Ultrahigh-Pressure Fluidjet Technology (Updated January 2012)

(Peter) H.-T. Liu

 

Introduction

 

The technology to form ultrahigh-speed fluidjets using ultrahigh-pressure (UHP) pumps was originally developed in the 1970s at Flow Industries, Inc [1].  The author has not only witnessed the maturing of the technology but also participated in various stages of its development.  Subsequently, he switched his R&D focus in advancing UHP fluidjet technology in 1995.

To form a fluidjet, working fluid such as water or liquid nitrogen is pumped to ultrahigh pressures and expelled through a nozzle consisting of a sapphire or diamond orifice to form a coherent, high-speed cutting jet.  The two most popular UHP pumps currently in use are of the intensifier and crankshaft types.  At 60 ksi (414 MPa), the resultant jet, which varies in diameter from 0.003 to 0.070 inch (0.076 to 1.78 mm), exits the nozzle at speeds of up to 3,000 ft/sec (914 m/s), or over 2.5 times the speed of sound.  The kinetic energy associated with these high-velocity jets can be used to cut virtually any material.  Pure waterjets (WJs), without abrasives, are commonly used to cut materials such as paper products, cloth, leather, wood, and soft rocks.  For harder materials, such as glass, metal, hard rocks, and most composites, abrasive particles are introduced into the waterjet stream to form an abrasive-waterjet (AWJ).  The most commonly used abrasive material is 50- to 320-mesh garnet.  Cutting, or controlled depth penetration of target materials, occurs as a result of erosion, shearing, failure under rapidly changing localized stress fields, or micromachining effects, depending on the specific properties of the material being cut.  Advantages of AFJs over conventional machining methods include:

·       Cutting tool does not become dull

·       Minimal force is exerted onto the target material

·       The target material is not heated or hardened during machining

·       Nearly any type of material can be machined with a single AFJ

·       Compatible with underwater operations

·       Cryogenic FJs are generally safe for handling explosives and hazardous materials

The AFJ can be readily deployed to cut, drill, mill, and turn almost any material.  Fundamentally, the AFJ is a cold process; unlike laser and EDM, it generates very little heat to cause damage to the target materials while machining.  For machining materials with different mechanical properties, various abrasive types with a wide range of hardness and specific gravities are available commercially.  In essence, versatility and material independence are two of the above-mentioned advantages of the AFJ over conventional machining tools.  Note that objects of certain geometries in terms of dimensions (e.g., hole size and aspect ratio) and orientations (e.g., angle of inclination) simply cannot be machined with conventional tools, especially when the workpieces are made of glass, ceramics, inconel, titanium, and other delicate materials that tend to be damaged by excessive mechanical and/or thermal stresses induced by the machining tools.  For example,   AFJs cut titanium 34% faster than stainless steel and there is little difference in cutting hardened and raw steel.  Cutting pre-hardened steel prevents deformation of parts induced by the heat treatment process.  In essence, the AFJ has the following "5M" advantages, - "Machining Most Materials from Macro to Micro scale".

Principle of Operation

Figure 1 shows schematics of an UHP intensifier pump and FJ and AFJ nozzles using water as the working fluid.  As shown in Fig. 1a, 3-ksi (21-MPa) pre-pressurized water (blue) is fed into the pump and is further pressurized to 55 ksi (380 MPa) by the reciprocal action of a piston with an area ratio of 20 to 1.  The UHP water (red) is then forced through 2 sets of check valves at the 2 ends of the piston.  An accumulator is generally used to store the UHP water to smooth out the pressure spikes before feeding into the WJ/AWJ nozzle.  Multi-plunger, crankshaft-driven pumps, as illustrated in Fig. 1b have become popular nowadays because of their high efficiency, compactness, and long operating life (http://www.omax.com/waterjets/direct-drive-pumps). 

