EP2799165B1

EP2799165B1 - Method for molding aluminum alloy powder, and aluminum alloy member

Info

Publication number: EP2799165B1
Application number: EP13800173.0A
Authority: EP
Inventors: Hideaki Matsuoka; Mikio Kondoh
Original assignee: Toyota Central R&D Labs Inc
Current assignee: Toyota Central R&D Labs Inc
Priority date: 2012-06-08
Filing date: 2013-05-28
Publication date: 2016-06-29
Anticipated expiration: 2033-05-28
Also published as: JP2013256678A; WO2013183488A1; EP2799165A1; JP5772731B2; EP2799165A4

Description

[Technical Field]

The present invention relates to an aluminum alloy member comprising a formed body obtained through compression forming of an aluminum alloy powder, and also to a method for forming an aluminum alloy powder suitable therefor.

[Background Art]

To reduce the weight and enhance the performance of various members, aluminum alloy materials have been widely used as substitute for conventional steel/iron materials and cast iron materials because they are lightweight and have excellent practical strength. There are various methods for producing a member comprising such an aluminum alloy, among which powder-metallurgical method using aluminum alloy powder is a method which can produce a member having a complex shape at relatively low cost. According to this powder-metallurgical method, a product can directly be obtained to have a shape close to a final shape, so that the material yield can considerably be enhanced and the process cost required for shape creation (referred to as "creation cost) can drastically be suppressed.
In the above context, there is a variety of proposals for the powder-metallurgical method using aluminum alloy powder, which include descriptions relevant to the following patent literature (PTL), for example.

[Citation List]

[Patent Literature]

[PTL 1]
JP63-190102A
[PTL 2]
JP3-120301 A
[PTL 3]
JP4-365832A
[PTL 4]
JP7-197167A
[PTL 5]
JP7-224341A
[PTL 6]
JP2006-316312A
[PTL 7]
JP Patent No. 3845035

US 5,372,775 discloses a method of preparing an aluminum matrix particle composite alloy containing dispersed ceramic particles, the method comprising the following steps: (a) providing a molten metal of an aluminum alloy, containing said ceramic particles, (b) disintegrating, by atomization, said aluminum alloy molten metal containing said ceramic particles to prepare a powder of composite grains containing said particles being not more than 20 µm in mean particle diameter, and (c) warm-forming and solidifying said powder of composite grains without melting said composite grains, by powder forging said powder by annealing said powder at a temperature in a range between 200 °C and 450 °C, cold compression-molding said annealed powder to form an initial compact having a true density ratio of at least 70 percent, and warm-molding and compacting said initial compact at a temperature in a range between 400 °C and 550 °C to form a final compact having a true density ratio of at least 99 percent.

[Summary of Invention]

[Technical Problem]

However, the powder-metallurgical methods described in the above PTL are all such that the aluminum alloy powder is compressively molded to form a compressed powder body (or preform) and the compressed powder body thus obtained is heated to a high temperature which is not lower than a solidus temperature determined depending at least on the alloy composition. In other words, all of the above conventional powder-metallurgical methods are liquid-phase sintering methods in which liquid phases are formed at the surfaces (or interfaces) of powder particles so that the powder particles are bonded to one another.
According to such a liquid-phase sintering method, however, the compressed powder body may be held under a high temperature environment for a long period of time. Therefore, even though the powder particles constituting the compressed powder body originally had a rapidly-solidified fine structure, the fine structure may not be maintained, and the high properties possessed intrinsically by the powder particles may not be effectively exploited. In addition, when the liquid-phase sintering method is employed, an abnormal structure may be made such that a molten portion caused at the time of sintering is re-solidified, and this abnormal structure may appear locally between the powder particles (at the interface between the particles). This may result in an inhomogeneous structure of the liquid-phase sintered body, and the characteristics thus tend to fluctuate.
As described above, according to the conventional liquid-phase sintering method, the composition of aluminum alloy powder to be used may be limited; the structure of the obtained sintered body tends to be inhomogeneous; and heating may be necessary at a high temperature for a long period of time. It may thus be difficult to produce an aluminum alloy member of high properties at low cost.
It is to be noted that the sintering method itself generally includes, in addition to the liquid-phase sintering method, a solid-phase sintering method in which solid-phase diffusion is caused between contacting powder particles so that the powder particles are bonded to one another. However, the surfaces of powder particles of aluminum alloy are covered by oxide films which are significantly stable and strong up to a high temperature. Therefore, when aluminum alloy powder is used, it may actually be impossible to sinter the powder particles with one another in solid-phase without causing liquid phases at the surfaces of the powder particles (i.e., at a lower temperature than the solidus temperature). In this regard, liquid-phase sintering may simply be referred to as "sintering" in the present description unless otherwise stated.
Meanwhile, the section of the scope of claim(s) in the above PTL 2 includes a recitation that a formed body is obtained by performing compression forming of a preform "at a temperature that is not higher than a temperature at which sintering does not start." In addition, as apparent from the section of Examples in PTL 2, a preform is heated to 500 degrees C, for example, and the preform comprises an aluminum alloy powder having an alloy composition of Al-19.5%Si-5.3%Fe-4.9%Cu-1.2%Mg-0.3%Mn (wt%) (First Example).
However, the solidus temperature of an aluminum alloy having that alloy composition may be about 490 degrees C in an equilibrium state. If so, the recitation in the scope of claim(s) of PTL 2 appears to conflict with the description of Examples, and the preform may actually be heated at a temperature at which a liquid phase occurs in the preform or the formed body. This can be found from the fact that some dimensional change occurs in the preform of 500 degrees C described in the section of Examples in PTL 2 (Second Table). Therefore, it may be apparent that the "formed body" referred to in PTL 2 is actually a liquid-phase sintered body in which a liquid phase caused between the powder particles is re-solidified so that the powder particles are bonded one another. In the first place, PTL 2 is not to avoid the occurrence of liquid-phase sintering, and aims to suppress the coarsening of primary crystals Si included in the aluminum alloy powder particles.
The present invention has been created in view of such circumstances, and an object of the present invention is to provide a method for forming an aluminum alloy powder in which, unlike in the conventional liquid-phase sintering method or the like, the powder particles can be bonded (fixed) to one another in a solid phase state. Another object of the present invention is to provide an aluminum alloy member obtained therethrough.

