Forged aluminum alloy material

Abstract

The present invention provides a forged aluminum alloy material which is capable of achieving both higher strength and improvement of stress corrosion cracking resistance. A forged aluminum alloy material (11) contains 0.50 mass% to 1.25 mass% inclusive of Mg, 0.40 mass% to 1.40 mass% inclusive of Si, 0.50 mass% to 1.00 mass% inclusive of Cu, 0.05 mass% to 0.40 mass% inclusive of Fe, 0.005 mass% to 0.10 mass% inclusive of Ti, and at least one element selected from among Mn and Cr within the range of 0.50 mass% to 1.20 mass% inclusive of Mn and within the range of 0.20 mass% to 0.33 mass% inclusive of Cr, with the balance being made up of Al and inevitable impurities. The average value (∆Vmax average) of the maximum potential differences between the matrix and Al-(Fe, Mn, Cr)-Si compounds having an equivalent circle diameter of 1.0 µm or more is 144 mV or less.

Description

Aluminum alloy forgings

[0001] The present invention relates to an aluminum alloy forging.

[0002] There has been a demand for improved occupant safety in vehicles, and for this purpose, the strength of vehicle bodies has been improved. Meanwhile, against the backdrop of worsening issues such as global warming, efforts to improve automobile fuel efficiency are accelerating. It is known that reducing the weight of vehicle bodies is an effective way to improve fuel efficiency. Therefore, 6000 series aluminum alloy forgings, which have high strength and toughness and relatively excellent corrosion resistance, have been used as materials for suspensions and other undercarriage parts. In recent years, in order to further reduce the weight of vehicle bodies, there has been a demand for the members used in vehicle bodies to be thinner than before, while also having higher strength and toughness.

[0003] For example, Patent Documents 1 to 5 propose automobile suspension parts or aluminum alloy forgings in which the content of Cu, an additive element effective in improving strength, is appropriately controlled.

[0004] Japanese Patent No. 5110938 Japanese Patent No. 5863626 Japanese Patent No. 5837026 Japanese Patent No. 5901738 Japanese Patent No. 6445958

[0005] However, the parts or forged materials described in Patent Documents 1 to 5 have a problem in that stress corrosion cracking resistance deteriorates as strength increases, making it difficult to obtain members that are excellent in both strength and stress corrosion cracking resistance.

[0006] The present invention has been made in consideration of the above problems, and aims to provide an aluminum alloy forging material that is suitable for use in strength members of transportation equipment and the like, particularly automobile undercarriage members, and that can achieve both high strength and improved stress corrosion cracking resistance, thereby achieving improved fuel efficiency through weight reduction.

[0007] The above object can be achieved by the aluminum alloy forging material according to the present invention as set forth in [1] or [2] below.

[0008] [1] An aluminum alloy forging containing Mg: 0.50% by mass or more and 1.25% by mass or less, Si: 0.40% by mass or more and 1.40% by mass or less, Cu: 0.50% by mass or more and 1.00% by mass or less, Fe: 0.05% by mass or more and 0.40% by mass or less, Ti: 0.005% by mass or more and 0.10% by mass or less, and at least one selected from Mn and Cr in the ranges of Mn: 0.50% by mass or more and 1.20% by mass or less, Cr: 0.20% by mass or more and 0.33% by mass or less, with the balance being Al and unavoidable impurities, characterized in that the average value of the maximum potential difference [ΔVmax average] between the matrix and an Al-(Fe, Mn, Cr)-Si-based compound having a circle equivalent diameter of 1.0 μm or more is 144 mV or less.

[0009] [2] An aluminum alloy forging containing Mg: 0.50% by mass or more and 1.25% by mass or less, Si: 0.40% by mass or more and 1.40% by mass or less, Cu: 0.50% by mass or more and 1.00% by mass or less, Fe: 0.05% by mass or more and 0.40% by mass or less, Ti: 0.005% by mass or more and 0.10% by mass or less, and at least one selected from Mn and Cr in the ranges of Mn: 0.50% by mass or more and 1.20% by mass or less, Cr: 0.20% by mass or more and 0.33% by mass or less, the balance being Al and unavoidable impurities, wherein the aluminum alloy forging is an Al-(Fe, Mn, Cr)-Si based compound having an equivalent circle diameter of 1.0 μm or more, and wherein at least one of the following (Condition 1) and (Condition 2) is satisfied: (Condition 1) The average Mn concentration [Mn]av calculated by the following formula (1) is more than 0.465. Formula (1) (Condition 2) The average Cr concentration [Cr]av calculated by the following formula (2) is 0.125 or more and less than 0.235. ...Equation (2) where, in the above equations (1) and (2), Fe(i) is the value of the average Fe concentration in the ith region (1≦i≦n) expressed in atomic % when n regions having an equivalent circle diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si compounds identified in a predetermined measurement region. Mn(i) is the value of the average Mn concentration in the ith region (1≦i≦n) expressed in atomic % when n regions having an equivalent circle diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si compounds identified in a predetermined measurement region. Cr(i) is the average Cr concentration in the ith (1≦i≦n) region, expressed in atomic %, when n regions having a circle equivalent diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si-based compounds identified in a predetermined measurement region.

[0010] Furthermore, a preferred embodiment of the present invention relating to an aluminum alloy forging relates to the following [3].

[0011] [3] The aluminum alloy forging material according to [1] or [2], characterized in that the 0.2% yield strength is 390 MPa or more.

[0012] According to the present invention, it is possible to provide an aluminum alloy forging material that can achieve both high strength and improved stress corrosion cracking resistance, thereby achieving improved fuel efficiency through weight reduction.

[0013] Fig. 1A is a plan view showing the position where test pieces were taken from an aluminum alloy forging. Fig. 1B is a cross-sectional view taken along line A-A in Fig. 1A. Fig. 2 is a plan view showing the shape of a tensile test piece taken from the aluminum alloy forging. Fig. 3 is a cross-sectional view showing the shape of a stress corrosion cracking test piece taken from the aluminum alloy forging.

