1、 Conventional brazing process for brazing joints of refrigeration equipment

Brass and copper brazing joints used in the air conditioning/refrigeration industry, such as pipe joint components, diverter (distributor) components, four-way valve end cover welds, globe valve outlet pipe welds, solenoid valve connecting pipe welds, etc., are usually brazed using flame brazing or conventional induction brazing (also known as conventional high-frequency welding). The following is an introduction using the commonly used brass pipe joint and copper pipe in the air conditioning industry as an example.

2、 Problems with using conventional brazing processes

1. Using the current conventional induction brazing method, after welding the brass pipe joint components in the refrigeration industry, there will be obvious oxidation and blackening appearance defects in the base material and weld seam

2. The base material and solder are easily oxidized during the heating process, which seriously affects the flow and spread of the solder, and the brazing quality cannot be guaranteed;

3. Welds, pipe joints and copper pipes are oxidized and blackened, and the color and appearance of welds are consistent with that of base metal. Operators cannot clearly identify and detect the filler metal filling, and can not visually detect whether there are slight welding defect such as pores, cracks and sand holes in welds, which may lead to leaking defective parts flowing into customers and after-sales quality problems;

4. In order to improve the flowability of the brazing material, a large amount of brazing flux is usually applied to the pipe joint components or welding rings before brazing, resulting in severe flux residue in the weld seam and nearby areas after welding, which affects the inspection and appearance quality of the weld seam;

5. Untreated pipe joints, workpieces stored, placed or used under humid conditions, may have appearance defects such as copper green near the weld seam, which affects the performance and reliability of the weld seam;

6. Poor appearance quality due to oxidation blackening can lower the overall product grade;

7. Oxidized blackening layer. If used directly without treatment, in the later use process, the oxidized blackening layer may corrode first, accelerate the corrosion of welds, shorten the service life of brazed joints, and thereby affect the service life of the entire machine or cause the machine to malfunction;

8. If mechanical or chemical methods are added to treat the oxidized black layer after welding, it will increase the operating procedures, personnel, equipment, and materials, increase production costs, and reduce production efficiency.

3、 Analysis on the Causes of Black and Long Copper Green After Welding in Conventional Brazing Joints

After analysis and verification, it can be concluded that the main reasons for the problems of oxidation blackening/copper green after welding of the above-mentioned pipe joint components when using flame brazing or conventional induction brazing are as follows:

1. Pipe joints, copper pipes, and brazing materials are all heated under atmospheric conditions, usually at brazing temperatures above 700 ° C. During the heating process, the base material and solder come into full contact with oxygen in the air, producing oxides such as copper oxide, resulting in the appearance of black substances on the surface of the joint.

2. The base metal and solder are highly susceptible to oxidation during the heating process, forming a dense oxide film on the surface of the base metal and solder, which seriously affects the flow and spreading of the melted solder.

To ensure the fluidity of the solder, during flame brazing or conventional induction brazing, operators will add a large amount of brazing flux to the workpiece and solder surface to fully remove the oxide film on the base metal surface. During the brazing process of copper and copper alloys, the mechanism of removing various brazing fluxes from the workpiece and the oxide film on the surface of the solder is as follows:

1) The Film Removal Mechanism of Boric Acid Solder Fluxes

Boric acid (H3BO3) decomposes into boron liver (B2O3) when heated. The generated boron liver (B2O3) has a melting point of 580 ℃ and forms a fusible borate with copper oxide.

2) The mechanism of removing film of borax type brazing flux

Borax (Na2B4O7 · 10H2O melts at 741 and decomposes into boron liver (B2O3) and sodium metaborate (Na2B4O7) in liquid state.

Boron liver and metal oxides form easily soluble borate, while metaborate and borate form composite compounds with lower melting temperatures.

The generated composite compound is easy to float to the surface of the brazing seam, and borax has a stronger film removal ability than boric acid. However, borax has a higher melting point, high viscosity below 800 ℃, and poor flowability.

3) Film removal mechanism of gas flux and acid free flux

Trimethyl borate (C3H9BO3) is mixed with oxygen and burned to form boron liver, which reacts with copper oxide to form borate

4) Demembraning of potassium tetrafluoroborate

The addition of potassium fluoride can reduce the melting temperature of boric acid and boron liver, as well as the viscosity of brazing flux. When the content of KF is 40%, the melting point of boron liver can be reduced to 600 ℃. To further reduce the melting temperature of boron liver and improve its activity, KBF4 can be added, with a melting point of 540 ℃. After melting, it decomposes into BF3, which has strong antioxidant ability.

As is well known, the oxide film on the surface of the base material and solder can seriously hinder the flow, filling, spreading, and diffusion of molten solder in the brazing gap. From the mechanism of removing the oxide film by the aforementioned brazing flux, it can be concluded that without the brazing flux, the oxide film on the surface of the base material and solder cannot be removed, and thus a good brazing joint cannot be formed.

Using conventional flame brazing and induction brazing, the base metal and brazing material are always in contact with oxygen in the air throughout the entire heating process, making them highly susceptible to oxidation. With the extension of heating time and the increase of heating temperature, the thickness and weight of the surface oxide film will accelerate, resulting in a serious oxide film layer.

Easy to understand: The thicker the surface oxide film, the greater the amount of flux required. The increase in the amount of brazing flux brings about problems such as difficulty in removing it after welding, severe residual brazing flux, corrosion of welds, and the generation of copper green.

