Laser Beam Welding
Laser welding is a high energy beam process and in this regard is similar to electron beam. With that exception they are unlike one another. The energy density of the laser is achieved by the concentration of light waves not electrons. The laser output is not electrical, does not require electrical continuity, is not influenced by magnetism, is not limited to electrically conductive materials and in fact can interact with any material whether it be metal, plastic, wood, ceramic, etc. Finally its function does not require a vacuum nor are x-rays produced.
The focal spot (thousandths of an inch in diameter) is targeted on the weld joint surface or by focal length selection above or below it. At the surface the enormous concentration of light energy is converted to thermal energy. Surface melting occurs and progresses through the weld joint by thermal conductance. For welding, beam energy is maintained below the vaporization temperature of the weld joint material. For hole drilling or cutting vaporization is required. Because weld joint penetration is dependent on conducted heat the thickness of materials to be welded is generally less than .080 inches if the ideal metallurgical and physical characteristics of laser welding are to be realized. These benefits are narrow welds, no distortion, minimal heat affected zones and excellent metallurgical quality.
As with electron beam the intense, concentrated energy produces melting and coalescence before a substantial heat affected zone can develop. Because the welds are narrow and therefore are of correspondingly low volume there is a minimal reservoir of heat for conductance into the adjacent area. When materials to be welded are thick and particularly if they have high thermal conductance (aluminum for example) this important metallurgical advantage of minimal heat affected zone can be detrimentally affected. It is claimed that since the source of energy is light of a specific wavelength contaminants in the weld pool or on the facing surfaces of the joint may be preferentially vaporized by their particular light absorbing characteristics resulting in a kind of weld purification. The excellent fatigue strength of laser welds is sometimes attributed to this purifying phenomenon.
Energy distribution across the beam is generated by the design of the resonant cavity, including mirror curvatures or shape and their relative arrangement. This combination results in photon oscillation within the cavity producing specific output beam energy distributions or patterns. These patterns are labeled transverse energy modes (TEM) and have specific identifying numbers. We cannot within this seminar describe their variety and effects. However, we will point out that the Gaussian mode TEM 00 is often preferred for welding inasmuch as its peak energy is in the center of the beam feathering off to its periphery. It might be likened to a pointer. The symmetry and profile of the Gaussian Beam is particularly suited for welding.
We have learned how light energy is amplified in the solid state laser cavity and how the laser beam and its unique characteristics are formed. It is important to note before proceeding that the function of all lasers whether they be gas (carbon dioxide, helium neon, etc.) or other lasing sources is based on the principle of the excitation of atoms by means of intense light, electricity, electron beam, chemicals, etc., and the spontaneous and stimulated emission of photons. Depending on the lasing source, output frequencies differ widely and are capable of a great number of applications. These range from welding to critical surgery, resistance trimming, communication, etc.
As a clear demonstration of the effect of light wave frequency, the beam of a neodymium YAG laser (l.06 micron wavelength) will pass through quartz lenses, clear plastic or glass and other transparent materials. However a carbon dioxide laser emitting a beam with a wavelength of 10.6 microns will not pass through the quartz lens etc. but rather will be absorbed by those materials resulting in their destruction. Carbon dioxide lasers must achieve focusing either by converging reflective optics or special salt based lens materials such as zinc selenide.
We have discussed the role of the objective focusing lens and how it concentrates the beam energy into a focal spot as small as .005 inches in diameter or less. We have also reviewed how a laser weld is produced by conducted heat and the excellent quality of the weld.
Because the energy density is so intense, in fact second only to the electron beam, the laser is capable of vaporizing metals such as tungsten or non-metallic materials such as ceramics. In fact, in conductance welding, care must be taken to prevent this vaporizing action. However, as with electron beam, lasers can produce deep penetration welds by the keyholing technique. Laser keyholing is limited to perhaps 3/4 to 1 1/2 inch thickness and for these depths a multi-kilowatt laser, such as the carbon dioxide type, must be used.