[0014] Several theories have been proposed regarding the mechanism of stress corrosion cracking (SCC) in aluminum alloys. These include the anodic dissolution theory, in which corrosion occurs preferentially along grain boundaries; the hydrogen embrittlement theory, in which corrosion occurs due to embrittlement caused by hydrogen diffused to grain boundaries; and the non-conductive film theory, in which corrosion occurs due to destruction of a passive film along grain boundaries (Mita Tadashi and Totsugi Yoichiro: Light Metals, 72 (2022), 431-440). The present inventors speculated that corrosion occurs due to the anodic dissolution theory, and conducted extensive research into the structural factors that affect stress corrosion cracking resistance (SCC resistance). As a result, they discovered that the potential difference between the compound and the aluminum parent phase (matrix) and the morphology of the crystal grains affect SCC resistance.

[0015] The anodic dissolution theory is believed to be based on the electrochemical factor of pitting corrosion, which is the initiation point of SCC, occurring when a potential difference of a certain value or greater exists between a compound and the aluminum matrix. Furthermore, it is believed that a mechanical factor of stress concentration at the site of pitting corrosion causes cracks to initiate and propagate, resulting in SCC. Therefore, the present inventors investigated the relationship between the rupture life, which indicates stress corrosion cracking resistance, and the potential difference between the compound and the aluminum matrix. As a result, they found that a smaller average value of the maximum potential difference [ΔVmax average] between a region where a specific compound is formed and the aluminum matrix tends to result in a longer rupture life, thereby achieving both high strength and SCC resistance. The present inventors also found that specifying the Mn and Cr concentrations within a specific region is effective in improving SCC resistance. The present invention was made based on the above findings.

[0016] [Aluminum alloy forging] The aluminum alloy forging according to this embodiment contains the specific elements shown below in predetermined content ranges. The elements contained in the aluminum alloy forging according to this embodiment and the reasons for limiting the contents thereof will be described in detail below. In the following description, the aluminum alloy forging may be simply referred to as a forging.

[0017] (Mg: 0.50 mass % or more and 1.25 mass % or less) Mg is added together with Si and Cu by artificial aging hardening treatment. ２ Si precipitates as β" phase, β' phase, and Q phase. Therefore, Mg is an essential element for increasing the 0.2% yield strength of the forged material. If the Mg content in the forged material is less than 0.50% by mass, the amount of age hardening decreases, and the 0.2% yield strength of the forged material decreases. In addition, the SCC resistance also decreases. Therefore, the Mg content in the forged material is set to 0.50% by mass or more, preferably 0.80% by mass or more, and more preferably 0.90% by mass or more, based on the total mass of the forged material.

[0018] On the other hand, if the Mg content in the forged material exceeds 1.25 mass%, the 0.2% yield strength becomes too high, and the forgeability of the ingot deteriorates. ２ Si and Q phases tend to precipitate. ２ The average particle size of the Si and Q phases and the Al-Fe-Si-(Mn,Cr)-based compounds in which Al, Si, Mn, Cr, and Fe are selectively bonded does not decrease, and the average spacing between these compounds cannot be increased. As a result, the corrosion resistance of the forged material deteriorates. Therefore, the Mg content in the forged material is set to 1.25 mass% or less, preferably 1.20 mass% or less, and more preferably 1.10 mass% or less, based on the total mass of the forged material.

[0019] (Si: 0.40 mass % or more and 1.40 mass % or less) Si, together with Mg and Cu, is also added by artificial age hardening treatment. ２ Si is an element that precipitates as Si (β" phase, β' phase) and is an essential element for increasing the 0.2% yield strength of a forged material. If the Si content in a forged material is less than 0.40% by mass, the amount of age hardening decreases, resulting in a decrease in the 0.2% yield strength of the forged material and a decrease in corrosion resistance. Therefore, the Si content in a forged material is set to 0.40% by mass or more, preferably 0.80% by mass or more, and more preferably 0.90% by mass or more, based on the total mass of the forged material.

[0020] On the other hand, if the Si content in the forged material exceeds 1.40 mass%, coarse particles of simple Si crystallize and precipitate during forging and quenching after solution treatment. ２ The average particle size of the Si and Q phases and the Al-Fe-Si-(Mn,Cr) compounds in which Al, Si, Mn, Cr, and Fe are selectively bonded does not become small, and the average spacing between these compounds cannot be increased. As a result, similar to the case of Mg, the corrosion resistance of the forged material decreases. Therefore, the Si content in the forged material is set to 1.40 mass% or less, preferably 1.30 mass% or less, and more preferably 1.20 mass% or less, based on the total mass of the forged material.

[0021] (Cu: 0.50% by mass or more and 1.00% by mass or less) Cu is an element that has the effect of improving the 0.2% proof stress of a forged material through solid solution strengthening. Cu also forms a Q phase together with Mg and Si during artificial age hardening, thereby significantly promoting the age hardening of the forged material. If the Cu content in the forged material is less than 0.50% by mass, the above effects cannot be fully achieved, and the 0.2% proof stress decreases. Therefore, the Cu content in the forged material is set to 0.50% by mass or more relative to the total mass of the forged material. To stably achieve these effects, the Cu content in the forged material is preferably more than 0.60% by mass, more preferably 0.65% by mass or more, relative to the total mass of the forged material.

[0022] On the other hand, if the Cu content in the forged material exceeds 1.00 mass%, the susceptibility of the structure of the forged material to stress corrosion cracking and intergranular corrosion becomes significantly high, and the corrosion resistance of the forged material decreases. Therefore, the Cu content in the forged material is set to 1.00 mass% or less, preferably 0.90 mass% or less, and more preferably 0.85 mass% or less, based on the total mass of the forged material.

