Corrosive agent for steel / copper foil dissimilar metal laser welding lap joint microstructure

Abstract

The invention discloses a steel / copper foil dissimilar metal laser welding lap joint microstructure corrosive, and relates to the technical field of metallographic corrosion. The corrosive agent is composed of 100-200 mL of ethyl alcohol, 50-100 mL of hydrochloric acid and 5-10 g of copper sulfate powder, synchronous apparent corrosion of grain boundaries on the two sides of a steel / copper foil dissimilar metal interface is achieved through an optimized proportion, and the corrosive agent is particularly suitable for microstructure characterization of a steel / copper foil lap joint formed through laser welding. The corrosive agent disclosed by the invention can complete double-phase grain boundary corrosion in a normal-temperature environment, synchronously shows grain morphology of a weld zone, a heat affected zone and a base material within 20-40 seconds, and solves the problem of structure contrast distortion caused by traditional step-by-step corrosion; and a key analysis means is provided for researching metallurgical reaction, grain growth orientation and element diffusion behavior of the dissimilar metal welding interface. The method has the characteristics of convenience in operation, low cost, good reproducibility and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of metallographic corrosion, and particularly to a grain boundary etchant for steel / copper foil lap joints. Background Art

[0002] In the precision manufacturing fields such as the connection of the tabs of new energy power batteries and microelectronics packaging, laser lap welding of dissimilar metals of steel / copper foil has been widely used due to the need for high thermal conductivity matching. However, the accurate characterization of the gradient structure (such as the columnar grain / equiaxed grain transition zone on the copper side and the martensitic transformation zone on the steel side) formed during the rapid solidification process of this heterogeneous interface is restricted by the limitations of traditional metallographic corrosion techniques.

[0003] The existing microscopic analysis of dissimilar metal joints generally adopts the sectional etching method: for the copper matrix, an FeCl3-HCl solution is mostly used, while for the steel matrix, it is necessary to switch to a nitric acid alcohol etchant. However, this method has many defects. For example, step-by-step etching leads to repeated oxidation of the tissue in the heterogeneous interface transition zone and distortion of the grain boundary contrast; the copper foil specimen is prone to mechanical damage during multiple corrosion-cleaning cycles; the observation fault of the element diffusion channel is caused by the time difference of two-phase corrosion.

[0004] Especially for the unique micro-melting pool interface of laser welding, due to the difference in the intergranular corrosion sensitivity of copper / steel, traditional etchants often cause over-etching of the copper matrix or insufficient display of the grain boundaries on the steel side. The present invention constructs a hydrochloric acid-copper sulfate-ethanol composite etching system to achieve synchronous etching of the two-phase grain boundaries on both sides of the heterogeneous interface, providing key technical support for analyzing the heterogeneous interface metallurgical behavior of steel / copper foil dissimilar metals under laser welding. Summary of the Invention

[0005] In order to overcome the deficiencies in the prior art, the present invention provides a grain boundary etchant for steel / copper foil lap joints. This etchant can effectively etch the grain boundaries of copper and steel simultaneously and can clearly display the grain boundary structure of the material, thus facilitating the study of the joint characteristics of steel / copper foil dissimilar metals.

[0006] The present invention provides a grain boundary etchant for steel / copper foil lap joints, and its formula includes the following components: ethanol, hydrochloric acid, copper sulfate powder. For every 100 - 200 mL of ethanol, 50 - 100 mL of hydrochloric acid and 5 - 10 g of copper sulfate powder are corresponding.

[0007] The mass fraction concentration of the hydrochloric acid is 36% - 38%, and the purity of the copper sulfate powder is 99.9%. In specific operations, the specific content of each component can be enlarged or reduced according to the actual situation of the surface of the specimen to be etched.

[0008] The grain boundary etching method for the steel / copper foil lap joints of the present invention includes the following steps:

[0009] (1) Inlay: The lap joint specimens of steel / copper dissimilar metals are subjected to inlay treatment;

[0010] (2) Grinding: After the inlaid samples are cooled, they are ground successively with sandpaper from low grit to high grit;

[0011] (3) Polishing: The ground specimens are polished to ensure a smooth and flawless surface;

[0012] (4) Primary cleaning: The specimens are cleaned with anhydrous ethanol;

[0013] (5) Corrosion: The cleaned specimens are immersed in the above-mentioned corrosive agent and corroded at room temperature for 40 - 50 seconds;

[0014] (6) Secondary cleaning: After corrosion, they are cleaned again with anhydrous ethanol;

[0015] (7) Drying: The specimens are dried with a hair dryer.

