Structural member and preparation method therefor, and electronic device

Abstract

This application provides a mechanical part, a preparation method therefor, and an electronic device. The preparation method for the mechanical part includes: performing n times of conversion coating on a mechanical part base, where n≥2; and after an ith time of conversion coating and before an (i+1)th time of conversion coating, performing thinning on a film converted through the ith time of conversion coating, where i is any integer from 1 to n-1, and a manner of the thinning includes chemical thinning or physical thinning. According to the foregoing preparation method, a film structure with high density and few defects can be formed on the mechanical part, and has stable low resistance.

Description

[0001] This application claims priority to Chinese Patent Application No. 202210237506.9, filed with the China National Intellectual Property Administration on March 11, 2022 and entitled "METHOD AND ELECTRONIC DEVICE", and to Chinese Patent Application No. 202210599198.4, filed with the China National Intellectual Property Administration on May 30, 2022 and entitled "MECHANICAL PART, PREPARATION METHOD THEREFOR, AND ELECTRONIC DEVICE", which are incorporated herein by reference in their entireties.TECHNICAL FIELD

[0002] Embodiments of this application relate to the field of electrical connection technologies, and in particular, to a mechanical part, a preparation method therefor, and an electronic device.BACKGROUND

[0003] To improve working performance of an electronic device, reduce radio frequency interference between components, increase isolation between components, reduce an electronic static electricity risk, and the like, the electronic device usually needs to be electrically connected to the ground, and each component in the electronic device also needs to be electrically connected to the ground. For example, a camera module needs to be electrically connected to the ground, to avoid interference from radio frequency signals from antennas; and a display assembly also needs to be electrically connected to the ground, to prevent electromagnetic noise from affecting normal operation of the display assembly.

[0004] A magnesium alloy has advantages such as low density and high strength, and can be used as a mechanical part of an electronic device. A conductive anti-corrosion coating is constructed on a surface of the magnesium alloy to improve corrosion resistance of the magnesium alloy and ensure good conductivity of the surface, for electrical connection to a conductive component in the electronic device and grounding. A chemical conversion coating film converted through chemical conversion coating (which may be referred to as "conversion coating" for short) is a conductive anti-corrosion coating with low costs. However, the current coating solution in practice has poor coating quality, with many defects and low density, resulting in a high coating resistance and making it unable to be directly applied to electrical connections or grounding.SUMMARY

[0005] In view of this, embodiments of this application provide a mechanical part, a preparation method therefor, and an electronic device, to resolve problems of poor film quality and excessively high resistance of a chemical conversion coating film on a surface layer of the mechanical part.

[0006] Specifically, a first aspect of embodiments of this application provides a preparation method for a mechanical part. The preparation method includes the following steps: performing n times of conversion coating on a mechanical part base, where n≥2; and after an i th< time of conversion coating and before an (i+1) th< time of conversion coating, performing thinning on a film converted through the i th< time of conversion coating, where i is any integer from 1 to n-1, and a manner of the thinning includes chemical thinning or physical thinning.

[0007] In this application, the n times (n≥2) of conversion coating are performed on the mechanical part base, and thinning is performed on a film converted through previous conversion coating between any two adjacent times of conversion coating. In this way, a defective film can be peeled off through chemical thinning, and a film of good quality can be retained as a film-forming nucleus for next conversion coating. Alternatively, physical thinning is used to increase density of metal atoms arranged on a surface of the mechanical part base and a dense oxide film formed accordingly, to reduce hydrogen evolution reaction in next conversion coating. Therefore, this implements good film preparation with high density and low defects, ensures stable and low resistance of an obtained film, and facilitates the mechanical part in electrical connection to the ground.

[0008] In an implementation of this application, the thinning includes: performing overall thinning on the film converted through the i th< time of conversion coating, or performing local thinning to remove a film in a preset region.

[0009] In some implementations of this application, a manner of the overall thinning includes the chemical thinning, and a manner of the local thinning includes the physical thinning.

[0010] In some implementations of this application, the chemical thinning is alkaline cleaning thinning, an alkaline thinning agent in use is a strong alkaline aqueous solution with pH≥12, and the alkaline thinning is performed for a time period more than 60s at a temperature within a range of 50°C to 90°C.

[0011] In an implementation of this application, a manner of the physical thinning includes laser engraving or ion beam thinning.

[0012] In some implementations of this application, the local thinning is performed through the laser engraving.

[0013] In some implementations of this application, the preparation method further includes: forming an alloy layer on the mechanical part base converted through the n times of conversion coating are performed, to cover a film converted through an n th< time of conversion coating. The alloy layer is in good contact with the film, with low contact resistance. This can further improve wear resistance and strength of an entire film layer.

[0014] A second aspect of embodiments of this application provides a mechanical part, obtained by using the preparation method according to the first aspect of embodiments of this application.

[0015] A third aspect of embodiments of this application provides a mechanical part. The mechanical part includes a mechanical part base and a first film structure disposed on a surface of the mechanical part base. The first film structure is located in a preset region of the mechanical part base, the preset region has a plurality of dents, and the first film structure is disposed on a surface of the dent.

[0016] In some implementations of this application, the mechanical part base further includes a non-preset region, a second film structure is disposed in the non-preset region, and a thickness of the second film structure is greater than that of the first film structure.

[0017] In some implementations of this application, an alloy layer is further disposed on the first film structure.

[0018] A fourth aspect of embodiments of this application provides a mechanical part, including a mechanical part base, and a first film structure and an alloy layer that are sequentially stacked on a surface of the mechanical part base.

[0019] In some implementations of this application, the first film structure completely covers a side surface of the mechanical part base.

[0020] In some other implementations of this application, the first film structure partially covers a side surface of the mechanical part base, the first film structure is located in a preset region of the mechanical part base, the preset region includes a plurality of dents, and the first film structure is disposed on a surface of the dent.

[0021] A fifth aspect of embodiments of this application provides an electronic device, including the mechanical part according to the second aspect, the third aspect, or the fourth aspect of embodiments of this application.BRIEF DESCRIPTION OF DRAWINGS

[0022] FIG. 1 is a diagram of a structure of an electronic device according to an embodiment of this application; FIG. 2 is a diagram of a complete technological process of performing one time of conversion coating on a mechanical part base according to some implementations of this application; FIG. 3 is a diagram of a complete technological process of performing one time of conversion coating on a mechanical part base according to some other implementations of this application; FIG. 4 is a diagram of a complete technological process of forming a film structure on a mechanical part base according to an embodiment of this application; FIG. 5 is a diagram of a preparation process of a mechanical part according to an embodiment of this application; FIG. 6 is a diagram of a preparation process of a mechanical part according to another embodiment of this application; FIG. 7 is a diagram of a preparation process of a mechanical part according to still another embodiment of this application; FIG. 8a, FIG. 8b, FIG. 8c, and FIG. 8d are diagrams of several structures of a mechanical part according to an embodiment of this application; FIG. 9 is a comparison diagram of surface micromorphologies and energy spectrum analysis results of a primary film converted through a first time of conversion coating and a secondary film converted through a second time of conversion coating after the film is thinned according to Embodiment 1 of this application; FIG. 10 provides a surface micromorphology diagram of a preset region of a base obtained through laser engraving according to Embodiment 2; and FIG. 11 provides surface micromorphology diagrams of a secondary film converted through second-time conversion coating on the basis of FIG. 10. DESCRIPTION OF EMBODIMENTS

[0023] The following describes the technical solutions in this application with reference to accompanying drawings in embodiments of this application.

