Method for analyzing residual stress in cast parts

Abstract

To provide a method for analyzing residual stress in a cast part, capable of improving calculation accuracy.SOLUTION: A method for analyzing residual stress in a cast part includes a step of obtaining a stress-strain curve of an alloy at each temperature range. The analysis method includes a step of acquiring temperature distribution data showing the time variation of the temperature distribution of the casting part during the period from when a molten alloy is poured into a mold to when the casting part is demolded from the mold. The analysis method includes a step of calculating the residual stress at a plurality of points of the casting part based on the acquired temperature distribution data and the stress-strain curve.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This specification discloses a method for analyzing residual stress in cast parts.

Background Art

[0002] Patent Document 1 discloses a method for analyzing residual stress in cast parts. Specifically, a reference temperature higher than the solidus temperature and lower than the liquidus temperature of the alloy is set. Also, the temperature change at each point of the cast part after pouring the molten metal into the mold is obtained. Then, based on the cooling rate when the temperature at each point passes through the reference temperature, the residual stress value at each point is calculated.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the technique of Patent Document 1, it was difficult to consider the residual stress generated in the process from pouring to mold release for the points that pass through the reference temperature after the cast part is removed from the mold. Therefore, there was a risk that the calculation accuracy of the residual stress value would decrease.

Means for Solving the Problems

[0005] The method for analyzing residual stress in cast parts disclosed in this specification includes a step of obtaining the stress-strain curve of the alloy in each temperature range. The analysis method includes a step of obtaining temperature distribution data indicating the time change of the temperature distribution of the cast part during the period from pouring the molten alloy into the mold to removing the cast part from the mold. The analysis method includes a step of calculating the residual stress at a plurality of points of the cast part based on the obtained temperature distribution data and the stress-strain curve.

[0006] According to the above configuration, residual stress can be calculated based on temperature distribution data during the period from pouring molten metal into the mold to demolding the cast part. This allows for the reliable calculation of residual stress generated during the process from pouring to demolding. Because the influence of residual stress generated from pouring to demolding can be considered, the accuracy of residual stress calculation can be improved. [Brief explanation of the drawing]

[0007] [Figure 1] This is a flowchart explaining the method for analyzing residual stress in cast parts. [Figure 2] This is an example of temperature dependence data for a stress-strain curve. [Figure 3] This is an example of temperature distribution data for a cast part. [Modes for carrying out the invention]

[0008] (Configuration of a residual stress analysis system for cast parts) Cast parts are components formed by pouring molten alloy into a mold. Examples of cast parts include box-shaped components that make up various types of cases. Examples of alloys include aluminum alloys.

[0009] The residual stress analysis system for cast parts disclosed herein comprises calculation means and storage means (not shown). The analysis system may be, for example, computer-aided engineering (CAE).

[0010] The memory means stores the analysis model, temperature dependence data of stress-strain curves, and various programs (e.g., residual stress analysis program). The analysis model is a numerical model that approximates the shape of the mold cavity using multiple elements. The analysis model can be created in advance using the calculation means of the residual stress analysis system. The analysis model may also be created by dividing CAD data using, for example, automatic mesh generation software. The temperature dependence data of stress-strain curves will be described later.

[0011] The arithmetic means executes various programs stored in the storage means. As a result, the arithmetic means performs calculations according to the programs based on the data stored in the storage means.

[0012] (Method for analyzing residual stress) The flowchart in Figure 1 illustrates the method for analyzing residual stress in cast parts. This flow begins when the calculation means of the residual stress analysis system executes the residual stress analysis program. Hereafter, "Step 10" will be abbreviated as "S10".

[0013] In S10, a process is carried out to obtain the stress-strain curve of the alloy used for casting. Specifically, the calculation means reads out temperature dependence data of the stress-strain curve, which is pre-stored in the storage means, from the storage means.

[0014] The temperature dependence data of stress-strain curves will be explained using the example in Figure 2. In Figure 2, the horizontal axis represents strain, and the vertical axis represents stress. The temperature dependence data of stress-strain curves includes multiple stress-strain curves for each temperature range used in casting. The temperature dependence data of stress-strain curves in Figure 2 includes stress-strain curves C1-C4. Stress-strain curves C1-C4 are curves for 22°C, 100°C, 300°C, and 450°C, respectively.

[0015] Stress-strain curves can be obtained in advance by performing tensile tests on alloy specimens. Temperature-dependent data can also be obtained in advance by performing multiple tensile tests at different temperatures. Alternatively, temperature-dependent data for stress-strain curves may be obtained in advance through simulation.

[0016] The temperature range and temperature step size for acquiring stress-strain curves can be freely set. The temperature range should be within the range used in the actual casting process. The temperature step size should be determined according to the required accuracy for residual stress analysis. For example, the temperature step size for the temperature range in which you want to analyze residual stress with high accuracy may be smaller than that for other temperature ranges. For example, the temperature range from when the molten metal is poured until the cast part is demolded may be the temperature range in which you want to analyze residual stress with high accuracy.

[0017] In S20, the process of acquiring the analysis model is performed. Specifically, the calculation means reads the analysis model, which is pre-stored in the storage means, from the storage means.

[0018] In S30, temperature distribution data is acquired that shows the time change in the temperature distribution of the cast part during a specific period SP. The specific period SP is the period from when the molten alloy is poured into the mold until the cast part is demolded from the mold. More specifically, the specific period SP is the period from immediately after the completion of pouring until immediately before demolding. The temperature distribution data of the cast part can be obtained, for example, by performing molten metal flow analysis and solidification analysis using an analytical model.

