Tribology in Abrasive Waterjets

Waterjet machining can be referred to as a material removal process which uses a high velocity stream of water or a mixture of water and abrasives. Waterjet machining can be categorized into: [1][3]

  1. Pure water jet: Soft materials such as paper products, sealing materials, plastics, foams, textiles, and food can be cut via pressurized stream of water.
  2. Abrasive water jet: Very hard materials such as glass and metal are difficult for a waterjet to penetrate on its own. Abrasive water jet uses a mixture of water and abrasive to more effectively cut through materials.

Working procedure of waterjets: A high-pressure water pump is used to pressurize the water. This pressurized water then travels through a high pressurized plumbing into the nozzle. At the nozzle, abrasives can be introduced which later on gets mixed into the mixing tube and ejects out of the nozzle at high speed. This high speed abrasive/pure water jet can be used for various purposes. [1][3]

Applications of waterjet technology [1][3]

  1. Rock cutting
  2. Cutting in manufacturing industries (cut thin non-metallic sheets, machining circuit boards)
  3. Surface treatment (to remove coatings, paint, deposits, or even rust) and in
  4. Food industry (cutting meats and fish, frozen foods, vegetables)

Main tribological components of abrasive waterjet

  1. Mixing tube: A critical component of AWJ systems is the mixing tube where the high-velocity jet of water is mixed with the cutting abrasive. The mixing chamber is susceptible to normal wear and has about a 500-hour lifespan. Wearing of the mixing tube is a serious problem in abrasive water jet machining. Attention should be paid on selection of mixing tube material as it has a direct effect on the performance of AWJ. [2][4]
  • In one of the research work, Eric Ness and Ron Zibbellb conducted ASTM abrasion and erosion tests on tungsten carbide/cobalt, boron carbide, and composite carbide, which are commonly used for the mixing tube. The results showed that Boron carbide, with the highest hardness, was better than tungsten carbide/cobalt in erosion. Tungsten carbide/cobalt, with the highest toughness, was better than boron carbide in abrasion. [7]
  • In another research work done by M. Hashish, it was found that the mixing tubes manufactured with tungsten carbide grades exhibited greater longevity than the harder ceramics, such as boron carbide, when garnet abrasives were used. The reverse trend was observed with aluminum oxide abrasives. Wear trends suggested that the wear mechanisms along the mixing tube change from erosion by particle impact at the upstream sections to abrasion at the downstream sections. [8]
  1. Nozzle: Increased wear of the AWJ nozzle makes the clearance between the abrasive mixture and the nozzle larger which causes incomplete mixing of the abrasive particles with the high velocity waterjet which results in deterioration of AWJ performance and workpiece geometry. In order to reduce nozzle wear, the outlet of the convergent section and the focusing section are constructed of wear resistance materials, such as sintered diamond. A worn nozzle should be replaced with the new one at the right time. Wear mechanism of abrasive water jet nozzle includes erosion and abrasion. [2][4][5]
  • Unand and Katz (2003) have come out with the method to prevent the nozzle from wear using porous lubricated nozzle. The nozzle was made of porous material and was surrounded by a reservoir containing a high viscosity lubricant. Continuous flow of lubricant helped in formation of a thin film of high viscosity fluid on the interior walls of the nozzle which protected the walls of the nozzle from the abrasive wear. [6]
  1. Pneumatic valves: The pneumatic on/off valve opens and closes to control water flow to the cutting head. When the valve is worn, water will drip from the cutting head. Thin hard surface coating have proved to be beneficial in reducing wear of pneumatic valves. [2][4]
  1. Pipelines and fittings: The high-pressure lines and fittings experience normal wear and have to be replaced at regular intervals. Safety of the operators could be at risk considering the pressure being pumped through it. Ceramic pipe and fittings prove to be extremely wear resistant and highly corrosion-resistant. [2][4]
  1. Pump: Friction and wear between the cylinder and the piston are important problems that have significant influence on the efficiency, reliability, and lifetime of the high-pressure pump. High pressure pumps are subjected to various types of wear processes such as erosion and corrosion –
  • Erosion is the wear and tear of the pump internal parts by suspended solid particles contained in the fluid being pumped. The most affected parts are: wear rings, shaft sleeves, packing, mechanical seal faces, lip seals, the pump casting, and the impeller. Elimination of presence of hard water and proper water conditioning could solve these issues. [9]
  • Corrosion is caused by a chemical or electrochemical attack on the surface of the metals. It is increased when there is an increase in temperature and/or presence of oxygen in the fluid or the surface of the fluid. Cavitation, erosion, and high fluid velocity advance the corrosion process. [9]

