Nitriding heat treatment method for magnesium alloy

Abstract

The invention discloses a nitriding heat treatment method for magnesium alloy. The nitriding heat treatment method comprises the following steps: S1, removing an oxide layer on the surface of a magnesium alloy matrix and processing to obtain a finished product; s2, putting the magnesium alloy matrix into a quartz crucible, and uniformly covering the surface of the magnesium alloy matrix with ammonium chloride powder; s3, the magnesium alloy matrix is put into a vacuum tube furnace, and air in the furnace is pumped out through a vacuum pump; introducing high-purity nitrogen, and carrying out nitriding treatment; s4, repeatedly introducing nitrogen for multiple times in the heating process; and S5, when nitriding heat preservation is finished, a power source of the heating furnace is cut off, positive pressure in the vacuum tube furnace is maintained, furnace cooling is conducted to the room temperature, and the nitrided part is taken out. According to the nitriding heat treatment method provided by the invention, a specific nitriding temperature is adopted for a magnesium alloy matrix, and a strengthened layer which takes MgO as a dominant material and contains a new alloy phase Zn1. 7GeN1. 8O is formed, and the strengthened layer can effectively improve the surface mechanical and tribological properties of the magnesium alloy; and the preparation scheme has the advantages of simple process, low cost and short preparation time.

Description

Technical Field

[0001] The invention belongs to the technical field of nitriding treatment, and in particular relates to a nitriding treatment method for magnesium alloy. Background Art

[0002] The main nitriding process methods include gas nitriding, ion nitriding, and liquid nitriding. The temperature of nitriding heat treatment is usually carried out at 460℃-650℃. A nitrogen-containing magnesium alloy strengthening layer will be formed on the surface of the treated metal substrate. The formation and growth rate of the nitriding strengthening layer depends on factors such as the nitrogen potential of the atmosphere, the absorption capacity of the magnesium alloy surface for nitrogen, and the diffusion of nitrogen in the magnesium alloy. The higher the nitrogen potential of the ambient atmosphere, the stronger the ability of the substrate surface to adsorb nitrogen atoms, thereby accelerating the diffusion rate of nitrogen.

[0003] At present, nitriding treatment is mainly used for strengthening the surface of titanium alloys, but the strengthening effect of nitriding treatment on the surface of titanium alloys is limited, and the weight of titanium alloys still restricts their promotion and use in high-end fields such as aviation.

[0004] Magnesium alloy is one of the lightest metal structural materials at present. It has the advantages of high specific strength and specific stiffness, good machinability, high electromagnetic shielding and high recycling rate. It is used as a supporting and connecting component in the fields of aerospace, automobile manufacturing and electronic communications. However, magnesium alloy has a close-packed six-bit structure and the surface oxide film formed is a loose structure, which leads to poor friction and wear performance of magnesium alloy. In recent years, research on improving surface friction and wear performance has mainly focused on surface modification, among which nitriding treatment is undoubtedly a good choice. However, due to the high temperature required for conventional nitriding process, long heating time, high nitrogen consumption and high cost, the gas nitriding process for magnesium alloy AZ91D material is still blank. Summary of the invention

[0005] The present invention aims to solve at least one of the technical problems in the related art to a certain extent. To this end, the main purpose of the present invention is to provide a nitriding heat treatment method for magnesium alloys, aiming to solve the problems of high temperature, long heating time, large nitrogen consumption and high cost in the existing nitriding process.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] A nitriding heat treatment method for magnesium alloys comprises the following steps:

[0008] S1. Remove the oxide layer on the surface of the magnesium alloy substrate and process it into a finished product;

[0009] S2, placing the magnesium alloy substrate obtained in step S1 into a quartz crucible, and uniformly covering the surface of the magnesium alloy substrate with ammonium chloride powder;

[0010] S3, placing the magnesium alloy substrate obtained in step S2 into a vacuum tube furnace, using a vacuum pump to extract the air in the furnace; introducing high-purity nitrogen gas, extracting the gas, repeating the process several times, maintaining the vacuum state in the furnace, and performing nitriding treatment as the furnace temperature rises;

[0011] S4, repeatedly introducing nitrogen during the heating process;

[0012] S5. When the nitriding insulation is finished, disconnect the power supply of the heating furnace, maintain the positive pressure in the vacuum tube furnace, and take out the nitrided parts as the furnace cools to room temperature.

