Welding Metallurgy. Sindo Kou

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should be noted that the melting efficiency cannot be increased indefinitely by increasing the welding speed without increasing the power input. To do so, the power input must be increased along with the welding speed. It should also be noted that in the presence of a surface‐active agent such as sulfur in steel, the weld pool can become much deeper even though the welding parameters and physical properties in Eq. (2.5) remain unchanged (Chapter 3).

      2.1.3 Power Density Distribution of Heat Source

      2.1.3.1 Effect of Electrode Tip Angle

Schematic illustration of the effect of electrode tip angle on shape and power density distribution of gas–tungsten arc. Schematic illustration of the effect of electrode tip angle on shape of gas–tungsten arc.

      Source: Glickstein [15]. Welding Journal, August 1976, © American Welding Society.

Schematic illustration of the effect of electrode tip geometry on shape of gas–tungsten arc welds in stainless steel.

      Source: Drawn from photos of Key [17]. Welding Journal, December 1980, © American Welding Society.

      2.1.3.2 Measurements

Schematic illustration of the measured power density distributions.

      Source: Lu and Kou [3]. Welding Journal, February 1988, © American Welding Society.

      2.2.1 Response of Material to Welding Heat Source

Schematic illustration of the heat-affected zones thermal cycle: (a) top view of weld pool, fusion zone and heat-affected zones, (b) thermal cycle recorded with a thermocouple inside HAZ during welding.

      A thermocouple is prepositioned inside the HAZ. As shown in Figure 2.14b, it detects a temperature well below T H at time t 1, a peak temperature between T L and T H at time t 2, and a temperature below T H at time t 3. The temperature‐time curve experienced at a location in the workpiece during welding, such as that shown in Figure 2.14b, is called the thermal cycle. The thermal cycle of the HAZ provides information such as the peak temperature (how close to T L), the duration above T H and the cooing rate, which are useful for better understanding of the HAZ microstructure and properties. Likewise, the thermal cycle of the fusion zone, which shows a peak temperature above T L, provides information useful for studying the fusion‐zone solidification microstructure and properties.

Schematic illustration of the coordinate system (x, y, z) moving with heat source.

      Source: Kou and Le [24]. © TMS.

      2.2.2 Rosenthal's Equations

      Rosenthal [25]

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