Brazing
Brazing is a metal-joining process in which two or more metal items are joined by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.
During the brazing process, the filler metal flows into the gap between close-fitting parts by capillary action. The filler metal is brought slightly above its melting temperature while protected by a suitable atmosphere, usually a flux. It then flows over the base metal and is then cooled to join the work pieces together.
Brazing differs from welding in that it does not involve melting the work pieces. In welding, the original metal pieces are fused together without additional filler metal.
Brazing differs from soldering through the use of a higher temperature and much more closely fitted parts. The principle of joining with filler metal is the same, but solder has a specific composition and lower melting point allowing work on delicate components such as electronics with minimal metallurgic reaction. The joints from soldering are weaker.
Brazing joins the same or different metals with considerable strength.
Process
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 joint without the need for secondary finishing. Additionally, dissimilar metals and non-metals 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. Welded joints must sometimes be ground flush, a costly secondary operation that brazing does not require because it produces a clean joint. 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.One of the main disadvantages is the lack of joint strength as compared to a welded joint due to the softer filler metals used. The strength of the brazed joint is likely to be less than that of the base but greater than the filler metal. Another disadvantage is that brazed joints can be damaged under high service temperatures. 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 from that of the base metal, creating an aesthetic disadvantage.
High-quality brazed joints require that parts be closely fitted with base metal surfaces exceptionally clean and free of oxides. In most cases, joint clearances of are recommended for the best capillary action and joint strength; in some brazing operations, however, it is not uncommon to have joint clearances around. Cleanliness of the brazing surfaces is also important, 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 important 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.
Another consideration 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, several factors influence the joint designer's temperature selection. The best temperature is usually selected to:
- Minimize braze temperature
- Minimize any heat effects on the assembly
- Minimize filler metal/base metal interaction
- Maximize the life of any fixtures or jigs used
Techniques
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:- Torch brazing
- Furnace brazing
- Induction brazing
- Dip brazing
- Resistance brazing
- Infrared brazing
- Blanket brazing
- Electron beam and laser brazing
- Braze welding
Torch brazing
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, it accounts for a majority of the brazing taking place. There are three main categories of torch brazing in use: manual, machine, and automatic torch brazing.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 whether 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. 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. Torch brazing of copper can be done without the use of flux if it is brazed with a torch using oxygen and hydrogen gas, rather than oxygen and other flammable gases.
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. 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 almost 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 the part preparation.
Furnace brazing
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. The process also offers the benefits of a controlled heat cycle 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.There are four main types of furnaces used in brazing operations: batch type; continuous; retort with controlled atmosphere; and vacuum.
A batch type furnace has relatively low initial equipment costs, and can heat each part load separately. It can be turned on and off at will, which reduces operating expenses when it's not in use. These furnaces are suited to medium to large volume production, and offer a large degree of flexibility in type of parts that can be brazed. 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. These furnaces are often conveyor fed, moving parts 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. Due to the high temperatures involved, the retort is 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 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, the 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 0.13 pascals to 0.00013 Pa or lower. Vacuum furnaces are most commonly batch-type, and they are suited to medium and high production volumes.