Edited by Sara Dorfner
International Copper Assoc.
New York, N.Y.
Anyone who works on cars is all too familiar with gray aluminum radiators and their traditional fin design. That image is changing, at least in the trucking industry where radiators composed of copper fins brazed to brass tubes and headers are increasingly common.
Engineers are discovering that copper and brass parts, and the brazing process used to join them, have several advantages. The cuprobraze process, for example, is simpler and less costly than manufacturing aluminum radiators because of higher throughput rates, fewer production steps, and lower scrap rates. The process is designed to produce radiators ranging in size from light trucks to delivery vehicles. Automobile radiator manufacturers are also looking into this process.
Cuprobraze radiators can be built with only minor retooling of existing aluminum lines. Existing tube mills, fin machines, and assembly equipment can also be used with minor adjustments. Investments in new equipment and operator retraining are minimal.
Brazing gives the copper-brass radiators stronger joints than those on brazed aluminum or soldered copper-brass models. Moreover, joints are stronger despite use of thinner fin and tube material. Brazed copper fins are 0.050-mm thick or less and brazed brass tubes are 0.100-mm thick. For most aluminum fins and tubes, the figures are 0.127 mm and 0.406 mm, respectively.
Cuprobraze radiators are also more efficient at shedding heat, which lets engineers lower cooling module costs and weight. It also means less parasitic engine losses and greater fuel economy. Or, manufacturers can give the cuprobraze radiator a small frontal area but the same air-side pressure drop as a comparable aluminum model.
A stronger bond
Cuprobraze radiators use a nontoxic alloy that melts at low temperature. The alloy is 75% copper, 5% nickel, 15% tin, and 5% phosphorous. As with other materials in the CuNiSnP system, it is self-fluxing. The brazing compound has no lead or other dangerous substances, and there’s no need to rinse after brazing. It works well in either conventional vacuum-brazing furnaces back-filled with nitrogen or in controlled-atmosphere brazing furnaces. Brazing typically takes place at 620 to 635°C.
There are several ways of spreading the brazing paste. For example, specially designed rollers in the fin machine can apply quick-drying brazing paste. This method applies the exact amount of brazing material. The tubes can receive a spray of water-based brazing paste as they leave the forming mill. Hot air then dries the coating.
Temperature is a key distinction between controlled-atmosphere brazing used with aluminum radiators and the cuprobraze process. Copper-brass brazing takes place at hundreds of degrees below the melting temperature of brass, but only 40°C separates the melting and brazing temperatures for aluminum. The larger margin for brass minimizes the risk of overshooting the melting temperature. This means the cuprobraze oven can ramp-up to temperature quickly and oven times can be relatively short. Because flux is unneccesary, costs are lower still. Elimination of fluxing and the obligatory rinsing step makes additional floor space available for other uses.
Brazed copper-brass joints are stronger than soldered metal and do not suffer from galvanic corrosion. Anneal-resistant header, fin, and tube materials strengthen the radiator cores.
The most critical connection on the radiator tube is where it joins the header, because coolant flows through the header from the tank to the tubes. Manufacturers have several options to ensure solid attachment. One method pours a slurry of brazing powder and alcohol on the air side of an assembled core. The alcohol evaporates, leaving brazing material at critical joints. A second method places a thin string of brazing paste on the air side of the header. The header is heated from the coolant side so the binder melts and paste flows into the joints.
Brazed cores are two to three times stronger in torsion and tension than soldered cores. This is partially because brazed joints resist corrosion better. Brazed joints between tubes and fins have experienced minimal attacks during testing which simulates typical road environments.
The finishing touch
Electrophoretic coating, widely used on automotive components, replaces traditional spray painting on cuprobraze radiators. This coating protects the radiator from external corrosion by providing an even distribution of paint over the entire outside surface. Conventional spray painting is largely cosmetic in comparison and actually accelerates corrosion. Most important, electrophoretic coating works with fins as thin as 0.025 mm. Cuprobraze radiators that have been coated show excellent corrosion resistance in laboratory testing, including seams and sharp edges. In addition, coating doesn’t affect heat transfer.
During electrophoretic coating, a thin film of paint — one-half to one-third the thickness of paint applied by conventional methods — accumulates on the radiator. The thin paint layer causes an electrical insulation that restricts further buildup and lets the coating reach all areas, including the dense inner core.
After electrophoretic coating, the radiator bakes in a curing oven at 150 to 177°C. The development of low-temperature curing makes this form of coating suitable for radiators fitted with plastic tanks and gaskets. Electrophoretic coating also has other benefits. It is highly automated, so it can be integrated with other production operations. In addition, it utilizes between 95 and 99% of the sprayed material compared with 30 to 50% for spray painting.
Down the road
Engineers are investigating several different technologies which could further improve brazed copper/brass radiators. For example, laser-welded brass tubes have shown promise. Laser welding lets manufacturers use thinner tube material (0.004 in.) similar to that of lock-seam welding while also keeping the well-defined shape of high-frequency welded tubes. Laser welding likewise lets new tube designs, such as a one-row radiator, replace the standard two-row design.
Three new alloys could increase the overall strength and durability of cuprobraze radiators. They include an anneal-resistant fin material that maintains strength after brazing and an anneal-resistant tube alloy that retains a fine grain structure after brazing. Fine-grain structure provides ductility and fatigue strength in brazed radiator cores, and lets tubes weld and form as easily as brass. The third alloy, used for brass headers, has forming qualities equal to or better than conventional materials and also retains its original structure after brazing.
Header-joint design is another area being improved. Use of round or oval tube ends makes possible a tube-touching concept. In this design, ferrules in the header sit so the edges of the tubes touch, leading to smaller fin depths. Tubes that touch make for efficient use of total fin area, limiting loss in air-side performance.
In addition, the copper from cuprobraze radiators can be recycled and fabricated directly into new radiator tube strip. Unlike conventional soldered radiators, brazed radiators contain no lead/tin solder. Aluminum radiators, by contrast, can only be recycled into less critical casting alloys because they contain silicon.