TWI Knowledge Summary

Joining of ceramics by friction heating and forging

The joining of ceramic parts directly to each other by the friction heating and forging process is not possible. However, certain joint configurations can be joined by using an aluminium interlayer.

The friction heating and forging process relies, fundamentally, on plastic deformation of one or both workpieces to cause rupture and dispersion of the oxides which are present at the faces to be joined. Once these oxides are dispersed and an appropriate temperature is attained, the joint faces are then forged into such intimate contact that molecular bonding occurs in the solid-phase (without melting). Unfortunately, due to their nature, ceramics can fracture under compressive forces which are well below those necessary to achieve plastic deformation. They may also fracture when exposed to rapid changes in temperature over short distances.

Aluminium is considerably softer than ceramic and easily deformed at quite low temperatures (e.g. 400°C). When rubbed against a ceramic surface, sufficient friction heating can be developed to enable plastic deformation of the aluminium. This can be achieved with a compressive rubbing force below that which causes the ceramic to fracture. Aluminium, especially pure aluminium, will bond to aluminium oxide and magnesium or yttrium oxide stabilised zirconium oxide ceramics ^[1] . There is some conjecture as to whether the bond is molecular (a solid-phase weld) or mechanical. During limited research at TWI, moderate strength bonds have been made in low density (open structure) ceramics. In contrast, the strength of bonds made in high density ceramics with polished surfaces were poor. This suggests that the aluminium may be mechanically keyed into the open pores in the case of the low density ceramic.

The fact that aluminium can be joined to ceramic means that two ceramic parts may be joined using an aluminium interlayer, provided that the joint geometry is feasible and that the parts can withstand the stresses and thermal cycle of the friction and forging process. For example, two round ceramic rods could be butt jointed by rotary friction welding. The operation would, of course, require two welds to be made. For such joints the final thickness of the aluminium interlayer would need to be carefully established for the particular service requirement.

No industrial applications of friction butt jointed ceramic rods using an aluminium interlayer are known to TWI. However, in feasibility studies, aluminium studs have been successfully rotary friction welded to the backs of ceramic printed circuit boards to act as heat sinks.

Reference

You can use the Weldasearch literature database to supplement what you find in JoinIT.

For further information contact philip.threadgill@twi.co.uk.