Surface treatment method for automobile part die-casting aluminum alloy

Abstract

The invention provides a surface treatment method for an automobile part die-casting aluminum alloy, and belongs to the technical field of metal surface treatment.The surface treatment method comprises the following steps that S1, a die-casting aluminum alloy substrate is pretreated; s2, preparing an acryloyl chloride modified cellulose nanocrystal through the reaction of acryloyl chloride and the cellulose nanocrystal; s3, the acryloyl chloride modified cellulose nanocrystals are dispersed in cationic polyacrylate, dopamine hydrochloride and water are added, and electrophoretic deposition suspension liquid is formed; s4, a pretreated die-casting aluminum alloy substrate is placed in the electrophoretic deposition suspension liquid to be subjected to electrophoretic deposition, and a composite coating is formed; and S5, the composite coating is subjected to curing treatment. According to the preparation method, the acryloyl chloride modified cellulose nanocrystals are introduced into a cathode electrophoretic deposition system, and the stable nano composite coating is constructed by utilizing uniform dispersion and interface chemical bonding of the acryloyl chloride modified cellulose nanocrystals in cationic polyacrylate.

Description

Technical Field

[0001] This invention belongs to the field of metal surface treatment technology, and specifically relates to a surface treatment method for die-cast aluminum alloys for automotive parts. Background Technology

[0002] Die-cast aluminum alloys are widely used in the automotive industry for manufacturing complex parts such as engine housings and structural supports due to their excellent casting fluidity, high specific strength, and good lightweight properties. However, die-cast aluminum alloys have problems such as porosity and compositional segregation on their surface, and the natural oxide film provides limited protection, making them susceptible to pitting and intergranular corrosion in corrosive environments such as humidity and salt spray, which seriously affects the service life and safety reliability of the parts.

[0003] To improve corrosion resistance and surface mechanical properties, a protective coating is typically applied to the surface of die-cast aluminum alloys. Cathodic electrophoretic coating technology, due to its excellent coverage, high utilization rate, and low volatile organic compound emissions, has become one of the mainstream coating processes for automotive metal parts. This technology involves applying a DC electric field to an aqueous dispersion system, causing positively charged polymer colloidal particles to deposit on the workpiece surface, which acts as the cathode, forming a uniform and dense coating.

[0004] However, existing single polymer electrophoretic coatings have several inherent defects: First, their hardness, wear resistance and scratch resistance are limited, making it difficult to meet the high requirements of automotive parts for surface scratch resistance; second, although the adhesion between the coating and the metal substrate can meet general standards, the adhesion may still decrease or even peel off under harsh conditions such as long-term thermal cycling, vibration or corrosive media penetration. Summary of the Invention

[0005] In view of the above situation and to overcome the defects of the prior art, the purpose of the present invention is to provide a surface treatment method for die-cast aluminum alloys for automotive parts, so as to at least partially solve the problems mentioned in the background art.

[0006] The technical solution adopted in this invention is as follows: This invention proposes a surface treatment method for die-cast aluminum alloys used in automotive parts, comprising the following steps: S1. Pre-treat the die-cast aluminum alloy substrate; S2. Acryloyl chloride-modified cellulose nanocrystals were prepared by reacting acryloyl chloride with cellulose nanocrystals; S3. The acryloyl chloride modified cellulose nanocrystals are dispersed in cationic polyacrylate, and dopamine hydrochloride and water are added to form an electrophoretic deposition suspension. S4. The pretreated die-cast aluminum alloy substrate is placed in the electrophoretic deposition suspension and electrophoretic deposition is performed to form a composite coating. S5. The composite coating is cured.

[0007] In some embodiments of the present invention, step S1 includes the following preprocessing: After alkaline cleaning of the die-cast aluminum alloy substrate, a zinc phosphate treatment agent is used for surface conversion treatment by spraying or immersion for 1-3 minutes at a temperature of 15-30℃.

