CN118788924A - A low-cost method for controlling the surface recrystallization of single crystal directional solidification blades - Google Patents

Abstract

The present invention relates to a low-cost method for controlling the surface recrystallization of a single crystal directional solidification blade, comprising the following steps: S1, wax pressing; S2, seeding bar; S3, mold shell preparation; S4, preparing the required directional single crystal blade casting blank. The present invention adopts a method of changing the seeding bar structure, so that the seeding bar will spontaneously break from the seeding bar notch during the solidification and shrinkage of the alloy component, thereby destroying the integrity between the single crystal alloy component and the seeding bar, so that the probability of mutual extrusion between the single crystal alloy component and the mold shell during the solidification and shrinkage process is greatly reduced, thereby fundamentally solving the surface recrystallization defects of the single crystal alloy component caused by the solidification shrinkage stress during the casting process, and effectively improving the production qualification rate of single crystal alloy structure castings.

Description

A low-cost method for controlling the surface recrystallization of single crystal directional solidification blades

Technical Field

The invention relates to a method for controlling the surface recrystallization of a single crystal directional solidification blade at low cost, belonging to the field of casting.

Background Art

40% to 50% of the metal materials in modern aircraft engines and gas turbines are nickel-based high-temperature alloys, mainly the third generation. Since single-crystal high-temperature alloys reduce grain boundary strengthening elements, once any grain boundary and recrystallized grains perpendicular to the stress principal axis are generated, they will become weak areas in the performance of single-crystal high-temperature alloy components. Once surface recrystallization occurs in a single-crystal component, its performance is greatly reduced, which may cause premature failure of the component. Therefore, the formation mechanism of single-crystal high-temperature alloy surface recrystallization and the control technology to prevent or eliminate single-crystal surface recrystallization are bottleneck problems that need to be urgently solved in the engineering application of single-crystal alloys.

According to the relevant technical research and engineering application of single crystal high temperature alloy components in China, many difficult problems of single crystal high temperature alloy components are mainly concentrated in the relatively serious surface recrystallization structure that occurs locally during the development of single crystal components. In fact, the surface recrystallization of nickel-based single crystal high temperature alloy components during the production process can be divided into two categories: one is due to the complex structure of the component or the hot forming process, which leads to large residual stress after hot forming of the component, and then surface recrystallization occurs in the subsequent heat treatment; the other is due to local stress concentration caused by external factors (such as bumps, sand blowing, polishing, grinding, etc.), which produces surface recrystallization in the subsequent heat treatment. According to statistics, during the hot forming preparation (casting, brazing, thermal spraying) of single crystal components, the probability of blade failure due to non-continuous structures such as surface recrystallization is more than 30%. Since single crystal alloy components often have large residual stress after casting, when the stress reaches a certain value, recrystallized structure is generated on the surface in the subsequent heat treatment.

In the actual production process, in order to ensure that the single crystal component can be prepared normally, the entire component needs to be kept at a temperature above 1500℃ for dozens of minutes to complete the crystal growth, which is a great challenge for the melt mold shell. In order to enable the crystal pulling process to proceed normally and orderly, the mold shell needs to have sufficient strength to withstand the long-term immersion of the high-temperature alloy liquid. At the same time, with the increase in blade size and the complexity of the shape, the solidification defects of high-temperature alloys become more and more serious. Among them, due to the change in the cross-sectional size of the blade (such as the protruding platform or corner), isolated supercooling zones are formed in some areas of the casting, and the nucleation is premature, forming miscellaneous crystals. The grain boundaries between the miscellaneous crystals and the matrix grains destroy the integrity of the single crystal and greatly reduce the mechanical properties of the single crystal blade. Although grain boundary strengthening elements are added to the alloy, crack sources are still easy to form in the grain boundaries, causing the blade to fail. Therefore, how to eliminate and control the formation of miscellaneous crystals and ensure the integrity of the single crystal has become an important link in the preparation technology of single crystal blades. As a relatively mature process, the technology of adding auxiliary seeding rods is often used in the directional solidification process of large-scale blades such as gas turbine blades. The process feature is that an auxiliary guide rod is introduced to guide the matrix dendrites to the isolated undercooling zone, forcing them to be filled with single crystal structure before cooling to the critical nucleation undercooling degree, thereby limiting the formation of impurity crystals.

Although the problem of impurity crystals in single crystal components can be solved by adding auxiliary seeding bar technology, the use of seeding bars will make the shape of alloy components more complicated. This complex configuration will inevitably aggravate the accumulation of shells in the wax mold of the component during the shell making stage, causing the shells to be too thick in some areas. In the later solidification and shrinkage process of the alloy component, the shells squeeze each other and bear the force, which makes the surface of the single crystal component alloy have large residual stress after the final solidification. When this part of the stress reaches a certain value, a recrystallized structure will be generated on the surface of the single crystal casting during the subsequent heat treatment. Experts in related fields have also conducted many studies on the surface recrystallization of alloy components caused by solidification shrinkage during the casting process. For example, the possibility of recrystallization of nickel-based single crystal high-temperature alloys during solution treatment is suppressed by pre-recovery heat treatment; or the deformation layer on the surface of the sample is removed by electrochemical corrosion when the degree of plastic deformation of the alloy component is small. However, none of these solves the problem from the source.

