JPS6316215B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a method for manufacturing a casting mold that prevents the occurrence of shrinkage cavities, and more specifically, performs numerical calculations on an analytical model that combines a mold and molten metal in its cavity. The present invention relates to a method for preventing the occurrence of shrinkage cavities by finding a mold shape and structure that satisfies required conditions, that is, a mold plan.

It is widely known that in cast products, shrinkage cavities occur in areas where it is difficult to replenish the molten metal or where it solidifies last during the process of solidifying the molten metal in the mold, and various countermeasures have been taken. That is, a feeder is provided above or outside the area where shrinkage cavities are likely to occur, and molten metal is supplied to the location where shrinkage cavities occur.
Various measures can be taken, such as cutting off the feeder part after solidification, accelerating solidification in areas where there is a risk of shrinkage cavities, and adding extra wall to constricted areas that may impede the supply of molten metal to delay solidification. This method has been adopted, and empirical formulas compiled based on experience have been proposed, making it possible to calculate the size of the riser, how to add extra thickness, etc.

However, even when producing a cast product with a constant shape, the initial process is to repeat casting tests by trial and error to find a mold structure that prevents the occurrence of shrinkage cavities, in other words, a mold method. It is a common practice to check for shrinkage cavities inside a cast product by providing an excessively large feeder or extra wall, or by X-ray inspection, and to repair the product if it can be repaired by welding.

On the other hand, with the spread of computers, attempts have been made to find mold structures and shapes that do not generate shrinkage cavities through numerical calculations of the solidification process of cast products.

For example, when a feeder is installed at the top of a casting, the solidification time of each part inside the casting is calculated and an equal solidification time curve is drawn so that solidification progresses sequentially from the bottom of the casting to the feeder at the top. Methods have been attempted to prevent the occurrence of shrinkage cavities by determining mold conditions, and when the occurrence of shrinkage cavities is predicted, some success has been achieved by taking steps in the molding strategy. However, in the method of preventing the occurrence of shrinkage cavities by changing the structure and shape of the mold based on the equal solidification time curve, the calculation results show that the equal solidification time curve is drawn almost parallel to the riser above, and solidification occurs toward the riser. It is known that shrinkage foci occur even when the disease appears to be progressing.

As a result of research on the relationship between numerical calculations using an analytical model consisting of a mold and the molten metal in its cavity and the occurrence of shrinkage cavities in actual cast products, the inventor found that the solid phase of the element assumed in the section of the analytical model is as described below. It has been learned that shrinkage foci occur when the value of the rate gradient F is below a certain value. Based on this knowledge, the present invention calculates the shrinkage of a cast product by finding a casting mold plan in which the molten metal elements on each section of the analytical model consisting of the mold and molten metal have a solid fraction gradient value F exceeding a certain value. In order to provide a method for preventing the formation of cavities, an analysis model consisting of a mold and the molten metal in the mold cavity is divided into one or a plurality of parallel sections, and each section is divided into squares or triangles. Divide into elements, give each element a material, divide it into mold elements and molten metal elements, give each element an initial temperature, and give boundary conditions to each boundary, and calculate the flow of heat between each element at each minute time. Convert the temperature change of each element from
The solid fraction f _i of the molten metal element at the above temperature is determined by the following formula, solid fraction f _i = 1.0 x liquidus temperature - element temperature / liquidus temperature - solidus temperature for each molten metal element. The solid fraction gradient F between the molten metal element and the surrounding elements at the end of solidification is determined by the following formula: F= _o 〓 ⁱ⁼¹ 1.0−f _i /△l _i where △l _i = element nodal distance, F≦0.2 When this occurs, the analysis model is recreated and the above calculation is repeated, and the solid phase gradient F of each molten metal element is set to a value exceeding 0.2 when all molten metal elements are completely solidified. Relating to a method of manufacturing a casting mold to prevent

Next, the method of the present invention will be explained using a compressor vane as an example with reference to the accompanying drawings.

If an isosolidification time curve is obtained according to the conventional method for the vane 1 shown in FIGS. 1 and 2, for example, the cross section will be as shown in FIG. It can be seen that the solidification progresses over time, and no shrinkage nests are expected to be generated inside the blade portion 4. However, as shown in FIG. 4, in the actual cast product, it was confirmed by X-ray photography that a shrinkage cavity 5 was generated approximately in the center of the blade 4.

