Voltage is responsible for:

Voltage plays a critical role in controlling the fluidity of the molten puddle during the welding process. When the voltage is increased, it can lead to a more fluid puddle, allowing for better movement and spreading of the molten metal. This is important for achieving good fusion between the base materials being welded, as a more fluid puddle can help cover gaps and create a stronger joint.

An optimal voltage is essential because if it's too low or too high, the resulting puddle characteristics can negatively impact the quality of the weld. A well-controlled voltage helps create a stable arc, which in turn results in a uniform and consistent heat distribution across the workpiece, encouraging effective puddle fluidity.

While other variables such as arc stability and penetration depth are also influenced by voltage, the specific focus of the question on the molten puddle's characteristics makes it clear that fluidity is a primary factor directly affected by voltage adjustments in the welding process.