legged fillet, the joint is now 4.5 × 4.5 mm. That means to address the legs alone, the volume of material deposited must increase by 3.24 times the “no-root opening” condition. In turn, if the welding speed is to remain constant, then the weld wire feed speed must increase by that factor (with an increase in current) or the welding speed must decrease. Assuming the user is GMA welding at 17 mm/s (40 in./min), then the wire feed speed for a no-root opening situation would be 34 mm/s (118 in./min) for 1.2-mm- (0.045-in.-) diameter wire and 112 mm/s (472 in./min) with a 2-mm root opening. These speeds are well within typical wire feed and corresponding welding parameters for typical equipment. However, for a typical 6-kW-laser HLAW process, the welding speed would be expected to be 50 mm/s (120 in./min). This means to achieve the same fillet size, the wire-feed speed for zero root opening would be 161 mm/s (354 in./min) and 523 mm/s (1418 in./min). As an alternative to these parameters, it may be possible to decrease the travel speed. Unfortunately, if the speed is decreased much below 42 mm/s (100 in./min), the travel speed will be very close to speeds achievable with tandem GMAW, which is less expensive to acquire and operate. Other Considerations In considering the use of HLAW vs. GMAW, the total number of welds per part and the length of each of these welds must be considered. As an example, for many automotive parts, the GMA welding that is accomplished is done as small stich welds that may be only 50 to 75 mm (2 to 3 in.) in length. While these welds may only be made at a quarter to onethird of the speed of HLAW, the “air time” of the robot constitutes most of the cycle time. Therefore, moving from GMAW to HLAW will not substantially decrease the cycle time. For parts with a large number of short welds, the cycle time and, therefore, the number of welding cells required to meet a production rate, is not going to be changed dramatically by the higher welding speed of the HLA process. It may actually be less expensive and faster to add another GMA welding robot than to convert to HLAW. As with any fusion process, the material that may be added to the weld pool and the amount of heat determines the properties of the entire weldment, or weld metal and heat-affected zone (HAZ). For a typical GMA weld, the filler material constitutes the majority of the weld metal and therefore the mechanical properties. And because the wire has been developed around certain wire-feed rates and heat input, the properties are fairly well understood. As for the HLAW, depending on the weld joint, a number of conditions may exist that differ from typical GMAW. As the HLAW approaches an “autogenous” weld (no filler), the microstructure of the weld metal will approach that of the cast (and rapidly cooled) base material. For many moderate to highly alloyed steels, for example, this may mean a weld with very high hardness and very little ductility. The result of this would be that the weld may be more prone to cracking. Therefore, for welding of these alloys with HLAW, a welding wire with less alloying may be required. Added to any issues with the weld metal will be the concern over the HAZ of the weld joint. In GMAW or HLAW, the chemistry of the HAZ material cannot be changed. The only option with the HAZ is to alter the thermal cycle that the material goes through during the welding process. What happens in the HAZ is highly influenced by the alloy that is being welded and the “condition” the material is in when welded. As an example, if the material being welded is a quench and tempered steel that has been thermally treated to have a high hardness, the HAZ of a GMA weld will depend on the chemistry, parameters, and procedures used. The material can be overtempered in the HAZ and result in a “softened” zone. If the same alloy in the same condition is HLA welded, the HAZ may be reaustenized and requenched, resulting in a HAZ with very high hardness. Actually, due to the higher heat input from the HLAW vs. the autogenous laser welding, the HLAW may actually have a lower HAZ hardness. Another scenario would be in materials that are precipitation hardened. Like the quench and temper materials, the GMAW may over “heat treat” these materials and result in a softer HAZ, especially if multipass GMAW is needed to make the joints. However, for HLAW, the thermal cycle may be so short that there may not be a noticeable HAZ. So, for some materials, the use of HLAW may actually offer benefits over welding with GMAW. While these examples and considerations show where HLAW may not always be the process of choice, there are applications where HLAW will be preferred. A general statement is that the HLAW process should not be substituted into most typical joints designed for GMAW. In some cases, it may be better to redesign the joint specifically for the HLAW process or to improve the joint to take advantage of the low heat input and high welding speed associated with HLAW. Also, the greatest advantages for HLAW are realized in very long welds and situations where distortion of the part is very critical. Conclusions In summary, HLA welding is not a magic bullet that can be used on any and all applications presently being GMA welded. In most cases, substituting HLAW directly into a GMAW application is usually unsuccessful. However, when the application is selected properly, possibly requiring a part/joint redesign for HLAW, changes to the procedures, and even the materials, it may be possible to successfully implement HLAW into production.♦ References 1. Bruck, G. J., et al. 1998. Method of Welding. U.S. Patent No. 4,737,612. 2. Diebold, T. P., and Albright, C. E. 1984. Laser-GTA welding of aluminum Alloy 5052. Welding Journal 63(6): 18−24. 3. Personal communication with Dr. Frank Roland, former managing director, Center of Maritime Technologies, Hamburg, Germany, and former manager for Advanced Welding Technologies, Meyer Werft Shipbuilding. WELDING JOURNAL 41 Fig. 4 — Image of HLAW edge fillet weld. Leg length L is much less than material thickness T. However, the penetration P is much greater than T, so the weld strength is greater than if L was equal to T.
Welding Journal | January 2013
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