Aluminum alloy with reduced residual stress, and manufacturing method therefor

Abstract

The present invention relates to an aluminum alloy with reduced residual stress, and provides an aluminum alloy which is an Al—Cu alloy containing a transition metal, and which has, during post-solution-annealing quenching, residual stress lower than that of an Al—Cu alloy containing no transition metals. The alloy of the present invention may inhibit dimensional deformation and damage, which may be caused by residual stress.

Description

BACKGROUNDField of the Disclosure

[0001] The present disclosure relates to an aluminum alloy with reduced residual stress and, more specifically, to reducing the residual stress by including a transition metal in the aluminum alloy.Description of Related Art

[0002] In order to relieve residual stress during preparing of a component, not only a method of slowly cooling an Al—Cu—Mg-based alloy, but also a method of compensating for lost mechanical properties by adding elements to the alloy via controlling the contents of Fe and Mn in the Al—Cu—Mg-based alloy are known. However, these methods are limited to plates and cannot be applied to parts with complex shapes. In addition, these methods have a limitation in that cracking and warpage of the components caused by macro-residual stress are not suppressed.

[0003] In addition, in order to prevent residual stress from being generated inside complex parts, there is a method of removing residual stress in the parts by coating a soft material mixed with wax on the surface of a casting frame to flexibly receive deformation due to macroscopic residual stress during cooling. However, when the temperature of the surface of the casting frame is kept high to keep the wax in a soft state, the cooling rate is very lowered such that mechanical properties are deteriorated, and post-treatment is required to remove the wax adhered to the surface of the component. Further, in order to flexibly receive the deformation due to cooling, this scheme is not applicable to the component requiring high dimension accuracy.

[0004] In order to suppress dimensional deformation and breakage of a thin part caused by the residual stress due to a local difference in a cooling rate during a casting process of a component having a complex shape such as an automobile component, a transition metal is added. However, the addition of the transition metal results in an increase in yield strength of a cast material, thereby causing the deformation.

[0005] In accordance with the present disclosure, a method for controlling residual stress of an aluminum casting material by adding a transition metal will be described.SUMMARY OF THE INVENTION

[0006] A purpose of the present disclosure is to provide an aluminum alloy to which a transition metal is added in order to provide the aluminum alloy in which residual stress is reduced.

[0007] Another purpose of the present disclosure is to provide a method for preparing an aluminum alloy in which residual stress is reduced by adding a transition metal thereto.

[0008] According to an aspect of the present disclosure, there is provided an Al—Cu alloy having reduced residual stress, wherein the Al—Cu alloy contains a transition metal, wherein the residual stress of the Al—Cu alloy containing the transition metal is lower that residual stress of an Al—Cu alloy free of the transition metal in a rapid cooling process after a solution heat treatment.

[0009] In the Al—Cu alloy having reduced residual stress, the reduced residual stress of the aluminum alloy may be in a range of −90 to −40 MPa.

[0010] In the Al—Cu alloy having reduced residual stress, the solution heat treatment process may include a heat treatment performed at 500 to 600° C. for 5 to 7 hours.

[0011] In the Al—Cu alloy having reduced residual stress, in the rapid cooling process, the heat-treated alloy may be continuously immersed in water at a temperature of 20 to 30° C. after the solution heat treatment. In the rapid cooling process, the temperature may be lowered to room temperature rapidly.

[0012] The transition metal may be added to the aluminum casting material to suppress dimensional deformation and breakage due to residual stress generated during the casting process based on the improvement of yield strength. In general, the macroscopic residual stress that causes deformation in the cast during the casting process is generated due to thermal stress due to the local difference in the cooling rate. When the stress is higher than the yield strength of the cast material, it leads to deformation. When the phenomenon occurs in rapid cooling after solidification of molten aluminum in the casting process and a solution treatment in the heat treatment process, the thin part having a small thickness in the complex shape of the component may be maximized in deformation according to this phenomenon.

[0013] Therefore, the alloy design is performed to improve the yield strength of the cast material capable of suppressing the deformation of the thin part which is vulnerable to deformation among the cast parts. The reinforcing effect capable of improving the yield strength in the process of solidifying molten aluminum includes the effect of refining the crystal grains using a peritectic reaction using a transition metal and strengthening the solid solution using a replacing element. The transition metal may be classified into an eutectic element having a relatively high solubility and a peritectic element having a low solubility in an aluminum alloy. The eutectic element may have a solid solution strengthening effect based on the high solubility, and the peritectic element may have a grain refinement effect, so that the yield strength of the casting material immediately after casting may be improved based on a combination of different types of elements.

