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INTRODUCTION
Laser-Assisted Machining (LAM)
Laser-assisted machining (LAM) is an alternative method of shaping difficult-to-machine materials and uses a laser beam to heat (without necessarily melting or evaporating material) a workpiece while a conventional cutting tool removes material (see Figure). Laser-assisted machining shows promise in reducing the cost of shaping structural ceramics such as silicon nitride, zirconias and mullite.
Thermally-assisted machining (TAM), of which LAM is a subset, relies on an intense, localized heat source (e.g. plasma torch, induction heating, oxy-acetylene torch) to raise the temperature of the workpiece to a level at which the strength of the material is significantly lower than at room temperature, enabling cutting with lower forces, less tool wear and higher material removal rates. Increasing the temperature of ceramic material will lower the yield strength below the fracture strength such that material removal is effected by plastic deformation rather than brittle fracture. LAM has been demonstrated successfully on silicon nitride, in which heating softens the glassy phase at the grain boundaries, thereby significantly decreasing the yield strength of one of the material's constituents and allowing quasi-plastic deformation of the workpiece (König and Zaboklicki, 1993; Rozzi et al., 1998; Lei et al., 1999). Without thermal (laser) assistance, the workpiece and cutting tool would fail catastrophically when trying to turn silicon nitride with a polycrystalline cubic-boron nitride insert.
Volumetric shrinkage during net-shape manufacturing restricts ceramic parts to simple geometries, and some machining is often required to meet the stringent requirements on dimensional accuracy (Solomah, 1993). Grinding and diamond machining are currently the only economically viable material removal methods capable of providing the required accuracy for many current products (Stinton, 1988; Wobker and Tönshoff, 1993); however, both of these processes are limited by small material removal rates and expensive grinding wheels. For example, depending on the amount of material removed, grinding of ceramic materials accounts for 60 - 90% of the final part cost (Wobker and Tönshoff, 1993; Yonushonis, 1994). The high material removal rate of LAM promises economic and time savings in the manufacturing cycle which could reduce part costs by 50%. Laser-assisted machining (LAM) of ceramics as an alternative method of shaping difficult-to-machine materials is vital for expanding the available manufacturing tools and adding a heretofore unknown flexibility to the production of ceramic components.
Structural Ceramics
The growing need for energy conservation and hence more efficient power cycles, as well as longer lasting components, has increased interest in structural ceramics. Structural ceramics have many desirable properties which include high strength to weight ratio, excellent wear resistance, improved toughness over traditional ceramics, electrical and thermal insulation (this is a generalization and an exception is found in silicon carbide and aluminum nitride which are both used for their high thermal conductivity), chemical stability and the ability to retain their strength and many of their other properties at elevated temperatures. Typical ceramics are very hard and brittle due to the localized covalent-ionic bonding and well organized crystal structure (Schwartz, 1992). The brittleness and hardness of ceramics make them difficult to machine, as stresses can cause brittle fracture or undesirable surface finish. Structural ceramics are often used as cutting tools because of their hardness, however, these properties in a workpiece material can result in very rapid wear of even the most advanced tool. Shaping structural ceramics is both difficult and expensive, resulting in a major barrier to the more rapid application of ceramics components. It is estimated that the U.S. market for structural ceramics will display an average annual growth rate of 8.2 percent, from $365 million in 1996 to $542 million in 2001 (Anonymous, 1999). The 1997 world market for finished structural ceramics components was approximately $3.75 billion (Anonymous, 1999). If the knowledge of how to machine structural ceramics in a cost effective manner existed, the market's annual growth rate would far exceed 8.2 percent and superior products could be put to market.
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BENEFITS OF LAM
- Laser-assisted machining offers a flexibility in manufacturing which is currently not available for medium to high volume ceramic machining.
- Compared to grinding, LAM offers:
- higher material removal rates
- little or no sub-surface damage (Lei et al. 1999)
- faster set-up
- more flexibility in using multiple inserts and changing part programs
- An average surface roughness of 0.3 microns can be achieved during LAM of silicon nitride (Rozzi et al., 1998).
- Laser heat source
- easily controlled: power, beam diameter (i.e. heat flux), targeting
- continuous wave or pulsed modes
- variety of wavelengths to choose from
- A single laser can be time-shared by multiple machining stations.
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REFERENCES
- Anonymous, 1999, "Overview of the Advanced Ceramics Market," The American Ceramic Society, Ceramic Information Center, Westerville, OH.
- König, W. and Zaboklicki, A. K., 1993, "Laser-Assisted Hot Machining of Ceramics and Composite Materials," Proceedings, International Conference on Machining of Advanced Materials, Gaithersburg, MD, NIST Special Publication 847, pp. 455-463.
- Lei, S., Shin, Y. C. and Incropera, F. P., 1999, "Experimental Investigation of Thermo-Mechanical Characteristics in Laser-Assisted Machining of Silicon Nitride Ceramics," Proceedings, ASME International Mechanical Engineering Congress and Exposition, Nashville, TN, MED-Vol. 10, pp. 781-788.
- Rozzi, J. C., Pfefferkorn, F. E., Incropera, F. P. and Shin, Y. C., 1998, "Experimental Evaluation of the Laser Assisted Machining of Silicon Nitride Ceramics," Proceedings, 1998 ASME International Mechanical Engineering Congress and Exposition, Anaheim, CA, MED-Vol. 8, pp. 229-239.
- Schwartz, M., Ed., 1992, "Handbook of Structural Ceramics," , McGraw-Hill, New York.
- Solomah, A. G., 1993, "Laser Machining of Silicon Nitride Ceramics," International Conference on Machining of Advanced Materials, NIST special publication 847, Gaithersburg, MD, pp. 543-547.
- Stinton, D. P., 1988, Assesment of the State of the Art in Machining and Surface Preparation of Ceramics, ORNL-TM-10791, Oak Ridge National Laboratory.
- Wobker, H. G. and Tönshoff, H. K., 1993, "High Efficiency Grinding of Structural Ceramics," International Conference on Machining of Advanced Materials, NIST special publication 847, Gaithersburg, MD, pp. 455-463.
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