CN102925851B - Two-section gas nitridation method for surfaces of aluminum and aluminum alloy - Google Patents

Abstract

The invention relates to a two-stage gas nitriding method on the surface of aluminum and aluminum alloy. It includes two steps of surface oxide film removal and surface nitriding treatment. Step 1 includes: pretreatment; first zinc immersion; water washing; zinc stripping; second zinc immersion; water washing and drying. Step 2 includes: Exhaust in the furnace, the ammonia decomposition rate is less than 10%; once the temperature is raised to 570~590°C, the ammonia decomposition rate is 15~25%; one-stage nitriding: the furnace temperature is 580~640°C, and the ammonia decomposition rate is maintained at 15~25% ;Secondary heating: 620~640℃, ammonia decomposition rate 40~55%; Second stage nitriding: 620~640℃, ammonia decomposition rate 40~55%; Nitrogen removal, furnace temperature 580~640℃, ammonia decomposition rate control At 70~80%. The advantages are: it can effectively eliminate the dense oxide film on the surface of aluminum and aluminum alloys, which is conducive to the infiltration of nitrogen atoms, and can be used for nitriding treatment in ordinary gas nitriding furnaces. The production cost is low, the operation is simple and easy, and the adaptability is wide. Good quality.

Description

Two-stage gas nitriding method for aluminum and aluminum alloy surface

technical field

The present invention relates to a heat treatment process for aluminum alloys, in particular to a two-stage gas nitriding method for the surface of aluminum and aluminum alloys.

Background technique

Aluminum and aluminum alloys are widely used in engineering structures and machine manufacturing, especially in the automotive industry, because they have high specific strength and small specific gravity, which can reduce the weight of engineering structures and complete machines without reducing safety and stability. At the same time, aluminum and aluminum alloys also have good machinability. However, with the development of industrial technology, the lower hardness, lower wear resistance and corrosion resistance of aluminum and aluminum alloys make it unable to meet the harsh service conditions such as high relative movement between components, high mass flow and high corrosion. requirements.

At present, the commonly used surface strengthening methods for aluminum and aluminum alloys include: anodic oxidation (hard anodic oxidation, micro-arc oxidation), laser and electron beam alloying, ion sputtering, surface hard chrome plating and some composite treatments. However, the oxide film formed by anodic oxidation (hard anodic oxidation) is prone to thermal cracks, which reduces the anti-corrosion effect. At the same time, the hardness difference between the aluminum and aluminum alloy oxide layers is large, and there are conditions for high stress load work, and the oxide layer will appear. Shedding phenomenon. Other strengthening methods are more expensive and not suitable for mass production.

In recent years, increasing the hardness and wear resistance of aluminum and its alloys by nitriding aluminum and aluminum alloys to form a high-hardness aluminum nitride layer on the aluminum surface has attracted more and more people's interest.

However, due to the active chemical properties of aluminum, there are natural oxide films with good stability on the surface of pure aluminum and aluminum alloys. According to the moisture content in the environment, the thickness of the oxide film can be up to about 0.1 μm, which seriously hinders the diffusion of nitrogen atoms into the aluminum matrix. This is also the difficulty of surface nitriding of aluminum and aluminum alloys. It is difficult to achieve nitriding on the surface of aluminum and aluminum alloys.

At present, the main researched and adopted aluminum and aluminum alloy nitriding process is glow discharge plasma nitriding.

The conventional DC glow discharge plasma nitriding process generally adopts a bipolar system, the workpiece is the cathode, and the vacuum container is the anode. A DC voltage is applied between the cathode and the anode to make the gas glow discharge generate plasma, and the positive ions are biased externally. Under the action of pressure, it accelerates towards the cathode workpiece, collides with the surface of the workpiece and penetrates into the surface to complete the nitriding process.

However, this method often requires pre-sputtering before nitriding to eliminate the oxide film. At the same time, the temperature is high, and some even reach 650°C, causing local melting of the aluminum alloy and due to the high resistance of AlN, in the ordinary DC glow discharge nitriding, with the thickening of the AlN layer, in order to maintain the discharge process, it must be changed Process parameters, thus increasing the difficulty of nitriding.

