Are the body shops ready as zero gap laser welding of zinc-coated steel sheets evolves to maturity?
Mariana G. Forrest and Feng Lu
The vision of using lasers to build car bodies emerged in the late 1970s as part of early concept development work by the R & D group at Ford Motor Co.1 Since then, the advantages of laser welding in automotive Body-In-White (BIW) assembly processing have been better understood, and most automotive OEMs have studied, developed, and implemented successful production applications.2-9
FIGURE 1. Power consumption as a function of penetration depth for each beam.
Among the advantages and opportunities offered by BIW laser welding are: significant product improvements such as increases in vehicle stiffness, design flexibility, dimensional control, and weight reductions; and improvements in productivity and cost reduction.
Compared to the currently used Resistance Spot Welding (RSW) process, the economic feasibility of laser welding has been proven by the continuously growing number of BIW laser welding applications in Europe and Japan, and more recently in the U.S.10 However, weld quality limitations and tooling and process costs still prevent the widespread introduction of laser welding in automotive body shops. Once these challenges are eliminated, it is anticipated that lasers will revolutionize the way automotive bodies will be designed and built.
The most common materials currently used in body construction are zinc-coated steels, and the most common joint type is the lap joint configuration. The problem of laser welding zinc-coated steels in lap joint configurations without gap control at the interface is related to the formation of pores and spatter caused by the zinc-gas expulsion. The expulsions are due to the significant difference between the zinc boiling point (~906° C) and the steel melting temperatures (~1500º C), which in turn causes zinc gas to be trapped in the molten weld pool, if no mechanism is provided for this gas to escape.
From the beginning the major challenge has been to produce high-quality welds on coated steels, without the added cost of special tooling or pre-processing, in order to ensure economic feasibility.
For this reason lasers have been slow in gaining acceptance in the automotive body shop. Furthermore, challenges such as high capital investment, availability of a trained workforce, and general resistance to change contributed to the implementation delays.11
Because coated steels are used widely in automotive body constructions for their corrosion resistance properties, any technical solution should cope with the presence of zinc coating and be compatible with the various types of coatings specified by automotive designers, within the tolerances provided by the steel supply industry.
FIGURE 2. The formation of concavity in the rear keyhole wall due to larger inter-beam distance.
The main approaches used in current body laser welding applications include: interfacial gap control, coating type and thickness control, or surface pre-processing techniques. In general, each of these approaches has disadvantages.
The most used approach of gap control requires complex tooling/fixtures, which lead to added cost and design limitations driven by accessibility requirements. Further, gap control methods are sensitive to gap variability, which often results in process instability leading to weld quality problems. Despite these difficulties, a large majority of current European production applications use controlled gaps, and a well developed tooling supply base was established based on this approach.
The second approach relies on limiting the applications to uncoated sheets, single-side coated sheets, lighter coatings, or special coatings. These generally lead to design and material selection limitations, and can also affect corrosion resistance properties.
Finally, pre-processing of the sheets’ weld interface surfaces has been demonstrated as a viable and robust technical solution, but unfortunately is less desirable as it adds cost and processing time.
With full awareness of the technical challenges related to weldability of coated steels and the importance of finding a robust solution to this problem, DaimlerChrysler’s Chrysler Group devoted years of internal effort to find a production viable solution by developing an advanced welding process capable for use in BIW and stamping assembly production applications.
FIGURE 3. New dual beam configuration proposed with backward inclined trailing beam.
A review of early work by others clearly suggested that conventional, single beam welding had severe limitations that required one of the costly mitigation solutions discussed above. It was found that alternative solutions discussed in previous research publications, such as laser knurling surface pre-processing, elliptical beam, copper foil insert at interface, and dual beam, also had disadvantages and limitations.
