A Further Investigation of DIAjet Cutting

by:

David A. Summers, Whang-Zhong Wu, Jianchi YaoCurators’ Professor, Associate Professor, Graduate Student

University of Missouri-RollaHigh Pressure Waterjet Laboratory

Rolla, Missouri 65401 U.S.A.

INTRODUCTION

High Pressure Waterjets have become an increasingly common method of cutting inindustrial use. In many applications, however, it has been found necessary to include ameasured amount of abrasive into the waterjet stream in order to effectively cut throughharder materials. The historic method of including this abrasive has been through themixing of the abrasive with the waterjets in a special mixing chamber, after the water hasbeen accelerated to final velocity.

It has been shown by Mazurkiewicz et al. (Reference 1) and confirmed by others, thatwhen the conventional method of abrasive mixing is used much of the abrasive isfragmented in the mixing process. This is not surprising given that the jet is hitting theparticles of abrasive at a velocity which is effective in cutting many geological materials.Because the cutting effectiveness of the jet has been shown to be a function of particlesize by Yazici (Reference 2) among others (Figure 1), the reduced particle size hitting thetarget makes the abrasive jet less effective than it might otherwise be. An alternativemixing method will therefore have considerable potential advantage.

Figure 1. Depth of Cut as a Function of Particle Size (after Yazici et al.).(#3 has an average size of 383 micron, #4 averages 452 micron and #5 515 micron)

DIAJET CUTTING

At the 1986 International Symposium on Jet Cutting Fairhurst et al. (Reference 3) firstdescribed the method of abrasive injection developed at BHRA. This system uses acharged pressure vessel, located between the pressurizing pump and the cutting nozzle, tometer abrasive into the waterjet stream (Figure 2). By using this process and meteringroughly the same concentration of abrasive, but at significantly higher volume flow rates,the resulting jet will cut material at much lower jet pressures. \

Figure 2. Schematic Representation of the DIAjet Equipment.

This technique has considerable potential advantage for use in geotechnical applicationssince it overcomes the high cost and equipment fragility associated with the higherpressure conventional abrasive cutting systems. The lower pressure pumps required withthis approach are relatively inexpensive and can also handle much dirtier water than thatrequired with the higher pressure operations.

The design and use of abrasive jet cutting systems requires, however, that the parameterswhich control this system be at least partially understood before the equipment isdeveloped, and that the parameters be optimized, if the equipment is to perform to its bestlevel. This paper serves to report on a continuing investigation of the parameterscontrolling DIAjet cutting, with particular emphasis on the drilling and cutting of rock.

DIAJET EQUIPMENT

The equipment to be used in this work is a conventional commercially available directinjection of abrasive batch unit which is normally operated at jet pressures of up to 35MPa. Water to the injection system was supplied from a 90 liter/min positivedisplacement high pressure pump. Because of the volume flow rate, the nozzle sizes usedwith the direct injection system are significantly larger than those of the conventionalhigher pressure abrasive cutting units. This has the subsidiary advantage of allowing theuse of larger abrasive particles which, in turn, improves preliminary cutting rates. This isnot an absolute benefit since the larger particles are more likely to fracture during thecutting process. Smaller particles retain their shape after impact, to a significantly largerextent, allowing the abrasive to be recycled. In earlier cutting trials of a waterjet drillingunit, the abrasive has been recycled up to 10 times. At that point, the abrasive was soladen with cuttings and finely pulverized abrasive that the cutting performance wassignificantly reduced.

REACTION FORCE ANALYSISOne of the many advantages cited for the use of high pressure waterjet equipment is therelatively low force required to hold and move the nozzle. This has made it a cutting toolof choice to add to a robot arm in many operations. This feature also makes it possiblefor manual operation of many cleaning guns, in what remains the largest sector ofwaterjet use to date.

Several different methods can be used to calculate the force of reaction, but thesegenerally relate only to the reaction force from a plain waterjet and do not include thechange in force induced when abrasive is metered into the stream. An experiment wasfirst set up, therefore, to measure the reaction force induced by jet action (Figure 3).

