Weldbonding

Weldbonding is also called spot-weld adhesive bonding. It is a method of fabricating hardware that uses both welding and adhesive bonding techniques. A layer of adhesive, either in paste or in film form, is applied to one of the metal members to be joined. The other metal member is placed on top, forming a lap-type joint, and the assembly is clamped or resistance 

---

Winans, A.M. Shibley J.R. Hall, Weldbonding in the United States An Annotated Bibliography and History , PLASTEC Note N26 (1974) 45) J.B. Titus, Trade Designations... 

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... 

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... 

Flow-in Method. In the flow-in method of weldbonding, spot welding precedes the adhesive application. A normal substrate separation will result from the weld nugget buildup in the joint (Fig. 14.8a). A low-viscosity adhesive is then used to infiltrate the clearance between the substrate members, forming a sealed bond line. The adhesive may be drawn into... 

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. 

Most adhesives used for weldbonding are arbitrarily selected from adhesives that were developed for other purposes, resulting in a compromise when used for weldbonding. Adhesives specifically developed for weldbonding should have the following characteristics in addition to those mentioned above ... 

An adhesive layer with appropriate thickness and elastic modulus is necessary to obtain reasonable distribution of stresses in the region of a weldbond joint. A thin adhesive layer of high elastic modulus improves the fatigue properties of weldbonded joints.43... 

Film adhesives can be cut out to provide for the absence of adhesive at the weld locations. However, often these adhesives do not flow sufficiently at the cure temperature without pressure (as would be the case in weldbonded joints) to wet the substrates. 

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. 

A commonly used adhesive in weldbonding applications is a modified epoxy, one- or two-component paste containing conductive metal filler. Other fillers commonly used in... 

Joint alloy Adhesive Curing condition Spot weld, lb Adhesive bond, lb Weldbond, lb... 

To assess the advantages of the weldbonded joint, one must look at the properties of the spot weld alone, the adhesive bond alone, and compare these to the properties of the weldbonded joint. One must also be aware of the physical and environmental effects on the joint. Studies show that weldbonded joints can be stronger than joints that are only spot welded or only adhesively bonded. However, metal thickness, surface preparation, adhesive flow and cohesion, and weld quality can influence the results. 

Weldbonded aluminum joints (1-mm aluminum alloy 2036-T4) where compared with spot welded aluminum (1-mm aluminum alloy 2036-T4) and spot welded steel (1-mm steel 1010) joints.45,46 Weldbonded 2036-T4 joints made with a vinyl plastisol adhesive and with a one-part modified epoxy had a fatigue strength about twice that of the spot welded aluminum alloy and approximately the same as that of the spot welded steel. The fatigue strength of the weldbonded aluminum joints with polysufide-epoxy adhesive or with high-peel-strength epoxy adhesive is higher than that of steel spot welds alone. 

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. 

Weldbonding automatically achieves joint sealing, which in mechanically fastened joints requires an additional process. 

Adherend stresses in weldbonded joints are lower and more uniform than those for comparable spot welded joints. This provides increased in-plane tensile shear and/or compressive buckling load-carrying ability for a given joint design. The presence of the spot weld provides enhanced out-of-plane load-carrying capability compared to adhesive bonding only. 

Anonymous, Weldbonding Joins Auto Body Sheet, Welding Design Fabrication, May 1979,... 

Lohman, R. J., Weldbonding Leads Growth of Structural Adhesives in Auto Market, Adhesives Sealants Industry, May 1997, pp. 30-34. 

---