We need to mention that although there are many laser types, the Nd:YAG and carbon dioxide lasers (CO2) are most common in production metal working. Carbon dioxide lasers utilize a combination of carbon dioxide, the primary lasing source, helium and nitrogen. The gas mixture circulates through a bank of electrodes, which is the energy source. The output wavelength is 10.6 microns. Carbon dioxide lasers have been developed with outputs exceeding 25 kilowatts. This high output of CO2 lasers is possible since they can be efficiently cooled. In contrast, cooling the solid state YAG laser crystal is difficult and critical. Considerable design attention is directed towards cooling, excitation lamps, their reflectors, cavity shape, materials, plating of reflectors, lens anti-reflection coatings, etc. This includes power supplies, which may be designed for continuous or for a pulsing output. Pulse repetition rates and pulse shaping are programmable.
We must now continue our initial discussion of the interaction of the laser beam with metals. As stated, heat is generated by the conversion of light energy. All metals reflect light to some degree, with gold and silver high on the list and carbon steel low on the list. Gold, silver, copper, and aluminum are therefore difficult to weld requiring intense energy usually available from high energy peaking pulses or resorting to light absorbing coatings such as graphite on the weld joint surfaces to reduce their reflectivity. The 1.06 micron wavelength of the Nd:YAG laser is more readily absorbed than the longer 10.6 micron wavelength of the CO lasers, therefore, in this respect more suited for welding highly reflective materials. However though metallic reflectivity is a factor, once melted, the reflectivity essentially disappears at the curie temperature (about 1425 degree F). Therefore most metals are readily welded. The intense energy of the beam quickly melts the surface, from which thermal conductance progresses to achieve penetration.
Because the beam can be reflected from mirrored surfaces (reflective at the laser wavelength) it follows that beam manipulation is almost unlimited. It is this feature that makes marking or engraving lasers possible. Holes can be drilled or cut as square, round, any geometric pattern, size or dimensional proportions by mirror manipulation. Beam energy can be tailored to produce strategic pulse profiles. Energy can be continuous, or weld seams may be produced by overlapping individual pulses which tend to reduce heat input by the brief cool cycle between pulses, an advantage for producing welds in heat sensitive materials. A third arrangement is a continuous output with pulsing action superimposed by an acousto-optic (Q) switch located in the cavity. This device is capable of generating pulse rates in the tens of thousands and can increase cavity energy by interrupting the output thus causing a brief period of gain or storage in the laser crystal. Considering that photon oscillations within the cavity occur at the speed of light even a brief interruption of the output is extremely effective for increasing the gain.
The manipulative ability of the laser establishes it as ideal for automation and robotics. Fiber optics dramatically adds to this versatility. Utilizing this capability, production assemblies on trays, fixtures or shuttles can be conveyorized while the laser focusing optics, incorporating the necessary axis of motion (x, y, z) including targeting and scanning, can track and follow the weld joint. This flexibility combined with motion and parameter programming is seemingly unlimited. Inert gas shielding of the weld is usually incorporated coaxially with the laser. However, inert gas trailers, underbead coverage and other strategic, and beneficial inert gas applications are easily adopted. If necessary assemblies can be placed in a vacuum chamber and the laser beam introduced through a quartz window. The raw beam can be focused through optics within the chamber or may be focused external to the chamber utilizing the appropriate focal distance. Alternately fiber optics may be utilized and routed through appropriate hardware in the chamber walls. The fiber optics can be terminated with focusing optics internal to the chamber. Many arrangements are possible.
The disadvantages of lasers include its high capital cost, the need for clean environment (to protect the optics), and the safety considerations. The latter two are most often resolved by the installation of the laser in a laser room. Warning signs, sounds and/or flashing lights are employed to signal when the laser is on. An additional disadvantage is though maintenance is generally minimal, laser operation requires training and experience. Maintenance and machine operator personnel must become aware of the subtleties that can influence the laser output. In this regard, the laser is somewhat unlike the electron beam, which has with rather positive switch controlled reactions.
We will next discuss how a laser beam is generated. It is necessary to start with atoms. Depending on the particular element there may be one, two or more electrons in single or multiple energy orbits circling the nucleus of the atom. In their normal quiescent or ground state the orbits are at discrete energy levels or distances from the nucleus that are characteristic of the specific atom. All of the atoms of a given element share this identical behavior.
