Plastic processing method for metals that are difficult to process

Abstract

To provide a plastic working technique capable of increasing deformation performance of cold compression plastic deformation for a working-resistance metal of low ductility.SOLUTION: A plastic working method for a working-resistance metal, which plastic deforms the working-resistance metal as a material to be worked through cold compression, compresses the material to be worked at a prescribed speed v (mm / sec.) from a vertical direction, and plastic deforms the material to be worked cold by simultaneously adding a compression force to the material to be worked and reciprocating and vibrating the material to be worked with prescribed amplitude α(°) at a prescribed rotational speed ω(° / sec.) around a rotational axis of the material to be worked to add both-oscillation torsion vibration caused by a both-oscillation torsion force.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method and apparatus for plastically processing metals that are difficult to process, and more particularly to a method for plastically processing metals that are difficult to process, in which metals that are difficult to process and have low ductility are plastically processed (forged) by compressive plastic deformation at room temperature (cold). [Background technology]

[0002] In recent years, attention has been focused on the properties of magnesium alloys (Mg alloys) and titanium alloys (Ti alloys), which have a relatively low specific gravity but high strength, and the possibility of plastically deforming them and applying them to transportation equipment such as automobiles, aerospace, ships, and railways, as well as electrical and electronic equipment, has been investigated.

[0003] However, metal materials such as the above-mentioned Mg alloys and Ti alloys have low ductility in cold working and almost no elongation, so that cold plastic working is currently difficult (for this reason, these metal materials are called "metals that are difficult to plastically work").

[0004] In other words, because Mg alloys and Ti alloys have a close-packed hexagonal lattice structure (hexagonal crystal structure: hcp structure), they are unable to secure a sufficient number of slip systems, and even a small degree of processing reaches the limit of plastic deformation, leading to the occurrence of cracks.

[0005] Specifically, in hcp metals such as Mg and Ti alloys, the deformability, or the limit of plastic deformation before cracking (ductile fracture) occurs, is determined by the axial ratio. However, in Mg alloys, the c-axis is relatively elongated, the distance between basal planes (planar spacing) is large, and the structure favors basal slip. Therefore, only two basal slip systems are active during cold working, leading to twinning and making plastic processing difficult. In contrast, in Ti alloys, the c-axis is short, resulting in a structure in which basal slip is difficult to activate, and slip occurs mainly on prismatic planes, making cold plastic processing difficult as well.

[0006] Because the number of active slip systems increases in the high-temperature range, plastic processing of metals that are difficult to process is generally performed at a temperature elevated to a high temperature range where the required ductility can be obtained. However, plastic processing at high temperatures requires heating equipment, which increases costs. Furthermore, if metal materials are exposed to the atmosphere at high temperatures, there is a risk of the metal materials being oxidized.

[0007] Therefore, various techniques have been proposed for cold plastic working of metals that are difficult to work with plastically, in order to activate slip systems that are difficult to activate in cold working.

[0008] For example, a technology has been proposed in which plastic processing is performed under high pressure by applying back pressure or hydrostatic pressure of several hundred MPa to several GPa in a closed mold. However, this technology requires special processing machines and molds, which incurs extra costs. In addition, the plastic processing is performed at a low speed, which causes problems with productivity. Furthermore, plastic processing under high pressure also poses the problem of ensuring safety.

[0009] In addition, a technique has been proposed in which the metal structure (crystal grain size, shape, texture, etc.) is adjusted in advance to increase ductility at room temperature. However, with this technique, the range of adjustment for the metal structure is narrow, and the appropriate metal structure differs depending on the processing conditions, and the appropriate metal structure is often unknown.

[0010] Furthermore, all of the above techniques have significant limitations on processing conditions (shape, etc.), and therefore cannot be said to be suitable for practical use or mass production.

