Laser Beam Welding Pdf Download
Lee Stlaurent
Laser beam welding (LBW) is a welding technique used to join pieces of metal or thermoplastics through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume and precision requiring applications using automation, as in the automotive and aeronautics industries. It is based on keyhole or penetration mode welding.
Like electron-beam welding (EBW), laser beam welding has high power density (on the order of 1 MW/cm2) resulting in small heat-affected zones and high heating and cooling rates. The spot size of the laser can vary between 0.2 mm and 13 mm, though only smaller sizes are used for welding. The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximized when the focal point is slightly below the surface of the workpiece
A continuous or pulsed laser beam may be used depending upon the application. Millisecond-long pulses are used to weld thin materials such as razor blades while continuous laser systems are employed for deep welds.
LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium. Due to high cooling rates, cracking is a concern when welding high-carbon steels. The weld quality is high, similar to that of electron beam welding. The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the workpieces. The high power capability of gas lasers make them especially suitable for high volume applications. LBW is particularly dominant in the automotive industry.[1][2]
A derivative of LBW, laser-hybrid welding, combines the laser of LBW with an arc welding method such as gas metal arc welding. This combination allows for greater positioning flexibility, since GMAW supplies molten metal to fill the joint, and due to the use of a laser, increases the welding speed over what is normally possible with GMAW. Weld quality tends to be higher as well, since the potential for undercutting is reduced.[5]
Although laser beam welding can be accomplished by hand, most systems are automated and use a system of computer aided manufacturing based on computer aided designs.[6][7][8] Laser welding can also be coupled with milling to form a finished part.[9]
In 2016 the RepRap project, which historically worked on fused filament fabrication, expanded to development of open source laser welding systems.[10] Such systems have been fully characterized and can be used in a wide scale of applications while reducing conventional manufacturing costs.
Gas lasers use high-voltage, low-current power sources to supply the energy needed to excite the gas mixture used as a lasing medium. These lasers can operate in both continuous and pulsed mode, and the wavelength of the CO2 gas laser beam is 10.6 μm, deep infrared, i.e. 'heat'. Fiber optic cable absorbs and is destroyed by this wavelength, so a rigid lens and mirror delivery system is used. Power outputs for gas lasers can be much higher than solid-state lasers, reaching 25 kW.[11]
Modern laser beam welding machines can be grouped into two types. In the traditional type, the laser output is moved to follow the seam. This is usually achieved with a robot. In many modern applications, remote laser beam welding is used. In this method, the laser beam is moved along the seam with the help of a laser scanner, so that the robotic arm does not need to follow the seam any more. The advantages of remote laser welding are the higher speed and the higher precision of the welding process.
Pulsed-laser welding has advantages over continuous wave (CW) laser welding. Some of these advantages are lower porosity and less spatter.[12] Pulsed-laser welding also has some disadvantages such as causing hot cracking in aluminum alloys.[2] Thermal analysis of the pulsed-laser welding process can assist in prediction of welding parameters such as depth of fusion, cooling rates, and residual stresses. Due to the complexity of the pulsed laser process, it is necessary to employ a procedure that involves a development cycle. The cycle involves constructing a mathematical model, calculating a thermal cycle using numerical modeling techniques like either finite elemental modeling (FEM) or finite difference method (FDM) or analytical models with simplifying assumptions, and validating the model by experimental measurements.
Not all radiant energy is absorbed and turned into heat for welding. Some of the radiant energy is absorbed in the plasma created by vaporizing and then subsequently ionizing the gas. In addition, the absorptivity is affected by the wavelength of the beam, the surface composition of the material being welded, the angle of incidence, and the temperature of the material.[12]
The physics of pulsed laser can be very complex and therefore, some simplifying assumptions need to be made to either speed up calculation or compensate for a lack of materials properties. The temperature-dependence of material properties such as specific heat are ignored to minimize computing time.
Laser welding or laser beam welding (LBW) is a process that uses a concentrated heat source in the form of a laser to melt the materials, which fuse together as they cool down. It is a versatile process since it can weld thin materials at rapid welding speeds while running narrow and deep welds for thicker materials.
The energy enters the weld zone only by heat conduction. This limits the welding depth and thus the process is great for joining thin materials. Heat conduction welding is often used for visible weld seams which need to be aesthetically pleasing.
Running the process in keyhole welding (deep penetration) mode creates deep, narrow welds with uniform structure. For metals, power densities of about 1 megawatt per square centimetre are applied. This does not only melt the metal but vapourises it, creating a narrow vapour-filled cavity.
This is called a keyhole cavity or vapour capillary and is filled with molten metal as the laser beam advances through the workpiece. Keyhole welding is a high-speed process and thus, the distortion and the formation of a heat-affected zone are kept to a minimum.
Laser beam welding works on the principle of using a laser with high power density to apply heat to a joint between the surface of two metals. The material melts at the joint, and it permits fusion between the metals as it solidifies.
The welding process can be performed under atmospheric conditions but for more reactive materials inert gas shielding is recommended to eliminate the risk of contamination. Similarly to electron beam welding, laser welding could be carried out in a vacuum but it is not deemed economically feasible. Thus, laser welders come equipped with gas nozzles that supply inert gas to the weld area.
Many laser welding applications are carried out without the need for additional filler material. However, some challenging materials and applications require filler material to produce satisfactory welds. Adding filler material improves the weld profile, reduces solidification cracking, gives the weld better mechanical properties and allows for more precise joint fit-up. The filler material can come in powder form or as filler wire but since powders are generally more expensive for most materials, using wire feedstock is more common.
A CO2 laser source is a mixture of gases with CO2 being the main component alongside nitrogen and helium. These lasers can operate in a continuous or pulsed mode at a low current and high voltage to excite the gas molecules. Carbon dioxide lasers are also used in special circumstances, such as in dual-beam laser welding, wherein two beams are produced and arranged either in tandem or side-by-side.
Laser-hybrid welding combines the concepts of electric arc and laser beam. The two simultaneously act in the same welding zone, complimenting each other and creating a unique welding process. Although laser welding can be used in conjunction with virtually any arc welding process, there are some that stand out and are used more commonly.
The hybrid welding process offers deep penetration brought by laser welding and a weld cap profile achieved comparable to arc welding processes. Using protective shielding gases and other arc welding consumables offer greater control over the weld characteristics than laser welding would allow just by itself. Laser-hybrid welding is definitely a process that is on the rise and will be utilised more and more in the shipbuilding, railroad, automotive industries and large-scale pipe welding projects in the future.
Laser welding operates in two fundamentally different modes: conduction limited welding and keyhole welding. The mode in which the laser beam will interact with the material it is welding will depend on the power density across the beam hitting the workpiece.
Conduction limited welding occurs when the power density is typically less than 105W/cm2. The laser beam is absorbed only at the surface of the material and does not penetrate it. Conduction limited welds often then exhibit a high width to depth ratio.
Laser welding is more usually accomplished using higher power densities, by a keyhole mechanism. When the laser beam is focused to a small enough spot to produce a power density typically > 106-107 W/cm2, the material in the path of the beam not only melts but also vaporises, before significant quantities of heat can be removed by conduction. The focused laser beam then penetrates in to the workpiece forming a cavity called a 'keyhole', filled with metal vapour (which in some cases can even be ionised, forming a plasma).
Furthermore, the coupling of the laser beam in to the workpiece is improved dramatically by the formation of this keyhole. Deep penetration welding is then achieved by traversing the keyhole along the joint to be welded or moving the joint with respect to the laser beam. This results in welds with a high depth to width ratio.
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