Spontaneous Emission
The basis for laser action occurs when an atom (for this example, neodymium) is excited by an external energy source. The absorbed energy will cause the atom's electrons to move from their ground state to one of the discrete and exact energy orbits, characteristic of the specific atom. Therefore when these orbiting electrons return spontaneously to their ground state, they release the energy difference as a photon. Since all of the photons originate from electrons in the same energy orbit, they have the same wavelength.
Stimulated Emission
Einstein theorized and proposed that a photon passing near an excited electron
of the same energy would cause the approached electron to return to its ground
state and in doing so release its photon of light. Two identical photons would now exist. The two photons would 
travel as a coherent pair and in the exact same direction. This phenomena would be repeated over and over again as each of the triggered photons approached other excited electrons in the same energy orbits. Groups of photons, depending on the emanated direction of their original source.
When an external source of energy, whether it be intense light, chemical, electrical, etc. is absorbed by the atom, its excited electrons will move to a new energy orbit but only after they have absorbed a specific amount of energy. They do not move half way or one and one-half times out but rather to a discrete new energy orbit characteristic of the atom. An analogy would be stepping from stone to stone across a stream. Too short or too long a step will result in a soaking. There is no margin for error. Another analogy is throwing a ball through a distant basket or loop. It requires a specific amount of energy to succeed. Too little, too much result in misses. This relationship between atoms, their electrons and energy is an atomic law. Therefore, the orbiting electron must absorb a specific energy value before they move to that very discrete and distinct new orbit. The electrons of every atom of a specific element excited by a common energy source contain the same energy inasmuch as they are in the same energy orbit and will release this energy as photons when they randomly drop back to their ground level, or normal state, an action called spontaneous emission. The released photons, therefore contain the same energy and, as a result, the same light wavelength.
Based on this fundamental atomic action a laser crystal (ground rod) containing an element such as neodymium, which is capable of releasing photons, when appropriately energized can become the basis for laser activity. All of the neodymium (Nd) electrons in their identical energy orbits will randomly return to their ground state, collectively releasing enormous quantities of photons each containing the energy difference between the excited orbit and the ground state. Their light wavelength is equivalent to this energy. In the instance of neodymium, the light wavelength is 1.06 microns. Other photon emitting elements can have other wavelengths. It should be noted there are numerous elements capable of emitting photons but many are ruled out for laser use because of the difficulty in acquiring, their instability, etc. To return to the foregoing, a neodymium: yttrium, aluminum garnet (Nd:YAG) solid state laser consists of the element neodymium dispersed in the host yttrium aluminum garnet (YAG) crystal.
Lets proceed to the next important phase of laser beam generation. In fact, the term laser (light amplification by stimulated emission of radiation) is the function we are about to discuss. As a matter of interest this theory was postulated by A. Einstein contributing to the development, design and fabrication of the first research lasers.
We have learned the atoms of certain elements when they absorb energy move their electrons to a new and discrete orbiting energy level. On their random, spontaneous return to their ground or quiescent state they emit photons of light. We may think of the photons as particles. The photons contain the energy difference between the excited and ground state orbits. Their specific wavelength is a result of and proportional to this energy.
The photons move in random spatial directions at the speed of light and of a specific wavelength. However, Einstein postulated that when a photon passes close to an excited electron of equal energy, it would trigger the electron spontaneous emission of a photon. There would now be two. Interestingly, the second photon as we now know will contain the same energy, therefore wavelength as the first triggering photon. To continue the phenomena the second triggered photon will travel in the exact same spatial direction as the first. As these two paired photons continue on they will trigger other electrons by stimulated emission creating an enormous amplification of photons traveling in the exact same direction dependent on their origin. Needless to say all of the photons have the same light wavelength.
Characteristics of the laser beam
Monochromatic
All of the photons which compose the beam are of the same energy and therefore the same wavelength. If the laser beam was directed through an optical prism it could not split up into the separate colors representing the wavelengths of the optical spectrum.
Coherent
The light waves are in phase (in step).
