Aluminum alloy for low-pressure casting

Abstract

An aluminum alloy for casting, made of an Al—Si—Cu—Mg alloy which consists of specific amounts of Si, Cu, and Mg, in addition to specifically desired amounts of titanium, phosphorus, boron, and optional additional chemical elements sodium and strontium, with the balance of the aluminum alloy comprising aluminum and any impurities. When a content of phosphorus is defined as X mass %, the content of phosphorus, a content of Y mass % of sodium, and a content of Z mass % of strontium satisfy the following relationships: 0.45Y+0.24Z+0.003≤X≤0.45Y+0.24Z+0.01; 0≤Y≤0.01; and 0≤Z≤0.03. The aluminum alloy ensures surface smoothness of a cast article by specifying the phosphorus content. This minimizes a surface segregation layer, even in production of a cast article using a molten metal containing a eutectic structure modifier such as sodium.

Claims

1. An aluminum alloy for casting, comprising an Al—Si—Cu—Mg alloy, wherein the aluminum alloy consists of 8.0 to 12.6 mass % of Si; 1.0 to 2.5 mass % of Cu; 0.3 to 0.8 mass % of Mg; and 0.2 mass % or less of Ti, 0.003 to 0.01 mass % of P, 0.003 to 0.005 mass % of B, optional chemical elements Y mass % of Na and Z mass % of Sr, with the balance of the aluminum alloy being aluminum and any impurities, and wherein, when a content of P is defined as X mass %, the content of P, a content of Na, and a content of Sr satisfy all of the following relationships: 0.45Y+0.24Z+0.003≤X≤0.45Y+0.24Z+0.01; 0≤Y≤0.01; and 0≤Z≤0.03.

2. An aluminum-alloy cast article comprising the aluminum alloy for casting according to claim 1, wherein an area ratio of a shrinkage cavity defect having a depth of 20 μm or more on a surface of the aluminum-alloy cast article is 1% or less per 100 mm.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a shape of a plaster mold used in examples and an external shape of each aluminum-alloy casted article produced.

MODES FOR CARRYING OUT THE INVENTION

(2) As described above, the aluminum alloy for low-pressure casting according to the present invention contains: 8.0 to 12.6 mass % of Si; 1.0 to 2.5 mass % of Cu; 0.3 to 0.8 mass % of Mg; and 0.2 mass % or less of Ti. The aluminum alloy further contains X mass % of P, Y mass % of Na, and Z mass % of Sr, with the balance including Al and inevitable impurities. The content of P, the content of Na, and the content of Sr (X, Y, Z) satisfy all of the following relationships: 0.45Y+0.24Z+0.003≤X≤0.45Y+0.24Z+0.01; 0≤Y≤0.01; and 0≤Z≤0.03. An embodiment of the present invention will be described below. It is noted that the present invention will not be limited to the following embodiment, and it will be appreciated that the present invention may be practiced in various other embodiments without departing from the scope of the present invention. The following description gives a chemical composition of the aluminum alloy according to the present invention; an alloy casted article produced from this aluminum alloy; and a method for producing the alloy casted article.

(3) <Chemical Composition>

(4) First, names and contents of the alloy components of the aluminum alloy for low-pressure casting according to the present invention will be described, with reasons why the contents of the alloy components are thus limited.

(5) Si:

(6) The Si content is 8.0 to 12.6 mass %. At below 8.0 mass %, Si's fluidity degrades, causing molten metal mis-running. An Si content of 12.6 mass % or more is not preferable, either, in that a hyper-eutectic composition results, causing many coarse Si particles to crystallize and resulting in degraded strength. A preferable range of the Si content is 8.6 to 9.4 mass %.

(7) Cu:

(8) The Cu content is 1.0 to 2.5 mass %. Cu causes AlCu.sub.2 to deposit in aging process and thus increases the matrix strength. At less than 1.0 mass %, this effect weakens, while in excess of 2.5 mass %, an Al—Cu—Mg intermetallic compound and a Cu—Mg intermetallic compound crystallize, resulting in degraded strength. A preferable range of the Cu content is 1.5 to 2.0 mass %.

(9) Mg:

(10) The Mg content is 0.3 to 0.8 mass %. Mg deposits as Mg.sub.2Si in aging process and thus increases the matrix strength. If the Mg content is less than 0.3 weight %, the amount of Mg.sub.2Si to deposit in aging treatment is small, making Mg less influential for increased strength. In contrast, if the Mg content is in excess of 0.8 weight %, many Mg oxides occur at the molten metal holding time and the casting time, causing extension and fatigue strength to degrade.

