Vacuum brazing defects and their solutions

Release date: 2017-10-27 Source: led network share:

Vacuum brazing is to heat and keep the structural parts under vacuum, so that the brazing material melts in a suitable temperature and time range, fully infiltrating, dissolving, diffusing and welding together with the solid metal under the action of capillary force. An advanced welding method for welding purposes.

The outstanding advantages of vacuum brazing are that it can be connected with different metals and realize the simultaneous welding of complex structures. The welded joints after welding are smooth and compact, have small deformation and have excellent mechanical properties and corrosion resistance.

1 solder layer thickness

When the thickness of the solder layer is too thin, it is easy to cause welding defects such as low welding strength, poor soldering, and under-pressure. When the thickness is too thick, the thickness of the core alloy is too thin, the pressure is not up to standard, and even the corrosion occurs. The phenomenon causes a leak. Therefore, the thickness and uniformity of the solder layer is an important indicator to measure its quality, and it is also one of the important factors affecting the quality of brazing.

2 Other quality requirements

Intrinsic defects such as pores, slag inclusions, and poor soldering of the solder layer of the core alloy; external defects are not clean, but also bruises and scratches during processing, when the depth exceeds the braze When the thickness of the layer is reduced, the continuity of the metal is directly destroyed, resulting in a decrease in pressure bearing capacity.

3 vacuum brazing process system

Even a small temperature difference at high temperatures requires high thermal conduction, ie the higher the vacuum heating temperature, the greater the amount of heat that needs to be transferred. It shows that under the same conditions, the heating rate in the vacuum furnace is much slower than other heating methods. The time required for vacuum heating is approximately three times that of an air furnace and six times that of a salt bath furnace. Therefore, when the vacuum brazing furnace heating process system is established, the heating process system of the air furnace, the salt bath furnace and the atmosphere furnace cannot be copied. The above formula also states that during the vacuum brazing process, it should be heated as slowly as possible to keep the temperature inside and outside the brazing product consistent, otherwise it will directly affect the brazing quality.

Preheating, temperature setting, heat preservation, energy storage and temperature setting, heat preservation, brazing temperature setting, heat preservation and power failure and temperature reduction in industrial production are effective processes that can achieve the above objectives and improve production efficiency, wherein vacuum brazing temperature And holding time is the key to affect the quality of brazing.

(1) Vacuum brazing temperature:

1 When the temperature is low, the brazing material has not reached the necessary temperature, and the fluidity and wettability of the brazing filler metal are poor, and it is easy to produce defects such as internal pores in the brazing joint, discontinuous brazing joint, and virtual welding, so that the strength of the brazed joint is lowered. If the pressure capacity is not up to standard, a leak will occur, and even if it is severe, it will tear;

2 When the temperature is high, the brazing material is completely melted and the fluidity is too large, and the brazing material is easily oxidized to form pores and the capillary force acting on the weld seam is deteriorated, resulting in defects such as loss of solder, corrosion, and product bending. A suitable fixed temperature should focus on the flow point of the solder. Usually, the melting point of the solder should be about 60 ° C lower than the melting point of the soldered metal. At this time, the liquid solder has good wettability and fluidity to the metal to be welded, can better fill the brazing gap under the action of capillary force, and can form a good alloying action with the metal to be welded to form a high-strength joint.

(2) Vacuum holding time:

The wetting and joint formation of the brazing material takes about 1 s to 2 s during brazing, so the holding time is mainly determined by the time required for the core temperature of the brazed product to reach the brazing temperature and the time required for the oxide film layer to dissipate. If the holding time is too short, the temperature of the center of the product to be brazed does not reach the brazing temperature; if the time is too long, the liquid solder tends to erode the metal to be welded.

2.4 vacuum brazing furnace vacuum

When the degree of vacuum at a high temperature is low, an oxidizing gas such as O2 or H2O remaining in the furnace easily reacts with the product metal to form a hard oxide film. The oxide structure is dense, stable, and has a high melting point. It is not easily decomposed under ordinary vacuum brazing temperature, and the fluid wettability is deteriorated after the solder is oxidized; the solder metal is oxidized and becomes difficult to infiltrate, resulting in a solder and a substrate. The welding performance deteriorates. Therefore, it is necessary to increase the degree of vacuum during vacuum brazing as much as possible, reduce the content of oxidizing gases such as O2 and H2O, and control the amount of metal oxide formed. Generally, the vacuum brazing furnace adopts multi-temperature zone temperature control, the furnace temperature uniformity is ±3 °C, the working vacuum degree should be guaranteed to be no more than 2.0×10-3 Pa, and the pre-vacuum vacuum limit must be in the order of 10-4. .

2.5 Environmental conditions during vacuum brazing

Humidity in the environment can affect the quality of the heat sink braze. The metal products assembled under high humidity are brazed in a vacuum furnace, the water will evaporate and release more gas, and the evaporation of water inside the radiator and the release of gas are a slow process. Moisture requires a large amount of heat of evaporation, which affects the temperature inside the heat sink; moisture also affects the degree of vacuum; moisture will exacerbate the oxidation of the metal, thereby affecting the quality of the braze. Therefore, before the surface treatment, assembly and brazing of the radiator member, a certain environmental humidity should be maintained, or an oven should be used for drying and drying to control the surface moisture content of the radiator member due to the environmental humidity.

The main phenomena of vacuum brazing defects are:

1 flow

The flooding is a thin solder coating formed by the solder flowing through the brazing joint on the base material during brazing.

