Aluminum alloy forgings and their manufacturing method

Abstract

Provided are a 6000-series aluminum alloy forging material having high strength and exceptional toughness (excellent ductility), and an efficient method for manufacturing the same. This aluminum alloy forging material is characterized by being formed from a 6000-series aluminum alloy, having a Cu content of 0.2-1.0 wt.%, the composition of the 6000-series aluminum alloy satisfying relational expressions (1) and (2), and having deposits at the base metal crystal grain boundary, specifically Al-(Fe,Mn,Cr)-Si-type crystalline deposits at the base metal crystal grain boundary. (1) Si (at%)≥2Mg (at%) (2) 0.2≤surplus Si (wt%)+Mn (wt%)+Cr (wt%)≤1.7

Description

[Technical Field]

[0001] The present invention relates to an aluminum alloy forging and a method for producing the same, and more particularly to an aluminum alloy forging that can be suitably used for automobile suspension parts and the like, and a simple and efficient method for producing the same. [Background technology]

[0002] The 6000 series aluminum alloys are Al-Mg-Si aluminum alloys containing mainly Mg and Si. In addition to excellent formability and corrosion resistance, they exhibit moderate age hardening and good strength, and forged components are widely used as structural components for transportation equipment, including automobiles.

[0003] However, in recent years, there has been an increasing demand for lighter weight transportation equipment in order to improve fuel efficiency and reduce CO2 emissions, and there is a strong demand for 6000 series aluminum alloy forged members with higher strength and toughness. In particular, when 6000 series aluminum alloy forged members are used for automobile suspension parts, etc., it is essential to impart high reliability to them.

[0004] In contrast to this, for example, Patent Document 1 (JP 2017-155251 A) describes an aluminum alloy forging containing, by mass%, 0.7 to 1.5% Si, 0.6 to 1.2% Mg, and 0.01 to 0.5% Fe, and further containing one or more of 0.05 to 1.0% Mn, 0.01 to 0.5% Cr, and 0.01 to 0.2% Zr, with the balance being Al and unavoidable impurities. The dislocation density measured by X-ray diffraction on the observation surface of the center of the thickest part of the forging is 1.0 × 10 on average. 14 ~5.0×10 16 / m 2 The average proportion of small-angle grain boundaries with a tilt angle of 2 to 15 degrees of crystal grains with a misorientation of 2 degrees or more measured by the SEM-EBSD method is 50% or more, and the average number density of precipitates measurable by TEM at a magnification of 300,000 times is 5.0 × 10 2 pieces / μm3 An aluminum alloy forging material having excellent strength and ductility, characterized by the above, is disclosed.

[0005] In the aluminum alloy forgings described in Patent Document 1, when a 6000 series aluminum alloy forging is subjected to solution and quenching treatment, and then subjected to warm working to impart processing strain, and then artificial aging treatment is performed, both the strength and ductility are improved (higher strength and higher ductility) compared to the usual case in which processing strain is not imparted. In order to achieve or guarantee this effect, the average dislocation density, the average proportion of low-angle grain boundaries, and the average number density of precipitates are each specified for the structure at the center of the thickest part of the forging after artificial aging treatment.

[0006] Furthermore, Patent Document 2 (JP 2008-163445 A) discloses an automobile suspension part made of an aluminum alloy forging containing, by mass%, 0.5 to 1.25% Mg, 0.4 to 1.4% Si, 0.01 to 0.7% Cu, 0.05 to 0.4% Fe, 0.001 to 1.0% Mn, 0.01 to 0.35% Cr, 0.005 to 0.1% Ti, with Zr restricted to less than 0.15%, and the remainder being Al and unavoidable impurities, wherein the density of crystallized precipitates observed in the widthwise cross-sectional structure at a maximum stress generation position is 1.5% or less in average area ratio, and the spacing between grain boundary precipitates observed in the structure at a cross-sectional region including a parting line generated during forging is 0.7 μm or more in average spacing.

