Brazing Handbook - Fabrication and assembly of components

Fabrication and assembly of components

All furnace brazing operations, among them CuproBraze, require narrow tolerances. Generally, closer tolerances and well-defined joint gaps result in better and stronger joints, see figure 19.

Figure 19. Diagram showing filling length in gap with different
clearance.

Another factor to take into account is that the brazing alloy is in powder form and it builds up a thicker layer than a solid metal. The geometry of the tubes and fins and the tube pitch in the header should be adjusted accordingly.

Tube fabrication

Several types of brass tubes can be used to manufacture CuproBraze heat exchangers. These tubes are uniformly made from strip because thin gauges are required for lightness and efficient heat exchange. Tube fabrication requires that the edges of the strip are reliably bonded together. The tube seams can be sealed during the brazing process or they can be welded prior to the brazing process.
Tubes for use in the CuproBraze process should be specified with a crown that is higher compared to the crowns of tubes for use in a soldering process. The crown (see Figure 20) should be 0.4mm to 0.8mm for tube widths 12mm to 25mm. A higher crown results in a more consistent bond between tube and fin, see chapter 11.

Figure 20. Definition of the crown on the tube. Crown is W₂ – W₁.

Important : Tubes for use in the CuproBraze process should be specified with a crown that is higher compared to the crowns of tubes for use in a soldering process.

HF-welded tubes

High frequency (HF) welded tubes are most commonly used today for CuproBraze heat exchangers because their contoured shape is uninterrupted around the circumference of the tube. As a result, a consistent gap can be achieved between the tube and the header. HF-welded tubes are commercially available. When welding tubes of brass strip for CuproBraze, note the differences in the material performance as described in part 2.2. as this can effect the settings of the welding parameters. When bare CuproBraze tubes are produced, careful rinsing and drying are needed to avoid discoloration.

Important: Note the differences between normal and CuproBraze tube materials when HF-welding

Folded tubes

Folded tubes can be made of thinner brass strips (gauges down to about 0.080 mm). In the solder process the most common folded tube uses the lock-seam fold. This type of tube can also be used for the CuproBraze process but new tube designs offer advantages over the lock-seam design. The folded design (called snap-over) and B-fold design are just two types of tubes being tested for CuproBraze heat exchangers. Spraying of paste on the surfaces of the tubes does not result in a leak-tight seal at the seams. Folding methods include injecting the overlapping parts with a bead of brazing paste in the tube mill. Paste injection has to be performed correctly for optimal brazing results. To get optimal fatigue properties of the tubes, the paste injection should secure complete joints on the water side of the tubes. Figure 21 shows cross section of one type of folded tube.

Figure 21. Cross section of one type of folded tube after forming.

Fins

Consistency in the fin amplitude is also important. Inconsistency in the fin height can result in a gap that is too large between fin tip and tube, and a low percentage of correctly brazed tube-to-fin bonds. Also unnecessary amounts of brazing paste on the tubes will be used. Variations from fin-tip to fin-tip should be at an absolute minimum.

Headers

The holes in the header can be designed and manufactured in different ways. (See Figure 22.) For the CuproBraze process, pierced holes are recommended using a two-stage operation, which creates a continuous collar of contact surface area between the header and the subsequently inserted tube. This shape draws braze alloy from the surface of the header into the joint by capillary action. The optimum capillary action is reached at 0.05 mm joint clearance which means that the optimal size for the gap between header and tube is 0.05mm and this gap should not exceed 0.1mm. No tears are allowed in the joint section.
Stiffening ribs on the header are also beneficial (See Figure 23). Besides functioning as reinforcement, ribs lower the slurry consumption. Paste flowing into the wells around the tubes is not wasted on areas between tubes. When header gauges are smaller than 0.8mm (0.03in), extra care is recommended with tabbed header design from the strength point.
The type of oil used in the stamping process normally leaves unwanted residues after brazing, this negatively influences the brazing of the tube-header joints.

Important : If the stamping oil gives this problem, which can be seen as dark surface of the header after brazing, the headers have to be degreased before assembling.

Figure 22. Pierced holes (left and middle) in header are
recommended. Lanced holes (right) are used for soldering but are
not recommended for brazing.