Figures 1c and 1d illustrate the schematics of the WJ/ASJ and AWJ, respectively.  The WJ and ASJ are respectively one-phase and two-phase flows at the nozzle exits and subsequently turned into two-phase and three-phase flows after air is entrained into the jets.  The AWJ is a three-phase flow at the nozzle exit as both abrasives and air are entrained at the feed port before entering into the mixing tube.   The coexistence and interaction among water, air, and abrasives in the AWJ and ASJ create complex three-phase flows.  Further complication results as the three-phase flows impinge onto the target surface and remove materials from that surface.

The characteristics of the AFJ, particularly the mechanisms of material removal, are far from fully understood.   As shown in Fig. 1d, abrasives entrained through a feed port are accelerated by the ultrahigh-speed WJ.  Once the threshold speed of a particular material is exceeded, the abrasives begin to remove material from the target surface at the point of contact.  The threshold speed is defined as the speed below which no material is removed by the WJ/AWJ.  For a given material, the threshold speed of the WJ is considerably higher than that of the AWJ [2].  The current AWJ nozzle configuration based on the entrainment process is by no means optimized in accelerating abrasives.  Even in the absence of abrasives, the performance of the WJ deteriorates as soon as the abrasive feed port is open.  Evidently, the entrained air reduces the coherency of the jet and causes it to spread rapidly [2].  A part of the kinetic energy in the WJ is consumed in accelerating the entrained air.  The interaction between the entrained air and the WJ causes premature breakup of the WJ and results in increasing the drag in the two-phase flow.  When abrasives are added, the entrained abrasives are not accelerated uniformly in the mixing tube by the WJ, leaving a large percentage of abrasives moving at speeds below the threshold speed for removing the target material.

Crankshaft pump

a. Intensifier Pump

b. Crankshaft Pump

c. Waterjet and Abrasive Slurry Jet Nozzle

d. Abrasive Waterjet Nozzle

Fig. 1.  Schematics of Key Elements of UHP WJ, ASJ, and AWJ


To improve the rate of material removal, an abrasive suspension or slurry jet (ASJ) was developed by direct pumping abrasive slurry through the nozzle [3].  Abrasives in the ASJ are accelerated more efficiently and uniformly than those in the AWJ.  See in Fig. 1c for a schematic of an ASJ nozzle.  It has been demonstrated that the ASJ is capable of achieving up to 5 times the material removal rate of the AWJ operating at the same pressure [5].  At present, high-pressure ASJs up to 20 ksi (138 MPa) are commercially available.  At pressure higher than 20 ksi (138 MPa), however, the abrasive slurry tends to wear out various components, such as check valves and mixing tubes, too fast to meet the reliability standards of an industrial tool.  Availability of super wear resisting materials is the key to commercialize rugged and reliable UHP ASJs.  Under certain conditions when wetting is not tolerable or cryogenic temperature is desirable, liquid nitrogen is used as the working fluid to form a cryogenic jet (CJ) or an abrasive-cryogenic jet (ACJ) [4].  For these jets, thermodynamics plays an important role in forming a coherent jet for effective machining. 

For precision machining and users friendliness, considerable progress has been made to automate the AFJ machining processes.  Waterjets are generally amenable to 3D machining with certain limitations. Spent abrasives still possess considerable residual erosive power to cause damage to the rest of the workpiece being cut, particularly if the waste stream comes in contact with other parts of the workpiece. For 3D parts with complex geometry, materials downstream and along the path of the cutting AWJ must be protected by a “catcher” to capture the spent abrasives, or by sacrificial pieces to absorb or dissipate the residual erosion power of spent abrasives. For complex 3D parts, however, there is usually not enough room to place the catcher or sacrificial pieces where they are needed. Most commercial AWJ systems machine 2D and 2D+ parts, although there are commercial 5+-axis AWJ systems available for machining simple 3D parts, as illustrated in Fig. 2.

PaR_man

Fig 2. Computer-Controlled 5-Axis AWJ Machining Workstation [5]

 

Alternatively, novel accessories for 2D AWJ systems have been developed to take into account the jet geometry and imperfection of raw materials for improving cutting accuracy, facilitating the fabrication of nearly 3D parts and multimode machining (http://www.omax.com/waterjet-cutting-accessories/A-Jet/163).  Figure 3 illustrates a 6-step procedure for machining a 3 dimensional honeycomb panel.  The author has used this system to machine titanium honeycomb cores as one of the prospective lightweight material for building the High Speed Civil Transport (HSCT) [5].   