Culminated
The laser beam does not diverge. It can be projected great distances without significant spreading. For example, it is used for topographical surveys where elevations miles away can be measured from a single, central location. Culmination makes is possible for laser beams to target satellites, etc. at great distance. Because of these three characteristics the laser beam can be precisely focused to very small diameters, resulting in an enormous increase in energy density.
From here we will proceed to understand how all of this activity i.e. the triggered release of photons by stimulated emission and the cascading effect resulting from the stimulating action of photons approaching other excited electrons of equal energy become the basis of the laser. The question now is how to harness, organize, control and concentrate the spatial motion of photons into a controlled beam of light capable of being projected without significant divergence, for miles and to contain a level of concentrated energy capable of vaporizing such high temperature materials as metals and ceramics.
The atoms of reodymium are a stable source of lasing action. The "YAG" crystal which is grown as a boule is doped with the element neodymium. This crystal is precisely ground to a rod configuration. When assembled in a resonant cavity it becomes the basis for solid state laser action, emitting a laser beam having a light wavelength of 1.06 microns.
The diagram shows a neodymium doped YAG crystal absorbing energy from an intense light source resulting in the release of photons in random spatial directions by the combined mechanism of spontaneous and stimulated emission.
Next, another view of the crystal with mirrors added to each end to produce a resonant cavity. By coincidence, the spatial direction of some of the photon groups will cause them to travel along the longitudinal axis of the cavity. The result is the impingement of these photons on the mirrors located at the ends of the crystal from where they will be returned to the crystal by reflection and will continue to stimulate the emission of other photons. This activity creates an enormous amplification of photons traveling back and forth between the mirrors, continually stimulating and aligning their travel direction. One of the mirrors designated the front mirror is deliberately designed to allow a controlled leakage or transmission of light -up to 60%. This transmission is the raw laser beam. The beam is pure light since it consists of a single wavelength (monochromatic) and in addition is both coherent (in phase) and collimated (low divergence).
The basic solid state neodymium "YAG" laser cavity consists of the ND:YAG crystal, an energy source, a 100% reflective rear mirror and a front or output mirror which is up to 60% light transmissive. The cavity may also contain accessories such as shutters, apertures and electro optical mechanisms.
A laser beam is generated when photons traveling in a direction along the longitudinal axis of the crystal are reflected and returned to the crystal by the end mirrors where they continue to amplify the output through the phenomena of stimulated emission. Since the front mirror is up to 60% light transmissive a laser beam is emitted. This beam of light is monochromatic, collimated and coherent. Because of these three characteristics and resultant low divergence, the beam is capable of traveling great distances with minimal energy loss.
These characteristics are extremely important. The ability to deliver this pure beam of light through an optical system and project it for miles or to focus it to so small a diameter its energy density can vaporize ceramics, is dependent on these three characteristics. It is timely to mention there are many lasers emitting pure light beams of different frequencies depending on their atomic origin. Special applications require particular light frequencies to be efficiently absorbed by the characteristics of the target material. Eye surgery or measurement operations, etc. require different light frequencies than those we commonly use for metal working. We must not neglect to add the cavity and optical system just described includes other items, apertures, collimators, safety shutters, beam splitters, viewing optics etc. to fine tune and control the beam.
After leaving the cavity through the partially transmissive front mirror, the beam continues through a safety shutter followed by an up collimator. The latter, a kind of reverse telescope, expands the beam, further improves the collimation and prepares it for the final optics. After leaving the up collimator, the beam proceeds to a turning or beam bending mirror where it is usually directed vertically downward through the final, objective focusing lens. Between the turning mirror and final lens, other accessories such as a trepanning device may be introduced. Depending on the system the beam, after leaving the up collimator, may enter a fiber optic coupler rather than the hard optics just described. Fiber optics terminated by focusing optics provide complete mobility of beam direction.
The final lens focuses and concentrates the raw laser beam into the desired spot diameter. In addition, it establishes the focal distance between the lens and workpiece and relative to this produces a specific depth of field within which distance there are negligible changes in focal spot diameter. Short or long focal distances have their corresponding short or long depth of fields. In all of the foregoing arrangements there are numerous variations; lens design, lens combinations, beam splitting, trepanning heads, power sampling, apertures, up collimator ratios, etc. intended to provide particular performance and control characteristics.