JP2013542328A - Magnesium alloy sheet having improved room temperature formability and method for producing the same - Google Patents

Abstract

Disclosed are magnesium alloy sheet materials having improved room temperature formability and methods for producing the same. A method for manufacturing a magnesium alloy sheet with improved room temperature formability according to an embodiment of the present invention includes a first step of performing a pretreatment to add compressive residual stress to the surface of a magnesium alloy base material.
[Selection] Figure 1

Description

  The present invention relates to a magnesium alloy sheet and a method for producing the same, and more particularly to a magnesium alloy sheet having improved room temperature formability and a method for producing the same.

  Magnesium materials are not only 2/3 the density of aluminum and 1/4 the level of iron, but when alloyed with other metals, the thermal conductivity is 1.2 times that of zinc and 2 times that of iron. Compared to plastic, etc., it has excellent heat dissipation characteristics. In addition, because of its excellent electromagnetic shielding properties, vibration absorption properties, corrosion resistance, etc., it has been attracting attention as a material for automobile parts and portable electronic products that require weight reduction.

  However, the magnesium material (pure magnesium or magnesium alloy) is different from the production and processing of existing steel plates and aluminum alloy plates, which means that magnesium has a low elongation at room temperature. Magnesium has a hexagonal close-packed structure (HCP), and since it does not have a sufficient slip system for free deformation at room temperature, it is very brittle and has poor moldability. It is pointed out as a limit.

  Due to the limitation of such room temperature molding of magnesium material, conventionally, magnesium material has been mainly molded by using a casting method such as die casting. However, the casting method not only increases the manufacturing time and manufacturing cost because the mold has to be manufactured separately, but also has a disadvantage that the quality is non-uniform due to a high defect rate and it is difficult to manufacture a thin plate shape.

  On the other hand, in order to manufacture a magnesium material in a complicated shape, there is a method of forming through a press process or the like. In the press forming method, since a magnesium material is easily destroyed when it is molded at room temperature, warm forming is performed in which the temperature of the material and the mold is in the range of 200 to 300 ° C.

  However, in the case of the above-mentioned warm forming, not only is it difficult to perform a continuous process for mass production, but there is a problem that it is not suitable for regular use due to problems such as cost.

  Accordingly, there is an increasing demand for a magnesium material having improved room temperature formability so that it can be molded and processed through press working or the like even at room temperature.

  Embodiments of the present invention seek to provide a magnesium alloy sheet material having improved room temperature formability by applying compressive residual stress to the surface of the magnesium alloy and a method for producing the same.

  According to one aspect of the present invention, there is provided a method for producing a magnesium alloy sheet material having improved room temperature formability, including a first step of performing a pretreatment for adding compressive residual stress to a surface of a magnesium alloy base material. Can be provided.

  The first step may be any one of sand blast, shot peening, laser peening, and ultrasonic peening.

  In addition, the method may further include an annealing step of annealing the magnesium alloy base material before the first step.

  According to another aspect of the present invention, a magnesium alloy sheet material having improved room temperature formability is manufactured using the method for manufacturing a magnesium alloy sheet material having improved room temperature formability according to one aspect of the present invention. Can be provided.

  The Example of this invention can improve the low moldability in normal temperature of a magnesium material by performing the pre-processing process which adds a compressive residual stress to the surface of a magnesium alloy raw material.

  In addition, by providing a magnesium alloy sheet having improved room temperature formability, a continuous process for mass production is possible, and large area parts of a large area magnesium alloy can be produced.

It is a flowchart which shows the manufacturing method of the magnesium alloy board | plate material which improved the normal temperature formability by one Example of this invention. It is a graph which shows the grade of the residual stress measured by the drilling method of the comparative example 1- comparative example 3. FIG. It is a graph which shows the grade of the residual stress measured by the drilling method of Example 1- Example 3. It is a graph which shows the result value of the tension test of the magnesium alloy board | plate material applicable to Comparative Examples 1-3 and Examples 1-3. It is a graph which shows the result value of the Eriksen test of the magnesium alloy board | plate material which was pre-processed by the comparative example 2 and the comparative example 3, and the Example.

  FIG. 1 is a flowchart illustrating a method for manufacturing a magnesium alloy sheet having improved room temperature formability according to an embodiment of the present invention.

  Referring to FIG. 1, the method for manufacturing a magnesium alloy sheet with improved room temperature formability includes a first step (S200) of performing a pretreatment to add compressive residual stress to the surface of the magnesium alloy base material.