Summary of the invention

The purpose of the present invention is to overcome the above-mentioned shortcomings and provide a low-cost method for controlling the surface recrystallization of a single crystal directionally solidified blade.

The object of the present invention is achieved in that:

A low-cost method for controlling the surface recrystallization of a single crystal directional solidification blade, characterized by:

The steps include:

S1, wax pressing;

Inject the wax material into the casting and runner wax mold at a temperature of 50-90°C and an injection pressure of 0.5-10Mpa to obtain a part wax mold, a pouring cup wax mold, a chassis wax mold and a pouring system wax mold;

S2, seeding bar;

Use self-breaking seed bars during wax pattern preparation;

S3, mold shell preparation;

A casting shell with a thickness of 5-20 mm is coated on the surface of the wax mold module, and then dewaxed at 120-200° C., and calcined at 800-1050° C. for 1-5 hours to obtain a casting ceramic shell for single crystal orientation;

S4. Place the ceramic mold shell prepared in S3 into a directional solidification furnace, set the parameters to the mold shell insulation temperature of 1450-1600℃, remelt the master alloy ingot and pour it into the mold shell after reaching the temperature, select the pouring temperature at 1450-1600℃, let it stand for 50-150 seconds after pouring, and then pull it out. The pulling speed is set at 2.5-10mm/min. After the crystal pulling is completed, cool it in the furnace for 8-20 minutes and then take it out. Cut off the pouring system and polish it to prepare the required directional single crystal blade casting blank.

Compared with the prior art, the beneficial effects of the present invention are:

The present invention adopts a method of changing the structure of the seeding bar, so that during the solidification and shrinkage process of the alloy component, the seeding bar will spontaneously break from the notch of the seeding bar, thereby destroying the integrity between the single crystal alloy component and the seeding bar, so that the probability of mutual extrusion between the single crystal alloy component and the mold shell during the solidification and shrinkage process is greatly reduced, thereby fundamentally solving the surface recrystallization defects of the single crystal alloy component caused by the solidification shrinkage stress during the casting process, and effectively improving the production qualification rate of single crystal alloy structure castings.

DETAILED DESCRIPTION

The following examples are used to further describe the specific implementation of the present invention. The following examples are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

The present invention provides a low-cost method for controlling the surface recrystallization of a single crystal directional solidification blade, comprising the following steps:

S1, wax pressing;

Inject the wax material into the casting and runner wax mold at a temperature of 50-90°C and an injection pressure of 0.5-10Mpa to obtain a part wax mold, a pouring cup wax mold, a chassis wax mold and a pouring system wax mold;

S2, seeding bar;

Use self-breaking seed bars to replace traditional seed bars during wax pattern preparation;

S3, mold shell preparation;

A 5-20 mm thick casting shell is coated on the surface of the wax mold module, and then dewaxed at 120-200°C, and calcined at 800-1050°C for 1-5 hours to obtain a single crystal oriented precision casting ceramic shell;

S4. Place the above ceramic mold shell into a directional solidification furnace, set the parameters to the mold shell insulation temperature of 1450-1600℃, remelt the master alloy ingot and pour it into the mold shell after reaching the temperature, select the pouring temperature at 1450-1600℃, let it stand for 50-150 seconds after pouring, and then pull it out. The pulling speed is set at 2.5-10mm/min. After the crystal pulling is completed, cool it in the furnace for 8-20 minutes and then take it out. Cut off the pouring system and polish it to prepare the required directional single crystal blade casting blank.

The invention is easy to operate, does not require changes to existing equipment and production processes, and is easy to put into production on a large scale; the method is pollution-free to castings and furnace bodies and has high safety.

In the above embodiments, the present invention is only exemplarily described, but those skilled in the art may make various modifications to the present invention without departing from the spirit and scope of the present invention after reading this patent application.

Claims (1)

1. A method for controlling the surface recrystallization of a single crystal directional solidified blade at low cost, characterized in that it comprises the following steps: S1, wax pressing; Inject the wax material into the casting and runner wax mold at a temperature of 50-90°C and an injection pressure of 0.5-10Mpa to obtain a part wax mold, a pouring cup wax mold, a chassis wax mold and a pouring system wax mold; S2, seeding bar; Use self-breaking seed bars during wax pattern preparation; S3, mold shell preparation; A casting shell with a thickness of 5-20 mm is coated on the surface of the wax mold module, and then dewaxed at 120-200° C., and calcined at 800-1050° C. for 1-5 hours to obtain a casting ceramic shell for single crystal orientation; S4. Place the ceramic mold shell prepared in S3 into a directional solidification furnace, set the parameters to the mold shell insulation temperature of 1450-1600℃, remelt the master alloy ingot and pour it into the mold shell after reaching the temperature, select the pouring temperature at 1450-1600℃, let it stand for 50-150 seconds after pouring, and then pull it out. The pulling speed is set at 2.5-10mm/min. After the crystal pulling is completed, cool it in the furnace for 8-20 minutes and then take it out. Cut off the pouring system and polish it to prepare the required directional single crystal blade casting blank.