Laser Beam Machining
Another thermal removal process is laser beam machining. In laser beam machining, an intense beam of collimated, single wave-length, in-phase light is focused by an optical lens onto the workpiece point to be machined. The light absorbed by the workpiece is converted to heat, which melts and vaporizes the workpiece material. Molten material is evacuated from the cut by the adjacent vaporization turbulence which typically occurs in drilling operations, or by the use of an "assist gas" in cutting operations (see Figure 16.15). Laser beam machining uses a directional, coherent, monochromatic beam of light to achieve precision in cutting and drilling. The intensity of this light produces a tremendous amount of heat at the point of application to the workpiece, and laser beam machining can take place at relatively high speeds.
A number of different types of lasers are used in laser beam machining, each with certain advantages for different operations or applications. The most commonly used lasers for machining are Nd:YAG lasers, which have certain advantages for hole drilling due to their higher pulse energy, and CCh gas lasers, which have certain advantages in cutting since they are capable of delivering much higher average power. Lasers may be operated in either pulsed or continuous-wave (CW) modes. The most powerful CCb lasers, however, are operable only in CW mode. CCb lasers can have output power generally ranging from 100 to 2000 W when pulsed and from 250 to 50(K) W in CW mode. Some lasers are capable of an output power of 25,000 W.
A 1250-W CO2 laser can cut mild steel at speeds ranging between 40 and 140 crrr/min, depending on material thickness. At a thickness of 12 mm, the cut can be 40 enrr/min. At a thickness of 2 mm, a 1250-W CO2 laser can cut at a rate of 140 enrr/min. Aluminum is generally cut at about one-half the speed of carbon steel with a CO2 laser because of the high thermal conductivity of the aluminum. In cutting applications, the laser beam may be "transmitted" and "switched" by using mirrors to manipulate the beam, or with Nd.YAG lasers by the use of a combination of fiber optic cables and switches. Below is the typical average power range and maximum pulse energy for the two types of lasers most commonly used in laser beam machining.
In the application of laser beam machining, it is possible to perform the operations of drilling and cutting with speed and precision. For instance, percussion (i.e., repeated pulse) drilling of Inconel 718 with a 250-W Nd:YAG laser can produce accurate holes 12 mm deep in under 10 sec and holes 25 mm deep in 40 sec. Length-to-diameter ratios are limited to about 30 or 40 to 1 with conventional Nd: YAG lasers. However, other laser technologies allow the length-to-diameter ratio to be higher. Frequently, gas is used to assist laser beam machining. A coaxial columnar flow of gas (oxygen, air, or inert gas) at pressures ranging from 1 to 6 bar expels molten metal from the cut. Oxygen assists in cutting steel and other materials at an increased rate because of the oxidation reaction with the metals.
Laser beam machining uses thermal energy to drill and cut with speed arid precision that traditional machining methods often cannot duplicate. Its applications are becoming more varied as new techniques are developed.

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