  In the present invention, a magnesium alloy means an alloy containing magnesium as a main component. For example, Mg-Al-Zn, Mg-Al, Mg-Zn, Mg-Mn-based AZ31, AZ61, AZ80, AZ91, AM100 alloy and the like can be mentioned. ZK-based high-strength alloys containing Zr; EZ, EK, HK, HZ, MH-based heat-resistant alloys containing Th, Ce; AS21, AS41, heat-resistant cast alloys containing Al, Si; Ag, R.A. QE22, QH21, WE54, which are heat-resistant alloys containing E, Th, Zr, etc .; and LA91, L14A1 alloys containing Li, and the like can also be included.

  A 1st step (S200) means performing the process which can add a compressive residual stress to the surface of a magnesium alloy in the pre-processing of a magnesium alloy raw material.

  Compressive residual stress refers to the stress remaining in the material even after the external force is completely removed after the material is deformed by plastic deformation.

  The process of adding compressive residual stress has been known to be performed to increase the fatigue life of metals. However, the inventors of the present invention have confirmed that, in the pretreatment process of the magnesium alloy base material, when compressive residual stress is applied to the surface of the magnesium alloy base material, the room temperature formability of the magnesium alloy can be improved.

  Accordingly, the present invention is characterized in that the low formability of the magnesium alloy at room temperature is greatly improved by adopting a step of applying compressive residual stress as a pretreatment step of the magnesium alloy base material.

  The first step (S200) is any one of a sand blasting process, a shot peening process, a laser peening process, and an ultrasonic peening process. Also good.

  Shot peening is a kind of cold working in which a steel ball called a shot ball is projected onto a metal surface at a high speed to hammer the metal surface. The shot ball collides with the surface of the metal at high speed, and the kinetic energy of the shot ball instantaneously causes plastic deformation on the surface of the material and comes off from the surface.

  Sand blasting is a process of applying compressive residual stress by jetting sand onto the surface of a material via compressed air.

  In laser peening, a layer made of an opaque coating and a transparent coating is covered on the surface of a material, and then a plasma pressure generated by irradiating a laser beam having a focal point diameter of about 2.5 mm to 25 mm. This is a step of applying compressive residual stress through shock waves caused by the above.

  Ultrasonic peening is a process in which an iron ball is resonated using ultrasonic waves, and the iron ball is dispersed on a metal surface to apply compressive residual stress.

  On the other hand, the step of applying compressive residual stress may be any step as long as it can apply compressive residual stress to the surface of magnesium, and is not limited to the specific example described above. However, in the following, for convenience of explanation, the explanation will be focused on the case where the process of applying compressive residual stress is a shot peening process.

  The method for improving the formability of a magnesium alloy at room temperature according to an embodiment of the present invention further includes an annealing step (S100) of annealing the magnesium alloy base material before the first step (S200). And

  The inventors of the present invention, in the pretreatment process of the magnesium alloy source material, after annealing the magnesium alloy source material, when compressive residual stress is applied to the surface of the magnesium alloy source material, room temperature molding of the magnesium alloy It was further confirmed that the property could be improved.

  The annealing means a heat treatment method in which the magnesium alloy base material is heated to a certain temperature and then gradually cooled to make the internal structure uniform.

  In the annealing step (S100), the heat treatment temperature may vary depending on conditions (type of material, previous state), and is not particularly limited. For example, the magnesium alloy can be air-cooled after heat treatment at 300 to 400 ° C.

  The heat treatment time is not particularly limited. For example, to optimize time and cost efficiency, the heat treatment time can be performed within 30 minutes.

  As described above, the embodiment of the present invention can improve the low formability at room temperature, which is a conventional disadvantage of magnesium material, by performing a pretreatment step of adding compressive residual stress to the surface of the magnesium alloy raw material. There is an effect.

  In addition, the magnesium alloy sheet material having improved room temperature formability manufactured using the method according to the embodiment of the present invention enables a continuous process for mass production and can produce a large area magnesium alloy sheet material. Has a positive effect.

Hereinafter, the present invention will be described in more detail by way of comparative examples and examples. However, the following comparative examples and examples are merely illustrative for the purpose of explaining the present invention in detail, and it is clear that the scope of rights of the present invention is not limited.
(Example)
<Preparation of the original magnesium alloy material>

  Under the conditions shown in Table 1 below, raw materials corresponding to Comparative Examples 1 to 3 and Examples 1 to 4 were prepared. The basic material is AZ31B, the thickness is 1.6 mm, and the plate material is manufactured by a strip casting process. Strip casting refers to a process in which a plate material is produced by cooling while passing directly between rolling rolls with a molten metal.

  A shot peening process was used as a process for applying compressive residual stress. As performance conditions, shot balls (Aluminia Powder # 240) were sprayed for 60 seconds to 120 seconds under conditions of a pressure of 0.8 MPa and a spray distance of 15 cm.

  On the other hand, annealing was maintained for 20 to 60 minutes at each temperature, and then air-cooled.

