A method for preparing a TEM sample of an A356 series aluminum alloy

Abstract

The application belongs to the technical field of TEM sample preparation, and particularly relates to a TEM sample preparation method of A356 series aluminum alloy, which comprises the following steps: firstly, pretreating and punching A356 series aluminum alloy sheet, then performing pit treatment on the punched and pretreated aluminum alloy, then performing electrolytic double spraying on the pit treated aluminum alloy in a low temperature environment, finally performing ion thinning and finishing on the electrolytic polished sample, and finally obtaining a TEM sample. The application can improve the accuracy and success rate of TEM sample preparation of A356 series aluminum alloy with uneven local microstructure composition, the accuracy is reflected in improving polishing effect, reducing dislocation introduction, improving the adverse effects of eutectic Si and precipitated second phase, and simultaneously improving the success rate of TEM sample preparation from 50% of the traditional method to 95% and above.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of TEM sample preparation, and particularly relates to a TEM sample preparation method of A356 aluminum alloy. BACKGROUND

[0002] A356 aluminum alloy belongs to Al-Si-Mg aluminum alloy, is a traditional casting aluminum alloy, and has good casting fluidity, high strength, good corrosion resistance, good machining performance, small shrinkage rate and hot crack tendency, and is widely applied to various fields to become a hypoeutectic material widely used in automobile and motorcycle hubs. Material characterization means become an indispensable step for material research. The microstructure characterization means of A356 aluminum alloy mainly include optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Since the magnification of A356 under the optical microscope is limited, only the basic organization and distribution can be observed, and some nanoscale precipitates cannot be observed under the scanning electron microscope, so TEM becomes an indispensable means for further characterization and analysis of microstructure. The current TEM sample preparation methods mainly include electrolytic double spraying, ion thinning and electrolytic double spraying followed by ion thinning. The electrolytic double spraying has high efficiency and short time, ion thinning is to thin the sample by argon ion bombardment, and ion thinning takes a long time, and the argon ion bombardment of the sample during ion thinning causes stress in the sample, thereby affecting the preparation of the sample.

[0003] The microstructure of A356 mainly consists of alpha-Al, eutectic silicon and second phase, and the local microstructure itself is not uniform, and A356 is further caused to have uneven microstructure distribution under some treatment means and process treatment, such as adding refining agents AlTiB, AlTiC, AlTiCB and other series of refining agents and heat treatment after adding the refining agents, so as to cause the second phase to precipitate at the grain boundary and the matrix, thereby further changing the uniformity of the local microstructure. The electrolytic polishing means is used, and due to the unevenness of the matrix structure, eutectic silicon and precipitated phase in the local microstructure, a potential difference is formed in the electrolytic polishing process, the second phase and eutectic silicon are preferentially corroded than the matrix structure, a sieve hole is formed, and the polishing effect is not uniform and ideal. At the same time, the ion thinning method is used alone, which takes a long time and causes high cost, and the ion thinning causes the thin area to easily produce amorphous regions, introduces dislocations and stress, and thus affects the experimental results; therefore, the electrolytic polishing and ion thinning methods are combined, and the concave means is used to assist, so as to reduce the sample preparation time, and finally the TEM sample is successfully obtained. SUMMARY

[0004] The electrolytic double spraying method is used for preferentially etching the second phase and eutectic silicon in the Al-Si-Mg aluminum alloy, and the single ion thinning method is time-consuming, high in cost and has an influence on the experimental results, and the problems of the TEM sample preparation of the Al-Si-Mg aluminum alloy with uneven local microstructure and composition are solved, and the TEM sample preparation method suitable for the A356 aluminum alloy, especially the A356 aluminum alloy with uneven local microstructure and composition is provided. In addition, the preparation method has a short time, compared with the ion thinning for 4-5 hours, the time is at least reduced by more than half, the efficiency is greatly improved, and the success rate of the TEM sample preparation is also greatly improved compared with the traditional electrolyte.

[0005] The technical scheme provided by the application is as follows:

[0006] The TEM sample preparation method of the A356 aluminum alloy comprises the following steps:

[0007] 1) The A356 aluminum alloy is linearly cut to obtain an aluminum alloy sheet;

[0008] 2) The aluminum alloy sheet is pretreated and then punched;

[0009] 3) The punched and pretreated aluminum alloy is processed in a pit;

[0010] 4) The pit-processed aluminum alloy is punched in a low-temperature environment by using the electrolytic double spraying method;

[0011] 5) The sample after the electrolytic double spraying is refined by ion thinning;

[0012] 6) The refined and thinned sample is placed in a sample box and vacuumized for storage.

