Simple Rules for Welding or Joining Metal Alloy Components
For many people, welding is welding; it really doesn’t matter what alloy they are welding or how they are welding it. Nothing could be farther from the truth.
Each metal alloy presents its own unique problems for welding, some of which are serious, while others are easy to deal with. For this reason, it’s very important to know exactly what alloys you are joining together, and how those alloys react when welded. By doing so, you can be sure of a solid weld joint that’s much less likely to fail.
While it is impossible to deal with each individual alloy and its properties in an article like this, there are a few simple rules which are common to all metal alloy compounds.
Avoid Dissimilar Alloys
Welding dissimilar alloys can be a disaster. Not only do they often have different melting temperatures, but they also require different welding rod or wire.
When dissimilar alloys have to be used, they should be joined together by other means, such as bolting. In cases where the ionic action between the alloys can cause galvanic corrosion, an insulator pad should be used between the pieces.
Avoid Dissimilar Thicknesses
This is a basic rule of any welding. When parts of dissimilar thickness are welded together, the chance of burn through is greatly increased.
The solution when parts with dissimilar thicknesses have to be welded is to proportion the heat application to the metal thickness. In other words, if two pieces of metal are being welded together, and one is twice as thick as the other, twice as much heat should be applied to the thicker piece of metal. This is accomplished by allowing the welding rod or wire to linger longer on that thicker piece.
Clean the Parts Before Welding
Foundry slag on the surface of cold-rolled steel isn’t a problem for welding, as it melts at a lower temperature than the steel. This isn’t necessarily true for all alloys. Stainless steel foundry slag actually melts at a higher temperature than the steel blank, impeding proper heat transfer to the part. This can cause a cold weld joint or improper heat penetration. Inadequate heat penetration is the number one cause of failed weld joints.
Likewise, paint, oil and other impurities can weaken a weld joint. If the weld temperature is sufficiently high to totally burn these up, it reduces the risk of problem. However, any residue, whether from plating, painting or just machine oil can reduce the strength of the weld.
Stagger the Weld
Not all metal alloys react the same to heat. Aluminum tends to shrink while being welded. Stainless steel parts will pull together, closing off the weld gap and misaligning the parts. To prevent this, tack the parts together with a staggered pattern before welding.
Start by tacking both ends, and then split the difference and tack in the middle. After that, split the differences again, tacking between the existing weld tacks. Repeat this process, until there are enough weld tacks to provide adequate structural stability while welding.
Match the Gas to the Alloy
Not only should the welding wire or rod be matched to the metal alloy being welded, but the shielding gas as well. Some alloys weld better with CO2 than Nitrogen. If you are unsure of what welding gas to use for your alloy, check with your welding supplier.
Use the Lowest Welding Temperature for Good Heat Penetration
While good heat penetration is essential for a good weld, excessive heat can cause problems for some alloys. Besides the obvious problem of burn through, excessive heat can reduce the temper of alloys and change the metallurgical properties of the alloy. The solution is to use just enough heat to ensure proper weld penetration, without overheating the parts.
- Never weld dissimilar metal alloys, fasten them by alternate methods.
- Know what alloys you are welding and the proper materials to weld them with.
- Clean your parts of plating, foundry scale, paint and oil to ensure the best heat penetration.
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