[0014] Regarding the deformation due to rapid cooling after the solution treatment, the yield strength may be improved using an appropriate composition of the peritectic element. The peritectic elements are required to be subjected to the solution treatment in order to form fine dispersoids due to low diffusivity inside aluminum. Based on the increase in yield strength through the dispersoids enhancement effect generated by this process, the deformation generated in the rapid cooling process after the solution treatment process may be suppressed.

[0015] Regarding the transition metal, preferably, Cr or Mn may be added alone. In addition, addition of the combination of the transition metals having different sizes may reduce the compressive residual stress generated in the rapid cooling, via the distortion of the lattice structure. Preferably, a combination Cr—Zr, a combination Mn—Zr, a combination Cr—Ti or a combination Cr—V may be added.

[0016] In the Al—Cu alloy having reduced residual stress, a total content of the transition metal added to the alloy may be in a range of 0.1 to 0.5 wt %.

[0017] In the Al—Cu alloy having reduced residual stress, the Al—Cu alloy may include a Al-6 Cu alloy.

[0018] According to another aspect of the present disclosure, there is provided a method for preparing an aluminum alloy, the method including: a first step of melting Al, Cu and an added transition metal; a second step of leaving a molten metal of the first step to remove gas and impurities from the molten metal; and a third step of casting the molten metal of the second step to produce a Al—Cu alloy ingot, wherein the residual stress of the Al—Cu alloy containing the transition metal may be lower that residual stress of an Al—Cu alloy free of the transition metal in a rapid cooling process after a solution heat treatment.

[0019] In the method, the reduced residual stress of the aluminum alloy may be in a range of −90 to −40 MPa.

[0020] In the method, the solution heat treatment process includes a heat treatment performed at 500 to 600° C. for 5 to 7 hours.

[0021] In the method, in the rapid cooling process, the heat-treated alloy may be continuously immersed in water at a temperature of 20 to 30° C. after the solution heat treatment. In the rapid cooling process, the temperature may be lowered to room temperature rapidly.

[0022] In the method, the addition of the transition metal includes: addition of Cr or Mn alone; or addition of a combination Cr—Zr, a combination Mn—Zr, a combination Cr—Ti, or a combination Cr—V.

[0023] In the method, a total content of the transition metal added to the alloy may be in a range of 0.1 to 0.5 wt %.

[0024] In the method, the second step may include maintaining the molten metal at 700 to 800° C. for 20 to 40 minutes and maintaining the molten metal in Ar gas for 30 minutes.

[0025] Dimensional deformation and breakage that may be generated due to the residual stress due to the local difference in the cooling rate may be suppressed based on an increase in yield strength of an aluminum cast alloy material and an effect of curing the solid solution of the element via the addition of the transition metal.BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 is a flowchart illustrating a method for casting an aluminum alloy of the present disclosure.

[0027] FIG. 2 is a graph showing a result of measurement of residual stress of an Al—Cu alloy based on addition of a transition metal to the Al—Cu alloy.DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The present disclosure may be subjected to various changes and may have various forms. Thus, particular embodiments will be illustrated in the drawings and will be described in detail herein. However, this is not intended to limit the present disclosure to a specific disclosed form. It should be understood that the present disclosure includes all modifications, equivalents, and replacements included in the spirit and technical scope of the present disclosure. While describing the drawings, similar reference numerals are used for similar components.

[0029] The terminology used herein is directed to the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular constitutes “a” and “an” are intended to include the plural constitutes as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise”, “including”, “include”, and “including” when used in this specification, specify the presence of the stated features, integers, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, operations, elements, components, and / or portions thereof.

[0030] Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this inventive concept belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0031] The present disclosure relates to an aluminum alloy, and more specifically, to an aluminum alloy in which strength is improved and deformation due to residual stress is alleviated by the addition of various transition metal elements thereto, and to use of the aluminum alloy as a material for automobile parts.

[0032] The residual stress may be controlled via the addition of transition metals. Further, the stress remaining due to the mismatch of characteristics during interaction in the material from the macroscopic or microstructural point of view may be reduced by adding the characteristics of the transition metal to the material to reduce the mismatch.

[0033] Hereinafter, the present disclosure will be described in detail together with a specific example.Example: Method for Preparing Al—Cu-Based Alloy

[0034] FIG. 1 is a flowchart showing a method of casting an aluminum alloy of the present disclosure, and FIG. 2 is a graph showing a result of measurement of the residual stress of the Al—Cu alloy via the addition of a transition metal of the Al—Cu alloy.

[0035] Referring to FIGS. 1 and 2, in order to prepare a Al—Cu alloy and a transition metal additive alloy, Pure Al (99.8 wt %), a Al-30 wt % Cu alloy, a Al-5 wt % Cr alloy, a Al-10 wt % Mn alloy, a Al-5 wt % Zr alloy, a Al-5 wt % Ti alloy, and a Al-5 wt % V alloy were cast and prepared according to each combination.

