Aluminum alloys are widely used in aerospace, construction, electrical appliances, automobiles, ships and other sectors because of their light weight, good corrosion resistance, excellent thermal conductivity and other characteristics, and their consumption is increasing. Aluminum alloys have long been considered as structural materials for making heat exchangers, waveguides and many other complex components. The research on Brazing of aluminium alloy focused on its metallurgical properties at first, and then on the role of adding elements in brazing process. At present, the research on Brazing of aluminium alloy for special purposes is mainly focused on the welding of aluminium alloy. Many special purpose aluminium alloys are being developed and applied in engineering practice. LT-3 aluminium alloy is designed for vacuum brazing of aluminium alloys. It is made of 4004 aluminium alloy rolled on the base of 3003 aluminium alloy by double bread cladding. The cladding material plays the role of solder in the brazing process. This material is widely used in the manufacture of radiator and condenser. The vacuum brazing scheme of LT-3 coated aluminium alloy was introduced, and the corresponding welding process was formulated.

1. Introduction of Vacuum Brazing Scheme for Aluminum Alloy

1.1 principle

Vacuum brazing is carried out in vacuum furnace, and no flux is used in the brazing process. Although there is no chemical action of flux and reduction effect of reducing atmosphere, the oxide film on the surface of welded parts can also be removed to protect the welded parts from oxidation due to the reduction of oxygen partial pressure in the brazing area.

1.1.1 Mechanisms of Removing Oxide Film on Metal Surface

A. The oxide film can be decomposed by itself in high temperature and high vacuum: decomposition pressure of oxide > partial pressure of oxygen in vacuum.

B. Magnesium vapor destroys oxide film on metal surface. In order to prevent the corrosion pollution of flux and cleaning after brazing, vacuum brazing usually uses metal instead of flux to replace oxygen, which is more active than aluminium. Magnesium metal is mostly used. However, with the addition of Mg, the solder has a serious corrosion effect on the matrix metal, and the content of Mg in the general solder is controlled by 1.0%-2.5%.

C. The expansion of aluminium force aluminium trioxide to crack first, so that the liquid filler metal from cracks into the oxide film, uncovering the oxide film.

1.1.2 Requirements for Brazing Alloys

The filler metal used for vacuum brazing is silicon-aluminium, which requires strict composition. The filler metal should not contain elements with high vapor pressure and elements with significant decomposition of surface oxide at vacuum and high temperature. Otherwise, the vacuum brazing process and the quality of vacuum brazing will be affected.

1.2 Vacuum Brazing Technology

Careful cleaning is required before vacuum brazing. Generally, oil stain treatment is enough for welding parts, but in order to remove the thick oxide film formed during heat treatment, sometimes cleaning agent is also needed. Temperature period depends mainly on the shape of the weldment in various factors, because heat is transmitted in the form of radiation in vacuum, and the parts with different working thickness will be heated unevenly. Because the difference between solids and brazing temperature of aluminium alloy is often small, the thinner part of the workpiece is easy to overheat, resulting in surface melting. So there is

It is necessary to preheat the workpiece, especially the larger one, to ensure that the parts are uniformly heated before reaching the brazing temperature. Because the removal of oxide film on the surface of Vacuum Brazed aluminium alloy mainly depends on Mg activator, in order to ensure that the base metal is sufficiently affected by Mg vapor, some domestic research institutes often adopt supplementary technology measures of partial shielding, and have achieved very good results. The most common method is to put the welded parts into stainless steel box (called process box) and then place them in vacuum furnace to heat brazing, which can significantly improve the quality of brazing, in which the formation of Mg vapor can be used by adding solid Mg particles into the filler metal or directly introducing Mg vapor. This can greatly reduce the pollution of Mg and other elements on the furnace during charging and discharging, thus reducing the cleaning times of vacuum brazing furnace and improving the service life of vacuum brazing furnace.

Vacuum brazing is the most important and difficult process parameter to control. In order to obtain excellent joints, the vacuum degree in the vacuum brazing furnace depends to a great extent, so the leakage rate of vacuum equipment is very low. During the heating process, the furnace is pumped to a certain vacuum, and the requirements for aluminium and its alloys are 0.0013Pa to 0.O133Pa. It is generally believed that when the vacuum brazing furnace is opened, water vapor in ambient temperature air is absorbed, which greatly prolongs the vacuum time before the next vacuum brazing temperature rise. According to experience, if vacuum brazing furnace is not used for a long time, vacuum brazing furnace should be used after baking. When using, especially in batch production, the time interval between two entries and exits should be as short as possible, so that the vacuum degree of vacuum brazing furnace can easily and quickly meet the required requirements, thereby improving production efficiency. The heating speed, stabilization temperature and holding time of vacuum brazing furnace temperature depend on the material, shape, structure, size, brazing solder form and the range of brazing solder crystallization temperature.

