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How to use the process to solve the pitting and pockmarking in investment casting

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Melting mold casting pockmarks and pits is common and more difficult to solve the casting defects, try the surface layer using special corundum sand refractory materials, after pouring buckle box, slurry with graphite, after pouring the shell local air cooling, after pouring the shell local water cooling and other process measures to continuously improve the process conditions, to find an effective way to overcome the blade castings on the pockmarks, pits.

Mold casting stainless steel casting surface is most likely to produce pitting, pockmarks and other defects, improve these defects is more difficult problem, to have a continuous process to improve the process.

The inner and outer shape of this blade is geometrically curved, the overall area of 303c㎡, the maximum wall thickness of 20mm, the thinnest wall thickness is 2.8 mm.

The blade material is similar to the Japanese grade SCS1, chemical composition (mass fraction, %): C: 0.08 ~ 0.10 (outside Commercial requirements 0.08) Mn:0.60~0.80, Si:0.20~0.50, Cr:12.0~13.0, Ni:0.4~0.6, Al:0.1~0.3, P:0.04,S:0.03.

Gas blade castings using investment casting full silica sol process production, castings in the trial production found pitting, pitting defects. Because such defects often involve the system of casting process links, so must continue to improve the process, continuously analysis to the problem, practice, and find ways to solve the problem.

Impeller blades

Pouring system design

Although the local wall thickness of the gas engine blade is relatively disparate, but on the whole, or belong to the thin-walled parts. After analysis, the blade’s process program for the bottom injection type slit within the gate, the blade’s thick wall at the top, and the horizontal sprue connected, forming a strong vertical direction pressure, in order to facilitate the complementary shrinkage.

After production practice, using the system casting production of blade castings have not seen internal defects, proving that the pouring system process design is reasonable and feasible. To solve the pockmark, pockmark defects, but also from the shell, roasting and pouring and other production links to start.

Refractory material preference test

Zircon sand materials are difficult to distinguish between good and bad

The first shell-making process of the blade is as follows: the surface layer of silica sol and zirconia powder powder-liquid mass ratio of 3.4:1, slurry viscosity value of 40 ~ 42s, scattering zirconia sand; the second and third layers do pre-wet, using mullite powder slurry, scattering mullite sand; the fourth and fifth layers do not do pre-wet, using mullite material; finally sealed with mullite slurry; mold shell roasting temperature of 1100 ℃; pouring temperature of 1650 ~ 1670 ℃, lifting package Pouring.

After several test castings, the defects of pockmarks and pits on the blade always existed seriously, and we began to suspect that the zircon sand material might have problems, due to the lack of analytical instruments, it was difficult to know the specific chemical composition of the zircon sand material, and the basis of quality control could only be based on the supplier’s paper “warranty”. As a result, we chose another channel to purchase zirconia powder and zirconia sand, prepared new slurry, made shells, roasted and poured according to the original process, and reduced the pitting and pockmarks, but did not completely eliminate them. Obviously, the composition of zircon sand material is not up to standard, which is one of the reasons affecting the quality of castings.

Over the years, the application of zirconia sand material for face shell making has become a mainstream process, but it is important to note the current tendency of irregularities in the zirconia sand material market. When using zirconia sand materials, attention should be paid to whether the composition is in accordance with the standards. When the ZrO2 content in zircon sand is <65% and it contains Ca, Mg, K, and Na oxides, the refractoriness will drop sharply, and zircon contains Fe which will produce pockmarks and hole defects, and it has become the consensus of casting workers.

Speaking of Si, zircon is a ZrO2-Si2 binary phase mineral, high temperature will decompose and precipitate amorphous SiO2, or called ZrO2-SiO2 “silicon precipitation”, such oxides have a fairly high activity, will react chemically with some heavy metal elements in the metal liquid, is an important casting surface pockmark, etc. One of the reasons. The surface of the shell is made of poor quality zirconium quartz powder slurry ten scattered zirconium quartz sand, before and after roasting coating density of the situation.

Domestic aviation standards for zirconium sand material SiO2 content is not specified, while the United States will be SiO2 as impurities, silicon shall not exceed 33.02%, Fe2O3 shall not exceed 0.03% (is the domestic standard 1 / 10), Al2O3 shall not exceed 1.6%, TiO2 shall not exceed 0.25%. From the blade process improvement practice to experience, zirconium material grade, the origin and price of the different, performance and effect there is a considerable difference.

In recent years, Europe and the United States industrial developed countries face shell material use of zirconium silicate greatly reduced, to the use of fused silica, and electric fusion corundum, so as to partially or fully improve the shell performance, further improve the surface quality of castings, this information coincides with this test.

Shape Forming

The advantages of corundum materials come to the fore.

A few years ago we had used ethyl silicate hydrolysis solution ten corundum powder, surface layer sprinkled white corundum sand, back layer sprinkled brown corundum sand production of high-temperature alloy aviation products and precision military products, the effect is very good, so the corundum material used in investment casting is quite optimistic. The face of zircon sand material does not adapt to the blade material characteristics of the case, we decided to choose corundum material for the second process test.