[0023] (Fe: 0.05% by mass or more and 0.40% by mass or less) Fe is an element added to an ingot to improve productivity during casting and to suppress recrystallization. However, when Fe is contained in a forged material, the amount of Al ７ Cu ２ Fe, Al １２ (Fe, Mn) ３Cu ２ , (Fe,Mn)Al ６ , Al-(Fe, Mn, Cr)-Si-based crystallized particles and precipitates are generated. These compounds become the starting points for fracture and corrosion, deteriorating toughness, fatigue properties, stress corrosion cracking resistance, and the like. In particular, when the Fe content in the forged material exceeds 0.40 mass%, the average particle size of the Al-(Fe, Mn, Cr)-Si-based compounds present on the grain boundaries increases, and the average spacing between the compounds decreases. As a result, toughness and corrosion resistance decrease. Therefore, the Fe content in the forged material is set to 0.40 mass% or less, and preferably 0.30 mass% or less, based on the total mass of the forged material.

[0024] On the other hand, if the Fe content in the forged material is less than 0.05% by mass, cracks may occur during casting, abnormal structures may occur, etc. Therefore, the Fe content in the forged material is set to 0.05% by mass or more, and preferably 0.10% by mass or more, based on the total mass of the forged material.

[0025] (Ti: 0.005% by mass or more and 0.10% by mass or less) Ti is an element added to an ingot to refine the crystal grains of the ingot and improve workability during forging. If the Ti content in the forged material is less than 0.005% by mass, the effect of refining the crystal grains cannot be obtained. Therefore, the Ti content in the forged material is set to 0.005% by mass or more, and preferably 0.01% by mass or more, based on the total mass of the forged material.

[0026] On the other hand, if the Ti content in the forged material exceeds 0.10 mass%, coarse crystallized particles are formed, which reduces workability during forging. Therefore, the Ti content in the forged material is set to 0.10 mass% or less, and preferably 0.05 mass% or less, based on the total mass of the forged material.

[0027] In this embodiment, in addition to the above-mentioned Mg, Si, Cu, Fe, and Ti, at least one element selected from Mn and Cr is contained within a predetermined range. The reasons for limiting the contents of Mn and Cr will be explained below.

[0028] (Mn: 0.50 mass % or more and 1.20 mass % or less) Mn forms an Al-(Fe, Mn, Cr)-Si compound during homogenization treatment, and further forms Al-(Fe, Mn, Cr)-Si-based compounds that become dispersed particles during homogenization treatment and subsequent hot forging. ６ Mn is formed. Such dispersed particles have the effect of preventing grain boundary migration, thereby enabling the formation of fine crystal grains and subgrain structures. The inclusion of a predetermined amount of at least one of Mn and Cr (described later) in the ingot prevents the migration of crystal grain boundaries and subgrain boundaries, resulting in the refinement of crystal grains and the formation of subgrains. As a result, the fracture toughness and fatigue properties of the forged material can be improved. Furthermore, Mn has the effect of reducing the potential difference between the Al-(Fe, Mn, Cr)-Si compound and the aluminum matrix. Reducing this potential difference suppresses the occurrence of pits and pitting corrosion at the interface between the Al-(Fe, Mn, Cr)-Si compound and the aluminum matrix, which are the initiation sites of stress corrosion cracking in corrosive environments, thereby improving SCC resistance.

[0029] If the Mn content in the forged material is less than 0.50% by mass, the effect of reducing the potential difference between the compound and the aluminum matrix cannot be sufficiently obtained, and SCC resistance deteriorates. Therefore, when Mn is contained in the forged material, the Mn content in the forged material is set to 0.50% by mass or more, preferably more than 0.60% by mass, and more preferably 0.70% by mass or more, based on the total mass of the forged material.

[0030] On the other hand, when the Mn content in the forged material exceeds 1.20 mass%, coarse Al ６ Mn, (Fe, Mn)Al ６ , Al—(Fe, Mn, Cr)—Si, and other crystallized products, which become the starting points for fracture, thereby reducing the toughness of the forged material. Therefore, the Mn content in the forged material is set to 1.20 mass% or less, preferably 1.10 mass% or less, and more preferably 1.00 mass% or less, based on the total mass of the forged material.

[0031] (Cr: 0.20% by mass or more and 0.33% by mass or less) Cr forms Al-(Fe, Mn, Cr)-Si compounds during homogenization treatment, and further forms dispersed particles such as Al-Cr-based particles during homogenization treatment and subsequent hot forging. These dispersed particles have the effect of preventing grain boundary migration after recrystallization, thereby enabling the formation of fine crystal grains and subgrain structures. The inclusion of a predetermined amount of at least one of Mn and Cr in the ingot can improve the fracture toughness and fatigue properties of the forged material. Furthermore, Cr has the effect of reducing the potential difference between the Al-(Fe, Mn, Cr)-Si compounds and the aluminum matrix. Reducing this potential difference suppresses the occurrence of pitting and pitting corrosion at the interface between the Al-(Fe, Mn, Cr)-Si compounds, which are the initiation sites of stress corrosion cracking in corrosive environments, and the aluminum matrix, thereby improving SCC resistance.

[0032] If the Cr content in the forged material is less than 0.20 mass%, the effect of reducing the potential difference between the compound and the aluminum matrix cannot be sufficiently obtained, and SCC resistance deteriorates. Therefore, when Cr is contained in the forged material, the Cr content in the forged material is set to 0.20 mass% or more, preferably 0.22 mass% or more, and more preferably 0.25 mass% or more, based on the total mass of the forged material.

[0033] On the other hand, if the Cr content in the forged material exceeds 0.33 mass%, the effect of lowering the potential of the above compounds decreases, and the potential difference between the Al-(Fe, Mn, Cr)-Si compounds and the aluminum matrix increases, resulting in a deterioration in SCC resistance. Therefore, the Cr content in the forged material is set to 0.33 mass% or less, preferably 0.32 mass% or less, and more preferably 0.30 mass% or less, based on the total mass of the forged material.

[0034] In this embodiment, at least one element selected from Mn and Cr is contained within the above range, and of Mn and Cr, it is more preferable to contain Mn.

