Joining and Assembly Methods
Joining and Assembly Methods
The exam tests welding, soldering, brazing, adhesives and fasteners, and the reasons for choosing one method over another.
Core rules
- Welding melts the parent metal. MMA uses a flux-coated electrode; MIG uses a continuous wire and shielding gas; TIG uses a non-consumable tungsten electrode and usually a separate filler.
- Soldering joins below about 450 °C with a filler that melts and wets the joint; brazing is above 450 °C and relies on capillary action between closely fitting parts. Neither melts the parent metal.
- Adhesives need clean, roughened surfaces, correct gap and clamp time. They suit dissimilar materials and thin sheet but are weak in peel.
- Fasteners: bolts clamp parts; torque produces preload. Washers spread load; locking devices resist vibration. Use the correct thread and grade.
- Dissimilar metals can form a galvanic cell; insulate or choose compatible materials.
Worked example
A copper pipe in a domestic water line and a steel bracket are to be joined. The copper pipe is soldered: clean the pipe and fitting, apply flux, heat until the solder is drawn into the capillary gap, then cool and clean. The steel bracket is welded or bolted because it carries structural load. Soldering the bracket would not provide the required strength.
Traps
Using solder for a structural joint; brazing with excessive clearance so capillary action fails; under-tightening a bolt; mixing stainless and mild steel fasteners without isolation; failing to clean adhesive surfaces.