Summer 2011 (contents):

1. A Road Less Traveled

2. Cold Metal Transfer - An Oxymoron?

3. CCC? What is That?

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A Road Less Traveled

"Two roads diverged in a yellow wood, and sorry I could not travel both...." is the beginning of the classic poem titled A Road Not Taken by Robert Frost which was written nearly a century ago. Frost describes having to make the choice of taking one road over the other and how that has made all the difference. In the world of welding, the welding energy often confronts a similar decision. But alas, being energy, it chooses the path of least resistance, much to the chagrin of the welding engineer. The problem of energy diversion is quite common. For example, in battery tab resistance welding, current that flows from one electrode to the other has multiple choices: either travel through the weld projections/dimples and make a good weld, or take a path of lesser resistance and go around the tab itself. Based on welding force and part design, one or the other paths could be more or less resistive. The welding engineer has to make a careful choice to make a robust process where majority of current consistently flows through the weld projections; not so easy. This problem of energy diversion affects practically all welding processes. In arc welding, the arc can wander due to disturbances in magnetic fields induced during welding or due to presence of magnetic parts and fixtures in the vicinity. Welding in confined spaces or sharp fillets can also give the opportunity for the current to wander from its original target. Laser welding poses unique challenge in that the easiest travel path for light is a straight line. So, if one or both parts are not placed in the line of fire, one could get a very weak weld. A unique condition that can plague laser welding is the deflection of the laser beam by the laser plume; especially in seam welding but can also affect pulsed welding. The laser plume above the weld can disperse the incoming laser beam causing a drop in intensity at the weld resulting in shallow weld depth. Proper alignment of shielding gas can effectively control this problem. As a welding engineer, you need to have a good understanding of the process or else, before you know it, your welding energy can take a walk.

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Cold Metal Transfer - An Oxymoron?

In arc welds made with filler wire, and arc is struck between the filler wire and the parts to be welded. Heat generated in the arc melts the tip of the wire and the molten metal is then transferred to the parts being welded. Depending on the welding conditions of voltage, current, wire speed, and diameter, the process can be setup such that the tip of the wire shoots out molten metal droplets towards the weld joint while the arc is continuously on. A lot of heat is generated in the arc and helps to make a deep and strong weld. The down side is that the heating process can easily melt away thinner sections. Secondly, high heat can melt a substantial amount of parent metals on both sides making it difficult to weld dissimilar metals. ( Spring 2010 Weld Nugget). In an effort to control the heat input, one can setup welding conditions such that instead of the wire shooting out droplets into the weld, the wire is fed right into the molten pool of the weld. When the tip of the wire makes contact with the weld pool, the arc is extinguished and causes a short-circuit condition, hence the name short-circuiting transfer. As the arc is extinguished, the molten droplet at the tip of the wire is transferred to the weld and again opens up a gap between the solid tip of the wire and the weld pool. Presence of gap re-establishes the arc that helps to melt the tip and the process repeats. Since the arc is frequently extinguished, heat input into the weld is minimized.

A recent development in short-circuit welding is the additional control on the wire feeding process. As the wire moves forward and short-circuits, the wire is immediately pulled back; pulling the wire back helps with detachment of the molten droplet during short circuit. This back-and-forth movement of the wire provides additional level of control for the process and further minimizes heating of the weld - and hence the name Cold Metal Transfer (CMT - Fronius ) Reduced heat input provides benefits such as reduced spatter, weld sheets as thin as 0.3 mm, and the ability to weld dissimilar metals and alloys.

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CCC? What is that? 

CCC is an acronym for Copper Contamination Cracking which affects austenitic and structural steels. When steel components are held in copper clamps, cooling fixtures, or brushed against copper components, a thin layer of copper is transferred to the steel surface. If this location of contamination is in proximity (a few millimeters) of a weld such that the surface temperature rises above melting point of copper, the molten copper wets the grain boundaries and penetrates into the steel structure. The weakened grain boundaries can easily separate under tensile stress to form cracks. CCC can be easily identified by observing the fracture surface in an electron microscope equipped with EDAX; it can be avoided by plating fixtures with chromium or nickel.

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