JP7575739B2 - Aluminum-based alloys - Google Patents

Description

The present invention relates to an aluminum-based alloy.

Extruded aluminum-based alloys can be formed into complex cross-sectional shapes, and 6000-series aluminum-based alloys in particular have excellent extrusion workability and formability, and are used in construction and transportation materials (see, for example, Patent Document 1). This extruded aluminum-based alloy has a large recrystallized texture that is divided into two layers, the surface and the interior, and in the interior texture, the <100> crystal orientation is formed in the Cube orientation or Goss orientation that is parallel to the extrusion direction.

International Publication No. WO2007/111002

However, conventional aluminum-based alloys such as those described in Patent Document 1 have anisotropy in bendability that differs greatly between the extrusion direction (RD) and the perpendicular direction (TD) in the range where the yield strength exceeds 250 MPa, due to differences in the texture between the surface and the interior, even if the yield strength is about the same, and this has the problem of making the alloy susceptible to fracture during processing.

The present invention was made with a focus on these problems, and aims to provide an aluminum-based alloy that has small anisotropy in bendability and is less likely to break during processing.

In order to achieve the above object, the aluminum-based alloy according to the present invention is an aluminum-based alloy obtained by extrusion processing, comprising 0.75 wt% to 1.0 wt% Mg, 0.3 wt% to 0.8 wt% Si, 0.15 wt% to 0.4 wt% Cu, 0.10 wt% to 0.25 wt% Fe, 0.07 wt% or less Cr, 0.025 wt% or less Mn, with the balance being Al and unavoidable impurities, and characterized in that, from the surface in a depth direction, there is a surface layer portion in which a maximum principal stress is compressive and an inner layer portion in which a maximum principal stress is tensile, the direction of the maximum principal stress in the surface layer portion being 5° or more and 40° or less with respect to the extrusion direction of the extrusion processing, and the direction of the maximum principal stress in the inner layer portion being 50° or more with respect to the extrusion direction of the extrusion processing.

In the aluminum-based alloy according to the present invention, the surface layer is preferably distributed to a depth of 0.10 mm to 0.15 mm from the surface. The inner layer preferably has a deepest part that is 0.3 mm or more deep from the surface. The inner layer preferably has a maximum von Mises stress of 150 MPa or more. In the surface layer, 80% or more of the crystal grains preferably have a grain size of 150 μm or less and a preferred crystal orientation in the <110> direction.

The present invention provides an aluminum-based alloy that has small anisotropy in bendability and is less likely to break during processing.

