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Silver-Based Brazing Processes Explained

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Silver-based brazing is a type of brazing process. During brazing, the workpieces are joined together by the solidification of the melted filler metal, creating a connection between the two pieces.

Currently, our company utilizes the oxy-fuel brazing method. In oxy-fuel brazing, the filler metal is either pre-coated on the interface surface or the brazing rod is heated, coated with flux, and then brought to the preheated surface of the workpiece at the brazing temperature. The filler metal evenly fills the gaps to achieve the welding objective.

I. Raw Materials

1. Base Material

The workpieces used by our company are copper or copper alloys. They should have good material quality and be free from impurities.

2. Filler Metal

Commonly known as brazing wire, the filler metal is the material used during brazing to fill the joint. Silver-based brazing alloys primarily consist of silver-copper and silver-copper-zinc alloys.

The silver content is the main factor, and different specifications and grades are available based on the silver content. For our regular copper products, a filler metal with a silver content of 30% (BAg30Cu) can be used.

3. Flux

Commonly known as brazing flux, it is a solvent used during brazing. Flux is initially in powder form and needs to be mixed with a solvent before use.

Its role is to remove oxides from the surface of the filler metal and base material, protect the workpiece and liquid filler metal from oxidation during brazing, and improve the wetting properties of the liquid filler metal on the workpiece.

There are two common grades of flux used for brazing copper and copper alloy with silver-based brazing alloys: QJ101, which has a brazing temperature range of 550-850°C, and QJ102, which has a brazing temperature range of 650-850°C.

Flux should be stored in a metal container and mixed with hot water to form a paste-like consistency. The water and flux do not need to be in precise proportions.

However, it is important to ensure that the mixture has a paste-like consistency and is not too diluted, as it may affect the welding effect. Stir the mixture while adding water, and if necessary, use gentle heat to facilitate dissolution.

II. Welding Equipment

1. Torch

Also known as a welding gun, a general-purpose torch or a specialized torch can be used. To ensure even heating of the workpiece, a specialized multi-flame nozzle or a fixed multi-head welding nozzle can be used.

2. Oxygen Cylinder and Acetylene Cylinder

Oxygen-acetylene flame brazing is the most commonly used brazing method. Regular inspections should be conducted to check for gas leaks, and precautions should be taken to avoid backfires, explosions, and other safety accidents.

3. Fixtures and Clamps

Most of the silver-based brazing products required by our company are bathroom accessories, which are generally small products. This requires fixtures and clamps to ensure accurate positioning while ensuring safety and high efficiency.

Several aspects need to be considered: the lightweight nature of the fixtures (they should be simple and practical without being too heavy), the ability of a set of fixtures to simultaneously position multiple pieces of the same product (usually around 5-6 pieces, but for very small and simple welding products, 10 pieces or more can be considered), and the ability of the fixtures to provide a certain angle on the welding surface during positioning, facilitating the smooth flow of the filler metal.

III. Human Resources

Silver-based brazing requires strict requirements for operators. They should have at least one year of practical brazing experience or receive training to reach a certain level of proficiency.

They should be able to skillfully adjust the welding torch and gas, control and master the heat. They should also have a certain understanding of product quality requirements and be responsible.

They should conscientiously conduct regular self-inspections of the products to ensure the production of high-standard and high-quality products.

IV. Brazing Process

1. Operation

In the silver-based brazing process, the operation is crucial, and the mastery of heat is of utmost importance as it directly affects production efficiency and product quality.

Based on experience, the welding torch should be approximately 30-40mm away from the workpiece. The flame should gently sway back and forth and from side to side on the surface to be brazed, ensuring that the heat is slightly dispersed rather than concentrated in one spot to avoid localized melting of the workpiece.

After uniformly heating the surface of the workpiece to the brazing temperature, the filler metal can be added. Otherwise, the filler metal will not be able to uniformly fill the gaps.

According to experience, when the product turns red due to heating, the filler metal should be promptly placed above the interface (above the surface to be brazed after fixture positioning) so that it can quickly flow along the interface.

Placing the filler metal before the product turns dark red or after it has turned deep red will not result in good brazing effects and may lead to defects.

The heating and cooling rates are also important process parameters during brazing. Rapid heating can result in uneven temperature distribution and internal stress in the workpiece.

Slow heating can cause harmful processes, such as grain growth in the base material, evaporation of low-boiling-point components in the filler metal, oxidation of metals, and decomposition of flux, to develop rapidly.

