JP6675753B2 - Processing method of magnesium or magnesium alloy - Google Patents

Description

  The present invention relates to a method for processing magnesium or a magnesium alloy with high strength.

  Magnesium alloys (including magnesium; the same applies hereinafter) are expected to be applied to next-generation lightweight structural materials because they are lightweight and have high specific strength. However, when a work hardening treatment is performed to achieve high strength, cracks and defects are easily generated by a general processing method, and it has been considered that high strength processing is difficult.

  Therefore, as a method of strengthening a magnesium alloy, a method of adding a transition metal and a specific rare earth metal has been proposed (see Non-Patent Documents 1 and 2). However, in these methods, it is necessary to add a rare earth metal element to the alloy in an amount of 5% to 7% or more by weight, and the expensive rare earth metal is used, so that the use is limited to high value-added products. Had become.

  Therefore, the present inventor has invented a method of manufacturing a high-strength magnesium alloy by a method of performing uniaxial forging while suppressing deformation of the magnesium alloy (see Patent Document 1). This involves forging uniaxially a magnesium alloy contained in a mold that suppresses deformation in order to avoid breakage that occurs during plasticity of the magnesium alloy (hereinafter referred to as “high load pressing under deformation restraint” or “DRF”). In some cases.) By such a working method, the amount of plastic deformation introduced during forging is made as small as possible, and at the same time, the introduction of deformation twins and the increase in dislocation density are promoted during working, thereby increasing the strength. In other words, the high density of twin deformation is introduced, the initial grain structure is divided, and the dislocation density increases due to multiple slip deformation, and at the same time, work hardening is brought about. Further, the strength was further enhanced by the development of a bottom surface texture that aggregates the bottom surfaces of the dense hexagonal crystal, which is the crystal lattice of magnesium.

WO2013 / 002082

Y. Kawamura, M. Yamasaki, Mater. Trans., Vol.48, pp.2986-2992 (2007) Nobuto Kawamura, Kenji Higashida, "Strengthening mechanism of high-strength magnesium alloy with long-period stacking structure", Research Report of the Japan Institute of Light Metals, 2010

  As described above, the high-load pressing under deformation constraint method can produce a high-strength magnesium alloy. However, the bottom surface of the crystal lattice (dense hexagonal) of magnesium is subjected to high-load pressing under deformation constraint. Since the rods are aligned in the vertical direction with respect to the forged shaft at the time, the yield strength in the longitudinal direction of the rod shape was not always high.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a processing method for improving yield strength in a longitudinal direction while maintaining strength by high-load pressing under deformation constraint. It is.

  Therefore, the present invention provides a first step of subjecting a workpiece made of magnesium or a magnesium alloy having an appropriate cross section and an appropriate length to high-load pressing under deformation restraint, and before or after the first step, A second step of deforming so as to reduce the cross section of the workpiece, wherein the first step comprises forging and compressing the workpiece in a longitudinal direction while suppressing the cross section of the workpiece from expanding. 1) σp> σf (where σf is the compressive rupture stress (MPa) of the workpiece), (2) plastic deformation rate 10% or less, and (3) strain rate 0.1 / sec or less. The second step is to cause a local or total plastic flow with a shape change.

  According to the above configuration, the deformation processing for reducing the cross-section in the second step (hereinafter, sometimes referred to as “cross-section reduction processing”) causes crystal rotation during the cross-section reduction processing, and a dense hexagonal bottom surface is possible. It can be made as parallel as possible to the longitudinal direction. Thereby, the strength in the longitudinal direction can be improved. In addition, the reduction of the cross-section promotes the increase in dislocation density and the introduction of deformation twins, so that a magnesium alloy having higher strength can be manufactured.

  In the invention having the above configuration, the workpiece is a rod-shaped member, and the second step includes an extrusion process, a drawing process, a wire drawing process, a swaging process, a groove roll rolling process, a compression process, a rolling process, and a rolling process. At least one type of processing selected from the processing may be performed.

  In the case where the workpiece comes out of a rod-shaped member as in the above configuration, a general-purpose processing method such as swaging can be employed, and the method can be appropriately selected and used depending on the shape of the workpiece. can do. Since these processing methods can be performed by a general-purpose machine, they can be processed at low cost.

