Principle of Process
The heat generated melts a portion of the tip of the electrode, of its coating and of the base metal in the immediate area of arc. A weld forms after the molten metal, a mixture of the base metal (workpiece), the electrode metal and substances from the coating of the electrode, solidifies in the weld area. The electrode coating deoxidizes the weld area and provides a shielding gas to protect from oxygen in the environment.
A bare section at the end of the electrode is clamped to one terminal of the power source, while the other terminal is connected to the workpiece being welded. The current usually used in ranges between 50 A and 300 A, power requirement are less than 10kW. The current maybe AC or DC. For sheet metal welding, DC is preferred because of the steady arc is produced.
The polarity of the DC current, the direction of the current flow is very important because the selection is depends on the type of electrode to be welded and the depth of the heated zone. In straight polarity the workpiece is positive and the electrode is negative because it is preferred for sheet metals to produce shallow penetration and for joint with very wide gaps. In reverse polarity, the electrode is positive and deeper weld penetration is possible. In the AC method is suitable for welding thick section and for large diameter electrodes at maximum currents.
Figure 1: Principle of Shielded Metal Arc Welding (SMAW) Process
Figure 2: SMAW circuit
Equipment
Shielded metal arc welding equipment typically consists of a constant current welding power supply and an electrode, with an electrode holder, a work clamp, and welding cables (also known as welding leads) connecting the two.
Evaluation of Result
The preferred polarity of the SMAW system depends primarily upon the electrode being used and the desired properties of the weld. Direct current with a negatively charged electrode (DCEN) causes heat to build up on the electrode, increasing the electrode melting rate and decreasing the depth of the weld. Reversing the polarity so that the electrode is positively charged and the work piece negatively charged increases the weld penetration. With alternating current the polarity changes, creating an even heat distribution and providing a balance between electrode melting rate and penetration.
Figure 3: 1) DCEN welding result, 2) DCEP welding result
The quality problems associated with SMAW include weld spatter, porosity, poor fusion, and shallow penetration. Weld spatter, while not affecting the integrity of the weld, damages its appearance and increases cleaning costs. It can be caused by excessively high current, a long arc, or arc blow, a condition associated with direct current characterized by the electric arc being deflected away from the weld pool by magnetic forces. Porosity, often not visible without the use of advanced nondestructive testing methods, is a serious concern because it can potentially weaken the weld. Another defect affecting the strength of the weld is poor fusion, though it is often easily visible. It is caused by low current, contaminated joint surfaces, or the use of an improper electrode. Shallow penetration, another detriment to weld strength, can be addressed by decreasing welding speed, increasing the current or using a smaller electrode. Any of these weld-strength-related defects can make the weld prone to cracking, but other factors are involved as well.
Advantages of the Shielding Arc Welding
a) The welding process is very simple and versatile.
b) The process is suitable for workpiece thickness of 3mm-19mm.
c) The process does not required high skills welder.
Limitations of the Shielding Arc Welding
a) The welding processes are required to clean the slag after each weld bead.
b) The solidified slags are cause of several corrosion of the weld area and lead to failure of the weld.
c) The cost and material in the welding process are very high.
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