aluminum brazing presentation

8/10/2019 Aluminum Brazing Presentation

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Aluminum Radiator Manufacturing

Since aluminum is a cheaper metal than copper ( Al LME 1750 copper LME 7000 usd ) there has been a

shift to aluminum as the preferred metal for manufacturing of automotive heat exchangers including

Radiators, Charge air coolers, oil coolers, condensers and evaporators.

Not only is aluminum cheaper but is also lighter almost 30% ( 2.7gm/cc against 8.96 gm /cc ) of the

weight of copper. This also drives the cost down. But the initial investment cost in machines for

aluminum radiators manufacturing is much higher offsetting some of the savings.

Aluminum as a metal has a less thermal conductivity compared to copper ( 118 against 223 Btu/hr F ft )

but the solder ( lead ) used in copper radiators acts a barrier for heat transfer. Thus aluminum radiators

tend to dissipate almost the same amount of heat if not more compared to copper radiators.

Aluminum is a difficult metal to braze

a.

Aluminum oxide layer. Aluminum is an active metal and it reacts with oxygen in the air toproduce thin hard film of aluminum oxide which needs to be removed for proper brazing or

welding.

b.

Before melting the filler metal, this oxide film must be destroyed to allow the free flow of

molten filler metal.

c.

This in Controlled Atmosphere Brazing ( CAB ) process is done by a flux ( NOCOLOK ) and a

Nitrogen atmosphere prevents the reformation of the oxide layer. There are strict controls on

the amount of oxygen that can be allowed in the brazing furnace atmosphere.

d.

The brazing temperatures are higher than copper brass radiators. The core aluminum alloy

would melt at 640 C whereas the clad ( filler metal ) will melt at 577 to 613 deg c. Since the filler

metal melting range is too close to the solidus temperature of the base metal, the brazing

furnace has to be have a very good control on the temperature (within 3 deg C ) This requires

that the brazing heat/time profiles are properly set for different mass of radiators and then

stored by the computer on the brazing furnace. Also too fast a heating and the flux evaporates

without forming a joint.

e.

Therefore the brazing furnace temperature needs to be well controlled so as to melt the clad

but not melt the core. Uniformity of temperatures on the product is very important other wise

some parts of the product will get brazed while some other parts might melt.

f.

The fits between the parts are also critical for the flow of the filler metal. Too tight a fit or too

loose a fit both result into non brazing. This requires that the height of the fin is more

accurately controlled usually within 0.02mm. Also the header slots for the tubes need to be

accurately punched within 0.02mm. The challenge is that these fit ups must be consistently

maintained.


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g.

The application of the flux has to be right. Some areas like tube to fin joint need less flux where

as the tube to header joint needs more flux.

Due to the above reasons machines used in Aluminum radiators are much more complex than

those used for copper brass radiators.

1.

The Brazing furnace is much bigger and requires much more controls. Also it has systems

for generating the Controlled atmosphere using Nitrogen. Hence a continuous aluminum

brazing furnace is a very large equipment and can be as long as 20 meter including

equipment like Thermal degreaser, fluxer, and Dryoff oven. Compared to this the copper

brass oven is 6m in length.

2.

The fin machines need to be able to produce very consistent fin height as opposed to copper

fins.

3.

Tube plate dies need to be dedicated to each model only then the close tolerance can be

achieved. In copper brass flexible dies are used and hence few dies are required but in

aluminum each model has its own header plate dies and it is a 3 step process requiring 3

dies per header.

4.

Tanks are plastic moulded. Plastic moulds are much more precise than the open dies used

for brass tanks as the molten plastic is injected at high pressure and temperature into the

mould. The precise moulds also need to be made out of high temperature steels as it is a

hot process and have water cooling passages. Hence a plastic mould for a truck radiator

tank would cost about 25K usd against a brass tank die of 6K USD.

5.

Brass tanks can be made in multiple dies like for eg the inlet out let pipe holes are punched

in separate dies and pipes are brazed later. So for the same tank shape but with a different

position of let pipe brass does not need a new die. But this is not the case with plastic tanks

as change in position or angle of the let pipe requires a new mould.

CAB Brazing

The controlled atmosphere brazing ( CAB ) process heats a product to brazing temperature while

maintaining uniform temperatures within the product in an oxygen free nitrogen atmosphere. During

furnace brazing, a brazing sheet of aluminum / silicon alloy ( cladding ) is heated to a liquid state and

flows to form aluminum joints or fillets.


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Heat exchangers such as radiators, condensers, oil coolers, evaporators, heaters and charge air coolers

are made by this process.

Picture of Seco Warwick CAB brazing furnace line


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There are other methods of Aluminum Brazing.


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Vacuum Brazing Versus CAB

Vacuum brazing was developed before the NOCOLOK flux CAB brazing. It is a flux less brazing

process. Here, the filler alloy contains 1.0-2.0% magnesium, which diffuses on the surface

during the brazing cycle and then vaporizes at 600C due to low pressure of 10-5 mbar. The Mg

vapour disrupts the oxide layer and thus enables the filler alloy to flow.

In controlled atmosphere brazing a non corrosive, non hygroscopic flux is employed to dissolve

and break up the oxide layer before the filler alloy melts. A Mg-content of over 0.3% in the

molten clad reduces the performance of the flux due to the formation of high melting K-Mg-F-

compound. These compounds reduce the viscosity of the liquid filler and results in poor brazing

results.

Parts produced in vacuum brazing are more bright. This process is batch type and generally

suitable for low to medium volume productions. But this process requires much tighter

tolerance on furnace atmosphere ( < -60 C dew point ), surface cleanliness and fit-up.

Dip Brazing

The earliest brazed aluminum heat exchangers employed a chloride flux, a mixture of chloride

salts with minor additives of fluorides. The units were immersed in a molten salt bath where the

salt acted as a flux and a means of raising the unit to brazing temperature.

However, this technique left a hygroscopic corrosive residue on the heat exchanger.

The brazed unit required extensive post braze treatment in the form of water washing, pickling

and passivation of the pickled surface to prevent further corrosive action

aluminum brazing presentation

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