The addition of filler wire during laser welding may be required to:
- Improve the joint fit-up tolerance (air gaps, mismatch, etc.) of the parts to be welded.
- Eliminate solidification cracking during welding. For some aluminium alloys wire is used to replace the low melting temperature alloys and reduce the freezing point during cooling. For welding 6XXX series aluminium alloys, high silicon content wire such as 4043 or 4047 leads to reduced cracking and improved mechanical properties of these weld.
- Modify the chemical composition or the microstructure of the weld metal to obtain suitable mechanical properties.
- Improve the weld profile such as to avoid undercut at the top and bottom bead. Excessive undercut can act as stress raiser which can reduce the mechanical properties of the weld during the service.
Laser welding with the filler wire is a multi-parameter process and there are a number of laser and filler wire parameters which determine the quality of the resultant weld. A discussion of the most important parameters follows.
Welding and filler wire speed: The wire feed rate for a given weld joint gap and plate thickness is an important parameter and depends on welding speed, the cross sectional area of the gap between the joint face, and cross sectional area of the filler wire. The relationship is expressed as follows:
Use of filler wire generally results in a 10% to 20% decrease in welding speed, for a given laser power, to compensate for the laser energy required to melt the wire.
If the filler wire feed rate is too low, the amount of heat generated from the laser beam will affect the wire and the material being welded may be able to melt a bigger section of the wire end. This may result in breaking a liquid metal bridge formed during the process, the formation of a drop at the end of the wire, and momentary disturbance of the process stability.
Too high filler wire feed rate causes the energy supplied to the welding area to be insufficient for stable and permanent wire melting. The volume of liquid metal at the end of the wire and in the liquid metal bridge increases thus flooding the air gap. Additionally, non-melted wire enters the back area of the pool, pushing out the liquid metal, which, by solidifying, forms characteristic humps of the weld surface and porosity at the root of the weld. Excessive wire speed can also reduce the penetration depth, weld width, and top bead height.
Laser beam – filler wire interaction: An exposed length of wire that is too short prevents the wire from being melted at the initial area of the bead, and the laser beam directly affects the material to be melted. In turn, an exposed length of wire that is too long causes the extended wire end to be pressed against the plate surface, and as a result, at the initial stage, the laser beam melts the wire through, dividing it into two parts. In consequence, the spot at which the process starts is covered with a wire end welded onto the surface that is difficult to remove. In an extreme case, the welded-on wire end could cause a collision with the gas shielding nozzle, disturbing or even eliminating the gas shielding.
Wire feed delivery angle. Angles between 30 and 60 degrees from the vertical can be used and 45 degree tends to be typical, as it simplifies setting the required wire intersection position with laser beam centerline. Angles greater than 60 degrees makes the latter difficult and angles less than 30 degrees causes the wire to intersect a large area of the laser beam, causing melting and vaporization of the wire without incorporating the metal into the weld pool.
Focused spot size: The spot size should be close to the filler wire diameter. A laser spot size that is too small compared to the wire diameter can lead to welds with porosity because the filler wire has not melted properly.
Optimization and control of these parameters will lead to a consistent welding process and high quality weld.
If you are a LASERDYNE system user and would like advice in developing a laser welding process, please feel free to contact LASERDYNE Sales at email@example.com.