2. Molten weld pools are dynamic
Liquid in weld pool is acted upon by several strong forces
This can result in high velocity fluid motion
Fluid flow velocities exceeding 1m/s have been observed in
GTAW
Fluid flow affects weld shapes and is related to a variety of
welding defects
Moving liquid also transports heat and dominates heat transport in
the weld pool
3. Heat transport by fluid depends on direction and speed of the fluid
Fluid flow in the weld pool can alter the weld shapes
Temperature gradients are also altered by fluid flow which can
affect the weld microstructure
In GTAW fluid flow can cause defects such as
1. Lack of penetration
2. Top bead roughness
3. Humped beads
4. Finger penetration
5. Undercutting
Instabilities in liquid film around keyholes in EBW and LBW can
lead to uneven penetration (spiking)
4. High velocity gas motion occurs in and around arc during welding
The gas motion can be partially due to cover gas flow
More importantly, it is driven by electromagnetic forces associated with the
arc
In GMAW the liquid filler metal is also being transported through the arc
from electrode to the workpiece
The addition of sulphur can substantially increase the depth to width ratio
in a weld
5. Forces driving fluid flow in GTAW include
1. Surface tension gradients (Marangoni convection)
2. Electromagnetic or Lorentz forces
3. Buoyancy forces
4. Aerodynamic drag forces caused by passage of arc plasma
over the weld pool. Also referred to as impinging or friction
force, it includes impacts from electrons, ions and additions to
the weld as well as photons (in LBW)
Surface tension forces drive fluid flow in GTAW welds
These can be drastically altered by addition of trace elements
Surface tension is temperature dependent and there are large
temperature gradients in the weld pool
Surface tension will be greatest in the coolest part of the weld
(edges) and lowest in the centre of the weld which is the hottest part
under the arc
6. Such a surface gradient produces outward fluid flow
This fluid flow pattern transfers heat efficiently from the hottest part
of the weld pool (near the centre) to the edge and produces a wide
and shallow weld (Fig. 2a)
7. Surface active elements segregate to surface of liquid metal
They can also change temperature dependence of surface tension
For a limited temperature range above Tm the surface tension
increases with temperature so the sureface tension will be highest near
the centre where the temperature is maximum
8. Such a surface tension gradient will produce a fluid flow inward along
the surface of the weld pool and then down
This fluid flow pattern transfers the heat efficiently towards the bottom of
the weld pool
This results in a relatively deep and narrow weld
Addition of S, O, Se, Te to SS in low concentrations (150 ppm)
substantially increases d/W ratio in GTAW welds.
If elements that can react with these trace elements are present or
added the D/W ratio decreases
Al reacts with O. Ce reacts with both S and O. This leads to shallower
and wider pools
The D/W ratio therefore decreases
9.
10. This effect is also observed in other welding processes eg. EBW
11.
12. In dissimilar welding two steels can have different penetration
characteristics. The weld pool is displaced towards the material
with low D/W. The low D/W material has a low concentration of
surface active elements and thus a higher surface tension. This
produces a fluid flow towards the low D/W steel
13. When oxygen is added to the shielding gas the D/W ratio passes through a
maximum and then decreases. The surface of welds made with torch gas
concentration mor than D/W maximum were heavily oxidized. At higher
oxygen concentrations, a liquid oxide film is formed on the pool
surfacealtering the surface tension gradients
14. Effects of high Current:
Electromagnetic (Lorentz) stirring becomes more important at high
currents. The Lorentz forces produce a deep penetration pattern.
At sufficiently high current, Lorentz forces dominate the fluid flow.
In addition, plasma jet becomes stronger producing a depression
of the pool surface
The plasma jet forces are resisted by the surface tension
For a travelling weld, the radial pressure gradient from the plasma
jet tends to transport liquid from the front to the rear of the weld
For a sufficiently high pressure gradient PB the liquid metal may
be pushed to the bottom of the pool
15. Addition of surface active elements lowers the surface tension and
increases the effect of the plasma jet.
In addition, a surface depression changes the energy input by the
arc to the weld pool
At very high currents, a vortex may form in the centre of the weld
pool
16. Keyhole formation
EBW and LBW offer very high power densities over a
small area
As the power density of the heat source increases, the
peak surface temperature of the weld pool increases
The vapour pressure of metal rises exponentially with
temperature and becomes appreciable at 0.8Tb
The liquid metal under the power source becomes
depressed by the vapour pressure
As the liquid moves away from the power source, the
surface is depressed and a cavity is formed
This leads to formation of a keyhole
17. Fluid flow in the keyhole
The keyhole is a cavity having roughly the size and shape of the
beam. Thus it is cylindrical in shape.
There is fluid flow in the thin layer around the cavity
In a travelling weld, the material at the front wall of the cavity is
transported to the back wall where it eventually solidifies
This liquid moves around the walls of the cavity as a thin, high
velocity layer
There is substantial temperature difference between front and rear
walls of the cavity. This gives rise to large surface tension
gradients.
This is the driving force for this fluid motion
18. In GMAW metal is transferred from torch to the weld pool.
In spray transfer mode the impact of droplets on the pool forms a
substantial depression
The momentum of these droplets determines the penetration of
the weld
19. In submerged arc welding high welding currents in excess of
1000A are employed. Fluid flow velocities up to 4m/s reported.
The Lorentz forces combined with the radial pressure gradient in
the moving torch transport the liquid to the rear of the weld cavity.