[0008] In some embodiments of the present invention, step S2, the preparation method of the acryloyl chloride modified cellulose nanocrystals specifically includes the following steps: Cellulose nanocrystals and acryloyl chloride were esterified in an organic solvent in the presence of a catalyst at a temperature of 40-60℃ for 10-14 hours. After the reaction, unreacted substances were removed by centrifugation and washing, and then freeze-dried to obtain acryloyl chloride-modified cellulose nanocrystals.

[0009] In some embodiments of the present invention, the catalyst is 1-methylimidazole, the organic solvent is 1,4-dioxane, and the amount of the 1-methylimidazole catalyst is 10%-20% of the mass of the cellulose nanocrystals.

[0010] In some embodiments of the present invention, the mass ratio of acryloyl chloride to cellulose nanocrystals is (2-3):1.

[0011] In some embodiments of the present invention, in step S3, the electrophoretic deposition suspension comprises the following components, the amounts of which are based on the total mass of the suspension: Acryloyl chloride modified cellulose nanocrystals: 0.05wt% to 0.5wt%; Dopamine hydrochloride: 0.05wt% to 0.15wt%; Cationic polyacrylate: 5wt% to 15wt%; The remainder is water.

[0012] In some embodiments of the present invention, step S3 further includes: ultrasonic treatment of the electrophoretic deposition suspension for 20-30 minutes and a power of 200-300W.

[0013] In some embodiments of the present invention, in step S4, the electrophoretic deposition is performed under a DC electric field, wherein: The electrophoresis voltage is 20-40V, and the deposition time is 30-90 seconds.

[0014] In some embodiments of the present invention, step S5, the curing process includes: First, air dry at room temperature for 20-40 minutes; Then cure in a vacuum environment at 150-170℃ for 30-50 minutes.

[0015] In some embodiments of the present invention, after the electrophoretic deposition described in step S4 and before the curing process described in step S5, the method further includes rinsing and pre-drying the substrate on which the composite coating is deposited. The rinsing process involves immersing the rinsing 3-5 times in deionized water. The pre-drying process involves letting the food stand in a circulating air environment at 30-50℃ for 10-20 minutes.

[0016] The beneficial effects achieved by this invention are as follows: This invention introduces acryloyl chloride-modified cellulose nanocrystals into a cathodic electrophoretic deposition system, utilizing their uniform dispersion and interfacial chemical bonding in cationic polyacrylate to construct a stable nanocomposite coating. The modified cellulose nanocrystals not only significantly enhance the coating's cohesive strength and interfacial bonding through covalent cross-linking, but also form a dense nano-reinforcing network. This allows the coating to maintain good adhesion while significantly improving its hardness and corrosion resistance, providing a surface solution for automotive die-cast aluminum alloys that combines high strength and long-term protection. Detailed Implementation

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to this invention. The preferred embodiments and materials described herein are for illustrative purposes only and do not limit the scope of this application.

[0019] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0020] To address the problems raised in the background art, this invention provides a surface treatment method for die-cast aluminum alloys used in automotive parts, comprising the following steps: S1. Pre-treat the die-cast aluminum alloy substrate; S2. Acryloyl chloride-modified cellulose nanocrystals were prepared by reacting acryloyl chloride with cellulose nanocrystals; S3. Acryloyl chloride-modified cellulose nanocrystals are dispersed in cationic polyacrylate, and dopamine hydrochloride and water are added to form an electrophoretic deposition suspension. S4. Place the pretreated die-cast aluminum alloy substrate in an electrophoretic deposition suspension and perform electrophoretic deposition to form a composite coating. S5. Curing treatment is performed on the composite coating.

[0021] This invention provides a surface treatment method for die-cast aluminum alloys for automotive parts. This method introduces cellulose nanocrystals modified with acryloyl chloride as a nano-reinforcing phase into a cathodic electrophoretic deposition system, thereby constructing a synergistic composite system with a cationic polyacrylate matrix, which significantly improves the overall performance of the coating.

