BRAZING
General
Brazing is a thermal method for permanently joining metallic parts. Brazing operations are characterized by the fact that the edges of the pieces to be joined are never brought to the liquid state. For this reason, the base metal must always have a higher melting point than the filler metal. Brazing is used to assemble hard to reach joints that are too difficult to weld. For thin pieces, brazing has a clear advantage since the thermal energy required for brazing is relatively low. Peak temperatures are therefore fairly low, thus keeping distortion and reduction of the piece’s mechanical properties to a minimum. Brazing is mainly used to join cast iron, steel, copper, nickel and their alloys.
Types of brazing
There are three types of brazing : soldering, brazing and braze welding.
In braze welding, the filler metal is melted then deposited inside a specially-shaped joint similar to the type used in preparation for conventional arc welding. The metal-to-metal joint is obtained through the wetting (”tinning”) action between the base and filler metals accompanied by a certain amount of diffusion between the atoms in the base metal and filler metal. Braze welding requires a filler metal with a melting point (liquidus) greater than 840°F (450°C).
Brazing is characterized by a filler metal melting point that is greater than 840°F (450°C), as in braze welding; the difference between the two is in the joint configuration and the binding mechanism. Filler metal in the liquid state infiltrates into the joint between the two pieces through capillary action.
Soldering, uses a filler metal with a melting point below 840°F (450°C). The binding mechanism and wetting (”tinning”) phenomenon are the same as in brazing.
Joining
Joint surface preparation
A clean, oxide-free surface is vital for brazing. All traces of oil, grease, dirt and oxide must be removed. Any of these items can negatively affect capillary action and prevent good filler metal adhesion. Surfaces can be cleaned mechanically or chemically.
Chemical cleaning is more effective in removing grease and oil. The nature of the contaminant will determine the chemical to be used to clean the pieces. However, some oxides and scale cannot be removed chemically; only mechanical means will remove such surface asperities. Milling, wire brushing or any type of machining are very effective mechanical means for surface cleaning and preparation.
Some brazing products come with a flux that de-oxidizes and strips the piece during heating. Nevertheless, it is advisable to clean the part lightly with a stainless steel wire brush to facilitate joining.
Spacing the parts to be joined
Capillary action plays a significant role in soldering and brazing. To facilitate capillary action, it is important to maintain a slight gap between the pieces to be joined. Too much of a gap will cause lack of metal inside the joint. In addition, shrinkage of the metal during solidification will be greater and could distort the assembly and produce residual stresses. A gap of 0.001 to 0.003 inch (0.03 to 0.08 mm) between the pieces is usually recommended. It is important to keep in mind that the joint will expand when heated and that it can close back up again. The following table shows the recommended gaps between pieces as a function of the type of metal and its coefficient of thermal expansion.
Sodel Product | Product Type | Joint Spacing |
Al – Si (a) | 0.006-0.024 inch (0.13-0.61 mm) | |
Mg | 0.004-0.010 inch (0.10-0.25 mm) | |
Cu -P | 0.001-0.005 inch (0.03-0.13 mm) | |
47, 66, 660 | Cu – Zn | 0.002-0.005 inch (0.05-0.13 mm) |
Ag | 0.002-0.005 inch (0.05-0.13 mm) | |
Sn – Ag | 0.003-0.008 inch (0.08-0.15 mm) |
If it is deeper, the gap should be 0.010-0.024 inch (0.25-0.61 mm)
Types of joints for brazing
It is important that assembly joints have as much surface area as possible for better mechanical strength. The following diagrams illustrate the preferred joint designs.
Controlling heat input
The dimensional aspects of the joint will directly determine the heat input needed for proper filler metal melting and fluidity. Less heat is needed for grooved joints, tee joints and lap joints than for butt joints or lap joints where the filler alloy must flow through capillary action.
There are two ways to apply alloys :
-by forming a bead through wetting (”tinning”);
-via capillary action.
The first technique (braze welding) is used for filling grooves or rebuilding missing sections and requires less heat than the second (soldering or brazing), which is used to assemble well-fitted joints. The low heat input improves working conditions and is better in terms of weldability. It prevents base metal melting and reduces risks of distorsion, warping and overheating.
Types of flames
Three types of flames are used with an oxyacetylene torch:
-neutral flame;
-reducing flame;
–oxidizing flame.
A neutral flame, also called a normal flame, is a flame whose oxygen supply equals its acetylene supply. This is the type of flame used most of the time for welding, brazing and heating. It represents perfect combustion since it is neither oxidizing nor reducing, and it protects against the effects of ambient air oxidation and the oxygen contained in the gas mixture. The inner flame is bright, white, well-defined and almost only visible through the welding goggles. Not all metals can be welded or brazed with a neutral flame. Some alloys require an oxidizing or reducing flame.
To obtain a reducing flame (also known as carburizing flame), which is colder, adjust the gas flow so that there is an excess of acetylene compared to a neutral flame. A halo will appear over the normal inner flame; it will sometimes be irregularly shaped and will lengthen as the acetylene is increased. It has a high carbon content. It is used for metals like aluminum, magnesium and cobalt because it is cooler and minimizes oxide layer formation due to its lower oxygen content.
