Novel magnesium alloy hot galvanizing solution as well as preparation and application thereof

Abstract

The invention discloses a novel magnesium alloy hot galvanizing solution and preparation and application thereof, and belongs to the technical field of magnesium alloy surface treatment. According to the surface treatment method, physical and mechanical polishing without pickling and plating assistance is adopted, the high activity of the magnesium alloy material is utilized, the surface treatment method of self-activation plating assistance is achieved, the obtained plated part is smooth in surface and good in pretreatment effect, and the surface pretreatment mode suitable for magnesium alloy hot dipping is prepared; a low-temperature plating solution capable of realizing magnesium alloy hot dipping is designed, and a hot-dip galvanizing layer with uniform and controllable thickness and excellent corrosion resistance is prepared through combined regulation and control of the dipping temperature, the dipping time, the dipping rate, the extraction rate, post-plating treatment and other process parameters.

Description

Technical Field

[0001] The invention belongs to the technical field of magnesium alloy surface treatment, and specifically relates to a novel magnesium alloy hot-dip galvanizing solution and a preparation method and application thereof. Background Art

[0002] As the lightest metal structural material, magnesium alloy is widely used in the fields of automobile, aerospace, electronic communication, etc., with broad application prospects, but its poor corrosion resistance seriously affects the large-scale application of its structural parts. At present, many surface protection technologies can improve the corrosion resistance of magnesium alloys to a certain extent, but there are still many problems such as high cost, high equipment requirements, thin surface film layer and poor protection, which cannot provide efficient and low-cost protection for magnesium alloy structural parts. Therefore, the research and development of new surface protection technology has always been the direction of efforts for large-scale application of magnesium alloys.

[0003] As the most widely used and effective surface protection method for steel materials, hot-dip plating technology has significant advantages such as simple process operation, good protection and low cost. The hot-dip plating process covers a series of process steps such as pretreatment of plated parts, preparation of plating solution, specific immersion plating process system and post-plating treatment. Although the hot-dip plating process of steel materials has been widely studied, the related process research on hot-dip plating of magnesium alloy materials is still in the initial exploration stage and has not yet formed a mature theoretical system and application practice. In addition, the high activity and low melting point of magnesium make hot-dip plating of its alloy materials extremely difficult. For example, the high activity of magnesium alloy makes it easy to form an oxide film on its surface, which not only hinders the metallurgical bonding between the substrate and the plating solution, but also poses a challenge to the adhesion and bonding of the coating. Magnesium and its alloy materials with active chemical properties are very easy to react violently with acid and salt solutions. Therefore, the pretreatment methods in the hot-dip plating process of steel are not applicable to magnesium alloy materials. At the same time, the low melting point characteristics of magnesium alloys further put forward strict requirements on the composition of the plating solution and the control of the hot-dip process window. The melting point of the plating solution and the immersion temperature cannot be too high to avoid melting of the base material, thereby ensuring the integrity of the plated structure.

[0004] Since the hot-dip plating process of magnesium alloys shows great development potential for the surface protection of magnesium alloys and can effectively protect magnesium alloys efficiently and inexpensively, researchers are committed to developing hot-dip plating technology suitable for magnesium alloys, thereby providing a highly efficient, low-cost and protective surface treatment method for magnesium alloys to solve the problem of large-scale magnesium alloy applications.

[0005] In 2007, Ji Fang et al. first attempted to prepare an aluminum alloy coating on the surface of AZ61 magnesium alloy by hot-dip coating. This process included strict pretreatment of the substrate material and short-term preheating of the substrate before dipping. The specific dipping temperature was 780℃~800℃, the dipping time was 2s~6s, and the holding time was 20min~60min. However, the coating effect was not ideal, and the overall protection effect and surface quality of the coating were poor. [1] In 2010, Chen Zhichao and others proposed an improved hot-dip plating method for the high activity and poor heat resistance of magnesium alloys. The process of chemically plating nickel on the surface of AZ91 magnesium alloy first includes pretreatment of the substrate and cleaning of nano-SiC. Then, chemical plating is performed using a pre-prepared plating solution, and ultrasonic and magnetic stirring are used for 50 to 60 minutes. During plating, continuous stirring is maintained to prevent the tin layer from directly forming compounds with the substrate. Subsequently, a pure tin and lead-based babbitt alloy coating is successfully obtained on the surface of the magnesium alloy. [2] However, this study is limited to pure tin coating and lead-based babbitt coating, and there are still challenges such as insufficient coating thickness, limited protection effect and complex process. The current technology still cannot meet the market demand for efficient, economical and simple magnesium alloy hot-dip plating.

[0006] Chen Zuquan and others conducted in-depth research on the hot-dip process of AZ31 magnesium alloy and used Al-Si multi-component aluminum alloy as a new bath alloy. The wettability between the magnesium alloy substrate and the bath was improved by pre-plating a layer of low-melting-point metal tin before hot-dip plating. The cross-sectional structure of the coating is shown in the attached figure. Figure 1 The surface flatness of the coating prepared in this study is low, with obvious pits, ridges and excessive substrate corrosion, and there are also defects such as holes inside the coating. At the same time, the cost and complexity of the process need to be further reduced to meet the actual needs of industrial applications. [3] .

[0007] At present, most of the reports on hot-dip plating processes are limited to steel materials, while there are few reports on hot-dip plating processes for magnesium alloys as base materials. There are only a few attempts to study the hot-dip plating process for magnesium alloys, and the surface quality of the obtained coatings is poor, the coating bonding is poor, the process window is narrow and cannot be applied on a large scale. There has also been no accurate process parameters to guide production. Therefore, the hot-dip plating process for magnesium alloys has not been actually applied and promoted in industry so far.