1 is a process chart showing a manufacturing method of an aluminum-based alloy according to an embodiment of the present invention. FIG. 1 shows, after bending test, (a) a metallurgical microscope photograph of a test piece in the L direction, (b) a metallurgical microscope photograph of an enlarged portion of (a), (c) a test piece in the LT direction, and (d) a metallurgical microscope photograph of an enlarged portion of (c), of an aluminum-based alloy according to an embodiment of the present invention. FIG. 1 shows, after bending test, (a) a metallurgical microscope photograph of a test piece in the L direction, (b) a metallurgical microscope photograph of an enlarged portion of (a), (c) a test piece in the LT direction, and (d) a metallurgical microscope photograph of an enlarged portion of (c), for comparative examples of an aluminum-based alloy according to an embodiment of the present invention. 4 is a graph showing the relationship between displacement and bending stress in a bending test for the aluminum-based alloy shown in FIG. 2 and the comparative example of the aluminum-based alloy shown in FIG. 3 . 1A-1B are inverse pole figure orientation maps (shown in black and white) illustrating crystal orientation in (a) the RD and (b) the TD of an aluminum-based alloy according to an embodiment of the present invention. FIG. 1 shows an inverse pole figure orientation map (left, shown in black and white), a [100] inverse pole figure (top right, shown in black and white), and pole figures (three in the bottom right, shown in black and white) showing crystal orientation in the RD of an aluminum-based alloy according to an embodiment of the present invention. FIG. 1 shows an inverse pole figure orientation map (left side, shown in black and white), an inverse pole figure of [100] (top right, shown in black and white), and pole figures (three in the bottom right, shown in black and white) showing crystal orientation in RD for a comparative example of an aluminum-based alloy according to an embodiment of the present invention. FIG. 1A is a perspective view showing the direction of stress components for a sample of an aluminum-based alloy according to an embodiment of the present invention; and FIG. 1B is a graph showing residual stress distribution in the depth direction, the graph showing the extrusion direction (σy), the direction perpendicular thereto (σx), the shear direction therebetween (σxy), the graph showing the maximum principal stress (σ1), the graph showing the minimum principal stress (σ2), and the graph showing the von Mises stress. FIG. 1A is a perspective view showing the direction of stress components for a sample of a comparative example of an aluminum-based alloy according to an embodiment of the present invention; and FIG. 1B is a graph showing residual stress distribution in the depth direction, the extrusion direction (σy), the direction perpendicular thereto (σx), the shear direction therebetween (σxy), the maximum principal stress (σ1), the minimum principal stress (σ2), and the von Mises stress. FIG. 1A is a three-dimensional graph showing the direction of principal stress at each depth, (b) an inverse pole figure orientation map of RD (shown in black and white), and (c) a graph showing the depth distribution of von Mises stress for an aluminum-based alloy according to an embodiment of the present invention; and (d) a three-dimensional graph showing the direction of principal stress at each depth, (e) an inverse pole figure orientation map of RD (shown in black and white), and (f) a graph showing the depth distribution of von Mises stress for a comparative example of an aluminum-based alloy according to an embodiment of the present invention. 11 is a graph showing the change in maximum principal stress direction with respect to depth for the aluminum-based alloy shown in FIG. 10 and a comparative example of the aluminum-based alloy. FIG. 1A shows an inverse pole figure orientation map showing crystal orientation in the RD, (b) a KAM map, and (c) a graph showing the depth distribution of von Mises stress of an aluminum-based alloy according to an embodiment of the present invention; and (d) an inverse pole figure orientation map showing crystal orientation in the RD, (e) a KAM map, and (f) a graph showing the depth distribution of von Mises stress of a comparative example of an aluminum-based alloy according to an embodiment of the present invention. FIG. 1A is a graph showing the depth distribution of von Mises stress for an aluminum-based alloy according to an embodiment of the present invention; (b) a graph showing the distribution of Σ of corresponding grain boundaries at a depth of 30 μm, (c) a depth of 170 μm, and (d) a depth of 260 μm; and (e) a graph showing the depth distribution of von Mises stress for a comparative example of an aluminum-based alloy according to an embodiment of the present invention; and (f) a graph showing the distribution of Σ of corresponding grain boundaries at a depth of 30 μm, (g) a depth of 105 μm, and (h) a depth of 220 μm. FIG. 1A is a graph showing the distribution of Σ of corresponding grain boundaries in the depth range of 0 to 200 μm and (b) in the depth range of 0 to 400 μm for an aluminum-based alloy according to an embodiment of the present invention; and FIG. 1C is a graph showing the distribution of Σ of corresponding grain boundaries in the depth range of 0 to 200 μm and (d) in the depth range of 0 to 400 μm for a comparative example of an aluminum-based alloy according to an embodiment of the present invention.

Hereinafter, the embodiment of the present invention will be described based on examples.
The aluminum-based alloy according to the embodiment of the present invention is obtained by extrusion processing and contains 0.75 wt % to 1.0 wt % Mg, 0.3 wt % to 0.8 wt % Si, 0.15 wt % to 0.4 wt % Cu, 0.10 wt % to 0.25 wt % Fe, 0.07 wt % or less Cr, 0.025 wt % or less Mn, and the balance being Al and unavoidable impurities.

The aluminum-based alloy according to the embodiment of the present invention has, from the surface to the depth direction, a surface layer in which the maximum principal stress is compressive, and an inner layer in which the maximum principal stress is tensile. In the surface layer, 80% or more of the crystal grains have a grain size of 150 μm or less, and the crystal orientation has a preferred orientation in the <110> direction.

The surface layer is distributed from the surface to a depth of 0.10 mm to 0.15 mm. The Euler angle of the maximum principal stress direction in the surface layer is 5° to 40° with respect to the extrusion direction of the extrusion process. In addition, the surface layer has stable grain boundaries with Σ values of 3 to 13 of 10% or more, and in particular, twin crystals with Σ3 of 5% or more, and the proportion of crystal grains with an average KAM value of 1.0 deg or less, which is proportional to intragranular strain, is 40% or more.

