diamond, coated carbide and carbide cutters. Most of their tests were concerned with end milling in the speed range of 152–610 m/min. The authors presented the effect of coolant pressure and coolant aiming location on tool-life. Ginting and Nouari, 2006, Ginting and Nouari, 2007 investigated ball end milling of Ti–6242S and concluded that in the cutting speed range of 60–160 m/min carbide tooling is the most suitable. Other studies on ball end milling (Che Haron et al., 2007) and end-milling (Lopez de Lacalle et al., 2000) with uncoated and coated carbide tools concluded that in the cutting speed range of 40–125 m/min the coated carbide tool out-performed uncoated carbide in terms of maximizing the tool life. In studies conducted on alternative means of improving the tool life, Su et al. (2006) investigated the performance of compressed cold nitrogen gas and oil mist cooling in end milling of Ti–6Al–4V with tool life almost 2.7 times that of dry cutting and 1.9 times that of a coolant system with only nitrogen-oil-mist at cutting speeds of 400 m/min. Hong et al. (1998) demonstrated the capability of cryogenic cooling in milling titanium showing promising results.

Surface integrity, which includes surface roughness, microstructure and residual stress after machining, is an important aspect of successful titanium machining. Good surface integrity is especially important in various engineering applications requiring high reliability and resistance to failure. It is generally understood that the feed and tool nose radius play the most significant role in surface finish. In end-milling of titanium, Nurul Amin et al. (2007) showed the dependence of surface roughness on the tool material. The surface roughness increased only marginally with an increase in tool wear for uncoated carbides, while this effect was pronounced when PCD inserts were used due to higher chatter observed at high cutting speeds. Contradictory results were reported by Elmagrabi et al. (2008), who conducted slot milling experiments of titanium. The authors concluded that for both uncoated and coated carbide tools the surface roughness was highly dependent on the feed while tool wear did not seem to affect the surface roughness. It has been shown that in milling of titanium compressive residual stresses are generally observed as demonstrated by Mantle and Aspinwall (2001) and Sun and Guo (2009). Mantle and Aspinwall (2001) concluded that the resultant compressive residual stress was dependent on flank wear and cutting speed. Increased tool wear resulted in marginally higher compressive values of residual stress, while increased cutting speed reduced the compressive residual stress. The contradictory effect of tool wear on surface roughness and the lack of experimental data quantifying the residual stress as a function of the tool wear suggests further research is needed to clarify this issue.

Compared to empirical or analytical methods, the advent of computers has allowed researchers to study machining through sophisticated numerical techniques. In numerical studies the primary focus has been on predicting the chip formation process and cutting forces during turning of titanium. Umbrello (2008), Bäker et al. (2002), Bäker (2006), Özel et al. (2010a) and Calamaz et al. (2008) all have conducted 2D orthogonal FEM studies on turning of Ti–6Al–4V, focusing on prediction of cutting force and chip formation. Amongst them, Bäker et al. (2002), Özel et al. (2010a) and Calamaz et al. (2008) focused on development of new material models, while Hua and Shivpuri (2005) developed a tool wear prediction model. For milling, Ginting and Nouari (2006) conducted a numerical study of dry milling in 2D approximation by qualitatively comparing the predicted chip shape with experimental results for varying cutting speeds and predicting the temperature using the machining simulation software AdvantEdge™ (Third Wave Systems Inc, 2008). Recently, 3D FEM simulations have gained popularity in modeling machining processes such as turning, milling, drilling, etc. Li and Shih (2006) simulated 3D turning of Ti–6Al–4V using AdvantEdge™, focusing on the prediction of cutting forces, temperature and the curling of the chip. Recently, Dandekar et al. (2010) conducted 3D FEM turning simulations of Ti–6Al–4V to successfully predict the crater tool wear rate and the cutting forces. In another study on turning, Özel et al. (2010b) carried out 3D FEM simulations to predict the cutting forces, chip morphology, temperatures and tool wear. Klocke et al. (2002) addressed some of the challenges involved in selecting the right cutting parameters through machining modeling for practical applications. The authors suggested the utilization of the modeling outputs of stresses, relative velocities, temperature and strains to arrive at the possible tool wear rate.

The majority of the work on 3D FEM of machining titanium is focused primarily on turning and not on milling. Additionally, most of the milling work on Ti–6Al–4V has focused on end-milling, while little has been reported on other popular milling processes such as face milling. Past studies have focused on tool wear and machinability issues with little reported on surface integrity resulting from face milling of titanium alloys. The objective of this research is to study face milling of Ti–6Al–4V by uncoated carbide cutters via experiments and numerical modeling. The investigation is carried out in terms of measurement of the specific cutting energy and mechanics, surface integrity and tool wear. The experimental results are supplemented by numerical simulation results based on a 3D FEM face milling model capable of predicting the specific cutting energy, tool–chip contact length, stress and temperature distributions and are used in the parameterization of a tool wear model. Tool wear is predicted based on 3D FEM face milling simulations, which has not been reported. Prediction of tool wear benefits the optimization of cutting parameters and also assists in designing better tooling and cooling systems thereby improving the machinability of titanium.

Section snippets

Experimental equipment

Face milling tests were performed using a Mazak VQC-15/40 milling center with the maximum speed of 5000 rpm. These tests were conducted on 50 mm × 50 mm square blocks of Ti–6Al–4V titanium alloy whose properties are listed in Table 1. The hardness values listed in this table were obtained from a Mitutoyo hardness tester (model ATK F1000). A standard face-milling cutter (Kennametal KDPR4SP430MB, lead of 30°, axial rake of 5° and radial rake of 2°) was used in the parametric study with uncoated

3D FEM machining simulations

The machining simulation software AdvantEdge™ was used in modeling of face milling using a single insert cutter. The 3D face-milling module with an indexable tool and focused cooling option was used in the simulations, as this best describes the experimental process (Fig. 2(a)). The updated-Lagrangian finite element method along with continuous remeshing and adaptive meshing techniques was applied in the model. 4-node, 12 degree-of-freedom tetrahedral finite elements were used to model the

Experimental matrix

In literature, use of positive rake angles for machining titanium alloys has been suggested by earlier investigators (Barnett-Ritcey et al., 2001). Accordingly preliminary milling tests were conducted using a positive rake angle cutter together with optimal cutter entry and exit angles (Fig. 1) representing climb cutting. These tests allowed for machining to be carried out with increased cutting speeds showing minimal tool wear. Therefore, based on the results of the preliminary investigation,

Mechanics of face milling of Ti–6Al–4V

The cutting forces measured during the cutting tests were transformed to the coordinate system rotating locally with the insert as shown by Shin and Waters (1997) and Jensen and Shin (1999). The specific cutting energy obtained from the tangential cutting force is plotted in Fig. 4 as a function of cutting speed. Specific cutting energy was utilized as a metric to demonstrate its constant behavior over the range of cutting speeds studied. Specific cutting energy is a function of the cutting

Conclusions

An experimental and numerical study of the face-milling of Ti–6Al–4V titanium alloy with uncoated carbide tool materials was undertaken. The following conclusions can be drawn from this research:

  • 1.

    There was very little variation in the specific cutting energy and hence the cutting force and friction coefficient within the range of cutting speeds studied in this research. In addition, very little variation in the shear stresses and temperatures in the primary shear zone has been observed from 3D

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