Brazing Handbook - Filler Materials

Filler materials

On the market, there are many well-known filler metals for normal brazing of copper and brass. Figure 9 shows melting ranges of some brazing-alloy families.
Cu-P alloys together with one or more other metals are a group of well known brazing materials for copper and copper alloys. A widely used brazing alloy for copper is CuP alloy with 6 %P. The melting range for this alloy is 707 – 850 ºC and could not be used for CuproBraze. Additions of silver and zinc in the CuP alloys decreases the melting point but still the temperatures are too high. Silver is also too expensive to be used for mass production of heat exchangers.
Except for the CuSnNiP-family, no filler metals have so far been found to be suitable for the CuproBraze process. The brazing filler metals used for joining CuproBraze fins and tubes belong to the lower temperature scale of the CuSnNiP-family. Table 5 lists the composition of the brazing alloys that are used in the CuproBraze process.
OKC600 alloy is patented by Luvata (U.S. Patent Number 5,378,294) but can be freely used for automotive and heavy-duty industrial heat exchanger applications. This alloy is mainly used for brazing powder. VZ 2255 is mainly used for brazing foil but can also be used as brazing powder.

Figure 9. Brazing alloy families and melting ranges.

Table 5.  Nominal composition of brazing filler metals for
CuproBraze.

Brazing powder

Cold forming of the filler metals in table 5 is virtually impossible, which means that roll bonding (cladding) of the filler metal on the tube or fin material is not possible. OKC600 filler metal alloy is only available as powder, which can be mixed into brazing-pastes.
The powder is produced by gas atomizing the molten material into spherically shaped, fine-grained powder. Atomization is normally performed using a protective gas such as nitrogen as atomizing media. The atomization parameters are set for a maximal particle size of about 90 µm. Water atomizing of this alloys is not possible due to reactions between the water and the particles forming hydroxides on the surfaces which make brazing impossible.
Depending on the powder manufacturer, the average particle size is normally 15 µm to 30 µm. In practice, each atomized lot is passed through a sieve to exclude particles exceeding 90 µm. Figure 10 shows a typical powder size distribution.

Figure 10. Typical particle size distribution and shape of
OKC600 brazing powder.

The fresh atomized powder is very sensitive to oxidation. Therefore, some powder manufacturers reduce the powder to remove surface oxidation. The powder must be protected against oxidation and humidity during manufacturing, transportation and storing. The product data sheets and storing instructions should be carefully followed. In case the powder oxidizes during transport or storage, reconditioning by reduction treatment might be possible by powder manufacturers. (Reduction is the reverse chemical reaction to oxidation).

Brazing foil

An organic free brazing foil (VZ2255) with a thickness from 20 µm to 40 µm is available for the CuproBraze process. The foil is made in one process step to the final dimension by a Rapid Solidification process (RS). Due to the amorphous structure the brazing foil is completely ductile meaning it could be bent during application without breaking.
In some cases the brazing foil can be more practical than paste and should be considered as an alternative or complementary brazing material to the paste - depending on the process requirements, heat exchanger design and production volume. Especially for brazing inner fin to tubes for charge air cooler and oil cooler and for multi-tube radiator designs the foil could be considered as an alternative brazing material.
The brazing result and joint properties of the foil will be the same as for OKC600 powder/paste. Brazing foil and brazing paste can be combined in one brazing procedure in the same heat exchanger.
The foil (figure 11) is available in the widths of 15-115 mm and a thickness of 20-40 µm. (Contact the foil manufacturer for updated dimensions) 

Figure 11: Brazing foil VZ2255 in different foil widths.

Brazing paste

To make it possible to apply the brazing powder for brazing of the cores, it is mixed with a binder to a suitable brazing paste. The binder is mostly specific for each paste manufacturer, which means that pastes from different manufacturers should not be mixed together. Application can then be done by means of conventional commercial application methods.
The binder is a chemical or a mixture of chemicals and is specific for each paste manufacturer. The binders can contain quite different kinds of chemical groups which mean that mixing of pastes with different kinds of binders can destroy the application and/or the brazing properties. The binders are chosen to decompose or evaporate cleanly below the brazing temperature and without leaving residues on the brazed samples. The binders are also chosen to be environmentally friendly.
All pastes are premixed and are ready to use after stirring. The stirring recommendations from the paste manufacturer should be followed in order to secure good paste applicability. There are pastes with different viscosities to be used at different kinds of joints, as well as different application methods. Contact the paste manufacturer to use the right kind of paste.
There are two main different types of binder system, solvent-based and thermoplastic. The solvent-based binders are dissolved in a solvent. The solvent is evaporated during drying, leaving a hard binder and if the binder is mixed with brazing powder, it will give a hard coating after drying which only could be re-dissolved in a solvent.
The pastes have normally long shelf life. For detailed information regarding the paste properties, contact the paste manufacturer.

Important: Do not make a mixture of pastes from different manufacturers, as they may not use the same binder system.