JP2009035766A - HIGH-FATIGUE-STRENGTH Al ALLOY AND PRODUCTION METHOD THEREFOR - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an Al alloy which exhibits high ductility considering its high strength, has high fatigue strength as well, and shows excellent reliability as a structural part or a member, and to provide a production method therefor. <P>SOLUTION: The Al alloy having the high strength, high ductility and also high fatigue strength is produced through refining crystallized products and precipitates which exist in the structure of Al-Zn-Mg-Cu 7000 series Al alloy obtained by a rapid cooling solidification method and tend to be coarsened by the segregation of alloy elements, by controlling the deposition rate, G/M ratio and the like of a sprayed molten metal by a spray forming operation with the use of an inert gas. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is an Al-Zn-Mg-Cu-based 7000-series Al alloy obtained by a rapid solidification method, and has high ductility for high strength, high fatigue strength, and is used as a structural component or member. The present invention relates to an Al alloy having excellent reliability and a method for producing the same.

  The Al alloy obtained by the rapid solidification method referred to in the present invention is an Al alloy obtained by solidifying a powder or preform obtained by rapidly solidifying an Al alloy molten metal by gas atomization. The solidified Al alloy is an Al alloy obtained by densifying rapidly solidified powder or preform by hot plastic working such as extrusion, forging, rolling, etc., and after densification, solution treatment, aging It is an Al alloy material having various shapes subjected to a tempering treatment such as a treatment. And depending on the use, it refers to an Al alloy material that is formed into a desired shape by using high ductility, such as cold, and is used as a desired member or component by using high strength.

  In recent years, high-strength and lightweight Al alloy materials are widely used in automobile parts, electronic material terminal machines, precision machine parts, and the like, which have been increasingly demanded for weight reduction.

  However, although the mechanical properties of Al alloys at room temperature have improved dramatically in recent years, they are still not sufficient compared to high-strength steels, and their use is also limited. For example, even the so-called A7000 Al alloy, which is widely used as a high-strength Al alloy, has insufficient strength and its use range is limited.

  On the other hand, the conventional melt cast alloy has a limit in dramatically improving the mechanical properties such as strength. For this reason, for the purpose of further increasing the strength of the A7000 series Al alloy, a method of obtaining rapidly solidified powder by an atomizing method has been proposed. According to this rapid solidification method, the content of the alloy element can be increased as compared with the melting cast Al alloy. Therefore, an Al alloy having a high strength can be obtained by pulverizing an Al alloy containing a large amount of these alloy elements by rapid solidification and solidifying the Al alloy.

For example, in Patent Document 1, a component composition of an A7000 series Al alloy is blended with a specific amount of Ag, and a rapidly solidified alloy powder of this component composition obtained by air atomization is solidified by powder metallurgy by extrusion. It is said. Incidentally, it is disclosed that the tensile strength of the molded body material after T6 tempering in which the molded body is homogenized and age-hardened is increased to about 900 MPa. In this Patent Document 1, as a more specific composition of the A7000 series Al alloy, Zn 5 to 11%, Mg 2 to 4.5%, Cu 0.5 to 2% and Ag 0.01 to 0.5% are included, and the balance is substantially In particular, an A7000 Al alloy rapidly solidified powder made of Al is disclosed.
In addition, it has been conventionally proposed to improve the fatigue strength of an Al alloy for structural members obtained by rolling an ordinary ingot instead of an Al alloy obtained by a rapid solidification method. For example, in Patent Document 2, in order to improve the fatigue strength of an Al-Cu-based A2000-based Al alloy for aircraft members and the like, the distance between crystallized substances generated during casting in this Al alloy structure is increased. It has also been proposed.
JP 7-316601 A Japanese Patent Laid-Open No. 8-283892

  Although this Patent Document 1 discloses an A7000 Al alloy having high strength, there is no disclosure of the elongation of this high strength Al alloy. However, even with the rapidly solidified powder of A7000 series Al alloy as in Patent Document 1, it is inevitable that the elongation decreases greatly as the strength increases. For example, as data published in the literature, the elongation when the tensile strength of the Al70Zn rapidly solidified powder solidified molding material of A7090 in the Al-Zn-Mg-Cu-based 7000-based Al alloy is about 625 MPa is about 6%. There is only. Moreover, even if it is an A7075Al alloy extrusion material which is a normal casting material, the total elongation when the tensile strength is 570 MPa is about 11%.

  With such a low elongation, the formability when cold forming into a desired member or part shape is extremely low depending on the application, and cold working becomes difficult. For example, cracks are particularly likely to occur during cold forming with a high processing rate such as rolling. For this reason, the actual situation is that the use of the high-strength 7000 series Al alloy has been greatly restricted due to such restrictions on cold forming.

