Chemistry Reference
In-Depth Information
One method involves a mixture of flux with silicon powder, the NOCOLOK
Sil Flux
process [95]. At brazing temperature, the silicon powder diffuses into the aluminium
substrate and generates Al-Si filler metal for joint formation. Sil flux can be applied
with a binder to specific component surfaces, e.g. extruded tubes. In this case, the filler
metal would be supplied from the tube and a clad fin sheet is not necessary.
In Composite Deposition technology (CD process) [96], the filler metal for joint
formation is derived from a composite powder, a compound consisting of potassium
fluoroaluminate flux and Al-Si alloy. The CD powder is 'deposited' selectively and
accurately on heat-exchanger components prior to assembly and brazing.
The common challenge for all cladless approaches is the development of a reliable
coating procedure.
7.17.13
Furnace Conditions
Specifications for acceptable CAB furnaces (with the use of 5 g/m
2
uniform flux load) are:
furnace atmosphere:
oxygen level below 100 ppm;
dew point below -40 C;
brazing temperature and time:
ideally a uniform 600 C - 5 C (heat exchanger surface temperature);
ideally 3 min - 0.5 min from 580 C heat up to 605 C on cool down.
Many production sites work with flux loads of 3 g/m
2
. As flux coating goes down,
furnace atmospheric conditions become more critical. Most CAB furnace manufacturers
produce furnaces which routinely operate in the 20-50 ppm oxygen and dew point levels of
-45 to -50 C (i.e., 92-67 ppm H
2
O). A low dew point will keep the formation of hydrogen
fluoride to a minimum [97].
A suitable treatment for the furnace exhaust (i.e. dry scrubber) is required.
7.18
Summary
Controlled atmosphere brazing using non-corrosive potassium fluoroaluminate flux is the
dominant process for making all aluminium heat exchangers.
There are several factors determining the success of aluminium brazing:
product fit-up and assembly;
component cleanliness;
flux application;
furnace atmosphere;
brazing temperature uniformity;
brazing time at temperature.
This technology provides robust and stable parameters. Over the years, a significant
optimization potential was achieved in all process steps as well as in equipment and
machinery.
