Welding, brazing, and soldering equipment is used to join two materials—often metals—together. The equipment itself may consist of a complete system or station, power source, gun or torch, or a monitor/controller.

The American Welding Society (AWS) defines welding as, "a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone and with or without the use of filler material." AWS distinguishes brazing and soldering as welding sub-operations, although they are usually considered completely different processes from welding.

The distinction between welding, brazing, and soldering depends primarily on process temperature and joining method.

Welding typically joins two workpieces by melting the pieces together; plastic welding using solvents is an exception to this rule. It may use various methods as described in the dedicated section below. Of the three processes, welding uses the highest process temperature and may or may not employ a filler alloy.

Brazing uses a filler alloy with a liquidus (the temperature above which it is completely liquid) above 450° C and below the solidus (the point below which a substance is purely solid) of the base materials. More simply, brazing joins two base pieces by melting a filler metal between them, and without melting them. Like welding, it may use one of several different heating methods.

Soldering is similar to brazing but differs in that the liquidus of the filler metal (solder) must be less than 450° C. Soldering may therefore be considered a type of low-temperature brazing.

Applications

Although welding, soldering, and brazing all typically join workpieces using heat, they are used in very different scenarios and applications. As implied in the table below, process selection factors to be considered include batch size (or need for automation), base thickness, base material, joint type, and overall assembly size.

Material Weldability

A material's weldability describes its capacity to be welded into a satisfactory structure. A base metal with good weldability results in a simpler fabrication process which yields welds with properties equal to or better than those of the metal itself. Materials with relatively poor weldability may require more complicated or expensive processes, and may require additional materials such as fluxes or special electrodes for satisfactory results.

When fabricating steels in particular, carbon content is of utmost importance. While the addition of carbon gives steels hardenability, it also causes them to become less ductile and brittle after heating and rapid cooling. The graph below establishes three zones related to carbon content and weldability. Steels within Zone I (the lowest portion of the graph below the horizontal black line) and Zone II have excellent and fair weldability, respectively, while those in Zone III are nearly unweldable without extensive heat treatment. Note that all steels with carbon concentrations of 0.1% or lower have excellent weldability regardless of high carbon equivalent.

Graph note: Carbon equivalent is a measure which considers both the percentage of carbon as well as weighted percentages of manganese, chromium, silicon, molybdenum, vanadium, copper, and nickel within an alloy.

The factors below influence the weldability of a metal. They should be carefully considered and, if possible, controlled.

• Chemical composition
• Effect of radical temperature change on alloy elements
• Expansion and contraction characteristics of base metal
• Filler metal
• Joint design
• Welding procedure

Joints

The success of welding and brazing depends on correct joints between base materials. Each process uses different joint types.

Welding and brazing may each use butt or lap joints, although butt types are more common in welding and lap joints are more common in brazing. Butt welds are used to join nearly parallel parts, and may involve beveling or flanging base part edges to simplify joint filling or compensate for warping. Lap joints overlap the two bases, resulting in a stronger joint (hence its popularity in brazing, which characteristically produces weaker joints than welding). The image below shows some common brazing joints, of which the butt and lap joints are also used in welding.

Welding joint types. Image credit: Denver Welding and Research

Welding and brazing may also employ unique joints beyond butt and lap types. For example, AWS recognizes corner, edge, and tee welding joints in AWS A3.0. An additional brazing joint is the scarf joint, which is similar to a butt joint but involves angled base edges to increase the bond area without increasing thickness.

Welding

Welding processes may use one of several technology types to generate the heat necessary for joining.

Arc welding comprises a number of different processes, all of which involve a consumable electrode to establish an electric arc. The arc melts the base metals at the welding point, effectively joining them. Arc welding requires a power supply, an electrode, an electrode holder or gun-style feeder, and sufficient cabling to connect both the electrode and the workpiece to the power source. This type also requires some type of protectant from oxidation caused by atmospheric contamination, which often takes the form of filler material (such as flux) or a noble gas.