[Solution to Problem]

As a result of intensive studies to solve the above problems, the present inventors have successfully obtained a formed body by forming an aluminum alloy powder at two different levels of pressures (or density ratios). In this formed body, the powder particles are metallically bonded to one another without causing a liquid phase between the powder particles (solid-phase bonding). Developing and generalizing this achievement, the present invention has been accomplished as will be described hereinafter.

«Method for forming aluminum alloy powder»

(1) The method for forming an aluminum alloy powder according to the present invention is characterized by the features according to claim 1.
(2) According to the method for forming an aluminum alloy powder (which may simply be referred to as "the forming method") of the present invention, a dense formed body can be obtained in which the powder particles (microparticles) constituting the aluminum alloy powder are metallically bonded to one another without causing a liquid phase at an interface between the particles (solid-phase bonding). This avoids the formation of a structure or the like which would be formed in the conventional liquid-phase sintered body such that a liquid phase caused at an interface between the particles is re-solidified. There can thus be stably obtained a formed body (aluminum alloy member) which is homogeneous as a whole and has high properties (strength, ductility and the like).
Moreover, since a liquid phase need not be caused between the powder particles, it is not necessary for the aluminum alloy powder to include low melting point elements (such as Si) and to be heated at a higher temperature than the solidus temperature for a long period of time. Thus, according to the forming method of the present invention, the alloy composition of the aluminum alloy powder can freely be selected depending on the required properties, and it is possible to effectively take advantage of the rapidly-solidified fine structure originally possessed by the powder particles. As will be understood, even when a complex shape is desired, a formed body having a near-net shape can be obtained, and it is possible to suppress working, such as cutting and grinding, thereby to significantly reduce the creation cost. In such a manner, according to the forming method of the present invention, even in a case of an aluminum alloy member which is required to have a complex shape and high properties, the aluminum alloy member can be efficiently produced at low cost.
(3) Incidentally, according to the forming method of the present invention, a formed body has been obtained in which the powder particles of aluminum alloy are metallically bonded to one another (solid-phase bonding), which would not have been realized heretofore. This mechanism is not necessarily sure, but it may be considered as follows.

First of all, the sparsely compact material obtained in the compacting step according to the present invention is such that the aluminum alloy powder is pressurized to be formed under a relatively low first pressure (P1). This sparsely compact material maintains its certain shape so that the powder particles (microparticles) of aluminum alloy are intertangled by plastic deformation, but in this state the microparticles merely abut and engage with one another via surface oxidation films, and a lot of spaces (gaps) remain between the microparticles. Such microparticles are still sufficiently allowed to cause relative displacement and/or plastic deformation due to external force applied.
Next, when the sparsely compact material comprising the microparticles in such a state is heated to a temperature not higher than the solidus temperature or to a temperature of 300 to 480 degrees C, and the second pressure (P2) higher than the above first pressure (P1) is applied in the forming step according to the present invention, the microparticles constituting the sparsely compact material may further be deformed to plastically flow. At this time, relative displacement may occur between the surfaces of the contacting microparticles (at the interface between the contacting microparticles), so that the thin oxidation films existing on the surfaces of the microparticles may be destroyed by physical (or mechanical) external force. This causes the abutting microparticles to be in a state in which newly-formed surfaces of aluminum alloy exposed due to the destruction of the oxide films are in contact with each other. Since the newly-formed surfaces of aluminum alloy are very active indeed, metallic bond occurs immediately between the contacting newly-formed surfaces, and the abutting microparticles are thus strongly bonded via the metallic bond. It can be considered that such bond between the microparticles occurs three-dimensionally, and a dense, homogeneous formed body can be obtained without a re-solidified phase or the like at the interface.
In addition, the phenomenon as described above can be utilized for various aluminum alloy powders because the phenomenon does not depend on the alloy composition of the microparticles and need not high temperature heating beyond the solidus temperature or the like. Therefore, it is possible to effectively take advantage of the rapidly-solidified structure originally possessed by the aluminum alloy powder. Moreover, it is not necessary to give a considerably large deformation as in an extrusion process and the like, and it may be enough if the second pressure is applied to the sparsely compact material to such an extent that the surface oxidation films on the microparticles can be destroyed. Thus, according to the forming method of the present invention, even in a case of a formed body (aluminum alloy member) having high properties and a complex shape, the formed body can be efficiently produced at low cost.

«Aluminum alloy member»

The present invention can be perceived not only as the above described forming method but as an aluminum alloy member. That is, the present invention can be understood as an aluminum alloy member characterized by comprising the formed body obtained through the above method for forming an aluminum alloy powder.

«Others»

(1) In the forming method of the present invention, respective steps may be continuously performed or otherwise intermittently performed (as in batch process).
(2) The "microparticles" according to the present invention are not limited in their shape and the like, but the particle diameter is 300 micrometers or less, and preferably 150 micrometers or less, as classification using sieves. In addition, the particle diameter of a large part of the microparticles may preferably be 5 micrometers or more, and more preferably 10 micrometers or more.
The formed body or the aluminum alloy member according to the present invention may preferably have a shape of a final product or a shape close to that of a final product (near-net shape), but may also be an intermediate product before a final product or a material thereof, thus the specific shape and the degree of working, etc. are not limited.
In the present invention, properties of the formed body or the aluminum alloy member obtained through the above-described forming method are not limited. However, the formed body or the aluminum alloy member may exhibit excellent mechanical properties, such as strength and ductility, because they have a homogeneous, dense structure in which constituent particles are metallically bonded. Furthermore, if the constituent particles are those subjected to precipitation strengthening by compound phases, there can be obtained not only excellent room temperature properties but also excellent high temperature properties (heat resistance).
(3) As described above, the "formed body" as referred to herein is in a state in which the constituent particles (microparticles) are bonded to one another, without causing a liquid phase at the interface therebetween, such that the newly-formed surfaces exposed due to the destruction of the surface oxide films are fixed (metallically bonded) to each other. In this regard, the formed body according to the present invention is different from the conventional liquid-phase sintered body in which the constituent particles are bonded to each other via liquid phases.
(4) Unless otherwise stated, a numerical range "x to y" as referred to herein includes the lower limit value x and the upper limit value y. Various numerical values or any numerical value included in numerical ranges described herein may be freely selected or extracted as a new lower limit value or upper limit value, and any numerical range such as "a to b" may thereby be newly provided using such a new lower limit value or upper limit value.

