25% from droplets, and 10% from cathode heating.
Figure 2.7 Power inputs during GMAW of aluminum: (a) measured results; (b) break‐down of total power input.
Source: Lu and Kou [10]. Welding Journal, September 1989, © American Welding Society.
The heat source efficiency can be very low in LBW because of the high reflectivity of metal surfaces to a laser beam – for instance, 98% for CO2 laser on a highly polished Al or Cu surface. The reflectivity can be found by determining the ratio of the reflected beam power to the incident beam power. Xie and Kar [14] show that roughening the surface with sandpapers and oxidizing the surface by brief exposure to high temperatures can reduce the reflectivity significantly.
2.1.1.3 Heat Source Efficiencies in Various Welding Processes
Figure 2.8 summarizes the heat source efficiencies measured in several welding processes. A few comments are made as follows:
LBW: The heat source efficiency is very low because of the high reflectivity of metal surfaces but can be significantly improved by surface modifications, such as roughening, oxidizing, or coating.
PAW: The heat source efficiency is much higher than LBW.
GTAW: The heat source efficiency for direct‐current electrode negative( DCEN) is slightly higher than that in PAW because of absence of heat losses from the arc plasma to the water‐cooled orifice gas nozzle and through the bottom of the keyhole.
GMAW, SMAW: Unlike in GTAW, heat transfer to the electrode can be transferred back to the workpiece through metal droplets, thus improving the arc efficiency.
SAW: Unlike in GMAW or shielded metal arc welding (SMAW), the arc is covered with a thermally insulating blanket of molten slag and granular flux, thus reducing heat losses to the surroundings and improving the arc efficiency.
EBW : The keyhole in EBW acts like a “black body” trapping the energy from the electron beam. As a result, the efficiency of the electron beam is very high.
Figure 2.8 Heat source efficiencies in several welding processes.
2.1.2 Melting Efficiency
DuPont and Marder [7] studied the ability of the heat source to melt the base metal (as well as the filler metal), as shown in Figure 2.9. Figure 2.9a shows the cross‐sectional area representing the portion of the weld metal contributed by the base metal, A base, and that contributed by the filler metal, A filler. One way to define the melting efficiency of the welding arc, η m , is as follows:
(2.4)
where V is the welding speed, H base the energy required to raise a unit volume of base metal to the melting point and melt it, and H filler the energy required to raise a unit volume of filler metal to the melting point and melt it. The quantity inside the parentheses represents the volume of material melted while the denominator represents the heat transfer from the heat source to the workpiece.
Figure 2.9 Melting efficiency: (a) transverse weld cross section; (b) lower heat input and welding speed; (c) higher at higher heat input and welding speed; (d) variation with dimensionless parameter ηEIV/Hαv.
Source: DuPont and Marder [7]. Welding Journal, December 1995, © American Welding Society
.
Figures 2.9b,c show the transverse cross‐sections of two steel welds differing in the melting efficiency [7]. Here, EI = 3825 W and V = 10 mm/s for the shallower weld of lower melting efficiency (Figure 2.9b) and EI = 10 170 W and V = 26 mm/s for the deeper weld of higher melting efficiency (Figure 2.9c). Note that the ratio EI/V is about the same in the two cases.
Fuerschbach and Knorovsky [5] proposed the following empirical equation for the melting efficiency:
(2.5)
where A and B are constants, H = H base + H filler, α is the thermal diffusivity, and v is the kinematic viscosity of the weld pool. The results of DuPont and Marder [7] shown in Figure 2.9d confirm the validity of Eq. (2.5). As the dimensionless parameter ηEIV/(Hαν) increases, the melting efficiency increases rapidly first and then levels off. If the arc efficiency η is known, ηEIV/(Hαν) is also known and the melting efficiency can be predicted from Figure 2.9. With the help of the following equation for determining A filler, A base can then be calculated from Eq. (2.4):
(2.6)
or
(2.7)
In the above equations, R filler and V filler are the radius and feeding speed of the filler metal, respectively. The left‐hand side of Eq. (2.6) is the volume of the weld metal contributed by the filler metal and the right‐hand side is the volume of filler metal used during welding.