[0016] Furthermore, in the step (1), cold inlay is carried out with epoxy resin powder at room temperature for 15 minutes.

[0017] Furthermore, in the step (2), sandpapers with grits of 240, 400, 600, 800, 1000, 1200, 1500, 2000, 3000, and 5000 are used to grind under running water successively.

[0018] Furthermore, in the step (3), first, rough polishing is carried out with a nylon polishing cloth and a diamond polishing spray with a particle size of 0.5 μm. After the grinding marks are polished off and the polishing marks are in the same direction, fine polishing is carried out with an imported velvet polishing cloth and a diamond polishing spray with a particle size of 0.25 μm until the surface shows a mirror-like luster and there are no polishing marks under an optical microscope.

[0019] Advantages of the present invention:

[0020] Combining the corrosive agent and the corrosion method in the present invention to corrode the lap joint of steel / copper foil has good corrosion effect, clear grain boundaries can be seen, and the observation effect under an optical microscope is good. It has low requirements for equipment, simple operation steps, effectively saves time cost and economic cost, and is conducive to studying the influence of the grain morphology and size of the steel / copper foil lap joint on the alloy performance. Description of the drawings

[0021] Figure 1 It is the metallographic structure photograph corroded in Example 2;

[0022] Figure 2 It is the metallographic structure photograph corroded in Example 2;

[0023] Figure 3 The metallographic structure photograph etched for Example 3.

[0024] Figure 4 The metallographic structure photograph etched for Comparative Example 1;

[0025] Figure 5 The metallographic structure photograph etched for Comparative Example 2. Detailed implementation manners

[0026] To make the objectives, technical solutions and advantages of the present invention clearer, the following will further describe in detail the implementation manners of the present invention in conjunction with the attached drawings. In all examples and comparative examples, the samples to be etched are laser dissimilar metal welded joints of low-carbon steel / pure copper foil.

[0027] (1) Grind the steel / copper foil lap joint specimen successively with 240# - 5000# silicon carbide sandpaper and mechanically polish it until the surface roughness Ra ≤ 0.05μm;

[0028] (2) Immerse the specimen in the etchant at 25 ± 2°C for 30 - 50 seconds;

[0029] (3) Immediately clean it with absolute ethanol after taking it out and blow it dry.

[0030] 4. The method according to claim 3, wherein the copper is rolled copper with a thickness of 5 - 300μm; the steel sheet is low-carbon steel with a thickness of 0.1 - 2mm.

[0031] Example 1

[0032] An etchant capable of simultaneously etching the grain boundaries of copper and steel, and its formula includes the following components: 100 mL C2H5OH, 50 mL HCl, 5 g CuSO4, and mix them evenly to obtain a light green No. 1 etchant.

[0033] Among them, the mass fraction of hydrochloric acid is adjusted between 36% - 38%, and the weight of CuSO4 is adjusted between 4 - 6 g, and an etchant with similar performance can also be obtained.

[0034] An etching method for the grain boundaries of copper and steel, including the following steps:

[0035] (1) Inlay the dissimilar metal joint of steel / copper foil, and use epoxy resin powder to cold inlay the copper foil / steel sheet lap joint specimen in an inlaying machine at room temperature for 15 minutes;

[0036] (2) Grind. After the inlaid sample cools down, grind the inlaid specimen with sandpaper from low mesh to high mesh; successively

[0037] Grind with sandpapers of 240 mesh, 400 mesh, 600 mesh, 800 mesh, 1000 mesh, 1200 mesh, 1500 mesh, 2000 mesh, 3000 mesh, and 5000 mesh under running water;

[0038] (3) Polishing: Polish the ground specimen. First, perform rough polishing using a nylon polishing cloth and a diamond polishing spray with a particle size of 0.5 μm. After the polishing marks are in the same direction, perform fine polishing using an imported velvet polishing cloth and a diamond polishing spray with a particle size of 0.25 μm until the surface shows a mirror-like luster and there are no polishing marks observed under an optical microscope;

[0039] (4) First cleaning: Clean the polished specimen with absolute ethanol;

[0040] (5) Corrosion: Dry the cleaned specimen, and then immerse it in the No. 1 etchant prepared in Example 1 for 45 s at room temperature;

[0041] (6) Second cleaning: Clean the corroded specimen with absolute ethanol again;

[0042] (7) Drying: Dry the specimen using a hair dryer.

