Even if you are new to welding, or have been at it for a while, you may have heard of the term ‘brazing’ or ‘braze welding,’ and may wondered what it is. There are different types of welding used for different things, and braze welding is one of these types.
What is braze welding?
Braze welding is a type of MIG/MAG welding where the large majority of the variables essential to the process are similar or identical to those of conventional MIG/MAG welding. However, in braze welding, the differences arise when it comes to the melting points of the filler wires.
In regular welding, this melting point is at a much higher temperature than it is in braze welding and closer to the melting point of the parent material. With brazing, this is not true.
With welding, the processes involved include melting the two metal workpieces before they are joined together. Brazing involves flowing a filler metal in at a much lower melting point than in other processes.
This is also different from soldering.
Differences between welding, brazing, and soldering
Soldering, brazing, and welding are all ways to join together two or more pieces of metals with other materials and are also used as methods to fill gaps in metal parts. However, despite the common purpose, the ways in which these processes are carried out differ from each other significantly.
When welding, the metal or thermoplastic parts that are to be joined together are supposed to be similar. That is, for example, copper cannot be welded to steel.
This is because welding uses high temperatures to melt the two pieces and weld them together simultaneously. Sometimes, filler metals are also used.
When this is done properly, the weld becomes as strong as the metal around it. However, this can easily go wrong if the process is not carried out properly.
If the two metals have melting points that are too far apart, the high heat can cause the metal’s properties to change, and this can result in a weaker weld. This is why the pieces to be welded together must be similar.
Brazing, on the other hand, does not require the two metals or thermoplastics to be melted together. Instead, it uses a filler metal, usually an alloy, that will bond to the two pieces and join them.
This filler metal will have a much lower melting point than the two metal pieces. Thus, since the two metals don’t have to be melted simultaneously, there is no risk of the metal’s properties being changed because of the high heat, and many dissimilar metals can also be joined together by brazing.
Brazing often uses flux, which is a liquid that promotes the wetting of the two metals and allows the filler metal to flow better over the two parts that are to be joined. Flux will also clean the parts of oxides and lets the filler bond more tightly to the metal parts.
Fluxes are also used in welding processes to clean surfaces. When carried out properly, brazed joints can be stronger than the two pieces that are being joined but usually not as strong as welded joints.
However, brazing does have minimal effects on the two metal parts on their own.
Soldering is another word that you may have heard in regard to welding. It is a low-temperature equivalent to brazing and takes place with fillers (known as solders) that have much lower melting points than those in brazing. Soldering can be done using various metals, such as gold, silver copper, etc.
The solder melts and bonds to the metal parts, causing them to join. However, because of the lower temperatures, it is not as strong as welded joints or brazed joints.
Solders were originally made only of lead, but due to the overall shift towards using environmentally friendly products, the industry is now looking for lead-free alternatives. Soldering also requires the use of flux just like in welding and brazing to clean the metal surfaces and to make it easier for the filler metal or solder to flow over the pieces that are to be joined.
Advantages of brazing
Strength of the joint
There are many significant advantages to brazing over other welding types. For one thing, brazed joints are strong joints.
In many cases, when a brazed joint is done right, it is either as strong or stronger than the filler metals used for joining.
Low temperatures
Brazing requires much lower temperatures than welding, which means that they are easier to reach, making the brazing process much simpler than welding. The lower heat also saves on costs and makes the process much more economical than regular welding.
Since high heat causes changes in the metal properties, brazing allows for the minimized danger of metal distortion or warping for the parent materials.
Metals are not melted
One of the most important advantages of brazing over other types of welding is that brazing does not require the two metals to be melted. Since the base metals are kept intact, they are able to retain most of their physical properties.
The integrity of the base metal is a characteristic of all brazed joints, including thin- and thick-section joints.
Ease of joining
Another main advantage of using brazing is how easy it is to join metals with dissimilar properties using flux or flux-coated alloys. Since the two metal workpieces don’t have to be melted for joining, it doesn’t matter what their melting points are.
This means that brazing steel and copper becomes as easy as joining steel to steel. Welding is significantly more difficult since it involves melting the two (or more) workpieces before they are joined together.
As mentioned earlier, this could lead to trouble in regard to the heat changing the properties of the metals, and in other cases, could also require much more expensive and sophisticated welding methods to be able to weld the two pieces together with minimal damage.
Brazing means that you can select any metals for your assembly depending on how well suited they are for the function, regardless of how different they are in their melting points, and know that you will be able to join them together with as much ease – possibly more – as conventional welding procedures.
Disadvantages of brazing
While brazing does offer a number of pros over fusion welding and other conventional processes, there are still some downsides to keep an eye out for.
Joint strength
Because the base metals are not being melted during the process, the joint that is formed is most likely not as strong as the base metals, though it is likely to be stronger than the filler metal used. This makes the weld joint less strong than those made through conventional fusion welding processes.
Joint damage
While joints can be considered strong on their own, they are easily damaged under high service temperatures. Brazed joints require a higher level of base-metal cleanliness in industrial settings, with flux being needed to clean the surface and the metals before the welding process takes place.
Color differences
Because the joint is made of the filler metal, the color and properties of the joint may be different from those of the base metals. While this is not a very large problem, in some cases, it is significantly noticeable and can cause aesthetic disadvantages.