[0011] As specific methods for improving the deformability of Mg alloys and improving their plastic workability by facilitating the activation of slip systems other than the basal plane during cold working, techniques have been proposed, such as changing the axial ratio of the close-packed hexagonal lattice structure by adding additive elements and performing high-temperature treatment, lowering the critical resolved shear stress (CRSS), adjusting the texture to bring the c-axis closer to a direction perpendicular to the compression direction, and refining the crystal grains (e.g., Patent Documents 1 and 2). However, these techniques do not currently provide sufficient improvement in deformability. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-280846 [Patent Document 2] Republished WO2017 / 154969 Summary of the Invention [Problem to be solved by the invention]

[0013] In view of the above-mentioned problems, an object of the present invention is to provide a plastic processing technique that can improve the deformability of a metal that is difficult to process and has low ductility in cold compressive plastic deformation. [Means for solving the problem]

[0014] The present inventors have conducted extensive research into solving the above problems and have found that the above problems can be solved by the invention described below, thereby completing the present invention.

[0015] The invention described in claim 1 is A method for plastically processing a metal that is difficult to process, in which the metal is used as a workpiece and is plastically deformed by cold compression, comprising: the workpiece is a metal material having a hexagonal close-packed lattice crystal structure, The aforementionedThe workpiece is compressed from above and below at a predetermined speed v (mm / sec), and a compressive force is applied to the workpiece. The workpiece is reciprocally vibrated around the rotation axis of the workpiece at a predetermined rotation speed ω (° / sec) and a predetermined amplitude α (°), and a reciprocating torsional vibration is applied to the workpiece by a reciprocating torsional force, The workpiece is subjected to cold plastic deformation. In this regard, The ratio ω / v (° / mm) of ω to v is 15 to 600° / mm. The present invention relates to a method for plastically processing metals that are difficult to process.

[0017] Claim 2 The invention described in The amplitude α (°) of the workpiece is 3° or more and less than 55°. 1 to This is a method for plastically processing the metal that is difficult to plastically process.

[0018] Claim 3 The invention described in 2. A method for compressing a workpiece from above and below, comprising the steps of: applying reciprocating torsional vibration to the workpiece from either or both compression surfaces of upper and lower rams; or Claim 2 to This is a method for plastically processing the metal that is difficult to plastically process.

[0019] Claim 4 The invention described in On the compression surface to which the reciprocating torsional vibration is applied, knurling grooves are provided so that a plurality of sets of linear grooves formed to the same depth and arranged at equal intervals in parallel intersect with each other at a predetermined angle, The method according to claim 1, wherein a compressive force is applied to the workpiece and a reciprocating torsional vibration is applied to the workpiece. 3 This is a method for plastically processing a metal that is difficult to plastically process, as described in 1.

[0020] Claim 5 The invention described in The knurled grooves have a depth of 0.2 to 0.6 mm, an apex angle of 60 to 120°, an interval of 0.4 to 0.8 mm, and an intersection angle of 60 to 90°. 4 This is a method for plastically processing a metal that is difficult to plastically process, as described in 1.

[0022] Claim 6 The invention described in The metal material having a hexagonal close-packed lattice crystal structure is a magnesium alloy. Any one of claims 1 to 5 This is a method for plastically processing the metal that is difficult to plastically process.

[0023] Claim 7 The invention described in The metal material having a close-packed hexagonal lattice crystal structure is a titanium alloy. Any one of claims 1 to 5 This is a method for plastically processing the metal that is difficult to plastically process.

[0024] Claim 8 The invention described in 10. The method according to claim 1, wherein the plastic deformation is plastic deformation by upset forging. 7 The present invention relates to a method for plastically processing a metal that is difficult to plastically process, comprising the steps of: [Effects of the Invention]

[0026] According to the present invention, it is possible to provide a plastic processing technique that can increase the deformability of metals that are difficult to process and have low ductility in cold compressive plastic deformation. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a schematic diagram illustrating the main components and operation of a plastic processing device used in one embodiment of the present invention. FIG. [Figure 2] FIG. 2 is a diagram illustrating the relationship between the ratio of compression speed to torsion speed and the compression rate (%) at which cracks (ductile fracture) occur in one embodiment of the present invention. [Figure 3]FIG. 1 is a diagram illustrating the relationship between axial compressive stress and torsion / compression speed in one embodiment of the present invention. [Figure 4] FIG. 1 is a diagram illustrating the relationship between axial compressive stress σZ and rω / v in a non-work-hardenable isotropic material. [Figure 5] 1 is a diagram illustrating the relationship between the axial compressive stress σZ (MPa) and the shear stress τZθ (MPa) when no torsion is applied, when one-way torsion is applied, and when reverse torsional vibration is applied. DETAILED DESCRIPTION OF THE INVENTION

[0028] [1] The process leading to the completion of this invention First, the process leading to the completion of the present invention will be described.

