Laser Beam Welding Characteristics
The laser beam welding characteristics of the laser and electron beam are similar. The concentration of energy by both beams is similar with the laser having a power density in the order of 10°W/cm². The power density of the electron beam is only slightly greater. It is compared to a current density of only 10 W/cm² for arc welding. Power level is up to 100kW. Welding speed is up to 250ft/min. It can weld the foil up to 1 inch plate.
Lasers are used for materials that are difficult to weld using other methods, hard to access areas and extremely small components. Inert gas shielding is needed for more reactive materials. Laser beam welding has a tremendous temperature differential between molten metal and base metal immediately adjacent to the weld. Heating and cooling rates are much higher in laser beam welding than arc welding and heat-affected zones are much smaller.
Primary Types of Lasers used in Welding
The various laser forms that are generally used are as follows:
- Liquid laser
- Gas laser
- Carbon dioxide laser
- Solid state laser
- Ruby laser, and
- Semi conductor laser.
(i) Liquid laser: A liquid-crystal laser is a laser that uses a liquid crystal as the resonator cavity to allow the selection of emission wavelength and polarization from the active laser medium. The lasing medium is usually a dye dopped into the liquid crystal. The tuning range is several tens of nanometers. Self-organization at micrometer scales reduces manufacturing complexity compared to using layered photonic meta-materials. Operation may be either in continuous wave mode or pulsed mode.
(ii) Gas lasers: A mixture of gases is used such as helium and nitrogen. There is also carbon dioxide (CO2) laser. These lasers use a low-current, high-voltage power source to excite the gas mixture using a lasing medium. They are operated in a pulsed or continuous mode.
(iii) Carbon dioxide lasers: They use a mixture of high purity carbon dioxide with helium and nitrogen as the lasing medium. CO2 lasers are also used in dual beam laser welding where the beam is split into two equal power beams.
(iv) Solid state lasers: YAG type and ruby lasers operate at 1 micrometer wavelengths. They can be pulsed or operate continuously. Pulsed operation produces joints similar to spot welds but with complete penetration. The pulse energy is 1 to 100 J. Pulse time is to 10 milliseconds.
(v) Ruby lasers: A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. Ruby lasers produce pulses of coherent visible light at a wavelength of 694.3 nm which is a deep red color. Typical ruby laser pulse lengths are on the order of a millisecond.
(vi) Semiconductor lasers: Semiconductor lasers are lasers based on semiconductor gain media where the optical gain is usually achieved by stimulated emission at an inter-band transition under conditions of a high carrier density in the conduction band. They consist of complex multi-layer structures requiring nanometer scale accuracy and an elaborate design.
Types of Laser Beam Welding
Pulsed laser beam welding : A pulse of focused laser energy beam when incident on a metallic surface is absorbed within a very small area and it may be treated as a surface heating phenomenon. Thermal response beneath the focused spot depends upon heat conduction. If the laser pulse is too short when compared to thermal diffusion time, the pulse energy remains at the surface and rapid localized heating occurs for little depth of penetration. This accumulation of heat at the surface causes metal to vaporize from the surface.
In this welding, the bottom lower surface should reach the melting temperature before the upper surface reaches the vaporization point. Therefore, thermal diffusivity and pulse duration control ensure the depth of successful porosity free welds.
Continuous wave laser beam welding : Lasers such as solid state and CO2 are capable of making high speed continuous metal welds. This continuous power supplied with continuous wave laser beam takes high power carbon dioxide laser with deep penetration capability. There is a precise control of energy delivery to highly localized regions. It is good for narrow gap geometrics and it permits welding without using filler metal. It reduces the amount of filler metal. Deep penetration
welds are similar to EBW process.
Advantages, Limitations and Applications of Laser Beam Welding
- There is no need of electrodes and power.
- Even very small holes can also be welded.
- There is no vacuum requirement such as electron beam.
- Accuracy is greater.
- There is no heat loss.
- Neat and clean surface finish can be obtained.
- Laser beam welding can be used to weld dissimilar metals which are difficult to weld.
- X-rays are not generated by the beam and hence it is safe.
- Laser beam can be manipulated using the principles of optics to permit easy automation.
- Cooling rates are high due to low energy inputs per unit weld length. Also, the problems associate with welding can be rectified by pre- or post-heat treatment processes.
- Ruby lasers are used for spot welding of thin gauge metals.
- Electrical efficiency of the process is 10-20% only.
- Better quality weld can be produced. It produces less tendency for incomplete fusion, spatter, porosity and distortion.
- It ensures precise working with exact placing of the energy spot welding of Tot complicated joint geometry.
- It produces low thermal distortion.
- It produces cavity-free welds.
- It needs low post weld operation times.
- Large working distance is possible.
- No filler metals are necessary.
- Works with high alloy metals without difficulty
- Welding process is slow.
- Limited depth of weld can be done.
- It is not suitable for large production.
- Capital cost for equipment is high.
- Optical surfaces of the laser are easily damaged.
- Maintenance cost is high.
- Rapid cooling rate may cause cracking in some metals.
- Thin metals about 0.5 mm to 1.5 mm thick can be welded. It includes welding of foils, stents, sensor diaphragms and surgical instruments.
- It can joint dissimilar metals such as copper, nickel, chromium, stainless steel, titanium and columbium.
- It is very much useful in electronic components welding.
- It is used in aircraft components joining.
- In automotive industry, it is mostly used for welding transmission components.
- It is very much useful in joining metal alloys.
- Laser beam welding of high-strength aluminum alloys is used for aerospace and automotive applications.
- With slight modifications, the process can be used for gas assisted cutting and surface heat treating and alloying applications.