[0024] Embodiments of this application provide a mechanical part with a chemical conversion coating film, a preparation method therefor, and an electronic device that can use the mechanical part. The technical solutions provided in this application are applicable to an electronic device that uses one or more of the following communication technologies: a Bluetooth (Bluetooth, BT) communication technology, a global positioning system (global positioning system, GPS) communication technology, a wireless fidelity (wireless fidelity, Wi-Fi) communication technology, a global system for mobile communications (global system for mobile communications, GSM) communication technology, a wideband code division multiple access (wideband code division multiple access, WCDMA) communication technology, a long term evolution (long term evolution, LTE) communication technology, a 5G communication technology, and other future communication technologies. An electronic device in embodiments of this application may be a mobile phone, a tablet computer, a notebook computer, a smart band, a smart watch, a smart helmet, a pair of smart glasses, or the like. The electronic device may alternatively be a cellular phone, a cordless phone, a session initiation protocol (session initiation protocol, SIP) phone, a wireless local loop (wireless local loop, WLL) station, a personal digital assistant (personal digital assistant, PDA), a handheld device having a wireless communication function, a computing device, another processing device connected to a wireless modem, a vehicle-mounted device, an electronic device in a 5G network, an electronic device in a future evolved public land mobile network (public land mobile network, PLMN), or the like. This is not limited in embodiments of this application.

[0025] FIG. 1 is an example diagram of a structure of an electronic device according to this application. In embodiments of this application, an electronic device is a mobile phone for description. As shown in FIG. 1, the electronic device 10 may include a cover glass (cover glass) 13, a display (display) assembly 15, a printed circuit board (printed circuit board, PCB) 17, a housing (housing) 19, and a rear cover (rear cover) 21.

[0026] The cover glass (cover glass, CG) 13 may be disposed in close contact with the display assembly 15, and is mainly configured to protect the display assembly 15 from dust. It should be understood that, in some embodiments, the cover glass 13 may alternatively be replaced with a cover made of another material, for example, an ultra-thin glass material cover or a PET (Polyethylene terephthalate, polyethylene terephthalate) material cover.

[0027] In an embodiment, the display assembly 15 may include a liquid crystal display (liquid crystal display, LCD), a light-emitting diode (light-emitting diode, LED) display panel, an organic light-emitting diode (organic light-emitting diode, OLED) display panel, or the like. This is not limited in this application.

[0028] The printed circuit board PCB 17 may be a flame-resistant material (FR-4) dielectric board, or may be a Rogers (Rogers) dielectric board, or may be a dielectric board mixing Rogers and FR-4, or the like. The FR-4 is a grade code name of a material that is flame resistant, and the Rogers dielectric board is a high frequency board. A metal layer may be disposed on a side that is of the printed circuit board PCB 17 and that is close to the housing 19, and the metal layer may be formed by etching metal on a surface of the PCB 17. The metal layer may be used for grounding an electronic component carried on the printed circuit board PCB 17, to prevent a user from an electric shock or prevent a device from being damaged. The metal layer may be referred to as a PCB floor. In addition to the PCB floor, the electronic device 10 may have another floor used for grounding, for example, a metal housing.

[0029] The electronic device 10 may further include a battery (not shown in the figure). The battery may be disposed in the housing 19, the battery may divide the PCB 17 into a main board and a sub-board, the main board may be disposed between the housing 19 and an upper edge of the battery, and the sub-board may be disposed between the housing 19 and a lower edge of the battery.

[0030] The housing 19 is mainly used to support the electronic device 10. The housing 19 may include a side frame 11. The side frame 11 may extend around a periphery of the electronic device 10 and the display assembly 15. The side frame 11 may specifically encircle four sides of the display assembly 15, to help fasten the display assembly 15. The side frame 11 may be made of a conductive material such as metal. In an implementation, the side frame 11 made of a metal material may be directly used as a metal side frame of the electronic device 10 to form an appearance of the metal side frame, and is applicable to a metal industrial design (industrial design, ID). In another implementation, an outer surface of the side frame 11 may alternatively be made of a non-metal material, for example, a plastic side frame, to form an appearance of a non-metal side frame. This is applicable to a non-metal ID.

[0031] The rear cover 21 may be a rear cover made of a metal material, or may be a rear cover made of a non-conductive material, such as a glass rear cover, a plastic rear cover, or another non-metal rear cover. The rear cover 21 is mainly configured to seal components such as a battery and a circuit board in an inner cavity of an enclosure, and protect each component of the electronic device.

[0032] It should be noted that FIG. 1 shows only some components included in the electronic device 10 as an example, and actual shapes, actual sizes, and actual structures of these components may not be limited by FIG. 1.

[0033] In an implementation of this application, the housing 19 may be made of a mechanical part with a chemical conversion coating film in embodiments of this application. The mechanical part includes a mechanical part base and a chemical conversion coating film disposed on the mechanical part base. A material of the mechanical part base may be magnesium or a magnesium alloy.

[0034] Because the magnesium or the magnesium alloy has low density, the magnesium or the magnesium alloy can be used in an electronic device to help reduce a weight of the electronic device. However, the magnesium or the magnesium alloy has high activity, and is easily oxidized and corroded during use. Therefore, surface treatment is required to improve corrosion resistance of the magnesium or the magnesium alloy without sacrificing surface conductivity of the magnesium or the magnesium alloy. In comparison with another surface protection technology, chemical conversion coating has advantages such as low costs and simple operations. A chemical conversion coating film formed on the mechanical part base through chemical conversion coating is a good conductive anti-corrosion coating, and may have good corrosion resistance and high electrical conductivity. In this way, when the mechanical part is used for the housing 19 of the electronic device, the chemical conversion coating film on a surface can provide an anti-corrosion protection function, and can be used for electrical connection to a conductive component in the electronic device 10, to implement grounding, thereby reducing an electronic static electricity risk, reducing radio frequency interference between components, and improving working performance of the electronic device 10. Specifically, grounding of the housing 19 may guide static electricity that enters a gap between the display assembly and the housing of the electronic device to the ground in a timely manner, to avoid damage to the display assembly caused by accumulation of static electricity, prevent electromagnetic interference, and the like. Therefore, the mechanical part provided in this application has good anti-corrosion performance, and can implement an electrical connection with high reliability at a preset position.

[0035] In embodiments of this application, the mechanical part may be prepared by using the following preparation method. The preparation method specifically includes: performing n times of conversion coating on the mechanical part base, where n≥2; and after an i th< time of conversion coating and before an (i+1) th< time of conversion coating, performing thinning on a film converted through the i th< time of conversion coating, where i is any integer from 1 to n-1, and a manner of the thinning includes chemical thinning or physical thinning.

