Space Frame Joining Techniques - Spot Welding

Space frames present a considerable challenge for fabrication in volume production and ultimately the success of this design approach depends heavily on how well associated fabrication processes lend themselves to volume production.

Some of the considerations that need to be taken into account when looking at the fabrication processes are: -

• Operating costs
• Cycle times
• Reliability
• Quality

Spot Welding

The conventional technology associated with steel monocoque designs in terms of joining techniques is spot welding. This technique has the benefits of being a very well known, extensively proven technology with which the industry is highly familiar and has considerable experience.

There are however significant problems with regard to spot welding aluminium in terms of accessing both sides of the joint and breaking down the surface oxide layers present on the aluminium.

For steel, a single phase 50-60Hz AC welder is sufficient to produce a high quality reliable joint.

For aluminium however there is a need for higher current devices with stiffer electrodes capable of delivering the higher forces necessary to break down the oxide film.  The consequence of this is that the welders are heavier and bulkier which in turn leads to accessibility problems when trying to position the equipment to the weld points on a spaceframe.

A further implication of the use of higher currents and electrode forces is the need for more frequent tip dressing as a result of rapid electrode wear due to over-heating.

Table showing the effect of material on single phase 50Hz AC resistance spot-welding parameters for 0.9mm body sheet Ref. Barnes T & Pashby I, (2000)

Parameters	Bare Aluminium	Bare Steel	Zinc Coated (hot-dipped)
Weld Time (50Hz cycles)	3	7 - 10	9 - 12
Current range (kA)	18.0 - 23.0	7.0 - 10.0	9.0 - 10.0
Force (kN)	4.1 – 5.0	1.9 – 2.6	2.2 – 2.9

Typically in automobile manufacture, spot welders are used in conjunction with highly versatile robot arms and also as part of large multi-welders capable of supplying simultaneously up to 240 welds. This can reduce fabrication time significantly but means that such equipment tends to be designed for use on specific parts and therefore cannot be readily adapted for use in fabricating spaceframes.

This information tends to suggest that mechanical fastening techniques and adhesive bonding would be a more practical route with respect to joining aluminium.