  On the other hand, in the invention having the above-described configuration, the second step is performed under the following conditions: (1) σp> σf (where σf is the compression of the workpiece) The stress may be carried out under conditions that satisfy (fracture stress (MPa)), (2) a plastic deformation rate of 10% or less, and (3) a strain rate of 0.1 / sec or less.

  According to the above configuration, since the cross-sectional reduction processing is performed in a state where the cross-section reduction is performed by the mold, the second step can be performed on a material having a shape that is not compatible with swaging processing, such as a deformed material. it can. Depending on the shape of the workpiece, local plastic flow may be caused. However, the local plastic flow enables work hardening and the introduction of deformation twins, thereby further strengthening the magnesium alloy.

  In the invention having the above configuration, the second step may be performed before and after the first step. In this case, the second step performed before and after the first step may be performed in a cross-section. The reduction ratio may be 10% or less.

  In the case of the above configuration, the dislocation density increases in both the first step and the second step, and deformation twins are introduced, so that the strength of the entire magnesium metal is improved. Then, in the subsequent second step, the strength in the longitudinal direction can be improved by aligning the bottom surfaces of the dense hexagonal crystals so as to be parallel to the longitudinal direction. In addition, setting the cross-sectional reduction rate in the second step to 10% or less means that the cross-sectional reduction processing is facilitated by aligning the bottom surfaces of the dense hexagonal crystals perpendicular to the longitudinal direction in the first step, thereby suppressing the processing. This is to prevent fracture during plasticity of the magnesium alloy.

  Further, in the invention having the above structure, the first step and the second step may be sequentially and alternately performed a plurality of times. In this case, the second step may be such that the cross-sectional reduction rate is 10%. It can be as follows.

  In the case of the above configuration, by performing the first step and the second step a plurality of times, the increase in the dislocation density and the introduction of the deformation twin are promoted, and the high strength is realized. The reason that the cross-sectional reduction rate in the second step is set to 10% or less is to prevent the magnesium alloy from being broken during plasticity, similarly to the above. Further, when the workpiece is a rod-shaped member, all of the cross-sectional reduction processes performed a plurality of times include extrusion, drawing, wire drawing, swaging, groove roll rolling, compression, and rolling. Alternatively, a single processing method selected from rolling processing may be performed, or a different processing method may be selected and combined for each individual cross-sectional reduction processing.

  Further, in the invention of each of the above structures, an aging heat treatment may be performed at a treatment temperature of 100 ° C. to 200 ° C. between the first step and the second step.

  In the case of the above configuration, the disappearance of the deformation twin can be suppressed by the aging heat treatment. That is, since the deformation twins introduced in the first step and the second step can be reversibly disappeared during repeated high loads, the precipitates are dispersed by performing aging heat treatment, The loss of reversible deformation twins can be suppressed.

  In the inventions having the above configurations, the workpiece may have a circular, elliptical, or polygonal cross-sectional shape. The present invention is not limited to a special shape, and in the case of a bar-shaped member, a bar-shaped member having a cross-sectional shape other than a round bar can be used, and even a plate material or a material having a different shape can be processed. Can be done.

  ADVANTAGE OF THE INVENTION According to this invention, crystal rotation of magnesium can be produced | generated by implementing both high load press work and deformation | transduction reduction under deformation constraint with respect to a magnesium alloy, and thereby, a crystal lattice (dense hexagonal Crystal) can be aligned either parallel or perpendicular to the longitudinal direction. Therefore, by aligning the bottom surfaces of the dense hexagonal crystals in the longitudinal direction, the yield strength in the longitudinal direction can be improved. It is to be noted that the first effect of the high-load pressing under the deformation constraint is that the cross-sectional reduction at room temperature can be easily performed.