Common brazing techniques
There are a number of brazing techniques available for different kinds of materials that are to be brazed together.
Torch brazing
Torch brazing is one of the most common types of brazing used today. It is used for smaller volumes of production, or for specialized operations, but in some countries, it accounts for a large majority of the brazing that takes place.
Torch brazing can be done using machines, manually or automatic torches.
Manual torch brazing
Manual torch brazing involves using heat from a gas flame that is placed near the brazing joint. This torch can be hand-held or fixed into position depending on the level of automation the process has.
Flux is required in manual torch brazing to reduce the amount of oxidation, though in some cases, like when brazing copper, this is unneeded.
Machine torch brazing
Machine torch brazing is often used where repeated torch brazing operations are being carried out. This is a mix of automated and manual operations with operators that will place the brazing material manually, while the machine does the actual brazing.
This method is advantageous in that it reduces the amount of labor required and the skill needed to do the actual brazing. There is no protective atmosphere, which means that flux is required as well.
Automatic torch brazing
Automatic torch brazing is a method that almost completely eliminates the need for manual labor, besides loading and unloading materials. This method implies a high production rate and better braze quality due to the uniformity of machine work and a reduced operating cost.
The machinery used is the same as that for machine torch brazing, but machinery also replaces the operator here.
Furnace brazing
Furnace brazing is another process that is mostly semi-automatic and is used widely for industrial brazing because of the fact that it can be used for mass production and does not require a lot of skilled labor. Furnace brazing has some advantages over other types.
For example, it can produce large quantities of small parts that are easily self-locating. There is also a controlled heat cycle and does not involve any post-braze cleaning.
There are four main types of furnaces used in brazing:
Batch type
Batch type furnaces have low equipment costs and are used to heat each part separately. It can be turned on and off whenever needed, which reduces operating costs when the furnace is not in use.
This is good for a medium to large volume production requirement and has advantages in that it has a greater degree of flexibility in terms of what parts can be brazed. Oxidation and cleanliness are controlled by the use of flux.
Continuous type
Continuous type furnaces are used for a steady flow of similar parts. These are often used with conveyors that carry materials through the hot zone at a speed that can be controlled.
These are often used with pre-applied flux or controlled atmospheres to manage oxidation levels and cleanliness. Because of the low requirement for manual labor, this type is often associated with large-scale production.
Retort type
Retort type furnaces use a sealed lining called a ‘retort,’ which makes them stand out from batch-type furnaces. A retort is filled with the desired atmosphere and welded shut, after which it is heated externally through conventional heating methods.
Because high temperatures are involved, retorts are usually heat resistant and are used in semi-continuous versions or batches.
Vacuum type
Vacuum furnaces are an economical way of preventing oxidation and are used for materials that have very stable oxides that are not capable of being brazed in furnaces with open atmospheres. Vacuum brazing is also used with refractory materials and exotic alloy combos that are unsuited to atmospheric furnaces.
There is no flux or reducing atmosphere in these furnaces, which makes cleanliness extremely important. Vacuum furnaces are most commonly used in batches and are suitable for medium to high production levels.
Silver brazing
Silver brazing is sometimes known as hard soldering and involves using a silver alloy filler metal for the brazing process. These silver alloys can have different percentages of various other metals.
Silver brazing is used for fastening ‘hard metal’ tips to tools like saw blades, for the tool industry. The braze alloy is melted onto the hard metal tip and placed next to the steel, which is then re-melted.
This is called ‘pre-tinning’ and is used to solve the problem of hard metals being difficult to wet, which would make the flow of the filler slow down. A special kind of silver brazing technique is called pin brazing and is used to connect cables to railway tracks.
Cast iron welding
Cast iron welding is actually a kind of brazing operation. The filler rod being used is made of nickel, although it can also be carried out through conventional welding processes by using cast-iron rods.
Cast iron welding is used to use electricity along the copper filler to keep the underground pipes warm in colder areas.
Vacuum brazing
Vacuum brazing is a joining technique that has a number of significant advantages. It is a process that is extremely clean and flux-free and has joints of the highest levels of strength and integrity.
Of course, because of the complexities with which it is carried out, the process can be expensive – that is, it is performed inside a vacuum chamber vessel.
Being inside a vacuum means that the uniformity of the temperature is maintained, and this reduces a lot of the problems that are otherwise associated with welding and brazing, such as residual stress because of slow heating and cooling.
Because of this, the thermal and mechanical properties of the workpieces are improved, and unique heat treatments are made possible. Because vacuum brazing is carried out in a furnace, several joints can be made at once because the entire furnace will reach the required temperature.
The heat is transferred through radiation since the existence of a vacuum means that many other methods cannot be used.
Dip brazing
Dip brazing is used for brazing aluminum because the air is excluded from the process, which means that oxides are less likely to be formed. The parts that are to be joined are fixtured, and the brazing compound is applied to the surface in a slurry form.
This assembly is then dipped into molten salt baths that act as a flux as well as a medium for the transfer of heat. This is a common method used in the aerospace industry.
Brazing is a common method used for joining many different kinds of materials that couldn’t be joined any other way and, thus, an important part of welding procedures.
Summary
There you have it. The complete guide on braze welding.
Who would have thought there could be so much to cover? Now you have everything you should need to get started with brazing, but it doesn’t hurt to brush up your skills with some of the best welding books available.