 

3D_prof_a

3D_prof_b

3D_prof_e

Step 1

Step 2

Step 3

3D_prof_c

3D_prof_f

3D_prof_d

Step 4

Step 5

Step 6

Fig. 3.  5-Step 3-D AWJ Profiling of Honeycomb Panel with Aluminum Facesheets and Core [5]


 

FJs/AFJs Applications

The WJ/AWJ has been applied successfully in factories and fields for machining and surface preparation.  In view of the advantages mentioned above, the WJ/AWJ has been applied for cutting disposable diapers, machining and lightweighting metals and composites, drilling small and large-aspect-ratio holes on glass and metals, to near-net shaping of optical glasses [3, 6, and 7].  Figure 4 displays several AWJ-machined samples to demonstrate its versatility as a machining tool.  Special processes have been and are developed for precision machining including drilling long-aspect-ratio and small-diameter holes on various materials and laminates, shape cutting silicon and graphite wafers, and lightweighting space mirrors.

Fig. 4. Examples of AFJ Machined Parts

One of the emerging developments is mAWJ technology through downsizing of AWJ nozzles for micromachining [8].  Cost effective and user friendly micromachine tools have been in great demand for various industrial, military, and biomedical applications from art and craft, jewel and watch making, singulation of SD chips of mobile phones, micro electromechanical systems (MEMS), to fabrication of various implants..  Such a demand is expected to ramp up significantly when commercial products of green energy and micro-nano technologies are brought online in the near future.  Products from green energy and micro-nano technologies have predicted to dominate the industrial, military, and consumer markets in the next several decades.  These technologies have received global attention and asserted R&D efforts and significant resources have been directed toward their commercialization.  Commercial products for consumer and industrial applications are expected to be available in three to five years [9].   One of the key bottlenecks for the commercialization is the lack of cost effective and versatile micromachining tools to manufacture affordable products, particularly for consumer applications.  Versatility is particularly important for machining nano devices as bottom-up nanotechnology is capable of integrating seamlessly and optimally various materials forming new classes of materials that would outperform conventional counterparts such as composites and laminates in terms of strength-to-weight and strength-to-cost for optimized performance.  As such, a single conventional machine tool that is material selective might have difficulty in cutting these nano components or devices.  For certain devices, more than one tool would have to be used to take into account drastic differences in mechanical and optical properties of their composition, significantly driving up the manufacturing costs.  Waterjet technology has the potential to be such a versatile tool that is cost effective, material independent, and capable of preserving the structural integrity of integrated materials.  AWJ micromachining tools would be expected to have broad applications that require top-down manufacturing from parting micro/nano product components to machining micro devices (components of MEMS, fuel cells, reactors, reformers, heat exchangers, and biomedical implants, etc.).  Figure 5 illustrates examples of AWJ/WJ machined meso- and micro-features for various materials.  The interlocking features in Fig. 5c and the miniature model of the space needle in Seattle were machined with the use of a Rotary Axis (http://www.omax.com/waterjets/rotary-axis).

image008.jpg

Fig. 5.  Meso- and micro-machining with miniature AWJ/WJ nozzles - M is the machineability of materials.  The scale is in millimeter

The prospect of developing low-cost mAWJ technology for various applications from microelectronics, orthopedics/prosthetics, green energy production, and others has led to the award of NSF SBIR Phase I and II grants for a feasibility and prototype development of mAWJ nozzles capable of machining features around 50 mm to 100 mm (Grant #0944239).  Laboratory models of mAWJ nozzles were developed to demonstrate the technical feasibility of the proposed development.  Figure 6 illustrates miniature composite and metallic samples machined with these models [10].  The mAWJ nozzle was also applied to fabricate orthopedic implants made of titanium that is known to be difficult to machine with conventional tools but not with AWJs.  Figure 7 illustrates several AWJ-machined orthopedic components used as trauma and reconstruction mesh implants [11].  During Phase I R&D, three patent applications were submitted for novel processes pertained to AWJ micromachining.

