Fiber laser welding continues to grow as a preferred process with improvements in weld quality, reliability and performance. Many fiber 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 beam welding applications are almost always autogenous for a wide variety of materials. However, certain challenging materials and difficult applications require the use of filler material in the welding process. In doing so, big improvements in the weld 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, wire is used to replace the low melting alloys and reduce the freezing point during cooling. Example: 6xxx series aluminum alloys, high silicon wire, i.e. 4043 or 4047 wire 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, i.e., to eliminate 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.
Choosing Between Powder And Wire Filler Material
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 is its lower cost. Usually, powder feedstock is more expensive than wire feedstock for most materials. For example, a typical cost of 0.9 mm diameter 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.
Parameters That Determine The Quality Of The Filler Wire Weld
As a multiparameter process, laser welding with filler wire is affected by several conditions which determine quality, process speed and cost.
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 section area of the gap between the joint face and cross sectional area of the filler wire. The relationship is expressed as follows:
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. Note that the lower speed tradeoff is offset by the increased benefits of utilizing filler wire. But it is important to use the correct filler wire rate. 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 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 rate can cause 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. A correct weld speed will ensure correct 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 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. 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, 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 makes the latter difficultand 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, that can lead to welds with porosity because the filler wire has not melted properly.
Positive Results With the LASERDYNE 795 With BeamDirector System
Prima Power Laserdyne has carried out detailed studies of 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 & 2b) and to improve the weld geometry (2c & 2d).
Extensive testing, as described above, 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 profile. The wide range of applications include aerospace, automotive and many industrial fabricating applications. 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 LASERDYNE’S 795 with BeamDirector fiber laser welding systems.
For more information about LASERDYNE products and services of Prima Power Laserdyne, call 763-433-3700, email: LDS.SALES@primapower.com, or visit www.primapowerlaserdyne.com. Prima Power Laserdyne is headquartered at 8600 109th Avenue North, #400, Champlin, Minnesota 55316.