RU2716612C1 - Method for hybrid treatment of magnesium alloys - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates to metallurgy, particularly to treatment of magnesium alloys, which can be used in production of structural or bioresorbable materials. Method of processing magnesium alloys involves homogenizing annealing, comprehensive isothermal forging and isothermal rolling. Homogenizing annealing is carried out at temperature of 350÷450 °C. All-around isothermal forging is performed in steps of 400÷300 °C with pitch 25 °C and with gradual increase of settling speed from 2 to 20 mm/min with provision of total true degree of deformation in range of 8÷10. Isothermal rolling is carried out at temperature of 300÷250 °C in several passes with degree of deformation in each pass of not more than 5 % and total degree of true deformation by rolling by about 1.EFFECT: improved ductility of magnesium alloys with simultaneous improvement of their strength and fatigue properties.1 cl, 3 dwg, 1 ex

Description

The invention relates to the field of engineering and aerospace industries, as well as medical materials science, where magnesium-based alloys can be used as structural or bioresorbable materials. A method for processing magnesium alloys includes homogenizing annealing, comprehensive isothermal forging, and isothermal rolling. Homogenizing annealing is carried out at a temperature of 350 ÷ 450 ° C. Comprehensive isothermal forging is carried out in steps in the temperature range of 400 ÷ 300 ° C in increments of 25 ° C and with a gradual increase in the precipitation rate from 2 to 20 mm / min, providing the total true degree of deformation in the range of 8 ÷ 10. Isothermal rolling is carried out at a temperature of 300 ÷ 250 ° C in several passes with a degree of deformation in each pass of not more than 5% and a total degree of true deformation by rolling of the order of 1. The technical result of the invention is to increase the ductility of magnesium alloys while increasing their strength and fatigue properties.

Interest in magnesium alloys is due to their favorable properties, the most important of which is their low specific gravity and high specific strength. If we compare magnesium with other metals, then its specific gravity is about a quarter of the specific gravity of steel, two-thirds of the weight of aluminum and almost two-fifths of the specific gravity of titanium. Therefore, at present, magnesium alloys, along with aluminum and titanium, are of great interest to the aviation and aerospace industries. The use of magnesium alloys in technology makes it possible to reduce the mass of the structure by 10–30%, which ultimately allows to significantly reduce both production and operational energy costs. In addition, magnesium has significantly better damping characteristics compared to aluminum and steel. These advantages, as well as the fact that magnesium is widely distributed in nature, expand the field of its use in technology.

Another promising and dynamically developing area of using magnesium and alloys based on it is their use in medicine in connection with the highest structural efficiency, expressed by an extremely attractive strength to density ratio and almost ideal biocompatibility: magnesium is an element involved in more than 300 biochemical reactions in the body, including processes that form bones and muscles. In addition, it is magnesium that is a unique material for medical use due to its gradual resorbability. It dissolves in the human body, forming fairly simple compounds (oxide and hydroxide), which are not only non-toxic, but even contribute to the healing of tissues.

Conducted in many countries of the world, such as the USA, Japan, Russia, China, Germany, Ukraine, Australia, etc., studies have shown that, along with the advantages of magnesium, there are also a number of disadvantages that limit its use in medicine. Firstly, pure magnesium has a high corrosion rate even in non-aggressive environments such as blood and other body fluids. In addition, the corrosion process is usually accompanied by active pitting, which negatively affects the mechanical properties of the product. To eliminate this drawback, magnesium is alloyed with various elements, such as calcium, zinc, lithium, silver, manganese, and some rare earth elements. The choice of an alloying system is complicated by the condition that the alloying element itself, as well as corrosion products formed subsequently, should not be toxic to the body. The second problem is that, although magnesium has a level of mechanical properties close to the level of bone tissue (Young's modulus is 5 ÷ 55 MPa and 45 MPa for bone tissue and magnesium, respectively), in practice this may not be enough, since for its successful applications as orthopedic implants and fastening structural elements, significantly higher strength characteristics are desirable - at the level of 400 MPa and even higher, depending on the specific application. Therefore, there is a need for hardening of magnesium alloys. Alloying, performed to improve corrosion resistance, also increases the mechanical characteristics to some extent, but their necessary level can be achieved by grinding grain up to an ultrafine-grained (UFG) structure. The formation of the UFG structure, in contrast to the usual grinding of grain to sizes above 1 ÷ 2 μm, leads not only to a significant hardening of magnesium alloys, but also often does not worsen, and in some cases improves the corrosion resistance of magnesium alloys. Therefore, obtaining the UFG structure in magnesium alloys is a promising and relevant direction in physical materials science.

