Laser Beam Welding (LBW)

INTRODUCTION

Laser beam welding (LBW) is a welding technique used to join pieces of metal or thermoplastics through the use of a laser. The beam provides a concentrated heat source, allowing for narrow, deep welds and high welding rates. The process is frequently used in high volume applications using automation, such as in the automotive industry. It is based on keyhole or penetration mode welding.

DEFINITION

The laser is a device that produces monochromatic (all the radiation is in the same wavelength), coherent (all the radiation waves are in phase) light. Therefore electromagnetic radiation (light) can be amplified unidirectional and with low divergence.

The LASER is an acronym for Light Amplification by Stimulated Emission of Radiation  

PRINCIPLE OF OPERATION

1. Laser Beam Welding (LBW) is a fusion joining process that produces coalescence of materials with the heat obtained from a concentrated beam of coherent, monochromatic light impinging on the joint to be welded.
2. In the LBW process, the laser beam is directed by flat optical elements, such as mirrors, and then focused to a small spot at the workpiece using either reflective focusing elements or lenses. 
3. LBW is a non-contact process and thus requires that no pressure be applied. Inert gas shielding is generally employed to prevent oxidation of the molten puddle, and filler metal may occasionally be used. 

LASER WELDING

When using the laser beam for welding, the electromagnetic radiation impinges on the surface of the base metal with such a concentration of energy that the temperature of the surface is melted vapor and melts the metal below. One of the original questions concerning the use of the laser was the possibility of reflectivity of the metal so that the beam would be reflected rather than heat the base metal. It was found, however, that once the metal is raised to its melting temperature, the surface conditions have little or no effect.

The distance from the optical cavity to the base metal has little effect on the laser. The laser beam is coherent and it diverges very little. It can be focused on the proper spot size at the work with the same amount of energy available, whether it is close or far away. 

With laser welding, the molten metal takes on a radial configuration similar to conventional arc welding. However, when the power density rises above a certain threshold level, keyholing occurs, as with plasma arc welding. Keyholing provides for extremely deep penetration. This provides a high depth-to-width ratio. Keyholing also minimizes the problem of beam reflection from the shiny molten metal surface since the keyhole behaves like a black body and absorbs the majority of the energy. In some applications, an inert gas is used to shield the molten metal from the atmosphere. The metal vapor that occurs may cause a breakdown of the shielding gas and creates a plasma in the region of high-beam intensity just above the metal surface. The plasma absorbs energy from the laser beam and can actually block the beam and reduce melting. Use of an inert gas jet directed along the metal surface eliminates the plasma buildup and shields the surface from the atmosphere.

The welding characteristics of the laser and of the electron beam are similar. The concentration of energy by both beams is similar to the laser having a power density in the order of 106 watts per square centimeter. The power density of the electron beam is only slightly greater. This is compared to a current density of only 104 watts per square centimeter for arc welding.

Laser beam welding has a tremendous temperature differential between the molten metal and the base metal immediately adjacent to the weld. Heating and cooling rates are much higher in laser beam welding than in arc welding, and the heat-affected zones are much smaller. Rapid cooling rates can create problems such as cracking in high carbon steels.

Experimental work with the laser beam welding process indicates that the normal factors control the weld. Maximum penetration occurs when the beam is focused slightly below the surface. Penetration is less when the beam is focused on the surface or deep within the surface. As power is increased the depth of penetration is increased.

TYPES OF WELDING 

1. Conduction mode welding
2. Conduction/penetration mode
3. Penetration or keyhole mode

• Conduction Welding: Performed at lower energy levels forming a wide and shallow weld nugget.There are two modes:

1. Direct heating: heat 􀁼ow is governed by classical thermal conduction from a surface heat source. The weld is made by melting portions of the base material. Can be made using pulsed ruby and CO2 lasers using a wide range of alloys and metals. Can also use Nd: YAD and diode lasers.
2. Energy transmission: energy is absorbed through novel inter-facial absorption methods.An absorbing ink is placed at the interface of a lap joint. The ink absorbs the laser beam energy, which is conducted into a limited thickness of surrounding material to form a molten interfacial 􀁻lm that solidi􀁻es as the welded joint. Butt welds can be made by directing the energy towards the joint line at an angle through the material at one side of the joint, or from one end if the material is highly transmissive.

• Conduction/penetration welding occurs at a medium energy density and results in more penetration.

• Penetration or keyhole mode:
  The keyhole mode welding creates deep narrow welds. In this type of welding, the laser light forms a filament of vaporized material known as a "keyhole" that extends into the material and provides a conduit for the laser light to be efficiently delivered into the material. 
  The direct delivery of energy into the material does not rely on conduction to achieve penetration, so it minimizes the heat into the material and reduces the heat affected zone. 
  The laser forms a hole that is sealed by the molten material behind the laser. The result is called a keyhole weld.

PROCESS PARAMETERS
                                                    

 Arc Augmented Laser Welding Diagram

1. In arc augmented laser welding the arc from a TIG or MIG torch is mounted close to the laser beam interaction point. The TIG torch will automatically lock onto the laser generated hot spot.
2. The temperature required for this phenomenon is around 300C above the surrounding temperature. The effect is either to stabilize an arc which is unstable due to its traverse speed or to reduce the resistance of an arc which is stable.
3. The locking only happens for arcs with a low current and therefore slow cathode jet for currents less than 80A. The arc is on the same side of the workpiece as the laser which allows doubling of the welding speed for a modest increase in the capital cost.

LBW Process Advantages: 

Major advantages of Laser Beam Welding include the following: 

1. Heat input is close to the minimum required to fuse the weld metal, thus heat affected zones are reduced and workpiece distortions are minimized. 
2. 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. 
3. No electrodes are required; welding is performed with freedom from electrode contamination, indentation or damage from high resistance welding currents. 
4. LBM is a non-contact process, distortions are minimized and tool wears are eliminated. 
5. Welding in areas that are not easily accessible with other means of welding can be done by LBM since the beams can be focused, aligned and directed by optical elements. 
6. The laser beam can be focused on a small area, permitting the joining of small, closely spaced components with tiny welds. 
7. A wide variety of materials including various combinations can be welded. 
8. Thin welds on small diameter wires are less susceptible to burn back than is the case with arc welding. 
9. Metals with dissimilar physical properties, such as electric resistance can also be welded. 
10. No vacuum or X-Ray shielding is required. 
11. Laser welds are not influenced by magnetic fields, as in arc and electron beam welds. They also tend to follow weld joint through to the root of the work-piece, even when the beam and joint are not perfectly aligned. 
12. Aspect ratios (i.e., depth-to-width ratios) of the order of 10:1 is attainable in LBM. 

Limitations

1. Rapid cooling rate may cause cracking in some metals
2. High capital cost for equipment
3. Optical surfaces of the laser are easily damaged
4. High maintenance costs