JP4632949B2 - Processing method of Mg alloy - Google Patents

Description

  The present invention relates to a magnesium (Mg) alloy processing method and an Mg alloy.

Mg alloys have the lowest density among practical metals and are therefore lightweight. Furthermore, Mg alloys are excellent in terms of specific strength, electromagnetic shielding properties, recyclability, and the like.
However, Mg alloys have few slip systems because they have a close-packed hexagonal lattice structure. For this reason, the Mg alloy has low ductility and is a difficult-to-process material.
In this specification, the Mg alloy means an alloy having Mg as a maximum component and a close-packed hexagonal lattice structure, and includes pure Mg. The shape of the Mg alloy does not matter.

The present invention has been made in view of the above circumstances. An object of the present invention is to provide a processing method capable of improving the workability of an Mg alloy and a highly workable Mg alloy.
The Mg alloy processing method according to the present invention comprises the following steps:
(1) applying a strain in at least one direction to the Mg alloy while maintaining the temperature of the Mg alloy at 473 K or higher;
(2) After that, lowering the temperature of the Mg alloy and applying strain in a direction different from the one direction to the Mg alloy.
After the step (2), the following steps may further be provided:
(3) A step of further reducing the temperature of the Mg alloy and applying a strain to the Mg alloy in a direction different from the strain direction in the steps (1) and (2).
The strain directions in the steps (1) to (3) may be substantially orthogonal to each other.
After the steps (1) to (3), any or all of the steps (1) to (3) may be repeated.
The true strain amount in each strain is preferably 0.3 or more.
It is preferable to add the strain in the next step within 3 minutes after adding the strain.
The temperature of the Mg alloy in step (1) is preferably in the range of 500K to 700K.
The temperature of the Mg alloy in the step (2) is preferably in the range of 400K to 600K.
The temperature of the Mg alloy in the step (3) is preferably in the range of 300K to 500K.
The strain rate at each strain is preferably in the range of 1 × 10 ⁻³ to 1 × 10 ⁺¹ s ⁻¹ .
Each strain is a compressive strain, for example.
The Mg alloy according to the present invention is processed by any one of the processing methods described above.
The workpiece according to the present invention is obtained by processing the Mg alloy.
The Mg alloy according to the present invention is an alloy having Mg as a maximum component and a fine hexagonal lattice structure,
(1) applying a strain in at least one direction to the Mg alloy while maintaining the temperature of the Mg alloy at 473 K or higher;
(2) Thereafter, reducing the temperature of the Mg alloy and applying strain to the Mg alloy in a direction different from the one direction;
(3) further reducing the temperature of the Mg alloy and applying strain to the Mg alloy in a direction different from the strain direction in the steps (1) and (2);
By executing the above, the crystal grain size may be 1 μm or less.
The crystal grain size of this Mg alloy may be 0.2 μm or less.
Furthermore, this Mg alloy may have a configuration in which the Vickers hardness at room temperature is 1000 MPa or more and the elongation at break in a tensile test at 150 ° C. is 300% or more.
The Mg alloy according to the present invention is an alloy having a fine hexagonal lattice structure with Mg as a maximum component, the crystal grain size of which is 1 μm or less, the Vickers hardness at room temperature is 1000 MPa or more, and at 150 ° C. The elongation at break in the tensile test may be 300% or more.
The Mg alloy according to the present invention is an alloy having a fine hexagonal lattice structure with Mg as a maximum component,
(1) applying a strain in at least one direction to the Mg alloy while maintaining the temperature of the Mg alloy at 473 K or higher;
(2) Thereafter, reducing the temperature of the Mg alloy and applying strain to the Mg alloy in a direction different from the one direction;
By running
The crystal grain size may be 1 μm or less, the Vickers hardness at room temperature may be 1000 MPa or more, and the elongation at break in a tensile test at 150 ° C. may be 300% or more.

FIG. 1 is a block diagram showing an outline of a processing apparatus according to an embodiment of the present invention.
FIG. 2 is an explanatory view showing an Mg alloy to be processed.
FIG. 3 is a flowchart for explaining a processing method according to an embodiment of the present invention.
FIG. 4 is a graph showing experimental results in an experimental example of the present invention. In this graph, the horizontal axis represents cumulative strain and the vertical axis represents temperature (K).
FIG. 5 is a graph showing experimental results in an experimental example of the present invention. In this graph, the horizontal axis represents cumulative strain and the vertical axis represents true stress.
FIG. 6 is a graph showing experimental results in an experimental example of the present invention. In this graph, the horizontal axis represents cumulative strain (logarithmic display), and the vertical axis represents processing stress (steady state stress) (logarithmic display).
FIG. 7 is a graph showing experimental results in an experimental example of the present invention. In this graph, the horizontal axis represents the crystal grain size D as D− ^{1 / 2} . The vertical axis represents room temperature Vickers hardness.
FIG. 8 is a graph showing experimental results in an experimental example of the present invention. In this graph, the horizontal axis is nominal strain, and the vertical axis is true stress.
FIG. 9 is an optical micrograph of the Mg alloy obtained in the experimental example of the present invention. Figure (a) is after the first stage (cumulative strain = 0.8), Figure (b) is after the second stage (cumulative strain = 1.6), and Figure (c) is after the fourth stage (cumulative). It is a photograph of the structure | tissue of distortion | strain = 3.2).

