AWJM of Mg-based composites with different nanoparticle contents

This paper investigates the inuence of jet traverse speed and water jet pressure of abrasive water jet machining (AWJM) on the magnesium-based metal matrix nanocomposites (Mg-MMNC) reinforced with 50 nm (average particle size) 0.66 and 1.11 wt.% Al 2 O 3 .The extent of grooving caused by abrasive particle and irregularities in AWJ machined surface with respect to traverse speed and cutting depth was investigated. The nanoindentation results hows softening of the material defended by higher reinforcement content nanocomposite due to presence of sucient amount of nanoparticles protecting the surface from being damaged. The values of selected amplitude parameters viz. average roughness (Ra), maximum height of peak (Rp), maximum depth of valleys (Rv) presenting the comparatively smooth surface nish in 1.11 wt.% reinforced composite at high (500 mm/min) speed. Moreover, the high water pressure AWJM produces the better surface quality due to sucient material removal and proper cleaning of debris from the machining zone as compare to the low water pressure, low transverse speed and low abrasive mass ow rate.


Introduction
MMCs reinforced with particulate are of exceptional relevanceas these poses superior elastic modulus, higher strength to weight ratio, high-temperature resistant, near-isotropic and tailorable behaviour 1 .
However, MMCs with bigger reinforcements fails prematurely as these reinforcements are susceptible to crack during mechanical loading [2][3][4] . The increase in particle size may also reduce the tensile strength MMCs when the content of reinforcement is higher 5,6 . The properties of MMCs are seen to enhance by decreasing particulate size to nano-meter level.These are second generation MMCs and called as metal matrix nano-composites (MMNCs).
There are primary investigations on machining of MMNCs, such as Teng et al., 7 investigated milling of magnesium based MMCs dispersed with Ti and TiB 2 particles of 50 nm. Though Mg/TiB 2 MMCs exhibit better machinability, abrasion and chip adhesion affected the cutting edges signi cantly, while the reinforcement content and reinforcement materials affect the severity and wear type. In addition, the depth of cut and speed have a considerable effect on the surface nish. The worse machined surface and higher cutting force were observed at the smaller feed (0.15 to 0.5 µm/tooth) shows stronger size effect. Gopalakannan et al., 8 applied electrical discharge machining to 1.5 wt% SiC reinforced (50 nm) Al alloy using copper tool electrode. The pulse current was the prominent factor in uencing all the machining performances for example, electrode wear rate, material removal rate and surface nish. The optimized combination of variables was pulse current 8.00 A, voltage 50.00 V, pulse off time 9.00 µs and Pulse on time 8.00 µs. Li et al., 9,10 investigated the machining characteristics of Mg-MMCs reinforced with 20 nm SiC particles (0, 5, 10, 15 vol.%) by using micro-milling process at various feeds and speeds.
The force during cutting raised with the rise of speed, feed and the reinforcement content. The huge increase in slope was noted when the content increase from 5 to 10, which is acceptable with the signi cant variation of material properties with the content of around 5 to 10 vol.%. Nevertheless, the rate of increase in cutting forces is much larger in case of 60,000 rpm, when comparing with 20,000 rpm and 40,000 rpm.
Since the magnesium metal matrix nanocomposites (Mg-MMNCs) demonstrate excellent mechanical properties that can be used for various applications, so it is essential to develop appropriate machining techniques, especially in micro-scale, to promote their industrial applications 9 . However, there is not much information on machining of this material as discussed above. The traditional machining technique causes serious tool wear during processing of MMCs and the higher surface roughness which restrics the applications of MMCs. Therefore, non-traditional machining processes such as, laser-beam, electrodischarge, abrasive water jet and electro-chemical are also be used for MMCs 11 . Abrasive water jet machining (AWJM)or abrasive slurry jet machining (ASJM), is an important machining process where pressurized water is used to force the suspended abrasive particles for example, silicon carbide, aluminium oxide (Al 2 O 3 ) or garnet, the material removal takes place by erosion without changing material properties due to minor heat generation 12 . It has been considered as a key enabling technology for processing micro features on hard-to-machine materials without excessive forces or thermal damages [13][14][15] .
Having said these, this paper investigates the AWJM of nano-particle reinforced MMCs with different reinforcement content where jet pressure, abrasive ow rate and traverse speed were varied. This will provide a better understanding on machinability of this material and improve the machining process which will be bene cial to researchers and professionals in the relevant areas.

