JP2014104475A - Method for evaluating strength of aluminum die cast component, and aluminum die cast component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an aluminum die cast component strength evaluation method that can properly evaluate the strength of an actual aluminum die cast component.SOLUTION: An aluminum die cast component strength evaluation method includes the steps of: performing ultrasonic flaw detection of a predetermined range of internal defects in a key lock part 5 being a high stress part of a column housing (aluminum die cast component) 1 in an electric power steering device, the high stress part that is found by a stress analysis beforehand; finding a defect rate obtained by dividing the total area of the predetermined range of internal defects by the total flaw detection area; finding a casting variable consisting of a product value of a molten metal temperature, an injection speed and a filling rate in the casting of the column housing 1; finding a casting variable specified value in which the defect rate specified value becomes lower than the defect rate set beforehand from the relationship between the defect rate and the casting variable; and evaluating that the column housing 1 has a predetermined strength when the casting variable in the casting is higher than the casting variable specified value.

Description

  The present invention relates to a method for evaluating the strength of an aluminum die-cast part and an aluminum die-cast part, and is suitable for, for example, a part used in an electric power steering device for a vehicle.

  For example, a column housing used for an electric power steering device of a vehicle is made of an aluminum die cast part. As a method for evaluating the strength of such an aluminum die-cast part, for example, there are methods described in Patent Document 1 and Patent Document 2 below. Among them, in Patent Document 1, for example, an aluminum die cast part is irradiated with ultrasonic waves, and a cast hole and a fractured chill layer of the aluminum die cast part are detected on the basis of sound wave information from the aluminum die cast part. The distribution data is acquired, X-ray CT measurement is performed on the same aluminum die-cast part, the cast hole of the aluminum die-cast part is detected from a plurality of cross-sectional images of the aluminum die-cast part, the second internal defect three-dimensional distribution data is acquired, The three-dimensional distribution data of one internal defect and the second three-dimensional distribution data of the second internal defect are compared to obtain the three-dimensional distribution data of the fractured chill layer of the aluminum die cast part. Further, in Patent Document 2, ultrasonic flaw detection is performed on an aluminum die-cast part having a composite member therein, and the distance from the surface to the composite member, the state of impregnation of the molten aluminum with respect to the composite member, the state of cracking, etc. are measured. A pass / fail judgment is made.

JP 2005-91288 A Japanese Patent Laid-Open No. 2004-144489

  However, the method for evaluating the strength of an aluminum die-cast part described in Patent Document 2 merely measures the degree of impregnation and cracking of an aluminum die-cast part having a composite member therein, and does not have a composite member. It does not evaluate the strength of die-cast parts. On the other hand, the strength evaluation method for aluminum die-cast parts described in Patent Document 1 enables evaluation of aluminum die-cast parts without composite members. For example, all of aluminum die-cast parts and complex aluminum die-cast parts can be evaluated. It is practically difficult to inspect the part. An actual aluminum die-cast part cannot avoid an internal defect such as a cast hole, and sometimes breaks starting from the internal defect. In addition, aluminum die-cast parts often have a complicated shape, and it is difficult to detect internal defects by, for example, ultrasonic flaw detection, and it is clear which part should be evaluated for strength. Absent.

  The present invention has been made by paying attention to the above-described problems, and it is possible to appropriately evaluate the strength of an actual aluminum die-cast part and to obtain an aluminum die-cast part having a predetermined strength. An object of the present invention is to provide a die cast part strength evaluation method and an aluminum die cast part.

  In order to solve the above problems, the aluminum die cast part strength evaluation method of the present invention is a method for evaluating the strength of an aluminum die cast part, and for the high stress part obtained by performing stress analysis on the aluminum die cast part in advance, Ultrasonic inspection of a predetermined range of internal defects in the high-stress part, and dividing the total area of the internal defects of the predetermined range by the total inspection area to obtain the defect rate, and the molten metal temperature at the time of casting the aluminum die-cast part and A casting variable consisting of a product value of an injection speed and a filling rate is obtained, and a casting variable prescribed value is obtained from the relationship between the defect rate and the casting variable so that the defect rate is equal to or less than a preset defect rate prescribed value. When the casting variable is equal to or greater than the casting variable prescribed value, it is evaluated that the aluminum die cast part has a predetermined strength.

