Laser welded solutions meet targets for cost, weight, and safety performance for 21st century vehicles
Jim Degen, Steve Jansen, Paul McKune
Ever increasing demands on automotive body design engineers to reduce weight and cost and at the same time improve safety and vehicle performance have resulted in an increasing number of laser welding applications for vehicle body construction. This article focuses on recent advances made in the development of laser welded blanks and developments in laser welded structural tube applications. Both these applications provided a means to successfully meet new targets for cost, weight, and safety performance on recently introduced vehicle products.
With respect to laser welded blanks, an expected six blanks per vehicle this year to an anticipated eight blanks per vehicle in 2008 highlights the growing trend for use of this technology. New developments in the use of nonlinear welding and the production of blanks with exposed class “A” properties are helping to propel this growth.
Exposed laser welded blanks provide the design engineer with a means to reduce weight, part complexity, and improve safety. A recent application that has been in volume production over the last 24 months involves the use of an exposed laser welded blank for a truck door opening panel (DOP). The benefits from this technology have been enormous and will be highlighted below. The challenges of processing an exposed material involve keeping the class A surface intact-free from scratches and dents. This challenge was met by re-engineering all the process equipment to minimize sliding and part-to-metal contact. Robotic material handling was maximized and in-process surface inspection stations added to ensure all parts delivered are free of defects. To date the process has yielded a level below 10 ppm to the stamping plant for surface defects.
By providing the ability to utilize an exposed grade of steel, body design engineers were able to design a part that had a steel usage savings over the conventional design of 36.46 kilograms per vehicle by reducing the steel usage per side from five to one part.
This technology provided the design engineer with a method to reduce part weight and steel usage-a major factor in reducing part cost. The nesting benefits of using multiple components to make up the blank drove the steel usage to a minimum. Numerous blank optimization efforts occurred during draw die development further reducing steel usage. A scrap portion of the exposed crew cab blank is reclaimed and used for other parts in the vehicle to further reduce the steel requirements for this program.
The vehicle body design engineers were also able to improve the NVH (noise, vibration, harshness) quality of the vehicle by eliminating reinforcement brackets and by placing the higher gauge steel only where it was required for strength benefits. The thin gauge exposed material is an electrogalvanized EESK, 1.0mm-thick Mittal provided material welded to a 2.0 dent-resistant 210 HD Mittal unexposed material. Each material is processed on an exposed blanking line, which cleans the material and provides the prescribed level of blank pre-lube following the washing process. The material requirements and blank die design occurred concurrently during the initial system engineering.
A cross-functional team consisting of members from the steel company, blanking supplier, the laser blank welder, forming die supplier, and the end user developed the parameters for this process. Numerous forming simulations were done to optimize the blank configuration and these results were then verified and further optimized during the forming die tryout phase of the program.
A key focus during the die tryout involved effectively managing the weld seam location during the initial draw die forming. This location and the resulting movement was managed by changing the final blank die definition until the required movement parameters were achieved. The seam locations are visible in the finished product and position is critical to maintain the aesthetic level of the vehicle. A picture of the formed panel with a summary of the benefits is shown in Figure 1.
FIGURE 1. Formed laser welded blank with summary of challenges and opportunities.
The recent development of curvilinear welding has provided forming and product design engineers with another means to optimize forming performance and strategically locate the gauge and alloy steel where it most benefits the part. The technology has the added complexity of using a multi-axis beam delivery and motion system versus the traditional single-axis or fixed beam welding systems typically used in linear laser welding.
The benefits of curvilinear welding are tremendous. Even the detrimental features of nonlinear welding, where multiple weld seams come together at a specific point producing stress risers, can be eliminated with the use of a curvilinear radius. This modified weld line results in a continuous weld that can be optimized for forming. A forming simulation of a proposed vehicle lift gate is shown in Figure 2, highlighting the associated benefits of the curvilinear process.
FIGURE 2. Curvilinear application for a lift gate inner.
The curvilinear process has been most prevalent in laser welded rear door inner panels. The curved radius of the thick gauge blank eliminates the stress riser during welding and provides the body designer with a solution that enables the inner door frame to be made of one steel thickness-thus reducing wind noise and other door seal issues. As body designers continue to look for ways to improve the door seal quality and reduce overall part weight, we see the trend moving to localized welding of multi-radius thick gauge pieces that meet the homogeneous thickness door seal requirement yet provide the strength in the door hinge area. Figure 3 illustrates the growing trend towards the use of the curvilinear process in the door inner part family and points to potential directions for the future.
FIGURE 3. Trends in tailored blanks include the curvilinear rear door inner.
The development of the laser welded structural tube provides design engineers with another means to improve vehicle weight, cost, and safety. Tubes can be welded from laser welded blanks providing additional capability in the product design by utilizing different alloys and gauge thicknesses.
By using a patented press forming process, tubes can be formed that completely eliminate or optimize the level of hydroforming required to produce a formed tube. Multi-diameter tubes can be formed for net shape applications or can be used in conjunction with hydroforming to increase diameter-to-thickness ratios enabling the design engineer to widen the application range for structural tubular products. Further design flexibility can be provided by using a laser welded blank to take advantage of local thickness and alloy combinations.
FIGURE 4. Press-formed multi-diameter laser welded tube.
Figure 4 highlights the potential use of a press-formed laser welded tube constructed from a laser welded blank and used for an instrument panel beam. This application is accomplished purely with the press form process and eliminates the need for hydroforming.
FIGURE 5. Tubular laser welded crush tips
Figure 5 illustrates a structural tube safety application for press formed front and rear frame member crush tips. These parts are currently hydroformed or produced as a clam shell assembly using a MIG welding process for joining. By using laser welding for joining and the patented press forming operation, a crush tip can be produced that provides weight, safety, and cost benefits. By using different alloy and gauge thickness of steels for the initial blank, the crash event can be further optimized to achieve required properties for crush at the various crash speeds and mass parameters for a series of vehicles. Thus a common shape tube utilizing different alloys and gauge thicknesses can be used across multiple vehicle lines.
In summary with recent technology developments in laser welding, body design engineers are now provided with additional tools to meet the ever increasing demands on the product. By using an exposed laser welded blank or a curvilinear laser welded blank the design engineer can further reduce part complexity, save weight, improve safety, and reduce cost. As these technologies continue to grow, more and more vehicles will take advantage of these processes. The advent of the press formed laser welded tube also provides the design engineer with a means to further improve vehicle safety and take advantage of structural benefits of the laser welded tube technology. These applications can also be integrated with advances in the laser welded blank technology-particularly the use of new advanced high strength steels to further improve the performance of vehicles designed for the 21st century.
Jim Degen is the vice president of engineering, Steven Jansen is the engineering project manager for Advanced Tubular Products, and Paul McKune is an application engineer, all with Noble Metal Processing, Detroit, MI. For more information, visit www.nobelintl.com. This article was adapted with permission from a paper they presented at the Advanced Laser Applications Conference, ALAC 2005 (www.alac-iluc.org).