A method for eliminating grain boundary cracks in large-sized directional hollow blades

Abstract

This invention belongs to the field of blade investment casting technology and relates to a method for eliminating grain boundary cracks in large-sized directional hollow blades. It is used for casting directional columnar crystal working blades for gas turbines and other applications with a transverse dimension exceeding 130mm. The method is achieved through the following steps: During wax pattern finishing, a cylindrical wax rod is adhered to the core positioning point extending from the trailing edge to enhance the coating properties of the mold slurry and increase the thickness of the mold shell at this location; during mold assembly, a wax pattern baffle is placed outside the leading and trailing edge positions to reduce the heat dissipation rate of the leading and trailing edges; during directional solidification, the settling time after pouring the alloy liquid is extended to slow down the solidification rate at the blade tip initiation position, thereby eliminating cracks at the directional grain boundaries caused by excessive transverse solidification stress and improving the yield rate of casting large-sized directional gas turbine working blades.

Description

Technical Field

[0001] This invention belongs to the field of blade investment casting technology and relates to a method for eliminating grain boundary cracks in large-sized directional hollow blades. Background Technology

[0002] High-temperature alloy castings are widely used in aero engines and gas turbines. In the field of gas turbines, internationally advanced units have reached power outputs of over 300MW, single-unit efficiencies of over 40%, and combined cycle efficiencies of 60%. Turbine inlet temperatures are generally between 1300-1500℃. Turbine blade materials have evolved from the initial equiaxed high-temperature alloys to the current directional and single-crystal high-temperature alloys. Similarly, forming processes have evolved from the initial ordinary casting to the current directional solidification / single-crystal casting. Turbine blade structures have evolved from solid structures to hollow air-cooled structures and then to film cooling structures.

[0003] As power designs increase and blade sizes grow, the demands on equipment and processes become more stringent. Directional solidification processes typically require a good temperature gradient and relatively straight isotherms along the longitudinal direction of the blade to control the sequential solidification along this dimension. Generally, the central region of the blade has a thicker wall and a higher metal content, while the wall thickness and metal content gradually decrease towards the leading and trailing edges. Simultaneously, the central region tends to have a planar structure, while the leading and trailing edges are often transitional or pointed structures. This results in a thicker mold at the planar structure, where coating is better, compared to the pointed structure, under the same mold processing conditions. These differences in metal content and mold thickness in different regions lead to a higher heat dissipation rate at the leading and trailing edges compared to the central region. This lateral temperature difference causes excessive lateral solidification rate variations, creating casting stress. Furthermore, due to equipment size constraints, increased blade size reduces module space utilization, and the larger empty areas near the leading and trailing edges further exacerbate the lateral temperature difference, leading to even greater casting stress. For oriented blades, although the grain size qualification rate is higher than that of single-crystal blades, the oriented grain boundaries are relatively weak links. During the casting process, the casting stress in the above-mentioned transverse dimension will concentrate at the grain boundaries and cause cracks to appear, becoming a bottleneck restricting the qualification rate of oriented blades.

[0004] Taking a directional hollow blade for a gas turbine as an example, the blade's dimensions are approximately 300mm*150mm*60mm, with maximum and minimum wall thicknesses of about 7mm and 1mm respectively, a difference of about 6mm. The thickness gradually decreases from the center to the trailing edge. The larger metal content in the central region further slows down heat dissipation, creating a greater lateral temperature difference with the leading and trailing edge regions during directional solidification. Furthermore, due to the difference in cross-sectional shape, the coating of the mold slurry on the curved surface of the central region is much better than that on the tip of the leading and trailing edge regions, resulting in a much thicker mold shell in the central region than at the leading and trailing edge. The mold shell, being a material with poor thermal conductivity, also contributes to a greater lateral temperature difference. The blade is assembled using wax molds on a 400mm diameter chassis. Excluding the module support structure of the central column tube, each group can only hold two blades, resulting in a space utilization rate of only about 60%. The excessive and uneven heat dissipation space leads to significant temperature differences at different locations on the blade, generating casting stress and ultimately causing crack defects concentrated at the grain boundaries. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of uneven heat dissipation during the directional solidification process of large-size directional gas turbine hollow blades, which leads to grain boundary cracks due to transverse temperature differences. This invention provides a method to eliminate grain boundary cracks in large-size directional gas turbine hollow blades, thereby solving the metallurgical problems encountered in large-size directional gas turbine hollow blades (blade dimensions: length ≥ 180 mm, width ≥ 130 mm, thickness ≥ 50 mm, wall thickness difference ≥ 5 mm).

