1. Dissimilar Metal Welding - Mixing it up!
2. Friction Stir Welding
3. Oxygen - The Kiss of Death
Dissimilar Metal Welding - Mixing it up!
There are many situations where a weld is required to be made between metals/alloys that are different. In fusion welds, mixing two dissimilar alloys results in the formation of a third alloy which may have properties that are vastly different from the two parent materials. At one extreme, the fused material can be so brittle and prone to cracking that the combination is deemed unweldable; Titanium and Steel would be a good example. In some situations, the mixture can be tailored to minimize the harmful effects of those welds such as in welding of carbon steel to 304 SS. Here an equal parts mixture will be martensitic and quite brittle. It is possible to tailor the chemistry of the fusion melt by biasing the weld energy towards the 304 SS or by possibly adding a filler alloy (309 SS) that will move it further towards 304 composition.
There are instances where one has to intentionally choose dissimilar alloys to avoid problems with welding. A good example is welding of 6061 Al; fusion of two components of 6061 produces a melt that will crack on cooling. The solution is to weld the 6061 to a 4047 component; the mixture has sufficient silicon to provide fluidity to heal the cracks. There are very few opportunities where dissimilar material are completely soluble in one another and pose no problems with welding; an example would be welding nickel to copper. In situations where welding is necessary but a fusion weld would be detrimental or unacceptable, an option is to go for a solid-state weld, such as a resistance weld. In solid-state resistance welding, the parts to be welded are softened and pushed together without actually melting and mixing the two. Since there is no fusion, the weld can avoid problems related to embrittlement.
However, theory sometimes does not accurately predict the outcome. Case in point is welding of steel to tantalum. One would imagine a brittle weld since those two do not want to mix. The trick here is to realize that steel and tantalum have vastly different melting points. A weld can be made in this system where the steel is forced to melt and flow over tantalum. Molten steel effectively acts like a braze alloy and bonds to tantalum which never actually melts! So sometime you just have to keep those books down, go to the lab, and just give that weld a shot. You might be pleasantly surprised.
Friction Stir Welding
Friction stir welding is the newest process in the field of welding. Invented and patented in the UK by TWI (The Welding Institute) in the early nineties, is ideally suited for welding structural aluminum alloys. The weld is made with a non-consumable rotating tool, similar to a spinning top. Heat is created by friction between the shoulder surface of the top and the two parts to be welded which are in a butt weld configuration. Friction heat softens the matrix which is then stirred together by the rotating pin; the height of the pin is almost equal to the thickness of the two parts being welded. The stirring action mixes the two materials almost like mixing cookie dough.
FSW is ideally suited for welding aluminum - a soft material with fairly low melting point. A hardened steel tool can be used without much wear from the friction process. Since FSW is a solid-state process which avoids problems with liquation cracking, solidification cracking, and porosity. These welds often have better mechanical properties compared to fusion welds and also have reduced distortion and residual stress due to lower heat input. On the downside, FSW requires a butt weld configuration and application of very heavy force between the tool and the components to produce frictional heat. The components to be welded also have to be sufficiently rigid and hence FSW cannot be used for thin foils and sheets.
Oxygen - The Kiss of Death
Even though most known life forms depend on oxygen for survival, welding does not seem to appreciate the beauty of oxygen. Oxygen is a reactive gas that will bond with most structural metals to form brittle oxides in the fusion zone. Even if present in small quantities, oxygen can dissolve in the matrix and increase hardness of the fusion zone. Oxygen is usually introduced in the fusion zone from the environment; it is not typically dissolved in any significant quantities in the metal/alloy itself. For metals such as Titanium, one has to go to extreme lengths to keep out oxygen from the weld metal. In the medical device industry, it is quite common to weld Ti inside a glove box where oxygen and moisture levels are controlled at ppm (parts per million) levels. For majority of fusion welding applications, oxygen can be kept out of the weld by flooding the weld area with a shielding gas such as Helium, Argon, or Nitrogen. Solid state welding processes such as resistance welding and ultrasonic welding have the benefit of a welding force that pushes the two parts together and hence keeps oxygen out of the weld zone. Shielding gas can be used in these applications but only to avoid discoloration for cosmetic purposes.