[0013] The technical scheme has the following advantages:

[0014] The electrolytic double spraying in the low-temperature environment can prevent the sample from being oxidized, improve the electrolytic polishing effect, expand the range of the thin area, and reduce the generation of stress and the introduction of dislocation in the process of double spraying thinning, and reduce the adverse effects of eutectic silicon and the second phase;

[0015] The specific electrolytic polishing solution can improve the adverse effects of the large potential difference between the eutectic silicon and the second phase and the matrix, increase the success rate of the TEM sample preparation, and reach 95% or more;

[0016] The combination of the electrolytic double spraying of the improved double spraying electrolyte under the low-temperature condition and the ion thinning can shorten the sample preparation time, and the refined sample after the electrolytic double spraying can maximize the accuracy and success rate of the TEM sample preparation.

[0017] Specifically, the A356 aluminum alloy is a locally microstructure component non-uniform A356 aluminum alloy, and the local microstructure is mainly composed of alpha-Al (50-200 pm), eutectic silicon (5-10 pm), smaller second phase precipitation (0.1-10 pm), and larger second phase precipitation (50-70 pm).

[0018] The A356 aluminum alloy belongs to a cast aluminum alloy. Due to the difference in solubility and binding capacity between elements and the matrix Al element during casting, different phases are formed; mainly the matrix alpha-Al, eutectic silicon, and some precipitated second phases. Due to the non-uniformity of the local microstructure composition, the TEM sample prepared by the traditional electrolytic double jet exists the phenomenon that the eutectic silicon and the second phase preferentially corrode the matrix structure, and the thin area cannot be observed or can be observed better. The ion thinning time is long and affects the eutectic silicon area; therefore, the improved electrolytic double jet is combined with ion thinning under low temperature conditions to improve the accuracy and success rate of the prepared TEM sample.

[0019] Specifically, in step 1), the aluminum alloy is cut by an electric spark wire cutting machine to obtain the aluminum alloy sheet, wherein the length of the aluminum alloy sheet is controlled to be 20-30 mm, the width is controlled to be 10-20 mm, and the thickness is controlled to be 4-8 mm.

[0020] Specifically, in step 2), the surface of the aluminum alloy is ground with 800-2000 mesh sandpaper, and uniformly ground for 60-80 pm.

[0021] Specifically, in step 2), during the grinding process, the sample is rotated by 90 degrees every 20-30 seconds to ensure the uniformity of the sample structure.

[0022] Specifically, in step 2), the pretreated sample is punched to obtain an aluminum alloy TEM circular sheet sample.

[0023] Specifically, in the pit treatment of step 3), the pit thinning radius is 70-80%, and the pit reduces the sample thickness by 20-30%.

[0024] Specifically, in the electrolytic double jet hole of step 4), the low temperature environment is controlled to be-30 to-20℃.

[0025] In the above technical solution, the process is carried out in a low temperature environment. Because the polishing speed of the second phase can be reduced under low temperature conditions compared to normal temperature conditions, the polishing effect is improved, and the sample is prevented from being oxidized, so the thin area range can be increased.

[0026] Specifically, in the electrolytic double spray hole of step 4), the electrolytic polishing solution is composed of methanol, hydrofluoric acid and nitric acid, and the volume ratio of each raw material is as follows: the volume ratio of methanol is 70-75%, the volume ratio of hydrofluoric acid is 3-7%, and the volume ratio of nitric acid is 23-25%, and the total volume ratio of each raw material is 100%.

[0027] Specifically, in the electrolytic double spray hole of step 4), the electrolytic double spray polishing current range is 150-200 mA, the electrolytic double spray polishing voltage is 16-20 V, and the time length is 30-60 s.

[0028] Specifically, in step 4), the sample preparation of the electrolytic double spray hole is to place the sample in the sample clamp, start the equipment, control the electrolyte flow rate until the electrolyte on both sides can be opposite, the electrolyte flow rate is about 28-32, the transmittance is about 10-15%, and the scan function is turned on to obtain the appropriate polishing voltage of the sample;

[0029] Specifically, in step 4), after the polishing of the electrolytic double spray hole is finished, the sample is first placed in an ethanol solution and shaken for cleaning, and then placed in distilled water for cleaning, and the process is repeated 3-5 times.

[0030] Specifically, in step 4), the sample cleaned after electrolytic polishing is blown dry, and the blowing dry temperature is in the range of 50-80°C.