[0043] Place the sample corroded by the above method under a microscope for observation. The obtained metallographic structure is as Figure 1 shown. It can be seen that the grain boundaries of the two metals, copper and steel, are clear and complete, the corrosion effect is good, and it is easy to observe.

[0044] Shortening the corrosion time of this example to 40 s can also obtain a good corrosion effect, that is, within the range of 40 - 45 s of corrosion time, a good corrosion effect can be achieved.

[0045] Example 2

[0046] An etchant capable of simultaneously etching the grain boundaries of copper and steel, the formula of which includes the following components: 100 mL C2H5OH, 50 mL HCl, 8 g CuSO4, and mix them evenly to obtain a light green No. 2 etchant.

[0047] Among them, the mass fraction of hydrochloric acid is adjusted between 36% - 38%, and the weight of CuSO4 is adjusted between 8 - 9 g, and an etchant with similar performance can also be obtained.

[0048] A corrosion method for the grain boundaries of copper and steel, including the following steps:

[0049] (1) Inlaying a dissimilar metal joint of steel / copper foil: Cold-inlay the copper foil / steel sheet lap joint specimen in an inlaying machine using epoxy resin powder at room temperature for 15 min;

[0050] (2) Polishing: After the sample to be inlaid has cooled down, polish the inlaid sample with sandpaper from a lower grit to a higher grit; successively

[0051] Use sandpaper with grits of 240, 400, 600, 800, 1000, 1200, 1500, 2000, 3000, and 5000 to polish under running water;

[0052] (3) Polishing: Polish the polished sample; first use a nylon polishing cloth and a diamond polishing spray with a particle size of 0.5 μm for rough polishing. After the polishing marks are in the same direction, then use an imported velvet polishing cloth and a diamond polishing spray with a particle size of 0.25 μm for fine polishing until the surface shows a mirror-like luster and there are no polishing marks observed under an optical microscope;

[0053] (4) First cleaning: Clean the polished sample with absolute ethanol;

[0054] (5) Corrosion: Dry the cleaned sample, and then immerse it in the No. 2 etchant prepared in Example 2 for 45 s, and the corrosion temperature is room temperature;

[0055] (6) Second cleaning: Clean the corroded sample with absolute ethanol again;

[0056] (7) Drying: Use a hair dryer to dry the sample.

[0057] Place the sample corroded by the above method under a microscope for observation. The obtained metallographic structure is as Figure 2 shown. It can be seen that the grain boundaries of the two metals, copper and steel, are clear and complete, the corrosion effect is good, and it is easy to observe.

[0058] Shortening the corrosion time of this example to 40 s can also obtain a good corrosion effect, that is, within the range of 40 - 45 s for the corrosion time, good corrosion effects can be achieved.

[0059] Example 3

[0060] An etchant capable of simultaneously etching the grain boundaries of copper and steel, whose formula includes the following components: 100 mL of C2H5OH, 50 mL of HCl, and 10 g of CuSO4. Mix them evenly to obtain a light green No. 3 etchant.

[0061] Among them, the mass fraction of hydrochloric acid is adjusted between 36% - 38%, and the weight of CuSO4 is adjusted between 10 - 11 g, and etchants with similar performance can also be obtained.

[0062] An etching method for the grain boundaries of copper and steel, including the following steps:

[0063] (1) Inlayed steel / copper foil dissimilar metal joint. The copper foil / steel sheet lap joint specimen is cold inlaid with epoxy resin powder in an inlaying machine. The inlaying temperature is room temperature and the time is 15 min.

[0064] (2) Grinding. After the inlaid sample cools down, the inlaid specimen is ground with sandpaper from low grit to high grit; sequentially

[0065] Use sandpaper with grits of 240, 400, 600, 800, 1000, 1200, 1500, 2000, 3000, and 5000 to grind under running water.