[Brief Description of Drawings]

[Fig. 1]
Fig. 1 is a graph illustrating a relationship between a first pressure in a compacting step and a density ratio of a sparsely compact material.
[Fig. 2]
Fig. 2 is graph illustrating a relationship between a pressurizing time in a forming step and a density ratio of a formed body.
[Fig. 3A]
Fig. 3A is a SEM photograph showing a fracture surface of a formed body.
[Fig. 3B]
Fig. 3B is an enlarged SEM photograph showing a part of the fracture surface.
[Fig. 4A]
Fig. 4A is a photograph of appearance showing one example of a sparsely compact material (preform).
[Fig. 4B]
Fig. 4B is a photograph of appearance showing one example of a formed body comprising the preform.
[Fig. 4C]
Fig. 4C is a photograph showing an appearance when a cross section of the formed body is color checked.
[Fig. 5]
Fig. 5 is a graph illustrating one example of a particle size distribution of aluminum alloy powder used in examples.

[Embodiments for Carrying out the Invention]

The present invention will be described in more detail with reference to embodiments of the invention. The content described herein may cover not only a method for forming an aluminum alloy powder but an aluminum alloy member obtained therethrough. Features regarding a manufacturing method, when understood as a product-by-process claim, may also be features regarding a product. One or more features freely selected from the description herein may be added to the above-described features of the present invention. Which embodiment is the best or not may be different in accordance with objectives, required performance and other factors.

«Compacting (roughly forming (molding)) step»

The compacting step is a step that applies a first pressure (P1) to an aluminum alloy powder to obtain a sparsely compact material (preform) in which spaces remain.

(1) The first pressure (P1) for performing the compacting step is not particularly limited, but this first pressure may be adjusted thereby to control an amount of remaining spaces in the sparsely compact material or a density ratio of the sparsely compact material.
If the first pressure is unduly low, the amount of spaces remaining between the powder particles is excessively large (the density ratio of the sparsely compact material is excessively small), so that an amount of oxygen involved in the sparsely compact material increases. This may lead to a risk that the newly-formed surfaces of the powder particles exposed at the time of the forming step are oxidized before being metallically bonded, thereby to inhibit the metal bond between the microparticles. In addition, when the first pressure is unduly low, damages may readily occur at corners of the sparsely compact material, etc. and the handling ability may thus deteriorate because the mechanical bond force between the powder particles will be weak.
If, in contrast, the first pressure is unduly high, the amount of spaces remaining between the powder particles is excessively small (the density ratio of the sparsely compact material is excessively large). This may result in that the relative displacement between the constituent particles is excessively small when the forming step is performed and that the spaces become closed pores to make the involved oxygen, etc. difficult to exit outside. Also in such cases, the metal bond promoted by the newly-formed surfaces may be inhibited. In fact, researches by the present inventors have revealed that, when the first pressure is unduly high (the density ratio of the sparsely compact material is unduly large), cracks tend to readily occur at an outer circumferential portion of the formed body.
Therefore, the first pressure may preferably be 100 to 650 MPa, more preferably 140 to 600 MPa, and most preferably 180 to 400 MPa. In addition, a sparsely compact material density ratio, which is a ratio of a bulk density of the sparsely compact material to a true density of the aluminum alloy, may preferably be 0.7 to 0.95, and more preferably 0.74 to 0.9.
(2) A temperature at which the compacting step is performed (first temperature /T1) is not limited. The compacting step may be performed at a temperature within a room temperature range or at a temperature within a warm temperature range. Note, however, that a uniform sparsely compact material having a desired density ratio may easily be obtained when the die for sparsely forming is heated to 200 degrees C or lower, or to 150 degrees C or lower, and the aluminum alloy powder is formed at a temperature within a warm temperature range. It is also to be noted that, although depending on the properties of a form release agent to be used for the compacting step, if the aluminum alloy powder is excessively heated, then adhesion, etc. may readily occur between the aluminum alloy powder and the cavity surface of a die for the sparsely forming thereby to result in increase in the extracting force, decrease in the lifetime of a die, and the like, thus being undesirable.

«Forming step»

The forming step is a step that applies a second pressure (P2) to the sparsely compact material to obtain a formed body in which the microparticles constituting the sparsely compact material are metallically bonded to one another.

