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15/07/2009
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Graham Pitcher looks at ways of fastening the high strength materials becoming commonly used in the automotive sector
 The use of high strength materials – such as dual phase and residual austenitic steels – within the automotive sector is increasing: a trend driven by the need for greater passenger protection, increased fuel economy and reduced carbon emissions. A consequence of this is the use of stronger, but lighter, components in safety critical areas – notably in side impact beams and in the A and B pillar areas.
An example is the Honda Civic, which has seen an increase of more than 500% in the use of 600MPa steel in its construction within one development generation. Elsewhere, 800MPa and 1200MPa materials are in production, with materials such as Usibor 1500 – a hardenable ultra high strength boron steel – being explored.
But the increasing use of high strength materials brings with it a challenge for designers: how do you fasten components made from them?
Typically, three types of mechanically mounted threaded fastener are used in automotive sheet metal applications: self pierce and rivet; rivet forming; and pressed insert.
Self piercing fasteners incorporate a cutting edge which, in combination with the insertion tool's bottom die, pierces a hole for the fastener to be located and riveted in position. Rivet type nuts and studs are simply inserted into pre-formed holes and their shanks are formed into a rivet head by the lower tool. Insert type fasteners, meanwhile, rely on the base material flowing into undercuts and recesses in the fastener to achieve the required location and retention.
Adrian Ellis of Profil UK said: "When compared to other assembly methods – notably welding – the attractiveness of pierce, riveting or insert fastening technology can be encapsulated within the term 'cost effectiveness'."
If you can't stand the heat …
Parts made from Usibor are formed and shaped in hot presses and their speed of cooling determines the creation of a hard martensite phase. Tensile strengths in the order of 1600MPa in the heat treated state can be obtained. If studs and nuts were inserted during the hot forming process, they would not meet ISO 898 and ISO 20898 requirements, so can only be inserted after the material is in its hardened state.
Because the strength of the base material exceeds that of even the highest property class fasteners (class 12 for nuts, 12.9 for studs), self piercing and pressed insert fastening techniques cannot be used. This means fasteners with a riveting action, in conjunction with pre-pierced components. must be applied.
To avoid the risk of cracking, the pre-pierced hole should be produced prior to heat treatment as part of the component's forming process.
Ellis observed: "While riveting of the fastener can be carried out in the component's hardened state, best results have been achieved by modifying the preparation of the hole to include raised pips or dimples, which can engage more effectively with the fastener's anti-rotational elements."
And it's the formability of the base metal that determines the suitability of mechanical fasteners for use with high strength steels – the fastener needs to engage with sufficient base metal in order to prevent spinning during final assembly. However, mechanically applied fasteners can, in principle, be used with all high strength steels.
Ellis said: "This is in contrast to welding, which becomes more troublesome as the specific strength of the steel increases. These problems are amplified when aluminium and high strength steel components are to be joined."
Mechanical fasteners also support better positional accuracy, said Ellis. "In most applications, tolerances of +0.20mm can be maintained, while a figure of three times this value might be expected for corresponding welded parts. Furthermore, the integrity of pierce/rivet and pressed insert methods are inherently greater than for welding, which is prone to material variations, surface contamination and operator skill levels."
There are probably as many mechanically secured fasteners available as as the applications to which they are put, Ellis claimed. Alongside standard products, many suppliers can develop specialist features or even custom engineered products for specific application requirements.
"If you require threaded fasteners in pressed or sheet metal components," he concluded, "pierce, rivet and insert assembly methods offer a combination of accuracy, strength, safety and cost effectiveness that is simply unmatched by other fastening processes."
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Audi uses Profil's high strength rivet nuts as part of a safety-critical bonnet latch mounting application on its latest A4 and A5 vehicles. The fasteners are installed in 0.8mm thick high tensile strength steel beams that are subsequently overmoulded in 30% glass reinforced Polyamide 6 to form an integral part of each vehicle's impact crumple zone.
Pointers
* The use of high strength steels is increasing in automotive applications, bringing fastening challenges to designers
* Mechanical fasteners can support location tolerances of ±0.2mm
* The tensile strength of high speed steels may exceed that of the fastener, meaning a riveting action is required for joining
* Mechanical fasteners can be a more cost effective method of joining
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Author
Graham Pitcher
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