[0035] (Balance: Al and inevitable impurities) The balance of the aluminum alloy forging according to this embodiment is Al and inevitable impurities. Examples of inevitable impurities include B, C, Na, Ni, Hf, V, Cd, Pb, Sn, Zn, and Zr. Each of these inevitable impurities is preferably 0.05 mass% or less relative to the total mass of the forging. Furthermore, the total amount of the inevitable impurities is preferably 0.15 mass% or less relative to the total mass of the forging. As long as the content of the inevitable impurities is within the above range, the effects of the present invention are not hindered.

[0036] Next, the reason for limiting the potential difference between the predetermined compound and the aluminum parent phase (matrix) in the aluminum alloy forging according to this embodiment will be described in detail.

[0037] ([ΔVmax average]: 144 mV or less) Mainly, Al-Fe-Si compounds containing Fe, which has a high standard electrode potential, have a large potential difference with an aluminum parent phase, which has a low standard electrode potential. Therefore, in a corrosive environment, pitting corrosion is likely to occur at the interface between the compound and the aluminum parent phase. Therefore, in this embodiment, the average value [ΔVmax average] of the maximum potential difference between the matrix and an Al-(Fe, Mn, Cr)-Si compound having an equivalent circle diameter of 1.0 μm or more is specified.

[0038] If the average value of the maximum potential difference [ΔVmax average] exceeds 144 mV, pits or pitting corrosion will occur at the interface between the compound and the aluminum parent phase, causing stress corrosion cracking in a corrosive environment. Therefore, the average value of the maximum potential difference [ΔVmax average] between the matrix and the Al-(Fe, Mn, Cr)-Si-based compound is set to 144 mV or less, preferably 140 mV or less, and more preferably 135 mV or less.

[0039] Here, a method for associating the potential difference distributions of the matrix and the Al-(Fe, Mn, Cr)-Si-based compound with the compounds will be described. The method for associating the potential difference distributions with the compounds is not limited to a specific method and various modes are possible, but for example, it can be performed by the following steps (I) to (III).

[0040] (I) Identifying compounds using data acquired by an electron probe microanalyzer (EPMA) and machine learning. (II) Evaluating the potential difference distribution using data acquired by a Kelvin Force Microscope (KFM). (III) Correlating the potential difference distribution with compounds.

[0041] The specific steps (I) to (III) above are as follows:

[0042] (I) Identification of Compounds Using Data Obtained by EPMA and Machine Learning Element concentration distribution is measured using EPMA, and compounds and aluminum matrices are identified using machine learning based on the element concentration distribution data obtained by EPMA. The method for identifying compounds using data obtained by EPMA and machine learning can be carried out in accordance with the "acquisition step" described in paragraphs

[0024] to

[0030] , the "element region identification step" described in paragraphs

[0031] to

[0035] , and the "impurity region identification step" described in paragraphs

[0036] to

[0041] of JP 2023-103800 A. Note that, in this embodiment, of the compounds of various sizes, taking into consideration resolution and the like, attention is focused only on Al—(Fe, Mn, Cr)—Si-based compounds having a circle-equivalent diameter of 1 μm or more.

[0043] (II) Evaluation of potential difference distribution using data acquired by surface potential microscope (KFM: Kelvin Force Microscope) The potential difference distribution is evaluated using data acquired by KFM. The evaluation method can be carried out following the <Acquisition step> described in paragraphs

[0024] to

[0030] and the <Potential difference region identification step> described in paragraphs

[0042] to

[0045] of Japanese Patent Application Laid-Open No. 2023-103800. Regarding the evaluation of potential difference distribution using KFM, see "3. Experiment" on pages 73-74 of "Analysis of corrosion reaction on metal surfaces using KFM" by Hiroyuki Masuda, Surface Science, Vol. 1.18, No. 2, pp. 72-78, 1997, and "Y. Takara, T. Ozawa, M. This is described in "2.2 Characterization" of "Yamaguchi, Japanese Journal of Applied Physics, 61, 2022", SL1008, pSL1008-2.

[0044] (III) Correspondence between potential difference distribution and compound The compound identified in (I) above is correlated with the potential difference distribution obtained in (II) above. The correlation is performed by adjusting the element concentration distribution map and the potential difference distribution map so that they have the same coordinates, following the <Coordinate System Coordination Step> described in paragraphs

[0046] to

[0052] of JP 2023-103800 A.

[0045] As described above, known methods can be used to measure element concentration distribution using EPMA and to evaluate potential difference distribution using data acquired by KFM. Furthermore, while data acquisition and analysis are performed using machine learning in the above process, manual methods can also be used. When using manual methods, the coordinates of each element constituting an Al-(Fe, Mn, Cr)-Si-based compound are identified, and only regions of the Al-(Fe, Mn, Cr)-Si-based compound with a circle-equivalent diameter of 1 μm or more are extracted. Then, the numerical data of the maximum potential difference is read from the potential in the region of this compound and the potential in the matrix region, and the average value of the maximum potential difference obtained for each compound is calculated. In this way, the average value of the maximum potential difference can also be calculated using manual methods.

[0046] Furthermore, the reasons for limiting the Mn concentration and Cr concentration as specified in (Condition 1) and (Condition 2) in the aluminum alloy forging according to this embodiment will be described in detail.

[0047] As described above, a compound containing Fe, which has a high standard electrode potential, has a large potential difference with an aluminum matrix phase, which has a low standard electrode potential. Therefore, by dissolving Mn or Cr, which has a standard electrode potential between Fe and Al, in an Al-Fe-Si compound, the potential difference between the compound and the aluminum matrix phase can be reduced. Specifically, by controlling the Mn and Cr compositions in the compound to fall within a predetermined range, the potential difference between the compound and the aluminum matrix phase can be reduced, thereby improving SCC resistance. In this embodiment, an Al-(Fe, Mn, Cr)-Si compound having an equivalent circle diameter of 1.0 μm or more satisfies at least one of the following (Condition 1) and (Condition 2).

