JPH10219413A - Production of high strength aluminum alloy excellent in intergranular corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high strength aluminum alloy excellent in intergranular corrosion resistance and extrudability. SOLUTION: This aluminum alloy is the one having a compsn. contg., by weight, 0.5 to 1.2% Mg, 0.5 to 1.4% Si, and the balance Al with inevitable impurities, and it is cast and is subjected to hot extrusion into a prescribed shape, which is thereafter subjected to solution heat treatment at the temp. T (K) satisfying the relational inequalities of Mg (%)<-5/9Si (%)+0.23exp (T-673)/100}+0.25 and Mg (%)<-3Si (%)+0.78exp (T-673)/100}+0.42 to <843K for 10sec to 2hr, is cooled from this temp. to a temp. range till 373K at a rate of >=10K/sec and is subjected to aging treatment. This alloy can be incorporated with small amounts of Cu, Zn, Cr, Fe, Mn, Zr, V and Ti. In this way, the high strength aluminum alloy excellent in intergranular corrosion resistance and extrudability can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength aluminum alloy excellent in grain boundary corrosion resistance and extrudability and suitable for structural members of automobiles, vehicles, electric equipment, buildings and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, Al-Mg-Si based alloys have been widely used in the fields of vehicles, ships, construction, etc. because they have excellent extrudability and high strength can be obtained by heat treatment. In recent years, from the viewpoint of reducing the weight of automobiles, extruded profiles of aluminum alloys have been applied to structural members of automobiles, and the above-mentioned Al-Mg-Si-based alloy has attracted attention as a material for such materials, and has been used for strength members. Have been. However,
This alloy has the disadvantage that it often undergoes intergranular corrosion in a corrosive environment, particularly in an environment containing Cl ^- ions, leading to deterioration of mechanical properties and sometimes even to failure. Until now, to prevent such intergranular corrosion,
JISA6063 alloy having a low alloy concentration has been widely used. However, since the present alloy has low strength, it cannot be applied to members requiring high strength.

[0003]

Accordingly, there is a need for an aluminum alloy for extrusion which is hard to cause intergranular corrosion even in a corrosive environment and has high strength. An object of the present invention is to provide a method for producing a high-strength aluminum alloy having excellent intergranular corrosion resistance and excellent extrudability, with the above circumstances as a background.

[0004]

It is considered that intergranular corrosion of an Al-Mg-Si alloy occurs due to preferential dissolution of β phase (Mg ₂ Si) or Si phase crystallized at the grain boundary. (For example, Aluminum Handbook (No. 5
Edition), edited by The Japan Institute of Light Metals (1994), p. 53). Thus, the present inventors have investigated in detail the relationship between the alloy composition and the solution treatment temperature or the preheating temperature during extrusion molding, which affects the generation limit of grain boundary crystal precipitates in an Al-Mg-Si-based alloy. By specifying the relationship between the amount of Si and the solution treatment temperature or the preheating temperature during extrusion molding, and by limiting the cooling rate after solution treatment or extrusion molding, it is possible to suppress the generation of grain boundary crystal precipitates. It was found that good intergranular corrosion resistance was obtained.

That is, the present invention provides: (1) Mg: 0.5 to 1.2%, Si: 0.
After casting an aluminum alloy containing 5-1.4% and the remainder consisting of Al and impurities and hot-extrusion into a predetermined shape, Mg (%) <− 5 / 9Si (%) + 0.23exp
{(T-673) / 100} +0.25 and Mg (%) <-3Si (%) + 0.78exp} (T−
673) /100°+0.42. Solution treatment is performed at a temperature not less than T (K) satisfying the relational expression of not more than 843K and not less than 10 seconds and not more than 2 hours,
A method for producing a high-strength aluminum alloy excellent in intergranular corrosion resistance, characterized by cooling a temperature range from that temperature to 373K at a rate of 10K / sec or more and further performing natural or artificial aging treatment. Here, Mg (%) and Si
(%) Indicates the weight percentage of Mg and Si, respectively.

(2) The method for producing an aluminum alloy according to the above (1), further comprising:
Cu: 0.05 to 1.0%, Zn: 0.05 to 0.6
%, Mn: 0.03 to 0.5%, Cr: 0.03 to 0.
5%, V: 0.03 to 0.3%, Fe: 0.05 to
0.5%, Zr: 0.03-0.3%, Ti: 0.00
A method for producing a high-strength aluminum alloy excellent in intergranular corrosion resistance containing at least one of 5 to 0.3%.

