Fall 2009 (contents):
1. Electron Beam Welding
2. Difference between brazing and soldering
3. All that "soot" is not Carbon
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Electron Beam Welding
Electron Beam Welding, or EBeam welding as it is usually referred to, was developed and commercialized in the sixties. In EBeam welding, a beam of high energy electrons are generated in the electron gun and accelerated towards the work piece. As the electrons encounter the metal to be welded, they decelerate and convert their kinetic energy into welding heat. Depending on the accelerating voltage, which will typically range from 25 kV to 200 kV, the electrons can achieve speeds that can be up to 70% of the speed of light. The number of electrons in the beam is controlled by the beam current which can range from 50 to 1000 mA. The electrons are focused into a small spot by electromagnetic focusing lenses and can achieve spot sizes of the order of 0.25mm to 0.75 mm. Combination of voltage, current, and spot size can produce energy densities of the order or 105 W/mm2, which is higher than any other welding process. Such high energy densities and narrow beam divergence in the work piece allow EBeam welding to produce very high aspect ratios (narrow and deep welds) with minimal heat affected zones (HAZ) and part distortion. Welding in a high vacuum environment limits beam divergence and maintains a clean weld metal without atmospheric contamination. Such attributes make EBeam welding ideal for nuclear industry and aerospace applications.
Limitations of using EBeam welding includes expensive equipment and having to place the work pieces in vacuum that increases costs and reduces throughput. Additionally, to make a narrow aspect ratio weld requires that the work pieces be precisely machined and aligned. Since the beam of electrons is negatively charged, difference in magnetic characteristics of the two parts being welded can affect beam deflection. Rapid solidification rates common in EBeam welding can cause cracking in some alloys. Interaction of the electron beam with the work piece generates x-rays and requires adequate personnel protection. Given these constraints, EBeam has always been a niche process and will continue to do so while maitaining the pole position as the porcess that can produce the highest aspect ratio fusion welds.
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Brazing or Soldering - What is in the name?
Brazing and soldering are practically identical processes where a filler alloy is added to form a metallurgical bond between two components. A flux is usually used to clean the surfaces and process temperatures are set such that the filler melts but the parts to be joined do not. Then why do we have two different names for these processes? We can blame it on history. The earliest solders were based on tin and its alloy which are low melting while brazes were based on copper-zinc alloys which have a higher melting point. The word braze is derived from an old English word braes meaning to cover with brass. Where as the word solder is derived from an old French word soudure which means to make solid (probably referring to an assembly process of making a solid by joining two pieces). Both, brazing and soldering, form a metallurgical bond with the base metal. A braze will usually dissolve the base metal and form a solid solution (atomic level mixture of base metal and braze) while a solder will typically react with the base metal to form a thin layer of intermetallics to which the remaining solder; however, there are exceptions to these rules.
There is no real significance to the temperature of 450 C (840 F) which is typically used to distinguish between soldering (below 450) and brazing (above 450 C); it is just a matter of semantics.
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All that Soot ain't not Carbon
Formation of black residue, often referred to as soot, near the weld zone of laser welds is a common occurrence. Given that this powder is typically black in color, it is very easy to jump to the conclusion that it must be some form of carbon. At that point, there is often a witch hunt to locate the source of any organics, the presumed source of carbon, and can include shielding gas, material contamination, and surface impurities from prior cleaning processes. Even though it is recommended that such organic contamination be avoided as much as possible, the black residue might not necessarily be carbon. The residue appears black because of the specific particle size that absorbs visible light. A client of mine once collected enough amount of this "carbon" and analyzed it under an SEM and found out that its chemical composition was identical to the alloy they were welding! This "carbon" is essentially metal (perhaps partly oxidized) powder that vaporized during welding and then condensed on the cooler metal near the weld.
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