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

[0007] A method for eliminating grain boundary cracks in large-size oriented hollow blades, used for casting oriented columnar working blades, the method being as follows:

[0008] When finishing the pressed blade wax mold, a cylindrical wax rod is glued to the positioning point of the core extending from the tail edge of the blade wax mold (called the tail edge core) to enhance the coating properties of the shell slurry and increase the shell thickness at this point; when the blade is placed vertically for module assembly, a wax mold baffle is set outside the front and tail edge positions to form a shell baffle during shell making; during directional solidification, the standing time after the alloy liquid is poured is extended before the shell is pulled out.

[0009] The specific steps are as follows:

[0010] The diameter of the wax rod is 3 to 4 times the thickness of the positioning core, typically 8 to 10 mm, and the bonding embedding depth is 0.5 to 1 mm. The bonding embedding operation involves machining the outer surface of the wax rod into a flat surface or groove to form surface-to-surface contact with the tail edge core, making bonding easier.

[0011] The axis of the bonded wax rod should be located as close as possible to the extension line of the tail core to ensure the symmetry and uniformity of the bonded structure; the wax mold baffle is a curved structure with a radius of curvature slightly less than 1 / 2 of the blade width, which is 1 / 3 to 1 / 2 of the blade width.

[0012] The blade's lateral dimension 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 and minimum wall thickness of the blade. Typically, the maximum wall thickness at the centerline of the blade width can reach 7mm, gradually decreasing towards the trailing edge. This results in excessive differences in solidification rates at different locations, creating casting stress and leading to directional grain boundary cracks, requiring special process control methods.

[0013] A partition is left between the two adhesive wax rods to expose the core, which serves as a core positioning window for contact with the shell during preparation and for positioning purposes.

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

[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. This structural size of the baffle effectively blocks heat dissipation from the leading and trailing edges during the directional solidification process, while also ensuring the space for sand pouring during shell making on the concave surface.

[0016] The installation position requirements for the wax mold baffle are as follows:

[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 edges on the chassis is maintained between 25 and 30 mm. The shell baffle is formed during shell making to reduce the heat dissipation rate at the front and rear edges during directional solidification. The wax mold baffle should be installed perpendicular to the chassis, and its width should cover the thickness of the front and rear edges of the blade wax mold.

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

[0019] During directional solidification, the settling time after casting the molten alloy is set to ≥3 minutes. Extending the settling time after casting the molten alloy before shell pulling, usually by no less than 3 minutes, reduces the temperature difference between the molten metal at the blade tip and the water-cooled plate it contacts. This causes a slow cooling process at the blade tip before the shell pulling begins, thus slowing down the solidification rate at the blade tip and reducing the stress generated during rapid solidification of the molten metal.

[0020] Furthermore, the edges of the adhesive wax stick are trimmed to create a slope of no less than 45° 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 this invention are:

[0023] In a method for eliminating grain boundary cracks in large-sized directional hollow blades according to the present invention, a cylindrical wax rod is bonded to the outside of the tail edge core positioning during wax mold finishing to enhance the coating properties of the shell slurry and increase the shell thickness at this location, thereby reducing the shell thickness difference at various locations on the blade cross section. During module assembly, a wax mold baffle is set outside the leading and trailing edge positions to reduce the heat dissipation rate of the leading and trailing edge tip structure during directional solidification. During directional solidification, the metal at the blade tip position will first come into contact with the lower-temperature water-cooled plate. This temperature difference will also create a large casting stress. Therefore, the blade tip is a common crack initiation location. When the shell pulling action begins after a short settling time, the blade tip position will be placed in the low-temperature region more quickly, resulting in greater casting stress. Therefore, appropriately extending the settling time after alloy liquid casting to 3 minutes will allow the blade tip position to undergo a slow cooling process, which can avoid excessive transverse solidification stress caused by excessively rapid cooling of the molten metal, leading to grain boundary cracks. Attached Figure Description

[0024] To more clearly illustrate the technical solutions implemented in this invention, the accompanying drawings used in the embodiments of this invention will be briefly explained below. Obviously, the drawings described below are merely some embodiments of this invention. Those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0025] Figure 1 This is a schematic diagram of the blade wax model dimensions in an embodiment of the present invention.

