Frequently Asked Questions
Hydroformed tubes are increasingly being applied in automotive high volume production. The hydroforming process allows complex, three-dimensional components to be made with relative ease and these can be used to replace components made from pressed and spot welded sheet, reducing the number of process steps.
Two typical examples of hydroformed components used for lightweight vehicle body manufacture are roof rails and windscreen reinforcement pillars. For these sorts of body-in-white applications, body panels made of thin sheet have to be joined to the tubes. The absence of flanges and the lack of access to the internal surface of the tube, or the back of the joint, necessitate the application of joining techniques termed as single-sided. Conventional spot welding, as traditionally used in vehicle body manufacture, is generally not easily applied to hydroformed components, but a number of other joining techniques can be employed.
Variations of resistance spot welding
In general, spot welding is feasible if the tube wall thickness is at least twice that of the sheet, and the structure has enough strength to tolerate the electrode force. Direct welds can then be made with a standard spot welding electrode in contact with the sheet and the return electrode on the opposite side of the tube providing it is of a shape suitable to be clamped between the electrodes. Alternatively, a contoured backing electrode can be used which reduces the risk of denting or collapse of the tube, or of the tube slipping from between the electrodes.
If no access to the back of the tube is available, series spot welding can be used, again assuming the tube walls have sufficient strength to react to the electrode force. In this process variant both electrodes are applied to the same surface, making two spot welds simultaneously.
Arc welding processes
Techniques offering continuous welding (as opposed to point joints) whilst applying little pressure to the structure are preferred for hydroformed components in automotive production. Arc welding processes such as MIG/MAG, plasma or TIG fall into this category and are generally used for the attachment of small components such as brackets. They are, however, not normally used for thin sheet, because the relatively high heat input can lead to unacceptable thermal distortion of the component. MAG brazing can be used with less heat input, but this is at the expense of joint strength and may therefore not be suitable for high strength steel grades.
The process most commonly used is, therefore, either CO2 or Nd:YAG laser welding, as these are both non-contact joining processes capable of producing welds at high travel speeds and with low heat input, generally without the need for filler addition. Alternatively, particularly for visible joint locations, laser brazing may be used. This can reduce the heat input even further and, especially in combination with post-weld machining, give excellent cosmetic appearance.
All the techniques discussed above are easily automated making them suitable for joining of three-dimensional components. Nd:YAG lasers are the easiest type of laser to robotise since the beam can be transmitted to the workpiece via flexible fibre optic cables.