The second shell process is as follows: silica sol + special white corundum powder, the surface layer of the slurry of the powder-liquid mass ratio of 3.15:1, viscosity value of 33 ~ 36s, the surface layer of sand sprinkling 80 ~ 120 mesh special white corundum sand; the second layer to do pre-wet, using the first layer of slurry, viscosity 17 ~ 18s, sprinkling 80 ~ 100 mesh special white corundum sand; the third layer to do pre-wet, using the first layer of slurry, viscosity 18 ~ 19s, sprinkling 40 ~ 70 mesh special white corundum sand; the third layer to do pre-wet, using the first layer of slurry, viscosity 18 ~ 19s 19s, sprinkle 40-70 mesh special white corundum sand; the fourth layer does not do pre-wetting, using mullite slurry, sprinkle 30-60 mesh mullite sand; the fifth layer does not do pre-wetting, mullite slurry, viscosity 14s, sprinkle 16-30 mesh mullite sand; seal the layer with mullite slurry.

The process scheme after pouring, only in the complementary shrinkage gate position of the blade wall thickness of the thickest local appear a small amount of pockmark, see Figure 2-22. obviously than the use of zirconium-alloy material-based process on the defect has a great improvement. At this point in the experiment, two new insights were gained in the process improvement practice.

On the one hand, α-Al2O3 electrofused white corundum is alkaline or neutral at high temperature, strong resistance to the action of acids and bases, does not change under the action of oxidizing agents, reducing agents and metal liquids, has excellent chemical stability and control the oxidation of metal liquids.

On the other hand, the impurities Fe2O3, active metal oxides CaO, MgO and alkaline oxides NK2O and TiO2 in mullite have stronger penetration into the opposite layer and deteriorate the surface quality of the casting.

Improvement test of oxidation conditions

Button box with waste wax after pouring.

The generation of pockmarks and pockmarks defects will generally analyze the problem from the perspective of the chemical reaction between the metal liquid and the coating to implement relevant process measures.

In the first process using zirconia sand material for the surface layer, considering the SCS1 material liquid phase line is higher, the blade wall thickness and thin, pouring temperature is bound to be higher, easy to induce pitting and pockmarking, so after pouring, the implementation of the buckle box with waste wax improvement measures. The method is to take two sets of mold shells as a box, scatter 50g of waste wax chips on the shell, the iron box upside down to close the shell in a reducing carbon monoxide atmosphere in slow cooling.

In the second process, when the special corundum material is used as the surface layer, the test of adding waste wax to the buckle box as described above was also done.

Practice has proved that the initiative of buckle box plus wax, the low carbon content of similar SCS1 stainless steel blade surface to eliminate pitting, pockmarking effect is very little.

Reinforcement layer slurry with graphite.

Theoretically, after the casting shell, reducing atmosphere on the casting surface refinement and prevention of defects is beneficial. Although the “buckle box with wax” measures have little effect, in the continuous process improvement, but still did the “slurry with graphite” test.

In the first shelling process, graphite with a mass fraction of 5% was added to the third layer of mullite slurry and stirred well, because the particle size of graphite powder was 270-320 mesh, it blended well with the mullite slurry, and the slurry had no lumps, no precipitation, no reaction, no effect on the coating, and did not hinder the determination of the viscosity value. In the second shell making process, graphite powder was added to the special corundum powder slurry in the third layer as well, and the properties of the slurry were the same as above.

After the actual casting test, we found that the method of “adding graphite to the slurry” has little effect on improving the pitting and pockmarking defects. We will “buckle box with wax” and “slurry with graphite” methods used at the same time, quality inspection results, the blade surface pitting, and pockmark defects elimination is still very little effect.


Cooling conditions to improve the test

After the oxidation condition improvement test, think the defect problem also from other aspects. In the second shell process, using corundum material coated with the first 3 layers, pitting and pockmarking defects are greatly reduced, only a small amount of concentration in the thickest part of the blade, and is in the lower part of the near shrinkage gate, it is obvious that this is a local overheating caused, imagine, if the attention to the blade wall thickness difference, improve the cooling conditions of the thick wall parts, I do not know how the effect.

To the thick wall at the blowing air cooling.

Still use the second shell process, use special corundum material coated with the first 3 layers of the shell, pouring finished, with 48mm rubber tube into the compressed air, aimed at the thickest part of the blade blowing air, for local forced cooling, blowing time 1min.

The blade casting quality inspection shows that the thick wall parts only a very small number of pitting and pockmarks. Blowing cooling on the pitting and pockmarking defects to improve the effect, the defect is still in the thickest part of the blade, the problem is not completely solved.

After pouring, water is sprayed to cool the thick wall area.

After the idea of local forced cooling was verified, we further adopted the method of water spray cooling for the test. After pouring, with Φ8 rubber tube into the tap water, aimed at the thickest blade shell parts spray water, increase the local forced cooling strength, spray water time 1min.

The quality inspection of the blade casting shows that the concave surface of the blade, the convex surface, especially the thick wall parts completely eliminated the pockmarks and pits, the scrap rate caused by the pockmarks and pits defects is reduced to 0.

Process improvement results

Through the blade pockmark defects continue to improve the work practice, the author experience, according to the different structural characteristics of the parts of the product, to be good at analyzing the essence of the problem, to dare to doubt and challenge the traditional process methods, so that not only conducive to solving problems but also fewer detours.

For this molten die castings, the choice of high-purity corundum to do the surface layer, two layers, or three layers of shell refractory materials, with local cooling measures, change the design of the pouring system, and ultimately can completely solve the special martensitic stainless steel blade castings pockmark, pockmark casting defects.


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