(11) Ti:

(12) The Ti content is more than 0 mass % and 0.2 mass % or less. Ti is used to make crystal grains fine. If the Ti content is in excess of 0.2 mass %, a coarse TiAl.sub.3 compound is formed at the casting time, causing the strength of the final product to degrade.

(13) It is noted that in the present invention, not only Ti but also B may be contained, in the form of Ti—B. This increases the effect of making crystal grains fine. When Ti—B is contained, preferable ranges of Ti and B are respectively 0.1 to 0.2 mass % and 0.003 to 0.005 mass %. If the contents of Ti and B are less than lower limits of their respective ranges, that is, if the contents of Ti and B are respectively less than 0.1 mass % and less than 0.003 mass %, the capability of making crystal grains fine is insufficient. If the contents of Ti and B are respectively in excess of 0.2 mass % and 0.005 mass %, no more effect of making crystal grains fine can be obtained. In addition, the resulting compound may be coarse enough to cause degraded strength.

(14) P:

(15) As has been described hereinbefore, the present invention ensures surface smoothness of a casted article by specifying an appropriate range of the P content. P reacts to Al to form AlP, which serves as the nucleus of Si particle formation, including a eutectic Si phase. In this respect, in specifying the P content in accordance with the present invention, the inventors have determined 0.003 to 0.01 mass % as a reference P content that serves as a basis of forming effective AlP for inducing a eutectic Si phase.

(16) The P content range of reference values, 0.003 to 0.01 mass %, will be described. First, P's solid solubility limit in an aluminum alloy is 0.0003 mass %. That is, at 0.0003 mass % or less, P is entirely consumed in a solid solution with aluminum, becoming less influential in inducing a eutectic Si phase. In this case, the effects of the present invention are not expected. If the P content is in excess of 0.0003 mass % but less than 0.003 mass %, AlP can be formed but the number of nuclei of AlP is small, with AlP dispersed unpreferably. In this case, with small pieces of AlP roughly dispersed, the number of eutectic cells is at a level that has an adverse effect on the efficiency with which molten metal is supplied. This causes a surface segregation layer to be formed, inducing a local shrinkage cavity.

(17) According to the inventors, in order to sufficiently increase the effective nucleus count of AlP, 0.003 mass % or more of P is necessary. In this case, the amount of AlP formed is sufficient enough to increase the number of eutectic cells. This shortens the time before the subsolidus phase state is reached and causes a solidified shell to be formed earlier in the outer layer, making the surface of the casted article smooth, without a surface segregation. It should be noted, however, that this effect obtained when P is 0.003 mass % or more remains unchanged in excess of 0.01 mass %. In light of these findings, the inventors determined the range of 0.003 mass % or more and 0.01 mass % or less as a reference P content that serves as a basis of forming effective AlP for ensuring surface smoothness of the casted article.

(18) In the present invention, an approximate P content is set while the contents of the eutectic structure modifiers Na and/or Sr is taken into consideration. The chemical elements Na and Sr, which are contained in Al—Si alloys as eutectic structure modifier, are not always added intentionally in alloy production processes. That is, it is possible for Na and Sr derived from raw material to contaminate Al—Si alloys. Thus, Na and/or Sr get contained in alloys, especially in production of a wide variety of Al—Si alloy casted articles. In the present invention, the P content is set while the content of Na and/or Sr in an alloy taken into consideration, irrespective of whether Na and/or Sr have been intentionally added.

(19) As described above, Na and Sr react to P to form compounds (such as Na.sub.3P and Sr.sub.3P.sub.2). In light of this, in the aluminum alloy according to the present invention, the P content after reaction to Na or Sr needs to be set within the above-described reference P content range (0.003 mass % or more and 0.01 mass % or less).

(20) Specifically, the P content (X mass %) in the aluminum alloy according to the present invention relative to the Na content (Y mass %) and the Sr content (Z mass %) is 0.45Y+0.24Z+0.003≤X≤0.45Y+0.24Z+0.01. In this relational expression, coefficient 0.45 of the amount of Na (Y) and coefficient 0.24 of the amount of Sr (Z) are values calculated according to stoichiometric ratios of the compounds Na.sub.3P and Sr.sub.3P.sub.2, which are formed as a result of reaction to P. Also in the above relational expression, the amount of P (0.45Y+0.24Z) calculated based on the amount of Na (Y) and the amount of Sr (Z) indicates an amount of P cancellation caused by reaction to these eutectic structure modifiers.