[0008] Moreover, with the deepening of research, the current magnesium alloy surface hot dip plating technology still has the following problems:

[0009] 1. Surface pretreatment process: Magnesium alloys are chemically active and easily react with water, oxygen, acid and salt solutions. The hot-dip surface pretreatment of steel materials uses a strong alkaline NaOH solution to remove oil, a hydrochloric acid solution to remove oxides, a zinc chloride ammonia solution to assist plating and activation, and then immersion plating. However, magnesium alloys react severely with these alloy materials and form a large number of corrosion products on the surface, thereby hindering the bonding of the coating with the substrate. The surface treatment methods of some magnesium alloys are cumbersome and have poor treatment effects. They use complex surface pretreatment methods such as hydrofluoric acid and chromic acid, or electroplating, and the treatment process is cumbersome. A large amount of wastewater, waste residue and other pollutants containing chromium ions will be generated in the treatment project. Therefore, conventional chemical surface pretreatment methods are not suitable for magnesium alloy materials, and it is difficult to find a suitable surface treatment method to treat the surface of magnesium alloys.

[0010] 2. Unknown composition of plating solution: The melting point of magnesium alloy is generally around 650℃, the melting point of steel material is generally 1500℃, and the immersion temperature of hot-dip plating of steel is generally 450℃~550℃, which mainly depends on the composition of plating solution. Therefore, how to find a plating solution with low melting point, good fluidity, and metallurgical bonding reaction with magnesium alloy without causing corrosion is an important key to realize the hot-dip plating process of magnesium alloy, and also one of the biggest difficulties.

[0011] 3. Dipping time, dipping temperature, dipping rate, withdrawal rate and post-plating treatment are all key factors affecting the success of hot-dip plating of magnesium alloys. They are also important factors affecting the surface quality and protectiveness of the coating. However, there is currently no data reference for research in this area, and different matrix magnesium alloy materials have different physical and chemical properties, and their process parameters and dipping effects are also significantly different. Therefore, how to coordinate and combine the range of various process parameters is an important key issue in achieving hot-dip plating of magnesium alloys.

[0012] 4. The hot-dip plating technology of magnesium alloys has not conducted any experimental research on the physical and chemical properties of different matrix magnesium alloy materials (such as second phase distribution, matrix phase structure distribution, matrix melting point and plating solution composition, etc.), nor has there been any relevant process parameters to guide the hot-dip plating process research of different matrix magnesium alloy materials.

[0013] References [1] Ji Fang. Preparation and performance study of aluminum coating on magnesium alloy surface [D]. Xi'an University of Architecture and Technology, 2007.

[0014] Reference [2] Zeng Bing, Ge Xiaolan, Chen Zhichao. Corrosion resistance of Ni-P-nano SiC composite chemical coating on magnesium alloy surface [J]. Materials Protection, 2010, 43(07): 5-7+76. DOI: 10.16577 / j.cnki.42-1215 / tb.2010.07.017.

[0015] Reference [3] Chen Zuquan. Study on the process and microstructure of hot-dip aluminum alloy on magnesium alloy surface [D]. Chongqing University, 2013. Summary of the invention

[0016] In view of the problems of low universality of plating solution, high cost, poor corrosion resistance of coating layer and the like in the hot-dip plating process of magnesium alloy, the present invention provides a novel magnesium alloy hot-dip galvanizing plating solution and its preparation and application. The present invention uses the high activity of magnesium alloy material itself to achieve a surface treatment method of "self-activation plating" by physical and mechanical polishing without pickling plating, and the obtained plated parts have a smooth surface and good pretreatment effect. A surface pretreatment method suitable for hot-dip plating of magnesium alloy is prepared, and a low-temperature plating solution capable of hot-dip plating of magnesium alloy is designed according to the physical and chemical properties of magnesium alloy. By combining and controlling the process parameters such as immersion plating temperature, immersion plating time, immersion rate, withdrawal rate and post-plating treatment, a hot-dip galvanized layer with uniform and controllable thickness and excellent corrosion resistance is prepared.

[0017] To achieve the above object, the present invention adopts the following technical solutions:

[0018] A novel magnesium alloy hot-dip galvanizing bath, used for magnesium alloy hot-dip galvanizing, wherein the components in the hot-dip galvanizing bath are as follows by mass percentage: Al 0.1% to 7%, Mg 0.1% to 4%, Sn 0 to 7%, Li 0.1% to 0.4%, and the balance is Zn and unavoidable impurities;

[0019] The magnesium alloy is a magnesium rare earth alloy or a conventional magnesium alloy; wherein the magnesium rare earth alloy is one of Mg-Gd, Mg-Y, Mg-Gd-Y and Mg-Zn-Y, and each alloying element in the conventional magnesium alloy satisfies the element composition range of the corresponding alloy grade;

[0020] In the Mg-Gd alloy, the mass percentage of Gd is 0.1% to 12%; in the Mg-Y alloy, the mass percentage of Y is 0.1% to 12%; in the Mg-Gd-Y alloy, the combined mass fraction of Gd and Y is 0.1% to 20%; in the Mg-Zn-Y alloy, the sum of the contents of Zn and Y is 0.1% to 20%;

[0021] The hot-dip galvanizing bath has a melting point lower than 320°C.

[0022] The method for preparing the hot-dip galvanizing solution comprises the following steps:

[0023] S1. Place the zinc ingot in a smelting device and heat it to 540℃~560℃ with the furnace. After the zinc ingot is completely melted, a zinc liquid is obtained; Al is added to the zinc liquid and the temperature is controlled at 540℃~560℃. After Al is completely melted in the zinc liquid, the temperature is kept at 10min~20min; then Sn is added to the zinc liquid. After it is completely melted, the temperature is controlled at 500℃~520℃ and the temperature is kept at 10min~20min;

[0024] S2. After the temperature of the zinc solution is reduced to 440°C to 460°C, Mg is added to the zinc solution and kept warm for 10min to 20min; after the temperature of the plating solution is reduced to 400°C to 420°C, Li is added to the zinc solution and stirred until Li is completely melted;

[0025] S3. Finally, a hot-dip galvanizing bath with a special ratio is obtained, and the temperature of the bath is maintained at 360°C to 420°C for standby use.

