Cesium Tetrafluoroaluminate (CsAlF4) 

Aluminium brazing is now the preferred process for the production of automotive heat exchangers such as radiators, condensers, evaporators and heater cores. Good corrosion resistance in standard applications, formability and high thermal conductivity make aluminium an ideal material for the construction of these heat exchangers.

Aluminium brazing involves joining of components with a brazing alloy, that is an aluminium alloy (Al-Si) whose melting point is appreciably lower than that of the components. This brazing alloy is usually placed adjacent to or in between the components to be joined and the assembly is then heated to a temperature where the brazing alloy melts but not the components. Upon cooling, the brazing alloy forms a metallurgical bond between the joining surfaces of the components.

In automotive heat exchanger applications, this filler metal is supplied via a thin sheet or clad on a core alloy. The core provides structural integrity while the low melting point Al-Si cladding alloy melts and flows during the brazing process, to provide upon cooling a metallic bond between the components.

Air-conditioning market has been evolving dramatically over the past decades. The most important factors on system design and alternative technologies are:

• Legislation and phase-out schedules for ozone depleting refrigerants
• Increased demand for comfort cooling
• Rising energy costs
• Regulations on energy efficiency

There are multiple ways to improve the efficiency of a given air-conditioning system. A minimization of the temperature difference between the airflows and the phase change temperatures of the refrigerants can be achieved by increasing the heat transfer surface area or by improving the heat transfer efficiency of the heat exchangers. Brazed microchannel heat exchangers have already proven that they are able to provide an elegant and cost effective solution for the utilization of this optimization potential – while also providing a number of other benefits.

Why brazed heat exchangers?

• Improved heat transfer performance
• Pressure drop reduction
• Refrigerant charge reduction
• Reliability in performance
• Recycling advantages
• Noise reduction
• Lower weight

Flame brazing aluminium to copper is common in the refrigeration industry where copper tubes are brazed to aluminium roll-bond panels or tubes. Brazing copper to aluminium is very similar to brazing aluminium to aluminium, but some precautions are necessary. In flame brazing the inter-diffusion of copper and aluminium can be halted rapidly by simply removing the heat source - in this case simply removing the flame is sufficient to allow the joint to cool quickly.

It is possible to braze aluminium and steel or stainless steel and this is practiced in the household appliance industry for the manufacturing of pots an pans for example.

In the manufacturing of pots and pans where there is a large surface area between the aluminium base plate and the pot, a mixture of filler metal powder and flux is often used. In aluminium tube to steel or stainless steel tube joining, conventional flame brazing techniques can be used. Filler metal wire, either pre-placed or fed into the joint must be used.

The first step in any successful brazing operation is the removal of residual lubricants and forming oils. With a view towards more environmentally friendly degreasing methods, an aqueous degreasing station is available. Here various cleaning methods and aqueous degreasing agents can be evaluated to suit your particular needs. A drying furnace normally used for predrying fluxed parts can also be used to evaluate thermal degreasing methods.

Purpose
To pre-flux sample parts – either on the complete surface or selected surface areas. Different types of flux and binder mixtures can be used. Manual and semi-automated pre-fluxing is possible. Optimal binder and mixture compositions for customer specific parts and brazing configurations can be evaluated.


Procedure
The components are pre-fluxed using manual or semi-automatic equipment which ensures uniform coating and control of the load. By changing the rpm of the rotating table the spraying time for the parts can be varied, thus changing the paint load. Constant rpm and the fixed setting of the spray gun ensure repeatability during testing. The design of the equipment makes it possible to paint large quantities of parts, even up to a few hundred per working day.

The batch brazing furnace meets all the requirements of full production capacity tunnel furnaces in terms of heating rate, atmosphere purity and temperature uniformity.

The maximum size for the component to be brazed is 600 mm width x 600 mm height x 900 mm depth. The maximum heatable mass is 60 kg aluminium plus an additional 10 kg steel.

