Method for eliminating grain boundary cracks of large-size directional hollow blade

Abstract

The invention belongs to the technical field of blade investment casting, and relates to a method for eliminating grain boundary cracks of a large-size directional hollow blade, which is used for casting directional column crystal working blades of gas turbines and the like with the transverse size exceeding 130mm. The method comprises the following steps of: when a wax mold is trimmed, adhering a cylindrical wax stick to a mold core positioning position extending out of a tail edge; the coating property of the enhanced shell slurry is used for increasing the thickness of the shell at the position; when the modules are combined, wax pattern baffles are arranged outside the front and tail edges, so that the heat dissipation speed of the front and tail edges is reduced; and during directional solidification, the standing time of the alloy liquid after casting is prolonged so as to slow down the solidification rate of the initial position of the blade tip, so that cracks at the directional grain boundary caused by overlarge transverse solidification stress are eliminated, and the casting qualification rate of the large-size directional gas turbine working blade is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of blade investment casting and relates to a method for eliminating grain boundary cracks of a large-size directional hollow blade. Background Art

[0002] High-temperature alloy castings are widely used in aircraft engines and gas turbines. In the field of gas turbines, the international advanced units have a power of more than 300MW, a single unit efficiency of more than 40%, and a combined cycle efficiency of 60%. The inlet temperature of the turbine is generally 1300-1500℃. The material of the turbine blades has developed from the initial equiaxed crystal high-temperature alloy to the current directional and single crystal high-temperature alloy. Similarly, the molding process has also developed from the initial ordinary casting to the current directional solidification / single crystal casting. The turbine blade structure has evolved from a solid structure to a hollow air-cooled structure and then to an air film cooling structure.

[0003] As the power design becomes higher and higher, the blade size becomes larger and larger, and the requirements for equipment and processes become higher and higher. The directional solidification process usually requires the module to have a good temperature gradient in the longitudinal direction and a relatively straight isotherm to control the sequential solidification of the longitudinal dimension of the blade. Usually, the wall thickness of the central area of ​​the blade is thicker and the metal content is more, while the wall thickness of the front and trailing edge area gradually decreases, and the metal content also decreases accordingly; at the same time, the shape of the central area of ​​the blade tends to be a plane structure, while the shape of the front and trailing edge area is mostly a transition and tip structure. This leads to the shell thickness of the plane structure with better coating properties being greater than that of the tip structure under the same shell process. The difference in metal content and shell in the above different areas will cause the heat dissipation rate in the front and trailing edge area to be greater than that in the central area. This temperature difference in the lateral dimension will cause the lateral solidification rate difference to be too large to form casting stress. At the same time, due to the limited size of the equipment, the increase in the blade size will lead to a decrease in the space utilization of the module. The excessive vacant area near the front and trailing edge will further aggravate the temperature difference in the lateral dimension of the blade and thus form greater casting stress. For oriented blades, although their grain size qualification rate is higher than that of single crystal blades, the oriented grain boundary is a relatively weak link. During the casting process, the casting stress in the above-mentioned lateral dimension will concentrate at the grain boundary and cause the appearance of cracks, becoming a bottleneck restricting the qualification rate of oriented blades.

[0004] Taking a gas turbine directional hollow blade as an example, the blade body size is about 300mm*150mm*60mm, the maximum and minimum wall thickness values ​​are about 7mm and 1mm, the wall thickness difference is about 6mm, and it gradually decreases from the center to the front and trailing edge area. The larger amount of metal in the center area slows down its heat dissipation rate, forming a larger lateral temperature difference with the front and trailing edge area during the directional solidification process; and due to the difference in its cross-sectional shape, the shell slurry coating of the curved surface structure in the center area is much greater than the tip structure in the front and trailing edge area, resulting in the shell thickness in the center area being much greater than the front and trailing edge position. As a material with poor thermal conductivity, the shell will also cause a larger lateral temperature difference. The blade is wax molded on a 400mm diameter chassis. Excluding the module support structure of the center column tube, only 2 blades can be placed in each group, and the space utilization rate is only about 60%. The heat dissipation space is too large and uneven, which leads to a large temperature difference at different positions of the blade, resulting in casting stress, and finally concentrated crack defects at the grain boundary. Summary of the invention

[0005] The purpose of the present invention is to provide a method for eliminating grain boundary cracks in the above-mentioned large-size directional hollow blades of directional combustion engines, in view of the disadvantage that uneven heat dissipation during directional solidification leads to lateral temperature differences and causes grain boundary cracks, so as to solve the metallurgical technical problems of large-size directional hollow blades of directional combustion engines (blade dimensions: length ≥180mm, width ≥130mm, thickness ≥50mm, wall thickness difference ≥5mm).