  Note that the molten metal temperature at the time of casting of the aluminum die-cast part indicates the molten aluminum temperature of the holding furnace, for example, and if the molten metal temperature is high, the molten metal part touching the sleeve portion is difficult to solidify, which leads to reduction of fracture chill. 650 degreeC or more is preferable. When obtaining the casting variables, a value obtained by subtracting the control lower limit value from the molten metal temperature during casting is used in the calculation. In addition, the injection speed at the time of casting aluminum die-cast parts indicates, for example, a high-speed injection speed when the molten metal is pushed out from the sleeve with a plunger. If the injection speed is high, the molten metal is injected into the cavity before it cools down. The pressure transmission is improved and the nest is reduced. Moreover, when the injection speed is high, the broken chill is pulverized at the gate, leading to reduction of the coarse broken chill, and usually 1.5 m / s or more is preferable. Further, the filling rate indicates the amount of molten metal poured / sleeve volume. If the filling rate is high, the temperature is difficult to cool, so that it is difficult to produce a rupture chill, which leads to reduction of the rupture chill, and is usually 20 to 50%.

Further, the predetermined range of the high stress portion is a stress range of 50% or more of the maximum stress of the high stress portion.
The aluminum die cast part of the present invention committee is an aluminum die cast part whose strength is evaluated by the aluminum die cast part strength evaluation method, wherein the defect rate is 0.5% or less. .
Further, the aluminum die-cast part is a column housing used for an electric power steering device of a vehicle, and the high stress portion is a key lock portion of the column housing.

  Thus, according to the aluminum die-cast part strength evaluation method of the present invention, for the high-stress part of the aluminum die-cast part obtained in advance by stress analysis, ultrasonic inspection is performed for an internal defect in a predetermined range of the high-stress part. A defect rate obtained by dividing the total area of internal defects in a predetermined range by the total flaw detection area, and a casting variable consisting of a product value of a molten metal temperature, an injection speed, and a filling rate at the time of casting of an aluminum die cast part are obtained. A casting variable prescribed value is obtained from the relationship with the casting variable so that the defect rate is less than or equal to a preset defect rate prescribed value.When the casting variable at the time of casting is equal to or greater than the prescribed casting variable value, the aluminum die cast part has a predetermined strength. Since it has been evaluated that it has, it is possible to appropriately evaluate the strength of the actual aluminum die cast part.

Moreover, the strength of the aluminum die cast part can be more appropriately evaluated by setting the predetermined range of the high stress part to be a stress range of 50% or more of the maximum stress of the high stress part.
Further, according to the aluminum die cast part of the present invention, the strength rate is evaluated by the aluminum die cast part strength evaluation method of the present invention, and the defect rate obtained by dividing the total area of internal defects within a predetermined range of the high stress portion by the total flaw detection area is 0. By setting it to 5% or less, an aluminum die cast part having a predetermined strength can be obtained.

It is explanatory drawing which shows one Embodiment of the aluminum die-casting component strength evaluation method of this invention. It is explanatory drawing of the internal defect flaw detection in the aluminum die-casting component strength evaluation method of FIG. It is explanatory drawing of the high stress part of an aluminum die-casting part. It is explanatory drawing of the internal defect flaw detection range of the high stress part of an aluminum die-casting part. It is explanatory drawing of the image obtained by the internal defect flaw detection of FIG. It is a flaw detection image obtained by the internal defect flaw detection of FIG. It is the image which binarized the flaw detection image of FIG. It is the image which extracted the internal defect area by image analysis from the binarized flaw detection image of FIG. It is a graph which shows the relationship between the defect rate of an Example and a comparative example, and the frequency | count of the twist at the time of a twist test. It is explanatory drawing which shows the relationship between the defect rate of the internal defect in the flaw detection range of FIG. 4, and a casting variable.

