Which type of welding uses a filler metal that melts at a lower temperature than the base metal?

Brazing is a welding process that uses a filler metal with a melting point lower than that of the base metals being joined. This technique involves heating the metals to a temperature above the melting point of the filler material but below the melting point of the base metals. This allows the filler to flow into the joint by capillary action and bond the materials together without melting the base metals themselves.

In brazing, the filler typically has a melting point above 450°C (approximately 840°F), which is lower than the melting points of most metals including steel and aluminum. This characteristic makes brazing suitable for joining dissimilar metals or for applications where it is critical to maintain the integrity of the base materials.

The other welding techniques listed do not fit this criterion. Soldering involves a filler that melts at an even lower temperature than brazing and is primarily used for electronic applications rather than structural joins. Arc welding and gas welding involve processes where the filler metals and base metals are often melted together, eliminating the scenario described in the question. Thus, the use of a filler metal with a lower melting point than the base materials is a defining characteristic of brazing, making it the correct choice.