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CFD Modeling of Abrasive-Fluid Jet–for Precision Machining of Most Materials from Macro to Micro Scales

Peter H.-T. Liu, Senior Scientist, OMAX.

OMAX Workshop Documents

Problem Description

Since commercialized in the mid-1980s, abrasive-fluid jet (AFJ)[1] technology has progressed significantly in terms of reliability, machining precision, cost effectiveness, and user friendliness. This emerging technology has been elevated from merely a rough cutting tool to a precision machine tool, competing in equal footing with laser, EDM, CNC machine tools, and others for various applications. AFJ has several technological and manufacturing merits that are unmatched by most established machine tools. For example, it is material independent (reflective, heat-treated, and nonconductive materials) and preserves the structural and chemical integrity of raw materials. It is capable of machining a wide range of part sizes from macro to micro scales (http://cdn.intechweb.org/pdfs/27087.pdf).

FJ is a very complex flow phenomenon—multi-phase and multi-parameter flows (fluid, abrasives, and/or air) moving at supersonic speeds.[2] AFJ cutting involves fluid-fluid, fluid-solid, and solid-solid interaction in a rapidly changing spatial environment. Because of the complexity, most of the advancements in AFJ technology have been achieved by physical modeling and/or trial and error, which are time consuming and by no means exhaustive. To achieve optimum performance, the AFJ must be collimated at the exit of the nozzle with the abrasives accelerated to the maximum attainable speed.

With the advent of CFD, it may be possible to model important aspects of the AFJ phenomenon. A canned CFD package like COMSOL or Fluent may play certain roles in the modeling effort. We are however flexible to new computational approaches to the complex flow phenomenon and the machining processes. The solutions would greatly facilitate process optimization to accelerate the advancement of AFJ technology for precision machining of most materials over a wide range of scales.

We are interested in CFD modeling of the following subtopics that are intimately related:

  • Physics of multi-phase flows through various AFJ nozzles:
    • Gravity-dominated flows in large nozzle for macro machining
    • Capillary-dominated microfluidics in micro AFJ nozzles for micromachining
    • Acceleration of abrasives by high-speed water droplets forced through the orifice in an AFJ operating in the entrainment mode (e.g., optimum geometry of mixing tube)
  • Feeding of abrasives (particularly fine abrasives for micromachining) to achieve constant feed rate—essential for precision machining
  • Wear of AFJ components:
    • Entrainment AFJ: components such as orifice, mixing chamber, and mixing tube
      • Relation between trajectories of abrasives in mixing tube and wear pattern
      • Optimum mixing tube configuration to minimize wear and increase operating life
    • Abrasive slurry jet: all components exposed to the high-speed abrasive slurry
    • Effect of abrasive type on mixing tube wear (garnet: 40-100 hours; silicon carbide: minutes)
  • AFJ piercing of delicate materials to mitigate piercing damage in materials with low tensile strength (composites, laminates and brittle materials) due to buildup of static pressure inside AFJ-drilled blind holes before breakthrough
  • AFJ precision machining to enhance cutting efficiency and accuracy, remove taper, and minimize surface roughness (burr and chipping)

[1] Several working fluids such as water (with and without additives), liquefied nitrogen/carbon dioxide, and others may be used for the AFJ. The AFJ may be operating in the following modes: water only, abrasive entrainment, and abrasive slurry.

[2] Phases of AFJ: waterjet–single phase; abrasive slurry jet–two phases; and abrasive-waterjet–three phases.

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