Competing or complementary joining methods?
May 8, 2007
Which joining processes—laser welding, brazing, or adhesive bonding—are most suitable for different automotive product attributes and regions of an automobile? This article answers some of those questions.
Selecting the best joining method for state-of-the-art body-in-white (BIW) engineering and manufacturing is a delicate task. Considering that, in principle, a car body engineer has only three parameters to combine for an optimal solution—the geometric shape, material, and the joining method—it is essential to choose the appropriate joining technique. This explains why new assembly technologies have received a lot of attention lately and are gaining ground steadily at the expense of more traditional welding methods.
Joining methods currently used in the body-in-white assembly process are numerous and varied.
Considering the palette of available joining methods (see Figure 1), it is easy to understand how problematic it can be for a design engineer to select the most suitable joining method for each application. Therefore, it may be useful to review what a BIW joint's purpose is.
Simply put, joining's main objective is to attach separate components to a functional structure. Furthermore, the joint must balance the properties of the materials it connects so that the assembly becomes as strong as possible, thereby reducing vehicle weight and optimizing fuel efficiency.
Making recommendations about which joining method to use for different car body components and assemblies is a challenging task. Throughout the years, attempts have been made to create general rules. For example, for the 1999-2000 vehicle releases, Volvo Cars was using traditional spot welding as the reference standard from which to evaluate the positives or negatives of alternative joining methods on various body properties and attributes.
Two joining methods that are being used with increasing frequency are laser beam welding and structural adhesive bonding. They both offer long, continuous bonding lines and therefore can be considered competing methods.
Understanding the pros and cons of these two technologies makes it possible to decide which body-in-white applications are more suitable for laser welding or adhesive bonding, resulting in design guidelines and recommendations for the engineer.
Assembling car body parts using either laser welding or structural adhesive bonding provides some advantages. Because they generate a continuous bond line, they create a larger area for load transfer between parts. This larger load transfer results in better crash performance, increased durability, and improved torsion and bending stiffness. That, in turn, makes it possible to down-gauge material thickness and reduce weight, while maintaining good car body performance.
In the case of traditional unitized car body structures, using both laser welding and adhesive bonding can contribute to the weight savings realized with the use of high-strength steel (HSS).
Today most car bodies made by automakers around the world have at least a couple of laser welded components (see Automakers Using Laser Welding).
Single-sided Access. One of the main benefits of laser welding is that it requires only single-sided access to the joining location, as opposed to classic resistance spot welding (RSW), which operates with upper and lower electrodes. This opens up possibilities for completely new design solutions; however, to really utilize these new design solutions for laser welding also requires an open-minded design approach.2 For example, laser welding a typical RSW flange does not make sense, because that type of layout is optimized for spot weld gun access and does not benefit from the possibility of narrow laser welds. Single-sided welding must take into consideration the necessity to reach fixture arrangements, such as pressure devices and fixed clampings, to create the smallest weld gap between sheet components in an overlap configuration.
Adhesive trap designs (left to right—flat, right-angle, inclined) prevent the risk of adhesive washoff during the cleaning steps preceding the paint and curing processes.
Invisible Joints. By positioning the laser weld on the sheet edge, or by using laser brazing technology in combination with some type of beam trap configuration (see Figure 2), it is possible to create nearly invisible joints. This gives the impression of craftsmanship in areas such as door, trunk lid, and tailgate apertures.
Material Reduction. Another advantage of laser welding is that it requires smaller flanges than spot welding requires for apertures such as side door openings. A reduction from 16 mm to 6 mm—a 10-mm reduction—is feasible with the correct flange design. Not only do such narrow weld flanges reduce weight and improve visibility, they also allow for increased volume for pillar, cantrail, and sill sections in confined spaces, which increase stiffness and strength. The latter, of course, is also a result of the continuous weld line, which, in this respect, is superior to intermittent RSW spacing. This, in turn, enables sheet components to be down-gauged while maintaining comparable performance to heavier spot welds.
From a manufacturing perspective, the high process speed is favorable because it improves productivity, reduces cycle time, and can be used to reduce costly floor space in the body shop. But the high process speed also results in less heat input and lower part distortion, which, in turn, help improve accuracy and quality of the final product.
High Investment Cost. Although it may sound as though laser welding could be the ultimate tool for BIW assembly, some drawbacks exist. One obvious drawback is the high investment cost of installing fully equipped laser welding cells. Therefore, it is important that the body design is such that the enhanced properties and performance can justify the higher investment costs.
Tight Tolerances. It is also important to bear in mind that laser welding puts more stringent requirements on parts and positioning tolerances to ensure a successful result. Overlooking this in early automotive laser history resulted in many failures.
Specific Repair Methods. Finally, because laser welding is a relatively new joining method, repair techniques specifically designed for laser welding must be used.
Adhesive bonding seems to be the topic of the day among European automotive manufacturers. In reality, they are addressing the subject of welding/bonding, in which the adhesive application is followed by spot welding to position and fixture the parts together accurately before the heat-curing adhesives harden to full strength during the paint process.
