Method for preparing interface metallography of magnesium alloy / duplex stainless steel laminated composite material

Abstract

The present application relates to the technical field of metallographic detection, and more particularly to a preparation method of magnesium alloy / dual-phase stainless steel layered composite interface metallography. The method comprises the following steps: (1) annealing; (2) preparation of a metallographic test block; (3) polishing of the metallographic test block; (4) mechanical polishing of the metallographic test block; (5) electrolytic polishing of a magnesium alloy layer; (6) metallographic observation of the magnesium alloy layer; (7) dissolution removal of the magnesium alloy layer; (8) mechanical polishing of a dual-phase stainless steel layer; (9) corrosion of the dual-phase stainless steel layer; (10) metallographic observation of the dual-phase stainless steel layer; and (11) synthesis of magnesium alloy / dual-phase stainless steel layered composite interface metallography. The metallograph prepared according to the method provided by the present application is not only uniformly corroded, but also has particularly obvious contrast of the dual-phase structure corroded.

Description

Technical Field

[0001] This invention relates to the field of metallographic testing technology, and more specifically, to a method for preparing the metallographic structure of a magnesium alloy / duplex stainless steel layered composite material interface. Background Technology

[0002] Layered metal composites possess numerous advantages, including saving rare and precious metals, reducing production costs, and improving the thermal expansion, strength, and wear resistance of single-metal materials. They can solve technical problems that single metals cannot address, thus belonging to the category of new materials. Therefore, as a novel structural and functional material that can both utilize the functions of rare metals and conserve resources, layered metal composites have a wide range of applications. Layered composites made by explosive welding of stainless steel and magnesium alloys can leverage the corrosion resistance of stainless steel to address the poor corrosion resistance of magnesium alloys. Magnesium / steel composite plates combine the lightweight properties of magnesium with the corrosion resistance of stainless steel, making them high-performance structural and functional integrated materials with broad application prospects in the automotive, aerospace, and defense industries.

[0003] Interface characterization is essential for studying the microstructure of composite materials. However, magnesium alloys and stainless steel have significant differences in physicochemical properties, making it difficult to obtain the grain morphology of the magnesium / steel interface using the same etchant. Furthermore, magnesium and steel are of different hardness; during grinding, the magnesium side wears faster, leading to an uneven interface and affecting subsequent metallographic observation. Explosive welding, a high-energy-rate, large-deformation welding technique, refines the grains at the interface of composite materials, making it difficult to obtain their grain morphology. Additionally, the large difference in the thermal expansion coefficients of the two materials results in highly concentrated stress at the interface after explosive welding, making it difficult to simply obtain the microstructure through interfacial metallographic etching.

[0004] To further investigate the metallurgical bonding mechanism and interfacial morphology of magnesium alloy / duplex stainless steel exploded welded layered composite materials, it is urgent to find a suitable method to characterize the metallographic structure of the magnesium alloy / stainless steel bonding interface. Summary of the Invention

[0005] In view of this, the present invention provides a method for preparing the metallographic structure of the interface of magnesium alloy / duplex stainless steel layered composite material. The method includes the following steps: (1) Annealing: annealing the magnesium alloy / duplex stainless steel layered composite material; (2) Preparation of metallographic specimens: preparing metallographic specimens by cutting magnesium alloy / duplex stainless steel layered composite material along the cross section using wire cutting; (3) Polishing of metallographic specimens: polishing the surface to be observed of the metallographic specimens with water sandpaper; (4) Mechanical polishing of metallographic specimens: mechanically polishing the surface to be observed of the metallographic specimens; (5) Electrolytic polishing of magnesium alloy layer: preparing magnesium alloy layer electrolyte, and electrolytic polishing the magnesium alloy layer of the metallographic specimens in the magnesium alloy layer electrolyte using an electrolytic corrosion instrument; (6) Metallographic observation of magnesium alloy layer: observing the metallographic structure on one side of the magnesium alloy layer in the composite interface under a microscope, and making positioning marks at the same time to prepare for subsequent image stitching. (7) Dissolution and removal of magnesium alloy layer: Prepare magnesium alloy layer etching solution, place metallographic specimen in magnesium alloy layer etching solution to completely dissolve magnesium alloy layer; (8) Mechanical polishing of duplex stainless steel layer: Mechanically polish the remaining duplex stainless steel layer to be observed in metallographic specimen; (9) Etching of duplex stainless steel layer: Prepare duplex stainless steel layer etching solution, use duplex stainless steel layer etching solution to etch the duplex stainless steel layer to be observed; (10) Metallographic observation of duplex stainless steel layer: Observe the metallographic structure of duplex stainless steel layer under microscope, and make positioning marks at the same time to prepare for subsequent image splicing; and (11) Synthesis of metallographic structure of magnesium alloy / duplex stainless steel layered composite material interface: Using the fully automatic microscope synthesis function, according to the positioning marks, synthesize magnesium alloy layer metallographic structure and duplex stainless steel layer metallographic structure to obtain metallographic structure of magnesium alloy / duplex stainless steel layered composite material interface.