[0036] FIG. 4 is a diagram of a complete technological process of forming a film structure on the mechanical part base by using n=2 as an example. The technological process shown in FIG. 4 includes the following steps: (1) First, one time of conversion coating is performed on the mechanical part base, to form a primary film on a surface of the mechanical part base. (2) Then, thinning is performed on the primary film. (3) Conversion coating is performed again on the mechanical part base with the thinned primary film (that is, a 2 nd< time of conversion coating is performed). Afterward, the mechanical part base obtained through the second time of conversion coating is water cleaned and dried in sequence for future use. The drying may be performed at a temperature of about 80°C.

[0037] In an implementation of this application, a material of the mechanical part base may include but is not limited to magnesium or a magnesium alloy, aluminum or an aluminum alloy, zinc or a zinc alloy, titanium or a titanium alloy, or the like. It is particularly necessary to process the magnesium or the magnesium alloy with high chemical activity by using the foregoing method provided in this application. For example, a designation of the magnesium alloy may be AZ91D, AZ31B, or the like.

[0038] In an implementation of this application, before the 1 st< time of conversion coating is performed on the mechanical part base, pre-processing (as shown in FIG. 4) is further performed on the mechanical part base. A pre-processing manner is not limited in this application, and may include a combination of acid pickling and alkaline cleaning, or may include a combination with another processing manner, or the like. In some implementations, as shown in FIG. 2, the pre-processing includes: pre-degreasing / degreasing, acid pickling, and alkaline etching. It may be understood that, each step of pre-processing should be followed by water cleaning, to prevent a residual reagent from the previous processing step from interfering with the next processing step. Each step of water cleaning can be performed at a room temperature for 1 minute to 2 minutes. The pre-degreasing / degreasing processing may be performed for 5 minutes to 10 minutes at a temperature of 55°C to 65°C by using an alkaline degreaser, or the processing may be referred to as "alkaline cleaning". The acid pickling may be performed for 0.5 minutes to 2 minutes at a temperature of 45°C to 55°C by using an organic acid pickling solution, and the alkaline etching may be performed for 3 minutes to 10 minutes at a temperature of 80°C to 90°C by using a strong alkaline solution. After the three-step pre-processing, dirt on the surface of the mechanical part base (especially a magnesium alloy die casting) can be effectively removed, and an active clean surface is obtained, which provides a good prerequisite for subsequent conversion coating. In some other implementations, as shown in FIG. 3, the pre-processing sequentially includes: alkaline cleaning, acid pickling, and surface conditioning. The alkaline cleaning is to remove oil stains on the surface. The acid pickling may be performed in a solution of citric acid, acetic acid or tartaric acid at 20°C to 30°C. The acid pickling may remove oxides, metal impurities, and the like on the surface of the mechanical part base. A surface conditioning solution may be a mixed solution including sodium hydroxide, sodium carbonate, sodium phosphate, and water. Surface conditioning of the mechanical part base may be implemented by placing the acid-pickled mechanical part base into the surface conditioning solution. In some other implementations, as shown in FIG. 4, the pre-processing before the 1 st< time of conversion coating may sequentially include cold acid pickling, neutralization, hot acid pickling, and neutralization. Although not shown in FIG. 4, it may be understood that each step of pre-processing should be followed by water cleaning, to prevent a residual reagent from the previous processing step from interfering with the next processing step. The cold acid pickling may be performed by using a fluorine-containing inorganic acid, and the hot acid pickling may be performed by using an organic weak acid (such as citric acid, acetic acid, or tartaric acid), and a temperature of the hot acid pickling is higher than that of the cold acid pickling. The two steps "neutralization" may be performed by using a strong alkaline solution independently.

[0039] Conversion coating means forming a surface film (that is, a film) on a surface of a base in a chemical or electrochemical manner. A type of the film converted through conversion coating can be determined based on a chemical conversion coating solution in use. Common films include but are not limited to a phosphide conversion coating film, a chromate conversion coating film, a stannate conversion coating film, a zirconate conversion coating film, a molybdate conversion coating film, a vanadate conversion coating film, a titanate conversion coating film, a permanganate conversion coating film, and the like. The chemical conversion coating solution (which may also be a "film agent") used for the conversion coating may include but is not limited to a phosphate system, a chromate system, a stannate system, a zirconate system, a molybdate system, a vanadate system, a titanate system, a permanganate system, a hydrotalcite system, a rare-earth salt system, a vegetable acid system, and a fluoride, or any combination of the foregoing systems.

[0040] For the magnesium or the magnesium alloy, a common film on a surface is usually a phosphate conversion coating film. In some implementations, a chemical conversion coating solution used for the 1 st< time of conversion coating on the mechanical part base may be a P-Ca-V composite phosphating solution whose pH is 2.2 to 3.2, and composition of the chemical conversion coating solution may be: 85% phosphoric acid: 4 ml / L to 20 ml / L, sodium dihydrogen phosphate: 5 g / L to 30 g / L, calcium nitrate: 10 g / L to 40 g / L, sodium benzenesulfonate: 0.5 g / L to 10 g / L, and ammonium metavanadate: 0.5 g / L to 5 g / L, with the remainder being water. The chemical conversion coating solution is used for performing processing at a temperature of 18°C to 60°C. In some other implementations, composition of the chemical conversion coating solution may include sodium dihydrogen phosphate, phosphoric acid, calcium salt, water, and rare-earth metal. The pH of the chemical conversion coating solution is 1.8 to 2.2, and the mechanical part base may be processed at 12°C to 15°C. In some other implementations, the chemical conversion coating solution used for the conversion coating includes a phosphate, a transition metal salt, and the like.

[0041] In this application, in the n times of conversion coating performed on the mechanical part base, chemical conversion coating solutions used in each time of conversion coating may be the same or different. A time period for each time of conversion coating may be the same or different. Generally, a time period for the 1 st< time of conversion coating is longer than that for other times of conversion coating.

[0042] In an implementation of this application, the thinning may include: performing overall thinning on the film converted through the i th< time of conversion coating, or performing local thinning to remove a film in a preset region. "Overall thinning" may make the film converted through the i th< time of conversion coating thinner in a thickness direction of the film, and each region that is of the film and that is away from the surface of the mechanical part base may be thinner in a direction of the mechanical part base, and thicknesses of the regions to be thinned may be the same or different. A manner of the overall thinning or local thinning includes chemical thinning or physical thinning. In general, the chemical thinning is more suitable for overall thinning, and the physical thinning is more suitable for local thinning. A manner of thinning a film converted through any i th< time of conversion coating from the 1 st< time to the (n-1) th< time may be the same or different.