[0026] In S1, the mass purity of the zinc ingot is ≥99.995%, the mass purity of the magnesium is ≥99.95%, the mass purity of the aluminum is ≥99.95%, the mass purity of the tin is ≥99.99%, and the mass purity of the lithium is ≥99.99%;

[0027] In S2, when adding Mg and Li to the zinc solution, a pressure spoon is used to press the corresponding elements into the zinc solution and the mixture is stirred thoroughly;

[0028] In the method, after each component in the hot-dip galvanizing bath is completely melted, it is required to be fully stirred at a rate of 80 rpm to 120 rpm and then kept warm.

[0029] The application of the hot-dip galvanizing solution in the magnesium alloy hot-dip plating process comprises the following steps:

[0030] Step 1. Surface pretreatment of plated parts

[0031] Step 1.1: using a degreasing agent to alkaline-wash and degrease the magnesium alloy plated part, and then performing physical and mechanical polishing to obtain a polished magnesium alloy plated part;

[0032] Step 1.2: soaking the polished magnesium alloy plated part in an acetone solution for cleaning and then removing it, and drying it in a dry hot air environment to obtain a pretreated magnesium alloy plated part;

[0033] Step 2. Hot-dip galvanizing process

[0034] Step 2.1 fix the pretreated magnesium alloy plated part in a lifting device, place the plated part perpendicular to the surface of the hot-dip galvanizing solution, immerse the plated part in the hot-dip galvanizing solution at a set speed for immersion plating, and obtain a plated part;

[0035] Step 2.2: The immersion-plated part is taken out at a set speed at an angle of 70° to 80° to the fresh hot-dip galvanizing solution, and is successively air-cooled and water-cooled to room temperature to obtain a hot-dip galvanized magnesium alloy plated part;

[0036] When immersing or taking out, the surface needs to be scraped to remove the oxide slag on the surface of the plating solution to expose a fresh and bright surface of the plating solution.

[0037] In step 1.1, the degreasing agent is a sodium carbonate solution with a mass concentration of 60 g / L to 100 g / L; the alkaline washing degreasing process is to immerse the magnesium alloy plated part in the degreasing agent at room temperature for 3 min to 5 min;

[0038] The physical mechanical grinding is to use a grinding wheel machine to mechanically grind it to remove surface oxides, pollutants and impurities to expose the magnesium alloy plated substrate; the surface roughness of the magnesium alloy plated after grinding meets Ra=0.4μm~6.3μm;

[0039] In step 1.2, the cleaning temperature is 40°C to 80°C, the cleaning time is 1 min to 5 min; the drying temperature is 120°C to 150°C;

[0040] In step 2.1, the immersion speed is 0.05m / s to 0.15m / s;

[0041] When the alloy is a magnesium rare earth alloy, the immersion plating temperature is 360°C to 420°C, and the immersion plating time is 0.5min to 20min;

[0042] When the alloy is a conventional magnesium alloy, the immersion plating temperature is 380°C to 400°C, and the immersion plating time is 0.1min to 2min;

[0043] In step 2.2, the removal speed is 0.05 m / s to 0.15 m / s; and the air cooling time is 1 min to 10 min.

[0044] The hot-dip galvanizing low-temperature plating solution of the present invention is suitable for hot-dip galvanizing processes of various magnesium alloys, including but not limited to magnesium rare earth alloys (such as Mg-Gd, Mg-Y, Mg-Gd-Y and Mg-Zn-Y, etc.) and conventional magnesium alloys (such as AZ31, AZ80 and ZK60, etc.). According to the physical and chemical properties of different magnesium alloys, the specific components are Mg-Gd alloy, Mg-Y alloy, Mg-Gd-Y alloy and Mg-Zn-Y alloy; wherein, in the Mg-Gd alloy of the magnesium rare earth alloy, the mass percentage of Gd is 0.1% to 12%; in the Mg-Y alloy, the mass percentage of Y is 0.1% to 12%; in the Mg-Gd-Y alloy, the combined mass fraction of Gd and Y is 0.1% to 20%; in the Mg-Zn-Y alloy, the combined mass fraction of Zn and Y is 0.1% to 20%. The alloying elements in conventional magnesium alloys meet the element composition range of the corresponding alloy grade. It is particularly suitable for hot-dip galvanizing of rare earth magnesium alloys.

[0045] In the process of preparing the hot-dip galvanizing bath, the temperature at which the zinc ingot is completely melted is set to 540°C to 560°C, which cannot be too high or too low. If it is too high, the surface of the zinc liquid will be severely oxidized, and zinc ash or zinc slag will appear; if it is too low, it will not provide enough activation energy to melt the elements added later.

[0046] In the process of preparing hot-dip galvanizing bath, the metal elements are added to the melted zinc bath in the order of Al, Sn, Mg and Li, and the corresponding bath temperature during the addition needs to be carried out according to the requirements of steps S1 and S2 according to the physical metallurgical reaction between each element and the Mg element. When Al is added, the bath temperature is controlled at 540°C to 560°C; when Sn is added, the addition temperature is 500°C to 520°C; when Mg is added, the bath temperature is controlled at 440°C to 460°C, and finally Li is added, and the bath temperature is 400°C to 420°C.

[0047] In the process of preparing the hot-dip galvanizing bath of the present invention, the melting point of Al is relatively high, about 660°C, and a relatively high bath temperature is required to melt it first. A thin and dense aluminum oxide film is formed on the surface of the bath after adding Al, and its oxidation potential is greater than that of other subsequent elements, and it can be preferentially oxidized, effectively preventing subsequent oxidation of metal elements to form oxidized inclusions.