[0031] Specifically, in step 5), the ion thinning feature is that ion thinning is performed for small angle finishing, and the finishing stage voltage is 2-4 KV and the time is 10-30 min. In the present application, the finishing voltage is controlled to be small after the hole is out, so the finishing stage voltage is adjusted to 2-4 KV and the time is 10-30 min.

[0032] Specifically, in step 6), the TEM sample is placed in a TEM sample box and vacuumized for vacuum preservation until the vacuum degree is less than 10 -3 MPa.

[0033] Specifically, electrolytic double spraying is performed under low temperature conditions, and the steps are as follows:

[0034] 1) Configure electrolytic double spraying liquid, which is composed of nitric acid, methanol and hydrofluoric acid. Configure according to a certain proportion, the content of methanol is 70-75%, the content of hydrofluoric acid is 3-7%, and the content of nitric acid is 23-25%. Low proportion and low acidity can avoid oxidation of the sample after electrolytic double spraying;

[0035] 2) Pour the prepared electrolyte into the electrolysis tank, pour appropriate amount of liquid nitrogen into the electrolysis tank, cover the electrolysis tank cover, wait until the temperature of the thermometer above the electrolysis tank is reduced to -30 to -20℃, and the low temperature environment can reduce the potential difference and increase the thin area;

[0036] 3) Place the sample in the sample holder and control the flow rate of the electrolyte before electrolysis. The electrolyte flow rate should be 28-32 until the electrolyte achieves the flushing effect. Set the transmittance to 10-15%.

[0037] 4) The electropolishing voltage is obtained by scanning the electropolishing instrument. Due to individual differences in samples, the scanning function is used to control the electropolishing voltage more precisely, keeping the voltage between 16 and 20V and the current between 150 and 200mA.

[0038] 5) After electropolishing, quickly immerse the sample in an ethanol solution for cleaning, and then rinse it in distilled water. The concentration of the ethanol solution is 75%. Repeat this process 3 to 5 times to remove the residual polishing solution and at the same time remove the stress distribution on the sample surface.

[0039] The method for preparing Al-Si-Mg alloy TEM samples provided by this invention is characterized by ion thinning.

[0040] The beneficial effects of this invention are as follows:

[0041] 1) Due to the relatively soft texture of aluminum alloy, the pretreatment of the sample involves polishing the surface of the aluminum alloy with 800-2000 grit sandpaper. The finer sandpaper prevents deep scratches and deformation of the aluminum alloy. Polishing is carried out to a thickness of 60-80 μm. At the same time, it is necessary to ensure that the sample has a large thickness inhomogeneity during the polishing process. During the polishing process, the sample is rotated 90 degrees every 20-30 seconds. A micrometer is used to measure the polished aluminum alloy in all directions to ensure that the error in each direction is less than 5 mm.

[0042] 2) Measuring the aluminum alloy in each direction is to prevent uneven grinding, which would lead to uneven punching and sampling in the later stage, thus affecting subsequent experiments. The ground aluminum alloy is then punched to obtain a more uniform circular aluminum alloy sheet.

[0043] 3) The voltage range for electropolishing was determined using the scan function of the electrolytic dual-jet instrument, which in turn determined the current range. Due to regional differences in the same sample, the voltage of the A356 electrolytic dual-jet will vary slightly, allowing for more precise voltage determination. Simultaneously, performing electrolytic dual-jet under low-temperature conditions prevents sample oxidation, improves polishing effect, and expands the range of thin areas. Furthermore, no stress is generated during the dual-jet thinning process, and the introduction of dislocations is reduced, thereby increasing the accuracy of TEM sample preparation.

[0044] 4) Compared with the traditional A356 electrolytic double spray electrolyte composed of methanol and nitric acid, the hydrogen fluoride is added in the electrolytic double spray electrolyte. Since the eutectic silicon and the precipitated second phase have a large potential difference compared with the matrix, a preferential corrosion state will be generated, thereby causing the appearance of the sieve hole. Therefore, in the case of electrolytic double spray, a small amount of hydrogen fluoride is added, which can improve the adverse effects of the eutectic silicon and the precipitated second phase;

[0045] 5) The electrolytic double spray sample is placed on the sample holder, and the flow rate of the electrolyte is controlled before the electrolytic double spray until the electrolyte reaches the hedging effect. The flow rate of the electrolyte is 28-32; the light transmittance is set to 10-15%, and the electrolytic double spray ends when the alarm sounds;