 

IMG_1616_C

IMG_1557_c

a. G-10 (3.2 mm)

b. Carbon epoxy (5 mm)

c. Alumina (3.2 mm)

mini metal parts

d. Various metal parts

e. Interlocking link

 

Fig. 6.  Miscellaneous miniature parts machined from various materials - scale in mm.  Numbers in parentheses are the material thickness.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 7.  AWJ-machined orthopedic components – scale in mm.

For machining miniature mechanical components, the 256mm nozzle was used to cut a set of planetary gears.  The set consists of seven gears [a sun gear (9.68 mm OD), a ring gear (19.05 mm OD), and five small planetary gears (3.55 mm OD)], a gear mounting plate, a gear carrier. Figure 18a illustrates the components of the gear set.  Also included are the tool paths of the seven gears corresponding to the screen display of the PC-based CAD program – LAYOUT (http://www.omax.com/waterjets/layout-software).  Note that tool paths with a magenta color represent an edge quality of 3 out of 5, with 5 as the best edge quality.  The ring and sun gears were nested to save material.  The LAYOUT diagram was then transferred to the CAM program, MAKE, for machining the planetary gear set made of 0.62 mm thick stainless steel plate using the 256mm nozzle (http://www.omax.com/waterjets/make-software).  Figure 18b illustrates the assembled planetary gear set driven by a micro spur gear head motor (Solarobotics, Model GM14a) powered by a single AAA battery.

P1050325_d

a. Components and tool paths of planetary gear

P1050354_c2

b. Assembled

Figure 8.  AWJ-Machined Planetary Gear Set


 Under the support of NSF SBIR Phase I and II grants (#0512066 and #0620277), a prototype flash abrasive-waterjet (FAWJ) was developed for mitigating piercing damage in delicate materials such as laminates, composites, and extremely brittle materials (patent pending [12]).  The FAWJ is a low-cost emulation of the phase change process of an abrasive cryogenic jet that uses liquefied nitrogen (LN2) as the working fluid [13, 14].  Instead of using liquefied nitrogen, the FAWJ superheats the high-pressure water just upstream of the nozzle.  This superheated water, just like the LN2, evaporates upon exiting the nozzle and escapes before entering the blind hole, significantly reducing the buildup of piercing pressure.  As such, piercing damage is mitigated in most delicate materials.   Piercing tests on a collection of industrial and aerospace materials with weak tensile strengths were conducted with good results [14].  In essence, the FAWJ has helped realize the full potential of waterjet technology as a material independent machine tool that cannot be matched by most existing tools.  Figure 9 illustrates samples of selected delicate materials machined with the FAWJ and conventional AWJ, preservation of structural integrity of the FAWJ by mitigating delamination, chipping, and cracking is clearly evident. 

image018.jpg

Figure 9.  Samples Machined with FAWJs and AWJs/w Pressure Ramping and Vacuum Assist

(Number in parentheses is the sample thickness)

The many advantages of the WJ/AWJ over conventional cutting/machining methods have led to numerous field applications.  For example, as illustrated in Fig. 10, specialized field deployable tools have been developed for scarifying worn-out road surfaces, decontaminating and decommissioning hazardous sites, and rejuvenating nuclear power plants for extended operations.  When operating in a pulsed mode, the WJ can be used as an efficient tool for breaking rocks.

One of the recent developments was a patented NewjetTM nozzle for surface preparation [15].  The NewjetTM nozzle incorporates a multi-staged acceleration mechanism; compressed air followed by a UHP WJ was used to accelerate the abrasives to speeds considerably higher than those achieved with its single-staged counterpart – the sand-blasting nozzle.  As a result, the NewjetTM technology is superior to sand blasting for surface preparation in terms of cleaning rate and cost effectiveness.  Because the NewjetTM is inherently a wet process, it produces little to no airborne dust and is therefore environmentally friendlier than sand blasting.   The spent abrasives of the AWJ can be readily collected and recycled to reduce the amount of hazardous waste and the associated costs for its treatment and disposal.  Figure  and 11 shows 2 examples using the NewjetTM for removing corroded paint on a ship hull and an offshore platform.