＜ビッカース硬さ試験（Ｖｉｃｋｅｒ ｈａｒｄｎｅｓｓ ｔｅｓｔ）＞

<Vickers hardness test>

  A Vickers hardness test was performed on Comparative Examples 1 and 2 and Example 2 in order to confirm whether or not compression residual stress was applied to the surface of the magnesium alloy source material. The Vickers hardness test was carried out using a Micro Vickers FM 700 hardness tester, and the test conditions were a test load of 500 gf and a dwell time of 5 sec. In Table 2 below, the hardness changes according to the thickness direction of the magnesium alloy base material corresponding to Comparative Examples 1 and 2 and Example 2 are arranged. On the other hand, in the case of Example 2, the hardness was measured by dividing the magnesium alloy base material into an upper part (surface), an inner part, and a lower part.

  Referring to Table 2 above, the hardness when the raw material was not pretreated (Comparative Example 1) was measured as Hv 65.8 (average value, hereinafter the same). Further, when only the annealing treatment is performed on the original material (Comparative Example 2), the hardness is measured as Hv 58.5, and it can be confirmed that the original material is further softened.

On the other hand, when the original material is annealed and compressive residual stress is applied (Example 2), the hardness of the upper part (surface) of the original material increases to Hv 90.5 (maximum Hv 98.6), It can also be confirmed that the hardness has increased to Hv 73.43 (maximum Hv 75.1). That is, in Example 2, both the surface and internal hardness increased compared to Comparative Example 1 or 2. Therefore, it can be seen that the surface of the magnesium plate material is plastically deformed and hardened by the compressive residual stress adding step (shot peening), and at the same time, the compressive residual stress remains.
<Measurement of Residual Stress by Hole Drilling Method>

For the comparative examples and examples, compressive residual stress was measured by a hole drilling method (Hole Drilling Method) according to ASTM E837-08. That is, a quantitative result was derived for the presence or absence of residual stress presented from the Vickers hardness test.
The perforation method is based on the theory that when a material with residual stress is perforated, the surrounding constraints are released to reach the equilibrium state of the stress. To do.

  The equipment used for this measurement was RS-200 (drilling equipment, Vishay, USA), P-3 (Strain Indicator, Vishay, USA), H-drill ver 3.10 (analysis S / W, Vishay, USA). The strain gauge used was CEA-06-06UL-120 (Vishay, USA).

  On the other hand, for the measurement of residual stress, the center of the test piece was determined to be No. 2 gauge, and the holes of No. 1 and No. 3 gauges were positioned 20 mm away from the left and right with reference to this.

  FIG. 2 a is a graph showing the degree of residual stress measured by the drilling method of Comparative Examples 1 to 3.

  The (−) direction from the reference point indicates that compressive residual stress exists in the test piece, and the (+) direction indicates that tensile residual stress exists in the test piece. On the other hand, N1, N2, and N3 indicate the number of tests.

  Referring to FIG. 2a, it can be confirmed that compressive residual stress exists when the raw material is not pretreated (Comparative Example 1), and when only the annealing treatment is performed (Comparative Examples 2 and 3) It can be confirmed that there is residual stress.

  FIG. 2 b is a graph showing the degree of residual stress measured by the drilling method of Examples 1 to 3. S1, S2, and S3 indicate the number of tests.

  Referring to FIG. 2b, when compressive residual stress is added to the original material (Example 1), it can be confirmed that the compressive residual stress is increased as compared with Comparative Example 1 described above. In addition, when compressive residual stress was applied after the annealing treatment (Examples 2 and 3), the tensile residual stress when only the annealing treatment was performed (Comparative Examples 2 and 3) was changed to the compressive residual stress state. Can be confirmed.

Therefore, it can be seen that compressive residual stress was generated on the surface of the magnesium plate material by the compression residual stress adding step (shot peening).
<Tensile test>

  A uniaxial tensile test was performed on Comparative Example 1 and Examples 1 to 3. In the tensile test, an ASTM-E8M standard test piece (gauge length 25 mm) was used in an INSTRON-4206 apparatus, and the test speed was 75 mm / min.

  FIG. 3 is a graph showing results of tensile tests of magnesium alloy sheet materials corresponding to Comparative Examples 1 to 3 and Examples 1 to 3. Referring to FIG. 3, the graph is a stress-strain curve, which is a curve showing the degree of deformation of the test piece with respect to the stress applied to the test piece.

  Elongation means a numerical value obtained by dividing the length of the test piece in the stress-strain curve by the initial length of the test piece, and is a general measure of the formability of the material. .

  First, in Comparative Example 1 in which the raw material was not pretreated, the tensile strength was 285.69 MPa, and the elongation was measured to be 15%.

  In comparison with this, as shown in the graph, in Example 1 in which only the process of applying compressive residual stress to the original material was performed, the tensile strength was 276.58 Mpa and the elongation was 17.62. It can be seen that there is a slight improvement. In Example 2 where the raw material was annealed at 345 ° C. and subjected to the step of applying compressive residual stress, it was confirmed that the tensile strength was 262.46 Mpa and the elongation was improved to 23.16%. can do.

