Brazing is one of three joining techniques in our manufacturing world that uses heat and a molten filler metal to create complex assemblies from simple starting pieces. The other two processes are welding and soldering.
When brazing, the base material being joined is not melted. Only the brazing filler metal (BFM) that is added to the joint is melted, and this molten BFM is then drawn into the joint by capillary action. Effective brazing requires heat, cleanliness of the parts being joined and, in the case of torch brazing by hand, special operator skills are needed.
Torch brazing, or flame brazing, as some people prefer to call it, involves the use of a non-oxidizing flame. The flame should wrap around the entire joint (if possible) in order to evenly heat the base metal in the entire joint area so that the heat in the base metal is able to melt a BFM that is touched to its surface rather than having the torch flame melt the BFM.
This process of torch brazing has proven to be useful in joining a wide range of base metals, including aluminum, copper, copper alloys and many other nonferrous metals, in addition to the many ferrous alloys on which it is commonly used. When this heating is done properly, the molten BFM can be drawn completely into and through the joint, the evidence for which should be a tiny meniscus (fillet) of BFM at both ends of the joint.
Figure 1 shows a typical example of a torch-brazing operation in which the torch flame is held at a convenient distance from the part so that the flame will wrap around the joint as much as possible and not just heat one small spot. Then, as the flame is moved slowly down the fitting, the molten BFM will also be drawn down through the fitting by capillary action because BFM likes to follow the heat and flow toward it.
A typical brazing torch might look like the one in Fig. 2 and generally consists of a metal mixer-body into which two different gases are fed. These two gases are typically oxygen (or perhaps just compressed air) and a combustible fuel gas (e.g., acetylene, propane, etc.). They enter the torch body through separate gas lines/hoses and are then blended before exiting out of the curved tip of the torch body, where that blended gas is then ignited and burned (Fig. 1). The gas flow rates are controlled by the two knobs on the mixer body.
Torch Brazing vs. Flame Brazing
Many people prefer to use the term “flame brazing” instead of “torch brazing” when they are describing this brazing technique. That’s OK. These two terms, referring to the same process, can be used interchangeably. The term “torch brazing” places emphasis on the tool being used (a torch), whereas the other term emphasizes the means of heating (a flame). Here in the U.S., we tend to emphasize the equipment (i.e., torch braze, furnace braze, etc.).
It should be noted that in some other countries the word “torch” is often used to refer to a flashlight. So, perhaps to avoid any confusion about the use of the word “torch,” they may choose to refer to this brazing process as flame brazing. In this article, I use the term torch brazing when referring to this joining technique.
Surfaces Must be Clean Before Brazing
Here is a rule of brazing: BFM will not flow over or bond to oil, dirt, greases or oxides. Any of those materials contaminating a surface can, and will, prevent proper brazing from happening. Do not depend on the torch heat to burn off the surface contaminants, and brazing flux will not clean off those contaminants.
Torch Not Used to Melt BFM
Too many torch brazers make the mistake of using the torch flame to directly heat and melt the BFM. This molten BFM then sits on the outside of the joint and, because the inside of the joint has not been heated sufficiently, the BFM may start to run down over the outside surfaces of the metal or merely form a cap on the outside of the joint when the torch heat is removed. These so-called “cold joints” do not properly seal the joint and may leak in service. Remember that the torch should not be used to melt the BFM but to heat the joint so that the heat inside the metal will melt the BFM.
A torch tip may have a single hole in it (Fig. 2) or perhaps several holes in its tip (Fig. 3). The dual-tipped torch shown in Fig. 3 uses multi-flame (“rosebud”) tips that allow much more even heating of all the joint surfaces of a component since the joint can now be heated from two sides at the same time.
Figures 4 and 5 show photos of unique torch tips that can literally apply torch flames completely around the periphery of the assembly being brazed to uniformly heat the entire joint up to brazing temperature.
Correct Torch-Brazing Sequence
It is very important that specific steps be taken to ensure that you will have good success when brazing with a torch. These steps include the following:
- The joint surfaces should fit together with a smooth slip-fit. It can be difficult to try to braze parts that have either a very sloppy (loose) fit or one that has such a tight press-fit that the BFM cannot get into the joint.
- The surfaces being joined must be thoroughly cleaned prior to being assembled.
- All surfaces should be coated with flux, which keeps clean surfaces clean and prevents those surfaces from oxidizing during heating.
- Proper torch tips should be used for the type of gases being used.
- The torch flame should be adjusted so that it is non-oxidizing.
- The torch flame should be used to heat the base metals to brazing temperature rather than merely trying to melt the BFM with the flame.
- Sufficient BFM should be fed into the joint during heating to completely fill it.
- The flux should be removed after brazing by thoroughly rinsing the joint surface with hot water.
- Visually inspect the joint to be sure the BFM has flowed around and through it and the joint meniscus is small and concave in shape.
Brazing with a hand-held torch can be very effective when done properly and when performed by someone who is well trained in the technique. Torch training is available and torch-certification courses exist to help people gain the expertise to conduct this process well. But please be aware that not everyone is suited to handling a torch or comfortable doing so. As described in the sidebar, it is very important to find the people in your shop who can be “torch-brazing athletes.” Focus your training on them. You’ll be glad you did.
Become a Torch-Brazing "Athlete"
Effective torch brazing – like professional sports – requires skill, dexterity, fearlessness, quickness in learning and the ability to effectively apply what has been learned. In sports, we see that the very skilled players are the ones out on the field or court, whereas the want-to-be players are sitting on the bench. The rest of us who may not have those skills are content to be spectators in the stands.
In a similar way, not everyone is capable of being a good, effective torch brazer. Treat torch brazing as a special skill that must be learned and should only be done by people who are very comfortable with holding and using a torch. They should be fearless (but respectful) of the hot flame only a few inches from their hands and be able to easily and quickly grasp how to manipulate the torch to effectively heat up a component assembly without overheating it, while at the same time feeding BFM into the joint with the other hand. A torch-brazing athlete should be able to judiciously and comfortably balance those tasks so that the molten BFM will be evenly drawn into the joint by capillary action and effectively fill the entire joint area being brazed.
Yes, it takes a torch-brazing athlete to do all this effectively. It should not, in my opinion, be done by someone who may have become “qualified” for the torch-brazing job strictly through seniority in their shop. Torch brazing is a unique skill, and those doing it should have become “qualified” only through actual training and thorough testing that verifies they are indeed expert and comfortable when performing that task.
Torch brazing allows effective localized heating and joining of metal components without the need for heating up the entire assembly. But when heating in localized areas, special skills are required to properly heat those areas uniformly (without distortion) so that good-looking, strong, permanent bonds are created between those metal components being joined. For people with the proper skills and with the proper training, torch brazing is a wonderfully effective joining process.