[0048] In the process of preparing the hot-dip galvanizing bath, the metal elements Mg and Li added in the present invention have active chemical properties and strong oxidizing properties, and are easy to generate more oxide slag under long-term high temperature, and the density is relatively light. The oxide slag is easy to float on the surface of the bath, and is easy to affect the immersion plating operation process during the immersion and withdrawal process. Therefore, the addition sequence is prevented after Al and Sn elements, and the immersion plating temperature is relatively low, which can greatly avoid the formation of oxide slag, stabilize the composition of the bath and the immersion plating operation, and is conducive to obtaining a good surface coating.

[0049] When adding corresponding elements (Mg and Li) to the zinc liquid, the present invention uses a pressure spoon to quickly press the corresponding elements into the zinc liquid, and fully stirs the corresponding elements to completely immerse them in the zinc liquid, thereby accelerating the melting reaction of the corresponding elements and the zinc liquid, while avoiding excessive contact with oxygen to cause excessive burning.

[0050] In the hot-dip galvanizing process provided by the present invention, the immersion temperature of the plating solution is maintained at 360°C to 420°C. The lower immersion temperature provides a wider process window range for low-melting-point magnesium alloy plated parts, has a longer immersion time tolerance range, and is convenient for process operation control. The lower immersion temperature is due to the composition of the plating solution under a special ratio of components.

[0051] The present invention utilizes the high activity of the magnesium alloy material itself to stimulate the subsequent hot-dip plating process and achieve the effect of "self-activation plating assistance".

[0052] The surface roughness of the plated part treated by the present invention needs to meet Ra=0.4μm~6.3μm. The roughness will have a significant impact on the subsequent hot dip plating operation. If the roughness is too large, the specific surface area of ​​the plated part will increase, the contact with the plating solution will be more sufficient, and the metallurgical reaction between the plating solution and the plated part will be too intense, which will produce a serious matrix ablation effect. If the roughness is too low, the surface of the plated part will be smooth, the wettability of the interface with the plating solution is poor, the wetting angle is less than 90°, and a good metallurgical bond cannot be formed, so obvious leakage plating will occur on the surface of the plated part.

[0053] The present invention places the plated part after physical and mechanical grinding in an acetone solution for cleaning, thereby removing particulate impurities or oxides, oil stains and the like remaining in the mechanical grinding process, so as to facilitate subsequent surface cleaning of the plated part.

[0054] The immersion plating temperature and immersion plating time ranges used in the present invention are very important process parameters in the hot-dip plating process of magnesium alloys, and according to the different base materials of the plated parts and the different physical properties of the base materials, appropriate immersion plating temperatures and immersion plating times are selected for hot-dip plating operations. Among them, the immersion plating temperature range of magnesium rare earth alloys is preferably 360°C to 420°C, and the immersion plating time is 0.5min to 20min. The conventional immersion plating temperature range is preferably 380°C to 400°C, and the immersion plating time is 0.1min to 2min.

[0055] In the process of hot-dip galvanizing the magnesium alloy, the galvanizing temperature and galvanizing time directly affect the protective effect of the coating. If the process parameters are not within this range, obvious serious defects such as missing plating, corrosion and zinc nodules will appear on the coating surface, thereby losing the protection of the substrate.

[0056] The immersion rate and withdrawal rate of the present invention are both 0.05m / s to 0.15m / s. When immersing or withdrawing the plating solution, the surface needs to be slag-stripped to remove the oxide slag on the surface of the plating solution to expose a fresh and bright plating solution surface. If the slag-stripping is not performed, a thicker oxide film is formed on the surface of the plating solution due to oxidation. After the plated part is immersed in the plating solution, the plating solution is withdrawn, and the oxide film adheres to the surface of the plated part, which seriously affects the surface quality and performance of the coating. In the present invention, the immersion angle is vertically immersed in the plating solution, and the immersion angle is 90° with the plating solution. When the plating solution is withdrawn, the withdrawal angle is 70° to 80° with the plating solution, which is convenient for controlling the rate at which the plating solution flows downward. When the withdrawal angle is too large, the downward flow rate of the plating solution is too fast, the thickness of the alloy layer on the surface of the coating is reduced, and the protectiveness of the coating is reduced. When the withdrawal angle is too small, the downward flow rate of the plating solution is too slow, which causes the zinc liquid on the surface of the plated part to stagnate on the surface of the plated part, thereby causing the surface thickness of the coating to be large and uneven, and failure behaviors such as coating peeling are prone to occur.

[0057] After the plated part is taken out of the plating solution, the present invention places it in the air to cool for 1min to 10min. The cooling time is adjusted accordingly according to different immersion plating process parameters. When the immersion plating temperature is low or the immersion plating time is short, a shorter air cooling time followed by water cooling is adopted. When the immersion plating temperature is high or the immersion plating time is long, a longer air cooling time followed by water cooling is adopted. The cooling time is mainly determined according to the solidification state of the coating surface.

[0058] In the process of hot-dip galvanizing the magnesium alloy, the present invention air-cools the plated parts after hot-dip galvanizing and then water-cools them, which is beneficial to improving the surface quality of the coating, improving the surface flatness of the coating, refining the surface microstructure of the coating, and improving the protectiveness of the coating.

[0059] The magnesium alloy hot-dip galvanizing process provided by the present invention can be used for various magnesium alloys of different systems, including magnesium rare earth alloys (such as Mg-Gd, Mg-Y, Mg-Gd-Y and Mg-Zn-Y, etc.), conventional magnesium alloys (such as AZ31, AZ80 and ZK60, etc.), and provides a magnesium alloy hot-dip galvanizing solution with a specific element ratio and accurate process parameters, and the obtained coating has uniform thickness, high uniformity, good protectiveness, and strong adhesion between the coating and the substrate. The hot-dip galvanizing process of the magnesium alloy has the characteristics of simple process, easy control, strong processability, low cost, etc., has a broad prospect for industrial promotion, and greatly improves the protectiveness of the magnesium alloy.

