Aluminum alloy wrought material

Abstract

This invention provides an aluminum alloy forging material that exhibits excellent strength, ductility, fatigue properties, and corrosion resistance even when containing Fe greater than 0.4% by mass. The invention relates to an aluminum alloy forging material containing Si: 0.7% by mass or more and 1.5% by mass or less, Fe: greater than 0.4% by mass or less and 0.67% by mass or less, Cu: greater than 0.4% by mass or less and 0.8% by mass or less, Mg: 0.85% by mass or more and 1.3% by mass or less, Ti: 0.005% by mass or more and 0.07% by mass or less, Zn: 0.25% by mass or less, and further comprising at least one selected from the group consisting of Mn: 0.1% by mass or more and 0.95% by mass or less, Cr: greater than 0.1% by mass or less and 0.4% by mass or less, and Zr: 0.05% by mass or more and 0.3% by mass or less, with the balance comprising unavoidable impurities and Al, a crystal area fraction of 3.2% or less, and an average crystal size of 8 μm or less.

Description

Technical Field

[0001] This invention relates to aluminum alloy forgings used as automotive chassis parts, etc. Background Technology

[0002] Aluminum alloy forgings used in automotive chassis parts such as upper or lower control arms require high strength, high ductility, high toughness, high corrosion resistance, fracture toughness, and fatigue properties, and have been subject to various development efforts.

[0003] For example, Patent Document 1 describes an automobile chassis part made of aluminum alloy forging material, wherein the aluminum alloy forging material contains Mg: 0.5-1.25 wt%, Si: 0.4-1.4 wt%, Cu: 0.01-0.7 wt%, Fe: 0.05-0.4 wt%, Mn: 0.001-1.0 wt%, Cr: 0.01-0.35 wt%, Ti: 0.005-0.1 wt%, and Zr is limited to less than 0.15 wt%, with the balance consisting of Al and unavoidable impurities. In the automobile chassis part, in the cross-sectional microstructure of the section where the maximum stress occurs, the density of crystals observed in the microstructure of the section where the maximum stress occurs is less than 1.5% in terms of average area ratio, and the spacing between each grain boundary precipitate observed in the microstructure of the section including the parting line generated during forging is more than 0.7 μm in terms of average spacing.

[0004] According to the invention described in Patent Document 1, even forged automotive chassis parts with a lightweight shape can be made to have high strength, high toughness and high corrosion resistance, but the Fe content is limited to less than 0.40% by mass.

[0005] On the other hand, Patent Document 2 describes an aluminum alloy forging material characterized by its ability to increase the proportion of recycled ingots from commercially available waste materials and to use new ingots with low purity, which contain Si: 0.4% by mass or more and 1.5% by mass or less, Fe: more than 0.4% by mass and 1.0% by mass or less, Cu: 0.40% by mass or less, Mg: 0.8% by mass or more and 1.3% by mass or less, Ti: 0.01% by mass or more and 0.1% by mass or less, and Zn limited to 0.05% by mass or less. Furthermore, it contains at least one selected from the group consisting of Mn: 0.01% by mass or more and 1.0% by mass or less, Cr: 0.1% by mass or more and 0.4% by mass or less, and Zr: 0.05% by mass or more and 0.2% by mass or less, and the hydrogen content is limited to 0.25 ml / 100g Al or less. The balance includes unavoidable impurities and Al, with an average grain diameter of less than 50 μm, a crystal area ratio of less than 3%, and an average crystal size of less than 8 μm.

[0006] According to the invention described in Patent Document 2, although it is recorded that an aluminum alloy forging material with the same fracture toughness and fatigue properties as an aluminum alloy forging material with an Fe content of 0.4% by mass or less is manufactured, the strength is slightly inferior because the Cu content is limited to 0.40% by mass or less.

[0007] Furthermore, given the current state of resource depletion, various recycling initiatives have been launched, and the recovery of large quantities of consumed metals has already begun.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2008-163445

[0011] Patent Document 2: Japanese Patent Application Publication No. 2011-214093 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] High-strength, high-ductility, and highly corrosion-resistant aluminum alloy forgings are typically manufactured from virgin aluminum ingots obtained through bauxite refining. However, bauxite refining requires a large amount of electricity, which is supplied by thermal power plants. Therefore, using virgin aluminum ingots to manufacture high-strength, high-ductility, and highly corrosion-resistant aluminum alloy forgings presents the challenge of emitting significant amounts of CO2.

[0014] However, it is known that reducing the use of new aluminum ingots and increasing the use of recycled ingots increases the amount of impurities such as Fe in the molten aluminum, leading to the crystallization of intermetallic compounds during casting. This negatively impacts the strength, ductility, and corrosion resistance of aluminum alloy forgings made from these raw materials. Therefore, there is a challenge that, in order to limit the Fe content, it may be necessary to use new aluminum ingots produced by emitting large amounts of CO2.

[0015] The present invention addresses the aforementioned issues and aims to provide an aluminum alloy forging material that, even when the Fe content is higher than 0.4% by mass, still possesses the same strength, ductility, fatigue properties, and corrosion resistance as aluminum alloy forging materials with an Fe content of less than 0.4% by mass.

[0016] Problem-solving methods

[0017] In order to solve the aforementioned problem, the inventors conducted detailed and repeated studies on the chemical composition of aluminum alloy forgings. As a result, they discovered that by keeping the contents of various additive elements, such as Fe (which affects tensile properties, ductility (elongation)), fatigue properties, corrosion resistance, and microstructure), Ti (which is usually added to refine the casting structure), and Cu (which affects strength and corrosion resistance), within a specified range, aluminum alloy forgings with excellent strength, ductility, fatigue properties, and corrosion resistance can be obtained even with Fe content exceeding 0.4% by mass. This invention was thus created.

