Patent Application
20140147331
The present invention relates to a method for producing a high-strength magnesium alloy material.
Magnesium alloys (including magnesium metal) are lightweight and have high specific strength. As such, they are expected to be widely used as next-generation lightweight structural materials.
On the other hand, magnesium alloys are hard-to-work materials that are known to easily crack or produce defects in the case where conventional processes such as a rolling process or forging are used. Thus, improving the strength of a magnesium alloy material through a work hardening process has been a challenge, and application fields of magnesium alloy materials have been limited to small electronic equipment components and similar applications in which material strength is not such an important factor.
In recent years, techniques have been disclosed for improving the strength of magnesium alloys by adding transition metals and certain rare earth metals to magnesium (see e.g., Non-Patent Documents 1 and 2).
The magnesium alloys described in Non-Patent Documents 1 and 2 are also referred to as KUMADAI magnesium alloy. In the KUMADAI magnesium alloy, alloy strength is improved by adding rare earth metal elements and causing the development of a special atomic structure (long-period stacking ordered structure) within the alloy structure.
However, to produce the KUMADAI magnesium alloy, rare earth metal elements have to be added at a weight ratio of approximately 5% to 7% or higher to control the alloy composition. Also, these rare earth metal elements are generally expensive, and in recent years, stable supply of these elements is becoming an issue. Accordingly, applications of the magnesium alloy materials disclosed in Non-Patent Documents 1 and 2 may be limited to high-quality value-added products.
In view of the above, it is an object of at least one embodiment of the present invention to provide a comparatively simple and inexpensive method for producing a high-strength magnesium alloy material.
According to one embodiment of the present invention, a method for producing a high-strength magnesium alloy material includes:
(a) a step of preparing a magnesium alloy workpiece having a top face and a side face; and
(b) a step of applying a compressive load σp (MPa) from the top face side of the workpiece and performing a uniaxial forging process on the workpiece;
wherein step (b) is performed while suppressing deformation of the workpiece widening outward under conditions including
(i) σp>σf (where σf is the compressive breaking stress (MPa) of the workpiece),
(ii) a plastic deformation rate is less than or equal to 10%, and
(iii) a strain rate is less than or equal to 0.1/sec.
Note that the plastic deformation rate is defined by a change ratio of the volume of the workpiece before and after the forging process. Also, the strain rate is defined by the initial strain rate.
In one preferred embodiment of the method according to the present invention, σp≧2.4σf.
In another preferred embodiment, a mold having an inner space for accommodating the workpiece is used in step (b), and the inner space is formed by an inner wall of the mold. Assuming L denotes the maximum dimension of the top face of the workpiece, and P denotes the maximum gap between the side face of the workpiece and the inner wall of the mold, the ratio (L:P) may be within a range from 20:1 to 600:1.
In another preferred embodiment, the inner space of the mold is formed by assembling a plurality of mold members.
In another preferred embodiment, the inner space does not have to penetrate through the mold.
In another preferred embodiment, a size of the inner space may vary along its depth direction.
According to another embodiment of the present invention, a magnesium alloy rod has a longitudinal direction substantially parallel to the c-axis direction.
According to another embodiment of the present invention, a magnesium alloy material produced by one of the above methods of the present invention is provided. The magnesium alloy material may have the shape of a rod, a plate, a block, or a pellet, or a tube.
According to an aspect of the present invention, a comparatively simple and inexpensive method for producing a high-strength magnesium alloy material may be provided.
In general, magnesium alloy materials have poor workability so that they may easily crack or incur defects when conventional work processes such as forging or a cold rolling process are performed thereon. Thus, in the case of working a magnesium alloy material, a large amount of distortion cannot be introduced, and improving the strength of the magnesium alloy material through a work hardening process has been difficult.
In recent years, techniques have been disclosed for increasing the strength of a magnesium alloy by adding rare earth metal elements in the alloy and developing a long period stacking ordered structure within the alloy structure (KUMADAI magnesium alloy).
