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Lasers and Their Emission Characteristics
Published in F.J. Duarte, Tunable Laser Optics, 2017
Emission characteristics of atomic pulsed metal vapor lasers are given in Table 9.4. Perhaps, the most well-known member of this subgroup is the copper laser, which is also referred to as the copper vapor laser. These lasers have found numerous applications due to their ability to emit large average powers in the green at λ = 510.554 nm and, at their secondary emission wavelength, at λ = 578.21 nm. CVLs use a buffer gas, such as He or Ne, and operate at a high prf in order to attain the necessary metal vapor pressure. Excitation of the upper 2P state occurs mainly via direct electron excitation (Harstad 1983). Output parameters from individual Cu lasers have been reported to cover a wide range of values. At pulsed energies of up to several millijoules per pulse, these lasers can emit pulses in the 10–60 ns range, at prfs from 2 to 32 kHz (Webb 1991). For instance, a specific CVL can yield an average power of 100 W, at 20 mJ per pulse, and a prf of 5 kHz (Webb 1991). Integrated CVL systems have been reported to yield average powers of up to 7 kW at a prf of 26 kHz (Bass et al. 1992). Copper lasers can also be operated at low repetition rates using copper halides to attain the necessary vapor pressures at relatively low temperatures (Piper 1978; Brandt and Piper 1981).
Drilling of titanium alloy (Ti6Al4V) – a review
Published in Machining Science and Technology, 2021
Chua Guang Yuan, A. Pramanik, A. K. Basak, C. Prakash, S. Shankar
Aside from rotary RUD, RUAD and LFVAD, laser drilling is another type of drilling mechanism classified as non-CD method, which is capable to form high aspect ratio holes (Gautam and Pandey, 2018). Unlike CD, RAD and RUAD, laser drilling does not remove workpiece material mechanically by cutting tool. In fact, this drilling method is one of the methods, which utilizes radiation heat generated from laser beam to melt and/or vaporize workpiece material (Dhaker and Pandey, 2018, 2019b). Cutting tool and workpiece does not come in contact throughout the whole laser drilling process (Bandyopadhyay et al., 2002; Shuja and Yilbas, 2014). Due to this characteristic of laser drilling, several problems existed in tool-based drilling methods can be eliminated. Rapid tool wear in tool-based drilling is one of the reasons causing detrimental effects on burrs and surface roughness of drilled holes. Furthermore, the issue of tool bending and fracture is enhanced when extremely thin drill bits are used to perform micro-drilling (Bellows and Kohls, 1982; Chatterjee et al., 2018). Elimination of drill bits solves some limitations of CD, RUD and RUAD. These specialties of laser drilling favor the drilling process for hard-to-cut material such as titanium alloy, nickel alloy, ceramics, composites (Dhaker and Pandey, 2019a). There are several types of lasers used to perform laser drilling. Those are neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, ultraviolet yttrium aluminum garnet (UV:YAG) laser (Yung et al., 2002), ruby laser and copper vapor laser (Luft et al., 1996; Chatterjee et al., 2018). Due to exceptional beam quality along with excellent peak power, Nd:YAG laser is widely adopted for laser drilling process as compared with other lasers (Goyal and Dubey, 2016). Throughout the years, numerous researches had been done to determine parameters that are affecting quality of laser-drilled holes. Laser drilling can be classified into two categories, laser beam drilling (LBD) and laser trepan drilling (LTD). Both LBD and LTD utilize heat generated from laser beam to melt workpiece material. Due to the efficiency in terms of time and cost, LBD is widely used in laser drilling industries. The key difference between LBD and LTD is the hole forming method as illustrated in Figure 25. Despite much higher processing efficiency of LBD compared with CD, it is prone to several geometrical and metallurgical defects such as tapered holes, spatter area, heat-affected zone (HAZ) (Prithpal Singh et al., 2020). Researchers found that these defects though not eliminated can be relatively reduced when LTD is used. Some data from laser drilling of Ti6Al4V from different articles is presented in Table 3 for quick reference.