[0048] (Condition 1): Average Mn concentration [Mn]av: greater than 0.465) In this embodiment, the average Mn concentration [Mn]av is a value calculated by the following formula (1).

[0049] ...Formula (1)

[0050] In the above formula (1), Fe(i) is the value expressed in atomic % of the average Fe concentration in the ith region (1≦i≦n) when n regions having an equivalent circle diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si compounds identified in the measurement regions in (I) and (II) above. Mn(i) is the value expressed in atomic % of the average Mn concentration in the ith region (1≦i≦n) when n regions having an equivalent circle diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si compounds identified in the measurement regions in (I) and (II) above. Cr(i) is the average Cr concentration in the ith (1≦i≦n) region, expressed in atomic %, when n regions having an equivalent circle diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si-based compounds identified in the measurement regions in (I) and (II) above.

[0051] When the average Mn concentration [Mn] calculated by the above formula (1) exceeds 0.465, the potential of the region in the Al-(Fe, Mn, Cr)-Si-based compound is lowered, and the potential difference between the compound and the aluminum matrix can be reduced. As a result, improved SCC resistance can be achieved. Therefore, the average Mn concentration [Mn] calculated by the above formula (1) is set to exceed 0.465, preferably 0.480 or more, and more preferably 0.500 or more.

[0052] ((Condition 2) Average Cr concentration [Cr]av: 0.125 or more and less than 0.235) In this embodiment, the average Cr concentration [Cr]av is a value calculated by the following formula (2).

[0053] ...Formula (2)

[0054] In the above formula (2), Fe(i), Mn(i), and Cr(i) are the same as in the above formula (1).

[0055] When the average Cr concentration [Cr]av calculated by the above formula (2) is 0.125 or more, the potential of the region in the Al-(Fe, Mn, Cr)-Si compound can be lowered, and the potential difference between the compound and the aluminum parent phase can be reduced. As a result, improved SCC resistance can be achieved. Therefore, the average Cr concentration [Cr]av calculated by the above formula (2) is set to 0.125 or more, preferably 0.130 or more, and more preferably 0.135 or more.

[0056] On the other hand, if the average Cr concentration [Cr] is 0.235 or more, the effect of reducing the potential difference between the compound and the aluminum matrix phase is no longer obtained. Although the reason for this is unclear, it is speculated that an increase in the Cr concentration in the compound changes the structure of the compound, resulting in a corresponding change in potential. Therefore, the average Cr concentration [Cr] calculated by the above formula (2) is set to less than 0.235, preferably 0.230 or less, and more preferably 0.220 or less.

[0057] (0.2% proof stress: 390 MPa or more) Conventional forged materials have a problem in that stress corrosion cracking resistance deteriorates as strength increases. In contrast, the aluminum alloy forged material according to this embodiment aims to achieve both high strength and improved stress corrosion cracking resistance. Specifically, according to this embodiment, even if the 0.2% proof stress is in the range of 390 MPa or more, the SCC resistance can be improved to a desired value. The 0.2% proof stress of the forged material is preferably 390 MPa or more, and more preferably 395 MPa or more.

[0058] As described above, according to this embodiment, it is possible to achieve both high strength and improved stress corrosion cracking resistance in a lightweight aluminum alloy forging material. Therefore, the material can be suitably used for strength members of transportation equipment, particularly automobile undercarriage members, and the weight reduction can improve fuel efficiency.

[0059] [Method for manufacturing aluminum alloy forgings] Next, a method for manufacturing an aluminum alloy forging according to the present embodiment will be described. The method for manufacturing an aluminum alloy forging according to the present invention is not particularly limited, and various embodiments are possible. The manufacturing method shown below is an example of a preferred manufacturing method.

[0060] <Casting Process> The casting process is a process of producing an ingot by casting a molten metal melted and adjusted to the above-mentioned chemical composition. As a casting method, a conventional melting and casting method such as a continuous casting method, a semi-continuous casting method (DC casting method), a hot-top casting method, etc. can be used, and an appropriate casting method is selected from these casting methods for casting. The shape of the ingot may be an ingot such as a round bar, a slab, etc., and is not particularly limited.

[0061] <Soaking step> The soaking step is a step of subjecting the ingot obtained by the casting step to a homogenization treatment. As a furnace for the homogenization treatment, an air furnace, an induction heating furnace, a saltpeter furnace, or the like can be appropriately used. The holding temperature when the ingot is homogenized is preferably 400°C to 570°C. Furthermore, the holding time is preferably 3 hours or more.

[0062] <Heating Step Before Forging> It is also desirable to perform a second heating step on the ingot after the soaking step before the forging step. This heating step further promotes the solid solution of Mn and Cr atoms in the Al-Fe-Si compounds, thereby reducing the potential difference between the matrix and the Al-(Fe, Mn, Cr)-Si compounds. As a result, an aluminum alloy forging with high strength and excellent stress corrosion cracking resistance can be produced. Setting the temperature in the heating step before forging to a temperature higher than the temperature reached in the soaking step can achieve even greater effects. Furthermore, a two-stage heat treatment step in which the ingot is cooled once and then reheated after the soaking step may be performed, or a two-stage heat treatment step in which the ingot is maintained at a predetermined temperature in the soaking step and then further increased in temperature may be performed.

[0063] <Forging Process> The forging process is a process in which an ingot (forging material) is heated after a soaking process or after a heating process before forging, and hot forged. In the forging process, a forging material such as an ingot or an extruded rod is hot forged using a mechanical press, a hydraulic press, or the like. If the starting temperature (forging start temperature) when hot forging the forging material is 450°C or higher, the proportion of subgrain structure in the forged structure increases and the subgrain size becomes finer, thereby improving the strength, toughness, and corrosion resistance of the aluminum alloy forged material. Therefore, it is preferable that the forging start temperature be 450°C or higher.