In order to obtain a better nitriding effect, many researchers have improved the process and equipment, and successively proposed enhanced plasma ion nitriding, laser-induced plasma nitriding, ECR (electron cyclotron resonance) microwave plasma nitriding, radio frequency nitriding, etc. (High-frequency) plasma nitriding, etc., but the above-mentioned nitriding processes of aluminum and its aluminum alloys have in common: high equipment requirements and production costs, complicated operation and difficult control, and it is difficult to obtain ideal for large-sized and complex-structured workpieces. Uniform nitrided layer.

Contents of the invention

The object of the present invention is to overcome the deficiencies of the prior art, and provide a method for nitriding aluminum and aluminum alloys with a uniform and controllable nitrided layer, low cost, and a simple and feasible process. The specific steps of the two-stage gas nitriding method on the surface of aluminum and aluminum alloy are as follows:

1) Remove surface oxide film

a. Pretreatment and cleaning of aluminum and aluminum alloy workpieces;

b. Dip zinc for the first time, immerse the cleaned workpiece in a zinc dipping solution at room temperature for 50 to 70 seconds;

c. Washing, washing the workpiece after the first galvanizing with water;

d. Zinc stripping, immersing the workpiece after washing in water at room temperature to strip zinc in _HNO3 aqueous solution;

e. Zinc dipping for the second time, immersing the stripped workpiece in a zinc dipping solution at room temperature for a second time of zinc dipping for 25 to 35 seconds;

f. Washing and drying, cleaning the workpiece after the second galvanizing with water and drying or drying;

2) Surface nitriding

a. Exhaust: After placing the workpiece with the surface oxide film removed above in the nitriding furnace, start feeding ammonia gas to remove the air in the furnace until the ammonia decomposition rate is less than 10%;

b. The first temperature rise: raise the furnace temperature to 570~590°C, and gradually reduce the flow rate of ammonia gas during the temperature rise process until the ammonia decomposition rate reaches 30~45%;

c. The first stage of nitriding: keep the furnace temperature at 570~590°C for 5~20 hours, and keep the ammonia decomposition rate at 15~25% during this process;

d. For the second temperature rise, after the first stage of nitriding is completed, the furnace temperature is raised to 620~640°C, and the ammonia gas flow rate is reduced at the same time, so that the ammonia decomposition rate at this temperature reaches 40~55%;

e. The second stage of nitriding: keep the furnace temperature at 620~640°C for 5~30 hours, and keep the ammonia decomposition rate at 40~55% during this process;

f. For nitrogen removal, keep the furnace temperature at 620~640°C, reduce the flow rate of ammonia gas, so that the ammonia decomposition rate reaches 70~80%, and keep this state for 1~2 hours.

The zinc dipping solution is obtained by dissolving the following mass compounds in 1 liter of water:

Zinc oxide ZnO 10~30 g,

Sodium hydroxide NaOH 50~70g,

Potassium sodium tartrate KNaC ₄ H ₄ O ₆ 50 ~90g.

The nitric acid _HNO3 aqueous solution used in the dezincification process in the step of removing the surface oxide film is a mixed solution of water and nitric acid _HNO3 with a volume ratio of 1:0.8 to 1:1.1, and the time for the workpiece to be immersed in the solution is 6 to 12s .

The pretreatment and cleaning of the surface of the workpiece in the step of removing the surface oxide film includes the sub-steps of chemical degreasing, hot water washing, cold water washing, acid etching and water washing carried out in sequence.

In the degassing step of the surface nitriding, after the workpiece is placed in the nitriding furnace, the temperature of the furnace is raised, and when the temperature of the furnace rises to 300° C., ammonia gas is introduced.

