Weldbonding Metals

Lockheed workers Kizer and Grosko have studied weldbonding using four different surface preparation techniques on aluminum. A technique developed for aluminum by the Northrop Grumman Corporation for the Air Force follows. It was found to be superior to the previously recommended FPL etch plus 60-min dichromate seal. 

Vapor degrease in trichloroethane vapor for 60 s, followed by spray rinse of condensed trichloroethane fluid for an additional 60 s. All parts must be free of water prior to vapor degreasing 

Alkaline clean in Cleaner 85 solution. (Cleaner 85 is available from Turco Products Division of Henkel Surface Technologies, Madison Heights, MI.) 

Immediately spray-rinse in cold deionized water for at least 5 min and inspect for water-break-free condition. Should water breaks occur, repeat the above steps 

Immerse the alkaline-cleaned parts in a deoxidizer solution consisting of 8—16% by volume of nitric acid and 20.1—24.6 g/L Amchem 7 for 7—10 min at room temperature. When chemical addition is required to maintain the strength of the solution, use Amchem 17 instead of Amchem 7. (These products are available from Amchem Products, Inc., Ambler, PA, www.amchemproducts.com) 

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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... 

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. 

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... 

The most suitable adhesive for general weldbond applications is modified-epoxy one-part paste type containing metal filler. 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 show improvements over general-purpose adhesives. ... 

Fillers for epoxy adhesives used in weldbonding include silica (Cab-O-Sil, 7% by wt) to prevent run-off, 3% strontium chromate to provide corrosion inhibition, and conductive metal powders. Viscosity of adhesives in paste form has an important effect on the weld and bonded joint. Viscosity must be low enough to allow the force of the electrodes to push the adhesive out of the interface contact area, yet sufficiently high or thixotropic so that it will not flow out of the joint during the cure cycle. 

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. ... 

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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