[0018] That is, the present invention relates to the following.

[0019] [1]

[0020] An aluminum alloy forging material containing

[0021] Si: ≥0.7% by mass and ≤1.5% by mass

[0022] Fe: above 0.4% by mass and below 0.67% by mass

[0023] Cu: above 0.4% by mass and below 0.8% by mass

[0024] Mg: ≥0.85% by mass and ≤1.3% by mass

[0025] Ti: ≥0.005% by mass and ≤0.07% by mass

[0026] Zn: less than 0.25% by mass; in addition,

[0027] It also contains at least one selected from the group consisting of Mn: 0.1% by mass or more and 0.95% by mass, Cr: more than 0.1% by mass and 0.4% by mass, and Zr: 0.05% by mass or more and 0.3% by mass.

[0028] The balance contains unavoidable impurities and Al.

[0029] The area fraction of the crystals is less than 3.2%, and the average crystal size is less than 8 μm.

[0030] [2]

[0031] According to the aluminum alloy forging material described in [1], the length of the large-angle grain boundary with an inclination angle of 15° or more, measured by SEM-EBSD in a range of 150μm×150μm at the center of the wall thickness of the aluminum alloy forging material perpendicular to the metal flow, is 4.15mm or more.

[0032] [3]

[0033] According to the aluminum alloy forging material described in [1] or [2], the 0.2% yield strength is above 345 MPa and the elongation is above 12.5%.

[0034] The effects of the invention

[0035] According to the present invention, it is possible to provide an aluminum alloy forging material that exhibits excellent strength, ductility, fatigue properties and corrosion resistance even when the Fe content is higher than 0.4% by mass. Attached Figure Description

[0036] Figure 1 The graph shows the relationship between Fe content and elongation in the aluminum alloy forgings of the examples and comparative examples. Detailed Implementation

[0037] [Aluminum alloy forgings]

[0038] The aluminum alloy forging material of the present invention contains

[0039] Si: ≥0.7% by mass and ≤1.5% by mass

[0040] Fe: above 0.4% by mass and below 0.67% by mass

[0041] Cu: above 0.4% by mass and below 0.8% by mass

[0042] Mg: ≥0.85% by mass and ≤1.3% by mass

[0043] Ti: ≥0.005% by mass and ≤0.07% by mass

[0044] Zn: less than 0.25% by mass, in addition,

[0045] It also contains at least one ingredient selected from the group consisting of Mn: 0.1% by mass or more and 0.95% by mass, Cr: more than 0.1% by mass and 0.4% by mass, and Zr: 0.05% by mass or more and 0.3% by mass.

[0046] The balance contains unavoidable impurities and Al.

[0047] The area fraction of the crystals is less than 3.2%, and the average crystal size is less than 8 μm.

[0048] The constituent elements of the aluminum alloy forging material of the present invention will be described below.

[0049] (Si: 0.7% by mass or more and 1.5% by mass or less)

[0050] Si is an essential element that contributes to high strength (yield strength). If the Si content is too low, the grains become coarse, and sufficient strength (tensile strength and 0.2% yield strength) cannot be obtained through artificial aging treatment. On the other hand, if the Si content is too high, corrosion resistance decreases. Furthermore, lower elongation also hinders processability. Therefore, the Si content is 0.7% by mass or more and 1.5% by mass or less, preferably 0.8% by mass or more and 1.3% by mass or less, and more preferably 0.9% by mass or more and 1.1% by mass or less.

[0051] (Fe: above 0.4% by mass and below 0.67% by mass)

[0052] Fe, together with Mn and Cr, forms dispersed particles (dispersed phase), which hinders grain boundary migration after recrystallization, thus preventing grain coarsening and refining the grain size.

[0053] In the case of existing aluminum alloy forgings with an Fe content of 0.4% by mass or less, if high-temperature heating treatment is performed, the solid solution of dispersed particles will also occur, and therefore, the grains will easily become coarser due to recrystallization.

[0054] If the Fe content is higher than 0.4% by mass, as in this invention, the density of the dispersed particles increases, thus suppressing recrystallization even under high-temperature heating. Furthermore, the forging process refines and rounds the Fe-based crystals. This grain refinement also suppresses fatigue crack propagation and improves fatigue properties.

[0055] If the Fe content is too low, these effects are absent. On the other hand, if the Fe content is too high, the crystals of Al-Fe-Si intermetallic compounds, etc., become coarser. Coarse crystals deteriorate elongation, fatigue properties, corrosion resistance, etc. The inventors investigated the relationship between Fe content and elongation, and established a regression equation ( Figure 1 Therefore, it can be seen that sufficient elongation can be obtained if the content is 0.67% by mass or less. Details are explained in the items of the embodiments. Therefore, the Fe content is higher than 0.4% by mass and lower than 0.67% by mass. Preferably it is higher than 0.40% by mass and lower than 0.65% by mass, more preferably it is higher than 0.40% by mass and lower than 0.55% by mass.

[0056] (Cu: above 0.4% by mass and below 0.8% by mass)

[0057] Cu contributes to increased strength through solid solution strengthening and also significantly promotes age hardening of the final product during aging treatment. If the Cu content is too low, these effects are absent. Conversely, if the Cu content is too high, the susceptibility of the aluminum alloy forging to stress corrosion cracking and intergranular corrosion is significantly increased, reducing its corrosion resistance and durability. Therefore, the Cu content is higher than 0.4% by mass and lower than 0.8% by mass, more preferably higher than 0.4% by mass and lower than 0.5% by mass.