Above-mentioned technical scheme of the present invention produces following benefit effect:

1) After galvanizing treatment, it can effectively eliminate the dense oxide film on the surface of aluminum and aluminum alloy and prevent its re-formation, which is conducive to the infiltration of nitrogen atoms;

2) Nitriding of aluminum and aluminum alloys can be realized by using ordinary gas nitriding furnaces or gas carburizing furnaces capable of nitriding treatment. The requirements for nitriding equipment are not high, the production cost is low, and the operation is simple and easy;

3) Wide adaptability, can handle workpieces of various shapes and sizes;

4) The quality of the infiltrated layer is good, and an aluminum nitride compound layer with uniform thickness can be obtained on all surfaces of the workpiece.

Detailed ways

The present invention will be further described below in conjunction with specific examples.

The following examples are carried out in RQ3-35-9 gas carburizing furnace.

Example 1

The workpiece material is 1050 pure aluminum, and its chemical composition is aluminum Al 99.50%, Si0.25%, Cu0.05%, Mg0.05%, and a very small amount of Zn, Mn, etc. First, the pure aluminum workpiece is treated to remove the surface oxide film, which is specifically completed by the following steps:

1) Pretreatment, chemical degreasing, hot water washing, cold water washing, acid etching and water washing are carried out on the surface of pure aluminum workpieces sequentially. Among them, chemical degreasing and acid etching are treated according to the existing general industrial pure aluminum surface treatment methods. Let me repeat.

2) For the first zinc immersion, prepare the zinc immersion solution first, and prepare according to the proportion relationship of dissolving the following quality compounds in 1 liter of water:

Zinc oxide ZnO 10 g,

Sodium hydroxide NaOH 50g,

_Potassium Sodium Tartrate _{KNaC4H4O6 50g} ,

At room temperature (5-35°C, the same below), immerse the above-mentioned cleaned workpiece in the prepared zinc immersion solution for the first immersion zinc, and the immersion time is 50s.

3) Water washing, washing the workpiece after the first galvanizing with water.

4) Dezincification, firstly prepare a dezincification solution, and prepare it according to the volume ratio of water and nitric acid HNO ₃ 1:1, immerse the above-mentioned washed workpiece in the dezincification solution, and carry out the dezincification, the temperature is room temperature, and the immersion time is 6s.

5) For the second zinc immersion, immerse the stripped workpiece in the zinc immersion solution at room temperature for 25 seconds; the second immersion zinc solution is the same as the first immersion zinc solution, and this Examples share the same tank solution.

6) Wash and dry, wash the workpiece after the second galvanizing with water and dry.

After the above steps are completed, the dense oxide layer on the surface of the workpiece is removed, and an extremely thin layer of zinc coating is obtained on the surface of the aluminum metal to prevent the oxide film of aluminum from being produced. Next, nitriding treatment is carried out on the surface of the workpiece, which is specifically completed by the following steps:

1) Exhaust: first open the furnace cover, place the workpieces with the surface oxide film removed above in the nitriding furnace, cover the furnace cover, raise the furnace temperature, and start feeding ammonia when the furnace temperature rises to 300°C Oxygen in the furnace is eliminated through the action of ammonia and oxygen in the air in the furnace. The air removal status in the furnace can be obtained by monitoring the ammonia decomposition rate. The less air in the furnace, the smaller the ammonia decomposition rate, and vice versa. The ammonia decomposition rate of this embodiment is controlled below 10%, and the control of the ammonia decomposition rate is realized by controlling the flow rate of ammonia gas.

2) The first temperature rise: raise the furnace temperature to 570°C, gradually reduce the flow rate of ammonia gas during the temperature rise process, and control the ammonia decomposition rate at 15%.

3) The first stage of nitriding: keep the furnace temperature at 570°C for 5 hours, and keep the ammonia decomposition rate at 15% during this process.

4) The second temperature rise: After the first stage of nitriding is completed, the furnace temperature is raised to 620°C, and the ammonia gas flow rate is reduced at the same time, so that the ammonia decomposition rate at this temperature reaches 40%;

5) Nitriding in the second stage: keep the furnace temperature at 620°C for 5 hours, and keep the ammonia decomposition rate at 40% during this process;

6) Nitrogen removal, keep the furnace temperature at 620°C, reduce the flow rate of ammonia gas, so that the ammonia decomposition rate reaches 70%, and keep this state for 1 hour.

