The Weldbonding Process

Two general processing approaches prevail for weldbonding, and minor variations are possible within each approach. The first and most common method is to apply the adhesive and weld through the joint. This process is sometimes referred to as the weld-through method. The second approach, sometimes referred to as the flow-in method, is to form the welded joint first and then allow a low-viscosity adhesive to flow into the spaces between the joint 

FIGURE 14.8 Schematic illustrations of the two fundamental approaches for producing weldbonded joints a) filling adhesive into the spaces between previous made spot welds and (b) spot welding through preapplied adhesive.40 

In these approaches, several different forms of welding can be utilized such as resistance spot welding, gas tungsten spot welding, laser spot welding, and electron beam spot welding. However, resistance spot welding is the most popular and well accepted of these methods. 

Weldbonding process specifications have been developed for use by both manufacturers and government agencies. These process specifications give detailed steps to provide optimum weldbonds. The descriptions below provide a generic summary of these processes. 

Weld-Through Method. Since this is a hybrid process, techniques common to both welding and adhesive bonding are modified to fit the combined process. The essential steps of the weld-through method of weldbonding are 

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Many different adhesives have been tested and used in weldbonding. These include both high-strength structural adhesives, such as modified epoxies, and relatively low-strength adhesives, such as vinyl plastisols or epoxy-polysulfides that are commonly used for sealing and vibration damping. Table 14.10 shows that a variety of adhesives and substrates are compatible with the weldbonding process. 

High-quality welds can be made in high-strength aluminum alloys up to 72 hours after lay-up of the parts with adhesive at the parts interface. A substantial production cost savings is realized when compared to other methods of joining because the weldbond process is highly adaptable to mechanization and automation. 

Kizer, J.A., Development of weld-thru weldbond process surface preparation. Proceedings of the 5th National SAMPE Technical Conference, Vol. 5, Kiamesha Lake, New York, pp. 124-130, October 9-11,1973. 

Kizer, J. A. and Grosko, J. J. (1972) Development of the weldbond joining process for aircraft structures. Processing for Adhesive Bonded Structures, Bodnar, M. J., Ed., Wiley, New York, p. 353. 

Important Processing Issues. As with conventional adhesive bonding, there are several important issues that cannot be overlooked with weldbonding. Two of the most important issues are joint design and surface preparation. 

The surface preparation method must be carefully considered, especially if the completed weldbond is to have long-term durability to hostile environments. The surface preparation should provide an optimal surface for both adhesion and welding. Thus, the choice of surface treatment is crucial, and there can be a conflict of requirements. The spot welding process requires a low electrical surface resistance, and many adhesive surface preparation processes provide a high surface resistance because of oxide layer buildup. When it is impossible to harmonize on a surface treatment, current practice tends to favor treatments that yield good weld nuggets at the expense of the adhesive bond. 

Good process control is required with weldbonding to ensure correct joint filling of the adhesive and to avoid weld quality problems. The process needs to be carefully controlled so that health and safety requirements are met. Welding through the adhesive may create hazardous fumes, and little information is available as to the organic compounds that are produced. Suitable ventilation and fume extraction equipment should be provided. 

The degree of acceptance of weldbonding applications has been increasing, as the process has been understood and its mechanical properties developed.47 The principal advantages that have been claimed for weldbonding include the following. 

Film adhesives have been found by some workers to be impractical because spot-welds could not be made consistently, due to the carrier.On the other hand, film adhesives have been found to provide substantial benefits because they improve workmanship in the production facility. The first requirement for any adhesive for the weld-through process of weldbonding is that the adhesives have the capability of being moved under pressure of the welding electrodes in order for metal-to-metal contact to occur at the joint interface. The second requirement is that the heat resulting from the spot weld cause only limited detrimental effect on the strength of the bond. ... 

The extent of process and quality control used in weldbonding (Chapter 8) must be based on the end use of the hardware being bonded. Methods in current use should be selected to fit a specific application. Consistent joint strength can be assured by evaluating cured weldbond tensile-shear specimens, cleaned with each batch of parts, for strength and bond quality. Consistent weld quality can be assured by hourly evaluation of uncured tensile shear and macro specimens for strength and weld quality. Even higher assurance can be obtained from the use of an in-process weld monitor that... 

Weldbonding A process developed in the former USSR and introduced in the USA by the Air Force Materials Laboratory. Developed for the aerospace industry, the process combines spot welding with adhesive bonding of aluminum structures. It has provided an economical and efficient means of laying up and oven curing large, epoxy-bonded assembhes. 

Figure 8.5 shows schematic (not to scale) details of spot-weld and the surrounding adhesive-bonded region of a single-lap weldbond joint. In the most common process, a paste adhesive is applied to the metal sheet material and the metal is then spot-welded through the uncured adhesive. 

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