[0022] First, the acryloyl chloride modification process employed in this invention introduces acryloyl groups onto the surface of cellulose nanocrystals through surface esterification. This modification process offers dual advantages: on the one hand, it significantly improves the dispersibility and compatibility of cellulose nanocrystals in the hydrophobic matrix of cationic polyacrylate, effectively preventing nanoparticle aggregation; on the other hand, the introduced acryloyl groups contain unsaturated double bonds, which can participate in the crosslinking reaction of polyacrylate during subsequent curing, forming a strong covalent bond with the matrix. This chemical bonding fundamentally solves the key problem of weak interfacial bonding in traditional physically blended nanocomposites.

[0023] Secondly, during electrophoretic deposition, acryloyl chloride-modified cellulose nanocrystals, due to their surface properties, form stable composite colloidal particles with cationic polyacrylate through electrostatic interactions and potential hydrogen bonding. This pre-constructed synergistic structure ensures that the cellulose nanocrystals and polymer can be co-deposited under an electric field, thereby forming a uniform three-dimensional reinforcing network within the coating. Compared to systems using only unmodified cellulose nanocrystals or simple physical mixing, the strong interfacial bonding established by chemical modification in this invention greatly promotes the effective transfer of stress from the softer polymer matrix to the high-modulus cellulose nanocrystals, laying a structural foundation for improving the mechanical properties of the coating.

[0024] Subsequently, during the curing stage, the copolymerization reaction between the double bonds on the surface of acryloyl chloride-modified cellulose nanocrystals and the polyacrylate matrix proceeded fully. This process not only strengthened the interfacial bonding between the cellulose nanocrystals and the matrix but also transformed the cellulose nanocrystals into strong nodes in the cross-linked network, significantly enhancing the cross-linking density of the coating. This structural optimization directly manifested as a significant increase in the macroscopic hardness and scratch resistance of the coating. Simultaneously, the uniformly dispersed cellulose nanocrystal nanoparticles acted as a physical barrier in the coating, synergistically extending the penetration path of corrosive media within the coating in conjunction with the dense cross-linked network, thereby greatly improving the barrier protection performance of the coating.

[0025] In summary, this invention introduces acryloyl chloride-modified cellulose nanocrystals into a cathodic electrophoretic deposition system, utilizing their uniform dispersion and interfacial chemical bonding within cationic polyacrylate to construct a stable nanocomposite coating. The modified cellulose nanocrystals not only significantly enhance the coating's cohesive strength and interfacial bonding through covalent cross-linking but also form a dense nano-reinforcing network. This allows the coating to maintain good adhesion while significantly improving its hardness and corrosion resistance, providing a surface solution for automotive die-cast aluminum alloys that combines high strength with long-term protection.

[0026] In some embodiments, in step S1, the pretreatment includes: after alkaline cleaning of the die-cast aluminum alloy substrate, surface conversion treatment is performed by spraying or immersion using zinc phosphate treatment agent for 1-3 minutes at a temperature of 15-30°C.

[0027] After being sprayed or impregnated with a zinc phosphate treatment agent at 15-30℃ for a short time, a uniform and dense zinc phosphate conversion film with a micro-nano structure can be rapidly formed on the surface of die-cast aluminum alloy. This conversion film not only effectively removes oil and natural oxide layers from the substrate surface, but also significantly increases the contact area between the coating and the metal. Furthermore, by forming strong physical anchoring points and promoting interfacial chemical bonding, it greatly enhances the mechanical interlocking force and interfacial adhesion between the coating and the substrate.

[0028] In some embodiments, the preparation method of acryloyl chloride modified cellulose nanocrystals in step S2 specifically includes the following steps: Cellulose nanocrystals and acryloyl chloride were esterified in an organic solvent in the presence of a catalyst at a temperature of 40-60℃ for 10-14 hours. After the reaction, unreacted substances were removed by centrifugation and washing, and then freeze-dried to obtain acryloyl chloride-modified cellulose nanocrystals.