However, to produce the KUMADAI magnesium alloy, rare earth metal elements have to be added at a weight ratio of approximately 5% to 7% or higher to control the alloy composition. Also, these rare earth metal elements are generally expensive. Thus, magnesium alloys obtained using the above techniques may become expensive as well. Further, the use of rare earth metal elements is not very favorable from the standpoint of securing a stable supply of materials.
On the other hand, as described in detail below, a method for producing a high-strength magnesium alloy material conceived by the inventors of the present invention does not require adding such expensive rare earth metal elements to control the alloy composition. Also, in the present invention, a high-strength magnesium alloy may be produced through a forging process. In this way, a high-strength magnesium alloy may be produced by a comparatively simple and inexpensive method.
According to one embodiment of the present invention, a method for producing a high-strength magnesium alloy material includes:
(a) a step of preparing a magnesium alloy workpiece having a top face and a side face; and
(b) a step of applying a compressive load σp (MPa) from the top face side of the workpiece and performing a uniaxial forging process on the workpiece;
wherein step (b) is performed while suppressing deformation of the workpiece widening outward under conditions including
(i) σp>σf (where σf is the compressive breaking stress (MPa) of the workpiece),
(ii) a plastic deformation rate is 10% or less, and
(iii) a strain rate is 0.1/sec or less.
In the above method for producing a high-strength magnesium alloy material, a heavy compressive load σp that satisfies formula (1) indicated below is applied to the workpiece.
σp>σf (1)
Note that of represents the compressive breaking stress of the workpiece in the application direction of the compressive load σp in the case where the workpiece is free of deformation constraints.
Forging processes are generally not performed under the above condition on workpieces made of hard-to-work materials. That is, when a heavy compressive load σp as described above is applied to the workpiece, the workpiece is prone to break.
However, in the method according to the present embodiment, a heavy compressive load σp satisfying the above formula (1) may be applied to the workpiece without causing the magnesium alloy material workpiece to break. In the present embodiment, this is achieved by performing a forging process “slowly” while the side face of the workpiece is “constrained” and the plastic deformation rate is restricted to a small value.
That is, in the present embodiment, the side face of the workpiece is “constrained,” the strain rate is adjusted to be less than or equal to 0.1/sec, and the plastic deformation rate is adjusted to be less than or equal to 10%. In this way, a uniaxial forging process may be performed on the workpiece while preventing the workpiece from cracking or breaking even when applying a heavy compressive load σp satisfying the above formula (1) to the workpiece.
Note that in the descriptions below, “constraint” of the side face of the workpiece or to “constrained” deformation of the side face of the workpiece refers to suppressing free deformation of the side face of the workpiece during a forging process. For example, the expression may refer to suppressing deformation of the side face of the workpiece widening outward from its original position.
According to an aspect of the present invention, after the forging process is performed, a large number of deformation twins may be introduced into the crystal structure and dislocation density may be improved by slip deformation. In this way, work hardening through the forging process may be enabled and the strength of the workpiece may be increased.
Note that the compressive load σp applied to the workpiece may be any value that satisfies formula (1). However, the compressive load σp is preferably set as high as possible to obtain greater strength improvement effects. For example, in one preferred embodiment, the compressive load σp may be arranged to be σp≧2.4σf, and more preferably σp≧3σf.
However, when the compressive load σp is increased to an excessively high value, the workpiece may be prone to cracking or breaking even when the forming process is performed under conditions (ii) and (iii) described above. Thus, in a preferred embodiment, the compressive load σp is arranged to satisfy formula (2) indicated below.
σp<10σf (2)
(Specific Configuration of Method According to the Present Embodiment)
In the following, the method according to the present embodiment is described with reference to the accompanying drawings.
As illustrated in
(a) a step of preparing a magnesium alloy workpiece having a top face and a side face (step S110); and
(b) a step of applying a compressive load σp from the top face side of the workpiece and performing a uniaxial forging process on the workpiece (step S120); wherein step (b) is performed under the conditions indicated below
(i) σp>σf (where σf is the compressive breaking stress (MPa) of the workpiece),
(ii) plastic deformation rate is 10% or less, and
(iii) strain rate is 0.1/sec or less
while suppressing deformation of the workpiece widening outward.
In the following, the above process steps are described in greater detail.