Next, an embodiment of an aluminum die cast component strength evaluation method of the present invention will be described with reference to the drawings.
FIG. 1 is an explanatory view of a six-axis movable ultrasonic flaw detector used in the aluminum die cast component strength evaluation method of the present embodiment, FIG. 1a is an overall view of the device, and FIG. 1b is a detailed view of an inspection object and a turntable. FIG. 1c is an explanatory diagram of internal defect flaw detection. Reference numeral 1 in the drawing is an aluminum die-cast part whose strength is to be evaluated in this embodiment, for example, a column housing of an electric power steering apparatus.

  In this embodiment, an aluminum die-cast part (column housing) 1 is mounted on the turntable 2, and the probe (probe) 3 is moved downward from above while rotating the turntable 2. Inspection of the inside of the housing 1 in a spiral manner. Since the aluminum die-cast part (column housing) 1 of this embodiment has a cylindrical portion, a high stress portion (to be described later) of the cylindrical portion is flawed by an ultrasonic flaw detector to detect internal defects.

  In the flaw detection, as shown in FIG. 2a, an evaluation gate was set between the surface echo and the bottom surface echo of the aluminum die cast part (column housing) 1. The evaluation gate means a flaw detection range in the ultrasonic irradiation direction or the reflection direction. In ultrasonic flaw detection, ultrasonic waves oscillated from the probe 3 are reflected by the surface and bottom surface of the aluminum die-cast part (column housing) 1 and returned. The reflected waves become the surface echo and the bottom echo, respectively. As shown in FIG. 2b, when the internal defect 4 exists in the flaw detection range of the aluminum die cast part (column housing) 1, a defect echo appears between the surface echo and the bottom echo, that is, in the evaluation gate range. Since the time of the surface echo and the time of the bottom echo are known in advance, an echo existing between the two times becomes a defect echo. Then, a method of illustrating the largest defect echo inside the aluminum die cast part (column housing) 1 was used. In addition, the internal diameter of the cylindrical part of the column housing which is the aluminum die-casting part 1 of this embodiment is 38 mm, and the flaw detection range was set to a range of 12 mm in the axial direction of the cylindrical part as described later. In addition, when the inner peripheral surface of the cylindrical portion of the aluminum die cast part (column housing) 1 is turned, it becomes easier to detect defect echoes because ultrasonic waves are less likely to be irregularly reflected on the inner peripheral surface.

  Prior to flaw detection, stress analysis was performed under the same load conditions as in the torsion test in which a key was inserted into the key lock portion of the column housing, which is the aluminum die cast part 1 of this embodiment, and a load in the torsion direction was applied. The portion A shown, that is, the key lock portion was found to be the portion with the highest stress. Further, even in the conventional torsion test, breakage occurs at the key lock portion of the column housing. That is, in this high-stress part (key lock part), for example, the internal defect of the high-stress part is flaw-detected because breakage is likely to occur starting from the internal defect.

  FIG. 4 shows the flaw detection range of the high stress portion (key lock portion) of the aluminum die cast component (column housing) 1 of the present embodiment. In the present embodiment, internal flaw detection is performed in a range of 12 mm in the axial direction of the cylindrical portion of the column housing, including the portion with the highest stress described above. This internal flaw detection range is a stress range of 50% or more of the maximum stress. Ultrasonic flaw detection is performed inside the aluminum die cast part (column housing) 1 within the internal flaw detection range including this high stress part (key lock part) A, and the aluminum die cast part (column housing) 1 is slit as shown in FIG. An internal defect is illustrated on a diagram in which the inside is developed by cutting from the portion.

