JP2009208095A - Aluminum alloy die-casting component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum alloy die-casting component in which the reduction of production cost can be achieved, and which is produced at high strength and high quality even if a casting defect is generated. <P>SOLUTION: Disclosed is an aluminum alloy die-casting component produced by a common die casting method using an aluminum alloy as the material. This component is produced in such a manner that Rockwell hardness is controlled to HRB≥50 in a state where thickness is controlled to ≤5 mm. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an aluminum alloy die-casting part suitable for a column housing, a rack housing, and a gear housing of a vehicle steering apparatus.

Aluminum alloy casting parts are used in a wide range of fields such as transportation, electricity, aviation, and construction because of their high shape accuracy and low cost.
In particular, in a vehicle steering apparatus that is required to be light in terms of energy saving and low fuel consumption, it is possible to reduce the thickness and weight of parts and obtain a clean casting surface even if the shape is complicated. For this reason, column housings and rack housings made of aluminum alloy castings are manufactured by ordinary die casting.

Aluminum alloy casting parts (column housing, rack housing, gear housing) manufactured by ordinary die casting method are a method of injecting molten aluminum alloy into a mold at a high speed. Inevitable, it is difficult to guarantee strength.
In recent years, vehicle steering systems have been improved in function such as higher torque and higher output, and a larger load than before has been applied to cast aluminum alloy parts (column housing, rack housing, gear housing). The aluminum alloy casting parts are required to have high physical strength.

In order to obtain such an aluminum alloy casting part having a stable strength, it is necessary to control casting defects affecting the strength. At present, various elemental technologies such as a special die casting method have been developed as techniques for reducing casting defects. However, these techniques have various problems such as high manufacturing costs and limited product shapes.
In order to grasp the actual strength of an aluminum alloy cast part, it is necessary to investigate the strength including the influence of casting defects mixed in the product using the product of the manufactured part.

For example, an aluminum alloy cast part manufactured by a normal die casting method has a Vickers hardness and tensile strength of less than 10% in the area ratio of casting defects (coarse α phase, cast defect). There is a correlation between the two. However, if the area ratio of casting defects exceeds 10%, there is no correlation between Vickers hardness and tensile strength.
Here, as a technique for evaluating the influence of casting defects on the mechanical properties of aluminum alloy castings, for example, Patent Document 1 discloses the area ratio of holes combined with shrinkage cavities and gas defects and the equivalent circle diameter of the holes. There is what we have specified.
JP 2001-288547 A

  However, this patent document 1 is directed to an aluminum alloy casting that is difficult to form a nest, and a normal die casting method in which a molten aluminum alloy is injected into a mold at a high speed and casting defects (coarse α phase, voids) are likely to occur. The influence of casting defects on the mechanical properties cannot be evaluated in the same way for column housings, rack housings and gear housings of vehicle steering systems manufactured in

  The present invention has been made in view of the above-described circumstances, and the object thereof is suitable as a column housing, a rack housing, and a gear housing of a vehicle steering apparatus, can reduce the manufacturing cost, and has a casting defect. An object of the present invention is to provide an aluminum alloy die cast part that can be produced with high strength and high quality even if it occurs.

Accordingly, the aluminum alloy die cast part according to claim 1 of the present invention is manufactured by a normal die casting method using an aluminum alloy as a material, and has a Rockwell hardness of 5 mm or less. It is an aluminum alloy die-casting part manufactured as HRB50 or more.
According to a second aspect of the present invention, in the aluminum alloy die cast part according to the first aspect of the invention, the area ratio of the coarse α phase existing in an equivalent circle of a predetermined reference area of the base structure is 20% or less. It is a thing.

According to a third aspect of the present invention, in the aluminum alloy die-cast casting part according to the first or second aspect of the present invention, the projected area ratio in the tensile axis direction of a casting hole defect existing in the base structure is 15% or less. It is a thing.
Furthermore, the invention according to claim 4 is the aluminum alloy die cast part according to any one of claims 1 to 3, wherein the aluminum alloy is an Al-Si-Cu-based aluminum alloy ADC12 of JIS H5302.

