id="ulink_60daaffd-e585-5e54-a5ff-f5181dfbf55b">Figure 3.12 Comparison of measured weld cross‐section with those calculated including and neglecting the influence of aluminum vapor.
Source: Murphy et al. [13]. © Elsevier.
Although the total power generated in the arc (electric current × arc voltage, e.g. 855 W) is not affected by the presence of Al vapor in the arc, its distribution is affected. Al vapor increases the electrical conductivity of the arc plasma at low temperatures. This means electric current can now also be conducted through the outer cooler portion of the arc. Since a larger horizontal cross‐section of the plasma can conduct the electric current, the current density is reduced, so is the heat flux density.
3.3 Arc Power‐ and Current‐Density Distributions
This split‐anode method has been used to determine the power‐ and current‐density distributions in the arc at the anode surface [14–17]. A block of Cu with a flat top surface is split in half, so that a narrow gap exists between the two halves to electrically and thermally isolate them from each other. Water cooling prevents melting of the Cu anode, maintains the narrow gap, and also allows the heat transfer to each half to be determined from the water temperature rise. The anode is moved slowly under the arc horizontally in the direction normal to the gap, while the current and heat conducted to each half are constantly measured. Obviously, the split Cu anode differs from the real anode in GTAW because there is no metal evaporating from the water‐cooled Cu.
Figure 3.13 shows such distributions for a 100 A, 2.7 mm long gas–tungsten arc measured by Lu and Kou [17] using the split‐anode method. These distributions are often approximated by the following Gaussian distributions:
(3.1)
(3.2)
where q is the power density, Q the power transfer to the workpiece, a is the effective radius of the power‐density distribution, j is the current density, I the welding current and b the effective radius of the current‐density distribution. The effective radius represents the location where q or j drops to 5% of its maximum value. Equation (3.1) is identical to Eq. (2.12). Lee and Na [6] measured and calculated the power‐ and current‐density distributions in GTAW arcs. Figure 3.14 shows that the power‐ and current‐density distributions flatten and widen as the arc length increases.
Figure 3.13 Gas–tungsten welding arc: (a) power‐density distribution; (b) current‐density distribution.
Source: From Lu and Kou [17]. Welding Journal, February 1988, © American Welding Society.
Figure 3.14 Effect of arc length on gas‐tungsten welding arcs: (a) power‐density distributions; (b) current‐density distributions.
Source: Lee and Na [6]. Welding Journal, September 1996, © American Welding Society.
The power‐ and current‐density distributions measured by the split‐anode method (Figures 3.13 and 3.14) do not show the effect of metal vapor in the arc. The vapor can flatten the distributions, because it can increase the electrical conductivity of the cooler outer portion of the arc (Figure 3.8), and hence reduce the weld depth.
3.4 Fluid Flow in Weld Pools
3.4.1 Driving Forces for Fluid Flow
The driving forces for fluid flow in the weld pool include the buoyancy force, the Lorentz force, the shear stress induced by the surface tension gradient at the weld pool surface, and the shear stress acting on the pool surface by the arc plasma. The arc pressure is another force acting on the pool surface but its effect on fluid flow is small especially below 200 A [18, 19], which is usually the case for GTAW. The driving forces for fluid flow in the weld pool, shown in Figure 3.15, are explained as follows:
Buoyancy force. The density of the liquid metal (ρ) decreases with increasing temperature (T). Because of the heat source is located above the center of the pool surface, the liquid metal is warmer at point a and cooler at point b. Point b is near the pool boundary, where the temperature is lowest at the melting point. As shown in Figure 3.15a, gravity causes the heavier liquid metal at point b to sink. Consequently, the liquid metal falls along the pool boundary and rises along the pool axis and, as shown in Figure 3.15b.
Lorentz force. GTAW with DC electrode negative is used as an example for the purpose of discussion. The electric current in the workpiece converges toward the tungsten electrode (not shown) and hence near the center of the pool surface. This converging current field, together with the magnetic field it induces, causes a downward and inward Lorentz force, as shown in Figure 3.15c. As such, the liquid metal is pushed downward along the pool axis and rises along the pool boundary, as shown in Figure 3.15d. The area on the pool surface where the electric current goes through is called the anode spot (πb2 where b is the effective radius of the current‐density distribution). The smaller the anode spot is, the more the current field converges from the workpiece (through the weld pool) to the anode spot, and hence the greater the Lorentz force becomes to push the liquid metal downward.
Shear stress induced by surface tension gradient. In the absence of a surface‐active agent, the surface tension (γ) of the liquid metal decreases with increasing temperature (T), namely, ∂γ/∂T < 0. As shown in Figure 3.15e, the warmer liquid metal with a lower surface tension at point a is pulled outward by the cooler liquid metal with a higher surface tension at point b. In other words, an outward shear stress is induced at the pool surface by the surface tension gradient along the pool surface. This causes the liquid metal to flow from the center of the pool surface to the edge and return below the pool surface, as shown in Figure 3.15f. Surface‐tension‐driven convection is also called thermocapillary convection or Marangoni convection.
Shear stress induced by
