TSINGHUA SCIENCE AND TECHNOLOGY IS SN l l 1 0 0 7 - 0 2 1 4 l l 1 5 / 2 1 l l p p 7 5 - 7 8 Volume 14, Number S2, December 2009

Fabrication of WC Matrix Composite Tool Material and Its Cutting Performance in Machining Titanium Alloys*

YANG Fazhan (���)1,2,**, MENG Guangyao (��)1, ZHAO Jun (� �)2, AI Xing (� �)2

1. School of Mechanical Engineering, Qingdao Technological University, Qingdao 266033, China;

2. School of Mechanical Engineering, Shandong University, Jinan 250061, China

Abstract: Titanium alloys are used widely for their high strength-to-weight ratio and good corrosion resis-tance characteristics. However, these alloys have been classified as ‘difficult-to-machine’ materials. In order to improve the cutting efficiency in machining the titanium alloys, a new WC matrix composite tool material was prepared by hot pressing technology in this research. The cutting performance of the newly developed tool material in machining titanium alloy Ti6Al4V was investigated. The factors which influence the wear modes of the tool were investigated and the wear mechanisms were analyzed. Results show that the high temperature and mechanical friction are the main factors in the wear process. The wear mechanisms of the tool in machining titanium alloys are adhesion wear and abrasive wear.

Key words: tool materials; cutting performance; Ti6Al4V; wear mechanisms

Introduction

Aerospace materials, such as titanium alloys (alpha- beta-based alloy Ti6Al4V) are attractive materials due to their unique combination of strength and lightness[1]. However, there are considerable problems in machin-ing process because of their poor machinability[2-7]. For example, low thermal conductivity, high hardness and strength at elevated temperatures may all aggravate the tool wear. Therefore, these characteristics cause high cutting temperature and resultant tool damage even at low cutting speeds.

Today, metal machining is still the major shaping process used in the production of engineering compo-nents. Even though new advanced manufacturing methods are being developed all the time, up to now, no serious challenger has emerged which could replace machining as the main shaping process within the next

decade. In cutting process, the tool materials play an important role to some extent in determining the ma-chining performance. At present, high speed steel and cemented carbide tool are used widely in machining titanium alloys. However, the use of high speed steel, for instance, is limited by the fast crater wear. And the cemented carbide tool is restricted by the diffusion and oxidation of the binding phase Co. It is a significant work in high speed machining fields to exploit a new tool material to enhance the titanium alloys machining efficiency.

In this paper, a new WC matrix tool material is de-veloped by hot-pressing technology and the cutting performance of the insert is investigated. The work piece material is titanium alloy Ti6Al4V. Mechanical properties and cutting performance of the tool material are analyzed. Wear mechanisms and failure mecha-nisms of the tools are investigated too.

1 Experiment 1.1 Fabrication of tool material and its

mechanical properties

The WC matrix composite used in this experiment is a

Received: 2009-05-08; revised: 2009-06-20

* Supported by the National Basic Research (973) Program of China(No. 2009CB724402) and the National Natural Science Foundationof China (No. 50575126)

** To whom correspondence should be addressed. E-mail: fazhany@163.com

Tsinghua Science and Technology, December 2009, 14(S2): 75-78 76

new tool materials with the following chemical com-position: 85.5-91.5 wt% WC, 8-14 wt% ZrO2, and 0.5 wt% VC. In the composite, VC is used as an inhibitor to suppress the grain growth. Table 1 lists the charac-teristics of the raw powders provided by the suppliers.

The tool materials with large additions of ZrO2 were fabricated by uniaxial hot-press sintering at sintering temperature 1630�, sintering pressure 32 MPa, and soaking time 25-35 min.

Table 1 Powders used to prepare WC matrix composite tool material

Powders Particle size ( m) Purity (%) Other elements Powder supplier WC 0.40 99.4 Si! 0.005% Xiamen, China ZrO2 0.04 97.7 Si! 0.100% Nanjing, China VC 2.00 98.5 Fe! 0.048% Changsha, China

The sintered materials are cut into two kinds of sam-ples: the dimension of first sample is 4 mm×5 mm× 30 mm (sample A), the other is 18 mm×18 mm×7 mm (sample B). Sample A is ground to standard sample (3 mm×4 mm×30 mm) to measure the mechanical properties and Sample B is ground to prepare the tool inserts (16 mm×16 mm×6 mm). The Vickers hardness, the bending strength, and the fracture toughness of the tool materials are 19.3 GPa, 856 MPa, and 7.9 MPa m1/2, respectively.

1.2 Machining tests

A series of turning tests were conducted using a CA6140 lathe. The cutting parameters were as follows: the feed rate f was 0.1 mm/r and the depth of cut ap

was 0.2 mm. The experiment was carried out without any cutting fluid. Tool material was WC matrix tool material. The variable that performed in the tests is shown as follows: cutting speed is 50 m/min, 65 m/min, and 80 m/min. The work-piece material used in the experiments was a bar of Ti6Al4V with a diameter of 90 mm and a length of 400 mm. The cutting tempera-ture was measured by the thermal infrared imager (TH5104R). Mechanical properties of the work-piece are shown in Table 2.

Wear photographs of the inserts were taken by scan-ning electron microscopy (SEM). Tool microscope JGX-1 optical microscope was used in order to meas-ure the mean flank wear values (VB) of each insert.