  Furthermore, in the invention according to claim 5, the aluminum alloy die-cast casting part according to any one of claims 1 to 4 is manufactured as a column housing, a rack housing, or a gear housing of a vehicle steering apparatus.

  According to the aluminum alloy die casting part according to the present invention, since it is manufactured by a normal die casting method, it is possible to reduce the thickness and weight, and to obtain a clean casting surface at a low cost even if the shape is complicated. it can. In addition, since the Rockwell hardness is set to HRB50 or more in a state where the wall thickness is set to 5 mm or less, high strength can be ensured even if a casting defect occurs.

Hereinafter, the characteristics of an aluminum alloy die cast part (hereinafter referred to as a die cast part) according to the present invention will be described with reference to the drawings.
The die-casting part of this embodiment is manufactured by a normal die-casting method using JIS H5302 Al—Si—Cu-based aluminum alloy ADC12 as a material.
In addition, the die cast part of the present embodiment has a Rockwell hardness (HRB hardness) of 50 or more with a wall thickness of 5 mm or less, and an area ratio of coarse α phase present in the metal structure is 20% or less. Furthermore, it is manufactured so that the defect projected area ratio in the direction of the tensile axis of the defect in the mold cavity existing in the metal structure is 15% or less.

Here, the critical significance of each numerical value of the above-described die cast part of this embodiment will be described.
(1) The HRB hardness of the surface and the inside of the specimen collected with a wall thickness of 5 mm or less is 50 or more.
As a result of intensive studies on the prediction of the strength value of die cast parts, the present inventors have found that the Rockwell hardness (HRB hardness) is larger than the Vickers hardness, so the plastic range is wide, and the HRB hardness is It was found that the strength including the influence of internal defects can be correctly evaluated.

  Therefore, the inventors manufactured an Al—Si—Cu-based aluminum alloy ADC12 by a normal die casting method, and formed a test piece having the shape shown in FIG. 1 (total length L1: 230 mm, central parallel portion length L2: 60 mm, standard 16 points having a distance L3 of 50.0 ± 0.1 mm and a parallel width w of the center portion of 6.4 ± 0.1 mm) with varying thickness t were prepared. Tensile strength (breaking strength) of each test piece And HRB hardness was measured.

  The tensile strength of each test piece is measured by setting each test piece on an universal testing machine manufactured by Instron and conducting a tensile strength test under the test condition of crosshead moving speed: 0.5 mm / min. Was measured with a video extensometer. And the stress-strain curve was created from the tensile stress applied to the test piece and the measured value of the video extensometer, and the tensile strength was obtained from the obtained stress-strain curve. Moreover, the measurement of the HRB hardness of each test piece was performed in the part of the grip part of the test piece after a tensile strength test.

  Table 1 shows the HRB hardness and tensile strength of each test piece, and FIG. 2 shows the correlation between the HRB hardness and tensile strength as a curve.

  As is clear from FIG. 2, when the thickness of the test piece is 5 mm or less, a correlation is confirmed between the HRB hardness and the tensile strength. When the HRB hardness is 50 or more, the tensile strength is 250 MPa. It satisfies the above and exhibits excellent mechanical properties.

  On the other hand, when the thickness of the test piece exceeds 5 mm, there is no correlation between the HRB hardness and the tensile strength. This is because when the thickness of the test piece is increased, structure information not included in the HRB hardness plastic region is generated. Further, when the thickness of the test piece is increased, the cooling rate at the time of solidification of the molten aluminum alloy is reduced, and shrinkage cavities are likely to be generated, which is not preferable.

  Therefore, the die-cast casting part of the present embodiment is manufactured with excellent mechanical properties even when casting defects occur when the thickness is 5 mm or less and the Rockwell hardness (HRB hardness) is 50 or more. be able to.

(2) The area ratio of the coarse α phase existing in the base organization is 20% or less.
In addition, as a result of studying the factor of strength reduction of die cast parts, the present inventors have found that a casting hole defect existing in the base structure greatly affects the strength.

