Tool yields high repeatability without rework or post-processing
In today's automotive industry and its high-volume production environment, laser welding has become a well-established joining technology. High productivity, repeatability, low heat input, and fast welding speed are some of the main advantages of laser welding compared to conventional joining technologies such as gas metal arc welding (GMAW) or resistance spot welding (RSW).
One of the advantages of RSW is the integrated clamping technology—the part to be welded is held together under pressure exerted by electrodes, and then joined by the heat obtained from the resistance between the electrodes and the magnitude and duration of the current. In recent years, car manufacturers started using more materials like aluminum and high-strength steel. This exposed some of the weaknesses of RSW such as low electrode life and high heat input, resulting in lower strength welds and low overall equipment efficiency (OEE).
The Laser Seam Stepper (LSS) developed by IPG Photonics combines the advantages of RSW and conventional laser welding. The parts are pressed together by one (Poker) or two (C-Gun) pressure pieces with a controlled clamping force up to 3000 N (FIGURE 1). Laser welding then takes place inside the pressure pieces, providing a Class 1 safety enclosure. In this way, no additional safety enclosure or clamping fixture is necessary, saving cost and valuable floor space. Total weld length can be selected from 1 to 40mm, with or without an additional weave. The frequency of the weave can be programmed between 1 and 25Hz. An additional fume exhaust makes sure that all fumes are extracted from the process. If it is desired to achieve welds free of oxidation, shield gas (e.g., argon and nitrogen) can also be added to the process. To move the LSS to each weld location, it can be mounted on a six-axis robot (minimum 30kg handling capacity) or gantry system.
|FIGURE 1. The Laser Seam Stepper (LSS) is available in two versions: the C-Gun version for two-sided access (a) and the Poker version for one-sided access (b).|
After finalizing the first LSS prototype in 2010, a long-term production benchmark test (more than 18 months) was successfully performed in a fully automated car plant. By October 2013, more than 20 seam steppers had manufactured more than 550,000 car bodies in highly automated production lines.
Laser welding offers significant advantages over resistance spot welding, especially in body-in-white (BIW) applications:
- Higher process speeds (shorter cycle time);
- Increased component strength by longer seams with higher torsional stiffness;
- Smaller flange width;
- Single-sided access;
- Repeatable high-quality weld results; and
- Low heat input (low distortion).
The implementation of high-strength materials in the automotive industry and the increasing demand for higher stiffness and rigidity require larger weld interface areas and low heat input during welding. In many cases, this cannot be achieved by conventional resistance spot welding mainly due to the recommended minimum distance between spot welds and the high heat input, which negatively affects the characteristics of the welded material. On Volkswagen's current Golf VII model, LSS welding was implemented in various applications (FIGURE 2). Twenty-six resistance spot welds were replaced by nine laser seam stepper welds joining the B-pillar to the rocker panel. On the roof frame, four laser welds are now applied where 10 RSW used to be required. Besides the more than 50% cycle time reduction, crash-test performance was also significantly improved due to the low heat input and bigger weld interface.
|FIGURE 2. LSS welding of triangle window (a) and roof frame (b).|
To meet federally mandated fuel economy standards, car manufacturers are using more and more aluminum for body panels, engine components, and structural parts to dramatically reduce vehicle weight. Due to the high thermal and electrical conductivity of aluminum compared with steel, RSW requires much higher welding currents and contact pressure, resulting in high contact heat between the electrodes and the part to be welded. Thus, the electrode tips rapidly deteriorate, affecting the quality of the weld if not frequently dressed or replaced. In high-volume production, this can be a crucial problem. With the LSS, excellent weld results can be achieved on aluminum. The quality of the welds is very repeatable and not dependent upon the condition of the tip. FIGURE 3 shows a 3-T lap joint welded with the LSS. The laser power can be precisely programmed to either result in a full- or partial-penetration weld.
|FIGURE 3. 3-T aluminum joint, with each layer measuring 1.5mm (a); high-strength steel weld (b); aluminum weld (c); and stainless steel weld (d).|
Based on the experience of more than five years in production within a fully automated car plant, various new applications with different material combinations were developed with the LSS. Typical materials can be zinc-coated or high-strength steel, as well as stainless steel or aluminum. Overlap welds can be performed in stacks of multiple layers and are not restricted to 2T configurations. The unique design of the upper and lower pressure pieces allow a reduction of the flange width from 15mm (required for RSW) down to 10mm or even 6mm. Assuming a total contour length of 14,200mm on all four door frames on a midsize car, a flange reduction by 6mm will result in a weight reduction of approx. 4% and an approx. increase of the entrance area by 8%.
Large part implementations
In some industries where large metal sheets are welded, the single-sided seam stepper holds a big advantage over conventional welding technologies. Implemented in several rail car, agriculture, and shipyard applications, LSS showed excellent weld results due to the low distortion and elimination of any post-processing on the backside of the part, which in many cases is visible. Boat hulls are currently manufactured using the one-sided access picker version by a European ship manufacturer welding 4mm thick stiffening structures to the outer skin panel. This process used to be performed with metal inert gas (MIG) welding, where a costly clamping fixture and post-processing was required. Due to the implementation of laser welding, this was eliminated and the overall weld quality significantly improved (FIGURE 4).
|FIGURE 4. LSS-welded boat hull (a) and railway carriage panel (b).|
Railway carriages are mostly made of mild steel, stainless steel, or aluminum sheet panels with reinforced profiles on the inside. These reinforcements are commonly welded to the panels using GMAW, resulting in clearly visible and significant distortion. An additional complex straightening post-process is necessary. When this manufacturing process is performed with the LSS, it can be mounted on a robot or gantry system to move the weld head to each weld location. The picker then presses the reinforcement onto the panel and starts the welding process inside the light-tight pressure piece. Besides serving as a hold-down device to minimize the gap, the pressure piece is the safety enclosure for deflected laser radiation. Additional light tight safety cells or post-processing is not required.
Resistance spot welding was compared with laser welding using the LSS. In cooperation with INPRO, an innovative company for advanced production systems in the automotive industry in Berlin and a joint venture of Daimler, Volkswagen, Siemens, ThyssenKrupp, and SABIC, process characteristics, tensile strength, crash-test performance, and fatigue life were compared along with economical aspect. The overall technical results show that the performance of the laser seam is comparable to or better than resistance spot weld, but completes welding tasks in half the production time. The results of economic comparison show a total cost reduction of 6–10%, assuming fully automated production of 800 units in a three-shift operation.
As a result of close cooperation with different manufacturers, the LSS has become a highly reliable laser welding tool with an uptime availability of 99.9%. Due to the high repeatability of the complete system (fiber laser and LSS module), excellent production quality can be guaranteed without the need for any rework or post-processing.