Despite the well-documented success of laser welding in the automotive, medical device, electronics, and many other industries, laser welding has remained a niche application in the aero-engine industry. This has been largely due to the difficulty and cost of introducing new manufacturing processes for existing designs of components into the highly regulated aerospace industry.
However, with the launch of new engine designs, designers and manufacturers have taken opportunity to explore and implement new processes, including laser welding, where appropriate.
Examples of successful application of laser welding include production of compressor stator cascades and joining cover plates to the cast cores of high-pressure and low-pressure turbine blades.
Laser weld requirements
Laser welding – the process and its requirements – are in many ways quite different from the more familiar laser cutting and drilling.
The goal of laser cutting and drilling is to remove material as cleanly as possible. These processes typically benefit from a small spot size and high intensity to produce holes and cuts with minimum metallurgical effects (recast layer thickness, heat affected zone).
Contrast this with laser welding which ideally involves only melting without removing material. Consequently, the laser welding process benefits from lower intensity and, most often, a larger spot size.
To produce mechanically sound joints, the weld seam should have the following general features:
- No top and under (bottom) bead undercut
- Top bead seam of a specified width
- Width of the weld at the interface (lap weld) or waist (butt weld) of a specified dimension.
- Bottom bead seam of a specified width
Lap welds produced with the same optical setup as for the laser drilling and cutting will have a narrow width at the interface of the two components. The width of the interface reflects the diameter of the focused spot, which is influenced by lens focal length and laser beam quality (M2).
A narrow interface in a lap weld will generally mean lower strength (due to the lower crossectional area) compared to welds with larger crossectional area produced with lower energy density arc or resistance welding processes. For this reason, it is common to specify that the width of the interface for a lap weld must be greater than the thickness of the thinnest section of material being welded.
For butt joints, the focused spot size and accompanying fusion zone produced with an optical configuration for laser drilling and cutting may be too small for the typical gaps in the joints of sheet metal parts. As indicated in Table 1 of Joint Design and Fit-up Guidelines for Laser Welding, a weld gap for butt joints should be less than 10% of the thinnest section of the weld joint.
Wobbling helps meet the requirements
Wobbling effectively increases the beam diameter during laser welding to increase the width of the weld while maintaining the high efficiency of deep keyhole welding. The increase in effective beam diameter occurs by superimposing movement of the laser beam in a linear or circular pattern onto the normal motion required to follow the weld joint.
Tests recently completed at Prima Power Laserdyne have documented the effect and potential benefits of wobbling (1) on weld dimensions and (2) for welding dissimilar metals.
Results of wobbling with circular motion
Pictured below are results using wobbling to control weld dimensions. For these tests, wobbling involved circular motion (orbiting) of the laser beam relative to the motion path at a frequency of 6-10 Hz.
Results related to dissimilar metal welding will be reported in a separate article.
Tables 1 and 2 show the effects of wobbling on key dimensions of LAP and BUTT welds respectively.
Figure 1 through 3 show some examples of LAPP and BUTT welds produced using no wobble (Figure 1) and with different amounts of wobbling (Figures 2 and 3).
Table 1: Effect of wobbling on welding geometry for LAP WELD of 2 mm thick Inconel 625. Other parameters include: average laser power – 1000 W; welding speed – 1000 mm/min; spot size – 0.167 mm; shield gas – Nitrogen.
|Wobble||Top Bead Width (mm)||Interface Width (mm)1||Bottom Bead Width (mm)||Gap (mm)|
|0.3 mm orbit radius||2.28||1.06||1.47||0.2|
|0.6 mm orbit radius||3.01||2.13||2.29||0.2|
1. Width of the weld at the interface of the two thicknesses of metal.
Table 2: Effect of wobbling on welding geometry for BUTT WELD of 2 mm thick Inconel 625. Other parameters: average laser power – 1000 W; welding speed – 1000 mm/min; spot size – 0.167 mm; shield gas – Nitrogen.
|Wobble||Top Bead Width (mm)||Bottom Bead Width (mm)||Waist (mm)1||Gap (mm)|
|0.3 mm orbit radius||2.13||1.41||0.70||0.2|
|0.6 mm orbit radius||2.32||2.25||1.56||0.2|
1. Waist is the width at the center (mid-thickness) of the weld.
Wobbling is effective in controlling dimensions of both lap and butt welds in Inconel 625. We expect similar results in other nickel-based superalloys.
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