The main features of rivets used in self-piercing riveting are:
- shank diameter and rivet length
- shape of rivet head and tail design
- rivet material and hardness
- type of coating/plating
Typically, the shank diameter is 3 or 5mm. Rivet length is chosen to be approximately equal to the combined thickness of the sheets to be joined, plus 75% of the rivet shank diameter. In most cases, the head diameter is 2 to 3mm greater than shank diameter.
The rivet head can be countersunk or raised with a pan or brazier head (raised heads having a flat contact with the sheet). The tail can have a square edge or an internal chamfer to assist flaring. The wall thickness, shape and length of the hollow tail of the rivet can be chosen to suit the rivet length, the materials being joined and ease of manufacture.
The shape of the transition (corner radius) from a countersunk head into the shank can influence the setting force required and the joint properties achieved, particularly the fatigue strength.
Hardness is an important property: the rivet must be hard enough to pierce the top sheet but sufficiently ductile to flare properly in the bottom sheet. The required hardness, therefore, depends on the hardness of the materials to be joined and is chosen to provide the correct button formation. If the hardness is too low, the rivet may collapse during the process; if it is too high, it may cause thinning or piercing of the bottom sheet, or cracks in the flared rivet tail. Rivets are available in a number of discrete hardness levels, appropriate to the manufacturer.
Typically, rivets are made from medium carbon or boron treated steel wire using a multi-blow cold forming process. Stainless steel, aluminium or copper may also be used in special circumstances.
Choice of rivet material depends mainly on:
- required hardness of rivet (hardness is also controlled by heat treatment of rivet)
- material costs
Choice of rivet coating
material depends mainly on:
- required resistance to corrosion (resistance to galvanic corrosion is an important issue in automotive applications which involve joining aluminium alloys using steel rivets)
- required appearance (e.g. colour matching of rivet head if no painting/coating after riveting is to take place)