Abstract:
A flash vaporizing liquid jet cutting tool and method for piercing with minimal damage to the cut material. The liquid is preferably superheated water, typically with abrasive particles added after the jet is expressed through a nozzle (abrasive water jet, AWJ) or with abrasive particles added before the jet is expressed through a nozzle (abrasive slurry jet, ASJ). In piercing, only a portion of water that has not changed phase enters into the cavity or must leave the cavity and the piercing pressure, which can damage the material, is therefore reduced.

Description:
[0001]    This application claims priority (with insignificant added new matter) from U.S. provisional application 60/843,806, the contents of which are incorporated by this reference. 
     
     BACKGROUND 
       [0002]    In hole drilling or slot machining, it has been discovered that the essentially incompressible jet of the abrasive water jet and the abrasive slurry jet (AWJ/ASJ) builds up an extremely high piercing pressure at the bottom of the blind hole or slot (hereafter referred to as the cavity) before break through. The piercing pressure build up is a direct consequence of deceleration and reversal of the AWJ as the bottom of the cavity is approached. For delicate target materials such as composites and laminates, surface/subsurface damages and delamination may result when the piercing pressure exceeds the tensile strength of the materials or the binding strength of the adhesive of the laminates. Furthermore, the large difference in the density between the water and abrasives lead to a lag of the abrasives&#39; trajectories behind the streamline of the water as the return slurry turns around and reverses its course at the bottom of the cavity. In the return slurry, the spent abrasives that still possess considerable erosive power are forced toward the wall of the cavity, particularly near the cavity entrance where the slurry exits. As a result, the spent abrasives (typically 12% by weight and 3% by volume) are forced toward the wall of the cavity and induce excessive wear on the wall near the cavity entrance, leading to nonuniformity in the hole diameter. 
         [0003]    Recent development of abrasive slurry jets or abrasive suspension jets (ASJ) by directly pumping an abrasive slurry through a nozzle has further improved the erosive power the UHP technology. It has demonstrated that under identical hydraulic and abrasive conditions, the two-phase ASJ consisting of water and abrasives has erosive power up to five times higher than that of the three-phase AWJs consisting of water, air, and abrasives. Evidently, the momentum transfer from the ultrahigh-speed water is more efficient in the ASJ with direct pumping of the slurry than in the AWJ with entrainment of abrasives downstream of the jet orifice. At present, the maximum pressure used in commercial ASJ systems is limited to 15,000 to 20,000 psi (103 to 138 MPa) due to lack of materials capable of resisting the erosive power of the ASJ at pressures higher than the above range. With the advent of development of advanced materials, ASJs operating at pressure comparable to that of AWJs are expected to become a superior machine tool to AWJs for various applications. However, the ASJ would be more problematic than the AWJ in terms of surface/subsurface damage. Because of the lack of entrained air in the two-phase slurry of the ASJ, the ASJ jet material will be less compressible than that of the three-phase slurry of the AWJ, creating still higher piercing pressures because they are proportional to the incompressibility of the fluid inside a blind cavity. Therefore using flash vaporization of the jet is even more effective in an ASJ than in an AWJ for mitigating surface/subsurface damage of delicate materials. 
         [0004]    For hydroscopic materials where the use of water jets is undesirable or unacceptable, a UHP abrasive cryogenic jet (ACJ) using liquefied nitrogen (LN 2 ) as the working fluid has been developed for coating removal and machining advanced/delicate materials. One of the key differences of AWJs/ASJs and ACJs is that the LN 2  in ACJs changes phase after exiting the mixing tube whereas water in AWJs/ASJs does not. When drilling holes or slots into a target material to form a cavity, the cavity size increases with time by the erosive action of the abrasives. As the ACJ jet is entering the cavity, the N 2  gas evaporated from the liquid N 2  escapes easily from the cavity. As a result, the piercing pressure of the ACJ inside the cavity is considerably weaker than that of the AWJ/ASJ. Surface/subsurface damages are mitigated provided the reduced piercing pressure is weaker than the tensile strength of the materials or the binding strength of the adhesive of the laminates. As the LN 2  entering the cavity continues changing into N 2 , the return flow consists mostly of dry abrasives and gas instead of a slurry as in the AWJ/ASJ. In other words, the return flow is considerably less organized and coheres less for the ACJ than for the AWJ/ASJ. The trajectories of the return spent abrasives in the ACJ are random in nature as they collide with the incoming abrasives and the side wall on their way out. The benefits of the phase change of the working fluid are therefore to mitigate surface/subsurface damage by reducing the piercing pressure inside the cavity and minimize nonuniform secondary damage by transforming the return flow from an abrasives slurry with liquid to dry abrasives and gas. 
