Summer 2008 (contents):
1. Weld Cracks - Engineer's Worst Nightmare
2. Explosive Welding - Doing it with a Bang!
3. Tungsten - Metal that cried "Wolf"
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Weld Cracks - engineer's worst nightmare
There are a variety of physical defects such as undercut, insufficient fusion, excessive deformation, porosity, and cracks that can affect weld quality. Of those defects, cracks are considered to be the worst since even a small crack can grow and lead to failure. All welding standards show zero tolerance for cracks where as the other defects are tolerated within certain limits. There are three requirements for cracks to form and grow: a stress-raising defect, tensile stress, and material with low fracture toughness. Microscopic defect locations are available in practically all welds including geometric features and weld chemistry that can raise the local stress enough to induce a crack. That leaves the engineer to work with the stress environment and toughness: if either of the two can be effectively controlled then cracks can be prevented from initiating and growing. Toughness is a measure of resistance to crack growth; resistance can be provided by blunting of the crack tip in ductile materials. However, if applied strain rate is very high (as would be the case when a spot weld cools at the end of the pulse) and the stress field is multi-axial, even ductile materials exhibit poor toughness and produce rapid crack growth. Hard materials, such as martensite formed during cooling of steels, are brittle and have poor toughness. Toughness can be improved by controlling alloy chemistry and post-weld heat treatment. Stresses can be reduced by changing the joint design to ensure that the weld is under very low tensile load, and preferably, have a compressive load at possible crack locations. Joint designs and fillet shapes can be controlled to minimize stress concentrators that assist in initiation of cracks.
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Explosive Welding - doing it with a bang!
Explosive welding has its roots in serendipitous observations during WWII when pieces of shrapnel were found stuck to armament. The high impact from the flying shrapnel was able to soften and deform the interface and bring atoms across the interface into intimate contact thus forming a solid-state bond. The process was further researched and patented by DuPont Corp. in 1962 and was used to produce clad metals needed by US mint for new coins. In the next three years, DuPont made over 70 million pounds of dime, quarter, and half-dollar blanks.
Explosion welding is now recognized as a unique process to weld dissimilar metals. The two metals to be joined typically start off as sheets that are pushed together with intense force by a pressure wave created by rapidly igniting explosives covering the top sheet. There is no diffusion or mixing of the metals even at the atomic level thus avoiding any formation detrimental intermetallics. Explosion welding has been successfully used to bond dissimilar metals that would otherwise be considered unweldable including mixed pairs of alloys of aluminum, titanium, nickel, steels, and copper. Applications include pressure vessels, busbars, and heat exchangers. Even though it is an industrial process, it is quite difficult to implement and control and hence most users will typically buy the welded bimetal sheets from specialty manufacturers of explosion welded materials
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Tungsten - metal that cried "WOLF"
Discovery of Tungsten dates back to the 17th century Europe. Miners noticed that certain ores disturbed the reduction of cassiterite, a tin mineral, and induced heavy slagging. "They tear away the tin and devour it like a wolf devours sheep," wrote a contemporary author using descriptive language of those times. The miners gave this annoying ore a German nickname, "Wolfrahm," which means wolf froth. This is the reason for the symbol "W" for Tungsten in the periodic table.
In 1758, the Swedish chemist and mineralogist Axel Fredrik Cronstedt, discovered and described an unusually heavy mineral that he called "tung-sten", which is Swedish for "heavy-stone." Pure tungsten is a shiny white metal and, in its purest form, is quite pliant and can easily be processed. However, it usually contains small amounts of carbon and oxygen, which give tungsten metal its considerable hardness and brittleness.
Reference: Advanced Materials and Processes, April 2005.