Fall 2011 (contents):
1. Elemental Attractions (science fiction)
2. Magnetic Pulse Welding
3. A Titanic Problem
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Elemental Attaractions
It's a material world. No doubt about it. Everything we use is made of some material - plastic, metal, ceramic, semiconductor, or some combination of those. Using materials requires an intimate knowledge of their properties, often times down to the atomic level. Welding is no different; knowledge of material properties is important to understanding and developing a good weld. Most engineers approach welding from the process and equipment perspective. While routine welds can be dealt with that way, more complex applications require a thorough understanding of materials.
We can go into details about grain boundaries, defects, brittle phases, and on and on. But I thought it would be fun to put a lighter spin on it. Many years ago, while in grad school, I wrote a story about a Sodium atom and his passion for lady Chlorine ( Click this link to download the whole story). The love story also includes other fictional characters and incorporates many properties and applications of materials. If you have a background in materials science you will get all the nuances; if not, here is the key:
- Nucleus of an atom (insides) is made of protons and neutrons.
- Meson are unstable subatomic particles; no idea where they are supposed to be but just sounds cool.
- Most atoms can lose/gain electrons to build a charge (positive or negative) which results in a voltage (or potential).
- Strongly ionic elements have high affinity for the opposite charged ionic elements. For example positively charged ions such as sodium and potassium have strong affinity for negatively charged ions such chlorine and iodine
- Some elements form a covalent bond by sharing electrons - Silicon Carbide is a good example.
- Chlorine gas is greenish-yellow in color (the word "Chloros" in Greek means greenish-yellow).
- Platinum is a commonly used catalyst to split compounds such as water to release hydrogen and oxygen.
- Copper has multiple oxidation states and can bond with chlorine to form CuCl or CuCl2.
- Argon is an inert gas and does not react with other elements.
- Some metals, like Nickel and Aluminum, can react to form intermetallic bonds.
- Molten sodium is used as a coolant in some nuclear reactors.
Now you know what I did in grad school :-)
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Magnetic Pulse Welding
Magnetic Pulse Welding is a one of the many cold welding processes; any heat generated is incidental. It is almost like explosive welding (see earlier newsletter on this topic) except that it uses a very strong pulse of energy to slam together the two parts being welded. It is ideally suited for lap welding tubes (one tube end inside the other) but can be used to weld strips as well; however strips could be better welded with roll cladding. The two tubes being welded, one inside the other, are placed inside a coil which is energized by a bank of capacitors. Then, as the capacitors are discharged, a sudden burst of energy produces very high currents in the coil, which in turn produces very high forces on the outer tube, which then collapses onto the inner tube to form a solid-state bond (no melting, see earlier newsletter on this subject). Energy of the order of 50 kJ producing 1 million-amps of current that thrust the parts towards each other at 600 m/sec is common. Energy required is a function of material properties (conductivity and yield strength) and size. Of the commonly used structural materials, welding aluminum requires the least amount of energy, followed by copper, and then steel. Welds can be made between dissimilar materials as long as the softer material is on the outside (the one which is accelerated).
Since this is a solid-state process, one does not have to worry about intermetallics or any third component such as solder or braze alloy. Intermetallics as well as solder/braze alloy are much more resistive than the parent metals. This process is ideally suited for applications where the goal is to make strong bonds that are very conductive. The process is easiest to setup for tubes; if flat pieces are required a tube weld could be made first and then the tubes cut open along the length and rolled flat. The requirement that one tube be bigger than the other is a limitation - essentially no butt welds. If expectations are not so stringent, other cheaper processes such as soldering, brazing, or arc welding could be utilized.
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A Titanic Problem
A material world can also cause a material disaster. Almost all man-made disasters can be ultimately traced back to faulty or improperly specified materials; from plane crashes to car accidents to sinking of the Titanic. One of the many things that followed Murphy's Law that fateful night included rivets used to hold the ship together. Turns out that some of the rivets (most were made of steel) were specified to be made of wrought iron of grade defined as "best". Now what's wrong with that? Apparently, an even better grade called "best-best" (I didn't know they had marketing MBAs back then J) was available but not used. The lower "best" grade had greater amount residual slag that can make the rivets brittle and susceptible to catastrophic failure. Similar brittleness and weakness can be found in welds with higher levels of impurity elements. A common occurrence is in Titanium, where strict control of moisture and oxygen is required to maintain sufficient ductility in the weld metal. So if you don't want your welding project to sink, always go for the "best-best".
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