RU2251596C2 - Method for coating articles of aluminum silicon-containing alloys - Google Patents

Abstract

FIELD: machine engineering, possibly improving operational properties of surfaces of aluminum and aluminum-silicon alloys.

SUBSTANCE: method comprises steps of preliminary treatment of alloy and micro-arc oxidizing. Preliminary treatment is realized until formation of such alloy structure in which particles of silicon-containing phases are in the form of separate inclusions and (or) their strings and are arranged in such a way that on flat cut of material mean distance between boundaries of adjacent silicon-containing inclusions is no more than 5% of mean linear size of inclusions. Invention provides comparatively low energy consumption process for depositing high quality ceramic coating by micro-arc oxidizing on aluminum alloys with silicon content more than 3%.

EFFECT: enhanced adhesion of coating and alloy, possibility of optimizing according to concrete demands micro-hardness, porosity, roughness and thickness of uniform-thickness coating due to lowered screening effect of silicon-containing phases and changed structural parameters of alloy.

2 cl, 4 tbl, 9 dwg

Description

The field of technology.

The invention relates to the field of mechanical engineering and materials science and can be used to improve the operational properties of the surfaces of aluminum products, including aluminum-silicon alloys. The invention may find application, in particular, in mechanical engineering and other industries where aluminum alloys are used. Cast and wrought aluminum alloys, due to their physicochemical and mechanical properties, are high-tech materials and have already found wide application in the manufacture of parts in devices where reducing the total weight of the product is extremely desirable, for example, in the automotive industry. At the same time, the task of increasing the wear resistance of these parts and their working surfaces is always urgent. One of the modern ways of hardening work surfaces is the application of ceramic coatings by oxidation.

The prior art.

A known method of hardening the surface of parts from valve, including aluminum, alloys based on microarc oxidation (MAO), which consists in the fact that before the MAO the surface of the product is preliminarily subjected to surface plastic deformation to a shear strain of 0.3-0.9. This increases the thickness of the coating, microhardness, decreases the roughness (Patent RU No. 2085615). However, this method does not provide a high-quality coating with good adhesion to the oxidizable alloy with a silicon content of more than 3% in view of the strong shielding effect from silicon-containing phases (CF).

There is also a method of producing hard protective coatings on products from aluminum alloys, including anodic-cathodic oxidation in an alkaline electrolyte with a temperature of 15-50 ° C, using alternating current frequency of 50-60 Hz, in which at the initial stage of the process for 5-90 seconds, oxidation is carried out at a current density of 160-180 A / dm ² , and then the current density is reduced to 3-30 A / dm ² and the process is conducted in the mode of spontaneous reduction of power consumption until a specified coating thickness is obtained (WO 99/31303). With MAO of silicon-containing aluminum alloys, the initial sharply increased current mode is necessary to suppress the screening effect from CF.

However, this method requires high energy costs, especially in the initial stage, to achieve a current density of 160-180 A / dm ² , therefore, its use in conditions of large volumes of output of products with a protective coating is economically inexpedient.

Disclosure of the invention.

The objective of this invention is to develop a relatively non-energy-intensive method for producing a high-quality ceramic coating by microarc oxidation on aluminum alloys containing mainly more than 3% silicon, which provides increased adhesion to the oxidizable alloy and the possibility of optimization for specific operational requirements (conditions) of microhardness, porosity , the roughness and thickness of the coating, as well as its equal thickness (waviness of the “coating - base” interface) by reducing the screening effect of silicon-containing phases due to changes in the structure parameters of the oxidized alloy.

The problem is solved using the method of coating on aluminum products containing silicon alloys, including preliminary processing of the alloy and microarc oxidation, in which the preliminary processing of the alloys is carried out until a structure is formed in which particles of silicon-containing phases are arranged as separate inclusions and / or their chains so that on a flat cut of the material, the average distance between the boundaries of adjacent silicon-containing inclusions is more than 5% of the average a certain size of inclusions.

