5Fiber laser welding continues to grow as a preferred process with improvements in weld quality, reliability and performance. Many laser welding applications are autogenous where the
weld is formed entirely by melting parts of the base metal and no additional filler wire or powder is used. Laser welding applications are almost always autogenous for a variety of materials. However, certain challenging materials and difficult applications require the use of filler material in the laser welding process. In doing so, big improvements in the process are possible.
Application improvements include:
- Better joint fit-up tolerance (air gaps, mismatch, etc.) of the parts to be welded.
- Elimination of solidification cracking during the welding. For some aluminum alloys, filler wire is used to replace the low-melting alloys and reduce the freezing point during cooling. For example, for 6XXX series aluminum alloys, high-silicon wire (4043 or 4047) is used to reduce cracking and improve the mechanical properties of these alloys.
- Modifying the chemical composition or the microstructure of the weld metal to obtain suitable mechanical properties.
- Improving the weld profile by eliminating undercut at the top and bottom of the weld bead. Excessive undercut can act as stress raiser, which can reduce the mechanical properties of the weld during the welding process.
Laser welding with filler material can be done with powder or wire (see Figure 1). However, the majority of industrial laser welding applications use wire. This article focuses on fiber laser welding with wire.
It should be noted that one of the reasons wire is preferred over powder is because typically, powder feedstock is more expensive for most materials. For example, a typical cost of 0.9-mm-dia. Inconel 625 wire is $26/lb. compared to $48/lb. for powder of the same material. For that reason, powder is primarily used in additive manufacturing applications and not for welding.
Filler wire parameters
There are parameters that determine the quality of the filler wire weld. As a multiparameter process, laser
welding with filler wire is affected by several conditions that determine quality, process speed and cost.
Welding and filler wire feed rate: The wire feed rate for a given air gap and plate thickness is an important parameter and is dependent on the welding speed, the cross-section area of the gap between the joint face and the cross-section area of the filler wire. The relationship is expressed as:
Wire feed rate (m/min) = welding speed (m/min) * cross-section area of gap (mm2)/cross-section area of wire (mm2)
The use of filler wire generally results in a 10 to 20 percent decrease in welding speed for a given laser power to compensate for the laser energy that has to be used to melt the wire. The lower speed tradeoff is offset by the increased benefits of utilizing filler wire. But it is important to use the correct filler wire feed rate.
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 by being able to melt a larger section on the wire end. This may result in breaking a liquid metal bridge formed during the process and the formation of a drop at the end of the wire and momentary disturbance of the process stability.
A too-high filler wire feed rate can cause the energy supplied to the weld 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. A correct welding speed ensures correct penetration depth, weld width and top bead height.
Laser beam and filler wire interaction: An exposed length of filler 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. At the initial stage, the laser beam melts the wire through, dividing it into two parts. As a result, the spot at which the process started was covered with a wire end that is welded to 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. The control features of the Laserdyne 795 with BeamDirector ensures correct laser beam and filler wire interaction.
Wire feed delivery angle: Angles between 30 and 60 degrees from the vertical can be used, and 45 degrees tends to be the norm as it simplifies setting the required wire intersection position with the laser beam centerline. Angles greater than 60 degrees make the latter difficult and angles less than 30 degrees cause the wire to intersect a large area of the laser beam, causing melting and vaporization of the wire without incorporating it 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 compared to the wire diameter, it can lead to welds with porosity because the wire has not melted properly.
Extensive testing has shown that fiber laser welding using filler wire has been proven effective in producing high-quality, robust welds with improved fit-up, reduced weld cracking and
better weld profiles. The wide range of applications include aerospace, automotive and many industrial fabricating applications.
Prima Power Laserdyne has carried out detailed studies of fiber laser welding with filler wire with its Laserdyne 795 with BeamDirector system. Both weld and filler wire parameters were developed and optimized to produce good quality welds without cracking or porosity with correct weld geometry. Figure 2 highlights welds made with filler wire to eliminate cracks and porosity (2a and 2b) and to improve the weld geometry (2c and 2d). It shows transverse sections of the filler wire welds made with 2-kW to 3-kW average power and nitrogen shield gas.
Control of the laser weld metallurgy and dimensions are possible with the enhanced control of fiber laser welding process parameters that are made to happen using the Laserdyne fiber laser welding system.