Figure 3. Schematic of the Reaction Force Measuring Equipment.

The equipment was designed so that it could be operated either with high pressureconventional abrasive jetting equipment or with one of the DIAjet units available at theUniversity. Simplistically, the unit consists of a metal frame through which a highpressure lance is passed. The nozzle mounting is held below a reaction plate by a loadingcell, previously calibrated. An initial load is applied to this device through a springmounted above the reaction plate. Waterjet pressure just behind the nozzle, and the valueof the reaction force are monitored through a computer. The amount of water leaving thenozzle, and the amount of abrasive used, are established by collecting both volumes overa timed interval and measuring them.

The first tests carried out were with plain water. The reaction force equation given by theWater Jet Association in the United States (Reference 4) is:

(1)

where Q is the flow in gpmP is the Pressure in psi

A second equation, proposed in the UK by Swan (Ref 5) is

(2)

where HP is the horse power of the prime mover

However, the horsepower of the unit can be given by the expression

(3)

This can be used to convert Swan’s equation to the same form as that of the Americanexpression.

(4)

The data has been converted to SI units.Tests were carried out at three nozzle diameters,(0.15, 0.25 and 0.35 mm) using the high pressure, low flow rate unit in the Laboratory,and at a single nozzle diameter (1.65 mm) with a larger flow rate system. Because thepressure readout from the transducer at the nozzle was at some distance from the pressurecontrol valve, exactly reproducible values were not obtained for the pressures used in thetests.

force (1b) = 0.052 ⋅ Q P

force (1b) = 66 ⋅ HP

P

HP = Q⋅ P

1 7 1 4

force (lb) = .0385 ⋅ Q ⋅ P

Data obtained from the initial experiments in which no abrasive was used have beensummarized and exemplified (Figures 4 & 5). Although it is not possible to drawabsolutely consistent conclusions from the data it is clear that the force of reaction issomewhat larger than that predicted by either of the more common equations used in itsderivation, where the flow rate is large (Figure 4). In contrast the reaction force is lowerthan that predicted when the jet flow rate is on the order of 1 gpm or less (Figure 5).Nevertheless, the values obtained are relatively close to the numbers predicted by theseequations. This suggests that the equations can be used to a first order in order toestablish the reaction forces from a jet system when designing waterjet cuttingequipment.

It is interesting to observe the change in the reaction force with the addition of abrasive tothe jet stream. Again only representative figures are presented. A 0.35 mm nozzle wasused for the water acceleration nozzle in the conventional injection system whose resultsare shown in Figure 6.

The significant reduction in the overall reaction force is perhaps, explained by theincreased absorption of energy by the abrasive as the concentration increases. It is alsospeculated that the increased fragmentation of abrasive particles at the higher jet pressurewill also reduce the energy levels available for the combined jet as it leaves the secondnozzle.

In contrast with the power loss of up to 30% with the higher pressure conventionalsystem, the DIAjet system because of its method of water and particle acceleration at thesame time, does not show such a reduction in jet energy with abrasive concentration(Figure 7).

Figure 4. Predicted and Actual Reaction Forces for Flowthrough a 1.65 mm Diameter Jet Nozzle (with the WJTA equation).

Figure 5. Predicted and Actual Reaction Forces for Flowthrough a 0.35 mm Diameter Jet Nozzle (using the WJTA equation).

Figure 6. Reaction Forces Measured at Two Jet Pressures, with Increasing AbrasiveFlow, using a Conventional Design with a Waterjet Nozzle Diameter of 0.35 mm.

Figure 7. Reaction Force as a Function of Increasing Abrasive Flow, using a DIAjetDesign with a Waterjet Nozzle Diameter of 2.84 mm. (The jet pressure increasedslightly from 30 to 32 MPa as the concentration of abrasive was increased).

