TWI Industrial Member Report Summary 695/2000
C A Olivier
Resistance spot welding has been the basic technique used for over 50 years for the assembly of steel based automotive bodies and sub-frames. However, there is widespread interest in the application of mechanical fastening processes (such as clinching and self-piercing riveting) and adhesive bonding, both as individual and combined (hybrid) techniques. These joining processes are expected to find greater use for applications where the use of resistance welding is limited.
Mechanical fastening can be used where manufacturers need:
- To use pre-painted material thereby eliminating post-painting or other finishing processes after joining
- To eliminate fumes and weld splash
- To reduce/eliminate thermal distortion in thin sheets products
Self-piercing riveting normally involves driving a semi-tubular rivet into the materials to be joined by a hydraulic operated punch and die set. The rivet is consequently flared and a 'button' is formed below the lower sheet. Hence, a pre-drilled hole is not required. The rivet material and shape, and the die form are designed to suit the materials being joined.
Clinching is a similar operation except that the rivet is replaced by a punch which forms the two sheets into a die. A button is also formed below the lower sheet, providing an interlock between the sheets due to local plastic deformation.
It is possible to benefit from the advantages of adhesive bonding (uniformity of load transfer, elimination of stress concentration, no thermal effects on the substrates, suitability for a wide range of metallic and non-metallic substrates, joint sealing to improve corrosion resistance) and compensate for its disadvantages (low resistance to peel stresses, limited resistance to heat, low energy absorption failure). This is done by combining adhesive bonding with point joining techniques and is referred to as hybrid joining. While weld-bonding is an established technique in the automotive industry, the combination of self-piercing riveting or clinching with adhesives is more recent.
Work has previously been carried out at TWI as part of its Core Research Programme to examine the static and dynamic performance in shear of:
- Mechanically fastened joints and spot welds in 1.2mm thick galvanneal coated (iron-zinc) low carbon steel and in 1.2mm thick 5754-O aluminium alloy.
- Mechanically fastened and hybrid 0.85mm thick galvanneal coated (iron-zinc) low carbon steel to 1.2mm thick 5754-O aluminium alloy joints.
The study concluded that mechanical fasteners performed better in fatigue compared to spot welds, particularly in the case of aluminium to aluminium joints.
In the case of steel to aluminium joints, the presence of adhesive at the joint resulted in a significant improvement in the performance of the hybrid joints in static or fatigue loading compared to that of the mechanically fastened joints.
The work presented in this report has extended the scope of the study to examine joining a range of steel types and thicknesses and cover resistance welding, clinching, self-piercing riveting, clinch-bonding, riv-bonding and weld-bonding. Static shear and peel testing of the samples were both carried out.
- To determine the suitability of point joining and hybrid joining techniques for joining a wide range of steel types and thicknesses.
- To evaluate the static performance of all point and hybrid joints in monotonic tensile shear and T-peel configurations.
- To compare the static performance of all point joining and hybrid joining techniques.