For HLAW, developed around the fact that a laser is a high-power-density process that can achieve high aspect ratio welds, using this process to melt material for a weld joint primarily focused on the size of the fillet greatly diminishes the advantages of the HLAW process. So butt joints are more “efficient” for the laser than a true “fillet” weld where the laser is only assisting in the melting of the wire/filler material. Specifications There are limited industry specifications that address HLAW. In the case of the Meyer Werft application, the shipyard worked closely with Det Norske Veritas (DNV) to develop specifications for HLAW as it applied to ships. ASME has developed a specification for hybrid welding, and the American Welding Society is in the process of forming Subcommittee C7D to develop specifications for HLAW. The U.S. Navy has accepted HLAW processing on a very limited basis. In most cases, these specifications detail the parameters that should be considered critical and do not address the performance of the weld joint. In many cases, this is left to the company that is using the process. For those companies that are directly substituting the HLAW for an arc process, especially those with fillets, the specifications would normally include the leg and throat size and would not consider the penetration along an interface. This use of arc-based specifications would often also specify a penetration into the base material. This is important because the welds may have too little heat input and result in incomplete fusion. This is less of an issue for the laser; in fact, the more energy from the laser used to melt the base material, the less efficient the laser/HLAW process. For those reasons, the most efficient joint (heat input vs. the area of the weld) is a square butt joint. For example, the HLA weld shown in Fig. 4, with an interface between the two parts greater than 1.5 times the thinnest member, would not pass many company specifications because the vertical leg is less than the thinnest member and the throat is not sufficient. Prewelding Steps While a square butt or similar joint is the most efficient, it may not be practical for many potential users of HLAW to alter their designs to a butt joint. Many potential users of HLAW presently use 40 JANUARY 2013 Fig. 2 — Diagram of the hybrid laser-gas metal arc welding process. sheared edges or possibly thermally cut edges for their arc process. These cutting processes usually result in larger root openings and mismatch than usually recommended for laser or HLAW. Without investing in improved cutting methods and/or clamping techniques, it may not be practical to use HLAW. In the Meyer-Werft applications, the joints were machined to improve fitup. This ensured minimal root opening and, therefore, minimal filler metal addition. It also meant that the process did not have to be adjusted for changing conditions. In many applications, it is not practical to machine the edges that would be HLA welded. However, with incorrect weld joint design, dealing with a too large root opening can sometimes make the process impractical. As an example, a typical joint that is being considered for HLAW would be an edge fillet in two materials 2.5 mm thick. If the GMAW joint specifications are going to be used, that normally would mean that the weld nugget must have a leg equal to the thickness of the plate (2.5 mm) and a throat thickness 60% of the thickness. If these parts are stamped and sheared, it is not unusual to have root openings equal to 2 mm, so the new leg would be 4.5 mm. Assuming an equal Fig. 3 — Illustration of how a large root opening greatly increases the quantity of additional filler metal required.
Welding Journal | January 2013
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