[0026] Figure 2 This is a schematic diagram of the dimensions of the adhesive wax stick, where (a) is a schematic diagram and (b) is a partial schematic diagram.

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

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

[0029] Figure 5 This is a schematic diagram showing the required distance between the cross sections of the blades and module structures.

[0030] Figure 6 These are typical blade grain boundary cracks; the cracks are shown in the boxes in the figure, (a) is a crack that runs through the blade tip and blade body, (b) is a crack at the blade tip, and (c) is a crack in the blade body.

[0031] Figure 7 These are grain boundary crack-free blades prepared by the process of this invention;

[0032] 1. Wax model body; 2. Tail edge positioning core; 3. Adhesive wax rod; 4. Core positioning window; 5. Wax model baffle; 6. Baffle support rod; 7. Sprue cup. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] The features of various aspects of the embodiments of the present invention will now be described in detail. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention can also be practiced without these specific details. The following description of the embodiments is merely intended to provide a better understanding of the invention by illustrating examples. The invention is not limited to any specific setups and methods provided below, but covers all improvements, substitutions, etc., to product structures and methods without departing from the spirit of the invention.

[0035] In the accompanying drawings and the following description, well-known structures and techniques are not shown to avoid unnecessarily obscuring the invention. This invention provides a method for eliminating grain boundary cracks in large-sized directional hollow blades. During wax pattern finishing, a cylindrical wax rod is adhered to the outer side of the tail-edge core positioning area to enhance the coating properties of the shell slurry and increase the shell thickness at this location, thereby reducing the shell thickness difference at various locations on the blade cross-section. During mold assembly, a wax pattern baffle is provided outside the leading and trailing edge positions to reduce the heat dissipation rate of the leading and trailing edge tip structure during directional solidification. During directional solidification, the settling time after casting the alloy liquid is extended. This method is used for casting large-sized directional hollow blades, and the dimensions of the hollow blade wax pattern body 1 are as follows... Figure 1 As shown, the blade is 400mm long, 120mm wide, and 80mm thick, with a wall thickness difference of 6mm. The trailing edge positioning core 2 is 280mm long and extends 3mm beyond the blade body along its extension line (as shown in the figure) for reinforcement and positioning. The thickness d of the positioning core 2 is 2mm (as shown in the figure). Figure 2 As shown in (b).

[0036] The implementation method includes the following steps:

[0037] A wax rod 3 with a diameter of 10mm is glued to the outside of the tail edge core, with the axis of the wax rod 3 kept on the extension line of the tail edge positioning core 2. Figure 2 As shown in (b), the diameter of the adhesive wax rod 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, resulting in excessive heat dissipation and causing cracks in the blade.

[0038] The curved surface of the adhesive wax roller 3 is machined into a plane with a width approximately equal to the thickness of the tail-edge positioning core 2 to facilitate bonding. Three 80mm long adhesive wax rollers 3 are evenly bonded to the outer side of the tail-edge positioning core 2, leaving two 20mm long core positioning windows 4 in the middle. Figure 2 As shown in (a).

[0039] Make a 45° bevel on the end face of the adhesive wax rod 3 on both sides of the empty core positioning window 4 for a smooth transition.

[0040] Fill the bonding edges of the adhesive wax rod 3 and the tail edge positioning core 2 with paraffin wax to transition them, and then assemble the modules.

[0041] The assembled wax model assembly is cleaned and inspected before the shell is prepared.

[0042] The wax model baffle 5 is bonded and assembled on the outer side of the front tail edge of the assembled blade wax model body 1, with the concave surface of the wax model baffle 5 facing the blade.

