Method for revealing four-zone structure of a rolled aluminum alloy welded joint at room temperature

Abstract

The present application is suitable for the technical field of microstructure sample preparation of metal material welded joint, and provides a method for showing four-zone microstructure of rolled aluminum alloy welded joint at normal temperature, comprising the following steps: after mechanical grinding, mechanical polishing is performed on the welded joint sample, and then electrolytic polishing is performed; HF-EDTA surface activation liquid is used to activate the polished surface; after activation treatment, the sample is immersed in a developing solution for normal temperature developing, the developing solution comprises KMnO4 and NaOH; the developing deposit is cleaned with an amino sulfonic acid solution, and then the sample is rinsed and dried, and observation is performed by using an optical microscope. The present application is suitable for aluminum alloy material welded joint, can simultaneously show four microstructure regions of weld, fusion line, heat affected zone and base material region under an optical microscope, and has good developing contrast.

Description

Technical Field

[0001] This invention belongs to the field of microstructure preparation technology for welded joints of metallic materials, and particularly relates to a method for revealing the four-zone microstructure of a rolled aluminum alloy welded joint at room temperature. Background Technology

[0002] Aluminum alloys are widely used in lightweight structures in aerospace, rail transportation, and automobiles, and welding is a key joining method in the manufacturing process. Due to the intense thermal cycle of welding, four regions with significant differences in microstructure and properties are usually formed at the joint: the weld, the fusion line, the heat-affected zone, and the base metal.

[0003] Existing metallographic etching methods, such as Keller's reagent, Barker's reagent coating method, and Weck's reagent development method, have the following drawbacks when developing welded joints of rolled aluminum alloys:

[0004] The development contrast is weak, and only some areas, such as the base material or weld, can be displayed;

[0005] Thermal development requires operation at 80℃, which demands sophisticated equipment and is difficult to control.

[0006] The boundaries of multiple zones are unclear, making them difficult to use for welding metallurgical analysis and quality assessment;

[0007] Poor visibility of grain boundaries limits subsequent structural characterization such as EBSD or image analysis.

[0008] Therefore, there is an urgent need to develop a new etching and developing process that can be operated at room temperature, produces clear results, and has controllable steps. Summary of the Invention

[0009] The purpose of this invention is to provide a method for revealing the four-zone microstructure of a rolled aluminum alloy welded joint at room temperature, thereby addressing the problems mentioned in the background art.

[0010] The present invention is implemented as follows: a method for revealing the four-zone microstructure of a rolled aluminum alloy welded joint at room temperature includes the following steps:

[0011] The welded joint sample was mechanically ground, then mechanically polished, and then electrolytically polished.

[0012] The polished surface was activated using HF-EDTA surface activation solution;

[0013] The activated sample is immersed in a developing solution and developed at room temperature of 10-30℃. The developing solution includes KMnO4 and NaOH.

[0014] The developed deposits were cleaned with sulfamic acid solution, and then the sample was rinsed and dried. It was then observed using an optical microscope, which simultaneously revealed four microstructure regions: weld, fusion line, heat-affected zone, and base material.

[0015] The present invention provides a method for displaying the four-zone microstructure of a rolled aluminum alloy welded joint at room temperature. HF-EDTA is used as a surface activating solution before room temperature development, KMnO4 and NaOH are used as developing solutions, and aminosulfonic acid is used as a selective cleaning solution for MnO2 deposition. While maintaining mild reaction conditions, the method significantly improves the contrast of welded microstructure zones and the visibility of grain boundaries, and has high practical and promotional value.

