Shenyang Beizhen Vacuum Technology Co., Ltd. specializes in the production of vacuum furnaces. It is mainly engaged in vacuum brazing furnaces, vacuum sintering furnaces, hydrogen furnaces and other heat treatment furnaces. It is one of the domestic large-scale manufacturers. For free consultation, please call: 13998872066
Vacuum brazing can obtain precision and bright joints, and the joints have excellent mechanical properties and corrosion resistance. Compared with other brazing methods, they have obvious advantages in the control of part oxidation and part deformation. These characteristics are for air cooling. The water-cooled cold plate and the box and box with special requirements are very suitable. The high-precision parts can be directly obtained, and the quality is easy to control. Now the application of vacuum aluminum brazing in the electronic product cooling system has been extensive. The main processes of vacuum brazing include: part processing, pre-brazing, part assembly and fixing, brazing, brazing and quality inspection. Each process will affect the final brazing quality. The following discussion focuses on vacuum. Factors that must be considered when designing and selecting aluminum brazing fixtures.
1 Brazing fixture and its general design principles
Brazed parts shall be assembled and positioned during brazing to ensure the mutual position of the parts and the joint clearance required for brazing. 2. The brazing fixture involves the whole process of fixing, assembling, feeding, brazing and discharging the brazed parts. Whether the fixture is reasonable and practical directly affects the quality of the brazing. The principles and requirements of the common fixture design are also applicable to the design of the brazing fixture. For example, the fixture system must have a certain precision; the fixture system must have a certain rigidity to overcome the deformation during the machining process; the fixture system should have a compact structure and a simple shape. In addition to the above principles, the brazing fixture has its own characteristics. Generally speaking, the brazing fixture design must also consider the following factors: (1) The fixture material can withstand the brazing temperature without losing strength. , does not deform and release gas, is not easy to produce alloy reaction with the group weldment; (2) pay attention to the temperature expansion and contraction of the fixture and parts, at the brazing temperature, to ensure that the brazed parts have suitable joint clearance; (3) fixture To ensure heat transfer in the brazed zone, the resulting heat transfer has minimal interference with the stable heating and cooling of the part and does not interfere with the flow of the solder.
2 vacuum aluminum brazing fixture design
2.1 Selection of fixture materials The choice of fixture materials is to ensure that the rigidity of the fixture material is greater than that of the brazed parts at the brazing temperature, so that the deformation of the weldment is always limited by the fixture during the brazing process. The weldment and its mating size can be guaranteed by fixture assembly. The working temperature of vacuum aluminum brazing is about 600 °C, the brazing time is 3-6 hours. Generally, 1Crl8Ni9Ti can be used to meet the requirements of use. 1Crl8Ni9Ti is austenitic stainless steel, the oxidation resistance is above 700 °C, and the important is the brazing temperature. There is still enough heat strength, the rigidity is much larger than that of aluminum alloy, the structure is stable, and the long-term rotation will not be embrittled. Therefore, the basic frame and main working parts of the brazing fixture are very suitable for 1Crl8Ni9Ti; for some matching dimensions and precision For parts with high requirements, the design of the fixture must involve the matching of the thread pair and the positioning hole shaft. The material selection is not suitable, the structure is close, the thermal stability is poor, and there is a “seizure” between the fixture parts under the brazing temperature. lCrl8Ni9 Fi can be used to solve this problem better.
2.2 to consider the impact of thermal deformation
The influence of brazing fixtures and parts on the brazing process caused by thermal deformation at the brazing temperature should be considered. Due to the different materials of the fixture and the brazed parts, the coefficient of thermal expansion is different. In the case of heating, the difference in the amount of expansion will occur, and the brazed parts will be deformed. The purpose of the fixture is to deform the brazed parts with the deformation of the fixture. Said, mainly for brazing parts flatness, verticality, weld gap requirements. For other parts that are not very strict, the forced deformation fixture can meet the requirements of use; however, for some parts with higher dimensions and matching dimensional accuracy, the use of forced deformation clamp can not meet the requirements of use, and must be considered in the forced deformation fixture. On the basis of the design, part of the elastic component is composed of a flexible clamp system. The elastic component is generally a high temperature spring or an elastic collet. The deformation of the elastic component is controlled by the calculation of the difference in the amount of expansion, so that the necessary pressure can be maintained on the brazed component. The problem of the difference in the amount of expansion is also solved.
As shown in Figure 1,
The parts are box-type brazing parts, and the upper and lower rail plates and the front and rear support frames form a box body with a pluggable insert, and a corrugated plate is placed between the upper and lower cover plates and the upper and lower rail plates to dissipate heat. In general, this part has a significant requirement, high dimensional accuracy and assembly precision requirements. After molding, it is not processed. The positioning accuracy between the upper and lower rail plates is not more than 0.15mm, and the upper and lower spacing dimensions are not more than 0.2 mm. The plugin needs to be plugged in freely. In the actual production, the thermal expansion of the fixture and the weldment material is accurately analyzed and calculated, and the elastic stainless steel pad and the steel C-clip are designed to eliminate the influence of the difference in thermal expansion on the brazing. As shown in the fixture system of Fig. 1, the periphery of the box is clamped by a small rigid C-clip and a pad. The upper and lower corners of the box are integrally clamped by C-clamps and elastic members over the entire height. The difference in the amount of expansion produced is calculated according to the coefficients of thermal expansion of the two materials.