  Further, in the case of a 7000 series Al alloy having the largest amount of alloy elements such as Zn, Mg, Cu, etc., among other Al alloys, when manufactured by the rapid solidification method, other Al alloys obtained by the spray forming method have other There is a problem peculiar to the 7000 series Al alloy that these alloy elements are more easily segregated than the Al alloy series. Furthermore, even if it is the same 7000 series Al alloy, the 7000 series Al alloy obtained by the spray forming method is more alloy element depending on the cooling conditions than the casting of the Al alloy obtained by normal ingot rolling. There is also a peculiar problem that segregation tends to occur.

  When alloy elements are segregated in this way, crystal precipitates generated during rapid solidification and precipitates generated during heat treatment, such as crystal precipitates generated in the 7000 series Al alloy structure (crystallized products and precipitates). Is a general term). That is, coarse crystal precipitates exceeding several μm are present in the 7000 series Al alloy structure produced by the rapid solidification method. For this reason, even if the strength and ductility as static mechanical properties are high, for example, in dynamic mechanical properties such as fatigue strength, coarse crystal precipitates become the starting point of fracture and fatigue. The strength is lowered. As a result, the reliability of the 7000 series Al alloy produced by the rapid solidification method as a structural component or member is reduced.

  The present invention has been made in view of such problems, and provides an Al alloy having high ductility for high strength, high fatigue strength, and excellent reliability as a structural component or member, and a method for producing the same. For the purpose.

  In order to achieve this object, the gist of the high fatigue strength Al alloy of the present invention is an Al alloy obtained by a rapid solidification method, and is in mass%, Zn: 5-12%, Mg: 2-4% , Cu: each containing 1 to 2%, the balance is made of Al and inevitable impurities, and the area of each crystal precipitate observed by SEM with a magnification of 5000 times by the backscattered electron method of this Al alloy structure is equivalent Of the size converted into the diameter of the circle, the maximum size is 0.5 μm or less.

  The high-strength and high-ductility Al alloy of the present invention may further contain 0.01 to 0.1% by mass of Ag in order to increase the strength. Further, Si, Fe, Mn, Cr, One or more selected from Co, Ni, Zr, Ti and V may be contained in a total amount of 0.1 to 0.5% by mass.

Moreover, in order to achieve the said objective, the summary of the manufacturing method of the high fatigue strength Al alloy of this invention is mass%, Zn: 5-12%, Mg: 2-4%, Cu: 1-2%. In addition, 0.01 to 0.1% by mass of Ag, or one or two or more kinds selected from Si, Fe, Mn, Cr, Co, Ni, Zr, Ti and V in total are 0.1. A molten Al alloy containing selectively 0.5% by mass and the balance consisting of Al and inevitable impurities was sprayed with an inert gas having a G / M ratio in the range of ⁴ to 15 Nm ³ / kg. Spray forming was performed at a deposition rate of the molten metal on the preform in the range of ¹ to 2.5 g / mm ² / s, and the preform body thus obtained was placed in a metal container and vacuum-sealed, and then hot extrusion was performed. Processed and solidified, then tempered to obtain an Al alloy The maximum size of each crystal precipitate observed in an SEM with a magnification of 5000 times by the backscattered electron method of this Al alloy structure is 0.5 μm or less in terms of the area converted into the equivalent circle diameter. It is to be.

  In the present invention, the maximum size of the specific crystal precipitate in the Al-Zn-Mg-Cu-based 7000-series Al alloy structure obtained by solidifying a powder or preform that has been rapidly solidified by gas atomization is regulated. As a result, the 7000 series Al alloy structure does not include coarse crystal precipitates exceeding several μm, and the crystal precipitates included in the structure are refined.

  As described above, the 7000 series Al alloy produced by the rapid solidification method tends to segregate the alloy elements, and the coarsening of crystal precipitates (generic name for crystallization and precipitates) generated in the 7000 series Al alloy structure during the production process. Has a problem peculiar to this alloy. And if the 7000 series Al alloy structure manufactured by the rapid solidification method contains coarse crystal precipitates exceeding several μm, it becomes the starting point of fracture, and dynamic mechanical properties such as fatigue strength are lowered, The reliability of the Al alloy as a part or member for use is reduced. Moreover, elongation is reduced and the moldability in cold working is also inhibited.

  On the other hand, if such coarse crystal precipitates are not included, the crystal precipitates contained in the 7000 series Al alloy structure produced by the rapid solidification method are refined, so that fatigue strength as a dynamic mechanical property is obtained. Therefore, it is possible to provide an Al alloy having excellent reliability as a structural component or member. At the same time, the strength and ductility as static mechanical properties can be increased.

  Quantitatively explaining the effect of improving the fatigue strength of the present invention, in the 7000 series Al alloy obtained by the conventional rapid solidification method, the fatigue strength is about 30% of the tensile strength and 40% of the tensile strength. No more. On the other hand, according to the present invention, the fatigue strength of the 7000 series Al alloy obtained by the rapid solidification method can be remarkably improved to about 40 to 50% of the tensile strength.

