Advanced Welding Processes: Technologies and Process Control

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Advanced welding processes
rate 1 ). The relationship between wire feed speed and current, which is given
by equation (7.7), is usually shown graphically in the form of burn-off curves
of the type shown in Fig. 7.18 and this allows the appropriate wire feed
speed to be selected for a given mean current.
7.5.2 Voltage–current characteristics
The voltage developed between the end of the contact tip and the workpiece
in the GMAW process is the sum of the resistive drop in the wire extension
plus the voltage fall across the arc. Calculation of the resistance of the
electrode stick-out is complicated by the temperature dependence of resistivity
and the steep temperature gradient which exists in the wire. Measurements
of the total voltage drop under a range of operating conditions show that the
relationship between mean current and voltage in the free-flight operating
modes of the GMAW process is very similar to the characteristic of a GTAW
arc. In the working range, the arc has a positive resistance and for any
shielding gas–filler wire combination at a fixed arc length the voltage increases
linearly with current.
In dip transfer, the mean current–voltage characteristic represents the
average of the short-circuit resistance and the arc resistance and follows the
same trend. In both dip and free-flight transfer, the relationship between
mean current and voltage may be expressed in an equation of the form:
Wire feed speed (m min –1 )
12
10
8
6
4
2
0
0 100 200 300 400
Current (A)
Wire diameter
1.2 mm 0.8 mm 1.6 mm
7.18 Burn-off characteristics for plain carbon steel filler wire.
1 The term melting rate is usually used to describe the mass of electrode material consumed
per unit time. Burn-off rate is the rate at which the wire is consumed or the wire feed
speed. Melting speed is sometimes used to describe the speed at which the melting
isotherm or solid–liquid boundary travels along the electrode wire.