[0036] For each alloy, about 5 kg of alloy material was melted inside a graphite crucible using an electric furnace. The molten metal was maintained at 750° C. for 30 minutes and the gas and impurities inside the molten metal were removed with high purity Ar gas for 30 minutes. Finally, molten aluminum was cast to make an ingot.

[0037] The transition metals selected for controlling the residual stress of the Al—Cu alloy are Cr, Mn, Zr, Ti, and V, and the optimal combination conditions were analyzed based on a measuring result of the residual stress via the single and combination addition of the corresponding elements.

[0038] In the case of the addition of the transition metal, the addition was performed in a very small amount (total 0.4 wt % or smaller) in order not to affect the characteristics such as heat treatment and castability of the existing alloy.

[0039] Table 1 as set forth below indicates the composition (wt %) of the Al—Cu alloy according to the addition of the transition metal.TABLE 1AlloyNo.designationsCuCrMnZrTiVFeSiAlExample 1Al—Cu6—————0.120.08Bal.Example 2Al—Cu—Cr60.25————0.120.08Bal.Example 3Al—Cu—Mn6—0.25———0.120.08Bal.Example 4Al—Cu—Cr—Mn60.1250.125———0.120.08Bal.Example 5Al—Cu—Zr6——0.15——0.120.08Bal.Example 6Al—Cu—Cr—Zr60.25—0.15——0.120.08Bal.Example 7Al—Cu—Mn—Zr6—0.250.15——0.120.08Bal.Example 8Al—Cu—Cr—Ti60.25——0.15—0.120.08Bal.Example 9Al—Cu—Cr—V60.25———0.150.120.08Bal.

[0040] In order to identify the effect of relieving the residual stress according to each alloy composition, the residual stress was generated by performing the same rapid cooling process after the solution heat treatment known as a process in which a lot of residual stress is generated in the industry. The solution heat treatment was performed as follows: the alloy was maintained at 540° C. for 6 hours and then was subjected to rapid cooling. Finally, the surface residual stress of each alloy in which residual stress was generated was measured by X-ray diffraction (XRD) to identify the residual stress relaxation effect according to the transition metal combination.

[0041] The results of measurement of residual stress according to the transition metal addition conditions of the Al—Cu alloy may be identified in Table 2 as set forth below.

[0042] It was identified that for all alloy conditions, compressive residual stress was generated due to shrinkage according to rapid cooling after the solution heat treatment (540° C.+6 h), and the residual stress value varied depending on the transition metal combination. In the case of the Al—Cu alloy free of the transition metal, the residual stress was −105 MPa. In contrast, the residual stress of the single Cr-added Al—Cu alloy was measured to be −84 MPa, and the residual stress of the single Mn-added Al—Cu alloy was measured to be −87 MPa. It was found that the residual stress of the aluminum alloy having the transition metal added thereto was relieved compared to the aluminum alloy free of the added transition metal. However, it was identified that in the case of the Zr single addition condition, the residual stress relaxation effect was absent.

[0043] Regarding the addition of the combination of the transition metals, in the Cr—Zr combination addition, the residual stress as measured was −44 MPa, in the Mn—Zr combination addition, the residual stress as measured was −57 MPa, in the Cr—Ti combination addition, the residual stress as measured was −78 MPa, and in the Cr—V combination addition, the residual stress as measured was −81 MPa. It was identified that in all of the combination additions, the residual stress as measured was lower than the residual stress of the Al—Cu alloy to which the transition metal was not added.

[0044] In addition, it was identified that in the single addition condition of Cr and Mn, and the combination addition condition of Cr+Mn, the residual stress was slightly alleviated compared to the Al—Cu alloy to which the transition metal was not added.

[0045] It was identified that when Zr was combined with Cr / Mn which was effective under a single addition condition, the residual stress was relieved by about 50% compared to that of the basic Al—Cu alloy.

[0046] Further, it was found that the residual stress was most relieved under addition of the combination of Cr and Zr.

[0047] It was identified that the residual stress value in the combination addition conditions of Cr and Ti, and Cr and V was substantially similar to that in the Cr single addition condition, and thus there was no residual stress relaxation effect according to addition of Ti and V.

