Oxide dispersion strengthened (ODS) alloys offer an attractive combination of properties, but they are difficult to join. TWI technologists have examined the potential of solid phase bonding, brazing, and TIG and laser fusion welding.
ODS alloys were first developed in the 1960s, and have aroused considerable interest in applications demanding high temperature mechanical properties and resistance to the operating environment. They are made by powder metallurgical techniques.
Various matrix alloys have been developed, including systems based on iron, nickel or intermetallic alloys. The material is rolled or extruded to its final shape.
Joining these materials satisfactorily has been a problem for many years. Conventional fusion welding techniques have been unsuccessful, as the alloys normally have very high occluded gas contents which lead to gross porosity, and the oxide dispersion is lost during fusion.
Alloys which are virtually gas free have since been developed, and these offer greater potential for fusion welding as the occluded gases can be diffused out at a temperature below that at which the strain energy will anneal out. However, fusion welding releases all the strain energy in the fused area, and the resulting grain structure, which is not ideal for creep resistance, cannot be easily modified. The mechanical properties of the joint will therefore be lower than those of the parent material, at ambient and elevated temperatures.
Solid phase bonding has shown considerable promise, particularly where the bonding temperature is below the annealing temperature. This allows subsequent recrystallisation to occur epitaxially across the interface. Work at TWI has developed diffusion bonding methods for iron based alloys in which the subsequent recrystallisation can be controlled to give substantial epitaxial growth across the interface, and in which the bond line becomes extremely difficult to detect microscopically. This technique has proved unsuccessful in nickel based alloys where the diffusion rates are very much lower. However, some success has been obtained in joining nickel based alloys by brazing, and allowing the braze interlayer to diffuse away during recrystallisation, and there is certainly scope for improving this process. Joints of the quality shown in Figure 1 can be easily achieved.
Considerable success has been obtained for many ODS alloys with friction welding. As a solid state process, this has obvious advantages.
Success has been obtained in fusion welding of substantially gas free, iron based alloys using TIG and laser welding. Although it has not yet been possible to induce epitaxial grain growth across the interface, this may not be necessary in many industrial applications where the alloy is employed for its resistance to oxidation and sulphidation, rather than for its creep resistance. The appearance of a laser weld in recrystallised material is shown in Figure 2, which confirms the absence of gross porosity.
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