[0006] To solve this technical problem, the technical solution of the present invention is:

[0007] A method for eliminating grain boundary cracks of a large-sized directional hollow blade, used for casting directional columnar crystal working blades, the method comprising:

[0008] When trimming the intact blade wax mold, a cylindrical wax stick is bonded to the positioning position of the core (called the trailing edge core) extending from the trailing edge of the blade wax mold to enhance the coating of the shell slurry and increase the thickness of the shell at this location; when the blade is placed vertically for module assembly, a wax mold baffle is set outside the front trailing edge position to form a shell baffle when making the shell; during directional solidification, the standing time after the alloy liquid is cast is extended before the shell is pulled.

[0009] The specific operations are as follows:

[0010] The diameter of the wax stick is 3 to 4 times the thickness of the positioning core, generally 8 to 10 mm, and the bonding embedding depth is 0.5 to 1 mm. The bonding embedding operation is to process the outer side of the wax stick into a plane or a groove to form a surface contact with the trailing edge core, which is easier to bond.

[0011] The axis line of the bonded wax stick is located on the extension line of the trailing edge core as much as possible to ensure the symmetry and uniformity of the bonding structure; the wax mold baffle is a curved structure with a curvature radius slightly smaller than 1 / 2 of the blade width, which is 1 / 3 to 1 / 2 of the blade width.

[0012] The transverse dimension of the blade is ≥130mm, and the wall thickness difference is greater than or equal to 5mm. The wall thickness difference refers to the difference between the maximum wall thickness and the minimum wall thickness of the blade. Usually, the maximum wall thickness at the midline of the blade width can reach 7mm, and gradually decreases toward the front and trailing edges. This causes a large difference in solidification rate at different positions, forming casting stress and directional grain boundary cracks, which require special process methods to control.

[0013] A partition is left between the two bonding wax sticks to expose the core as a core positioning window, which is used for contacting with the shell during preparation to play a positioning role.

[0014] The core positioning window is made of paraffin wax with a free end having a thickness of 0.1 to 0.3 mm, so as to avoid stress caused by the core breaking caused by the tight contact between the core and the shell during the directional solidification process.

[0015] The height of the wax mold baffle is 1 / 3 to 1 / 2 of the blade height, and the width is equivalent to the blade thickness. The baffle of this structural size can effectively block the heat dissipation of the leading and trailing edges during directional solidification, while also ensuring the sand spraying space of the concave surface during shell making.

[0016] The installation position requirements of the wax mold baffle are:

[0017] After the wax mold baffle is bonded to the chassis, the projection distance between the wax mold baffle and the outermost structure of the front and rear edge on the chassis is maintained between 25 and 30 mm, and the shell baffle is formed during shell making to reduce the heat dissipation speed at the front and rear edge during directional solidification; the wax mold baffle should be installed perpendicular to the chassis, and its width range should cover the thickness of the front and rear edge of the blade wax mold;

[0018] The projection distance between the outermost structure of the wax mold baffle and the edge of the chassis on the chassis shall not be less than 15mm, and this distance is reserved for the thickness of the shell.

[0019] During directional solidification, the standing time after alloy liquid casting is set to ≥3min. Extending the standing time after alloy liquid casting before shell drawing, usually not less than 3min, will reduce the temperature difference between the metal liquid at the tip and the water cooling plate in contact with it, and will cause a slow cooling process at the tip before the shell drawing begins, so as to slow down the solidification rate at the starting position of the blade tip and reduce the stress generated by the rapid solidification of the metal liquid.

[0020] Furthermore, the wax stick is bonded to a partition edge and trimmed to form a slope structure of not less than 45 degrees to transition the wax stick to the edge of the core.

[0021] Furthermore, the outer side of the wax mold baffle is connected to the chassis by a baffle support rod for oblique support.

[0022] The beneficial effects of the present invention are:

[0023] In a method for eliminating grain boundary cracks of large-sized directional hollow blades of the present invention, a cylindrical wax stick is bonded to the outer side of the trailing edge core positioning during wax mold trimming to enhance the coating property of the shell slurry to increase the shell thickness at this location, so as to reduce the difference in shell thickness at various locations of the blade cross section; when the module is assembled, a wax mold baffle is arranged outside the front trailing edge position to reduce the heat dissipation speed of the front trailing edge tip structure during directional solidification; during directional solidification, the metal at the blade tip position will first contact a water cooling plate with a lower temperature, and this temperature difference will also form a larger casting stress, so the blade tip is a common crack origin position, and when the shell pulling action is started when the standing time is short, the blade tip position will be placed in a low temperature area faster and form a larger casting stress, so appropriately extending the standing time after the alloy liquid is cast to 3 minutes will cause a slow cooling process at the blade tip position, which can avoid the metal liquid cooling too fast and generating excessive lateral solidification stress leading to grain boundary cracks. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions implemented in the present invention, the following is a brief explanation of the drawings required to be used in the examples of the present invention. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0025] Figure 1 It is a schematic diagram of the dimensions of the blade wax model of an implementation example of the present invention.

