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Aluminium Joining Systems

Joining Systems For Spaceframe Body Structures

This project offered a unique opportunity to design a vehicle for manufacture with few concept design constraints. In order to limit the project scope an existing prototype vehicle rear quarter structure was taken as the base and the aim was to make a similar structure as easily and cheaply as possible in an automated volume environment. The existing structure already used aluminium extrusions as its base but the processes used to join them were totally unsuitable for volume production. There were also a number of nodal joints, which were perceived to add nothing but cost and complexity to the structure.

A new structure has been designed to allow a large number of possible joining solutions to be tested and evaluated for cost, reliability of application, structural integrity and aesthetics. The structure is not intended to provide an optimum solution but has enabled a number of design guidelines to be produced as a basis for designers to be able to develop optimum solutions. Design guides have been produced for riveting, clinching, welding, adhesive injection, aluminium extrusion and Rivnuts. These take the form of single sheets of paper with drawings tables etc. forming a quick reference for how and where to apply different joining and part production methods. These also exist as powerpoint files on the project CD distributed to all collaborators.

In addition to the design data a number of novel joining techniques have been developed using existing technology as a base. A single sided clinching joint has been developed based on an existing process from the double glazing industry. The novelty is that the new process does not pierce the outer metal but merely forms a spherical depression into an orifice in the nodal joint. The joints produced compare favourably to spot welding or rivets but leave the outer aesthetic surface unblemished. This technique is suitable for all forms of auto closures and similar parts.

The welding work performed as a part of this work package identified two areas of concern for using MIG welding of aluminium in a production environment. The first of these is the control of penetration and the second is a failure to achieve penetration at the weld start. Solutions have been found to both of these. The answer to the first is to us a dual pulse welding system such that by using a combination of hot and cold pulses and varying the duty cycle of each a linear variable is created that controls penetration. This allows much finer control than trying to use pulse current or frequency. The answer to the lack of penetration at the start is to use a combined MIG and TIG torch and to use the TIG torch to preheat the weld start prior to using the MIG torch to perform the weld. This gives good penetration at the start but maintains the speed, gap fill capability and economics of MIG welding. The machine costs are increased slightly but in many cases it is believed that the manual rework using TIG, which is normally required, can be eliminated.