Laser Beam Welding
Laser is the abbreviation of light amplification by stimulated emission of radiation. It is very strong coherent monochromatic beam of light, highly concentrated with a very small beam divergence. The beam exiting from the laser source may be 1-10 mm in diameter, when focussed on a spot has energy density of more than 10 KW/mm2. Laser beam welding is a thermoelectric process accomplished by material evaporation and melting. Focussing is achieved by various lens arrangements while focusing of electron beam is achieved by electrostatic and magnetic means. Because of this focusing, high power densities are achieved by both the ‘electron’ and the ‘Laser’ beams.
• The process does not require a vacuum chamber, size of HAZ is smaller and the thermal damage to the adjascent part is negligible. Laser can be used to join dissimilar metals, difficult-to-weld metals e. g. copper, nickel, chromium, stainless steel, titanium and columbium. Currently the process is largely in use in aerospace and electronic industries.
• The principle of working of a Laser Welder is shown in Fig. 2.21(a). An intense green light is thrown on a speciai man-made ruby, 10 mm in diameter, containing about
0. 05% by weight of chromium oxide. The green light pumps the chromium atoms to a higher state of energy. Each of these excited atoms emits red light that is in phase with the colliding red light wave.
Fig. 2.21(b) Schematic diagram of laser welding
• Thus, the red light gets continuously amplified. To further enhance this effect the parallel ends of the rod are mirrored to bounce the red light back and forth within the rod. When a certain critical intensity of pumping is reached, the chain reaction of collisions becomes strong enough to cause a burst of red light. The mirror in the front of the rod is only a partial reflector, allowing the burst of light to escape through it.
• Lasers used for welding could be of two types:
1. Solid-state lasers
2. Gas Lasers (The chief gas Laser is CO2 laser)
Solid-state lasers are ruby, Nd : Glass and Nd : YAG. The last two are the Lasers in
which (Nd : Glass) or single crystals of Yttrium-Aluminium-Garnet (Nd : YAG) are doped with
Nd (neodymium) ions as the active medium. The chief gas laser is CO2 laser.
• Ruby and Nd: Glass are capable of high energy pulses but are limited in maximum repetition rate, Nd YAG and CO2 Lasers can be continuous wave or pulsed at very high repetition rate.
• Incident laser radiations do reflect back from metallic surfaces in appreciable amounts, sufficient energy is still absorbed to maintain a continuous molten puddle. Ruby and Nd: Glass lasers, because of their high energy outputs per pulses, overcome this reflectivity problem.
• Due to inherently low pulse rates 1-50 pulses per second, welding speeds for thin sheets are extremely slow. In contrast Nd : YAG and inparticular CO2 lasers are capable of very high continous wave outputs or they can be pulsed at several thousand pulses per second, giving rise to high speed continuous welding.
Pulsed Laser Beam Welding
A pulse of focussed laser energy beam when incident on a metallic surface is absorbed within a very small area and may be treated as a surface heating phenomenon. Thermal response beneath the focussed spot depends upon heat conduction. The depth ‘x’ to which the energy is
felt in time ‘t’ depends upon thermal diffusivity, k, and is given by - J^kt. This leads to the
concept of thermal time constant for a metal plate of thickness ‘x’.
x = 44kt
x2 = 4kt
This represents the pulse duration required for full panetration. (through melting). For
0. 13 to 0.25 mm metal sheets, thermal time constants are comparable to pulse duration. If the laser pulse is very short as compared to thermal diffision time, the pulse energy remains at the surface and rapid localized heating occurs with very little depth of penentration. This accumulation of heat at the surface causes metal to vaporize from the surface.
In laser beam welding the bottom lower surface of the sheet must reach the melting temperature before the upper surface reaches the vaporization point. Thus, thermal diffusiv - ity and pulse duration control the depth to which successful porosity free welds could be made. Typically a solid-state laser can be pulsed for an ‘on’ period of 10 milliseconds. This limits the depth of penetration to 1 mm.
Continuous Wave Laser Beam Welding
Lasers like Nd : YAG and CO2 are capable of making high speed continuous metal welds. Laser’s, more than 500 watts capacity are capable of welding steel sheets 0.25 mm thick at several mm/second. CO2 lasers of 10 kW continuous wave output power can produce deep penetration welds in 13 mm thick steel plates at 25 mm/s.
When heating or melting a metal with a Laser beam, the concept of energy absorbed per unit volume of metal becomes a controlling parameter. The energy absorbed can be written in dimensions of J/mm3. This parameter becomes a measure of power dersity/welding speed. For example
W/mm2 x S/mm = J/mm3
The focused spot size ‘d’ of a laser beam is given by
d = f 0
where f is the focal length of the lens and 0 is the full angle beam divergence. The power density, PD, at the focal plane of the lens is given by
PD = 4 Pl
where P1 is the input power, hence
п( f 0)2
Therefore power density depends upon the laser power and beam divergence. For a laser beam operating in the basic mode, the energy distribution across the beam is gaussian, the beam divergence is
0 - - Thus PD - 4Pl -
a n f2 a2
where a is a characteristic dimension of the laser beam and - is the wavelength of laser radiation. It can, therefore, be noted that the power density is inversely proportional to the square of the wavelength of the laser radiation.
This continuous power provided by continuous wave laser beam makes high power carbon dioxide laser with deep penetration capability. There is precise controt of energy delivery to highly localized regions. This is good for ‘‘narrow gap’’, geometries and permits welding without the need for filler metal. This results in savings in filler metal. Deep penetration welds made by this process are similar to the electron beam welds. The process offers the following advantages.
1. Vacuum environment is not required, reative metals can be protected from the atomosphere by inert gas shields.
2. X-rays are not generated by the beam.
3. Laser beam can be manipulated using the principles of optics. This permits easy automation.
4. Can successfully join a variety of metals and alloys.
5. Because of low energy inputs per unit weld length, the cooling rates are high. Cooling rates and associated problems could be modified by pre - or post heating.
Typical CO2 Laser Beam Welding Performance
6. Ruby lasers are used for spot welding of thin gauge metals, microelectronic components, tasks requiring precise control of energy input to work.
7. 100 kW pulses of one millisecond duration give a series of overlapping spot welds which could be used for special applications.
8. The electrical efficiency of the process is 10 - 20% only.
9. With slight modifications, the process could be used for gas assisted cutting and for surface heat treating and alloying applications.
10.Typically a solid state laser can be pulsed for an on period of 10 milliseconds. This limits the depth of penetration to 1.0 mm.
Table. Thermal time constants for laser beam welding, seconds