1. Trail Mix
2. Metal Transfer in MIG Welding
3. Party Poopers
In welding processes that make continuous fusion seam welds, as is common with CW (Continuous Wave) laser welding, most arc welding processes, and electron beam welding, the molten metal pool is carried along the weld seam. As the heat source (laser beam, electron beam, or electric arc) moves forward, the molten metal pool moves forward as well, consuming fresh parent metal in the front while allowing molten metal to solidify at the back. Under steady-state conditions (constant linear speed and heat balance), the molten metal pool stabilizes to a fixed shape. The leading (melting) end of the pool always has a circular shape but it is the trailing (solidifying) end which is affected by the welding speed and in turn can affect weld quality. At slow linear speeds, the molten pool may appear almost circular. As speed increases, the pool will become elongated and appear elliptical, with the long axis along the weld seam; one can also see circular ripple marks on the surface after the weld has cooled. At even higher speeds, the weld pool becomes tear-drop shape and leaves V-shaped ripple marks. However, the outer appearance of the ripple marks is very deceptive as the grain structure inside can be quite different.
As the weld metal solidifies, the higher melting components solidify first while pushing lower melting phases towards the solid/liquid interface. At slow welding speeds with an elliptical weld pool, the grains start solidifying along the parent metal boundary and curve towards trailing edge of the weld puddle such that solidifying tip of the grains is perpendicular to the pool boundary, as shown in the schematic above. The solidification boundary is circular and will push out the lower melting constituents into the weld pool. If there is sufficient fluidity and strong enough convection currents in the molten metal, the lower melting phases can continue to accumulate in the weld pool and are deposited at the end of the weld. In welds where the puddle is sluggish, the contaminants will freeze out along various grain boundaries at the circular cooling edge and will be distributed fairly uniformly in the fusion zone thus reducing the risk of cracking. As welding speed is increased, the weld pool becomes tear-drop shaped with a long tail. The solidifying grains grow from the parent metal boundary towards the tail of the teardrop, once again maintaining perpendicularity between the solidifying tip and the pool boundary. At very high welding speeds, the grains are almost perpendicular to the welding direction. In some alloy systems, the grains will meet along the centerline (as shown in the schematic above) where they deposit all the contaminants and low melting phases. Depending on the weld chemistry and strain on the weldment, the weld centerline could become a defect location. A weld engineer should keep in mind that productivity is not the only factor in defining weld speed, one should pay close attention to solidification on the welding trail; make sure it does not leave you in tears.
Metal Transfer in MIG Welding
In MIG welding, the filler metal wire also acts as an electrode ( Spring 2013 Weld Nugget). Consequently, the electrode has to serve a dual purpose of providing welding current as well as adding filler metal to the weld pool. The phenomenon by which material is deposited depends on the level of current being utilized. There are three modes of transfer, all of which are described below.
At low levels of current, the arc is not hot enough to transfer any molten metal. Instead, the wire tip gets hot only after it contacts the workpiece, hence the name Short-Circuiting Transfer, and is followed by a quick increase in current and tip temperature that leads to melting and separation of a drop of molten metal. Current profile has to be adjusted such that wire tip melts quickly and deposits the molten metal drop yet not hot enough to produce spatter. Once the drop separates, the arc is re-established till the next short-circuit occurs as the wire is continuously fed into the weld. This process is ideally suited for welding thin parts, welding out-of-position, or bridging large root openings.
As the welding current is increased, the arc is hot enough to produce a molten metal drop that is transferred from the wire tip to the workpiece without contact between the two. The drop is large enough, hence the name Globular Transfer, to be affected by gravity and hence this process is usually limited for welding in flat position only. Arc voltage has to be controlled such that it is not too low or else the molten drop at the wire tip might contact the workpiece causing short-circuit followed by a rapid increase in temperature and disintegration of the drop producing considerable spatter. Arc voltage cannot be too high or else there will be incomplete fusion and excessive reinforcement. Adjusting between the two is difficult and hence globular transfer is not commonly used in production.
Beyond a certain current level, the transfer mode transitions from globular transfer to one where the wire electrode melts and sends out a stream of droplets that are sprayed onto the weld, and hence the name Spray Transfer. The droplets are small enough and are accelerated by the arc voltage such that welding is independent of gravity and hence can be used in any position. Spray transfer produces a deep and narrow weld that is relatively free of spatter. New technology in power supplies that enables them to control a base current with a superimposed pulsation has benefited spray transfer. The pulsation mode can be tuned to meet the needs for welding in all positions and a wide variety of materials and thicknesses. Making the right choice of metal transfer mode will serve the engineer well in both meeting weld specifications and having sufficient throughput.
Helium is an inert gas that is lighter than air and is commonly used to fill balloons for kid's parties. Being lighter requires higher flow rates for sufficient shielding of the weld. In addition to being inert, it also has a high ionization potential thus requiring higher arc voltages compared to other gases. Helium is used commonly for welding with CO2 lasers where higher ionization potential is necessary to reduce plasma formation above the weld. It is also the shielding gas of choice for welding conductive alloys of Aluminum, Magnesium, and Copper, where a high energy density arc is preferred. It also gives an aesthetically pleasing appearance to welds on stainless steels. But alas, there is an apparent shortage of Helium which will get worse over the next few decades; welding uses 20% of all Helium consumed. Due to a supply constraints, management is encouraging welders to switch to alternatives such as Argon. Hah. Such party poopers!