[0058] (Mg: ≥0.85% by mass and ≤1.3% by mass)

[0059] Mg is an essential element that contributes to high strength (yield strength). If the Mg content is too low, the age hardening during artificial aging treatment is reduced. Furthermore, grain coarsening is more likely to occur. Corrosion resistance also decreases. On the other hand, if the Mg content is too high, the strength (yield strength) becomes excessively high, thus hindering forgeability. Additionally, corrosion resistance and other properties decrease. Therefore, the Mg content is 0.85% by mass or more and 1.3% by mass or less, preferably 1.0% by mass or more and 1.2% by mass or less.

[0060] (Ti: ≥0.005% by mass and ≤0.07% by mass)

[0061] Ti has the effect of refining the grain size of the ingot. If the Ti content is too low, this effect cannot be achieved. Furthermore, grain coarsening leads to reduced strength. On the other hand, if the Ti content is higher than 0.07% by mass, the grains of the forged material tend to coarsen after heat treatment, reducing fatigue properties. Coarse crystals become the starting point for fracture, reducing elongation. Therefore, the Ti content is 0.005% by mass or more and 0.07% by mass or less, preferably 0.01% by mass or more and 0.06% by mass or less.

[0062] (Zn: less than 0.25% by mass)

[0063] Zn, which is easily introduced as an impurity, is preferably 0% by mass. If the content is higher than 0.25% by mass, the strength, elongation, and corrosion resistance will decrease. Therefore, the Zn content is 0.25% by mass or less, preferably 0.05% by mass or less.

[0064] (Contains at least one selected from the group consisting of Mn: 0.1% by mass or more and 0.95% by mass, Cr: more than 0.1% by mass and 0.4% by mass, and Zr: 0.05% by mass or more and 0.3% by mass)

[0065] Among these components, Mn and Cr, during homogenization heat treatment and subsequent hot forging, will selectively combine with Fe, Mn, Cr, Si, Al, etc., depending on their content, to form dispersed particles (dispersed phases) composed of Al-Mn and Al-Cr intermetallic compounds.

[0066] Although it also depends on the manufacturing conditions, these dispersed particles formed by Mn and Cr have the effect of hindering grain boundary migration after recrystallization. Therefore, they can prevent grain coarsening.

[0067] If the contents of Mn and Cr are too low, these effects cannot be expected, resulting in coarsening of the grains and a decrease in strength. On the other hand, an excess of these elements can lead to a decrease in elongation. Therefore, it is preferable to contain at least one of Mn and Cr, with the Mn content being 0.1% by mass or more and 0.95% by mass or less, preferably 0.2% by mass or more and 0.6% by mass, and the Cr content being more than 0.1% by mass and less than 0.4% by mass, preferably more than 0.1% by mass and less than 0.3% by mass, and more preferably more than 0.1% by mass and less than 0.2% by mass.

[0068] Zr, like Mn and Cr, forms dispersed particles (dispersed phase). In cases where Zr is present, such as in the presence of Ti, depending on the casting conditions, it can actually hinder grain refinement in the ingot, becoming a factor contributing to grain coarsening. Furthermore, it also contributes to reduced fatigue properties. Therefore, it is desirable to add Zr within a range that does not cause grain coarsening during casting. Specifically, the Zr content is 0.05% by mass or more and 0.3% by mass or less, preferably 0.05% by mass or more and 0.1% by mass or less.

[0069] The aluminum alloy forging material of the present invention, by containing Si, Cu, and Mg within a certain range, can achieve the strength required for, for example, automotive chassis parts requiring high strength. By containing Ti within this range, the casting structure can be refined. Furthermore, by containing Mn, Cr, and Zr within this range, recrystallization during solution treatment can be suppressed, resulting in fine crystals. Therefore, high strength can be ensured. Moreover, in the present invention, even with a large amount of Fe, forging can reduce, refine, and round the Fe-containing crystals, achieving grain refinement and thus ensuring elongation, fatigue characteristics, and corrosion resistance.

[0070] (Balance: unavoidable impurities and Al)

[0071] The balance includes unavoidable impurities and Al. Examples of unavoidable impurities include C, Ni, Na, Ca, V, and Hf. These are easily mixed in as impurities and can hinder the properties of automotive chassis parts, so it is preferable that they are not present. However, it is permissible if each is less than 0.05% by mass and the total is less than 0.10% by mass.

[0072] In addition, boron (B) is also an impurity, but like titanium (Ti), it can refine the grain size of the ingot, improving its resistance to extrusion and machinability during forging. However, if the content exceeds 300 ppm, it will still form coarse crystalline precipitates, reducing the machinability. Therefore, the preferred boron content is 300 ppm or less.

[0073] <Crystals>

[0074] The aluminum alloy forging material of the present invention has a crystal area ratio of less than 3.2% and an average crystal size of less than 8μm.

[0075] (Area percentage of crystals: below 3.2%)

[0076] The crystallization area ratio is determined by the amount of added elements and their solid solution content. By keeping the crystallization area ratio at 3.2% or less, the decrease in elongation caused by the increase in crack propagation paths can be suppressed. In addition, the decrease in corrosion resistance caused by the easy corrosion of the parent phase around the crystals can be suppressed. The crystallization area ratio is more preferably 3.1% or less, and even more preferably 3.0% or less.

[0077] The crystallization area ratio can be calculated using a SEM image of a cross-section perpendicular to the metal flow of the forged material at the center of the wall thickness, and through image analysis. Specific calculation methods are shown in the examples.

[0078] (Average crystal size: below 8μm)

[0079] The average crystal size is determined by the amount of added elements and the solidification rate. By making the average crystal size 8 μm or less, it is possible to suppress the crystals from becoming the initiation point of cracks in the tensile test, thereby reducing the elongation. More preferably, the average crystal size is 7 μm or less, and even more preferably 6 μm or less.

