Method for manufacturing metal member having residual stress

Abstract

To provide a method for producing a metal member that is less susceptible to release of residual stress.SOLUTION: A method for producing a metal member includes the steps of: performing a first treatment to atomize crystal grains of a metal member; performing a second treatment, followed by the first treatment, to relax the residual stress of the metal member; and performing a third treatment, followed by the second treatment, to give the metal member residual stress.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a metal component having a residual stress imparted thereto. [Background technology]

[0002] Techniques for imparting residual stress to metal members are known in order to improve the fatigue strength, etc., of metal members. For example, Patent Document 1 describes a method of applying laser peening or the like to a suspension spring mounted on an automobile to impart compressive residual stress to the interior of the suspension spring, and then applying shot peening to the suspension spring to impart compressive residual stress to the surface layer portion of the suspension spring. In this way, by introducing compressive residual stress into the suspension spring from the surface layer to a certain depth, it is possible to maintain the fatigue strength of the suspension spring even if corrosion occurs on the surface of the suspension spring. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-121262 Summary of the Invention [Problem to be solved by the invention]

[0004] As described above, residual stress is imparted to a metal component by performing a peening treatment on the metal component. The residual stress imparted to the metal component is released and gradually alleviated over time. In particular, in aluminum alloys, β-phase compounds are precipitated over time, and as these β-phase compounds precipitate, they alleviate the internal strain of the aluminum alloy. Therefore, the residual stress of aluminum alloys is easily released over time. When the residual stress of a metal component is released over time in this way, it causes a decrease in the fatigue strength of the metal component.

[0005] Therefore, an object of the present disclosure is to provide a method for manufacturing a metal component in which residual stress is not easily released. [Means for solving the problem]

[0006] In one embodiment, a method for manufacturing a metal component imparted with residual stress is provided, which includes the steps of performing a first treatment to refine the crystal grains of the metal component, performing a second treatment after the first treatment to relieve the residual stress in the metal component, and performing a third treatment after the second treatment to impart residual stress to the metal component.

[0007] In the method according to the above aspect, the crystal grains of the metal component are refined by the first treatment. Here, since the reliability of X-ray residual stress measurement of the metal component depends on the number of crystals in the metal component, by refining the crystal grains of the metal component and increasing the number of crystals, it is possible to improve the reliability of X-ray residual stress measurement of the metal component. Then, by performing the second treatment and the third treatment after the first treatment, it is possible to effectively impart residual stress to the metal component while relatively suppressing the release of the residual stress.

[0008] In one embodiment, the metal member may be made of an aluminum alloy containing Mg, Cu, or Mn. Aluminum alloys containing Mg, Cu, or Mn are prone to precipitating β-phase compounds. When these β-phase compounds are precipitated, strain within the metal member is alleviated, and residual stress introduced into the metal member is released. In contrast, in the above embodiment, residual stress can be effectively imparted to the metal member by performing the second and third treatments after the first treatment. Meanwhile, because the amount of Mg, Cu, or Mn contained in the metal member is constant, the release of residual stress can be relatively suppressed.

[0009] In one embodiment, the first treatment may be a shot peening treatment. By subjecting the metal member to a shot peening treatment, the crystal grains of the metal member can be effectively refined.

[0010] In one embodiment, the third treatment may be a laser peening treatment. By performing the laser peening treatment on the metal member, residual stress can be effectively imparted to the metal member without significantly changing the crystal structure of the metal member.

[0011] In one embodiment, the step of performing the third treatment may include the steps of forming a sacrificial layer on the surface of the metal member and irradiating the sacrificial layer with laser light. By irradiating the sacrificial layer with laser light, it is possible to impart residual stress to the metal member with high uniformity while suppressing damage to the surface of the metal member.

[0012] In one embodiment, the second treatment may be a thermal refining treatment in which the metal member is heated for six hours or more at a temperature of 125° C. or higher and 170° C. or lower. By heating the metal member for six hours or more at a temperature of 125° C. or higher and 170° C. or lower, it is possible to reduce residual stress in the metal member while suppressing recrystallization of the metal member. [Effects of the Invention]

[0013] According to the present disclosure, it is possible to manufacture a metal component in which residual stress is not easily released. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a flowchart illustrating a method for manufacturing a metal member according to an embodiment. [Figure 2] 10 is a flowchart showing the flow of a laser peening process. [Figure 3] 1 is a graph showing changes in residual stress of a metal member over time. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following description, identical elements or elements having identical functions will be denoted by the same reference numerals, and redundant description will be omitted.