The above steps complete the surface nitriding treatment of the workpiece, turn off the power of the furnace, and let the workpiece cool with the furnace. During this process, the pressure in the furnace should be kept at positive pressure. When the temperature of the furnace drops to 150°C, open the furnace door and take out the workpiece , The depth of the infiltrated layer was measured to be 8 μm, and the microhardness of the workpiece surface was 64.5HV0.1.

Example 2

The workpiece material is 5050 aluminum alloy, its chemical composition and content (wt%) are: Si0.4, Fe0.7, Cu0.2, Mn1.1, Mg1.4, Zn0.25, other 0.15, and the balance is aluminum. First, the aluminum alloy workpiece is treated to remove the surface oxide film, which is specifically completed by the following steps:

1) Pretreatment, chemical degreasing, hot water washing, cold water washing, acid etching and water washing are carried out on the surface of the aluminum alloy workpiece in sequence. Let me repeat.

2) For the first zinc immersion, prepare the zinc immersion solution first, and prepare according to the proportion relationship of dissolving the following quality compounds in 1 liter of water:

Zinc oxide ZnO 20 g ,

Sodium hydroxide NaOH 60g,

Potassium Sodium Tartrate KNaC ₄ H ₄ O ₆ 70g,

Dip the above-mentioned cleaned workpiece into the prepared zinc dipping solution at room temperature for the first dipping of zinc for 60 seconds.

3) Water washing, washing the workpiece after the first galvanizing with water.

4) Dezincification, firstly prepare the dezincification solution, and prepare it according to the volume ratio of water and nitric acid HNO ₃ 1:1.1, immerse the above-mentioned washed workpiece in the dezincification solution, and carry out the dezincification, the temperature is room temperature, and the immersion time is 10s.

5) For the second zinc immersion, immerse the stripped workpiece in the zinc immersion solution at room temperature for a second immersion of 30 seconds; the second immersion zinc solution is the same as the first immersion zinc solution, and this Examples share the same tank solution.

6) Wash and dry, wash the workpiece after the second galvanizing with water and dry.

After the above steps are completed, the dense oxide layer on the surface of the workpiece is removed, and an extremely thin layer of zinc coating is obtained on the surface of the aluminum metal to prevent the oxide film of aluminum from being produced. Then carry out nitriding treatment on the surface of the workpiece, which is specifically completed by the following steps:

1) Exhaust: first open the furnace cover, place the workpieces with the surface oxide film removed above in the nitriding furnace, cover the furnace cover, raise the furnace temperature, and start feeding ammonia when the furnace temperature rises to 300°C Oxygen in the furnace is eliminated through the action of ammonia and oxygen in the air in the furnace. The air removal status in the furnace can be obtained by monitoring the ammonia decomposition rate. The less air in the furnace, the smaller the ammonia decomposition rate, and vice versa. The ammonia decomposition rate of this embodiment is controlled below 10%, and the control of the ammonia decomposition rate is realized by controlling the flow rate of ammonia gas.

2) The first temperature rise: raise the furnace temperature to 575°C, gradually reduce the flow rate of ammonia gas during the temperature rise process, and control the ammonia decomposition rate at 20%.

3) The first stage of nitriding: keep the furnace temperature at 575°C for 10 hours, and keep the ammonia decomposition rate at 20% during this process.

4) The second temperature rise: After the first stage of nitriding is completed, the furnace temperature is raised to 630°C, and the ammonia gas flow rate is reduced at the same time, so that the ammonia decomposition rate at this temperature reaches 50%;

5) Nitriding in the second stage: keep the furnace temperature at 630°C for 15 hours, and keep the ammonia decomposition rate at 50% during this process;

6) Nitrogen removal, keep the furnace temperature at 630°C, reduce the flow of ammonia gas, so that the ammonia decomposition rate reaches 75%, and keep this state for 1.5 hours.

