A Further Investigation of DIAjet Cutting. by:

Transcription

1 INTRODUCTION A Further Investigation of DIAjet Cutting by: David A. Summers, Whang-Zhong Wu, Jianchi Yao Curators Professor, Associate Professor, Graduate Student University of Missouri-Rolla High Pressure Waterjet Laboratory Rolla, Missouri U.S.A. High Pressure Waterjets have become an increasingly common method of cutting in industrial use. In many applications, however, it has been found necessary to include a measured amount of abrasive into the waterjet stream in order to effectively cut through harder materials. The historic method of including this abrasive has been through the mixing of the abrasive with the waterjets in a special mixing chamber, after the water has been accelerated to final velocity. It has been shown by Mazurkiewicz et al. (Reference 1) and confirmed by others, that when the conventional method of abrasive mixing is used much of the abrasive is fragmented in the mixing process. This is not surprising given that the jet is hitting the particles 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 particle size by Yazici (Reference 2) among others (Figure 1), the reduced particle size hitting the target makes the abrasive jet less effective than it might otherwise be. An alternative mixing 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)

2 DIAJET CUTTING At the 1986 International Symposium on Jet Cutting Fairhurst et al. (Reference 3) first described the method of abrasive injection developed at BHRA. This system uses a charged pressure vessel, located between the pressurizing pump and the cutting nozzle, to meter abrasive into the waterjet stream (Figure 2). By using this process and metering roughly 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 applications since it overcomes the high cost and equipment fragility associated with the higher pressure conventional abrasive cutting systems. The lower pressure pumps required with this approach are relatively inexpensive and can also handle much dirtier water than that required with the higher pressure operations. The design and use of abrasive jet cutting systems requires, however, that the parameters which control this system be at least partially understood before the equipment is developed, and that the parameters be optimized, if the equipment is to perform to its best level. This paper serves to report on a continuing investigation of the parameters controlling DIAjet cutting, with particular emphasis on the drilling and cutting of rock.

3 DIAJET EQUIPMENT The equipment to be used in this work is a conventional commercially available direct injection of abrasive batch unit which is normally operated at jet pressures of up to 35 MPa. Water to the injection system was supplied from a 90 liter/min positive displacement high pressure pump. Because of the volume flow rate, the nozzle sizes used with the direct injection system are significantly larger than those of the conventional higher pressure abrasive cutting units. This has the subsidiary advantage of allowing the use of larger abrasive particles which, in turn, improves preliminary cutting rates. This is not an absolute benefit since the larger particles are more likely to fracture during the cutting process. Smaller particles retain their shape after impact, to a significantly larger extent, allowing the abrasive to be recycled. In earlier cutting trials of a waterjet drilling unit, the abrasive has been recycled up to 10 times. At that point, the abrasive was so laden with cuttings and finely pulverized abrasive that the cutting performance was significantly reduced. REACTION FORCE ANALYSIS One of the many advantages cited for the use of high pressure waterjet equipment is the relatively low force required to hold and move the nozzle. This has made it a cutting tool of choice to add to a robot arm in many operations. This feature also makes it possible for manual operation of many cleaning guns, in what remains the largest sector of waterjet use to date. Several different methods can be used to calculate the force of reaction, but these generally relate only to the reaction force from a plain waterjet and do not include the change in force induced when abrasive is metered into the stream. An experiment was first set up, therefore, to measure the reaction force induced by jet action (Figure 3). Figure 3. Schematic of the Reaction Force Measuring Equipment.

4 The equipment was designed so that it could be operated either with high pressure conventional abrasive jetting equipment or with one of the DIAjet units available at the University. Simplistically, the unit consists of a metal frame through which a high pressure lance is passed. The nozzle mounting is held below a reaction plate by a loading cell, previously calibrated. An initial load is applied to this device through a spring mounted above the reaction plate. Waterjet pressure just behind the nozzle, and the value of the reaction force are monitored through a computer. The amount of water leaving the nozzle, and the amount of abrasive used, are established by collecting both volumes over a timed interval and measuring them. The first tests carried out were with plain water. The reaction force equation given by the Water Jet Association in the United States (Reference 4) is: force (1b) = Q P (1) where Q is the flow in gpm P is the Pressure in psi A second equation, proposed in the UK by Swan (Ref 5) is 66 HP force (1b) = P (2) where HP is the horse power of the prime mover However, the horsepower of the unit can be given by the expression HP = Q P 1714 (3) This can be used to convert Swan s equation to the same form as that of the American expression. force (lb) =.0385 Q P (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 the pressure readout from the transducer at the nozzle was at some distance from the pressure control valve, exactly reproducible values were not obtained for the pressures used in the tests.

