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Aluminum alloy and stainless steel vacuum brazing

With the acceleration of the energy saving and environmental protection process and the lightweight trend of various mechanical parts in the automotive and industrial fields, the development and research of aluminum alloy and stainless steel vacuum brazing technology is becoming more and more important. However, not only because of the strong oxide film and its melting point, The thermal physical properties are very different, and the formation of brittle intermetallic compounds makes it difficult to weld aluminum alloys and stainless steels. Even if a more suitable vacuum brazing method is used, there are only research attempts to select Al-Si solders. In the early stage of the study, it was difficult to form joints between aluminum alloy and stainless steel by vacuum brazing with Al-Si brazing filler metal. It is caused by the diffusion of Zn in the flux into the solder layer.

Based on the above research results, on the other hand of improving the vacuum brazing method, this research tried to add different amounts of Zn to the Al-Si-based solder, develop Al-Si-Zn solder, and evaluate its process performance. At the same time, the vacuum brazing property was investigated, that is, the vacuum brazing of aluminum alloy and stainless steel was carried out by using the newly developed Al-Si-Zn solder and zinc-free solder. While verifying that the zinc content has a great influence on the (rapid) isothermal solidification of Al-Si-based solder, the difficulty, microstructure, and Mechanical properties, etc., in order to predict and control the performance of the formed joint by controlling the zinc content.

1 Aluminum alloy and stainless steel vacuum brazing test method

The main materials used in the test are heat-resistant, high-strength, widely used austenitic stainless steel 304 and industrial aluminum alloy 6063 plate, BAI88Si brazing filler metal, and QFB-204 KF-ALF3 eutectic active flux. Auxiliary materials for smelting solder include pure zinc, potassium chloride, sodium chloride, etc.

1.2 Aluminum alloy/stainless steel vacuum brazing test using Al-Si-Zn solder

First, process the base metal for the test into a sample of 20mmx40mmx3mm, cut the newly smelted solder into (20mmx15 mmx1.5mm) the size consistent with the area of the overlapping base metal, and vacuum braze the surface of the base metal before vacuum brazing. After grinding, carry out ultrasonic cleaning and degreasing in acetone together with the brazing material, weld a thermocouple on the stainless steel side, clamp the brazing material between the two overlapping base materials and set it in a ceramic square cup, and adjust the brazing flux into a paste with water Shape, and then coated on the base metal and the vacuum brazing part of the brazing material, using a box-type resistance furnace to heat at a heating rate of about 773 K/300 s, and vacuum brazing in a vacuum brazing furnace. The vacuum brazing temperature is between 873-883K, and the vacuum brazing time is 50s. After the vacuum brazing is completed, the sample is air-cooled to about 300 C in the atmosphere and placed in boiling water for cleaning to remove residual flux and impurities.

After vacuum brazing, tensile tests were carried out on the brazed joints, and SEM structure observation, energy spectrum analysis, X-ray radiation analysis and other test and analysis methods were used to observe the elemental diffusion status of the base metal, solder and flux components , to study the tissue characteristics and mechanical properties of its joints.

2 Experimental conclusions of vacuum brazing of aluminum alloy and stainless steel

(1) Adding zinc to Al-Si-based high-temperature solder, and developing the smelting of Al-Si-Zn solder is quite complicated and difficult, because the solder is oxidized at high temperature very seriously, and it is easy to form a dense oxide film. Melting temperature is difficult to control. The measure to solve the problem is to add a certain proportion of sodium chloride and potassium chloride to the Al-Si based solder to form a protective film, thereby increasing the fluidity of the solder and making casting possible.

(2) The effect of adding Zn on the mechanical properties of Al-Si-based solder is greater than that on the process performance. With the increase of zinc content, the fluidity and wettability of the solder are improved. When w(Zn)15% When w(Zn) is 0.1%, the process performance is better but the mechanical property is the worst; and when w(Zn) is 0.1%, the tensile strength of the solder is the highest and the process performance is poor.

(3) The effect of the developed Al-Si-Zn solder on the properties of aluminum alloy/stainless steel joints is closely related to the amount of zinc added. With the increase of zinc content, although the wettability and spreadability of brazing filler metal to stainless steel has been improved, on the other hand, it promotes the wetting and absorption of Al-Si-Zn brazing filler metal and aluminum alloy base metal; In addition, the volatilization and diffusion of Zn is easy to produce pores, so it does more harm than good to the performance of the joint, so it eventually leads to a significant decrease in the strength of the joint. In the future, controlling w(Zn) at 1%~1.5% or using the rapid solidification method to refine the solder is expected to improve its process and mechanical properties.

(4) Al-Si-Zn solder with w(Zn) 15% has the worst joint performance in vacuum brazing stainless steel/aluminum alloy. This result verifies that with the increase of zinc content, Al-Si-Zn solder and The fact that the aluminum alloy undergoes a wetting reaction and accelerates the isothermal solidification process.

(5) The difficulty in vacuum brazing of aluminum alloy and stainless steel lies in how to ensure that the brazing filler metal fully wets the stainless steel (synchronized with the aluminum alloy), effectively controls the zinc content in the brazing filler metal, and controls and reduces the generation of brittle intermetallic compound FeAl as much as possible .