JP7580215B2 - Aluminum alloy for die casting and manufacturing method of cast product using same - Google Patents

Description

The present invention relates to an aluminum alloy suitable for die casting using a die casting machine and a manufacturing method for a cast product using the same by die casting, which is particularly capable of achieving both high strength and high toughness.

Die casting is a casting method in which molten metal is filled into a mold cavity at high speed and under high pressure. It is highly productive and is widely used for automobile parts, machine parts, etc.
For such aluminum alloys for die casting, Al-Si based alloys are used since they are required to have good flowability into a die. However, in recent years, there has been a demand for higher strength aluminum alloys due to the need for weight reduction. However, these alloys tend to have insufficient toughness, so there is a demand for aluminum alloys that are both high strength and high toughness.

Patent Document 1 discloses an aluminum alloy containing 3.5 to 7.5% Si, 0.45 to 0.8% Mg, 0.05 to 0.25% Cr, and 0 to 0.01% Cu, but describes that a heat treatment, known as a T6 treatment, is required after solution treatment to obtain a tensile strength of 320 MPa or more.
However, the T6 treatment is longer and less productive than the T5 treatment, and the Cu content is low at 0 to 0.01%, so it is presumed that sufficient strength cannot be obtained with the T5 treatment.

Patent Document 2 discloses an alloy containing 6.00-6.50% Si, 0.10-0.50% Mg, 0.30% or less Fe, 0.30-0.60% Mn, and 0.10-0.30% Cr in order to obtain high yield strength and high ductility, and further describes the inclusion of 30-200 ppm Sr, 1-50 ppm B, 0.05-0.20% Sb, and 0.05-0.30% Ti. However, the low Si content may result in poor melt flow, and only a small amount of Cu is added, so it is presumed that sufficient strength will not be obtained with T5 treatment.

Patent No. 5575028 Patent No. 5797360

The present invention aims to provide an aluminum alloy suitable for producing aluminum die-cast products with high strength and high toughness, and a manufacturing method for cast products using the same.

The aluminum alloy for die casting according to the present invention is characterized by containing, all in mass %, Si: 7.0-9.0%, Mg: 0.4-0.6%, Cu: 0.4-0.7%, Cr: 0.1-0.5%, Mn: 0.5% or less, [Cr+Mn]: 0.1-0.8%, Fe: 0.10-0.25%, Sr: 0.005-0.02%, and the balance being Al and impurities.
Furthermore, Ti may be contained in the range of 0.2% or less, if necessary.
Here, the impurities refer to unavoidable impurities that may be mixed in during the manufacturing process.

The reasons for selecting the ranges of ingredients in the present invention will be explained below.
<Si Component>
In die casting, Si needs to be 7.0 mass% or more (hereinafter simply expressed as %) to ensure fluidity. If it exceeds 9.0%, coarse primary crystals of Si are likely to precipitate, and there is a risk of reduced elongation. Therefore, the Si range is set to 7.0 to 9.0%.
Preferably, Si is in the range of 7.10 to 8.50%.
<Mg, Cu Components>
Mg and Cu are effective components for improving the strength of cast products. In the present invention, the Mg content is set to 0.4% or more and the Cu content is set to 0.4% or more so that high strength can be obtained in the heat treatment (T5 treatment) after casting.
However, in order to prevent the elongation from decreasing more than necessary, the Mg content is set to 0.4 to 0.6%, and the Cu content is set to 0.4 to 0.7%.
<Cr, Mn, Fe components>
In die casting, the Fe component is one of the components that is easily mixed in. If the Fe content exceeds 0.25%, the crystallized particles become coarse and the elongation decreases. Therefore, in the present invention, the Fe content is set in the range of 0.10 to 0.25%, and Cr is added to make the Fe-based crystallized particles finer.
If the Cr content exceeds 0.5%, there is a risk of coarse crystallization, so the Cr content is set in the range of 0.1 to 0.5%.
In the present invention, the Fe-based crystallized particles can be refined by adding Mn, and when Mn is added, the amount of Mn added is preferably 0.5% or less.
If the Mn content exceeds 0.5%, coarse crystallized particles are likely to crystallize, and the elongation decreases.
Furthermore, when both Cr and Mn are added, it is necessary to control the total amount, and the sum of Cr and Mn [Cr+Mn] is preferably in the range of 0.1 to 0.8%.
<Sr, Ti components>
A small amount of Sr added is effective in refining eutectic Si, and the Sr: range is set to 0.005 to 0.02%.
Furthermore, Ti is not an essential component in the present invention, but its addition is effective in refining the crystal grains of the aluminum alloy, and if added, the range of 0.01 to 0.2% is preferable.

The mold used in die casting consists of a movable die that is controlled to open and close relative to a fixed die. When the fixed die and the movable die are closed, a cavity is formed. Molten aluminum alloy poured into a sleeve is injected into the cavity at high speed and high pressure using a plunger to cast the alloy.
The injection conditions are preferably set so that the gate speed is 1 m/sec or less so that the cavity of the mold is filled with a laminar flow.
The mold used in the present invention is preferably a center gate type mold capable of radially injecting molten metal from the center.