[0013] In some specific embodiments, the magnesium alloy matrix includes the following components, by mass fraction: Al 8.5-9.5%, Zn 0.45-0.90%, Mn 0.17-0.4%, Si≤0.05%, Cu≤0.025, Ni≤0.001, Fe≤0.004, and the balance is Mg.

[0014] In some specific embodiments, removing the oxide layer on the surface of the magnesium alloy substrate in step S1 specifically comprises: polishing the magnesium alloy substrate with 400-3000 mesh sandpaper to remove the oxide layer on the surface of the magnesium alloy substrate, and then ultrasonically cleaning with anhydrous ethanol and drying.

[0015] In some specific embodiments, the purity of the ammonium chloride powder in step S2 is ≥ 95%.

[0016] In some specific embodiments, in step S3, the process parameters of the nitriding treatment are: 1) insulation temperature: 400-450° C., insulation for 10 hours, and gas pressure in the furnace of 0.06-0.08 MPa.

[0017] In some specific embodiments, the introduction of nitrogen in step S4 is performed by extracting the gas in the furnace every 0.5-1.5 hours and introducing nitrogen at the same time.

[0018] In some specific embodiments, maintaining the positive pressure in the vacuum tube furnace in step S5 specifically includes: after heating is completed, the gas in the furnace is extracted, and nitrogen is introduced to replace the gas in the furnace to maintain the positive pressure state in the furnace.

[0019] A part prepared by the aforementioned nitriding heat treatment method.

[0020] Compared with the prior art, the present invention has at least the following advantages:

[0021] 1) The nitriding heat treatment method for magnesium alloy provided by the present invention adopts a specific nitriding temperature for a specific magnesium alloy substrate, combined with other specific process steps and parameters, to form a strengthening layer dominated by MgO and containing a new alloy phase Zn1.7GeN1.8O, which can effectively improve the mechanical and tribological properties of the magnesium alloy surface; and the preparation scheme has the advantages of simple process, low cost and short preparation time;

[0022] 2) When magnesium alloy is applied to self-lubricating spherical bearings, its tribological properties are the key issues in preparation and use. The magnesium alloy parts prepared by the nitriding heat treatment scheme of the present invention have excellent friction resistance and excellent microhardness; the present invention can improve the theoretical system of magnesium alloy modification and improve the comprehensive performance of self-lubricating spherical bearings, which is of great significance to improving my country's aerospace industry and even strengthening national defense strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the specific embodiments of the present invention, the drawings required for use in the specific embodiments or the description of the prior art will be briefly introduced below.

[0024] Figure 1 A schematic diagram of changes in pressure in a furnace during operation of the nitriding heat treatment method for magnesium alloys provided by the present invention;

[0025] Figure 2 The tribological properties of the magnesium alloy and the substrate after being treated by the method provided by the present invention;

[0026] Figure 3 This is the physical phase test condition of the untreated magnesium alloy matrix in the present invention;

[0027] Figure 4 This is the physical phase test result of the magnesium alloy treated by the method provided by the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. The following embodiments are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0029] When a certain amount, concentration or other value or parameter is expressed in the form of a range, a preferred range, or a preferred upper and lower numerical limit, it should be understood that it is equivalent to specifically revealing any range by combining any pair of upper range limits or preferred numerical values ​​with any lower range limit or preferred numerical value, regardless of whether the range is specifically disclosed. Unless otherwise indicated, the numerical range values ​​listed herein include the endpoints of the range, and all integers and fractions within the range.

[0030] Unless otherwise indicated, all percentages, parts, ratios, etc. herein are by weight.

[0031] The materials, methods, and examples herein are illustrative and are not to be construed as limiting unless specifically stated.

[0032] In the following embodiments, the magnesium alloy used is AZ91 magnesium alloy produced by Huatai Metal Materials; the ammonium chloride used is ammonium chloride powder produced by Shanghai Bid Pharmaceutical Technology Co., Ltd., with a purity of 99.5% AR and a molecular weight of 53.49.

[0033] In the following implementation, the manufacturer of the vacuum tube furnace used is Tianjin Kaiheng Electric Heating Technology Co., Ltd., model KLG-12C;

[0034] The tribological properties and mechanical properties are reflected by testing the wear resistance, phase and hardness changes of each test sample respectively; the main properties tested in this application include tribological properties and mechanical properties.