[0006] Preferably, the annealing temperature in step (1) is 100–120°C, the heating rate is 20–30°C / s, and the holding time is 60–120 min. Preferably, the annealing temperature in step (1) is 100°C, 110°C, or 120°C, the heating rate is 20°C / s, 25°C / s, or 30°C / s, and the holding time is 60 min, 90 min, or 120 min.

[0007] Preferably, step (3) grinding of the metallographic specimen includes: grinding the surface to be observed of the metallographic specimen with 600#, 1000# and 2000# wet sandpaper in sequence. The first grinding direction is at -50° to 50° with the extension direction of the composite interface between the magnesium alloy layer and the stainless steel layer in the metallographic specimen. When changing sandpaper of different grits, the grinding direction is at 90° with the grinding direction of the previous sandpaper.

[0008] Preferably, step (4) mechanical polishing of the metallographic specimen specifically includes: sequentially polishing the surface of the metallographic specimen to be observed with 6μm diamond polishing agent, 2.5μm diamond polishing agent, and 0.5-1μm non-agglomerated alumina polishing powder, with a polishing disc rotation speed of 100-300 rad / min and a polishing time of 5-10 min. Preferably, the polishing disc rotation speed is 100 rad / min, 150 rad / min, 200 rad / min, 250 rad / min, or 300 rad / min, and the polishing time is 5 min, 6 min, 7 min, 8 min, 9 min, or 10 min.

[0009] Preferably, step (5) electrolytic polishing of the magnesium alloy layer specifically includes: preparing an electrolyte solution for the magnesium alloy layer using 1 mL of 50% hydrofluoric acid, 2 mL of 99% tartaric acid, and 98 mL of distilled water; electrolytic polishing of the magnesium alloy layer of the metallographic specimen using an electrolytic etching apparatus in the electrolyte solution; the cathode of the electrolytic etching apparatus being a stainless steel sheet connected to the negative terminal of a constant current and voltage regulated power supply; and the anode of the electrolytic etching apparatus being the metallographic specimen connected to the positive terminal of a constant current and voltage regulated power supply; the electrolytic etching voltage being 10–20V, the current being 0.6–0.9A, the electrode distance being 35–50mm, and the electrolytic etching time being 30–60s; during electrolytic polishing, the magnesium alloy layer electrolyte solution being magnetically stirred at a speed of 3–5r / s using a magnetic stirring device, and the temperature of the magnesium alloy layer electrolyte solution being maintained at 0℃ using a constant temperature water bath; and after electrolytic polishing, the metallographic specimen being washed with anhydrous ethanol and dried. Optionally, the electrolyte for the magnesium alloy layer can be prepared using 2 mg of tartaric acid powder with a purity higher than 99%. Preferably, the electrolytic corrosion apparatus has an electrolytic corrosion voltage of 10V, 15V, or 20V, a current of 0.6A, 0.7A, 0.8A, or 0.9A, an electrode distance of 35mm, 40mm, 45mm, or 50mm, and an electrolytic corrosion time of 30s, 40s, 50s, or 60s. Preferably, the magnetic stirring device rotates at a speed of 3r / s, 4r / s, or 5r / s.