As shown in FIG. 3, the coarse α phase is a matrix structure in which eutectic Si, a matrix α phase, and an intermetallic compound exist, and a structure called α phase close to the composition of pure aluminum is a thick and coarse dendrite arm. It is a form scattered in the state.
The coarse α phase is formed as a solidified layer by cooling the aluminum alloy molten metal supplied in the sleeve when the aluminum alloy molten metal is injected into the mold by a normal die casting method, This flows into the cavity and is heated by molten aluminum alloy having a high temperature to grow grains. The hardness is lower than that of the base structure, and the HRB hardness and strength are reduced by being scattered throughout the casting.

  Therefore, in the present embodiment, 13 test pieces having the same shape as that shown in FIG. 1 are produced, and evaluation is performed by observing the cross-sectional structure of the central parallel portion of the test piece after the tensile strength test. That is, as shown in FIG. 3, the area ratio of the coarse α phase existing in an equivalent circle having a predetermined reference area was measured by a two-dimensional image analysis process on the cross-sectional structure of each test piece. More specifically, the cross-sectional structure of each test piece was observed and the area ratio of the coarse α phase was measured using Image-Pro · Plus, an image analysis software manufactured by Nippon Roper.

  Table 2 shows the area ratio and tensile strength of the coarse α phase of each test piece, and FIG. 4 shows the area ratio and tensile strength of the coarse α phase as curves.

As is apparent from Table 2 and FIG. 4, if the area ratio of the coarse α phase existing in the equivalent circle having a predetermined reference area is 20% or less, there is no significant change in tensile strength. It can be seen that the strength decreases. Desirably, when the area ratio of the coarse α phase is 15% or less, a stable tensile strength can be obtained.
Therefore, the die cast part of the present embodiment exhibits excellent mechanical properties when the area ratio of the coarse α phase existing in the equivalent circular shape having a predetermined reference area in the base structure is 20% or less.

(3) The projected area ratio in the direction of the tensile axis of the defect in the mold cavity existing in the base structure is set to 15% or less.
In addition, the present inventors have obtained the knowledge that the defects in the mold cavity existing in the base structure greatly affect the strength.

  The cavity defect is a cavity inside the die-cast part, and the cavity shrinkage generated by the volumetric shrinkage of the molten aluminum alloy filled in the mold cavity during solidification, in the sleeve, or in the mold. Air, release agents, and lubricants are categorized as blowholes as holes generated by the aluminum alloy melt. When the number of defects in the cast hole increases, the density and specific gravity of the die cast part are lowered. It is known that hardness and density are generally correlated with each other, and hardness / strength decreases as the defects in the casting hole increase.

Therefore, 13 test pieces having the same shape as that shown in FIG. 1 were prepared, and before the tensile strength test, a cross-sectional CT scan of the central parallel portion was performed. As shown in FIG. Using the image analysis software, the area ratio of the defects in the mold cavity relative to the predetermined reference area in the base structure was measured.
Table 3 shows the area ratio and tensile strength of the defect in each test piece, and FIG. 6 shows the correlation between the area ratio of the defect and the tensile strength in a curve.

  As is apparent from Table 3 and FIG. 6, it can be seen that if the area ratio of the casting defect is 15% or less, the tensile strength does not change greatly, but if it exceeds 15%, the tensile strength decreases. Desirably, a stable tensile strength can be obtained when the area ratio of the defect in the cast hole is 10% or less.

Therefore, the die-cast casting part of this embodiment is an excellent machine when the area ratio of the casting hole defect to the predetermined reference area in the base structure (projected area ratio of the casting hole defect in the tensile axis direction) is 15% or less. The physical properties.
[Example]
In order to confirm the effect of the present invention, the present inventors use a JIS H5302 Al-Si-Cu-based aluminum alloy ADC12 to form a plurality of column housings 1 of the automobile steering system shown in FIG. Individually manufactured. And the test piece of the same shape as FIG. 1 was cut out from the part from which thickness differs. Moreover, about each test piece, the casting defect (coarse alpha phase, a casting hole defect) and mechanical properties (HRB hardness, tensile strength) were evaluated. Table 4 shows the casting defects and mechanical properties of each test piece described above, and FIG. 8 shows the correlation between the HRB hardness and tensile strength of each test piece.