(3) The aluminum alloy according to the above (1) or (2) is melted, cast into a billet having a predetermined shape, homogenized, and then Mg (%) <− 5 / 9Si (% ) + 0.23exp
{(T'-623) / 100} +0.25 and Mg (%) <-3Si (%) + 0.78exp} (T '
−623) /100°+0.42 After preheating at a temperature T ′ (K) or more and less than 793K which satisfies the relational expression, hot extrusion molding is performed,
This is a method for producing a high-strength aluminum alloy excellent in intergranular corrosion resistance, characterized by cooling at a temperature range of up to 73 K at a rate of 10 K / sec or more and further performing natural or artificial aging treatment.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
First, the reasons for limiting the component composition will be described. In addition, all% are weight%. Mg is a basic alloying element in the alloy of the system targeted in the present invention, and forms a compound with Si to contribute to an increase in strength. Mg content is 0.5%
If less than Mg, which contributes to an increase in strength by precipitation hardening
_{2 Since} the amount of generated Si is small, sufficient strength cannot be obtained. On the other hand, if it exceeds 1.2%, extrudability is reduced and grain boundary precipitates are obtained even when liquefied under the conditions described below. Cannot be suppressed, and the intergranular corrosion resistance decreases. Therefore, the amount of Mg was set in the range of 0.5 to 1.2%.

[0009] Si is also the basic alloy of the alloys of the present invention.
An element that forms a compound with Mg to improve strength.
Contribute to the top. If Si is less than 0.5%, it contributes to hardening
Mg _TwoSufficient strength is obtained because the amount of generated Si is reduced.
On the other hand, if it exceeds 1.4%, the intergranular corrosion resistance is low.
Not only drop but also coarse Si phase crystallizes during solidification
And reduce the extrudability. Therefore, the amount of Si is 0.5 to
It was within the range of 1.4%. Cu promotes age hardening,
It is an element that increases the strength of the alloy, but the content is 0.0
If less than 5%, the effect is not seen and exceeds 1.0%.
And the formation of grain boundary precipitates containing Cu in addition to the β and Si phases
And reduce the intergranular corrosion resistance. Therefore, the amount of Cu is
0.05 to 1.0%.

[0010] Zn is an effective element for improving the strength, but if its content is less than 0.05%, its effect is small, and 0.6%
If the ratio exceeds the above range, intergranular corrosion resistance and weldability will decrease. Therefore, the content of Zn is set to 0.05 to 0.6%. C
Each of r, Mn, Zr and V is an element having the effect of refining and stabilizing crystal grains and improving the strength, and one or more of them are added as necessary. In this case, if any of the elements is less than 0.03%, the above effects cannot be obtained. On the other hand, if Cr and Mn exceed 0.5% and Zr and V each exceed 0.3%, the above effects are saturated. On top, it reduces the extrudability. Therefore, the contents of Cr and Mn are set to 0.03 to 0.5%, and the contents of Zr and V are set to 0.03 to 0.3%.

Fe is essentially an unavoidable impurity, but has the same effect as Cr, Mn, Zr, V, etc.
Add as needed. In this case, if the content is less than 0.05%, the above effect cannot be obtained. If the content is more than 0.5%, the above effect is saturated, and an Al—Fe—Si based intermetallic compound is generated, thereby reducing the extrudability. Let it. Therefore, the content of Fe is 0.1.
0.5 to 0.5%. Ti is generally added alone or in combination with a small amount of B in order to refine the crystal grains of the ingot. In this case, if the content of Ti is less than 0.005%, the above effect cannot be obtained, and if it exceeds 0.3%, the effect is saturated. Therefore, the content of Ti is 0.005 to 0.5.
3%. The addition amount of B is advantageously 0.0005 to 0.03%.

In order to obtain excellent intergranular corrosion resistance even with a high-strength aluminum alloy having the above-mentioned composition, M
It is necessary to limit the relationship between the amounts of g and Si and the manufacturing conditions of the alloy. In order to suppress the formation of intergranular precipitates, which lower the intergranular corrosion resistance, at the time of solution treatment or extrusion molding, the solute elements Mg and Si are completely dissolved in Al and then cooled. What is necessary is just to cool at a sufficiently high rate so that these do not reprecipitate in the process. The present inventors first examined in detail the relationship between the amounts of Mg and Si and the solution treatment temperature, and found that the Mg (%) <− between the content of Mg and Si and the solution treatment temperature of the alloy. 5 / 9Si (%) + 0.23exp {(T-673) / 10 100} +0.25 ... and Mg (%) <- 3Si (%) + 0.78exp {(T-673) / 100} +0.42 When this was true, it was found that Mg and Si completely dissolved in Al. Therefore, in the alloy manufacturing process, it is necessary to perform the solution treatment at the temperature T (K) satisfying the above two equations or to give the alloy a heat history corresponding thereto.