[0016] In one embodiment, a metal component having residual stress is manufactured. Examples of the manufactured metal component include automobile, railway, and aircraft components, which require high fatigue strength and wear resistance. Examples of the metal component include, but are not limited to, an alloy containing iron as a main component, such as steel, or an aluminum alloy. For example, the metal component may be made of an aluminum alloy that precipitates β-phase compounds. An example of an aluminum alloy that precipitates β-phase compounds is an aluminum alloy containing at least one element selected from Mg, Cu, and Mn.

[0017] In this type of aluminum alloy, during the isothermal aging process, plate-shaped stable phase precipitates (β phase) are precipitated through a supersaturated solid solution, an acicular zone, and rod-shaped precipitates in that order. The total volume of the precipitated β phase decreases from the total volume of the supersaturated solid solution. If the energy generated by the decrease in total volume is E and the energy required for β phase precipitation, i.e., the generation of a new phase, is G, then G > E. Normally, G = E + H is calculated by adding thermal energy H. By using the strain energy ΔG contained within the material as a substitute for H, β phase can be precipitated even at room temperature. If the total content of Mg, Cu, and Mn is at least 0.2%, β phase compounds are formed.

[0018] Residual stress may be imparted to a metal component by peening. By imparting residual stress to a metal component, the fatigue strength of the metal component is improved. However, the residual stress imparted to the metal component is gradually released over time. In particular, when the metal component is made of an aluminum alloy containing elements such as Mg, Cu, and Mn, a β-phase compound is formed on the strain introduced into the metal component by the reaction between these elements and aluminum. At this time, the strain in the metal component is relaxed, and the residual stress is released. Below, a method for manufacturing a metal component in which residual stress is difficult to release is described.

[0019] 1 is a flowchart showing a method for manufacturing a metal member according to one embodiment. As shown in Fig. 1, in the method for manufacturing a metal member according to one embodiment, a metal member that is an object to be processed is prepared (step ST1).

[0020] The metal member prepared in step ST1 is, for example, an aluminum alloy member containing Mg, Cu, or Mn. More specifically, the metal member may be a wrought aluminum alloy or a cast aluminum alloy. The aluminum alloy member may be a bulk body that has been subjected to a thermal refining treatment such as annealing in advance. Note that the metal member may be made of a metal material other than an aluminum alloy, such as steel, as long as the material releases residual stress over time.

[0021] Next, a first process is performed to refine the crystal grains of the prepared metal member (step ST2). Examples of the first process include, but are not limited to, shot peening, Equal-Channel Angular Pressing (ECAP), High-Pressure Torsion (HPT), and cold forging. These methods can refine the crystal grains of the metal member by plastically deforming the metal member to change its crystal structure. For example, shot peening is a method of imparting compressive residual stress to the surface of the metal member by projecting a shot medium onto the metal member and causing it to collide with the metal member. When the surface of the metal member is plastically deformed by the impact of the shot medium, the crystal grains in the surface layer of the metal member are refined.

[0022] Known shot peening processes include air shot peening, in which shot media is sprayed using compressed air, and impeller shot peening, in which shot media is sprayed using the centrifugal force of a rapidly rotating impeller. Shot media made of, for example, zirconia, glass, or steel are used. The diameter of the shot media can be, for example, 20 μm or more and 2000 μm or less. The hardness of the shot media can be, for example, 250 HV or more and 700 HV or less.

[0023] When air shot peening is used as the first treatment, the injection pressure of the compressed air can be, for example, 0.05 MPa or more and 1 MPa or less. When impeller shot peening is used as the first treatment, the rotation speed of the impeller can be, for example, 20 m / s or more and 150 m / s or less. The coverage of the shot peening can be, for example, 80% or more and 500% or less. The conditions for shot peening are set appropriately depending on the material of the metal member.

[0024] As described above, the crystal grains of the metal member are refined by plastically deforming the surface of the metal member through the first treatment. The grain size of the refined crystal grains is, for example, 5 nm to 50 nm, and may be 5 nm to 20 nm. The refined crystal grains are present, for example, within a depth range of 50 μm from the surface of the metal member. Refining the crystal grains of the metal member increases the number of crystals in the metal member. Since the amount of strain that can be introduced into the metal member depends on the number of crystals, refining the crystal grains of the metal member through the first treatment increases the residual stress that can be introduced into the metal member.

[0025] In one embodiment, after the first treatment and before the second treatment described below, the compressive residual stress imparted to the metal member may be measured by X-ray residual stress measurement. For example, by confirming the occurrence of continuous Debye rings, it is confirmed that a certain number or more of crystal grains exist within the measurement range of the X-ray residual stress measurement.