[0007] In the automobile suspension part described in Patent Document 2 above, the widthwise cross-sectional structure of each specific part of the arm portion of the automobile suspension part, which has a lightweight shape, for example, the rib and web at the maximum stress generation part in the rib, is specified, and the components are adjusted and manufactured so that the widthwise cross-sectional structure of each specific part of the arm portion of the automobile suspension part, such as the rib, at the maximum stress generation part in the arm portion of the automobile suspension part after forging, the rib and web parts at the maximum stress generation part, can be made to have a predetermined structure, thereby making it possible to suppress coarsening of crystal grains in the rib and web parts of the arm portion of the automobile suspension part, which has a lightweight shape, during forging, particularly in the specific parts where the maximum stress occurs. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-155251 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-163445 Summary of the Invention [Problem to be solved by the invention]

[0009] The mechanical properties of 6000 series aluminum alloys are affected by precipitates at grain boundaries and precipitates within grains. However, the aluminum alloy forging material described in Patent Document 1 basically focuses only on precipitates within grains, and does not take into account the effect of precipitates at grain boundaries, which greatly contribute to toughness (ductility).

[0010] Furthermore, in the automobile suspension part described in Patent Document 2, the spacing between precipitates at the grain boundaries is specified, but the size, shape, and other characteristics of the precipitates that are extremely important in terms of the metal structure are not taken into consideration.

[0011] That is, from the viewpoint of achieving both high levels of strength and toughness in 6000 series aluminum alloy forgings, it is difficult to say that the precipitates at the grain boundaries and within the grains are in a sufficiently optimal state.

[0012] In view of the above-mentioned problems in the prior art, an object of the present invention is to provide a 6000 series aluminum alloy forging material having high strength and excellent toughness (good ductility), and an efficient method for producing the same. [Means for solving the problem]

[0013] In order to achieve the above object, the present inventors have conducted extensive research into the relationship between the composition and microstructure of 6000 series aluminum alloy forgings. As a result, they have found that it is extremely effective to add a sufficient amount of Si to form fine precipitates within crystal grains, and to add appropriate amounts of Mn and Cr to refine precipitates at grain boundaries, and have arrived at the present invention.

[0014] That is, the present invention provides: Made of 6000 series aluminum alloy, The Cu content is 0.2 to 1.0 wt%; The composition of the 6000 series aluminum alloy satisfies the following relational expressions (1) and (2): The material has precipitates at the grain boundaries and Al-(Fe, Mn, Cr)-Si crystal precipitates within the grains. The present invention provides an aluminum alloy forging material characterized by: Si(at%)≧ Mg / 2 (at%) (1) 0.2≦Excess Si(wt%)+Mn(wt%)+Cr(wt%)≦1.7 (2)

[0015] In the aluminum alloy forged member of the present invention, sufficient Si is added to form Mg2Si, resulting in the formation of fine and large amounts of precipitates within the crystal grains. In addition, by setting the total content of excess Si, Mn, and Cr to 0.2 to 1.7 wt%, Al-(Fe, Mn, Cr)-Si compounds are crystallized during casting and precipitated during homogenization heat treatment and forging preheating, thereby increasing the strength of the aluminum alloy forged member. In addition, the consumption of excess Si refines the precipitates at the grain boundaries. Here, the amount of excess Si (wt%) can be calculated by "Si amount (wt%) - (Mg amount (wt%) / 1.731".

[0016] In addition, the aluminum alloy forging material of the present invention contains 0.2 to 1.0 wt% Cu, which provides good mechanical strength and fatigue strength through the formation of Al, Mg, Si, and Cu-based quaternary precipitates (Q phase or Q' phase).

[0017] In the aluminum alloy forging of the present invention, the Si content is preferably 0.5 to 1.4 wt% and the Mg content is preferably 0.6 to 1.7 wt%, more preferably 0.9 to 1.2 wt%, and the Mg content is more preferably 0.8 to 1.2 wt%.

[0018] By setting the Si content to 0.5 wt% or more, solid solution strengthening and age hardening can be fully achieved, and by setting it to 1.4 wt% or less, it is possible to suppress a decrease in corrosion resistance and a decrease in ductility due to coarsening of crystallized particles and precipitates. Furthermore, by setting the Si content to 0.9 to 1.2 wt%, these effects can be obtained more reliably.

[0019] Furthermore, by setting the Mg content to 0.6 wt% or more, a sufficient amount of Mg-Si precipitates are formed, improving strength and fatigue properties, and by setting the Mg content to 1.7 wt% or less, the formation of coarse compounds that act as fracture initiation sites can be suppressed. By setting the Mg content to 0.8 to 1.2 wt%, these effects can be obtained more reliably.

