Spot weld bonding

In the future, pre-coated metal sheets could probably be used, but then the spot weld bonding would be no more possible. In addition, one has to keep in mind that the adhesive strength on such coatings is lower than that of high strength structural adhesives. Even the adhesion quality of a hot dip galvanized steel coating has to be tested since with some structural adhesive the zinc layer debonds from the metal surface. 

Real-life experience from cars shows that spot weld bonded areas provide a much better corrosion resistance than spot welded areas alone because the adhesive fills the joint preventing the intrusion of moisture. 

The diagram in O Fig- 46.18 illustrates this case. Spot weld bonded H-samples (see O Fig. 46.15) start the fatigue test at a stress level of around 60 kN and end after six million cycles at around 30 kN, which is the starting level of the Metal Active Gas (MAG)-welded sample. 

What is also obvious is how much improvement can be seen between the spot welded 2.0 mm aluminum and the 2.0 mm spot weld bonded aluminum. The distance between the spot welds is 50 mm which is longer than the usual 30 mm. With shorter distances between the spot welds the results for the spot welded samples would be better but will not reach the level of bonded samples. 

Stiffness increase of spot weld bonded cars measured before and after curing of the adhesive... 

This is not only observed with these test specimens, but it was also proven in the comparison of real spot weld bonded cars, as shown in O Fig. 46.20. 

In the diagram in Fig. 46.21, the strength of spot welded and spot weld bonded lap shear samples with 45 mm width and 16 mm overlap plus one spot weld is shown. It shows that strength level is only determined by adhesive bonding. After the failure of the bond the spot weld can still bear additional load as was shown in the research report from Hahn and his group (Hahn and Wipiing 2007). 

Failure diagram of bonded and spot weld bonded lap shear sample... 

And for the stifiEness, only the G-modulus is important and with the rather thin adhesive layers of 0.3-0.5 mm in a spot weld bonded flange, there wOl be no significant difference between an adhesive with 100 and 1,000 MPa G-modulus in the overall torsion stiffness of the car body. 

In the absence of an electric field, the dome-closed conformation must be the most stable tip structure, even when spot-welds are considered, since only the perfectly dome-closed tip has no dangling bonds (i.e., it is a true hemifullerene). At the 3000°C temperature of the arc, the rate of tip annealing should be so fast that it is sure to find its most stable structure (i.e., to close as a dome). Clear evidence of this facile closure is the fact that virtually all nanotubes found in the arc deposit are dome-closed. (Even stronger evidence is the observation of only dome-closed nanotubes made at 1200°C by the oven laser vaporization method.) Such considerations constituted the original motivation for the electric field hypothesis. 

Bonding enables stresses to be distributed over large areas in the joint, thus avoiding the local stress concentrations present in riveted or spot-welded joints which can reduce fatigue 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. 

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