(Step S110)
First, a magnesium alloy workpiece is prepared.
As illustrated in
Note that in the present descriptions, the terms “top face” and “side face” are used to describe relative locations of the workpiece. That is, the “top face” refers to a face of the workpiece that comes into contact with a press mandrel (member for applying a compressive load to the workpiece) while a forging process is performed on the workpiece. The “top face” is substantially perpendicular to the direction in which the compressive load is applied. The “side face” of the workpiece refers to a face that is adjacent to the “top face” of the workpiece.
Thus, for example, in a case where the workpiece is prismatic, and the workpiece is compressed in a direction parallel to the longitudinal direction of the workpiece, the “top surface” refers to one end face of the workpiece, and the “side face” refers to at least one of a plurality of faces extending in the longitudinal direction of the workpiece.
Also, for example, in a case where the workpiece is tubular, and the workpiece is compressed in a direction parallel to the longitudinal direction of the workpiece, the “upper face” of the workpiece refers to one end face of the work piece having a tubular opening, and the “side face” refers to an outer peripheral face and/or an inner peripheral face of tubular structure extending in the longitudinal direction.
The workpiece 110 is made of a magnesium alloy material. The material of the workpiece 110 is not particularly limited as long as it includes a magnesium alloy. For example, an AZ-based magnesium alloy (magnesium alloy containing zinc and aluminum), a rare-earth-element-doped magnesium alloy, or a Ca-doped magnesium alloy may be used as the material of the workpiece 110.
Further, the present invention may be applied to hard-to-work materials other than magnesium alloys including, but not limited to, titanium alloys, zirconium alloys, molybdenum alloys, and niobium alloys, for example.
(Step S120)
Next, a forging process is performed on the workpiece 110.
As illustrated in
The mold 220 has an inner wall 225 that forms the inner space 215.
Note that although the materials of the mold 220, the base member 230, and the press mandrel 240 are not particularly limited, materials having a high compressive strength including, but not limited to, steel materials for molds and super hard ceramics, for example, are preferably used.
Upon performing a forging process, the workpiece 110 is accommodated within the inner space 215 of the mold 220. In this case, the workpiece 110 is positioned within the inner space 215 of the mold 220 such that the bottom face 116 comes into contact with the base member 230 and the side face 114 faces the inner wall 225 of the mold 220. Also, during the forging process, the press mandrel 240 is arranged above the top face 112 of the workpiece 110.
Further, a small gap P is formed between the side face 114 of the workpiece 110 and the inner wall 225 forming the inner space 215 of the mold 220.
During the forging process, the press mandrel 240 is pressed against the top face 112 of the workpiece 110, and the press mandrel 240 moves along the longitudinal direction of the workpiece 110 (Z direction of
In the present embodiment, assuming of denotes the compressive breaking stress in the longitudinal direction of the workpiece 110, the compressive load σp (MPa) applied to the workpiece 110 satisfies formula (1) indicated below.
σp>σf (1)
Normally, a forging process under conditions satisfying the above formula (1) would not be performed on a workpiece that is made of a hard-to-work material. This is because the workpiece would most likely break when such a heavy compressive load σp is applied to the workpiece.
In the present embodiment, only a small gap is provided between the side face 114 of the workpiece 110 and the inner wall 225 forming the inner space 215 of the mold 220. Accordingly, even when the workpiece 110 receives compression deformation forces generated by the forging process, the side wall 114 of the workpiece 110 may be “constrained” by the inner wall 225 of the mold 220 or prevented from deforming outward to a large extent (such deformation being referred to as “constrained deformation” hereinafter). Also, during the forging process, the strain rate of the workpiece 110 is controlled to be less than or equal to 0.1/sec, and the plastic deformation rate of the workpiece 110 is controlled to be less than or equal to 10%. For example the plastic deformation rate of the workpiece 110 may be adjusted to be within a range from 2% to 8%.
By implementing the above-described measures, in the present embodiment, a heavy compressive load σp may be applied to the workpiece 110 without causing the workpiece 110 to break or incur defects.