[0043] In an implementation of this application, when chemical thinning is performed, the mechanical part base may be soaked in a chemical thinning agent, or a chemical thinning agent may be sprayed or coated on the film converted through the i th< time of conversion coating. That is, a specific implementation of chemical thinning includes but is not limited to soaking, spraying, or coating. The chemical thinning may specifically include but is not limited to alkaline cleaning thinning, acid pickling thinning, or the like. In other words, the chemical thinning agent includes an alkaline thinning agent or an acid thinning agent. In some implementations of this application, the chemical thinning is alkaline cleaning thinning, and an alkaline thinning agent in use may be a strong alkaline aqueous solution with pH≥12. A solute-strong alkali in the strong alkaline aqueous solution may be NaOH, KOH, or a combination thereof. Alkaline thinning may be performed for a time period more than 60s, for example, 60s, 80s, 90s, or 120s, at a temperature within a range of 50°C to 90°C, for example, 55°C, 60°C, 75°C, 80°C, or 85°C. In some specific embodiments, when the mechanical part base is soaked in the alkaline thinning agent to perform alkaline cleaning thinning, to ensure good thinning effect, an ultrasonic wave may be further applied in the process of alkaline cleaning thinning. That is, in the presence of the ultrasonic wave, the alkaline cleaning thinning is performed.

[0044] FIG. 5 is a diagram of a preparation process of the mechanical part by using an example in which a total quantity of times of performing conversion coating is n=2. In FIG. 5, ① after pre-processing is performed on the mechanical part base, a first time of conversion coating is performed on the mechanical part base, so that the surface of the mechanical part base is covered with a primary film; ② then, overall thinning is performed on the primary film through chemical thinning; and ③ a second time of conversion coating is performed on the mechanical part base with the thinned primary film, to form a secondary film on the basis of the thinned primary film, so as to obtain the mechanical part with a film structure.

[0045] The chemical thinning performed on the primary film can promote thinning / peeling-off of a film (for example, a surface film with low density and large porosity) in a film-forming defect region in the primary film. In this way, a good quality part (for example, a bottom film with high density) in the primary film can be retained and used as a crystal nucleus, and a crystal nucleus of good quality left after thinning can greatly reduce a requirement for oversaturation of a conversion coating solution during the second time of conversion coating, and guide secondary film growth, thereby improving film-forming quality (for example, improving film density and reducing a film defect) of the second time of conversion coating, and obtaining a film with high density, a low defect, and low film resistance. Further, because the retained crystal nucleus can reduce the oversaturation of the conversion coating solution used for the second time of conversion coating, a time period for performing the second time of conversion coating can be shortened, and a low-thickness film can be obtained.

[0046] Similarly, when n≥3, n-1 times of conversion coating are performed on the mechanical part base with a thinned primary film, and thinning is performed on a film converted through previous conversion coating between any two adjacent times of conversion coating, a technical effect similar to that when n=2 can also be achieved, and a crystal nucleus of good quality formed during the previous conversion coating can be retained, so as to guide rapid and dense deposition and growth of the film during next conversion coating. A total quantity of times of conversion coating may be determined based on corrosion resistance and a resistance requirement. In addition, when n≥3, an appearance part with an adjustable film color may be further prepared by adjusting and controlling a formula of a chemical conversion coating solution for conversion coating, to adapt to more application scenarios.

[0047] According to the foregoing technical solution, a film structure formed on the surface of the mechanical part base is obtained through superposing on the film obtained through thinning for the 1 st< time to the (n-1) th< time and the film obtained through conversion coating for the n th< time (as shown in FIG. 5). Because the film obtained through each time of thinning is a retained high-density film layer, and the film obtained through the last conversion coating is formed by seeding on the basis of the (n-1) th< film, density of the entire film structure in a thickness direction of the film structure is basically unchanged, and the film structure is a high-density film layer, unlike an existing primary film that has gradually decreasing density in a thickness direction (in other words, a bottom-up direction) away from the mechanical part base.

[0048] Chemical thinning is performed on the film converted through the i th< time (i is any integer from 1 to n-1) of conversion coating. In some implementations of this application, in the n times of conversion coating performed on the mechanical part base, at least one of the following conditions may be further controlled to be met, to implement preparation of a film structure with a low film thickness and few film defects: (a) A time period for performing the i th< time (i≥2) of conversion coating is less than a time period for performing the 1 st< time of conversion coating. (b) Concentrations of chemical conversion coating solutions used for n times of conversion coating show a gradient.

[0049] For the condition (b), n=4 is used as an example. Concentrations of the chemical conversion coating solutions used for the 1 st< time of conversion coating, the 2 nd< time of conversion coating, the 3 rd< time of conversion coating, and the 4 th< time of conversion coating may increase or decrease in gradient, for example, gradually decrease. In addition, a time period for each time of conversion coating may further meet the following condition: A time period for performing the (i+1) th< time of conversion coating is less than that for performing the i th< time of conversion coating. In this way, through a plurality of times of conversion coating in which the time periods for conversion coating are gradually reduced, a time period for obtaining the entire film structure and a thickness of the film can be further reduced, and film quality can be ensured. For example, two times of conversion coating are used. A time period for the 1 st< time of conversion coating may be 20s to 40s, and a time period for the 2 nd< time of conversion coating may be 1s to 10s.

[0050] In an implementation of this application, a manner of performing physical thinning on the film converted through the i th< time of conversion coating may include but is not limited to laser engraving or ion beam thinning. These physical thinning manners can generate enough heat to remelt a surface material of the mechanical part base, and a narrow processing technology depth does not over-destruct the base. In some implementations, the foregoing local thinning is performed in a physical thinning manner, to thin the film in the preset region. The preset region is usually a part of a region on a side surface of the mechanical part base.

[0051] In some implementations of this application, the local thinning is performed in a laser engraving manner. Laser power during laser engraving can be controlled within 25 W to 30 W (rated power of a laser engraving machine can be greater than or equal to 35 W), and a laser engraving speed is less than or equal to 3000 mm / s. This helps ensure that a primary film in the preset region is completely carved. In some implementations, laser engraving includes linear laser engraving, a line spacing of laser engraving is less than or equal to 0.05 mm, and laser engraving lines need to be crossed each other. In laser engraving line cross processing, metal elements on the surface of the mechanical part base can be melted more smoothly.

[0052] FIG. 6 is a diagram of another preparation process of the mechanical part by using an example in which a total quantity of times of performing conversion coating is n=2. In FIG. 6, ① after pre-processing is performed on the mechanical part base, a first time of conversion coating is performed on the mechanical part base, so that the surface of the mechanical part base forms a primary film; ② then, local thinning is performed on the primary film in a physical thinning manner such as laser engraving, to remove a film in the preset region; and ③ a second time of conversion coating is performed on the mechanical part base with the thinned primary film, and a secondary film whose thickness is thinner than that of a film in a non-preset region is formed in the preset region, to obtain the mechanical part.

[0053] After processing in step ②, there is no primary film in a region (that is, the preset region) in which physical thinning such as laser engraving is performed, and a primary film is further retained in another region (that is, the non-preset region) in which physical thinning is not performed. The retained primary film has a protection function. In a subsequent conversion coating process, it can be ensured that a chemical conversion coating solution basically does not cause a corrosion amount to the film in the region in which physical thinning is not performed. In this way, it is ensured that the film in the non-preset region is thick and has better corrosion resistance, and the film in the preset region is thin and has low resistance, so that the mechanical part implements a highly reliable electrical connection in the region.

