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The laser as a tool for the processing of materials

Lasers in materials processing

In 1921 Albert Einstein proposed in a scientific paper that light could be amplified by so-called stimulated emission. However, it was not until more than thirty years later that his theories began to be applied in practice.

Charles H. Townes, Nobel laureate for his role in the discovery of the laser, came up with the theoretical solution while strolling through a quiet park in Washington one morning. In 1960 T. Mayman was able to apply his theories and manufacture a practical device, called a laser, an acronym for Light Amplification by Stimulated Emission of Radiation, one of the most practically significant inventions of the second half of the 20th century. Here we will look at some of its applications.

A multi-purpose device

Laser applications cover more and more fields of technology. It is used in the manufacture of parts mainly due to its ability to concentrate energy. The ease with which it can be controlled electronically makes it very efficient in information processing, telecommunications, medicine, optics…

The photocopier and laser printer are based on projecting the beam onto a drum of the appropriate material, such as selenium, and electrostatically charging the illuminated area. This allows the carbon-plastic powder particles to be retained, which are then thermally fused and transferred to the paper.

Some lasers deliver pulses of enormous power, with the discharge occurring in times of less than a thousand trillionths of a second. Others provide a continuous beam of up to one hundred million watts per square millimetre. These characteristics make them very useful for materials processing.

They are used, for example, to coat a sheet metal with a highly resistant layer of another material, deposited on it as a powder and melted by laser light; they can bore a material to the desired diameter and depth without affecting the rest of the part, such as holes for car tail lights, or to configure electronic circuits. It is also used for heat treatments, as the extent and depth of thermal penetration is easily controlled on parts that require differentiated surface properties, such as aircraft engines or automotive diesels.

Some laser applications

Measuring distances

Since 1983, the standard metre has been defined as the length travelled by a laser light in a vacuum for 1/299,792,458 seconds (to understand the reason for such a fraction, it is sufficient to realise that the denominator is the value of the speed of light in a vacuum in metres per second). The laser generates a light that makes it possible to obtain length measurements with very high precision. For example, the Earth-Moon distance using a mirror placed there by the Apollo missions.

In industry, it is used to measure shapes, sizes or thicknesses of parts or layers, to instantly know the diameter of bars or tubes without touching them, or in three-dimensional vision systems with cameras. They are also used, among other applications, to read barcodes or to produce holograms.

Other applications are the measurement of velocities in fluids (e.g. wind tunnel experiments) or the measurement of deformations in solids.

Motion in a fluid. On loan from the Holography and Optical Metrology Group of the I3A.

Communications

Laser and fibre optics allow communication across the world. It is a fast and secure system. An optical fibre is less than the diameter of a human hair and a pair of fibres can support more than 700 communications simultaneously. As transmission occurs at the speed of light, it is possible to transmit a large amount of information in a very short time, for example, the contents of the Encyclopaedia Britannica in 5 seconds.

Image courtesy of the Photonic Technologies Group, Department of Applied Physics, University of Zaragoza.

Materials processing

In this unlimited race to control and improve the processing of materials, the use of lasers has moved from the research laboratory to industry in just a few years. Today, the use of lasers in the cutting of steel, textiles, plastics and welding is already massive. Moreover, its industrial use is undoubtedly opening up new perspectives in the surface treatment of steels and ceramics, deposition of hard, anti-thermal or anti-corrosion coatings on metals, engraving on metal, plastics, wood, textiles and ceramics.

A good oven

The laser is a source of coherent light, easy to manipulate optically, which makes it possible to concentrate high power in very small volumes and in areas that are difficult to access, with much greater control of time and power than any conventional electric or induction furnace.

Welding solidly and permanently joins two or more parts. The technique is simple: raising the temperature of the surfaces to be welded and bringing them into contact – whether or not a material similar to that of the part is used. The process is mainly applied to joining metals, but also to certain plastic materials. The laser welds materials with extremely high precision, e.g. for soldering electronic microcircuit terminals. It also cuts sheet metal, ceramics, glass, plastic, fabric or any other material, leaving cleaner edges, without burrs or other imperfections at the cut edge.

A laser for welding

Welding has always been associated with the repair of broken parts, but nowadays it has become very important in the construction of new parts. Without it, there would be no oil tankers, high-pressure tanks, bridges, aircraft, electronic devices…

Welding solidly and permanently joins two or more parts. The technique is simple: by raising the temperature of the surfaces to be welded and bringing them into contact – whether or not a material similar to that of the part is used. The process is mainly applied to joining metals, but also to certain plastic materials. The laser welds materials with extremely high precision, e.g. for soldering electronic microcircuit terminals. It also cuts sheet metal, ceramics, glass, plastic, fabric or any other material, leaving cleaner edges, without burrs or other imperfections at the cut edge.

Engraving

Laser engraving techniques are used in industry to mark expiry dates and to draw on metal parts, ceramics, wood, plastics…

Industrial engraving systems have direct applications on production lines, due to their low cost and high speed, as well as their ability to be integrated without modifying the production system. For example, the cans and bottles of many beverages, both aluminium, glass and plastic, in many cases contain the expiry date engraved with laser, at production or filling speed, and with immediate programmed adaptation to change the date or other data without any type of stoppage.

