Bonding, adhesive weldbonding

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

Mitchell, R.A., R.M. Woolley, and S.M. Baker, Component Parts Assembly with Joints Adhesive-Mechanical, Part 4. Analyses and Test of Bonded and Weldbonded Lap Joints, Picatinny Arsenal Technical Report (PATR) 4965, AD A029426, lYepared by NBS, March 197. 

The most suitable adhesive for general weldbond applications is a 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 weld-bonding. Adhesives specifically developed for weldbonding should show improvements over general-purpose adhesives [32]. 

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. 

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. 

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. 

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... 

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... 

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. ... 

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. 

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. 

Harris and Adams (1985) have examined the impact strength of lap-shear joints bonded with ductile adhesives. They also made open-ended cylinders of steel and aluminum alloy joined by spot welding and weldbonding with one of the ductile adhesives used for the lap-shear tests, and tested them under axial impact loading. The results indicated that the weldbonded joints had a small failure area and higher energy absorption than those joined only by spot welding. 

Weldbonding A process in which a joint is formed by spot welding through an uncured adhesive bond line, or by flowing an adhesive into... 

The extent of the inspection of the production item must be determined from or based on, the end use of the part, and will also be affected by the size and complexity of the part. If the parts are small, visual inspections for surface defects and surface adhesive irregularities may be adequate. If the parts are large and complex and the end use is critical, radiographic inspections may be used for determination of weld quality and ultrasonic inspection for determination of bond quality [39]. X-ray radiography will reveal weld nugget defects such as cracking, expulsion, and porosity [40], Infrared nondestructive methods cannot be used for weldbonded structural assemblies [41],... 

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