Computer modeling of arc welds to predict effects of critical variables on weld penetration Page: 4 of 21
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Surface Tension Gradient: Temperature-dependent surface tension gradients
can cause shear stress on the weld pool surface that can increase by an order of
magnitude the momentum transport rates at the surface and, consequently, the
transport of the heat. The spatial variation of surface tension causes the molten metal
to be drawn along the surface from a region of lower surface tension to a region of
higher surface tension. For pure metals and alloys, the temperature dependency of
surface tension (dy/dT) is negative. Therefore, surface tension would be highest at the
periphery of the pool where the temperatures are relatively lower, causing an outward
flow as shown in Fig. 1a. On the other hand, surface active elements, such as S, can
produce a positive dy/dT causing an inward flow as shown in Fig. 1b. Depending on
the welding conditions and the composition of the weld metal, the spatial variation of
surface tension can produce surface flows that are either (a) radially outward (for
negative dy/dT), (b) radially inward (positive dy/dT), or (c) a combination of a and b
which is schematically illustrated in Fig. 2.
Surface Curvature: The weld pool surface can undergo temporal and spatial
deformations, depending on the heating and the electromagnetic effects of the arc and
the heat transfer and the convection in the weld pool. Associated with the local
deformation conditions, a surface pressure is exerted on the free surface due to the
direct contribution of the surface tension.
In addition, at high welding currents, the arc pressure (due to the plasma arc)
and the shear force (due to the impinging shielding gas flow) may influence the fluid
flow in the weld pool. For normal welding conditions, surface tension forces dominate
all other forces in the weld pool, and buoyancy plays a very minimal role in the
development of the weld pool. Depending on the welding process and welding current,
electromagnetic forces can influence and, at high currents, dominate the development
of the weld pool.
VARIABLES AFFECTING WELD PENETRATION
During welding, there are several variables that influence the flow mechanisms
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Zacharia, T. & David, S.A. Computer modeling of arc welds to predict effects of critical variables on weld penetration, article, January 1, 1991; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1071861/m1/4/: accessed April 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.