Spring 2014 (contents):
1. Tack Welding
2. Active Metal Brazing
3. Manganese - The Cleanup Crew
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Tack Welding
One of the key requirements of any seam welding process is to ensure that parts to be welded are held in position throughout the welding cycle. Such requirements are commonly referred to as fitup which includes alignment and gap between parts. Parts are commonly held in a fixture to maintain required fitup during welding. Fixtures are designed to clamp on the parts close to weld interface to prevent loss of fitup due to thermal expansion stresses during welding, yet far enough to allow access for energy source to the weld seam. There are situations where use of fixtures is not feasible when a) parts have a complex profile, b) parts are too small and difficult to hold during welding, c) fixtures are not strong enough to maintain alignment during welding, or d) product volume is too low to justify making expensive fixtures. That is when tack welding comes in handy.
Tack welds are small welds made along the weld seam in order to provide in-situ fixturing to hold the parts together during welding. Tack welds come in wide variety of shapes and types. Some are single spot welds while others can be short seam welds - depending on strength requirements. Some are made during fitup while others are made on the automated machine prior to running the full seam weld. Some are made autogenously while others are made with filler alloy. There is no limit to how many tack welds can be made along the seam but usually a minimum number is chosen that will provide required strength. Even though it seems that the tack welds are lending a helping hand, there are risks associated with them. A tack weld or welds are made on the part while the part is at room temperature and not at steady state temperature as during the seam weld, thus exposing the tack weld to rapid cooling and related defects. Tack welds can also be exposed to additional stress when in some cases positioning fixtures are released prior to welding thus transferring all the fitup loads to the tack welds. Tack welds made autogenously may not have the right chemistry as compared to the actual seam weld made with filler. Under these circumstances, it is possible that the tack welds have defects such as cracks, porosity, or a brittle HAZ - defects that would be unacceptable if allowed to remain after seam welding. The best method to avoid any lingering issues from tack welding is to make a seam weld that is big enough to completely wipe out any memory of the tack weld - fusion zone, HAZ, undercuts, and all!
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Active Metal Brazing
Conventional brazing uses a combination of flux and filler metal in a two-step process that includes removal of surface contaminants and oxides by the flux followed by wetting of the base metal by the braze alloy to form a strong bond. This approach does not work on ceramics because the whole structure is made of oxides (or nitrides, carbides,..) so the flux has a never ending task, and then the molten braze is unable to wet the ceramic surface. One method used to braze ceramics is to apply a metallization layer on the surface followed by conventional braze, but then again it becomes a two step process. The other method, which is a one-step process, uses a special braze alloys that includes an active element such as titanium, and hence the name - active metal brazing.
Active metal brazing is a one-step process since there is no need for a flux. The braze alloy foil is sandwiched between parts to be joined (ceramic-to-ceramic or ceramic-to-metal), placed in an oven (vacuum or pure argon), and heated to a temperature above melting point of the braze alloy. Once the alloy melts, the titanium atoms migrate to the interface and react with the ceramic to form a layer of oxides (or nitrides, carbides,..) followed by a layer of complex intermetallics that have properties that are intermediate between metals and ceramics. The remainder of the braze alloy is now able to bond onto the intermetallic layer. This ability to react with the ceramic and form a layered structure at the interface is unique to the active metal brazing process. Other active elements such as zirconium and vanadium have been investigated but none are as effective as titanium. In addition to joining ceramics, active metal brazing is widely used to braze ceramics to metals and can be credited for expanding the use of ceramics into unique applications such as engine valves for high-performance cars and rotors for turbochargers.
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Manganese - The Cleanup Crew
Sulfur is a nasty element when present in steels since it can form iron sulfide that causes hot cracking during forging and welding processes. Completely removing sulfur from steels is expensive and hence manganese is added to steels to bind with sulfur. Manganese has a much stronger affinity for sulfur and forms manganese sulfide that rises up to the surface as slag during steel processing or is trapped as inclusions that have a very high melting point and prevent hot cracking. Manganese content of four times that of sulfur is sufficient to make sure all sulfur is cleaned up, but usually even greater amounts of manganese is added since it provides additional benefits including removal of oxygen and increased hardenability. Even though iron and carbon are considered to be the important elements of steel and the industrial revolution, progress would not have been possible without the cleaning powers of manganese.
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