[0026] Figure 2 Schematic diagram of the dimensions of the bonding wax stick, wherein (a) is a schematic diagram and (b) is a partial schematic diagram.

[0027] Figure 3 It is a schematic diagram of the blade wax model module.

[0028] Figure 4 It is a partial schematic diagram of the cross section of the blade and module structure.

[0029] Figure 5 It is a schematic diagram of the cross-sectional distance requirements for blades and module structures.

[0030] Figure 6 It is a typical blade grain boundary crack; the cracks are in the wireframe in the figure, (a) is an overall crack that runs through the blade tip and blade body, (b) is a blade tip crack, and (c) is a blade body crack.

[0031] Figure 7 The invention discloses a blade without grain boundary cracks prepared by the process of the invention;

[0032] 1. Wax mold body; 2. Tail edge positioning core; 3. Bonding wax stick; 4. Core positioning window; 5. Wax mold baffle; 6. Baffle support stick; 7. Pouring cup. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0034] The features of various aspects of the embodiments of the present invention will be described in detail below. In the detailed description below, many specific details are proposed to fully understand the present invention. However, it is obvious to those skilled in the art that the present invention can also be implemented without these specific details. The following description of the embodiments is only for a better understanding of the present invention by illustrating examples of the present invention. The present invention is not limited to any specific settings and methods provided below, but covers all product structures, any improvements, replacements, etc. of methods covered without departing from the spirit of the present invention.

[0035] In the various drawings and the following description, well-known structures and technologies are not shown to avoid unnecessary ambiguity of the present invention. The present invention provides a method for eliminating grain boundary cracks in large-sized directional hollow blades. When the wax mold is trimmed, a cylindrical wax stick is bonded to the outer side of the trailing edge core positioning to enhance the coating property of the shell slurry to increase the thickness of the shell at this location, so as to reduce the difference in shell thickness at various locations of the blade cross section; when the mold assembly is combined, a wax mold baffle is set outside the front and trailing edge position to reduce the heat dissipation rate of the front and trailing edge tip structure during directional solidification; during directional solidification, the standing time of the alloy liquid after casting is extended. For casting large-sized directional hollow blades, the size of the wax mold body 1 of the hollow blade is as follows: Figure 1 As shown, it is 400mm long*120mm wide*80mm thick with a wall thickness difference of 6mm. The length of the trailing edge positioning core 2 is 280mm. It extends 3mm (as shown in the figure w) from the blade body along the extension line direction (as shown in the figure) for reinforcement and positioning. The thickness d of the positioning core 2 is 2mm (as shown in the figure d). Figure 2 (b) as shown.

[0036] The implementation method includes the following steps:

[0037] A wax stick 3 with a diameter of 10 mm is bonded to the outside of the trailing edge core, and the axis of the wax stick 3 is kept on the extension line of the trailing edge positioning core 2. Figure 2 (b) The diameter of the bonding wax stick is 3 to 4 times the thickness of the positioning core, generally 8 to 10 mm. If the diameter is too small, the thickness of the blade trailing edge shell will be insufficient and the heat will dissipate too quickly, which will also cause cracks in the blade body.

[0038] The adhesive wax stick 3 is processed into a plane with a width equivalent to the thickness of the trailing edge positioning core 2 to facilitate bonding. Three sections of adhesive wax stick 3 with a length of 80 mm are evenly bonded to the outside of the trailing edge positioning core 2, leaving two core positioning windows 4 with a length of 20 mm in the middle. Figure 2 (a) shown.

[0039] The end faces of the bonding wax stick 3 on both sides of the vacated core positioning window 4 are made into 45° inclined surfaces for smooth transition.

[0040] The bonding edges between the bonding wax stick 3 and both sides of the trailing edge positioning core 2 are filled with paraffin wax for transition, and then the module is assembled.

[0041] The assembled wax mold set is cleaned and inspected before the shell is prepared.

[0042] The wax mold baffle 5 is bonded and assembled outside the front and rear edges of the assembled blade wax mold body 1, with the concave surface of the wax mold baffle 5 facing the blade.

[0043] The installation position of the wax pattern baffle 5 is as follows: Figure 4 As shown: the projection distance between the wax model baffle 5 and the outermost structure of the front and rear edge on the chassis is 25mm ( Figure 5 As shown in h2 in the figure, a shell baffle is formed during shell making to reduce the heat dissipation rate at the front and rear edges during directional solidification;

[0044] The projection distance between the outermost structure of the wax mold baffle and the edge of the chassis on the chassis shall not be less than 15mm ( Figure 5 This distance is reserved for the shell thickness.