[0029] In considering solutions to the above-mentioned problems, the inventors of the present invention considered that in conventional plastic processing of metals that are difficult to process, a load is applied to the workpiece by compressing it from only one direction (the axial direction), which generates a large compressive stress in the axial direction (axial compressive stress), and even a small plastic deformation (strain) exceeds the limit of the compressive stress, causing cracks (ductile fracture).

[0030] Therefore, the inventors considered that if the occurrence of the above-mentioned axial compressive stress could be suppressed during plastic processing, it might be possible to perform plastic processing that suppresses the occurrence of ductile fracture, and as a result, they came up with the idea of ​​adding a torsional force.

[0031] In other words, when a workpiece is rotated during plastic processing and a torsional force is applied from a direction (perpendicular to the axial direction) different from the main processing (axial direction), the two stress components generated by the torsional force, the circumferential shear stress and the axial compressive stress, are superimposed to become compressive stress on the workpiece, causing changes in the forming load, material flow characteristics, and strain distribution. This suppresses the occurrence of ductile fracture, makes it possible to control the material properties of the workpiece, and is thought to lead to reduced forming loads, the creation of complex shapes (net shaping) and material control (high functionality) of processed products.

[0032] Based on this idea, experiments were conducted and it was found that although the axial compressive stress can be reduced when a torsional force is applied in one direction, dislocations due to circumferential shear stress accumulate in the workpiece, leading to an increase in dislocation density and the occurrence of ductile fracture, and therefore sufficient plastic processing cannot be performed on the workpiece.

[0033] Therefore, next, we further investigated methods for suppressing the increase in dislocation density, and came up with the idea of ​​adding reversed torsional vibrations that repeatedly reverse back and forth during plastic processing. That is, by adding reversed torsional vibrations, dislocations caused by circumferential shear stress are periodically reversed and canceled out, making it possible to suppress the increase in dislocation density in the workpiece, and coupled with the fact that the addition of torsional force makes it possible to reduce axial compressive stress, we thought that cold plastic processing would be possible without causing ductile fracture.

[0034] Based on this idea, experiments were conducted and it was confirmed that when compression (compression speed) and reverse torsional vibration (rotation speed and amplitude) were appropriately combined, the direction of deformation could be changed moment by moment by adding reverse torsional vibration, even in cases where deformation occurs solely due to basal slip, such as in Mg alloys, or solely due to prismatic slip, such as in Ti alloys. This confirmed that large deformability could be obtained even in cold working, and that sufficient plastic processing was possible, leading to the completion of the present invention.

[0035] There have been previous research examples on compression processing with the addition of torsion, such as material property evaluation tests to evaluate stress-strain curves and deformability (cracking susceptibility), and severe strain processing tests to measure changes in metal structure due to the introduction of large strain. However, in all of these tests, unidirectional torsion is applied after the forging process is completed, and they do not perform compression processing by applying reversible torsional vibration simultaneously with compressive deformation during the forging process, as in the present invention.

[0036] [2] About the present invention The present invention was completed based on the above-mentioned concept and has the following features.

[0037] In other words, the method for plastically processing a metal that is difficult to process (hereinafter also referred to simply as the "plastic processing method") of the present invention is a plastic processing method in which a metal that is difficult to process is used as a workpiece and plastically deformed by cold compression, and is characterized in that the following two operations are performed simultaneously during the plastic processing.

[0038] The first operation is to compress the workpiece from above and below, applying a compressive force to the workpiece. The second operation is to vibrate the workpiece back and forth around its rotation axis at a predetermined rotation speed and amplitude, applying a reverse torsional vibration to the workpiece using a reverse torsional force. By performing these operations simultaneously, the workpiece is plastically deformed in the cold, performing plastic processing.

[0039] In this way, by applying compressive force and simultaneously applying reciprocating torsional vibrations to the workpiece, it is possible to perform cold forging and plastic processing with high deformability, even on metals that are difficult to process, such as Mg alloys and Ti alloys.

[0040] [3] Implementation form Specific embodiments of the present invention will be described below.

