TWI Frequently asked questions
Cold wire feeding techniques for laser welding are available and well established. In particular instances, the addition of filler wire might be necessary to:
- Avoid solidification cracking of the weld metal
- Modify the chemical composition or the microstructure of the weld metal to obtain suitable mechanical properties
- Improve the joint fit-up tolerance of autogenous laser welding
- Achieve multi-pass welds or fill a V-groove joint
The use of cold filler wire generally results in a 10% to 20% decrease in welding speed to compensate for the energy used to melt the wire. When welding thick section materials (> 6~ 10mm), uneven mixing of the filler wire can be observed along the depth of the weld. This requires additional attention, particularly when the filler is used to control the composition, the microstructure and/or the mechanical properties of the weld.
The filler wire nozzle is generally mounted onto the laser welding head so that the wire is aligned with the centre of the joint. In most cases, the wire should intersect the laser beam at the upper surface of the workpiece into the leading edge of the weld pool. Accurate positioning of the filler wire is essential to the success of the welding operation. Shielding of the filler wire itself is generally not necessary as laser welding already requires separate gas shielding of the workpiece.
The wire feed rate for a given plate thickness can be estimated from the cross-sectional area of gap, the welding speed and the cross-sectional area of the filler wire as follows:
Wire feed rate, (m/min) = (Welding speed x cross-sectional area of gap) / (cross-sectional area of filler wire)
If the gap width is variable along the joint length, the wire feed rate will need to be changed accordingly to maintain uniform gap filling. Consequently, some form of gap sensing system could be required to detect the gap width and automatically adjust the wire feed rate to suit.
Filler wires can also be added by using a hybrid laser/arc processing technique. Hybrid Laser-MAG and Laser-MIG processes are now being used in industry due to their combined advantages of both laser welding and arc welding.
