Sand irrigation with narrow inner cavity impeller

Figure 2-41 shows the stamped and welded turbine, and Figure 2-42 shows the partial state of stamped and welded blades. The shell of the turbine is made of 4mm steel plate, which is stamped and welded. The turbine blades are made of thin 1mm steel plates that are stamped and welded to the inner cavity of the casing. The semicircular ring is also a stamping part, which is welded to 21 blades. Obviously, the stamping component assembly welding method not only has a large workload, long production cycle and low efficiency, but also cannot meet the requirements of turbine design performance and working parameters. Therefore, it is required to change the turbine manufacturing method to investment casting.

Control of process details

Pressure type design.

 From the perspective of turbine modeling, the difficulties of investment casting focus on the inner cavity, and the curved blade is connected to a semicircle ring. It is problematic to press the wax mold at one time, so two pressure molds must be set up separately, and two wax molds must be pressed, and then combined. See Figure 2-43 to Figure 2-45.

• 2-43 Three-dimensional profile of blade and half ring anastomosis
• Figure 2-44 Three-dimensional diagram of blade state in the inner cavity of the housing
• Figure 2-45 Three-dimensional diagram of the position of the embedded ring in the blade

Mold preparation

1. Set three stages of wax filtration. The low-temperature moulds are dewaxed with hot water, and the wax liquid is filtered for the first time before it flows from the dewaxing tank to the treatment tank. After acid treatment, the wax solution is filtered for the second time before flowing into the stationary precipitation bucket. Strain the wax solution a third time before pouring into the mold.
2. Add wax planing process. Using casting mold ingot, a cylinder with a diameter of 450mm and a length of 800mm, placed on the wax planer to process into thin pieces of wax. Stirring the wax paste is fast, uniform and delicate, and there are no particles in the wax paste.
3. Cooling of wax mold. The temperature of the molding room shall be strictly controlled to be less than 25℃. After the wax mold of blade and ring is removed from the pressing mold, it shall not be put into water for cooling. The wax mold shall be stored on the plate in pairs, and the wax mold shall not be stacked.
4. Stitching of wax mold. The traditional welding process with ferrochrome is abandoned and bonded wax is adopted, as shown in Figure 2-46 and Figure 2-47. The heating temperature of bonding wax is generally 60°C, under which the bonding wax liquid is more dense, and a circle of “wax accumulation” often appears on the bonding surface when the wax mold is glued. Therefore, the heating temperature was raised to 70°C, and the semi-circular ring wax mold was immersed in the adhesive wax solution for less than 2s. After the wax was glued, the wax solution was not immediately glued. The wax solution was evenly divided with a row of brushes, and the wax solution was stopped for 5-7s. Then smoothly place the semi-circular ring wax mold into the blade wax mold, as shown in Figure 2-48.

• Figure 2-46 Blade wax mold
• Figure 2-47 semi-circular ring wax mold
• Figure 2-48 Bonded wax mold

Gating system design.

The design scheme of the first pouring feeding system is to use spherical riser and centrifugal pouring, following the radius of the riser, as shown in FIG. 2-49 and FIG. 2-50. The purpose of setting three thick feeding runner on the spherical riser is to facilitate wax discharge, exhaust, shell feeding and improve the shell rigidity of the module.

• Figure 2-49 Spherical die head
• Figure 2-50 Three exhaust bars on the spherical mold head

The design scheme of the second pouring feeding system, spherical riser with four inner runner centrifugal pouring, is shown in Figure 2-51.

• Figure 2-51 Connecting the runner to the half-ring
• Figure 2-52 Integrated inner runner

The third design scheme of pouring feeding system adopts an integral inner runner. The whole inner runner body is set at the top of the turbine housing, and the upper part is set with 5 liquid steel channels. The upper part is similar to the horizontal washing channel, which is made into an overall circle considering the characteristics of centrifugal pouring.

In order to ensure the complete filling of the semicircular ring in the turbine inner cavity, a filling mode combining inside and outside is formed. A direct washing channel is drawn from the center of the lower end of the circular cross runner, and a four-pronged inner gate is adopted to connect with the inner wall of the through-hole with a diameter of 74mm, as shown in Figure 2-53.

• Figure 2-53 four-pronged inner runner

Shell making process

Test process.

The first layer: Apply mullet powder for 35s, sprinkle 80-100 mesh mullet sand, dry for 10h, drying chamber temperature 23°C, relative humidity 65%.

The second layer: apply Mullet powder 22s, sprinkle 60-80 mesh mullet sand, dry for 12h, drying chamber temperature 23°C, relative humidity 65%.

The third layer: apply mullet powder for 15s, sprinkle 60-80 mesh mullet sand, dry for 12h, drying room temperature 23°C, relative humidity 50%, blow air, tie wire.

The fourth layer: apply mullet powder for 14s, sprinkle 30-60 mesh mullet sand, dry for 12h, drying room temperature 23°C, relative humidity 50%, blow.

The fifth and sixth layers: apply mullet powder for 14S, sprinkle 16-30 mesh mullet sand, dry for 12h, drying room temperature 23℃, phase: humidity 50%, blow.

Sealing layer: Apply Mo Shui slurry for 14s, dry for 16h, drying room temperature 23°C, relative humidity 50%, blow.

Current production process

 In order to facilitate sand cleaning, the first and second layers remain unchanged. Before coating the third layer, the inner cavity of the blade is filled with sand (60~80 mesh mullite) and closed sand (mullite slurry and mullite powder are mixed into mud and blocked), and then coated with the third, fourth and fifth layers and sealing layer. After pouring, the cleaning difficulty is significantly improved.

Module dewaxing

Aafter dewaxing, take out the shell, immediately rinse it with boiling water twice, and thoroughly remove the remaining wax and debris in the shell.

Two roasting of the mold shell

• Pre-roasting: The cross shell is pre-roasted in a baking furnace at 950℃. After the pre-calcined mold shell cools, the inner cavity of the mold shell is cleaned with water.
• Packing roasting: packing roasting means that the pre-roasted mold shell is put into a round iron box, which is filled with coarse sand, and the surface of the sand is brushed with a layer of thin silica sol. The purpose is to make the mold shell heated evenly and ensure that the casting temperature of the mold shell is increased, so as to facilitate the centrifugal casting.
• Roasting temperature and holding time. The roasting temperature of the mold shell is set at 1100~1150°C, the insulation temperature of the mold shell is 1100~1150°C, and the insulation time of the mold shell is ≥30min.

Smelting and pouring

Self-made centrifuge

The trial production practice has proved that the turbine must adopt centrifugal pouring to meet the requirement of complete filling. See Figure 2-55 for the self-made adjustable speed centrifuge.

• Figure 2-54 About to cast after packing roasting
• Figure 2-55 Self-made adjustable centrifuge