Method For Producing High Strength Aluminum Alloy Extruded Material With High SCC Resistance

Abstract

A method includes: casting a billet using an aluminum alloy comprising, by mass: 6.0 to 8.0% of Zn, 1.5 to 2.8% of Mg, 0.10 to 0.50% of Cu, 0.10 to 0.50% of Zr, 0.005 to 0.05% of Ti, 0.10 to 0.40% of Mn, 0.05% or less of Cr, with a total of Mn+Cr+Zr being 0.10 to 0.50%, and a balance of Al and inevitable impurities; subjecting the resulting casted billet to homogenization treatment at 470 to 560 C. for 1 to 14 hours, followed by cooling at a cooling rate of 50 C./hr or higher; and subjecting the billet to extrusion process, immediately followed by air cooling until the temperature of an extruded material drops to 300 to 480 C., and further cooling to 200 C. or less at a cooling rate within a range of 150 to 950 C./min, and thereafter subjecting the extruded material to artificial aging treatment.

Claims

1. A method for producing a high strength aluminum alloy extruded material with high SCC resistance, the method comprising: casting a billet using an aluminum alloy comprising, by mass: 6.0 to 8.0% of Zn, 1.5 to 2.8% of Mg, 0.10 to 0.50% of Cu, 0.10 to 0.50% of Zr, 0.005 to 0.05% of Ti, 0.10to 0.40% of Mn, 0.05% or less of Cr, with a total of Mn+Cr+Zr being 0.10 to 0.50%, and a balance of Al and inevitable impurities; subjecting the resulting casted billet to homogenization treatment at 470 to 560 C. for 1 to 14 hours, followed by cooling at a cooling rate of 50 C./hr or higher; and subjecting the billet to extrusion process, immediately followed by air cooling until the temperature of an extruded material drops to 300 to 480 C., and further cooling to 200 C. or less at a cooling rate within a range of 150 to 950 C./min, and thereafter subjecting the extruded material to artificial aging treatment.

2. The method for producing a high strength aluminum alloy extruded material with high SCC resistance according to claim 1, wherein the artificial aging treatment comprises: a first stage at 80 to 130 C. for 2 to 8 hours, and a second stage at 130 to 160 C. for 2 to 16 hours, and wherein the extruded material has a 0.2% proof strength of 460 MPa or more and high SCC resistance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 illustrates compositions of aluminum alloys used for evaluation.

[0008] FIG. 2 illustrates producing conditions of billets and extrusion materials used for evaluation.

[0009] FIG. 3 illustrates evaluation results of the extruded materials.

DETAILED DESCRIPTION

[0010] The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. These are, of course, merely examples and are not intended to be limiting. In addition, the disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Further, when a first element is described as being connected or coupled to a second element, such description includes embodiments in which the first and second elements are directly connected or coupled to each other, and also includes embodiments in which the first and second elements are indirectly connected or coupled to each other with one or more other intervening elements in between.

[0011] It is an object of the invention to provide a method for producing an aluminum alloy extruded material that is effective for achieving high strength while ensuring high SCC resistance.

[0012] A method for producing a high strength aluminum alloy extruded material with high SCC resistance in the invention is characterized by comprising: casting a billet with an aluminum alloy comprising, by mass: 6.0 to 8.0% of Zn, 1.5 to 2.8% of Mg, 0.10 to 0.50% of Cu, 0.10 to 0.50% of Zr, 0.005 to 0.05% of Ti, 0.10 to 0.40% of Mn, 0.05% or less of Cr, with a total of Mn+Cr+Zr being 0.10 to 0.50%, and a balance of Al and inevitable impurities; subjecting the resulting casted billet to homogenization treatment at 470 to 560 C. for 1 to 14 hours, followed by cooling at a cooling rate of 50 C./hr or higher; and subjecting the billet to extrusion process, immediately followed by air cooling until the temperature of an extruded material drops to 300 to 480 C., and further cooling to 200 C. or less at a cooling rate within a range of 150 to 950 C./min, and thereafter subjecting the extruded material to artificial aging treatment.

[0013] Here, the artificial aging treatment is performed at 80 to 130 C. for 2 to 8 hours in a first stage and at 130 to 160 C. for 2 to 16 hours in a second stage. The extruded material has a 0.2% proof strength of 460 MPa or more, preferably with high SCC resistance and an average crystal grain size of 250 m or less.

[0014] When a billet is cast using an AlZnMg based aluminum alloy, and the billet is subjected to extrusion process, the temperature of an extruded material immediately after extrusion is as high as 440 to 585 C. due to processing heat and the like.

