Cylindrical grinding of Al/SiC metal matrix composites

This paper deals with an experimental study on the grindability of Al/SiC metal matrix composites in cylindrical grinding. Machining of metal matrix composites (MMCs) is an area to be focused and finishing processes such as grinding to obtain a good surface finish and damage-free surfaces are crucial for the application of these materials. Nevertheless, grinding of MMCs has received little attention so far, thereby a detailed study on that has been carried out. In the present work, experiments are carried out to study the effect of grinding parameters; wheel velocity, work piece velocity, feed and depth of cut and SiC volume fraction percentage on the responses; grinding force, surface roughness and grinding temperature. Surface integrity of the ground surfaces is assessed using a scanning electron microscope (SEM). There are no cracks and defects found on the cylindrical ground surfaces at high wheel and work piece velocities, low feed and depth of cut.


INTRODUCTION
applying composites in different applications 4 . While machining of MMCs the subsurface damages are caused due to conventional and unconventional processes, making it inevitable for finishing processes such as grinding to improve the surface integrity 5 . Grinding is particularly needed to acquire high dimensional accuracy and surface finish. However, grinding of silicon carbide is difficult because of its low fracture toughness, making it very sensitive to cracking 6 . This makes the grinding of aluminium MMCs a difficult and unpredictable process. Unlike the investigations into the machining of traditional metallic materials, relatively little study has been carried out on machining advanced composite materials.
Previous studies on grinding of composites have shown that Al/SiC composites exhibit an improved grindability with respect to non-reinforced aluminium alloy for the better surface finish and the lower tendency to clog the wheel 7 . Sun et al. 8 studied the grinding characteristics of SiC particle reinforced Aluminium based MMCs. He reported that grinding is one of the final processes for finishing aluminium composites to obtain good surface quality and high machining accuracy. The wide application of MMCs will not be possible without the solution of the grinding problems. Zhaowei Zhong et al. 9 and N.P Hung et al. 10 presented the study on grinding of aluminium composites reinforced with SiC particles. They recommended that rough grinding with a SiC wheel followed by fine grinding with a fine-grit diamond wheel are required for the grinding of alumina/aluminium composites. Di Ilio 11 made a comparison between conventional abrasives and super abrasives in grinding of SiC-Aluminium composites. They revealed that among the types of grinding wheels employed in experimental tests, the wheels manufactured with conventional abrasives and open structure have given better performances than those with super abrasives in terms of low clogging, low grinding forces and better surface finish. Li et.al., 12 reported a study on grinding forces and force ratio of the unsteady-state grinding technique. He found that accurate measurement of the grinding force has great research value and practical significance on studies in the field of grinding. The properties of a ground surface depend on the grinding temperature, knowledge of its magnitude is important to establish the grinding conditions 13-15. However, reports on cylindrical grinding of composites are still very scarce. Therefore, a further study on the cylindrical grinding of these materials is an area to be focused to obtain damage-free surfaces for the application of these materials. Previous researches [5][6][7][8]

EXPERIMENTAL Parameter Study and Volume Fraction Study
In this section, experimental procedure is explained with all the details which require producing experimental data to use in the parameter study and volume fraction study. The Al/SiC composite specimens are cast aluminium alloys reinforced with 13 µm SiC particles of the size diameter 30 X 200 mm. The details of the specimens LM25/SiC/4p (4Vol% SiC) and LM25/SiC/2p-12p (2 to 12 Vol% SiC) are shown in Table 1. Grinding experiments are carried out on a horizontal spindle cylindrical grinding machine (Type -G13 P HMT).
The experiments for parameter study are planned using a complete 3 4 factorial design. Based on this, a total of 81 experiments, each having a combination of different levels of variables are carried out and the details are shown in Table 2.A vitrified-bonded white aluminium oxide grinding wheel (AA60K5V8) is used to grind the MMC specimens LM25/SiC/4p (4Vol % SiC) at low, medium, and high levels of grinding variables for parameter study. The specimens LM25/SiC/2p-12p (2 to 12 Vol% SiC) are also ground with white aluminium oxide grinding wheel for volume fraction study at medium level grinding parameters of Vs 2026 m/min, Vw 12.72 m/min, f 0.09 m/min and d 20 µm as shown in Table 3

Measurement of responses
The responses tangential grinding force (F T ), surface roughness (Ra) and grinding temperature (G T ) are monitored and measured while grinding Al/SiC composite specimens. A Variable Frequency Drive (VFD) (ACS 350-03E-12A5-4 ABB Make) is attached to the grinding wheel motor so that the wheel is capable of changing speed. A VFD is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. The tangential grinding force (F T ), tangent to the wheel-work contact, when multiplied by wheel speed (V S ) and a constant, determines the power used by the operation (ASM Metals Hand book: Machining 16 , Vol. 16,1989). The equation for Power is: Equation (1) for power is valid for horse power, using pounds of force and feet per minute for F T and V S , respectively. And the VFD is utilized to measure the power of the grinding wheel motor, so that the tangential grinding force (F T ) can be calculated from the above Eq. (1).
The surface roughness of the cylindrical ground specimens is measured in the direction perpendicular to the grinding direction using a surface roughness tester (Kosaka Make-Surfcorder-SE1200). The cut-off is 0.8 mm and evaluation length is 4 mm. On each ground surface three values are measured to calculate the average surface roughness value (Ra). An infra red non contact Laser thermo meter (METRAVI MT-9) is used to measure the temperature generated during cylindrical grinding with a standoff distance of 15 cm from the wheel-work interface and emissivity correction of 0.02. Before every grinding experiment, dressing is carried out. A single point diamond dresser is used for the dressing of Al 2 O 3 grinding wheels. Surface integrity of the cylindrical ground surfaces is assessed using a scanning electron microscope (SEM). The samples are observed in the as-ground condition.