On the other hand, in the case of a magnesium plate material, since it is often in a stress state of two or more axes, it is difficult to correctly determine formability only by a uniaxial tensile test. Therefore, the judgment of the formability of the magnesium sheet must be made together with the results of the Erichsen test and the deep drawing test described later together with the uniaxial tensile test.
<Formability test (Ericsen test)>

  A moldability test was performed on Comparative Examples 1 to 3 and Examples 1 to 4. The Erichsen test was used as a formability test method. The Eriksen test is a typical test method for determining the formability of a plate material. The height of the ball-shaped punch rising up and breaking the material while the material is pressed between the upper and lower dies is measured. It means to measure the formability of the plate material by measuring.

  The Erichsen test was performed on an Erichsen-142 / 40 apparatus, the test conditions were pressure 0.8 Mpa, punch speed 0.5 mm / s, blank holding force 3.5 kN.

  The Eriksen value (Index of Erichsen) derived from the Eriksen test is an index indicating the formability of the material without deforming, and is representative of the formability in a biaxial or higher stress state, unlike the uniaxial tensile test. Index. The higher the Erichsen value, the better the moldability.

  In Table 3 below, the Erichsen values of Comparative Examples 1 to 3 and Examples 1 to 4 are shown as the moldability test results described above. In this connection, FIG. 4 is a graph showing the Eriksen test result values of the magnesium alloy sheets pretreated by Comparative Examples 2 and 3 and Examples.

  In the graph, the Y-axis is force, and the X-axis indicates the degree of elongation of the magnesium alloy sheet. Therefore, by comparing the degree of elongation of the magnesium alloy sheet material with the same force, it is possible to compare the formability of the magnesium alloy sheet material between the comparative example and the example described above.

  Referring to Table 3 and FIG. 4 described above, compared with the case where the raw material is not pretreated (Comparative Example 1), whether the annealing treatment is performed (Comparative Example 2 and Comparative Example 3), or compressive residual stress on the raw material When the step of adding is performed (Example 1), excellent results are obtained in the formability test, and it can be seen that there is an effect of increasing the formability of the magnesium alloy sheet.

  Further, when the step of adding compressive residual stress is sequentially performed after the annealing treatment (Example 2 and Example 3), the magnesium alloy sheet is formed more than the case of performing only the annealing treatment or the step of applying compressive residual stress. It can be confirmed that there is an effect of increasing the sex.

However, even when the step of applying compressive residual stress was sequentially performed after the annealing treatment, it was confirmed that the formability was rather lowered when the annealing treatment temperature was 400 ° C. or higher.
<Deep drawing square cup molding>

  A deep drawn square cup was molded for Comparative Examples 1 and 2 and Example 2. Deep drawing is a typical forming method for making a seamless hollow container from a flat plate, and is a technique for making a side wall by moving between a punch and a die while shrinking a material on a die surface in a circumferential direction.

  In the case of Comparative Examples 1 and 2, the deep drawing rectangle was molded at a mold temperature of 100 ° C., a punch speed of 0.43 mm / s, a molding height of 7 mm, and was molded to a size of 40 × 60 mm. On the other hand, in the case of Example 2, the temperature of the mold is set at room temperature, and the remaining conditions are the same as described above.

  As a result of execution, in the case of the original material without pretreatment (Comparative Example 1) or the original material subjected to only annealing (Comparative Example 2), it was observed that the formation of the square cup failed due to breakage.

  However, in the case of the original material (Example 2) which has been subjected to the annealing and compressive residual stress addition process as a pre-treatment, the moldability is improved by successfully forming the square cup without breaking even though it is molded at room temperature. We were able to confirm that improved.

  As described above, the embodiments of the present invention have been described. However, those who have ordinary knowledge in the technical field can add components without departing from the spirit of the present invention described in the claims. The present invention can be variously modified and changed by changing, deleting, or adding, and it can be said that this is also included in the scope of the right of the present invention.

Claims (4)

  A method for producing a magnesium alloy sheet with improved room temperature formability, comprising a first step of performing a pretreatment to add compressive residual stress to the surface of a magnesium alloy base material. The first step includes
The room temperature molding according to claim 1, wherein the process is one of sand blasting, shot peening, laser peening, and ultrasonic peening. For producing a magnesium alloy sheet having improved properties. Before the first step,
The method for manufacturing a magnesium alloy sheet with improved room temperature formability according to claim 1, further comprising an annealing step of annealing the magnesium alloy source material.   A magnesium alloy sheet having improved room temperature formability, wherein the magnesium alloy sheet is manufactured using the method according to any one of claims 1 to 3.

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