(1) Causes of flooding The fixture must have a certain degree of elasticity and rigidity at the brazing temperature, so that there is a suitable gap at the weld joint to form a capillary phenomenon to absorb the molten solder. The tooling clamp is not tight, and the gap of the brazing assembly is too large to keep the solder, and the flow defects are generated. The tooling has a low degree of rigidity, and the thermal deformation and gravity caused by heating cause an increase in the joint gap of the brazing assembly, and the capillary phenomenon which cannot form the brazing material also causes the brazing material to flow.

Vacuum brazing is radiation heat transfer, the heat capacity of the fixture is large, the heating rate of the brazed parts is small, the residence time of the brazing material in the solid-liquid phase temperature range is long, the low melting point component of the brazing material is more volatile, and the flux is The action time is also long, which further destroys the surface tension of the liquid solder and excessively improves the wettability of the solder to the base material.

The amount of furnace is large, the temperature of the lifting and lowering is small, the holding time is long, and the heat capacity of the fixture is the same. The brazing material has a long residence time in the liquid state, and the slow cooling rate is equivalent to prolonging the holding time of the solder liquid, and also generates a flooding flow. The function of the flux is to reduce the oxide film on the surface, reduce the surface tension of the liquid solder, and improve the wettability of the solder to the base material.

The flux is used in a large amount, and the wettability of the solder to the base material is too good, resulting in the flow of the solder. The high temperature of the holding, the surface tension of the liquid solder is small, and the effect of the flux reducing the surface tension is enhanced, and the combined effect of these factors causes the flow of the solder. The workpiece stays in the solid-liquid phase temperature range of the solder for a long time and causes a flooding.

(2) Elimination measures It is possible to increase the clamping force of the tooling and reduce the connection gap of the brazing assembly. Improve the rigidity of the fixture, and ensure that the joint gap does not change when the state is hot. The hollowing out reduces the weight of the tooling or replaces part of the steel with graphite to reduce the heat capacity of the tooling. Reduce the amount of flux, and reduce the amount of flux in the case of continuous brazing. The staged lifting and lowering temperature is adopted to rapidly raise and lower the temperature in the solid solution phase of the brazing material, shorten the brazing holding time, reduce the brazing holding temperature and reduce the amount of furnace charging.

2 dissolution

Dissolution is a depression in which the surface of the base material is melted by the molten solder.

(1) Causes of dissolution The solder does not match the brazing base material, and the solder and the component in the base material form a low melting point phase, which lowers the solidus temperature in a portion of the base material. The large heat capacity of the tooling or the large amount of furnace loading results in a slow heating rate of the parts. The residence time in the solid-liquid phase temperature of the brazing filler metal is too long. At a certain temperature point, a certain melting point of the brazing filler metal and the parent phase synthesizes a low melting point. The phase causes the melting point of the partial region of the parent phase alloy to decrease and melt. The furnace temperature is not uniform, the local temperature of the brazing material is too high, and the brazing temperature is too high, so that the low melting point component of the base material in the diffusion region of the brazing material is melted. The temperature rises and falls slowly in the solid solution phase of the solder. Brazing heat preservation time is too long.

(2) Elimination measures The solution is generally to replace the solder grade. Or rapidly increase the temperature near the melting point of the solder, reduce the amount of furnace loading, reduce the weight of the tooling, reduce the brazing temperature, and shorten the holding time of vacuum brazing.

3 product brazing strength is low

(1) Cause The brazing time is short, and the diffusion time of a certain component to the base metal is short. In the solid phase of the solder, the heating time is too long, and the solder component is more volatile. The vacuum pressure is too high or the vacuum furnace leak rate is large, and the solder or base material is partially oxidized during heating. The oxide film is not completely removed. The solder or base material is over-etched during alkaline washing to change the composition of the solder. The amount of flux is small and the wettability of the solder is not good.

(2) Elimination measures Extend the holding time to fully complete the diffusion. The temperature rise between the partitions is used to rapidly increase the temperature in the solid phase of the solder to reduce the volatilization of the low melting point component of the solder. Reduce the vacuum pressure to prevent the solder or base material from re-oxidizing during heating. Check the pressure rise rate of the device. Increase the concentration or temperature of the alkali solution, or prolong the time of alkali etching to completely remove the oxide film. Reduce the concentration or temperature of the lye (generally controlled at 60 ° C), or shorten the alkali etching time, separate the brazing material or the base material by alkali washing to prevent the composition of the brazing material from changing during the alkaline washing. Increase the amount of flux to improve the wettability of the solder.

4 leakage welding

Leakage welding is an unwelded gap formed by the absence of solder or the melting loss of solder at the butt joint of the brazing material.

(1) Cause The amount of solder is not enough or the connection gap is large. If the heating rate of brazing is too large, the deformation of the part will increase, and the joint gap will increase, and the shape will not be capillary. The flux is used in a large amount, and the wettability of the brazing material is too good, resulting in loss of the brazing material or excessive brazing seam. In the solid-liquid phase interval of the brazing material, the heating rate is slow, and the volatilization of the low-melting component of the brazing material changes the brazing filler component, improves the melting point of the remaining brazing filler metal, and reduces the interdiffusion between the brazing filler metal and the parent metal. . The amount of furnace loading is large or the tooling design is unreasonable. The tooling is too heavy to absorb too much heat, which causes the heating rate to be slow. The long-term holding time or the cooling rate is slow, and the low melting point component of the brazing material is more volatile. The solder is over-etched, changing its composition and changing the melting point.

(2) Elimination measures Increase the amount of solder, increase the clamping force of the tooling and reduce the gap at the joint. Before the brazing, the stress-relieving annealing process of the brazing assembly is increased, or the temperature is raised in stages and the isothermal phase is set, and the temperature is rapidly increased above 500 °C. Reduce the amount of flux used.

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Label: vacuum brazing defects and their solutions