The gap P between the workpiece 110 and the inner wall 225 may vary depending on the plastic deformation rate and/or the maximum length of the top face 112 of the workpiece 110 (denoted as “L”). For example, a ratio of the gap P to the maximum length L of the top face 112 of the workpiece 110 (P:L) may be arranged to be within a range from 1:20 to 1:600. (Note that a total gap between the inner wall 225 and the workpiece 110 with respect to a direction parallel to the top face 112 (XY plane) equals 2P at the maximum.)
According to an aspect of the present invention, after a forging process is performed, a large number of deformation twins may be introduced into the crystal structure and dislocation density may be improved by slip deformation. In this way, work hardening through the forging process may be enabled and the strength of the workpiece 110 may be increased after the forging process.
Note that a workpiece made of an AZ-based magnesium alloy (8 wt % Al-wt % Zn—Mg) was used in the present example, and the strain rate of the workpiece was adjusted to 10−3/sec while the plastic deformation rate of the workpiece was adjusted to 3%. Also, the gap P was arranged so that the ratio (P:L)=1:102.
As can be appreciated from
The above results suggest that by slowly performing compression deformation while restricting the extent of deformation through “constrained deformation,” the workpiece may be prevented from breaking even when a heavy compressive load σp is applied to the workpiece during the forging process, and a large number of deformation twins may be generated.
As can be appreciated from
(Other Configuration of Apparatus Used in Method of Present Embodiment)
An example has been described above in which the apparatus 200 illustrated in
In the following, exemplary configurations of other molds that may be used in the present embodiment is described with reference to
As illustrated in
Note that the inner space 415 does not penetrate through the mold 420 so that one end of the inner space is closed. Thus, the mold 420 does not necessarily have to include a base member like the base member 230 illustrated in
In the case of performing a forging process on the workpiece 310 using the mold 420, a press mandrel 440 having a shape matching the shape of the top portion of the inner space 415 is used. By moving the press mandrel 440 along the longitudinal direction (Z direction of
As illustrated in
As illustrated in
By using such a “divided” inner mold 660, a workpiece may be easily removed from the mold 620 after the forging process.
Note that in the example illustrated in
Also, the number of mold members making up the inner mold 660 is not particularly limited. That is, the inner mold 660 may be formed by assembling three or more mold members, for example.
Further, the configurations of the press mandrel and/or the base member are not limited to those having flat contact faces that respectively come into contact with the top face and the bottom face of the workpiece.
As illustrated in
The press mandrel 940 with the above configuration may be suitably used in a case where the workpiece has a tubular shape.
As illustrated in
By applying a compressive load to the upper part 942 of the press mandrel 940 along the Z direction, the workpiece 710 may be compressively deformed.
Meanwhile, deformation of an outer periphery side face of the workpiece 710 is “constrained” such that the outer periphery side face of the workpiece 710 can only be deformed (widened) outward up to a point where the gap between the outer periphery side face of the workpiece 710 and the inner wall 825 closes. Similarly, deformation of an inner periphery side face of the workpiece 710 is “constrained” by the extension part 943 of the press mandrel 940 such that the workpiece 710 can only be deformed up to a point where a gap between the inner periphery side face of the workpiece 710 and the extension part 943 of the press mandrel 940 closes.
Thus, in the present example, “constrained deformation” may be implemented with respect to the overall configuration of the workpiece 710 during the forging process so that the through hole of the workpiece 710 may be prevented from closing and the overall strength of the workpiece 710 may be increased.
In the example illustrated in
Note that the apparatus used in the present embodiment may have numerous other configurations. For example, the inner space for accommodating a workpiece may be arranged to have a relatively simple configuration as described above, or alternatively, the inner space may have a more complicated configuration approximating the outer shape of a final molded product, for example. Also, the gap P between the side face of the workpiece and the inner wall of the mold may be arranged to vary in the depth direction (forging direction), for example.
In the following, practical examples of the present invention are described.
(Forging Process)
Disk-shaped samples were prepared from a commercially available AZ80 magnesium alloy rod produced by hot extrusion (by Osaka Fuji Corporation). The samples were arranged to have a diameter L of 25.5 mm and a total length of 16 mm.