[0026] After the first treatment, a second treatment is performed to relieve residual stress in the metal member whose crystal grains have been refined (step ST3). The second treatment may relieve residual stress in the metal member while maintaining the state in which the crystal grains have been refined by the first treatment. The second treatment is, for example, a thermal refining treatment in which the metal member is held at a temperature of 125°C or higher and 170°C or lower for one hour or longer, and then cooled to room temperature. The generation of β-phase compounds is promoted by heating the aluminum alloy in a temperature range of 125°C or higher. Furthermore, the recrystallization temperature of aluminum alloys is 180°C or higher. Therefore, by heating the metal member at a temperature of 125°C or higher and 170°C or lower, the residual stress in the metal member can be relieved while suppressing recrystallization of the metal member.

[0027] In the second treatment, the time (heat treatment time) for which the metal member is held at a temperature of 125°C or higher and 170°C or higher is at least 1 hour, preferably 6 hours or higher. By heat treating the metal member for 1 hour or longer, the residual stress of the metal member can be alleviated without significantly changing the crystal grain size of the metal member. Furthermore, by promoting the formation of β-phase compounds by the second treatment, the formation of β-phase compounds is suppressed after the second treatment, and therefore the release of residual stress in the metal member over time is suppressed.

[0028] The heat treatment time for the second treatment may be 72 hours or less, 48 ​​hours or less, or 24 hours or less. By setting the heat treatment time to 72 hours or less, recrystallization of the metal component can be more reliably suppressed and the metal component can be manufactured with high productivity. Furthermore, by temporarily relaxing the residual stress in the metal component in the second treatment, the introduction of residual stress into the metal component in the third treatment described below is promoted.

[0029] In one embodiment, the second treatment is performed using a device capable of maintaining a constant temperature, such as a drying oven or a muffle furnace. The second treatment is performed, for example, until the half-width of the X-ray diffraction peak of the metal component changes by 0.1 degrees or more, preferably 0.2 degrees or more, before and after the thermal treatment. The second treatment is also performed so that the change (absolute value) between the residual stress of the metal component within 24 hours after the thermal treatment and the residual stress of the metal component 24 hours or more after the thermal treatment is 50 MPa or less. This prevents the residual stress of the metal component from being released over time after the thermal treatment.

[0030] In one embodiment, after the second process and before the third process described below, the compressive residual stress imparted to the metal member may be measured by X-ray residual stress measurement. For example, by confirming the occurrence of continuous Debye rings, it is confirmed that a certain number or more of crystal grains exist within the measurement range of the X-ray residual stress measurement.

[0031] After the second process, a third process is performed to impart residual stress to the metal member (step ST4). The third process is a processing process different from the first process. Examples of the third process include, but are not limited to, laser peening, cavitation peening, water jet peening, and low plasticity burnishing (LPB). Unlike the first process, these third processes can impart residual stress to the metal member without significantly changing the crystal structure of the metal member. For example, shot peening deforms the crystal grains of the metal member, while laser peening changes the interlattice distance of the crystals without changing the crystal grains of the metal member. In other words, although both shot peening and laser peening are processes that impart residual stress to the metal member, the mechanisms by which the residual stress is generated are different.

[0032] Fig. 2 is a flowchart showing the process flow when laser peening is employed as the third process. As shown in Fig. 2, when laser peening is performed on a metal member, a sacrificial layer is first formed on the surface of the metal member that has been subjected to the first process in order to protect the surface of the metal member (step ST11). For example, a black film tape is used as the sacrificial layer. Next, a laser beam is irradiated onto the sacrificial layer using a laser peening device (step ST12). At this time, a pulsed laser having a power density sufficient to generate laser ablation is used as the laser beam irradiated onto the metal member.

[0033] When a sacrificial layer attached to the surface of a metal member is irradiated with laser light, the sacrificial layer is converted into plasma and a shock wave is induced. This shock wave propagates through the metal member, imparting residual stress to the metal member. At this time, since the residual stress has been released from the metal member by the second process, the residual stress is effectively introduced into the metal member by the third process. When performing laser peening, the metal member may be irradiated with laser light while the metal member is placed in water or with a water film formed on the surface of the metal member.

[0034] In one embodiment, the laser beam may be directly irradiated onto the surface of the metal member that has been subjected to the first treatment without forming a sacrificial layer on the surface of the metal member. Even when the laser beam is directly irradiated onto the surface of the metal member, it is possible to impart compressive residual stress to the metal member.

[0035] As described above, in the method for manufacturing a metal component according to one embodiment, a metal component having compressive residual stress can be manufactured by sequentially performing the first treatment, the second treatment, and the third treatment. The metal component manufactured in this manner has a property that the compressive residual stress is difficult to release.

[0036] The above describes various embodiments of the method for manufacturing a metal component, but the invention is not limited to the above-described embodiments and various modifications can be made within the scope of the invention without departing from its essence.

[0037] For example, in the above embodiment, the metal component is made of an aluminum alloy, but the metal component may be made of a metal material other than an aluminum alloy. Aluminum alloys have the property that compressive residual stress is easily released by the formation of β-phase compounds, but compressive residual stress is also released over time in metal materials other than aluminum alloys, such as steel. Even with these materials, a metal component in which residual stress is less likely to be released can be manufactured by sequentially performing the first, second, and third treatments described above.