FIG. 4 is an explanatory diagram illustrating a first step of the processing method according to the embodiment of the present invention. FIG. 4 is an explanatory view illustrating a second step of the processing method according to the embodiment of the present invention. FIG. 4 is an explanatory view showing another example of the first step of the processing method according to the embodiment of the present invention. FIG. 7 is an explanatory view showing another example of the second step of the processing method according to the embodiment of the present invention. (A) is an explanatory view showing the direction of the dense hexagonal crystal after the first step in the rod-shaped member, and (b) is the illustration showing the direction of the dense hexagonal crystal after the second step. (A) is a true stress-nominal strain curve showing the result of Experimental Example 1, (b) is a true stress-nominal strain curve showing the result of Experimental Example 2, and (c) is a result of Experimental Example 3. It is a true stress-nominal strain curve shown.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. According to the present invention, a magnesium alloy is subjected to high-load pressing under deformation constraint (first step) and cross-sectional reduction processing (second step). Therefore, a description will be given of a processing method of high-load press working under deformation constraint (first step) and cross-sectional reduction processing (second step). The first step utilizes the processing method described in Patent Document 1 mentioned above, but the present invention is also used for a plate material or a polygonal material.

  FIG. 1 shows a mold 1 and a press mandrel 2 for performing high-load press work under constraint on a workpiece A. As shown in this figure, the mold 1 has an internal space slightly larger than the workpiece A so that the workpiece A can be loosely fitted. 11, the press mandrel 2 can be housed in the mold 1 through the opening 11.

  The workpiece A accommodated in the mold 1 is in a state in which the periphery (entire side surface) has a small gap L in the internal space of the mold 1, and the upper surface is located slightly inside the opening 11. It has become. In this state, when high-load press working is performed by the press mandrel 2, the workpiece A receives a compressive load and tends to deform so as to enlarge the cross section, but the periphery is restrained by the mold 1. , Only slight deformation. At this time, the deformation rate (that is, the porosity formed by the gap L) is adjusted to be 10% or less of the entire volume of the workpiece A. Further, the load σp by the press mandrel 2 during the press working is a high load exceeding the compressive rupture stress of the workpiece A. The strain rate at that time is set to 0.1 / sec or less. However, particularly in the case of a soft magnesium alloy, there is a case where processing of 10% or more is possible under limited conditions.

  FIG. 2 shows, as a model diagram, an example of groove roll rolling performed as a second step. When the workpiece A passes between two opposed groove rolls 3 and 4, the rolls 3 and 4 are rolled. The cross section is reduced to the interval between the grooves formed on the surface. At this time, the diameter and interval of both grooves of the rolls 3 and 4 are adjusted so that the plastic deformation rate (cross-sectional reduction rate) is 10% or less.

  For the workpiece B having a polygonal cross section or a plate shape, as shown in FIG. 3, a mold 1 having a quadrangular internal space is used, and gaps are formed in the longitudinal and lateral directions of the workpiece A, respectively. Is provided to accommodate the workpiece A. The press mandrel 2 has a quadrangular shape according to the shape of the workpiece B and the shape of the opening 11 of the mold 1. By using the mold 1 and the press mandrel 2 as described above, it is possible to perform a high-load press working under constraint also on a square or plate-shaped workpiece B. In the case of a polygon, the shapes of the mold 1 and the press mandrel 2 are changed according to the shape.

  On the other hand, when the cross-sectional reduction processing is performed on the plate-shaped workpiece B, it is also possible to use roll rolling, but there is also a method using a high-load press machine under constraint as shown in FIG. obtain. Since this is the second step, it is used before or after the first step is performed by the processing machine shown in FIG. When a high-load press machine under constraint as shown in FIG. 4 is used, it is difficult to give plastic deformation as a whole, and therefore, a mold 5 having a gap M only in the longitudinal direction is used. . It is to be noted that the extremely small gaps in the periphery other than the longitudinal direction allow accommodation in the internal space, but these other gaps are much smaller than the longitudinal gap M. Then, by performing high-load press working from the upper opening by the press mandrel 6, a local plastic flow can be generated in the plate-shaped workpiece B.

  The processing method as described above is an example, and it is not necessary to individually enumerate all the processing methods. However, the key is that the mold 1 capable of restraining the plastic deformation of the workpiece A and the predetermined height are required. The use of the press mandrel 2 which generates a load compressive load makes it possible to perform high-load press working under constraint in the longitudinal direction. Further, the cross-sectional reduction processing can be performed by a swaging machine or a rolling machine.

