abrasive water jet machinig - process parameters

8/9/2019 Abrasive Water Jet Machinig - Process Parameters

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Abrasive water-jet controlled depth milling of Ti6Al4V alloy

an investigation of the role of jetwork piece traverse speed and

abrasive grit size on the characteristics of the milled material

Abrasive water-jetting (AWJ) is a technology for removal of material, where abrasive particles are entrained into ajet of water which is accelerated to high velocities by use of pressures in excess of 130MPa. The particle-ladenwater jet impinges onto the surface of the workpiece and material is removed by an erosion process.

SURFACE ROUGHNESS

Hashish [2,5] indicated that the milling of aluminium alloy with 180_m (80#) garnet grit resulted in asurface roughness of 20_m, whilst milling with 100_m (150#) grit resulted in a surface roughness of only13_m. Li et al. [7] also suggested that the use of smaller grit reduced the surface roughness (Ra) value.

They demonstrated that at impingement angles of 90 the surface roughness is at a maximum value for

most materials and decreases as the impingement angle decreases.

Surface roughness is also material dependent. Under fixed milling conditions, Li et al. [7] observed a

surface roughness (Ra) value of 4.0_m on an aluminium alloy, whereas that developed in a titanium alloywas only 3.0_m.

The traverse speed of the jet over the workpiece also has a strong influence on its surface finish. Ojmertz[9] has shown that low traverse speeds result in an irregular surface morphology of the milled area but thatdespite this, lowersurface roughness values are observed.

SURFACE WAVINESS

He also demonstrated that surface waviness improves significantly (i.e. is reduced) by increase oftraverse

speed up to 0.01ms1 (600mmmin1) but that further increases in traverse speed yield only small

improvements. However, Hashish [5] suggested that the traverse speed must exceed a higher critical value

(0.016ms1 (960mmmin1)) to achieve surface uniformity (i.e. a low surface waviness).MATERIAL REMOVAL RATE

Traverse speed also affects the volumetric material removal rate. Hashish [1,5] found that the material

removal rate in the milling of aluminium alloy was approximately constant for jet traverse speeds between

0.5 and 5ms1. Ojmertz [9] also reported a constant volumetric removal rate for a lower range of traverse

speeds between 0.005ms1 (300mmmin1) and 0.26ms1 (1560mmmin1). However, he demonstrated

that lower traverse speeds resulted in significantly increased material removal rates but also resulted in highsurface waviness. He concluded that the high traverse speeds required for controlling the depth and

waviness resulted in low volumetric removal rates.

The grit size also affects the material removal rate. Hashish [1] suggests that the volumetric removal rate is

independent of grit size for larger grits, but then decreases as the grit size falls to 100_mand below.


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Fig. 1 shows the effect of traverse speed on material removal rate for the two grit sizes. It can be seen that material

removal rate (Ed) for both grit sizes is high at the lowest traverse speed examined and decreases rapidly with

increasing traverse speed.

Fig. 3 shows the effect of jetworkpiece traverse speed on the surfacewaviness of the kerf for the two grit sizes. Itcan be seen that an increase in traverse speed results in a reduction in surface waviness for both grit sizes, with the

reduction being most significant for the larger grit (Fig. 3a). It can also be seen that at all traverse speeds, the surfacewaviness is greater with the larger grit than with the smaller grit.

Fig. 7 shows the effect of traverse speed on the surface roughness of the bottom of the kerf, with single jet passes in

the case of linear milling and twenty passes of the jet for the rotary milling. It can be seen that increasing thetraverse speed results in an increase in kerf roughness.

Fig. 8 shows the development of depth of cut with multiple passes of the nozzle across the workpiece with both

large and small grit sizes at two traverse speeds. It can be seen that in all cases, the cumulative depth of cut

increased linearly with number of passes. In a similar manner to Fig. 1, it can be seen that the material removal rate

is always higher with the larger grit size and is significantly higher at the lower traverse speed.


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Fig. 9 shows the development of surface waviness with multiple passes of the nozzle across the workpiece at two

traverse speeds with both large (Fig. 9a) and small (Fig. 9b) grit sizes. It can be seen that there is a general increase

in waviness with number of passes and thus with depth of cut. For both grit sizes, the waviness is significantlyhigher at the lower traverse speeds.

