Brazing and soldering are methods of joining two or more metals together. The process involves using heat to melt a brazing rod that forms a filler metal, thus joining the pieces. Usually, the filler material has a lower melting point than the materials it is joining. However, not any filler is valid for use in the process; there are specific requirements that it must meet.
First, it's melting point should be lower than those of the pieces it joins. The filler material must also flow readily after melting and should be able to form a strong bond with the pieces. Besides, the main objective of joining the metals won't be complete if the bond is not stable and durable. To help you get a better understanding of what the process involves, we look at how soldering and brazing work in details.
Filler Metal Solder
Soldering is one of the most used methods of joining metals together. The process involves melting and flowing a filler metal into the joint, forming a bond. It is a better option since the bonding creates a permanent connection between the pieces. Tin (Sn) is the primary element in solder filler metals as it has some affinity for Copper. It also adheres to the Copper fitting and alloy tube. However, using a pure tin can result in a weak joint. To enhance its strength, you can add other elements to alloy with tin. Doing that doesn’t just help in forming a stronger alloy, it makes working with the filler metal easier.
Due to the recent prohibition of using 50/50 (Sn/Pb, Tin/Lead) solder, experts have been developing many new and stronger alloys. What makes them better is that they don’t have lead elements, which makes them suitable for use in various soldering applications. The alloys comprise a combination of tin and other elements such as bismuth, copper, silver, and nickel.
Most people use brazed joints to help them achieve more fatigue resistance or higher joint strength. To do that, use filler metals that are stronger than the ones that contain tin only. The increase in power comes from filler metals comprising of materials that can melt if you subject them to high temperatures. The two most common brazing filler metals for joining fittings and copper tubes are:
- BCuP Alloy- The B in this represents brazing, Cu stands for Copper while P is for Phosphorous. This alloy contains approximately 0-30% Silver.
- BAg Alloy- This alloy contains other elements too, but the most dominant one is Silver. The percentages of Silver in it ranges between 24% and 93%.
Strengths and Joint Requirements
Whether you are using soldering or brazing technologies, there are some necessary steps you must follow to get healthy joints. In both techniques, fully insert the tube into the fitting. You should melt the filler metal into the capillary space. Doing so enables the metal to flow completely and easily to the back of your fitting cup and fill the space between the fitting and tube.
Even though some people might prefer 100% fill and penetration of the capillary space fitting, a solder joint of about 70% fill is usually stronger. Such a joint can withstand maximum pressure for both fitting systems and copper tubes.
The significant difference that you might observe after using soldering and brazing technologies is in the amount of fill or joint overlap necessary to develop the strength of your joint fully. When using brazing technology, it is highly recommendable that you fully insert the tube to the back of your fitting cup. However, filling the joint space throughout the entire length is not a must to get a healthy joint.
Due to the increased strength in alloys made using brazing, even the smallest amount of penetration can result in a joint that is stronger and more fabricated than the fittings or tubes. Unlike in soldering where the fillet provides little additional strength, in brazing, the joint need fabrication to get a more developed cap or net between the fitting and tube. The fillet allows the stresses developed by thermal expansion to distribute along the face of the cap.
In brazing, fabricating a joint without an adequately developed filler, then the stress will concentrate at the sharpest point of contact between the fitting, filler metal (braze alloy) and tube. It would form a stress fracture inside the tube. To minimize the possibility of this happening, create a fillet when fabricating the joints.
The most significant difference between using soldering and brazing technologies is the level of temperatures they use. For soldering, the necessary temperatures are about 840 degrees. If they go below that then the process changes to brazing, which results in a stronger joint.
When building most of the things that use metals from computers to submarines, joining the metals is a crucial step. Brazing and soldering are some of the effective ways that you can use to join the pieces. You only need a source of heat and filler material that will fill the gap to form a bond between the metals.