[0060] The novel magnesium alloy hot-dip plating process and the plating solution composition of the present invention have the following beneficial effects:

[0061] 1. The present invention proposes a new magnesium alloy hot-dip plating process technology and its plating solution composition applicable to various magnesium alloy materials according to the physical and chemical properties of various types of magnesium alloy plated materials, and provides a detailed process parameter range and detailed and practical process operation steps. The process is simple, has low requirements on equipment, and is low in cost. The process cost is significantly lower than other magnesium alloy surface treatment technologies, the process window range is wide, the process parameters are easy to control, and the obtained zinc-plated layer has excellent corrosion resistance and high hardness, which significantly improves the corrosion resistance and wear resistance of the magnesium alloy substrate. It is applicable to various types of magnesium alloy materials and provides accurate data reference and practical guidance for the industrial application research of magnesium alloy hot-dip galvanizing process.

[0062] 2. Aiming at the special physical and chemical properties of magnesium alloy materials, the present invention designs a special proportion of plating solution composition that can realize the hot dip plating process of magnesium alloy, namely, Zn-Al-Mg-Sn-Li alloy plating solution, and provides specific plating solution preparation process steps, wherein the order of adding various elements needs to be added in a specific order and at a specific temperature. Under this plating solution composition, it is suitable for low-melting-point magnesium alloy plating materials, and will not cause serious ablation effect on the plated parts. The prepared coating has good corrosion resistance and high hardness, significantly improves the protectiveness of the coating, has good bonding between the coating and the substrate, and forms excellent intermetallic compounds between the coating and the substrate. The intermetallic compounds formed are small and uniform in size and densely distributed between the coatings, effectively enhancing the comprehensive performance of the coating.

[0063] 3. The present invention provides a magnesium alloy surface pretreatment method that is particularly suitable for magnesium alloy hot dip plating process. The obtained plated surface is smooth and flat, with a roughness in the range of Ra = 0.4μm to 6.3μm, which can effectively promote the metallurgical reaction between the plated surface and the plating solution without excessive corrosion. This method uses a specific physical and mechanical polishing method to remove oxides on the surface of the plated part and obtain a smooth and bright metal matrix. It utilizes the high activity of the magnesium alloy itself to achieve self-activation plating of the magnesium alloy, and finally obtains good results. This surface pretreatment method has a simple process, does not require a large amount of chemical reagent solution to treat the magnesium alloy surface, and has a good plating effect, which is suitable for large-scale promotion.

[0064] 4. The present invention selects a special Zn-Al-Mg-Sn-Li according to the physical and chemical properties of magnesium alloys, and utilizes the combined effect of its multi-component metal elements to control the morphology, size and distribution of intermetallic compounds in the coating, thereby achieving a plating solution melting point below 320°C and good fluidity. Therefore, the present invention is particularly suitable for hot-dip plating processes of various magnesium alloy materials, and can prepare coatings with excellent protection, smooth coatings and good surface quality.

[0065] 5. The present invention provides a novel magnesium alloy hot-dip plating process and its plating solution composition, which can be applied to magnesium rare earth alloys (such as Mg-Gd, Mg-Y, Mg-Gd-Y and Mg-Zn-Y, etc.) and conventional magnesium alloys (such as AZ31, AZ80 and ZK60, etc.). According to the physical and chemical properties of different magnesium alloys, the specific composition is Mg-Gd alloy, Mg-Y alloy, and Mg-Gd-Y alloy; wherein, in the Mg-Gd alloy of the magnesium rare earth alloy, the mass percentage of Gd is 0.1% to 12%; in the Mg-Y alloy, the mass percentage of Y is 0.1% to 12%; in the Mg-Gd-Y alloy, the combined mass fraction of Gd and Y is 0.1% to 20%; in the Mg-Zn-Y alloy, the combined mass fraction of Zn and Y is 0.1% to 20%. Each alloying element in the conventional magnesium alloy meets the element composition range of the corresponding alloy grade.

[0066] 6. The present invention combines the cooling method of the plated part after immersion plating, utilizes the treatment method of air cooling for 1min to 10min and then water cooling, combines the immersion plating process parameters with the characteristics of the magnesium alloy material, adopts specific immersion rate, immersion angle, immersion time, withdrawal rate, withdrawal angle and other process parameters, controls the solidification state of the plating solution on the surface of the plated part, and finally prepares a coating with uniform and controllable thickness. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Figure 1 The effect diagram of the current hot-dip coating process for magnesium alloys; (a) is a macroscopic image of the hot-dip aluminum alloy coating, and (b) is a cross-sectional microstructure of the hot-dip aluminum alloy coating;

[0068] Figure 2 It is a schematic diagram of the hot-dip plating process of magnesium alloy of the present invention;

[0069] Figure 3 The magnesium alloy surface coating obtained in Example 1 has good quality;

[0070] Figure 4 It is a macroscopic image of the surface coating of the magnesium alloy in the comparative example. DETAILED DESCRIPTION

[0071] In the process of preparing the plating solution in the embodiment of the present invention, the raw materials selected are metal zinc, metal magnesium, metal aluminum, metal tin and metal lithium. Among them, the metal zinc is preferably a zinc ingot, the mass purity of the metal zinc is ≥99.995%, the mass purity of the metal magnesium is ≥99.95%, the mass purity of the metal aluminum is ≥99.95%, the mass purity of the metal tin is ≥99.99%, and the mass purity of the metal lithium is ≥99.99%.

[0072] The melting points of the hot-dip galvanizing baths in the embodiments of the present invention are all lower than 320°C.

