Abrasive Water Jet Machining process
Abrasive water jet (AWJ) has been finding extensive use in the manufacturing industries for machining wide range of materials such as metals and non-metals. The reason behind the selection of AWJ machining process is that does not generate heat at the cutting zone, but the heat is less while machining hard materials; ability to cut all kinds of materials such as metals, non-metals, composites, ceramics; a higher material removal rate than the Wire EDM process, and production of a better surface integrity than the laser machining process; cutting thick components in the range of 250 mm (depends on materials); absence of thermal distortion to work materials but ability to cut intricate shapes; existence of minimum cutting force on the work materials and yield of better dimensional accuracy due to insignificant deformation; rock drilling and surface cleaning.
Major influence of AWJ machining parameters
a) Water jet pressure: This is an important process parameter in the AWJ machining process. Kinetic energy of the AWJ depends on the pressure level of water. Pressure is less than the threshold pressure range when no material removal takes place. Similarly, the pressure equal to the critical pressure range represents the limit indicated for effective cutting. The machining process becomes ineffective if extended beyond this stage. Water jet pressure is directly proportional to the penetration depth and the material removal rate. It has influence on the distribution of water as well as abrasive particles in the jet. It is frequently denoted by MPa or bar or PSI.
b) Traverse rate: It determines the quality of the cut surfaces produced by the AWJ process. The major influence on the traverse rate on the AWJ machining process is the determination of the exposure time. A Lower traverse rate increases the surface quality by allowing a larger number abrasive particles to impinge on the target material surface. It also impacts on the cutting rate of the process. The traverse rate is indicated by mm3 /min.
c) Abrasives: Various types of natural (garnet) and artificial abrasives (silicon carbide, aluminium oxide) are used in the AWJ machining process. The abrasive particle size, shape and hardness have a significant influence on the AWJ cutting performance. The higher the hardness of the work material, the harder should be the abrasives to be used. An increase in the size of the abrasive particle increases the particle disintegration. A decrease in the depth of penetration and material removal rate is seen following an increase in the limit of the size of the abrasive particles, which happens due to a reduction in the impingement frequency on the target material surface. The range of the abrasive size is universally indicated by mesh size (#).
d) Abrasive mass flow rate: The flow rate of the abrasives along with the water jet pressure has an influence on the AWJ material removal rate. An optimum supply of abrasives yields a higher cutting performance with a better surface finish. The flow rate of the abrasives depends on the diameter of the focusing nozzle used in the AWJ machining process. A transformation of the jet momentum is required for ensuring effective acceleration of the abrasive mass flow rate. The rate of abrasive mass flow is usually expressed as kg/min.
e) Stand-off distance: It is defined as the distance between the target material and the nozzle. It is usually maintained by an optimal level of distance in mm, considering the large influence of SOD on the kerf profile produced by the AWJ.
Jet impingement angle: This angle is associated with the tilting of the cutting head. It is defined as the angle between the initial AWJ flowing direction and the target material surface. Any change in the jet impingement angle eventually changes the jet attack angle on the target material with an effect on the mode of erosion. The use of the changeover jet impingement angle has a great impact on the AWJ cutting performance without the involvement of any additional costs. There are two different types of jet impingement angles used in the machining process, namely, forward and backward directions. The advantages include lower kerf taper formation, less striation formation, less contaminated zone, and machining of very soft and fragile materials.
g) Work material: The work material desired in the AWJ machining process usually has any size and shape. The AWJ process uses hard materials like titanium alloy, tungsten carbide, ceramics, stainless steels, composites, etc. The major phenomena found in the work material after the machining process are abrasive contamination and striation formation. The abrasive contamination for soft materials is higher with the hard materials due to the hardness of the work materials. In contrast, striation formation is significantly present in the hard material cut surface rather than the soft materials. This happens due to the increase in the abrasive attack angle at a larger cutting depth, as the kinetic energy of the abrasive particles is reduced. This cutting energy is greatly reduced by machining the hardened material at a lower cutting region.