When the aluminum alloy according to the present invention is used, a cast product having high strength and high toughness, such as a tensile strength of 280 N/mm2 ^or more, a 0.2% yield strength of 180 N/mm2 ^or more, an elongation of 4% or more, and a Charpy impact value of 3 J/cm2 ^{or more} , can be obtained by performing heat treatment (T5 treatment) at 160°C to 200°C for 2 to 8 hours after die casting.
In addition, when the average area of the crystallized particles excluding eutectic Si in the metal structure of such a cast product is measured using an optical microscope, it is 2 ^μm2 or less.

When die casting is performed using the aluminum alloy according to the present invention, the subsequent T5 treatment results in a die cast product that is both high strength and has excellent toughness.

The chemical composition of the aluminum alloy used in the evaluation is shown below. The evaluation results are shown. 1 shows optical microscope photographs of the metal structures of Example 1 and Comparative Example 1.

Molten aluminum alloys having the chemical compositions shown in the table in FIG. 1 were prepared, and die-cast into cast products using a center-gate type die.
The casting conditions were a molten metal temperature of about 700° C. and an injection gate speed of 0.8 m/s.e.
After casting, the alloy was subjected to a heat treatment (T5) at 190° C. for 3 hours.
FIG. 2 shows the results of measuring the mechanical properties and the average area of crystallized particles excluding eutectic Si (average crystallized particle area).
FIG. 3 shows examples of optical microscope photographs of Example 1 and Comparative Example 1.

The test items and test methods are as follows:
<Tensile strength, 0.2% yield strength and elongation>
Based on JIS-Z2241, JIS-4 test pieces were cut out from the castings and subjected to tensile tests using a tensile testing machine conforming to the JIS standard to measure the tensile strength (MPa), 0.2% yield strength (MPa) and elongation (%).
<Charpy impact test>
Based on JIS-Z2242, JIS-4 Charpy impact test pieces were cut out from the castings, and measurements were made with a Charpy impact tester conforming to the JIS standard.
<Average crystallized area>
The cut surfaces of the castings were mirror-polished and observed and measured under an optical microscope.
Image processing was performed within a measurement area of 0.166 ^mm2 at a magnification of 1000 times, and the area of the crystallized matter in the portion excluding the eutectic Si was measured and the average value was calculated.

The following can be seen from the chemical composition in FIG. 1 and the evaluation results in FIG.
In the present invention, the targets are high strength with a tensile strength of 280 MPa or more and a 0.2% yield strength of 180 MPa or more, and high toughness with an elongation of 4% or more and a Charpy impact value of 3 J/cm ² or more.
All of Examples 1 to 11 have alloy compositions within the set ranges, and all targets for T5 treatment are met.
In contrast, in Comparative Example 1, the Cr content was 0.85%, exceeding 0.5%, so coarse Cr-based crystals were formed, the elongation was below the target, and the Charpy impact value also did not reach the target.
On the other hand, in Comparative Example 2, since Cr was not added, Fe-based crystallized particles became coarse, resulting in a low elongation value.
Since Comparative Example 3 had a small Cu content, Comparative Example 4 had a small Si content, and Comparative Example 5 had a small Mg content, the 0.2% yield strength of the tensile strength did not reach the target.
Comparative Example 6 had a high Cu content, Comparative Example 7 had a high Mg content, and Comparative Example 8 had a high Fe content, resulting in low elongation.
In Comparative Example 9, since Sr was not added, the elongation was poor.
Comparative Examples 10 to 14 are made of F material and therefore have low strength.
In Comparative Example 15, the total of [Cr + Mn] was 0.98%, exceeding 0.8%, so the crystallized particles became coarse and the average crystallized particle area exceeded 5.6 μm ² and 2 μm ² , resulting in low elongation.

Claims (2)

An aluminum alloy for die casting containing, by mass, Si: 7.0-9.0%, Mg: 0.4-0.6%, Cu: 0.4-0.7%, Cr: 0.1-0.5%, Mn: 0.5% or less, [Cr+Mn]: 0.1-0.8%, Fe: 0.10-0.25%, Sr: 0.005-0.02% , Ti: 0.01-0.2% , and the balance being Al and impurities ,
The die is filled with laminar flow at a gate speed of 1 m/sec or less by die casting.
A method for manufacturing a cast product, comprising the steps of: after die casting, performing heat treatment at 160°C to 200°C for 2 to 8 hours. 2. The method for manufacturing a cast product according to claim 1 , characterized in that the cast product has high strength and high toughness, with a tensile strength of 280 N/mm2 ^or more, a 0.2% yield strength of 180 N/mm2 ^or more, an elongation of 4% or more, and a Charpy impact value of 3 J/ ^cm2 or more.

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