(Al alloy composition)
The chemical composition (unit: mass%) of the Al alloy of the present invention will be described below, including the reasons for limiting each element. In addition,% display of content of each element means the mass% altogether.

  The chemical composition of the Al alloy of the present invention is the Al-Zn-Mg-Cu-based 7000-based Al alloy obtained by the rapid solidification method to be described later, in order to guarantee the fatigue strength and mechanical properties intended by the present invention. To be determined. From this point of view, the chemical composition of the Al alloy of the present invention includes, in mass%, Zn: 5 to 12%, Mg: 2 to 4%, and Cu: 1 to 2%, respectively, with the balance being Al and inevitable impurities. Shall be. As a selective additive element for this composition, Ag is further selected from Si, Fe, Mn, Cr, Co, Ni, Zr, Ti and V in the range of 0.1 to 0.01% Ag. Or you may contain 2 or more types in 0.1 to 0.5% of total, respectively.

(Zn, Mg)
The essential alloy elements Zn and Mg form a fine dispersed phase such as a GP zone or an intermediate precipitation phase such as MgZn ₂ and Mg ₃₂ AlZn ₄₉ after the T6 treatment to improve strength and fatigue strength. If the Zn and Mg contents are too small, such as Zn is less than 5% and Mg is less than 2%, these finely dispersed phases are insufficient and the strength and fatigue strength are reduced.

  On the other hand, if the Zn and Mg contents are too large, such as Zn exceeding 12% and Mg exceeding 4%, these elements cannot be dissolved in Al even if the molten metal undergoes rapid solidification. A crystallized substance is formed, which causes a decrease in strength and fatigue strength of the Al alloy. Also, cold workability is significantly reduced. Furthermore, if the content of Zn is too large, depending on the solution treatment temperature during the solution treatment, a liquid phase is likely to be generated, and it is necessary to lower the temperature to sacrifice the solution effect. The solution treatment itself becomes difficult. Therefore, these contents are made into the range of Zn: 5-12% and Mg: 2-4%.

(Cu)
Cu, which is an essential alloy element, improves the strength by solid solution strengthening. If the Cu content is less than 1% and the Cu content is too small, the amount of solid solution Cu decreases, and the strength and fatigue strength decrease. On the other hand, if the Cu content exceeds 2%, the precipitates become coarse, the corrosion resistance such as stress corrosion cracking resistance is remarkably lowered, and the strength and fatigue strength are also lowered. Therefore, the Cu content is in the range of 1 to 2%.

(Ag)
Ag, which is a selective additive element, has an effect of refining precipitates, and improves the strength and fatigue strength of the Al alloy. When it is contained in order to exert this effect, it is contained in an amount of 0.01% or more, and it is not necessary to contain more than 0.1%. Therefore, when Ag is selectively contained, the content is made 0.1 to 0.01%.

(Si, Fe, Mn, Cr, Co, Ni, Zr, Ti, V)
The selective additive elements Si, Fe, Mn, Cr, Co, Ni, Zr, Ti and V can improve the strength and fatigue strength of the Al alloy by the precipitation effect. When contained in order to exert this effect, one or more selected from Si, Fe, Mn, Cr, Co, Ni, Zr, Ti and V are contained in a total of 0.1% or more. . However, when these contents exceed 0.5% in total, coarse precipitates of these elements are formed, which rather causes a decrease in strength, fatigue strength, and ductility. Therefore, when one or more selected from these elements are selectively contained, the total amount (total amount) is set to a range of 0.1 to 0.5%.

(impurities)
Other elements other than the elements described above are basically unavoidable impurities, and are included in ordinary Al-Zn-Mg-Cu-based 7000-series Al alloys within a range that does not impair the intended characteristics of the present invention. To the extent allowed. However, it is preferable that oxygen be reduced as much as possible because oxygen-containing inclusions are generated in the Al alloy structure, which may become a starting point of fracture and reduce fatigue strength and elongation. For this purpose, in rapid solidification, as will be described later, an inert spray gas such as nitrogen, Ar or He is used as the spray gas without using air.

(Organization)
Assuming the above 7000 series Al alloy composition, in the present invention, in order to improve the fatigue strength, the maximum size of the crystal precipitates is set so as to suppress the coarse crystal precipitates of the Al alloy structure. regulate. That is, the maximum size of each crystal precipitate observed by SEM with a magnification of 5000 times by the backscattered electron method of this Al alloy structure is 0.5 μm in the size converted into the equivalent circle diameter. The following.