[0046] 6) The electrolytic double spray time is 30-60s, and after the double spray is finished, the sample is placed in a 75% ethanol solution for oscillation cleaning, and then placed in distilled water for oscillation cleaning. This process is repeated 3-5 times; the residual double spray liquid on the surface of the sample is removed, and the residual stress on the surface of the sample is eliminated;

[0047] 7) The A356 after the electrolytic double spray hole cleaning is subjected to ion thinning treatment, and the ion thinning is subjected to small angle finishing. The finishing stage voltage is 2-4KV, and the time is 10-30min. The finishing obtains the final TEM sample;

[0048] 8) Finally, the TEM sample after finishing is vacuumed and stored to prevent further oxidation. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 TEM sample after electrolytic double spray of methanol and nitric acid electrolyte of Example 1.

[0050] Figure 2 TEM sample after low-temperature electrolytic double spray of methanol, nitric acid and hydrogen fluoride electrolyte of Example 2.

[0051] Figure 3 TEM sample diffraction pattern after low-temperature electrolytic double spray of methanol, nitric acid and hydrogen fluoride electrolyte and ion thinning combination of Example 3.

[0052] Figure 4 High-resolution image of TEM sample after low-temperature electrolytic double spray of methanol, nitric acid and hydrogen fluoride electrolyte and ion thinning combination of Example 3. DETAILED DESCRIPTION

[0053] The principles and characteristics of the present application are described below, and the examples are used to explain the present application, but not to limit the scope of the present application.

[0054] The sample processed in each embodiment is an Al-Si-Mg series A356 aluminum alloy with refiner added and T6 heat treated, from prior art, A356 series products available from Tsinghua Deka Co., Ltd.

[0055] Example 1

[0056] The sample is A356 (produced by Tsinghua Deka Co., Ltd.) with 0.02wt% refiner Al5TiB added and T6 heat treated. The main steps for preparing the TEM sample are as follows:

[0057] Step 1: Use an electric spark wire cutting machine to cut aluminum alloy sheets, with the length controlled at about 20 mm, the width controlled at about 15 mm, and the thickness controlled at about 3 mm;

[0058] Step 2: Polish the surface of the aluminum alloy with about 1500 mesh sandpaper to about 60 μm, rotate 90 degrees every about 25 s during polishing, and measure the polished aluminum alloy in each direction with a screw micrometer to obtain an error of less than 2 mm in each direction. Punch the polished aluminum alloy to obtain relatively uniform circular aluminum alloy sheets;

[0059] Step 3: Perform pit processing on the aluminum alloy sheets, with the pits of the punched aluminum alloy thinned by about 30%, and the thinned diameter range accounting for about 80% of the circular hole sheet;

[0060] Step 4: Perform electrolytic double spraying on the punched circular sheets, with the steps as follows:

[0061] Prepare the electrolytic double spraying solution, which is composed of nitric acid and methanol. Prepare the solution according to a volume ratio of nitric acid to methanol of 3:1, and the total volume of the two electrolytic polishing solutions is 100%;

[0062] (1) Pour the prepared electrolyte into the electrolysis tank, and cover the tank cover;

[0063] (2) Control the flow rate of the electrolyte to be 30 before electrolytic double spraying until the electrolyte reaches the counter-punching effect, and set the transmittance to be 12%;

[0064] (3) Obtain the voltage of the electrolytic polishing through the scan function of the electrolytic double spraying instrument. Due to the individual differences of the samples, the scan function is used to more accurately control the voltage of the electrolytic double spraying, with the voltage controlled at about 16 V and the current controlled at about 150 mA;

[0065] (4) After the electrolytic polishing is completed, quickly place the sample in an ethanol solution for cleaning, and then place it in distilled water for cleaning. The concentration of the ethanol solution is 75%, and this process is repeated 5 times to remove the residual polishing solution and remove the stress distribution on the surface of the sample.