[0015] Therefore, the extruded material can be quenched by cooling it immediately after extrusion, which is called die edge quenching.

[0016] Two methods are known for this die edge quenching: air cooling by a fan, etc. and water cooling with water, etc.

[0017] In this case, the addition of transition elements such as Mn, Cr, and Zr in the aluminum alloy improves hardenability as well as crystal grain refinement.

[0018] Unfortunately, as disclosed in Documents 1 and 2, as the amount of Cr added to the extruded material increases, the quench sensitivity increases more sharply and sufficient quench effect can only be achieved by rapid cooling at the water cooling level.

[0019] However, water cooling of an extruded material at a high temperature state immediately after extrusion process causes cooling strain in the extruded material. This makes it difficult to ensure the shape quality of the extruded material and is a technical challenge that reduces the stress corrosion cracking resistance (SCC resistance).

[0020] The invention then suppresses the addition of Cr and controls the addition of 0.1to 0.40% of Mn and 0.10 to 0.50% of Zr, which have relatively less sharp quench sensitivity, and the total amount of Mn+Cr+Zr is in the range of 0.10 to 0.50%. This is characterized by air cooling from an extruded material temperature of 500 to 585 C. immediately after extrusion to 300 to 480 C. at a cooling rate of 50 to 150 C./min, followed by rapid cooling at a cooling rate of 150 to 950 C./min to suppress the reduction of the SCC resistance of the extruded material and increase strength thereof.

[0021] The reasons for selecting the composition of the aluminum alloy according to the invention are described below.

Zn, Mg

[0022] The Zn and Mg components contribute to the aluminum alloy obtaining high strength.

[0023] Among them, Zn can be added at high concentrations without relatively reducing extrudability. However, too much Zn addition will reduce SCC resistance, so the following mass percentages are preferred: 6.0 to 8.0% of Zn, 1.5 to 2.8% of Mg.

Cu

[0024] The Cu component is effective for increasing the strength by the solid solution effect. However, too much Cu content will reduce extrudability and general corrosion resistance, so 0.10 to 0.50% of Cu is preferable.

Mn, Zr, Cr

[0025] Mn, Zr, and Cr are all transition elements, and can suppress crystal grain refinement and surface recrystallization depth during extrusion.

[0026] In the invention, air cooling in the high temperature range and rapid cooling from 300 to 480 C. are performed on the extruded material, while hardenability is ensured by cooling immediately after extrusion. Considering this, 0.10 to 0.40% of Mn, 0.10 to 0.50% of Zr, and as little Cr as possible, preferably at least 0.05% or less.

[0027] It is also preferable that the total of Mn+Cr+Zr be controlled in the range of 0.10 to 0.50%.

Ti

[0028] The Ti component is effective in crystal grain refinement during billet casting.

[0029] Considering the dissolution of Ti, the range of 0.005 to 0.05% of Ti is preferred.

Fe, Si

[0030] The Fe and Si components are impurities in the invention, and are preferably suppressed to 0.4% or less of Fe and 0.3% or less of Si.

[0031] In the invention, quenching of the extruded material immediately after extrusion is performed by air cooling until the extrusion temperature is 300 to 480 C., then cooling strain can be suppressed by rapid cooling at the cooling rate of 150 to 950 C./min. This can both suppress the reduction in SCC resistance and achieve high strength.

[0032] Exemplary embodiments are described below. Note that the following exemplary embodiments do not in any way limit the scope of the content defined by the claims laid out herein. Note also that all of the elements described in the present embodiment should not necessarily be taken as essential elements.

[0033] A molten metal having an aluminum alloy composition as illustrated in FIG. 1 was adjusted, an 8-inch columnar billet was produced under producing conditions illustrated in FIG. 2, the billet was subjected to homogenization treatment, and then an extruded material was produced under producing conditions illustrated in FIG. 2.

[0034] Billet casting is produced by continuous casting process such as hot top casting process and heat insulating mold casting process.

[0035] At this time, when the casting speed is set to 50 mm/min or more, the casting structure of the billet becomes a fine structure having an average crystal grain size of 250 m or less.

[0036] The average crystal grain size shows the average at 33 points of the center and the midpoints between the center and the peripheries of the cross section, at the tip, the middle, the rear end of the billet.

[0037] Homogenization treatment (HOMO) of the billet is carried out to eliminate the micro segregation that occurred during the casting of the billet.

