The Weld Nugget (Winter 2013) - A Newsletter to inform, educate, and entertain


Stirred, Not Shaken

James Bond preferred his martinis be shaken, not stirred.  Fortunately, Mr. Bond was not in the soldering business, otherwise he would have quickly realized his folly.  In soldering, you want the solder to melt and allow the thermal gradients to stir the solder, and then as you allow the solder to cool, you want the melt to solidify without being shaken which can cause problems for non-eutectic tin-lead (Sn-Pb) solders.  Based on the ratio of Sn and Pb, solder can freeze from molten state instantaneously (if the ratio is a eutectic mixture) or can go through a "mushy" state - a highly technical term defining a viscous fluid that is partly solid and partly liquid.  Conventional Sn-Pb solders are usually Pb-rich non-eutectic compositions, which means that during solidification they go through a mushy state where part of the alloy starts solidifying in a Pb-rich phase while the composition of the liquid region keeps shifting toward the eutectic composition.  Once the composition of the liquid reaches eutectic composition, the alloy freezes instantaneously, like a pure element.

Both, eutectic and non-eutectic Sn-Pb compositions have pros and cons.  A eutectic 63-37 solder is very fluid down to its melting point at which it solidifies instantly.  Such compositions can be used when gaps between parts are small and consistent; eutectics are also immune to being shaken during cooling.  Non-eutectic compositions, such as 60-40 or 50-50, go through the aforementioned mushy phase as they cool from liquid to solid.  In the mushy phase they are viscous and hence able to fill wider gaps between parts to be soldered.  But there is a downside.  If the parts being soldered are disturbed (the term preferred by the soldering cognoscenti, though I think shaken conveys the real meaning) while in the mushy state, solid and liquid phases tend to move separately and produce a matt finish on the surface of the solder instead of typical shiny appearance.  You can produce a disturbed joint in manual soldering and is often a problem in automated soldering where machine vibrations can be an issue.  In extreme case, where the solid phase is segregated to the surface of the components being soldered, shaking can force the two parts being soldered to move away from each other while the remaining liquid solder is unable to bridge the gap resulting in a crack or sometimes a complete separation between the two parts; not a desirable outcome.  So as long as you are not trying to be a super-hero and just trying to make a good solder bond, it is better to have your solder stirred but not shaken.


Plasma Arc Welding

Plasma Arc Welding (PAW) can be thought of as a specialized version of the Gas Tungsten Arc Welding (GTAW) process ( Fall 2012 Weld Nugget).  While both processes use a non-consumable electrode, PAW adds a constricting nozzle around the electrode that forces the arc to a narrow cylindrical column instead of the wide arc seen in GTAW.  The arc in PAW is initiated with a high frequency spike of current between the negative electrode and the positive constricting nozzle.  Once the arc is established, the orifice gas flowing through the arc gets ionized, and produces a steady stream of plasma that flows out of the nozzle.  The arc between electrode and nozzle is maintained at low current levels and provides a pilot arc.  A larger shielding gas nozzle surrounds the constricting nozzle and protects the molten metal during welding.

At the time of welding, the arc is transferred by connecting the workpiece to the positive terminal.  The arc is now forced to extend from the electrode to the workpiece while still maintaining the constricted gas flow; combination of the two results in an intense arc of high energy density and high velocity plasma flow.  There are many benefits of the narrow and intense plasma beam including higher welding speeds, low distortion, stable arc, and ability to weld in keyhole mode ( Fall 2013 Weld Nugget).  Additionally, since the arc is a narrow cylindrical beam, it is not sensitive to torch standoff distance and hence PAW equipment does not usually require arc voltage control.  A generous standoff distance also makes it easier to introduce filler metal into the weld puddle without risk of electrode contamination.  On the downside, due to the narrow arc, the process has little tolerance for part misalignment.  Plasma gas flow also has to be carefully controlled to make sure the torch does not end up cutting rather than welding, which is especially an issue if the parts do not have good fitup.  Overall, PAW works to bridge the gap - pun intended - between GTAW and Laser Welding and often proves to be an economical option to Laser Welding.


Regular Unleaded

Lead has been known to humans for ages and is represented by the symbol Pb which is short for its latin name Plumbum.  Lead was the choice of metal for making pipes in the Roman Empire and hence the word plumber refers to anyone who works with lead pipes and fittings.  Concerns of lead poisoning were noticed way back in 2nd century BC by Greek botanist Nicander who described paralysis from lead poisoning.  With the advent of industrialization, use of lead increased significantly and was commonly found in batteries, bullets and shots, paint, gasoline, jewelry, and solder; most industries now limit the use of lead or have strict controls on recycling.  However, given the number of products still in use that contain lead, it will be a long time before the world goes "unleaded".