[0066] Example 2:

[0067] The sample is A356 (produced by CITIC Deka Co., Ltd.) after adding 0.02wt% refiner Al5TiB and T6 heat treatment. The main steps for preparing the TEM sample are as follows:

[0068] Step 1: Use an electric spark wire cutting machine to cut aluminum alloy sheets, with a length of about 25 mm, a width of about 15 mm, and a thickness of about 3 mm;

[0069] Step 2: Grind the surface of the aluminum alloy with 1500 mesh sandpaper to about 70 μm, rotate 90 degrees every 20 seconds during grinding, and measure the error in each direction with a screw micrometer. The error in each direction is less than 1.5 mm. Then punch the polished aluminum alloy to obtain uniform circular aluminum alloy sheets;

[0070] Step 3: Perform pit processing on the aluminum alloy sheet, and reduce the thickness of the punched aluminum alloy to about 35%, with a reduced diameter range of about 75% of the circular hole sheet;

[0071] Step 4: Perform electrolytic double spraying on the punched circular sheet, with the following steps:

[0072] (1) Configure the electrolytic double spraying solution, which is composed of nitric acid, methanol and hydrofluoric acid. Configure according to a certain proportion, with methanol content of about 75%, hydrofluoric acid content of about 3%, and nitric acid content of about 22%;

[0073] (2) Pour the prepared electrolyte into the electrolysis tank, cover the tank cover, pour liquid nitrogen into the electrolysis tank, and wait until the temperature of the thermometer above the electrolysis tank is reduced to about -25℃;

[0074] (3) Control the flow rate of the electrolyte to about 32 before electrolytic double spraying until the electrolyte reaches the counterflow effect, and set the transmittance to 10%;

[0075] (4) The voltage of electrolytic polishing is obtained by the scan function of the electrolytic double spraying instrument. Due to the individual differences of the sample, the scan function is used to more accurately control the voltage of the electrolytic double spraying, and the voltage of the electrolytic double spraying is controlled at 18V and the current is controlled at 160mA;

[0076] (5) After electrolytic polishing, quickly put the sample into ethanol solution for cleaning, and then put it into distilled water for cleaning. The concentration of the ethanol solution is 75%, and this process is repeated 5 times to remove the remaining polishing solution and remove the stress distribution on the surface of the sample.

[0077] Example 3:

[0078] The sample is A356 (produced by CITIC Deka Co., Ltd.) after adding 0.02wt% refiner Al5TiB and T6 heat treatment, and the main steps for preparing the TEM sample are as follows:

[0079] Step 1: use an electric spark wire cutting machine to cut aluminum alloy sheets with a length of about 30 mm, a width of about 25 mm, and a thickness of about 3 mm;

[0080] Step 2: polish the surface of the aluminum alloy with about 1500 mesh sandpaper to about 60 μm, rotate 90 degrees every 25 seconds during polishing, and measure the polished aluminum alloy in each direction with a screw micrometer to ensure that the error in each direction is less than 2 mm; then punch the polished aluminum alloy to obtain uniform circular aluminum alloy sheets;

[0081] Step 3: perform pit processing on the aluminum alloy sheets, thin the thickness of the punched aluminum alloy pits by 30%, and the thinned diameter range accounts for 80% of the circular hole sheet;

[0082] Step 4: perform electrolytic double spraying on the punched circular sheets, and the steps are as follows:

[0083] (1) configure electrolytic double spraying liquid, which is composed of nitric acid, methanol and hydrofluoric acid. Configure according to a certain proportion, the methanol content is about 72%, the hydrofluoric acid content is about 3%, the nitric acid content is about 25%, and the sum of the volumes of various electrolytic polishing liquids is 100%;

[0084] (2) pour the prepared electrolyte into the electrolysis tank, cover the tank cover, pour liquid nitrogen into the electrolysis tank, and wait until the temperature of the thermometer above the electrolysis tank decreases to about -26℃;

[0085] (3) control the flow rate of the electrolyte before electrolytic double spraying to be 30 until the electrolyte reaches the counter-punching effect, and set the transmittance to be 12%;

[0086] (4) obtain the voltage of electrolytic polishing through the scan function of the electrolytic double spraying instrument. Due to the individual differences of the sample, the scan function is used to more accurately control the voltage of the electrolytic double spraying, and the voltage of the electrolytic double spraying is controlled at about 16V and the current is controlled at about 150mA;

[0087] (5) after the electrolytic polishing is completed, quickly put the sample into an ethanol solution for cleaning, and then put it into distilled water for cleaning. The concentration of the ethanol solution is 75%, and this process is repeated 5 times to remove the remaining polishing liquid and remove the stress distribution on the surface of the sample;

[0088] Step 5: Thin the sample ions after double spraying. The key feature is that the ion thinning is refined. The refinement voltage is controlled at a relatively low level. Therefore, the voltage of the refinement stage is adjusted to about 2KV and the time is about 10min.

[0089] Step 6: Place the refined sample into the transmission chamber and evacuate until the vacuum level is less than 10. -3 Save the MPa value.

