Question

CODE 102_8 Welding Many welding processes require the application of heat and/or pressure to produce a suitable bond between the two parts being joined. There are a number of conventional and commercially practiced welding processes, which possess limitations in relation to material type and thickness, and in addition, the cost analysis may not allow for a process to be used on a specific application. The selection of suitable welding consumable (electrode or wire) is also an important factor in producing suitable weldments. The choice depends on the intended engineering stress on the weld and the welding process being used for the work. Great care is required in the selection process, and some engineering and commercial compromises are often necessary to arrive at a suitable choice. The term arc welding is used to classify a group of processes that use an electric arc as the source of heat to melt and join metals. A large number of welding processes use an electric arc as the source of heat for fusion. An electric arc consists of a relatively high current discharge sustained through a thermally ionized gaseous column, called plasma. The power of an arc may be expressed in electrical units as the product of the current passing through the arc and the voltage drop across it. In a typical example, 350 Amperes at 22 volts will dissipate 7,700 Watts of power. This will generate heat energy of about 7,700 J/s. It should be noted that not all the heat generated can be effectively utilized for welding, since some heat loss occurs, the effective heat is significantly lower than the generated heat calculated above; this general efficiency of arc is called 'arc efficiency'. The arc efficiency of a welding process is an important variable that needs to be considered in determining the effect of heat on the weld, and the heat affected zone. In general, the arc welding process can be described as the process in which a welding arc is struck between the work-piece and the tip of the electrode. The electrode may or may not be consumed in the joining (welding) process. Non-consumable electrodes are either carbon or tungsten. The arc welding process may provide the forward movement to electrodes by either manual or mechanical operations. Similarly, the filler material may be fed into the weld pool by either manual or mechanical means. If all these activities are manual, the process is termed a manual process; conversely if all the processes are mechanical, the process is called automatic. There is the possibility of mixing mechanical and manual operations; in that case the process is called semi-automatic, although some specifications have called them mechanized processes. The majority of semi-automated processes automate the wire-feed. The

Answer 1

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