This article is about the metal joining process. For the cooking technique, see
braising.
Brazing is a metal-joining process whereby a filler metal or alloy is heated to melting temperature above 450 °C (840 °F) and distributed between two or more close-fitting parts by capillary action. The filler metal is brought slightly above its melting (liquidus) temperature while protected by a suitable atmosphere or flux. It then interacts with a thin layer of the base metal (known as wetting) and is then cooled rapidly to form a sealed joint. By definition the melting temperature of the braze alloy is lower (sometimes substantially) than the melting temperature of the materials being joined. Brazed joints are generally stronger than the individual filler metals used due to the geometry of the joint as well as to the metallurgical bonding that occurs.[1][2]
Fundamentals
In order to obtain high quality brazed joints, parts must be closely fitted and the base metals must be exceptionally clean and free of oxides. In most cases joint clearances of .03 to .08 mm (0.002 to 0.003 inch) are recommended for the best capillary action and joint strength [3]. However, in some brazing operations it is not uncommon to have joint clearances on the order of .61mm (.024 inch). Cleanliness of the brazing surfaces is also of vital importance as any contamination can cause poor wetting. The two main methods for cleaning parts prior to brazing are chemical cleaning and abrasive or mechanical cleaning. In the case of mechanical cleaning, it is of vital importance to maintain the proper surface roughness as wetting on a rough surface occurs much more readily than on a smooth surface of the same geometry.[3]
Another consideration that cannot be over-looked is the effect of temperature and time on the quality of brazed joints. As the temperature of the braze alloy is increased, the alloying and wetting action of the filler metal increases as well. In general, the brazing temperature selected must be above the melting point of the filler metal. However, there are several factors that influence the joint designer’s temperature selection. The best temperature is usually selected so as to (1) be the lowest possible braze temperature, (2) minimize any heat effects on the assembly, (3) keep filler metal/base metal interactions to a minimum, and (4) maximize the life of any fixtures or jigs used.[3] In some cases, a higher temperature may be selected to allow for other factors in the design (e.g. to allow use of a different filler metal, or to control metallurgical effects, or to sufficiently remove surface contamination). The effect of time on the brazed joint primarily affects the extent to which the aforementioned effects are present; however, in general most production processes are selected to minimize brazing time and the associated costs. This is not always the case, however, since in some non-production settings time and cost are secondary to other joint attributes (e.g. strength, appearance).
Flux
In the case of brazing operations not contained within an inert or reducing atmosphere environment (i.e a furnace), flux is required to prevent oxides from forming while the metal is heated. The flux also serves the purpose of cleaning any contamination left on the brazing surfaces. Flux can be applied in any number of forms including flux paste, powder or pre-made brazing pastes that combine flux with filler metal powder. Flux can also be applied using brazing rods with a coating of flux, or a flux core. In either case, the flux flows into the joint when applied to the heated joint and is displaced by the molten filler metal entering the joint. Excess flux should be removed when the cycle is completed as flux left in the joint can lead to corrosion, impede joint inspection and prevent further surface finishing operations. Fluxes are generally selected based on their performance on particular base metals. To be effective, the flux must be chemically compatible with both the base metal and the filler metal being used. As a general rule, longer brazing cycles should use less active fluxes than short brazing operations.[4]
Filler materials
A variety of alloys are used as filler metals for brazing depending on the intended use or application method. In general, braze alloys are made up of 3 or more metals to form an alloy with the desired properties. The filler metal for a particular application is chosen based on its ability to: wet the base metals, withstand the service conditions required, and melt at a lower temperature than the base metals or at a very specific temperature.