It is important to ensure uniform heating while minimizing the heating time. Although the cooling of the workpiece occurs after the brazing process, the cooling rate also affects the joint quality.

Excessive cooling rates can cause the workpiece to crack due to excessive thermal stress or result in the formation of gas pockets due to rapid solidification of the brazed joint.

2. Filler Metal Usage

Using excessive amounts of filler metal and flux is not beneficial. The amount of filler metal used should be sufficient to completely fill the gaps and form smooth fillets along the edges.

Insufficient filler metal will result in poor fillet formation and may not fill the gaps completely. Excessive filler metal usage not only leads to wastage but also causes the base material to erode, surface contamination of the workpiece, and adhesion between the workpiece and fixtures.

It is ideal for a joint to be successfully brazed in one attempt. However, if the initial brazing is not entirely successful or there are instances of incomplete brazing, it may be necessary to perform additional brazing.

Additionally, it is important to note that the actual usage of filler metal should be greater than the calculated value based on the geometric dimensions of the joint, as some loss is inevitable during the heating and filling process.

3. Gap

During brazing, there will always be a certain gap between the joint surfaces of the workpieces, and the gap value has a direct impact on the performance of the brazed joint.

Within a certain range, the joint has maximum strength, often exceeding the strength of the original filler metal. When brazing copper and copper alloys with silver-based filler metals, the recommended gap value is 0.05-0.2mm. Joint strength decreases when the gap is either larger or smaller than this range.

5. Joint Defects and Preventive Measures

In many cases, soldered joints may have certain defects. These defects in soldered joints can be classified as unfilled gaps, solder porosity, solder slag inclusion, base material cracks, and base material erosion.

However, these defects can all be avoided. The causes of defect formation and preventive measures are shown in the table below:

Causes of Soldered Joint Defects and Preventive Measures

Types of DefectsCauses of Defect FormationPreventive Measures
Partial unfilled gaps1. Poor joint design (excessive or insufficient gap, poor assembly)

2. Improper selection of solder flux or filler material

3. Inadequate surface cleaning before soldering

4.Insufficient or uneven soldering temperature

5.Insufficient quantity of solder filler material 
 1. Ensure the correct clearance.

2. Choose an appropriate clearance.

3. Ensure an adequate amount of solder material.

4. Thoroughly clean the surface of the weldment.
Solder porosity1. The debonding effect of flux or the deoxidizing effect of protective gas is weak.

2. Gas precipitation in the base material or brazing material.

3. Overheating of the brazing material.
1. Thoroughly clean the surface of the weldment to remove any oxide film.

2. Choose suitable flux and brazing material.

3. Reduce the brazing temperature and shorten the holding time.
Solder slag inclusion 1. Improper joint clearance.Excessive or insufficient flux usage.

2. Excessive amount of brazing material or simultaneous addition from both sides of the joint.

3. Incompatibility in melting points between flux and brazing material.

4. Non-uniform heating.

5. Excessive density of the flux.
1. Choose an appropriate gap, avoiding it being too large or too small.

2. Use the flux with a moderate density and quantity.

3. Add the brazing material from one end.

4. Ensure the melting points of the flux and brazing material are matched.

5. Heating must be done uniformly.
Solder crack1. The joint design is not reasonable.

2. The cooling rate after brazing is too fast.

3. There is a significant difference in thermal expansion coefficients between the base material and the brazing material.

4. Excessive brittleness in the brazed joint. 
1. Choose base material and brazing material with similar thermal expansion coefficients.

2. Slow down the cooling rate after brazing.

3. Reduce the post-brazing temperature and shorten the holding time.
Base material crack1. Excessive overheating or burning of the base material.

2. Infiltration of brazing material into the grain boundaries of the base material.

3. Excessive internal stress caused by uneven heating or rigid clamping of the workpiece.

4. Hammering or scratching on the brazed surface.
1. Reduce the brazing temperature and shorten the holding time.

2. Prior to brazing, ensure thorough annealing.

3. Avoid hammering or scratching the brazing surface.
Base material erosion1. The brazing temperature is too high, and the holding time is too long.

2. Excessive reactivity between the base material and brazing material.

3. Excessive amount of brazing material.
1. Strictly control the brazing temperature and holding time.

2. Modify the composition of the brazing material to control the amount used.

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