Welding thick sections

When a leading manufacturer of vibration testing systems explored an alternative route for manufacture, it turned to TWI for assistance in identifying the right method for them.

LDS Test and Measurement is an international company with its UK base in Royston, Hertfordshire. It manufactures electro dynamic vibration test systems and supplies OEMs and test houses across the world.

A range of their products incorporate large vibration tables, which are often made to customer specifications. Therefore, a certain degree of flexibility is required in their manufacture. By their nature, these tables need to be lightweight but high strength, so the material chosen for this application is magnesium. Magnesium has the added benefit of having vibration damping properties - something that is particularly important for vibration testing equipment.

The drawback with this choice of material is that its supply is somewhat limited, with plate only rolled in certain widths, which are sometimes not appropriate for the LDS vibration tables. As a result, for large testing systems, LDS require that two plates are joined together to form the complete table.

The manufacturing process traditionally used by LDS to manufacture these tables had, in the past, not been completely reliable, and did not offer the full flexibility required by the company, so they requested assistance from TWI to identify a suitable alternative.

It was crucial that this alternative offered a high integrity solution, since the tables contain many machined features including long gun-drilled oil-ways necessary for oil circulation to bearings within the table.

These oil-ways and other features span the width of the component crossing the welded joint in several places. Any defects along the welded joint pose a potential risk of the high- pressure oil finding a way out.

It quickly became apparent that this was an ideal application for TWI's high force friction stir welding (FSW) machine, located at their Yorkshire Technology Centre. With proven capacity to weld 75mm thick aluminium plates in a single pass, the welding of 75mm thick magnesium plates was a natural progression. A series of welding trials demonstrated that the fabrication of the table by FSW was feasible, so TWI proceeded to the next stage, and welded two 1.3 m by 3 m magnesium plates together to form the table - a structure well within the working envelope of the TWI machine.

An additional benefit of TWI's high force FSW machine is that it has a very large flat machine bed, which enables the two sections of plate to be aligned accurately edge-to-edge, thus avoiding any steps in the joint.

The welding process proved to be a success, and the component was machined with no evident signs of the joint, demonstrating that the consolidated material along the weld line was free from porosity.

Not only did the welding process prove successful for the application, but it was shown to represent a significant cost saving over the traditional fabrication process.