Bringing different metals together creates a few challenges, one of which is sometimes addressed with the “sacrificial anode.” We have a few boaters on the staff here at Wiley Metal and this is a term they’re familiar with. Sacrificial anodes are used on boats, especially those that see salt water. They “attract” corrosion and so reduce the tendency of other connected metal pieces to corrode. This highlights that welding dissimilar metals creates problems that might not be obvious at the outset.
What do we mean by dissimilar?
If we suggested welding galvanized steel to 6260 aluminum a few people might chuckle. It can be done though, as photos on the EWI website show. Admittedly, that’s an extreme example, but we are receiving more requests to join different grades of the same metal type. Welding a low alloy steel to an austenitic stainless would be a good example.
Driven by cost
High strength metal is expensive, so designers only use it where needed. That however, requires the metal fabricator to join a high strength metal to one of lower strength. This is seen more and more in the auto industry where companies are turning to “tailor welded blanks” (TWB’s.)
A TWB is a stamping in a relatively low strength steel, to which high strength inserts are welded. The assembly then goes into another press that forms it into a structural body member. This lets the manufacturer achieve the strength needed while using the minimum amount of expensive high strength metal.
A less common driver is appearance. A designer might want stainless steel on the visible surfaces, but to save money will use a low alloy steel in hidden areas. For joining these there are several options: fasteners, adhesives, and of course welding.
There are three main problems with welding dissimilar metals:
- Different thermal expansion coefficients.
- Different metallurgy.
- Potential for galvanic corrosion.
After dissimilar metals are first preheated then welded, they contract by different amounts as they cool. Since welding has fused the pieces together the result is high residual stresses that will cause distortion and even cracking. Post-welding stress relief helps, but differential expansion rates is still something to consider.
The relative weldability of the two metals determines how readily they will join. Weldability is specified in various standards, and can also be expressed in terms of “carbon equivalency.” This complex formula lets the welder assess susceptibility to cracking and determine the preheat temperature.
Differences in metal strength influences the choice of filler wire. In general a welder will choose a filler with the same strength as the weaker metal, although service conditions of the finished fabrication, (particularly exposure to cyclic loads,) affect the decision.
Joining dissimilar metals and placing them in a wet environment completes an electrical circuit. Some metals will give up their electrons more easily than others, and this allows them to corrode faster. A good example is zinc and brass: zinc is the more “active” metal, so when joined with brass in a wet environment will corrode faster. That’s why, when brass is used on boats for various fittings, it’s connected to a zinc sacrificial anode which corrodes instead.
Whenever dissimilar metals are joined it’s important to consider galvanic corrosion. If the metals are close in terms of their willingness to yield electrons, (similarly “active,” as defined by the “galvanic series,”) it’s less of a problem than if they’re different.
With appropriate attention to weldability and in-service conditions it’s possible for welding dissimilar types of steel using gas metal arc techniques. As they become more different though other methods are needed. TWB’s are usually welded by laser for example, while for material combinations like aluminum and stainless newer and more exotic techniques like friction stir welding are used.