[0010] Preferably, step (7) dissolving and removing the magnesium alloy layer specifically includes: preparing an HNO3 ethanol solution with a volume fraction of 4-6% as the magnesium alloy layer corrosion solution, and placing the metallographic specimen in the magnesium alloy layer corrosion solution for 90-180 minutes to allow the magnesium alloy layer to completely dissolve. Preferably, the volume fraction of HNO3 in the HNO3 ethanol solution is 4%, 5%, or 6%, and the corrosion time of the metallographic specimen in the magnesium alloy layer corrosion solution is 90 minutes, 120 minutes, 150 minutes, or 180 minutes.

[0011] Preferably, step (8) mechanical polishing of the duplex stainless steel layer specifically includes: sequentially using 5μm diamond polishing agent and 1μm diamond polishing agent to mechanically polish the surface of the duplex stainless steel layer to be observed, with a polishing disc rotation speed of 100-300 rad / min and a polishing time of 5-10 s. Preferably, the polishing disc rotation speed is 100 rad / min, 200 rad / min, or 300 rad / min, and the polishing time is 5 s, 6 s, 7 s, 8 s, 9 s, or 10 s.

[0012] Preferably, step (9) of corrosion of the duplex stainless steel layer specifically includes: preparing a duplex stainless steel layer corrosion solution using 10-15 mg of potassium hydroxide, 10-30 mg of potassium ferricyanide, and 60 mL of distilled water; heating the duplex stainless steel layer corrosion solution in a constant temperature water bath to 30-60°C; wiping the surface of the duplex stainless steel to be observed with a cotton swab dipped in the corrosion solution; when the surface of the duplex stainless steel to be observed turns light yellowish-brown, immediately rinsing the surface of the duplex stainless steel to be observed with clean water, then washing away the water marks with ethanol, and finally drying with a hair dryer. Preferably, the duplex stainless steel layer corrosion solution is prepared using 10 mg, 11 mg, 12 mg, 13 mg, 14 mg, or 15 mg of potassium hydroxide, 10 mg, 15 mg, 20 mg, 25 mg, or 30 mg of potassium ferricyanide, and 60 mL of distilled water. Preferably, the duplex stainless steel layer corrosion solution is heated in a constant temperature water bath to 30°C, 40°C, 50°C, or 60°C.

[0013] Preferably, the magnesium alloy in the magnesium alloy / duplex stainless steel layered composite material is AZ31B, and the duplex stainless steel is SAF2205. Preferably, the magnesium alloy / duplex stainless steel layered composite material is an explosively welded magnesium alloy / duplex stainless steel layered composite material.

[0014] The present invention also provides an interfacial metallographic model of a magnesium alloy / duplex stainless steel layered composite material prepared according to the above method.

[0015] Compared with the prior art, the advantages of the present invention are as follows:

[0016] 1. The method provided by this invention can obtain the full metallographic profile of the interface of magnesium alloy / duplex stainless steel exploded welded layered composite material, so that the microstructure of the interface can be clearly displayed at the same time, without the problem of excessive corrosion or shallow corrosion of a certain phase.

[0017] 2. The metallographic structure obtained by this invention is not only uniformly etched, but also has a particularly obvious contrast in the etched two-phase structure, which facilitates subsequent analysis;

[0018] 3. The method provided by this invention utilizes the characteristics of magnesium alloy and duplex steel to perform metallographic characterization in steps and then synthesize them. This avoids the situation of one high and one low when observing soft / hard materials at the same time. Metallographic observation can be completed under a regular microscope without the need to use an expensive ultra-depth-of-field microscope.

[0019] 4. Before metallographic preparation, the present invention can effectively remove the stress at the interface by low-temperature stress-relief annealing. Low-temperature annealing does not affect the metallographic structure of the interface and preserves the original morphology of the interface of the explosively welded composite material. Attached Figure Description

[0020] Figure 1 It shows the interface morphology of the magnesium alloy / duplex stainless steel exploded welded layered composite material before corrosion.

[0021] Figure 2 This is the complete metallographic morphology of the interface of the magnesium alloy / duplex stainless steel exploded welded layered composite material prepared by the method provided in this invention. Detailed Implementation

[0022] The purpose of this invention is to address the shortcomings of the prior art by using low-temperature annealing to remove stress and taking advantage of the significant difference in corrosion resistance between magnesium and stainless steel. The metallographic characteristics of the magnesium-stainless steel interface are characterized step by step, and finally, interface synthesis is performed to obtain the full metallographic profile of the magnesium alloy / duplex stainless steel exploded welded layered composite material interface.

