Brazing Handbook - General

New brass and copper alloys offer high strength as well as excellent retention of strength at elevated operating temperatures. They can withstand high-temperature brazing processes without substantial loss in strength.
A Brazing Center has been established to build prototype heat exchangers based on these new alloys and to demonstrate the CuproBraze® process. As a result, CuproBraze® technology is now being applied globally in the manufacture of advanced heat exchangers using the new brazing process described in this manual.
Brazing furnaces have been developed for all capacities of production including batch, semi-continuous and continuous furnaces. This handbook provides an update on CuproBraze brazing technology in use today, and it will be regularly updated with the latest knowledge of the process.
It details trends in the selection of furnaces, the application of filler materials, the assembly of components and the control of brazing operations. The CuproBraze process was specifically developed for the manufacture of automotive and heavy-duty industrial heat exchangers. By using high-strength and high-conductivity copper and copper alloys, it is possible to manufacture strong, efficient and compact heat exchangers at a low cost with an environmentally friendly process.

Efficient heat exchangers

The high thermal conductivity and high strength of new copper and brass alloys have changed the rules of design for mobile heat exchangers.
The new brass-tube and copper-fin alloys offer high strength as well as excellent retention of strength at elevated operating temperatures. They make copper and brass extremely attractive once again for heat exchangers of all shapes and sizes, like mobile radiators, heaters and charge air coolers.
In recent years, designers have demanded lighter fins and tubes and hence stronger alloys for more-compact, lighter and higher-efficiency heat exchangers. An important advantage of thin gauge material is that, besides reducing weight, the lower cross-sectional area allows air to pass more freely through the core of the heat exchanger. The relative ease with which air flows through a radiator core is measured as a lower air pressure drop for a given performance. A low air pressure drop is highly desirable in advanced design of efficient compact heat exchangers for fuel-efficient vehicles.

Technology Development

The use of thin gauges in compact heat exchangers requires new processes. The International Copper Association responded to the industry need for a new generation of copper-brass radiators by developing CuproBraze technology, which is a new process now being applied globally in the manufacture of advanced heat exchangers. CuproBraze technology was specifically developed for application to automotive and heavy-duty industrial heat exchangers.
For example, it enables the manufacture of charge air coolers that can withstand higher temperatures than existing equipment, allowing the transportation industry to reduce emissions and increase fuel efficiency by replacing temperature-challenged aluminum charge air coolers with copper-brass counterparts.

Effects of annealing

The alloys used in conventional copper and brass radiators are designed for soldering below 450ºC. When subjected to high temperatures for long periods, these conventional alloys, soften due to annealing, a well understood metallurgical effect.
Annealing rearranges the positions of metal atoms in the metal lattice through solid-state diffusion effectively removing the deformations that would otherwise strengthen the alloys. The resulting decrease in yield strength is particularly steep for metals previously strengthened by rolling or other deformation-hardening processes. Annealing is time and temperature dependent. Because annealing is based on solid-state diffusion, metals and alloys can significantly lose strength well below the melting point; however, annealing is much more pronounced at temperatures close to the melting point.
Process engineers and radiator designers have long been confronted with an “either-or” type of dilemma. Brazing processes promised strong bonds at the joints but brazing weakened the bulk material because of softening. Heat-exchanger designers have been frustrated for several decades by these limitations. The industry had to wait for the development of anneal-resistant copper alloys before further advances could be made. As an example, figure 1 shows the softening properties for standard fin copper and the CuproBraze fin copper.

For decades, manufacturers avoided annealing effects in copper-brass radiators by using solders that melted well below annealing temperatures. These solders were used to bond copper fins to brass tubes and brass tubes to headers, which are the essential steps in radiator assembly. These methods are still widely employed today to make heavy-duty radiators for truck and off-road applications. A tremendous body of specialized manufacturing expertise and process knowledge that also includes many specialized machines and furnaces have been developed around this industry. The basic process consists of melting, flowing and solidifying the solder at the joint, typically forming a metallic bond with the soldered surfaces (or parent metals).
Soldering and brazing involve the same bonding mechanism except that soldering is defined as using filler metals that melt below 450ºC and brazing uses filler metals that melt above this temperature. In both soldering and brazing the bonding mechanism is a reaction between the filler metal and the parent metal or metals. Brazing and soldering usually result in alloying, i.e., a metallic-type bond forms at the interface.
Typically, the filler metal flows into the joint gap by capillary force , solidifies and forms a bond. The capillary force is dependent on the gap clearance, which means that the filler metal flows further into a closer than a wider gap. Oxide and contamination of the surface influence the capillary force negatively. Several factors affect the mechanical performance of the finished joint. For example, joint clearance and geometry are important. In general, the joint strength is higher for narrow joints. Other effects of geometry are the possibilities of slag entrapment and void formation in the joint. Interactions between the filler metal and the base metal take place in both soldering and brazing. Because of the higher temperatures for brazing, however, interactions are usually greater for brazing than soldering. The interactions are time and temperature dependent. To minimize interactions, the brazing temperature should be as low as possible, and the time period that the materials are held at the brazing temperature should be as short as possible.
For more detailed information regarding soldering and brazing in general, see references 8, 9 and 10. Many of the companies who sell brazing materials also have useful information regarding brazing in general.