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Eleven Defects and Their Reasons for Die Casting Parts (Part one)

Source: Junying   Author: Junying   Posted: 2017-05-30 16:03:09   Hits: 27

1. Oxide Inclusions
A. Defect Characteristics:
Oxide inclusions are mostly distributed on upper surfaces of die casting parts, and the corners where the casts are not ventilated. Fractures are mostly hoary or yellow, which can be found through x-ray, in the machining processes, in alkaline cleaning, pickling or anodizing.

B. Causes:
1. Charge is not clean and foundry returns are over used.
2. The gating systems have poor designs. 
3. The slags in the alloy liquid are not removed clearly.
4. Improper operation of pourings bring the slags.
5. The standing time is not enough after refining and modification.

C. Prevention Methods:
1. Sand blast the charges and appropriately reduce the usage amount of foundry returns.
2. Improve the design of the pouring systems to enhance their capacities of pushing off the slags.
3. Use suitable fluxes to remove the slags.
4. Pouring should be smooth and steady and we should pay attention to slags.
5. After refining and before pouring, alloy liquid should be put for a certain time.

2. Blowholes/Pinholes
A. Defect Characteristics: 
The inner wall blowholes of die casting parts are generally round or oval with smooth surfaces. They are often shiny oxide scales, sometime they appear in the color of butters. Blowholes and pinholes on the surfaces of die casting parts can be found by sand blasting, and inner blowholes and pinholes can be tested by x-ray fluoroscopy or machining processes, which appear in black in the X-ray films. 

B. Causes:
1. Casting alloys are not smooth and steady, which brings gases.
2. Molding core sands are mixed with organic impurities(such as slack coal, grass roots, horse manure, etc.)
3. The molds and sand cores have bad ventilation.
4. There is shrinkage on the surfaces of chills.
5. Pouring systems have bad designs.

C. Prevention Methods:
1. Correctly control the pouring rates and avoid bringing gases.
2. Molding core sands shall not be mixed with organic impurities to reduce the amount of molding material gas evolution.
3. Improve gas exhausting capacities of the core sands.
4. Have correct selection and treatment of chills.
5. Improve designs of pouring systems.

3. Shrinkage Porosity
A. Defect Characteristics:
The shrinkage porosity of aluminum castings generally occurs in the thick parts of feed head ends, the meeting points of thick wall thicknesses and thin wall thicknesses, large area of thin wall thicknesses near inner pouring gates. In the casting state, the fractures are gray, pale yellow after heat treatment or ash black which appear in cloudiness in the X-ray films. Serious filamentous shrinkage can be tested by X-ray and fluorescent low fracture.

B. Causes:
1. Have poor feeding effects of feed heads.
2. There are too many gases in the charges.
3. The nearby parts of ingates are overheated.
4. Sand molds have too much moisture, and sand cores aren't dry.
5. Alloys have big grains
6. Die casting parts have wrong positions in the molds
7. The pouring temperatures and the casting speeds are too high.

C. Prevention Methods:
1. Repour the molten metals from feed heads and improve designs of feed heads. 
2. The charges should be clean and free from corrosion.
3. Place feed heads with chills in the parts where shrinkage porosity occurs.
4. Control moisture of molding sands, and dry sand cores.
5. Take measures to refine the grains.
6. Improve the positions of castings in the molds to lower the casting temperatures and pouring rates.