[0054] Generally, in a molding process (for example, a die casting molding process) of the mechanical part base, a mold release agent usually needs to be introduced to help a molded part to be demolded. As a thickness of a mechanical part of an electronic device becomes thinner (for example, a thinnest part of a magnesium alloy mechanical part may reach 0.38 mm), an intrusion depth proportion of a mold release agent is increasing (for example, an intrusion depth exceeds 10 µm). The conventional technology cannot effectively remove the mold release agent with stable chemical properties. As a result, resistance of a region in which the mold release agent remains on the mechanical part base deteriorates sharply (for example, a resistance value exceeds 20 S2), which cannot meet a grounding requirement of a 3C electronic device. If prolonging processing of a time period of acid pickling is adopted, a corrosion amount of the mechanical part base will be increased and a yield rate will be reduced. However, in this application, physical thinning such as local laser engraving is performed on the mechanical part base with the primary film. Heat generated in the processing process may clear the primary film and the mold release agent on the surface of the preset region of the mechanical part base. In addition, remelting of metal elements (such as Mg and Al) of the mechanical part base may form a layer of dense oxide film in this region, the oxide film can slow down hydrogen evolution reaction on the surface of the mechanical part base in a film-forming process of the second time of conversion coating, and does not generate hydrogen gas during reaction between the oxide film and the chemical conversion coating solution, and provides metal ions used for film forming, thereby ensuring that a secondary film with high density, few defects, and low resistance is formed in the preset region. When the mechanical part base is magnesium or a magnesium alloy, the oxide film includes magnesium oxide, and may provide Mg 2+< for conversion coating use. In addition, when the mechanical part base is a magnesium alloy, during physical thinning such as laser engraving, Mg in the alloy is more likely to melt out than an Al element, a high-density aluminum segregation β-phase (that is, Mg 17 Al 12 ) is more likely to be formed, and existence of the aluminum segregation β-phase is more conducive to increasing density of the secondary film.

[0055] Furthermore, when physical thinning such as local laser engraving is performed on the mechanical part base with the primary film, a material on the surface of the mechanical part base can be melted to some extent. In a process of cooling the material into a solid, defects on the surface of the mechanical part base can be reduced, and metal atoms can be arranged more densely. In a salt spray test, an electrolyte solution is not easy to penetrate into the mechanical part base, thereby improving corrosion resistance or weather resistance of the mechanical part base.

[0056] It should be noted that, when the physical thinning is performed through laser engraving or ion beam thinning, a plurality of dents are generated in the preset region of the mechanical part base, the plurality of dents are usually continuously distributed, and adjacent dents are adjoined. When the second time of conversion coating is performed after physical thinning, a secondary film is formed on a surface of each dent.

[0057] Similarly, when the n (n≥3) times of conversion coating are performed on the mechanical part base and physical thinning is performed on the film converted through the i th< time of conversion coating, a technical effect similar to that when n=2 can also be achieved.

[0058] Similarly, physical thinning is performed on the film converted through the i th< time of conversion coating, and in the n times of conversion coating performed on the mechanical part base, at least one of the following conditions may be further controlled to be met, so as to implement preparation of a film structure with a low film thickness and few film defects: (a) A time period for performing the i th< (i≥2) time of conversion coating is less than a time period for performing the 1 st< time of conversion coating. (b) Concentrations of chemical conversion coating solutions used for the n times of conversion coating show a gradient, for example, a gradient increase or gradient decrease trend. In addition, a time period for each time of conversion coating may further meet the following condition: A time period for performing the (i+1) th< time of conversion coating is less than that for performing the i th< time of conversion coating. In this way, through a plurality of times of conversion coating in which the time periods for conversion coating are gradually reduced, a time period for obtaining the entire film structure and a thickness of the film can be further reduced, and film quality can be ensured.

[0059] In some implementations of this application, when local thinning is performed on the film converted through the i th< time of conversion coating in a physical thinning manner, n, a total quantity of times of conversion coating performed on the mechanical part base, may be equal to 2. In this case, the preparation method for the mechanical part can better balance short technology duration and low film resistance.

[0060] In some implementations of this application, when the local thinning is performed in a physical thinning manner, the preparation method further includes: forming an alloy layer on the mechanical part base converted through the n times of conversion coating, to cover a film converted through an n th< time of conversion coating. In this case, an alloy layer is formed on a surface (specifically, a surface of a film formed in the preset region after the n th< time of conversion coating is performed) of a film in the preset region, and there may be no alloy layer on a film in the non-preset region. For example, when n=2, as shown in FIG. 7, an alloy layer is formed on the secondary film located in the preset region.

[0061] The alloy layer may serve as a protective layer, and may be formed by spot coating a liquid low melting point alloy material. The alloy layer has good infiltration in a liquid state, and can be in profile tracing contact with an uneven surface of a film formed through physical thinning. In comparison with direct bonding or welding of a metal sheet on the film, the profile tracing contact can effectively increase a contact area with the film, reduce contact resistance, and improve bonding stability. Further, this can smooth a surface of the film with the alloy layer, which is more conducive to a subsequent stable electrical connection to a conductive component of the electronic device. In addition, after the alloy layer is attached to the film in the preset region, attachment strength of the film on the base is ensured, and wear resistance and strength of the entire film layer are significantly improved, so that the film can be adapted to more application scenarios while having effects of high wear resistance and stable low resistance.

[0062] In an implementation of this application, a melting point of the alloy layer is lower than that of the mechanical part base, and the melting point of the alloy layer is preferably obviously lower than a welding temperature of a soldering tin, for example, the melting point of the alloy layer is below 200°C. In addition, the wear resistance of the alloy layer is higher than that of the mechanical part base. In some implementations of this application, a material of the alloy layer may include but is not limited to an alloy of indium, tin, and bismuth, an alloy of indium, tin, and gallium, or the like.

[0063] Similarly, when the foregoing chemical thinning manner is used to perform overall thinning on the film converted through the i th< time of conversion coating, the preparation method may further include: forming an alloy layer on the mechanical part base after the n times of conversion coating are performed, to cover the film converted through the n th< time of conversion coating. In this case, the alloy layer may cover all surfaces of the film structure of the mechanical part base. The alloy layer is in good contact with the film structure, with low contact resistance. This can further improve wear resistance and strength of the film structure.

[0064] It should be noted that, in this application, manners of thinning the film converted through the i th< time (i is any integer from 1 to n-1) of conversion coating may be the same or different. For example, in another implementation of this application, local physical thinning may be performed on the film converted through the 1 st< time of conversion coating, then n-1 times of conversion coating are performed on the mechanical part base, and chemical thinning is performed on the film converted through the i th< time of conversion coating after the i th< time of conversion coating and before the (i+1) th< time of conversion coating, where i is any integer between 2 and n-1. Similarly, as described above in this application, gradient control may also be performed on the concentration of the chemical conversion coating solution used in the n times of conversion coating, and time periods for performing the 2 nd< and subsequent times of conversion coating are controlled to be less than the time period for performing the 1 st< time of conversion coating, and the like. Certainly, an alloy layer may also be formed on the mechanical part based on which the n times of conversion coating are performed, to cover the film converted through the n th< time of conversion coating.