(1) The second pressure for performing the forming step may preferably be a pressure determined such that the pressure ratio of the second pressure to the first pressure (P2/P1) is 1.2 or more, more preferably 1.4 or more, and most preferably 1.8 or more. If this pressure ratio is unduly small, the relative displacement between the constituent particles of the sparsely compact material (plastic flow) will be excessively small, so that the metal bond promoted by the exposure of the newly-formed surfaces may not be facilitated. The upper limit of the pressure ratio may be high, but the pressure ratio may preferably be 6 or less, and more preferably 5 or less, in consideration of the production efficiency, the lifetime of a die for forming, and the like.
Specific numerical value of the second pressure is not limited, but may preferably be 500 to 1000 MPa, and more preferably 550 to 800 MPa. A formed body density ratio, which is a ratio of a bulk density of the formed body to the true density of the aluminum alloy, may preferably be 0.97 or more, more preferably 0.98 or more, and most preferably 0.99 or more. It is thus preferable that the formed body has a formed body density closer to the true density (the formed body density is 1).
(2) The forming step is performed at a forming temperature at which no liquid phase occurs at the surfaces of the microparticles constituting the sparsely compact material. This forming temperature is set at a temperature lower than the solidus temperature of the aluminum alloy. By performing the forming step in which the sparsely compact material is heated to such a temperature (hot-forming step), the powder particles constituting the sparsely compact material are softened so that the surface oxidation films are easily destroyed to expose the newly-formed surfaces and the metallic bond between the newly-formed surfaces is facilitated. Moreover, a dense formed body according to the present invention can be obtained even when the second pressure is relatively low. Furthermore, when the forming step is performed at a temperature higher than a solution treatment temperature at which compound phases (strengthening phases) form solid solution in the microparticles or at a precipitation temperature at which such compound phases precipitate, the forming step can function also as a heat treatment.
Specific forming temperature differs depending on the alloy composition of the aluminum alloy powder. For example, when using an aluminum alloy powder having an alloy composition as will be described later (solidus temperature: 600 to 650 degrees C), the forming temperature may preferably be 350 to 480 degrees C, and more preferably 380 to 465 degrees C. If the forming temperature is unduly low, the precipitation strengthening will be insufficient and/or the forming efficiency will deteriorate, thus being undesirable. If the forming temperature is unduly high, the metallic structure (precipitated phases or crystal grains) of the formed body will be coarsened and/or the lifetime of a die for forming will be shortened owing to adhesion or the like, thus being undesirable. Note that the forming temperature according to the present invention is a temperature of a die. It is preferable that the temperature of the sparsely compact material is a temperature comparable to the forming temperature (a temperature within a range of -100 degrees C to 0 degrees C with respect to the forming temperature).
(3) Pressurizing time
A time for applying the second pressure to the sparsely compact material heated to a desired forming temperature (pressurizing time) may preferably be 3 to 30 seconds, and more preferably 5 to 20 seconds. If the pressurizing time is unduly short, the plastic flow of the constituent particles of the sparsely compact material will be insufficient, and it may not be possible to facilitate the metallic bond and/or densify the formed body. If, in contrast, the pressurizing time is unduly long, the high temperature state of the formed body will continue for a long time so that the precipitated phases or the like may be coarsened.

«Degassing step»

(1) After the compacting step and before the forming step, a degassing step may be performed for the sparsely compact material, as necessary, thereby to remove gasses such as oxygen and water involved in the sparsely compact material. This allows the metallic bond between the powder particles to be facilitated in the forming step, and there can be stably obtained a formed body and therefore an aluminum alloy member of high properties.
(2) The degassing step may be performed, for example, such that the sparsely compact material is placed in a vacuum atmosphere or in an inert gas (such as N₂ and Ar) atmosphere. In order to facilitate the degassing from the sparsely compact material, the sparsely compact material and/or the processing atmosphere may be heated. The heating temperature is lower than the solidus temperature of the aluminum alloy, and may preferably be equal to a solution treatment temperature or an aging temperature of the aluminum alloy, which are determined depending on the alloy composition. For example, in a case of the alloy composition as will be described later, the heating temperature may be 350 to 480 degrees C, and the heating time may be 0.5 to 2 hours.

«Aluminum alloy powder»

By the aluminum alloy powder comprising the following alloy composition according to the present invention there can be obtained a formed body and an aluminum alloy Kabushiki Kaisha Toyota Chuo Kenkyusho
member which are excellent not only in the strength and the ductility but also in the heat resistance, even without performing a heat treatment.
That is, the aluminum alloy according to the present invention has an alloy composition comprising, when whole thereof is assumed to be 100 mass% (referred simply to as "%", hereinafter), iron (Fe): 2-7%, zirconium (Zr): 0.6-1.5%, titanium (Ti): 0.5-1%, and optionally magnesium (Mg): 0.5-2.2%, and modifying elements including Cr, Mn, Co, Ni, Sc, Y, Ca, V, Hf and Nb, the balance being aluminum (Al) and inevitable impurities.

(1) Fe
Fe is an element that enhances the strength, the hardness and the like of aluminum alloy. When the whole of the aluminum alloy is assumed to be 100 mass% (this phrase will be omitted hereinafter), the content of Fe may preferably be 2-7%, more preferably 2.5-6.5%, and most preferably 3-6%. If the content of Fe is unduly small, there cannot be obtained sufficient strength and/or hardness, whereas, if the content of Fe is unduly large, the ductility may deteriorate and the strength may be excessively high so that the formability and/or the workability will be poor.
(2) Zr and Ti
Zr and Ti cooperate with Al to form a second compound phase that enhances the heat resistance of aluminum alloy. The content of Zr may preferably be 0.6-1.5%, more preferably 0.7-1.3%, and most preferably 0.8-1.2%. The content of Ti may preferably be 0.5-1%, and more preferably 0.7-0.9%. In the above cases, it may further be preferable that the mass ratio of Zr and Ti (Zr/Ti) is 1.1 to 1.5, or 1.15 to 1.4, because there can be formed a second compound phase that is stable up to a high temperature range.
If the content of Zr or Ti is unduly small, the above effect will not be obtained. If the content of Zr or Ti is unduly large, the melting temperature will be considerably high thereby to increase the production cost for the aluminum alloy powder, and a coarse crystallized substance or precipitated substance will be formed with Al, thus being undesirable. When the mass ratio Zr/Ti is unduly small or unduly large, the formation of a desired second compound phase will be difficult and a sufficient strength property cannot be obtained.
(3) Mg
Mg is an element that is effective to enhance the strength (in particular, room temperature strength) of aluminum alloy. The content of Mg may preferably be 0.5-2.2%, more preferably 1-2%, and most preferably 1.2-1.8%. If the content of Mg is unduly small, the above effect will not be obtained, whereas, if the content of Mg is unduly large, the formability of the aluminum alloy powder may deteriorate.
(4) The above modifying elements are elements, other than Al, Fe, Zr, Ti and Mg, which are effective to enhance the properties of aluminum alloy. Properties to be enhanced may be such as, but are not limited to, the strength, hardness, toughness, ductility and dimensional stability at a temperature within a high temperature range or room temperature range. Specific examples of such modifying elements include chromium (Cr), manganese (Mn), cobalt (Co), nickel (Ni), scandium (Sc), yttrium (Y), lanthanum (La), vanadium (V), hafnium (Hf), and niobium (Nb). Compounding or the like of each element may be freely performed, but the content thereof may be extremely small amount in general.
The inevitable impurities are impurities or the like, such as impurities contained originally in the raw material to be molten and impurities mixed during each step, which may be elements that are difficult to be removed for the cost or technical reason or other reasons. In the case of the above alloy composition, Si (silicon) or the like represents the inevitable impurities.
(5) Even in a case of being exposed for long time to an atmosphere of a high temperature such as 300 degrees C or higher or 400 degrees C, the formed body or the like comprising the aluminum alloy powder having the above alloy composition can exhibit excellent properties such as high strength and hardness, and deterioration in the strength and/or hardness due to thermal history may also be very small. Rather, heating may enhance the strength and/or hardness of the formed body or the like in contrast.