[0041] 1.Plastic processing equipment Since the present invention performs plastic processing (forging) by applying compressive forces to the workpiece from above and below while simultaneously applying bi-directional torsional vibrations from a direction different from the compressive direction, it cannot be performed with a normal press that applies compressive forces only from one axial direction. Therefore, in this embodiment, prior to carrying out the plastic processing method according to this embodiment, a plastic processing device compatible with this plastic processing method was designed and manufactured.

[0042] The inventors of the present invention considered that if one of the rams were to vibrate back and forth while sandwiching the workpiece between the upper and lower rams and compressing it from above and below, it would be possible to apply reciprocating torsional vibration to the workpiece.

[0043] Fig. 1 is a schematic diagram illustrating the main components of a plastic processing apparatus used in this embodiment and their operation. As shown in Fig. 1, in this embodiment, the plastic processing apparatus is equipped with two rams, one upper and one lower, so that the workpiece can be sandwiched between them from above and below and compressed by lowering the upper ram. The lower ram is equipped with a reciprocating torsional vibration applying mechanism (not shown) that applies reciprocating torsional vibration to the workpiece by rotating the lower ram in reciprocating directions around the rotation axis of the workpiece at a rotational speed of ω (° / sec) and an amplitude of α (°).

[0044] In the above description, the upper ram moves up and down and the lower ram vibrates back and forth, but as long as the workpiece can be compressed by the upper and lower rams and simultaneously rotated to impart bi-directional torsional vibration to the workpiece, either or both of the upper and lower rams may move up and down or vibrate back and forth.

[0045] In this embodiment, it is preferable that knurling grooves are formed on the compression surface on the side (the lower ram in this embodiment) where the workpiece sandwiched between the upper and lower rams is reciprocally vibrated to apply the reciprocating torsional vibrations, with multiple sets of linear grooves formed to the same depth and arranged at equal intervals in parallel, intersecting each other at a predetermined angle. The provision of such knurling grooves prevents the workpiece from slipping on the compression surface, so that the rotation of the lower ram can be reliably transmitted to the workpiece, applying a reciprocating torsional force and applying the reciprocating torsional vibrations to the workpiece.

[0046] From the viewpoint of appropriately preventing slippage of the workpiece, the knurled grooves preferably have a depth of 0.2 to 0.6 mm, an apex angle of 60 to 120°, an interval of 0.4 to 0.8 mm, and an intersection angle of 60 to 90°.

[0047] In addition, even in conventional end face restraint tests, grooves are sometimes formed on the compression surface, but these grooves are formed concentrically to prevent slippage in the radial direction, and usually multiple sets of linear grooves formed to the same depth as described above and arranged in parallel at equal intervals, intersecting each other at a predetermined angle, as in the case of knurling grooves, which prevent circumferential slippage of the workpiece, are not formed.

[0048] Furthermore, this type of plastic processing device can be implemented with relatively simple modifications, such as adding a torsional motion mechanism that functions as a torsional vibration adding mechanism to a conventional plastic forming device and replacing the corresponding processing program, thereby reducing the need for large costs. Furthermore, since it does not require high-temperature or high-pressure equipment, it is also preferable from a safety standpoint.

[0049] 2.Plastic working method Next, a plastic processing method using the above-mentioned plastic processing apparatus will be described. In the following, in order to facilitate a more concrete understanding of the contents of the present invention, an example will be described in which plastic processing is performed at room temperature using a cylindrical material obtained from an extruded bar of a commercially available AZ31B magnesium alloy (Mg-3 mass% Al-1 mass% Zn) as the workpiece, but the same can be applied to other metals that are difficult to process, such as Ti alloys.

[0050] As shown in Figure 1, the workpiece is sandwiched between the upper and lower rams, and then the upper ram is lowered in the -Z direction at a speed of v (mm / sec) and a stroke of s (mm) to compress the workpiece, thereby causing compressive deformation in the workpiece.

[0051] Meanwhile, the compression surface of the lower ram is provided with knurled grooves, and the lower ram is rotated in both directions around the Z axis at a rotational speed of ω (° / sec) and an amplitude of α (°). This allows a reciprocating torsional force to be applied to the workpiece without causing slippage between the compression surface and the workpiece, thereby adding reciprocating torsional vibration.

