Carbon-carbon (C/C) material brazing jigs are used for processing of aluminum and copper brazing of automotive components such as car radiators and oil coolers. These applications take full advantage of the excellent material properties of C/C.
The material properties of C/C when compared to metal include a high heat resistance over 2000˚C (3500˚F); low thermal expansion; light weight (20% the weight of metal); and excellent resistance to corrosion and radiation. Compared to ceramics, C/C has a high resistance against fracture, it does not bond and it can be made into complex shapes. When compared to graphite, C/C has high strength and rigidity and a high resistance to fracture.
The structural design and the major advantages of using C/C brazing jigs will be reviewed.
Figure 1 and 2 show examples of C/C brazing jigs. The jig is designed to hold the braze product between C/C plates. C/C coil springs are used to apply equal pressure during the brazing process.
Less Gross Weight Means Added Productivity
In heat-treating, including brazing, the furnace heating capacity and maximum load weight are two important factors that determine the amount of total load capacity (jig weight + parts weight) processed in a heat-treat cycle. Figure 3 illustrates one particular case in which C/C-material brazing jigs were used to increase production volume. C/C braze jigs of 66 kg (145 pounds) replaced stainless jigs weighing 200 kg (440 pounds) on a heat-treat furnace with total load capacity of 350 kg (772 pounds). The ratio of total jig weight to total load capacity was drastically reduced by 70%, which made it possible to carry more parts. Using C/C jigs, a significant increase is achieved in the quantity of parts that could be processed per cycle.
Improvements in Plant Operations
With a metal brazing-jig setup, a supplementary load is used (weights added) during cases where a constant load on the part is required. With C/C brazing jigs, the weights could be replaced with C/C coil springs to apply the load. Figure 4 shows an example when a 20-kg (44-pounds) load is needed on a part. With a metal jig setup, a 20-kg weight is used. But with a C/C brazing jig setup, only two C/C coil springs of 70 grams each are needed for an equivalent amount of load. A C/C leaf spring setup may also be used under the right conditions.
As can be seen, there is huge difference with using 20-kg weights as opposed to using 70-gram (2.5-ounce) C/C springs, which could result in improved working conditions and production performance.
Decrease in Defect Rate
With metal-jig brazing, when the parts are aligned too closely, the shadow of the supplementary weights have a tendency to block radiant heat waves. This, in addition to the heat capacity of the dead weights, compromises the heating uniformity and leads to defective brazed parts. To maintain heating uniformity in this situation, the parts are positioned with sufficient space, which limits the quantity of parts processed per cycle, even at less than load capacity.
With a C/C brazing-jig setup, the jig itself is very compact and does not encounter these problems. Hence, a decrease in the defect rate and an increase in process quantity is achieved.
Longer Service Life
After long-term service, the metal jigs tend to deform due to high-temperature deformation, which also has a negative effect on the distortion of the parts. On the other hand, C/C jigs maintain high strength from room temperature up to 2000°C (3500˚F), thereby assuring a positive effect on part distortion. Also, C/C has a very high deformation resistance, which leads to longer service life.
Increase in Production Volume
In addition to C/C braze jigs, base trays used to set the jigs could also be replaced with C/C material for further improvements in production efficiency. There have been reports that production volume has doubled merely from the optimized loading of C/C jigs and fixtures without any upgrades or increase to furnace load capacity. IH
For more information: Contact Lloyd Nagamine, sales engineer, ACROSS USA, Inc., 1480 Beachey Place, Carson, CA 90746; tel: 310-635-3555; fax: 310-635-9171; e-mail: firstname.lastname@example.org; web: www.acrosscc.com
Carbon Springs for Heat-Exchanger Production Brazing
Carbon springs are springs made of C/C composites (carbon-fiber reinforced carbon). C/C coil springs have the features of light weight, high strength and high modulus. Excellent spring characteristics are maintained even at temperatures higher than 1000˚C (1832˚F) with high toughness, excellent wear resistance and corrosion resistance.
A usage example would be brazing heat exchangers during the production process. After the parts are set up into the C/C jig (as shown), a constant force will be applied by the spring. The C/C spring maintains the same force as temperature increases, and brazing will start when the temperature reaches the melting point of the filler metal.
The benefits of C/C springs include:
• Drastic reduction in cost of energy. Each carbon spring can generate force up to 24.5 kg but weighs only 26-84 grams. This means we can get same effect with much smaller heat capacity.
• Speed up in processing time. Small heat capacity will allow faster heating time.
• Less need for maintenance/replacement due to fatigue. A C/C spring does not wear or deform in atmosphere without oxygen. Its spring constant is maintained under repeated heat cycles.