Table 2 Physical and chemical properties of Ti-6Al-4V[7]

Mat. Melting point (�) Thermal conductivity (W/(m K)) Hardness (HB) Density (g/cm3) Young modulus (N/mm2)Ti6Al4V 1670 7.1 350 4.43 115 000

2 Results and Discussions 2.1 Relationship between flank wear and cutting

distance

The relationship between flank wear and the cutting distance is shown in Fig. 1.

The flank wear value VB=0.3 mm is chosen as wear criterion according to international standard organiza-tion (ISO 3685)[8]. It can be seen from Fig. 1 that the flank wear is about 0.03 mm when cutting speed is 50 m/min and the cutting distance is about 190 m. Meanwhile, when the cutting speed is accelerated to 65 m/min, the flank wear is about 0.08 mm. The flank wear increases to 0.12 mm when the cutting speed is 80 m/min. It can be observed that in general, an increase of cutting speed leads to a higher wear rate.

Fig. 1 Relationship between flank wear and the cut-ting distance ( f = 0.1 mm/r, ap= 0.2 mm)

Additionally, it can be observed that when the cutting speed is higher than 65 m/min, the flank wear increased linearly with the cutting distance. At the

YANG Fazhan (���) et al.�Fabrication of WC Matrix Composite Tool Material and Its Cutting … 77

cutting distance 760 m, the tool (under the cutting speed 80 m/min) is worn out to excess (0.32 mm) and the other two inserts remained below the wear criterion.

This case can be attributed to the high temperature of the tool. When cutting speed increases, the cutting temperature increases quickly (It can be seen from Fig. 2 that the cutting temperature is almost up to 1100�). The high temperature may produce the inter-nal stress and expand the lacunae between the grains in the tool materials. This may worsen the grain bounda-ries strength in the tool materials. Meanwhile, high temperature can raise the chemical activity and accel-erate the adhesion wear between the tool and the workpiece material. It will be discussed in the follow-ing parts.

Fig. 2 Relationship between cutting temperature and cutting speed (f = 0.1 mm/r, ap= 0.2 mm)

As a conclusion, it can be assumed that the wear behavior of WC matrix composite tool material is re-lated to the cutting temperature tightly. Meanwhile, the cutting temperature is related nearly to the cutting speed. So, it can be deduced that in the same cutting distanced, a higher cutting speed leads to a higher wear rate.

2.2 Wear mode and wear mechanisms

To get a better insight into the wear of cutting tools at the edge of the wear tests for particular cutting edges, SEM micrographs are taken in this experiment.

Figures 3 and 4 show the morphologies of the worn surfaces of WC matrix composites tool material after machining 380 m at cutting speed of 50 m/min and 65 m/min, respectively.

Fig. 3 Wear mode of the tool (v = 50 m/min, L = 380 m)

Fig. 4 Wear mode of the tool (v = 65 m/min, L = 380 m)

It is not difficult to deduce the wear process of the composite from the SEM examinations. Due to the higher chemical affinity between Ti and Zr, a strong adhesion will occur when machining titanium. With the relative movement of WC matrix tool inserts and the new machined surfaces, the newly adhesion points will be torn off, resulting in the transfer of Ti on the surface of the inserts, as can be seen in Fig. 3 and Fig. 4. A powerful adhesion will be established between the chips and the flank face of the cutting edges. As a consequence of such an adhesion process, the wear process is accelerated.

In the SEM photograph (see Figs. 5 and 6) some ar-eas of heavy scars or scratches characteristic of the abrasion wear are evident. Furthermore, Ti element is detected on the surface of the insert. It is generally considered that the wear of the cutting tool is a result of mechanical (thermo-dynamic wear, due to loaded motions, i.e. abrasion, adhesion) and chemical (thermo-chemical wear, where elevated temperatures (see in Fig. 3) enhance the chemical processes, i.e. diffusion and oxidation) interactions between the tool

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Tsinghua Science and Technology, December 2009, 14(S2): 75-78 78

and work piece[9,10].

Fig. 5 Wear mode of the tool (v = 80 m/min, L = 760 m)

Fig. 6 Wear mode of the tool (v = 80 m/min, L = 950 m)

Additionally, the higher temperature may deteriorate the cutting condition and shorted the tool life. Micro cracks are detected around the tool tip shown in Fig. 6. However, the newly developed tool materials provide the possibility of raising the cutting speeds, which in-creases the cutting temperature significantly. Therefore, the appearance of the thermo-chemical wear becomes more evident, especially when the speeds increase into the high-speed-cutting range. It is believed that based on the present work, some interesting conclusions can be made. It is certain that during cutting the events happen only close to the surface of the tool-flank and rake face.

3 Conclusions

In this research, a new WC matrix composite tool ma-terial is developed and the inserts are prepared to ma-chine the titanium alloy Ti6Al4V. High speed cutting experiments have been carried out using the WC ma-trix composite tool. The wear mode and wear mecha-nisms are studied and some conclusions are deduced.

(1) Results show that the wear rate increases quickly with the increase of cutting speed (more than 65 m/min). It may be due to the coupling effect of thermal shock and mechanical stress.

(2) Due to the higher chemical affinity between Ti and Zr, a strong adhesion is occurred at the speed of 50 m/min and this behavior deteriorates the wear re-sistance of the tool and shortens the tool life.

(3) As the increase of cutting speed, the wear mode changes from initial adhesion to the mixture of adhe-sion, flank wear, and crater wear. The wear mecha-nisms are due to the nonuniform thermal-mechanical coupled intense stress field theory effect.

References

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