The mechanism that such excellent heat resistance as described above can be developed is not necessarily sure, but it may be considered at present as follows. First, an appropriate amount of Fe cooperates with Al to form an intermetallic compound (Al-Fe-based intermetallic compound: first compound phase) in a mother phase (alpha-Al phase). This first compound phase enhances the strength and/or hardness of the aluminum alloy. Here, this first compound phase may not necessarily be thermally stable, and phase transformation and/or shape variation (coarsening), etc. may occur if the first compound phase is exposed to a high temperature atmosphere for long time.
Next, an appropriate amount of Zr and Ti cooperates with Al to form an Al-(Zr, Ti)-based intermetallic compound (second compound phase) of L1₂-type structure. This intermetallic compound may be formed in the mother phase such that Zr and Ti having formed supersaturated solid solution in the mother phase precipitate in an ultrafine form (e.g., average size is about 1 to 30 nm) such as when the aluminum alloy is heated.
The second compound phase, which is a commensurate phase that is commensurate with the mother phase, may appear in the vicinity of a boundary (interface) between the Al-Fe-based intermetallic compound and the mother phase and may be stable up to a high temperature range. Accordingly, the second compound phase is unlikely to cause phase transformation and/or coarsening at least at a temperature not higher than the temperature at which the precipitation starts.
Therefore, the first compound phase may be responsible for the strength and/or hardness of the aluminum alloy, while on the other hand, the second compound phase, which is present in the vicinity of a site at which the first compound phase is in contact with the mother phase, may operate to suppress the phase transformation and/or shape variation (performs so-called pinning operation) at the time of high temperature. In other words, the properties such as strength exhibited by the first compound phase can be maintained up to a high temperature range by the second compound phase. It is thus considered that the first compound phase and the second compound phase operate synergistically thereby to allow the aluminum alloy member or the like comprising the above alloy composition to exhibit excellent heat resistance, which would not be expected by the conventional technique.
Meanwhile, it has also been found that the second compound phase has a nanoparticle-like shape in which the concentration of Zr is high at the central part while the concentration of Ti is high at the outer part. In other words, it has been found that each concentration of Zr and Ti in Al₃(Zr, Ti) has a gradient from the central part to the outer part. It is important for the formation of the second compound phase that Zr exists much more than Ti and the mass ratio of Zr to Ti (Zr/Ti) is within a predetermined range.
Further, in order for the second compound phase to be finely dispersed in the mother phase in the vicinity of the boundary with the first compound phase, it is also important that Zr and Ti form sufficient solid solution (supersaturated solid solution) and are precipitated afterward. Specifically, it may be necessary that, after rapid solidification is conducted to cause an appropriate amount of Zr and Ti to form supersaturated solid solution, some energy is imparted to generate a driving force for facilitating the precipitation. Examples of such energy include thermal energy applied such as by heat treatment and hot working, and strain energy applied such as by plastic working. For example, according to the forming step in the present invention, thermal energy and strain energy can be applied at the same time to accelerate the precipitation of the second compound phase, and there can thus be efficiently obtained a formed body or the like comprising the heat resistant, high strength aluminum alloy.
As is known in the art, atomizing method or the like may be employed to obtain aluminum alloy powder comprising particles in a state in which Zr and Ti form supersaturated solid state in an Al base. In the atomizing method, molten alloy comprising the above-described alloy composition may be rapidly solidified at a cooling rate not less than 300 degrees C per second. When the forming step according to the present invention is performed for the sparsely compact material comprising such aluminum alloy powder, a formed body (aluminum alloy member) having excellent heat resistance can easily be obtained such that not only the first compound phases but also a considerable number of ultrafine second compound phases are precipitated in the mother phase. In this case, it is not necessary to perform an aging treatment or the like which requires a long period of time for the precipitation of the second compound phases (non-heat treatment-type), and a formed body (aluminum alloy member) of high properties can thus be obtained efficiently at low cost. As will be understood, the second compound phases may also be precipitated using some heat treatment (e.g., aging treatment) and the like.

«Aluminum alloy member»

The aluminum alloy member comprising the formed body according to the present invention is not limited in its use application or the like, but may be suitable for members, such as a member having a complex shape, for which high properties (such as mechanical property and heat resistance) are required. Examples of such members include high strength members, such as a piston, inlet valve and con rod of an internal-combustion engine; a rotor (impeller) of a supercharger; a bladed wheel and piston of a compressor; screws; and an underbody component, shift fork and synchronizer ring of a car, which have been manufactured such as by forge processing and metallic forming casting and which have complex shapes and are to be used under an environment of high temperature or high load. Note that the aluminum alloy member of the present invention may be widely utilized not only as a member to be used at a temperature within a high temperature range but as other members such as a high strength member for which weight saving is required.

