What advances have there been in the manufacture of rivets which are used for joining for aircraft and rivet technology and materials used?
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Rivets need to be harder than the material being joined by them, and the growing use of high-strength alloys has made the use of rivets of the same material as the material being joined problematic. To set a rivet, it needs to have some plasticity, which in turn is greatly reduced when an alloy is optimized for high strength.
Initially, rivets were made from aluminium alloys, but have been replaced by fasteners, especially Hi-Loks (Hi-Lock is also used) which are (mostly) titanium or steel bolts with an aluminium or titanium nut. The nut has two parts; the lower part, the collar, has the internal thread and remains on the Hi-Lok after assembly while the upper part forms a hexagonal head for tightening with a special wrench. Between both is a groove which allows the head to shear off at the right torque. This ensures consistent torque and eliminates the need for torque inspection. The wrench needs a hex bolt at its center which serves as a socket wrench and engages with the shank of the fastener to keep it from rotating with the nut.
By using titanium nuts rather than steel or stainless steel nuts, we can save up to 110kg on a single aisle aircraft and 220 kg on a wide body aircraft.
Hi-Lok fastener (picture source)
Another advance are special bolts for joining composite parts. The head of a regular fastener is too small and runs the risk of being pulled through the composite sheet. Special bolts, washers and nuts for composites spread the load out.
Composite panel fastener (picture by Alcoa Fastening systems, source)