Joining Metal

Choose a joint around its load, access and need for disassembly

Joining parts is Fabricating. First identify the metals, thicknesses, loading, service environment and whether the assembly must come apart. Then check access for the joining equipment and for inspection.

A welded frame can avoid a large casting tool, but fit-up, fixtures and inspection still cost money. A bolt-on cover allows service access, but needs clearance for the fasteners and tools. See Fasteners and Galvanic Corrosion for mechanical assembly.

This is a process-selection overview. For safety requirements, use Personal Protective Equipment, Fire Safety, Chemicals and Safety Data Sheets and Equipment Operating Procedure.

Welding joins the base metal; melting is not required in every process

Fusion welding melts base metal at the joint, with or without added filler metal. The pool solidifies to form the joint. The common arc, oxy-fuel and resistance spot processes below are fusion processes.

Solid-state welding forms a bond without bulk melting of the base metals. Friction welding is one example. A solid-state process can still heat and alter nearby metal; “not melted” does not mean “unchanged.”

The heat-affected zone is nearby base metal whose structure or properties change because of the thermal cycle, without itself becoming weld metal. Selecting a process includes checking those changes and distortion, not merely whether a bead looks smooth.

Sources for fusion welding, solid-state welding and the heat-affected zone

TWI describes arc fusion welding and solid-state welding without melting the base metal. Its heat-affected-zone explanation identifies thermal changes in unmelted base metal.

Compare the heat source, filler and access of five welding processes

Starting candidates for process selection

Process	How it works and where it fits	Trade-off
Oxy-fuel welding	A fuel-and-oxygen flame heats the joint; a separate filler rod may be added. Oxyacetylene can suit selected thin-steel work and repair.	Flexible equipment, but slower and with more broadly distributed heating than many production arc processes.
SMAW / stick	An arc melts a flux-coated consumable rod. The coating supplies protection for the weld pool. Common for steel repair and construction work.	Portable equipment and flexibility; rods need replacement and slag needs removal between appropriate stages.
GMAW (MIG/MAG)	An arc melts continuously fed wire under external shielding gas. Common for repeated steel or aluminum fabrication with suitable wire, gas and settings.	Continuous wire supports production and automation; shielding and equipment access must suit the job.
GTAW / TIG	An arc from a non-consumable tungsten electrode heats the joint under inert gas. Filler, when needed, is supplied separately. Useful for controlled joints in thin stainless steel and aluminum.	Good control of heat and filler, but often slower than wire-fed production welding.
Resistance spot welding	Opposed electrodes press overlapping sheets together and pass current through them, forming a local fused nugget. Common in repeated sheet assemblies.	Rapid repeat joints with suitable equipment and fixtures; conventional arrangements require access to both sides. Separate spots do not make a sealed seam.

Why MIG is a familiar name but GMAW is the broader term

MIG refers to metal inert gas welding. GMAW also covers processes using active shielding gases or mixtures, often called MAG. Gas and filler selection depend on the alloy and process; the familiar shop name is not a complete welding specification.

Sources for the five process mechanisms

TWI compares SMAW and GTAW arc processes and explains MIG/MAG wire feed and shielding, oxy-fuel fusion welding, and resistance spot welding. These are process explanations, not approved settings or operating procedures for a particular job.

Practice choosing a joining method

Brazing and soldering melt filler while the base parts stay solid

In brazing and soldering, a molten filler wets the solid base parts. In a suitable close-fitting joint, capillary action draws filler into the gap. The usual dividing temperature is 450°C: the American Welding Society describes brazing fillers as having a liquidus above 450°C and soldering fillers below it. Liquidus is the temperature above which the filler is fully liquid. This distinction concerns the filler, not the flame temperature or the machine's setting. In both processes the base parts remain solid. Neither is simply a cooler version of fusion welding.

Brazing can join suitable dissimilar metals or attach a carbide tip to a steel tool body. Soldering is common in electrical connections. Choose filler and joint design for compatibility, temperature and loads; a soldered electrical connection is not automatically suitable for a structural load.

Joint clearance, surface condition and heating method matter. Brazing can be manual or performed in production equipment such as a furnace. Mechanization changes throughput and tooling costs; the process name alone does not identify the cheapest route.

Sources for the filler-temperature distinction and capillary joint

The AWS Brazing and Soldering Manufacturers Committee defines the liquidus distinction and explains wetting and applications. TWI's vacuum-brazing overview describes filler flow by capillary action while the parent materials stay below their solidus, the temperature at which melting begins.