JP2012132054A - Aluminum alloy casting and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum alloy casting formed of a fine structure having little casting defect and having excellent mechanical properties.SOLUTION: The aluminum alloy casting includes, assuming the whole as 100 mass%, 9-13 mass% Si, 1-5 mass% Cu, and the balance being Al and inevitable impurities. The aluminum alloy casting is constituted by a complex eutectic structure in a reticular form including Al-Si eutectic grains consisting of binary eutectic of Al and Si and a multi-eutectic matrix surrounding the Al-Si eutectic grains and consisting of multi-eutectic including Al, Si, and Cu, wherein the particle size of the Al-Si eutectic grains is as small as 1.5 mm or less. The aluminum alloy casting is mechanically excellent, so that it is constituted by the complex eutectic structure solidified into a glue-like state while being consisting of the alloy composition which is normally solidified in a manner of forming a skin, thereby inhibiting the occurrence of a large casting defect.

Description

  The present invention relates to an aluminum alloy casting having an Al-Si eutectic composition and a method for producing the same.

  In recent years, the demand for weight reduction has become stronger, and aluminum alloy castings (hereinafter simply referred to as “castings”) are used for a wide variety of members. Such castings are required to have few casting defects such as cast holes, shrinkage, and cracks, and have stable mechanical properties. Incidentally, a casting defect is caused by a molten metal with reduced fluidity not turning to the details of the mold or by solidification shrinkage when the molten metal changes from a liquid phase to a solid phase. Moreover, the mechanical properties (for example, strength, toughness, etc.) of castings are reduced due to uneven distribution of metal composition, structure, and the like due to casting defects.

  A casting method (a method for producing an aluminum alloy casting) for obtaining a casting with few casting defects and stable mechanical properties has been proposed in the following patent documents.

JP 2005-89827 A JP 2007-216239 A

Yoshiaki Osawa, Akira Sato: Foundry Engineering, 72 (2000), 733-738. "Refining of solidification structure by ultrasonic vibration"

  Patent Document 1 describes an aluminum alloy casting that has an Al—Si eutectic region composition and has a metal structure in which binary (Al—Si) eutectic grains are dispersed in a eutectic matrix, and a method for producing the same. ing. However, the Al—Si eutectic grains are relatively coarse with a grain size of about 2 to 3 mm.

  Patent Document 2 describes a casting method in which ultrasonic vibration is applied to an aluminum alloy molten metal having a liquidus temperature or higher to increase the number of embryos that are buds of crystal nuclei and to refine crystallized materials. However, Patent Document 2 is directed to a hypereutectic aluminum alloy casting having a considerably large amount of Si, and not a eutectic aluminum alloy (for example, JIS ADC12). Further, the formed metal structure is not a solidified solid.

  In Non-Patent Document 1, ultrasonic vibration is applied to a molten aluminum alloy having a hypoeutectic composition (Al-6% Si) or a hypereutectic composition (Al-18% Si) to refine the primary crystal. There is a statement to aim. This non-patent document 1 is not intended for an aluminum alloy casting having a eutectic composition, and the formed metal structure is not solidified in a saddle shape.

  The present invention has been made in view of such circumstances. That is, an object of the present invention is to provide an aluminum alloy casting that has a composition near the Al—Si eutectic and has few casting defects and excellent mechanical properties, and a method for producing the same.

  As a result of extensive research and trial and error, the present inventor applied ultrasonic vibration to a molten metal having an Al—Si eutectic composition before crystallization of primary Al (α-Al). Thus, the present inventors succeeded in obtaining an aluminum alloy casting made of a network-like metal structure (composite eutectic structure) in which fine Al—Si eutectic grains are dispersed in a multi-element eutectic matrix. By developing this result, the present invention described below has been completed.

《Aluminum alloy castings》
(1) The aluminum alloy casting of the present invention is 9 to 13% by mass of silicon (Si), 1 to 5% by mass of copper (Cu), and the balance of aluminum when the whole is 100% by mass. Al—Si eutectic grains composed of (Al) and inevitable impurities and / or modifying elements, and a binary eutectic of Al and Si, and surrounding the Al—Si eutectic grains, contain Al, Si, and Cu It has a network-like composite eutectic structure composed of a multi-element eutectic matrix composed of multi-element eutectic, and the Al—Si eutectic grains have a particle size of 1.5 mm or less.