[0016] Specifically, the method provided in this embodiment of the invention is completed entirely at room temperature (10-30℃), requiring no heating device and ensuring high safety. The activation solution enhances the selectivity of subsequent deposition zones, while the cleaning solution improves grain boundary resolution, comprehensively enhancing optical imaging quality. Furthermore, the effect of a certain orthogonal polarizer can be observed under a regular optical microscope. Under an optical microscope, four structural regions—weld, fusion line, heat-affected zone, and base material—can be simultaneously visualized, with clear boundaries between the developed areas and good contrast. It exhibits good repeatability and is suitable for standardized batch sample preparation processes. The steps are clearly defined and applicable to aluminum alloy welded joints of different thicknesses and welding methods (e.g., MIG welding, TIG welding, laser welding, laser-arc hybrid welding, etc.). Attached Figure Description

[0017] Figure 1 This is a grain morphology diagram of the weld zone of a 6082-T6 aluminum alloy MIG welding head provided in Embodiment 1 of the present invention;

[0018] Figure 2 This is a grain morphology diagram of the fusion line of the 6082-T6 aluminum alloy MIG welded joint provided in Embodiment 1 of the present invention;

[0019] Figure 3 This is a grain morphology diagram of the heat-affected zone of the 6082-T6 aluminum alloy MIG welded joint provided in Embodiment 1 of the present invention;

[0020] Figure 4 The image shows the grain morphology of the 6082-T6 aluminum alloy MIG welding head base material provided in Embodiment 1 of the present invention.

[0021] Figure 5 This is a grain morphology diagram of the weld zone of a 6082-T6 aluminum alloy MIG welding head provided in Comparative Example 1 of the present invention;

[0022] Figure 6 This is a grain morphology diagram of the fusion line of the 6082-T6 aluminum alloy MIG welded joint provided in Comparative Example 1 of the present invention.

[0023] Figure 7This is a grain morphology diagram of the heat-affected zone of the 6082-T6 aluminum alloy MIG welded joint provided in Comparative Example 1 of the present invention;

[0024] Figure 8 The image shows the grain morphology of the 6082-T6 aluminum alloy MIG welding head base material provided in Comparative Example 1 of this invention.

[0025] Figure 9 This is a grain morphology diagram of the weld zone of the 6082-T6 aluminum alloy MIG welding head provided in Comparative Example 2 of the present invention;

[0026] Figure 10 This is a grain morphology diagram of the fusion line of the 6082-T6 aluminum alloy MIG welded joint provided in Comparative Example 2 of the present invention.

[0027] Figure 11 This is a grain morphology diagram of the heat-affected zone of the 6082-T6 aluminum alloy MIG welded joint provided in Comparative Example 2 of the present invention;

[0028] Figure 12 The image shows the grain morphology of the 6082-T6 aluminum alloy MIG welding head base material provided in Comparative Example 2 of this invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0030] The perchloric acid used in this embodiment of the invention is a purchased finished product with a labeled concentration of 70wt%.

[0031] The specific implementation of the present invention will be described in detail below with reference to specific embodiments.

[0032] Example 1: A method for revealing the four-zone microstructure of a 6082-T6 aluminum alloy MIG welded joint at room temperature, comprising the following steps:

[0033] (1) Preparation of the medicine:

[0034] Dissolve perchloric acid in anhydrous ethanol and stir until homogeneous to prepare an electrolyte, wherein the volume ratio of perchloric acid to anhydrous ethanol is 1:9.

[0035] Dissolve EDTA-2Na powder in 70mL of deionized water and stir well. Add Tween-20 and continue stirring. Slowly add 1.25mL of 40% HF solution (equivalent to 0.5g HF), and add water to 100mL. Stir well to prepare a surface-activated solution. The mass ratio of HF, EDTA-2Na, Tween-20 and deionized water is 0.5:0.05:0.01:100.

[0036] Sodium hydroxide and potassium permanganate are dissolved in deionized water and stirred evenly to prepare a developing solution, wherein the weight ratio of sodium hydroxide, potassium permanganate and deionized water is 1:4:100.

[0037] Sulfamic acid is dissolved in deionized water and stirred evenly to prepare a cleaning solution, wherein the weight ratio of sulfamic acid to deionized water is 0.5:100.

