The simultaneous erosion by water jet as well as abrasive particles removes material during AWJM. The erosion by water jet results from high-speed impingements of jet or droplet (liquid streak) on a solid surface. Where material is removed progressively and fails subsequently. Micro-cracking is the initial response of the target which occurs due to microstructural irregularities, stress concentration around the slip steps and pre-existing flaws. Impacts of water jet induce localised plastic deformation and rough surface which initiate of micro-cracks in homogenous bulk materials 21. The water jet also generates cavitation erosion. The cavitation erosion process is described by cyclic deformation parameters 22 where damage in materials occurs through hydraulic penetration, stress wave propagation and lateral out flow jetting. The damage produced by these loading conditions on a material surface exposed to a single or multiple water drop impingements is responsible for initiating damage and subsequent material removal 22, 23. Material removal by abrasive particles in ductile erosion occurs due to cutting and deformation processes as in metal cutting or grinding. The impacting particle strikes the surface to develope an indentation and begins removing a chip of metal. The particle breaks due to impact and fragments projects radially from the primary site to develope secondary damage 24. Due to repeated strikes of abrasive particles deformation wear occurs on the target surface which work-harden the surface and initiate cracks. Propagation and distribution of the cracks resulted in material removal25, 26. There are investigations on target melting during erosion by abrasive which have been encouraged by studies related to heating and melting of ductile materials subjected to erosive particles27. However, melting is unlikely in view of high thermal conductivity of magnesium alloy and presence of high speed water at room temperature which is capable of taking away heat from the erosion zone which is generated because of deformation in the target material.
In AWJM the individual effects of water jet and abrasive erosions, as well as these erosions, are complementing each other. The water jet deforms material and induces cavitation which helps the particles to cut the material easily. On the contrary, the particles damage the surface by ploughing, indenting, embedding, work hardening and crack generating, then the high speed water jet easily removes material from the damaged areas. In addition to cooling the machining process, the water jet interacts with the microstructural irregularities and defects and induces stress concentration which produces tensile stress and initiates micro cracking to remove materials 28. A typical surface machined by abrasive water jet machining is presented in Fig. 11. The machining processes generated different features such as, (I)-cavitation erosion, (II)-embedded abrasive particle, (III)-ploughing by abrasives, (IV)-machined groove by abrasives, (V)-groove due to abrasive flow at transverse direction of jet, (VI)-material removed due to crack, (VII)-indentation by abrasives, (VIII)-almost separated material and (IX)-groove due to abrasives flow at an angle with jet direction on the machined surfaces.
The intensity of these different features depends on the machining condition. At high water pressure (i.e. high jet speed) and low transverse speed, the surface features are more uniform (Figs. 5a, b) compare to that of low water pressure, low transverse speed and low abrasive mass flow rate (Fig. 5d) for Mg-6Al/0.66 Al2O3 surface. This due to weaker process parameters which were able to damage the surface but could not clean it. However, smother surfaces (Figs. 5b, c) were generated at higher water jet pressure, transverse speed and abrasive mass flow rate. On the other hand, the nano MMC with higher reinforcement content (Mg-6Al/1.11 Al2O3) has greater ability to resist the erosion. In this case, smother surfaces were generated at lower transverse speeds (Figs. 6a, b). With the increase of transverse speed, surface damage due to abrasive particles reduces and effect of water jet cavity increases which is clearly visible in Fig. 6(c). With the reduction of water jet pressure and transverse speed, the damages in the surface were primarily due to abrasive particles. Due to ductile nature of Mg-6Al alloy, the embedment of the abrasive particle was noticed all over the machined surface irrespective of the machining conditions.
Most of the researchers have found that the centreline erosion rate because of AWJ machining decreases with increasing standoff. The reason behind this is the radial expansion of the jet spreads which reduces the number of strikes per unit area though this does not influence the velocity of particles significantly 29. However, very short standoff distance may impede abrasive flow from the tube. Significant variations in the abrasive flow rate are typical in AWJ process which affected both by the spreading of the divergent jet with respect to the standoff distance and, depth of workpiece which affects the flow limit and size of the stagnation zone 30.After exit of slurry from orifice, aslurry jet in air can be split into three distinct phase: (i) the starting phase, when the velocity in the potential core remains unchanged at its value at the exit of the orifice; (ii) the main phase in which the mean velocity of the flow decreases with distance from the orifice, and a surrounding mist phase arises; (iii) the diffused droplet phase, a comparatively low velocity phase included with the disintegration of jet into droplets 31, 32. In ASJM, strong deceleration of abrasive particles takes place due to the water stagnation zone near the target 14. Erosion rate decreases with the increase of workpiece depth because of jet spreading during an increase in distance from the end of the effective nozzle to the bottom of the machined surface. It is also reported that the central water jet splits up into droplets after a few standoff distance depending on the water jet velocity as the jet entrains air with the abrasives in the upstream of mixing tube 33, 34. Due to the spread of the jet, only a fraction of original jet reaches higher depth and this fraction decreased with the increase of the depth. In addition, particle velocity further decreases from drag within the stagnation zone close to the bottom of the channel 30.
The waviness of the machined Mg-6Al/0.66Al2O3 surface was maximum (Fig. 3c) at higher jet pressure, abrasive rate and transverse speed. This might be due to lower resistance of MMNC to abrasion and high transverse speed when the abrasive jet does not get enough time to reduce waviness of the machined surface. The maximum surface waviness of the MMNC with higher content of reinforcement (Mg-6Al/1.11Al2O3) is lower as shown in Fig. 4. The highest waviness of the machined Mg-6Al/1.11Al2O3 surface was noticed at lower jet pressure, abrasive rate and transverse speed (Fig. 4d). This uneven machining occurs due to lower abrasive rate and jet pressure as this material has higher resistance to erosion.
Machined surface at the top is exposed to less diverged and higher amount of abrasive jet compare to that at the bottom surface. This generates longer, straighter and sharper grooves at the top surface compare to that of bottom surface for both materials. The higher reinforcement content of Mg-6Al/1.11Al2O3 increases its erosion resistance which reduces the groove length in the top surface compare to that of Mg-6Al/0.66Al2O3.