The final (consumer) properties of materials are determined not only by their chemical composition, but also to a large extent by the design of the microstructure: grain size and distribution, phase particle distribution, crystallographic texture, etc. To obtain the necessary microstructure, a wide range of methods of deformation thermomechanical processing is currently developed. While traditional processing methods, such as extrusion and rolling, are convenient for obtaining semi-finished products with a strong crystallographic texture, the use of intensive plastic deformation methods allows not only to substantially grind the microstructure to submicron sizes and to achieve a much more uniform distribution of particles of the strengthening phases, but also to form significantly weaker texture. Hybrid technologies combining various combinations of deformation methods possess the greatest flexibility.

The choice of a deformation thermomechanical processing scheme is determined by both purely technological factors of the possibility of implementing one or another scheme for a given workpiece geometry (for example, given sizes of initial ingots), and the effectiveness of various schemes for the formation of a particular microstructure and crystallographic texture. There are a very large number of schemes for processing magnesium alloys, ranging from traditional ones such as direct and reverse extrusion and rolling, and ending with efficient schemes that allow obtaining very large degrees of deformation and a highly ground structure in workpieces - these are methods of intense plastic deformations, including torsion under hydrostatic pressure , equal channel angular pressing (ECAP), comprehensive isothermal forging (VIC), rotary forging (RC) and many others.

An example is the method of combined intensive plastic deformation of a metal plate (RU 2514239 C2, IPC В21С 25/00, application date 05/06/2012), which involves deformation of a plate by channel angular pressing by forcing a plate through intersecting first and second channels of the matrix with bending along its height in the first channel, the bottom of which is made wavy, and with a change in the shape of its cross section in the second channel having a cross section in the form of a corrugation. The uniformity of deformation over the cross section of the plate and the possibility of multiple hardening of the plate with an indirect profile are ensured by the fact that the front end of the plate is in the form of the bottom of the first channel, and repeated pressing cycles of the plate with a changed cross-sectional shape after the first cycle are performed using the device for repeated pressing cycles, the first channel of which is performed with a cross section similar to the cross section of said second channel.

A known method of combined intensive plastic deformation of billets (RU 2529604 C1, IPC B21J 5/06, C22F 1/18, B82B 3/00, application date 04/08/2013), which consists in the fact that to obtain nanocrystalline billets of metals and alloys with improved physical and mechanical properties produce equal channel angular pressing of a cylindrical workpiece. In this case, an ultrafine-grained structure with a grain size of 200–300 nm is formed in the workpiece metal. Then the workpiece is cut into disks, each of which is subjected to intense plastic deformation by torsion using two rotating strikers. Torsion deformation is carried out at room temperature under a pressure of 4-6 GPa with a number of revolutions of the strikers n≤2. This ensures the formation of a homogeneous nanocrystalline structure with a grain size of ≤100 nm. As a result, the physicomechanical properties of the processed metal are improved.

There is also a known method of processing a magnesium alloy of the Mg-Al-Zn system by rotational forging (RU 2664744 C1, IPC C22F 1/06, application filing date 28.11.2017), which includes preliminary heat treatment by homogenizing annealing at a temperature of 450 ÷ 500 ° C and rotational forging, and rotational forging is carried out stepwise in the temperature range 400 ÷ 350 ° C with a total true degree of deformation of 2.5 ÷ 3, while forging at each step is carried out at a temperature of 25 ° C below the previous step to obtain a structure consisting of grains medium size ohm less than 5 microns saturated with deformation twins. The technical result of the invention is to increase the strength of the magnesium-based alloy of the Mg-Al-Zn system with a simultaneous increase in its ductility.