[0073] The method for preparing the hot-dip galvanizing bath in the embodiment of the present invention comprises the following steps:

[0074] S1. First, place the zinc ingot in a smelting device and heat it to 540℃~560℃ with the furnace. After the zinc ingot is completely melted, a zinc liquid is obtained; Al is added to the zinc liquid and the temperature is controlled at 540℃~560℃. After Al is completely melted in the zinc liquid, keep it warm for 10min~20min; then, add Sn to the zinc liquid, control the temperature at 500℃~520℃ after it is completely melted, and keep it warm for 10min~20min;

[0075] S2. After the plating solution temperature is reduced to 440°C to 460°C, Mg is added to the zinc solution and kept warm for 10min to 20min; after the plating solution temperature is reduced to 400°C to 420°C, Li is added to the zinc solution and stirred thoroughly until Li is completely melted; when Mg and Li are added to the zinc solution, the corresponding elements are pressed into the zinc solution using a pressure spoon and stirred thoroughly; the stirring speed is 80rpm to 120rpm;

[0076] S3. Finally, a hot-dip galvanizing bath with a special ratio is obtained, and the temperature of the bath is maintained at 360°C to 420°C for standby use.

[0077] After each component in the hot-dip galvanizing bath in each embodiment of the present invention is completely melted, it needs to be fully stirred at a speed of 80 rpm to 120 rpm and then kept warm.

[0078] The magnesium alloy is a magnesium rare earth alloy or a conventional magnesium alloy; wherein the magnesium rare earth alloy is one of Mg-Gd, Mg-Y, Mg-Gd-Y and Mg-Zn-Y, and each alloying element in the conventional magnesium alloy satisfies the element composition range of the corresponding alloy grade. In the Mg-Gd alloy, the mass percentage of Gd is 0.1% to 12%; in the Mg-Y alloy, the mass percentage of Y is 0.1% to 12%; in the Mg-Gd-Y alloy, the combined mass fraction of Gd and Y is 0.1% to 20%; in the Mg-Zn-Y alloy, the sum of the contents of Zn and Y is 0.1% to 20%.

[0079] In the process of hot-dip galvanizing the magnesium alloy with the hot-dip galvanizing solution prepared by the present invention, when the alloy is a magnesium rare earth alloy, the immersion plating temperature is 360°C to 420°C, and the immersion plating time is 0.5min to 20min; when the alloy is a conventional magnesium alloy, the immersion plating temperature is 380°C to 400°C, and the immersion plating time is 0.1min to 2min.

[0080] The magnesium alloy hot dip plating process in the embodiment of the present invention is as follows: Figure 2 shown.

[0081] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0082] Example 1

[0083] The new magnesium alloy hot-dip galvanizing bath in this embodiment has the following composition elements by mass percentage: Al 0.1%, Mg 0.1%, Li 0.1%, and the balance is Zn and unavoidable impurities. The hot-dip galvanizing bath in this embodiment is used to perform hot-dip plating on Mg-Gd-Y magnesium rare earth alloy. In the Mg-Gd-Y magnesium rare earth alloy in the embodiment of the present invention, the mass fraction of Gd is 8%, and the mass fraction of Y is 4%.

[0084] The novel magnesium alloy hot-dip galvanizing bath in this embodiment is prepared according to the following method:

[0085] S1. According to the proportion of the plating solution, first place the zinc ingot in the smelting equipment, heat it to 550℃ with the furnace, and obtain zinc liquid after the zinc ingot is completely melted; add Al to the zinc liquid and control the temperature at 550℃. When Al is completely melted in the zinc liquid, stir it thoroughly and keep it warm for 10 minutes; then add Sn to the zinc liquid and control the temperature at 510℃ and keep it warm for 10 minutes;

[0086] S2. After the plating solution temperature is lowered to 450°C, Mg is added to the zinc solution, stirred thoroughly, and kept warm for 10 minutes; after the plating solution temperature is lowered to 410°C, Li is then added to the zinc solution, stirred thoroughly until Li is completely melted;

[0087] S3 finally obtains a hot-dip galvanizing bath with a special ratio, and the bath temperature is maintained at 380 ° C to obtain a hot-dip galvanizing bath;

[0088] After each component in the hot-dip galvanizing bath is completely melted, it is fully stirred at a rate of 80 rpm to 120 rpm and then kept warm.

[0089] The prepared hot-dip galvanizing solution is used to hot-dip galvanize the Mg-Gd-Y magnesium rare earth alloy according to the following steps:

[0090] Step 1. Surface pretreatment of plated parts:

[0091] Step 1.1. Using sodium carbonate with a mass concentration of 100 g / L as a degreasing agent, immerse the magnesium alloy in the degreasing agent and soak it at room temperature for 3 minutes. After alkali washing and degreasing the magnesium alloy plated part, it is physically polished with a grinding wheel to obtain a polished plated part; the surface roughness of the polished plated part is Ra = 4.3 μm.

[0092] Step 1.2. Soak the polished plated part in an acetone solution, wash it at 40°C for 1 min, take it out, and dry it with 120°C dry hot air to obtain a pre-treated plated part.

[0093] Step 2. Hot-dip galvanizing process

[0094] Step 2.1 Fix the pretreated magnesium alloy plated part in a lifting device, make the pretreated plated part contact with the hot-dip galvanizing solution at an immersion rate of 0.05 m / s, and immerse the plated part and the liquid surface of the hot-dip galvanizing solution vertically into the hot-dip galvanizing solution, and maintain the immersion temperature of the hot-dip galvanizing solution at 420°C. During the immersion process, the surface of the plating solution is slag-removed to remove the oxide slag on the surface of the plating solution, revealing a fresh and bright surface of the plating solution. After immersion for 0.5 minutes, the plated part is obtained;

[0095] Step 2.2: Take out the plated parts from the bath after immersion plating. During the process of taking out, the bath surface is slag-removed to remove the oxidized slag on the bath surface, exposing a fresh and bright bath surface. Take out the plated parts after immersion plating at a rate of 0.05 m / s and a 70° take-out angle with the fresh hot-dip galvanizing bath, place them in the air to cool for 1 min, and then water-cool them to obtain hot-dip galvanized magnesium alloy plated parts.

[0096] The present embodiment finally obtains a hot-dip galvanized layer with uniform coating thickness, good surface finish, strong corrosion resistance, strong adhesion between the coating and the substrate, and is not easy to fall off. The macroscopic morphology of the coating surface is as shown in the attached Figure 3 shown.