  Here, the change (crystal rotation) of the orientation of the magnesium crystal lattice (dense hexagonal crystal) by the above two types of processing will be described. FIG. 5 is a conceptual diagram showing the orientation of the workpiece and the orientation of the crystal lattice. A hexagonal column portion P in the figure indicates a dense hexagonal crystal, and the hexagonal plane Pa is defined as a bottom surface. In addition, the bottom surface Pa is formed on the upper surface and the lower surface of the hexagonal prism portion (dense hexagonal crystal) P, and both are referred to as the bottom surface.

  As shown in FIG. 5A, when a compressive load is applied to the workpiece A in the longitudinal direction, the bottom surface Pa of the dense hexagonal crystal P is perpendicular to the load direction (that is, the longitudinal direction of the workpiece A). Will be aligned in different directions. Strictly, not all crystal lattices coincide in the same direction, but a considerable proportion will be in the above orientation. Such a change in texture is brought about by large plastic deformation. However, even under high-constrained press working under constraint where plastic deformation does not occur, a strong bottom surface texture can be formed on the pressed surface, and crystal rotation can occur. This is because the crystal lattice bottom surface Pa, which is a slip surface, is arranged in the direction of plastic deformation due to plastic deformation that slightly expands in the peripheral direction. When the bottom surface P of the dense hexagonal crystal P is aligned in the direction orthogonal to the compressive load direction (longitudinal direction of the workpiece A), the workpiece A provides ductility in the longitudinal direction. Lower. On the other hand, cross-section reduction such as swaging is facilitated.

  On the other hand, as shown in FIG. 5B, when a compressive load is applied from the periphery of the workpiece A toward the axis by the cross-sectional reduction processing, the bottom surface Pa of the dense hexagonal crystal P is also perpendicular to the compression direction. As a result, they are aligned in a direction parallel to the longitudinal direction of the workpiece A. Strictly, even in this case, the directions of all the crystal lattices do not coincide with each other, and the crystal lattices are aligned in the same direction at a considerable ratio. By aligning the crystal lattice bottoms Pa in such a direction, the tensile strength in the longitudinal direction is increased, and the yield strength is improved.

  In both the high-load pressing under constraint and the cross-section reduction, deformation twins are introduced and the dislocation density increases, so that the overall strength increases. Therefore, by repeating the high-load press working under constraint and the cross-section reduction working, in addition to the introduction of the deformation twins and the increase of the dislocation density described above, the strength can be increased by the development of the texture of the crystal lattice bottom surface Pa. is there. When the final step is the cross-sectional reduction processing (second step), the crystal lattice bottom surface Pa is orthogonal to the longitudinal direction, and a metal material having an increased yield strength can be obtained.

  A strength experiment was conducted in the case of processing a magnesium alloy by combining the high-load pressing under constraint and the cross-section reduction, and will be described below.

<Experimental example 1>
The magnesium alloy to be used is a commercially available AZ80 (a magnesium alloy containing about 8% of aluminum and less than 1% of zinc). The hot extruded material is used as an initial material, and is processed by a predetermined processing process after a solution treatment. Was. As the processing process, (1) high-load press working under constraint (DRF) + extrusion processing (cross-section reduction processing), (2) extrusion processing + DRF + extrusion processing, (3) DRF + extrusion processing + DRF + extrusion processing, and a tensile test was performed. Was. For comparison, a similar tensile test was performed on two types of processed materials (A) only with DRF and (B) only with extrusion, and on a solution-treated material. FIG. 6A shows the experimental results.

As can be seen from the above experimental example 1, the above-mentioned processing processes (1), (2) and (3) all have a yield strength as compared with the DRF only (A), the extrusion processing only (B) and the material subjected to the embodying treatment. And excellent tensile strength. In particular, in the above-mentioned working processes (2) and (3), relatively high yield strength and tensile strength were achieved.

<Experimental example 2>
Further, using the same material as in Experimental Example 1, processing was performed by a combined process of DRF and swaging processing. At that time, the tensile strength and the yield strength when the compression load in the DRF was changed were measured. The result is shown in FIG. Note that “MPa” in the figure indicates the magnitude of the compressive load at the time of DRF, and “0 MPa” is simply a hot extruded material, and “0 MPa + Swinging” is a high-pressure press working under constraint. This shows a case where only swaging processing is performed without performing the above.