[0029] Esterification at 40-60℃ for 10-14 hours under mild conditions with the aid of a catalyst allows acryloyl groups to be stably grafted onto the surface of cellulose nanocrystals via covalent bonds. This surface modification not only significantly enhances the dispersion stability of cellulose nanocrystals in organic polymer matrices, but more importantly, introduces carbon-carbon double bonds that can participate in subsequent curing and crosslinking.

[0030] In some embodiments, the catalyst is 1-methylimidazole, the organic solvent is 1,4-dioxane, and the amount of 1-methylimidazole catalyst used is 10%-20% of the mass of cellulose nanocrystals. As a highly efficient organic base catalyst, 1-methylimidazole can significantly promote the esterification reaction of acryloyl chloride with the hydroxyl groups on the surface of cellulose nanocrystals within this specific dosage range, while effectively suppressing the occurrence of side reactions. 1,4-dioxane, as the reaction medium, not only provides suitable solubility and a suitable reaction environment but also ensures the stability of the reaction system.

[0031] In some embodiments, the mass ratio of acryloyl chloride to cellulose nanocrystals is (2-3):1. By setting the mass ratio of acryloyl chloride to cellulose nanocrystals within a suitable range, it is ensured that the acryloyl chloride fully meets the esterification reaction requirements of the hydroxyl groups on the surface of the cellulose nanocrystals, achieving a high grafting rate, while effectively avoiding side reactions caused by excess acryloyl chloride. In some embodiments, in step S3, the electrophoretic deposition suspension comprises the following components, the amounts of which are based on the total mass of the suspension: Acryloyl chloride modified cellulose nanocrystals: 0.05wt% to 0.5wt%; Dopamine hydrochloride: 0.05wt% to 0.15wt%; Cationic polyacrylate: 5wt% to 15wt%; The remainder is water.

[0032] By setting the ratio of the electrophoretic deposition suspension within a suitable range, acryloyl chloride-modified cellulose nanocrystals can be uniformly dispersed in a polyacrylate matrix. Meanwhile, the double bonds on their surface covalently cross-link with the amine groups of dopamine during curing, constructing a robust three-dimensional network structure. Simultaneously, the in-situ polymerization of dopamine during electrodeposition significantly enhances the interfacial adhesion between the coating and the substrate.

[0033] In some embodiments, step S3 further includes: ultrasonically treating the electrophoretic deposition suspension for 20-30 minutes at a power of 200-300W. Ultrasonic treatment effectively prevents secondary aggregation of acryloyl chloride-modified cellulose nanocrystals and promotes the uniform distribution of dopamine hydrochloride in the system.

[0034] In some embodiments, in step S4, electrophoretic deposition is performed under a DC electric field, wherein the electrophoretic voltage is 20-40V and the deposition time is 30-90 seconds. Setting the electric field conditions for electrophoretic deposition within a suitable range ensures sufficient migration and uniform co-deposition of acryloyl chloride-modified cellulose nanocrystals and cationic polyacrylate, while effectively controlling the deposition rate to avoid coating defects caused by excessively rapid deposition.

[0035] In some embodiments, step S5, the curing process includes: first air drying at room temperature for 20-40 minutes; then curing in a vacuum environment at 150-170°C for 30-50 minutes. The pre-drying stage at room temperature ensures the orderly evaporation of the solvent in the coating, avoiding surface defects caused by rapid drying; while the subsequent vacuum heat treatment significantly promotes the covalent cross-linking of the double bonds on the surface of the acryloyl chloride-modified cellulose nanocrystals with the polyacrylate matrix, while simultaneously driving further polymerization of dopamine.

[0036] In some embodiments, after electrophoretic deposition in step S4 and before curing in step S5, the substrate with the composite coating deposited is further subjected to rinsing and pre-drying steps. Rinsing involves immersing the substrate in deionized water 3-5 times; pre-drying involves allowing it to stand in a circulating air environment at 30-50°C for 10-20 minutes. Thorough rinsing effectively removes loosely adsorbed impurity particles from the surface of the deposited layer, avoiding coating defects caused by impurity encapsulation during curing. The subsequent pre-drying promotes the initial compaction of the coating structure through controlled moisture evaporation. This combined treatment significantly improves the coating uniformity and reduces the accumulation of internal stress during the curing process.

