Analysis of Defects of Investment Casting

Investment casting is characterized by complex process, many processes, many factors affecting casting quality and various raw and auxiliary materials. It is not easy to control the quality of investment casting and affects the quality of casting to different degrees.

1. Casting dimension is out of tolerance

The main factors affecting the size change of castings are the shape, size and structure of the castings, the pressing mold, the making of the fusible mold, the mold shell and the pouring process.

1.1 Influence of mold making process on casting size

(1) Injection time.The injection time includes three periods of filling, compaction and holding pressure.

(2) wax pressure temperature.The shrinkage rate of the wax mold decreases with the increase of the wax temperature when the injection time is short, but with the increase of the injection time, the shrinkage rate of the wax mold decreases.

(3) Injection pressure.The shrinkage rate decreases slightly with the increase of pressure, and the effect of the increase of pressure will be more obvious when the wax pressing temperature is high.

(4) Flow velocity.The shrinkage rate of wax mold is affected by changing the setting of mold flow rate of wax press, such as changing the cross-sectional area of wax injection mouth.

(5) Pressure injection equipment.The cooling system of the wax press may have an effect of 0.3% on the shrinkage of the wax mold.The pressure mold temperature is high, the wax mold cooling is slow, the shrinkage is large.

In order to reduce the impact of investment size deviation on casting size, attention should be paid to the following aspects: the use of line reduction of die material, and pay attention to maintain the process performance of die material.According to the structure, shape and size of the investment die, the reasonable injection process parameters are selected correctly.

1.2 Influence of shell making material and process on casting size

The thermal expansion of mold shell affects the size of casting, and the thermal expansion of mold shell is related to the material and process of shell making.

1.3 Influence of pouring conditions on casting size

The mold shell temperature, the molten metal pouring temperature and the position of the casting in the mold shell will affect the casting size.

2. The casting surface is rough

Casting surface roughness refers to the investment casting surface roughness is not up to the requirements.

2.1 Factors affecting the surface roughness of investment die

(1) the relationship between the investment surface roughness and the pressing surface roughness and the pressing mode.

(2) pressing process parameters.Mold temperature, mold temperature, injection pressure, holding time and so on all have great influence on the surface roughness of the investment when the paste mold is used to press the investment mold.

(3) methods to improve the investment surface quality: ① pay attention to the production quality of the pressure and strengthen the management of use and preservation;② according to the investment structure, size, shape of different, reasonable selection of pressing process parameters;③The size of the wax injection channel should be designed reasonably according to the size of the investment die.

2.2 Factors affecting the surface roughness of molded shells

(1) wettability of paint.In order to make the coating can be well wet investment, attention should be paid to the following aspects: ① the investment surface should be cleaned, remove oil and other substances;② to the water-based binder sodium silicate, silica sol coating to add the right amount of wetting agent;③ the surface coating can not be used immediately after preparation, after about 12 hours of recycling to make the binder and refractory powder fully wet before use.

(2) the influence of powder to liquid ratio of surface coating on the compactness of shell.Silica sol and ethyl silicate hydrolysate binder itself viscosity is small, so the preparation of surface coating powder is relatively high.

(3) Influence of drying and hardening conditions on the surface roughness of molded shells.

2.3 Influencing factors of precise reconstruction of metal liquid

High enough shell temperature, suitable pouring humidity and qualified molten steel.

2.4 Other factors affecting the surface roughness of the casting

In the process of casting and metal solidification, the surface of casting will be rough due to oxidation.

3. Casting defect analysis

3.1 Sand defects

(1).Sand is a common surface defect in investment casting.It is characterized by the adhesion of a layer of metal to a shell compound or shell material on the surface of the casting.

(2) Cause of formation.For silica sand powder on the surface of shell, casting steel, due to the shell of oxidizing atmosphere, make the metal surface oxidation produce ferrous oxide, with the interaction between the main mineral composition of silicate powder quartz of ferrous silicate melting point 1205 ℃, only when the temperature higher than the melting point, good liquidity, can dissolve wet quartz, shell surface is ferrous silicate after corrosion, pore is growing between particles, sand and casting bonding together after solidification, has formed chemical adhering sand.

(3) Preventive measures.Strictly control the content of impurities in surface coating and sanding.Correct selection of shell refractories, high manganese steel and high temperature alloy steel castings, surface coating, sand should choose neutral refractories is appropriate.

3.2 Shrinkage defects

(1).Large and concentrated holes with rough surface and irregular shape exist in the inner part of the casting.

(2) Cause of formation.During the liquid shrinkage and solidification of the alloy, a certain part of the casting cannot be fed by liquid metal in time, and the shrinkage cavity is formed.If the hot joint is too large and the pouring riser is not designed properly, it is not conducive to the sequential solidification, so that the hot joint of the casting can not be supplemented with metal liquid, the pouring temperature is too high, and the heat dissipation conditions are poor, resulting in local overheating.

(3) Preventive measures.Improved casting structure to make the wall thickness uniform and reduce hot spot, which is beneficial to sequential solidification.Reasonable setting of the sprue riser system to ensure sequential solidification.

3.3 Crack defects

Hot crack is one of the most common and dangerous defects in medium carbon steel and alloy steel.

(1).The external crack, the crack starts from the surface of the casting, and gradually extends to the inside with the surface wide and the inside narrow. The crack is oxidized and discolored.Internal cracks, which usually occur at the final solidification of the casting, sometimes occur under the shrinkage cavity and are irregular and bifurcated.

(2) Causes and influencing factors: ① the influence of casting structure;② the influence of casting alloy properties;③ the harm of gas and inclusions in alloy liquid;④ the influence of gating system and cooling rate.

(3) Preventive measures.Improve casting structure, reduce wall thickness difference, smooth over fillet, and reduce stress concentration.

3.4 Deformation defects

(1).When the geometry of the cast state is different from the drawing, it is called deformation or as-cast deformation.

(2) the formation of reasons: ① investment deformation;② deformation of shell;③ the deformation of casting during solidification and cooling;④ Improper cleaning and transportation operation causes casting deformation.

(3) Preventive measures.Carefully check the investment mold, found that the investment mold deformation should be scrapped or adopt corrective measures, and control the studio temperature.

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Post time: Feb-22-2021