[0035] 1) Tribological test

[0036] The friction and wear performance tester UMT-3 produced by the Center for Tribology Company of the United States was used to test the friction and wear performance. The tribological performance was tested using a linear reciprocating module in accordance with the "ASTM G133-05 Standard Test Method for Linear Reciprocating Ball Sliding Wear".

[0037] 2) Physical phase test

[0038] The phase analysis was performed using an Ultima IV multifunctional X-ray diffractometer produced by Rigaku Corporation of Japan. The test parameters used were: operating voltage 40 kV, operating current 40 mA, scanning angle 20-120°, and scanning speed 0.02 degrees / second.

[0039] 3) Hardness test

[0040] The microhardness of the strengthening layer was tested using a CPX-NHT2 nanoindentation tester produced by Anton Paar, Switzerland. The test parameters used were: indentation load of 10 mN, indentation time of 10 seconds, and measurement at three locations evenly spaced in a zigzag pattern and the average value was taken.

[0041] Example 1

[0042] This embodiment provides a nitriding heat treatment method for magnesium alloy, comprising the following steps:

[0043] S1. Grinding the AZ91 magnesium alloy substrate with sandpapers of 150 mesh, 400 mesh, 600 mesh, 1000 mesh, 2000 mesh and 3000 mesh in sequence, and then polishing with diamond abrasive pastes of 2.5 μm, 1.5 μm and 0.5 μm to make the substrate surface mirror-finished; removing the oxide layer and scratches on the surface of the magnesium alloy substrate, and then ultrasonically cleaning it in anhydrous ethanol for 30 minutes to obtain the treated magnesium alloy substrate;

[0044] S2, placing the magnesium alloy substrate treated in step S1 into a quartz crucible, and uniformly covering the surface of the magnesium alloy substrate with ammonium chloride powder;

[0045] S3, placing the magnesium alloy substrate with the ammonium chloride powder uniformly coated on the surface in step S2 into a vacuum tube furnace, using a vacuum pump to extract the air in the furnace, and then introducing high-purity nitrogen, and repeating the cycle several times to ensure that the furnace is full of nitrogen; the schematic diagram of the pressure change in the furnace during operation is as shown in Figure 1 As shown, specifically, during operation, the pressure in the furnace repeatedly increases from zero, remains for a long time, and then drops to zero. After ensuring the gas environment pressure in the furnace, the process parameters required for nitriding treatment are set: insulation temperature: 400±5℃, insulation for 10 hours, and gas pressure in the furnace 0.06MPa.

[0046] S4, repeatedly introducing nitrogen during the heating process;

[0047] S5. When the nitriding and heat preservation is finished, disconnect the power supply of the heating furnace, extract the gas in the furnace, introduce nitrogen to replace the gas in the furnace, maintain the positive pressure state in the furnace, and take out the nitrided magnesium alloy parts as the furnace cools to room temperature.

[0048] Example 2

[0049] This embodiment provides a nitriding heat treatment method for magnesium alloy, comprising the following steps:

[0050] S1. Grinding the AZ91 magnesium alloy substrate with sandpapers of 150 mesh, 400 mesh, 600 mesh, 1000 mesh, 2000 mesh and 3000 mesh in sequence, and then polishing with diamond abrasive pastes of 2.5 μm, 1.5 μm and 0.5 μm to make the substrate surface mirror-finished; removing the oxide layer and scratches on the surface of the magnesium alloy substrate, and then ultrasonically cleaning it in anhydrous ethanol for 30 minutes to obtain the treated magnesium alloy substrate;

[0051] S2, placing the magnesium alloy substrate treated in step S1 into a quartz crucible, and uniformly covering the surface of the magnesium alloy substrate with ammonium chloride powder;

[0052] S3. Place the magnesium alloy substrate whose surface is uniformly coated with ammonium chloride powder in step S2 into a vacuum tube furnace, use a vacuum pump to evacuate the air in the furnace, and then introduce high-purity nitrogen, repeat the cycle several times to ensure that the furnace is full of nitrogen; during operation, the pressure in the furnace repeatedly increases from zero, maintains for a long time and then decreases to zero. After ensuring the gas environment pressure in the furnace, set the process parameters required for nitriding treatment: insulation temperature: 430±5℃, insulation for 10 hours, gas pressure in the furnace 0.06MPa.

[0053] S4, repeatedly introducing nitrogen during the heating process;

[0054] S5. When the nitriding and heat preservation is finished, disconnect the power supply of the heating furnace, extract the gas in the furnace, introduce nitrogen to replace the gas in the furnace, maintain the positive pressure state in the furnace, and take out the nitrided magnesium alloy parts as the furnace cools to room temperature.