2 Vacuum Brazing Technology

2.1 Vacuum Brazing Welding Analysis

LT-3 aluminium clad plate is manufactured by cold rolling process. The core metal 3003 is first rolled into shape, and then the epithelial metal 4004 is rolled on the top of the core metal. Core metal 3003 belongs to Al-Mn alloy, which has good corrosion resistance and pressure workability.

Cortical metal 4004 is based on eutectic Al-Si alloy. It has good wettability and fluidity, and contains 1.0-2.0% Mg, which is helpful to remove oxide film. Vacuum brazed clad sheet is much more convenient than wire, strip or powder to use 304 and can be used to make complex parts. When the cladding metal on the brazing plate melts, it can wet the bonding base metal directly and fill the gap immediately.

Joints can be formed with only slight diffusion. The chemical composition and melting point temperature of LT-3 aluminium composite sheet are shown in the table below.

Chemical Composition Table of Core and Core Layer of LT-3 Aluminum Alloy

Melting Temperature/C of Material Si Fe Cu Mn Mg Zn Al

Core layer (3003 aluminium alloy) margin 643-654

Cortex (4004 aluminium alloy) 9.0- 0.1 1.0-2.0.2 residual 559-591

2.2 Welding Process of Vacuum Brazing

2.2.1 Cleaning before Vacuum Brazing

Brazing of aluminium and aluminium alloys requires high cleanliness of workpiece surface. In order to obtain good quality, oil stain and oxide film on the surface must be removed before brazing. The surface oil pollution can be removed by washing with Na2CO3 solution at 60-70 C for 5-10 minutes, then rinsing with clean water; the surface oxide film can be removed by soaking with NaOH solution at 20-40 C for 2-4 minutes, then rinsing with hot water; the workpiece after removing the surface oil pollution and oxide film can be washed in running water and finally dried by air. After treatment, braze within 6-8 hours and braze as soon as possible.

Because of the active nature of aluminium, it is easy to regenerate oxide film on the surface of metal if it stays too long. The workpiece surface should not be excessively polished before welding, so as to avoid the damage of the skin metal. It is also easy to destroy the smoothness of workpiece surface when grinding, which makes brazing difficult.

2.2.2 Vacuum Brazing Technology

Generally, vacuum brazing adopts stage heating mode to ensure uniform heating of workpiece. The technological parameters of vacuum brazing are as follows: setting temperature 620 C, holding 15 min-30 min, vacuum degree 3 x 10-3 PA and heating rate 10 C/min in the brazing temperature control meter. In the initial cooling stage, the brazing temperature is controlled at 10 C/min, ending at 450 C, and then cooling to room temperature with the furnace.

The vacuum brazing test of aluminium alloy radiator was carried out by using six different brazing holding time and temperature of 15min-90min and 595-620 C. The effect of vacuum brazing holding time and temperature on weld structure was determined by optical microscopy, scanning electron microscopy, X-ray energy dispersive spectrometer and hardness test. Relationship.

The results show that with the prolongation of vacuum brazing holding time and the appropriate increase of holding temperature, the element diffusion between welds becomes more and more sufficient, and the shape of Si segregation changes from dendritic to dotted. When vacuum brazing process is kept at 615 C for 75 minutes, only a small amount of dotted Si segregation structure exists in the weld structure, most of which are dotted Si segregation structure. Solid solution, solder has a little corrosion on the base metal, bonding more firmly, vacuum brazing process is better.

The requirements for vacuum brazing furnace are as follows:

(1) Vacuum brazing furnace needs sufficient heating rate and great pumping speed, which requires the limit vacuum degree to be less than 4*10-4 Pa, the working vacuum degree to be less than 3.0*10-3 Pa, and the brazing temperature fluctuation in the soaking zone to be less than 3 degrees Celsius;

(2) Vacuum brazing furnace should have advanced, reliable and safe electrical control and protection system;

(3) The size of heating chamber of vacuum brazing furnace can accommodate brazed components;

(4) Vacuum brazing furnace should have forced cooling function. First, it can meet the requirements of material vacuum brazing technology, and second, it can shorten the period of vacuum brazing. In order to prevent the occurrence of brazing defects such as non-penetration, melting and loss of filler metal, vacuum brazing was carried out before and after vacuum brazing.

The following points should be noted in the process:

(1) Strictly control the vacuum brazing temperature and holding time to ensure that the vacuum degree and temperature uniformity of the vacuum brazing furnace system meet the standard requirements.

(2) Before vacuum brazing, the oil stain and oxide film on the workpiece surface should be cleaned thoroughly, so that the liquid solder can wet the core layer completely.

(3) Workpiece assembly should be reasonable to ensure the normal filling of liquid filler metal. The fixture for vacuum brazing assembly should be designed reasonably to minimize the heat capacity of fixture and prevent brazing deformation of products.