[0066] (3) Polishing. The ground specimen is polished. First, use a nylon polishing cloth and diamond polishing spray with a particle size of 0.5 μm for rough polishing. After the polishing marks are in the same direction, then use an imported velvet polishing cloth and diamond polishing spray with a particle size of 0.25 μm for fine polishing until the surface shows a mirror-like luster and there are no polishing marks observed under an optical microscope.

[0067] (4) First cleaning. The polished specimen is cleaned with anhydrous ethanol.

[0068] (5) Corrosion. The cleaned specimen is dried and then immersed in the No. 3 corrosion agent prepared in Example 3 for 45 s. The corrosion temperature is room temperature.

[0069] (6) Second cleaning. The corroded specimen is cleaned again with anhydrous ethanol.

[0070] (7) Drying. Use a hair dryer to dry the specimen.

[0071] The sample corroded by the above method is placed under a microscope for observation. The obtained metallographic structure is as Figure 3 shown. It can be seen that the grain boundaries of the two metals, copper and steel, are clear and complete, the corrosion effect is good, and it is easy to observe.

[0072] Shortening the corrosion time of this example to 40 s can also obtain a good corrosion effect, that is, when the corrosion time is between 40 - 45 s, good corrosion effects can be obtained.

[0073] Comparative Example 1

[0074] A steel grain boundary corrosion agent, whose formula includes the following components: 100 mL C2H5OH, 4 mL HNO3, to obtain a colorless No. 4 corrosion agent.

[0075] Among them, the mass fraction of HNO3 is adjusted between 65 - 68%.

[0076] A corrosion method for corroding the steel grain boundary, including the following steps:

[0077] (1) Inlaying steel / copper foil dissimilar metal joints. The copper foil / steel sheet lap joint specimens are cold-inlaid with epoxy resin powder in an inlaying machine at room temperature for 15 minutes.

[0078] (2) Grinding. After the inlaid samples are cooled, the inlaid specimens are ground with sandpaper from low grit to high grit; successively

[0079] Use sandpapers with grits of 240, 400, 600, 800, 1000, 1200, 1500, 2000, 3000, and 5000 to grind under running water.

[0080] (3) Polishing. The ground specimens are polished. First, rough polishing is carried out using a nylon polishing cloth and a diamond polishing spray with a particle size of 0.5 μm. After the polishing marks are in the same direction, fine polishing is carried out using an imported velvet polishing cloth and a diamond polishing spray with a particle size of 0.25 μm until the surface shows a mirror-like luster and there are no polishing marks observed under an optical microscope.

[0081] (4) Primary cleaning. The polished specimens are cleaned with absolute ethanol.

[0082] (5) Corrosion. The cleaned specimens are dried, and then immersed in the No. 4 corrosion agent prepared in Comparative Example 1 for 10 s at room temperature.

[0083] (6) Secondary cleaning. The corroded specimens are cleaned again with absolute ethanol.

[0084] (7) Drying. Use a hair dryer to dry the specimens.

[0085] The samples corroded by the above method are placed under a microscope for observation. The obtained metallographic structure is as Figure 4 shown. It can be seen that the grain boundaries of steel are clear and complete, and the corrosion effect is good, but the grain boundaries on the copper side are not corroded out.

[0086] Shortening the corrosion time of this example to 5 s also gives the same corrosion effect, that is, within the range of 5 - 10 s of corrosion time, similar corrosion effects are obtained.

[0087] Comparative Example 2

[0088] A copper grain boundary corrosion agent, the formula of which includes the following components: 50 mL of H2O, 40 mL of HCl, and 10 g of FeCl3. After mixing, the brown No. 5 corrosion agent is obtained.

[0089] Among them, the mass fraction of hydrochloric acid is adjusted between 36% - 38%, and the purity of FeCl3 is 99.9%.