The inner layer has a depth of 0.3 mm or more from the surface at its deepest point. The direction of the maximum principal stress in the inner layer is at an angle of 50° or more to the extrusion direction of the extrusion process. The inner layer also has a maximum von Mises stress of 150 MPa or more.

An aluminum-based alloy was manufactured, and bending tests, crystal orientation analysis by EBSD method, stress measurements, etc. were performed on the manufactured alloy. Two types of alloy samples with the compositions shown in Table 1 were manufactured as the aluminum-based alloy. These alloy samples are A6000 series aluminum alloys used as structural materials, with alloy sample 1 corresponding to A6061 and alloy sample 2 corresponding to A6005C. Note that alloy sample 1 is an aluminum-based alloy according to an embodiment of the present invention, and alloy sample 2 is a comparative example.

As shown in Figure 1, the method for producing each alloy sample was as follows: alloy raw materials adjusted to each composition in Table 1 were heated to 700°C, melted, cast, cooled, and then homogenized at 570°C for 3.7 hours. After cooling, the alloy raw materials were heated to 480-500°C and extruded, and then water-cooled at 3000°C/min. Then, aging treatment was performed at 200°C for 2.5 hours to produce each alloy sample.

A bending test was conducted on each alloy sample produced. The bending test was conducted using a three-point bending tester in accordance with JIS Z2248. The test pieces for the bending test were 60 mm long, 20 mm wide, and 2.0 mm thick. One specimen with a length parallel to the extrusion direction (L direction) and one specimen with a length perpendicular to the extrusion direction (LT direction) were cut from each alloy sample. The radius of the indenter was changed during the bending test.

Figure 2 shows metallurgical microscope photographs (dark field images) of the cross sections of the test piece of alloy sample 1 in the L direction with an indenter radius of 4 mm (4R) and the test piece of the LT direction with an indenter radius of 5 mm (5R) among the alloy samples after the bending test. Figure 3 shows metallurgical microscope photographs (dark field images) of the cross sections of the test piece of alloy sample 2 in the L direction with an indenter radius of 4 mm (4R) and the test piece of the LT direction with an indenter radius of 9 mm (9R). As shown in Figures 2(a) and (b), it was confirmed that no cracks occurred in the bending test in the L direction of alloy sample 1. As shown in Figures 2(c) and (d), it was also confirmed that no cracks occurred in the bending test in the LT direction of alloy sample 1.

As shown in Figures 3(a) and (b), in the L direction of alloy sample 2, the bending test confirmed that many cracks (circles in Figure 3(b)) had occurred not only on the surface of the test piece, but also in the range of 100 to 400 μm deep from the surface. In addition, as shown in Figures 3(c) and (d), in the LT direction of alloy sample 2, the bending test confirmed that the test piece had cracked, and many cracks (circles in Figure 3(d)) had also occurred inside the test piece.

Figure 4 shows the relationship between the displacement and bending stress obtained by the bending test for each test piece. As shown in Figure 4, it was confirmed that each test piece of alloy sample 1 did not break even after the yield stress, and bending deformation continued slowly. In addition, for alloy sample 2, it was confirmed that the test piece in the L direction did not break even after the yield stress, but the bending stress dropped rapidly. It was also confirmed that the test piece in the LT direction of alloy sample 2 broke near the yield stress. From the above test results, it can be said that alloy sample 1 has small anisotropy in bendability and has excellent bending workability.

Next, crystal orientation analysis was performed on each alloy sample using the EBSD method. For the crystal orientation analysis, an OIM crystal orientation analyzer manufactured by TSL Solutions was used. Figure 5 shows the inverse pole figure orientation map of the rolling direction (RD) and the direction perpendicular to the rolling direction in the rolling plane (TD) of alloy sample 1. Figure 6 shows the inverse pole figure orientation map of RD, the inverse pole figure of [100], and the pole figure of alloy sample 1. Figure 7 shows the inverse pole figure orientation map of RD, the inverse pole figure of [100], and the pole figure of alloy sample 2. The rolling direction is the extrusion direction during extrusion processing.