(2) The aluminum alloy casting of the present invention (simply referred to as “casting” as appropriate) has a composition in the vicinity of an Al—Si eutectic (this is referred to as “eutectic composition”), so-called skin. It is not a formed solidified metal structure, but a cage-like solidified metal structure. Specifically, the Al—Si eutectic grains are composed of a complex eutectic structure dispersed in a network-like multi-element eutectic matrix. Moreover, the Al—Si eutectic grains in this composite eutectic structure are very fine with a particle diameter of 1.5 mm or less.

  The casting of the present invention comprising this composite eutectic structure has few large casting defects and is excellent in mechanical properties. This is considered to be due to the following reasons. In the case of the casting of the present invention, even if casting defects such as shrinkage nests and gas nests occur, these casting defects are dispersed in the Al-Si eutectic grain boundaries (that is, in or near the multi-eutectic eutectic matrix). It comes to exist. For this reason, casting defects do not grow into large defects that are three-dimensionally connected. In addition, the casting of the present invention is excellent in mechanical properties because the solidified structure is fine and homogeneous as a whole and there is little compositional or structural uneven distribution. Of course, since the casting of the present invention has a eutectic composition, it is also superior in terms of strength, wear resistance, heat resistance, etc., compared to a casting having a hypoeutectic composition.

  Note that the Al—Si eutectic grains referred to in the present invention are not apparent in the form of needles or dendrites, and the details thereof are not limited as long as they are granular. The multi-element eutectic matrix contains at least Al, Si, and Cu, but may contain other elements, and a compound other than the eutectic may be crystallized or precipitated.

<< Production Method of Aluminum Alloy Castings >>
(1) The present invention can be grasped not only as an aluminum alloy casting but also as a manufacturing method thereof. That is, the present invention provides a casting of an aluminum alloy comprising a melt preparation step of preparing a molten aluminum alloy, an injection step of injecting the molten metal into a mold, and a solidification step of cooling and solidifying the injected molten metal. A method for producing an aluminum alloy casting, characterized in that the molten metal preparation step includes a vibration step of applying ultrasonic vibration to the molten metal before crystallization of primary crystals, and the above-mentioned casting is obtained. It may be.

(2) The mechanism by which the casting of the present invention as described above is obtained by this vibration process is not necessarily clear, but at present, it is considered as follows. When the molten metal before crystallization of the primary crystal is vibrated with ultrasonic waves, it is considered that very fine aluminum phosphide (AlP) serving as a solidification nucleus is uniformly dispersed in the molten metal. When the molten metal in this state is further cooled, the AlP serves as a nucleus and fine Al—Si eutectic grains begin to crystallize in a bowl shape. Thus, a solid-liquid coexistence state (semi-solid state or semi-molten state) in which a solid phase composed of Al—Si eutectic grains and the remaining liquid phase are mixed is formed. When the molten metal in that state is further cooled and solidified, a composite eutectic structure in which fine Al-Si eutectic grains are dispersed in a network-like multi-eutectic matrix is formed, and the casting of the present invention is obtained. It is thought.

  AlP is an inevitable impurity that is a very small amount of P generally contained in molten aluminum alloy combined with Al, and crystallizes in a small amount from the molten metal before crystallization of the primary crystal (α-Al). obtain.

<Others>
(1) With regard to Al—Si eutectic grains and multi-element eutectic matrix, “eutectic” as used in the present invention does not mean a so-called academically defined eutectic consisting of only one composition, and a person skilled in the art Any material that can be generally determined as eutectic by observation is sufficient. A composition in which a eutectic in this sense is formed is referred to as “eutectic composition” in this specification.