Online health assessment of abrasive waterjets

  1. Marco Grasso and co-workers investigated a new approach for the online health condition assessment of both UHP pump and cutting head components by using a single type of information source, i.e., the plunger displacement signal. The results demonstrated that plunger displacement signals are suitable for detecting and identifying critical faults in WJ/AWJ cutting systems. [10]
  2. R Kovacevic and co-workers presented an on-line technique for monitoring the nozzle wear which is based on monitoring the acoustic signals generated by the abrasive waterjet. The autoregressive moving average (ARMA) spectra are used to estimate the nozzle wear. It has been shown that the ARMA spectra can reveal more features of the nozzle wear than the conventional fast Fourier transform (FFT) method. [11]

Knowledge of tribology can be applied in order to make more reliable, efficient and long lasting abrasive water jet machining systems

References:

  1. Wter Jet Cutting presentation made by onlinemetallurgy.com http://www.slideshare.net/umairbukhari3/wjc
  2. Day, Jeff. “What’s Involved in Abrasive Waterjet Maintenance?” The Fabricators. The Fabricator Magazine, 14 Oct. 2008. Web. 07 Feb. 2017 http://www.thefabricator.com/article/waterjetcutting/whats-involved-in-abrasive-waterjet-maintenancer
  3. Water Jet Cutter: A Tribological Tool for Various Uses in Industry, MANE 6963: Friction and Wear of Materials, Term Research Project, Joseph Giedra Full paper available online as PDF
  4. Shimizu, Seiji. “Tribology in Water Jet Processes.” New Tribological Ways (2011) Full paper available online as PDF
  5. Syazwani, H., G. Mebrahitom, and A. Azmir. “A Review on Nozzle Wear in Abrasive Water Jet Machining Application.” IOP Conference Series: Materials Science and Engineering 114 (2016): 1-8. Full paper available online as PDF
  6. Anand, Umang, and Joseph Katz. “Prevention of Nozzle Wear in Abrasive Water Suspension Jets (AWSJ) Using Porous Lubricated Nozzles.” Journal of Tribology 125.1 (2003): 168-80. Full paper available online as PDF
  7. Hashish, M. “Observations of Wear of Abrasive-Waterjet Nozzle Materials.” Journal of Tribology 116.3 (1994): 439-444. Abstract only
  8. Ness, Eric, and Ron Zibbell. “Abrasion and Erosion of Hard Materials Related to Wear in the Abrasive Waterjet.” Wear 196.1-2 (1996): 120-125 Abstract only
  9. “Pump Wear.” Pump Wear, LaBour Pump Company. http://www.peerlesspump.com/pump%20wear.pdf
  10. Grasso, Marco, Massimo Goletti, Massimiliano Annoni, and Bianca Maria Colosimo. “A New Approach for Online Health Assessment of Abrasive Waterjet Cutting Systems.” International Journal of Abrasive Technology 6.2 (2013): 158. Abstract only
  11. Kovacevic, R., L. Wang, and Y. M. Zhang. “Identification of Abrasive Waterjet Nozzle Wear Based on Parametric Spectrum Estimation of Acoustic Signal.” ARCHIVE: Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 1989-1996 (vols 203-210) 208.32 (1994): 173-81. Full paper available online as PDF

HARSHVARDHAN SINGH
About HARSHVARDHAN SINGH 17 Articles
Harshvardhan Singh is an Automotive Engineer and has good experience in lubrication science and experimental tribology. He loves to write about tribology and related fields such as coating technology, surface engineering and others.

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