[0038] (2) Mechanical pretreatment:

[0039] The welded joint samples were wet-polished with 800-grit, 1000-grit, 1500-grit, and 2000-grit silicon carbide sandpaper, respectively. Coarse and fine polishing were performed using 2.5-micron and 0.5-micron particle polishing agents (the polishing agent was diamond polishing agent, and the mechanical polishing speed was 300 r / min). The final requirement was to obtain a polished surface with a particle size of 0.5 microns.

[0040] (3) Electropolishing:

[0041] The welded joint sample was placed in an electrolyte solution composed of perchloric acid and anhydrous ethanol and anodic electrolytic polishing was performed to obtain a mirror effect. The electrolysis voltage was 30V, the electrolysis current was 0.3A, the electrolysis time was 8S, and the electrolyte temperature was 15℃.

[0042] (4) Surface activation treatment:

[0043] The electrolytic welded joint sample was immersed in a surface activation solution for 5 seconds at a temperature of 10-30℃ to remove the surface oxide film, expose and activate the clean metal substrate;

[0044] (5) Development process:

[0045] The welded joint sample was immersed in the developing solution for 12 seconds at a temperature of 10-30℃ for development treatment (the developing solution needs to be continuously stirred on a magnetic stirrer with a magnetic rotor speed of 300r / min).

[0046] (6) Development, deposition, and cleaning:

[0047] After removing the welded joint sample, wipe it with cleaning solution for 3 seconds to selectively remove the MnO2 products deposited on the surface;

[0048] (7) Rinse and dry:

[0049] After rinsing with running water and then with anhydrous ethanol and drying, the grain morphology was observed under a metallographic microscope.

[0050] The results are as follows Figures 1 to 4As shown, the grain boundary edges in each region are clearly visible, and grains with inconsistent grain orientation can be observed at the weld seam.

[0051] Comparative Example 1: A method for revealing the microstructure of a 6082-T6 aluminum alloy MIG welded joint, comprising the following steps:

[0052] (1) Preparation of the medicine:

[0053] A corrosive agent was prepared by sequentially adding 2 mL of hydrofluoric acid, 3 mL of hydrochloric acid, and 5 mL of nitric acid to 190 mL of deionized water.

[0054] (2) Mechanical pretreatment:

[0055] Welded joint samples were taken and water-polished with silicon carbide sandpaper of 800 grit, 1000 grit, 1500 grit and 2000 grit respectively. Coarse polishing and fine polishing were performed using 2.5 micron particle size polishing agent and 0.5 micron particle size polishing agent (polishing agent is diamond polishing agent, mechanical polishing speed is 300 r / min).

[0056] (3) Chemical corrosion:

[0057] Take the welded joint sample from step (2) and immerse it in the etchant from step (1) until it develops color, about 60s. Gently shake the welded joint sample during the immersion etching process. Then rinse the welded joint sample with running water, rinse with 20wt% nitric acid solution, rinse with running water, rinse with anhydrous ethanol and dry it. Observe the grain morphology under a metallographic microscope.

[0058] The results are as follows Figures 5 to 8 As shown, only the weld metallographic structure is normal, while in other areas, various precipitates are over-corroded, appearing as rod-shaped or dot-shaped black corrosion pits, to the point that the grain boundaries cannot be displayed.

[0059] Comparative Example 2: A method for revealing the microstructure of a 6082-T6 aluminum alloy MIG welded joint, comprising the following steps:

[0060] (1) Preparation of the medicine:

[0061] Dissolve perchloric acid in anhydrous ethanol and stir until homogeneous to prepare an electrolyte, wherein the volume ratio of perchloric acid to anhydrous ethanol is 1:9.

[0062] Sodium hydroxide and potassium permanganate are dissolved in deionized water and stirred evenly to prepare a developing solution, wherein the weight ratio of sodium hydroxide, potassium permanganate and deionized water is 1:4:100.