Fig. 10. Field Deployment of AFJ

Fig. 11.  Removal of Corroded Paint on a Ship Hull and Offshore Platform with NewjetTM

References

[1]   Olsen, J. H., Hurlburt, G. H., Kapcsandy, L. E. (1980) “Method of Making High Velocity Liquid Jet,” U.S. Patent No. 4,216,906.

[2]   Liu, H.-T., Miles, P., Hibbard, C. and Cooksey, N. (1999) “Measurements of Water-Droplets and Abrasive Speeds in Waterjets and Abrasive Waterjets,” Proc. 10th Amer. Waterjet Conf., Houston, Texas, August 14-17.

[3]   Liu, H.-T. (1998) “Near-Net Shaping of Optical Surfaces with Abrasive Suspension Jets,” Proc. 14th Int. Conf. on Jetting Technology, Brugge, Belgium, September 21 – 23, pp. 285-294.

[4]   Liu, H.-T., Fang, S., and Hibbard, C. (1999) “Enhancement of Ultrahigh-Pressure Technology with LN2 Cryogenic Jets,” Proc. 10th Amer. Waterjet Conf., Houston, Texas, August 14-17.

[5]   Hibbard, C., Liu, H.-T., Fang, S., and Miles, P. (2000) “Low-Cost Manufacturing of Lightweight Airframe Structures – Prototype Development,” Waterjet Technology Technical Report No. 810, February.

[6]   Miles, P. (1998) “Lightweighting Large Optics with Abrasive Waterjets,” Proc. Of SPIE: Novel Optical Systems and Large-Aperture Imaging, Vol. 3430, San Diego, California, July 20-21, pp. 304-312.

[7]   Liu, H.-T., Miles, P., and Veenhuizen, S. D. (1988) :CFD and Physical Modeling of UHP AWJ Drilling,” Proc. 14th Int. Conf. On Jetting Technology, Brugge, Belgium, September 21-23, pp. 15-24.

[8]   Liu, H.-T. (2008) “Waterjet Machining of Arrayed Microchannels” Proc. of Arrayed Microchannel Manufacturing Workshop, 5th Micro-Nano Breakthrough Conference, Vancouver, WA, September 8-10, 2008.

[9]   Liu, H.-T. and Schubert, E. (2012) “Micro Abrasive-Waterjet Technology (Chapter Title)”, Micromachining, Ed. Mojtaba Kahrizi, INTECH Open Access Publisher, ISBN 978-953-307-906-6, January.

[10] Liu, H.-T. and McNiel, D. (2010) “Versatility of waterjet technology: from macro and micro machining for most materials” Paper for Presentation at 20th Int. Conf. on Water Jetting, October 20–22, Graz, Austria.

[11] Liu, H.-T., Schubert E., and McNiel, D. (2010) “Development of Micro Abrasive-Waterjet Technology,” SciTopics, October, pp. 4 (http://www.scitopics.com/Development_of_Mirco_Abrasive_Waterjet_Technology.html).

[12] Liu, H.-T. (2007)  “Flash Vaporizing Water Jet and Piercing with Flash Vaporization,” US Patent Application number 20080060493, June (also PCT/US2007/019413)

[13] Liu, H.-T. and Schubert, E. (2008) “Piercing in Delicate Materials with Abrasive-Waterjets,” to appear in Int. J. of Adv. Manufacturing Tech (DOI: 10.1007/s00170-008-1583-5).

[14] Liu, H.-T. (2006) “Collateral Damage by Stagnation Pressure Buildup during Abrasive- Fluidjet Piercing,” Proc. 18th Int. Conf. On Water Jetting, Gdansk, Poland, September 13-15.

[15] Pao, Y. H., Madonna, P. L., Coogan, R. T. (2000) “Method and apparatus for producing a high-velocity particle stream,” U. S. Patent No. 6,283,833

  

 

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Fiberglass Composite (1.6)

G10-Al-G10

Laminate (7.9)