[0065] The surface of the mechanical part obtained by using the foregoing preparation method in this embodiment of this application has a film structure with high density, few defects, and low resistance. When the mechanical part needs to be electrically connected to the ground, existence of the film can avoid a problem of deterioration of lap resistance. In addition, when the film structure further has the alloy layer, wear resistance and contact strength of the entire film layer can be improved, and reliability of an electrical connection between the mechanical part and a conductive device is not affected.

[0066] An embodiment of this application further provides a mechanical part. A surface of the mechanical part base has a film structure. The mechanical part may be obtained by using the foregoing preparation method in embodiments of this application.

[0067] As shown in FIG. 8a, in some implementations of this application, the mechanical part 100 includes a mechanical part base 101, and a first film structure 201 is disposed on a side surface of the mechanical part base 101. The surface of the mechanical part base 101 may be completely or partially covered by the first film structure 201.

[0068] In some implementations, in a thickness direction of the first film structure 201, density of the first film structure 201 basically remains unchanged. In this way, the first film structure 201 basically has high density in the thickness direction of the first film structure 201, and there are few surface defects, to effectively resolve a problem of deterioration of lap resistance on a surface of the film caused by a film defect. However, in another conventional technology, density of the film structure on the surface of the mechanical part base gradually decreases in a direction shown by an arrow in FIG. 8a (to be specific, in a direction from the mechanical part base 101 to the film, that is, a direction from bottom to top). The foregoing "density basically remains unchanged" may be understood as follows: Porosity of the first film structure 201 in the thickness direction of the first film structure 201 also basically remains unchanged. A relative deviation degree k between porosity a on a side that is of the first film structure 201 and that is away from the mechanical part base 101 and porosity b on a side that is of the first film structure 201 and that is close to the mechanical part base 101 does not exceed 30%, that is, k=|a-b| / b, and k≤30%. Further, k≤20%, or k≤10%, or k≤5%, or even k≤2%, or the like.

[0069] Further, as shown in FIG. 8b, in some implementations, an alloy layer 30 is further disposed on the first film structure 201, and the first film structure 201 and the alloy layer 30 are sequentially stacked on the mechanical part base 101. Existence of the alloy layer 30 can ensure high wear resistance and good weather resistance of the entire film layer on the mechanical part base 101, and good contact strength and low contact resistance between the mechanical part 100 and a conductive component electrically connected to the mechanical part 100.

[0070] As shown in FIG. 8c, in some other implementations of this application, the provided mechanical part 100 includes the mechanical part base 101 and a first film structure 201' located on the side surface of the mechanical part base 101. The first film structure 201' is located in a preset region of the mechanical part base 101. The preset region may be all surface regions of the side surface of the mechanical part base 101, or may be some regions shown in FIG. 8c. The preset region has a plurality of dents, and the first film structure 201' is disposed on a surface of the dent.

[0071] In this case, a shape of the first film structure 201' is different from that of an existing chemical conversion coating film, quality of a film layer of the first film structure 201' is good, for example, with high density and few defects, and there is almost no chemically stable mold release agent in the preset region, so that a problem that lap resistance on a surface of the film deteriorates because the mold release agent cannot be cleaned in an existing film technology can be resolved. A depth of the dent may be 2 µm to 6 µm, for example, 3 µm, 4 µm, or 5 µm; and a diameter of the dent may be 30 µm to 60 µm, for example, 35 µm, 40 µm, or 50 µm.

[0072] Generally, the plurality of dents in the preset region are usually continuously distributed, and adjacent dents are adjoined and have no gap. The first film structure 201' is located on an inner wall of each dent. In some embodiments, the first film structure 201' may partially cover the inner wall of the dent. In some other embodiments, the first film structure 201' may be a continuous film layer structure, and may completely cover the inner wall of the dent, but the dent is partially filled with the first film structure 201', in other words, the dent is not filled and leveled up with the first film structure 201'. A surface that is of the first film structure 201' and that is away from the mechanical part base 101 may be a non-flat surface. The first film structure 201' may be in a continuous valley shape, and a depression direction of the first film structure 201' faces the mechanical part base 101.

[0073] Still refer to FIG. 8c. The mechanical part base 101 further includes a non-preset region, a second film structure 202 is disposed in the non-preset region, and a thickness of the second film structure 202 may be greater than that of the first film structure 201'. In this way, the film in the non-preset region of the mechanical part base 101 is relatively thick and has better corrosion resistance, and the film in the preset region is relatively thin and has low resistance, so that the mechanical part implements highly reliable electrical connection in this region. Therefore, the mechanical part 100 can have stronger market competitiveness.

[0074] Further, as shown in FIG. 8d, in some other implementations of this application, the alloy layer 30 is further disposed on the first film structure 201'. The alloy layer 30 may fill a depression of the first film structure 201' in the continuous valley shape. In some cases, the alloy layer 30 may completely fill and level up the depression of the first film structure 201' in the continuous valley shape, and cover the mechanical part base in the preset region. In this case, a surface that is of the alloy layer 30 and that is away from the first film structure 201' is a flat surface. This helps increase a contact force and a contact area between the mechanical part 100 and another conductive component subsequently, to implement a stable electrical connection. Similar to the foregoing, existence of the alloy layer 30 can ensure high wear resistance and high weather resistance of the entire film layer on the mechanical part base 101, and good contact strength and low contact resistance between the mechanical part 100 and a conductive component electrically connected to the mechanical part 100.

[0075] It should be noted that, when the mechanical part 100 in this embodiment of this application needs to be electrically connected to the ground, the mechanical part 100 may be directly electrically connected to an electronic component that needs to be grounded, or an elastic conductive medium (for example, conductive foam) that can absorb a structural tolerance may be filled between the mechanical part 100 and the electronic component that needs to be grounded, to ensure reliability of the electrical connection.

[0076] It should be noted that, unless otherwise specified and limited, the term "electrical connection" in this application should be understood in a broad sense. For example, in this application, "electrical connection" may be understood as physical contact and electrical conduction of components; or may be understood as a form in which different components in a circuit structure are connected by using a physical line that can transmit an electrical signal, such as a printed circuit board (printed circuit board, PCB) copper foil or a conducting wire, or may be understood as a coupled connection. "Communication connection" may refer to electrical signal transmission, including a wireless communication connection and a wired communication connection. The wireless communication connection requires no physical medium, and does not belong to a connection relationship that limits a product structure. Both "connected" and "connection" may refer to a mechanical connection relationship or a physical connection relationship. For example, A is connected to B or a connection between A and B may mean that there is a fastening component (for example, a screw, a bolt, or a nail) between A and B, or A and B are in contact with each other and A and B are difficult to be separated.

[0077] An embodiment of this application provides an electronic device, and the electronic device may include any mechanical part described in the foregoing embodiments.