[0080] The average crystal size can be obtained by taking a reflected electron image of a cross section perpendicular to the metal flow of the forged material at the center of the wall thickness using SEM, and then converting it into a circle of equal area using analysis software to calculate the average size.

[0081] (Average grain diameter: below 50μm)

[0082] The average grain diameter significantly affects mechanical properties. From the perspective of tensile and fatigue properties, the average grain diameter is preferably 45 μm or less, and more preferably 40 μm or less.

[0083] The average grain diameter can be calculated using the minor axis slicing method. That is, after etching the center of the wall thickness of a section perpendicular to the metal flow of the forged material, an optical microscope is used to take a picture, a straight line is drawn along the direction orthogonal to the major axis of the grain, the number of grains on the line is measured, and the average grain diameter is calculated by dividing the measured number of grains by the distance of the line.

[0084] (Large grain boundary length: 4.15 mm or more)

[0085] In the aluminum alloy forging material of the present invention, the length of the large-angle grain boundary with an inclination angle of 15° or more, measured by SEM-EBSD in a range of 150μm×150μm at the center of the wall thickness of the cross section perpendicular to the metal flow of the aluminum alloy forging material, is preferably 4.15mm or more.

[0086] Large grain boundary length significantly impacts mechanical properties. From the perspective of tensile and fatigue properties, a large grain boundary length of 4.2 mm or more is preferred, and even more preferably 4.3 mm or more is preferred.

[0087] (Small angle grain boundary length: ≥2.0mm)

[0088] Small-angle grain boundary length significantly affects mechanical properties. From the viewpoint of tensile and fatigue properties, the small-angle grain boundary length is preferably 2.0 mm or more, more preferably 2.2 mm or more, and even more preferably 2.5 mm or more.

[0089] SEM-EBSD (EBSP) is a crystal orientation analysis method that uses an electron backscatter diffraction pattern (EBSD) system mounted on an SEM.

[0090] Large-angle and small-angle grain boundary lengths can be determined using SEM-EBSD. Within a 150 μm × 150 μm area at the center of the wall thickness in a section perpendicular to the metal flow of the forged material, crystal orientation analysis is performed with an observation step size of 0.5 μm. Boundaries with an orientation difference of 15° or more between adjacent grains are defined as large-angle grain boundaries, and those between 5° and 15° are defined as small-angle grain boundaries. The lengths are automatically calculated using analysis software (TSL OIM Analysis 7×64). Data with a Confidence Index (CI) value below 0.1 are excluded from analysis.

[0091] <Tension Properties>

[0092] (0.2% yield strength: above 345MPa)

[0093] The 0.2% yield strength of the aluminum alloy forging material of the present invention is preferably 345 MPa or higher. If it is lower than 345 MPa, the necessary strength for the forging material used as an automotive chassis part may not be guaranteed. The 0.2% yield strength is more preferably 355 MPa or higher, and even more preferably 360 MPa or higher.

[0094] (Elongation rate: higher than 12.5%)

[0095] The elongation (%) of the aluminum alloy forging material of the present invention is preferably higher than 12.5%. If it is lower than 12.5%, the formability of the forging material used as an automotive chassis part may be insufficient. The elongation is preferably 13% or more, and more preferably 13.5% or more.

[0096] (Tensile strength: above 370MPa)

[0097] From the viewpoint of strength, the tensile strength of the aluminum alloy forging material of the present invention is preferably 370 MPa or more. More preferably, it is 380 MPa or more, and even more preferably 385 MPa or more.

[0098] The tensile and fatigue properties of aluminum alloy forgings can be adjusted by using the aforementioned chemical composition and, in addition, by using the manufacturing methods described later.

[0099] The elongation, 0.2% yield strength, and tensile strength of aluminum alloy forgings can be measured according to the tensile testing of metallic materials according to JIS Z 2241 (2011 revision). Specifically, tensile test pieces (test piece No. 4) based on JIS Z 2201 are prepared by cutting from the center of a cross-section at any point with the test piece axis parallel to the metal flow of the forging, and tensile tests are performed at room temperature (25°C). The N number for the mechanical property measurements is 2, and the values ​​are calculated as averages. In this way, the elongation, 0.2% yield strength, and tensile strength can be calculated.

[0100] <Fatigue Characteristics>

[0101] (Number of fracture repetitions under alternating stress of 170 MPa in fatigue tests using unnotched rotating bending fatigue test specimens: 1 × 10⁻⁶) 6 (more than one cycle)

[0102] The aluminum alloy forging material of the present invention, from the viewpoint of fatigue life, preferably has a fracture repeat count of 1×10⁻⁶ under alternating stress of 170 MPa in fatigue tests using unnotched rotational bending fatigue test specimens, which is suitable for use as a forging material for automotive chassis parts.6 More than one cycle. The number of fracture repetitions under alternating stress of 170 MPa in fatigue tests using unnotched rotary bending fatigue test specimens is more preferably 2 × 10⁻⁶. 6 More than one cycle, further preferably 3×10 6 More than one cycle.

[0103] (Number of fracture repetitions under alternating stress of 70 MPa in fatigue tests using notched rotary bending fatigue test specimens: 1 × 10⁻⁶) 6 (more than one cycle)

[0104] From the viewpoint of fatigue life, the number of fracture repetitions under alternating stress of 70 MPa in fatigue tests of the notched rotary bending fatigue test specimens of the aluminum alloy forging material of the present invention is preferably 1 × 10⁻⁶, based on the fatigue life of the forging material for use in automobile chassis parts. 6 More than one cycle. The number of repeated fracture cycles under alternating stress of 70 MPa in fatigue tests using notched rotary bending fatigue test specimens is more preferably 2 × 10⁻⁶. 6 More than one cycle, further preferably 3×10 6 More than one cycle.