[0063] (2) Mechanical pretreatment:

[0064] The welded joint samples were wet-polished with 800-grit, 1000-grit, 1500-grit, and 2000-grit silicon carbide sandpaper, respectively. Coarse and fine polishing were performed using 2.5-micron and 0.5-micron particle polishing agents (the polishing agent was diamond polishing agent, and the mechanical polishing speed was 300 r / min). The final requirement was to obtain a polished surface with a particle size of 0.5 microns.

[0065] (3) Electropolishing:

[0066] The welded joint sample was placed in an electrolyte solution composed of perchloric acid and anhydrous ethanol and anodic electrolytic polishing was performed to obtain a mirror effect. The electrolysis voltage was 30V, the electrolysis current was 0.3A, the electrolysis time was 8S, and the electrolyte temperature was 15℃.

[0067] (4) Development process:

[0068] The welded joint sample was immersed in the developing solution for 8 seconds at a temperature of 10-30℃ for development treatment.

[0069] (5) Rinse and dry:

[0070] After rinsing with running water and then with anhydrous ethanol and drying, the grain morphology was observed under a metallographic microscope.

[0071] The results are as follows Figures 9 to 12 As shown, due to the lack of surface activating liquid and cleaning liquid, the grain orientation difference in the weld area was not apparent, the fusion line and heat-affected zone were too darkly colored, the grain boundaries were not visible, while the grain boundaries in the base material area were clearly visible.

[0072] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of revealing a four-zone structure of a welded joint of a rolled aluminum alloy at room temperature, characterized in that, The method comprises the following steps: The welded joint sample is mechanically polished after mechanical grinding, and then electrolytic polishing is performed; HF-EDTA surface activation liquid is used for activation treatment of the polished surface, wherein the HF-EDTA surface activation liquid comprises HF, EDTA-2Na, Tween-20 and deionized water, and the mass ratio is 0.5:0.05:0.01:100; The sample after activation treatment is immersed in a developing solution, and developing is performed at room temperature of 10-30 DEG C, wherein the developing solution comprises KMnO4, NaOH and deionized water, and the mass ratio of NaOH, KMnO4 and deionized water is 1:4:100; The developing deposit is cleaned by using an amino sulfonic acid solution, and then the sample is rinsed and dried, and observation is performed by using an optical microscope, and four organizational regions of a weld, a fusion line, a heat affected zone and a base material zone are displayed.

2. The method of Claim 1, wherein In the step of mechanically polishing the welded joint sample after mechanical grinding, and then performing electrolytic polishing, the mechanical grinding is performed by using 800-mesh, 1000-mesh, 1500-mesh and 2000-mesh sandpaper for step-by-step polishing; The mechanical polishing is performed by using 2.5-micron polishing agent and 0.5-micron polishing agent for rough polishing and fine polishing; The electrolyte used for electrolytic polishing is a mixed solution of perchloric acid and anhydrous ethanol, the voltage is 30-35 V, the temperature is 10-15 DEG C, and the time is 5-10 s.

3. The method of claim 2, wherein, The concentration of the perchloric acid is 70 wt%, the concentration of the anhydrous ethanol is greater than or equal to 99.7 wt%, and the volume ratio of the perchloric acid to the anhydrous ethanol in the electrolyte is 1:

9.

4. The method of claim 1, wherein, In the step of using HF-EDTA surface activation liquid for activation treatment of the polished surface, the activation treatment time is 3-5 s.

5. The method of revealing a four-zone microstructure of a welded joint of a rolled aluminum alloy at room temperature according to claim 1, characterized in that In the step of immersing the sample after activation treatment in a developing solution and developing at room temperature of 10-30 DEG C, the developing time is 20-30 s.

6. The method of revealing a four-zone microstructure of a welded joint of a rolled aluminum alloy at room temperature according to claim 1, characterized in that In the step of cleaning the developing deposit by using an amino sulfonic acid solution, the concentration of the amino sulfonic acid solution is 0.5%.

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