Joining & Finishing Metals
Categories of Metal Joining
Metal components are joined either permanently (the joint is intended to be inseparable without destroying the parts) or temporarily (the joint can be disassembled and reassembled). Selecting the right joining method depends on the metals involved, the forces the joint must withstand, whether disassembly is needed for maintenance, and the required production speed.
Welding
Welding is the strongest permanent joining process for metals. It works by melting the parent (base) metal at the joint and, usually, adding a filler rod of similar composition. On cooling, the metals fuse into a single homogeneous joint. The three main arc-based welding processes are:
- MIG welding (Metal Inert Gas) — a consumable wire electrode is automatically fed through the welding gun while a shielding gas (pure argon is inert for true MIG; an argon/CO2 mix makes it MAG welding) protects the molten pool from atmospheric contamination. MIG is fast, easy to learn and suitable for steel and aluminium; widely used in automotive and general fabrication.
- TIG welding (Tungsten Inert Gas) — a non-consumable tungsten electrode creates the arc; filler rod is added separately by hand. TIG produces a very clean, precise weld bead with excellent control over heat input. It is used for thin sheet, stainless steel, aluminium and titanium where weld quality is critical (aerospace, food equipment, artistic metalwork). TIG is slower and requires greater skill than MIG.
- Arc welding (MMA — Manual Metal Arc) — a flux-coated electrode is used; the flux melts and forms a protective slag over the weld pool, eliminating the need for shielding gas. Simpler and cheaper equipment; suitable for outdoor use and for joining thick or rusty steel. The weld quality is generally lower than MIG or TIG and the slag must be chipped away after welding.
In welding, the base metal melts and fuses with a filler of the same composition — the joint becomes part of the parent material. In brazing and soldering, the base metal does NOT melt; only a lower-melting-point filler is melted and drawn into the joint by capillary action. Brazing uses filler melting above 450 °C; soldering uses filler melting below 450 °C.
Brazing and Soldering
Brazing uses a brass-based or silver-based filler alloy with a melting point above 450 °C (typically 700–900 °C). A flux is applied to prevent oxidation. The base metals are heated with an oxy-acetylene or propane torch until the filler melts and flows into the joint by capillary action. The joint is strong, neat and suitable for copper alloys and dissimilar metals. Brazed joints on copper pipe work and cycle frame lugs are common examples.
Soldering uses a tin-based filler alloy melting below 450 °C (soft solder melts at around 180–190 °C). It is used where low heat input is essential — electronic circuit boards (where high heat would destroy components), copper plumbing joints and jewellery. Flux is applied to clean the joint surfaces. Soldered joints are much weaker than brazed or welded joints and are not suitable where mechanical strength is the primary requirement. The critical difference from brazing is the lower temperature: in soldering the base metal remains entirely solid.
Riveting
Riveting is a permanent mechanical fastening method. A rivet is a short cylindrical pin with a preformed head. The plain (tail) end passes through pre-drilled holes in the parts being joined and is then deformed to form a second head, clamping the parts together. Riveting does not require heat and can join dissimilar metals. Pop rivets (blind rivets) can be set from one side only using a rivet gun; the mandrel is pulled through the rivet body, expanding the tail inside the joint, then snaps off. Pop rivets are widely used in aircraft skins, guttering and sheet metal enclosures. Traditional solid rivets (used in structural steelwork and older aircraft) require access to both sides and are stronger but slower to set.
Temporary Joining
Temporary joints can be disassembled without destroying either component. They are used wherever maintenance, adjustment or replacement of parts is required:
- Nuts and bolts — the bolt passes through clearance holes in both parts; a nut is tightened onto the threaded end. Can be locked against loosening by a lock nut, spring washer or thread-locking adhesive. Suitable for heavy-duty structural joints (machinery, motor vehicle components).
- Machine screws — similar to bolts but thread directly into a tapped (threaded) hole in one of the components, or into a threaded insert. Used in precision assemblies, electronics enclosures and consumer products where a clean appearance is needed (countersunk machine screws sit flush with the surface).
- Self-tapping screws — cut their own thread as they are driven into a pre-drilled pilot hole; used for joining thin sheet metal without the need for a separate nut.