Materials And Methods
The matrix alloy was made of 6% and 94% Al and Mg (99.9% pure) which supplied by Alfa Aesar (USA).
Approximately, 50 nm Al 2 O 3 nano-particles were used as reinforcement provided by Baikowski (Japan).
The amounts of particles were 0.66 % and 1.11 %by weight in MMCs. Disintegrated Melt Deposition (DMD) method was applied to fabricate the nano-particle reinforced MMCs 16 . This is a hybrid casting process where elements of conventional casting and spray casting are combined. This method involves adding reinforcement particles to the melt matrix by mechanical means. Afterwards melt slurry of the composite gets disintegrated by jets of inert gas, which is normally oriented to the stream of the matrix melt. Subsequently, this melt deposits in a metallic substrate. The compositions of the MMCs are listed in Table 1. The AWJM process was performed on PTV: CNC WJ2020B-1Z-D machine. The machining variables were chosen based on a pilot experimentation. The experimental variables are displayed in Table 2. The description of each experimentis listed in Table 3. A detailed sketch of cutting head of abrasive water jet machine is illustrated in Figs   The nano-indentation testing was carried out using XP-Nanoindenter (Agilent, USA) before machining.
After machining the testing was done on the Agilent G200 Nanoindenter. The details of nanoindentation testing before and after machining were discussed in our previous study [33].

Surface morphology
The pro les of the machined Mg-6Al/0.66 Al 2 O 3 surfaces at different machining conditions are presented in Fig. 3. The surfaces at all the machining conditions are wavy and, the widths and heights of the peaks and valleys vary along the length. Some of these are continuous, but few are not. All the peaks and valleys are inclined in the direction of the jet ow though these are not very parallel to each other in all the cases. Figure 3(a) gives the three dimensional surface pro le at a low transverse speed where maximum amplitude of peak and valley is 84.15 µm. Few tiny depressions are also noticed. The distance between height of the peaks and valley-depth decreases to 74.78 µm with the increase of transverse speed though the size and number of depressions is increased as demonstrated in Fig. 3(b). With the further increase of transverse speed (Fig. 3c), the distance increases signi cantly to 166.3 µm where most of the surface is depressed and higher areas on the surfaces exist as thin discontinuous lines with varying thickness.
However, at lower jet pressure, abrasive rate and transverse speed the distance between maximum peakheight and valley-depth becomes 78.17 µm as illustrated in Fig. 3(d).
Similar pro les are also noticed on machined surfaces of Mg-6Al/1.11 Al 2 O 3 at different machining conditions as shown in Fig. 4. In this case, very rare depressions are noticed on the machined surface at lower transverse speed ( Fig. 4(a)). Again, as the transverse speed increases the maximum distance along Z-axis decreased though the machined surface is full of depressions deeper than that at lower transverse speed as presented in Fig. 4(b). The higher areas are appeared as a continuous line of different thickness which is interrupted with depressions of different depths. The maximum variation in Z-axis becomes 108.0 µm with further increase of transverse speed (Fig. 4c) where the higher areas are in the form of short lines of different thickness which are interrupted and surrounded by depression of different depths.
The variation increased at reduced jet pressure, abrasive rate and transverse speed ( Fig. 4(d)).
The machined surfaces of Mg-6Al/0.66 Al 2 O 3 are composed of pronounced ploughing traces generated by abrasive grains as shown in Fig. 5. The ridges of the traces are very closely populated and sharp when the transverse speed is lower (Fig. 5a). At slightly higher speed there are also small oval depressions sparsely dispersed on the surface (Fig. 5b). The ridges atten and the distance between traces increases with the increase of transverse speed (Fig. 5c). A very irregular surface with random orientation of the traces is generated when the transverse speed, abrasive rate and jet pressure is low as shown in Fig. 5(d).
The traces of ploughing are also generated on the machined Mg-6Al/1.11 Al 2 O 3 surface as shown in In this case, the surface is full of randomly oriented distinct short lines generated due to AWJM. In addition, the bottom area of the machined surface i.e. jet exit region seems to be full of grooves and slots this could be attributed to jet spreading or non-uniformity of abrasive distribution in the jet.
The surface characteristic Mg-6Al/1.11 Al 2 O 3 is almost similar to that of Mg-6Al/0.66 Al 2 O 3 . In this case, the top region is full of straight and curved lines which are densely located. The randomness of these curves increases in the surface towards the bottom of the sample.

