Laser Welding

Laser welding utilizes a beam of laser light to melt and fuse material across the weld joint.  The molten metal solidifies on cooling to form a weld.  Laser welding is a non-contact process and does not require any electrical connection to complete the circuit as with arc welding.  As with all other welding processes, laser welders come in all shapes and sizes; from the jewelry welder where the parts are presented manually to the laser light to high-power 6 kW machines that can weld thick sheets of steel.  Compared to other welding processes, laser welding is relatively new and continues to gain ground with new opportunities and applications.  When it comes to welding of small parts, lasers are utilized for applications where access is difficult and contamination is unacceptable.  For large scale applications, laser welding works really well where distortion and welding speed are an issue.  There are multiple issues related to laser welding starting with fundamentals of laser welding discussed in the section below.

Fundamentals

In all welding processes, energy has to be supplied to heat parts to be welded and form a bond.  A beam of laser light is a source of such energy that can travel through air or vacuum.  As the beam interacts with the parts to be welded, the laser energy is converted to heat and used to fuse the parts to be welded.  As an option, laser soldering and laser brazing are also viable processes where the solder/braze is melted as opposed to the base metal in welding.  Most commonly used lasers are Nd:YAG, fiber lasers, and CO2 lasers.  YAG lasers are solid-state lasers and generate wavelength at about 1.064 microns.   While YAG energy is generated in a YAG rod, fiber lasers generate energy in a special doped fiber itself.  Benefit of using fiber for lasing is higher peak power at very small spot size and very high beam quality.  Even though initial cost of fiber laser may be a bit higher, they consume considerably less power and have a fairly small footprint. CO2 laser is a gas based laser and generates light of 10.64 microns.  Most metals are not good conductors of laser light and will reflect majority in incident light.  The small portion of light that is absorbed is converted to heat and raises local temperature of the intended weld spot.  As the temperature rises, the absorption coefficient increases as well and leads to greater absorption of incident energy followed by further increase in temperature.  The process leads to a thermal runoff and, with the right amount of laser power, can produce melting of parts within milliseconds.

As the weld spot melts, molten metal from both sides of the weld interface melt and mix, thus forming a weld fusion nugget at the interface.  Heat from the weld spot spreads and heats adjacent parts and structures.  The dynamic between rate of heat input and rate of heat dissipation will govern the weld spot size.  If the rate of heat input is too low, the weld spot may be too small or not form at all.  If the rate of heat input is too high, the molten metal can become too fluid and start to bubble out metal droplets being spit out of the weld.  Such droplets, or weld spatter, can cause a loss of volume in the weld and also cause collateral damage as the spatter deposits on the other components and machinery.  Control of the size and shape of the weld spot depends on multiple factors including peak power, pulse width, and spot size.  Fusion zone size can also be distinctly affected by type of energy delivery, either single spot or continuous.