How is the molten pool (the pool of hot iron) formed during welding?

The molten pool (the area of ​​molten iron) during welding is a liquid metal region formed by the localized melting and mixing of the base metal and filler material under the influence of the welding heat source. Its formation process directly determines the quality and performance of the weld.

The formation process of the molten pool:
* **Heat source action:** At the start of welding, a high-energy heat source such as an electric arc, laser, or flame concentrates heat at the joint of the weldment, rapidly raising the local temperature above the metal's melting point.
* **Melting of base metal and filler material:** Under high temperature, the edges of the base metal begin to melt, while the welding rod or wire, acting as filler material, also melts, forming droplets that transition into the welding area.
* **Mixing to form the molten pool:** The molten base metal mixes with the liquid metal from the welding rod/wire, together forming a liquid metal region with a specific geometric shape—this is the molten pool.
* **Dynamic existence and solidification:** As the heat source moves, the molten pool migrates forward, continuously undergoing metallurgical reactions (such as deoxidation and dehydrogenation) during this movement, eventually cooling and solidifying to form the weld.

Factors affecting molten pool formation:

Welding parameters: Current affects penetration depth and pool volume, voltage affects weld width, and welding speed alters pool shape (e.g., from elliptical to droplet-shaped);
Material composition: The chemical composition of the base metal and welding material affects melting rate and metallurgical behavior;
Spatial position: The molten pool is most stable during flat welding; during vertical and overhead welding, the flow of molten metal needs to be controlled by the welding rod movement;
Protection conditions: Gas shielding or flux covering can prevent air intrusion and reduce defects such as porosity and slag inclusions.

Key characteristics of the molten pool:
High temperature (approximately 1700–2000℃ for low-carbon steel), in a superheated state;
Small volume (typically only a few cubic centimeters), rapid cooling rate (solidification completes in about 10 seconds); Incomplete metallurgical reaction, prone to defects such as porosity and cracks, requiring metallurgical control through flux coating or shielding gas.