[0023] The design concept of this invention is as follows: Due to the difference in thermal expansion coefficients between magnesium and steel, the interfacial stress after explosive welding is relatively large. Low-temperature stress-relief annealing is used to eliminate the interfacial stress between magnesium and steel. The annealing temperature should not be too high to prevent grain growth. Electrolytic corrosion is performed on the magnesium alloy of the composite plate. A magnesium alloy electrolytic corrosion solution is prepared and electrolytically corroded on the metallographic specimen of the entire composite plate to characterize the microstructure of the magnesium alloy. Nitric acid alcohol solution can dissolve the magnesium alloy and simultaneously form micro-corrosion on the duplex steel without affecting the metallographic structure of the stainless steel. The magnesium alloy layer of the metallographic specimen is completely dissolved using the nitric acid alcohol solution, while the stainless steel layer is preserved. The metallographic structure of the stainless steel layer is prepared using a stainless steel corrosion solution to obtain the metallographic structure of the stainless steel layer. Finally, the magnesium alloy metallographic structure and the stainless steel metallographic structure are synthesized to obtain the complete metallographic structure of the magnesium alloy / duplex stainless steel explosively welded layered composite material interface.

[0024] The present invention will be described below with reference to specific embodiments. Those skilled in the art will understand that these embodiments are for illustrative purposes only and do not limit the scope of the invention in any way. Unless otherwise specified, the raw materials, instruments, etc., used in the following embodiments are commercially available products.

[0025] Example 1: Preparation of interfacial metallographic features of AZ31B / SAF2205 explosively welded layered composite material

[0026] (1) Annealing: The AZ31B / SAF2205 explosion-welded layered composite plate was annealed at a temperature of 100℃, a heating rate of 25℃ / s, and a holding time of 90min; (2) Preparation of metallographic specimens: Metallographic specimens were prepared by cutting the AZ31B / SAF2205 composite plate along the cross section using wire cutting. The specimen size was 30mm×10mm×5mm; (3) Grinding of metallographic specimens: The surface to be observed of the metallographic specimens was ground sequentially with 600#, 1000#, and 2000# wet sandpaper. The initial grinding direction was at -50° to 50° with the extension direction of the composite interface between AZ31B and SAF2205 in the metallographic specimen. When changing to different grit sandpaper, the grinding direction was at 90° with the grinding direction of the previous grit sandpaper; (4) Metallographic specimens Mechanical polishing: The metallographic specimen was mechanically polished with 6μm diamond polishing agent, 2.5μm diamond polishing agent and 0.5~1μm non-agglomerated alumina polishing powder in sequence. The polishing disc rotation speed was 150rad / min and the polishing time was 8min. (5) Electrolytic polishing of AZ31B layer: AZ31B layer electrolyte was prepared with 1mL of 50% hydrofluoric acid, 2mL of 99% tartaric acid and 98mL of distilled water. The AZ31B layer of the metallographic specimen was electrolytically polished in the AZ31B layer electrolyte using an electrolytic corrosion instrument. The cathode of the electrolytic corrosion instrument was a stainless steel sheet and connected to the negative terminal of the constant current and voltage regulated power supply. The anode of the electrolytic corrosion instrument was the metallographic specimen and connected to the positive terminal of the constant current and voltage regulated power supply. The electrolytic corrosion voltage was 15V and the current was 0.8A, electrode distance 40mm, electrolytic corrosion time 50s, during electrolytic polishing, the electrolyte of AZ31B layer is magnetically stirred at a speed of 4r / s using a magnetic stirring device and the temperature of the electrolyte of AZ31B layer is kept at 0℃ using a constant temperature water bath. After electrolytic polishing, the metallographic specimen is washed with anhydrous ethanol and dried; (6) Metallographic observation of AZ31B layer: the metallographic structure on one side of AZ31B layer in the composite interface is observed under a microscope, and positioning marks are made at the same time to prepare for subsequent image splicing; (7) A Dissolution and removal of Z31B layer: Prepare a 5% HNO3 ethanol solution as the AZ31B layer etching solution. Place the metallographic specimen in the AZ31B layer etching solution for 120 min to completely dissolve the AZ31B layer; (8) Mechanical polishing of SAF2205 layer: Mechanically polish the surface of SAF2205 layer to be observed with 5μm diamond polishing agent and 1μm diamond polishing agent in sequence. The polishing disc speed is 200 rad / min and the polishing time is 8s; (9) SA Corrosion of the F2205 layer: Prepare a corrosion solution for the SAF2205 layer using 12 mg of potassium hydroxide, 20 mg of potassium ferricyanide, and 60 mL of distilled water. Heat the corrosion solution in a constant temperature water bath to 50°C. Use a cotton swab dipped in the corrosion solution to wipe the surface of the SAF2205 to be observed. When the surface turns light yellowish-brown, immediately rinse the surface with clean water, then wash away the water marks with ethanol, and finally use a blower. (10) Metallographic observation of the SAF2205 layer: The metallographic structure of the SAF2205 layer was observed under a microscope, and positioning marks were made to prepare for subsequent image stitching; and (11) Synthesis of the metallographic structure of the AZ31B / SAF2205 exploded welded layered composite material interface: Using the fully automatic microscope synthesis function, the metallographic structure of the AZ31B layer and the SAF2205 layer were synthesized according to the positioning marks to obtain the metallographic structure of the AZ31B / SAF2205 exploded welded layered composite material interface.

