CN1895815B - Lost-wax casting process with contact layer - Google Patents

Abstract

A dewaxing casting technology using contact layer for preparing the multi-layer ceramic shell mould including a mother mould made of wax or its similar material and at least one contact layer includes such steps as dipping said mother mould in a modeling paint containing ceramic particles and adhesive to form a contact layer, depositing black particles on said contact layer, and drying.

Description

Lost Wax Casting Using Contact Layer

technical field

The invention involves making components, such as metal blades and shrouds, with complex geometries using a technique known as lost-wax casting.

Background technique

In the manufacture of turbojet engine blades and shrouds, such as rotor or stator components or structural components, using lost wax casting techniques, wax or similar materials that are easily disposed of in subsequent processes are used to form master patterns. Multiple masters can be grouped together if necessary. The master is quenched in a first build paint to form a first layer of material in contact with the surface of the master around which a ceramic mold is produced. The surface of this first layer is sandblasted for easier bonding to the subsequent layers, followed by drying (including the application of stucco and drying operations) in its entirety. Subsequent quenching operations are repeated on the styling paint, which may differ in composition, each quenching operation being followed by plastering and drying, so that a ceramic shell consisting of several layers is formed. The shape coatings are admixtures of ceramic material particles, especially powders (for example alumina, mullite, zircon, etc.) with colloidal inorganic binders and (if necessary depending on the desired rheology) consist of. These blends are able to control and stabilize the properties of different types of ceramic layers without cracking due to the different physicochemical properties of the raw materials that make up the styling paint. They may include wetting agents, diluents, or texture modifiers for the desired deposit thickness.

The shell mold is then dewaxed, a step that removes the material that made up the original master mold. After removing the master mold, a ceramic mold is obtained whose cavity replicates all the details of said master mold. The mold is then subjected to a high-temperature heat treatment, or "fired," to impart the desired mechanical properties. The shell molds thus obtained can be used for casting metal parts.

After checking the internal and external integrity of the shell mold, the next step is to pour molten metal into the mold cavity, followed by solidification of the metal within the mold cavity. Currently in the field of lost wax casting, there are multiple solidification techniques, so the various casting techniques are classified according to the properties of the alloy and the properties required for the cast part. These techniques are respectively columnar structure directional solidification (DS), single crystal structure directional solidification (SX) or equiaxed solidification (EX). The first two categories of components (eg HP turbine blades) involve superalloys that enable the component to withstand the high loads, heat and mechanical strength found in turbojet engines.

After casting the alloy, shake out (shake out) to break the shell mold, the manufacture of metal parts.

During the casting step, various shells can be used in different ways. Each shell should have specific properties to enable the desired type of curing.

For example, for isometric curing, several different methods can be implemented, one using an ethyl silicate-based adhesive and another using a colloidal silica-based adhesive. For orientational curing, different batch materials (silica-alumina, silica-zircon or silica-based batches) can be used to form the shell.

The first layer of these shells is an important element that forms the interface between the shell mold and the casting alloy. It should be inactive to cast alloys in the case of columnar or monocrystalline solidification. In the case of equiaxed solidification, it should allow equiaxed grain growth. Among other things, the integrity of this contact layer determines the final quality of the cast part, especially the surface condition.

This first layer should actually meet certain requirements to avoid defects such as ceramic loss of adhesion and surface defects.

Loss of adhesion of contact layers before or during casting can create unwanted marks on the part.

Surface defects resulting from excessive microporosity in the contact layer can create unwanted protrusions on the part.

The main surface defects often originate from surface capillarity at the interface between the wax master and the first layer. After quenching the first layer, during the spreading of the grit, the grit will form a stack, forming many capillaries. Each capillary is equivalent to a suction cup, which forms a low pressure zone. The smaller the capillary, the greater the low pressure. This is equivalent to not having sufficient thickness for the first layer. The low pressure causes the styling paint to rise capillarily towards the plaster until the liquid column thus formed restores the pressure difference. This results in the formation of recessed regions with cavities leading to the formation of surface defects. This phenomenon is especially serious for first layers that are too thin.

These two defects (the main defects of casting) are related to the inherent resistance properties of the contact layer. In fact, to avoid loss of adhesion of the ceramic, it is necessary to obtain a thin and uniformly deposited first layer, while to avoid surface defects, the first layer should be deposited uniformly and thickly.

The properties of the contact layer should therefore be balanced with said resistance properties so that all imperfections of the part will not cause cracks.

Contents of the invention

The present invention achieves this object by the following methods.

A method for the manufacture of a multilayer ceramic shell mold comprising the manufacture of a contact layer on a master mold of wax or other similar material, the method comprising stepping The master is cooled to form the contact layer, black particles are deposited on this layer and the contact layer is dried. According to the invention, the method is characterized in that the ceramic particles of the styling paint are mullite particles. More specifically, the admixture includes a wetting agent, a diluent and a texturing agent.

Due to the composition of the modeling paint, it is possible to achieve the purpose set for all casting molds, so that the mold properties meet the casting conditions, especially the requirements of the DS and SX curing methods. Specifically, the contact layer does not react with the cast superalloy.