Fig. 10 shows the development of surface roughness with multiple passes of the nozzle across the workpiece at two

traverse speeds with both large (Fig. 10a) and small (Fig. 10b) grit sizes. For both sizes of grit, the surface roughness

does not change significantly with number of passes and thus with depth of cut. The surface roughness is not

strongly dependent on traverse speed; however, the roughness developed with the 180_m (80#) grit is around 5_mwhereas that developed with the 75_m (200#) grit is round 3.5_m.

It has been shown that the traverse speed, grit size and number of passes of the jet allinfluence changes in the way that the material is removed in the AJW milling of titanium,

and that in selecting parameters, some compromise must be made, primarily betweenmaximization of material removal rate and minimization of surface waviness.

At low jet traverse speeds, the jet cuts primarily on the leading edge of the kerf and is

then channelled along the slot to produce a directional morphology; this results in highmaterial removal rates but also in high surface waviness.

Surface waviness is observed to increase significantly with number of passes of the jet atlower traverse speed, while the surface roughness is independent of number of passes.

High traverse speeds result in lower material removal rates as material is removed

primarily by high angle impingement. Here, little directionality in the surface

morphology is observed on the bottom of the kerf; the surface roughness is higher, butthere is a significant reduction in the surface waviness compared to milling at low

traverse speed.

For the two grit sizes examined, the material removal rate, surface roughness and surfacewaviness was lower for smaller grit. However, when cuts were made to the same depth

with the two grit sizes, the differences in waviness for the two sizes was less at the higher

traverse speeds than at the lower traverse speeds. The smaller grit size gave a surfacewaviness value between 55 and 70% of that of the larger grit.


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A study of taper angles and material removalrates of drilled holes in the abrasive water jetmachining process

Effect of variation of S-O-D on material removal.

Effect of variation of S-O-D on taperness.


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Effect of variation of chemical concentration on material removal.

Effect of variation of chemical concentration on taperness

(1) The material removal increases with the increase in S-O-D, up to certain limit and further

increase in the S-O-D beyond the limit results in decrease of the material removal.(2) The material removal was found to be more in presence of chemically active liquids such as

acetone and phosphoric acid rather than plain water in the slurry.

(3) The material removal was identified to be the highest in the case of a slurry mixed with

polymer (polyacrylamide) rather than other two chemical environments used in the experiments.

(4) The slurry with a polymer combination shows a continuous increase in material removal witha variation in the chemical concentration.

(5) The chemical concentration was observed to be having an influence over the taper of theholes produced. The hole taper in case of polymer combination showed almost nil taper.


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Abrasive jet polishing on SKD61 mold steel usingSiC coated with WaxThe results show that the use of wax-coated abrasive particles reduces the polishing time and achieves animproved surface finish.

Fig. 6(b) shows that irrespective of the amount of machining oil added to the pure water-solvent, the materialremoval rate is significantly lower than that achieved using pure water only. This suggests that the addition ofmachining oil reduces the cutting force applied to the workpiece surface by the abrasive particles. This is beneficialfrom a polishing point of view since it not only reduces the depth of the scratches produced on the workpiecesurface during the blasting process, but also delays the onset of the surface hardening effect with the result that animproved surface quality can be obtained.

Fig. 9 Influence of Si abrasive particle diameter on polished surface roughness.Fig. 9 shows that the #2000SiC abrasive particles achieve a better surface roughness improvement than the#3000SiC particles. This result is most reasonably attributed to the larger diameter of the #2000SiC particles,which increases the cutting force developed by the particles upon impact and therefore enhances the materialremoval rate.


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Fig. 12 Influence of compound additive composition (water wax:SiC:water) on polished surfaceroughness.Fig. 12 shows that when the water wax content of the additive is very low compared to that of the pure water (i.e.250:1750), apoor surface roughness occurs since the water wax is stripped from the particles under the influence of the supplypressure. As a result, the sliding/grinding effect of the abrasive particles is reduced, and thus a higher total surfaceroughness is obtained. It can be seen that the best polishing performance is obtained using the compound additivemixed in a ratio of 500:1000:1500 (water wax:SiC particles:pure water).

Fig. 13 Influence of abrasive particle diameter on polished surface roughness.Fig. 13 shows that after 60 min, the surface roughness achieved using the #3000SiC abrasive particles (Ra =0.054_m) is significantly lower than that achieved using the #2000SiC or #8000SiC abrasives. Of the threeabrasives, the #8000SiC particles have the smallest diameter, and hence generate a lower cutting force.Conversely, the #2000SiC particles have the largest diameter, and thus develop the greatest cutting force.