[0037] The present invention will be further described below by way of specific embodiments.

[0038] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods already existing in the art; unless otherwise specified, the experimental materials used in the following embodiments are all purchased from commercial channels.

[0039] Example 1: S1. After alkaline cleaning of the die-cast aluminum alloy substrate, surface conversion treatment is carried out by immersion with zinc phosphate treatment agent for 1 minute at a temperature of 15°C.

[0040] S2. Cellulose nanocrystals and acryloyl chloride were subjected to an esterification reaction in a 1-methylimidazolium catalyst (10% of the mass of cellulose nanocrystals) and a 1,4-dioxane solvent at a reaction temperature of 40°C for 10 hours. The mass ratio of acryloyl chloride to cellulose nanocrystals was 2:1. After the reaction, unreacted materials were removed by centrifugation and washing, and the nanocrystals were freeze-dried to obtain acryloyl chloride-modified cellulose nanocrystals.

[0041] S3. The acryloyl chloride modified cellulose nanocrystals (0.05 wt% of the total mass of the suspension) are dispersed in cationic polyacrylate (5 wt%), and dopamine hydrochloride (0.05 wt%) and water are added to form an electrophoretic deposition suspension. The suspension is then ultrasonically treated for 20 minutes at a power of 200W.

[0042] S4. The pretreated die-cast aluminum alloy substrate is placed in the electrophoretic deposition suspension and electrophoretic deposition is performed under a DC electric field. The electrophoretic voltage is 20V and the deposition time is 30 seconds. After deposition, the substrate is rinsed three times with deionized water and pre-dried by standing in a circulating air environment at 50°C for 10 minutes.

[0043] S5. The composite coating is first air-dried at room temperature for 20 minutes, and then cured in a vacuum environment at 150°C for 30 minutes.

[0044] Example 2: Same as Example 1, except that: In the S1 pretreatment, the zinc phosphate treatment time is 3 minutes and the treatment temperature is 30℃.

[0045] In S2, the esterification reaction temperature is 60℃, the reaction time is 14 hours, the mass ratio of acryloyl chloride to cellulose nanocrystals is 3:1, and the amount of 1-methylimidazole catalyst is 20% of the mass of cellulose nanocrystals.

[0046] In S3, the amount of acryloyl chloride modified cellulose nanocrystals is 0.5wt%, the amount of dopamine hydrochloride is 0.15wt%, the amount of cationic polyacrylate is 15wt%, the ultrasonic treatment time is 30 minutes, and the power is 300W.

[0047] In S4, the electrophoresis voltage is 40V, the deposition time is 90 seconds, and the pre-drying is carried out in a 30℃ environment for 20 minutes.

[0048] In S5, air dry at room temperature for 40 minutes, vacuum cure at 170℃ for 50 minutes.

[0049] Example 3: Same as Example 1, except that: In the S1 pretreatment, the zinc phosphate treatment time is 2 minutes and the treatment temperature is 22℃.

[0050] In S2, the esterification reaction temperature is 50℃, the reaction time is 12 hours, the mass ratio of acryloyl chloride to cellulose nanocrystals is 2.5:1, and the amount of 1-methylimidazole catalyst is 15% of the mass of cellulose nanocrystals.

[0051] In S3, the amount of acryloyl chloride modified cellulose nanocrystals is 0.25wt%, the amount of dopamine hydrochloride is 0.1wt%, the amount of cationic polyacrylate is 10wt%, the ultrasonic treatment time is 25 minutes, and the power is 250W.

[0052] In S4, the electrophoresis voltage is 30V, the deposition time is 60 seconds, and the pre-drying is carried out in a 40℃ environment for 15 minutes.

[0053] In S5, air dry at room temperature for 30 minutes, vacuum cure at 160℃ for 40 minutes.