Surface roughness
The roughness parameters such as Ra (average), Rp (maximum height of peak) and Rv (maximum depth of valley) are used to analyze the surface characteristics. These parameters are compiled along ten different parallel lines across the AWJ ow. Figure 9 presents the effect of varying the transverse speed on roughness at a constant abrasive ow rate. As expected, it is showing higher roughness when higher transverse speed (500 mm/min) is used. This could be interpreted by the matter off act that less number of abrasive particles comes in connection with cutting surface at higher speed due smaller interaction time between abrasive jet and work-piece. Whereas, at lower speeds more number of abrasive particles get connected with cutting surface in view of more interaction time between abrasive jet and work-piece. Therefore, it produces a smooth surface nish. Figure 9 (a-f) clearly show the smoother surface at lower traverse speed and rougher surface at higher traverse speed at same abrasive ow rate. It is also evident that for both the materials, the roughness at 20 and 250 mm/min are almost similar. But the difference in Ra, Rp and Rv can be clearly seen at higher speed (500 mm/min). The metal matrix nano composites (MMNC) with higher reinforcement content shows lower roughness than that of MMNC reinforced with lower reinforcement content at higher traverse speed. This is due to the higher resistance of MMNC (1.11% reinforcement) which protects the surface from abrasive jet to produce irregular peaks and valleys. This has also been demonstrated in Fig. 4(c) and 6(c). It is believed that the roughness of the MMNC is mainly in uenced by micro effects of each impacting particles 18 . Since reinforcing particles (50 nm) in this composite are much smaller than the abrasive particle (177 µm). So, the nanoparticles will have little or no individual effect on machined surface nish. However, the combined effects of nanoparticles have in uenced the abrasive water jet machinability of MMNCs in this case. Figure 9 (a) and (d) show that the average surface roughness (Ra) does not very notably with the directions of the measurements for the both materials and all the transverse speeds. The maximum height of peak (Rp) gradually increase from direction L1 to L10 for transverse speed 250 and 500 mm/min ( Fig. 9 (b)) but it was almost constant with a peak and valley at low transverse speed (20 mm/min) for MMNC with lower reinforcement content. When the reinforced content is higher, a sudden peak of Rp has noticed along line 5 direction at all the speeds (Fig. 9 (e)). In addition, a valley was noticed for speed 20 and 500 mm/min along line 9 direction. A peak of Rp indicates that the surface is very high on the other hand the valley of Rp indicates that there is a depression on the surface. All these indicate the presence of transverse traces of particle ow on the machined surface across the direction of AWJ ow.

Mechanical properties
The change in hardness and elastic modulus along the depth from the machined surface for both materials are presented in Fig. 10

Discussion
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 aws. 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 ow 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 removal 25,26 . 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 particles 27 . 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 ow 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 ow 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 ow rate (Fig. 5d) for Mg-6Al/0.66 Al 2 O 3 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 ow rate. On the other hand, the nano MMC with higher reinforcement content (Mg-6Al/1.11 Al 2 O 3 ) 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 in uence the velocity of particles signi cantly 29 . However, very short standoff distance may impede abrasive ow from the tube. Signi cant variations in the abrasive ow 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 ow limit and size of the stagnation zone 30 .After exit of slurry from ori ce, 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 ori ce; (ii) the main phase in which the mean velocity of the ow decreases with distance from the ori ce, 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/0.66Al 2 O 3 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/1.11Al 2 O 3 ) is lower as shown in Fig. 4. The highest waviness of the machined Mg-6Al/1.11Al 2 O 3 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/1.11Al 2 O 3 increases its erosion resistance which reduces the groove length in the top surface compare to that of Mg-6Al/0.66Al 2 O 3 .

Conclusions
The following points can be drawn based on the analysis of the practicability of AWJM of MMNCs: 1. The surface generated from AWJM ensures the regular surface topography at 20 mm/min and 250 mm/min speeds for material A and material B whereas at 500 mm/min speed the surface nish becomes worsens in material A compare to material B due to less resistance to the abrasive particles. 2. Based on analysis of three dimensional pro les it could be concluded that the depth of valleys as well as the size of depressions enhanced with transverse speed but in material B at 20 mm/min speed no remarkable depressions were seen.
3. The surfaces examined at three different regions can be explained as the striations in the AWJ machined surfaces increases from jet inlet to jet exit regions. The possible reason behind this fact is unsteady jet penetration process and non-uniformity of abrasive distribution in the jet at the exit region. 4. The values of selected amplitude parameters (Ra, Rp, Rv) increases from lower to higher traverse speed in case of both materials type. However, there is a large difference in values for material A and material B at 500 mm/min speed. At some extent, it ascertains the better surface quality of material B even at higher speed. 5. The results from nanoindentation testing convey the softening of the AWJ machined surface up-tothe depth of 20-25 µm in the case of material A whereas no signi cant variations in hardness and modulus values and softening phenomenon was observed in material B.