5 Data obtained from the initial experiments in which no abrasive was used have been summarized and exemplified (Figures 4 & 5). Although it is not possible to draw absolutely consistent conclusions from the data it is clear that the force of reaction is somewhat larger than that predicted by either of the more common equations used in its derivation, where the flow rate is large (Figure 4). In contrast the reaction force is lower than 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 these equations. This suggests that the equations can be used to a first order in order to establish the reaction forces from a jet system when designing waterjet cutting equipment. It is interesting to observe the change in the reaction force with the addition of abrasive to the jet stream. Again only representative figures are presented. A 0.35 mm nozzle was used for the water acceleration nozzle in the conventional injection system whose results are shown in Figure 6. The significant reduction in the overall reaction force is perhaps, explained by the increased absorption of energy by the abrasive as the concentration increases. It is also speculated that the increased fragmentation of abrasive particles at the higher jet pressure will also reduce the energy levels available for the combined jet as it leaves the second nozzle. In contrast with the power loss of up to 30% with the higher pressure conventional system, the DIAjet system because of its method of water and particle acceleration at the same time, does not show such a reduction in jet energy with abrasive concentration (Figure 7). Figure 4. Predicted and Actual Reaction Forces for Flow through a 1.65 mm Diameter Jet Nozzle (with the WJTA equation).

6 Figure 5. Predicted and Actual Reaction Forces for Flow through a 0.35 mm Diameter Jet Nozzle (using the WJTA equation). Figure 6. Reaction Forces Measured at Two Jet Pressures, with Increasing Abrasive Flow, using a Conventional Design with a Waterjet Nozzle Diameter of 0.35 mm.

7 Figure 7. Reaction Force as a Function of Increasing Abrasive Flow, using a DIAjet Design with a Waterjet Nozzle Diameter of 2.84 mm. (The jet pressure increased slightly from 30 to 32 MPa as the concentration of abrasive was increased). The absolute values of the abrasive quantities used are significantly higher with the DIAjet system but the relative proportion of abrasive to water has a maximum value of 5310 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 in force at the higher pressure is still, at 15%, greater than the reduction of 8% with the DIAjet 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 high pressure waterjets remove material. In seeking to develop low pressure abrasive jet cutting, it is of value to examine this discussion, as it relates to both water and abrasive removal of material. Where plain waterjet impact occurs, in many cases, the removal process is one of failure by crack extension. The initial arrival of the waterjet on the surface will cause water to penetrate into surface cracks and flaws. Subsequent jet impact will pressurize the small fluid wedges which will induce the cracks to grow further and ultimately lead to material removal. This mechanism does not prevail in abrasive jet impact where there is considerable evidence that the vast majority of the

8 material removal occurs due to abrasive impact, with relatively little occurring due to the action of the water. Previously investigators have also shown that dry abrasive removal of material from a surface is controlled by the brittleness or ductility of the target material which significant differences between the two mechanisms. In order to verify this process in wet abrasive cutting, microphotographs have been taken of the surface under impact by high pressure abrasive jets. Initially, tests were carried out on steel to represent ductile material, and glass representing a more brittle response. METAL SAMPLES Small coupons of steel were cleaned for 15 seconds in acetone before being tested. In order to more easily examine the mechanism of material removal, microphotographs were taken outside the zone where the majority of the cutting occurred so that the effects of individual particle impact could be more clearly discerned. Pressures were at a lower level than normally used in order to reduce the amount of damage and allow a clearer picture of the removal mechanisms. To evaluate the role of water in the cutting of the steel, initial tests were made with waterjets at driving pressures of and 210 MPa. At the two lower pressures there was little significant material removal, but by 210 MPa the jet passage left a clear track across the sample, with pitting of the surface and evidence of waterjet penetration along crystal boundaries into the material (Figures 8 and 9). These results would suggest, as have other data, that at the lower jet pressures commonly used with DIAjet cutting that the role of water in the material removal process is relatively insignificant by itself. In contrast when a simple abrasive laden air stream at 0.75 MPa was directed at the sample, significant material was removed from the surface, which was left pitted over the entire area. This was because, in order to reduce material removal rates, the sample was placed some 27.5 cm from the nozzle. Some particles of the sand used for this blasting test were also found embedded in the surface, although these were generally significantly damaged (Figures 10 and 11).

