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Practical guide to torch brazing with brazing alloys in the form of wires or flux-cored rods.
This article is intended as a general guide to aluminium brazing practices. Hence the subject matter is of a general nature, not taking into account the various variables that can be encountered during work in specific applications.
In recent years, aluminium has taken the place of copper and brass, especially in the automotive industry, given that its low specific weight makes it much lighter than the above-mentioned metals and it has excellent corrosion resistance properties and high ductility.
In the automotive industry, products are generally made with automated processes using mainly brazing furnaces and automatic torch systems, while in the refrigeration and HVAC sector, manual torch brazing is the main process used, with the resulting need for brazers to also be skilled in brazing aluminium.
In addition, fairly frequently there is the need to repair brazed copper-to-aluminium or aluminium-to-aluminium joints.
Aluminium brazing requires specific skills and greater care since, unlike copper, the metal does not change color when heated. This makes it hard for the operator to determine when the base material has reached the right temperature for introducing the filler material.
The process we will be looking at in this article solves this issue since, during the heating stage, as the rod is brought close to the workpiece to be brazed, once you see the flux starting to flow and wetting the workpiece, it means that the alloy to be applied has reached its working temperature and it is thus time to introduce the filler material.
Another feature of this type of brazing is that aluminium, which typically comes in the form of an alloy, has a melting range that is very close to that of the filler materials that can be used, making torch brazing a rather delicate process. Hence, controlling brazing temperature becomes key to a successful brazing outcome.
In addition, it is important to bear in mind that aluminium brazing needs parts to be perfectly clean and free of grease, otherwise it becomes extremely difficult to produce the joint.
In aluminium-to-aluminium brazing, if the correct procedures are followed, you will get a quality result with the brazed joint proving stronger than with the base materials.
In the case of aluminium-to-copper brazing, it is worth taking into account that the finished braze will not be as strong as with aluminium-to-aluminium brazing.
Brazing and soldering alloys and fluxes for aluminium
When it comes to aluminium brazing and soldering, the market offers alloys designed specifically for this purpose. The Saldobrase catalogue, for example, includes products AlSi 12, AlZn98 and AlZn78 that have been designed to make the brazer’s job easier by virtue of their flux core, which also acts as a temperature indicator.
The products that can be used are:
AlSi 12 alloy flux cored ISO 17672: 2016 – Al 112
Melting range: 575°-585°
Alloy consisting of 12% Silicon and 88% aluminium.
With a melting temperature close to aluminium’s own melting point, it is hard to use, especially when brazing thin sections. This alloy consists almost entirely of aluminium meaning that, generally speaking, there is no risk of porosity or microporosity. While widely used in the automotive industry, alloys with lower melting temperatures are preferred in the refrigeration sector.
AlZn78 alloy flux cored
Melting range: 440°-470°
Alloy consisting of 78% Zinc and 22% aluminium.
The low melting temperature makes it ideal for brazing aluminium, even when dealing with thin sections. This type of alloy is a better choice than the AlZn98 alloy since, once you have finished heating the area, it solidifies immediately, unlike the AlZn98 alloy, which instead tends to boil and stay liquid for a few seconds after the heat source is removed. It can also be used to braze copper to aluminium.
ALZn98 alloy flux cored
Melting range: 380°-405°
Alloy consisting of 98% Zinc and 2% aluminium.
Ideal for soldering joints with tight clearances or extremely thin sections, or whenever a low melting temperature is a priority. Its low melting temperature makes it ideal for soldering even aluminium workpieces with thin sections. Given the high zinc percentage, special care must be taken not to overheat the soldering alloy as otherwise there is the risk of microporosity and resulting tiny leaks. It can also be used to solder copper to aluminium.
All three solutions have a non-corrosive flux inside: while the Alsi 12 alloy contains a Nocolok flux with a working range from 570° to 660°, the other two alloys use a special flux called CsAlF with a working range from 420° to 480°.
Special care must be taken with the base materials to be brazed as non-corrosive fluxes can be used with aluminium alloys with a Magnesium (Mg) content of no more than 0.9%.
The first step when adjusting the flame is to use a neutral flame setting that is not too aggressive on the materials.
Where possible, it is best that the flame not be highly concentrated, favouring a wider flame: using propane instead of acetylene might make the operation easier.
In addition, you must keep the inner cone away from the workpiece and bring the torch to the joint gradually, without rushing, especially when performing this type of brazing for the first time.
The design of a joint brazed using aluminium alloys generally requires more joint clearance than when brazing and soldering capillary joints, which have very tight clearances; while it is also advisable not to have too much lap between the components.
VIDEO: Aluminium-to-aluminium torch brazing technique
Once you have cleaned the parts to be brazed thoroughly, bring the flame close, being careful not to linger over a single point for too long, keeping the torch moving at all times. At the same time, move round the joint to be brazed, placing the rod on it every now and then to check whether the flux contained starts to flow, wetting the joint.
Once the flux starts to flow, deposit the filler metal, being extra careful not to aim the flame straight at the brazing alloy, instead melting it by conduction.
Once the alloy melts, move the flame around the joint so that the filler material follows the heat and fills the joint.
Use of preforms
Brazing processes can be automated with the aid of preformed products like rings/preforms, which are designed and produced based on the geometry of the joint to be produced.
Preforms can be made from cored wire or solid wire, applying the flux before doing the brazing.