[0038] Next, the effects of the above-described method for manufacturing a metal member will be described based on examples and comparative examples, but the present invention is not limited to the following examples.

[0039] First, in Example 1, a sample part made of a wrought aluminum alloy specified in JIS (Japanese Industrial Standards) A7075 was prepared. Next, this sample part was subjected to a shot peening treatment as a first treatment. A steel shot medium (particle size: 0.10 mm) was used as the shot material for the shot peening treatment. The treatment conditions for the shot peening treatment were as follows: (Shot peening conditions) Injection pressure: 0.2MPa Shot media injection rate: 9.0 kg / min Coverage: 300% Arc height: 0.118mA

[0040] Next, in Experimental Example 1, as the second treatment, the shot-peened sample part was heated at 150°C for 6 hours and then cooled to room temperature, thereby releasing the compressive residual stress in the sample part without recrystallizing the crystal grains.

[0041] The sample parts with the released compressive residual stress were then subjected to laser peening as the third treatment to impart compressive residual stress to the sample parts. The treatment conditions for the laser peening were as follows: (Laser peening conditions) Laser spot diameter: 0.4 mm Power density: 12GW / cm 2 Pulse energy: 100mJ ·Irradiation density: 56Pulses / mm 2

[0042] In contrast, in Comparative Example 1, only shot peening was performed on the same sample part as in Experimental Example 1. The shot peening conditions in Comparative Example 1 were the same as those in Example 1. In Comparative Example 2, only laser peening was performed on the same sample part as in Experimental Example 1. The laser peening conditions in Comparative Example 2 were the same as those in Example 1. Then, the residual stress of the sample parts obtained in Experimental Example 1, Comparative Example 1, and Comparative Example 2 was measured. The residual stress was measured using an X-ray residual stress measurement device μ-X360s manufactured by Pulstec Industrial Co., Ltd.

[0043] Fig. 3 is a graph showing the change over time in residual stress of the sample parts obtained in Experimental Example 1, Comparative Example 1, and Comparative Example 2. The vertical axis of Fig. 3 represents the residual stress of the sample parts, and the horizontal axis represents time (days). In Fig. 3, tensile residual stress is represented as a positive value, and compressive residual stress is represented as a negative value.

[0044] As shown in Figure 3, the sample part of Comparative Example 1 exhibited a residual stress of -250 MPa immediately after shot peening, but the residual stress changed to -210 MPa within 10 days after shot peening. In other words, the compressive residual stress of the sample part of Comparative Example 1 was released over time. Furthermore, the compressive residual stress of the sample part of Comparative Example 1 continued to be released even after 10 days after shot peening, and changed to -190 MPa within 120 days.

[0045] The residual stress of the sample part of Comparative Example 2 was -210 MPa immediately after the laser peening treatment, but the residual stress changed to -190 MPa within 10 days after the laser peening treatment. In other words, it was confirmed that the compressive residual stress of the sample part of Comparative Example 2 was also released over time.

[0046] In contrast, it was confirmed that the residual stress of the sample part of Example 1 remained constant at around -200 MPa for 120 days. This result confirmed that the compressive residual stress was difficult to release in the sample part manufactured in Example 1.

Claims

1. A method for manufacturing a metal member made of an aluminum alloy containing Mg, Cu, or Mn, comprising: a step of performing a first treatment of plastically deforming the metal member to refine the crystal grains of the metal member; After the first treatment, a second treatment is performed in which the metal component is heated at a temperature of 125°C or higher and 170°C or lower, thereby reducing residual stress in the metal component while suppressing recrystallization of the metal component and promoting the generation of β-phase compounds; performing a third treatment after the second treatment, which is laser peening, cavitation peening, water jet peening, or low-plasticity burnishing, to impart residual stress to the metal member; A manufacturing method comprising:

2. The manufacturing method according to claim 1 , wherein the first treatment is a shot peening treatment.

3. The manufacturing method according to claim 1 or 2, wherein the third treatment is a laser peening treatment.

4. The step of performing the third treatment includes: forming a sacrificial layer on the surface of the metal member; irradiating the sacrificial layer with laser light; The method of claim 3, comprising:

5. 5. The manufacturing method according to claim 1, wherein the second treatment is a thermal refining treatment in which the metal member is heated at a temperature of 125° C. or higher and 170° C. or lower for 6 hours or longer.

6. The manufacturing method according to any one of claims 1 to 4, wherein the second treatment is a thermal refining treatment in which the metal member is heated at a temperature of 125°C or higher and 170°C or lower for 1 hour or higher and 72 hours or lower.

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