Ch 33

Filler metal

A concern with brazed joints is their enhanced susceptibility to corrosion. Because the filler metal is of different composition from the materials being joined, the brazed joint is actually a localized galvanic corrosion cell.

Fluxless brazing

A flux is not required. Controlled furnace atmospheres can make a flux unnecessary by reducing existing oxides and preventing the formation of new ones

Vapor-phase soldering

A product with prepositioned solder is passed through a chamber containing hot, saturated vapors, which condense on the cooler product, transferring the heat of vaporization

Assist gas

A stream of assist gas blows the molten metal through the cut, cools the workpiece, minimizes the heat-affected zone, and may participate in a combustion reaction with the material being cut.

Lead-free solders

At present, none of the lead-free solders meet all of these requirements, and most are deficient in more than one area

Hardfacing

Because the deposited surfaces are generally harder than the base metal, the process is often called hardfacing

Vapor-phase reflow soldering

Because the solder is prepositioned, this process is also known as vapor-phase reflow soldering

Torch-brazing

Common source of heat for brazing is the gas-flame torch. In the torch-brazing procedure, oxyacetylene, oxyhydrogen, or another gas-flame combination can be used.

Metallizing

If a nontransferred arc is used, only a mechanical bond is produced, and the process becomes a form of metallizing

Furnace brazing

If the flux and the filler metal can be preloaded into the joints and the part can endure uniform heating, a number of assemblies can be brazed simultaneously in controlled atmosphere or vacuum furnaces, a process known as furnace brazing

Capillary action

In brazing, bonding requires capillary action to distribute the filler metal between the closely fitting surfaces of the joint.

Autogenous welds

Laser-beam welding is most effective for simple fusion welds without filler material, called autogenous welds.

Hybrid processes

Lasers have also been used in hybrid processes that combine laser welding with arc welding, with both operating in one process zone and producing one weld pool.

Molding plates

Liquids are confined in electroslag welding to the region between the materials being joined by means of sliding water-cooled molding plates that are usually made of copper.

Joint clearance

Of all the factors contributing to joint strength, joint clearance is the most important. There must be sufficient clearance for the braze metal to wet the joint and flow into it under the force of capillary action.

Endothermic cutting

See exothermic cutting

Fluxless soldering

Several fluxless soldering techniques have been developed using controlled atmospheres, thermomechanical surface activation, or protective coatings that prevent oxide formation and enhance wetting

Corrosive or noncorrosive, rosin

Soldering fluxes are generally classified as corrosive or noncorrosive. The most common noncorrosive flux is rosin (the residue after distilling turpentine) dissolved in alcohol

Laser-beam welding

The beam of coherent light can be focused to a very small diameter, providing a huge power density. The high-intensity beam can be used to simply melt the material at the joint, but more often, it produces a very narrow column of vaporized metal ( a "keyhole") with a surrounding liquid pool.

Electron-beam welding (EBW)

The metal to be welded is heated by the impingement of a beam of high-velocity electrons

Salt-bath brazing

Third type of heating is salt-bath brazing, where the parts are preheated and then dipped into a bath of molten salt that is maintained at a temperature slightly above the melting point of the brazing material

Soldering

a brazing-type operation where the filler metal has a melting temperature below 450 C

Fluidity

a measure of the flow characteristics of the molten braze metal and is a function of the metal, its temperature, surface cleanliness, and clearance.

Thermit

a mechanical mixture of about one part (by weight) finely divided aluminum and three parts iron oxide plus possible alloy additions

Flash welding

a process used to produce butt welds between similar or dissimilar metals in solid or tubular form

Wave soldering

a process used to solder wire ends, such as the multiple connectors that protrude through holes in electronic circuit boards

Wettability

a strong function of the surface tensions between the braze metal and the base alloy. Generally, the wettability is good when the surfaces are clean and the two metals can form intermediate diffused alloys.

Electroslag welding

a very effective process for welding thick sections of steel plate. There is no arc involved (except to start the weld)

Silver solders

alloys based on silver and copper, have brazing temperatures significantly below that of pure copper and are used in joining steels, copper, brass, and nickel.

Upset welding

also similar to flash welding, but there is no period of arcing. The heating is achieved through electrical resistance.

Thermit welding

an extremely old process in which superheated molten metal and slag are produced from an exothermic chemical reaction between a metal oxide and a metallic reducing agent

Percussion welding

as similar process to flash welding, in which a rapid discharge of stored energy produces a brief period of arcing, which is followed by the rapid application of force to expel the molten metal and produce the joint. In percussion welding, the duration of the arc is 1 to 10 ms.

Laser-beam cutting

begins by "drilling" a hole through the material and then moving the beam along a programmed path.

Laser spot welding

can be performed with access to only one side of the joint. It is a noncontact process and produces no indentations

Resistance brazing

can be used to produce relatively simple joints in metals with high electrical conductivity

Lap joints

can provide bonding areas that are considerably larger than butt configuration. They are often preferred when maximum strength is required.

Braze welding

differs from straight brazing in that capillary action is not required to distribute the filler material

Butt joints

do not require additional thickness in the vicinity of the joint and are most often used where the strength requirements are not that critical.

Kerf

narrower than with any other thermal cutting process

Thermal spray

offer a means of applying a coating of high-performance material to more economical and more easily fabricated base metals

Brazing fluxes, flux

play an important part in the process by: (1) dissolving oxides (2) preventing the formation of new oxides (3) lowering the surface tension between the molten brazing metal and the surface to be joined. Ideally, the flux will melt and become active at a temperature below the solidus of the filler metal, yet remain active throughout the entire range of temperatures encountered while making the braze

Exothermic cutting

produces an oxidized edges, while endothermic cutting (also called clean cutting) results in oxide-free surfaces

Dip brazing

the assemblies are immersed in a bath of molten brazing metal

Brazing

the permanent joining of similar or dissimilar metals or ceramics through the use of heat and a filler metal whose melting temperature is above 450C but below the melting point of the materials being joined.

Surface of thermal cladding

the process of depositing a layer of weld metal on the surface or edge of a different composition base material

Jigs and fixtures

used to hold the components during the heating

Induction brazing

utilizes high-frequency induction currents as the source of heat and is therefore limited to the joining of electrically conductive materials

Dip soldering

where the entire piece is immersed in molten metal, has been used to produce automobile radiators and "tinned" coatings.