Special features such as viewing ports allow a first hand look at brazing in progress.

Temperature Monitoring System "Datapaq"
One important factor in aluminium brazing is monitoring the component temperature.

For successful brazing of heat exchangers, it is necessary to precisely maintain the temperature profile in the furnace zones during the entire brazing cycle. A uniform temperature distribution throughout all parts of the units is essential for good joint formation.

The control of temperature and time profiles during brazing can be realized by using a system (Datapaq) which records all information with eight precision thermo-couples. This heat-insulated device is passed through the brazing cycle together with the heat exchanger. Thereafter, the data is processed and transferred by a computer program.



 

Purpose
Testing the characteristics and properties of different material and flux combinations under flame brazing conditions.

Procedure
The components to be joined are first cleaned and then assembled using holding fixtures. Filler alloy is added in the form of brazing paste or brazing rings. Fluxing paste is applied manually by brush to the joint area.

After fluxing, the parts are heated with an open propane-butane flame. Operation of the flame is manual. Several different material combinations like Al-Al, Al-Cu, Al-Steel are possible.

Purpose
Testing brazing characteristics and properties for different combinations.

Procedure
Two different parts made of aluminium alloys (coupon 25 mm x 25 mm and angle shaped stripe) are coated with flux, dried and brazed in a laboratory glass furnace. The brazing process can be filmed (video recorded) upon request.
The furnace heating control allows different linear heating rates and dwell times at any temperature to be applied.

Standard brazing heating profile:
Heating rate – 30 °C/min up to 605 °C Dwell time – 2 minutes at 605 °C, Cooling rate – approx. 30 °C/min

The single unit test involves the following:
Preparation of brazing sample (one coupon and one angle), coating of sample with flux (Solvay’s material and the clients materials can be used), brazing , optional recording by real time movie, and report. Brazing is evaluated by an arbitrary scale from 1 to 5, where 5 is very good.

 

Purpose
To observe under an optical microscope metallographic structures of investigated materials, for example to evaluate brazing quality, joint geometry, progress of corrosion, etc.

Procedure
Depending on the sample, an appropriate cross section is chosen and cut either with a band saw or with a high precision cutting wheel machine. The cut out samples are embedded in epoxy resin and gradually polished mirror to obtain a like surface.

The cross section can be observed and photographed under either non etched or etched conditions. Optionally, special electrochemical etching can be carried out to reveal the grains.

The results of optical microscope observations are summarized in a report including some interpretations of the observed microstructure features.

Purpose
Brazing trials for single or small numbers of units, e.g. prototypes or mock up components. It offers a means of simulating the whole production process starting with part assembly, fluxing (wet and electrostatic methods are possible) and brazing. The size of the brazing chamber and other equipment makes it possible to braze standard-size heat exchangers.

Procedure
The components are cleaned by aqueous washing, then assembled and fluxed in either a wet spray or electrostatic chamber. Other fluxing alternatives like pre-fluxing with a mixture of flux and binder are also possible. The details of the fluxing process are individually chosen depending on the part design and customer requirements.

Brazing is performed under nitrogen atmosphere in a batch type furnace. The length of time the parts are heated closely matches typical heating rates seen in industry.

Purpose
To check corrosion resistance and behaviour of real parts as well as mock ups and laboratory samples.

Procedure
Testing is done in a spray type chamber (approx. 1000x60x25 cm). Standard testing usually follows ASTM no G85 – SWAAT Test, however, other methods such as the Salt Spray Test are also possible.

You can buy and download the ASTM G85 - 09 Standard Practice for Modified Salt Spray (Fog) Testing on the ASTM website:
ASTM-Website

Purpose
To investigate sample surface morphology and to determine presence and distribution of different chemical elements.

Procedure
Procedure covers: preparation of the sample (cutting), electron microscope analysis, element mapping, report

 Classified as hazardous according to the European regulation (EC) 1272/2008

Safety recommendations and additional information can be found in the Material Safety Data Sheet (MSDS).