  As a result, the crystal precipitates contained in the 7000 series Al alloy structure produced by the rapid solidification method are refined, the fatigue strength as dynamic mechanical characteristics is increased, and the reliability as a structural component or member is improved. Can be increased. As described above, the fatigue strength of the 7000 series Al alloy obtained by the conventional rapid solidification method is about 30% of the tensile strength and never exceeds 40% of the tensile strength. On the other hand, according to the present invention, the fatigue strength of the 7000 series Al alloy obtained by the rapid solidification method can be remarkably improved to about 40 to 50% of the tensile strength.

  At the same time, the strength and ductility as static mechanical properties can be increased. According to the present invention, the mechanical properties at room temperature of the 7000 series Al alloy obtained by the rapid solidification method have a tensile strength of 600 MPa or more for increasing the strength. As the strength-ductility balance, when the tensile strength is 600 MPa or more and less than 800 MPa, it has an elongation of 15% or more. In addition, when the tensile strength is in a high strength range of 800 MPa or more and 950 MPa or less, it has an elongation of 10% or more. This strength-ductility balance is quite epoch-making for a high-strength Al-Zn-Mg-Cu-based 7000-based Al alloy.

  Usually, even if the strength-ductility balance is a rapidly solidified powder of an Al-Zn-Mg-Cu-based 7000-based Al alloy, as in Patent Document 1, usually, the higher the strength, the greater the decrease in elongation. Is inevitable. Therefore, the conventional high-strength Al—Zn—Mg—Cu-based 7000-series Al alloys including the above-mentioned Patent Document 1 have extremely poor cold workability. On the other hand, in the present invention, in the Al-Zn-Mg-Cu-based 7000 series Al alloy, the elongation when the tensile strength is 600 MPa is about 15% or more from the conventional about 6%, and the tensile strength is 800 MPa or more. In this case, the elongation can be drastically improved from about 2-3% to 10% or more. This means that rigorous cold working such as rolling makes it impossible to do so far. Therefore, it can be said that such an effect is epoch-making based on the common knowledge that the elongation of the Al—Zn—Mg—Cu-based 7000-based Al alloy rapidly solidified powder that cannot be avoided as the strength becomes higher.

(Method for measuring the size of crystal precipitates)
The size of the crystal precipitates is measured by observing the Al alloy structure with an SEM (scanning electron microscope) with a magnification of 5000 times by the backscattered electron method. In the SEM with a magnification of 5000 times by this backscattered electron method, crystal precipitates (crystallized products and precipitates) present in the matrix are observed as small white irregularly shaped patterns scattered on the solid matrix. This is image-processed, and about 50 visual fields are observed with the size of each crystal precipitate (each white pattern) observed in the visual field converted into a diameter of a circle with an equivalent area. If none of these visual fields has a maximum size exceeding 0.5 μm, the maximum size is 0.5 μm or less. (Within the present invention). On the other hand, if at least one of the visual fields has a maximum size exceeding 0.5 μm, it is determined that the maximum size exceeds 0.5 μm (outside of the present invention).

  Here, the crystal precipitates referred to in the present invention are, in terms of crystal precipitate composition, intermetallic compounds such as alloy elements such as Zn, Mg, and Cu (the finely dispersed phase of Zn and Mg, and the intermetallic compounds with Al). Compound). Further, when selective additive elements such as Ag, Si, Fe, Mn, Cr, Co, Ni, Zr, Ti, and V are contained, the intermetallic compound includes these. However, in the present invention, regardless of the type of these crystal precipitates, or whether the crystal precipitates are precipitates, white irregular shapes scattered on a plain matrix in the SEM with a magnification of 5000 times by the above-described backscattered electron method. All of these observed as small patterns are treated as crystal precipitates that particularly affect fatigue strength.

(Production method)
Below, the manufacturing method of this invention Al alloy is demonstrated. The Al-Zn-Mg-Cu-based 7000-based Al alloy of the present invention is produced by a rapid solidification method instead of the usual melt casting method in order to precipitate a large amount of Zn and Mg-based intermetallic compounds and increase the strength. To do. This rapid solidification method solidifies a powder or preform obtained by rapidly solidifying an Al alloy molten metal by gas atomization. This solidification is densification, and the rapidly solidified powder or preform is processed into various shapes by hot plastic processing such as extrusion, forging and rolling. Then, after this densification (solidification), tempering treatment such as solution treatment and aging treatment is performed.

Rapid solidification method:
The rapid solidification method has a much faster cooling and solidification rate than a normal melt casting method (ingot making), and therefore, a fine intermetallic compound (the dispersed phase) can be formed with high density. Moreover, the strength of the Al—Zn—Mg—Cu-based 7000-based Al alloy can be further improved by aging precipitation hardening of the dispersed phase. Furthermore, by rapidly solidifying the Al alloy melt, crystallization and segregation of the alloy elements can be suppressed, and as much as possible can be dissolved in Al (the alloy element solid solution range is greatly expanded to the high concentration side). From this aspect, the strength of the Al alloy can be further improved.