Braze alloy is generally available as rod, ribbon, powder, paste, cream, wire and preforms (such as stamped washers). (ref p 131-160) Depending on the application, the filler material can be pre-placed at the desired location or applied during the heating cycle. For manual brazing, wire and rod forms are generally used as they are the easiest to apply while heating. In the case of furnace brazing, alloy is usually placed beforehand since the process is usually highly automated.[5] Some of the more common types of filler metals used are:
Common techniques
Torch brazing
Torch brazing is by far the most common method of mechanized brazing in use. It is best used in small production volumes or in specialized operations and in some countries accounts for a majority of the brazing taking place. There are three main categories of torch brazing in use:[7]
Manual torch brazing is a procedure where the heat is applied using a gas flame placed on or near the joint being brazed. The torch can either be hand held or held in a fixed position depending on if the operation is completely manual or has some level of automation. Manual brazing is most commonly used on small production volumes or in applications where the part size or configuration makes other brazing methods impossible.[7] The main drawback is the high labor cost associated with the method as well as the operator skill required to obtain quality brazed joints. The use of flux or self fluxing material is required to prevent oxidation.
Machine torch brazing is commonly used where a repetitive braze operation is being carried out. This method is a mix of both automated and manual operations with an operator often placing brazes material, flux and jigging parts while the machine mechanism carries out the actual braze.[7] The advantage of this method is that it reduces the high labor and skill requirement of manual brazing. The use of flux is also required for this method as there is no protective atmosphere and it is best suited to small to medium production volumes.
Automatic torch brazing is a method that eliminates the need for manual labor in the brazing operation except for loading and unloading of the machine. The main advantages of this method are a high production rate, uniform braze quality and reduced operating cost. The equipment used is essentially the same as that used for Machine torch brazing, with the main difference being that the machinery replaces the operator in part preparation.[7]
Furnace brazing
Furnace brazing schematic
Furnace brazing is a semi-automatic process used widely in industrial brazing operations due to its adaptability to mass production and use of unskilled labor. There are many advantages of furnace brazing over other heating methods that make it ideal for mass production. One main advantage is the ease with which it can produce large numbers of small parts that are easily jigged or self locating.[8] The process also offers the benefits of a controlled heat cycle (allowing use of parts that might distort under localized heating) and no need for post braze cleaning. Common atmospheres used include inert, reducing or vacuum atmospheres all of which protect the part from oxidation. Some other advantages include: low unit cost when used in mass production, close temperature control and the ability to braze multiple joints at once. Furnaces are typically heated using either electric, gas or oil depending on the type of furnace and application. However, some of the disadvantages of this method include: high capital equipment cost, more difficult design considerations and high power consumption.[8]
There are 4 main types of furnace used in brazing operations: batch type, continuous, retort with controlled atmosphere, and vacuum.
Batch type furnaces have relatively low initial equipment costs and heat each part load separately. It is capable of being turned on and off at will which reduces operating expenses when not in use. These furnaces are well suited to medium to large volume production and offer a large degree of flexibility in type of parts that can be brazed.[8] Either controlled atmospheres or flux can be used to control oxidation and cleanliness of parts.
Continuous type furnaces are best suited to a steady flow of similar sized parts through the furnace.[8] These furnaces are often conveyor fed, allowing parts to be moved through the hot zone at a controlled speed. It is common to use either controlled atmosphere or pre-applied flux in continuous furnaces. In particular, these furnaces offer the benefit of very low manual labor requirements and so are best suited to large scale production operations.
Retort-type furnaces differ from other batch type furnaces in that they make use of a sealed lining called a ‘retort’. The retort is generally sealed with either a gasket or is welded shut and filled completely with the desired atmosphere and then heated externally by conventional heating elements.[8] Due to the high temperatures involved, the retort usually made of heat resistant alloys that resist oxidation. Retort furnaces are often either used in a batch or semi-continuous versions.
Vacuum furnaces is a relatively economical method of oxide prevention and is most often used to braze materials with very stable oxides (aluminum, titanium and zirconium) that cannot be brazed in atmosphere furnaces. Vacuum brazing is also used heavily with refractory materials and other exotic alloy combinations unsuited to atmosphere furnaces. Due to the absence of flux or a reducing atmosphere part cleanliness is critical when brazing in a vacuum. The three main types of vacuum furnace are: Single wall hot retort, double walled hot retort and cold wall retort. Typical vacuum levels for brazing range from pressures of 1.3 to .13 Pascals (10-2 to 10-3 Torr) to .00013 Pa (10-6 Torr) or lower.[8] Vacuum furnaces are most commonly batch type and are suited to medium and high production volumes.
