A Study on Abrasive Water Jet Machining of Aluminum with Garnet Abrasives

MATERIALS AND METHODS

The experiments were carried out in an abrasive water jet machine of model WJ 4080. The machine was equipped with the controller type Acramatic 2100 CNC control. The work material used was aluminum with the following main properties: tensile strength-90MPa, modulus of elasticity-69 GPa and density-2.71 g cm^-3. The abrasive used was garnet with mesh size of 80 and hardness of 7.5 Mohs. The cutting parameters investigated were stand-off distance of the nozzle from the work (from 1 to 5 mm), feed rate of the work table (from 10 to 50 mm min^-1) and jet pressure (from 12 to 50 ksi). The jet was perpendicular to the work surface and the nozzle diameter was 0.75 mm. Small rectangular aluminum blocks were cut by abrasive water jet half way along the length. This was done to measure the width of cut at the top and bottom of the block. After measuring the width of cut, the blocks were cut to the full length and the surface roughness was measured on the vertical cut surface. Surface roughness was measured by a surface roughness measuring equipment model SURFPAK SV-514. Surface roughness was measured at distances of 10, 20, 30 and 40 mm from the top of the surface.

RESULTS AND DISCUSSION

Effect on width of cut
Effect of stand off distance on width of cut: Figure 1 shows the effect of stand-off distance on widths of cut (B_top-width at the top and B_bottom-width at the bottom) of the vertical cut surface of the aluminum blocks. During the cutting process the jet pressure was 30 ksi and the work feed rate was 15 mm min^-1. B_top has increased by 0.542 mm due to the increase in stand-off distance from 1 to 2 mm. Then, it has changed its trend and width of cut decreases until the stand-off of 5 mm. It can be concluded that increasing the standoff distance will increase the width of cut up to the stand-off distance of 2 mm, because of the taper shape of the jet. Some researchers^[7,9] also concludes that width of cut increases with nozzle distance. But as the stand-off distance increases beyond 2 mm, the stream becomes weaker and as a result, the width of cut decreases.

Effect of pressure on width of cut: Figure 2 shows the relationship between width of cut and abrasive water jet pressure ranging from 10 to 50 ksi. Both B_top and B_bottom show the same pattern.

Fig. 1:	Effect of stand-off distance on width of cut

Fig. 2:	Effect of pressure on width of cut

Their increasing trend is almost parallel, starting from 10 to 50 ksi. It is obvious that with an increase in jet pressure the energy of the stream increases which causes the widening of the kerfs. Similar results were obtained by Momber^[9].

Effect on taper of cut: The taper of cut (T_R) is defined as a non-dimensional ratio between the top cut width and the bottom cut width. It can be calculated as follows:
T_R = (B_top-B_bottom)/2. Machining parameters should be selected so that the taper of cut is as small as possible.

Effect of stand-off distance on taper of cut: Figure 3 shows the trend of change of taper of cut with the change in stand-off distance of the nozzle from the work. The taper of cut gradually reduces to the minimum value as the stand-off distance is increased from 1 to 5 mm. At low stand-off distance the jet is strong and it has a diverging shape. That’s why the top width of the cut is smaller than that of the lower and the taper angle is negative.

Fig. 3:	Effect of stand-off distance on taper of cut

Fig. 4:	Effect of feed rate on taper of cut

Fig. 5:	Effect of pressure on taper of cut

But as the stand-off distance is increased, the jet becomes weaker and near the exit the jet looses its kinetic energy and the width of cut at the bottom becomes smaller than that of the top one. The negative values indicate that the upper width of cut is lower than bottom width of cut.

Effect of feed on taper of cut: Figure 4 shows the relationship between taper of cut and feed rate. The general trend of the curve shows that increase in work feed rate results in increase in taper of cut. Rankin and Kavocevic^[8] also shows some interesting relationship between feed rate and depth of cut. Momber^[9] showed that with increase in traverse rate, the top width of cut decreases. But the decrease in width of cut with increase in work feed rate is more prominent near the exit of the slot since at the exit the jet becomes weaker. As a result taper of cut gradually increases with increase in work feed rate.