[0097] Example 2

[0098] This embodiment is the same as Embodiment 1, except that:

[0099] 1. The composition of the hot-dip galvanizing bath is, by mass percentage, Al 7%, Mg 4%, Sn 7%, Li 0.4%, and the balance is Zn and unavoidable impurities; the zinc ingot is placed in a smelting device and heated to 560°C with the furnace;

[0100] 2. Hot-dip galvanizing bath is used for hot-dip plating of Mg-Zn-Y magnesium rare earth alloy;

[0101] 3. During the hot-dip galvanizing process, the surface roughness of the polished plated parts is Ra=6.3μm; the plated parts are cleaned in an acetone solution at 80℃ for 5min; the drying temperature of the dry air is 150℃; the immersion rate of the plated parts is 0.15m / s, the immersion temperature is 360℃, and the immersion time is 20min; the plated parts after immersion are taken out at an angle of 80° to the fresh hot-dip galvanizing solution at a rate of 0.15m / s; the air cooling time is 10min.

[0102] This embodiment finally obtains a hot-dip galvanized layer with uniform coating thickness, good surface finish, strong corrosion resistance, strong adhesion between the coating and the substrate, and is not easy to fall off.

[0103] Example 3

[0104] This embodiment is the same as Embodiment 1, except that:

[0105] 1. The composition of the hot-dip galvanizing bath is as follows by mass percentage: Al is 4%, Mg is 3%, Sn is 6.8%, Li is 0.3%, and the balance is Zn and unavoidable impurities; the zinc ingot is placed in a smelting device and heated to 560°C with the furnace;

[0106] 2. Hot-dip galvanizing bath is used for hot-dip plating of AZ31 conventional magnesium alloy;

[0107] 3. During the hot-dip galvanizing process, the surface roughness of the polished plated parts is Ra=5.5μm; the plated parts are cleaned in an acetone solution at 60℃ for 2min; the drying temperature of the dry air is 130℃; the immersion rate of the plated parts is 0.10m / s, the immersion temperature is 390℃, and the immersion time is 10min; the plated parts after immersion are taken out at a rate of 0.10m / s at an angle of 75° to the fresh hot-dip galvanizing solution; the air cooling time is 8min.

[0108] In this embodiment, the AZ31 conventional magnesium alloy plated part is immersed in the plating solution at a temperature of 390°C for 10 minutes, then taken out of the plating solution, cooled in the air for 8 minutes, and then water-cooled to finally obtain a hot-dip galvanized layer with uniform coating thickness, good surface finish, strong corrosion resistance, strong adhesion between the coating and the substrate, and not easy to fall off.

[0109] Example 4

[0110] This embodiment is the same as Embodiment 1, except that:

[0111] 1. The composition of the hot-dip galvanizing bath is as follows by mass percentage: Al is 6%, Mg is 3%, Sn is 2%, Li is 0.2%, and the balance is Zn and unavoidable impurities; the zinc ingot is placed in a smelting device and heated to 560°C with the furnace;

[0112] 2. Hot-dip galvanizing bath is used for hot-dip plating process of ZK61 conventional magnesium alloy;

[0113] 3. During the hot-dip galvanizing process, the surface roughness of the plated parts after grinding is Ra=6.0μm; the plated parts are cleaned in an acetone solution at 50℃ for 2min; the drying temperature of the dry air is 120℃; the immersion rate of the plated parts is 0.15m / s, the immersion temperature is 380℃, and the immersion time is 5min; the plated parts after immersion are taken out at a rate of 0.15m / s at an angle of 70° to the fresh hot-dip galvanizing solution; the air cooling time is 1min.

[0114] In this embodiment, the ZK61 conventional magnesium alloy plated part is immersed in the plating solution at a temperature of 380°C for 5 minutes, then taken out of the plating solution, cooled in the air for 1 minute, and then water-cooled to finally obtain a hot-dip galvanized layer with uniform coating thickness, good surface finish, strong corrosion resistance, strong adhesion between the coating and the substrate, and not easy to fall off.

[0115] Comparative Example 1

[0116] This comparative example is the same as Example 1, except that:

[0117] During the hot-dip galvanizing process, the surface roughness of the polished plated parts is Ra=12.5μm; the plated parts are cleaned in an acetone solution at 40°C for 1 min; the dry air drying temperature is 120°C; the plated parts are immersed at a rate of 0.05m / s, the immersion temperature is 350°C, and the immersion time is 5min; the plated parts after immersion are taken out at an angle of 70° to the fresh hot-dip galvanizing solution at a rate of 0.05m / s; the air cooling time is 1min.

[0118] In Comparative Example 1, the Mg-Gd-Y magnesium rare earth alloy plated part was mechanically polished to a roughness of Ra = 12.5 μm, and was immersed in the plating solution for 5 min at a temperature of 350°C, then taken out of the plating solution, cooled in the air for 1 min, and then water-cooled. The final coating showed obvious leakage plating, uneven thickness of the coating surface, and obvious defects. The appearance of the coating is as shown in the attached figure. Figure 4 .

[0119] The physical mechanical grinding method uses a grinding wheel to grind it, and has specific requirements for the surface roughness of the plated parts, requiring the surface roughness of the plated parts to be Ra=12.5μm; the roughness significantly affects the hot dip plating process. If the roughness is too high, the specific surface area of ​​the plated parts increases, resulting in closer contact with the plating solution, thereby causing excessively intense metallurgical reactions between the plating solution and the plated parts, which may cause severe melting of the substrate. On the contrary, if the roughness is too low, the surface of the plated parts is too smooth, the wettability of the interface with the plating solution is insufficient, and the wetting angle is less than 90°, which will hinder the formation of a good metallurgical bond, resulting in obvious leakage on the surface of the plated parts.