(2) The “particle diameter” of the Al—Si eutectic grains is determined by observing Al—Si eutectic grains present in a 20 mm × 20 mm measurement region arbitrarily extracted from the cut surface of the casting with an optical microscope. It is an average particle diameter obtained by image analysis. Here, the average particle diameter is an arithmetic average value of diameters obtained by assuming that the shape of each eutectic grain included in the measurement region is circular. Specifically, from the optical micrograph of the mirror-polished casting cut surface, the part of Al-Si eutectic grains is extracted, the number and area of Al-Si eutectic grains contained in the measurement region are obtained, and the diameter The average particle diameter is obtained by calculating the arithmetic average value.

  The particle diameter of the Al—Si eutectic grains is preferably 1.5 mm or less, 1.4 mm or less, 1.3 mm or less, 1.2 mm or less, or 1.1 mm or less.

(3) “Inevitable impurities” are impurities contained in the raw material (mother alloy), impurities mixed in at each step, etc., and are elements that are difficult to remove due to cost or technical reasons. For example, there are magnesium (Mg), zinc (Zn), iron (Fe), manganese (Mn), nickel (Ni), tin (Sn), phosphorus (P), and the like.

  These elements are, for example, Mg: 0.5% by mass or less, further 0.3% by mass or less, Zn: 1.2% by mass or less, or 1% by mass when the entire aluminum alloy casting is 100% by mass. % Or less, Fe: 1.5 mass% or less, further 1.3 mass% or less, Mn: 0.7 mass% or less, further 0.5 mass% or less, Ni: 0.7 mass% or less, further 0.5 What is necessary is just to set it as the mass% or less, Sn: 0.5 mass% or less further 0.2 mass% or less, P: 0.1 mass% or less further 0.05 mass% or less.

  “Modifying element” is an element effective for improving the characteristics of an aluminum alloy casting. Properties that are improved include, for example, mechanical properties (such as strength, toughness, wear resistance or heat resistance), castability or microstructure refinement. The modifying element may exist in a state of being dissolved in eutectic grains or a eutectic matrix, or may be present by crystallization or precipitation by forming an intermetallic compound with Al, Si, or the like.

  Typical examples of the modifying element include Fe, Mg, Mn, and Cu, as well as strontium (Sr) and phosphorus (P) that are effective in improving the crystallization form and refinement of the metal structure of the casting. Further, there are also titanium (Ti) of 0.3% or less, further 0.15% or less, and sodium (Na) of 0.02% or less, further 0.01% or less.

  The modifying element and the inevitable impurities are not generally distinguished. For example, there are elements such as P that are originally inevitable impurities but contribute to the refinement of the complex eutectic structure.

(4) The “aluminum alloy casting” may be a final product, a basic material such as a bar, a pipe, or a plate, or an ingot as a casting raw material, regardless of the form.

(5) Unless otherwise specified, “x to y” in this specification includes a lower limit value x and an upper limit value y. A range such as “ab” can be arbitrarily configured by combining various numerical values and arbitrary numerical values included in the numerical range described in the present specification.

Sample No. 2 is an X-ray CT image photograph showing a cross section of a casting according to 1. FIG. It is a metal micrograph showing the microstructure. Sample No. It is a X-ray CT image photograph which shows the cross section of the casting concerning C1. It is a dispersion | distribution figure which shows the relationship between the temperature at the time of the completion | finish of the vibration of the molten metal by an ultrasonic wave (vibration completion temperature), and the particle size (eutectic particle size) of an Al-Si eutectic grain.

  The present invention will be described in more detail with reference to embodiments of the invention. In addition, the content demonstrated by this specification including the following embodiment can correspond not only to the aluminum alloy casting which concerns on this invention but the manufacturing method. Therefore, one or more configurations selected from the description in the present specification can be arbitrarily added to the configuration of the present invention described above. Under the present circumstances, if the structure regarding a manufacturing method is understood as a product by process, it can also become a structure regarding a thing (aluminum alloy casting). Which embodiment is the best depends on the target, required performance, and the like.

