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The heat source efficiency η is defined as

(2.1)

where Q is the rate of heat transfer from the heat source to the workpiece, Q _nominal the nominal power of the heat source, and t _weld the welding time. A portion of the power provided by the heat source is transferred to the workpiece and the remaining portion is lost to the surroundings. Consequently, η < 1. If the heat source efficiency η is known, the heat transfer rate to the workpiece, Q, can be easily determined from Eq. (2.1).

In arc welding with a constant voltage E and a constant current I, the arc efficiency can be expressed as

(2.2)

Equation (2.2) can also be applied to electron beam welding (EBW), where η is the heat source efficiency. In laser beam welding (LBW), Q _nominal in Eq. (2.1) is the power of the laser beam, for instance, 2500 W.

It should be noted that in welding the term heat input often refers to Q _nominal (or EI in the case of arc welding) and the term heat input per unit length of weld often refers to the ratio Q _nominal/V (or EI/V in the case of arc welding) where V is the travel speed of the heat source.