(21) If the amount of P excluding the amount of P cancellation caused by the reactions to the eutectic structure modifiers is less than 0.003 mass %, AlP is roughly dispersed, resulting in a eutectic cell count that can adversely affect the efficiency with which molten metal is supplied. This causes a surface segregation layer to be formed, inducing a local shrinkage cavity. In contrast, if the amount of P excluding the amount of P cancellation caused by the chemical reactions to the eutectic structure modifiers is 0.003 mass % or more, the effective nucleus count of AlP increases sufficiently enough to increase the number of eutectic cells. This, as a result, shortens the time before the subsolidus phase state is reached and causes a solidified shell to be formed earlier in the outer layer. This prevents a shrinkage cavity from occurring, resulting in a smooth surface. The upper limit of the P content excluding the amount of P cancellation is 0.01 mass %. In excess of this upper limit, the effects of P remain unchanged. The above relational expression indicates these technically significant effects.

(22) As described later, the upper limit value of the amount of Na (Y) is 0.01 mass %, and the upper limit value of the amount of Sr (Z) is 0.03 mass %. With this point taken into consideration, in the present invention, all of the relationships≤Y≤0.01 and Z≤0.03 needs to be satisfied, in addition to the above relational expression being satisfied.

(23) Thus, the present invention is characterized by adjusting the P content based on whether the eutectic structure modifiers Na and/or Sr are added or not and based on how much they are contained. As described above, an Al—Si alloy is generally obtained by combining an aluminum base metal and an Al—Si mother alloy and dissolving the combination. In this manner, an alloy whose composition is adjusted as desired is obtained. Even though the aluminum base metal and the Al—Si mother alloy are combined and dissolved, there may be a deficiency in the P content. In light of the circumstances, it is preferable to adjust the P content by adding an appropriate amount of P in the alloy solution (for example, adding P in the form of Cu—P mother alloy).

(24) Modifier (Na, Sr):

(25) In the present invention, the eutectic structure modifiers Na and Sr are optional chemical elements. Therefore, at least one of the contents of Na and Sr may be 0 mass %. It should be noted, however, that at least one of Na and Sr may be contained. When at least one of Na and Sr is contained, it is preferable that the content of Na be 0.01 mass % or less, and the content of Sr be 0.03 mass % or less. These contents are added-amounts in general hypo-eutectic Al—Si alloys, and the present invention also employs these ranges of contents. Na and Sr react to P to respectively form Na.sub.3P and Sr.sub.3P.sub.2, and these compounds remain in the molten metal as impurities. In the present invention, a comparatively large amount of P is contained. Therefore, if the contents of Na and Sr are greatly varied, more impurities may possibly occur. More impurities cause mechanical properties such as fatigue strength to degrade. Also, as described above, excessive addition of Na and Sr serves as a factor that causes the fluidity of molten metal to degrade. In light of the circumstances, the general usage upper limits, Na: 0.01% and Sr: 0.03%, also apply in the present invention. Na and Sr may be added in the alloy by utilizing a molten metal containing modifiers, in particular, an aluminum alloy scrap in which modifiers are contained, as practiced in production sites. It should be noted, however, that the addition of the eutectic structure modifiers Na and/or Sr is optional, as described above.

(26) Other Chemical Elements:

(27) Other chemical elements than the above-described chemical elements may basically be Al and inevitable impurities. Still, other chemical elements than the above-described chemical elements added in the aluminum alloy are generally tolerated within ranges that will not greatly influence the characteristics and properties of the aluminum alloy.

(28) <Surface Quality of Aluminum-alloy Casted Article>

(29) The above-described aluminum alloy according to the present invention is suitable for production of aluminum-alloy casted articles by low-pressure casting methods. After casting, many of these casted articles are used without surface treatment and surface cutting. In light of the circumstances, such aluminum-alloy casted articles are preferably without a shrinkage cavity defect having a depth of 20 μm or more on the surfaces of the aluminum-alloy casted articles. Specifically, the area ratio of a shrinkage cavity having a depth of 20 μm or more on each of the surfaces is preferably 1% or less per 100 mm.sup.2. This is because if a shrinkage cavity on a surface of a casted article is in excess of 20 μm and extends inward, it is highly possible for the defect to develop into a crack, resulting in a broken casted article.