[0055] Example 3

[0056] This embodiment provides a nitriding heat treatment method for magnesium alloy, comprising the following steps:

[0057] S1. Grinding the AZ91 magnesium alloy substrate with sandpapers of 150 mesh, 400 mesh, 600 mesh, 1000 mesh, 2000 mesh and 3000 mesh in sequence, and then polishing with diamond abrasive pastes of 2.5 μm, 1.5 μm and 0.5 μm to make the substrate surface mirror-finished; removing the oxide layer and scratches on the surface of the magnesium alloy substrate, and then ultrasonically cleaning it in anhydrous ethanol for 30 minutes to obtain the treated magnesium alloy substrate;

[0058] S2, placing the magnesium alloy substrate treated in step S1 into a quartz crucible, and uniformly covering the surface of the magnesium alloy substrate with ammonium chloride powder;

[0059] S3. Place the magnesium alloy substrate whose surface is uniformly coated with ammonium chloride powder in step S2 into a vacuum tube furnace, use a vacuum pump to evacuate the air in the furnace, and then introduce high-purity nitrogen, repeat the cycle several times to ensure that the furnace is full of nitrogen; during operation, the pressure in the furnace repeatedly increases from zero, maintains for a long time and then drops to zero. After ensuring the gas environment pressure in the furnace, set the process parameters required for nitriding treatment: insulation temperature: 450±5℃, insulation for 10 hours, gas pressure in the furnace 0.06MPa.

[0060] S4, repeatedly introducing nitrogen during the heating process;

[0061] S5. When the nitriding and heat preservation is finished, disconnect the power supply of the heating furnace, extract the gas in the furnace, introduce nitrogen to replace the gas in the furnace, maintain the positive pressure state in the furnace, and take out the nitrided magnesium alloy parts as the furnace cools to room temperature.

[0062] Comparative Example 1

[0063] This comparative example provides a nitriding heat treatment method for magnesium alloy, and its preparation method is basically the same as that of Example 2, except that the process parameters of the nitriding treatment in step S3 are: holding temperature: 360±5°C, holding time 10 hours, and gas pressure in the furnace 0.06MPa.

[0064] The performance of the magnesium alloy prepared in the comparative example was tested. The results showed that within this temperature range, the amount of nitrogen atoms decomposed was small, the strengthening layer formed on the surface of the sample was thin and not evenly covered, the decomposition rate of ammonium chloride was relatively slow, and the generated chlorine gas failed to corrode the scratches on the surface of the magnesium alloy. The surface roughness and friction coefficient were relatively high, and the tribological performance was relatively poor.

[0065] Comparative Example 2

[0066] This comparative example provides a nitriding heat treatment method for magnesium alloy, and its preparation method is basically the same as that of Example 2, except that the process parameters of the nitriding treatment in step S3 are: holding temperature: 480±5°C, holding time 10 hours, and gas pressure in the furnace 0.06MPa.

[0067] The performance of the magnesium alloy prepared by this comparative example was tested, and the results showed that within this temperature range, the strengthening layer was continuously corroded, and nitrogen atoms regenerated a new strengthening layer on the corroded surface, resulting in discontinuous accumulation on the surface of the matrix, and a large number of ridge-like accumulations and damage, resulting in increased surface roughness, increased friction coefficient, and reduced tribological properties.

[0068] Performance test case

[0069] This application takes Example 2 as an example to perform performance tests on the nitrided magnesium alloy parts prepared therefrom, specifically:

[0070] 1) Tribological performance test

[0071] The UMT-3 friction and wear tester produced by the Center for Tribology Company of the United States was used. The test parameters were load 20N, stroke 6mm, frequency 2Hz, and total stroke 21.6m. The grinding pair used in the test was Gcr15 steel ball with a small ball diameter of 9.35mm. The tribological properties of S3 nitrided magnesium alloy were tested. The results are as follows Figure 2 As shown in the figure, the friction coefficient of the strengthening layer is greatly reduced compared with the AZ91 magnesium alloy substrate. Under the same friction test conditions, the wear amount of the substrate is about 7.1 mg, and the wear amount of the strengthening layer is about 2.3 mg, which is 67% less than that of the substrate.

