Summer 2012 (contents):

1. Hocus Focus

2. Stick with it?

3. Terra Firma

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Hocus Focus

All of us have at some point marveled at the ability of a lens to focus light to small spot and tried to use that phenomena to try to burn a dry leaf or boil a droplet of water. It is evident that the lens is able to focus sunlight down to a much smaller spot. The smaller spot has all the energy focused into a higher energy density that is sufficient to cause some damage. The lens' ability to focus a beam of light to a small spot is function of its geometry, refractive index of the lens material, beam quality, and range of wavelengths being focused. For any given lens, the smallest spot is obtained by a high quality (parallel) beam of monochromatic (single wavelength) light.

Due to aberrations in the focusing lens, all the light rays are not focused to the same spot: some are focused further away and some at a shorter distance. This leads to the concept of "depth of focus", a range where the spot size does not change appreciably leading to a robust manufacturing process when welding is done at focus. Depth of focus is usually of the order of 1-3 mm. The spot size at focus is often not very small, and thankfully so. Energy density values required for welding are of the order of 100,000 W/sq.cm. A much smaller spot size could end up drilling/vaporizing the material instead of melting it. For most welding applications, a spot size of the order of 0.3 to 1.0 mm is common.

Even though the setup may be able to get a smaller spot, there are situations where we actually want a bigger spot size, often to bridge a bigger gap or to compensate for poor alignment between the laser and parts being welded. The quickest way to increase spot size (without changing the hardware) is to defocus the beam. When trying to defocus the beam from a focused position, you should move the lens towards the parts being welded so that the actual focus is now below the weld surface. However, as opposed to process robustness at focus due to depth of focus, there is a risk with defocusing the beam. Now the user must make sure that the defocus setting is controlled accurately, down to less than 0.25 mm, since even a small change in defocus distance can lead to a big change in energy density and change in weld quality. So user beware - you can lose your focus but make sure you don't lose control.

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Stick with it!

Stick welding is the simplest and most robust of the arc welding processes. The stick electrode, which has a metal rod core and looks like a small stick, has a coating on the surface except for a short length at the back end which allows for making electrical connection with stick. The techno-types like to call it by the official name of SMAW, or Shielded Metal Arc Welding. To make a weld, the operator grips the metal end in an electrode holder, brings down the mask in front of the eyes, and strikes an arc with the workpiece. Then it is a matter of expertise to maintain an arc and move it along the weld joint. Heat generated in the arc melts the electrode core and the drops the metal along the weld.

One of the key differences between stick welding and other arc welding processes is the presence of electrode coating which has multiple functions. During welding, some components of the coating evaporate and produce a gas that shields the molten metal from exposure to air. Additionally, deoxidizers in the coating react with and reduce the level of dissolved gaseous elements in the weld and help reduce porosity. Part of the coating forms a slag that covers the molten metal and further shields the solidified metal as it cools down. Coating can also include metal powders such as iron, added to change weld chemistry including control of grain growth and improve mechanical properties of the weld. Some components of the coating also work to change the weld pool viscosity, wetting behavior, and shape of the weld pool. Electrodes designed to be used with an AC source contain potassium compounds to make it easier to re-establish the arc every half cycle. The coating's ability to provide such a variety of functions in an easy-to-use delivery system makes for a very robust welding process.

Other benefits of stick welding include low-cost power supplies, no need for shielding gas supply, and ability to weld in practically any position and anywhere. Almost all welding that one can see being done outdoors is stick welding. Plenty of good reasons why welders all over the world are sticking with it.

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Terra Firma

Terra Firma is latin for solid ground. During any arc welding process, the parts being welded have to be directly or indirectly (through the fixtures) connected to same ground as your power supply. In automated equipment, it is often not easy to figure out if you indeed have a good ground connection and should be the first thing to check while debugging any arc process. It is always better to visually trace the ground path rather than just measuring voltage or resistance between ground and the part/fixture. All contact surfaces through which current will flow from the part to the ground should be free or oil, dirt, oxides, or paints. It is always better to use a copper contact shoe clamped securely to the workpiece. For rotating parts, contact should be made through sliding shoes. Poor ground can cause arc instability and can even lead to porosity in the weld. So get your weld on terra firma before you look for other causes for your welding problems.

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