The closest in essence to our invention can be considered a method of processing a magnesium alloy of the Mg-Y-Nd-Zr system by the method of equal-channel angular pressing (RU 2678111 C1, IPC C22F 1/06, filing date 05/21/2018), including homogenizing annealing at a temperature of 500 ÷ 530 ° C for 7 ÷ 9 hours followed by cooling in air and equal channel angular pressing, which is carried out stepwise in the temperature range 425 ÷ 300 ° C with a total true degree of deformation of 6.0 ÷ 8.0, with equal channel angular pressing at each stage lyayut at a temperature of 25 ° C below the temperature of the preceding stage to obtain a structure consisting of a grain size less than 1 micron. The technical result of the invention is to increase the ductility of the alloys of the Mg-Y-Nd-Zr system while maintaining sufficient strength by changing the preferred deformation mechanism from basic to prismatic sliding.

All the mentioned methods of processing magnesium alloys have a significant drawback - they are not universal relative to the product range. Almost every new type of product requires the manufacture of a new type of equipment and the attraction of additional technological equipment. In addition, when processing large ingots with such methods of deformation processing as, for example, ECAP, there are insurmountable difficulties at the current technical level due to the need to use huge efforts in the press.

The aim of the present invention is to provide a method for the hybrid processing of magnesium alloys with a sufficiently broad technological versatility, providing increased ductility of magnesium alloys while increasing their strength and fatigue properties.

The goal is achieved in that the hybrid processing method of magnesium alloys according to the invention includes homogenizing annealing, comprehensive isothermal forging and isothermal rolling. Homogenizing annealing is carried out at a temperature of 350 ÷ 450 ° C. Comprehensive isothermal forging is carried out in steps in the temperature range of 400 ÷ 300 ° C in increments of 25 ° C and with a gradual increase in the precipitation rate from 2 to 20 mm / min, providing the total true degree of deformation in the range of 8 ÷ 10. Isothermal rolling is carried out at a temperature of 300 ÷ 250 ° C in several passes with a degree of deformation in each pass of not more than 5% and a total degree of true deformation by rolling of the order of 1. The technical result of the invention is to increase the ductility of magnesium alloys while increasing their strength and fatigue properties.

As a specific example of the implementation of the method, we present the results of a study of one of several magnesium alloys, namely Mg-1,0Zn-0,18Са.

In order to reduce dendritic segregation, the alloy was first annealed at 400 ° С for 4 hours, followed by cooling in air. After homogenization, scalping of the ingot, and removal of the shrink shell, a billet with dimensions ∅ 58 × 153 mm was obtained.

In total, 5 VIC cycles were carried out. Cracks formed during forging were polished. The total exposure time at a temperature of 400 ° C ÷ -1.5 hours, 375 ° C ÷ -1 hour, 350 ° C - 1.5 hours, 325 ° C - 1.5 hours, 300 ° C - 1.5 hours. A workpiece with dimensions ∅ 63 × 129 mm was obtained. The degree of deformation per VIK cycle was e ~ 1.82, and the total degree of deformation was e ~ 9.1.

The VIK blank was cut in half: one half was left in this state (marking VIK1), and the second was deposited to a height of ~ 8.8 mm (e ~ 2) at 300 ° C. The resulting disk had dimensions ∅ 160 × 8.8 mm.

For subsequent isothermal rolling, two billets 115 × 56 × 8.8 mm in size were cut from the disk. The billets were heated to a temperature of 300 ° C for 15 minutes and rolled at a speed of 2.4 mm / sec. The degree of deformation per passage did not exceed 5%. After each pass, the billets were heated for 5 minutes (temperature stabilized) in a furnace heated to the rolling temperature. The total degree of deformation was e ~ 0.84, and the final sheet thickness was ~ 3.8 mm. The total residence time of the workpieces at 300 ° C during rolling was 3 hours. Alloy marking with combined deformation processing (VIK + isothermal rolling) - VIK1P.