As shown in Figures 5 and 6, in alloy sample 1, it was confirmed that more than 80% of the crystal grains in the RD and TD from the surface to a depth of 0.5 mm had a <110> preferred orientation in the extrusion direction. Furthermore, these crystal grains had a nearly uniform grain size of 150 μm or less. In contrast, as shown in Figure 7, in alloy sample 2, it was confirmed that about 30% of the crystal grains in the RD and TD from the surface to a depth of 0.5 mm had a nearly uniform grain size of 150 μm or less and had a <110> preferred orientation in the extrusion direction. Furthermore, it was confirmed that many coarse grains were present in the range of 0.1 to 0.3 mm deep from the surface in both the RD and TD. This region with many coarse grains was not observed in alloy sample 1 (see Figures 5 and 6).

Next, stress measurements were performed on each alloy sample. During stress measurements, samples were cut from the surface of each alloy sample, immersed in a 6 vol% perchloric acid ethanol solution, and the sample was sequentially electrolytically polished from the surface at 0°C and DC 15V, while measuring the residual stress distribution in the depth direction from the surface. Sequential electrolytic polishing was performed for 20 minutes each, and the sample was polished about 25 μm at a time from the surface. Residual stress was measured using a portable X-ray residual stress measurement device "μ-X360s" manufactured by Pulstec Industrial Co., Ltd., using the cos α method (ray source: φ2 mm Cr-kα, 30 kV, 1.5 mA, diffraction surface: Al (311), 2θ = 139.32°) while oscillating ψ, φ, and ω at 10° in three axes.

The residual stress distribution in the depth direction of alloy sample 1 is shown in Figure 8. The residual stresses were obtained in the extrusion direction (σy), the perpendicular direction (σx), the shear direction between them (σxy), the maximum principal stress (σ1), the minimum principal stress (σ2), and the von Mises stress. Similarly to alloy sample 1, the residual stress distribution in the depth direction of alloy sample 2 is shown in Figure 9. The direction of the principal stress for each depth of alloy sample 1 is shown in Figures 10(a) to 10(c) together with the inverse pole figure orientation map of the RD shown in Figures 5 and 6 and the depth distribution of the von Mises stress shown in Figure 8(g). The direction of the principal stress for each depth of alloy sample 2 is shown in Figures 10(d) to 10(f) together with the inverse pole figure orientation map of the RD shown in Figure 7 and the depth distribution of the von Mises stress shown in Figure 9(g). In addition, the change in the maximum principal stress direction with respect to the depth and the extrusion direction for alloy samples 1 and 2 shown in Figure 10 is summarized and shown in Figure 11.

As shown in Figures 8, 10, and 11, in alloy sample 1, in the region (surface layer) up to a depth of 80 μm where the von Mises stress is small and the maximum principal stress is compressive, the Euler angle in the principal stress direction was confirmed to be 9° to 35° with respect to the extrusion direction. In the region (inner layer) deeper than 150 μm where the maximum principal stress is tensile, the Euler angle in the principal stress direction was confirmed to be 50° or more with respect to the extrusion direction. In the tensile region, it was confirmed that the maximum von Mises stress of 150 MPa or more was obtained at a depth of 170 μm. It was also confirmed that the same crystal structure and stress distribution were obtained even if the added elements Cr were reduced to 0.03 wt% and Mn to 0.01 wt%. In addition, the yield strength was 270 MPa or more in both RD and TD, and the bendability was good with R2 to 3 mm in both RD and TD, and it was confirmed that the anisotropy was small.

As shown in Figures 9 to 11, in the alloy sample 2, in the region up to a depth of 50 μm where the von Mises stress is small and the maximum principal stress is compressive, the Euler angle in the principal stress direction was confirmed to be 15° to 70° with respect to the extrusion direction. It was also confirmed that the maximum principal stress becomes tensile near a depth of 100 μm and the Euler angle in the principal stress direction is around 50° with respect to the extrusion direction. The von Mises stress was confirmed to be approximately 120 MPa at a depth of 105 μm. It was also confirmed that the maximum principal stress becomes compressive again at a depth of 150 to 250 μm and the Euler angle in the principal stress direction is 20° or less with respect to the extrusion direction. It was also confirmed that the maximum principal stress becomes tensile again at a depth of 250 μm or more and the Euler angle in the principal stress direction is 35° to 40° with respect to the extrusion direction. It was confirmed that the maximum von Mises stress is 50 MPa or more at a depth of 300 μm. The yield strength was 270 MPa or more in both RD and TD, but the bending property was R3 mm in RD and R10 mm in TD, confirming that the material has anisotropy.

