10 May 2017

Laser beam welding

Introduction :

Laser beam welding is a technique of welding which used to join multiple pieces of metal through the use of laser. It is frequently used in high volume applications using automation, such as in the automotive industries.

Now we all know that laser is a concentrated beam of coherent monochromatic radiation. Normal white light consists of a number of colours and a number of waves. As a result, it is not possible to use a monochromatic source as laser, provided all waves are of a single phase. This is achieved by means of stimulation. Because of coherency, it is possible to concentrate the laser beam by means of an optical lens to a spot of any desired size without appreciably losing any of its intensity. Thus, the laser beam is a high-energy source of heat to melt a joint for fusion welding in laser beam welding.

Laser beam welding operates in two fundamentally different modes :

  • Conduction limited welding 
  • Keyhole welding 
The mode in which the laser beam will interact with the material it is welding will depend on the power density of the focus laser spot on the workpiece.

How it works?

There are generally two types of laser which are used in welding operation such as solid-state lasers and gas lasers.

In solid-state lasers, light is emitted from a glass on a single crystal that is doped with transition elements such as chromium for ruby. When a beam of normal white light impinges on the crystal, the outer shell electrons of the dope elements go to a higher energy metastable state. They return to the normal state after emitting the extra energy in the form of a photon. All the photons that are stimulated to emit at a given instant will form coherent radiation, which can be concentrated by optical lenses. Thus, the output would be normally in pulses. The power ratings of such units may be up to 2 kW.

In gas lasers, the gas such as carbon dioxide molecules is excited to the higher vibrational energy level by means of an electric discharge. The transition from this high-energy level to the normal level generates the radiation which is coherent and gets focused by means of the usual optical lenses. Continuous-wave gas lasers using carbon dioxide gas with powers up to 20 kW are used for laser beam welding. 

Laser beam welding
With low power lasers typically less than 1 kW, the penetration would not be much and the weld is obtained by means of complete welding of the joint near the surface. But as the power increased because of that the large heat density obtained. That large density causes the metal at the centre of the jet to be vaporized with a keyhole being formed similar to that of electron beam welding. The temperature within this keyhole can reach as high as 25000 0C. This technique permits large welding speeds depending on laser size.  

There are three types of lasers that are commonly used in welding operation such as CO2, Nd: YAG and fibre laser. The Nd: YAG laser light is absorbed quite well by conductive materials, with a typical reflectance of about 20 to 30 % for most metals. Using standard optics, it is possible to achieve focused spot sizes diameter as small as 0.025 mm. On the other side, the COlaser has an initial reflectance of about 80 to 90 % for most metals and requires special optics to focus the beam to a minimum spot size of about 0.075 to 0.100 mm diameter. However, whereas the Nd: YAG lasers might produce power outputs up to 500 watts, the CO2 system can easily supply 10 kW of power and more.

Advantages of laser beam welding :

  • Heat input is close to the in the minimum required to fuse the weld metal, thus heat-affected zones are reduced and workpiece distortions are minimized.
  • Time for welding thick sections is reduced and the need for filler wires and elaborate joint preparations is eliminated by employing the single pass laser welding procedures.
  • No electrodes are required.  
  • LBM being a non-contact process so distortions are minimized and tool wears are eliminated.
  • Welding in areas that are not easily accessible with other means of welding can be done by LBM.
  • The joining of small spaced components with tiny welds very easily because of laser beam can focused on a small area.
  • Wide variety of materials including various combinations can be welded very easily.
  • Thin welds on small diameter wires are less susceptible to burn back than is the case with arc welding. 
  • Metals with dissimilar physical properties, such as electric resistance can also be welded by LBW.
  • No vacuum or X-Ray shielding is required. 
  • Welds magnetic materials also. 
  • Aspect ratios means depth-to-width ratio of the order of 10:1 are attainable in LBM.
  • Faster welding rate.
  • No flux or filler metal required.
  • Single-pass two-side welding. 
  • Shorter cycle and higher up times. 
Limitations of laser beam welding :
  • Joints must be accurately positioned laterally under the beam.
  • Final position of the joint is accurately aligned with the beam impingement point.
  • The maximum joint thickness that can be welded by laser beam is somewhat limited. 
  • The materials have high thermal conductivity and reflectivity like Al and Cu alloys can affect the weldability with lasers. 
  • An appropriate plasma control device must be employed to ensure the weld reproducibility while performing moderate to high power laser welding. 
  • Lasers tend to have low energy conversion efficiency less than 10 percent. 
  • Some weld-porosity and brittleness can be expected, as a consequence of the rapid solidification characteristics of the LBM. 
Laser beam welded parts are often required additional steps before and after the actual weld. we can see this additional steps below :

Process before welding :
  • CAD/CAM product design and weld design.
  • Tooling design and fabrication.
  • Parts cleaning.
  • Strategic Sourcing and Subcontractor Contract Management
Process after welding :
  • Leak test.
  • Metallurgic evaluations.
  • Post weld thermal treatment.
  • Not destructive testing.
You also check it out the :

  • laser beam welding application