The results of tensile tests showed that the alloy after comprehensive isothermal forging (VIK1) demonstrates lower strength (~ 200 MPa) compared to, for example, an extruded state, but significantly greater ductility (~ 26%). The combination of VIK with isothermal rolling (VIK1P) allows you to increase the strength to ~ 260 MPa without a significant decrease in ductility (~ 21%).

Qualitative and quantitative analysis of the microstructure was carried out on optical microscopes “Nikon L150” and Axiovert 40 MAT, as well as a Tescan Lyra3 scanning electron microscope on thin sections made by mechanical grinding and polishing according to the standard procedure. The grain structure was detected by chemical etching for 5 seconds. in a reagent of the following composition: 75 ml ethyl alcohol, 2 g picric acid, 37.5 ml acetic acid, 20 ml distilled water. Then the samples were washed for 5 s in a 10% solution of nitric acid.

The structure of the alloy in the delivery state is a typical coarse-grained cast with a relatively uniform distribution of excess phases (Fig. 1 - microstructure of the alloy in the delivery state). After comprehensive isothermal forging, the structure becomes uniform fine-grained with a grain size of about 4 microns. In this case, the structure becomes homogeneous both at the micro and macro levels (Fig. 2 - microstructure of the alloy after VIK (VIK1)). Rolling practically does not change the dispersion and uniformity of the structure (Fig. 3 - microstructure of the alloy after VIC, precipitation and isothermal rolling (VIK1P)).

In addition, microstructural studies were performed by scanning electron microscopy in conjunction with the backscattered electron diffraction (EBSD) method using a Carl Zeiss Sigma scanning electron microscope equipped with InLens and SE detectors.

The sections of the VIK1P thin section were studied in the ED and TD directions. The topography of surfaces began to appear at magnifications of the order of 10,000, but no microstructural features were detected. A uniform distribution of chemical elements without any signs of the formation of specific phases was established, with the exception of individual calcium and zinc inclusions in the main magnesium matrix.

The texture of the deformed alloy was studied by EBSD using EBSD scans obtained using a ZEISS SIGMA scanning electron microscope with a field cathode and a Hikari 5.0 EDAX / TSL detector.

In the initial cast state, the alloy structure is homogeneous, the texture is close to random. After a comprehensive isothermal forging, a very uniform completely recrystallized structure with a sufficiently fine grain is realized. In the plane parallel to the axis of the workpiece, there is a texture characteristic of ECAP, but with a more diffuse distribution of the basal planes relative to the poles, which is an advantage. In this case, the maximum value of the texture is relatively small and is 6.5. There are no deformation twins. After isothermal rolling of alloy samples that have undergone comprehensive isothermal forging, a characteristic rolling texture is formed in the material with basal planes oriented perpendicular to the rolling direction.

It follows from the foregoing that, both from the point of view of microstructure and texture, the proposed method for hybrid processing of magnesium alloys is very promising, which allows processing workpieces of a wide range of sizes to very large degrees of deformation and to produce semi-finished products of various shapes. Its application provides a very homogeneous fine-grained structure with less texture sharpness compared, for example, with extrusion and ECAP, which, in turn, allows one to obtain sufficiently high values of strength and ductility, as well as reduced asymmetry of mechanical behavior and, as a result, increased fatigue characteristics.

Claims (1)

A method of hybrid processing of magnesium alloys, including homogenizing annealing, comprehensive isothermal forging and isothermal rolling, characterized in that homogenizing annealing is carried out at a temperature of 350 ÷ 450 ° C, comprehensive isothermal forging is carried out stepwise in the temperature range 400 ÷ 300 ° C with a step of 25 ° C and with a gradual increase in the precipitation rate from 2 to 20 mm / min, ensuring the total true degree of deformation in the range of 8 ÷ 10, and isothermal rolling is carried out at a temperature of 300 ÷ 250 ° C in several passes with Degree of deformation in each pass is not more than 5% and a total rolling degree of true strain of about 1.

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