Silver brazing
If silver alloy is used, brazing can be referred to as 'silver brazing'. These silver alloys consist of many different percentages of silver and other compounds such as copper, zinc and cadmium. Colloquially, the inaccurate terms "silver soldering" or "hard soldering" are used, to distinguish from the process of low temperature soldering that is done with solder having a melting point below 450 °C (842 °F), or, as traditionally defined in the United States, having a melting point below 800 °F (427 °C). Silver brazing is similar to soldering but higher temperatures are used and the filler metal has a significantly different composition and higher melting point than solder. Silver brazing requires a gap not greater than a couple hundred micrometres or a few mils for proper capillary action during joining of parts. (Soldering also uses capillary action to fill small spaces, although the need for small gap distances may be less critical than in brazing.) This often requires parts to be silver brazed to be machined to close tolerances.
Brazing is widely used in the tool industry to fasten hardmetal (carbide, ceramics, cermet, and similar) tips to tools such as saw blades. “Pretinning” is often done: the braze alloy is melted onto the hardmetal tip, which is placed next to the steel and remelted. Pretinning gets around the problem that hardmetals are hard to wet.
Brazed hardmetal joints are typically two to seven mils thick. The braze alloy joins the materials and compensates for the difference in their expansion rates. In addition it provides a cushion between the hard carbide tip and the hard steel which softens impact and prevents tip loss and damage, much as the suspension on a vehicle helps prevent damage to both the tires and the vehicle. Finally the braze alloy joins the other two materials to create a composite structure, much as layers of wood and glue create plywood.
The standard for braze joint strength in many industries is a joint that is stronger than either base material, so that when under stress, one or other of the base materials fails before the joint.
One special silver brazing method is called Pinbrazing or Pin Brazing. It has been developed especially for connecting cables to railway track or for cathodic protection installations.
The method uses a silver and flux containing brazing pin which is melted down in the eye of a cable lug. The equipment is normally powered from batteries.
Braze welding
In another similar usage, brazing is the use of a bronze or brass filler rod coated with flux together with an oxyacetylene torch to join pieces of steel. (For brazing small or thin pieces where the heat is lost slowly, a "swirljet" propane torch with propane or hotter burning MPS gas can be sufficient.) The American Welding Society prefers to use the term braze welding for this process, as capillary attraction is not involved, unlike the prior silver brazing example. Braze welding takes place at the melting temperature of the filler (e.g., 870 °C to 980 °C or 1600 °F to 1800 °F for bronze alloys) which is often considerably lower than the melting point of the base material (e.g., 1600 °C (2900 °F) for mild steel).
In Braze Welding or
Fillet Brazing, a bead of filler material reinforces the joint. A braze-welded tee joint is shown here.
Braze welding has many advantages over fusion welding. It allows you to join dissimilar metals, to minimize heat distortion, and to reduce extensive pre-heating. Another side effect of braze welding is the elimination of stored-up stresses that are often present in fusion welding. This is extremely important in the repair of large castings. The disadvantages are the loss of strength when subjected to high temperatures and the inability to withstand high stresses.
The equipment needed for braze welding is basically identical to the equipment used in brazing. Since braze welding usually requires more heat than brazing, an oxyacetylene or oxy-mapp torch is recommended.
‘Braze welding’ is also used to mean the joining of plated parts to another material. Carbide, cermet and ceramic tips are plated and then joined to steel to make tipped band saws. The plating acts as a braze alloy.
Cast iron "welding"
The "welding" of cast iron is usually a brazing operation, with a filler rod made chiefly of nickel being used although true welding with cast iron rods is also available.