[0064] Furthermore, when hot forging is performed on the forging material, plastic working at high temperatures can promote dynamic recovery and reduce the dislocation density after working. As a result, grain coarsening due to recrystallization can be suppressed. If the end temperature (forging end temperature) when hot forging the forging material is 400°C or higher, dynamic recovery is suppressed, and the strength, toughness, and corrosion resistance of the Al alloy forged material are improved. Therefore, it is preferable that the forging end temperature be 400°C or higher.

[0065] <Tempering Process> The tempering process is a process in which the member after the forging process is subjected to a solution treatment and artificial aging hardening. The holding temperature for the solution treatment is preferably in the range of 520°C to 570°C. The holding time for the solution treatment is preferably 20 minutes to 20 hours, and the temperature increase rate is preferably 100°C / hr or more. A quenching process may be performed after the solution treatment. Examples of quenching methods include immersing the member after the solution treatment in water or hot water. The cooling rate during the quenching process is preferably 40°C / second or more.

[0066] Specific examples of artificial age hardening treatments include T6 (artificial age hardening treatment to obtain maximum strength after solution treatment at 520 to 570°C), T7 (excessive aging treatment in which treatment is carried out in excess of the artificial age hardening conditions to obtain maximum strength after the above solution treatment), and T8 (artificial age hardening treatment to obtain maximum strength after cold working after the above solution treatment). The artificial age hardening treatment is preferably selected from the range of 4 to 9 hours at a temperature of 170 to 200°C, taking into consideration the conditions (maximum strength) for obtaining the above T6, T7, and T8 tempered treated materials.

[0067] The present embodiment will be described in more detail below with reference to examples, but the present invention is not limited to these examples, and modifications can be made within the scope of the spirit of the present invention, and all such modifications are included in the technical scope of the present invention.

[0068] <Production of Aluminum Alloy Forgings> First, cylindrical samples with a diameter Φ of 23 mm and a height h of 23 mm were cut out from ingots of aluminum alloys having various compositions. Next, the obtained samples were subjected to a homogenization treatment and then air-cooled to room temperature. Thereafter, a second heating step was performed, followed by water cooling. Further, the samples were subjected to compression processing in the cylindrical axial direction by hot working, and immediately water-cooled to room temperature. Thereafter, the samples were subjected to a solution treatment as a tempering treatment, followed by immediate cooling in hot water at 40°C, and then artificial aging hardening treatment, thereby obtaining cylindrical aluminum alloy forgings. The contents of each component in the aluminum alloy forgings are shown in Table 1 below, and the conditions for the artificial aging hardening treatment are shown in Table 2 below.

[0069] (Homogenization treatment) Heating rate: 167°C / hour Reached temperature: 500°C Holding time: 4 hours

[0070] (Second heating step) Heating rate: 167°C / hour Reached temperature: 520°C Holding time: 1 hour

[0071] (Hot working) Heating rate: 10°C / sec. Reached temperature: 450°C. Holding time: 300 sec. Strain rate: 6.5 / sec. Working rate: 74%

[0072] (Tempering treatment) Heating rate in solution treatment: 167°C / hour Temperature reached in solution treatment: 555°C Holding time in solution treatment: 3 hours

[0073] <Test Measurements> (Tensile Test) Tensile test specimens were taken from the obtained aluminum alloy forgings. Fig. 1A is a plan view showing the position where the test specimens were taken from the aluminum alloy forgings, and Fig. 1B is a cross-sectional view taken along line A-A in Fig. 1A. Fig. 2 is a plan view showing the shape of the tensile test specimen taken from the aluminum alloy forgings.

[0074] As shown in Figures 1A and 1B, the resulting aluminum alloy forging 11 had a circular diameter Φ of approximately 45 mm in plan view and an axial (compression) height T1 of approximately 6 mm. A tensile test specimen 14 (shown in Figure 2) was taken from a region R1 3 mm away from the central axis O of the aluminum alloy forging 11 in the metal flow direction (L direction). The tensile test specimen 14 was cut from the center of the aluminum alloy forging 11 in the compression direction (ST direction) so that the tensile direction was perpendicular to the L direction (LT direction) in plan view. The tensile test specimen 14 had a thickness of 1.6 mm, a width in the L direction at the parallel portion 14a of 4 mm, a length in the LT direction of 10 mm, and a width in the L direction at the gripped portion 14b of 6 mm.

[0075] Thereafter, a tensile test was carried out using the collected tensile test piece 14 with a distance between marks of 10 mm and a tensile speed of 1 mm / min, and the 0.2% proof stress was measured.

[0076] (Stress corrosion cracking test (SCC test)) Stress corrosion cracking test specimens were taken from the obtained aluminum alloy forgings. FIG. 3 is a cross-sectional view showing the shape of a stress corrosion cracking test specimen taken from an aluminum alloy forging. As shown in FIGS. 1A and 1B , a test specimen 13 for an SCC test having a U-shaped cross section shown in FIG. 3 was taken from the radially outer side of region R1 in an aluminum alloy forging 11. The position from which the specimen was taken was designed so that a curved portion 13b serving as the bottom was on the side of the central axis O of the aluminum alloy forging 11 and ends of a pair of parallel portions 13a were on the radially outer side of the aluminum alloy forging 11.

[0077] The thickness of the test piece 13 was 0.5 mm, the length of the parallel portion 13a in the L direction was 5 mm, the inner radius of curvature r of the curved portion 13b was 2 mm, and a hole 13c for inserting a jig was provided at a position 2.5 mm from the end of the parallel portion 13a.

[0078] In the SCC test, a bolt or the like was first attached to the hole 13c of the test piece 13, and a stress of 80% of the 0.2% proof stress was applied in the ST direction, and the test piece was then immersed in this state in an aqueous chromic acid solution at 95° C. The aqueous chromic acid solution was prepared by mixing 3 g / L of NaCl, 33 g / L of chromium oxide, and 30 g / L of potassium dichromate.