[0072] 2) Physical phase test

[0073] The present application conducted a phase test on the nitrided magnesium alloy parts prepared in Example 2, and at the same time introduced a magnesium alloy part that had not been nitrided to conduct a phase test. The results are as follows: Figure 3 Figure and Figure 4 As shown, there are only two single phases, Mg and MgO, in the non-nitrided magnesium alloy parts, and the characteristic peak height of Mg is 80,000, which shows that it occupies a dominant position in the non-nitrided magnesium alloy parts, and the content of this crystal phase is relatively high. However, a new alloy phase Zn1.7GeN1.8O is formed in the nitrided magnesium alloy parts in this application. In addition, the characteristic peaks of the alloy phase MgO increase, and the characteristic peak ratio of Mg decreases significantly, which can indicate that Mg is oxidized in large quantities after strengthening, and a new phase is formed.

[0074] 3) Hardness test

[0075] The present application conducts a hardness test on the nitrided magnesium alloy parts prepared in Example 2. In terms of hardness, the microhardness of the matrix is ​​about 713 MPa, and the microhardness of the strengthening layer is about 3493 MPa. After nitriding, the microhardness of the magnesium alloy is greatly improved, which is 3.8 times that of the unstrengthened matrix.

[0076] In summary: The nitriding heat treatment method of the present application prepares a strengthening layer on the surface of the AZ91 magnesium alloy. Compared with the conventional nitriding process, the surface strengthening method has a lower temperature and has the advantages of simple process, low cost and short preparation time. There is a lack of comprehensive research on the tribological properties and wear mechanism of magnesium alloys after material strengthening. The present invention selects the widely used AZ91 magnesium alloy as the material for kinematic pair friction, and studies the preparation process of the strengthening layer on its surface, which is of great significance to the application of magnesium alloys in friction.

[0077] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. These modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and specification of the present invention.

Claims

1. A nitriding heat treatment method for magnesium alloy, characterized in that: The steps include: S1. Remove the oxide layer on the surface of the magnesium alloy substrate and process it into a finished product; S2, placing the magnesium alloy substrate obtained in step S1 into a quartz crucible, and uniformly covering the surface of the magnesium alloy substrate with ammonium chloride powder; S3, placing the magnesium alloy substrate obtained in step S2 into a vacuum tube furnace, using a vacuum pump to extract the air in the furnace; introducing high-purity nitrogen gas, extracting the gas, repeating the process several times, maintaining the vacuum state in the furnace, and performing nitriding treatment as the furnace temperature rises; S4, repeatedly introducing nitrogen during the heating process; S5. When the nitriding insulation is finished, disconnect the power supply of the heating furnace, maintain the positive pressure in the vacuum tube furnace, and take out the nitrided parts as the furnace cools to room temperature.

2. The nitriding heat treatment method for magnesium alloy according to claim 1, characterized in that: The magnesium alloy matrix comprises the following components, by mass fraction: Al 8.5-9.5%, Zn 0.45-0.90%, Mn 0.17-0.4%, Si≤0.05%, Cu≤0.025, Ni≤0.001, Fe≤0.004, and the balance is Mg.

3. The nitriding heat treatment method for magnesium alloy according to claim 1, characterized in that: The step S1 of removing the oxide layer on the surface of the magnesium alloy substrate is specifically as follows: polishing the magnesium alloy substrate with sandpaper of 400-3000 mesh to remove the oxide layer on the surface of the magnesium alloy substrate, and then ultrasonically cleaning with anhydrous ethanol and drying.

4. The nitriding heat treatment method for magnesium alloy according to claim 1, characterized in that: The purity of the ammonium chloride powder described in step S2 is ≥95%.

5. The nitriding heat treatment method for magnesium alloy according to claim 4, characterized in that: In step S3, the process parameters of the nitriding treatment are: holding temperature: 400-450°C, holding time: 10 hours, and gas pressure in the furnace: 0.06-0.08 MPa.

6. The nitriding heat treatment method for magnesium alloy according to claim 1, characterized in that: The nitrogen is introduced in step S4 by extracting the gas in the furnace every 0.5-1.5 hours and introducing nitrogen at the same time.

7. The nitriding heat treatment method for magnesium alloy according to claim 1, characterized in that: In step S5, the positive pressure in the vacuum tube furnace is maintained as follows: after heating is completed, the gas in the furnace is extracted, and nitrogen is introduced to replace the gas in the furnace to maintain the positive pressure state in the furnace.

8. A part prepared by the nitriding heat treatment method according to any one of claims 1 to 7.

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