[0054] Example 4: Same as Example 1, except that: In the S1 pretreatment, the zinc phosphate treatment time is 1.5 minutes and the treatment temperature is 20℃.

[0055] In S2, the esterification reaction temperature was 45℃, the reaction time was 11 hours, and the mass ratio of acryloyl chloride to cellulose nanocrystals was 2.2:1.

[0056] In S3, the amount of acryloyl chloride modified cellulose nanocrystals is 0.1 wt%, the amount of dopamine hydrochloride is 0.07 wt%, and the amount of cationic polyacrylate is 8 wt%.

[0057] In S4, the electrophoresis voltage is 25V and the deposition time is 45 seconds.

[0058] In S5, the vacuum curing temperature is 155℃ and the curing time is 35 minutes.

[0059] Example 5: Same as Example 1, except that: In the S1 pretreatment, the zinc phosphate treatment time is 2.5 minutes and the treatment temperature is 28℃.

[0060] In S2, the esterification reaction temperature was 55℃, the reaction time was 13 hours, and the mass ratio of acryloyl chloride to cellulose nanocrystals was 2.8:1.

[0061] In S3, the amount of acryloyl chloride modified cellulose nanocrystals is 0.4 wt%, the amount of dopamine hydrochloride is 0.13 wt%, and the amount of cationic polyacrylate is 14 wt%.

[0062] In S4, the electrophoresis voltage is 35V and the deposition time is 75 seconds.

[0063] In S5, the vacuum curing temperature is 165℃ and the curing time is 45 minutes.

[0064] Example 6: Same as Example 1, except that: In S2, the mass ratio of acryloyl chloride to cellulose nanocrystals is 2:1, and the amount of 1-methylimidazole catalyst is 10% of the mass of cellulose nanocrystals.

[0065] In S3, the amount of acryloyl chloride modified cellulose nanocrystals is 0.08 wt%, the amount of dopamine hydrochloride is 0.06 wt%, and the amount of cationic polyacrylate is 7 wt%.

[0066] In S4, the electrophoresis voltage is 22V and the deposition time is 50 seconds.

[0067] In S5, the vacuum curing temperature is 150℃ and the curing time is 40 minutes.

[0068] Comparative Example 1: Similar to Example 1, except that the zinc phosphate surface conversion treatment step in S1 is omitted, and electrophoretic deposition is performed directly after cleaning.

[0069] Comparative Example 2: Consistent with Example 1, except that no esterification reaction was performed in S2, and unmodified raw cellulose nanocrystals were used directly.

[0070] Test method: Thickness testing: A TT260 eddy current thickness gauge was used for measurement. According to GB / T4957-2003 "Eddy current method for measuring the thickness of non-conductive coatings on non-magnetic substrate metals", 10 points were randomly selected on the surface of each sample for measurement, and the average value was calculated. The test results are shown in Table 1.

[0071] Adhesion test: The test was conducted according to GB / T9286-2021 "Paints and Varnishes - Cross-cut Test". A grid was created on the coating surface using a cross-cut tester with a 1mm spacing, and then 3M 600 tape was used to peel it off. The adhesion level was then assessed. The test results are shown in Table 1.

[0072] Corrosion resistance: The YWX-750 salt spray test chamber was used for testing. The time when white rust first appeared on the coating surface was recorded according to the neutral salt spray test standard of GB / T10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test". The test results are shown in Table 1.

[0073] Coating hardness: According to GB / T6739-2006 "Determination of hardness of paint film by pencil method", a pencil was used to perform a scratch test from low to high, and the highest hardness grade of the unscratched coating was taken as the result. The test results are shown in Table 1.