Examples

Examples will be mentioned to more specifically describe the present invention. «Production of samples»

(1) Preparing step for raw powder

Molten metal of aluminum alloy comprising each of various alloy compositions listed in Table 1A and Table 1B (referred collectively to as "Table 1") was prepared. The molten alloy was atomized in vacuum atmosphere, and air atomized powder (aluminum alloy powder) was thus obtained. The obtained air atomized powder was classified using a sieve to have a particle diameter of 106 micrometers or less and then used as raw powder. Fig. 5 shows one example of a particle size distribution of the raw powder after the classification (Sample 15 shown in Table 1A).
There is known a relationship between the size of powder particles (microparticles) obtained by air atomizing and the cooling rate, and it is found that the above atomized powder comprises particles which were rapidly solidified at a cooling rate of 10⁴ degrees C or more per second. Note that commercially available non-heat treatment-type aluminum alloy (JIS A5052) was used as Sample 31. The alloy composition thereof is aboutAl-2.6Mg-0.2Cr (mass%). Samples 30 and 31 are comparative examples.

(2) Compacting step

The cavity of a die heated to 150 degrees C was filled with the atomized powder, and compression forming was performed at each of various first pressures (P1) listed in Table 1. In such a manner, a cylindrical preform (sparsely compact material) having a diameter of 30 mm, 35 mm or 39 mm was obtained (see Fig. 4A). The relative density of each preform (sparsely compact material density ratio) is also listed in Table 1. Each relative density is a value obtained through dividing a bulk density (rho) by a true density (rho0) which is obtained from each aluminum alloy composition, wherein the bulk density (rho) is obtained through dividing a weight of the preform by its volume. Note that the above compression forming was performed using a die warm compaction method in which lithium stearate (lubricant) was sprayed to the inner surface of the heated die before the die was filled with the atomized powder. Details of this die warm compaction method are described in Patent No. 3309970 (International Publication WO01/43900 ).

(3) Degassing step

The following degassing step was performed for some of preforms. In this degassing step, each preform was put into a heating furnace and held for 1 hour in nitrogen gas at an atmosphere temperature listed in Table 1. At that time, the nitrogen gas flow rate in the furnace was 10 L/min.
During the degassing step, swelling such as blister did not occur in each preform. Moreover, change in dimension also did not occur at all in any preform before and after the degassing step. From these results, it can be said that a liquid phase was not caused between powder particles in each preform when heating was performed at a temperature of about 350 to 450 degrees C. This applies to the forming step to be described below.

(4) Forming step

Hot forming was performed such that second pressures (P2) listed in Table 1 were applied to respective preforms after the degassing step for a predetermined pressurizing time using Hot Die Coining (HDC). Before this hot forming, each preform was preliminarily reheated to each heating temperature for sparsely compact material listed in Table 1. In addition, at least a part of the die (die and punch) to be in contact with the preform was caused to have a die temperature listed in Table 1. Further, molybdenum disulfide (lubricant) was applied to the die surface to be in contact with the preform.
In such a manner, there was obtained each formed body (aluminum alloy member) protruding like a circular truncated cone from a base portion having a diameter of 40 mm (see Fig. 4B). Except for Sample C3, any of samples was a dense formed body having a formed body density ratio of 0.999 or more obtained in the same manner as that for the preform. Note that the formed body of Sample C3 had a formed body density ratio of 0.989.

«Measurement of samples»

Tensile test in conformity with JIS Z2241 was performed using a tensile test piece cut out from each sample. From the obtained stress-strain diagram (SS diagram), 0.2% proof stress, breaking strength and breaking elongation of each sample were obtained. Results thereof are also listed in Table 1.

«Observation of samples»

Fig. 3A is a photograph (SEM image) obtained by observing a fracture surface of the tensile test piece of Sample 15 using a scanning-type electron microscope (SEM), and Fig. 3B is an enlarged photograph of a part thereof.

«Evaluation of samples»

(1) Sparsely compact material density ratio

The same atomized powder as that for Sample 15 was used to make sparsely compact materials (preforms) in which the first pressure in the above-described compacting step was changed to various pressures. The density ratio of each sparsely compact material was obtained using the above-described method. Fig. 1 illustrates a relationship between the obtained sparsely compact material density ratio and the first pressure.
As apparent from Fig. 1, it has been confirmed that the first pressure and the sparsely compact material density ratio are in a relationship of monotonic increase. It has also been found that the sparsely compact material density ratio can be 0.65 to 0.95 when the first pressure is 100 to 650 MPa. In particular, it has been found that, when the first pressure is about 150 to 400 MPa, a sparsely compact material can be obtained which has a density ratio of about 0.7 to 0.87 and which is thus suitable for the forming method of the present invention. In addition, it has also been confirmed that the increase in the sparsely compact material density ratio is very small even when the first pressure is increased above 650 MPa.

(2) Formed body density ratio

The same atomized powder as that for Sample 15 was used to obtain preforms (sparsely compact material density ratio: 0.756) for which the above-described compacting step (first pressure: 196 MPa) and a degassing step (450 degrees C × 1 hour) were performed. These preforms were used to make formed bodies for which the pressurizing time at the time of the forming step was changed to various time periods. The density ratio of each formed body was obtained using the above-described method. Fig. 2 illustrates a relationship between the obtained formed body density ratio and the pressurizing time. Note that the second pressure in the forming step was 588 MPa or 784 MPa, and both the sparsely compact material heating temperature and the die temperature were 450 degrees C.
As apparent from Fig. 2, it has been confirmed that, regardless of the second pressure, the formed body density ratio is around 0.990 when the pressurizing time is 3 seconds; the formed body density ratio is 0.999 when the pressurizing time is 5 seconds; and the formed body density ratio is approximately 1 when the pressurizing time is 10 seconds or more, thus a dense formed body having substantially the true density is obtained.

(3) Mechanical properties

The followings are found from the mechanical properties of the formed bodies as listed in Table 1. Note that symbols shown in "determination" section of Table 1 are as follows:

AA: breaking elongation is 2% or more;
A: breaking elongation is 2% or less;
B: breaking elongation is 2% or more but breaking strength is relatively slightly low;
C: breaking elongation is 2% or less and breaking strength is relatively low;
D: breaking occurs when plastic yield starts; and
E: breaking occurs when elastic deformation occurs.