Today the most commonly used adhesives are epoxy-based, but as environmental concerns become more pronounced, intensive R&D is ongoing to try to develop more environmentally friendly, rubber-based alternatives. The presence of about 132 to 231 feet (40 to 70 meters) of epoxy adhesive in a modern car body structure is considered more or less "state-of-the-art." Current examples include the Audi A4® and A6® models, the BMW 5- and 7-Reihe®, and the Mercedes E-, S- and CLK-Klasse®.
Preventing Cracks. Adhesive bonding is a well-known complement to help resolve problems with cracks around spot welds occurring as a result of fatigue loads. Distributing the stress forces over a larger area reduces the stress concentration on the spot welds. Therefore, it is possible to reduce sheet component thickness without risking fatigue performance, and to reduce the number of spot welds. This, in turn, will require fewer welding robots in the body shop, resulting in a substantial cost reduction—both in terms of investment in new automation equipment and utilities such as electricity and cooling water.
NVH. Replacing otherwise necessary sealant materials with adhesive automatically achieves a double functionality. Doing so not only seals out water, it also improves noise, vibration, and harshness (NVH). Here, the adhesive not only dampens airborne noise distribution, it also can help minimize body vibrations for critical frequencies because of its stiffness.
Performance. Tailor-made adhesives are being developed that could be attractive from an engineering standpoint, for example, to improve fatigue and crash performance.
Environmental Impact. The drawbacks of adhesive usage today are mainly linked to the working environment, because the hazardous long-term effects of adhesives, which initially raised a number of questions, seem to have been validated. One concern about using epoxy adhesives is that they can cause skin allergies. Environmentally conscious automakers have fully robotized the application of structural adhesives. Through an exact extrusion, excessive amounts of adhesives that could contaminate equipment and personnel can be avoided. Postheating operations, such as repair welding, in areas where adhesives are present should be avoided.
Section height within a restricted space can be gained by using minimum flange length and laser fillet welding.
Washoff. Another disadvantage is the risk of so-called washoff of the adhesives during the various cleaning steps preceding the paint and curing processes. As the adhesive has not cured at this state, it is important to trap it in place to maintain the intended product improvements. This can be done either by smart adhesive trap designs (see Figure 3) or, for especially critical applications, by placing the adhesive-joined parts in pregel ovens in the body shop.
Taking into consideration the advantages and disadvantages of laser welding and adhesive bonding that have been outlined, some obvious recommendations can be given as to when to use each method. However, in some situations the choice is not so clear-cut, especially with complex loading and when counteracting product properties exist, such as the cross section of structural members.
So far there has not been evidence of improvements from a continuous laser weld for dynamic crash loads. Where loads are parallel to the weld seam—which is the case in the axial compression of front side members at frontal (head-on) or offset (40 percent overlapping of the impacting car's front) accidents, for example—this is easy to understand. The forces are concentrated at the very small weld width at the start or end of the laser weld, so the risk that the narrow weld may just unzip is obvious.[image8]
However, not even using spot welds to decrease stress concentration at the ends of the laser weld has improved crash performance.5 The only remaining alternative, then, is to configure the laser weld pattern by using a remote welding technique,6 so that the crash loads can be absorbed in a more optimized and controlled manner. However, further research has to be conducted before these assumptions can be relied on.
What is a little more difficult to explain is the laser weld's poor behavior compared with adhesively bonded flanges in four-point dynamic bending (see Figure 4). The "single-hat" beams used in this study had a 17-mm, flat flange, which is the standard for a spot-welded flange. They also were wide enough to be joined securely with an adhesive application 10 mm wide.
Looking at the laser-welded components only showed that continuous laser welds behave better than intermittent stitch welds, and a fully penetrated overlap weld performs better than a fillet weld. What is perplexing, however, is that the spot-welded component (with 40-mm spacing) is almost as strong as the continuous laser-welded version. Equally perplexing is that the adhesively bonded beams perform better in this loading situation, even when the bond width is only 50 percent of the value that has been stipulated.
It can be concluded that for structural beams or members with equal cross sections, adhesive bonding achieves a larger load distribution area, so it will always be superior to laser welding in terms of durability and crashworthiness.[image9]
However, as indicated earlier in this article, the geometric shape is another way to improve the body performance. A larger cross section of a structural member will always provide better stiffness, fatigue, and energy absorption than a continuous joint. This has been successfully achieved by laser welding the assembly of the floor sill of the Volvo C70® convertible (see Figure 5). With laser fillet welding, the upper and lower weld flanges have been minimized, so it is possible to increase the height of the sill section without interfering with surrounding exterior design areas. This has contributed to the superior global torsion stiffness—12.7 kilonewton meters (kNm)/deg.—of this vehicle.
To summarize the recommendations for using either laser welding or adhesive bonding, the following rules of thumb can be given:
The rules can be simplified even more:
The latter, of course, depends on the assembly sequences in the body shop—and perhaps to some extent is application-specific.
On one hand, as adhesive bonding always requires a certain flange width to secure a strong connection, it should be used for underbody applications where the flange width reduction is of minor concern. Too, limiting the use of adhesives to the underbody flanges prevents operators and equipment from getting contaminated from uncured adhesives being squeezed out, as they could from upper body flanges.
On the other hand, the ability to laser weld narrow flanges offers the strongest benefits in the upper structure, where visibility and access ergonomics are of prime interest. Also, concentrating the laser welding operations on the upper structure limits the number of laser stations needed, which can help contain laser investment costs.