Before MAO products from aluminum alloys containing predominantly more than 3% silicon, by the action of energy, physical, mechanical, chemical, metallurgical or other methods or their combination at separate stages of the technological process, an oxidizable alloy structure is formed in which CF particles are arranged as separate inclusions or their chains so that the average distance A (see FIG. 1) between the nearest boundaries of various inclusions observed on a flat cut of the material (thin section), taking into account x distribution in the plane of the section was more than 5% of the average of the maximum linear dimensions in these inclusions, where A and B are defined as follows:

1) segments are drawn that connect the boundaries of the various inclusions closest to each other in the shortest possible way;

2) segments are drawn that connect the boundaries of the inclusions closest to each other and included in the various groups of inclusions formed after the first operation (p. 1) (the second operation is repeated until there are separate inclusion groups left on the analyzed section);

3) the value of A is defined as the average length of the segments obtained after previous steps;

4) the value of B is determined as the average value of the maximum linear dimensions of inclusions measured in the thin section plane.

After preliminary processing of the alloy, microarc oxidation is carried out. The electrolyte used is mainly an aqueous solution of potassium hydroxide (1-2 g / l) and water glass Na ₂ SiO ₃ (2-4 g / l).

As a result, a coating is formed with the adhesion and mechanical adhesion required under specified operating conditions with the oxidizable alloy, microhardness, cohesion, porosity, roughness, thickness and uniform thickness of the coating, as well as the undulation of the “coating-base” interface.

The following is an example of the implementation of this method when developing a technology for hardening the working surface of an aluminum cylinder block.

The experimental samples from AK6M2 aluminum casting alloy (GOST 1583; ASTM standard analogue: SG 64D, Table 1) were made of chill mold (casting size: diameter 45 mm, height 15 mm). Then the samples were subjected to microarc oxidation both in the initial state and after various heat treatments (table 2). Next, microstructural analysis of the base and coatings obtained on these samples by the central heating center method was carried out on transverse sections. The alloy structures (bases) are shown in Fig.2-6.

Table 1 The chemical composition of the alloy AK6M2 Element Mg% Si% Mn% CU% Ti% Ni% Fe% Zn% Pb% Sn% Other according to GOST 1583 0.30-0.45 5.5-6.5 0.10 1.8-2.3 0.10-0.20 0.05 0.60 0.06 - - - in fact 0.44 6.5 0.05 2.28 0.11 - 0.54 - - - - table 2 Heat treatments before microarc oxidation of AK6M2 alloy Designation in the text Heat treatment mode T2 aging 4 hours 235 ± 5 ° C T6 (1) homogenizing annealing for 4 hours at 515 ± 5 ° С followed by quenching from 515 ± 5 ° С in water heated to Т> 80 ° С + aging 4 hours at 235 ± 5 ° С T6 (2) homogenizing annealing for 8 hours at 515 ± 5 ° С followed by quenching from 515 ± 5 ° С in water heated to Т> 80 ° С + aging 4 hours at 235 ± 5 ° С T6 (3) homogenizing annealing for 12 hours at 515 ± 5 ° С followed by quenching from 515 ± 5 ° С in water heated to Т> 80 ° С + aging 4 hours at 235 ± 5 ° С

In general, the microstructure of all the studied variants is characterized by the presence of reinforcing intermetallic compounds in the α phase and eutectic (α + CF). The main difference is observed in the shape and size of CF inclusions. For samples without heat treatment and processed according to T2, the structure does not have significant metallographic differences (Figs. 2 and 3): a finely dispersed eutectic containing CF is located along the grain boundaries of the α phase in the form of solid sections. After heat treatment according to T6, coagulation and spheroidization of CF is observed, and the development of these processes is stronger, the longer the duration of homogenizing annealing before quenching (Figs. 4-6). That is, in comparison with T2, heat treatment according to T6 and an increase in the duration of homogenizing annealing at T6 lead to a decrease in the continuity of eutectic sections along the grain boundaries of the α phase.

The results presented below prove that the properties of ceramic coatings obtained on samples of the same alloy under the same MAO regime are determined by its initial structure, which forms depending on the heat treatment mode, which is due to the screening effect from CF, which is expressed in the fact that CF prevents coating growth during MAO (see below).