First, as apparent from Sample 1 to Sample 31, it is found that, even though the alloy composition of the aluminum alloy powder changes to various compositions, formed bodies having excellent mechanical properties of high strength (0.2% proof stress and breaking strength) and high ductility (breaking elongation) can be obtained when a forming step is performed such that the pressure ratio (P2/P1) is 1.5 to 4; the forming temperature (sparsely compact material heating temperature and die temperature) is 400 to 450 degrees C (lower than solidus temperature); and the pressurizing time is 7 to 30 seconds, for each low-density sparsely compact material having a sparsely formed density ratio of 0.93 or less. Note that the aluminum alloy powder used in the present examples is a rapidly solidified powder (air atomized powder) obtained such that the alloy elements are forced to form solid solution so that no substantial segregation occurs in the constituent particles. Therefore, the solidus temperature of the aluminum alloy powder is substantially the same as the solidus temperature as referred to in the equilibrium diagram. In this regard, the "solidus temperature" as used herein may be defined as the solidus temperature in the equilibrium diagram. Note that the solidus temperature is 450 degrees C even in a case of Al-Mg binary system of which the solidus temperature is lowest. Therefore, any forming temperature lower than 450 degrees C is lower than the solidus temperature of all the samples.
Next, the followings are found by comparing Samples C1 to C6 and Samples 9 to 24 which all have the same alloy composition of aluminum alloy powder. First, it is found that the strength and/or ductility of the formed bodies are considerably deteriorated when the pressure ratio (P2/P1) is unduly small, i.e., 1 or less, as in the cases of Samples C1 and C2. It appears that this is because the constituent particles of the sparsely compact material cannot sufficiently be metallically bonded to one another since the first pressure is high so that the density of the sparsely compact material is already high and the constituent particles are less likely to move such as by plastic flow even when the second pressure is applied in the forming step.
When the pressurizing time in the forming step is unduly short, i.e., 3 seconds, as in Sample C3, the strength and/or ductility of the formed body are also considerably deteriorated, and the formed body is to break during the elastic deformation like Sample C2. It appears that this is because the formed body density ratio is insufficient as seen from the previously-described results of Fig. 2 so that breaking easily occurs from spaces as points of origin remaining in the formed body.
In contrast, when the pressurizing time in the forming step is unduly long as in Sample C4, sufficient strength cannot be obtained. It appears that this is because the metallic structure (such as precipitated phases) is coarsened even though a dense formed body can be obtained in which the constituent particles may sufficiently be metallically bonded to one another.
Further, when the forming temperature is unduly low as in Sample C5, there occurs a state (underaged state) in which precipitation and the like of the compound phases to be the strengthening phases are insufficient. Also in this case, a formed body of sufficient strength cannot be obtained. On the other hand, when the forming temperature is unduly high as in Sample C6, the precipitated compound phases (precipitated phases) and the like may be coarsened like in the case in which the pressurizing time is unduly long. Also in this case, a formed body of sufficient strength cannot be obtained.
Meanwhile, when the sparsely compact material heating temperature is 530 degrees C (higher than the solidus temperature) as in Sample C7, the formed body (which may substantially be a liquid-phase sintered body) breaks when elastic deformation occurs, and the strength is also considerably low. It appears that this is because locally molten portions are caused at the surfaces and the like of the constituent particles during the forming step, and the molten portions are re-solidified to form an abnormal structure in the formed body (liquid-phase sintered body).

(4) Observation of cross sections

As apparent from Fig. 3A and Fig. 3B, in the sample according to the present invention, the fracture faces appear in the constituent particles rather than at grain boundaries of the constituent particles. This teaches that the formed body according to the present invention is such that the constituent particles are unified with one another via the metallic bond, and is homogeneous as a whole.

Fig. 4C shows an appearance when a cross section obtained by cutting the formed body of Sample 15 is color checked. This also teaches that a dense, homogeneous formed body can be obtained without cracks according to the forming method of the present invention.

[Table 1A]

Sample No.	Aluminum alloy powder		Production conditions								Mechanical properties
	Alloy composition (mass%)	Solidus temperature in equilibrium diagram (°c)	Compacting step		Degassing step	Forming step				Pressure ratio (P2/P1)	Formed body			Determination
	Alloy composition (mass%)	Solidus temperature in equilibrium diagram (°c)	First pressure P1 (MPa)	Sparsely compact material density ratio	Atmosphere temperature (°C)	Sparsely compact material heating	Die temperature (°C)	Second pressure P2 (MPa)	Pressurizing time (seconds)	Pressure ratio (P2/P1)	0.2% proof stress (MPa)	Breaking strength (MPa)	Breaking elongation (%)	Determination
1	Al-3Fe-1Zr -0.8Ti-1.5Mg	633	196	0.767	450	450	450	588	10	3	440	495	6.2	AA
2			392	0.881	450					1.5	435	490	4.8	AA
3			196	0.767	- (None)					3	439	498	5.0	AA
4			392	0.881	- (None)					1.5	440	490	4.0	AA
5	Al-5Fe-1Zr -0.8Ti-0.7Mg	642	196	0.762	450	450	450	588	10	3	455	510	4.5	AA
6			392	0.862	450					1.5	448	490	3.2	A
7			196	0.762	-					3	447	495	4.0	AA
8			392	0.862	-					1.5	442	490	2.8	A
9			196	0.744	400	400	400	784	10	4	478	520	4.8	A
10			196	0.744	-	400	400	784	10	4	470	510	3.6	A
11			196	0.758	450	450	450	588	7	3	482	530	4.0	AA
12			588	0.936	450					1.5	480	505	3.0	A
13			196	0.758	-					3	479	528	4.5	AA
14			588	0.936	-					1.5	477	510	3.3	A
15			196	0.744	450	450	450	784	10	4	481	545	4.5	AA
16	Al-5Fe-1Zr -0.8Ti-1.5Mg	628	392	0.862						2	478	518	3.5	A
17	Al-5Fe-1Zr -0.8Ti-1.5Mg	628	588	0.929						1.33	481	530	2.8	AA
18			196	0.744	-					4	480	520	4.0	A
19			392	0.862						2	475	535	3.0	AA
20			588	0.929						1.33	476	520	2.5	A
21			196	0.744	450	450	450	784	30	4	469	528	5.2	AA
22			588	0.929	450					1.33	465	521	4.5	A
23			196	0.744	-					4	465	529	4.8	AA
24			588	0.929	-					1.33	460	522	3.9	A