[0052] In this example, the workpiece was made of the above-mentioned AZ31B steel, and knurled grooves with a depth of 0.4 mm, an apex angle of 60°, and intervals of 0.8 mm were formed on the compression surface of the lower ram. Compression was initiated at a speed v of 0.005 to 0.1 mm / sec, and after the stroke s reached 0.3 mm, a reversed torsional force was applied around the central axis of the workpiece while maintaining the compression at a rotational speed ω of 0.5 rpm and an amplitude α of 3 to 45° (frequency: 0.02 to 0.25 Hz), thereby adding reversed torsional vibration.

[0053] The relationship between the ratio of twist to compression rate, which can be expressed as ω / v (° / mm), and the compression rate (%) at which cracking (ductile fracture) occurs, which can be expressed as s / h0, was determined. The results are shown in Figure 2. In Figure 2, the horizontal axis is ω / v (° / mm) and the vertical axis is s / h0 (%). The results for compression only (dashed line) and for torsion (55°) applied in only one direction are also shown.

[0054] Figure 2 shows that as ω / v increases, s / h0 increases, and that adding alternate torsional vibration improves it by up to approximately 1.8 times compared to compression alone without adding torsion.

[0055] In addition, ω / v and axial compressive stress σ Z The relationship between the ω / v (° / mm) and the σ (MPa) was calculated. The results are shown in Figure 3. In Figure 3, the horizontal axis is ω / v (° / mm) and the vertical axis is σ Z (MPa), and the results when a twist (55°) is applied in only one direction are also shown.

[0056] As can be seen from Figure 3, as ω / v increases, σ Z It can be seen that the increase in s / h0 mentioned above is due to the change in the stress-strain state caused by the addition of reverse torsional vibration.

[0057] This result is based on the assumption of a non-work-hardening isotropic material, and is expressed as the axial compressive stress σ in the Z direction in the rθz coordinate system. Zand shear stress τ in the θ direction Zθ Or τ rθ When considering the case where both operate simultaneously (τ Zθ =τ rθ σ is obtained by assuming that the plastic strain increments in the Z and θ directions, the equivalent stress, and the equivalent strain increments follow the flow rule and the Mises yield criterion. Z This is also in good agreement with the relationship between ω and rω / v (see Figure 4).

[0058] That is, Fig. 4 suggests that applying torsion in the θ direction while plastically deforming in the Z direction reduces the load in the Z direction and increases the applied strain, which is in good agreement with the results of this embodiment shown in Fig. 3. Furthermore, the temperature rise in the workpiece due to plastic working was kept below 15°C, confirming that the decrease in axial compressive stress shown in Fig. 3 is not due to thermal softening of the workpiece.

[0059] And, from Figures 2 and 3, σ Z Since it was found that the deformability varies greatly depending on the (MPa) and ω / v (° / mm), the axial compressive stress σ Z (MPa) and shear stress τ Zθ The relationship between the tensile strength and the compressive strength (MPa) was determined.

[0060] The results are shown in Figure 5. In Figure 5, the horizontal axis represents the axial compressive stress σ Z (MPa), the vertical axis is the shear stress τ Zθ The values ​​are in MPa. Circles represent data for no torsion, squares for one-way torsion, and triangles for double-ended torsional vibration. Black indicates cracking, and white indicates no cracking. The dashed line represents the forging limit, and the dashed-dotted line represents the yield stress and shear yield stress.

[0061] From Figure 5, the axial compressive stress σ Z (MPa) and shear stress τ ZθIt can be seen that if the axial compressive stress σ (MPa) is within the area surrounded by the dashed line and the dashed line, plastic working can be performed without generating cracks (ductile fracture). In the case of no torsion or one-way torsion, it hardly falls within this area, while in the case of reversing torsional vibration, it Z (MPa) and shear stress τ Zθ (MPa) remains within the above-mentioned range with an appropriate balance, so that plastic working can be performed without causing cracks.