[0105] The fatigue characteristics of aluminum alloy forgings can be measured using the rotary bending fatigue test for metallic materials according to JIS Z 2274. Specifically, for unnotched rotary bending fatigue test pieces, a rotary bending fatigue test piece (test piece No. 2) based on JIS Z 2274 is prepared by cutting at any position parallel to the metal flow of the aluminum alloy forging, and a rotary bending fatigue test is conducted at an alternating stress of 170 MPa. Similarly, for notched rotary bending fatigue test pieces, a test piece with an annular semi-circular groove (where ρ is 0.31 mm as the groove angle R) based on JIS Z 2274 is prepared by cutting at any position parallel to the metal flow of the aluminum alloy forging, and a rotary bending fatigue test is conducted at an alternating stress of 70 MPa. The N number for fatigue characteristic measurement is 2, and the minimum value is calculated. In this way, the fatigue characteristics can be calculated.

[0106] The fatigue properties of aluminum alloy forgings can be adjusted by using the chemical composition described above, and furthermore, by using the manufacturing methods described later.

[0107] The aluminum alloy forging material of the present invention described above can have the same strength, elongation, fatigue characteristics and corrosion resistance as aluminum alloy forging materials with an Fe content of 0.4% by mass or less.

[0108] In other words, the aluminum alloy forging material of the present invention can contain more than 0.4% by mass of Fe, which has an adverse effect on strength and elongation. Therefore, when manufacturing the aluminum alloy forging material, the amount of new aluminum ingots used can be reduced, and CO2 emissions can be significantly reduced.

[0109] [Manufacturing method of aluminum alloy forgings]

[0110] Next, the manufacturing method of the aluminum alloy forging material of the present invention will be described.

[0111] The manufacturing method of the aluminum alloy forging material of the present invention is not particularly limited. The aluminum alloy forging material can be properly manufactured by a manufacturing method that sequentially includes a casting process, a homogenization heat treatment process, a heating process, a forging process, a solution treatment process, a quenching process, and an artificial aging treatment process.

[0112] Furthermore, in the above-described method for manufacturing aluminum alloy forgings, any step that does not impede the intended effect of the present invention is permitted to be included. Examples of such steps include, for instance, an extrusion step performed between a homogenization heat treatment step and a heating step, and a roll forging step performed between a heating step and a forging step.

[0113] (Casting process)

[0114] The casting process is the process of casting an ingot of aluminum alloy having the aforementioned composition. The composition has already been described in detail, therefore, the description is omitted.

[0115] The casting process is preferably carried out under conditions of heating temperature of 710-810°C and cooling rate of 7°C / sec or higher to the liquidus temperature.

[0116] If the heating temperature in the casting process is above 710°C, the melting time can be shortened, enabling high-efficiency operation. Furthermore, if the heating temperature in the casting process is below 810°C, the formation of slag as an oxide can be suppressed, metal loss reduced, and thus, ingots can be obtained efficiently.

[0117] If the cooling rate to the liquidus temperature is less than 7°C / sec, the crystals will become coarse, and the crystal area ratio will not reach below 3.2%.

[0118] Furthermore, the heating temperature is preferably 710–750°C, and the cooling rate to the liquidus temperature is preferably 10°C / sec or higher.

[0119] Casting can be carried out through melting and casting methods such as continuous casting, semi-continuous casting, and hot-top casting, with continuous casting being the preferred method.

[0120] (Homogenization heat treatment process)

[0121] The subsequent homogenization heat treatment process involves subjecting the ingot cast in the casting process to a homogenization heat treatment at 420–560°C for 2.5–8 hours. By maintaining a heating temperature of 420°C or higher and a heating time of 2.5 hours or higher in the homogenization heat treatment process, the crystals are fully dissolved, and the area ratio of the crystals is reduced, thus ensuring the elongation of the product. On the other hand, by maintaining a heating temperature of 560°C or lower and a heating time of 8 hours or lower in the homogenization heat treatment process, the coarsening of dispersed particles can be suppressed, thus enabling them to be dispersed uniformly, finely, and with high density. In other words, a grain refinement effect is easily obtained, and the average grain diameter is reduced.

[0122] Furthermore, the heating temperature in the homogenization heat treatment process is preferably 500–540°C, and the heating time is preferably 4–8 hours.

[0123] (Heating process)

[0124] The next heating process involves heating the homogenized heat-treated ingot at 400–545°C for at least 0.5 hours.

[0125] As described in this invention, Fe, together with Mn and Cr, forms dispersed particles (dispersed phase), which have the effect of hindering grain boundary migration after recrystallization. Therefore, even with a large addition of Fe and a thorough heating process, the number and density of dispersed particles can be made to the same level as existing materials, preventing grain coarsening and thus maintaining the grains in a fine state. Therefore, tensile properties can be maintained to the same level as existing materials. This effect can be achieved by thoroughly heating the ingot during this heating process before the forging process, causing the Fe-based crystals to dissolve and decrease, further refining them.

[0126] If the heating temperature in the heating process is above 440°C and the heating time is above 0.5 hours, even in the case of aluminum alloy forgings containing a large amount of Fe, as in this invention, solid solution of Fe-based crystals can proceed, maintaining the elongation at the same level as existing materials. On the other hand, if the heating temperature is below 545°C, eutectic melting caused by heat during processing can be suppressed, making it difficult for voids to form. Mechanical properties are less likely to decrease. In addition, the coarsening and dedensification of dispersed particles caused by heat treatment are less likely to occur, making it easier to achieve a grain refinement effect.