As can be appreciated from
Next, an apparatus similar to the apparatus 200 illustrated in
First, the sample was arranged within an inner space of a mold. The inner space penetrates through the mold and has a circular disk shape with a diameter of 26 mm and a total length of 16 mm. When the sample was arranged within the inner space, the gap P between the side face of the sample and the inner wall of the mold was 0.25 mm. Thus, L:P=25.5:0.25=102:1.
Next, a press mandrel was placed above the sample. The press mandrel has a diameter of 25.5 mm.
In this state, a compressive load σp was applied to the sample via the press mandrel, and the sample was compressed along its longitudinal direction. Note that the initial strain rate was adjusted to 1×10−3/sec, and the plastic deformation rate was adjusted to 3%.
The compressive load σp was varied with respect to each testing sample. Specifically, the compressive load σp was adjusted to 566 MPa, 754 MPa, 943 MPa, 1320 MPa, and 1509 MPa. The above compressive loads correspond to cases where the ratio σp/σf is approximately 1.4, approximately 1.9, approximately 2.4, approximately 3.3, and approximately 3.8, respectively. In the following descriptions, “sample 1” refers to the sample processed under the condition σp/σf=approximately 1.4, “sample 2” refers to the sample processed under the condition σp/σf=approximately 1.9, “sample 3” refers to the sample processed under the condition σp/σf=approximately 2.4, “sample 4” refers to the sample processed under the condition σp/σf=approximately 3.3, and “sample 5” refers to the sample that is processed under the condition σp/σf=approximately 3.8.
After testing, the samples 1-5 were visually inspected, and it was confirmed that all the samples were free of cracks or defects.
(Evaluation)
The structures of the samples 1-5 after forging processes were performed thereon were observed using an optical microscope.
As can be appreciated from these observation results, deformation twins introduced into the structure may be increased, as the compressive load σp during the forging process is increased.
As can be appreciated from
On the other hand, as can be appreciated from
The above results suggest that crystal rotation occurs as a result of implementing the method according to the present embodiment. It is quite common for the (0001) plane texture to be formed on a working surface. However, in the initial hot-extruded rod, the c-axis is oriented in a direction perpendicular to the longitudinal direction of the rod. On the other hand, the processed rod obtained by implementing the present embodiment has a texture with the c-axis oriented parallel to the longitudinal direction.
Normally, such a crystal rotation may be triggered only when substantial plastic deformation occurs in a material. Thus, in a hard-to-work material, such crystal rotation could only be observed in a broken sample. However, by implementing the method according to the present embodiment, a forging process may be performed on a workpiece without breaking the workpiece, and crystal rotation may occur after the forging process.
Next, tensile testing at room temperature was performed on the samples 1-5 to evaluate their strengths. The tensile test was performed using test equipment by Illinois Tool Works Inc. (Instron), and the initial strain rate was adjusted to 1×10−3/sec.
As can be appreciated from these results, even in sample 1 that is processed under the condition σp/σf=approximately 1.4 (σp/σf≈1.4), the maximum tensile stress and the yield stress is substantially improved compared to the pre-forging sample. Further improvements in the maximum tensile stress and the yield stress can be observed in samples 3 (σp/σf≈2.4) through sample 5 (σp/σf≈3.8) compared to the pre-forging sample.
Also, the maximum tensile strength of each of the above samples exceeds 400 Mpa and is improved compared to the maximum tensile strength of the pre-forging sample (maximum tensile strength of approximately 350 Mpa). Further, the yield stress of each of the above samples is greater than or equal to 250 Mpa and is improved from the yield stress of the pre-forging sample (yield stress of approximately 100 MPa)
It can be confirmed from the above results that a high-strength magnesium alloy material can be produced by the method according to the present embodiment. Also, the elongation of each of the above samples was approximately 6% indicating that desirably high workability may be achieved by implementing the method according to the present embodiment.
The present application is based on and claims the benefit of priority of Japanese Patent Application No. 2011-143042 filed on Jun. 28, 2011, the entire contents of which are herein incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-143042 | Jun 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/065666 | 6/19/2012 | WO | 00 | 12/27/2013 |
