To join a pair of sheet metal, various methods are available such as welding (e.g. resistant spot welding), adhesive bonding, mechanical joining, etc. Mechanical joining includes bolting, riveting, and form joining. Welding has been most widely used because it is fast and easy to automate. In addition, it provides high joining strength. It has, however, difficulty in joining a pair of dissimilar or coated sheet metal. Sheet metal bonding simply with adhesives (e.g. epoxy) shows high bonding strength under the shear loading condition, but the strength drastically decreases under peeling and cleavage. On the other hand, the form-joining process (or clinching) uses a set of die and punch to impose the plastic deformation-induced geometric constraint on a sheet metal pair. Thus, it is able to join unweldable sheet metals with practiced ease. Several form-joining processes (e.g. TOX, Eckold) are commercially available, although their joining strength ranges 50-70 percent of that of the resistance spot welding.
In this work, a new form-joining process with the aid of an adhesive is proposed in which an epoxy adhesive is applied to a sheet metal pair, and before it cures the pair is clinched to cause the geometric constraint in the form of a protrusion. The bonding of a sheet metal pair using an epoxy adhesive was experimentally investigated to obtain the relationship between the bonding strength and the diameter of the epoxy spread on the sheet. In cases that an epoxy adhesive was applied to the TOX process and the two-step form-joining process, the peak load of failure on the tensile-shear test surpassed that of the resistance spot welding. In order to reduce the forming load and the height of protrusions, a new die and punch set with a very small clearance was devised to reduce the depth of drawing and the forming load. Taguchi method was employed to find the optimal values of design parameters. To implement each case of the orthogonal array, the finite element method was used. The experiments showed that on the tensile-shear test, the bonding strength of the new form-joining process with an epoxy adhesive is approximately the same as that of the resistance spot welding; and in comparison with the other two form-joining processes with an epoxy adhesive, the height of protrusions was reduced by more than 65 percent; and the forming load by 50 percent.