  In the rapid solidification method, as described above, it is important to reduce oxygen or not increase oxygen. In order to reduce oxygen, whether by the spray forming method or the atomized powder method (quenched powder metallurgy method), as a premise, the atomizing gas does not use air, such as nitrogen, Ar or He, Use an inert atomizing gas. When air is used as the atomizing gas, the strength can be increased, but oxygen cannot be reduced as in the present invention, and the elongation cannot be improved. In addition, when nitrogen is used as a spray gas, nitrogen (N) is contained in the Al alloy during the spraying process, so that in addition to high strength and high ductility, the toughness of the Al alloy can be further improved. it can. When nitrogen is used as the atomizing gas, the amount of nitrogen contained in the Al alloy is generally in the range of 0.0005 to 0.01% by mass, although it depends on the gas / metal ratio (G / M ratio) described later. is there. Nitrogen contained in the Al alloy is finely precipitated as AlN. In the Al alloy manufacturing process such as deaeration, solution treatment, and artificial aging heat treatment, the Al alloy crystal grains are prevented from coarsening and fine crystallized. It is assumed that the grain structure is improved and the toughness of the Al alloy is improved.

Atomized powder method:
When the Al alloy of the present invention is manufactured by the atomized powder method (rapidly cooled powder metallurgy method) which is one of the rapid solidification methods, the atomized powder method itself can be manufactured according to a conventional method. For example, an Al alloy having the composition according to the present invention is melted and discharged at a temperature of 800 to 1100 ° C. in a high-frequency melting furnace. This Al alloy molten metal is poured into a crucible and guided from the opening at the bottom of the crucible to the molten metal outlet of the atomizing nozzle for atomization.

  Therefore, immediately before the Al alloy molten metal reaches the atomized nozzle molten metal outlet, an inert spray gas such as high-pressure nitrogen, Ar, or He is ejected from the nozzle hole, and the pressure of this gas comes out of the molten metal outlet. Finely pulverize the molten Al alloy. The molten metal finely pulverized in this way is immediately cooled and solidified by a high-pressure gas and / or atmosphere to obtain an Al alloy rapidly solidified powder.

  The atomized Al alloy powder is sieved according to the application. At this time, it is preferable to classify and use fine particles having an average particle size of 150 μm or less, preferably 100 μm or less. The Al alloy of the present invention can be easily obtained by solidifying and molding only such fine particles by CIP or HIP. Coarse atomized powder having an average particle size exceeding 20 μm has a slow cooling rate, so that a solid solution amount of Cu, Ag or the like cannot be secured, and if used, strength may not be improved.

Spray forming method:
When obtaining the alloy of the present invention, the spray forming method is more preferable than the atomizing powder method (quenching powder metallurgy method). The spray forming method, which is one of the rapid solidification methods, has the same mechanism as the atomized powder method in that gas is ejected and sprayed by the pressure of the gas. However, in the atomized powder method, even when an inert atomizing gas is used during atomization, the powder must be handled in the atmosphere, and oxygen in the Al alloy tends to increase due to oxidation. In contrast, in the spray forming method, even if the handling is performed in the atmosphere, a preform body having a certain density is obtained, and it is difficult to oxidize and oxygen in the Al alloy hardly increases.

  The spray forming method is advantageous over the atomized powder method in that a preform body having a certain density can be obtained and preliminary solidification molding with CIP or HIP is unnecessary. The atomized powder needs to be preliminarily solidified with CIP or HIP before solidifying. Furthermore, the spray forming method has an advantage that the structure (intermetallic compound phase) can be made finer because the cooling solidification rate can be increased as compared with the atomized powder method.

  However, even in this spray forming method, the spray gas to be used is an inert spray gas such as high-pressure nitrogen, Ar, or He, and the spray gas containing oxygen such as air is used to increase oxygen in the Al alloy. Do not use. It is also necessary to optimize the cooling and solidification rate. In a preferred embodiment by the spray forming method, the Al alloy having the above-described component composition of the present invention is melted at 800 to 1100 ° C., and spraying with the inert gas of the molten metal is started in this temperature range by the spray forming method. A preform body is produced on the floor.

(G / M ratio)
The cooling and solidification rate in spray forming (during the spray process) is first controlled by the gas / metal ratio [G / M ratio: the amount of gas blown onto the molten metal per unit mass (kg) (Nm ³ )]. In the present invention, the higher the G / M ratio, the faster the cooling rate, the finer intermetallic compound can be obtained, and a predetermined amount of Cu and Ag can be dissolved in the metal Al matrix.

