Why is it essential to apply heat to the lap joint during brazing?

Applying heat to the lap joint during brazing is essential primarily to melt the filler metal. In the brazing process, the filler metal must be heated to its melting point and drawn into the joint by capillary action. This ensures that the filler metal flows into the space between the joined pieces, forming a strong bond as it cools and solidifies.

In addition to melting the filler metal, adequate heat distribution to the joint components is vital for achieving the desired flow and successful bonding. If the heat is insufficient, the filler metal cannot melt and thus won't effectively penetrate the joint, compromising the integrity of the bond.

Melting the filler metal is crucial because it dictates the success and effectiveness of the brazing process, leading to a strong, durable joint that can withstand operational stresses.