Figure 23. Stiffening ribs on header.

Important : Optimal size for the gap between header and tube is 0.05mm, this gap should not exceed 0.1mm.

Surface conditions

At the brazing temperature, the surfaces of the components to be joined, as well as the brazing powder must be free from any non-metallic films, such as organic residues and metal oxides. In the brazing process, there is nothing, which can remove any dirt, heavy oils and oxides. To be able to wet and alloy with the components, they have to be clean before brazing. All storing of the components must be done in such a way that no contamination of any dust, chemicals and oxides take place, especially the paste coated parts that are very sensitive.
Most types of oils on copper and brass surfaces will form black or even invisible organic residues after heating in nitrogen atmosphere. The residues will be very thin films, which sometimes can be difficult to remove. Oils with low boiling point do not normally form this kind of organic residues.

Important :The oil on the surfaces after fin and tube production, are normally not harmful.

Brazing fixtures and assemblies

As previously mentioned, the brazing powder builds up a thicker layer than a solid metal of the same weight. This extra thickness must be taken into account when specifying tube pitch in headers and brazing-fixture devices.
The tube pitch (see figure 24) in the headers is a function of the tube width and fin amplitude with an allowance for a brazing paste layer. The allowance for brazing paste in turn depends on the tube dimension, the core width and the fin design. The brazing paste allowance has to be determined with actual components. As a guideline, increasing the pitch by 0.10mm often works well, resulting in a complete brazed joint between fin and tube. But for optimal brazing result, it is recommended to check the brazing result between the tubes and the fins for the first sample before further work. Figure 25 illustrates the shape changes of the tubes during compression of the core. See also figures 20 and 23.
The virgin tube has a convex belly, when the tube is compressed it starts flattering. If it is compressed too much, the tube walls will be bent inwards in the centre and become concave.

Figure 24. Definition of the tube pitch.

Figure 25. The figure illustrates what happens to the tubes during
compression of the core. The virgin tube (A) is convex, when the core
is compressed to right dimension the tube sides are flat (B). If the
core is compressed too much, the tube is formed like a “dog-bone”
(C).

In the CuproBraze process the parts go through a temperature-cycle from room temperature up to 650ºC, which means that the differences in the thermal expansion will influence the fixture design. Table 8 shows the thermal expansion coefficient and the expansion for 1 meter of the material from 25 ºC to 650 ºC.

Table 8. The heat expansion and the increase in length (∆l ) from
25ºC to 650ºC for 1m long object.

The assembly and brazing-fixture system should allow for the convex shape of the core before brazing, due to build-up of brazing paste and the heat expansion of the fixture. For a 500mm x 500mm core, a typical dimension measured in the middle of the core is 502.5mm to 503mm.
Another requirement of the brazing fixture is the demand for a reduced mass that allows the fixture to follow the temperature of the radiator core as closely as possible. This arrangement prevents differences in dimensions due to temperature variations that could lead to permanent deformation of fins or tubes. For the same reason, the fixture material should have a thermal expansion coefficient as close as possible to that of brass, favouring stainless steel instead of plain steel.
A slight flexibility in the fixture (to allow it to follow the core when the brazing paste melts) is also recommended, especially for larger cores.
Due to the differences of the heat expansion for header brass and the steel in the fixtures, especially careful work has to be done when designing the fixtures in the corner area of the cores. Figure 26 shows schematically what happens in this area during brazing.

Figure 26. Principles for the influence of the heat expansion in the
corner of the core. At room temperatures (left) and at brazing
temperatures (right).

In figure 26, lh is the distance between the outer tubes and lw is the length of the steel fixture. At brazing temperature, the steel fixture is shorter than the distance between the outer tubes. If the steel part is too short from the beginning or the fixture is placed too close to the header, the outer tubes will be crushed and leakage in the tube- header in the corner area will appear.
It is preferable to design with side supports that are mechanically attached to the header prior to brazing. These supports provide a well-defined gap during the entire brazing cycle for the outermost fins that are close to the header. In this manner, brazing voids or deformation of the fin or tube in the corner region can be avoided.