Spot-weld adhesive bonding

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... 

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... 

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. 

The heat created by the spot welding process should not degrade the adhesive so as to weaken the final bond strength or create thermal decomposition products, which can contaminate and weaken the weld area. 

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. 

There is improved energy absorption compared to either spot welding or adhesives bonding alone. 

Cost savings can be achieved compared to adhesive bonding alone. The spot weld clamps the joint so that expensive fixturing is not required and the joints can be cured in commercial ovens rather than autoclaves. 

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, Spot Welding Teams Up with Adhesives for Stronger Metal-to-Metal Bonds, Product Engineering, May 1975. 

High levels of noxious odors and concerns over worker safety have resulted in acrylic adhesives being removed from consideration in automotive hem bonding plans (9). They were rejected despite a five-year development program that showed that replacing spot welding with a rapidly fixturing adhesive would substantially lower hood and lid assembly costs. However, an aerobic acrylic similar to the B formulation shown in Table 5 has been approved for trial production because of its low odor and nonflammable properties. 

Acrylic adhesives can be used to replace spot welding where immediate handling of the joined metal parts is required. Another broad area of application is for bonding dissimilar substrates, including metals and other materials with different coefficients of expansion. 

Figure 8.1 is a schematic (not to scale) detail of a spot-weld and the surrounding adhesive bonded region of a single-lap weldbonded joint. In the... 

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. ... 

It has been shown that in comparison with spot welds, rivets and a variety of selfpiercing mechanical fastenings, toughened structural adhesives may yield single lap shear joints of superior energy absorption when used to bond aluminium alloy sheet the findings were confirmed in tests on full-size bonded vehicles (see Toughened adhesives, Structural adhesives). 

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. 

J. D. Minford, Comparative effect of surface contamination on the strengths and performance of aluminum spot-welded or adhesive-bonded joints, ASM/ADDRG Conf. Tech. Impact of Surfaces Relationship to Forming, Welding, and Painting, Dearborn, Michigan (April 1981). 

The principal advantage of adhesives is in the ability to reduce stress concentration in joints by transferring load via the whole of the bonded area, as opposed to the discrete load points introduced by bolts or spot welds. In addition, the low density of adhesives imparted by their polymeric nature makes their use in joining technology attractive to the designers of weight-efficient structures and their uptake has therefore been widespread within the aerospace industry. 

Typical Use Automotive (hem-flange). Bonding of body panels (together with spot-welding). Good adhesion to oily steel (2 - 3 g.m-. ... 

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