An Mg alloy processing method according to an embodiment of the present invention will be described with reference to the accompanying drawings. First, the outline | summary of the processing apparatus used for this method is demonstrated. This apparatus includes a processing unit 1, a heater 2, and a control unit 3. The processed part 1 can apply a compressive force to the Mg alloy A (see FIG. 2) from above. In this apparatus, by changing (for example, rotating) the position of the Mg alloy A, strain can be applied from three directions of the Mg alloy (X, Y, and Z directions on the Mg alloy). The heater 2 heats the Mg alloy A to a predetermined temperature. The control unit 3 reduces the heating temperature of the Mg alloy A by the heater automatically or manually when the processing direction (that is, the strain direction) to the Mg alloy A by the processing unit 1 changes.
Next, an Mg alloy processing method in the present embodiment will be described.
(Step 3-1 in FIG. 3)
First, the temperature of the Mg alloy is set to 473 K or higher by the heater 2. More preferably, the temperature of the Mg alloy is in the range of 500K to 700K. If the temperature of the Mg alloy is sufficiently high in the initial state, there is no need to heat with the heater 2. Next, the processed portion 1 applies a compressive stress in the X-axis direction in FIG. 2 to the Mg alloy to generate strain. This strain is a compressive strain in this embodiment. Strain ^{rate, 1 × 10 -3 ~1 × 10} +1 in the range of ^{s -1,} more preferably in the range of ^{1 × 10 -2 ~1 × 10 +1} s -1 are preferred. The true strain amount is preferably 0.3 or more, more preferably 0.4 or more. By making the strain conditions within these ranges, good refinement of crystal grains after processing can be expected.
(Step 3-2 in FIG. 3)
Next, after the compression in the X-axis direction by the processed portion 1 is released, the temperature of the Mg alloy is lowered. However, the load stress may be released during the temperature drop. The temperature of the Mg alloy here is preferably in the range of 400K to 600K. Next, a strain on the Mg alloy is applied to the Mg alloy from the Y-axis direction (that is, a direction substantially orthogonal to the X-axis direction). The strain conditions are the same as in step 3-1. It is preferable that the time until the strain is applied in Step 3-2 after the strain processing in Step 3-1 is within 3 minutes, more preferably within 2 minutes. The same applies to the subsequent steps.
(Step 3-3 in FIG. 3)
Next, after releasing the compression in the Y-axis direction, the temperature of the Mg alloy is further lowered. However, the load stress strain may be released during the temperature drop. The temperature of the Mg alloy here is preferably in the range of 300K to 500K. Next, a strain on the Mg alloy is applied to the Mg alloy from the Z-axis direction (that is, a direction substantially orthogonal to the X-axis and Y-axis directions). The strain conditions are the same as in step 3-1.
Thereafter, after releasing the load stress in the Z-axis direction, the temperature of the Mg alloy may be further lowered, and strain from the X or Y-axis direction may be applied to the Mg alloy.
According to the method of the present embodiment, the crystal grains become finer as the number of processing steps increases. An Mg alloy having a large strength and elongation of the product can be obtained.
In this embodiment, since the strain is applied to the Mg alloy from three directions, the crystal grain size can be further refined. Thereby, the anisotropy of deformation (strain against stress) in the processed Mg alloy can be suppressed. However, if anisotropy can be tolerated, it is possible to adopt a method of applying strain from only two directions. Moreover, it is good also as a structure which adds the distortion from 4 directions or more to Mg alloy. Even in that case, it is considered necessary to lower the temperature of the Mg alloy every time it goes through the stage.
In addition, the strain directions at each stage do not necessarily have to be orthogonal to each other. In short, any direction that contributes to refinement of the crystal grain size in the Mg alloy may be used.
(Experimental example)
In accordance with the method of the embodiment, the experiment was performed under the following conditions.
(Experimental conditions)
Material: Mg alloy AZ31 (Mg alloy containing 3% by mass of Al and 1% by mass of Zn)
Forming method of Mg alloy: cut out from a round bar extruded material Shape of Mg alloy (test piece): a rectangular parallelepiped shape having a height of 13 mm, a length of 12 mm, and a width of 7 mm (the length direction is defined as the extrusion direction)
Strain rate ε: 3 × 10 ⁻³ s ⁻¹ (constant)
Compression true strain amount at each stage Δε: 0.8 (constant)
Compression temperature T at the first stage (X-axis direction) strain: 673K
Compression temperature T at the second stage (Y axis direction) strain: 523K
Compression temperature at the third stage (Z-axis direction) strain T: 473K
Compression temperature at the fourth stage (X-axis direction) strain T: 433K
That is, in this embodiment, after Step 3-1, strain was once again applied to the Mg alloy from the X-axis direction. The relationship between temperature and cumulative strain is shown in FIG. In this embodiment, water quenching is performed at the end of each stage, and the test piece is further polished and molded, and then about 0.6 to 0.78 ks (kiloseconds) under the temperature conditions in the next stage. After reheating, the specimen was strained.
The results are shown in FIGS. FIG. 5 shows the relationship between accumulated strain and stress when strain is applied. At each stage, a large strain of Δε = 0.8 is obtained. Further, even at a low heating temperature such as T = 473K or 433K, a large strain can be obtained. The stress at this time is considerably high but does not break. The broken line in FIG. 5 is a comparative example. When machining is started at T = 473K, the stress rapidly increases and breaks with a true strain of about 0.3.
FIG. 6 shows the relationship between the crystal grain size at each stage and the stress at that time. Reference numerals (a) to (d) in the figure correspond to the processing in the first to fourth stages. Here, the crystal grain size is an average value of crystal grain sizes in a certain range in the structure. The crystal grain sizes in the examples (a) to (d) are as follows.
(A) 3.5 μm
(B) 1.9 μm
(C) 0.55 μm
(D) 0.18 μm
In the figure, ○ represents data on the Mg alloy processed from only one axial direction.
From these, it can be seen that in this experimental example, the crystal grain size is much smaller than in the prior art. It can also be seen that the higher the stress, the smaller the crystal grain size.
FIG. 7 shows the relationship between the crystal grain size and room temperature hardness. In the horizontal axis in the figure, it means that the crystal grain size is smaller toward the right. According to this, it can be seen that the smaller the crystal grain size is, the more the hardness is increased and the strength of the Mg alloy is doubled by refining. For example, depending on the particle size, a Vickers hardness of 1000 MPa at room temperature can be obtained. Thereby, it turns out that the intensity | strength of the workpiece using this alloy can be improved.
FIG. 8 shows the results of a tensile test on the Mg alloy that has been subjected to the above-described four-stage processing. In this test, there were ^three conditions of strain rate ε: 8.3 × 10 ⁻³ s ⁻¹ , 8.3 × 10 ⁻⁴ s ⁻¹ , and 8.3 × 10 ⁻⁵ s ⁻¹ . As a result, for example, at a strain rate of 8.3 × 10 ⁻⁵ s ⁻¹ , a very high elongation of nearly 300% could be obtained. The temperature condition at this time is 423 K (that is, about 150 ° C.). Further, even at a high strain rate of 8.3 × 10 ⁻³ s ^−1, an elongation of about 100% could be obtained. From this result, it can be seen that the Mg alloy obtained in accordance with the present invention has the characteristics capable of plastic working. In other words, plastic working that has been difficult in the prior art is possible with Mg alloys.
FIG. 9 is a photograph of the crystal structure. It can be seen that the crystal structure is fine.
The description of the embodiment is merely an example, and does not indicate a configuration essential to the present invention. The configuration of each part is not limited to the above as long as the gist of the present invention can be achieved.
Further, the functional blocks in the processing apparatus may be combined and integrated into one functional block. Further, the function of one functional block may be realized by cooperation of a plurality of functional blocks.