  If this G / M ratio is too low, the cooling and solidification rate is insufficient. For this reason, the intermetallic compound by an alloy element also becomes coarse, and intensity | strength is insufficient. On the other hand, if the G / M ratio is too high, the yield of the preform (melting efficiency of the molten metal) decreases, the amount of inert gas used increases, and the manufacturing cost increases. In addition, alloy elements of the 7000 series Al alloy are easily segregated, and crystal precipitates generated in the 7000 series Al alloy structure during the manufacturing process are likely to be coarsened. As a result, the produced 7000 series Al alloy structure contains coarse crystal precipitates exceeding several μm, which is a starting point of fracture, and dynamic mechanical properties such as fatigue strength are lowered, and structural parts and The reliability of the Al alloy as a member is lowered. Moreover, elongation is reduced and the moldability in cold working is also inhibited.

The G / M ratio that satisfies these conditions is preferably in the range of 4 to 15 Nm ³ / kg. The lower limit of G / M ratio is preferably 4 Nm ³ / kg or more, more preferably 6 Nm ³ / kg or more, the upper limit of the G / M ratio, 15 Nm ³ / kg or less, preferably to less 13 Nm ³ / kg Is recommended.

(Deposition rate)
Further, spray forming can be performed in such a range that the deposition rate of the sprayed molten metal on the preform is in the range of ¹ to 2.5 g / mm ² / s by the inert gas having such a G / M ratio range. preferable. The “deposition rate” defined in the present invention is a rate at which the sprayed molten metal is deposited as a preform, and is deposited per unit area / unit time of a portion where the deposition of the preform is progressing. It is expressed by the weight (g / mm ² / s) of the Al alloy. When this deposition rate exceeds 2.5 g / mm ² / s, the molten metal deposited one after another makes it difficult to cool the previously deposited molten metal and the preform, and the cooling rate is significantly reduced. As a result, the alloy elements of the 7000 series Al alloy are easily segregated, and the crystal precipitates generated in the 7000 series Al alloy structure during the manufacturing process are likely to be coarsened. Therefore, the produced 7000 series Al alloy structure includes coarse crystal precipitates exceeding several μm, and the maximum size of the crystal precipitates of this Al alloy structure is 0.5 μm or less. I can't. For this reason, dynamic mechanical characteristics such as fatigue strength are lowered, and the reliability of the Al alloy as a structural component or member is lowered. Moreover, elongation is reduced and the moldability in cold working is also inhibited.

On the other hand, when the deposition rate is less than 1 g / mm ² / s, the efficiency of spray forming is reduced, and the yield of the preform (the weight of the Al alloy obtained as the preform is divided by the weight of the dissolved Al alloy). The calculated melt deposition efficiency is reduced, the amount of inert gas used is increased, and the production cost is increased.

(Solidification, densification)
The Al alloy preform obtained by the spray forming method under such conditions can have a porosity of, for example, about 10% by volume. Incidentally, since the preform has a high porosity and lacks toughness, it is necessary to deaerate the preform or to solidify the preform so that the pores of the preform are crushed and densified.

  As the solidification method, it is preferable to place the preform in a metal container such as Al and vacuum-enclose it, and then extrude it hot to solidify (densify). In this case, in order to prevent oxidation of the Al alloy and maintain a low oxygen state, the preform body is not directly hot-worked, but a metal container such as pure aluminum or an appropriate aluminum alloy. It is preferable to perform hot working after putting in a vacuum.

  In addition, the preform body and the powder obtained by the rapid powder metallurgy method may be solidified (densified) by forging, rolling, or hot working appropriately combined with extrusion, forging, and rolling.

  At this time, before the hot working, the preform body and the powder obtained above are sealed in a vacuum container, etc. and subjected to CIP or HIP treatment to solidify (vacancy, pore crushing), You may densify beforehand (preliminarily). However, HIP treatment or the like exposes the Al alloy (preform body) to a high temperature for a long time, so that the intermetallic compound is easily coarsened. For this reason, as described above, it is preferable not to perform preliminary densification processing such as HIP processing in the preform body by the spray forming method.

  It is preferable that the processing temperature in the hot processing of the forging, extrusion, and rolling described above is relatively low in the range of 425 to 500 ° C. When hot working in such a working temperature range, the intermetallic compound including the Al-based intermetallic compound phase is refined and uniformly dispersed. If the processing temperature in the hot processing is too high, the intermetallic compound becomes coarse. On the other hand, if the processing temperature is too low, densification by hot processing cannot be achieved.

  For the same purpose, the processing rate in these hot workings is made as large as possible. In the case of hot extrusion, the extrusion ratio is 6 or more, preferably 8 or more, more preferably 10 or more. In the case of hot rolling or hot forging, the processing rate is 70% or more. If the extrusion ratio and the processing rate are too small, it is highly possible that densification by hot working cannot be achieved.