<Aluminum alloy composition>
(1) Si
Si is an element necessary for forming an Al—Si eutectic grain or a multi-element eutectic matrix. Si is preferably contained in an amount of 9 to 13% by mass, more preferably 10 to 12.5% by mass, based on 100% by mass of the entire casting. When Si is too small, the composition becomes hypoeutectic and primary Al (α-Al) is easily crystallized. If Si is excessive, the composition becomes hypereutectic and primary Si is easily crystallized. In either case, the composite eutectic structure referred to in the present invention is hardly formed. However, since the composition range referred to in the present invention is not a strict eutectic composition, primary Al (α-Al) or primary Si may be slightly scattered in the multi-element eutectic matrix.

(2) Cu
Cu can finely crystallize Al—Si eutectic grains and improve the mechanical properties of the casting by solid solution strengthening. Cu is preferably contained in an amount of 1 to 5% by mass, more preferably 1.5 to 3.5% by mass, based on 100% by mass of the entire casting. If the amount of Cu is too small, the effect is poor, and if the amount of Cu is excessive, granulation of the Al—Si eutectic grains can be hindered. In particular, when Cu is excessive, it is not preferable because even when Sr or the like is added, it does not cause saddle-like solidification but skin formation type solidification occurs.

(3) Sr
Sr promotes crystallization of Al—Si eutectic grains from the molten metal. That is, the solidification form of the casting is changed from a skin-forming mold to a bowl shape, and the formation of a complex eutectic structure is facilitated. Sr is 0.003 to 0.3% by mass (30 to 3000 ppm) and further 0.01 to 0.1% by mass (100 to 1000 ppm) when the entire casting is 100% by mass. It is preferable to include it in the molten metal. If the amount of Sr is too small, the effect is poor. If the amount of Sr is too large, the Al—Si eutectic does not crystallize in a granular form, resulting in a rough needle-like structure, which can lower the mechanical properties of the casting.

<< Production Method of Aluminum Alloy Castings >>
(1) Molten metal preparation process The molten metal preparation process of the present invention has an oscillating process of applying ultrasonic vibration to the molten metal before crystallization of the primary crystal when preparing a molten aluminum alloy. As a result, fine AlP that becomes solidification nuclei prior to the primary crystal is crystallized uniformly in the molten metal. As a result, Al—Si eutectic grains are easily crystallized in a bowl shape with AlP as a nucleus.

  Here, the type, characteristics, and the like of the ultrasonic wave applied in the vibration process are not limited. For example, the ultrasonic wave applied to the molten metal may be a longitudinal wave or a transverse wave. Further, as long as the ultrasonic vibration is propagated throughout the molten metal, the arrangement of the excitation source is not limited.

  However, the frequency of the ultrasonic wave is preferably 60 kHz or less, more preferably 40 kHz or less. When the frequency of the ultrasonic wave is excessive, the wavelength becomes short, and the vibration to the molten metal becomes insufficient. Further, the output of the ultrasonic wave is preferably 100 W or more, more preferably 150 W or more.

  The vibration process of the present invention is performed at a temperature higher than the temperature at which the primary crystal begins to crystallize (primary crystal start temperature), but when the vibration process ends at a temperature considerably higher than the primary crystal start temperature, the above-described effects are obtained. I don't get much. Therefore, in the vibration process, the vibration end temperature, which is the temperature of the molten metal at the end of vibration by ultrasonic vibration, is determined from the initial crystal start temperature (Ts), which is the temperature at which crystallization of the primary crystal starts. It is preferable that the temperature is 70 ° C. or even 60 ° C. higher than the crystal start temperature (Ts + 70 ° C. or Ts + 60 ° C.). More specifically, the excitation end temperature is preferably 590 to 640 ° C, more preferably 600 to 630 ° C.

(2) Injection process and solidification process The mold for injecting the molten metal may be a mold or a sand mold, and the injection of the molten metal may or may not be pressurized. The method of cooling the molten metal poured into the mold may be natural cooling or forced cooling.

  Accordingly, the production method of the present invention may be any of die casting, sand casting, die casting, low pressure casting, gravity casting and the like. However, since the aluminum alloy composition described above is similar to the ADC12 alloy (JIS) frequently used for die casting, the manufacturing method of the present invention is particularly suitable for the die casting method.