  In Table 4, Examples 1 to 4 and Comparative Examples 1 to 4 compare effects on the wall thickness t with respect to HRB hardness and tensile strength. According to this, as is clear from the comparison between Embodiments 1 and 2 and Comparative Examples 1 and 2, even when the HRB hardness is 50 or more, the thickness t exceeds 5 mm. 8 does not match the relationship between the regular HRB hardness and the tensile strength shown in FIG. 8, and the tensile strength decreases and the variation increases.

In addition, as shown in Embodiments 3 and 4 and Comparative Examples 3 and 4, when the thickness t is 5 mm or less, there is a relationship between normal HRB hardness and tensile strength, and when the HRB hardness is 50 or more, the tensile strength is improved. To do. As Comparative Examples 6 and 7 show, when the HRB hardness is less than 50, the tensile strength is greatly reduced.
Further, as apparent from comparison between Embodiments 5 and 6 and Comparative Examples 7 and 8, when the area ratio of the coarse α phase exceeds 20%, the tensile strength is lowered. At this time, in Comparative Examples 7 and 8, the HRB hardness is also affected and decreases. As shown in the fifth and sixth embodiments, when the area ratio of the coarse α phase is less than 20%, the tensile strength is improved.

In addition, as apparent from the comparison between Embodiments 7 and 8 and Comparative Examples 9 and 10, when the area ratio of the defect in the casting hole exceeds 15%, the tensile strength and the HRB hardness are also affected and lowered.
Therefore, as apparent from Embodiments 9 and 10, both the tensile strength and the HRB hardness were excellent when the area ratio of the coarse α phase was 20% or less and the area ratio of the casting hole defect was 15% or less. Indicates the value.

  Here, as is clear from Comparative Examples 11 and 12, when the area ratio of the coarse α phase exceeds 20% and the area ratio of the casting defect exceeds 15%, the values of tensile strength and HRB hardness are caused by the interaction. Is much lower than it affects alone.

It is a figure which shows the test piece used for the strength test of aluminum alloy die-casting casting components. It is a figure which shows the relationship between the HRB hardness (Rockwell hardness) of a test piece, and tensile strength. It is the figure which showed typically the coarse alpha phase which exists in the base structure of an aluminum alloy die-casting casting part. It is a figure which shows the relationship between the area ratio of the coarse alpha phase of a test piece, and tensile strength. It is the figure which showed the cross section of the test piece of an aluminum alloy die-casting casting part by CT scanning. It is a figure which shows the relationship between the area ratio of the casting hole defect of a test piece, and tensile strength. It is a figure which shows the column housing of the motor vehicle steering device manufactured by the normal die-casting method. It is a figure which shows the relationship between the HRB hardness of the test piece cut out from the column housing shown in FIG. 7, and tensile strength.

Explanation of symbols

    1 ... Column housing

Claims (5)

It is manufactured by ordinary die casting method using aluminum alloy as a material,
An aluminum alloy die-casting part produced by producing Rockwell hardness of HRB 50 or more with a wall thickness of 5 mm or less.   2. The aluminum alloy die cast part according to claim 1, wherein the aluminum alloy die cast part according to claim 1 is manufactured so that an area ratio of a coarse α phase existing in an equivalent circle having a predetermined reference area of a base structure is 20% or less.   The aluminum alloy die-casting part according to claim 1 or 2, wherein the aluminum alloy die-casting part is manufactured so that a projected area ratio in a tensile axis direction of a casting hole defect existing in a base structure is 15% or less.   The aluminum alloy die-casting part according to any one of claims 1 to 3, wherein the aluminum alloy is an Al-Si-Cu-based aluminum alloy ADC12 of JIS H5302.   5. The aluminum alloy die cast part according to claim 1, wherein the aluminum alloy die cast part is manufactured as a column housing, a rack housing, or a gear housing of a vehicle steering apparatus.

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