         [0005]    Although the advantages of ACJs over AWJs/ASJs for machining delicate materials have been demonstrated, there is considerable trade off in terms of economical and technical issues to be overcome before ACJs can be commercialized as a machine tool. ACJs are bulky, expensive to maintain, and difficult and hazardous to operate. First of all, the LN 2  requires a very large cryogenic storage and delivery facility. To ensure that no phase change takes place inside the UHP pump, an inline subcooler is often required just upstream of the pump to lower the temperature of the LN 2 . The cryogenic temperature presents an extremely hostile environment to components such as the seals and valves of the pump and significantly reduces their operating life. Equally import, the spent LN 2  and N 2  must be vented properly to prevent unacceptable dilution of the O 2  in the work space. 
       SUMMARY OF THE INVENTION 
       [0006]    The invented system emulates the phase changing characteristics of the abrasive cryogenic jet (ACJ) with a flash vaporizing abrasive water jet (AWJ) or abrasive slurry jet (ASJ) (FAWJ/FASJ) by superheating the water in a AWJ/ASJ. The superheated water flashes and changes into steam as soon as the jet exits the mixing tube. As a result, only a portion of water that has not changed phase enters into the cavity or must leave the cavity and the piercing pressure is therefore reduced. As the superheated water in the AWJ/ASJ continues evaporating into steam after entering the cavity, the return flow consists of wet abrasives and gas rather than a slurry of abrasives and liquid. Unlike a returning liquid slurry, the wet abrasives are not forced by the incoming stream toward the wall of the cavity on their way out. The flow characteristics of the FAWJ/FASJ inside the cavity are similar to that of the ACJ. Consequently, the FAWJ/FASJ achieves the benefits of the ACJ in terms of mitigating surface/subsurface damage and minimizing nonuniform secondary damage to the side wall of the cavity. The key advantage of the FAWJ/FASJ over the ACJ is that superheating the water in the AWJ can be achieved readily with inexpensive and simple set ups such that the FAWJ/FASJ will be considerably more portable and cost effective and safer to operate and maintain than the ACJ. 
         [0007]    In one aspect, the invention is a jet cutting jet system using a hot liquid where a portion of the jet vaporizes after exiting a nozzle. The system includes a reservoir containing a liquid fluid that is a liquid in a range of 0 degrees C. to 50 degrees C. and earth atmospheric pressures; coupled to, such that the fluid may flow into a pump that pressurizes the fluid to a pressure sufficient keep the fluid in liquid form at a temperature that would produce a gas within the range of earth atmospheric pressures; coupled to, such that the fluid may flow into a nozzle which allows the fluid to be expressed in a jet into an atmosphere at a pressure within the range of earth atmospheric pressures. The system further comprises a heater that heats the fluid to a temperature that would produce a gas in the range of earth atmospheric pressures such that a portion of the fluid vaporizes after exiting the nozzle. 
         [0008]    The fluid may be water. The system may further comprise an abrasive supply system that adds abrasive particles to the fluid before the jet strikes a workpiece. The system may further comprise a secondary nozzle that accelerates the fluid jet with propulsion provided by expansion of the fluid as a portion of it vaporizes. The heater may be coupled between the pump and the nozzle or between the pump and the reservoir or may be placed to heat the jet after it exits the nozzle and before it strikes a workpiece. The heater may heat the workpiece which heats the jet as it strikes the workpiece. 