There are two methods for producing an aluminum alloy: a method of performing aging treatment after forming into a predetermined shape and then performing a solution treatment, and a method of performing aging treatment as it is after high-temperature forming such as hot extrusion molding. Is manufactured. When the solution treatment is performed after forming into a predetermined shape, the temperature T (K) or more that satisfies the above two formulas and 843K
The solution treatment may be performed at a temperature less than. When the solution treatment is performed at a temperature that does not satisfy either one of the above two formulas, Mg and Si do not form a solid solution in Al, and form grain boundary crystal precipitates to reduce the grain resistance. Interfacial corrosion is reduced. In this case, if the solution treatment time is less than 10 seconds, even if the solution treatment is performed at a temperature that satisfies the above two formulas,
In addition, since the solid solution of Si and Si does not sufficiently occur, and the crystal grains become coarse when the time exceeds 2 hours, in any case, not only the intergranular corrosion resistance but also the ductility is reduced. Therefore, the solution treatment time was set to 10 seconds or more and 2 hours or less.

On the other hand, when the solution treatment temperature exceeds 843K, partial dissolution occurs. Therefore, the upper limit of the solution treatment temperature was set to 843K. After the solution treatment, the temperature range up to 373K is cooled at a rate of 10K / sec or more. If the cooling rate is less than 10 K / sec, the second phase precipitates during cooling,
In addition to reducing the intergranular corrosion resistance, the amount of Mg and Si precipitated by the aging treatment decreases, and the strength decreases. Therefore, the cooling rate from the solution treatment temperature to 373 K was set to 10 K / sec or more. After cooling, natural or artificial aging treatment is performed by a general method to secure the strength. In addition,
The manufacturing process from melting and casting the aluminum alloy to before the solution treatment, that is, the process until the aluminum alloy is formed into a predetermined shape, may be a conventional general aluminum alloy product manufacturing method. For example, a method of producing an ingot by a DC casting method, performing a homogenizing treatment, and forming a predetermined shape by hot extrusion or the like can be used.

On the other hand, after extrusion molding, no solution treatment is performed,
When the aging treatment is performed as it is, the preheating of the ingot must also serve as the solution treatment. Generally, at the time of high-temperature molding such as hot extrusion molding, the temperature of the alloy rises by 50 to 100 K from the preheating temperature due to the heat generated during processing.
Therefore, it is sufficient that the preheating temperature is lower than the temperature T (K) defined by the above two equations by 50K or more. That is, between the amounts of Mg and Si and the preheating temperature T ′ (K) of the alloy, Mg (%) <− 5 / 9Si (%) + 0.23 exp {(T′-623) / 10 100} +0.25 ... and Mg (%) <-3Si (%) + 0.78exp {(T'-623) / 100} + 0.42 ··················· When preheating is performed at a temperature at which either one of the above two formulas is not satisfied, Mg and Si do not form a solid solution in Al, and form grain boundary crystal precipitates to prevent grain boundary resistance. Corrosion is reduced.

If the preheating temperature exceeds 793K, the temperature of the alloy exceeds 843K due to the heat generated during processing.
Partial dissolution occurs. Therefore, the upper limit of the preheating temperature is 7
93K. For the hot extrusion, a usual extrusion method such as a direct extrusion method or an indirect extrusion method can be used. After hot extrusion, 1
The temperature range up to 00 ° C. is cooled at a rate of 10 K / sec or more. If the cooling rate is less than 10 K / sec, the second
A phase is precipitated to lower the intergranular corrosion resistance, and the amount of Mg and Si precipitated by the aging treatment is reduced to lower the strength. Therefore, after extrusion molding, the cooling rate in a temperature range up to 373 K was set to 10 K / sec or more. After cooling, natural or artificial aging treatment is performed by a general method to secure the strength.

As the ingot for extrusion, one obtained by casting the above aluminum alloy to a predetermined size by a general method, for example, a DC casting method, and then homogenizing the same is used. As described above, in the present invention, it is possible to suppress the formation of grain boundary crystal precipitates by appropriately adjusting the component composition of the alloy and defining the amounts of Mg and Si in the alloy and the manufacturing conditions of the alloy. As a result, an aluminum alloy having excellent intergranular corrosion resistance and excellent extrudability and high strength can be obtained.

