Joining & Finishing — Permanent and Temporary Fixings
Why Joining Matters
Very few engineering products are made from a single piece of material. Almost every product — from a bicycle frame to a circuit board — is an assembly of parts that must be joined together. The method of joining has a direct effect on the strength of the assembly, how easy it is to take apart for maintenance, and what materials can be joined. Choosing the wrong joining method can result in a product that fails in service or cannot be repaired.
Permanent and Temporary Fixings
Permanent fixings are used when the joint will not need to be dismantled. Welding, brazing and riveting all produce permanent joints. A riveted aircraft fuselage panel is not intended to be removed — the rivet must be drilled out if the panel needs replacing, which damages the rivet and sometimes the surrounding material.
Temporary fixings allow parts to be dismantled and reassembled. Nuts and bolts are the most common example: tightening the nut clamps the parts together; loosening it allows disassembly. Screws, machine screws and set screws also provide temporary fixings. These are essential in products that need regular maintenance — a car engine must be taken apart for servicing, so its major components are bolted together, not welded.
The key question when choosing between permanent and temporary fixings is: “Will this joint ever need to be taken apart?” If the answer is no, a permanent method may be stronger and more reliable. If maintenance or replacement is expected, a temporary fixing preserves the ability to disassemble without damage.
Soldering
Soldering uses a low-melting-point filler alloy to join metal parts. The key characteristic of soldering is that the filler melts below 450°C and the parent metals do not melt at all — they remain solid throughout. A soldering iron heats the joint and melts the solder, which flows into the gap by capillary action and solidifies as a permanent joint on cooling. Soldering is used widely in electronics to join components to printed circuit boards, and in plumbing for copper pipe joints.
A flux is applied to the joint area before soldering. Flux prevents oxidation of the metal surfaces during heating, which would prevent the solder from bonding. Without flux, the solder will bead up on an oxidised surface rather than flowing into the joint.
Brazing
Brazing is similar in principle to soldering, but the filler alloy has a melting point above 450°C. This higher temperature produces a stronger joint than soldering. In a school workshop, brazing is carried out on a brazing hearth. The parts are heated by a gas torch until the parent metal is hot enough to melt the brazing rod when it is touched to the joint. The filler flows by capillary action into the gap. Flux is also required in brazing to prevent oxidation.
Because brazing temperatures are higher than soldering temperatures, it is important that the parent metals have melting points well above the brazing filler — otherwise the parent metal itself would melt. For this reason, brazing is more suitable for steel and copper components than for aluminium, which has a lower melting point.
Soldering: Filler melts below 450°C. Parent metals stay solid. Used for electronics and light copper plumbing. Produces the weakest joint of the three.
Brazing: Filler melts above 450°C. Parent metals stay solid. Produces a stronger joint than soldering. Used for steel and copper fabrication in a workshop setting. Requires a gas torch and brazing hearth.
Welding: The parent metals themselves are melted together (with or without a filler rod). Produces the strongest joint because the parent metals fuse. MIG welding (Metal Inert Gas) uses a continuously fed wire electrode and a shielding gas to protect the weld pool. Welding requires more specialist equipment and produces more intense heat, UV radiation and fumes than brazing or soldering.
Exam trap: students often state that brazing “melts the metals together” like welding. This is wrong — in brazing the parent metals do not melt. The filler melts and bonds to the solid parent metal surfaces.
Heat Treatment
Heat treatment changes the properties of a metal by carefully controlling heating and cooling. Three common heat-treatment processes are:
- Annealing — the metal is heated to a high temperature and then allowed to cool very slowly (often left inside the furnace or buried in sand). This softens the metal and removes internal stresses, making it more ductile and easier to work. Copper work-hardens as it is hammered; annealing restores its softness.
- Hardening — steel is heated until it reaches the correct temperature range (the metal glows a specific colour) and then quenched (plunged rapidly into water or oil). This rapid cooling ‘traps’ the structure of the steel in a hard but brittle state. Hardening is used for cutting tools that need to resist wear.
- Tempering — hardened steel is often too brittle for practical use. Tempering reheats the hardened steel to a lower temperature and then allows it to cool in air. This reduces brittleness while keeping much of the hardness gained during hardening. Tool steel is hardened and then tempered to give the correct balance of hardness and toughness.
Finishing processes protect the surface of a material or improve its appearance. Examples include painting, plating and polishing.
Painting: A coating of paint applied to protect against corrosion and to provide colour. Steel must be painted or coated to prevent rusting in damp conditions. The surface must be clean and free from oil before painting, otherwise the paint will not adhere properly.
Plating: A thin layer of one metal is deposited onto the surface of another. Electroplating uses an electric current to deposit the plating metal; hot-dip galvanising coats steel with zinc by dipping it in molten zinc. Zinc plating (galvanising) is widely used on steel components that will be used outdoors because zinc corrodes preferentially to steel, protecting the steel underneath.
Polishing: Abrasive polishing smooths and brightens a metal surface. It may be functional (removing surface roughness to reduce friction) or cosmetic (improving appearance). A buffer or polisher machine uses a rotating abrasive pad or cloth wheel to achieve a high-gloss finish.
Chemical treatment (etching): Chemical etching is used in electronics manufacturing to remove unwanted copper from a PCB (printed circuit board), leaving only the required circuit tracks. It is also used to create a key (rough surface) on metals before painting.
Hardening and tempering are always done in sequence: harden first (heat and quench), then temper (reheat to a lower temperature and air-cool). Tempering without prior hardening does not give the desired result. The purpose of tempering is to reduce the brittleness introduced by rapid quenching.