         [0009]    In another aspect the invention is a method in a jet cutting system for reducing lateral pressure on side walls of cuts when making piercing cuts by using a vaporizing jet. The method comprises having a jet cutting system like the one described above, operating the system with a fluid that is a gas in the range of earth atmospheric pressures such that a portion of the fluid vaporizes after exiting the nozzle, and using the system and the fluid to make a piercing cut in a workpiece. 
         [0010]    This method may be employed with a system that further comprises an abrasive supply system that adds abrasive particles to the fluid before the jet strikes the workpiece. The system may further comprise a secondary nozzle that accelerates the fluid jet with propulsion provided by expansion of the fluid as a portion of it vaporizes. The fluid may be a gas when above 0 degrees C. at earth atmospheric pressures and may comprise molecules of two nitrogen atoms. The fluid may be a liquid in a range of 0 degrees C. to 50 degrees C. and earth atmospheric pressures, such as water, and the system may further comprise a heater that heats the fluid to a temperature that would produce a gas in the range of earth atmospheric pressures such that a portion of the fluid vaporizes after exiting the nozzle. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows a typical FAWJ which operates by superheating the water between the pump and the nozzle exit. 
           [0012]      FIG. 2  shows a resistive method for heating the water. 
           [0013]      FIG. 3  shows a conductive method for heating the water. 
           [0014]      FIG. 4  shows an inductive method for heating the water. 
           [0015]      FIG. 5  shows a supersonic FAWJ acceleration nozzle attachment. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A FAWJ/FASJ may use any of several methods, either applied individually or combined, to superheat the water in the AWJ/ASJ. The temperature of the water must be sufficiently high to cause the water to evaporate or flash soon after the FAWJ/FASJ exits the mixing tube, similar to the LN 2  in the ACJ. The optimal locations at which the water of the FAWJ/FASJ flashes depends on the required enhancement for various machining applications. 
         [0017]    In one embodiment, the temperature measured with a thermocouple attached to the nozzle was between 180 to 200 degree C. when the effects of mitigating of piercing damage in many delicate materials were demonstrated at 40 ksi (276 MPa) pressure upstream of the nozzle. The objective is to raise the temperature sufficiently high to reduce the piercing pressure to below the tensile strength of the materials or the binding strength of laminates. In practice, it is desirable to minimize the electrical power required to superheat the water. Tests suggest that the preferred temperature is be around 250 degree C. for most materials. At that temperature, most of the superheated water would be evaporated before entering into the blind hole. In rapid heating the water through a steel high-pressure tube, we must limit the temperature of the high-pressure tube to 600 degree F. such that the strength of the stainless steel would not be compromised. 
         [0018]      FIG. 1  is a sketch of a typical FAWJ which operates by superheating the water between the ultra high pressure (UHP) pump and the nozzle exit, which is just upstream of the abrasive feed port  5 . Similar methods may be used for the FASJ. The difference between the two is that, in the abrasive slurry jet, a slurry of water and abrasive particles is pumped through the jet orifice within the nozzle, and in the abrasive water jet, the abrasive particles are added to a high velocity stream of water after it is expressed through a jet orifice. To protect the seals and the pressure vessels, it is preferable to apply heating downstream of the UHP pump or the accumulator (for an intensifier pump). Examples of heating methods, individually or combined, include:
   Wrap heating tapes  2  around the UHP tubing  1  upstream of the AWJ/ASJ nozzle  4 .   Apply inductive heating  3  around the mixing tube  6  from which the jet  7  exits.   Place the target workpiece  8  on a heated plate  9 .     
         [0022]    Optional heating methods may also be used to superheat the water.  FIGS. 2 ,  3 , and  4  illustrate three such methods via resistive ( FIG. 2 ), conductive ( FIG. 3 ), and inductive ( FIG. 4 ) heating. These methods are used to heat the water in a section of the high-pressure tubing just upstream of the nozzle. To increase the length of time that the water is heated as it passes through the pipe, the UHP tubing is bent into tightly wound coils. 
