Aluminum tube+stainless steel induction brazing

High frequency induction brazing of aluminum and stainless steel pipes

The high-frequency induction welding process for carbon steel pipes is mature, easy to maintain, and widely used. For various types of pipes with large quantities, the welding speed of high-frequency induction welding makes this process far superior to other welding methods. However, high-frequency induction welding of aluminum and stainless steel pipes is relatively difficult. In developed industrial countries abroad, the high-frequency induction welding technology for aluminum and stainless steel pipes has become very mature. Similar research has been conducted in China in the past few years, but it has never been widely promoted. The implementation of high-frequency induction welding for aluminum and stainless steel pipes involves many factors, and only by comprehensively understanding and mastering relevant technologies can stable welding be achieved. In summary, the following factors have a significant impact on the high-frequency induction welding of aluminum and stainless steel pipes.

1. Physical properties of materials

Compared with carbon steel pipes, the MAX hindrance of high-frequency induction welding for aluminum and stainless steel pipes is the influence of oxides. Due to the fact that the melting point of iron oxide (FeO 1420 ℃; Fe2O3 1565 ℃; Fe 1537 ℃) is similar or lower than that of iron elemental, the oxide basically reaches the melting point at the same time or before the base metal during welding. This makes the removal of oxides easier. This situation is completely different when welding aluminum and stainless steel pipes. Firstly, aluminum is highly susceptible to oxidation into refractory AL2O3, although its thickness is only 0.1-0.2 μ M. But the melting point is as high as 2050 ℃, while the melting point of pure aluminum is 658 ℃. During welding, the oxide film hinders the melting and fusion of the base material, making it prone to incomplete penetration defects. The density of the oxide film is higher than that of aluminum, and it is not easy to float off the surface of the molten pool, which can easily form slag inclusion defects in the weld seam. Similar to aluminum, whether it is chromium based stainless steel or nickel based stainless steel, there is a problem of oxide melting points much higher than the base material. When the chromium content in stainless steel ω When the chromium content is greater than 12%, chromium preferentially combines with iron to form a dense oxide film Cr2O3 on the surface of the base material, with a melting point of 2265 º C. And the melting point of nickel oxide (NiO) is 2090 º C, which is much higher than the melting point of nickel at 1446 º C. So the key factor affecting the welding of aluminum and stainless steel pipes is the influence of oxides. Due to the high electrical resistivity and uneven distribution of aluminum oxide, if the cutting quality of the strip is poor, the distribution of the oxide is also extremely irregular. It is difficult to achieve complete removal during high-frequency induction welding. Another influencing factor is that liquid aluminum absorbs hydrogen severely, while in solid state, hydrogen is almost insoluble. Therefore, during the rapid cooling, solidification, and crystallization process of the welding pool, if hydrogen does not escape from the surface of the pool in time, it will form pores in the weld seam. Causing the finished pipe to fail the pressure testing. Due to the easy decomposition of water into hydrogen and oxygen at high temperatures, the presence of water can also have adverse effects on welding.

2. Characteristics of forming processes for different materials

Due to differences in the strength and resilience of materials, there must be certain design characteristics in the forming process. The coefficient of linear expansion and crystallization shrinkage of aluminum are twice that of steel, which can easily cause significant welding deformation and internal stress. The coefficient of linear expansion of stainless steel is also higher than that of carbon steel, for example, the coefficient of linear expansion of austenitic stainless steel is 40% higher than that of carbon steel. The tensile strength, yield strength, and elongation of aluminum and stainless steel are significantly different from those of carbon steel. Practice has proven that austenitic stainless steel has good adaptability to dual radius forming. The forming should adopt the comprehensive bending deformation method, and the deformation process is shown in the figure. Austenitic stainless steel strips should undergo solid solution treatment before forming to reduce hardness and deformation resistance. Stainless steel strips have strong cold work hardening and large rebound under the action of forming rollers. But as long as the roller system design is correct, it is convenient to adjust the roller system gap and extrusion pressure. This problem can be solved very well.

The following table shows the mechanical properties of some commonly used stainless steel materials.

Specifications, mechanical properties, tensile strength, yield strength, elongation, hardness MPaMPa% HV0Cr18Ni9 ≥ 520 ≥ 210 ≥ 35 ≤ 20000Cr19Ni10 ≥ 480 ≥ 180 ≥ 35 ≤ 2000Cr17Ni12Mo2 ≥ 520 ≥ 210 ≥ 35 ≤ 20000Cr17Ni14Mo2 ≥ 480 ≥ 180 ≥ 35 ≤ 200

Reasonable molding process design can utilize products obtained with fewer roller systems.

3. Machining accuracy

Unlike welding carbon steel pipes, the units and molds used for welding aluminum and stainless steel pipes require high machining accuracy. To ensure axial and radial runout during equipment operation. Any significant periodic vibration may cause non ideal states during the welding process, resulting in welding defects. The machining accuracy of foreign machines can generally be controlled within 0.01mm of radial runout, while in China, the maximum machining level can reach 0.03mm. To ensure the stable operation of the unit, high machining accuracy is required, while also placing high demands on the accuracy of all bearings on the unit. Rough mold processing may result in situations as shown in Figure A. Figure B shows the processing conditions that meet the requirements.

4. High frequency welding machine and unit operation control accuracy

Due to the narrow plastic range of aluminum alloy and stainless steel materials, it is necessary to control the input heat. Any large power fluctuation may cause slag inclusion and porosity in the weld seam. Generally, the output ripple of the power supply is required to be below 1%. The better the output smoothness, the more precise the welding power adjustment can be provided, which will affect the appearance and mechanical performance of the weld seam. It is not difficult to understand that the control accuracy of the unit's operating speed should also reach below 3%, otherwise it will cause significant fluctuations in the equivalent power of the welding area. To achieve the control accuracy of the power supply, specific control indicators should generally be proposed for the source effect and load effect of the power supply.

There are many varieties and grades of aluminum alloy and stainless steel materials. The high-frequency welding performance of each material varies. Generally, pure aluminum and non heat treated reinforced deformed aluminum alloys have good weldability, while heat treated deformed aluminum alloys have poor weldability. Austenitic stainless steel in the chromium nickel series has good high-frequency weldability. But there may be some problems with scraping the weld nodules. Magnetic ferritic stainless steel also has good high-frequency weldability and is easy to scrape weld nodules. Martensitic stainless steel with high carbon content has poor high-frequency weldability. The above is just a brief summary of some common influencing factors. It is worth mentioning that high-frequency induction welding of aluminum and stainless steel pipes is a systematic project that requires all aspects to be done well, otherwise it is difficult to succeed.

The high-frequency induction welding of aluminum and stainless steel pipes is not as difficult as people imagine. I believe that with the deepening of people's theoretical understanding and the development of technology, the use of high-frequency welding technology to produce aluminum and stainless steel pipes in China will become increasingly common.