FIG. 6 is a flaw detection image obtained by ultrasonic flaw detection in the flaw detection range of the aluminum die cast part (column housing) 1 described above. As described above, since the flaw detection range is 12 mm in the height direction and the entire inner circumference of the cylindrical portion, the horizontal axis of the image is 119 mm (inner circumference of inner diameter φ38 mm), and the vertical axis is 12 mm. FIG. 7 shows the flaw detection image, which is binarized by 50% echo intensity as a threshold value and binarized above and below. The light gray portion in the figure is an internal defect having an echo intensity of 50% or more. The 50% echo intensity is an echo intensity when a master test piece having a defect area of 0.2 mm ² is adjusted so as to have the same area in image analysis described later. Further, FIG. 8 shows an image analysis of the area of each internal defect having an echo intensity of 50% or more shown in FIG. A pixel number of 20 pixels or less (area 0.2 mm ² or less) was judged to be satisfactory in terms of strength, and was excluded from the evaluation target of internal defects.

Table 1 below shows the area of all internal defects analyzed in this way. The total area of these internal defects 4.99mm ^2, the total flaw area is 1400 mm ^2. In the present embodiment, the total area of internal defects is divided by the total flaw detection area, the value is defined as the defect rate, and the internal defect state and the strength of the aluminum die cast part are evaluated based on the size of the defect rate. In this example, the defect rate is 4.99 / 1400 × 100 = 0.36%.

  In this way, for the seven aluminum die-cast parts (column housing) 1, internal defects in the high stress part (key lock part) are similarly detected by ultrasonic flaw detection, and the area of the detected internal defect is obtained by image analysis. The defect rate was determined by dividing the total area of these internal defects by the total flaw detection area. Further, a twist test was performed on these aluminum die-cast parts (column housing) 1 to detect the number of twists until breakage. This torsion test is added to the technical standards for locking devices such as four-wheeled vehicles in the Road Transport Vehicle Law, Road Transport Vehicle Safety Standards, Article 11, 2nd Detail Notification, Appendix 1, Section 7 Attachment 7. Attachment 2 The test was performed based on the test procedure of the locking device that acts on the steering device provided with the torque limiting device. However, in the present embodiment, the number of twists until the destruction was repeated by repeating the cycle was determined by looking at the safety factor and setting the torque value to 200 Nm, which is twice the prescribed 100 Nm. Table 2 below shows the relationship between the defect rate of internal defects in the high stress part (key lock part) 5 and the number of twists at the time of the torsion test destruction. FIG. 9 is a graph showing the results of Table 2. As is clear from FIG. 9, it was found that when the defect rate exceeds 0.5%, the number of twists at the time of destruction in the twist test is greatly reduced. That is, if the defect rate is 0.5% or less, it can be evaluated that the aluminum die cast part (column housing) 1 has a predetermined strength.

  On the other hand, the casting conditions that may cause internal defects during casting of the aluminum die cast part (column housing) 1 include the melt temperature during casting, the injection speed of the melt, and the filling rate into the mold. As described above, the molten metal temperature at the time of casting of the aluminum die-cast part indicates, for example, the molten aluminum temperature of the holding furnace, and if the molten metal temperature is high, the molten metal portion that is in contact with the sleeve portion is difficult to solidify, which leads to reduction of fracture chill. Usually, 650 ° C. or higher is preferable. Moreover, in this embodiment, in order to raise the influence degree with respect to a casting variable, the actual performance management lower limit 640 degreeC was reduced and used from the molten metal temperature, for example. In addition, the injection speed at the time of casting of aluminum die-cast parts indicates the high speed injection speed when extruding the molten metal from the sleeve with a plunger, for example. If the injection speed is high, the molten metal is injected before it cools down. Is effective, and has the effect of reducing the nest. Further, if the injection speed is high, the breaking chill is pulverized by the gate, leading to reduction of the coarse breaking chill. Therefore, the gate speed is preferably 20 to 60 m / s and the gate thickness is preferably 2.5 mm or less. In addition, the filling rate indicates the amount of molten metal poured / sleeve volume. If the filling rate is high, the temperature is difficult to cool down, so that it is difficult to break chill, pressure transmission in the molten metal is improved, and the nest is reduced. Further, when the injection speed is high, the fracture chill is crushed at the gate, and the pressure transmission in the molten fracture chill of the coarse fracture chill is improved, and the nest is reduced. The filling rate is usually preferably 20 to 50%. In this embodiment, the product value of the molten metal temperature, the injection speed, and the filling rate is used as a casting variable, and the relationship between the casting variable and the defect rate of the above-described internal defects is examined.