[0078] It should be noted that, although the foregoing embodiments of this application are described by using an example in which the mechanical part is used as a housing of an electronic device such as a mobile phone, it may be understood that the mechanical part provided in embodiments of this application may be further used as an enclosure of another electronic device. Other feasible electronic devices may be the foregoing examples in this application, but a scope of the electronic device in this application is not limited to the foregoing examples.

[0079] Because the electronic device uses the mechanical part with good conductivity and excellent anti-corrosion performance in the foregoing embodiments of this application, quality and reliability of the electronic device are high, and use experience and market competitiveness of the electronic device can be improved.

[0080] In embodiments provided in this application, it should be understood that the disclosed system, apparatus, and method may be implemented in another manner. For example, the described apparatus embodiment is merely an example. For example, division into the units is merely logical function division and may be other division in actual implementation. For example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections may be implemented through some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic or other forms.

[0081] Embodiments of this application are further described below by using a plurality of embodiments. Embodiments of this application are not limited to the following specific embodiments.Embodiment 1

[0082] As shown in FIG. 4 and FIG. 5, a preparation method for a mechanical part is provided, and the preparation method includes the following steps. (1) The following pre-processing is first performed on a mechanical part base whose material is a magnesium alloy AZ91D and whose thickness is greater than or equal to 0.2 mm: Cold acid pickling is performed on the mechanical part base by using a fluorine-containing inorganic acid at a temperature of 0°C to 10°C; neutralization is performed by using a water solution such as NaOH; and then hot acid pickling is performed by using an organic weak acid such as citric acid at a temperature higher than that of the cold acid pickling, for example, 50°C to 90°C. Then, a chemical conversion coating solution such as a phosphate system is used to perform a first time of conversion coating on the mechanical part base, and a conversion coating time period may be 20s to 40s. A surface of the magnesium alloy base is covered with a phosphate film of a specific thickness. (2) Thinning of a primary film: The magnesium alloy mechanical part base obtained in step (1) is soaked in a NaOH solution with pH>12 to perform overall thinning for at least 50s; during the thinning process, an ultrasonic wave is applied to control the temperature at 10°C to 90°C; and after the thinning is completed, water cleaning is performed. (3) A second time of conversion coating is performed on the magnesium alloy mechanical part base with the thinned primary film to form a secondary film. A concentration of a solution used for the second time of conversion coating may form a gradient with a concentration of the solution used for the first time of conversion coating, and a time period for secondary film-forming may be controlled within 1s to 10s. Then, the mechanical part base obtained through the second time of conversion coating is water cleaned and dried in sequence (the temperature may be 80°C).

[0083] According to the scheme in Embodiment 1, a high-density and low-thickness film can be generated on the magnesium alloy mechanical part, so that stable low resistance of the film in a preset region and a low corrosion amount on the mechanical part are implemented. When a resistance test is performed on the mechanical part prepared in Embodiment 1 by using a gold-plated ball-tip probe with a diameter of 1.4 mm (an elastic force of the probe tip is about 0.7 N), it is measured that a resistance value of a film on the mechanical part is about 0.2 Ω, and a corrosion amount against the mechanical part is about 5 µm (the corrosion amount is obtained by comparison with the mechanical part base before the first time of conversion coating). However, for a film made in the conventional technology (that is, a film formed by performing one time of conversion coating on the mechanical part base), it is measured by using the foregoing test method that a resistance value of the film is greater than 5 Ω, and a corrosion amount against the mechanical part is about 10 µm. This indicates that the scheme in Embodiment 1 of this application can lower resistance of the film of the magnesium alloy mechanical part and a corrosion amount of the mechanical part.

[0084] FIG. 9 summarizes a scanning electron microscope (Scanning Electron Microscope, SEM) diagram and an energy dispersive X-ray spectroscopy (Energy Dispersive X-Ray Spectroscopy, EDX) diagram of a film converted through a first time of conversion coating is performed on the magnesium alloy mechanical part base in Embodiment 1, and a SEM diagram and an EDX diagram of a secondary film converted through a second time of conversion coating after the film is thinned. After comparison and analysis, it is found that primary film samples are distributed in irregular fragments and have obvious gully-shaped cracks, and an average crack width is ≥1 µm.A surface of the secondary film obtained through thinning shows continuous network distribution, with a grain boundary aluminum segregation β-phase (Mg 17 Al 12 ) protruding, and only microcracks exist in intra-granularity with an average width of ≤0.5 µm. This indicates that density of the secondary film obtained through thinning increases greatly. At the same time, phosphorus content in the secondary film obtained through thinning is greatly reduced, which indirectly indicates that there are fewer reaction products from the chemical conversion coating, that is, the film layer becomes thinner. Therefore, after the initial primary film is thinned, a non-dense part of the surface is removed, and the obtained secondary film has high density, and the film layer is relatively thin. This helps improve salt spray resistance of the film, and meets a requirement of a low resistance value by using a relatively thin thickness of the secondary film.

[0085] The foregoing film features are formed mainly due to the fact that alkaline cleaning thinning after the first time of conversion coating can promote hydrolysis of monohydrogen phosphate, dihydric phosphate or phosphate in the primary film, and can also promote peeling-off of hydrogen phosphate in some film-forming defect regions, thereby achieving thinning of the film and retaining some crystal nuclei with good film quality in the first time of conversion coating. Using a retained primary film of good quality as a nucleus can shorten a time period for performing the second time of conversion coating and improve film quality. When the time period for performing the second time of conversion coating is shortened, a high-density and low-thickness magnesium alloy film can be obtained, so that stable low resistance of the magnesium alloy film and a low corrosion amount on the mechanical part are implemented.Embodiment 2

[0086] A preparation method for a mechanical part is provided. Refer to the process in FIG. 6. The preparation method includes the following steps. (1) Pre-processing and a first time of conversion coating are first performed on a mechanical part base made of a magnesium alloy to form a primary film with a specific thickness. For specific operation steps, refer to Embodiment 1. (2) Thinning of the primary film: Local high-power laser engraving is performed on a preset region of the magnesium alloy mechanical part base processed in step (1), to thin off the primary film in the preset region, and the primary film is retained in a region that is not laser engraved. Laser engraving is specifically linear laser engraving, and a line spacing of laser engraving is less than or equal to 0.05 mm. Laser engraving lines need to be crossed each other. A laser engraving speed is less than or equal to 3000 mm / s, and laser power during laser engraving is 25 W to 30 W. After laser engraving is completed, a surface of the mechanical part base is cleaned. (3) A second time of conversion coating is performed on the magnesium alloy mechanical part base obtained through laser engraving, to form a secondary film in a laser engraving region, and then water cleaning and drying are performed on the mechanical part base obtained through the second time of conversion coating. A concentration of a solution used for the second time of conversion coating may be the same as that of a solution used for the first time of conversion coating, and a time period for the second time of conversion coating is 5s to 8s. During the second time of conversion coating, the primary film in the region that is not laser engraved basically remains unchanged.