As shown in Figures 10 and 11, in alloy sample 2, the direction of the maximum principal stress changes from compression to extrusion as the depth increases, and then becomes compressed again in the region where many coarse grains are present (depth 150 to 250 μm), which suggests that the stress is released by recrystallization at this depth. In contrast, in alloy sample 1, the direction of the maximum principal stress changes from compression to extrusion as the depth increases, but does not become compressed again, and no region where many coarse grains are present is observed, which suggests that recrystallization is suppressed.

Next, the KAM map of each alloy sample was obtained by crystal orientation analysis using the EBSD method, and is shown in Figure 12. In alloy sample 1, as shown in Figure 12(b), the KAM values are distributed relatively uniformly, whereas in alloy sample 2, as shown in Figure 12(e), it was confirmed that large KAM values are concentrated in the area where many coarse grains are present. From this, it is believed that the distortion is large in the coarse grains of alloy sample 2.

Next, the correspondence grain boundary Σ, which is an index of grain boundary stability, was examined for each alloy sample. The distribution of Σ of the correspondence grain boundary near the depth where the von Mises stress shows the compression and tension peaks for each alloy sample was obtained by crystal orientation analysis using the EBSD method, and is shown in Figure 13. In addition, the distribution of Σ values in the depth ranges of 0 to 200 μm and 0 to 400 μm for each alloy sample is summarized and shown in Figure 14. Comparing the distribution of Σ of alloy sample 1 shown in Figures 13(b) to (d) and Figures 14(a) and (b) with the distribution of Σ of alloy sample 2 shown in Figures 13(f) to (h) and Figures 14(c) and (d), it was confirmed that alloy sample 2 has more correspondence grain boundaries of Σ3, 5, and 7 than alloy sample 1 at a depth shallower than 200 μm, but that alloy sample 1 has more correspondence grain boundaries of Σ3, 5, and 7 than alloy sample 2 at a depth deeper than 200 μm. 14(a) and (b) of alloy sample 1, the correspondence grain boundaries of Σ3, 5, and 7 hardly change with depth, whereas the correspondence grain boundaries of Σ3, 5, and 7 decrease with depth in the distribution of Σ of alloy sample 2 shown in Figures 14(c) and (d). From these results, it is considered that alloy sample 1 has an interface that is more energetically stable (stronger interface) than alloy sample 2.

Claims (4)

An aluminum-based alloy obtained by extrusion processing,
0.75 wt% to 1.0 wt% Mg, 0.3 wt% to 0.8 wt% Si, 0.15 wt% to 0.4 wt% Cu, 0.10 wt% to 0.25 wt% Fe, 0.07 wt% or less Cr, 0.025 wt% or less Mn, with the balance being Al and unavoidable impurities;
The steel sheet has a surface layer portion in which the maximum principal stress is compressive and an inner layer portion in which the maximum principal stress is tensile, from the surface to the depth direction;
An aluminum-based alloy having a yield strength exceeding 250 MPa, characterized in that the direction of the maximum principal stress in the surface layer portion is 5° or more and 40° or less with respect to the extrusion direction of the extrusion processing, and the direction of the maximum principal stress in the inner layer portion is 50° or more with respect to the extrusion direction of the extrusion processing. The aluminum-based alloy according to claim 1, characterized in that the inner layer is distributed to a depth of 0.3 mm or more from the surface. The surface layer portion is distributed from the surface to a depth of 0.10 mm to 0.15 mm,
3. The aluminum-based alloy according to claim 1, wherein the inner layer portion has a maximum von Mises stress of 150 MPa or more. The aluminum-based alloy according to any one of claims 1 to 3, characterized in that 80% or more of the crystal grains in the surface layer have a grain size of 150 μm or less and have a preferred crystal orientation in the <110> direction.