[0020] In the aluminum alloy forging of the present invention, it is preferable that the average grain size of the precipitates at the matrix grain boundaries is 50 nm or less. By setting the average grain size of the precipitates at the matrix grain boundaries to 50 nm or less, the aluminum alloy forging can be endowed with good ductility (toughness). Here, the average grain size of the precipitates may be calculated as the circle-equivalent diameter.

[0021] In the aluminum alloy forging of the present invention, it is preferable that the aspect ratio of the precipitates at the matrix grain boundaries is 5 or less. By setting the aspect ratio of the precipitates at the matrix grain boundaries to 5 or less, the proportion of the matrix grain boundaries occupied by the precipitates is reduced, and the distance between the precipitates can be increased. As a result, good ductility (toughness) can be imparted to the aluminum alloy forging.

[0022] In the aluminum alloy forging of the present invention, it is preferable that the width of the precipitate-free zone centered on the base material grain boundary is 100 nm or less. By making the width of the precipitate-free zone at the base material grain boundary 100 nm or less, it is possible to impart high strength and good ductility to the aluminum alloy forging.

[0023] Furthermore, the aluminum alloy forging of the present invention preferably has a 0.2% yield strength of 350 MPa or more and an elongation of 10% or more. When the aluminum alloy forging has a 0.2% yield strength of 350 MPa or more and an elongation of 10% or more, it can be suitably used for structural members that require high reliability.

[0024] The present invention also provides an automobile suspension part made of the aluminum alloy forging of the present invention. The aluminum alloy forging of the present invention has good strength and ductility, and the automobile suspension part of the present invention can be suitably used in cases where high strength and reliability are required.

[0025] Furthermore, the present invention provides A method for producing an aluminum alloy forging of the present invention, comprising: The Cu content of the aluminum alloy forging is 0.2 to 1.0 wt%, a hot forging preheating process for preheating the aluminum alloy material; a hot forging step of hot forging the preheated aluminum alloy material obtained in the hot forging preheating step, The preheating temperature in the hot forging preheating process is 300 to 550 ° C., and the preheating time is 1 to 3 hours. The composition of the aluminum alloy satisfies the following relational expressions (1) and (2): Also provided is a method for producing an aluminum alloy forging, characterized by: Si(at%)≧ Mg / 2 (at%) (1) 0.2≦Excess Si(wt%)+Mn(wt%)+Cr(wt%)≦1.7 (2)

[0026] In the method for producing an aluminum alloy forging of the present invention, a hot forging preheating step is performed at a preheating temperature of 300 to 550°C for a preheating time of 1 to 3 hours to precipitate Al-(Fe, Mn, Cr)-Si compounds, thereby increasing the strength of the aluminum alloy forging. In addition, the precipitates at the grain boundaries are refined by consuming excess Si.

[0027] Furthermore, in the method for producing an aluminum alloy forging of the present invention, it is preferable to have a homogenization heat treatment step of the aluminum alloy material before the hot forging preheating step, and to set the temperature of the homogenization heat treatment step to 500 to 550°C and the holding time to 5 to 10 hours.

[0028] By performing homogenization heat treatment at 500 to 550°C for 5 to 10 hours, Al-(Fe, Mn, Cr)-Si compounds are more reliably precipitated, increasing the strength of the aluminum alloy forged component. In addition, the excess Si is consumed, resulting in finer precipitates at the grain boundaries. [Effects of the Invention]

[0029] According to the present invention, it is possible to provide a 6000 series aluminum alloy forging material having high strength and excellent toughness (good ductility), and an efficient method for producing the same. [Brief explanation of the drawings]

[0030] [Figure 1] 1 is a schematic diagram of the microstructure of an aluminum alloy forging of the present invention. [Figure 2] 1 shows the results of TEM observation of the vicinity of grain boundaries in the aluminum base material of the aluminum alloy forging of Example 1. [Figure 3] 1 shows the results of TEM observation of the interior of aluminum base material crystal grains in the aluminum alloy forging of Example 1. [Figure 4] 4 is a TEM-EDS spectrum of the crystal precipitate shown in FIG. 3. [Figure 5] 1 shows the results of TEM observation of the vicinity of grain boundaries in the aluminum base material of the aluminum alloy forging of Comparative Example 5. [Figure 6] 1 shows the results of TEM observation of the vicinity of grain boundaries in the aluminum base material of the aluminum alloy forging of Comparative Example 1. [Figure 7] 1 shows the results of TEM observation of the vicinity of grain boundaries in the aluminum base material of the aluminum alloy forging of Comparative Example 4. [Figure 8] 1 shows the results of TEM observation of the interior of aluminum base material crystal grains in the aluminum alloy forging of Comparative Example 5. DETAILED DESCRIPTION OF THE INVENTION

