Evolving laser processing

A better understanding of material interactions, applying this to optimized processes, and better integration of these processes with other manufacturing techniques will expand markets for industrial lasers

Robust CO₂ (<50kW) and Nd:YAG (<6kW) lasers are the workhorses for laser-aided material processing, and diode-pumped solid-state lasers with improved beam quality and pointing stability are ideal for microfabrication. Ultrashort pulse widths for precision processing and high power or repetition rates for production speed are available. Frequency doubling and tripling or even quadrupling provides wavelengths spanning the near infrared to the ultraviolet. Users can select a wide range of processing conditions in terms of power, wavelength, repetition rate, pulse energy, and pulse width to address a plethora of processing needs.

Laser-processed items are everywhere today: Nd:YAG laser spot-welded razor blades, CO₂ marked buttons, excimer laser processed electronic chips, CO₂ laser cut and welded car inner doors, laser cladded valves in high-performance engines, CO₂ laser heat-treated cylinder walls in railroad engines, and laser-scored plastic packaging for easy tearing are just a few examples.

Laser cladding, cutting, drilling, heat treating, and welding and laser microfabrication have become common and mature applications. However, for a mature technology to advance and to find new applications, it has to improve and evolve. Applications that are common or mature can be improved or made more efficient; new and innovative applications need to be explored to advance the science and technology of laser processing. To this goal, let's examine the basics of laser processing and assess its use in novel or innovative applications as well as improving or extending present applications.

Laser processing

To comprehend and utilize laser processing one has to understand the effects of laser beam-material interaction. A laser beam is a heat source that can be controlled over a wide range of intensity; hence it's utility. For many processes concern is the depth of the heat-affected zone (HAZ) in the workpiece, which depends on the thermal diffusivity of the material and interaction time of the applied beam. Subpicosecond interaction times tend to have negligible HAZ and millisecond times tend to produce HAZ of approximately 1mm for metals.

Figure 1. Mapping of laser processes according to irradiance and interaction times.

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The map in Figure 1 representing the laser beam-material interaction requirements for a particular application is an extension of Steen's original map to higher irradiance. ¹

The product of the irradiance and the interaction time is the energy supplied per unit area. There are three regimes: heating, melting, and vaporization. Short interaction times achieved with sub-picosecond and femtosecond laser pulses result in insignificant melt zones as depicted on the upper left of Figure 1. As the interaction time increases, the melting regime grows. For drilling, high irradiance and short interaction times are required to minimize melt whereas for transformation hardening (heat treating) the irradiance and interaction is limited to prevent melting. In general, laser processing deals with heating, melting, or vaporizing the material but minimizing the heat effects on the base material, such that undesirable effects are minimized. For brevity, the effects of plasma are neglected.

Laser selection

In practice, users are constrained by the characteristics of the laser used. The available laser power limits the process rate but the characteristics of the beam impact the quality of the process. The unabsorbed energy is reflected or transmitted and may affect the quality and efficiency of the process.

The absorptivity of a material during processing is a function of the temperature, wavelength of the laser, angle of incidence of the laser beam, and surface conditions (chemical composition and roughness) of the workpiece.^3,4 Absorptivity of many materials, particularly metals, increases with temperature. For metals, the absorptivity tends to decrease with wavelength and a Nd:YAG beam can heat treat steel without the addition of an absorptive coating that is often required when CO₂ beams are used. A rough surface enhances the absorption.

Short wavelengths have higher photon energy. The molecular bonds of a material can be broken directly by using a wavelength that has comparable photon energy. Consequently, UV or excimer lasers can process silicon nitride and carbide with insignificant heat effects and similarly for silicon and silicon oxide with fluoride lasers. For metals, the ionization energies are too high for even these lasers, and femtosecond beams with ultrahigh irradiances have to be used.

Figure 2. A typical heat treating laser beam profile and two critical irradiances delineating the case depth and melting thresholds.

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The beam profile and quality also impact processing. The beam quality is a measure of the focusability of the beam, with a Gaussian beam having the smallest spot size at the focus of a lens. When irradiance is critical and power is lacking, beam quality becomes an important parameter. In general, it is a combination of the irradiance requirement and the profile that determines the efficiency and quality of the process. Figure 2 depicts a beam profile used in a heat-treating application. The slight peak in the center of the beam caused undesirable melting of the metal. Good case depth is obtained between the two indicated intensities but the tails of the profile are wasted energy. The ideal profile is top hat with no tails and peaks maximizing the useful energy delivered. Similar beam profile requirements apply to welding and cutting when there is sufficient irradiance for the required penetration.