[0019] Figure 3 shows an example of temperature distribution data for a cast part. Figure 3 shows the temperature distribution data obtained from the molten metal flow analysis and solidification analysis described above. The horizontal axis represents elapsed time, and the vertical axis represents temperature. Figure 3 shows the temperature change at representative measurement points M1-M3 of the cast part. Time t1 is the time when pouring is completed. At time t1, the alloy is liquid. Time t2 is the time when the cast part is demolded. At time t2, the alloy is solid. The period from time t1 to t2 corresponds to the specific period SP. During the specific period SP, a portion of the molten metal has solidified, and the state is one in which liquid and solid are mixed.

[0020] From time t3 to t4, it is the heat treatment period P1. During the heat treatment period P1, the cast part is heat-treated by heating means (e.g., heater and air blowing). From time t4 to t5, it is the rapid cooling period P2. During the rapid cooling period P2, the cast part is cooled by cooling means (e.g., air blowing). At time t5, the temperature of the cast part drops to almost normal temperature. From time t5 to t6, it is the pressing period P3. During the pressing period P3, trimming press and shot blasting are performed.

[0021] As shown in FIG. 3, the temperature change slope at time t1 - t2 (specific period SP) is larger than the temperature change slope at time t2 - t4. Also, when demolding at time t2, the cast part is in a high-temperature state. This is because, for mass production, the time from the completion of pouring to the start of demolding is shortened.

[0022] In S40, a process of calculating the residual stress at multiple points of the cast part is performed. The calculation process is executed based on the temperature distribution data obtained in S30 and the temperature dependence data of the stress-strain curve obtained in S10. Also, in the calculation process, the residual stress generated during the specific period SP and the residual stress generated during the rapid cooling period P2 are calculated. A specific example will be described below.

[0023] First, the residual stress generated during the specific period SP is calculated. For each of the measurement points M1 - M3, the corresponding stress-strain characteristics are calculated based on the temperature distribution data. For example, the stress-strain characteristics may be calculated by interpolating between the stress-strain curves C1 - C4 in FIG. 2. Then, based on the temperature distribution data and the stress-strain characteristics, the residual stress values at the measurement points M1 - M3 are calculated by a general CAE analysis method. The residual stress values may be calculated, for example, using general-purpose casting simulation software.

[0024] Next, the residual stress generated during the rapid cooling period P2 is calculated. Note that the processing during the rapid cooling period P2 is the same as that during the specific period SP described above, so the explanation is omitted. Finally, the residual stress generated during the specific period SP is added to the residual stress generated during the rapid cooling period P2.

[0025] (effect) Generally, it is believed that residual stress occurs in aluminum alloys during the process of temperature decrease from 300°C to room temperature. This is based on the understanding that above 300°C, the aluminum alloy is in a soft state where it is a mixture of liquid and solid, making it difficult for residual stress to occur. Therefore, conventionally, residual stress analysis of cast parts was performed based on the temperature change during the period from when the alloy solidified and was demolded until it cooled to room temperature. For example, in Figure 3, only the residual stress generated during the rapid cooling period P2 was analyzed. However, the inventors discovered that the residual stress generated during a specific period SP, when the alloy is in a mixed state of liquid and solid, is a value that cannot be ignored. One reason for this is the large slope of the temperature change during the specific period SP. Therefore, in the technology described herein, residual stress can be calculated based on the temperature distribution data during the specific period SP (S40). Then, the residual stress generated during the specific period SP can be added to the residual stress generated during the rapid cooling period P2. This makes it possible to consider the influence of residual stress generated in the mixed state of liquid and solid (specific period SP), thereby improving the accuracy of residual stress calculation. As a result, when performing various machining processes such as cutting on cast parts, residual stress can be accurately considered, making it possible to improve the machining accuracy of cast parts.

[0026] Conventionally, there is a technique for performing residual stress analysis based on the temperature conditions between the solid and liquid phases by understanding the relationship between the solidus and liquidus temperatures of an alloy. However, this technique is difficult because it requires distinguishing between solid and liquid phases. On the other hand, the technique described herein allows for residual stress analysis based on temperature distribution data and stress-strain curves, eliminating the need to distinguish between solid and liquid phases. This makes it possible to easily obtain the results of residual stress analysis.

[0027] (modified version) There are various ways to define the specific period SP. For example, the period from the completion of pouring (time t1) to just before the start of heat treatment (time t3) may be defined as the specific period SP.

[0028] The process of acquiring temperature distribution data of the cast part (S30) can be carried out in various ways. For example, temperature distribution data may be acquired by temperature measurement experiments of a verification workpiece. [Explanation of symbols]

[0029] C1-C4: Stress-strain curve t1-t6: Time SP: Specific period P1: Heat treatment period P2: Rapid cooling period P3: Pressing period

Claims

[Claim 1] A method for analyzing residual stress in cast parts, A process for obtaining the stress-strain curve of the alloy in each temperature range, A step of acquiring temperature distribution data showing the time change in the temperature distribution of the cast part during the period from immediately after the completion of pouring to immediately before demolding, from the time of pouring the molten alloy into the mold until the cast part is demolded from the mold. A step of calculating residual stress at multiple points of the cast part based on the acquired temperature distribution data and the stress-strain curve, Equipped with, The process for calculating the residual stress is as follows: A step of calculating the first residual stress that occurs during the period from immediately after the completion of pouring to immediately before demolding, A step of calculating the second residual stress generated during the rapid cooling period after demolding the cast part, A step of determining the residual stress of the cast part by applying the first residual stress to the second residual stress, A method for analyzing residual stress in cast parts, comprising the following features.

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