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1 (a) No, with 2D heat flow the temperature distribution changes little in the depth direction.

2 (b) With 3D heat flow, the temperature distribution changes significantly in the depth direction.

3 (c) 2D equation works better because the weld width is essentially uniform in the depth direction, suggesting 2D heat flow.

4 (d) 3D equation works better because the weld width changes significantly in the depth direction, suggesting 3D heat flow.

5 (e) Increasing preheating temperature increases the weld width but decreases the cooling rate.

Example 2.3 Consider the transverse cross‐section of the weld pool based on Rosenthal's 3D heat flow equation. What is the shape of the transverse cross‐section of a weld based on Rosenthal's 3D equation?