Optimizing applications

Femtosecond lasers are capable of micromachining any solid with negligible HAZ.⁵ Manufacturing requires speed as well as quality, therefore the femtosecond laser, with the available power of only a few watts, is mainly suitable for microprocessing. Consequently, conventional lasers are used

to optimize the process quality. Specifically required for drilling are:

Beam irradiance £2 GW/cm² to minimize plasma6,
Short pulse width to minimize HAZ,
Short wavelength (high photon energy) for minimal heat effects,
Trepanning for improved quality, precision, and hole roundness.

When processing silicon carbide nanosecond pulsed UV and excimer lasers or picosecond pulsed lasers are required to obtain holes with minimal or negligible HAZ. The shorter wavelengths of these lasers are absorbed better than those of CO₂ and Nd:YAG lasers. High fluence usually causes substantial melting and spatter. By limiting the fluence through the use of trepanning where the deposition of energy is controlled by the trepanning speed, spatter can be controlled and roundness of the hole improved compared to percussion drilling. However, for metals with high ionization energies, the best holes can only be obtained with femtosecond lasers. The knowledge of beam-material interactions can be similarly applied to optimize other laser processes.

Improved and innovative applications

The competitive demands of the marketplace require more efficient manufacturing to produce goods with a lower cost and better quality. For current laser-based methods more efficient or faster processing with incremental increase in cost are needed. For new applications, unique capabilities or more efficient means are desired. Through the understanding of material properties and their interaction with laser beams, improved and new applications can be developed. Optimal parameters can then be developed to process the material. The flexibility of laser processing is in the precision, control, and dynamic range of this energy source. Irradiances can exceed terawatts and interaction times as small as a few femtoseconds. An important property of laser beams is their absorptivity by materials. Their high absorptivity by certain materials but low absorptivity by others can be very advantageous. An example is in the use of diode lasers for welding of plastics where the beam can be transmitted through the transparent plastic to the position where the weld is desired by the use of an absorptive dye.4 Another example is the use of fragmentation at low irradiance to increase rock drilling efficiency compared to melting and vaporization. ²

Hybrid techniques can be used to increase speed and efficiency as demonstrated by combining laser with arc for welding. ⁷ In photochemical drilling, the action of the beam is enhanced by the photochemical reactions with an added compound. ⁸ An analogy is the use of oxygen assist gas in laser cutting. Similarly, laser cladding can be carried out and selective etching of some of the constituents used to obtain the desired structure. ⁹

Conclusions

Laser processing has progressed from the initial applications of cutting and welding to today's proliferation of applications in manufacturing parts and components in diverse fields. The speed, precision, and low HAZ have made laser processing the optimal manufacturing tool. Photolithography as well as laser cladding, cutting, surface treatment, and welding have become mature or standard-manufacturing techniques while the development of more novel and hybrid applications opens new application areas. Several factors to optimize and foster the evolution of laser processing include:

developing an improved understanding of laser-material interactions with a comprehensive knowledge of the material constituents and their thermophysical properties,
applying the knowledge to optimize the processing or develop novel methods of processing materials, and
integrating laser processing with other manufacturing techniques to develop hybrid methods that are faster or more efficient.

As researchers and users grow in number, knowledge will expand, which in turn helps to accelerate the growth and evolution of laser processing.

References

W.M. Steen, Laser Material Processing, Springer-Verlag London, 1994.
K.H. Leong, Z. Xu, and C.B. Reed, "Laser drilling of rocks," Industrial Laser Solutions, March, 2003.
C.J. Nonhof, Material Processing with Nd-Lasers, Electrochemical Publications Limited, Ayr, Scotland, 1988.
J.F. Ready, Editor, LIA Handbook of Laser Materials Processing, Laser Institute of America, 2001.
K.H. Leong, A.A. Said, and R. L. Maynard, "Femtosecond micromachining of electro-optic components," 51st Electronic Components & Technology Conference, May 29-June 1, 2001, Lake Buena Vista, Florida.
J.J. Chang, B.E. Warner, E.P. Dragon, and M. W. Martinez, "Precision micromachining with pulsed green lasers," J. Laser Applications, 10, 285-291, 1998.
E. Beyer, B. Brenner, and R. Poprawe, "Hybrid laser welding techniques for enhanced welding efficiency," Proc. ICALEO 1996.
C. Achara, et al., "Laser photochemical drilling of stainless steel," Proc. ICALEO 2003.
H.C. Man, et al., "Surface and adhesion characteristics of laser surface textured titanium alloys," Proc. ICALEO 2003.