The absolute values of the abrasive quantities used are significantly higher with theDIAjet system but the relative proportion of abrasive to water has a maximum value of5310 gm/gal with the conventional injection and only 838 gm/gal with the DIAjet.However even when similar concentrations of abrasive are compared the reduction inforce at the higher pressure is still, at 15%, greater than the reduction of 8% with theDIAjet system. In the latter case the reaction force is considerably more stable therefore,over the range of abrasive investigated.

MECHANISMS OF MATERIAL REMOVAL

There has been much discussion over the years in regard to the mechanism by which highpressure waterjets remove material. In seeking to develop low pressure abrasive jetcutting, it is of value to examine this discussion, as it relates to both water and abrasiveremoval of material. Where plain waterjet impact occurs, in many cases, the removalprocess is one of failure by crack extension. The initial arrival of the waterjet on thesurface will cause water to penetrate into surface cracks and flaws. Subsequent jetimpact will pressurize the small fluid wedges which will induce the cracks to growfurther and ultimately lead to material removal. This mechanism does not prevail inabrasive jet impact where there is considerable evidence that the vast majority of the

material removal occurs due to abrasive impact, with relatively little occurring due to theaction of the water.

Previously investigators have also shown that dry abrasive removal of material from asurface is controlled by the brittleness or ductility of the target material which significantdifferences between the two mechanisms. In order to verify this process in wet abrasivecutting, microphotographs have been taken of the surface under impact by high pressureabrasive jets. Initially, tests were carried out on steel to represent ductile material, andglass representing a more brittle response.

METAL SAMPLES

Small coupons of steel were cleaned for 15 seconds in acetone before being tested. Inorder to more easily examine the mechanism of material removal, microphotographswere taken outside the zone where the majority of the cutting occurred so that the effectsof individual particle impact could be more clearly discerned. Pressures were at a lowerlevel than normally used in order to reduce the amount of damage and allow a clearerpicture of the removal mechanisms.

To evaluate the role of water in the cutting of the steel, initial tests were made withwaterjets at driving pressures of 70. 140 and 210 MPa. At the two lower pressures therewas little significant material removal, but by 210 MPa the jet passage left a clear trackacross the sample, with pitting of the surface and evidence of waterjet penetration alongcrystal boundaries into the material (Figures 8 and 9).

These results would suggest, as have other data, that at the lower jet pressures commonlyused with DIAjet cutting that the role of water in the material removal process isrelatively insignificant by itself.

In contrast when a simple abrasive laden air stream at 0.75 MPa was directed at thesample, significant material was removed from the surface, which was left pitted over theentire area. This was because, in order to reduce material removal rates, the sample wasplaced some 27.5 cm from the nozzle. Some particles of the sand used for this blastingtest were also found embedded in the surface, although these were generally significantlydamaged (Figures 10 and 11).

Figure 8. 210 MPa Waterjet Traverse over Steel at 50X Magnification.

Figure 9. 210 MPa Waterjet Traverse over Steel at 2,000X MagnificationShowing Grain Penetration.

Figure 10. Conventional Sandblasted Surface at 300X Magnification.

Figure 11. Detail from a Sandblasted Surface at 1000X MagnificationShowing a Sand Grain.

Figure 12. Steel Surface Impacted by a 7 MPa DIAjet at 300X Magnification.

A 7 MPa DIAjet cutting on the same material left a somewhat different pattern which hasnot yet been fully explained (Figure 12).

As with the sandblasting, individual particles of the abrasive could be seen in the finalsurface of the target. When the edge of the surface was examined, the cutting andshearing action of the individual abrasive particles could be clearly identified (Figure 13),this was also evident on examination of the central zone of material removed (Figure 14).

Figure 13. Single Particle Impact on the Edge of the Cutting Zone.

Figure 14. Multiple Particle Impacts in the Center of the Cutting Zone.

These photographs confirm that the major mechanism for material removal underabrasive waterjet impact appears to follow that for dry abrasive impact and thus it can beanticipated that the same rules will apply for ductile material removal under DIAjetimpact as hold true under dry abrasive impact.