Effect of pressure on taper of cut: Figure 5 shows the trend in change in taper of cut with increase in jet pressure during machining of aluminum. As the jet pressure is increased from 10 ksi, the taper of cut gradually increases to a maximum value at a jet pressure of 30 ksi. This trend is similar to the results obtained by Momber and Kovacevic^[9]. But the taper angle reduces again with further increase in jet pressure. This is due to the more straightening of the jet at a higher pressure.

Effect on surface finish
Effect of stand off distance on surface finish: Figure 6 shows the effect of stand-off distance of the nozzle from the work surface and the surface roughness of the vertical cut surface at different depths from the top surface. Jet pressure and work feed rate in this illustration are 30 ksi and 15 mm min^-1, respectively. The general trend of increase in surface roughness with increase in stand-off distance is obvious. It is also to be observed that work surface roughness increases rapidly beyond a stand-off distance of 3 mm. At a stand-off distance of 4 mm it is clear that further the surface is from the top, rougher it is.

Effect of feed rate on surface finish: Figure 7 shows the effect of feed rate on roughness of the vertical machined surface at various depths from the top surface. In the Fig. 7 the jet pressure and the stand-off distance of the nozzle were 30 ksi and 3 mm, respectively. Increasing the feed rate causes the increasing of striation formation of the jet and hence the surface roughness increases with increase in feed rate. Thus at a feed rate of 50 mm min^-1 the value of surface roughness at a depth of 40 mm from the top of the surface is 11.84 μm where as the value is 5.58 μm at a feed rate of 10 mm min^-1 at the same depth. It can also be observed from the Fig. 7 that in general the machined surface is smoother near the top of the surface and becomes rougher at greater depths from the top surface.

Fig. 6:	Effect of stand-off distance on surface finish

Fig. 7:	Effect of feed rate on surface roughness

Fig. 8:	Effect of pressure on surface roughness

Effect of pressure on surface finish: Figure 8 shows the effect of jet pressure on surface roughness at different depths from the top surface (at a work feed rate of 15 mm min^-1 and a stand-off distance of the nozzle of 3 mm). It can be observed from the Fig. 8 that as the pressure increases, the surface roughness gradually decreases. This result is similar to those obtained by Kovacevic^[10]. With increase in jet pressure, brittle abrasives break down into small ones. As a result of reduction of size of the abrasives the surface becomes smoother. Again, due to increase in jet pressure, the kinetic energy of the particles increases which results in smoother machined surface. It can also be observed that the highest value of surface roughness is at the greatest depth of the machined surface.

CONCLUSIONS

In the present study experimental investigations have been carried out for specific cutting parameters of jet pressure, stand-off distance of the nozzle and feed rate. It was observed that for a moderate pressure of 30 ksi and feed rate of 15 mm min^-1, the width of cut increases with increase in stand-off distance, but further increase in stand-off distance causes narrowing the slot due to a weak jet. Again, both the top and the bottom widths increase with the increase in pressure due to higher kinetic energy of the abrasives and water. Therefore, pressure should be kept as low as possible in order to reduce the wastage of material, though it would reduce the productivity of machining.

Taper of the kerf is another important factor in machining accuracy and it should be as small as possible. Experimental investigations reveal that a stand-off distance of around 4 mm (at a feed rate of 15 mm min^-1 and a jet pressure of 30 ksi) produces the smallest width of cut. Again, in order to minimize the width of cut (at a jet pressure of 30 ksi and a stand-Off distance of 3 mm) feed rate should be kept within 40 mm.

Surface finish is of great concern for AWJM. It was found that the cut surface is smoother near the top surface and gradually becomes rougher at higher depths. Surface roughness increases with increase in stand-off distance and the results show that the stand-off distance should be kept within 3 mm (at a pressure of 30 ksi and a work feed rate of 15 mm min^-1) in order to obtain a reasonably smooth surface. Similarly, increase in work feed rate results a poorer surface. It is advisable to limit the feed rate within 30 mm min^-1 (at a jet pressure of 30 ksi and a stand-off distance of 3 mm) which gives a good surface finish.

Jet pressure plays an important role in surface finish. As the jet pressure increases, surface becomes smoother. This is due to fragmentation of the abrasive particles into smaller ones as a result of high pressure and small particles give a smoother surface. Again, surface is smoother near the top of the work surface and gradually it becomes rougher at higher depths.