Brazing is a permanent bonding technique, excellent for all kinds of metals, where welding is not useable because of the high working temperature and its limitation when joining different base metals. The brazing filler metal (BFM) penetrates the joint by capillary action and diffuse the base metal. During brazing the base metal is only heated slightly above the BFM melting temperature. This temperature is always below the melting point of the base metals. In most brazing procedures, fluxes are being used either separately or integrated to remove oxides from the surfaces, to enable the BFM to wet. The flux and BFM are sucked into the joint by capillary action, filling the entire joint. Depending on brazing process time, base metal alloy and the BFM’s melting range, different fluxes are available. Brazing has a big advantage when it comes to joining hidden joints, when there is a risk of deformation of the base metal, temperature sensitive joints and from environmental aspects.
There are three categories of brazing; Soldering, hard brazing and high temperature brazing.
Soldering is mostly used as sealing or creating permanent bonds with demands on conductivity. Soldering is being performed with a working temperature up to 450°C. Filler metals/solders being used are normally tin-alloyed with addition of copper, silver or lead and both with/without flux core. Typical applications are: Electronic components, cables, roofing components, electrical componets etc.
Includes all joints performed at a working temperature from 450°C to 1000°C. Brazing filler metals commonly used are silverbased, silver-copper-phosphorus, aluminum, or brass(sometimes called weld-brazing). When the right joint design is made the joint becomes as strong as the base metal, and proof of Hydrogen and Helium. Typical applications: (see page 28; Applications)
High temperature brazing
High temperature brazing includes all brazing performed over 1000°C. The high temperature brazing is done in controlled athmosphere in enclosed chambers where vacuum or protection gas prevents the parts from oxidizing so that no fluxes are normally not needed, and therefore requires no after treatment. A high temperature brazed joint will be stronger than the base metal. The filler metals are often copper and nickel based but special silver alloys without zink are also used. This type of brazing can join difficult to join materials like aluminium, titanium, zirkonium, superalloys and ceramics. Typical applications: Aerospace, automotive, turbines, medicinal equipment etc.
The fundametal 7 steps of brazing
1. SELECTION OF FILLER METAL AND FLUX
This step is determined by what type of base materials to be joined, process requirements and the final parts exposure to different media.
2. CLEARANCES AND FIT OF THE JOINT
Since the BFM penetrates the joint by capillary action its important to have the right clearances and joint design (see page24; Joint designs) so that the capillary action can work most effectively. If the gap is too tight the brazing filler metal cannot penetrate the joint and if the gap is too wide there will be no capillary action and the BFM will only plate the surfaces. Important also is to calculate the joints in brazing temperature and not room temperature. Different metals have different coefficient of thermal expansion. So if you join dissimilar metals in ex tubular designs this is most important to take this into consideration.
For the lap (lenght) of the bond area or the ”joint lenght” you are normally safe to use the thickness of the thinner base metal times three. So if the thinnest pipe in the assembly is 1mm the lap of the joint gap should be 3mm. If you choose a shorter lap you will get a weaker joint and if you choose a longer lap you will may waste brazing filler metal without gaining lap strenght. You may use 4-6 times on special applications.
3. CLEANING OF BASE MATERIALS
In order to get a good brazing result the base metal surfaces need to be clean from dirt, oil, scale and other residues. These can be removed chemically or mechanically. Oil and grease first needs to be removed with a suitable degreasing solvent and if there is rust and scale this needs to be removed by abrasive materials. Here we have optimal cleaning cloths for this operation. After cleaning operation you should braze as soon as possible in order to not allow any new oxides to form on the surface of the base metal.
Oxides form when heating the base materials combined with oxygen from the air. These oxides prevents us from wetting and bonding the surfaces proparly and to protect the surface against oxides we need to apply flux. Flux is used on all open athmosphere brazing operations. Besides from protecting the surface the flux is also used as a temperature indicator and to help the filler metal flow. Fluxes comes in following forms: Powder and paste, flux coating on rods/wires and flux inside rods/ wires. The latter is the most recent development in brazing technology. If you use too much flux you have a larger risk of flux inclusions inside the joint and if you use to little it may lead to not getting enough filler metal inside the joint. This is why the flux cored technology is developed to make life easier for the operators. (read more on page 28; Flux cored technology)
5. FIXTURING, ASSEMBLY
Next step is fixturing of your parts. Its important that they have a good alignment during the brazing cycle so that the clearances of the joint do not shift and the capillary action can work optimal. You also need to consider that no heat absorbing materials should have contact with the joint area. A good solution is to use ceramics or other low heat absorbing materials like inconel or non magnetic steels to avoid the heat from conducting away from the base metals. The easiest way is to use gravity to hold your parts together or add additional weight to hold the down.
6. HEATING METHOD – BRAZING TECHNIQUE
The most commonly used heating sources for brazed joints are: induction, torches, protection gas furnaces and vacuum furnaces. The different heating sources have different advantages depending on the quality of the joint. But the most common heating source is torch brazing. (See page 23; heating sources) The goal is to have a uniform brazing temperature on both parts of the assembly. Larger parts require more heat than thinner, smaller material so if you have one large and one small component you can put the heat on the large one and the small one will absorbe this heat. Overheating the part for too long may cause cracks and poors in the joint as well as scaling to remove after brazing. When your material is heated enough you add the filler metal and see it flow into the joint.
7. CLEANING FLUX RESIDUES
After brazing is finished you can cool of the parts in water, this will also make the flux residues crack and easily remove. You can also use a wet cloth. The importance of clening is due to following resons:
1. If you have an unsuccessfull joint you may have too much flux residues inside the joint and this need to be removed in order to inspect the result.
2. The residues will lead to corrosion if they are not removed and come in contact with water.
3. If you have painting or coating as next step the residues will prevent them from sticking