[0127] The preferred heating temperature in the heating process is 440–545°C.

[0128] (Forging process)

[0129] The subsequent forging process involves forging the heated ingot from the heating process at a forging end temperature of 350°C or higher and a reduction rate of 50-95%, thereby obtaining a forged material of a specified shape. Let the height of the raw material before forging be L0, and the height after forging be L1. The reduction rate is defined as (1 - (L1 / L0)) × 100.

[0130] If the forging end temperature in the forging process is above 350°C, the residual strain is minimal, thus recrystallization is unlikely to occur, and grain coarsening is difficult. Furthermore, if the reduction rate in the forging process is above 50%, casting defects can be compacted, and grains and crystals can be sufficiently reduced. If the reduction rate is below 95%, the processing rate will not become excessive, making grain coarsening due to recrystallization unlikely. Also, as long as the forging end temperature does not exceed the heating temperature, it is preferable to have the highest possible temperature.

[0131] The preferred forging end temperature is above 370℃, and the preferred reduction rate is 70-90%.

[0132] Forging under these conditions can be carried out, for example, by mechanical presses and hydraulic presses.

[0133] (Solution treatment process)

[0134] The subsequent solution treatment process involves treating the forged material obtained in the forging process at a temperature of 480–580°C for more than 0 hours but less than 24 hours. This solution treatment facilitates the solution treatment of additives used to increase strength during the subsequent artificial aging process, or improves elongation due to the refinement of crystals.

[0135] If the heating temperature in the solution treatment process is above 480°C and the heating time is above 0 hours, the solution treatment is sufficient, thus achieving good elongation and strength (tensile strength and 0.2% yield strength). On the other hand, if the heating temperature in the solution treatment process is below 580°C and the heating time is below 24 hours, the grains are less likely to coarsen, the average grain diameter is less likely to increase, and good strength (tensile strength and 0.2% yield strength) can be obtained.

[0136] Furthermore, the heating temperature in the solution treatment process is preferably 540–560°C, and the heating time is preferably 2.5–8.0 hours.

[0137] (Quenching process)

[0138] The next step, quenching, involves quenching the forged material that has undergone solution treatment in the aforementioned solution treatment process at a temperature below 75°C. Quenching increases the strength of the forged material.

[0139] If the quenching temperature in the quenching process is below 75℃, it can be fully quenched, and the strength can be fully improved in the artificial aging process described later.

[0140] Furthermore, the lower limit of the quenching temperature is simply the room temperature of the water used for quenching, which is 20±15℃ (5~35℃) as specified in JIS Z 8703.

[0141] (Artificial aging process)

[0142] The next artificial aging process involves aging the forged material, which has been quenched in the quenching process, at 160–250°C for 0.5–20 hours. Furthermore, the treatment from the solution treatment process up to this artificial aging process is called artificial age hardening treatment. Through this artificial aging process, strength required for, for example, automotive chassis parts can be obtained.

[0143] If the heating temperature in the artificial aging process is above 160°C and the heating time is above 0.5 hours, sufficient strength, fatigue properties, and corrosion resistance can be obtained. On the other hand, if the heating temperature in the artificial aging process is above 250°C and the heating time is above 20 hours, it becomes an excessively over-aged state, and sufficient strength and elongation cannot be obtained.

[0144] Furthermore, the heating temperature in the artificial aging process is preferably 170–250°C, and the heating time is preferably 3–12 hours.

[0145] According to the above-described method for manufacturing aluminum alloy forgings, it is possible to manufacture aluminum alloy forgings with an Fe content higher than 0.4% by mass, but with the same strength, elongation, fatigue characteristics, and corrosion resistance as aluminum alloy forgings with an Fe content of less than 0.4% by mass.

[0146] Example

[0147] The following describes embodiments of the present invention in detail. However, the scope of the present invention is not limited thereto.

[0148] (Production of forging materials)

[0149] The aluminum alloy forgings No. 1 to 6 are manufactured using aluminum alloys with the chemical composition shown in Table 1, under the following conditions.

[0150] First, the molten aluminum alloy is gravity-cast using a mold to produce an ingot. The casting conditions are: heating temperature 720°C, and cooling rate of 10–15°C / sec up to the liquidus temperature.

[0151] Each casting block is surface-cut to φ55mm × length 100mm, and then subjected to homogenization treatment, heating, hot die forging using a hydraulic press, solution treatment, quenching, and artificial aging treatment under the conditions shown in Table 2 below to produce aluminum alloy forgings with various numbers.

[0152] Table 1

[0153]

[0154] Table 2

[0155]

[0156] Table 3

[0157]

[0158] The tensile properties, fatigue properties, average grain diameter (μm), large-angle grain boundary length (mm), small-angle grain boundary length (mm), crystal area ratio (%), average crystal size (μm), corrosion rate (mm / y) as a general corrosion resistance, and stress corrosion cracking resistance (SCC resistance) of forgings No. 1 to No. 6 were evaluated. Furthermore, as tensile properties, tensile strength (MPa), 0.2% yield strength (MPa), and elongation (%) were measured. Regarding fatigue properties, the number of cycles of fracture under alternating stress of 170 MPa in fatigue tests using unnotched rotary bending fatigue test specimens and the number of cycles of fracture under alternating stress of 70 MPa in fatigue tests using notched rotary bending fatigue test specimens were measured.

[0159] These evaluations were conducted in the following manner.

[0160] <Tension Properties>

[0161] Tensile properties are measured by cutting and fabricating tensile test pieces (test piece No. 4) from any position parallel to the metal flow of the aluminum alloy forging, according to JIS Z 2201, and measuring them using a tensile testing machine according to JIS Z 2241.

