Brazing Aluminium and Copper
2010
Brazing aluminium to copper is common in the refrigeration industry where copper tubes are brazed to aluminium roll-bond panels or tubes. To join aluminium and copper using brazing technology and standard NOCOLOK® Flux, flame brazing would be applicable (as well as using a low-melting flux with a low-melting filler metal). It is very similar to brazing aluminium to aluminium, but some precautions are necessary.
However, when copper is brazed to aluminium and the heating process takes too long, the copper will diffuse into the aluminium at the joints. A low melting Al-Cu alloy (Al-Cu33 eutectic temperature 548°C) is thus formatted, and this could lead to erosion by perforation.
Therefore, during the brazing process, the flame should never be directly applied to the joint, because the heat should be transferred by conduction through the parts to be brazed. As soon as the filler metal begins to melt, the flame must be quickly removed.
A second issue with brazing copper to aluminium is that the aluminium has a much lower melting point than copper (Al: app. 650°C; and Cu: above 1000°C). Therefore, the flame is usually directed on the copper. Nevertheless, once the heat transferred from the copper to the aluminium reaches the melting range of aluminium, it will start to burn down very fast, while the copper is still taking the heat. The formation of the above mentioned low melting Al-Cu alloy accelerates the destruction of the aluminium components.
Consequently, flame Brazing of aluminium to copper is a delicate process and requires some experience. But it is used by many companies for large scale production. But it is next to impossible in furnace brazing. There are no conventional furnace designs which will cool quickly enough to halt the continual formation of the aluminium-copper eutectic. For this reason, brazing copper to aluminium in a furnace is not practiced.
There are three different ways to provide (or generate) filler metal in 
flame brazing of aluminium to copper.
- Use of Al-Si filler alloy (Al-Si 12 – AA4047). Standard procedure like in flame brazing of aluminium to aluminium – just a little bit faster to avoid burn-through.
- Rely on the formation of Al-Cu alloy during the brazing cycle. If this method is used, a support provided by a thin Stainless Steel tube along the interior joint area can provide additional structural integrity.
- A pre-heated copper tube is inserted very fast into an aluminium tube. The mechanical energy released will generate additional heat. Abrasion of surface oxide by the inserted tube promotes the formation of Al-Cu filler alloy. This process works with and without flux (however, results are better with flux).
July 16th, 2010 at 15:18
Dear
we want to make joint between two Al tubes of dia 6.35 mm OD ant 0.5 thickness and second tube 2 mm od
Pls suggest suitable brazing rod for thesame.
July 19th, 2010 at 10:08
The first you need to do is form a capillary joint between the two tubes by crimping down the 6.35 tube to the 2mm tube. Parts should be clean and heated uniformly. Most common Brazing rod would 4047 and flux would be non-corrosive applied as a flux paste or combined with alloy in the form of fluxed cored rod. Brazed paste (alloy and flux) may also be used but joint design may not be suitable.
Zn/Al alloys are also availble used with a different type of non-corrosive flux and again can be used with flux paste or flux cored options.
Final selction depends on specific application details
September 14th, 2010 at 14:20
I revised the article and add some additional information.
September 23rd, 2010 at 11:41
We want to make joints ( Lap & Butt) between Al to copper , Al to brass strips of thicness 3.5 mm & width 12mm. Pl.suggest the filler metal, brazing rods for the same.& availability of the material in India.
September 24th, 2010 at 07:59
Zn/Al 78/22 alloy and cesium flux is the combination used to braze Al to Cu and Al to brass. Al to Cu is most common but Al to brass can be done but tensile strength not very high, but maybe enough depending on application. Difference of mass is important make sure joint is brought to temp evenly before applying wire.
Chemical composition of brass can impact on results, lead and sulfur are both coming to surface during heating and will effect action of the flux so wettability is not so good.
I would recomend you first have a go with some seperate pieces of material simulating joint dimensions and masses and check the joint quality.
October 14th, 2010 at 14:26
We want to make pipe joints between Al to copper through brazing with tube dia
Al Tube Dia 9.5 with Cu tube Dia 9.5
Al Tube Dia 12.5 with Cu tube Dia 12.5.
plz.suggest the filler metal, brazing rods for the same.& availability of the material in India
October 16th, 2010 at 10:14
Flame brazing with Zn/Al filler metals and Cesium flux is currently the most popular method to join Copper and Aluminium. There are several different alloy flux combinations available and selection isdependant on application details such as joint design, heating method etc.
If you forward your full contact details via my web site http://www.flamebrazing.com or e-mail on derekdavies@ukonline.co.uk then I can forward details of suppliers in India.
February 24th, 2011 at 15:42
Thanks for posting this. I have been trying to read up on information about aluminum brazing, aluminum bonding and soldering so this was very useful. I am looking to join copper and aluminum so any help is always appreciated.
February 25th, 2011 at 07:47
For joining copper with aluminum, Al/Si filler alloy /AA4047) and NOCOLOK® Flux is suitable. However, most companies use Zn/Al filler alloys (there are several alloy combinations commercially available – from 2 to 25% Al content). When Zn/Al filler alloy is used, the flux must be a low melting flux (i.e. Cs-Al-F flux as non-corrosive flux: CsAlF Complex). A low melting Zn/Al filler alloy with a low melting flux is often preferred, because the brazing window (i.e. the temperature between filler metal melting and melting of the base alloys is broader when compared to Al/Si filler alloy.
In flame brazing applications, pastes are the most popular choice. There are also filler alloy rings available which are coated or filled with flux (i.e. flux coated – resp. flux cored rings and wires).
Several companies are offering such products.
September 12th, 2011 at 04:32
WE WOULD LIKE TO JOIN (BRAZE) ALUMINIUM PIPE WITH COPPER T SIZE IS 25.4 MM DIA .PLEASE SUGGEST US THE PROCESS DETAILS AND THE FLUX MATERIAL +ROD ETC TO BE USED
September 13th, 2011 at 10:46
As mentioned on the previous replies, the most common alloy choice for flame brazing copper to aluminium is the Zn/Al range 98/02, 85/15 and 78/22.
Zn78/Al22 is increasingly becoming more popular due its melting range being simliar to the Cesium Complex flux.
These materials are available in separate solid wire and flux pastes and also flux cored wire.
As in all flame brazing applications it is important to heat the joint uniformly. This becomes more challenging as the diameter increases. For 25mm dia joint multiple gas/air flames would provide better heating and control than use of a single torch flame.
Specific application details such as, joint design, joint length, accessibility for heating equipment etc also need to be considered when developing the process.
regards
Derek Davies
derekdavies@flamebrazing.com