[0090] A corrosion method for corroding steel grain boundaries, including the following steps:

[0091] (1) Inlay steel / copper foil dissimilar metal joint. The copper foil / steel sheet lap joint specimen is cold inlaid in an inlaying machine using epoxy resin powder. The inlaying temperature is room temperature and the time is 15 min;

[0092] (2) Grinding. After the inlaid sample cools down, the inlaid specimen is ground with sandpaper from low grit to high grit; successively

[0093] Use sandpapers with grits of 240, 400, 600, 800, 1000, 1200, 1500, 2000, 3000, and 5000 to grind under running water;

[0094] (3) Polishing. The ground specimen is polished. First, rough polishing is carried out using a nylon polishing cloth and a diamond polishing spray with a particle size of 0.5 μm. After the polishing marks are in the same direction, fine polishing is carried out using an imported velvet polishing cloth and a diamond polishing spray with a particle size of 0.25 μm until the surface shows a mirror-like luster and there are no polishing marks observed under an optical microscope;

[0095] (4) First cleaning. The polished specimen is cleaned with absolute ethanol;

[0096] (5) Corrosion. The cleaned specimen is dried, and then it is immersed in the No. 5 corrosion agent prepared in Comparative Example 2 for 30 s. The corrosion temperature is room temperature;

[0097] (6) Second cleaning. The corroded specimen is cleaned again with absolute ethanol;

[0098] (7) Drying. Use a hair dryer to dry the specimen.

[0099] The sample obtained by corrosion using the above method is placed under a microscope for observation. The obtained metallographic structure is as Figure 5 shown. It can be seen that the grain boundaries of copper are clear and complete, and the corrosion effect is good, but the grain boundaries on the steel side are not corroded out.

[0100] Shortening the corrosion time of this example to 20 s can also obtain a similar corrosion effect, that is, within the range of 20 - 30 s of corrosion time, similar corrosion effects can be obtained.

[0101] The corrosion agent of the present invention can complete duplex grain boundary corrosion at room temperature, and simultaneously show the grain morphologies of the weld zone, heat affected zone, and base metal within 20 - 40 seconds, solving the problem of tissue contrast distortion caused by traditional step-by-step corrosion, and providing a key analysis means for studying the metallurgical reaction, grain growth orientation, and element diffusion behavior at the dissimilar metal welding interface. It has the characteristics of convenient operation, low cost, and good reproducibility; it is especially suitable for the microstructure characterization of steel / copper foil lap joints formed by laser welding.

[0102] Although embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to the embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A microscopic structure etchant for a lap joint of dissimilar metals of steel / copper foil by laser welding, characterized in that Its formula includes the following components: ethanol, hydrochloric acid, and copper sulfate powder. For every 100 - 200 mL of ethanol, it corresponds to 50 - 100 mL of hydrochloric acid and 5 - 10 g of copper sulfate powder.

2. The microstructure etchant for the lap joint of steel / copper foil dissimilar metals according to claim 1, characterized in that, The mass fraction concentration of the hydrochloric acid is 36% - 38%.

3. A microstructure etchant for a steel / copper foil dissimilar metal laser welding lap joint according to claim 1, characterized in that, The purity of the copper sulfate powder is 99.9%.

4. A method for intergranular corrosion of a steel / copper foil lap joint using the etchant according to any one of claims 1-3, characterized in that, It includes the following steps: (1) Inlaying: The steel / copper dissimilar metal laser welding lap joint specimen is inlaid. (2) Grinding: After the inlaid sample cools down, it is ground successively with sandpapers from low grit to high grit. (3) Polishing: The ground specimen is polished to ensure a smooth and flawless surface. (4) First cleaning: The specimen is cleaned with anhydrous ethanol. (5) Corrosion: The cleaned specimen is immersed in the above corrosion agent and corroded at room temperature for 40 - 50 seconds. (6) Second cleaning: After the corrosion is completed, it is cleaned again with anhydrous ethanol. (7) Drying: The specimen is dried with a hair dryer.

5. The method according to claim 4, characterized in that, In the step (1), cold inlaying is carried out using epoxy resin powder, and the inlaying is at room temperature for 15 minutes.

6. The method according to claim 4, characterized in that, In the step (2), sandpapers with 240 mesh, 400 mesh, 600 mesh, 800 mesh, 1000 mesh, 1200 mesh, 1500 mesh, 2000 mesh, 3000 mesh, and 5000 mesh are used to grind under running water successively.

7. The method according to claim 4, characterized in that, In the step (3), first, rough polishing is carried out using a nylon polishing cloth and diamond polishing spray with a particle size of 0.5 μm. After the grinding marks are polished off and the polishing marks are in the same direction, fine polishing is carried out using an imported velvet polishing cloth and diamond polishing spray with a particle size of 0.25 μm until the surface shows a mirror-like luster and there are no polishing marks observed under an optical microscope.

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