Clinching and self-piercing riveting for sheet joining

Fastening technology is forming an increasing part of the activities in sheet joining at TWI. This article covers some of the results of work conducted at TWI to-date involving these processes.

Static performance of clinched and self-piercing riveted joints

This study covered the evaluation of the performance of a number of commercial self-piercing riveting and clinching systems on a range of material combinations, and comparison was made with spot welding where applicable.

Experimental details

Samples were produced using five different materials in fifteen combinations. Two low carbon steels were used, 0.8 and 3.0mm thick, and one hot dip zinc (HDZ) coated low carbon steel, 0.8mm thick with a nominal zinc thickness of 20 µm on both sides. The two other materials were 1.6mm thick 5182 aluminium alloy and a 1.3mm thick proprietary aluminium/polypropylene/aluminium (Al/PP/Al) sandwich material. The skin material was a modified 5182 alloy. Three samples were produced with a single joint for each of the fifteen different material combinations in both the tensile and peel joint test configurations, as shown in Fig.1.

Hydraulic press tools were used by the equipment suppliers for the clinching and self-piercing riveting of the samples provided, although details of the equipment were not supplied. The tool designs and rivet types varied between both the material combinations joined and the suppliers, based on their respective range of designs. The rivets were all carbon steel.

Where appropriate, comparative resistance spot welded samples were produced. Standard equipment was used and welding conditions, based on recommended values were set up to produce welds of 5 √t mm diameter (where t = sheet thickness) when peel or chisel testing welded samples.

Each sample was visually examined with regard to joint quality and the dimensions of the joints were measured. A sample of each joint type and material combination was sectioned and photographed. Mechanical tests were carried out with peel and shear loading and the load to failure and failure mode were recorded.

Results

A summary of the results of the shear and peel mechanical tests on selected material combinations is presented in the Table and the main features of the material groups outlined below.

Table: Summary of mechanical test results on press joints, self-piercing rivets and spot welds in selected sheet material combinations

Steel joints

The mechanical test results and joint appearance varied appreciably with joint type for the 0.8mm thick steels. While spot welds generally had the highest shear loads for these materials, one of the riveting systems produced the highest peel load to failure. The most limiting property was the generally low peel loads to failure of the clinched joints. Typical examples are shown in Fig.2.

When joining 3mm uncoated steel, the riveted joints were typically half and the clinch joints about a quarter of the strength of spot welds in both peel and shear. However, the mechanically fastened joints performed better in the dissimilar thickness combinations (0.8-3.0mm steel) as the clinched joints were similar in shear and the riveted joints stronger in peel compared to the spot welds.

Aluminium alloy and dissimilar material joints

In materials which are more difficult, or combinations impossible to spot weld, riveted joints gave particularly good strength results. Both peel and shear loads generally exceeded those of spot welds as coated steel rivets were used throughout. The clinched joints gave strengths typically half those of spot welds.

The dissimilar material joints were broadly similar in strength to joints in the 0.8mm steels, and in some cases the aluminium alloy, but direct comparison was difficult. In many cases a common rivet or clinching tool design was not used for each material combination. Examples of a clinched and riveted joint are shown in Fig.3.

Joints involving Al/PP/Al sandwich material

In joining the aluminium sandwich material to itself, both the clinched joints and self-piercing rivets split the aluminium skins of the sandwich. Joint strength was low, particularly in peel.

When joining the sandwich material to the other materials, the riveted joints gave the best results with up to 2.5kN shear load. The joints were of good appearance, with the sandwich material retained underneath the rivet head, see Fig.4.

Discussion

The results obtained for the clinched joints depended to a degree on the style of tooling chosen. Shear strength was frequently higher for a larger diameter punch and the tooling designs which gave greater sheet interlocking, or a deeper indentation, improved the peel strength. The mechanical performance of the riveted joints was clearly influenced by the choice of rivet size and die shape. As a general rule, the shank diameter controls the joint shear strength, while the head diameter controls the peel strength. Refinements in rivet design also enabled breakthrough of the lower sheet to be avoided.

For joining organic coated or sandwich materials and dissimilar material combinations, mechanical fasteners (possibly in combination with adhesives) are the only available option. However, where spot welding, clinching and self-piercing rivets can be considered, the choice of joining techniques will depend on both technical and economic considerations. At present, this remains to be solved on a case-by-case basis.

Fatigue properties

Work continued on selected clinched and self-piercing riveted joints in 1.6mm 5182 and 1.2mm iron/zinc alloy coated low carbon steel (IZ), with an examination of fatigue properties. Comparison was again made with spot-welding. In an effort to employ a test sample more representative of a structural section, H-section multi-point samples were made, as used by a number of workers for automotive studies, see Fig.5.

Initial results indicate the potential improvement in fatigue that can be achieved from mechanically fastened joints in comparison to spot welds. Figure 6 shows an example of the results so far.

The full results will be published as a TWI members report.

Hybrid joints

Weldbonding, the combination of spot-welding with adhesives, provides an attractive enhancement of joint properties, particularly in automotive application where aluminium alloys and thinner steel sheet are options for reducing vehicle weight. The mechanical fastening techniques may also be combined with adhesives and these hybrid joints form the basis of current Core Research work. Such samples have also been produced for sponsored project work in conjunction with the system manufacturers. The potential of these fastening techniques is their ability to cope with special sheet surface treatments designed to impart improved durability to adhesive bonds, but which are not readily spot-welded. This is important as the concern about durability is probably the greatest barrier to more widespread use of structural adhesives in the vehicle industry.

Promotion of fastening technology

As a means of increasing the awareness of fastening techniques and providing guidance on a broader range of joining techniques for TWI members, a number of initiatives have been set up.

The Joining Forces technology transfer programme has supported the following on-going activities:

• Establishment of a demonstration centre, supported by equipment manufacturers, to demonstrate and use fastening processes in research (Fig.7-9 below).
• Seminars, conferences and workshops including those targeting specific industries such as automotive, furniture and domestic appliance.
• Trade shows such as Fastenex.
• A clinching and self-piercing riveting applications seminar plus demonstration area, scheduled for late 1996.