[0031] Representative embodiments of the aluminum alloy forging material and its manufacturing method according to the present invention will be described in detail below with reference to the drawings, but the present invention is not limited to these. In the following description, the same or equivalent parts will be designated by the same reference numerals, and duplicated explanations may be omitted. Furthermore, since the drawings are intended to conceptually explain the present invention, the dimensions and ratios of the components shown may differ from the actual dimensions.

[0032] 1. Aluminum alloy forgings (1) Composition The aluminum alloy forging is made of a 6000 series aluminum alloy, and the contents of Si, Mg, Mn, and Cr are optimized to impart high strength and toughness (ductility) to the aluminum alloy forging. Each of the characteristic component elements of the aluminum alloy forging of the present invention will be described below.

[0033] Cu: 0.2 to 1.0 wt% The Cu content is 0.2 to 1.0 wt%. Cu has the effect of increasing mechanical strength and fatigue strength by forming Al, Mg, Si, and Cu-based quaternary precipitates (Q phase or Q' phase). If the Cu content is less than 0.2 wt%, these effects cannot be fully obtained, and the yield strength of the aluminum alloy forging material cannot be increased to 350 MPa or more. On the other hand, if the Cu content exceeds 1.0 wt%, there is a risk of reducing corrosion resistance.

[0034] Si: 0.5 to 1.4 wt% The Si content is preferably 0.5 to 1.4 wt%. By setting the Si content to 0.5 wt% or more, solid solution strengthening and age hardening can be fully achieved, and by setting the Si content to 1.4 wt% or less, it is possible to suppress a decrease in corrosion resistance and a decrease in ductility due to coarsening of crystallized particles and precipitates. Furthermore, the Si content is more preferably 0.9 to 1.2 wt%. By setting the Si content to 0.9 to 1.2 wt%, these effects can be obtained more reliably.

[0035] Mg: 0.6 to 1.7 wt% The Mg content is preferably 0.6 to 1.7 wt%. By setting the Mg content to 0.6 wt% or more, a sufficient amount of Mg-Si precipitates are formed, improving strength and fatigue properties, while by setting the Mg content to 1.7 wt% or less, the formation of coarse compounds that serve as fracture initiation sites can be suppressed. Furthermore, a more preferable Mg content is 0.8 to 1.2 wt%. By setting the Mg content to 0.8 to 1.2 wt%, these effects can be obtained more reliably.

[0036] Mn: 0.1 to 0.8 wt% The Mn content is preferably 0.1 to 0.8 wt%. By setting the Mn content to 0.1 wt% or more, the strength of the aluminum alloy forging can be increased by forming Al-(Fe, Mn, Cr)-Si compounds. Furthermore, by setting the Mn content to 0.8 wt% or less, the formation of coarse Al-(Fe, Mn, Cr)-Si compounds that reduce toughness and ductility can be suppressed.

[0037] Cr: 0.1 to 0.8 wt% The Cr content is preferably 0.1 to 0.8 wt%. By setting the Cr content to 0.1 wt% or more, the strength of the aluminum alloy forging can be increased by the formation of Al-(Fe, Mn, Cr)-Si compounds. Furthermore, by setting the Cr content to 0.8 wt% or less, the formation of coarse Al-(Fe, Mn, Cr)-Si compounds that reduce toughness and ductility can be suppressed.

[0038] Fe: 0.05 to 0.3 wt% The Fe content is preferably 0.05 to 0.3 wt%. By setting the Fe content to 0.05 wt% or more, the strength of the aluminum alloy forging can be increased by forming Al-(Fe, Mn, Cr)-Si compounds. Furthermore, by setting the Fe content to 0.3 wt% or less, the formation of coarse Al-(Fe, Mn, Cr)-Si compounds that reduce toughness and ductility can be suppressed.

[0039] Additionally, Cu, Zn, Ti, etc. may be contained within the composition ranges specified for various 6000 series aluminum alloys (Al-Mg-Si alloys).

[0040] Furthermore, the constituent elements of the aluminum alloy forged material of the present invention must satisfy the following two requirements.

