Brazing Handbook - Copper alloys for CuproBraze

Conventional deformation-hardened alloys soften when exposed to brazing temperatures. Fortunately, researchers faced this challenge and developed strong materials that could withstand the temperatures of brazing up to 670ºC and remain strong. Anneal resistant copper alloys are strengthened by mechanisms other than merely deformation hardening. In this handbook the basics for the materials for Cuprobraze are described.

Fin material

The basic alloying element in this anneal resistant copper is chromium. The mechanism of the alloying is that it forms copper-chromium intermetallic compounds. During the casting operation most of the chromium is dissolved in the copper matrix. Before the final rolling procedure the material is annealed and some of the precipitates will be dissolved. In this state there is a lot of chromium in solid solution, and with the consequence that the electrical conductivity is much lower than for standard copper. The electrical conductivity is around 60% IACS, heat conductivity is proportional to the electrical conductivity. The material is delivered to customers in this condition. After the fins have been formed and assembled with the tubes and headers to a core, brazing is done in a furnace at 640°C to 660°C. The brazing process can be regarded as the last heat treatment for this copper alloy. During brazing the chromium in the solid solution precipitates out of solution from the copper matrix. The precipitates that have the greatest effect in preventing softening are 3 nanometers (0.000003 mm) in size. The result is a material that now exhibits an electrical conductivity of around 90% IACS and with the retained strength.
The most used temper for material for normal corrugated fins is called standard temper. For more complicated formed fins, copper in soft temper is recommended. Nominal properties of the copper-fin alloy before and after brazing are listed in Table 1.

Because soldering temperatures are not high enough to raise the thermal conductivity, this new copper-alloy fin material must not be used to make conventional soldered radiators. It should only be used for CuproBraze heat exchangers. The CuproBraze brazing operation is needed to restore the thermal conductivity. Additional physical properties for copper fin material are listed in Table 3.
The copper material for Cuprobraze will soften at a higher temperature compared with normal fin copper. Figures 2 and 3 show the softening for the copper fin material for CuproBraze at different holding times and temperatures.

 Figure 2. Yield and tensile stresses for standard temper at different
temperatures and holding times.

Figure 3. Yield and tensile stresses for soft temper at different
temperatures and holding times.

Tube Material

The conventional brass that is used for radiator tubes is of composition 65 -70% copper and 30-35% zinc. The brass alloy that has been developed for higher temperature joining purposes is basically a brass composed of 85% copper and 14% zinc. Figure 4 compares the softening of the yield strength for normal 1070 brass and the anneal resistant brass. To achieve anneal resistance, a mechanism had to be introduced in the material to avoid re- crystallization. The principle that is utilized in this alloy is the use of precipitates that prevent the material to re-crystallize. The brass is alloyed with about 1% iron.

Figure 4. Comparison of SM 1070 brass with the anneal resistant
brass.

The iron forms particles that are about 0.2 micrometers in size. That fact, in combination with a very small grain size gives a very high resistance to re-crystallization. Nominal mechanical properties for brass tube materials, before and after annealing, are listed in Table 2. Additional physical properties for the tube material are listed in Table 3. The basic contribution to softening resistance is the fine grain size coupled with the grain size retention even after being subjected to temperatures as high as 670°C. The grain size of the base material is adjusted to 3 micrometers (0.003 mm).

Table 2 -- Nominal mechanical properties of brass tube material  for
CuproBraze .

Table 3 - Nominal physical properties of copper and brass materials
for CuproBraze.

When using the tube material for HF-welding, note the differences in melting temperatures and melting ranges (A and B) compared with normal tube brass. See Figure 5 and table 3.

Table 3. Melting properties for tube brasses.

Figure 5. The binary phase diagram copper-zinc showing the
differences in melting temperatures and melting range between
normal tube brass (SM 1070 dashed lines) and the CuproBraze®
tube alloy (SM 2385 solid lines).

This means that higher energy input in the welding coil is needed. The smaller melting range implies a closer control of the welding parameters. Thus the welding parameters have to be adjusted relative Cu70Zn30 brass. In figure 6 the softening properties for the yield and tensile stresses at different temperature and holding times are shown.

Figure 6. Yield and tensile stresses for tube brass at different
temperatures and holding times.

The header brass material is a Cu64ZnNi3 (C74400) brass. It was originally developed for lamp socket production requiring multiple forming processes. The material exhibits excellent forming properties and good mechanical properties even at elevated temperatures. The alloy is solution hardened by the nickel addition and that accounts for the retained mechanical properties after the brazing operation. The mechanical data before and after the brazing process are shown in Table 4. The physical properties of the header brass are shown in Table 3.

Table 4 -- Nominal mechanical properties of brass header material
for CuproBraze .

In figure 7 the softening properties for the yield and tensile stresses at different temperature and holding times are shown.

Figure 7. Yield and tensile stresses for header brass at different
temperatures and holding times.

Strength at elevated temperatures

Besides being anneal resistant, the new copper-fin and brass-tube alloys have high strength at elevated temperatures. For example, when the operating temperature is increased from 0°C to 300°C the tensile strength of the brass-tube alloy only decreases from 400 N/mm2 to 260 N/mm2, and the tensile strength for copper-fin alloy only decreases from 350 N/mm2 to 260N/mm2. Similarly, the fin and tube alloys retain much of their yield strength at 300°C. Figure 8 illustrates the strength at the specific temperature from room temperature to 300°C for the two alloys. New generations of charge air coolers need to operate at temperatures around 300 ºC. The copper fins and brass tubes described here are well suited for such high-temperature service.

Figure 8. Tensile strength (Rm) and yield strength (Rp) of copper
and brass materials for CuproBraze® at elevated temperatures.