Dissimilar metals are commonly welded using fusion and pressure welding processes. The major Difficulties encountered are as follows :
1. Differences in physical and mechanical properties.
2. Dilution of deposited filler material.
3. Formation of intermetallic compounds at the interface causing embrittlement of the joint. To eliminate this or reduce, heat input to the weld is reduced.
7.5.1 Guidelines for Welding Dissimilar Metals
In the welding of dissimilar metals the following guidelines are helpful:
1. Minimise heat input to minimise dilution and restrict diffusion.
2. Choose proper filler material compatible with both materials being welded.
3. Reduce dilution by controlling welding process variables related to penetration. Thus minimise penetration. In GMA welding reduce current density so that dip. transfer of metal occurs.
4. Dilution and formation of intermetallic phases can be minimized by applying a layer of compatible material on both the joint faces.
5. In case of the welding of heat treated steels appropriate heat treatment should be used. If one plate is hardenable low-alloy steel, appropriate pre and post weld heat treatment should be used.
If for some reasons heat-treatment is not possible, ductile austenitic filler material must be used (for hardenable materials). This will compensate for lack of ductility in the HAZ.
7.5.2 Tips for Joining Certain Combinations
1. Joining alloy Steels
Joining 2.25 Cr-1 Mo. steel with 1 Cr-0.5 Mo steel or 0.5 Mo steel with plain carbon steel can be best done by using a filler that matches with the lower alloy for good weldability.
2. Joining ‘Ferritic steel’ with Austenitic steel
This is best done by using austenitic filler rod. Filler metal should have a composition that will stabilize austenite even after dilution, otherwise the carbon will migrate from ferrite and alloy elements from the other plate to form a crack susceplible zone.
3. Joining highly Austenitic Materials
This is successfully done by using a filler material which is highly ferritic such as electrode type 312 (29 Cr—9 Ni). This will leave sufficient ferrite in the weld metal to avoid hot cracking. If one base metal is highly ferritic then a highly austenitic electrode (310) can be used to avoid weld which will contain large quanties of ferrite.
4. Joining stainless steel to plain carbon steel
Plain carbon steel is first coated with a layer of austenitic steel like 309 (25 Cr-12 Ni) using TIG or MMA processes with usual precautions. In service, problems arise, due to different thermal expansion coefficients of plain carbon and stainless steels. Large thermal stresses are built-up due to unequal expansions and contractions. Because of high solubility of carbon in austenitic stainless steels, carbon from low alloy steel will have a tendency to migrate during welding to austenite regions. This will result into decarburized zone in ferritic steel just adjascent to the interface. This may lead to service failures.
5. Welding of aluminium to steel
This is a very common situation in industrial applications. The steel part is first coated with aluminium and the joint is completed using TIG welding using aluminium based filler
wires. The arc is directed towards the aluminium member during welding. The molten weld pool flows over the aluminium coating on steel without melting too much of the steel. Thus the formation of intermetallic compounds can be eliminated. The aluminium coating on steel should be thick enough to avoid burning near the edges.
6. Applications of explosive and friction welding
Explosive and friction welding can avoid the formation of intermetallic compounds and are used for dissimilar metals welding. Similarly flash butt welding has the advantage that the intermetallic phases are squeezed out of the joint while in the molten state.