[0018]

Next, the present invention will be described with reference to examples. (Example 1) Each alloy having the chemical components shown in Table 1 was melted and cast by a conventional method, subjected to facing and homogenizing treatment to obtain a material for hot extrusion. After preheating these materials at 773K for 5 minutes, hot extrusion was performed at a speed of 20 m / min to produce extruded profiles. The shape of the profile is 2mm thick, 40mm on each side
It is a square shape of mm. After extrusion, a test piece having a length of 50 mm was cut out, subjected to a solution treatment of 823 K × 30 minutes, quenched in ice water from that temperature, and then subjected to an aging treatment of 453 K × 8 hours. For each alloy obtained in this way,
The mechanical properties and intergranular corrosion resistance were evaluated. The mechanical properties were determined by a tensile test according to a test method specified in JIS. In addition, the intergranular corrosion resistance is 3
A test piece of 0 mm × 50 mm was cut out and evaluated by performing a salt spray test (960 hours) in accordance with the test method specified in JIS.

Evaluation of the intergranular corrosion resistance was as follows: ：: excellent (no intergranular corrosion) 〇: good (0.1 mm or less of intergranular corrosion depth) Δ: slightly poor (0.1 to 0 of intergranular corrosion depth) ×: poor (grain boundary corrosion depth 0.5 mm or more). At the time of extrusion molding, from the viewpoint of extrusion resistance, surface properties and tool wear, the extrudability of each alloy was compared with that of the comparative material A6063.
It was represented by a numerical value (extrusion index) relative to the alloy as 100. If the extrusion index is 80 or more, it can be said that the extrudability is good. Table 2 shows the evaluation results. As is clear from Table 2, the aluminum alloy according to the present invention is excellent in intergranular corrosion resistance and extrudability as compared with the aluminum alloy of the comparative material, and has a high tensile strength of 320 MPa or more. You can see that it is.

[0020]

[Table 1]

[0021]

[Table 2]

Example 2 Alloy N among the alloys shown in Table 1
o9 was melted and cast by a conventional method, subjected to facing and homogenizing treatment to obtain a raw material for hot extrusion. 773K of these materials
After preheating for 5 minutes, hot extrusion was performed at a speed of 20 m / min to produce an extruded profile. The shape is 2m thick
m, 40 mm on each side. After the extrusion, a test piece having a length of 50 mm was cut out, subjected to a solution treatment for 5 seconds to 3 hours at each temperature of 793 to 853 K, quenched in ice water from that temperature, and then subjected to an aging treatment of 453 K × 8 hours. Was. The mechanical properties and intergranular corrosion resistance of each alloy thus obtained were evaluated in the same manner as in Example 1. Table 3 shows the results.

[0023]

[Table 3]

As is evident from Table 3, when the solution treatment was performed at a temperature not satisfying the above formula or the formula, and even when the solution treatment was performed at a temperature satisfying the formula, the solution treatment time was 10 seconds. In the comparative method in which the temperature is less than 10% or more and the solution treatment is performed at a temperature of less than 843K for 10 seconds or more and less than 2 hours, both the intergranular corrosion resistance and the tensile strength satisfy the conditions of the present invention and the formula. It turns out that it is inferior compared. In the case of the comparative material having a solution heat treatment temperature of 853K, the test piece could not be collected due to partial dissolution, and 8
The solution-treated material of 23K × 3 hours has poor intergranular corrosion resistance and low ductility. Thus, the above-mentioned and the formulas within the range of the present invention are satisfied, and excellent intergranular corrosion resistance and high strength of 320 MPa or more in tensile strength in solution treatment for 10 seconds to 2 hours at a temperature of 843 K or less. It can be seen that both are compatible.

(Embodiment 3) Of the alloys shown in Table 1, alloy N
o9 was melted and cast by a conventional method, subjected to facing and homogenizing treatment to obtain a raw material for hot extrusion. 773K of these materials
After preheating for 5 minutes, hot extrusion was performed at a speed of 20 m / min to produce an extruded profile. The shape is 2m thick
m, 40 mm on each side. After the extrusion, a test piece having a length of 50 mm was cut out, subjected to a solution treatment of 823 K × 30 minutes, and cooled by changing an average cooling rate from the temperature to 373 K from 1 K / sec to 100 K / sec. After that, aging treatment of 453K × 8 hours was performed. The mechanical properties and intergranular corrosion resistance of each alloy thus obtained were evaluated in the same manner as in Example 1.
Table 4 shows the results. As is clear from Table 4, when the temperature range from the solution treatment temperature to 373 K is cooled at a cooling rate of 10 K / sec or more, which is the condition of the present invention,
Although excellent intergranular corrosion resistance and high strength of tensile strength of 320 MPa or more are obtained, when cooled at 10 K / sec or less as a comparative method, intergranular corrosion resistance is inferior and sufficient strength is obtained. It turns out that it cannot be obtained.