Claims

1. A novel magnesium alloy hot-dip galvanizing bath, characterized in that: The hot-dip galvanizing solution is used for hot-dip galvanizing of magnesium alloys; The hot-dip galvanizing bath comprises the following components by mass percentage: Al 0.1% to 7%, Mg 0.1% to 4%, Sn 0 to 7%, Li 0.1% to 0.4%, and the balance is Zn and unavoidable impurities; The hot-dip galvanizing bath has a melting point lower than 320°C.

2. A novel magnesium alloy hot-dip galvanizing bath according to claim 1, characterized in that: The magnesium alloy is a magnesium rare earth alloy or a conventional magnesium alloy; wherein the magnesium rare earth alloy is one of Mg-Gd, Mg-Y, Mg-Gd-Y and Mg-Zn-Y, and each alloying element in the conventional magnesium alloy satisfies the element composition range of the corresponding alloy grade; In the Mg-Gd alloy, the mass percentage of Gd is 0.1% to 12%; in the Mg-Y alloy, the mass percentage of Y is 0.1% to 12%; in the Mg-Gd-Y alloy, the combined mass fraction of Gd and Y is 0.1% to 20%; in the Mg-Zn-Y alloy, the sum of the Zn and Y contents is 0.1% to 20%.

3. The method for preparing the novel magnesium alloy hot-dip galvanizing bath according to claim 1 or 2, characterized in that: The following steps are involved: S1. Place a zinc ingot in a smelting device and heat it to 540°C to 560°C with the furnace. After the zinc ingot is completely melted, a zinc liquid is obtained. Al is added to the zinc liquid. After Al is completely melted in the zinc liquid, the temperature is kept at 540°C to 560°C. Subsequently, Sn is added to the zinc liquid. After Sn is completely melted, the temperature is controlled at 500°C to 520°C and the temperature is kept. S2. After the temperature of the zinc liquid is reduced to 440°C to 460°C, Mg is added to the zinc liquid, and the temperature is kept after Mg is completely melted; after the temperature of the zinc liquid is reduced to 400°C to 420°C, Li is continuously added to the zinc liquid, and the mixture is stirred until Li is completely melted; S3. Finally, a hot-dip galvanizing bath with a special ratio is obtained, and the temperature of the bath is maintained at 360°C to 420°C for standby use.

4. The method for preparing a novel magnesium alloy hot-dip galvanizing bath according to claim 3, characterized in that: In the method, after each component in the hot-dip galvanizing solution is completely melted, it is required to be fully stirred at a rate of 80 rpm to 120 rpm and then kept warm for 10 min to 20 min.

5. The method for preparing a novel magnesium alloy hot-dip galvanizing bath according to claim 3, characterized in that: In S1, the mass purity of zinc ingot is ≥99.995%, the mass purity of magnesium is ≥99.95%, the mass purity of aluminum is ≥99.95%, the mass purity of tin is ≥99.99%, and the mass purity of lithium is ≥99.99%; In S2, when Mg and Li are added to the zinc liquid, a pressure spoon is used to press the corresponding elements into the zinc liquid and stir it thoroughly.

6. Application of the novel magnesium alloy hot-dip galvanizing solution according to claim 1 or 2 in a magnesium alloy hot-dip galvanizing process, characterized in that: The following steps are involved: Step 1. Surface pretreatment of plated parts Step 1.1: using a degreasing agent to alkaline-wash and degrease the magnesium alloy plated part, and then performing physical and mechanical polishing to obtain a polished magnesium alloy plated part; Step 1.2: soaking the polished magnesium alloy plated part in an acetone solution for cleaning and then removing it, and drying it in a dry hot air environment to obtain a pretreated magnesium alloy plated part; Step 2. Hot-dip galvanizing process Step 2.1 fix the pretreated magnesium alloy plated part in a lifting device, place the plated part perpendicular to the surface of the hot-dip galvanizing solution, immerse the plated part in the hot-dip galvanizing solution at a set speed for immersion plating, and obtain a plated part; Step 2.2: The immersion-plated part is taken out at a set speed at an angle of 70° to 80° to the fresh hot-dip galvanizing solution, and is successively air-cooled and water-cooled to room temperature to obtain a hot-dip galvanized magnesium alloy plated part; When immersing or taking out, the surface needs to be scraped to remove the oxide slag on the surface of the plating solution to expose a fresh and bright surface of the plating solution.

7. The use of the novel magnesium alloy hot-dip galvanizing solution according to claim 6 in a magnesium alloy hot-dip galvanizing process, characterized in that: In step 1.1, the degreasing agent is a sodium carbonate solution with a mass concentration of 60 g / L to 100 g / L; the alkaline washing degreasing process is to immerse the magnesium alloy plated part in the degreasing agent at room temperature for 3 min to 5 min; The physical mechanical grinding is to use a grinding wheel machine to mechanically grind it to remove surface oxides, pollutants and impurities to expose the magnesium alloy plated substrate; the surface roughness of the magnesium alloy plated after grinding meets Ra=0.4μm~6.3μm.

8. The use of the novel magnesium alloy hot-dip galvanizing solution according to claim 6 in a magnesium alloy hot-dip galvanizing process, characterized in that: In step 1.2, the cleaning temperature is 40°C to 80°C, the cleaning time is 1 min to 5 min; the drying temperature is 120°C to 150°C.

9. The use of the novel magnesium alloy hot-dip galvanizing solution according to claim 6 in a magnesium alloy hot-dip galvanizing process, characterized in that: In step 2.1, the immersion speed is 0.05m / s to 0.15m / s; When the alloy is a magnesium rare earth alloy, the immersion plating temperature is 360°C to 420°C, and the immersion plating time is 0.5min to 20min; When the alloy is a conventional magnesium alloy, the immersion plating temperature is 380° C. to 400° C., and the immersion plating time is 0.1 min to 2 min.

10. The use of the novel magnesium alloy hot-dip galvanizing solution according to claim 6 in a magnesium alloy hot-dip galvanizing process, characterized in that: In step 2.2, the removal speed is 0.05 m / s to 0.15 m / s; and the air cooling time is 1 min to 10 min.

Images