  From the results of Experimental Example 2, a tensile strength of 417 MPa and a yield strength of 365 MPa can be obtained, and the yield strength can be clearly increased as compared with a hot extruded material.

<Experimental example 3>
Next, processing was performed by a combination process in which DRF + swaging processing was repeated twice. The magnesium alloy to be used is the same as that in Experimental Example 1. In the DRF, the compressive load was set to 673 MPa, and one of the two was finally used with an elliptical swaging die, and was subjected to swaging again. The result is shown in FIG. Note that “e” in the drawing indicates that an elliptical swaging dice was used, and “p” indicates that a perfect circular swaging dice was used.

From the results of Experimental Example 3, it was possible to obtain a magnesium alloy having a tensile strength of 495 MPa, a yield strength of 480 MPa, and a composition elongation of about 4%. The material obtained by this processing is superior to the mechanical properties of general rare earth-added magnesium alloys. Such a processing method is an easy method, but a processing method capable of obtaining a high-strength magnesium alloy. It is. The following table summarizes a comparison with the case of using only the rare earth-added magnesium alloy and DRF.

  As is clear from the above table, in the case of the above-mentioned method, the tensile strength can obtain a strength comparable to conventional rare earth-added magnesium alloys and high-strength magnesium alloys by DRF, and in terms of yield strength, The results exceeded those of these conventional magnesium alloys.

  Although the embodiment and the experimental example of the present invention are as described above, in the experimental example, only the extrusion process or the swaging process is employed as the cross-sectional reduction process, but this is a process for increasing the strength of the magnesium alloy. In consideration of the above, the present invention is not limited to these, and similar effects can be obtained by other methods.

1,5 Mold 2,6 Press mandrel 3,4 Groove roll A, B Workpiece L, M Gap P between mold and workpiece Crystal lattice (dense hexagonal)
Pa Bottom of crystal lattice (dense hexagonal)

Claims (7)

A first step of subjecting a workpiece made of magnesium or a magnesium alloy having an appropriate cross-section and an appropriate length to high-load pressing under deformation restraint;
Before or after the first step, a second step of deforming the cross section of the workpiece to be reduced at room temperature ,
In the first step, forging compression is performed in the length direction in a state where the cross section of the workpiece is suppressed from expanding. (1) σp> σf (where σf is the compressive rupture stress of the workpiece) (MPa)), (2) the plastic deformation ratio of 10% or less, and, (3) strain rate 0.1 / sec or less, which is carried out at room temperature under conditions satisfying,
The method for processing magnesium or a magnesium alloy, wherein the second step causes a local or total plastic flow accompanied by a shape change.   The workpiece is a rod-shaped member, and the second step includes at least one selected from extrusion, drawing, drawing, swaging, groove roll rolling, compression, rolling, and rolling. The method for processing magnesium or a magnesium alloy according to claim 1, wherein the method is for performing various types of processing.   In the second step, (1) σp> σf (where σf is the compressive rupture stress (MPa) of the workpiece) in a state where the shape of the workpiece is suppressed by a mold having a different degree. 2. The method for processing a magnesium or magnesium alloy according to claim 1, wherein the method is carried out under a condition that a plastic deformation rate is 10% or less and (3) a strain rate is 0.1 / sec or less. The second step is performed before and after the first step,
The method for processing magnesium or a magnesium alloy according to any one of claims 1 to 3, wherein each of the second steps performed before and after the first step has a cross-sectional reduction rate of 10% or less.   The method according to claim 1, wherein the first step and the second step are sequentially and alternately performed a plurality of times, and each of the second steps has a cross-sectional reduction rate of 10% or less. A method for processing the magnesium or magnesium alloy according to the above.   The method for processing magnesium or a magnesium alloy according to claim 1, wherein an aging heat treatment is performed at a processing temperature of 100 ° C. to 200 ° C. between the first step and the second step.   The method for processing magnesium or a magnesium alloy according to any one of claims 1 to 6, wherein the workpiece has a cross-sectional shape of a circle, an ellipse, or a polygon.