[0048] As a result, it was identified that the condition for effectively mitigating the residual stress of the Al—Cu alloy was the addition of each of a combination Cr+Zr and a combination Mn+Zr. Further, it was found that the synergy effect for mitigating the residual stress resulting from the addition of the combination of Cr and Zr was slightly greater.TABLE 2Alloys conditionResidual stress(MPa)Base alloyAl—Cu alloy−105Single transitionAl—Cu—Cr alloy−84element alloyingAl—Cu—Mn alloy−87Al—Cu—Zr alloy−144Mutli transitionAl—Cu—Mn—Cr alloy−92element alloyingAl—Cu—Cr—Zr alloy−44Al—Cu—Mn—Zr alloy−57Al—Cu—Cr—Ti alloy−78Al—Cu—Cr—V alloy−81

[0049] The present disclosure relates to a technique for finally suppressing dimensional deformation and breakage of components by controlling the local difference in the cooling rate according to a complex shape in the aluminum alloy casting process and the residual stress generated in the rapid cooling process after heat treatment. In this technique, the residual stress may be relieved by adding the transition metal element to the Al—Cu-based alloy which is a representative industrial aluminum alloy. In addition, since a very small amount of the element is added in order not to reduce the castability and heat treatment effect of the existing aluminum alloy, the existing advantages of the alloy may be maintained. This may provide a method for preparing an aluminum alloy and the aluminum that may be directly applied to the actual casting industry.

[0050] An appropriate amount of elements may be added in order not to reduce the castability and heat treatment effect of the existing aluminum casting material for automobile parts, thereby maintaining the existing advantages of the casting material. Thus, a method for preparing an aluminum alloy and an aluminum alloy which may be directly applied to an actual casting industry material may be provided.

[0051] In addition, in the case of automobile powertrain components used in a high-temperature environment, there is a problem in that the strength is reduced due to a low durability limit temperature of the aluminum casting material. However, in accordance with the present disclosure, the internal residual stress reduction and the improvement in the high-temperature characteristics may be achieved based on element solution effect and the precipitate strengthening effect via the addition of an appropriate amount of transition metal and optimization of heat treatment.

[0052] Although the present disclosure has been described above with reference to the preferred embodiments of the present disclosure, those skilled in the art will understand that the present disclosure may be variously modified and changed within the scope not departing from the spirit and scope of the present disclosure described in the following patent claims.

Claims

1. An Al—Cu alloy having reduced residual stress, wherein the Al—Cu alloy contains a transition metal,wherein the residual stress of the Al—Cu alloy containing the transition metal is lower that residual stress of an Al—Cu alloy free of the transition metal in a rapid cooling process after a solution heat treatment.

2. The Al—Cu alloy having reduced residual stress of claim 1, wherein the reduced residual stress of the aluminum alloy is in a range of −90 to −40 MPa.

3. The Al—Cu alloy having reduced residual stress of claim 2, wherein the solution heat treatment process includes a heat treatment performed at 500 to 600° C. for 5 to 7 hours.

4. The Al—Cu alloy having reduced residual stress of claim 3, wherein in the rapid cooling process, the heat-treated alloy is continuously immersed in water at a temperature of 20 to 30° C.

5. The Al—Cu alloy having reduced residual stress of claim 3, wherein the addition of the transition metal includes:addition of Cr or Mn alone; oraddition of a combination Cr—Zr, a combination Mn—Zr, a combination Cr—Ti, or a combination Cr—V.

6. The Al—Cu alloy having reduced residual stress of claim 5, wherein a total content of the transition metal added to the alloy is in a range of 0.1 to 0.5 wt %.

7. The Al—Cu alloy having reduced residual stress of claim 1, wherein the Al—Cu alloy includes a Al-6Cu alloy.

8. A method for preparing an aluminum alloy having reduced residual stress, the method comprising:a first step of melting Al, Cu and an added transition metal;a second step of leaving a molten metal of the first step to remove gas and impurities from the molten metal; anda third step of casting the molten metal of the second step to produce a Al—Cu alloy ingot,wherein the residual stress of the Al—Cu alloy containing the transition metal is lower that residual stress of an Al—Cu alloy free of the transition metal in a rapid cooling process after a solution heat treatment.

9. The method of claim 8, wherein the reduced residual stress of the aluminum alloy is in a range of −90 to −40 MPa.

10. The method of claim 9, wherein the solution heat treatment process includes a heat treatment performed at 500 to 600° C. for 5 to 7 hours.

11. The method of claim 10, wherein in the rapid cooling process, the heat-treated alloy is continuously immersed in water at a temperature of 20 to 30° C.

12. The method of claim 10, wherein the addition of the transition metal includes:addition of Cr or Mn alone; oraddition of a combination Cr—Zr, a combination Mn—Zr, a combination Cr—Ti, or a combination Cr—V.

13. The method of claim 12, wherein a total content of the transition metal added to the alloy is in a range of 0.1 to 0.5 wt %.

14. The method of claim 8, wherein the second step includes maintaining the molten metal at 700 to 800° C. for 20 to 40 minutes and maintaining the molten metal in Ar gas for 30 minutes.

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