[0162] Regarding tensile properties, for the 0.2% yield strength, a value above 345 MPa is acceptable, while a value below 345 MPa is unacceptable. Regarding elongation, a value above 12.5% ​​is acceptable, while a value below 12.5% ​​is unacceptable.

[0163] <Fatigue Characteristics>

[0164] In fatigue characteristics, for the unnotched rotating bending fatigue test specimens, rotating bending fatigue test specimens (No. 2 specimens) based on JIS Z 2274 were cut from any position in a manner parallel to the metal flow of the aluminum alloy forgings and evaluated at an alternating stress of 170 MPa. For the notched rotating bending fatigue test specimens, specimens with an annular semi-circular groove (where ρ as the groove angle R is 0.31 mm) based on JIS Z 2274 were cut from any position in a manner parallel to the metal flow of the aluminum alloy forgings and evaluated at an alternating stress of 70 MPa.

[0165] The number of fracture repetitions at an alternating stress of 170 MPa in the fatigue test using the unnotched rotating bending fatigue test specimens and the number of fracture repetitions at an alternating stress of 70 MPa in the fatigue test using the notched rotating bending fatigue test specimens were both 6 more than 1×10

[0166] cycles, and were considered qualified. Figure 1 Also,

[0167] (Average grain diameter)

[0168] The average grain diameter (μm) was calculated by etching the center of the wall thickness of the cross-section perpendicular to the metal flow of the aluminum alloy forgings, taking a 400-fold photograph with an optical microscope, drawing a straight line along the direction orthogonal to the long axis of the grains, measuring the number of grains on this straight line, and dividing the measured number of grains by the distance of the straight line.

[0169] If the average grain diameter is higher than 50 μm, the tensile characteristics and fatigue characteristics will decrease. Therefore, an average grain diameter of 50 μm or less is considered qualified, and higher than 50 μm is considered unqualified.

[0170] (Length of large-angle grain boundaries)

[0171] The SEM-EBSD (EBSP) method is a crystal orientation analysis method in which a backscattered electron diffraction image [EBSD: Electron Back Scattering (Scattered) Diffraction Pattern] system is mounted on the SEM.

[0172] Regarding the large-angle grain boundary length, crystal orientation analysis was performed using SEM-EBSD at a 150 μm × 150 μm range with an observation step size of 0.5 μm at the center of the wall thickness in a section perpendicular to the metal flow of the forged material. Grain boundaries with an orientation difference of 15° or more adjacent to this data point were defined as large-angle grain boundaries, and the length was automatically calculated using analysis software (TSL Solution OIMAnalysis ver.7). Data with a Confidence Index (CI) value below 0.1 were excluded from the analysis.

[0173] Regarding the length of large-angle grain boundaries, in the microstructure of the central part of the wall thickness of aluminum alloy forgings, when the length of large-angle grain boundaries with an inclination angle of 15° or greater, measured by SEM-EBSD in a range of 150μm×150μm, is less than 4.15mm, the fatigue characteristics decrease. The number of fracture repetitions under alternating stress of 170MPa in fatigue tests using unnotched rotating bending fatigue test specimens and the number of fracture repetitions under alternating stress of 70MPa in fatigue tests using notched rotating bending fatigue test specimens cannot be guaranteed to be 1×10⁻⁶. 6 More than one cycle. Therefore, regarding the length of large-angle grain boundaries, the length of large-angle grain boundaries with an inclination angle of 15° or more, measured by SEM-EBSD in a range of 150μm×150μm at the center of the wall thickness of the cross section perpendicular to the metal flow of the aluminum alloy forging, is considered acceptable if it is 4.15mm or more.

[0174] (small angle grain boundary length)

[0175] SEM-EBSD (EBSP) is a crystal orientation analysis method that uses an SEM equipped with an Electron Back Scattering (Scattered) Diffraction Pattern (EBSD) system.

[0176] Regarding the small-angle grain boundary length, crystal orientation analysis was performed using SEM-EBSD at the center of the wall thickness in a 150 μm × 150 μm range with an observation step size of 0.5 μm in a section perpendicular to the metal flow of the forged material. Grain boundaries with an orientation difference of 5° to 15° adjacent to this data point were defined as small-angle grain boundaries, and the length was automatically calculated using analysis software (TSL OIM Analysis 7×64). Furthermore, data with a Confidence Index (CI) value below 0.1 were excluded from the analysis.

[0177] Regarding the length of small-angle grain boundaries, in the microstructure of the central part of the wall thickness of aluminum alloy forgings, when the length of small-angle grain boundaries with an inclination angle of 5° to 15°, measured by SEM-EBSD in a range of 150μm × 150μm, is less than 2.0mm, the fatigue characteristics decrease. The number of fracture repetitions under alternating stress of 170MPa in fatigue tests using unnotched rotating bending fatigue test specimens and the number of fracture repetitions under alternating stress of 70MPa in fatigue tests using notched rotating bending fatigue test specimens cannot be guaranteed to be 1×10⁻⁶. 6 More than one cycle. Therefore, regarding the small-angle grain boundary length, at the center of the wall thickness of the section perpendicular to the metal flow of the aluminum alloy forging, the length of the small-angle grain boundary with an inclination angle of 5° to 15°, measured by SEM-EBSD in a range of 150μm × 150μm, is considered acceptable if it is 2.0mm or more.

[0178] <Crystals>

[0179] (Area ratio of crystals)

[0180] The area ratio of crystals (%) was calculated by taking a 400x reflected electron image of the center of the wall thickness of a cross section perpendicular to the metal flow of the forging material using a SEM (JEOL, JSM-IT700HR), and by analyzing the image (WinROOF2018, Ver.4.7.0) to calculate the area of ​​white contrast, which was then divided by the total area of ​​the image analysis performed.

