Vacuum purging:Progressive Innovation for

CAB equipment and processes

P. Skarbiński, A. Bielewicz, SECO/WARWICK, Poland

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Typical Processes for Aluminum Brazing

/ Controlled Atmosphere Brazing (CAB) In this process, the role of the flux is to dissolve the oxide film on the Al surfaces to be joined and prevent re-oxidation during brazing.

Typical brazing conditions:

- flux load : 2 - 5 g/m2

- fast heat-up rates (> 20°C/min)

- nitrogen atmosphere oxygen < 100 ppm; dew point < -40°C

- average demanding deoiling – (assembled heat exchangers cleaned in TD)

- max. joint gaps ≤ 0.15 mm

/ Vacuum Aluminum Brazing (VAB) When an aluminum alloy is heated, the oxide layer cracks, due to differential expansion. In vacuum conditions the presence of Mg vapor enhances filler metal flow and wetting of aluminum alloy surfaces.

Typical brazing conditions:

- alloys containing magnesium are required

- pressure (vacuum) < 1x10-4 mbar

- very demanding deoiling – very clean surface required! (single parts cleaned before assembly or washed in special cleaners with solvents)

- good fit-up (0.05 to 0.10 mm)

http://www.alueurope.eu/wp-content/uploads/2012/01/AAM-Joining-4-Brazing.pdf

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Why use vacuum purging in the CAB process?

A Vacuum Purging cycle provides:

/High brazing quality of products – products will be braze in perfect brazing atmosphere level of 10ppm of O2.

/A perfect brazing atmosphere within the internal parts of the heat exchanger./Shiny silver surface after brazing – results comparable with vacuum brazing.

with vacuum purging standard process

/Saves nitrogen – only a single backfilling is required./Saves time – 8 cycles/volumes of standard purging compare to vacuum purging - some vacuum purging systems can reduce purge time to 4 minutes.

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Vacuum – what is it?

/ The definition of vacuum is:

- “the state of a gas at which its pressure in a vessel and therefore its particle density is lower than that of the ambient surrounding atmosphere or in which the pressure of the gas is lower than 300 mbar,“.

/ There are several levels of vacuum defined:

Pressure range in vacuum technology - Pfeiffer Vacuum 2013 [1]

VAB

CAB

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Vacuum purging for CAB process

/ Vacuum pumping level at 0,25 mbar (25 Pa) will allow the system toachieve 50 ppm after refilling with pure nitrogen (5 ppm of Oxygen).

/ A Typical (properly sized) Medium Vacuum (MV) systemguarantees a short purging time of 7 minutes – the time required to get to 0,25 mbar vacuum (50 ppm of Oxygen) for a chamber volume of 5 m3.

Data from Oerlikon Vacuum technology, 2007 [2]

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Conventional chamber purging

/ Purity at level of 50ppm of Oxygen required ~ 8 purging chamber volumes changes of nitrogen.

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Pumping Systems for VAB

/ For a Vacuum Aluminum Brazing furnace (High Vacuum – HV), a high vacuum system is required. An HV system is more demanding and expensive compared with a standard MV system.

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Pumping Systems for CAB

/ For a CAB system with the vacuum purging option (to prepare the brazing atmosphere) a lower pressure range system also known as Medium Vacuum – MV is required.

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Pumping Systems

/ The differences between MV and HV pumping systems:

High Vacuum – HV (HV system includes MV system)

Medium Vacuum – MV

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CAB System Configurations available with vacuum purging

/ Batch – one chamber

/ Batch – two chamber

/ Semi-continuous Active Only10

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CAB batch furnace with vacuum purging

Medium VacuumSystem

Brazingchamber

VacuumPurging/Cooling

chamber

Thermaldoor

Vacuumdoor

Integrated single body design

Load area size 1100 x 1100 x 1500 mm. Load mass 250 kg. Output – 1 cycle per hour.

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CAB Line with vacuum purged batch furnace

Batch BrazingFurnace

DryerDebinder

Loadingtable

TransferSystem

Cooling - unloadingSystem

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Vacuum Purging / Cooling chamber

Heat exchanger

Recirculation fan

Nitrogeninlet

Medium VacuumSystem

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Brazing chamber

Heating Elements

ScrubberRecirculation

fan

Nitrogeninlet

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Vacuum purge option on a semi-continuous Active Only furnace

Loadingtable

DryerDebinder

VacuumPurging

BrazingChamber

CoolingWith

Vacuumpurging

AirCooling

Unloadingtable

Load area size 1500 x 500 x 3000 mm. Load mass 250kg. Output – 3 cycles per hour.

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Dryer / Debinder

Exhaust Fan

Gasburner

Recirculation fan

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Vacuum purge chamber with preheating

MediumVacuumSystem

Heating Elements

Brazing chamber

Heating Elements

Recirculation fan

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Cooling chamber with vacuum purging

MediumVacuumSystem

Recirculation fan

HeatExchanger

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Load area size 250 x 2100 x 1350 mm, Load mass 400 kg. Output – 4 cycles per hour.

Vacuum purge option on a semi-continuous furnace for vertical core position

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/ Temperature survey for plate and Bar from the system done on maximal size load - 160mm thickness.Brazing point with +/- 3 ◦C uniformity was reached in the brazing chamber within 10 minutes.

CAB Line with vacuum purge option on a semi-continuous furnacefor vertical core position

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/ Working area size of 900 x 1900 x 2000 mm; Load mass - 100 kg. Output - 1 cycle per hour.

Single chamber vacuum purged Batch CAB Furnace for vertical core position

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/ Single Chamber furnace with working area size 500 x 300 x 500 mm. Load mass 2 kg. Laboratory furnace with CAB process simulation system.

Laboratory Batch vacuum purged furnace

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Summary / Conclusions

/ By utilizing the vacuum purging option to the CAB furnace system savings in utility consumption as well as an improvement in the brazing quality can be achieved.

/ For semi-continuous and batch brazing operations a single Nitrogen backfill instead of approx. 8 exchanges of the purging chambers is required.

/ While reducing the Nitrogen consumption, excellent brazing atmosphere purity is achieved in the whole preheating, brazing and initial cooling phases of the process, usually on the level of 10 ppm of oxygen. This provides the highest quality of brazing atmosphere for forming the joints and excellent appearance of the cores after brazing.

/ The internal volume of the core is purged equally as well as the external surfaces of the core.

/ CAB process with Vacuum purging, in some cases, may be considered as a lower cost alternative to VAB. Savings may be achieved due to less demanding tolerances of the heat exchanger components, simpler cleaning process, and elimination of magnesium containing alloys..

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This presentation was developed by SECO/WARWICK Europe for the International Aluminium Brazing Congress organized by the DVS – German Welding Society on 3 – 5 June, 2014 in Düsseldorf/Germany. 

To learn more about the CAB Brazing process & equipment, visit www.secowarwick.com