The method of the present invention is used to find a mold structure and shape that can prevent the occurrence of shrinkage cavities as follows.

First, an analytical model as shown in FIGS. 6 to 8 consisting of the mold 6 and the molten metal 7 in its cavity is set up, and this is subjected to numerical calculations according to the usual procedure. If the cast product is almost symmetrical, in order to simplify the calculation, for example, in the case of the vane in this example, the vane is divided into left and right sections along the cross-section shown by the dashed-dotted line in Figure 3.
Numerical calculations may be performed on either one of the cross sections as a heat insulating surface.

This analytical model can be used to calculate sections S1, S2, S3 as shown in FIGS. Divide into ,……. It is preferable to select each section so that the entire cast product can be seen with a small number of sections when examining the calculation results. If the vane in this example has a nearly flat shape, it is easier to see inside the cast product by taking a section parallel to the vane than by taking a section perpendicular to the vane. be understood. Note that the number of sections is one in the case of two-dimensional analysis, and multiple in the case of three-dimensional analysis.

Next, each section is divided into square or triangular elements 8 as shown in FIG. The smaller the size of each element 8, the higher the calculation accuracy will be, but the time required for calculation will increase. On the other hand, if it is too large, the calculation accuracy will decrease, but the calculation time will be short. Therefore, depending on the shape of the cast product, it is generally divided into rectangular pieces ranging from several millimeters to several centimeters in size.

Next, each element 8 is given a material. That is, each element can be distinguished by a symbol of mold material × or molten metal ○. The thermal conductivity, specific gravity, and specific heat of each element vary depending on the material.

Next, write in the symbol for the boundary condition, ie, insulation + or air △.

Examples of sections of the analytical model subjected to the above processing are illustrated in FIGS. 10 and 11.

For each element of each section treated by the well-known method as described above, the solid fraction gradient with respect to surrounding elements is determined as follows. To do this, first set the initial temperature of each element (for example, 1600℃ for the molten metal element, and 1600℃ for the mold element.
1050°C), let the node at the center of the element represent the element temperature, and calculate the temperature change by determining the flow of heat between each element at every minute time Δt from time zero using the following equation (1).

Amount of heat = area between elements x temperature difference x △t/thermal resistance... (1) Thermal resistance is determined by each material (thermal conductivity), distance between nodes, and heat transfer coefficient.

The change in temperature of each element due to the transfer of this amount of heat is calculated using the following equation (2). However, if the density of the element is d, the specific heat is c, and the volume is V, then temperature change = amount of heat/d x c x V... (2) Next, find the solid fraction of the molten metal element. The temperature of each element decreases over time, but the molten metal element has a solidus ratio f _i of 12 between the liquidus temperature T _L and the solidus temperature T _S.
As shown in the figure, it is assumed that the value changes from zero to 1 in proportion to the drop in temperature. Therefore the temperature of element i
The solid fraction f _i at T _i can be determined by the following equation (3).

f _i =1.0×T _L −T _i /T _L −T _S ……(3) Next, for each molten metal element, when that element has finished solidifying, the solid fraction gradient F between it and the surrounding elements is as follows. of(4)
Obtained by the formula. However, Δl _i is the distance between the node of the i element and the contact point of the adjacent element.

F= _o 〓 ⁱ⁼¹ 1.0−f _i /△l _i ...(4) In the formula, n is 4 in two-dimensional calculation and 6 in three-dimensional calculation when the element is a rectangle.

Each time each molten metal element finishes solidifying, the solid phase gradient F
If calculated, each molten metal element will have its own F value as illustrated in FIG.

Since the solid fraction gradient F is considered to be a parameter representing the difficulty of supplying molten metal to the solidified portion, the smaller the value of F, the more likely shrinkage cavities will occur.

The inventor investigated the relationship between the F value and the occurrence of shrinkage foci and found that when F≦0.2, shrinkage foci occur there. Therefore, create an analytical model for a cast product of the desired shape by changing the mold material, thickness, and boundary conditions, and repeat the above calculation to find out that F≦0.2 does not hold for all molten metal elements, that is, F>
By finding the mold conditions and boundary conditions that give 0.2 and casting with such molds and boundary conditions, it is possible to prevent the occurrence of shrinkage cavities.