(30) <Method for Producing Aluminum-Alloy Casted Article>

(31) The aluminum alloy obtained in the present invention can be made into an aluminum-alloy casted article by being dissolved into a molten metal of a desired chemical composition and then being poured into a mold and formed into a desired shape.

(32) The molten metal that has been poured into the mold is cooled in a direction from a chill plate disposed above the mold toward the sprue of the mold. At the same time, the molten metal is applied an air pressure of more than 0 and 1 or less. Then, the formed article is subjected to solutionizing treatment, hardening, and artificial aging treatment. In this manner, the formed article is imparted a strength.

EXAMPLES

(33) In the following description, examples of the present invention will be described in comparison with comparative examples, so as to prove the effects of the present invention. These examples are provided as examples of one embodiment of the present invention and will not limit the present invention.

(34) In the examples, aluminum-alloy molten metals adjusted to chemical compositions listed in Table 1 were produced. Then, according to an aluminum-alloy molten metal low-pressure casting method, each molten metal at 750° C. was poured into a plaster mold of 200° C., and solidified using an iron chill plate of 200° C. In this manner, an aluminum-alloy casted article was obtained. FIG. 1 illustrates a shape of the plaster mold used here and an external shape of the aluminum-alloy casted article produced. Then, the aluminum casted article produced was evaluated in terms of surface structure and mechanical properties according to the following methods.

(35) TABLE-US-00001 TABLE 1 Composition (mass %) Si Cu Mg Ti B P Na Sr Al Example 1 8.10 1.80 0.50 0.12 0.0032 0.0033 — — Balance Example 2 12.50 1.50 0.60 0.10 0.0046 0.0052 — — Balance Example 3 9.20 1.00 0.80 0.15 0.0030 0.0044 — — Balance Example 4 9.00 2.40 0.40 0.18 0.0035 0.0071 — — Balance Example 5 8.40 1.60 0.30 0.16 0.0031 0.0069 — — Balance Example 6 9.30 1.20 0.80 0.15 0.0038 0.0037 — — Balance Example 7 9.00 1.10 0.60 0.02 0.0049 0.0058 — — Balance Example 8 8.70 1.70 0.50 0.20 0.0042 0.0049 — — Balance Example 9 10.20 1.90 0.50 0.14 0.0045 0.0032 — — Balance Example 10 11.60 1.60 0.70 0.15 0.0039 0.0217 0.010 0.030 Balance Example 11 9.10 1.90 0.60 0.12 0.0005 0.0037 — — Balance Example 12 9.30 1.10 0.50 0.10 0.0046 0.0051 0.002 — Balance Example 13 9.00 1.90 0.60 0.15 0.0035 0.0072 — 0.010 Balance Example 14 10.30 1.30 0.70 0.16 0.0033 0.0093 0.010 — Balance Example 15 9.70 1.80 0.50 0.18 0.0041 0.0127 — 0.030 Balance Example 16 11.10 1.50 0.40 0.11 0.0022 0.0198 0.010 0.030 Balance Comparative 5.00 1.30 0.60 0.12 0.0024 0.0036 — — Balance example 1 Comparative 15.00 1.50 0.70 0.15 0.0041 0.0082 — — Balance example 2 Comparative 9.50 0.50 0.50 0.12 0.0042 0.0071 — — Balance example 3 Comparative 8.80 3.52 0.60 0.19 0.0032 0.0048 — — Balance example 4 Comparative 8.20 1.90 0.20 0.11 0.0038 0.0033 — — Balance example 5 Comparative 10.30 1.30 1.20 0.08 0.0021 0.0058 — — Balance example 6 Comparative 11.40 1.50 0.80 0.23 0.0015 0.0097 — — Balance example 7 Comparative 9.60 1.60 0.40 0.13 0.0045 0.0011 — — Balance example 8 Comparative 9.10 1.40 0.70 0.1 0.0006 0.0015 0.002 — Balance example 9 Comparative 10.20 1.90 0.60 0.11 0.0019 0.0028 — 0.010 Balance example 10 Comparative 9.20 1.90 0.60 0.15 0.0031 0.0081 0.008 0.015 Balance example 11 Comparative 9.00 1.90 0.50 0.15 0.0032 0.0168 0.015 — Balance example 12 Comparative 9.10 1.10 0.50 0.12 0.0046 0.0196 — 0.040 Balance example 13
<Evaluation of Surface Structure>

