Optimization of machinability parameters of aluminium/alumina metal matrix composites.

Yuvaraja, C. ; Sharma, K.V.

Introduction

Applications of composite materials are among the most important developments in material engineering in recent years. MMCs have emerged as an important class of materials and are increasingly utilized in various engineering applications, such as aerospace, marine, automobile and turbine-compressor engineering, which require materials offering a combination of light weight with considerably accelerated mechanical and physical properties such as strength, toughness, stiffness and resistance to high temperature. However, the applications of MMCs are limited by their poor machinability which is a result of their highly abrasive nature that causes excessive tool wear in cutting with tungsten carbide tools and even with diamond tools. The tremendous engineering potentials of MMCs will always be undermined by their high machining costs unless the economical and efficient machining mechanism for MMCs is developed.

Previous investigations by Tomac and Tonnessen [1] on the machinability of the MMCs with respect to tool wear has revealed that the cutting speed while machining Al-alumina MMCs with polycrystalline diamond (PCD) and coated tungsten carbide tools reduces the tool life due to excessive flank wear. Further their investigation also reveals the high feed rate reduces tool wear due to the improvement in the conduction of heat from the cutting zone onto the workpiece that causes softening of the metallic matrix enabling alumina particle to be embedded into the machined workpiece. Work done by Yuan et al. [2] shows that when a PCD cutting tool is used, the cutting depth has no significant effect on the surface roughness of the machined workpiece. However, it was reported by Lane [3] that the tool life of the PCD cutting tool was found to be inversely proportional to radial depth of cut. Weinert [4] showed that instead of improving the tool life, the application of water and oil as lubricants generated more tool-wear for both PCD and coated carbide cutting tools.

Lane [5] reported that a 50% drop in volume percentage of the particulate reinforcement in MMCs can improve the tool life by 21% and a 27% decrease in the particulate reinforcement's diameter can improve tool life by 500%. This shows that the size of the reinforcing particles has a much stronger effect than the population of reinforcing particles. As particulate volume percentage rises, more cracks and scales will be produced on the machined surface. This is due to the culmination of higher stress concentration on the cutting layer during cutting as well as the occurrence of non-homogeneous plastic deformation. Further the selection of proper heat treatment condition for the MMCs workpiece, could improve the tool life 100 and 350%.

The wear process at the rake face and the clearance face of a cutting tool can be classified as sliding wear. The reasons for the occurrence of tool wear while machining MMCs is the direct contact between the particles and the cutting edge. Therefore, the hardness of the reinforcement should be the dominant factors for tool wear. The study of the topography of the worn cutting edges led to the conclusion that the main tool wear mechanism is due to abrasion. Fang [6] observed that the tool wear produced while machining Al-alumina MMCs was very symmetrical at the clearance face. The characteristics of the tool wear were deep grooves, which were mostly parallel to the cutting direction. Further he attributed the creation of these grooves is due to the phenomenon of a cutting tool coming in contact with hard and brittle particles which tends to move the particles rather than cutting or breaking them.

The present study intends to enhance the understanding of the machinability of MMCs by investigating the details of the tool wear mechanism while cutting the MMCs, in particular the effects of the percentage reinforcement in the MMCs on tool wear. For such a purpose, an experimental study of cutting of Al-alumina MMCs using tungsten carbide tools is conducted. The result shows that the main mechanism of tool wear while cutting the MMCs is due to two-body abrasion and three-body abrasion. The abrasive wear is accelerated when the percentage of reinforcement in the MMCs exceeds a critical value. The wear acceleration varies with the densities of the reinforcement and the matrix. The size of the reinforcement particles and the tool geometry was developed and verified by comparison with experimental results.

Experimental Studies

Composite preparation

In the present study, liquid metallurgy technique is adopted to prepare the composites. Al-6061, which exhibits excellent casting properties and reasonable strength, is selected as the base alloy. This alloy is suitable for mass production of lightweight castings and can be either sand cast or die cast.

A vortex method is adopted to prepare the composite specimens. Alumina particles of size 30-50 [micro]m varying from 5 to 15 wt% are preheated and introduced into the vortex created in the molten alloy in vacuum. The vortex is created using an aluminite coated mechanical impeller. The coating of aluminite is necessary in order to prevent the migration of ferrous ions from the stirrer into the matrix alloy melt. The melt is thoroughly stirred, degassed subsequently and poured into a preheated split-type permanent mould. The samples of length 178 mm and diameter 22 mm were developed for experimental work.

Machinability test

All the cutting tests were performed on a HMT CNC Lathe. In the cutting tests for tool wear mechanisms, coated carbide is inserted in a tool holder of 6[degrees] rake angle and 95[degrees] approach angle was used. In the cutting tests for correlation between tool wear and percentage of alumina in the MMCs, coated carbide is inserted in a tool holder of 8[degrees] rake angle, 45[degrees] approach angle and 0[degrees] inclination angle were used. The tool wear measurements were performed on an olympus measuring microscope with a resolution of 0.0001 mm.

Results

Tool abrasive wear

A microscopic analysis to verify the presence of abrasive wear on the tool in cutting of Al-alumina MMCs is conducted. The result show two-body abrasive wear and three-body abrasive wear. Two-body abrasive wear is caused by rubbing of a softer surface by a hard rough surface while three-body abrasive wear is caused by hard particles entrapped between two sliding surfaces.