7 shows the microstructure of coatings obtained by the MAO method on samples AK6M2 with various preliminary heat treatment. It is seen that when preparing microsections on samples without heat treatment, cracking is observed (Fig. 7a) and coating peeling off (Fig. 7b). On samples with heat treatment according to T2, a main crack is observed along the entire coating – base interface (Fig. 7c), which indicates poor coating adhesion. On samples with heat treatment according to T6 with different durations of homogenizing annealing, cracking and peeling of the coatings are not observed, the interface “coating - base” is continuous, without cracks (for example, for heat treatment according to T6 with a short exposure time during homogenizing annealing, see fig.7g) .

Table 3 presents the results of measurements of the thickness and hardness parameters of coatings obtained by the MAO method. Note that for samples without heat treatment during the preparation of the thin section, it is not possible to preserve the coating continuity (Figs. 7a-b), which indicates its weak adhesive and cohesive properties (the coating is chipped at a length of up to 95% of the total length of the interface “coating - metal"). Therefore, the thickness for the sample without heat treatment was measured only in the remaining area with the coating. Nevertheless, for samples without heat treatment, even in the remaining area, it should be noted that the coefficient of variation of the coating thickness varies sharply, which indicates the greatest difference in thickness compared to other investigated options. We also note that the smallest thickness difference is observed for the variant with preliminary heat treatment according to T6 (2).

Table 3 Coating Properties Heat treatment options for AK6M2 Thickness mm Coefficient of variation of thickness Hardness, HVO, 1 Coefficient of variation of hardness without heat treatment 0.148 ± 0.076 0.632 758 ± 53 0.161 T2 0.116 ± 0.008 0.195 566 ± 73 0.294 T6 {1) 0.106 ± 0.006 0.161 784 ± 67 0.195 T6 (2) 0.109 ± 0.005 0.134 809 ± 7b 0.215 T6 (3) 0.131 ± 0.010 0.231 731 ± 91 0.285

The results of measuring the hardness HV0.1 and the coefficient of variation of the hardness of the coating presented in the table indicate a large influence of the preliminary heat treatment modes on these values. For coatings obtained by the MAO method, it is known that the greater the coefficient of variation of microhardness, the greater the porosity. Thus, according to the table, it can be concluded that coatings obtained on samples with heat treatment according to T6 (1) and T6 (2) have the least porosity. Coatings obtained on samples with heat treatment according to T2 and T6 (3) have approximately equal porosity at different hardness, which indicates other structural differences between these options.

It was also found that the roughness on the AK6M2 alloy samples coated with the MAO method, necessary to ensure the required wear resistance of the friction pair “upper piston compression ring - sleeve”, can only be obtained after heat treatment according to T6 (1) and T6 (2).

·100%≈45%>5%, что подтверждает эффективность указанного в формуле изобретения критерия получения качественного покрытия методом МДО на апюминиево-кремниевых сплавах.Figure 1 shows a schematic structure of the sample with heat treatment according to T6 (1), obtained by computer processing of the corresponding real microstructure (figure 4). On the schematic structure, CF particles (gray areas) are visible. Also, the schematic structure shows the segments indicating the distance between the particles of CF in accordance with the algorithm given in p. 3 of the description. According to this algorithm, for the schematized structure shown in Fig. 7, we have

· 100% ≈45%> 5%, which confirms the effectiveness of the criterion specified in the claims for obtaining high-quality coating by the MAO method on apuminium-silicon alloys.

Thus, the modes of preliminary heat treatment of the AK6M2 aluminum alloy are crucial for the quality of coatings obtained by the MAO method. This changes the thickness, hardness, porosity, uniformity, adhesion and cohesion, as well as the achievable level of roughness of the coating and, ultimately, the wear resistance of the coating and counterbody. A preliminary heat treatment mode was found that allows one to form a discontinuous structure of CF chains and obtain a high-quality MAO coating with good adhesion and the required level of roughness after finishing machining. In this case, the wear of the counterbody paired with the coating obtained by the MAO method is less or at least comparable to its wear paired with a specimen of Gh 190 V (normal Fiat-VAZ 52205 for casting cylinder blocks). In order to test on real parts of the piston group, the liners for the cylinder block were made of AK6M2 alloy with a structure optimized for MAO according to this method. The liners were pressed into a cast-iron block and oxidized, which simulated a monoblock of aluminum alloy (Fig. 8). Further, this unit was installed on the engine and passed bench tests. The oxidized working surface of the liners practically did not wear out, and the wear of the piston rings was less or, in some cases, at the level of their wear when working with a cast-iron block. The method of influencing the structure presented in the above example can be attributed to energy.