9 Figure MPa Waterjet Traverse over Steel at 50X Magnification. Figure MPa Waterjet Traverse over Steel at 2,000X Magnification Showing Grain Penetration.

10 Figure 10. Conventional Sandblasted Surface at 300X Magnification. Figure 11. Detail from a Sandblasted Surface at 1000X Magnification Showing a Sand Grain.

11 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 has not yet been fully explained (Figure 12). As with the sandblasting, individual particles of the abrasive could be seen in the final surface of the target. When the edge of the surface was examined, the cutting and shearing 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.

12 Figure 14. Multiple Particle Impacts in the Center of the Cutting Zone. These photographs confirm that the major mechanism for material removal under abrasive waterjet impact appears to follow that for dry abrasive impact and thus it can be anticipated that the same rules will apply for ductile material removal under DIAjet impact as hold true under dry abrasive impact. GLASS TARGET When the DIAjet was used to cut a brittle material, the mechanism of material removal changes. It has been shown by earlier investigators that in brittle impact, circular fractures occur around the impact point generating cracks which grow into the material (Figure 15). These cracks are frequently referred to as Hertzian ring fractures (Reference 5). Material is, therefore, removed by a fracture growth mechanism from these initial cracks. 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 all abrasive cutting. It is important, therefore, as efforts continue to develop theoretical explanations for abrasive removal of material to confirm the mechanism of material removal in brittle targets. A glass sample was, therefore, cut by a DIAjet at a pressure of 1,000 psi. The material removal mechanism within the major area of cutting is not clearly discernable because of the overlapping nature of the impact phenomenon. However, where the view is moved to the 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 of cracks from a central highly fractured impact zone out into the material. The conchoidal

13 nature of these surfaces confirms that these are cracks extended out from the initial fractures 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.

14 COMMENTS This study of DIAjet behavior indicates that the abrasive laden stream is acting very much along the same lines as those occurring under dry abrasive impact. Two separate mechanisms for material removal can be identified and the target material response must be incorporated in any model of material removal. It will also likely play a part in the selection of the most effective abrasive material for different target surfaces. Particularly, in more brittle materials, it is likely that the abrasive particle shape will have a 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 readily exploit the fractures induced in the brittle material surface (and, if so how?) remains to be determined. Ductile material removal under abrasive impact has been seen to occur through a process which requires that the metal melt to deform. This metal melting which is evident in microphotographs means that the particles of metal removed from the surface will have instantaneously a very hot outer surface. This is evidenced by the sparking often seen in abrasive jet cutting of metal. Given that the high temperature is a part of the material removal process, it cannot be completely removed. However, the presence of high volumes of water, in the vicinity of the cutting process, is usually going to be sufficient to rapidly cool the surface and to remove sufficient heat that the ignition of even combustible mixtures of gas in the vicinity becomes less likely. ACKNOWLEDGEMENTS This work is being carried out, in part, under funding from the Generic Minerals Technology Center program of the U.S. Bureau of Mines through the Mine Systems Design and Ground Control Center at Virginia Polytechnic Institute. Part of this work is also being carried out in cooperation with the National Association of Corrosion Engineers. This interest and assistance is greatly appreciated. This work was carried out with the assistance of Mr. J. Kaufmann and with the advice of Dr. O Keefe and Dr. Hale. This is gratefully acknowledged. REFERENCES Mazurkiewicz, M, Olko, P., and Jordan, R., Abrasive Particle Distribution in a High Pressure Hydroabrasive Jet, International Waterjet Symposium, Beijing, China., September Yazici, S., Abrasive Jet Cutting and Drilling of Rock. Ph.D. Thesis, University of Missouri-Rolla, 1989.

15 Fairhurst, R.M., Heron, R.A., and Saunders, D.H., Diajet - a new abrasive water jet cutting technique, 8th International Symposium on Jet Cutting Technology, Durham, England, September, Water Jet Technology Association, Recommended Practices for the use of Manually Operated High Pressure Water Jetting Equipment Evans, A.G., Impact Damage Mechanisms: Solid Projectiles in Erosion a Treatise on Materials Science and Technology, ed C. M. Preece, Academic Press, 1979.