(36) First, each casted article was evaluated as to whether there were surface defects on the surfaces of the casted article. Specifically, liquid penetrant testing was conducted according to JIS Z 2342 to check whether there was, throughout the surfaces of the casted article, a fluorescent point that had a depth of 20 μm or more and that extended inward from each surface. When there was a fluorescent point (shrinkage cavity), the area of the fluorescent point was measured and the area ratio per 100 mm.sup.2 was calculated. When the area ratio was in excess of 1%, the fluorescent point was determined as a surface defect.

(37) <Evaluation of Mechanical Properties>

(38) Mechanical properties, namely, tensile strength, proof strength, and extension were measured. In the measurement of these mechanical properties, a round bar tensile test piece specified by JIS Z 2201 was cut out of a center portion of each casted article, and the round bar tensile test piece was subjected to the measurement according to a JIS Z 2241 test method at room temperature. Then, the measured tensile strength, proof strength, and extension were checked as to whether they were equal to or more than values (tensile strength: 370 MPa, 0.2% proof strength: 270 MPa, and extension: 7% or more) measured from an Al—Si aluminum alloy for low-pressure casting that was produced according to a conventional technique that involved adding Na.

(39) Evaluation results of the surface structure and the mechanical properties of the aluminum casted articles produced in the examples are listed on Table 2.

(40) TABLE-US-00002 TABLE 2 P content [wt %] Lower limit value based on Surface TS [MPa] YS [MPa] EI [%] expressions Content defect Example 1 411 ∘ 308 ∘ 9.2 ∘ 0.0030 0.0033 None Example 2 380 ∘ 301 ∘ 7.6 ∘ 0.0052 None Example 3 372 ∘ 280 ∘ 9.9 ∘ 0.0044 None Example 4 423 ∘ 311 ∘ 7.2 ∘ 0.0071 None Example 5 381 ∘ 296 ∘ 8.8 ∘ 0.0069 None Example 6 401 ∘ 302 ∘ 9.6 ∘ 0.0037 None Example 7 397 ∘ 288 ∘ 8.3 ∘ 0.0058 None Example 8 408 ∘ 297 ∘ 10.0 ∘ 0.0049 None Example 9 403 ∘ 305 ∘ 9.2 ∘ 0.0032 None Example 10 401 ∘ 302 ∘ 8.1 ∘ 0.0147 0.0217 None Example 11 401 ∘ 302 ∘ 8.1 ∘ 0.0030 0.0037 None Example 12 372 ∘ 290 ∘ 7.6 ∘ 0.0039 0.0051 None Example 13 388 ∘ 288 ∘ 8.0 ∘ 0.0054 0.0072 None Example 14 415 ∘ 291 ∘ 7.5 ∘ 0.0075 0.0093 None Example 15 399 ∘ 276 ∘ 7.9 ∘ 0.0102 0.0127 None Example 16 405 ∘ 275 ∘ 7.7 ∘ 0.0147 0.0198 None Comparative 363 x 250 x 9.4 ∘ 0.0030 0.0036 Identified example 1 Comparative 345 x 210 x 1.3 x 0.0082 None example 2 Comparative 310 x 234 x 8.1 ∘ 0.0071 None example 3 Comparative 422 ∘ 283 ∘ 3.3 x 0.0048 None example 4 Comparative 351 x 280 ∘ 8.9 ∘ 0.0033 None example 5 Comparative 388 ∘ 278 ∘ 6.6 x 0.0058 None example 6 Comparative 381 ∘ 281 ∘ 4.2 x 0.0097 None example 7 Comparative 391 ∘ 304 ∘ 8.9 ∘ 0.0011 Identified example 8 Comparative 376 ∘ 281 ∘ 8.2 ∘ 0.0039 0.0015 Identified example 9 Comparative 395 ∘ 267 ∘ 8.8 ∘ 0.0054 0.0028 Identified example 10 Comparative 388 ∘ 314 ∘ 5.5 x 0.0102 0.0081 Identified example 11 Comparative 372 ∘ 274 ∘ 6.5 x 0.0098 0.0168 None example 12 Comparative 375 ∘ 277 ∘ 6.3 x 0.0126 0.0196 None example 13 TS (tensile strength): 370 MPa or more was accepted (∘). YS (0.2% proof strength): 270 MPa or more was accepted (∘). EI (extension): 7% or more was accepted (∘). Surface defect: A shrinkage cavity having a depth of 20 μm or more and having an area ratio over 1% was “identified” as a surface defect.