[0062] As described above, in this embodiment, as ω / v (° / mm) increases, σ Z (MPa) decreases and s / h0 increases. At this time, if ω / v (° / mm) is greater than 0°, the effect of improving deformability is exhibited, but in order to achieve a sufficient improvement in deformability, it is preferable that it be 15° / mm or more. On the other hand, if ω / v (° / mm) exceeds a certain value, the improvement in deformability saturates, so in practice, it is preferable that it be 600° / mm or less. That is, from the viewpoint of appropriately improving deformability, ω / v (° / mm) is preferably 15 to 600° / mm. At this time, since increasing ω may cause slippage, it is preferable to maintain ω at a constant value and decrease v to increase ω / v.

[0063] In the above, the plastic processing of an Mg alloy, which deforms only by basal slip, has been described as an example. However, as mentioned above, the same can be said for a Ti alloy, which deforms only by prismatic slip. By constantly changing the deformation direction by applying reverse torsional vibration, a large deformability can be obtained.

[0064] As described above, the plastic processing method according to this embodiment is a much simpler method than conventional methods, in that it applies alternating torsional vibration simultaneously with compression, and is expected to be applicable not only to metal materials with a close-packed hexagonal lattice structure, but also to the plastic processing of metal materials that are difficult to process, such as high-strength steel. Furthermore, it is expected to be applicable not only to forging, but also to the compression of metal materials with an axisymmetric shape.

[0065] Furthermore, metal materials that have been plastically processed using the plastic processing method according to this embodiment can be easily and clearly distinguished from conventional plastically processed materials by observing their macroscopic structure.

[0066] In other words, in this embodiment, because plastic working is performed by applying reverse torsional vibration, the size of the area in the workpiece where deformation is concentrated is significantly different from that in conventional plastic working. This difference can be clearly distinguished when observing the macroscopic structure of the plastically worked metal material, and therefore, by observing this cross-sectional structure, it can be easily determined whether the plastic working method used is a conventional plastic working method or the plastic working method according to this embodiment.

[0067] Although the present invention has been described above based on the embodiments, the present invention is not limited to the above-described embodiments. Various modifications can be made to the above-described embodiments within the scope of the same or equivalent to the present invention.

Claims

1. A method for plastically processing a metal that is difficult to process, in which the metal is used as a workpiece and is plastically deformed by cold compression, comprising: the workpiece is a metal material having a hexagonal close-packed lattice crystal structure, The workpiece is compressed from above and below at a predetermined speed v (mm / sec) to apply a compressive force to the workpiece, and at the same time, The workpiece is reciprocally vibrated around the rotation axis of the workpiece at a predetermined rotation speed ω (° / sec) and a predetermined amplitude α (°), and a reciprocating torsional vibration is applied to the workpiece by a reciprocating torsional force. When the workpiece is subjected to cold plastic deformation, A method for plastically working a metal that is difficult to plastically work, characterized in that the ratio ω / v (° / mm) of ω to v is 15 to 600° / mm.

2. 2. The method for plastically processing metals that are difficult to plastically process according to claim 1, wherein the amplitude α (°) of the workpiece is 3° or more and less than 55°.

3. 3. A method for plastically processing difficult-to-plastically process metals according to claim 1 or claim 2, characterized in that reciprocating torsional vibrations are applied to the workpiece from either or both compression surfaces of the upper and lower rams that compress the workpiece from above and below.

4. On the compression surface to which the reciprocating torsional vibration is applied, knurling grooves are provided so that a plurality of sets of linear grooves formed to the same depth and arranged at equal intervals in parallel intersect with each other at a predetermined angle, 4. The method for plastically working a metal that is difficult to plastically work as set forth in claim 3, wherein a compressive force is applied to the workpiece, and reciprocating torsional vibrations are applied to the workpiece.

5. The method for plastically processing a difficult-to-plastically process metal according to claim 4, characterized in that the knurled grooves have a depth of 0.2 to 0.6 mm, an apex angle of 60 to 120 °, an interval of 0.4 to 0.8 mm, and an intersection angle of 60 to 90 °.

6. 6. The method for plastically processing a metal that is difficult to plastically process according to claim 1, wherein the metal material having a close-packed hexagonal lattice crystal structure is a magnesium alloy.

7. 6. The method for plastically processing a metal that is difficult to plastically process according to claim 1, wherein the metal material having a close-packed hexagonal lattice crystal structure is a titanium alloy.

8. 8. The method for plastically working a metal that is difficult to plastically work as set forth in claim 1, wherein the plastic deformation is plastic deformation by upset forging.

Images