[0079] The occurrence of cracking was judged by visually observing the test piece 13 for stress corrosion cracking every 2 hours, 4 hours, 6 hours, and 24 hours after immersion in the chromic acid aqueous solution. Furthermore, before immersion, a strain gauge was attached to the R portion of the test piece 13, and the time change in strain during the test was measured with a data logger. The time when a sudden change in strain occurred at the time of cracking, which was observed up to the time when stress corrosion cracking occurred visually, was defined as the time to fracture.

[0080] (Measurement of Average Mn Concentration [Mn]av and Average Cr Concentration [Cr]av) (Measurement of Average Value of Maximum Potential Difference [ΔVmaxavg]) Using the curved portion 13b of the test piece 13 for the SCC test, the average Mn concentration [Mn]av and the average Cr concentration [Cr]av, as well as the average value of the maximum potential difference [ΔVmaxavg], were measured by the following steps (I) to (III).

[0081] (I) Identification of Compounds Using Data Obtained by EPMA and Machine Learning Following the procedure of the <Acquisition Process> described in paragraphs

[0024] to

[0030] of Japanese Patent Publication No. 2023-103800 (Patent Document 1), the distribution of element concentrations (atomic%) was acquired as image data using an EPMA (JXA8530F PLUS manufactured by JEOL Ltd.). The test piece used was the test piece 13 shown in FIG. 3, and measurements were taken of the curved portion 13b indicated by the thin hatching. The image range was 120 μm × 120 μm, and the step was 0.24 μm, and six pieces of image data were obtained for Fe, Mg, Si, Cu, Mn, and Cr.

[0082] Next, focusing on compounds containing Fe, the compounds were identified using machine learning following the procedure of the <Element Region Identification Step> described in paragraphs

[0031] to

[0035] of Japanese Patent Publication No. 2023-103800 (Patent Document 1). The compounds were identified by citing a dimensionality reduction method using nonnegative matrix factorization soft orthogonal constraints (NMF-SO) as a technique. NMF-SO is described in "S. Muto and M. Shiga, Microscopy, Volume 69, Issue 2, April 2020, pp. 110-122". As a result, compounds such as Al-Fe-Si, Al-Fe-Mn-Si, Al-Fe-Mn-Cr-Si, and Al-Mg-Si were identified. It was found that Cu was almost in a solid solution state and did not form a compound. Of the regions where these compounds were identified, regions containing Fe, i.e., regions where Al-Fe, Al-Fe-Si, Al-Fe-Mn, Al-Fe-Cr, Al-Fe-Mn-Cr, Al-Fe-Mn-Si, Al-Fe-Cr-Si, and Al-Fe-Mn-Cr-Si compounds were formed, were extracted.

[0083] Next, the center of gravity and the circle-equivalent diameter were determined for each region, and from the obtained values, only regions where the circle-equivalent diameter was 1.0 μm or more were extracted. Thereafter, for each region where the circle-equivalent diameter was 1.0 μm or more, the average value Fe(i) of the Fe concentration, the average value Mn(i) of the Mn concentration, and the average value Cr(i) of the Cr concentration were calculated and expressed in atomic %. Note that i represents the i-th region (1≦i≦n) out of n regions where the circle-equivalent diameter was 1.0 μm or more and where Al—(Fe, Mn, Cr)—Si-based compounds were identified in each test specimen.

[0084] That is, Fe(i) is the value expressed in atomic % of the average Fe concentration in the ith region (1≦i≦n) when n regions having an equivalent circle diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si compounds identified in the measurement region. Mn(i) is the value expressed in atomic % of the average Mn concentration in the ith region (1≦i≦n) when n regions having an equivalent circle diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si compounds identified in the measurement region. Cr(i) is the value expressed in atomic % of the average Cr concentration in the ith region (1≦i≦n) when n regions having an equivalent circle diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si compounds identified in the measurement region.

[0085] Furthermore, the obtained Fe(i), Mn(i), and Cr(i) were used to calculate the average Mn concentration [Mn]av according to the following formula (1) and the average Cr concentration [Cr]av according to the following formula (2).

[0086] ...Formula (1)

[0087] ...Formula (2)

[0088] (II) Evaluation of potential difference distribution using data acquired by KFM Following the <Acquisition process> described in paragraphs

[0024] to

[0030] of Japanese Patent Application Laid-Open No. 2023-103800, the potential difference distribution was evaluated using KFM (AFM5300, manufactured by Hitachi High-Tech Science Corporation). The potential difference distribution was measured in the same field of view as the test piece in which the compound was identified by EPMA, with a step size of 0.23 μm. Thereafter, one image data sheet was acquired for each test piece, using the potential distribution as image data. Furthermore, a potential difference distribution map was obtained by subtracting the matrix potential from the obtained potential value.

[0089] (III) Correspondence between Potential Difference Distribution and Compounds Following the "Coordinate System Matching Step" described in paragraphs

[0046] to

[0052] of Japanese Patent Application Laid-Open No. 2023-103800, the compound identified in (I) above was matched to the potential difference distribution obtained in (II) above (coordinate system matching) using the element concentration distribution of Fe. Specifically, the image data of the element concentration distribution obtained in (I) above and the image data of the potential difference distribution obtained in (II) above were translated, rotated, and scaled to match the two-dimensional coordinate systems of both. The method for matching the two-dimensional coordinate systems was performed using the A-KAZE algorithm by the Perona and Malik diffusion method. A-KAZE is described in "ALCANTARILLA et al. : Fast Explicit Diffusion for Accelerated Features in Nonlinear Scale Spaces”.

[0090] By aligning the two-dimensional coordinate systems as described above, six element concentration distribution maps (Fe, Mg, Si, Cu, Mn, Cr) were associated with one corresponding potential difference distribution map. Specifically, the regions in which Al-Fe, Al-Fe-Si, Al-Fe-Mn, Al-Fe-Cr, Al-Fe-Mn-Cr, Al-Fe-Mn-Si, Al-Fe-Cr-Si, and Al-Fe-Mn-Cr-Si compounds having a circle equivalent diameter of 1.0 μm or more, as specified in (I), were formed were associated with the potential difference distribution map measured in (II). Then, for each region in which the compound was formed, the maximum potential difference [ΔVmax] (mV) between the matrix and the region was determined. Furthermore, the average of the maximum potential differences [ΔVmax average] (mV) in each test piece was calculated from the maximum potential differences [ΔVmax] (mV) in the n regions.