[0074] Table 1

[0075] Referring to the test results in Table 1, the adhesion of all examples reached 5B, and the salt spray corrosion resistance time exceeded 700 hours. This demonstrates the synergistic effect of the zinc phosphate conversion layer and the modified cellulose nanocrystal-reinforced polyacrylate coating, forming a protective layer that is firmly bonded to the substrate and has extremely strong shielding properties. The coating thickness was between 13.2 and 16.8 μm, and the hardness was between 4H and 5H. Examples 2 and 5, due to the use of higher acryloyl chloride-modified cellulose nanocrystals and higher curing temperatures, had higher crosslinking densities, resulting in better coating hardness and corrosion resistance. Comparative Example 1 showed a significant decrease in adhesion (2B) and corrosion resistance (250 hours), demonstrating that the zinc phosphate conversion film is crucial and indispensable for providing a strong bond between the coating and the metal substrate. Comparative Example 2 showed a significant decrease in adhesion (3B), corrosion resistance (320 hours), and hardness (2H), indicating that acryloyl chloride modification can improve the dispersion and interfacial compatibility of cellulose nanocrystals in the resin, thereby enhancing the coating's adhesion, corrosion resistance, and hardness.

[0076] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0077] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the protection scope of the present invention.

Claims

1. A surface treatment method for die-cast aluminum alloys for automotive parts, characterized in that, Includes the following steps: S1. Pre-treat the surface of the die-cast aluminum alloy substrate; S2. Acryloyl chloride-modified cellulose nanocrystals were prepared by reacting acryloyl chloride with cellulose nanocrystals; S3. The acryloyl chloride modified cellulose nanocrystals are dispersed in cationic polyacrylate, and dopamine hydrochloride and water are added to form an electrophoretic deposition suspension. S4. The pretreated die-cast aluminum alloy substrate is placed in the electrophoretic deposition suspension and electrophoretic deposition is performed to form a composite coating. S5. The composite coating is cured.

2. The method according to claim 1, characterized in that, In step S1, the preprocessing includes: After alkaline cleaning of the die-cast aluminum alloy substrate, a zinc phosphate treatment agent is used for surface conversion treatment by spraying or immersion for 1-3 minutes at a temperature of 15-30℃.

3. The method according to claim 1, characterized in that, In step S2, the preparation method of the acryloyl chloride modified cellulose nanocrystals specifically includes the following steps: Cellulose nanocrystals and acryloyl chloride were esterified in an organic solvent in the presence of a catalyst at a temperature of 40-60℃ for 10-14 hours. After the reaction, unreacted substances were removed by centrifugation and washing, and then freeze-dried to obtain acryloyl chloride-modified cellulose nanocrystals.

4. The method according to claim 3, characterized in that, The catalyst is 1-methylimidazole, the organic solvent is 1,4-dioxane, and the amount of 1-methylimidazole catalyst used is 10%-20% of the mass of cellulose nanocrystals.

5. The method according to claim 3, characterized in that, The mass ratio of acryloyl chloride to cellulose nanocrystals is (2-3):

1.

6. The method according to claim 1, characterized in that, In step S3, the electrophoretic deposition suspension contains the following components, the amounts of which are based on the total mass of the suspension: Acryloyl chloride modified cellulose nanocrystals: 0.05wt% to 0.5wt%; Dopamine hydrochloride: 0.05wt% to 0.15wt%; Cationic polyacrylate: 5wt% to 15wt%; The remainder is water.

7. The method according to claim 1, characterized in that, Step S3 also includes: ultrasonic treatment of the electrophoretic deposition suspension for 20-30 minutes at a power of 200-300W.

8. The method according to claim 1, characterized in that, In step S4, the electrophoretic deposition is performed under a DC electric field, wherein: The electrophoresis voltage is 20-40V, and the deposition time is 30-90 seconds.

9. The method according to claim 1, characterized in that, In step S5, the curing process includes: First, air dry at room temperature for 20-40 minutes; Then cure in a vacuum environment at 150-170℃ for 30-50 minutes.

10. The method according to claim 1, characterized in that, After the electrophoretic deposition described in step S4 and before the curing process described in step S5, the process further includes rinsing and pre-drying the substrate with the composite coating deposited on it. The rinsing process involves immersing the rinsing 3-5 times in deionized water. The pre-drying process involves letting the food stand in a circulating air environment at 30-50℃ for 10-20 minutes.

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