<Application>
(1) Various uses of the casting of the present invention are conceivable. For example, in the field of automobiles and motorcycles, engine members such as engine blocks and cylinder heads, body structural members, chassis members, wheels, space frames, steering wheels (core bars), seat frames, suspension members, transmission cases, pulleys, oils Pan, shift lever, instrument panel, door impact panel, intake surge tank, pedal bracket, front shroud panel, etc.

(2) The casting of the present invention exhibits excellent strength, wear resistance, heat resistance, etc. even in an as-cast state in which heat treatment is not performed after casting, but heat treatment may be performed after casting.

The present invention will be described more specifically with reference to examples.
<Production of sample>
(1) Molten metal preparation step JIS ADC12 alloy (manufactured by Fukuoka Aluminum Industry Co., Ltd./component composition: Al-11.26% Si-1.85% Cu-0.28% Mg-0.31 as a raw material (mother alloy) % Mn-0.84% Fe-0.85% Zn) was prepared. The unit (%) of the component composition is mass% when the entire target alloy is 100 mass% (the same applies hereinafter).

  This raw material was put in a No. 6 graphite crucible (manufactured by Nippon Crucible Co., Ltd./inner diameter φ100 mm × depth 140 mm) and melted at 750 ° C. in an electric furnace. Sr raw material (component composition: Al-10% Sr / manufactured by Foseco Japan Limited) was added to the molten Al alloy. Under the present circumstances, it prepared so that Sr amount might be 0.03% with respect to the whole Al alloy molten metal. This was held at 750 ° C. for 10 minutes in the electric furnace.

  A horn (material SUS304, φ20 × L170) connected to an ultrasonic vibration excitation apparatus (Sonopet 303S manufactured by Seidensha Electronics Co., Ltd.) was immersed in the upper center of the molten Al alloy taken out from the electric furnace. This horn had a tip end surface made of a flat cylinder, and the distance (immersion depth) from the surface of the Al alloy molten metal to the tip end surface of the horn was 10 mm.

  The molten metal was naturally cooled to 600 ° C. in the air at room temperature. During this cooling process, ultrasonic vibration was applied from the horn to the molten Al alloy (vibration process). The frequency of the ultrasonic vibration at this time was 28.5 kHz and the output was 250 W. In addition, the molten metal temperature (vibration start temperature) when the excitation by the ultrasonic vibration was started was set to about 740 ° C. in any case. On the other hand, as shown in Table 1, the molten metal temperature (excitation end temperature) when the excitation was finished was changed for each sample. For comparison, a molten metal that was not subjected to vibration by ultrasonic vibration was also prepared (Sample No. 8).

(2) Pouring process (injection process)
The molten Al alloy (melt temperature: about 600 ° C.) prepared as described above was poured into a shell cup for thermal analysis (SG cup-A, manufactured by Nakayama Co., Ltd., inner diameter φ30 × depth 50 mm) as a mold.

(3) Solidification step The shell cup was cooled in an air atmosphere to solidify the molten Al alloy. Coagulation began at 570 ° C. for all samples and was completed at 490 ° C.

<< Observation >>
(1) The cross section of the casting of each sample was observed by an X-ray CT (Computed Tomography) image. As an example, sample no. An X-ray CT image (photograph) of No. 1 is shown in FIG. In FIG. 1, the cell-like gray portion is an Al—Si eutectic grain made of Al—Si binary eutectic, and the white portion surrounding it is a multi-eutectic matrix made of multi-element eutectic. The black part to be performed is a casting defect (cast hole). The multi-element eutectic matrix appears white because the CuAl ₂ phase, which is a component of the matrix, hardly transmits X-rays.

  In the case of this casting, casting defects (casting holes) are small, isolated and dispersed, and have not developed into a continuous large casting hole. On the other hand, FIG. 3 shows an X-ray CT image (photograph) of a casting (sample No. C1) manufactured without adding Sr to the molten Al alloy in the molten metal preparation step. In this case, unlike the above-described casting, a large casting defect with a continuous casting cavity was generated, and it did not become a network-like metal structure (slag-like solidified structure).