  The solidified (densified) Al alloy after the hot working is further subjected to a solution treatment at about 480 to 520 ° C. for about 2 to 8 hours and an age hardening treatment at about 100 to 150 ° C. for about 10 to 50 hours. A treatment (tempering treatment) is performed to obtain an Al-Zn-Mg-Cu-based 7000-series product Al alloy (material for parts, members, etc.), which is an Al alloy of the present invention.

  This product Al alloy is formed into a desired member or part shape by cold rolling such as rolling according to the use such as automobile parts, terminal machines for electronic materials, precision machine parts, etc. Or parts.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Spray forming is performed on Al-Zn-Mg-Cu-based 7000-series Al alloy melts having the component compositions shown in Table 1 below. As shown in Table 2 below, the type of spray gas, G / M ratio, deposition rate, etc. The effect of the Al alloy on the mechanical properties and fatigue strength was investigated and evaluated by changing the spray conditions to control and change the size of crystal precipitates in the finally obtained Al alloy structure.

  Specifically, the melting temperature of 1000 ° C. is commonly used for the molten Al alloy of each component composition from A to T shown in Table 1 below (A to M are composition examples of the invention and N to T are comparative example compositions). Dissolved and spray-formed.

  Each preform body obtained in this manner is not subjected to preliminary densification treatment such as HIP treatment, but is placed in an aluminum container and vacuum-sealed, and then the processing temperature is 460 ° C. and the extrusion ratio is 15. Then, it was directly hot-extruded and solidified to obtain a 10 mmφ round bar. Thereafter, this Al alloy round bar is also commonly treated by solution treatment at 500 ° C. for 5 hours, and then subjected to T6 treatment (tempering treatment) for age-hardening treatment at 125 ° C. for 30 hours. -Zn-Mg-Cu based 7000 series product Al alloy was obtained.

  Test specimens were collected from the obtained Al alloys, and the sizes of crystal precipitates contained in these Al alloys were investigated, and the mechanical properties and fatigue strength of these Al alloys were as follows. investigated. These results are shown in Tables 2 and 3, respectively.

(Crystal precipitate)
The Al alloy structure of the test piece was observed in 50 fields of view with an SEM (scanning electron microscope) with a magnification of 5000 by the reflection electron method described above, and each crystal precipitate observed in the field of view was image-processed to obtain individual sizes. Table 2 and Table 3 show the maximum size of the converted size (μm) as the maximum dimension.

(Strength, elongation)
In each case, a room temperature tensile test in the extrusion direction (longitudinal direction of the test piece) of a round bar test piece of 8.0 mmφ and 90 mm length obtained by cutting the obtained Al alloy was performed, and the tensile strength (MPa). The total elongation (%) was measured. The room temperature tensile test was performed at room temperature of 20 ° C. based on JIS Z2241 (1980). The tensile speed was 5 mm / min, and the test was performed at a constant speed until the test piece broke.

(Fatigue strength)
In each example, a 8.0 mmφ, 90 mm long round bar obtained by cutting the Al alloy obtained above was provided with a 20 mm length at the center and a taper (each 5 mm long) from both sides. The test piece was cut to a small diameter of 5.0 mmφ. When this specimen is rotated by 10 ⁷ rotations while bending the specimen (loading stress) with the Ono rotary bending tester, which is widely used for evaluating the durability of materials, the minimum value that does not break. Stress (MPa) was defined as fatigue strength.

  As is apparent from Tables 1 and 2, each of Invention Examples 1 to 18 uses Al alloys (molten metal) A to M of the present invention composition, except for Invention Examples 16 and 19, and a preferable G / M ratio by nitrogen gas, Spray forming is performed under spray conditions such as deposition rate.

  As a result, in each of Invention Examples 1 to 18, the maximum size of the crystal precipitates contained in the Al alloy is 0.5 μm or less. Therefore, as the fatigue strength of the Al alloy, the minimum stress that does not break is 350 MPa or more, which is remarkably improved to nearly 50% (about 48%) of the tensile strength. In addition, each of Invention Examples 1 to 18 has a tensile strength of 700 MPa or more as mechanical properties at room temperature, and has an elongation of 15% or more when the tensile strength is 700 MPa or more and less than 800 MPa. At the same time, when the tensile strength is 800 MPa or more, it has an elongation of 10% or more. Therefore, the strength and fatigue strength required for the structural material, or the cold workability to the structural material can be satisfied.

  However, among the inventive examples, the inventive example 14 with a relatively low G / M ratio during spray forming and the inventive example 18 with a relatively high deposition rate are compared with other inventive examples in terms of fatigue strength. Low. Inventive Examples 16 and 19 are also excellent in fatigue strength and mechanical properties at room temperature, but Inventive Example 16 is too high when the G / M ratio is preferable, and Inventive Example 19 is a condition where the deposition rate is preferable. Therefore, it is too slow and productivity is low.