GLASS TARGET

When the DIAjet was used to cut a brittle material , the mechanism of material removalchanges. It has been shown by earlier investigators that in brittle impact, circularfractures occur around the impact point generating cracks which grow into the material(Figure 15). These cracks are frequently referred to as Hertzian ring fractures (Reference5). Material is, therefore, removed by a fracture growth mechanism from these initialcracks.

This is obviously a different mechanism from that of the ductile response. It is, therefore,unlikely to be describable by a common equation seeking to combine both and cover allabrasive cutting. It is important, therefore, as efforts continue to develop theoreticalexplanations for abrasive removal of material to confirm the mechanism of materialremoval in brittle targets.

A glass sample was, therefore, cut by a DIAjet at a pressure of 1,000 psi. The materialremoval mechanism within the major area of cutting is not clearly discernable because ofthe overlapping nature of the impact phenomenon. However, where the view is moved tothe edge of the cutting zone, individual particle impacts and their results can be seen.Close examination of one of these impact points indicates (Figure 15) the extension ofcracks from a central highly fractured impact zone out into the material. The conchoidal

nature of these surfaces confirms that these are cracks extended out from the initialfractures generated on impact.

Figure 15. Schematic Representation of Brittle Damage Under Abrasive Impact.

Figure 16. Damage from a DIAjet Particle Impact on Glass 1000X Magnification.

COMMENTS

This study of DIAjet behavior indicates that the abrasive laden stream is acting verymuch along the same lines as those occurring under dry abrasive impact. Two separatemechanisms for material removal can be identified and the target material response mustbe incorporated in any model of material removal. It will also likely play a part in theselection of the most effective abrasive material for different target surfaces.

Particularly, in more brittle materials, it is likely that the abrasive particle shape will havea greater significance on the material removal rate than occurs in ductile materials.Whether, as waterjet pressure is increased, the water pressure will be able to more readilyexploit the fractures induced in the brittle material surface (and, if so—how?) remains tobe determined.

Ductile material removal under abrasive impact has been seen to occur through a processwhich requires that the metal melt to deform. This metal melting which is evident inmicrophotographs means that the particles of metal removed from the surface will haveinstantaneously a very hot outer surface. This is evidenced by the sparking often seen inabrasive jet cutting of metal. Given that the high temperature is a part of the materialremoval process, it cannot be completely removed. However, the presence of highvolumes of water, in the vicinity of the cutting process, is usually going to be sufficient torapidly cool the surface and to remove sufficient heat that the ignition of evencombustible mixtures of gas in the vicinity becomes less likely.

ACKNOWLEDGEMENTS

This work is being carried out, in part, under funding from the Generic MineralsTechnology Center program of the U.S. Bureau of Mines through the Mine SystemsDesign and Ground Control Center at Virginia Polytechnic Institute. Part of this work isalso being carried out in cooperation with the National Association of CorrosionEngineers. This interest and assistance is greatly appreciated.

This work was carried out with the assistance of Mr. J. Kaufmann and with the advice ofDr. O’Keefe and Dr. Hale. This is gratefully acknowledged.

REFERENCES

Mazurkiewicz, M, Olko, P., and Jordan, R., “Abrasive Particle Distribution in a HighPressure Hydroabrasive Jet”, International Waterjet Symposium, Beijing, China.,September 1987.

Yazici, S., Abrasive Jet Cutting and Drilling of Rock. Ph.D. Thesis, University ofMissouri-Rolla, 1989.

Fairhurst, R.M., Heron, R.A., and Saunders, D.H., “Diajet” - a new abrasive water jetcutting technique”, 8th International Symposium on Jet Cutting Technology, Durham,England, September, 1986.

Water Jet Technology Association, Recommended Practices for the use of ManuallyOperated High Pressure Water Jetting Equipment. 1987.

Evans, A.G., “Impact Damage Mechanisms: Solid Projectiles” in Erosion a Treatise onMaterials Science and Technology, ed C. M. Preece, Academic Press, 1979.