The choice between nuts and bolts, machine screws and self-tapping screws depends on the forces involved, the thickness of the material and whether a nut or tapped hole is feasible. Nuts and bolts provide the highest clamping force and can be torqued to a precise specification, making them suitable for safety-critical structural joints such as engine mountings. Machine screws suit precision enclosures where a flush appearance matters and the wall thickness allows tapping. Self-tapping screws offer the fastest assembly in thin sheet metal because no nut or separate tapping operation is needed.
The three thermal joining processes are frequently confused. The key distinguishing variable is whether the parent metal melts. In welding (MIG, TIG, arc) the parent metal reaches its melting point and the filler is the same or similar alloy; the cooled joint is essentially continuous with the parent material. In brazing the parent does not melt (it is kept below its melting point); the filler (brass or silver alloy, above 450 °C) flows into the gap by capillary action and bonds metallurgically. In soldering the parent again does not melt; the filler (tin-based, below 450 °C) flows in by capillary action and bonds by adhesion and limited diffusion. Joint strength decreases in the sequence welding > brazing > soldering.
Temperature comparison: arc welding steel reaches approximately 3,500 °C at the arc; brazing with a propane torch might heat the joint to 750 °C; soldering a circuit board operates at around 250 °C. These temperature differences explain why soldering is safe for delicate electronic components but welding would destroy them instantly.
Surface Finishes and Corrosion Protection
Most ferrous metals corrode when exposed to moisture and oxygen; many non-ferrous metals also deteriorate in certain environments. Surface finishes serve two main purposes: protecting the metal from corrosion and improving its appearance. Common surface treatments include:
- Painting — the simplest and most versatile finish. A primer is applied first to improve adhesion and provide an initial corrosion barrier; topcoat paint provides colour and further protection. Breaks in the paint film expose the metal to corrosion, so surface preparation (cleaning, degreasing, sanding or shot-blasting) is critical for paint adhesion.
- Powder coating — dry polymer powder is electrostatically applied to the metal surface and then cured in an oven at 180–200 °C, fusing into a tough, uniform film. Provides better impact resistance, corrosion resistance and consistency of finish than liquid paint. Used on bicycle frames, garden furniture and electrical enclosures.
- Electroplating — a thin layer of a corrosion-resistant or decorative metal (chrome, nickel, zinc, gold) is deposited onto the base metal using an electrolytic process. Chromium plating on taps and car trim provides a bright, hard, corrosion-resistant surface; zinc electroplating (electro-galvanising) provides sacrificial protection to steel.
- Anodising — an electrochemical process specific to aluminium. The aluminium is made the anode in an electrolytic bath; this thickens the natural oxide layer on the surface, greatly improving corrosion and abrasion resistance. The porous anodised layer can be dyed in a wide range of colours before sealing. Used on bicycle components, laptop cases and sporting equipment.
- Galvanising — steel is dipped in molten zinc (hot-dip galvanising), coating it with a tough zinc layer. The zinc provides both a physical barrier and sacrificial protection: zinc is more reactive than iron, so it corrodes preferentially if the coating is damaged, protecting the underlying steel. Used on fence posts, crash barriers, steel road signs and structural steelwork.
- Dip coating (plastic dip coating) — the metal is preheated and dipped into a tank of powdered or liquid thermoplastic (often PVC or polyethylene). The plastic melts onto the surface and solidifies as a thick, flexible, grip-able coating. Used on tool handles, garden tools, dishwasher baskets and wire shelving. Provides excellent grip, insulation and corrosion protection.
- Polishing — progressive abrasion using finer and finer abrasives, finishing with a buffing compound, produces a mirror-like surface on metals such as stainless steel, aluminium and copper. Used for decorative products (trophies, jewellery, cookware). Polishing is a surface preparation, not a corrosion barrier — a polished surface must still be lacquered or otherwise protected if corrosion resistance is required.
Galvanising applies to steel and uses a zinc coating that provides sacrificial protection (the zinc corrodes first). Anodising applies only to aluminium and is an electrochemical process that thickens the existing oxide layer; it does not involve a separate metal coating. Confusing the two is a frequent exam error.