To assess the “effect of structural modification by introducing special alloys on the oxidizability of aluminum-silicon alloys, the study was carried out on samples of AK12MMgN alloy (table 4) obtained by casting in a chill mold (casting size: diameter 45 mm, height 15 mm). Next, both samples were subjected to heat treatment according to the T6 mode (1) (see table 1.) Fig. 9 shows the structures of samples of the AK12MMgN alloy after T6 (1), of which the first sample was cast without the use of special structure modifiers (Fig. 9a), and the second sample - with the introduction of raspl in the ligature AdSr10 (Fig.9b).

Table 4 The chemical composition of the alloy AK12MMgN Element Mg% Si% Mn% Cu% Ti% Ni% Fe% Zn% Pb% Sn% Other according to GOST 1583 0.8-1.3 11.0-13.0 0.2 0.5-1.5 0.2 0.8-1.3 0.7 0.2 0.05 0.01 -

From a comparison of the microstructures presented in Fig. 10, it can be seen that, from the point of view of the screening effect of CF during MAO, when a modifier is introduced, the structure becomes more transparent.

Next was the MAO. As in previous cases, the adhesion of the coating turned out to be higher with greater, from the point of view of the screening effect from KF, transparency of the structure, which in this case corresponds to the alloy AK12MMgN with AlSr10 alloy (Fig.10b).

To check the complex effect of the treatments on increasing the transparency of the structure from the point of view of the screening effect of KF, AK12MMgN alloy was used (see table 4), which was cast without the use of special modifiers and had a needle-like structure of silicon-containing phases. After the manufacture of steep samples from castings, they were surface treated by deformation by a roller with slipping. After that, the treatment was performed according to T6 (1) (table 2). As a result, we obtained a refinement of the structure in the near-surface region, an increase in the uniformity of the distribution of CF and the degree of its spheroidization, which led to an increase in the transparency of the structure from the point of view of the screening effect from CF. The coating quality was obtained at the coating level on samples cast using AlSr10 modifying ligature (see above).

Other previous MDO methods of influencing the structure are also possible, which allow optimizing it in the direction required to create high-quality coating. These are physical methods (for example, ultrasonic action during crystallization of the melt, leading to grinding of the structural components of the material), mechanical (for example, volumetric or surface deformation by various methods, including with complex movement of the deforming tool, leading to grinding and redistribution of the structural components in the original material), metallurgical (for example, in addition to those described above, holding the metal before crystallization for a long time in the molten state, which drive um to grinding the structure) and other methods. A combination of different methods is possible. For example, surface deformation for grinding particles of CF with their subsequent heat treatment for spheroidization of the crushed particles, which leads to an increase in the ratio A / B.

Industrial applicability.

The proposed method can find application in the application of wear-resistant coatings to parts made of aluminum containing silicon alloys operating in abrasive and aggressive environments, in particular to pistons and cylinder liners of internal combustion engines, working parts of compressor equipment, bearings, shutoff valves, heat exchangers, etc. .P. EFFECT: invention improves adhesion and mechanical adhesion of a coating with an oxidizable alloy and optimizes the alloy for specific operational requirements at low energy costs.

Claims (2)

1. A method of obtaining a coating on products made of aluminum containing silicon alloys, including pretreatment of the alloy and microarc oxidation, characterized in that the pretreatment of the alloys is carried out before the formation of such a structure in which particles of silicon-containing phases are arranged as separate inclusions and / or chains thereof so that on a flat cut of the material, the average distance between the boundaries of adjacent silicon-containing inclusions is more than 5% of the average linear size of the inclusions. 2. The method according to claim 1, characterized in that the preliminary processing of the alloys is carried out by exposure to energy, physical, mechanical, chemical, metallurgical or other methods, or a combination thereof.

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