Скачать или смотреть What is Welding: Mastering the Solidification Process for Stronger Joints

Solidification Process of the Weld Puddle
The weld puddle forms when heat from the welding process melts the base metal and any filler material. As the heat source moves away or is removed, the molten metal cools and begins to solidify. The process typically involves:

Nucleation: Solidification begins at small, localized points (nuclei) within the weld puddle or along the boundaries of the molten and solid material.
Growth: The nuclei grow into dendrites (tree-like structures), extending outward as the temperature drops below the melting point. These structures grow in the direction of heat extraction, typically opposite the heat source.
Grain Formation: The dendrites eventually collide, forming grains. The grain size and shape are influenced by factors like cooling rate, heat input, and metal composition.
Solidification Process of the Weld
The weld solidification process involves transitioning from liquid metal in the weld pool to a solid weld joint. Key steps include:

Cooling and Solidification: As the molten metal cools, it passes through its freezing range, solidifying from the edges toward the center of the weld bead.

Metals with a narrow freezing range solidify quickly, forming smaller grains.
Metals with a wide freezing range may experience segregation and solidification cracking if cooling is uneven.
Crystallization: The liquid metal forms crystalline structures. The type of structure (equiaxed, columnar, etc.) depends on cooling rates and thermal gradients.

Columnar grains: Form when the cooling rate is slow, and the heat gradient is steep.
Equiaxed grains: Form under rapid cooling or low thermal gradients.
Post-Solidification: The newly solidified weld may undergo transformations, such as phase changes or precipitation of secondary phases, depending on the material's composition and cooling rate.

The solidification process directly impacts the weld's strength, ductility, and susceptibility to defects. Proper control of heat input and cooling rates is critical for achieving high-quality welds.