[0181] If the area ratio of the crystals is higher than 3.2%, the elongation will decrease and cannot be guaranteed to be higher than 12.5%. Therefore, a crystal area ratio below 3.2% is considered acceptable, while a ratio above 3.2% is considered unacceptable.

[0182] (Average crystal size)

[0183] The average crystal size (μm) was obtained by taking a 400x reflected electron image of the center of the wall thickness of a cross section perpendicular to the metal flow of the forging material using a SEM (JEOL, JSM-IT700HR), and then converting it into a circle of the same area using analysis software to calculate the average size.

[0184] If the average crystal size is higher than 8 μm, the elongation will decrease and cannot be guaranteed to be higher than 12.5%. Therefore, an average crystal size below 8 μm is acceptable, while an average crystal size above 8 μm is unacceptable.

[0185] <General corrosion resistance>

[0186] (Corrosion rate)

[0187] Regarding the corrosion rate (mm / y), a flat plate is made of forged aluminum alloy material and is carried out in accordance with the provisions of the alternate immersion method of ASTM G47. Here, the corrosion rate (mm / y) represents the corrosion amount in mm for 1 year (365 days). The test conditions are to repeat soaking and removing in salt water for 30 days, measure the weight before and after the test, and calculate through the following formula (1). Here, M represents the corrosion reduction amount (g / 30 days), ρ represents the density of 2.7 (g / cm 3 ), and S represents the area of the evaluation part (cm 2 ).

[0188]

Formula 1

[0189]

[0190] When the corrosion rate exceeds 0.12 mm / y, the function of the forged aluminum alloy material used as an automotive chassis part cannot be satisfied. Therefore, a corrosion rate of 0.12 mm / y or less is qualified, and a corrosion rate higher than 0.12 mm / y is unqualified.

[0191] <Stress corrosion cracking resistance>

[0192] The evaluation of stress corrosion cracking resistance (SCC resistance: Stress Corrosion Cracking) follows the provisions of JIS H8711. A C-shaped test piece is made of forged aluminum alloy material and is carried out by the alternate immersion method.

[0193] The SCC test assumes use as an automotive chassis part and is carried out under the conditions of a load stress of 180 MPa (tensile) and a period of 30 days. After the test, observe whether stress corrosion cracking occurs on the test piece.

[0194] Regarding stress corrosion cracking resistance, if no stress corrosion cracking occurs on the test piece visually, it is qualified; if stress corrosion cracking occurs, it is unqualified.

[0195] The tensile properties of the forged materials of No.1 to 6, the number of fracture repetitions (cycles) under an alternating stress of 170 MPa in the fatigue test using a non-notch rotating bending fatigue test piece, the number of fracture repetitions (cycles) under an alternating stress of 7 MPa in the fatigue test using a notch rotating bending fatigue test piece, the average grain diameter (μm), the length of large-angle grain boundaries (mm), the length of small-angle grain boundaries (mm), the area ratio of crystallites (%), the average crystallite size (μm), the corrosion rate (mm / y) as general corrosion resistance, and the SCC resistance are shown in Table 1.

[0196] As shown in Table 1, the forged aluminum alloy material of No.4 fully meets the requirements of the present invention, and thus can obtain good evaluation results.

[0197] On the other hand, the Fe content of forging materials No.1 and No.2 is lower than the lower limit.

[0198] Forged material No. 3, due to its Ti content exceeding the upper limit, exhibited a smaller large-angle grain boundary length. Consequently, it failed the fatigue tests using unnotched rotary bending fatigue test specimens with an alternating stress of 170 MPa and the fatigue tests using notched rotary bending fatigue test specimens with an alternating stress of 70 MPa.

[0199] Forged materials No. 5 and 6 have higher Fe content than the upper limit, resulting in coarser Al-Fe-Si crystals and a larger crystal area ratio. Consequently, their elongation and corrosion rate are substandard.

[0200] Industrial availability

[0201] According to the present invention, it is possible to provide an aluminum alloy forging material that still exhibits excellent strength, ductility, fatigue properties and corrosion resistance even when the Fe content is higher than 0.4% by mass.

[0202] The present invention has been described in detail with reference to specific embodiments, but various changes and modifications can be made without departing from the spirit and scope of the invention, which will be apparent to those skilled in the art.

[0203] This application is based on Japanese patent applications filed on January 15, 2024 (Takumi 2024-003898) and October 3, 2024 (Takumi 2024-174169), the contents of which are incorporated herein by reference.

Claims

1. An aluminum alloy forging material, comprising Si: ≥0.7% by mass and ≤1.5% by mass Fe: above 0.4% by mass and below 0.67% by mass Cu: above 0.4% by mass and below 0.8% by mass Mg: 0.85% by mass or more and 1.3% by mass or less Ti: ≥0.005% by mass and ≤0.07% by mass Zn: less than 0.25% by mass, and, It also contains at least one ingredient selected from the group consisting of Mn: 0.1% by mass or more and 0.95% by mass, Cr: more than 0.1% by mass and 0.4% by mass, and Zr: 0.05% by mass or more and 0.3% by mass. The balance contains unavoidable impurities and Al. The area fraction of the crystals is less than 3.2%, and the average crystal size is less than 8 μm.

2. The aluminum alloy wrought material of claim 1, wherein, At the center of the wall thickness of the cross section perpendicular to the metal flow of the aluminum alloy forging, the length of the large-angle grain boundary with an inclination angle of 15° or more, measured by SEM-EBSD in a range of 150μm×150μm, is 4.15mm or more.

3. The aluminum alloy wrought material of claim 1 or 2, wherein, The yield strength at 0.2% is above 345 MPa, and the elongation is above 12.5%.

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