Two surfaces P and Q are to be joined together. In which of the given joining operation(s), there is no melting of the two surfaces P and Q for creating the joint?

The correct options are Brazing and Adhesive bonding.

To understand why these joining operations do not involve the melting of the two surfaces to be joined (surfaces P and Q), let us analyze each of the given processes:

1. Brazing: In brazing, the two base surfaces P and Q are placed close to each other, and a filler metal with a lower melting point than the base metals is introduced into the joint. The assembly is heated above the melting point of the filler metal but below the melting point of the base metals P and Q. The molten filler metal flows into the gap by capillary action and solidifies to form a strong joint upon cooling. Since the temperatures reached are lower than the melting points of P and Q, the base surfaces P and Q do not melt.
2. Adhesive bonding: In this process, a non-metallic substance (the adhesive) is applied between the surfaces P and Q. The joint is created by surface adhesion and cohesion within the adhesive layer. This process is typically performed at or near room temperature (or moderate curing temperatures) and relies purely on chemical and physical bonding forces, meaning there is absolutely no melting of the base materials P and Q.
3. Arc welding and Spot welding: Both of these are fusion welding processes. They require heating the joint interface of the base metals P and Q to their melting points to fuse them together (often along with a filler material in arc welding, or through electrical resistance and pressure in spot welding). Thus, both welding methods involve the melting of surfaces P and Q.