Zero gap welding
Because use of surface pre-processing and add-on materials was not preferred for volume production, the choice was made to focus on exploring best keyhole geometry for enabling zinc gas evacuation. Chrysler Group participated in the study of Dual Beam Zero Gap Laser welding-an EWI Group Sponsored Project completed in 2001, where a parallel beam configuration with equal power assignments and diameters to each of the two beams was investigated. While promising, the results reported were neither fully understood nor robust for production applications. To this end, Chrysler started to internally develop a proprietary modeling methodology, based on fundamental understanding of the process physics. This enabled the understanding of the laser beam to material and coating interaction, the prediction of keyhole profile under different process parameter combinations, and evaluation of zinc gas escape mechanisms. Modeling results were validated through experimentation, and the insights gained from modeling were used to design experimental process parameters to achieve larger welding windows.
Technical milestone highlights
The novel methodology discussed above, first used for an in-depth analysis of the EWI - GSP study results, revealed that, for the case of the parallel dual beam configuration with equal power and beam diameter between the two beams, the leading beam must have a higher power than the trailing beam. The calculated power consumption as a function of penetration depth is shown in Figure 1. It can be noticed that the leading beam is unable to fully penetrate the joint stack-up, while the trailing beam power is not fully utilized. Obviously, this equal power assignment scenario is not an efficient way in terms of energy utilization. Furthermore, the model results suggest a step-like structure in the front keyhole wall, which is not favorable for smooth fluid flow, leading to unstable welding process, as validated by experimental results. After assigning the leading beam more power, very good weld quality was obtained within a larger welding speed window on a variety of hot dipped galvanized (HDG) steels up to a total stack-up thickness of 1.2 mm/1.2 mm. However, after extensive experimental work, three difficult problems still remained to be solved: high sensitivity to the inter-beam distance parameter with poor robustness (small window); difficulty to weld thicker stack-ups even with higher power levels; and finally severe difficulty to weld galvanneal (GA) coated steels.
FIGURE 4. The second developmental dual beam laser welding head with trailing beam shape adjustability.
These difficulties were also revealed in part by the predicted keyhole profiles, as shown in Figure 2, where the inter-beam distance was limited to small values, in order to avoid the generation by the trailing beam of a concavity at the rear keyhole wall within the weld pool. Obviously, this is not a desired geometry for stable fluid flow, leading to spatter and poor weld quality, as confirmed by experimental results.
In order to resolve the keyhole concavity problem and further improve the process, an inclined trailing beam configuration was proposed12,13 as shown in Figure 3. The idea behind this proposal was that by aligning the trailing beam with the rear keyhole wall profile generated by the leading beam, a stable keyhole structure could be achieved by eliminating the concavity problem at larger inter-beam distances.
This idea was first explored through use of a twin head system, by combining two laser welding heads, each delivering a laser beam from its own, separate laser source. The leading beam (perpendicular to sheet surface) was from a 4.5KW Rofin Sinar Nd:YAG laser, delivered with two different size fibers, 400 and 600 µm. The trailing beam was obtained from a 3.0KW TRUMPF Nd:YAG laser, which could only be delivered with a 600 µm fiber. Due to the precision fixturing limitations related to the size of the focusing heads, the minimum angle achievable between the two beams was 21.5˚. All trials were conducted at this fixed angle, and led to very encouraging results. This set-up offered the needed robustness to inter-beam distance and, as a result, thicker joint stack-ups (1.4 mm/1.4 mm HDG) were successfully welded up to 3.5 m/min.
FIGURE 5. Cross-section of 0.76mm/0.76mm/0.76mm HDG DQSK steels derived by second developmental head.
Following the success gained from the twin head system described above, a new, integrated laser welding head for use in process development was built, which was able to deliver the same beam configuration from a single fiber input power. This developmental head provided continuous adjustability in a wide range for most of the parameters: inter-beam distance, inter-beam angle, leading to trailing beam power ratio, and etc.14
Although successful in further improving the process robustness to inter-beam distance variations, this first developmental head still could not resolve welding GA steels, and in addition was unable to weld three-layer joints (3T overlapped HDG steel sheets). To solve these problems, efforts were devoted to further enhance the keyhole shape so that a desired aerodynamic system could be achieved to favor zinc gas evacuation through the keyhole. This was achieved by further optimization of the optical systems. A second developmental laser head prototype with this improved capability was designed and built (see Figure 4). This head inherits all the adjustability features from the first laser head and is also equipped with a pressure wheel system to ensure focus position robustness and allow for production applications development.