[0091] The average Mn concentration [Mn]av, average Cr concentration [Cr]av, and average value of maximum potential difference [ΔVmaxaverage] for compounds having an equivalent circle diameter of 1.0 μm or more, as well as the measurement results from each test, are shown in Table 2. As shown in Table 2, in this example, the number n of compounds extracted from each test material was between 30 and 74.

[0092]

[0093]

[0094] As shown in Tables 1 and 2 above, in Invention Examples 1 to 4, the contents of each component in the forged material were within the ranges specified by the present invention, and the average values ​​of the maximum potential differences [ΔVmax average] for compounds having an equivalent circle diameter of 1.0 μm or more were within the ranges specified by the present invention. Furthermore, in Invention Examples 1 to 4, at least one of the average Mn concentration [Mn]av and the average Cr concentration [Cr]av was within the ranges specified by the present invention. Therefore, while maintaining a 0.2% yield strength of 390 MPa or more, the fracture time was longer than that of the Comparative Example, and aluminum alloy forgings were obtained that satisfied both improved strength and stress corrosion cracking resistance.

[0095] On the other hand, in Comparative Example No. 1, the Cu content was less than the lower limit specified in the present invention, and the Mn and Cr contents were both less than the lower limit specified in the present invention. The average value of the maximum potential difference [ΔVmax average] exceeded the upper limit specified in the present invention. Furthermore, the average Mn concentration [Mn]av was equal to or less than the lower limit specified in the present invention, and the average Cr concentration [Cr]av was less than the lower limit specified in the present invention. Therefore, the 0.2% yield strength in particular was reduced, and the time to fracture was also shorter than that of the invention examples.

[0096] In Comparative Example No. 2, the Mn and Cr contents were both less than the lower limit specified in the present invention. The average value of the maximum potential difference [ΔVmax average] exceeded the upper limit specified in the present invention. Furthermore, the average Mn concentration [Mn]av was equal to or less than the lower limit specified in the present invention, and the average Cr concentration [Cr]av was less than the lower limit specified in the present invention. Therefore, the time to fracture was shortened, and the SCC resistance was significantly reduced.

[0097] In Comparative Example No. 3, the Mn content was less than the lower limit specified in the present invention, and the Cr content exceeded the upper limit specified in the present invention. The average value of the maximum potential difference [ΔVmax average] exceeded the upper limit specified in the present invention. Furthermore, the average Mn concentration [Mn]av was equal to or less than the lower limit specified in the present invention, and the average Cr concentration [Cr]av exceeded the upper limit specified in the present invention. Therefore, the time to fracture was shortened, and the SCC resistance was significantly reduced.

[0098] Although various embodiments have been described above with reference to the drawings, it goes without saying that the present invention is not limited to such examples. It is clear that a person skilled in the art can conceive of various modifications or alterations within the scope of the claims, and it is understood that these also naturally fall within the technical scope of the present invention. Furthermore, the components of the above-described embodiments may be combined in any manner without departing from the spirit of the invention.

[0099] This application is based on a Japanese patent application (Patent Application No. 2024-087138) filed on May 29, 2024, the contents of which are incorporated herein by reference.

[0100] 11 Aluminum alloy forging 13 Test piece 14 Tensile test piece

Claims

1. An aluminum alloy forging containing Mg: 0.50% by mass to 1.25% by mass, Si: 0.40% by mass to 1.40% by mass, Cu: 0.50% by mass to 1.00% by mass, Fe: 0.05% by mass to 0.40% by mass, Ti: 0.005% by mass to 0.10% by mass, and at least one selected from Mn and Cr in the ranges of Mn: 0.50% by mass to 1.20% by mass, Cr: 0.20% by mass to 0.33% by mass, with the balance being Al and unavoidable impurities, characterized in that the average value of the maximum potential difference [ΔVmax average] between the matrix and an Al-(Fe, Mn, Cr)-Si compound having a circle equivalent diameter of 1.0 μm or more is 144 mV or less.

2. An aluminum alloy forging containing Mg: 0.50% by mass to 1.25% by mass, Si: 0.40% by mass to 1.40% by mass, Cu: 0.50% by mass to 1.00% by mass, Fe: 0.05% by mass to 0.40% by mass, Ti: 0.005% by mass to 0.10% by mass, and at least one selected from Mn and Cr in the ranges of Mn: 0.50% by mass to 1.20% by mass, Cr: 0.20% by mass to 0.33% by mass, the balance being Al and unavoidable impurities, wherein the aluminum alloy forging is an Al-(Fe, Mn, Cr)-Si compound having an equivalent circle diameter of 1.0 μm or more, and wherein at least one of the following (Condition 1) and (Condition 2) is satisfied: (Condition 1) The average Mn concentration [Mn]av calculated by the following formula (1) is more than 0.

465. Formula (1) (Condition 2) The average Cr concentration [Cr]av calculated by the following formula (2) is 0.125 or more and less than 0.

235. ...Equation (2) where, in the above equations (1) and (2), Fe(i) is the value of the average Fe concentration in the ith region (1≦i≦n) expressed in atomic % when n regions having an equivalent circle diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si compounds identified in a predetermined measurement region. Mn(i) is the value of the average Mn concentration in the ith region (1≦i≦n) expressed in atomic % when n regions having an equivalent circle diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si compounds identified in a predetermined measurement region. Cr(i) is the average Cr concentration in the ith (1≦i≦n) region, expressed in atomic %, when n regions having a circle equivalent diameter of 1.0 μm or more are measured among the Al—(Fe, Mn, Cr)—Si-based compounds identified in a predetermined measurement region.

3. An aluminum alloy forging according to claim 1 or 2, characterized in that the 0.2% yield strength is 390 MPa or more.

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