(41) Table 2 shows that in Example 1 through Example 16, the components Si, Cu, Mg, and Ti are within the respective ranges specified in the present invention. Also, the P content is appropriately adjusted. As a result, the aluminum-alloy casted articles of the examples had no defects of 20 μm or more on the surfaces of the aluminum-alloy casted articles, having satisfactory surface smoothness. Also, the mechanical properties, namely, tensile strength, proof strength, and extension satisfied the respective reference values.

(42) In contrast, in Comparative example 1 through Comparative example 7, the components Si, Cu, Mg, and Ti were outside their respective corresponding ranges specified in the present invention, and thus were inferior in the smoothness of the casted article surfaces or in the mechanical properties. Specifically, the following results were obtained.

(43) In Comparative example 1, there was a deficiency in Si, causing tensile strength and proof strength to be equal to or less than their respective corresponding reference values. Further, because of insufficient fluidity, there was a defect of 20 μm or more on the casted article surface. Thus, Comparative example 1 was rejected.

(44) In Comparative example 2, there was an excessive amount of Si, resulting in a hyper-eutectic alloy whose tensile strength, proof strength, and extension were all below their respective corresponding reference values of an aluminum alloy for low-pressure casting. Thus, Comparative example 2 was rejected.

(45) In Comparative example 3, there was a deficiency in Cu, causing tensile strength and proof strength to be equal to or less than their respective corresponding reference values. Thus, Comparative example 3 was rejected. In contrast, in Comparative example 4, there was an excessive amount of Cu, causing extension to be equal to or less than its corresponding reference value. Thus, Comparative example 4 was rejected.

(46) In Comparative example 5, there was a deficiency in Mg, causing tensile strength to be equal to or less than its corresponding reference value. Thus, Comparative example 5 was rejected. In contrast, in Comparative example 6, there was an excessive amount of Mg, causing extension to be equal to or less than its corresponding reference value. Thus, Comparative example 6 was rejected.

(47) In Comparative example 7, there was an excessive amount of Ti, causing extension to be equal to or less than its corresponding reference value. Thus, Comparative example 7 was rejected.

(48) In comparative examples 8 to 11, the P contents were lower than the lower limit value that is based on the relational expressions of the present invention (comparative example 8: 0.003 mass %, Comparative example 9: 0.0039 mass %, Comparative example 10: 0.0054 mass %, and Comparative example 11: 0.0102 mass %). The alloys of these comparative examples had defects of 20 μm or more on the surfaces of the alloys. Thus, these comparative examples were rejected. The P contents in these comparative examples were in excess of their solid solubility limit in Al—Si alloys, and were lower than the lower limit value specified in the present invention. This led to the assumption that while an excess of P beyond its solid solubility limit formed AlP, the number of eutectic cells was at a level that had an adverse effect on the efficiency with which molten metal was supplied, causing a surface segregation layer to be formed, which induced a shrinkage cavity.

(49) In Comparative examples 12 and 13, Na and Sr were in excess of their respective upper limits (Na: 0.01 mass %, and Sr: 0.03 mass %), causing extension to be equal to or less than its corresponding reference value. Thus, these comparative examples were rejected. These comparative examples contained comparatively large amounts of P, and it is assumed that this P reacted to Na or Sr to form Na.sub.3P or Sr.sub.3P.sub.2, which remained in the molten metal as impurities. These comparative examples contained large amounts of impurity compounds, which presumably led to the degraded extension of the alloy casted articles produced.

INDUSTRIAL APPLICABILITY

(50) In the aluminum alloy for low-pressure casting according to the present invention, the P content is appropriately controlled with the contents of Na and/or Sr taken into consideration. This enables an aluminum-alloy casted article with improved surface smoothness to be produced. The aluminum-alloy casted article made of the hypo-eutectic Al—Si alloy produced in the present invention is superior in mechanical properties and has a smooth surface, without a surface shrinkage cavity throughout the surface. The present invention, taking advantage of its mechanical properties, has utility in engine parts and/or similar parts.