Welding parameters for Gas Metal Arc Welding
In arc welding processes a number of welding parameters exist that can effect the size, shape, quality and consistency of the weld. The major parameters that affect the weld include weld current, arc voltage, and travel speed. The sizes and types of electrodes for shielded metal arc welding define the arc voltage requirements and the amperage requirements. The current may be either alternating or direct, but the power source must be able to control the current level in order to respond to the complex variables of the welding process itself. As direct current electrodes perform well at low amperage, they are often selected for welding thin metals. Most covered electrodes operate best with electrode positive (reverse polarity), which produces the deepest penetration. Electrode negative operation might produce a higher melting rate. The secondary variables include the angle of the electrode to the work, the angle of the work itself, the thickness of the flux layer, and the arc length.
How to determine the correct amperage for a certain electrode will depend on the size and classification of the electrode. Even the type of joint and welding position must be considered. The process requires sufficient electric current to melt both the electrode and a proper amount of base metal. The higher the current, the deeper the penetration. Using to high amperage may cause problems such as excessive spatter, electrode overheating and cracking.
The arc voltage is varied within narrower limits than welding current. It has an influence on the bead width and shape. Higher voltages will cause the bead to be wider and flatter. Extremely high arc voltage should be avoided, since it can cause cracking. The low arc voltage produces a stiffer arc that improves penetration. If the voltage is too low, a very narrow bead will result.
The speed the electrode travels along the joint has a direct influence on bead shape, depth of fusion, cosmetic appearance and heat input into the base metal. Faster travel speeds produce narrower beads that have less penetration. This can be an advantage for sheet metal welding where small beads and minimum penetration are required. Travel speed also affects heat input, which in turn influences the metallurgical structure of the weld metal. The cooling rate increases or dereases proportionately with the travel speed. Also, the heat-affected zone will increase in size and the cooling rate decreases. If speeds are too fast, however, there is a tendency for undercut and porosity, since the weld freezes quicker.
Arc length refers to the distance from the molten tip of the electrode core to the molten weld pool. Generally, arc length increases as the size of the electrode and amperage increase. Limiting arc length to the diameter of the core rod of the electrode is a good guideline.