Carbides are well known for their high wear resistance and hardness, especially hot hardness. Due to these properties they have gained popularity as a tool material. They are extensively used for making dies and cutting tools. Hardened materials as well as difficult-to-machine materials can be machined by carbides. These carbides, made by powder metallurgy are difficult to machine by conventional techniques due to their high hardness. In order to cut carbide, first a fracture is to be developed. A fracture can be developed in the material if the force applied is more than the inter-granular bonding force. It was suggested that inter-granular fracture dominates the material removal at perpendicular impact angles[1,2]. Guinot et al. and Magnnsson and Ohlsson also worked on force parameters involved in abrasive water jet machining. In the present work an attempt has been made to machine carbides with high pressure water jet. Carbide is a brittle material, but water is a soft material and produces a smooth surface. Again, since water jet machining is a cool machining technique without much heat generation, no metallurgical changes occur on the machined surface and the surface is almost free of residual stress. It was stated by Tikhomirov et al. that material removal rate decreases almost linearly with increase in work material hardness. Since carbides are very hard materials, only high pressure water jet can be used to machine them. In the present work it was found that the minimum water pressure required to machine carbides is 35 ksi. Due to high pressure the abrasive grains are broken into small elements. Ohlsen introduced a new character called disintegration number to quantitatively evaluate the process of abrasive particle disintegration. He found that disintegration number increases with increase in pump pressure, increase in abrasive flow rate and decrease in focal length of the jet. Literature survey shows that a lot of works have been done on machining of different metallic, non-metallic and composite materials with water jet machining, but a little research was found on machining of carbides. In the present research, investigations has been done on surface finish produced by abrasive water jet machining with different pressure, abrasive flow rate and work feed rate.
MATERIALS AND METHODS
The abrasive used in the present study was garnet with the mesh size of 80.
The work material was carbide of P 25 grade. Its approximate composition is
78.5% tungsten carbide, 8% tantalum carbide and 10.5% cobalt. Its hardness is
about 1400 HV50 and density is 13.5 g cm-3. The size of the carbide
pieces was 13x13x3.18 mm. The cutting operations were performed on a water jet
machine of model WJ 4080. The machine was equipped with the controller type
2100 CNC Control. The nozzle used for the abrasive water jet was of carbide
with the orifice diameter of 0.1 mm. During cutting operations, the jet was
held perpendicular to the work surface.
In the present study the machining variables were the jet pressure, abrasive flow rate and the work feed rate. The range of pressure was from 35 to 50 ksi, the range of abrasive flow rate was from 135 to 175 g min-1 and the range of work feed rate was from 1.36 to 5 mm min-1. The output parameter of the work was surface finish produced by the abrasive water jet. Surface roughness was measured at the depths of 0.5, 1.5 and 2.7 mm from the jet entrance. Surface roughness was measured using a surface roughness measuring equipment model SURFPAK SV-514. The cut surface was also observed under a microscope.
RESULTS AND DISCUSSION
Influence of pump pressure on surface roughness: Carbide is a very hard
material and the strength of bonding among the particles is very high.
|| Surface roughness at different depths
||Serration of the machined surface at various jet pressures
As a result, a very high force is to be applied to overcome the bonding strength.
In the present study the minimum pressure required for cutting carbide was 35
ksi. The effect of pressure on surface roughness at different depths from the
jet entrance is illustrated in Fig. 1 and Fig.
2. During these investigations the work feed rate was 1.36 mm min-1,
stand-off distance of the nozzle from the work surface was 2 mm and the abrasive
flow rate was 135 g min-1. From Fig. 1 it is obvious
that with increase in pressure, surface roughness gradually decreases. Due to
increase in pressure, kinetic energy of the particles increases which enhances
their cutting ability. Again, at a higher pressure the abrasive particles break
down into smaller particles that produce a smoother surface. Similar result
was found by Kovacevic et al..