Experiments conducted with the second head showed significant weld quality improvements over the first head. Three layers of HDG sheets could be welded with good quality, with zero intended gaps, provided that sufficient power was available. Figure 5 shows a weld cross-section for 0.76mm/0.76mm/0.76mm 3T stack-up.15
For the case of GA steel, significant improvement was also achieved with this second head. Figure 6 shows a cross-section from a 1mm/1mm GA steel stack-up. Due to the wide use of GA coated steels, at this moment, significant effort is being devoted to further enhance the GA coated steel weld quality.
An additional benefit in improved structural performance was revealed when comparing welds with controlled gap at interface versus those with zero intended gaps. Specifically, preliminary mechanical testing on welds derived from 1.4 mm/1.4 mm HDG DP980 steels with and without an intended gap at the interface suggested that, although the tensile strengths were similar regardless of the existence of any gap, fatigue performance was really deteriorated by the presence of the interfacial gap. For this reason, this new process may prove advantageous to product durability improvements.
The work to date fully demonstrates the basic capability of this novel laser welding technology to produce quality welds on various materials, coatings and joint stack-ups. Steels such as DQSK, IF, DP600, DP800 and DP980, with thicknesses ranging from 0.76 mm to 2.0 mm in 2T and 3T lap joint configuration with intended zero gap at the interface were successfully welded with this new technique, at speeds ranging from 1.3 m/min to 6.1 m/min, depending on stack-up thickness and available power. In addition to coated steels, this new process also shows promise in high-speed welding of lightweight materials such as magnesium castings, which are currently becoming of high interest for future lightweight vehicles.
FIGURE 6. Cross-section of 1mm/1mm HDG DQSK steels derived by second developmental head.
Ongoing efforts are focused to further optimize the welding process for galvanneal coated steels, as well as the robustness and cost efficiency of production tooling, with plans to transfer to production applications as soon as feasible.
When considering the major benefits of BIW laser welding (increased structural rigidity, design and processing flexibility, and increased productivity), it is expected that the improved weld quality and performance offered by this new welding technique will contribute to the rapid increase in the number of successful laser welding applications in automotive BIW and stamping sub-assembly operations.
Drs. Mariana Forrest (email@example.com) and Feng Lu are with the Advanced Joining Technology Group of Materials Engineering Department, DaimlerChrysler Corporation, Auburn Hills, MI.
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- Forrest, M. “Laser Welding vs. Other Joining Technologies in the U.S. Automotive Body Shops-The Resistance is Not Only in the Process,” AutoMan Global Conference 2004, June 8-9, 2004, Dearborn, MI, USA.
- Forrest, M. G. and Lu, F., “Advanced Dual Beam Laser Welding of Zinc-Coated Steel Sheets in Lap Joint Configuration with Zero Gap at the Interface,” ICALEO’04, October 4-7, 2004, San Francisco, California, USA.
- Forrest, M. G. and Lu, F., “Development of A Novel Dual Beam Configuration Leading to Significant Improvements in Process Robustness for Laser Lap Joining of Zinc Coated Steel Sheets with Zero Gap at the Interface,” ALAC 2004, September 20-22, 2004, Ann Arbor, Michigan, USA.
- Forrest, M.G. and Lu, F., “Advanced Dual Beam Laser Head for Robust Lap Welding of Zn-Coated Steel Sheets - Design and Preliminary Experimental Results,” ALAW 2005, April 13-14, 2005, Plymouth, Michigan, USA.
- Forrest, M. G. and Lu, F. “Development Of Advanced Dual-Beam Laser Welding Head For Lap Joining Of Zinc-Coated Steel Sheets Without Intended Gap At The Interface,” SMWC 2006, May 10-12, 2006, Novi, MI, USA.