[0087] A cross processing process of the laser engraving line can melt an alloy on the surface of the magnesium alloy mechanical part base, and in a process of melting and cooling the alloy into a solid state, metal defects on the surface of the magnesium alloy can be reduced, metals can be arranged more densely, and salt spray resistance of the magnesium alloy can be improved. After the laser engraving, a mold release agent and the primary film on the surface of the magnesium alloy mechanical part base in the corresponding laser engraving region are cleared, to form a dense oxide film. The oxide film can slow down hydrogen evolution reaction on the surface of the magnesium alloy in the second time of conversion coating process, and provide magnesium ions for film forming. This helps form a dense secondary film.

[0088] FIG. 10 provides a surface micromorphology diagram of the preset region of the base obtained through laser engraving according to Embodiment 2. FIG. 11 provides surface micromorphology diagrams of the secondary film converted through second-time conversion coating after laser engraving. The right diagram is an enlarged diagram of a part of a region on the left diagram. It can be learned from these diagrams that, after laser engraving, a plurality of continuously distributed dents are generated in the preset region of the mechanical part base. When the second time of conversion coating is performed on the mechanical part base after laser engraving, a secondary film is formed on the inner wall of the dent.

[0089] According to the scheme in Embodiment 2, a high-density and low-thickness film can be generated in the preset region of the magnesium alloy mechanical part, so that stable low resistance of the film in the preset region and a low corrosion amount on the mechanical part are implemented. When a resistance test is performed on the mechanical part prepared in Embodiment 2 by using a gold-plated ball-tip probe with a diameter of 1.4 mm (an elastic force of the probe tip is about 0.7 N), it is measured that a resistance value of a film in the preset region of the mechanical part is about 0.2 Ω, and a corrosion amount against the mechanical part is about 3 µm. However, for a film made in the conventional technology (that is, a film formed by performing one time of conversion coating on the mechanical part base), it is measured by using the foregoing test method that resistance values of all regions of the film are greater than 5 S2, and corrosion amounts against the mechanical part are about 10 µm.Embodiment 3

[0090] A preparation method for a mechanical part is provided. Refer to the process in FIG. 7. A difference between the preparation method and Embodiment 2 lies in that, after step (3), the following step (4) is further included: A layer of liquid low melting point alloy (a melting point is below 200°C) is plated on a secondary film corresponding to a local laser engraving region through glue dispensing, and the low melting point alloy is attached to the secondary film after cooling.

[0091] According to the scheme in Embodiment 3, a high-density and low-thickness film can also be generated in a preset region of a magnesium alloy mechanical part, so that stable low resistance of the film in the preset region and a low corrosion amount on the mechanical part are implemented. For specific effect, refer to the description in Embodiment 2.

[0092] In addition, an alloy layer is formed on the surface of the secondary film by using a point-low melting point alloy. When the alloy layer is in a liquid state, the alloy layer can have a profile tracing contact with a concave convex surface after laser engraving. The profile tracing contact can increase a contact area between the two and reduce contact resistance. In addition, after the alloy layer is attached to the secondary film, wear resistance and strength of the film layer can be improved significantly.

[0093] The foregoing descriptions are merely example implementations of this application, and descriptions thereof are relatively specific and detailed, but cannot be construed as a limitation on the patent scope of this application. It should be noted that, any variation or replacement readily figured out by a person skilled in the art within the technical scope disclosed in this application shall fall within the protection scope of this application. Therefore, the protection scope of this application shall be subject to the protection scope of the claims.

Claims

1. A preparation method for a mechanical part, wherein the preparation method comprises the following steps: performing n times of conversion coating on a mechanical part base, wherein n≥2; and after an ith time of conversion coating and before an (i+1)th time of conversion coating, performing thinning on a film converted through the ith time of conversion coating, wherein i is any integer from 1 to n-1, and the thinning comprises chemical thinning or physical thinning.

2. The preparation method according to claim 1, wherein the thinning comprises: performing overall thinning on the film converted through the ith time of conversion coating, or performing local thinning for film removal from a preset region.

3. The preparation method according to claim 2, wherein the overall thinning comprises the chemical thinning, and the local thinning comprises the physical thinning.

4. The preparation method according to any one of claims 1 to 3, wherein when the chemical thinning is used, the mechanical part base is soaked in a chemical thinning agent, or a chemical thinning agent is sprayed or coated on the film converted through the ith time of conversion coating.

5. The preparation method according to claim 4, wherein the chemical thinning is alkaline cleaning thinning, an alkaline thinning agent in use is a strong alkaline aqueous solution with pH≥12, and the alkaline thinning is performed for a time period more than 60s at a temperature within a range of 50°C to 90°C.

6. The preparation method according to any one of claims 1 to 3, wherein the physical thinning comprises laser engraving or ion beam thinning.

7. The preparation method according to claim 6, wherein laser power during the laser engraving is 25 W to 30 W, and a laser engraving speed is less than or equal to 3000 mm / s.

8. The preparation method according to claim 7, wherein the laser engraving comprises linear laser engraving, and a line spacing of the laser engraving is less than or equal to 0.05 mm.

9. The preparation method according to any one of claims 1 to 8, wherein in the n times of conversion coating performed on the mechanical part base, at least one of the following conditions is met: (a) when i≥2, a time period for performing the ith time of conversion coating is less than a time period for performing a 1st time of conversion coating; and (b) concentrations of chemical conversion coating solutions used for the n times of conversion coating show a gradient.

10. The preparation method according to any one of claims 1 to 9, wherein the preparation method further comprises: forming an alloy layer on the mechanical part base converted through the n times of conversion coating, to cover a film converted through an nth time of conversion coating.

11. The preparation method according to any one of claims 1 to 10, wherein before the 1st time of conversion coating is performed on the mechanical part base, the preparation method further comprises: performing pre-processing on the mechanical part base.

12. A mechanical part, obtained by using the preparation method according to any one of claims 1 to 11.

13. A mechanical part, comprising a mechanical part base and a first film structure disposed on a surface of the mechanical part base, wherein the first film structure is located in a preset region of the mechanical part base, the preset region has a plurality of dents, and the first film structure is disposed on a surface of the dent.

14. The mechanical part according to claim 13, wherein the first film structure completely covers an inner wall of the dent.

15. The mechanical part according to claim 13 or 14, wherein the mechanical part base further comprises a non-preset region, a second film structure is disposed in the non-preset region, and a thickness of the second film structure is greater than that of the first film structure.

16. The mechanical part according to any one of claims 13 to 15, wherein an alloy layer is further disposed on the first film structure.

17. A mechanical part, comprising a mechanical part base, and a first film structure and an alloy layer that are sequentially stacked on a surface of the mechanical part base.

18. The mechanical part according to claim 17, wherein the first film structure completely covers a side surface of the mechanical part base.

19. The mechanical part according to claim 17, wherein the first film structure partially covers a side surface of the mechanical part base, the first film structure is located in a preset region of the mechanical part base, the preset region comprises a plurality of dents, and the first film structure is disposed on a surface of the dent.

20. The mechanical part according to claim 18 or 19, wherein in a thickness direction of the first film structure, density of the first film structure basically remains unchanged.

21. An electronic device, comprising the mechanical part according to claim 12, the mechanical part according to any one of claims 13 to 16, or the mechanical part according to any one of claims 17 to 20.

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