Laser Technology

Laser Technology

 

the processing and welding of materials with laser radiation. Solid-state and gas pulsed and continuous-wave lasers are used. Most processes of laser technology use the thermal action of light, which is caused by its absorption in the material being processed. Optical systems are used to increase the radiation flux density and localize the zone of processing.

The distinctive features of laser technology are the high radiation flux density in the zone of processing, producing the necessary thermal effect in a short time (the pulse duration is 1 millisecond [msec] or less); the local nature of the action of the radiation, which results from the possibility of focusing it into light beams of extremely small diameter (of the order of the wavelength of the radiation); the small thermally affected zone, which is provided by the brief period of action of the radiation; the input of energy into the processing zone without contact; and the possibility of carrying out industrial processes in any transparent medium (vacuum, gas, liquid, or solid) through the transparent ports of industrial chambers or the shells of electrovacuum devices. The processes of welding, drilling, and cutting have been best studied and mastered.

Welding. Laser welding may be of the spot or seam type. Pulsed lasers, which provide the smallest thermally affected zone, are used in most cases. High-quality joining of parts made of stainless steel, nickel, molybdenum, Kovar, and other materials can be produced using laser welding. The high power of laser radiation makes possible the welding of materials with high thermal conductivity, such as copper and silver. Laser welding of materials that submit poorly to welding by other methods (tungsten with aluminum, copper with steel, and beryllium bronze with other alloys) is possible. The radiation flux density on the surface of the welded parts is 0.1–1.0 megawatts per sq cm (MW/cm2), depending on the materials. In welding with a pulsed solid-state laser the depth of penetration of the material may be 0.05–2.0 mm, and the ratio of the depth to the diameter of the weld spot or the seam width is 0.5–5.0. This makes possible reliable welding of parts 0.01–1.0 mm thick. Laser welding equipment operates with a radiation energy of 0.1–30.0 joules (J) in the pulse, a pulse duration of 1–10 msec, and a diameter of the light spot of 0.05–1.5 mm. Spot welding capacity is 60 operations per minute, and seam welding capacity is 1 m/min with a penetration depth of 0.5 mm. The use of a laser is most effective for welding in hard-to-reach areas of structures, to join easily deformed parts, under conditions of intensive heat transfer (for example, for materials with high thermal conductivity or at low temperatures) and in cases when it is necessary to ensure a minimal thermally affected zone. Substitution of laser welding for soldering of miniature parts is economically profitable, since in this case contamination of the welded parts with solder is eliminated, a joint of better quality is produced, and the structure weighs less. The areas of application of laser welding include the manufacture of electrovacuum and semiconductor devices, integrated circuits, and instruments for precision mechanics. Laser welding makes possible an increase in labor productivity by a factor of 3–5 over ordinary methods of welding and soldering.

Drilling. Holes may be drilled in any material with a laser. Pulsed lasers with a pulse power of 0.1–30.0 J, a pulse duration of 0.1–1.0 msec, and a radiation flux density in the processing zone of 10 MW/cm2 or more are generally used for this purpose. The maximum output is achieved when holes are drilled with one high-energy pulse (up to 30 J). In this case most of the material is removed from the hole in the molten state under the pressure of the vapor formed as a result of vaporization of a relatively small part of the substance. However, the precision of processing by the single-pulse method is low (10–20 percent of the diameter). Maximum precision (1–5 percent) and controllability of the drilling process are achieved when a series of pulses of relatively low energy (usually 0.1–0.3 J) and short duration (0.1 msec or less) is applied to the material (the multipulse method). The drilling of through and blind holes with various types of cross section (such as round and triangular) and longitudinal section (such as cylindrical and conical) is possible. The drilling of holes with a diameter of 0.003–1.0 mm and a ratio of depth to diameter of 0.5–10.0 has been mastered. The surface roughness of the hole walls corresponds to the sixth to tenth degrees of surface roughness (∇6-∇10), and the depth of the structurally altered, or imperfect, layer is 1–100 μ, depending on the mode of processing and the properties of the material. The capacity of laser drilling units is usually 60–240 holes per minute. The use of lasers to drill in materials that are hard to process by other methods (such as diamond, ruby, and ceramics), to produce holes with a diameter of less than 100 μ in metals, and to drill at an angle to the surface is most effective. In the USSR laser-beam drilling of holes has found especially wide application in the production of ruby watch pins and diamond draw plates. For example, the drilling of diamond draw plates can be accomplished successfully with a Kvant-9 unit using a neodymium-doped glass laser. The labor productivity of this operation has been increased by a factor of 12 over previously used methods.

Contactless removal of extremely small amounts of material by laser also is used in the dynamic balancing of gyroscope rotors and in precision adjustment of the balances of watch mechanisms. This makes possible a great increase in the precision of the operations and an increased output.