The above steps complete the nitriding treatment on the surface of the workpiece, turn off the power of the furnace, and the workpiece cools with the furnace. During this process, the pressure in the furnace should be kept at positive pressure. When the temperature of the furnace drops to 170°C, open the furnace door and take out the workpiece. The measured penetration depth is 25 μm, and the microhardness of the workpiece surface is 238HV0.1.

Example 3

The workpiece material is 2A12 aluminum alloy, its chemical composition and content (wt%) are: Cu4.3, Mg1.6, Mn0.6, Si0.4, and the balance is aluminum. First, the aluminum alloy workpiece is treated to remove the surface oxide film, which is specifically completed by the following steps:

1) Pretreatment, chemical degreasing, hot water washing, cold water washing, acid etching and water washing are carried out on the surface of the aluminum alloy workpiece in sequence. Let me repeat.

2) For the first zinc immersion, prepare the zinc immersion solution first, and prepare according to the proportion relationship of dissolving the following quality compounds in 1 liter of water:

Zinc oxide ZnO 30 g,

Sodium hydroxide NaOH 70g,

Sodium Potassium Tartrate KNaC ₄ H ₄ O ₆ 90g,

Dip the above-mentioned cleaned workpiece into the prepared zinc dipping solution at room temperature for the first dipping zinc, and the dipping time is 70s.

3) Water washing, washing the workpiece after the first galvanizing with water.

4) Dezincification, firstly prepare a dezincification solution, and prepare it according to the volume ratio of water and nitric acid HNO ₃ 1:0.8, immerse the above-mentioned washed workpiece in the dezincification solution, and carry out the dezincification, the temperature is room temperature, and the immersion time is 12s.

5) For the second zinc immersion, immerse the stripped workpiece in the zinc immersion solution at room temperature for 35 seconds; the second immersion zinc solution is the same as the first immersion zinc solution, and this Examples share the same tank solution.

6) Wash and dry, wash the workpiece after the second galvanizing with water and dry.

After the above steps are completed, the dense oxide layer on the surface of the workpiece is removed, and an extremely thin layer of zinc coating is obtained on the surface of the aluminum metal to prevent the oxide film of aluminum from being produced. Then carry out nitriding treatment on the surface of the workpiece, which is specifically completed by the following steps:

1) Exhaust: first open the furnace cover, place the workpieces with the surface oxide film removed above in the nitriding furnace, cover the furnace cover, raise the furnace temperature, and start feeding ammonia when the furnace temperature rises to 300°C Oxygen in the furnace is eliminated through the action of ammonia and oxygen in the air in the furnace. The air removal status in the furnace can be obtained by monitoring the ammonia decomposition rate. The less air in the furnace, the smaller the ammonia decomposition rate, and vice versa. The ammonia decomposition rate of this embodiment is controlled below 10%, and the control of the ammonia decomposition rate is realized by controlling the flow rate of ammonia gas.

2) The first temperature rise: raise the furnace temperature to 590°C, gradually reduce the flow rate of ammonia gas during the temperature rise process, and control the ammonia decomposition rate at 25%.

3) The first stage of nitriding: keep the furnace temperature at 590°C for 20 hours, and keep the ammonia decomposition rate at 25% during this process.

4) The second temperature rise: After the first stage of nitriding is completed, the furnace temperature is raised to 640°C, and the ammonia gas flow rate is reduced at the same time, so that the ammonia decomposition rate at this temperature reaches 50%;

5) Nitriding in the second stage: keep the furnace temperature at 640°C for 30 hours, and keep the ammonia decomposition rate at 55% during this process;

4) Nitrogen removal, keep the furnace temperature at 640°C, reduce the flow rate of ammonia gas, so that the ammonia decomposition rate reaches 80%, and keep this state for 2 hours.