  On the other hand, Comparative Examples 20 to 26 use Alloys N to T in Table 1 whose alloy composition is out of the range. Among these, the comparative example 20 uses the alloy N from which Zn falls outside the lower limit. Comparative Example 22 uses an alloy P in which Mg falls below the lower limit. In Comparative Example 24, an alloy R in which Cu falls below the lower limit is used. Moreover, the comparative example 21 uses the alloy O from which Zn remove | deviates from an upper limit. In Comparative Example 23, an alloy Q in which Mg deviates from the upper limit is used. Comparative Example 25 uses an alloy S in which Cu is out of the upper limit.

  For this reason, these comparative examples are manufactured by a preferable manufacturing method, and although the maximum size of the crystal precipitates contained in the Al alloy is 0.5 μm or less, the fatigue strength and mechanical properties at room temperature are inferior. Yes. That is, although the fatigue strength is close to 50% of the tensile strength, the tensile strength itself is less than 700 MPa and the fatigue strength is too low, less than 340 MPa. Therefore, these comparative examples do not satisfy the strength and fatigue strength required as a structural material.

  Moreover, the comparative example 26 uses the alloy T whose total content of Fe and Mn is too much even if it makes it contain positively. For this reason, although Comparative Example 26 is manufactured by a preferable manufacturing method, the maximum size of crystal precipitates contained in the Al alloy exceeds 0.5 μm due to the action of Fe and Mn. The mechanical properties of are inferior.

  Comparative Example 27 uses Alloy A in Table 1 whose alloy composition is within the range, but the G / M ratio is too low. For this reason, in Comparative Example 27, the maximum size of the crystal precipitates contained in the Al alloy exceeds 0.5 μm, and the fatigue strength and mechanical properties at room temperature are inferior.

  Comparative Example 28 uses Alloy A in Table 1 whose alloy composition is within the range, but the deposition rate is too fast. For this reason, in Comparative Example 28, the maximum size of the crystal precipitates contained in the Al alloy exceeds 0.5 μm and the tensile strength is high, but the fatigue strength is inferior.

  Comparative Example 29 uses Alloy A in Table 1 whose alloy composition is within the range, but uses air as the atomizing gas. For this reason, in Comparative Example 29, the conditions during spray forming are within a preferable range, and the maximum size of the crystal precipitates contained in the Al alloy is 0.5 μm or less. Is inferior.

  From the above results, the critical significance of the requirements and preferred requirements of the present invention for the Al-Zn-Mg-Cu-based 7000-based Al alloy to satisfy high strength, high ductility, and high fatigue strength is demonstrated. It is supported.

  As described above, the present invention is an Al-Zn-Mg-Cu-based 7000-series Al alloy obtained by solidifying a rapidly solidified powder or a preform body, and has high ductility for high strength and fatigue. It is possible to provide an Al alloy having high strength and excellent reliability as a structural component or member and a method for producing the same. Therefore, depending on its application, it is molded into a desired shape with high ductility and cold processed, etc., and is made into desired members and parts with high strength. Automotive parts, terminal machines for electronic materials, Suitable for precision machine parts.

Claims (4)

  An Al alloy obtained by a rapid solidification method and containing, by mass%, Zn: 5 to 12%, Mg: 2 to 4%, Cu: 1 to 2%, with the balance being Al and inevitable impurities The maximum size of each crystal precipitate observed in an SEM with a magnification of 5,000 times by the backscattered electron method of this Al alloy structure is 0.5 μm or less in terms of the area converted into an equivalent circle diameter. A high fatigue strength Al alloy characterized by   The high fatigue strength Al alloy according to claim 1, wherein the Al alloy further contains 0.01 to 0.1 mass% of Ag.   The Al alloy further contains 0.1 to 0.5% by mass in total of one or more selected from Si, Fe, Mn, Cr, Co, Ni, Zr, Ti and V. Or the high fatigue strength Al alloy according to 2; In mass%, Zn: 5 to 12%, Mg: 2 to 4%, Cu: 1 to 2%, respectively, and further Ag is 0.01 to 0.1% by mass, Si, Fe, Mn, Cr , Co, Ni, Zr, Ti and V selected from a total of 0.1 to 0.5% by mass, with the balance being Al and unavoidable impurities. With an inert gas having a G / M ratio in the range of ⁴ to 15 Nm ³ / kg, the deposition rate of the sprayed molten metal on the preform is in the range of ¹ to 2.5 g / mm ² / s. After forming, the preform body thus obtained is placed in a metal container and vacuum-sealed, and then hot-extruded and solidified, then tempered to obtain an Al alloy, and the reflected electrons of this Al alloy structure Crystallization observed by SEM with a magnification of 5000 times Things of, among area size in terms of the diameter of a circle equivalent, the method of producing a high fatigue strength Al alloy, characterized in that the maximum size and 0.5μm or less.