TWI Knowledge Summary

Arc weldability of dissimilar metals

When arc welding two dissimilar materials, there are a number of aspects that need to be addressed, in addition to those associated with welding similar materials.

Firstly, from a practical viewpoint, it may not be possible to make a fusion weld if the melting points of the two materials are too different, as it is essential to have controlled melting on both sides of the joint simultaneously. Secondly, even if this criterion is met, it may not be possible to produce an adequate joint if the two materials are metallurgically incompatible.

Metallurgical incompatibility may lead to uncontrollable weld metal/HAZ cracking or a weld metal microstructure that cannot provide adequate mechanical or corrosion performance, e.g. containing unacceptable martensite or intermetallic phases. Even when cracking can be avoided and suitable weld metal deposited, there are other potential problems. Adjacent to the fusion boundary there will be a band, typically very narrow, over which there may be a steep composition and melting point gradient. This fusion boundary region has a composition between that of the parent metal and weld metal and may contain microstructures that are unacceptable for service. In the case of welding highly alloyed steel to carbon steel, the fusion boundary may contain very hard martensite.

'Unmixed zones', or 'partially mixed zones' consisting of fused parent material, which has not mixed fully with the bulk weld metal, e.g. due to differences in melting point, may also form adjacent to the fusion boundary. These may similarly be undesirable. When arc welding dissimilar metals, 'arc blow' or uncontrollable deflection of the arc may occur due to the flow of thermoelectric currents between the hot and cold parts of the joint, and hence the development of magnetic fields, in a similar way to the operation of a thermocouple.

When two materials are metallurgically incompatible, it may be possible to make a satisfactory weld by addition of a suitable filler metal. This is exemplified by the joining of steels and stainless steels, when the Schaeffler diagram, or a modification of it, may be used to select a filler metal that is resistant to both solidification cracking and hydrogen cracking. Frequently, where a welding consumable with composition similar to one of the steels is not appropriate, a nickel based filler may be adopted. In addition, these may be used for welding of steels to some copper alloys due to their tolerance to dilution by a range of alloying elements without a phase change. However, care must be taken to avoid Fe/Ni ratios giving sensitivity to solidification cracking, e.g. approximately 30%Fe, 30%Ni is particularly susceptible.

Where no satisfactory fusion welding process exists, e.g. between Fe and Ti alloys or Cu and Al alloys, it may be possible to produce a weld by a solid state process such as friction welding or explosive bonding. Although less flexible than fusion welding, such processes may be used to produce a range of joint geometries.

Further information

Welding and fabrication of high temperature components for advanced power plant. available to Industrial Members only

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

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