[0041] (1)Si (at%) ≥ Mg / 2 (at%) Si and Mg are Si (at%) ≧ Mg / 2By satisfying the above condition (at %), sufficient Si is present for the formation of Mg2Si, and a large amount of fine precipitates can be formed within the crystal grains.

[0042] (2) 0.2≦Excess Si(wt%) + Mn(wt%) + Cr(wt%)≦1.7 By ensuring that the total content of excess Si, Mn, and Cr is 0.2 to 1.7 wt%, Al-(Fe, Mn, Cr)-Si compounds are crystallized during casting, and Al-(Fe, Mn, Cr)-Si compounds are precipitated during homogenization heat treatment and forging preheating. This not only increases the strength of the aluminum alloy forged component, but also refines the precipitates at the grain boundaries by consuming the excess Si.

[0043] (2) Organization Fig. 1 shows a schematic diagram of the microstructure of an aluminum alloy forging of the present invention. In the aluminum alloy forging of the present invention, precipitates 6 are formed at grain boundaries 4 of an aluminum base material 2. Furthermore, extremely fine Al-(Fe, Mn, Cr)-Si-based crystalline precipitates are dispersed within the crystalline grains of the aluminum base material 2. Note that the crystalline grains are not limited to being Al-(Fe, Mn, Cr)-Si-based crystalline precipitates; for example, the crystalline grains may be dispersed with a general β phase and its precursor phase, or a Q phase and its precursor phase, which are known as aging precipitate phases in Al-Mg-Si-based alloys.

[0044] The average grain size of the precipitates 6 at the grain boundaries 4 is preferably 50 nm or less. When the average grain size of the precipitates 6 at the grain boundaries 4 is 50 nm or less, good ductility (toughness) can be imparted to the aluminum alloy forging. The average grain size of the precipitates 6 is more preferably 40 nm or less, and most preferably 30 nm or less.

[0045] The aspect ratio of the precipitates 6 at the grain boundaries 4 is preferably 5 or less. By setting the aspect ratio of the precipitates 6 at the grain boundaries 4 to 5 or less, the proportion of the precipitates 6 that occupy the grain boundaries 4 is reduced, and the distance between the precipitates 6 can be increased. As a result, it is possible to suppress the propagation of cracks through the precipitates 6, and to impart good ductility (toughness) to the aluminum alloy forging. The aspect ratio of the precipitates 6 is more preferably 4 or less, and most preferably 3 or less.

[0046] Furthermore, the width of the precipitate-free zone centered on the grain boundary 4 is preferably 100 nm or less. By making the width of the precipitate-free zone at the grain boundary 4 100 nm or less, high strength and good ductility can be imparted to the aluminum alloy forging. The width of the precipitate-free zone is more preferably 90 nm or less, and most preferably 80 nm or less.

[0047] The aluminum alloy forgings have excellent tensile properties due to the above-mentioned composition. The aluminum alloy forgings preferably have a 0.2% yield strength of 350 MPa or more and an elongation of 10% or more. The aluminum alloy forgings 2 have a 0.2% yield strength of 350 MPa or more and an elongation of 10% or more, so that they can be suitably used for structural members that require high reliability. The aluminum alloy forgings 2 more preferably have a 0.2% yield strength of 360 MPa or more, and most preferably have a 0.2% yield strength of 370 MPa or more. The aluminum alloy forgings 2 also more preferably have an elongation of 12% or more, and most preferably have an elongation of 14% or more.

[0048] 2. Automotive suspension parts The automobile suspension part of the present invention is an automobile suspension part made of the aluminum alloy forging of the present invention.

[0049] Specific examples of automobile suspension parts include upper arms, lower arms, transverse links, and the like, which are automobile suspension parts.

[0050] 3. Manufacturing method of aluminum alloy forgings The method for producing an aluminum alloy forging of the present invention provides an effective method for producing the aluminum alloy forging of the present invention. The method for producing an aluminum alloy forging of the present invention includes a hot forging preheating step in which the Cu content of the aluminum alloy forging is 0.2 to 1.0 wt% and preheating the aluminum alloy material, and a hot forging step in which the preheated aluminum alloy material obtained in the hot forging preheating step is hot forged. Furthermore, other steps are not particularly limited as long as they do not impair the effects of the present invention, and various conventionally known steps used to produce 6000 series aluminum alloy forgings may be used as necessary. Below, the steps characteristic of the method for producing an aluminum alloy forging of the present invention will be described.