Another more recent application is the engraving of side mirrors on cars and perfume bottles, as well as the production of plastic signs. Various types of engraving are also carried out on wood, mainly for decorative purposes.

Coatings

Coatings of metal surfaces are useful because they can partially avoid the disadvantages of metals when working with them under extreme conditions such as high temperatures and corrosive media. In addition, a coated metal can be used in the machining of materials of similar or higher hardness. Surface coatings increase the performance of the metal part against corrosion, wear and temperature, but maintain the degree of ductility conferred by its metallic nature. This results in products with longer average lifetimes and features such as maintenance savings and increased reliability of finished products, factors that make it attractive to replace conventional parts.

Coatings on metals also provide access to other types of high-temperature applications, such as high-performance internal combustion engines, which must operate at 1,200 degrees, or engines, structures and protective surfaces in aviation and aerospace systems.

What do we do at INMA?

We sort out the microstructure

Most of the physical properties of a material depend on its microstructure. Thus, in order to improve the properties of the material, it is often subjected to different thermal and thermochemical treatments that modify it in a controlled way.

One way to improve performance is to make composites of two or more materials with different properties. Typical examples are glass fibre and resin composites, which are used in skis, or wood laminates, which are used in furniture.

Composite materials are now being developed at the microscopic level with sophisticated multilayer production techniques, i.e. by physically stacking different materials in alternating layers of microscopic dimensions or by inserting, for example, very fine fibres of one material into another that acts as a matrix.

A less costly method of microstructural ordering, which is followed at our Institute, is based on the directional solidification of eutectic (low melting point) mixtures. By means of the Zonal Laser Melting (ZLF) technique, thermal gradients of up to 1,000 degrees per millimetre are achieved, which allows the rapid growth of polyphase crystalline materials that have greater toughness and resistance to thermal shocks than each of the individual phases, improving their overall physical properties. The effect of the ordered microstructure on all kinds of properties opens up possibilities for the application of these materials in electronics, optics, fuel cells or structural materials for high-temperature applications, options that are currently being explored.

We decorate ceramics

Laser ablation is being successfully applied in the production of decorative reliefs on commercial ceramic surfaces (conventional stoneware floor tiles, porcelain tiles, etc.) in order to achieve improvements in this type of applications, which include good definitions in the drawing (with photographic quality). Thanks to the high calorific power achieved by the laser (at plasma level), high quality drawings and reliefs can be produced, in most cases impossible to obtain using conventional rotogravure methods. The laser can also be used to selectively melt, react and evaporate materials, with the additional advantage that these processes can be carried out in a very small space. The direct application of this process on commercial ceramic materials is the in-situ synthesis of pigments on substrates replacing conventional screen printing.

We develop applications for the restoration of artistic and cultural heritage

Aragonese Mudejar art has recently been included in the World Heritage List, and is currently the object of studies and important interventions for its conservation. The restoration of Mudejar brick is a complex task, as the type of contamination it suffers in its surface layers depends greatly on the (external) environment and the various actions it has undergone throughout its history. Today, the laser ablation technique is a very versatile and suitable cleaning tool for this type of surface, as it allows the removal of layers of dirt, of very diverse origin and composition, without damaging the substrate. In addition, it avoids contact with the surfaces and generates hardly any residue.

We manufacture superconductors in geometries of industrial interest

The directional solidification of high-temperature superconducting materials (below -173 degrees), processed by laser zone melting, changes their microstructure (producing a high preferential alignment) with a consequent increase in their performance. For example, a cylinder of pressed powders with a current-carrying capacity of less than 200 Amps per centimetre can be transformed by this technique into a textured cylinder of more than 5,500 Amps, which competes favourably with commercial products obtained by other techniques.

One type of material obtained by laser processing is the so-called Bi2Sr2CaCu2O8 materials textured by laser-induced zone melting. These materials can be used in the current supply elements of classical superconducting coils used for example in diagnostic MRI systems in medicine. These superconducting coils are capable of holding a man inside, operate in a bath of liquid helium and, for charging, require currents of several hundred amperes in connections ranging from room temperature to the coil at -269 degrees Celsius. Conventional electrical conductors are excellent thermal conductors and give a constant heat input from the outside, which increases helium evaporation and consumption. As these superconductors conduct much less heat and, being superconductors, do not dissipate energy, when used in power bars they reduce heat input and helium losses by orders of magnitude, reducing coolant costs and extending autonomy.

Another application of BSCCO’s textured cylinders that is being developed is the manufacture of current limiters, which basically function as reversible safety switches to protect electrical power distribution lines. These materials have two states, a superconducting state with no resistance and a normal state where they are resistive. When a short circuit occurs, the superconductor must be able to generate a lot of impedance to prevent the current from reaching very high values. Lasers are also used to machine these materials to increase the length of the superconductor, which is necessary to increase the resistance of these materials and improve the limiting capacity of these superconductors.

More information:

Laser processing of materials:
http://www.aragoninvestiga.org/investigacion/temas_detalle.asp?id_tema=91

Press release at Tercer Milenio, Heraldo de Aragón.
LASER-Heraldo.pdf

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Instituto de Nanociencia y Materiales de Aragón