         [0023]    As shown in  FIG. 2 , resistive heating is accomplished by applying AC current via power supply wires  24  to several coils  22  of stainless steel tubing between an inlet  23  to the tubing and an exit  21 . 
         [0024]    Alternatively, as shown in  FIG. 3 , the high-pressure coils  34  may be placed inside an electric melting pot  35  filled with a heat transfer fluid  33 . The heaters in the melting pot raise the temperature of a heat transfer oil  33  in which the high-pressure coils are submerged. High pressure water or slurry enters the coils at  32  and exits the coils at  31 . 
         [0025]    As shown in  FIG. 4 , inductive heating may be applied to the guard of the mixing tube  46  within the nozzle assembly to achieve localized heating. An electric coil  45  is wrapped around the mixing tube  46  and an alternating current is applied to the wire ends  44 , which induces an alternating magnetic field  41  which induces alternating currents shown by arrows  42  and  43  in the mixing tube  46  and its watery contents, heating them both. Water molecules, having di-polar moments, absorb high amounts of energy from oscillating electric fields that oscillate at the resonant frequency of the polar molecules, which is the frequency selected for microwave ovens for this reason. The same frequency is effective here for direct heating of the water molecules from the electric field and it may be applied with the same magnetron devices. 
         [0026]    To take advantage of the FAWJ/FASJ, additional hardware devices may be attached to the mixing tube to achieve specific enhancements ( FIG. 5 ). For example, if an objective is to take advantage of the expansion of the phase change as the water flashes to further accelerate the abrasive particles, it is preferable to have the water flash at the exit of the mixing tube  51 . A device  55  consisting of an expanded cavity  53  followed by a convergent  54  - divergent  56  (C-D) supersonic nozzle may be attached to the end of the mixing tube. The expanded cavity is designed to stimulate the jet  52  to flash. The flashed jet  57  consists of abrasives carried by a gaseous jet saturated with water vapors at an elevated temperature. As the greatly expanded jet moves through the supersonic nozzle, the jet accelerates in the convergent section of the nozzle, achieves a sonic speed at the throat of the nozzle, and further accelerates through the divergent section of the nozzle. The acceleration increases the material removal rate. The incorporation of the C-D nozzle  55  into the conventional FAWJ/FASJ nozzle takes advantage of a two-stage acceleration of the abrasives: first by the UHP superheated waterjet  52  followed by the flashing in which a part of the water changes into an ultrahigh-speed steam jet  57 . 
         [0027]    The described system will emulate the phase changing characteristics of the bulky, costly, hazardous, and technically challenging ACJ to enhance the performance of the UHP AWJ/ASJ in the following ways:
   The FAWJ/FASJ will minimize the piercing pressure build-up inside the cavity of the blind hole as a part of the water evaporates and escapes the cavity as a gas. This greatly reduces the damage to the target workpiece, particularly for surface/subsurface damage of composites and delamination of laminates.   A large percentage of the water in the FAWJ/FASJ flashes before entering the cavity of the blind hole and gas can flow easily out of the hole, therefore reducing the wearing on the wall of the cavity by the abrasives carried by the otherwise strong return slurry, improving the uniformity of the hole diameter and reducing the anomaly of a relatively large entry hole diameter.   The FAWJ/FASJ can increase the abrasive speed via two-stage acceleration (accomplished with the convergent/divergent nozzle attachment), thus improving the material removal rate and machining efficiency of the FAWJ/FASJ (as compared with the AWJ)   The FAWJ/FASJ emulates the advantages of the ACJ for mitigating surface/subsurface damage of delicate materials and laminates at a considerably lower cost, is more portable, and is safer to operate and maintain.   
 
         [0032]    Because many varying and different embodiments may be made within the scope of the inventive concept herein taught including equivalent structures or materials hereafter thought of, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense, the invention being specified in the following claims.