[0051] (1) Homogenization heat treatment process As a pretreatment for the hot forging process including the hot forging preheating process, it is preferable to subject the aluminum alloy material to hot forging to homogenization heat treatment. The homogenization heat treatment temperature is preferably 500 to 550°C, and the holding time is preferably 5 to 10 hours.

[0052] By performing homogenization heat treatment at 500 to 550°C for 5 to 10 hours, Al-(Fe, Mn, Cr)-Si compounds are more reliably precipitated within the crystal grains of the aluminum base material 2, increasing the strength of the aluminum alloy forged member. In addition, the excess Si is consumed, thereby making the precipitates 6 at the crystal grain boundaries 4 finer. As a result, the aspect ratio of the precipitates 6 can be reduced.

[0053] (2) Hot forging preheating process This is a pretreatment performed before the hot forging process. By subjecting an aluminum alloy material to a heat treatment at a preheating temperature of 300 to 550°C for a preheating time of 1 to 3 hours, Al-(Fe, Mn, Cr)-Si compounds are precipitated within the crystal grains of the aluminum matrix 2, which not only increases the strength of the aluminum alloy forged member, but also refines the precipitates 6 at the crystal grain boundaries 4 by consuming excess Si. As a result, the aspect ratio of the precipitates 6 can be reduced.

[0054] (3) Hot forging process The preheated aluminum alloy material may be hot forged using any of various conventionally known forging methods to form a desired shape, and the final shape may be formed into an upper arm, a lower arm, a transverse link, or the like, which are suspension parts for automobiles, to thereby obtain the automobile suspension parts of the present invention.

[0055] (4) Solution treatment and aging treatment The strength of the entire forged part can be improved by subjecting the forged part, which has been formed into its final shape by hot forging, to appropriate solution treatment and aging treatment.

[0056] The conditions for the solution treatment and aging treatment are not particularly limited, and various conventionally known solution treatments and aging treatments can be used as long as they do not impair the effects of the present invention. Note that the optimum conditions depend on the type of aluminum alloy, the shape and size of the forged part, etc., so it is preferable to observe the structure and evaluate the mechanical properties of the forged part after the solution treatment and aging treatment and select appropriate conditions as needed.

[0057] Representative embodiments of the present invention have been described above, but the present invention is not limited to these, and various design modifications are possible, all of which are included in the technical scope of the present invention. [Example]

[0058] Example Slabs of aluminum alloys having the compositions shown in Table 1 as examples were obtained by DC continuous casting. The components in Table 1 are shown in wt%. Table 1 also shows the values ​​of "excess Si (wt%)" related to relational formula (1) and relational formula (2) and "excess Si (wt%) + Mn (wt%) + Cr (wt%)" related to relational formula (2). All of the aluminum alloys according to the examples contain excess Si and 0.2 to 1.0 wt% Cu, and also satisfy the relationship 0.2≦excess Si (wt%) + Mn (wt%) + Cr (wt%)≦1.7.

[0059] [Table 1]

[0060] Next, the obtained slab was cut and subjected to a hot forging preheating process at 350°C or 500°C for 2 hours, followed by forging to a forging ratio of 60%, to obtain aluminum alloy forgings. Here, we investigated the cases where homogenization heat treatment was performed at 510°C for 6 hours or at 550°C for 10 hours before the hot forging preheating process, and the cases where homogenization heat treatment was not performed.

[0061] Next, the obtained aluminum alloy forged material was subjected to solution treatment at 550°C for 2 hours, followed by water cooling and aging treatment at 180°C for 8 hours.

[0062] The tensile properties and manufacturing conditions of each aluminum alloy forging obtained are shown in Table 2. The tensile test specimens used were No. 14A test specimens specified in JIS Z 2241, and the tension speed conformed to JIS Z 2241, being 2 mm / min up to 0.2% yield strength and 5 mm / min thereafter. As shown in Table 2, the aluminum alloy forgings of the present invention have both a 0.2% yield strength of 350 MPa and an elongation of 10% or more.

[0063] Furthermore, the average circle-equivalent diameter and aspect ratio of precipitates present at the grain boundaries of the aluminum base material were determined for several aluminum alloy forgings. Specifically, the circle-equivalent diameter and aspect ratio of precipitates were calculated for TEM observation photographs using image processing software (Image-Pro Premier V9.0, manufactured by MediaCybernetic, USA). The results are shown in Table 2. It can be seen that in the aluminum alloy forgings of the present invention, the average circle-equivalent diameter of precipitates present at the grain boundaries is 50 μm or less, and the aspect ratio is 5 or less.