Cutting. Laser cutting of materials is done in both the pulsed and continuous modes. In cutting in the pulsed mode a continuous cut is produced as a result of the superposition of successive holes. The cutting (milling) of thin-film passive elements for integrated circuits—for example, for precision adjustment of their resistance or capacitance—has become extremely widely used. Pulsed Q-spoiled yttrium aluminum garnet (YAG) lasers, as well as carbon dioxide lasers, are used for this purpose. The pulsed nature of the processing ensures a minimum depth of heating and eliminates damage to the substrate to which the film is applied. Various types of laser units make possible processing under the following conditions: radiation energy, 0.1–1.0 mJ; pulse duration, 0.01–100.0 μsec; radiation flux density, up to 100 MW/cm2; pulse recurrence frequency, 100–5,000 pulses per sec. In combination with automatic control systems, laser units designed for adjusting resistors provide a capacity of over 5,000 operations per hour. Pulsed YAG lasers also are used to cut semiconductor substrates for integrated circuits.

Continuous-wave carbon dioxide lasers with an output ranging from several hundred watts to several kilowatts are used for gas-laser cutting, in which a gas jet is fed into the zone affected by the laser beam. The gas chosen depends on the type of material to be processed. For cutting wood, plywood, plastics, paper, cardboard, and textiles, air or an inert gas is fed into the processing zone to cool the cut edges and inhibit combustion of the material and expansion of the cut. For cutting most metals, glass, and ceramics, the gas jet blows the molten material out of the affected zone. This makes possible the production of a surface with low roughness and ensures high precision of the cut. In the case of iron, low-carbon steels, and titanium, a stream of oxygen is fed into the heating zone. Additional heat is released as a result of the exothermic reaction of oxidation of the metal. This makes possible a great increase in cutting speed. The characteristic conditions of gas-laser cutting are as follows: radiation power, 300–1,000 W; radiation flux density in the processing zone, 100 kW/cm2; width of cut, 0.3–1.0 mm; and a thickness of the material being cut, up to 10 mm. The cutting speed depends on the thickness and properties of the processed material and may range from 0.5 to 10 m/min (or up to 50 m/min or more for thin materials, such as paper or fabric). The merits of gas-laser cutting are the simplicity of automation of the process, the small cutting width, the low depth of the affected zone, the absence of harmful by-products in the cutting of glass-fiber-reinforced plastics, and the fusion of the cut edges of synthetic textiles, which prevents unraveling.

Carbon dioxide lasers are used to cut brittle materials (such as glass and ceramics) by the method of controlled thermal cleavage. Upon local heating of the material, thermal stresses that exceed the breaking point of the material arise along the path of the beam. The crack that appears develops behind the beam, whose path may have a complex shape. The cutting speed may be as high as several meters per minute. Controlled thermal cleavage is used in cutting glass tubes in the production of electrovacuum devices and ceramic substrates for integrated circuits and in cutting sheet and shaped glass.

Uses in other areas. The thermal action of laser radiation may be used for casehardening (the tempering and “healing” of microscopic flaws by fusion) of fast-wearing metal parts—for example, the cutting tool used to make p-n junctions in the production of semiconductor devices. The action of a laser is used in the production of integrated circuits to achieve local thermal dissociation of certain metal-containing organic compounds in the manufacture of film elements for the circuits, to intensify the processes of local oxidation and reduction, and to produce thin films by vacuum vaporization of materials.

In the USSR, industry produces commercial laser units of various designs with neodymium-doped glass lasers, YAG lasers, carbon dioxide lasers, and lasers using other active mediums. A typical diagram of an industrial laser unit is presented in Figure 1.

Figure 1. Typical schematic diagram of an industrial laser unit with a solid-state laser: (1) charging unit, (2) capacitance accumulator, (3) control system, (4) firing unit, (5) laser head, (6) cooling system, (7) radiation energy stabilization system, (8) radiation energy sensor, (9) optical system, (10) focused laser beam, (11) part being machined, (12) coordinate table, (13) memory-stored control system

The future development of laser technology is associated with an increase in laser power, which will make possible the processing of increasingly thick materials. The tasks of laser technology in providing higher precision of processing are the development of effective methods of controlling radiation parameters, improving the uniformity of the distribution of radiation intensity over the cross section of the beam, increasing the stability of laser output parameters, and studying in detail the physical processes by which laser radiation affects materials in various modes of laser operation.

REFERENCES

Deistvie izlucheniia bol’shoi moshchnosti na metally. Moscow, 1970.
Lazernaia tekhnologiia. Moscow, 1970.
Tekhnologicheskoe primenenie gazovykh lazerov. Leningrad, 1970.
Lazernaia litografiia. Leningrad, 1971.

M. F. STEL’MAKH and A. A. CHEL’NYI

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