[Table 1B]

Sample No.	Aluminum alloy powder		Production conditions								Mechanical properties				Notes
	Alloy composition (mass%)	Solidus temperature in equilibrium diagram (°C)	Compacting step		Degassing step	Forming step				Pressure ratio (P2/P1)	Formed body			Determination
	Alloy composition (mass%)	Solidus temperature in equilibrium diagram (°C)	First pressure P1 (MPa)	Sparsely compact material density ratio	Atmosphere temperature (°C)	Sparsely compact material heating	Die temperature (°C)	Second pressure P2 (MPa)	Pressurizing time (seconds)	Pressure ratio (P2/P1)	0.2% proof stress (MPa)	Breaking strength (MPa)	Breaking elongation (%)	Determination
25	Al-6Fe-1Zr -0.8Ti-1.5Mg	628	196	0.726	450	450	450	784	10	4	505	560	2.5	AA
26	Al-6Fe-1Zr -0.8Ti-1.5Mg	628	196	0.726	-	450	450	784	10	4	490	548	3.0	A
27	Al-4Fe-1Zr -0.8Mg	640	196	0.804	450	450	450	784	10	4	435	465	4.0	AA
28	Al-4Fe-1Zr -0.8Ti-0.8Mg	640	196	0.763	450	450	450	784	10	4	445	495	4.5	AA
29	Al-2.5Fe-0.6Zr -0.2Ti-1.1 Co-0.8Mo	635	196	0.778	450	450	450	784	10	4	420	460	4.8	AA
30	Al-7.5Mg-2.5Fe -0.6Zr-0.2Ti-1.1Co -0.8Mo-0.5Cu	500	196	0.713	450	450	450	784	10	4	575	660	3.5	A
31	A5052 (JIS)	607	196	0.844	450	450	450	784	10	4	110	230	6.0	AA
C1			784	0.965	450	450	450	784	10	1	480	482	0.1	D	Pressure ratio (P2/P1) is unduly small
C2			784	0.965	450	450	450	588	10	0.75	-	390	(Unmeasurable)	E	Pressure ratio (P2/P1) is unduly small
C3	Al-5Fe-1Zr -0.8Ti-1.5Mg	628	196	0.744	450	450	450	588	3	4	-	420	-	E	Pressurizing time is unduly short
C4	Al-5Fe-1Zr -0.8Ti-1.5Mg	628	196	0.744	450	450	450	588	300	4	405	440	6.5	B	Pressurizing time is unduly long
C5			196	0.744	350	350	350	784	10	4	360	390	2.5	B	Forming temperature is unduly low
C6			196	0.744	-	500	500	784	10	4	390	430	7.0	B	Forming temperature is unduly high
C7	Al-12Si-5Cu-1.5Mg	510	196	0.795	-	530	530	784	10	4	-	278	-	E	Sparsely compact material heating temperature is higher than solidus temperature, i.e., unduly high

Claims

A method for forming an aluminum alloy powder, the method comprising:
a compacting step that applies a first pressure (P1) to an aluminum alloy powder to obtain a sparsely compact material, the aluminum alloy powder comprising microparticles of an aluminum alloy having a particle diameter of 300 micrometers or less, the sparsely compact material being such that the microparticles are intertangled and in contact with one another by plastic deformation while leaving spaces therebetween; and

a forming step that applies a second pressure (P2) to the sparsely compact material at a forming temperature to obtain a dense formed body in which the microparticles are metallically bonded to one another, the forming temperature being a temperature at which no liquid phase occurs at surfaces of the microparticles, a pressure ratio of the second pressure (P2) to the first pressure (P1) (P2/P1) being 1.2 or more,
wherein the aluminum alloy has an alloy composition comprising, when whole thereof is assumed to be 100 mass% (referred simply to as "%", hereinafter),
iron (Fe): 2-7%,
zirconium (Zr): 0.6-1.5%,
titanium (Ti): 0.5-1%, and optionally
magnesium (Mg): 0.5-2.2%, and
modifying elements including Cr, Mn, Co, Ni, Sc, Y, Ca, V, Hf and Nb, the balance being aluminum (Al) and inevitable impurities.
The method for forming an aluminum alloy powder as recited in claim 1, wherein:
the compacting step is a step in which a sparsely compact material density ratio is 0.7 to 0.95, the sparsely compact material density ratio being a ratio of a bulk density of the sparsely compact material to a true density of the aluminum alloy; and

the forming step is a step in which a formed body density ratio is 0.97 to 1, the formed body density ratio being a ratio of a bulk density of the formed body to the true density of the aluminum alloy.
The method for forming an aluminum alloy powder as recited in claim 1 or 2, wherein:
the first pressure is 100 to 650 MPa; and

the second pressure is 500 to 1000 MPa.
The method for forming an aluminum alloy powder as recited in any one of claims 1 to 3, wherein the forming temperature is lower than a solidus temperature of the aluminum alloy.
The method for forming an aluminum alloy powder as recited in claim 1 or 4, wherein the forming temperature is a precipitation temperature or higher, the precipitation temperature being a temperature at which a compound phase precipitates in the microparticles.
The method for forming an aluminum alloy powder as recited in any one of claims 1 to 5, wherein the forming step is a step in which a pressurizing time is 5 to 30 seconds, the pressurizing time being a time during which the second pressure is applied to the sparsely compact material at the forming temperature.
The method for forming an aluminum alloy powder as recited in any one of claims 1 to 6, further comprising a degassing step that removes gas involved in the sparsely compact material after the compacting step and before the forming step.
The method for forming an aluminum alloy powder as recited in any one of claims 1 to 7, wherein the forming temperature is 350-480 degrees C.
An aluminum alloy member comprising the formed body obtained through the method for forming an aluminum alloy powder as recited in any one of claims 1 to 8.