In order to meet economic requirements related to waste, the styling paint preferably comprises 65-90% by weight of mullite powder, free of zircon. Also, the grit or "stucco" used for this contact layer consists of mullite particles rather than zircon particles.

The addition of the admixture to the styling paint can control the deposition on the wax and ensure optimum performance on the part with regard to thickness and distribution.

Preferably, in order to meet environmental requirements, the adhesive is a water-based colloidal solution, such as colloidal silicon dioxide, rather than an alcohol-based adhesive.

Deposition of the contact layer on wax and reinforcement by spreading mullite sand with a particle size distribution in the range of 80-250 microns gives a cast part with very good adhesion of the first layer and very good surface condition.

Detailed ways

The method is described in more detail below.

The method of making a shell mold includes a first step of making a master mold from wax or another similar material known in the art. The most commonly used is wax. Depending on the type of part, multiple masters can be grouped together to make multiple molds at the same time. The size and shape of the master tool is the same size and shape as the final part, taking into account the shrinkage of the alloy.

The manufacturing steps of the shell are preferably carried out by a robot whose actions are controlled so as to optimize the quality of the deposition and to make the blades and shrouds of different geometries non-cracking.

Simultaneous preparation of the styling paint and continuous quenching of the master tool or sets of master tools to deposit the ceramic material.

The composition of the first modeling paint by weight percentage is as follows:

- Mullite powder 65-80;

- colloidal silica binder 20-35;

- water 0-5%;

-Admixture of three materials: humectant, diluent and texture improver.

The three materials play the following roles respectively:

- The diluent enables the desired faster rheology during the manufacture of the layer. It acts as a dispersant. It is preferably selected from the following compounds: amino acids, ammonium polyacrylates or tricarboxylic acids with hydroxyl groups;

- The wetting agent aids in the coating of the layer during the quenching process. Wetting agents are preferably selected from the following compounds: polyalkylene fatty alcohols or alkoxylated alcohols;

- Texture modifiers enable layer optimization for proper deposition. It is preferably selected from ethylene oxide polymers, xanthan gum or guar gum.

Once the master has been removed from the impregnation step of the first styling paint, the master thus coated is drained and subsequently coated. The "plaster" particles (grains of sand) are then applied by spreading in order not to affect the thin contact layer. Using mullite, the particle size distribution of the mullite in the first layer is narrow, 80-250 microns. The surface condition of the final part is related in part to this particle size distribution.

Dry the first coat.

Tests have shown that it is advantageous to add the admixture even if it is not necessary in order to obtain satisfactory rheological properties.

A quenching step is then carried out in the second shape coat to form the so-called "intermediate" layer.

The "stucco" is deposited, as previously described, and subsequently dried.

The master is subsequently dipped in a third shape paint to form a third layer, which is the first layer of the so-called reinforcement layer.

The stucco is then applied, followed by drying. The third look coat quench, stucco application and drying steps are repeated to obtain the desired shell thickness. For the last layer, proceed to the glazing operation.

The second and third look paints may comprise 45-95% by weight of a mixture of alumina powder and mullite powder and 0-25% by weight of a mixture of mullite particles.

The quenching operation is different for different layers to obtain a uniform thickness distribution and (especially in the trapped area) to prevent the formation of air bubbles.

Dry the outermost layer last.

Thus, the shell contains 5-12 layers.

The firing cycle of the mold includes a heating phase for a set time, a holding time at the firing temperature, and a cooling phase. The firing cycle is chosen to optimize the mechanical properties of the shell, allowing it to be cold processed without risk of cracking and minimizing its susceptibility to thermal shock (which can occur at various casting stages).

The method of making a shell mold using the contact layer of the invention has been described above. This contact layer can be combined with all types of layers suitable for all conditions, even layers formed of zircon particles, if necessary.

Claims (6)

1. A method of manufacturing a multilayer ceramic shell mold for casting turbine components, said shell mold comprising at least one contact layer made from a wax master of the part to be manufactured, said method comprising the steps of: dipping the master in a first look paint comprising ceramic particles and a binder, forming said contact layer; depositing sand grains on the contact layer, the sand grains are composed of mullite particles, the grain size distribution of the sand grains is 80-250 microns; drying the contact layer; Wherein the ceramic particles of the modeling paint are mullite particles, the modeling paint includes a wetting agent, a diluent and a structure modifier, and the wetting agent is selected from polyalkylene fatty alcohols or alkoxylated alcohols class, the diluent is selected from amino acids, ammonium polyacrylates or tribasic carboxylic acids with hydroxyl groups, and the structure modifier is selected from ethylene oxide polymers, xanthan gum or guar gum. 2. The method according to claim 1, characterized in that the binder is based on a water-based inorganic colloid solution. 3. The method of claim 2, wherein the inorganic colloid is colloidal silicon dioxide. 4. The method of claim 1, wherein the grit is applied by spreading. 5. Use of the shell mold obtained by the method according to any one of the preceding claims in the manufacture of components by directional solidification of column structures. 6. Use of the shell mold prepared by the method according to any one of claims 1-4 in the manufacture of parts by directional solidification of single crystal structure.