Clinching (Figure 1) is a high-speed, mechanical fastening technique for point joining of formable sheet metal components that deforms one material into the other to form an interlock. The sheets are initially clamped between the blankholder and the die assembly. The punch is then forced onto the sheets, and locally pushes them into the die. As the deformed sheets touch the bottom of the die, further downward movement of the punch forces the material to flow radially and form a button, forming the mechanical interlock which holds the sheets tightly together.
The clinch riveting process (Figure 2) is an adaption of the conventional clinching process. The main innovation is that, instead of retracting the punch at the end of the process cycle, a consumable tip (or rivet) is used. This rivet remains in the joint giving extra reinforcement. Just like in the clinch joints, the punch pushes the rivet into the sheets. The material flow is like a clinch forming the interlock, but in this case the interlock is formed the rivet is part of the locking mechanism re-enforcing it.
Blind riveting (Figure 3) is a non-threaded mechanical fastening technique, particularly used when access to the joint is only available from one side or when the materials to be joined are not sufficiently formable for self-tapping technologies. The rivet is placed in a pre-formed hole and is generally of a tubular form with a head on one side, and a headed mandrel through it. As the mandrel is pulled back, it causes the tail of the rivet to flare against the reverse side of the sheets, providing the mechanical interlock between the original and the newly formed head. As the load on the mandrel increases, it breaks at a notch just behind the head, inside the rivet (closing off the hollow rivet). With some types, the mandrel is pulled through entirely (leaving an opening).
Self-Piercing Rivets (SPR)
Self-piercing riveting (SPR) (Figure 4) is a method of joining two or more pieces of sheet material, using a punching rivet. Unlike conventional (blind) riveting, SPR does not require a pre-drilled hole as the rivet makes its own hole as it is being inserted. Interlocking of the rivet into the material is considered to be the main parameter dictating joint strength, however the level of interlock required to achieve a good joint is highly dependent upon the ductility of the materials being joined.
Solid Punch Riveting
The solid punch riveting process (Figure 5) is an evolution of the established self-pierced riveting process. Instead of punching the upper sheet, and then relying on plastic deformation to form a locking joint into the lower sheet, a solid punch rivet punches out both sheets and fills the hole it has created. This offers a significant advantage if a joint is to be made in materials that have little or no ductility, such as high strength steels or higher strength aluminium grades.
Resistance Spot Welding
Resistance welding (Figure 6) can be defined as a process whereby a force is applied to sheet metal surfaces and in which the heat for welding is produced by the passage of electric current through the material at, and adjacent to, these surfaces. The process takes advantage of the natural electrical resistance of material to produce the heat. Sheet joining is thereby achieved by clamping of the sheet materials between copper alloy electrodes and convey the necessary electrical current through the work pieces. Heat is developed mainly at the interface between the sheets, eventually causing the material being welded to melt, forming a molten pool, the weld nugget. The molten pool is contained by the pressure applied by the electrodes and the surrounding solid metal.
When spot welding steel to aluminium, heat is generated in the higher resistance steel material and conduction into the aluminium melts the aluminium in contact with the steel, effectively forming a brazed joint. Hot dip galvanised steels (pure zinc coating) are the easiest to join to aluminium. Galvannealed steels (iron zinc coating) are more challenging and press hardening steels with an aluminium-silicon coating have not been successfully joined.
High Speed Tacking
High speed tacking (Figure 7) is a method where a hardened steel nail is driven through material plates by a high-speed punch. High speed joining is applied with extremely high forces and punches through multiple sheet combinations of both steel and aluminium. High speed joining has been demonstrated on materials of up to 1600MPa strength without the need for a pre-formed pilot hole, but can only be applied to very stiff assemblies, otherwise the forces exerted by the punch can distort the parts.
Flow Drill Screws (FDS)
The FDS process (Figure 8) uses a friction drilling mechanism to pierce a hole in formable materials (typically aluminium), before thread forming and locking the joint in place. This process is followed by:
- Initially the sharp point of the screw is rotated at high speed on the upper sheet surface;
- Friction heating softens the material and it begins to flow away from the screw tip;
- With heating, the screw penetrates deeper into the material;
- Once the screw has fully penetrated the materials, the rate of rotation is slowed down and the screw thread locks the screw in place;
- As the material cools, thermal contraction tightens the grip of the material onto the threaded part of the screw.
Friction Element Welding
Friction Element Welding (Figure 9) is an innovative mechanical fastening method that uses a friction drilling element which, when rotated at speeds of approximately 5000/6000 rev/min, can pierce soft materials. The same element can also be used to create a friction stud or rotary friction weld, when it comes into contact with a hard material plate. The process was designed specifically to allow high strength joining of aluminium to steel, with several sheets of aluminium being pierced to join to one sheet of steel below.
Resistance Element Welding
The resistance element welding process (Figure 10) is a novel two step method to produce reliable high strength joints between steel and aluminium, taking advantage of the good punching performance of most common automotive grades of aluminium.
Adhesive bonding is defined as a process relying upon a third material, ‘an adherent’, that is able to form a bond between mating surfaces. The bonds formed are by chemical bonding (van der Waals forces or hydrogen bonding) or by mechanical bonding to the surface features of the substrates. Adhesive bonding is probably the most important of all dissimilar joining technologies and it is very unlikely that any structural joint between dissimilar materials will be used in a car body without an adhesive. The adhesive serves the roles of:
- Increasing the joint area to give better load bearing and structural rigidity properties
- Providing a watertight seal, vital for dissimilar joints, where the galvanic coupling between the materials can lead to aggressive corrosion