(2) Sample No. A photograph obtained by microscopic observation of the metal structure (microstructure) of the casting of No. 1 is shown in FIG. As is apparent from this photograph, it can be seen that a network-like composite eutectic structure in which Al—Si eutectic grains are surrounded by a multi-element eutectic matrix is formed. Incidentally, the black portions in the photograph of FIG. 2 is a CuAl ₂ intermetallic compound.

<Measurement>
About each sample from which the vibration end temperature mentioned above differs, the optical microscope photograph was image | photographed, after mirror-polishing about the measurement site | part of 20 mm x 20 mm of sample longitudinal cross-section center part. This photograph was subjected to image analysis, and the average particle size was identified by measuring the number and total area of Al—Si eutectic grains contained in the measurement site. For image analysis, free software “ImageJ” manufactured by the National Institute of Health was used.

<Evaluation>
As is apparent from this result, it has been found that the size of the Al—Si eutectic grains formed in each casting varies greatly depending on the end temperature of vibration. Specifically, when the excitation end temperature is 650 ° C. or more, the effect of ultrasonic excitation is poor, the size of the eutectic grain size is around 2.5 mm, and the refinement of the composite eutectic structure is I could n’t.

  On the other hand, when the vibration end temperature was 640 ° C. or less, 635 ° C. or less, and further 630 ° C. or less, the eutectic particle size was rapidly reduced to 2 mm or less, 1.5 mm or less, or 1.2 mm or less. For this reason, applying ultrasonic vibration to the molten Al alloy in the temperature range close to the temperature at which the primary crystal begins to crystallize (primary crystal start temperature) is effective in refining the Al-Si eutectic grains and thus the composite eutectic structure. It was confirmed that

  Since the primary crystal start temperature (Ts) of the Al alloy melt used in this example is 570 ° C., if the excitation end temperature is within the range of Ts to Ts + 70 (° C.), the composite eutectic structure is fine. It can be said that ultrasonic vibration is effective for the conversion.

Claims (7)

When the total is 100% by mass, 9 to 13% by mass of silicon (Si), 1 to 5% by mass of copper (Cu), the balance aluminum (Al), unavoidable impurities and / or modifying elements And consist of
A network comprising Al-Si eutectic grains composed of binary eutectic of Al and Si and a multi-eutectic matrix composed of multi-eutectics surrounding Al-Si eutectic grains containing Al, Si and Cu Having a composite eutectic structure of
The aluminum alloy casting, wherein the Al-Si eutectic grains have a particle size of 1.5 mm or less.   The aluminum alloy casting according to claim 1, comprising 0.003 to 0.3% by mass of strontium (Sr) when the whole is 100% by mass.   The inevitable impurities or the modifying element is 0.5% by mass or less of magnesium (Mg), 1.2% by mass or less of zinc (Zn), and 1.5% by mass or less when the total is 100% by mass. Iron (Fe), 0.7 mass% or less manganese (Mn), 0.7 mass% or less nickel (Ni), 0.5 mass% or less tin (Sn), or 0.1 mass% or less phosphorus The aluminum alloy casting according to claim 1, wherein the casting is one or more of (P). A molten metal preparation step of preparing a molten aluminum alloy;
An injection step of injecting the molten metal into a mold;
A method for producing an aluminum alloy casting comprising a solidification step of cooling and solidifying the injected molten metal,
The molten metal preparation step has a vibration step of applying ultrasonic vibration to the molten metal before crystallization of primary crystals,
A method for producing an aluminum alloy casting, wherein the casting according to any one of claims 1 to 3 is obtained.   In the excitation step, an excitation end temperature, which is a temperature of the molten metal when the excitation by the ultrasonic vibration is ended, is determined from an initial crystal start temperature (Ts), which is a temperature at which the crystallization of the primary crystal starts. The method for producing an aluminum alloy casting according to claim 4, wherein the temperature is a temperature up to a temperature (Ts + 70: ° C.) 70 ° C. higher than the primary crystal starting temperature.   The method for producing an aluminum alloy casting according to claim 5, wherein the excitation end temperature is 590 to 640 ° C. 6.   The method for producing an aluminum alloy casting according to any one of claims 4 to 6, wherein the ultrasonic vibration applied in the vibration step has a frequency of 60 kHz or less.