[0038] The billet is held at 470 to 560 C. for 1 to 14 hours for homogenization treatment, and then cooled at a cooling rate of 50 C./hr or higher.

[0039] Then, the billet (BLT) is preheated to 400 C. or more, preferably 440 to 500 C., and then filled into an extruder container and extruded.

[0040] A direct extruder is used in the extrusion process.

[0041] The temperature of the extruded material (extruded shape) immediately after the extrusion process is 440 C. or more, preferably 480 to 510 C.

[0042] Immediately after extrusion from a die, the temperature of the extruded material is as high as 440 C. or more, but it is cooled by air at room temperature.

[0043] The invention is characterized by the following points. The extruded material is air cooled for about 0.1 min or more immediately after extrusion, and water cooling or strong fan air cooling is carried out from the stage where the temperature of the extruded material (extruded shape) reaches 300 to 480 C. until the temperature drops to 200 C. or less at a rate of 150 to 950 C./min.

[0044] Thus, in the die edge quenching during extrusion, adequate quench effect can be obtained by rapid cooling at a rate of 150 to 950 C./min when the temperature of the extruded material reaches 300 to 480 C. This prevents cooling strain from occurring in the extruded material compared to conventional water cooled die edge quenching.

[0045] Therefore, SCC resistance reduction can be suppressed while maintaining high strength.

[0046] This point is discussed below by comparing examples and comparative examples.

[0047] Then, the extruded material is subjected to artificial aging treatment at 80 to 130 C.2 to 8 hr in the first stage, and 130 to 160 C.2 to 8 hr in the second stage to obtain the extruded material with high SCC resistance and high strength.

[0048] In the invention, the target was T5 tensile strength of 480 Mpa or more, 0.2% T5 proof strength of 460 Mpa or more, and T5 elongation of 10% or more.

[0049] The target of stress corrosion cracking resistance (SCC resistance) was that cracking did not occur for 720 cycles or more under sample conditions described below.

[0050] In addition, the depth of the recrystallization layer formed on the surface of the extruded material was targeted at an average of 150 m or less.

[0051] The evaluation method of the extruded material will be described below.

[0052] 1. In advance, JIS-No. 5 tensile test piece was prepared along the extrusion direction of the extruded material in accordance with JIS-Z2241. Mechanical properties were measured in a tensile tester in accordance with JIS standards.

[0053] 2. In advance, the cross section of a sample cut from the extruded material was mirror polished and etched with a 3% NaOH sodium hydroxide solution.

[0054] The average depth of the recrystallized layer formed on the surface of the extruded material was measured from a 100-fold image of the metal structure by optical microscopy.

[0055] The SCC resistance target was achieved when cracking of the test piece occurred for 720 cycles or more under the following conditions with one cycle of stress applied at 80% of the proof stress.

One Cycle

[0056] The extruded material is immersed in 3.5% NaCl aqueous solution at 25 C. for 10 min, then left at 25 C. and 40% humidity for 50 min, and then dried naturally.

Evaluation Results

[0057] FIGS. 1 to 3 illustrate the producing conditions of Examples 1 to 15 and Comparative Examples 1 to 4 and the evaluation results of the extruded materials obtained thereby.

[0058] Examples 1 to 15 achieved the goal in all evaluation items, with tensile strength of 480 MPa or more, high strength of 0.2% proof strength of 460 MPa or more, and high SCC resistance.

[0059] In contrast, in Comparative Example 1, the quenching start temperature of the extruded material from 150 to 950 C./min was 500 C., which exceeded the upper limit of the standard of 480 C., resulting in the failure to achieve the target SCC resistance.

[0060] Comparative Example 2 had 0.70% of Mn, exceeding the upper limit of the standard of 0.40%, and the quenching start temperature was high, resulting in the failure to achieve the target SCC resistance.

[0061] In addition, Comparative Example 2 had less strength than Comparative Example 1.

[0062] Comparative Example 3 had Cu content of 1.60%, which exceeded the standard upper limit of 0.50%, and even at high strength, the SCC resistance was significantly reduced.

[0063] Comparative Example 4 had Mg content of 1.21%, which was lower than the standard lower limit of 1.5%, resulting in the failure to achieve the target strength.

INDUSTRIAL APPLICABILITY

[0064] By using the method for producing the aluminum alloy extruded material according to the invention, high strength extruded material with high SCC resistance can be obtained. This extruded material can be used for various structural members in vehicles, industrial machinery, and the like.

[0065] Although only some embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within scope of this invention.