TWI Frequently asked questions
by Steve Westgate
It is difficult to generalise the comparison of processes, as there are a number of factors that determine the best process for a particular application. As fitness-for-purpose is the main consideration, factors such as high jointstrength are not necessarily required in all cases. In addition, there are process variants in each case that modify the general capabilities. Some of the things that need to be considered and the relative comparisons for thin sheetmaterials are listed below.
| Factor | Clinching | Self-piercing riveting | Spot welding |
|---|
| Base material suitability | Most materials with reasonable ductility; harder materials use semi-piercing tooling | Most materials with reasonable ductility | Most sheet steels. Aluminium alloys more difficult |
| Metal coatings, e.g. zinc | Can be joined | Can be joined | More difficult for thick coatings and if passivated |
| Organic coatings/lubricants | Thick coatings can affect joint properties | Thick coatings can affect joint properties | Only thin, weldable coatings possible |
| Dissimilar metals | Possible | Possible | Metallurgical compatibility necessary
Al to steel not possible |
| Joint shear strength | Medium | High | High |
| Joint peel strength | Low | Medium | Medium |
| Fatigue performance | Medium | High | Low |
| Cosmetic appearance | Hole one side, button on other | Flush one side, button normally on other | Slight indent both sides
Coatings damaged |
| Corrosion issues | Low susceptibility | Low susceptibility but special coatings required on steel rivets | Low susceptibility but some damage to protective coatings |
| Basic equipment cost | Low to medium | Medium to high | Medium to high |
| Consumables | None | Rivets | Electrodes |
| Cost per joint | Low | High | Medium |
| Workplace environment | Good | Good | Possible weld splash and fume |
Choice of process, which will also be influenced by component design, will need to be made on a case-by-case basis. All processes can be operated as simple manual operations through to automated robotic or multi-headed, dedicatedmachines. Joint performance data and manufacturing options are available from system suppliers.
