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Laser-beam-welding,for High Quality Joints.SOLUTIONS with Effective, Powerful Advice
Laser-beam-welding is a high energy process. It has certain characteristics, in particular the extremely high energy density, in common with Electron Beam Welding. However being similar in applications, it is quite different in essence, physics, equipment and practice. Both processes were briefly introduced together in our page on High Energy Welding Processes.
The Laser is a high density coherent light source that is used in welding to heat and melt metals. Laser-beam-welding is possible because the coherent nature of the light beam allows it to be focused to a small spot where the energy density can reach Laser sources can be based on a solid, liquid, gas or a semi-conductor. Different sources emit different wavelength light. Most used for welding applications are CO2 sources which emit in the far infrared (10.6 micron) and can provide several kW of power. This beam will not pass through the quartz lens but rather will be absorbed by those transparent materials resulting in their destruction. Neodymium-doped yttrium aluminum garnet (Nd:YAG) emits its primary light output in the near-infrared, at a wavelength of 1.06 microns and is generally limited in power to a few kW. This beam will pass through quartz lenses, clear plastic or glass and other transparent materials. Carbon dioxide Laser-beam-welding equipment must achieve focusing either by converging reflective optics or by special salt based lens materials such as zinc selenide. The Laser-beam-welding output is not electrical, does not require electrical continuity, is not influenced by magnetism, is not limited to electrically conductive materials and can be used for plastics. Micro Laser-beam-welding is used for making delicate instruments and for jewelry. 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. Laser-beam-welding is especially dominant in the automotive industry.
Heat is generated by the conversion of light energy. All metals reflect light to some degree, with gold and silver in high amounts and carbon steel only somewhat. Gold, silver, copper, and aluminum are therefore difficult to weld, requiring intense energy usually available from high energy peaking pulses or using light absorbing coatings on the weld joint surfaces such as graphite 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 CO2 lasers. It is therefore, in this respect, more suited for Laser-beam-welding highly reflective materials. However though metallic reflectivity is initially a factor, once melted, the reflectivity essentially disappears and therefore most metals are readily fusion welded. The intense energy of the beam quickly melts the surface, from which thermal conductance progresses to achieve penetration. Limited thickness metals, more sensitive to surface reflectance, are weldable in the Convection Mode which is the factor affecting the geometry of the laser melt pool shape, aspect ratio and surface ripples. It can result in defects like variable penetration, porosity and lack of fusion. Convection generates mixing in the weld pool and therefore it affects its composition. In Deep Penetration Mode, the energy transfer for Laser-beam-welding occurs through keyhole formation, which is produced by a sufficiently powerful beam. Once the surface is melted and reflectivity ceases to be a factor, a column of ionized metal vapor forms below the beam impingement point. The hole acts as a black body, absorbing effectively the incoming laser energy, and distributing heat deep into the material. The pressure produced by the vapor in the hole displaces the molten metal upward along the walls. This makes narrow, deep welds with little heat input, more efficiently than processes where the weld shape is governed by thermal conduction and convection. In traditional processes on the contrary, energy is deposited on the surface, and is brought into the interior by conduction. Laser beams are delivered to the workpiece in two ways. In the first, beams are deflected and focused through the use of mirrors and lenses only, with practical limitations in the distance of the workpiece from the laser source. Also with some difficulties in finding the correct position and angle to perform the weld. The second method uses a fiber optic cable. For Laser-beam-welding, the energy is focused into one end of the cable and exits at the other end with negligible loss. The beam is then refocused onto the workpiece. This method delivers the beam precisely to the welding area, even with robots, by moving the focusing optics instead of, or in addition to, the workpiece itself. Among the Advantages of Laser-beam-welding one can list:
Limitations of Laser-beam-welding are:
Depending on applications one can select between continuous or pulsed beam. Concerns about common and detrimental issues of Laser-beam-welding results refer to cracking, burning, incorrect penetration and welds off the joint line. Of the many factors that can cause problems in a metal joining process, only some are under the control of the process itself. If successful Laser-beam-welding parameters are developed for an application, then results depend on suitable process control. Most up to date laser systems control automatically the power and duration of each laser pulse. Other parameters that must be monitored and taken care of throughout production runs are cleanliness of parts, shielding gas, tooling and motion control. Once absorptivity of the material is known or modified, and shielding gas, if necessary, is selected, to develop LBW procedures, after determining weld joint design and gap size, these parameters must be established:
The results to be evaluated are:
Laser-beam-welding hazards are different from those of traditional welding processes. Therefore it is most important to learn and apply the safety rules that govern their safe use. ANSI/LIA Z136.1-2007
A derivative of Laser-beam-welding, hybrid welding, combines the laser with an arc welding method such as Gas Metal Arc Welding (GMAW). This combination provides synergy allowing for greater positioning flexibility, since GMAW supplies molten metal to fill the joint, while laser increases the welding speed. Weld quality is higher as well, since the potential for defects is reduced. Click on Hybrid Welding to see a page dedicated to this combined process. A Middle Month Bulletin with Updated Resources on Hybrid Laser Arc Welding was recently released.
An Article on Laser Engineered Net Shaping was published (2) in Issue 98 of Practical Welding Letter for October 2011. An Article on How to develop a laser welding procedure was published (3) in Issue 109 of Practical Welding Letter for September 2012. An Article on Progress with Laser Orbital Welding was published (2) in Issue 118 of Practical Welding Letter for June 2013. An Article on a new laser arc method for obtaining Enhanced Diamond-like Coatings [that] boost Fuel Efficiency was introduced (11) in Issue 148 of Practical Welding Letter for December 2015. An Article on A new way to measure Laser Power was published (7) in Issue 152 of Practical Welding Letter for April 2016. An Article on Laser Welds in Vacuum and another one on Using Lasers for Welding, which deals with measuring the power density, were published (2) (3) in Issue 160 of Practical Welding Letter for December 2016. An Article on Laser Shot Peening was published (8) in Issue 166 of Practical Welding Letter for June 2017.
To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics.
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