Where: △ is the difference in the amount of expansion caused by thermal expansion; L is the clamping distance; ΔT is the brazing temperature minus room temperature, here 570 ° C ~ 580 ° C; a2 is the average thermal expansion coefficient of the clamping member, ° C; a1 is The average thermal expansion coefficient of the clamp, °C. The total height of the cabinet is 410 mm, and the clamping distance between the small chucks is 35-40 mm. After calculation, the amount of heat generated by the height of the box is 2.38 mm, and the amount of heat generated by the small chuck is 0.19 mm. (The amount of gap is negative, indicating that the length of the clamp is greater than that after heating and expansion. Chuck clamping spacing). After the small C-clamp clamping position is heated, the gap is 0.19 mm. According to the analysis, two brazing faces are included here, because the brazing filler fills the joint gap. There is a 0.10 mm collapse on each brazed surface, and the two are offset. It is feasible to use a rigid collet; the overall height of the box is 2.38 mm after heating, minus the collapse of the four brazed surfaces. The existing gap is about 2 mm. If the gap adjustment is too large, the clamp cannot be effectively clamped after being heated, the gap is not guaranteed by brazing, and the welding is not sufficient or the weld is obviously uneven; if the gap adjustment is too small, After the heating, the chuck shape variable can not meet the deformation requirements of the weldment, and the box body will be deformed due to the restriction of the clamp, that is, the four-corner support column is bent or twisted due to insufficient thermal expansion space, and the parts are scrapped. The problem is solved by the combination of the self-made stainless steel spring pad (the piece 10 in the figure) (the deformation of the spring pad is controlled at 2 mm). The brazing piece is welded sufficiently, including the weld seams of the upper and lower corners, which are consistent with the design. Claim.
2.3 Fixtures should fully consider the requirements of uniformity of heat dissipation of brazing parts. Heating and cooling speed of parts during brazing is also an important process parameter. Vacuum brazing of aluminum, vacuum degree is controlled at about 3×10ˉ3Pa, using radiant heating In order to ensure that the heat dissipation during the brazing process is a uniform process, the deformation of the weldment can be minimized, and the dimensional accuracy of the weldment is easily ensured.
As shown in Fig. 2, if the heating cycle temperature curve of aluminum brazing is too fast, the temperature inside the weldment will be uneven and internal stress will be generated. If the heating is too slow, for example, the parent metal grain will grow, and the low boiling point of the brazing material. The rapid development of harmful processes such as elemental evaporation and metal oxide flux decomposition, the cooling rate of the weldment has a direct influence on the joint quality. Slowly slow cooling may cause the base material to grow, and the accelerated cooling speed is beneficial to refine the brazing seam. The structure increases the strength of the joint, but if the cooling is too fast, the weldment may be cracked due to the formation of excessive thermal stress, or the joint may be too fast to solidify so that the gas does not escape and the pores are generated. In the temperature control of the weldment, based on the above curve, good results can be obtained in the brazing. The heating rate in the curve is mainly determined by the material shape and structural size of the weldment, and also in the form of using the brazing material. There is a direct relationship between the crystallization temperature range of the solder. However, if the brazing fixture neglects the heat dissipation requirement and limits the heat exchange of the parts, the quality of the brazed weld is difficult to guarantee.
As shown in Fig. 3, the brazed part is a box body which is connected by brazing and welding of the upper and lower rail plates and the left and right side plates. This part not only requires convenient insertion and removal of the insert after molding, but also has airtightness requirements for the box body, so that In addition to the high requirements for brazing precision, the welding standards of the welds on both sides are also improved. During the actual brazing process, it is found that the weldment has a gas-tight inspection failure, which means the quality of the weld. Not up to standard. After analysis, it is confirmed that the design of the brazing fixture is unreasonable. The fixture limits the heat conduction in the brazing zone and affects the quality of the weld. The fixture is designed without considering the heat dissipation factor. The weldment is surrounded by the front and rear positioning plates and the weight plate. Among them, affecting the heat exchange inside and outside the box, the heating and heat dissipation process of the brazing part is affected by the fixture. In this process, the temperature difference between the inside and outside of the box may cause inconsistent deformation during thermal expansion and cold shrinkage, and the process of brazing It is a rapid process. When the brazing material reaches the brazing temperature, the wetting of the brazing material and the formation of the joint require only a few seconds. The temperature inside and outside the brazing material is inconsistent, and it is easy to cause the weld to have pores and clips. The slag, even the weld seam is cracked, and the weldment is scrapped. In order to solve this problem, a pad is added between the front and rear positioning plates of the clamp and the box body. The positioning plate adds a heat dissipation hole, and the heat dissipation cavity and the heat dissipation hole are designed on the weight plate. This measure solves the brazing part. The problem of uniformity of heating and heat dissipation improves the quality of the weld and meets the requirements for airtightness of the parts.
3 small knots
The factors to be considered in the design of brazing fixtures in vacuum furnace are more complicated. Under the conditions of ensuring the above requirements, the influence of gravity should also be taken into consideration. After the weldments are heated, the joints will loosen due to expansion, and the brazing material will naturally flow downward. Therefore, attention must be paid to the clamping and placement of the fixture and the weldment; the fixture structure should be as simple as possible, the brazing furnace is a closed process, the specific process is not visible, the simpler the fixture, the less unforeseen problems, so that the fixture is reliable High sex and operability. In short, the brazing fixture design should focus on the following aspects: to ensure the joint gap of the brazing joint, for the assembly of aluminum and aluminum alloy, there is a gap of 0.05 ~ 0.10 mm in the joint, at this time the solder fluidity Best; pay attention to the expansion and contraction of the fixture and the group weldment at the brazing temperature, consider the difference of the expansion coefficient, and ensure the coordination of the parts; the influence of the fixture on the brazing process of the part is minimal, and the heat conduction of the brazing zone is not affected, Prevents the flow of solder.