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Fig. 14 Influence of wax-coating on polished surface roughness.Fig. 14 shows that thewax-coated particles reduce the surface roughness to a value of Ra = 0.049_m after 45 min.

This result isbetter than that obtained using the uncoated #3000SiC abrasives (Ra = 0.054_m; 60 min) and is most reasonablyattributed to the enhanced sliding/grinding effect induced by the wax lubrication layer on the abrasive particles.

Fig. 15 Influence of wax-coating on material removal rate.

Fig. 15 shows that the material removal rate of thewax-coated #3000SiC particles is significantly lower than that ofthe uncoated #3000SiC particles. Hence, it can be inferred that the wax layer reduces the cutting force applied bythe particles on theworkpiece surface.


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An investigation on kerf characteristics in abrasivewaterjet cutting of layered compositesIn abrasive waterjet machining, various machining parameters such as abrasive size, supply pressure, standoffdistance, mass flow rate and cutting speed can be adjusted to influence the depth of cut and surface quality of thecut material (Wang, 2003). However, the cutting capacity in terms of the kerf quality is one of the majorobstructions that limit its application.

Fig. 5 Effect of traverse speed on kerf taper angle.The increase in kerf taper angle is a direct result of the exposure time because at higher traverse, speedless time isavailable forcutting, leading to less overlapping of the jet on the target material.

Fig. 6 Effect of water pressure on kerf taper angle.Results indicate that, within the operating range selected, increase of water pressure results in decrease of kerftaper angles.


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When water pressure is increased, the jet kinetic energy increases that leads to a high momentum transfer of theabrasive particles, generating a wider-bottom kerf. Therefore, the difference in top and bottom kerf width isreduced, leading to a decrease in kerf taper angle.

Effect of standoff distance on kerf taper angleWith increase in standoff distance, the kerf taper increases within the range 25mm as shown in Fig. 7. Byincreasing the standoff distance the material surface is exposed to the downstream of the jet. At downstream, the

jet starts to diverge losing its coherence thereby reducing the effective cutting area that directly affects the kerftaper angle.

Fig. 8 Effect of abrasive mass flow rate on kerf taper angle.It is implicit that a critical energy transfer from the jet to the particles is needed to fracture the material, belowwhich any increase in abrasive mass flow rate does not have an effect on the kerf taper angle.


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In general, the effect of traverse speed and water pressure is pronounced higher compared to

standoff distance with the abrasive mass flow rate having minimal effect. It is recommended that

a combination of high water pressure, low traverse speed and short standoff distance be used toproduce more vertical kerf wall.

Performance of different abrasive materials

during abrasive water jet machining of glass

Fig. 2. Influence of SOD.

It can be observed from that the garnet abrasives produced the largest taper of cut followedby Al2O3 and SiC abrasives. Among the three types of abrasives used, SiC is the hardest

material and consequently it retains its cutting ability as it moves down.

Fig. 3. Influence of feed rate on taper of cut on taper of cut.


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For all types of abrasives the taper of cut shows an increasing trend with increase in workfeed rate. With increase in work feed rate the machining zone is exposed to the jet for ashorter time. Cutting process is less effective at the jet exit that results an increase in taperof cut. Garnet abrasives demonstrate a high taper of cut followed by SiC and Al2O3.

Fig. 4. Effect of pressure on taper of cut.

Taper of cut is smaller for SiC abrasives followed by Al2O3 and garnet. SiC abrasives beingharder than Al2O3 and garnet abrasives retain their sharp edges both at the entrance andthe exit of the jet and produce the smallest width of cut. On the other hand, garnet

abrasives being comparatively softer loose the sharpness of their cutting edges when theyare near the jet exit.


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It was found that SiC produced the widest slot followed by Al2O3 and garnet. This is by virtueof higher hardness of SiC that enables more effective material removal.


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From Figs. 57 it was observed that in all the cases the average width of cut produced bySiC was higher than those produced by Al2O3 and garnet abrasives. It can be concluded thathardness is a key property of abrasive materials.

A correlation for predicting the kerf profile fromabrasive water jet cutting

A study of abrasive water jet machining process onglass/epoxy composite laminate


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A Taguchi and experimental investigation intothe optimal processing conditions for theabrasive jet polishing of SKD61 mold steel


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abrasive water jet machinig - process parameters

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