Civil Engineering Reference
In-Depth Information
Unfortunately, an all-inclusive intelligent weld control review is beyond the scope of this chapter. Nev-
ertheless, numerous references at the end of this chapter are included for further investigation.
7.2
Welding and Bonding
Most welding processes require the application of heat or pressure, or both, to produce a bond between
the parts being joined. The welding control system must include means for controlling the applied heat,
pressure, and filler material, if used, to achieve the desired weld microstructure and mechanical properties.
Welding usually involves the application or development of localized heat near the intended joint.
Welding processes that use an electric arc are the most widely used in the industry. Other externally
applied heat sources of importance include electron beams, lasers, and exothermic reactions (oxyfuel gas
and thermit). For fusion welding processes, a high energy density heat source is normally applied to the
prepared edges or surfaces of the members to be joined and is moved along the path of the intended
joint. The power and energy density of the heat source must be sufficient to accomplish local melting.
7.3
Control System Requirements
Insight into the control system requirements of the different welding processes can be obtained by
consideration of the power density of the heat source, interaction time of the heat source on the material,
and effective spot size of the heat source.
A heat source power density of approximately is required to melt most metals [18]. Below
this power density the solid metal can be expected to conduct away the heat as fast as it is being introduced.
On the other hand, a heat source power density of or will cause vaporization of most
metals within a few microseconds, so for higher power densities no fusion welding can occur. Thus, it
can be concluded that the heat sources for all fusion welding processes lie between approximately
and heat intensity. Examples of welding processes that are characteristic of the low end of
this range include oxyacetylene welding, electroslag welding, and thermit welding. The high end of the
power density range of welding is occupied by laser beam welding and electron beam welding. The
midrange of heat source power densities is filled by the various arc welding processes.
For pulsed welding, the interaction time of the heat source on the material is determined by the pulse
duration, whereas for continuous welding the interaction time is proportional to the spot diameter
divided by the travel speed. The minimum interaction time required to produce melting can be estimated
from the relation for a planar heat source given by [18]
10
3
W
cm
2
10
6
10
7
W
cm
2
10
3
10
6
W
cm
2
2
t
m
[
K
p
d
]
(7.1)
where
p
is the heat source density (watts per square centimeter) and
K
is a function of thermal conduc-
d
cm
2
/s
5000 W
tivity and thermal diffusivity of the material. For steel, Eagar gives
K
equal to
. Using this
value for
, one sees that the minimum interaction time to produce melting for the low power density
processes, such as oxyacetylene welding with a power density on the order of , is 25 s, while
for the high energy density beam processes, such as laser beam welding with a power density on the order
of
K
10
3
W
cm
2
10
6
W
cm
2
, is 25
s. Interaction times for arc welding processes lie somewhere between these
extremes.
Examples of practical process parameters for a continuous gas tungsten arc weld (GTAW) are 100 A,
12 V, and travel speed 10 ipm (4.2 mm/s). The peak power density of a 100-A, 12-V gas tungsten arc
with argon shielding gas, 2.4-mm diameter electrode, and 50-degree tip angle has been found to be
approximately . Assuming an estimated spot diameter of 4 mm, the interaction time
(taken here as the spot diameter divided by the travel speed) is 0.95 s. At the other extreme, 0.2-mm
(0.008-in.) material has been laser welded at 3000 in./min (1270 mm/s) at 6 kW average power. Assuming
a spot diameter of 0.5 mm, the interaction time is
3
cm
2
8 0
W
s
4
3.94
10
.