In order to prevent the occurrence of shrinkage cavities in the vanes shown in FIGS.
As shown in Figure 4, a 12 mm thick ceramic wool insulation material 9 was wrapped around the outside of the mold 6' for the lower half of the blade. The mold 6 is made of zircon-shamotsu material and has a thickness of 8 mm. Solidification analysis was performed with such a mold structure, and the F value was determined for each molten metal element, and F = 0.2 and F = 0.3 for section 5.
An example of the equal distribution curve is shown in FIG. With this, there was still a region where F≦0.2, and the occurrence of shrinkage foci could not be prevented.

Therefore, we next performed a solidification analysis using a mold structure in which the thickness of the mold part 6'' in the upper half of the blade was reduced from 8 mm to halved to 4 mm, and among the results of determining the F distribution, the equal distribution curve for section 5 was obtained. For example, the 15th
As shown in the figure. As is clear from this figure, there is no place where F≦0.2 in the blade, and all F
>0.2, and it was predicted that the occurrence of shrinkage cavities could be prevented, so we actually made a mold with this structure,
The vanes were cast by injecting chromium-based stainless steel at 1,600°C, and X-ray inspection was conducted to confirm that no shrinkage cavities had occurred.

As explained above, according to the method of the present invention, it is possible to prevent the occurrence of shrinkage cavities that cannot be prevented using conventional molding methods based on experience or experimental methods even if extra thickness is added or the feeder is made excessively large. Can be done.

Calculations can be easily performed by using an electronic computer and using a pre-programmed program.
As a result, the most economical mold structure capable of preventing shrinkage cavities, and therefore the technically optimal molding method, can be found, and the practical effects thereof are extremely large.

In addition to changing the wall thickness of the mold, there are other ways or means to change the cooling rate of the molten metal to eliminate areas where F≦0.2, such as using heat insulating material, incorporating a heater to partially heat the mold, or blowing compressed air, etc. Conventionally known means can be used, such as forcing the mold to cool partially, using a chiller for a part of the mold, or providing a riser at a suitable location.

[Brief explanation of the drawing]

Fig. 1 is a front view and b plan view of a vane for explaining an embodiment of the method of the present invention, Fig. 2 is a side view thereof, Fig. 3 is an isosolidification time diagram according to a conventional method, and Fig. 4 5 is a sketch of an X-ray photograph showing the presence of shrinkage foci, FIG. 5 is a diagram showing an example of a constant solid fraction gradient line calculated by the method of the present invention, and FIG. 6 is a diagram to which the method of the present invention is applied. 7 is a sectional view taken in the same direction as shown in FIG. Graph showing an example of a section consisting of mold elements, No. 11
The figure is a graph showing an example of a section consisting of a mold element and a molten metal element, Figure 12 is a graph showing the relationship between the temperature of the molten metal element and the solid fraction, and Figure 13 is an example of the distribution of the molten metal element and the solid fraction gradient. Graph showing 14th
The figure is a constant solid fraction gradient distribution diagram of an example of an analytical model section where shrinkage cavities occur, and FIG. 15 is a uniform solid fraction gradient distribution diagram of an example in which shrinkage cavities are prevented from occurring. 1... Vane, 2... Vane tip, 3... Vane base, 4... Vane blade, 5... Shrinkage nest,
6... Mold, 7... Molten metal in cavity, 8... Element, 9... Ceramic wool.

Claims (1)

[Claims] 1. An analytical model consisting of a mold and the molten metal in the mold cavity is divided into one or a plurality of parallel sections, each section is divided into quadrangles or triangles as elements, and each element has its own material. is divided into mold elements and molten metal elements, giving an initial temperature to each element and a boundary condition to each boundary, and converting the temperature change of each element from the flow of heat between each element determined at each minute time. , The solid fraction f _i of the molten metal element at that temperature is determined by the following formula, solid fraction f _i = 1.0 x liquidus temperature - element temperature / liquidus temperature - solidus temperature For each molten metal element. The solid fraction gradient F with respect to the surrounding elements at the end of solidification of the molten metal element is determined by the following formula: F= _o 〓 ⁱ⁼¹ 1.0−f _i /△l _i where △l _i = nodal distance, F≦ When it becomes 0.2, the analysis model is recreated and the above calculation is repeated, and when all the molten metal elements have solidified, the solid fraction gradient F of each molten metal element is set to a size exceeding 0.2 to prevent the occurrence of shrinkage cavities. How to make a casting mold plan to prevent this.