Therefore, the oxidation of the base material and the surface layer of the brazing material during the heating process is an important reason for quality problems such as blackening, copper green, and severe flux residue in the brazing joints of the air conditioning/refrigeration industry after welding. In addition, the serious oxidation of the surface of the base metal and filler metal is also an important reason for welding defect such as local faulty welding of welds, poor filler metal molding and so on during the mass production of conventional flame brazing and conventional induction brazing.

4、 How to solve the problem of blackening/long copper green/solder residue?

To solve these problems, it is necessary to strictly control the following two aspects:

1. Prevent or minimize surface oxidation of the workpiece base material and brazing material during the heating process;

2. Do not use soldering flux or minimize the amount of soldering flux used.

Simply put, it is crucial to ensure that the base material and solder do not oxidize during the entire heating process, which is the key to obtaining high-quality brazed joints in refrigeration equipment.

So how do we do it?

The following methods can be used:

1. Place the base material and solder in an inert gas/vacuum atmosphere, so that they do not come into contact with oxygen during the entire brazing heating process;

2. Try to shorten the brazing heating time as much as possible to reduce the risk of contact between the workpiece and base material and air;

3. Try to reduce the heating temperature of brazing as much as possible, and reduce the risk of contact between the workpiece and base material and air;

4. Use self brazing filler metals with properties and materials that match current conventional brazing materials to reduce the amount of paste or liquid brazing flux used.

Through in-depth research, development, and extensive validation, Zhuhai Bairuisi Welding Technology Co., Ltd. (hereinafter referred to as Bairuisi BRZ) has developed a new automatic induction process for brazing joints made of certain materials such as brass and red copper in the air conditioning/refrigeration industry. By adopting this process, the surface of the base material/solder can be prevented from oxidation during the heating process, significantly reducing the amount of brazing flux used, ensuring that the brazing seam and base material can be used without oxidation, blackening, copper green, and separate post weld treatment after welding, achieving high-quality brazing joints with both internal and external brazing quality.

The new induction brazing process mainly includes:

1. Digital induction heating brazing system: stable output, high accuracy, easy operation, ensuring the stability, reliability, and consistency of output process parameters, and ensuring brazing quality;

2. Induction heating brazing module for preventing workpiece solder oxidation: It can avoid oxidation of the base material and solder during the heating process;

3. Induction heating mechanism: High heating efficiency, high dimensional accuracy, and high quality, which can shorten heating time;

4. Positioning fixture mechanism: High positioning accuracy, easy operation, convenient switching, and durability;

5. Supporting induction brazing process technology: ensuring uniform and consistent heating temperature;

6. Amorphous soldering filler metal foil: The filler metal is in the form of a foil shaped strip, and its material and properties match those of conventional copper phosphorus and copper phosphorus silver brazing materials. The thinnest thickness can reach 0.025mm. When brazing, placing the solder foil in the joint gap has the following beneficial effects on brazing:

1) As a brazing compensation piece, it reduces assembly gaps, improves the alignment and uniformity of the workpiece after piping, enhances the capillary force of the melted solder, and further reduces porosity;

2) The solder foil has a self brazing effect, improving the fluidity of the solder and reducing the amount of flux used;

3) The interface of the base material is covered with solder foil, reducing the entry of external gas and reducing the risk of surface oxidation of the base material;

4) When amorphous foil shaped solder melts, it instantly releases a large amount of heat, increases the temperature of the brazing interface, accelerates the melting of the solder, and reduces the input heat of induction heating equipment and the risk of overheating;

5) The melting point of amorphous foil solder can be as low as 600 ℃, which is about 100 ° C lower than conventional copper phosphorus silver or copper phosphorus solder, further reducing the brazing temperature.

After using amorphous brazing foil for the brazing joints of refrigeration equipment, the amount of brazing flux can be further reduced, and the brazing quality and brazing qualification rate can be improved.

5、 Advantages of BRZ Induction Brazing New Process

1. Due to the special protection process used during brazing, the base material and welding ring are all in an inert gas protection atmosphere throughout the entire brazing heating process. The base material and welding ring will not oxidize, ensuring that the melted brazing material can easily flow, spread, and fill in the gaps, ensuring brazing quality;

2. The brazed joint and base material will be in the original color of the base material after welding, without oxidation or blackening, and the internal and external quality of the weld seam is good;

3. Due to the fact that the base material and solder do not oxidize throughout the entire process, the amount of solder used can be significantly reduced. After comparison, using the BRZ new induction brazing process, the amount of brazing flux used for pipe joint components is only about one-third of that of conventional induction brazing, which can significantly save the cost of brazing flux and greatly improve or avoid the serious problem of residual brazing flux after welding using conventional induction brazing processes.

4. After welding, the appearance of the weld seam is smooth and clean, and the copper pipe, pipe joint, and brazing seam can be clearly distinguished. The operator can easily detect minor defects such as sand holes, air holes, and cracks through the naked eye, reducing or avoiding quality accidents caused by defective parts flowing into the customer;

5. There will be no appearance defects such as copper green;

6. After welding, there is no need to add an additional process to treat the surface oxidation and blackening layer. The post welding treatment process for brazing joints can be cancelled, improving efficiency, reducing personnel and working hours, reducing production costs, and shortening production cycles

7. Stable and reliable quality, high qualification rate;

8. Advantages such as low equipment investment, simple operation, and green environmental protection;

9. Strong process adaptability and low requirements for welding material quality. The new BRZ induction brazing process has no special requirements for brazing flux and solder, and the customer's existing flux grades and specifications can be adjusted to meet the requirements. In theory, adopting this new process can reduce the requirements for welding material quality (such as using silver free solder instead of silver based solder) and reduce production costs.