[0090] like Figure 1 As shown, Example 1 yielded the macroscopic morphology of the TEM sample after conventional electrolytic double spraying, compared with... Figure 2 and Figure 3 It can be seen that the traditional electrolytic dual-spray solution has the worst effect.

[0091] like Figure 2 The image shown is a TEM sample obtained in Example 2. It can be seen that the improved electrolytic dual-spray solution reduces the adverse effects of eutectic silicon and the precipitation of the second phase under low temperature conditions, and increases the position of the thin region.

[0092] like Figure 3 The image shown is a TEM image of the sample obtained in Example 3. It can be seen that an improved electrolytic dual-spray solution combined with ion thinning method was used, minimizing the adverse effects of eutectic silicon and the precipitation of the second phase, resulting in a smoother surface. Compared to the simply improved electrolytic dual-spray solution used at low temperatures, the prepared sample has a larger thin area, a higher success rate, and a smoother microstructure.

[0093] like Figure 4 The image shown is a high-resolution TEM image of the sample obtained in Example 3. It can be seen that, due to the combination of electrolytic double spraying and ion thinning, an ideal high-resolution image of the aluminum alloy can be obtained well and the crystal lattice can be clearly obtained.

[0094] 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, improvements, etc., 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 preparing a TEM sample of an A356 series aluminum alloy, characterized by, It comprises the following steps: 1) linear cutting of A356 aluminum alloy to obtain aluminum alloy sheet; 2) pretreatment of the aluminum alloy sheet, followed by punching treatment; 3) pit treatment of the punched and pretreated aluminum alloy; 4) hole etching of the pit-treated aluminum alloy in a low-temperature environment using electrolytic double spraying method; 5) ion thinning and finishing of the sample after electrolytic double spraying hole etching; 6) vacuum storage of the sample after finishing and thinning in a sample box; In step 2): Grind the surface of the aluminum alloy with 800-2000 mesh sandpaper, evenly grind to a thickness of 60-80 μm; Rotate 90 degrees every 20-30 seconds during grinding to ensure uniformity of sample thickness; In step 4) of electrolytic double spraying hole etching: The low-temperature environment is controlled at a temperature range of -30 to -20℃; The electrolytic polishing solution is composed of methanol, hydrofluoric acid and nitric acid, and the volume ratio of each raw material is as follows: methanol accounts for 70-75%, hydrofluoric acid accounts for 3-7%, and nitric acid accounts for 23-25%, and the total ratio of each raw material is 100%; The polishing current range of electrolytic double spraying is 150-200 mA, the polishing voltage of electrolytic double spraying is 16-20 V, and the time is 30-60 s.

2. The method of preparing a TEM sample of an A356 series aluminum alloy according to claim 1, wherein In step 1), the aluminum alloy is cut by an electric spark wire cutting machine to obtain the aluminum alloy sheet, wherein the length of the aluminum alloy sheet is controlled at 20-30 mm, the width is controlled at 10-20 mm, and the thickness is controlled at 4-8 mm.

3. The method of preparing a TEM sample of an A356 series aluminum alloy according to claim 1, wherein In step 3) of pit treatment:

4. The method of preparing a TEM sample of an A356 series aluminum alloy according to claim 1, wherein The pit thinning radius is 70-80%; the pit reduces the sample thickness by 20-30%. In step 4): Sample preparation for electrolytic double spraying hole etching is to place the sample in the sample holder, start the equipment, control the electrolyte flow rate until the electrolyte on both sides can be opposite, the electrolyte flow rate is 28-32, the transmittance is set to 10-15%, and the scan function is turned on to obtain the appropriate polishing voltage of the sample; After polishing of the electrolytic double spraying hole etching, the sample is first placed in an ethanol solution and shaken for cleaning, then placed in distilled water for cleaning, and this process is repeated 3-5 times; 5. The method of preparing a TEM sample of an A356 series aluminum alloy according to claim 1, wherein The sample cleaned after electrolytic polishing is blown dry, and the blowing temperature is in the range of 50-80℃.

6. The method of preparing a TEM sample of an A356 series aluminum alloy according to claim 1, wherein In Step 6), the TEM sample is placed in a TEM sample holder and vacuumed for vacuum preservation until the vacuum degree is less than 10 -3 MPa.

7. The method of preparing a TEM sample of an A356 series aluminum alloy according to claim 1, characterized by: In step 5), the ion thinning feature is that the ion thinning is performed at a small angle, and the finishing stage voltage is 2-4 KV, and the time is 10-30 min. The A356 aluminum alloy is a locally microstructurally inhomogeneous A356 aluminum alloy.

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