The above steps complete the surface nitriding treatment of the workpiece, turn off the power of the furnace, and let the workpiece cool with the furnace. During this process, the pressure in the furnace should be kept at positive pressure. When the temperature of the furnace drops to 150°C, open the furnace door and take out the workpiece , The measured penetration layer depth is 35μm, and the workpiece surface microhardness is 337HV0.1.

The present invention is by no means limited to the above-mentioned embodiments, and more implementations can be combined according to the technical solutions described in the present invention combined with existing technical means in the art, and these all fall within the scope of the technical solutions to be protected by the present invention. In addition, the carburizing furnace used in the present invention is not limited to the RQ3-35-9 gas carburizing furnace described in the above-mentioned embodiment, and other types of gas nitriding furnaces or gas carburizing furnaces can also follow the steps and process conditions of the above-mentioned embodiment to implement.

Claims (5)

1. A two-stage gas nitriding method on the surface of aluminum and aluminum alloys, comprising the following steps: 1) Remove surface oxide film a. Pretreatment and cleaning of aluminum and aluminum alloy workpieces; b. Dip zinc for the first time, immerse the cleaned workpiece in a zinc dipping solution at room temperature for 50 to 70 seconds; c. Washing, washing the workpiece after the first galvanizing with water; d. Zinc stripping, immersing the workpiece after washing in water at room temperature to strip zinc in _HNO3 aqueous solution; e. Zinc dipping for the second time, immersing the stripped workpiece in a zinc dipping solution at room temperature for a second time of zinc dipping for 25 to 35 seconds; f. Washing and drying, cleaning the workpiece after the second galvanizing with water and drying or drying; 2) Surface nitriding a. Exhaust: After placing the workpiece with the surface oxide film removed above in the nitriding furnace, start feeding ammonia gas to remove the air in the furnace until the ammonia decomposition rate is less than 10%; b. The first temperature rise: raise the furnace temperature to 570~590°C, and gradually reduce the flow rate of ammonia gas during the temperature rise process until the ammonia decomposition rate reaches 30~45%; c. The first stage of nitriding: keep the furnace temperature at 570~590°C for 5~20 hours, and keep the ammonia decomposition rate at 15~25% during this process; d. For the second temperature rise, after the first stage of nitriding is completed, the furnace temperature is raised to 620~640°C, and the ammonia gas flow rate is reduced at the same time, so that the ammonia decomposition rate at this temperature reaches 40~55%; e. The second stage of nitriding: keep the furnace temperature at 620~640°C for 5~30 hours, and keep the ammonia decomposition rate at 40~55% during this process; f. For nitrogen removal, keep the furnace temperature at 620~640°C, reduce the flow rate of ammonia gas, so that the ammonia decomposition rate reaches 70~80%, and keep this state for 1~2 hours. 2. the two-stage gas nitriding method of aluminum and aluminum alloy surface according to claim 1, is characterized in that described zinc dipping solution is the solution obtained by dissolving the proportioning relation of following quality compound by 1 liter of water: Zinc oxide ZnO 10~30 g, Sodium hydroxide NaOH 50~70g, Potassium sodium tartrate KNaC ₄ H ₄ O ₆ 50 ~90g. 3. The two-stage gas nitriding method for aluminum and aluminum alloy surfaces according to claim 1, characterized in that the nitric acid _HNO3 aqueous solution used in the dezincification process in the step of removing the surface oxide film is 1: 0.8～1: 1.1 volume A mixed solution of water and nitric acid _HNO3 , the time for the workpiece to be immersed in the solution is 6-12s. 4. The two-stage gas nitriding method on the surface of aluminum and aluminum alloy according to claim 1, characterized in that the pretreatment process in the step of removing the surface oxide film comprises sequential chemical degreasing, hot water washing, cold water washing, Sub-steps of acid etching and water washing. 5. The two-stage gas nitriding method for the surface of aluminum and aluminum alloy according to claim 1, characterized in that in the exhaust step of the surface nitriding, after the workpiece is placed in the nitriding furnace, the furnace is heated up , When the furnace temperature rises to 300°C, ammonia gas is introduced.