Gas welding uses burning fuel gas (often acetylene) and oxygen to join metals. This gas combination allows for flame temperatures of up to 3,500° C, almost twice as hot as a propane/air flame. Equipment involved includes two tanks and two valves—one each for the fuel gas and oxygen—hoses, and a handheld torch.

Resistance welding pairs two corresponding electrodes which pass a current of up to 100,000 A through two metals at the point of coalescence. The joining action is achieved by the small pools of molten metal which form as a result of heat buildup due to resistance. These welding systems can be classified as spot welders or seam welders, using pointed and wheel-shaped electrodes, respectively.

Laser and electron beam welding are newer processes which use highly focused lasers or electron beams. These systems are capable of deep weld penetration over a minimal joining area, and are fast and easily automated.

Solid state welding does not involve the melting of the base materials. For example, ultrasonic welding joins workpieces by vibrating them together at high frequencies and high pressure. Similar processes include friction welding and magnetic pulse welding.

Three common welding setups (left to right): shielded metal arc (SAW), gas, and resistance.

The table below provides a comprehensive overview of the three common welding technologies shown above.

Brazing

Equipment used in brazing operations is typically classified by the process heating method. Unlike welding, all brazing uses a filler material, frequently copper/brass, silver, or aluminum-silicon, to join two bases together.

Torch brazing involves heating and flowing filler using a gas flame near the joint; it represents the most common type of brazing in current use. It requires the use of protective flux to prevent oxidation and may be done manually, semi-automatically (machine brazing using manual loading of consumables), or fully automatically.

Furnace brazing is an easily automated process which uses a furnace to provide heat. This type may be done in batches or continuously. The joint may be protected by flux or a tightly controlled atmospheric environment.

Silver brazing employs a silver-alloy-based filler to join hard metal components (such as saw blades) to tools.

Braze welding is a high-temperature hybrid process which uses a brass or bronze filler coated with flux. It is similar to gas welding, in that it involves an acetylene-fueled torch to achieve elevated temperatures.

Induction brazing uses induction heating to melt a filler material. Common induction brazes include silver, silver-copper, aluminum, and copper.

Video credit: Ron Walker / CC BY-SA 4.0

Soldering

Soldering is essentially low-temperature brazing which is performed with a soldering iron, a hand tool with a heated metal tip. The iron is placed near both the filler alloy and the parts to be joined in order to melt and flow the solder into the joint in a process known as wetting. The solder itself was historically lead-based, and tin-lead solders are still in widespread use. However, recent lead-free directives such as RoHS have sparked an interest in the use of tin, copper, indium, bismuth, silver, and zinc to replace lead in solders. Like brazing and welding, soldering typically involves the use of flux to prevent oxidation.

Solder joint involving a through-hole lead and a PCB.

Joints formed by soldering are reversible, meaning that used solder can be melted off a joint and the connection can be resoldered if necessary. Flux will remain but can easily be cleaned off using abrasives or chemicals.

The video below provides a comprehensive overview of soldering equipment. Parts 2 and 3 of this excellent video series cover the electronics soldering process.

Video credit: EEVblog / CC BY-SA 4.0

Standards

Welding, brazing, and soldering standards may address terminology, equipment, or operations used with these processes. A list of example standards is found below.

ISO 9692 -- Welding and allied processes (series)

AWS D1.1 -- Structural welding code

AWS B2.2 -- Specification for brazing procedure and performance qualification

AWS BRH -- Brazing handbook

AWS B2.3 -- Specification for soldering procedure and performance qualification

References

GH Group—Brazing overview

GoWelding—Welding calculations

Henning Johansen/Gjøvik University College—Welding processes

Image credits:

Henning Johansen/Gjøvik University College | ESAB Welding and Cutting Products | Denver Welding and Research | Integrated Publishing | Weld Guru | Fabricators & Manufacturers Association, Intl. | Mechanical Engineer Topics | Roadkill Customs