[0045] For better results, Figure 4 As shown, a baffle support rod 6 with a diameter of 10 mm is used to connect the outer side of the wax mold baffle 5 to the chassis for oblique support.

[0046] During directional solidification, after the alloy liquid is filled and cast, the standing time is extended to 3 minutes, and the shell is drawn after the standing time. Figure 7 shown.

[0047] In order to illustrate the effect of this method, a comparative test was conducted with the prior art (not using bonding wax sticks and wax mold baffles, and short standing time), and the results are as follows:

[0048] like Figure 6(a) shows a blade prepared under the existing operation, in which no adhesive wax stick and wax mold baffle are used, and the standing time is not extended (standing time is 1 minute). This will cause the metal liquid at the tip to solidify too quickly to form a large stress, and the front and trailing edges of the blades to dissipate heat too quickly, which will lead to the appearance of grain boundary cracks.

[0049] like Figure 6 (b) shows a blade prepared under the operating conditions of using an adhesive wax stick and a wax mold baffle, but with insufficient standing time (standing time of 1 minute). The adhesive wax stick and the wax mold baffle slow down the heat dissipation rate at the leading and trailing edges of the blade, reducing the lateral temperature difference of the blade, thereby preventing excessive solidification stress from causing grain boundary cracks. However, due to insufficient standing time, the cooling rate at the blade tip is still too fast, resulting in a large solidification stress and grain boundary cracks at the blade tip.

[0050] like Figure 6 (c) shows a blade prepared with an extended standing time (standing time of 4 minutes) but without the use of adhesive wax sticks and wax mold baffles. Extending the standing time will alleviate the rapid solidification at the tip of the blade and eliminate the grain boundary cracks at the tip of the blade. However, not using adhesive wax sticks and wax mold baffles will cause grain boundary cracks to appear at the blade body.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the field can easily think of various equivalent modifications or substitutions within the technical scope disclosed by the present invention, and these modifications or substitutions should all be covered within the protection scope of the present invention.

Claims

1. A method for eliminating grain boundary cracks in large-sized directional hollow blades, characterized in that: For casting directional columnar crystal working blades, the method is: When trimming the pressed blade wax mold, a cylindrical wax stick is bonded to the core positioning position extending from the trailing edge of the blade wax mold to enhance the coating of the shell slurry and increase the thickness of the shell at this point; when the blade is placed vertically for module assembly, a wax mold baffle is set outside the front and trailing edge positions to form a shell baffle during shell making; during directional solidification, the static time after alloy liquid casting is extended before shell extraction; the specific operations are as follows: The diameter of the wax stick is 3 to 4 times the thickness of the positioning core, and the bonding embedding depth is 0.5 to 1 mm; The axis line of the bonded wax stick is located on the extension line of the trailing edge core to ensure the symmetry and uniformity of the bonding structure; the wax mold baffle is a curved surface structure with a curvature radius of 1 / 3 to 1 / 2 of the blade width.

2. The method according to claim 1, characterized in that: The transverse dimension of the blade is ≥130 mm, and the wall thickness difference is greater than or equal to 5 mm. The wall thickness difference refers to the difference between the maximum wall thickness and the minimum wall thickness of the blade.

3. The method according to claim 1, characterized in that: A partition is left between the two bonding wax sticks to expose the core as a core positioning window, which is used for contacting with the shell during preparation to play a positioning role.

4. The method according to claim 1, characterized in that: The core positioning window is made of paraffin wax as a free end with a thickness of 0.1 to 0.3 mm.

5. The method according to claim 1, characterized in that: The height of the wax mold baffle is 1 / 3 to 1 / 2 of the blade height, and the width is equivalent to the blade thickness. The baffle of this structural size can effectively block the heat dissipation of the leading and trailing edges during directional solidification, while also ensuring the sand spraying space of the concave surface during shell making.

6. The method according to claim 5, characterized in that: The installation position requirements of the wax mold baffle are: After the wax mold baffle is bonded to the chassis, the projection distance between the wax mold baffle and the outermost structure of the front and rear edges on the chassis is maintained between 25 and 30 mm, and the mold shell baffle is formed during shell making to reduce the heat dissipation speed at the front and rear edges during directional solidification; The projection distance between the outermost structure of the wax mold baffle and the edge of the chassis on the chassis shall not be less than 15mm, and this distance is reserved for the thickness of the shell.

7. The method according to claim 1, characterized in that: During directional solidification, the standing time of the alloy liquid after casting is set to ≥3 minutes.

8. The method according to claim 3, characterized in that: The wax stick is bonded to the partition edge and trimmed to a slope structure of no less than 45°. Transition is made between the wax stick and the edge of the core.

9. The method according to claim 5, characterized in that: The outer side of the wax mold baffle is connected to the chassis by a baffle support rod for oblique support.

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