Effect of grinding variables on responses
The significance of the cylindrical grinding variables on the selected responses is evaluated by conducting experiments and the results are represented by graphs. The effect of grinding variables on tangential grinding force (F T ), surface roughness (Ra) and grinding temperature (G T ) is shown in Figs. 1 to 3.It is observed from the results shown in Fig. 1 that the tangential grinding force decreases with an increase in wheel velocity and work piece velocity. This could be attributed to the thermal induced softened matrix at high speeds. As the grinding wheel velocity increases, the heat generated in the deformation zone increases and thereby softening the aluminium matrix thus reducing the force required to remove the material.   It is shown in Fig. 2 that the surface roughness decreases with an increase in wheel velocity and work piece velocity. This is due to the increase in relative velocity between the wheel and work piece and the reduction in contact time thereby reducing the chip thickness. It is also observed from the Figures 1 and 2 that the tangential grinding force and surface roughness increase with an increase in feed and depth of cut. When the feed and depth of cut are increased, the increase in material removal rate and the increase in chip thickness, account for the increase of the F T and Ra values.
The minimum and maximum values of F T and Ra are obtained as 16N, 0.171 µm and 39N, 0.893 µm respectively at wheel velocities between 2639 m/ min and 1414 m/min, work piece velocities between 26.72 m/min and 6.11 m/min, table feed between 0.06 m/min and 0.17 m/min and depth of cut between 10 µm and 30µm.
It is found from the results shown in Fig. 3 that the grinding temperature increases with an increase in wheel velocity (Vs), work piece velocity (Vw), feed (f) and depth of cut (d). The higher values of Vs, Vw, f, d results in higher grinding temperature due to the increase of the energy required to grind a unit volume of the metal. The G T values are scattered in the range of 740°C-856°C at the lower and higher levels of grinding variables. The results comply with the trends available in the literature (6,13).

Effect of SiC volume fraction on responses
The effect of percentage of SiC volume fraction on tangential grinding force (F T ), surface roughness (Ra) and grinding temperature (G T ) is shown in Figs. 4 to 6. It is found from these figures that the F T , Ra, G T values increase with an increase of percentage of SiC volume fraction. This is mainly due to the increased resistance to material removal which is a result of increased amount of SiC particles in unit volume of the material. The minimum and maximum values of F T , Ra and G T are obtained as 20 N, 0.187µm, 754°C and 39N, 0.606µm, 866°C respectively for the specimens of 2% and 12% SiC volume fractions at constant medium level grinding parameter of Vs 2026 m/min, Vw 12.72 m/min, f 0.09 m/min and d 20 µm. The results comply with the trends available in the literature (5).

SEM analysis of cylindrical ground surface
The surface texture of the cylindrical ground specimens are assessed using a scanning electron microscope. The SEM micro structure of the specimen LM 25/SiC/4p in Fig. 7 shows the uniform distribution of the SiC particles in the aluminium matrix before grinding. In general, the SiC particle distribution is nearly identical in all the specimens observed. It also shows the presence of Al-Si eutectic particles in spike form. Figure 8 shows the SEM micrograph of rough ground surface. This figure shows that the Al 2 O 3 grains of the wheel are embedded with the rough grinding marks on the surface of the work piece at low wheel and work piece velocities, high feed and depth of cut. Figure 9 shows the rough ground surface with high magnification (1500X). This micrograph clearly reveals that at low wheel and work piece velocities, high feed and depth of cut, the SiC particles are not ground but are fragmented and pulled out of the surface. Figure 10 shows the SEM micrograph of fine ground surface. The fine grinding marks shown on the SiC particles in this figure ensured that both the SiC particles and aluminium matrix are removed by cylindrical grinding at high wheel and work piece velocities, low feed and depth of cut. During the cylindrical grinding, the aluminium matrix has undergone plastic deformation and the SiC particles are covered by aluminium matrix. There are no cracks and defects found on the fine ground surfaces when observed with the SEM. Hence, the potential of using Al 2 O 3 wheels, for the cylindrical grinding of Al/SiC composites is high. Grinding parameters should be optimized to make the cylindrical grinding using Al 2 O 3 wheels more attractive.

CONCLUSIONS
The investigations of this study indicated that the grinding variables; wheel velocity, work piece velocity, feed and depth of cut are the primary influencing factors which affect the surface integrity of Al/SiC composites during cylindrical grinding. Based on the experimental results and discussions, the following conclusions are drawn: Better surface finish and damage free surfaces are obtained due to low grinding force at high wheel and work piece velocities with white Al 2 O 3 wheels during cylindrical grinding.
The surface finish and damaged surfaces are found to be high at high feed and depth of cut during cylindrical grinding. The experimental work demonstrates that the tangential grinding force developed during cylindrical grinding can be calculated from power measurements of the grinding wheel motor, using a Variable Frequency Drive (VFD). The approach presented in this paper for cylindrical grinding of Al/SiC composites can be extended with super abrasive grinding wheels like diamond and CBN.