[0026]

[Table 4]

Example 4 Alloy N among the alloys shown in Table 1
o9 was melted and cast by a conventional method, subjected to facing and homogenizing treatment to obtain a raw material for hot extrusion. 773-
After preheating at each temperature between 813K for 5 minutes, hot extrusion was performed at a speed of 20m / min, and the average cooling rate in the temperature range up to 373K was changed from 1K / sec to 100K / sec. After cooling, an extruded profile was produced. The shape of the shape is a square shape having a thickness of 2 mm and a side of 40 mm. After extrusion, 5
A test piece having a length of 0 mm was cut out and subjected to an aging treatment of 453 K × 8 hours. The mechanical properties and intergranular corrosion resistance of each alloy thus obtained were evaluated in the same manner as in Example 1. In addition, at the time of extrusion molding,
The extrudability was evaluated in the same manner as described above. Table 5 shows the results.
Shown in

[0028]

[Table 5]

As is evident from Table 5, the preheating temperature is a temperature satisfying the above conditions and the formula of the present invention, and the average cooling rate in a temperature range up to 373K after extrusion molding is 10K which is a condition of the present invention. / Sec or more, excellent intergranular corrosion resistance and high strength of 320 MPa or more in extrudability and tensile strength are obtained. In addition to the reduction in the properties, the extruded alloy has poor intergranular corrosion resistance and does not have sufficient strength. Even if the preheating temperature is within the temperature range that satisfies the above formula and the formula, if the average cooling rate in the temperature range up to 373 K after extrusion molding is less than 10 K / sec, the intergranular corrosion resistance is poor and sufficient. It can be seen that a high strength was not obtained. When the preheating temperature was 813K, a partial extruded material was generated and a sound extruded material could not be collected.

[0030]

As described above, the aluminum alloy according to the present invention is excellent in intergranular corrosion resistance and extrudability, and has a high tensile strength of 320 MPa or more. It can be widely used as a high-strength aluminum alloy for structural members such as electrical equipment and buildings.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification symbol FI C22F 1/00 640 C22F 1/00 640A 691 691C 691B 692 692A 692B

Claims (3)

[Claims] (1) Mg: 0.5-1.2% by weight, S
i: An aluminum alloy containing 0.5 to 1.4%, the balance being Al and impurities, was cast and hot-extruded into a predetermined shape, and then Mg (%) <-5 / 9Si (%) + 0.23exp
{(T-673) / 100} +0.25 and Mg (%) <-3Si (%) + 0.78exp} (T−
673) /100°+0.42. Solution treatment is performed at a temperature not less than T (K) satisfying the relational expression of not more than 843K and not less than 10 seconds and not more than 2 hours,
A method for producing a high-strength aluminum alloy excellent in intergranular corrosion resistance, characterized by cooling a temperature range from that temperature to 373K at a rate of 10K / sec or more and further performing natural or artificial aging treatment. Here, Mg (%) and Si
(%) Indicates the weight percentage of Mg and Si, respectively. 2. The method for producing an aluminum alloy according to claim 1, wherein the alloy further contains, by weight%, Cu: 0.05 to 1.0% and Zn: 0.05 to 0.6.
%, Mn: 0.03 to 0.5%, Cr: 0.03 to 0.
5%, V: 0.03 to 0.3%, Fe: 0.05 to
0.5%, Zr: 0.03-0.3%, Ti: 0.00
A method for producing a high-strength aluminum alloy excellent in intergranular corrosion resistance containing at least one of 5 to 0.3%. 3. The aluminum alloy according to claim 1 or 2 is melted, cast into a billet having a predetermined shape, homogenized, and then Mg (%) <− 5 / 9Si (%) + 0.23exp.
{(T'-623) / 100} +0.25 and Mg (%) <-3Si (%) + 0.78exp} (T '
−623) /100°+0.42 After preheating at a temperature T ′ (K) or more and less than 793K which satisfies the relational expression, hot extrusion is performed,
A method for producing a high-strength aluminum alloy having excellent intergranular corrosion resistance, characterized by cooling a temperature range up to 73K at a rate of 10K / sec or more and further performing natural or artificial aging treatment.