Question

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?

• A. Arc welding
• B. Brazing
• C. Adhesive bonding
• D. Spot welding

Hint:

Remember the key distinction:
- {Welding:} Usually melts the base metal(s).
- {Brazing/Soldering:} Melts a filler metal, but NOT the base metal. (Brazing>450°C, Soldering<450°C).
- {Adhesive Bonding:} No melting involved, uses chemical adhesion.

Answer 1

The Correct Option is B, C

Step 1: Understanding the Concept:
This question asks to identify joining processes that do not involve melting the base materials (surfaces P and Q). Joining processes can be broadly categorized based on the state of the materials involved: fusion welding (base metal melts), solid-state welding (no melting), and processes like brazing, soldering, and adhesive bonding which use filler materials or adhesives.
Step 2: Detailed Explanation:
Let's analyze each process:
- (A) Arc welding: This is a fusion welding process. An electric arc is created between an electrode and the base materials, generating intense heat that melts the workpieces at the joint. The molten materials fuse together to form the joint upon cooling. So, melting of P and Q occurs.
- (B) Brazing: This is a joining process where a filler metal with a melting point lower than the base materials is heated above its melting point and distributed between the two close-fitting surfaces by capillary action. The base materials (P and Q) are heated, but they do not melt. The joint is formed by the solidified filler metal. So, no melting of P and Q occurs.
- (C) Adhesive bonding: This process uses an adhesive (like epoxy or glue) to join two surfaces. The adhesive is applied, the surfaces are brought together, and the adhesive cures (hardens) through a chemical reaction, often at room temperature or with mild heating. There is no melting of the base materials P and Q.
- (D) Spot welding: This is a type of resistance welding, which is a fusion process. A large electrical current is passed through the workpieces where the join is to be made. The resistance to the current flow generates intense localized heat, causing the material at the interface to melt and form a small fused "nugget." So, melting of P and Q occurs.
Step 3: Final Answer:
The operations where the surfaces P and Q are not melted are Brazing and Adhesive bonding.
Step 4: Why This is Correct:
Brazing and adhesive bonding are fundamentally different from welding. Brazing relies on a molten filler metal with a lower melting point, and adhesive bonding is a chemical process. Both create a strong joint without melting the parent or base materials. Arc welding and spot welding are fusion processes that explicitly rely on melting and resolidifying the base materials.