Page 104 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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To limit the heat input to the work-piece, the current is low during part of the
current cycle. This part of the cycle is denoted “background pulse time”, and is
represented by T b. The current during this part of the cycle comprises background
current and is represented by I b. It must exceed a specified critical value in order to
obtain a stable arc, i.e. the arc must neither go randomly over the work-piece nor
be extinguished [24][25]. This lower current limit is defined by an empirical
constant K 1. During peak pulse time, T p, the current is high. The electrode is
molten, and a droplet is detached and transferred to the work-piece. The main force
for detaching a droplet and transferring it is the electromagnetic force induced by
the peak current, I p. The gravity of the droplet mass plays a minor role. The
criterion for detachment of one droplet per pulse is governed by the relationship

n
I ˜ T K 2 3.2
p
p

where K 2 is a constant depending on the material, and n § 2 [25]. To achieve a
better result, droplets should be of the spray type. If droplets grow larger than the
electrode diameter, a globular metal transfer results, leading to greater probability
of short-circuiting, spatter, uneven weld bead and other fusion defects. The
approximate droplet volume is given by

D V § K 3 A (I p T p + I b T b) 3.3

where K 3 is a constant and A is the electrode cross-sectional area. Wire feed rate,
W f should match the burn-off rate W b, so that a constant arc length can be
maintained. This is important to avoid burn-backs and stubbing-in, which can
cause defective welds. The mean current, I m, is expressed by

I ˜ T I b T ˜ b
p
p
I 3.4
m
T T p
b

and the wire burn-off rate is expressed by

W K 4 I ˜ m K 5 I ˜ 2 m l ˜ e 3.5
b

where K 4 and K 5 are empirical constants for given materials and sizes. The first
term describes the melting due to arc heat of the electrode tip and the second term
describes the joule heating of the electrode stick-out (l e) by the welding current
flowing between the weld table and the electrode wire tip.

3.4.1 Synergic Control

The peak current of the current source used during pulsed GMAW can be current-
controlled. This means that a preset current value will be given independently of
the impedance of the welding process. Some welding sources have an option