[0043] The installation position of the wax model baffle 5 is as follows: Figure 4 As shown: The projection distance between the wax mold baffle 5 and the outermost structure of the front and rear edges on the chassis is 25mm. Figure 5 As shown in h2), 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 distance between the outermost structure of the wax mold baffle and the projection of the chassis edge onto the chassis is not less than 15mm. Figure 5 (As shown in h1), this distance is reserved for the shell thickness.

[0045] For better results, such as Figure 4 As shown, a 10mm diameter baffle support rod 6 is used to connect the wax model baffle 5 to the chassis for oblique support.

[0046] During directional solidification, after the alloy liquid is filled and cast, the settling time is extended to 3 minutes, and the shell is then pulled out after the settling period. The result is as follows: Figure 7 As shown.

[0047] To illustrate the effectiveness of this method, a comparative experiment was conducted with existing techniques (which do not use adhesive wax rollers and wax mold baffles, and have a shorter settling time). The results are as follows:

[0048] like Figure 6(a) shows a blade prepared under the existing operation, in which the adhesive wax rod and wax mold baffle were not used, and the standing time was not extended (standing time 1 min). This will cause the metal liquid at the blade tip to solidify too quickly, resulting in large stress. The heat dissipation at the leading and trailing edges of the blade is too fast, which in turn leads to grain boundary cracks.

[0049] like Figure 6 (b) shows a blade prepared under the operating conditions of using a bonded wax rod and a wax mold baffle, but with insufficient settling time (1 min). The bonded wax rod and wax mold baffle slowed down the heat dissipation rate of the blade's leading and trailing edges, reducing the transverse temperature difference of the blade and thus preventing excessive solidification stress that could cause grain boundary cracks. However, due to insufficient settling time, the blade tip still experienced excessively rapid cooling, resulting in significant solidification stress and grain boundary cracks at the blade tip.

[0050] like Figure 6 (c) shows a blade prepared with an extended settling time (4 min) but without the use of a wax stick and wax mold baffle. Extending the settling time will alleviate the rapid solidification at the blade tip, thereby eliminating grain boundary cracks at the blade tip. However, without the use of a wax stick and wax mold baffle, grain boundary cracks will 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 person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should 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: The method for casting oriented columnar crystal working blades is as follows: When finishing the pressed blade wax model, a cylindrical wax rod is glued to the core positioning point extending from the tail edge of the blade wax model to enhance the coating properties of the shell slurry and increase the shell thickness at this point; when assembling the mold with the blade placed vertically, a wax model baffle is set outside the front and rear edges to form a shell baffle during shell making; during directional solidification, the settling time after the alloy liquid is poured is extended to ≥3 minutes before shell extraction; the specific operation is as follows: The diameter of the wax rod is 3 to 4 times the thickness of the positioning core, and the bonding embedding depth is 0.5 to 1 mm; The axis of the bonded wax rod is located on the extension line of the tail core; The wax mold baffle has a curved surface structure with a radius of curvature of 1 / 3 to 1 / 2 of the blade width, a height of 1 / 3 to 1 / 2 of the blade height, and a width that is equivalent to the blade thickness. The installation position requirements for the wax mold baffle are as follows: 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 tail edge on the chassis should be maintained between 25 and 30 mm, and the projection distance between the outermost structure of the wax mold baffle and the edge of the chassis on the chassis should be no less than 15 mm.

2. The method according to claim 1, characterized in that: The blade has a lateral dimension ≥130mm and a wall thickness difference ≥5mm; the wall thickness difference refers to the difference between the maximum and minimum wall thickness of the blade.

3. The method according to claim 1, characterized in that: A partition is left between the two adhesive wax rods to expose the core, which serves as a core positioning window for contact with the shell during preparation and for positioning purposes.

4. The method according to claim 3, characterized in that: The free end of the core positioning window is made of paraffin wax with a thickness of 0.1~0.3mm.

5. The method according to claim 3, characterized in that: The edges of the adhesive wax sticks are trimmed to create a slope of at least 45°; the wax sticks are then transitioned to the edges of the core.

6. The method according to claim 1, 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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