Vacuum brazing
Vacuum brazing is a materials joining technique that offers significant advantages: extremely clean, superior, flux-free braze joints of high integrity and strength. The process can be expensive because it must be performed inside a vacuum chamber vessel. Temperature uniformity is maintained on the work piece when heating in a vacuum, greatly reducing residual stresses due to slow heating and cooling cycles. This, in turn, can significantly improve the thermal and mechanical properties of the material, thus providing unique heat treatment capabilities. One such capability is heat-treating or age-hardening the workpiece while performing a metal-joining process, all in a single furnace thermal cycle.
Vacuum brazing is often conducted in a furnace; this means that several joints can be made at once because the whole workpiece reaches the brazing temperature. The heat is transferred using radiation, as many other methods cannot be used in a vacuum.
Heating methods
There are many heating methods available to accomplish brazing operations. The most important factor in choosing a heating method is achieving efficient transfer of heat throughout the joint and doing so within the heat capacity of the individual base metals used. The geometry of the braze joint is also a crucial factor to consider, as is the rate and volume of production required. The easiest way to categorize brazing methods is to group them by heating method. Here are some of the most common:[2][9]
- Torch brazing
- Furnace brazing
- Induction brazing
- Dip brazing
- Resistance brazing
- Infrared brazing
- Blanket brazing
- Electron beam and laser brazing
- Braze welding
Advantages and disadvantages
Brazing has many advantages over other metal joining techniques such as welding. Since brazing does not melt the base metal of the joint, it allows much tighter control over tolerances and produces a clean join without the need for secondary finishing. Additionally, dissimilar metals and metals and non-metals (i.e. metalized ceramics) can be brazed. In general, brazing also produces less thermal distortion than welding due to the uniform heating of a brazed piece. Complex and multi-part assemblies can be brazed cost effectively. Another advantage is that the brazing can be coated or clad for protective purposes. Finally, brazing is easily adapted to mass production and it is easy to automate because the individual process parameters are less sensitive to variation.[10][11]
One of the main disadvantages is the lack of joint strength as compared to a welded joint due to the softer filler metals used.[2] The strength of the brazed joint is likely to be less than that of the base metal(s) but greater than the filler metal.[citation needed] Another disadvantage is that brazed joints can be damaged under high service temperatures.[2] Brazed joints require a high degree of base metal cleanliness when done in an industrial setting. Some brazing applications require the use of adequate fluxing agents to control cleanliness. The joint color is often different than that of the base metal, creating an aesthetic disadvantage.
See also
References
- ^ Brazing VW autobodies http://www.sme.org/cgi-bin/get-press.pl?&&20021547&WN&&SME
- ^ a b c d e Groover 2007, pp. 746-748.
- ^ a b c Scwartz 1987, pp. 20-24.
- ^ Scwartz 1987, pp. 271-279.
- ^ Scwartz 1987, pp. 131-160.
- ^ Scwartz 1987, pp. 163-185.
- ^ a b c d Scwartz 1987, pp. 189-198.
- ^ a b c d e f Scwartz 1987, pp. 199-222.
- ^ Scwartz 1987, pp. 24-37.
- ^ Scwartz 1987, p. 3.
- ^ Scwartz 1987, pp. 118-119.
Bibliography
- Groover, Mikell P. (2007), Fundamentals Of Modern Manufacturing: Materials Processes, And Systems (2nd ed.), John Wiley & Sons, ISBN 9788126512669 .
- Schwartz, Mel M. (1987), Brazing, ASM International, ISBN 9780871702463 .
Further reading
- M.J.Fletcher, “Vacuum Brazing”. Mills and Boon Limited: London, 1971.
- P.M.Roberts, "Industrial Brazing Practice" CRC Press, Boca Raton, Florida, 2004.
- Kent White, "Authentic Aluminum Gas Welding: Plus Brazing & Soldering." Publisher: TM Technologies, 2008.
External links