  Therefore, ten aluminum die cast parts (column housings) 1 are cast under casting conditions where the casting variables are 4, 6, 10, and 20, respectively, and ultrasonic flaw detection is performed on them in the same manner as described above, and the calculated defect rate. And the relationship between casting variables. Table 3 shows the average value of + 3σ and the average value of −3σ of the defect rate in each casting variable. FIG. 10 is a graph of the results in Table 3. σ is a standard deviation. As described above, when the defect rate is 0.5% or less, it can be determined that the aluminum die cast part (column housing) 1 has a predetermined strength. Since the casting variable with a defect rate of 0.5% is 10, in this embodiment, it can be determined that the aluminum die cast part (column housing) 1 cast under casting conditions with a casting variable of 10 or more has a predetermined strength. .

  As described above, in the aluminum die cast component strength evaluation method of the present embodiment, an internal defect in a predetermined range of the high stress portion is ultrasonically detected with respect to the high stress portion of the column housing (aluminum die cast component) 1 obtained in advance by stress analysis. The defect rate is determined by dividing the total area of internal defects within the predetermined range by the total flaw detection area, and from the product value of the molten metal temperature, injection speed, and filling rate at the time of casting the column housing (aluminum die cast part) 1 When the casting variable is equal to or greater than the predetermined value of the casting variable when the casting variable is equal to or greater than the predetermined value of the casting variable. Since the column housing (aluminum die-cast part) 1 is evaluated as having a predetermined strength, the actual strength of the column housing (aluminum die-cast part) 1 is appropriate. It can be evaluated.

Further, the strength of the column housing (aluminum die-cast part) 1 can be more appropriately evaluated by setting the predetermined range of the high stress portion to be a stress range of 50% or more of the maximum stress of the high stress portion. .
Further, according to the aluminum die cast part of the present embodiment, the strength rate is evaluated by the above-described strength evaluation method, and the defect rate obtained by dividing the total area of internal defects within a predetermined range of the high stress portion by the total flaw detection area is 0.5% or less. As a result, a column housing (aluminum die-cast part) 1 having a predetermined strength can be obtained.

1 is aluminum die-casting part (column housing)
2 is a turntable 3 is a probe 4 is an internal defect A is a high stress part (key lock part)

Claims (4)

  A method for evaluating the strength of an aluminum die-cast part, wherein a high-stress part obtained by performing stress analysis on the aluminum die-cast part in advance is subjected to ultrasonic inspection for an internal defect in a predetermined range of the high-stress part. The total area of internal defects is divided by the total flaw detection area to obtain a defect rate, and a casting variable consisting of a product value of a molten metal temperature, an injection speed, and a filling rate at the time of casting of the aluminum die cast part is obtained, and the defect rate And determining the casting variable prescribed value at which the defect rate is equal to or less than a preset defect rate prescribed value from the relationship between the casting variable and the casting variable when the casting variable is equal to or greater than the prescribed casting variable value. A method for evaluating the strength of an aluminum die-cast part, wherein the strength is evaluated to have a predetermined strength.   The aluminum die cast component strength evaluation method according to claim 1, wherein the predetermined range of the high stress portion is a stress range of 50% or more of the maximum stress of the high stress portion.   An aluminum die-cast part whose strength has been evaluated by the aluminum die-cast part strength evaluation method according to claim 1 or 2, wherein the defect rate is 0.5% or less.   The aluminum die-cast part according to claim 3, wherein the aluminum die-cast part is a column housing used for an electric power steering device of a vehicle, and the high stress part is a key lock part of the column housing.

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