Vacuum Brazing Furnace-aluminum Alloy Brazing

1.  Cleaning and Assembly Before Welding

Aluminum and aluminum alloys will quickly form a strong adhesive and heat-resistant Al2O3 oxide film when exposed to the air. The oxide film easily absorbs moisture, which not only hinders the good bonding of the brazing seam, but is also the source of pores and slag inclusions. In order to ensure the quality of brazing, strict cleaning measures should be taken before welding to thoroughly remove the oxide film and oil on the surface of the base material and the brazing filler metal. The chemical cleaning process is as follows: first, etch the aluminum material with a solution of w (NaOH) 10% for 15 min and the brazing filler metal for 10 min. The temperature of the solution should be 40~60, and etch the aluminum material for 10 min in a nitric acid + hydrofluoric acid solution. 5min; The solution ratio is 15L for W(HNO3) 58% ~ 62% solution and 0.6L for w(HF) 48% solution. In the process of chemical cleaning, the heating temperature and solution concentration should not be too high, otherwise the chemical reaction will be too violent and a white film will be formed on the surface of the sample, which will affect the welding quality. After chemical cleaning, the residual solution on the sample must be rinsed with water, otherwise it will cause local pitting corrosion and reduce the service life of the weldment. It is best to braze the base material and the solder in time after cleaning, otherwise the oxide film will be regenerated during storage, so the time interval from cleaning to before welding should be shortened as much as possible, not more than 12h, otherwise it needs to be cleaned again. The reasonable assembly of brazing joints has an important influence on ensuring good brazing, manufacturability and the performance of aluminum-copper brazing joints. In the experiment, the aluminum-copper plate butt joint is used. The base material and solder are placed in the order of aluminum plate-sheet solder-copper plate. The assembly diagram of the aluminum-copper vacuum brazing joint is shown in Figure 1.

By using gaskets (the thickness of the gasket is 7.3~7.4mm) to control the gap between the brazing seams of aluminum and copper, the brazing gap is kept between 0.2~0.3mm. Within this gap value range, the brazing joints of aluminum and copper have the largest Bond strength. This is because the gap value range of 0.2 ~ 0.3mm ensures that the brazing filler metal is fully and densely filled, the base material has a good alloying effect on the brazing filler metal, and the base material has sufficient support for the brazing gold layer. The gap is too large or too large. If the gap is too small, it will affect the compactness of the brazing seam and the strength of the joint. If the gap is too small, it will be difficult to fill the gap with solder, and the gas in the gap will be more difficult to discharge, which will easily cause defects such as incomplete penetration and pores. When the gap is too large, the capillary action will weaken. It also makes the brazing filler metal unable to fill the gap, the alloying effect of the base material on the center area of the brazing filler metal disappears, the brazing seam crystallization generates columnar structure and dendrite flattening, and the support of the base metal to the brazing gold layer is weakened.

2. Points of Brazing Process

The main process parameters of aluminum-copper vacuum brazing include brazing heating temperature, heating rate, holding time and vacuum degree. The brazing heating temperature should be higher than the melting point of the solder and lower than the melting point of the base material to reduce the surface tension of the liquid solder, improve the wetting and joint filling ability, and enable the solder and the base metal to fully interact, which is beneficial to improve The strength of aluminum-copper joints. Because the vacuum furnace is heated by radiation, the furnace temperature is not exactly the same as the temperature of the workpiece. Therefore, controlling the heating rate is very important for the quality of the aluminum-copper brazed joint. The holding time determines the bonding strength of the aluminum-copper brazed joint. If the holding time is too short, the solder is not completely melted, and the combination with the base material is insufficient, resulting in a decrease in joint strength; if the holding time is too long, the solder will melt excessively, causing the base material Excessive reaction with the brazing filler metal (or even a direct reaction between the base metal) causes dissolution. Reasonable heat preservation time should make the surface layer of the solder micro-melt, and it can be combined with the base material. Vacuum is the most important and difficult to control process parameter in the aluminum-copper vacuum brazing process. To obtain high-quality aluminum-copper brazing joints, it largely depends on the degree of vacuum.In actual production, if the vacuum brazing equipment has not been used for a long time, the vacuum furnace should be operated for a few hours before use; in mass production, the time interval between two uses should be as short as possible to ensure the vacuum in the vacuum furnace Meet the requirements quickly. The main points of the process of vacuum brazing aluminum-copper dissimilar non-ferrous metals with aluminum-silicon-magnesium (Al-Si-Mg) solder are: the brazing heating temperature is 590 ~ 615 ℃; the heating rate should be faster, about 15 ~ 20 ℃/min; heat preservation The time should not exceed 5min; the vacuum degree should be maintained above 1×10-3Pa.