[0027] refer to Figure 1 and Figure 2 , Figure 1 The image shows the interface morphology of the AZ31B / SAF2205 exploded welded layered composite material before corrosion. Figure 2 This is the complete metallographic morphology of the interface of the AZ31B / SAF2205 exploded welded layered composite material prepared by the method provided in this invention. For example... Figure 2 As shown, the metallographic structure prepared by the method provided by the present invention can simultaneously and clearly display the microstructure around the interface of the two materials, without the problem of excessive or shallow corrosion of one phase. Furthermore, the metallographic structure prepared by the present invention is not only uniformly corroded, but also exhibits particularly obvious contrast in the corroded two-phase microstructure, which facilitates subsequent analysis.

[0028] While preferred embodiments of the invention have been shown and described herein, it will be apparent to those skilled in the art that these embodiments are provided by way of example only. Various changes, modifications, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein can be used to practice the invention. The scope of the invention is intended to be defined by the claims, and thereby covers the methods and structures within the scope of these claims and their equivalents.

Claims

1. A method for preparing the metallographic structure of a magnesium alloy / duplex stainless steel layered composite interface, characterized in that, Includes the following steps: (1) Annealing: The magnesium alloy / duplex stainless steel layered composite material is annealed; (2) Preparation of metallographic specimens: Metallographic specimens were prepared by cutting magnesium alloy / duplex stainless steel layered composite material along the cross section using wire cutting; (3) Polishing of metallographic specimens: Polish the surface of the metallographic specimens to be observed with water sandpaper; (4) Mechanical polishing of metallographic specimens: Mechanical polishing of the surfaces to be observed on the metallographic specimens; (5) Electrolytic polishing of magnesium alloy layer: Prepare magnesium alloy layer electrolyte with 1 mL of 50% hydrofluoric acid, 2 mL of 99% tartaric acid and 98 mL of distilled water. Electrolytic polish the magnesium alloy layer of the metallographic specimen in the magnesium alloy layer electrolyte using an electrolytic corrosion instrument. (6) Metallographic observation of magnesium alloy layer: Observe the metallographic structure of one side of magnesium alloy layer in composite interface under microscope, and make positioning marks at the same time to prepare for subsequent image stitching. (7) Dissolution and removal of magnesium alloy layer: Prepare an HNO3 ethanol solution with a volume fraction of 4-6% as the magnesium alloy layer corrosion solution. Place the metallographic specimen in the magnesium alloy layer corrosion solution for 90-180 min to completely dissolve the magnesium alloy layer. (8) Mechanical polishing of duplex stainless steel layer: Mechanical polishing of the remaining duplex stainless steel layer in the metallographic specimen; (9) Corrosion of duplex stainless steel layer: Prepare a duplex stainless steel layer corrosion solution with 10~15 mg of potassium hydroxide, 10~30 mg of potassium ferricyanide and 60 mL of distilled water, and use the duplex stainless steel layer corrosion solution to corrode the surface of the duplex stainless steel layer to be observed. (10) Metallographic observation of the duplex stainless steel layer: Observe the metallographic structure of the duplex stainless steel layer under a microscope, and make positioning marks to prepare for subsequent image stitching; and (11) Synthesis of the metallographic interface of magnesium alloy / duplex stainless steel layered composite material: Using the fully automatic microscope synthesis function, the metallographic interface of magnesium alloy layer and the metallographic interface of duplex stainless steel layer were synthesized according to the positioning mark to obtain the metallographic interface of magnesium alloy / duplex stainless steel layered composite material.