The presence of grooves, which were parallel to the cutting in Figure 1, indicates that two-body abrasion is the mechanism dominating the tool wear. As reported by Akbulut et al. [7], the characteristics of three-body abrasion is that it can create either scratches at low wear rate or craters at high wear rate. The scratches and craters found on the flank wear surface confirm the occurrence of three-body abrasion.

[FIGURE 1 OMITTED]

Tool Wear Acceleration

Cutting tests were performed to determine the effect of the Al-alumina MMCs alumina weight percentage on tool wear. Six Al-alumina MMCs workpiece, each with 5, 10 and 15 alumina weight percentage are turned with equal material removal at a cutting speed 65m/min, feed rate 0.1 mm/rev and depth of cut 0.5mm. In each of the cutting tests, the tool wear is measured after cutting off equal amount of workpiece material, therefore, the measured wear values actually represent the tool wear rates in the cutting processes. The tool wear values for equal workpiece material removal are then plotted against the alumina weight percentage of the workpiece. Keeping the other machining parameters unchanged, the turning tests are repeated at higher and lower cutting speeds of 88.61 and 24.26m/min, respectively, as well as higher and lower feed rates of 0.2 and 0.05 mm/rev, to examine the sensitivity of the results to the cutting speed and feed rate. The results obtained are shown graphically in Figure 1 and Figure 2. It is shown that the tool wear rate (tool wear for equal material removal) increases with the increase in alumina weight percentage. Detailed analysis of the graph reveals that there are two tool wear rate regions: one with relatively low tool wear rates when alumina weight percentage is below 10 % and another with high tool wear rates when alumina weight percentage is 15 % and above. This phenomenon can be seen during machining throughout the entire range of cutting conditions investigated. This indicates that the increase of tool wear with the alumina weight percentage is accelerated as the percentage of alumina in the MMCs exceeds a critical value, which is between 10 and 15%. Figure 1 and Figure 2 also show that the tool wear acceleration does not vary with cutting conditions.

[FIGURE 2 OMITTED]

Effect of Reinforcement Particle Length on the Tool Wear Acceleration

Cutting tests are further conducted to study the effect of the length of the reinforcement particles of the MMCs on the tool wear acceleration relating to reinforcement percentage of the MMCs. The diameters of the particle used in the MMCs specimens are 30, 40, 50 and 60 ?m. For each length, a set of MMCs specimens varying in the alumina weight percentage, namely 5, 10, and 15 % are fabricated. Since the tool wear acceleration does not vary with cutting conditions, the following cutting conditions are used i.e., cutting speed 64.67 m/min, feed rate 0.1 mm/rev and depth of cut 0.5mm. In each of the cutting tests equal amount of workpiece material was removed. The measured tool wear values representing the tool wear rates in the cutting tests are shown in Figure 3.

[FIGURE 3 OMITTED]

The results again reveal that while cutting the MMCs for each alumina particle size, there are two wear rates when alumina weight percentage is below a critical value band and another with high tool wear rates when alumina weight percentage is above the critical value band. However, the critical value band varies with the size of the reinforcement particles. The critical band values decrease with increase in the particle size.

Conclusions

The following conclusions can be drawn from the study.

1. The wear acceleration is caused by the interference between the reinforcement particles. The interference is associated with a critical weight percentage of the reinforcement in the MMCs.

2. The critical weight percentage of reinforcement is a function of the densities of the reinforcement and the matrix as well as the length of the reinforcement and the radius of the tool cutting edge.

3. The model can be used to develop MMCs machinability maps showing the critical reinforcement percentages of MMCs varying with the densities of the reinforcement and the matrix as well as the size of the reinforcement particles and the radius of the tool cutting edge.

References

[1] N. Tomac and K. Tennessen, Machinability of particulate aluminium matrix composites Ann. CIRP 41 (1) (1992), pp55-58.

[2] Z.J. Yuan, L. Geng and S. Dong, Ultra precision machining of Glass/A1 composites, Ann. CIRP 42 (1) (1993), pp 107-109.

[3] C. Lane, The effect of different reinforcements of PCD tool life for aluminium composites, in: Proceedings of the Machining of Composite Materials Symposium, ASM Material Week. Chicago, IL, 1-5 November 1992, pp. 17-27.

[4] K. Weinert, A consideration of tool wear mechanism when machining metal matrix composites (MMCs), Ann. CIRP 41 (1) (1993), pp 95-98.

[5] C. Lane, Machinability of aluminium composites as a function of matrix alloy and heat treatment, in: Proceedings of the Machining of Composite Materials Symposium, ASM Material Week, Chicago, IL, 1-5 November 1992, pp. 3-15.

[6] L. Fang, Q.D. Zhou and Y.J. Li, An explanation of the relation between wear and material hardness in three-body abrasion. Wear [5] (1991), pp 313-321.

[7] H. Akbulut et al, Materials Science and technology, vol.14, pp. 299-305.

* C. Yuvaraja and ** K.V.Sharma

* Department of Mechanical Engineering, K.S. Institute of Technology, K.R. Road, Bangalore -560 062,Karnataka, India E-mail: cyuvaraj1437@yahoo.co.in

** Department of Mechanical Engineering, University Visveswaraiah College of Engineering, K.R. Circle, Bangalore- 560 001, India E-mail: vinayaksharma33@yahoo.com