[0064] [Table 2]

[0065] Figure 2 shows the results of TEM observation of the vicinity of the grain boundaries of the aluminum base material of the aluminum alloy forging of Example 1 (homogenization heat treatment: 510°C, 6 hours, hot forging preheating step: 500°C, 2 hours). TEM observation was performed using a Tecnai series G2-F20 manufactured by FEI. Precipitates at the grain boundaries of the aluminum base material can be confirmed, and it can be seen that the precipitates are fine and granular. In addition, the precipitates are not in close contact with each other, which is an ideal state for imparting good toughness and ductility to the aluminum alloy forging. In addition, the width of the precipitate-free zone is 100 nm or less.

[0066] Figure 3 shows the results of TEM observation of the aluminum matrix crystal grains of the aluminum alloy forging of Example 1 (homogenization heat treatment: 510°C, 6 hours, hot forging preheating step: 500°C, 2 hours). It can be seen that a large amount of fine precipitates are dispersed within the aluminum matrix crystal grains. Figure 4 shows the TEM-EDS spectrum of the precipitates, which confirmed that the precipitates contained Al-(Fe, Mn, Cr)-Si-based precipitates.

[0067] Comparative Example Aluminum alloy forgings were obtained in the same manner as in the Examples, except that slabs of aluminum alloys having the compositions shown as Comparative Examples in Table 1 were used. The obtained aluminum alloy forgings were evaluated in the same manner as in the Examples.

[0068] The manufacturing conditions, tensile properties, and information on precipitates present at the grain boundaries of the aluminum base material for the aluminum alloy forgings obtained as comparative examples are shown in Table 3. Table 3 also shows the results of determining the average circle-equivalent diameters and aspect ratios of precipitates present at the grain boundaries of the aluminum base material for several aluminum alloy forgings.

[0069] [Table 3]

[0070] As shown in Table 3, the aluminum alloy forgings of the comparative examples were unable to achieve both high levels of strength and ductility. Comparative Examples 1 to 4, which did not contain excess Si, were insufficient in absolute strength, with the 0.2% yield strength being less than 350 MPa in all cases. On the other hand, Comparative Example 5, which contained excess Si but did not contain Mn and / or Cr, was poor in ductility, with elongation being less than 10% in all cases. Furthermore, Comparative Example 6, which contained Mn and Cr but did not contain excess Si, was insufficient in absolute strength, with the 0.2% yield strength being less than 350 MPa in all cases.

[0071] Regarding the precipitates present at the grain boundaries of the aluminum base material, when there is no excess Si, no coarsening or increase in aspect ratio is observed, but when there is excess Si (Comparative Example 5), the average circle-equivalent diameter is greater than 50 nm and the aspect ratio is also greater than 5.

[0072] Figure 5 shows the results of TEM observation of the vicinity of the grain boundaries of the aluminum base material of the aluminum alloy forging of Comparative Example 5 (homogenization heat treatment: 510°C, 6 hours, hot forging preheating step: 500°C, 2 hours). Precipitates were observed at the grain boundaries of the aluminum base material, and it was found that the precipitates were coarse and needle-shaped. In addition, the width of the precipitate-free zone was larger than that of the aluminum alloy forging obtained in the Examples.

[0073] The results of TEM observation of the vicinity of the grain boundaries of the aluminum base material of the aluminum alloy forging of Comparative Example 1 (homogenization heat treatment: 510°C, 6 hours, hot forging preheating step: 500°C, 2 hours) are shown in Figure 6. Precipitates at the grain boundaries of the aluminum base material can be confirmed, and it can be seen that the amount of said precipitates is smaller than in the case of the aluminum alloy forgings obtained in the Examples.

[0074] The results of TEM observation of the vicinity of the grain boundaries of the aluminum base material of the aluminum alloy forging of Comparative Example 4 (homogenization heat treatment: 510°C, 6 hours, hot forging preheating step: 500°C, 2 hours) are shown in Fig. 7. Precipitates at the grain boundaries of the aluminum base material can be confirmed, and it is clear that the precipitates are finer than in Comparative Example 1.

