High Power Laser Welding With Filler Material

Laser welding with the filler material can either be carried out with powder or wire (Figure 1) however, the majority of the laser welding applications in industry use wire, and hence this article will only discuss laser welding with wire. Normally the powder feedstock is more expensive than the wire feedstock across the board i.e. typical cost of 0.9 mm diameter Inconel 625 wire is $26/lb. compare to $48/lb. for powder for the same material. Therefore the powder is mainly used in additive manufacturing applications.

Laser beam welding is almost always autogenous for a wide variety of materials, however some of the materials and applications do require addition of filler material, i.e.

• 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 alloys and reduce the freezing point during cooling, i.e. for 6XXX series aluminium alloys high silicon wire i.e. 4043 or 4047 to reduce cracking and improve mechanical properties of these alloys
• Modify the chemical composition or the microstructure of the weld metal to obtain suitable mechanical properties
• Improve the weld profile i.e. no 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 multiparameter process and there are a number of laser and filler wire parameters which determine the quality of the resultant weld. Some of the important parameters are listed below.

Welding/filler wire speed: The wire feed rate for a given air gap and plate thickness is an important parameter and is dependent 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:

The 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 that has to be used to melt the wire.

If the filler wire 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.

A high filler wire 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 topbead 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 affected 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 started was covered with a wire end welded onto the surface and 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 vertical can be used and 45 degree tends to be norm, 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 into the weld pool.

Focused spot size: The spot size should be close to the filler wire diameter. If the laser spot size is too small compare to the wire diameter than this can lead to welds with porosity because the filler wire has not melted properly.

Prima Power Laserdyne has carried out a detailed study of laser welding with filler wire. Both weld and filler wire parameters were developed and optimized to produce good quality welds i.e. no cracking or porosity with correct weld geometry. Figure 2 highlights welds made with the filler material to eliminate cracks and porosity (2a & 2b) and to improve the weld geometry (2c & 2d).

4.0 mm thick 6065 aluminium alloy (Al-Mg-Si-Cu) welded with 0.8 mm diameter 6065 wire; double pass Tee joint. This alloy well suited for many applications in the aerospace industry, however is prone to solidification cracking without the filler material.

Fiber lasers welding with filler material can deliver a high quality weld consistently and is a good possible alternative to the manual welding.

If you are interested in the learning more about using a fiber laser with filler material for your welding application, please contact lds.sales@primapower.com