Joint Design and Fit-up Guidelines for Laser Welding
Laser welding requires precise control of the gap in weld joints. This is primarily a consequence of one of laser welding’s greatest strengths – the ability to concentrate high power into a relatively small area.
In fact, when considering laser welding, the design of the joint and, in some cases, the fabrication method of the finished product may need to be reviewed in order to gain the full advantage of the process.
Using filler material, typically in the form of wire, can widen the weld joint tolerance band. However, use of filler material is not common, at least for the majority of applications involving 3D parts.
Why?
First, a filler metal process requires additional hardware – a wire feeder as well as wire filler, both that add cost. Secondly, a wire feed welding process is typically slower which leads to a longer overall cycle time.
Welding without filler material
This article is focused on an ‘autogenous’ process, that is, one in which there is no filler material added to the joint. If you are interested in welding with filler metal, read the article titled “Laser Welding with Filler Material” from a previous issue of this newsletter.
Generally, the gap must be no more than 10% of the thickness of the thinnest component. This can be relaxed for materials thicker than 1 mm. On the other hand, the gap may need to be 5% or less for materials less than 0.2 mm thick.
It is also much more difficult to start a laser weld in an area with a large gap area compared to maintaining a seam weld through a short section of high gap area where bridging can sometimes be maintained.
In the autogenous process, whether spot or seam, the weld gaps must be filled with metal from the adjacent area. Bridging any gap requires extra laser energy. As the gap increases, the welds become increasingly concave until the gap fails to bridge and the joint is incomplete.
Design guidelines for several joint types
Table 1 highlights factors that affect the joint configurations for laser welding assuming a 0.4 to 0.6 mm laser beam diameter. Figures 1-3 show three different weld joint designs and crossections of typical welds.
Table 1: Factors affecting joint configurations for laser welding
(ST* = Sheet thickness; applies to sheet materials up to 6 mm thick)
| Factors | Joint Configuration | |||||
|---|---|---|---|---|---|---|
| Lap | Butt | Hem | Edge | Multi-layer | T-Butt | |
 |
 |
 |
 |
 |
 |
|
| Tolerance to gap between sheets | <10% ST | <10% ST | <10% ST | <10% ST | <10% ST for each layer | <10% ST |
| Tolerance to beam joint misalignment** | >1 mm | <0.3-0.5 mm | > 1 mm | <0.3-0. 5mm | >1 mm | <0.3-0.5 mm |
| Tolerance to beam focus position | ± 1 mm | ± 1 mm | ± 1 mm | ± 1 mm | ± 1 mm | ± 1 mm |
| Seam tracking requirement | No | Yes | No | Yes | No | Yes |
| Tolerance to edge preparation | Avoid burrs | <5% ST | Avoid burrs | <5% ST | Avoid burrs | <5% ST |
| Tolerance to coatings (e.g. Zinc) | Low | Medium | Low | Medium | Low | Medium |
*ST = Sheet thickness
** The smaller the focused spot size, the smaller the allowable misalignment. For reference, for a 400 µm spot size, misalignment of the laser beam to the joint should be less than 0.3 mm. Misalignment up to 0.5 mm is acceptable for a 600 µm spot size.
Figure 1: The rule of thumb for joint fit-up for a butt joint is that the maximum gap should be less than 10% of the lowest sheet thickness. The maximum tolerance for misalignment of the laser beam to the weld joint for a 0.6 mm focused laser beam is 0.5 mm. Weld strength is proportional to weld depth.
Figure 2: The maximum gap for a lap joint should be less than 10% of the thinnest sheet thickness. The maximum tolerance to beam joint misalignment can be large (1 mm) depending upon the application. Weld strength is directly proportional to weld width at the interface. Whenever possible, the thinnest sheet should be on the top.
Figure 3: The rule of thumb for T- and fillet joints is that the joint gap should be less than 10% of the thinnest sheet thickness. Misalignment of the beam to the joint must be less than 0.3 to up to 0.5 mm depending upon the focused beam diameter. Custom fixtures are often required to achieve maximum weld strength.
Don’t forget joint cleaning
In addition to the suitable preparation and alignment of the joint faces, edges to be welded must be free from contaminants such as grease, paint, dirt and oxide scales. Careful removal of any residue from chemical degreasing and cleaning is required to avoid weld spatter and porosity caused by these substances.
Questions? Comments?
Send your questions or comments on this article to lds.sales@primapower.com.
If this article was of interest…
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