Industrial applicability

  ADVANTAGE OF THE INVENTION According to this invention, the processing method which can improve the workability of Mg alloy can be provided. In addition, a highly workable Mg alloy can be provided.

Claims (12)

A method for processing an Mg alloy having a close-packed hexagonal lattice structure with Mg as a maximum component, characterized by comprising the following steps:
(1) applying a strain in at least one direction to the Mg alloy while maintaining the temperature of the Mg alloy at 473 K or higher;
(2) Thereafter, reducing the temperature of the Mg alloy and applying strain to the Mg alloy in a direction different from the one direction ;
(3) After the step (2), the temperature of the Mg alloy is further reduced, and strain in a direction different from the strain direction in the steps (1) and (2) is applied to the Mg alloy. Step to add . Direction of the strain in step (1) to (3), processing method according to claim 1, wherein the mutually orthogonal. After said step (1) to (3), the processing method according to claim 1 or 2, characterized in that repeating any or all of the steps of the step (1) to (3).   The processing method according to claim 1, wherein an amount of true strain in each strain is 0.3 or more. Wherein within 3 minutes after the addition of strain, processing method according to claim 1 any one of claims 4, characterized in that the addition of strain in the next step. Temperature of the Mg alloy in the step (1) is a processing method of any one of claims 1-5, characterized in that in the range of 500K～700K. The processing method according to any one of claims 1 to 6 , wherein the temperature of the Mg alloy in the step (2) is in a range of 400K to 600K. The processing method according to any one of claims 1 to 7 , wherein the temperature of the Mg alloy in the step (3) is in a range of 300K to 500K. The processing method of the strain rate according to any one of claims 1-8, characterized in that it is in the range of ^{1 × 10 -3 ~1 × 10 +1} s -1 in each strain. Processing method according to any one of claims 1-9, characterized in that each strain is compression strain. The Mg alloy is Mg-Al-Zn-based alloy, according to claim 1-10 processing method according. The Mg alloy is AZ31 alloy according to claim 1-10 processing method according.

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