2. The preparation method according to claim 1, characterized in that, The annealing temperature in step (1) is 100~120℃, the heating rate is 20~30℃ / s, and the holding time is 60~120 min.

3. The preparation method according to claim 1, characterized in that, Step (3) Grinding of the metallographic specimen includes: grinding the surface to be observed of the metallographic specimen with 600#, 1000# and 2000# wet sandpaper in sequence. The first grinding direction is -50° to 50° with the extension direction of the composite interface between the magnesium alloy layer and the stainless steel layer in the metallographic specimen. When changing sandpaper of different grits, the grinding direction is 90° with the grinding direction of the previous sandpaper.

4. The preparation method according to claim 1, characterized in that, Step (4) Mechanical polishing of the metallographic specimen includes: mechanically polishing the surface to be observed of the metallographic specimen in sequence with 6 μm diamond polishing agent, 2.5 μm diamond polishing agent and 0.5~1 μm non-agglomerated alumina polishing powder, with a polishing disc rotation speed of 100~300 rad / min and a polishing time of 5~10 min.

5. The preparation method according to claim 1, characterized in that, In step (5), the cathode of the electrolytic corrosion instrument is a stainless steel sheet and is connected to the negative terminal of the constant current and voltage regulated power supply. The anode of the electrolytic corrosion instrument is a metallographic sample block and is connected to the positive terminal of the constant current and voltage regulated power supply. The electrolytic corrosion voltage is 10~20 V, the current is 0.6~0.9 A, the electrode distance is 35~50 mm, and the electrolytic corrosion time is 30~60 s. During electrolytic polishing, a magnetic stirring device is used to magnetically stir the magnesium alloy layer electrolyte at a speed of 3~5 r / s, and a constant temperature water bath is used to keep the temperature of the magnesium alloy layer electrolyte at 0℃. After electrolytic polishing, the metallographic sample block is washed with anhydrous ethanol and dried.

6. The preparation method according to claim 1, characterized in that, Step (8) Mechanical polishing of the duplex stainless steel layer specifically includes: mechanically polishing the surface of the duplex stainless steel layer to be observed with 5 μm diamond polishing agent and 1 μm diamond polishing agent in sequence, with a polishing disc rotation speed of 100~300 rad / min and a polishing time of 5~10 s.

7. The preparation method according to claim 1, characterized in that, Step (9) Corrosion of duplex stainless steel layer specifically includes: placing duplex stainless steel layer corrosion solution in a constant temperature water bath and heating it to 30~60℃, using a cotton swab dipped in duplex stainless steel layer corrosion solution to wipe the duplex stainless steel surface to be observed, when the duplex stainless steel surface to be observed turns light yellow-brown, immediately rinse the duplex stainless steel surface to be observed with clean water, then wash away the water marks with ethanol, and finally dry it with a hair dryer.

8. The preparation method according to claim 1, characterized in that, The magnesium alloy in the magnesium alloy / duplex stainless steel layered composite material is AZ31B, and the duplex stainless steel is SAF2205.

9. The preparation method according to claim 1, characterized in that, The magnesium alloy / duplex stainless steel layered composite material is a magnesium alloy / duplex stainless steel layered composite material obtained by explosive welding.

10. A metallographic profile of the interface of a magnesium alloy / duplex stainless steel layered composite material prepared by the preparation method according to any one of claims 1 to 9.

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