Figure 2 shows that surface roughness rapidly increases at a higher depth. This is due to the fact that as the abrasive particles move down, they loose their kinetic energy and produce an inferior surface. It is obvious from the Fig. 3 that with increase in pressure the height of the zone before clear serration marks increases with increase in pressure.
||Surface roughness at different depths and abrasive flow rates
Effect of abrasive flow rate on surface roughness: It was found in the present situation that a low flow rate of abrasive particles cannot cut carbide at all. It needs a large number of impacts per unit area under certain pressure to overcome the bonding strength of carbide particles and cut the work material. In the present work, at a jet pressure of 45 ksi, it was found that the minimum abrasive flow rate required to machine carbide satisfactorily is 135 g min-1. In this case the jet pressure was 45 ksi and the stand-off distance of the nozzle from the work was 2 mm. It can be observed that with increase in abrasive flow rate surface roughness decreases. This is because of more number of impacts and cutting edges available per unit area with a higher abrasive flow rate. With an abrasive flow rate of 175 g min-1, the average value of surface roughness at a depth of 0.5 mm below the jet entrance was only 0.403 μm which may be considered to be a very good surface finish (Fig. 4).
Abrasive flow rate determines the number of impacting abrasive particles as
well as total kinetic energy available. Therefore, higher the abrasive flow
rate, higher should be the cutting ability of the jet.
||Serration of machined surface at different abrasive flow rates
But for higher abrasive flow rate, abrasives collide among themselves and
loose their kinetic energy. Again, kinetic energy available per particle is
also reduced due to increase in abrasive flow.
It is evident that the surface is smoother near the jet entrance and gradually the surface roughness increases towards the jet exit (Fig. 5).
Figure 6 shows the microscopic views of the machined surface, cut at a pressure of 45 ksi, work feed rate of 1.36 mm min-1 and at various abrasive flow rates.
||Surface roughness at different depths and feed rates
It can be observed that the radius of curvature of serration at the bottom zone of the machined surface is larger at higher abrasive flow rate.
Effect of feed rate: The jet pressure, abrasive flow rate and stand-off distance of the nozzle from the work were 45 ksi, 150 g min-1 and 2 mm min-1 respectively (Fig. 7). It can be concluded that surface roughness gradually increases with increase in feed rate. This is due to the fact that as the work moves faster, less number of particles are available that pass through a unit area. Therefore, less number of impacts and cutting edges are available per unit area, which results a rougher surface.
It is obvious that surface roughness increases rapidly towards the jet exit
(Fig. 8). Figure 9 shows the cut surface
produced by machining carbide at a constant pressure of 45 ksi, abrasive flow
rate of 165 g min-1 and at various feed rates ranging from 1.36 to
5 mm min-1. These photographs clearly show that the width of waviness
increases at a higher feed rate resulting a rough surface.
|| Machined surfaces at different feed rates
In the present investigation, the process failed to cut the work of 3.18 mm
thickness beyond a feed rate of 5 mm min-1.
From the above discussions the following conclusions can be drawn:
||A jet of very high pressure is required to machine carbide.
A jet pressure below 35 ksi failed to cut carbide under the conditions considered.
With increase in jet pressure, the kinetic energy of the abrasive particles
increases enhancing their cutting ability. As a result the cut surface becomes
smoother at a higher pressure.
||Minimum abrasive flow rate that made it possible to machine carbide was
135 g min-1. As abrasive flow rate is increased, more number
of cutting edges of the particles passes through unit area. As a result,
with increase in abrasive flow rate, surface roughness reduces.
||Work feed rate didnt show a prominent influence on surface roughness.
But with increase in work feed rate the surface roughness increased. Width
of serration waviness increases with increase in feed rate. Under the conditions
of the present investigations, the jet pressure failed to cut carbide at
a feed rate higher than 5 mm min-1.
||In all the investigations it was found that the machined surface is smoother
near the jet entrance and gradually becomes rougher towards the jet exit.
This is due to the fact that as the particles moves down, they loose their
kinetic energy and their cutting ability deteriorates.