[0075] The results of TEM observation of the interior of the aluminum matrix crystal grains of the aluminum alloy forging of Comparative Example 5 (homogenization heat treatment: 510°C, 6 hours, hot forging preheating step: 500°C, 2 hours) are shown in Fig. 8. No clear dispersion of crystal precipitates was observed within the crystal grains of the aluminum matrix.

[0076] Furthermore, the aluminum alloy forging of Comparative Example 7 contains sufficient Si and satisfies the relationship 0.2≦Excess Si (wt%)+Mn (wt%)+Cr (wt%)≦1.7, but the Cu content is less than 0.2 wt%, resulting in low values ​​for the tensile strength and 0.2% yield strength.

[0077] From the above results, it can be seen that the aluminum alloy forging material of the present invention has a large amount of fine Al-(Fe, Mn, Cr)-Si-based precipitates dispersed within the crystal grains of the aluminum base material, and the precipitates within the crystal grains are fine and have a shape that is close to granular, resulting in high strength and excellent toughness (good ductility). [Explanation of symbols]

[0078] 2. Aluminum base material, 4···grain boundaries, 6...Precipitate.

Claims

1. Made of 6000 series aluminum alloy, The Cu content is 0.2 to 1.0 wt%, the Si content is 0.5 to 1.4 wt%, the Mg content is 0.8 to 1.2 wt%, the Mn content is 0.1 to 0.8 wt%, the Cr content is 0.1 to 0.8 wt%, the Fe content is 0.05 to 0.3 wt%, the Ti content is 0.01 to 0.03 wt%, and the balance is Al and unavoidable impurities; The composition of the 6000 series aluminum alloy satisfies the following relational expressions (1) and (2): The alloy has precipitates at the grain boundaries of the base material and Al-(Fe, Mn, Cr)-Si-based precipitates within the base material grains, the average grain size of the precipitates at the base material grain boundaries is 50 nm or less, the aspect ratio of the precipitates at the base material grain boundaries is 5 or less, 0.2% yield strength is 350 MPa or more and elongation is 10% or more, The excess Si (wt%) in the relational expression (2) is set to a positive value calculated by "Si content (wt%) - Mg content (wt%) / 1.731", Aluminum alloy forgings characterized by: Si (at%)≧Mg / 2 (at%) (1) 0.2≦Excess Si (wt%)+Mn (wt%)+Cr (wt%)≦1.7 (2)

2. The content of Si is 0.9 to 1.2 wt %; 2. The aluminum alloy forging according to claim 1, wherein:

3. the width of the precipitate-free zone centered on the base material grain boundary is 100 nm or less; 3. The aluminum alloy forging according to claim 1 or 2, characterized in that:

4. The aluminum alloy forging material according to any one of claims 1 to 3 is used. An automobile suspension part characterized by the above.

5. A method for producing an aluminum alloy forging according to claim 1 or 3, the aluminum alloy material has a composition comprising a Cu content of 0.2 to 1.0 wt%, a Si content of 0.5 to 1.4 wt%, a Mg content of 0.8 to 1.2 wt%, a Mn content of 0.1 to 0.8 wt%, a Cr content of 0.1 to 0.8 wt%, an Fe content of 0.05 to 0.3 wt%, a Ti content of 0.01 to 0.03 wt%, and the remainder being Al and unavoidable impurities; a hot forging preheating step of preheating the aluminum alloy material; a hot forging step of hot forging the preheated aluminum alloy material obtained in the hot forging preheating step; and subjecting the aluminum alloy forged material obtained in the hot forging step to a solution treatment and an aging treatment, The preheating temperature in the hot forging preheating process is 300 to 550 ° C., and the preheating time is 1 to 3 hours. The composition of the aluminum alloy material satisfies the following relational expressions (1) and (2), The excess Si (wt%) in the relational expression (2) is set to a positive value calculated by "Si content (wt%) - Mg content (wt%) / 1.731", A method for manufacturing an aluminum alloy forging material, characterized by: Si (at%)≧Mg / 2 (at%) (1) 0.2≦Excess Si (wt%)+Mn (wt%)+Cr (wt%)≦1.7 (2)

6. A homogenization heat treatment step of the aluminum alloy material is performed before the hot forging preheating step, The temperature of the homogenization heat treatment step is 500 to 550 ° C., and the holding time is 5 to 10 hours.

6. The method for producing an aluminum alloy forging according to claim 5, wherein

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