The oxidizing flame is the hottest, it can be obtained by gently increasing the oxygen flow with respect to a neutral flame. This will shorten the inner flame and outer envelope. The inner flame will become duller, more bluish and pointed, and will emit a whistling sound, and the outer envelope will become brighter. This flame is primarily used with zinc-based alloys because the zinc oxidation that occurs during working prevents the zinc from evaporating. This application is most often encountered in braze welding of galvanized pipe or cladding.
Torch handling technique
The heat input must be controlled when brazing by holding the torch at a certain distance that varies according to the type of base metal. High-melting-point metals require the flame to be held closer to the piece to concentrate the heat. With a lower melting point metals, the flame must be held farther away to produce a lower heat. The following table shows typical spacing between the tip of the inner flame and the work piece.
Metal Melting Point | Distance between tip of flame and work piece |
500°F (260°C) | 2 inches (50 mm) |
1,000°F (540°C) | 1 inch (25 mm) |
1,300°F (700°C) | 1/2 inch (13 mm) |
1,800°F (980°C) | 1/4 inch (6 mm) |
It is most important to keep the torch constantly in motion in order not to overheat the piece. If heating is too localized, the section will not be completely heated to the ideal temperature, and this will cause lack of penetration and wetting problems. Begin by making circles with the torch; then, while the piece is heating, reduce the size of the circles with the torch; then, while the piece is heating, reduce the size of the circles slowly to concentrate the heat. Hold the torch and the filler metal at a 45° angle to the assembly piece so that there is a 90° angle between the filler metal and the torch. Deposit one drop of metal at a time, being careful to spread it out well each time before depositing a second drop. It is very important to keep the torch in constant motion during the entire operation.
If you are using powdered flux, you must mix it with water to make a paste before applying it to the piece. Paste fluxes must be applied to a clean piece prior to heating. If a bare filler metal is used, flux must be applied around the filler metal and on the assembly pieces. However, pure copper can be brazed without flux when using Sodel 473 and Sodel 474. The pieces should be clean even if you are using flux.
Filler metals
When assembling by brazing, several types of filler metals can be used to provide a joint of sufficient strength for the application in question. However, some alloys are more appropriate than others because of operational characteristics such as fluidity, melting point, corrosion resistance, color, etc. It is therefore very important to know the type of metal you wish to join in order to select the type of filler metal to use. To make the correct choice, you must know :
-the type of alloy to be joined;
-whether dissimilar metals are involved;
-the melting point of the base metal(s) and filler metal;
-alloy characteristics such as thermal conductivity, whether an oxide film forms on the alloy, etc;
-the thickness of the assembly pieces;
-the environment in which the alloy will be used; i.e., utilization temperature, corrosive potential of the products coming into contact with the pieces, mechanical loading (strength), etc.
The following table brazing 1 shows the compatibility of various base metals with specific filler metals. It is nevertheless important to evaluate the preceding items before making your selection.
PRACTICAL TIPS FOR BRAZING
1-Possibility of joining materials at a lower cost than conventional arc welding processes.
2-The equipment required for manual or automatic brazing is very simple. Brazing makes it possible to join materials with very different melting points.
3-Brazing produces less heat distortion and less stress than other fusion welding processes.
4-Brazing makes it possible to join dissimilar materials such as steel and carbides, as well as other composites.
5-Heating the piece uniformly will prevent distortion and warping caused by localized preheating.
6-With copper alloys, place the piece on insulating refractory materials to reduce heat losses through metal-to-metal contact.
7-For the best braze welding results, round off the chamfer angles slightly to allow the filler metal to take hold better.
8-A rough surface is better for anchoring purpose when braze welding.
9-Brazing is a fairly simple process to automate.
10-Brazing can alter the mechanical properties obtained through heat treatment. Annealed materials, on the other hand, are not affected by heating.
11-The width of the heat affected zone in brazing is wider and less well-defined than with another welding process, thus producing a less brittle joint.
12-When subsequent repairs are required, some brazed joints can be reheated and disassembled without damaging the part.
13-To join steel or copper to aluminum, the steel or copper piece must be buttered using a silver rod such as Sodel 472 or Sodel 475. Next, the aluminum piece is joined to the buttered part using Sodel 480 which is an aluminum alloy.
ADVANTAGES OF SILVER SOLDERING
1-Silver alloys provide excellent capillary action when joining some alloys that are hard to braze. Their fluidity, mechanical properties and corrosion resistance make them the rods of choice.
2-They have greater wetting and capillary action than bronze.
3-They have a lower melting point that reduces the width of the heat affected zone and minimizes distortion.
4-Alloys with high silver content have good electrical and thermal conductivity.
5-High silver content alloys are ideal for the food industry, hospital and other applications where hygiene is paramount.
6-The color deposit is comparable to that of stainless steel.
7-They allow you to join dissimilar metals.
8-They can braze dissimilar metals together.