During brazing, the components connected by the brazed parts must be fixed in the position for assembly. During the whole brazing process, the relative position should be fixed to ensure that the welding gap meets the design requirements and the gap is uniform. aluminum Brazing fixture and positioning tooling must be used to ensure the uniform assembly gap and position.
1、 Assembly and positioning of brazed parts
The assembly positioning of aluminum brazed parts depends on the brazing technology used, the materials to be brazed and the joint type of the brazed components. For different structures, the assembly and positioning methods are different. The specific examples are as follows:
1. For sheet metal components, spot welding is often used to locate the position, such as resistance intermittent spot welding, capacitor energy storage spot welding, argon arc welding and riveting.
2. Cylindrical parts, pipes and solid parts can be brazed by various methods. If possible, the assembly method used shall ensure that the joint gap is uniform throughout the brazing process. The methods used for assembling cylindrical parts are: pile shoulder, pipe expanding mouth, bell mouth, curling, upsetting, knurling and embossing.
3. For the assembly positioning of more complex structures, such as in the aviation industry, it is often necessary to do the assembly positioning with fine workmanship. The conditions are as follows:
- the qualified joint clearance of structural components must be maintained at the brazing temperature, which conforms to the characteristics of filler metal, basic metal and brazing method used;
- In many cases, air passage and cooling hole must be considered in the process of brazing and welding. After the solder is finished, the fixture shall not be adhered with solder;
- “self clamping” assembly structure is adopted for self positioning in the design of joint, so as to ensure the assembly clearance and uniformity. The specific structure examples are as follows:
2、 Positioning of brazing fixture
When aluminum brazing complex parts, it is often necessary to select fixture for positioning. In general, the following principles should be followed when selecting the positioning tooling and fixture for brazing:
1. The fixture should be as simple as possible. The fixture with thin section shall meet the requirements of rigidity and durability.
2. Avoid using bolts or screws in the fixture. Because they are easy to relax when heated, they often can not exert pressure on the brazing seam to maintain its installation position.
3. If the spring or clamp is used in the heating area, it must be able to withstand the brazing temperature, otherwise it will produce relaxation during brazing operation.
4. The thickness of the fixture should be uniform and uniform, and large thickness section should be avoided.
5. In most cases, it is necessary to avoid using dissimilar materials with different expansion coefficients to make fixtures, as they may affect the size of the components. However, when different thermal expansion has advantages, exceptions can be made.
6. Before being used as aluminum brazing equipment, the fixture should be subjected to the effect of brazing environment and temperature to ensure the dimensional stability and stress relief of the fixture.
7. The fixture structure sa hould be easy to check, and the size of the fixture must be checked frequently.
8. Aluminum and titanium nickel base heat-resistant alloy will produce protective oxide film when heated in air, which helps to prevent the parts from welding with the fixture.
9. The use of materials that react in close contact with brazed components at high temperatures should be avoided. For example, do not use nickel base alloy clamps when brazing titanium, and vice versa. Because of the formation of Ni Ti eutectic at 943 ℃, this temperature is usually lower than that of Ti or Ni based alloys.
10. By using flexible high melting point plated gaskets, balls or pins, the fixture can exert local pressure on the brazed member.
11. When the brazing components are required to maintain the precise shape tolerance, they need to have a very stable clamping effect. Metal fixture is often lack of stability after repeated use, so in order to maintain precision tolerance, ceramic fixture is usually used. Castable Ceramic materials, such as fused silica, are used with excellent dimensional stability and precision tolerances without machining. Alumina can also be used to make brazing clamps, but it is difficult to machine precise tolerances and poor heat shock resistance.
12. When selecting the fixture, the brazing method, brazing temperature, shielding gas and properties of base metal material should be considered simultaneously. Different brazing methods have different requirements. The specific examples are as follows:
1) In vacuum brazing, the only limitation of fixture material is that it must be stable at brazing temperature, has the expansion property matching with the brazed workpiece, and does not contain gasifying substances and substances that may pollute the brazing atmosphere.
2) In induction brazing, it is usually required to place the fixture near the inductor. If this is the case, the fixture material is limited to the material that is not affected by the induction field.
3) In salt bath brazing, the clamp used should be free of moisture and has clamping effect all the time, and the buoyancy effect of molten salt must be considered. The fixture material shall not react with molten salt.
4) Due to the large thermal expansion coefficient of aluminum and magnesium, the fixture for brazing such materials must be equipped with spring or some other adjustment devices. Generally, nickel base heat-resistant alloy and stainless steel can be used as spring materials.
5) The tooling for flame brazing shall not hinder the brazing flame, the welder’s sight or the application of brazing filler metal.