Abstract: A joint is produced in at least two overlapping workpieces (W1, W2) using a joining tool comprising a punch (16) reciprocally disposed in a cylinder. The workpieces (W1, W2) are disposed between the tool and a die (12) and the tool is used to apply a compressive force to deform the workpieces into a joint at a joining area between the tool and the die. Continuous ultra-sonic energy (A) is applied to at least one of the workpieces in the joining area for at least part of the time during production of the joint so as to increase the ductility of at least one of the workpieces in the joining area. This induces an acousto-plastic effect in the material being formed which temporarily reduces its strength so as to exhibit increased ductility. The joining method is used in self-piercing riveting and enables viable joints to be formed in high strength materials or thick sheets of material.

Abstract: A self-piercing rivet for forming joints in high strength steel has a partially hollow shank of a particular form in order to ensure that it has sufficient column strength to pierce the material. The thickness of the shank is such that adequate joint strength is achieved with a lower degree of flare than would be necessary for conventional riveted joints. The ratio of the thickness of the shank (outside diameter minus inside diameter) is in the range 0.47 to 0.52. The ratio of the cross sectional area of the shank in the region around the bore to the outside diameter of the shank is in the range 3.0:1 to 3.