The Common Defects of Aluminum Die Casting Parts (Part Three)
7. Segregation
Features: The chemical composition at the bottom and top of the crucible is uneven during the smelting process. The die casting part which solidifies first and the die casting part that solidifies later have uneven chemical composition.
Analysis of causes: The precipitated phase and the liquid phase contain different solute concentration during the solidification of alloy. In most cases solute of the liquid phase gathers and there is no enough time for it to spread, which will lead to the die casting part which solidifies first and the die casting part that solidifies later have uneven chemical composition. The chemical composition at the bottom part of the crucible is uneven in a certain temperature range during the smelting process, because compounds which are formed by some element have different density from mother liquor.
Prevention methods: 1. Stir the metal liquid more and put it aside appropriately during the smelting process. 2. Appropriately increase speeds of solidification and cooling.
8. Tumor forms of segregation
Features: The bump on the surface of the die cast has the shape of tumors.
Analysis of causes: When the central part of the die cast has not solidified, the liquid phase which doesn't solidify in the shrinkage center of the die cast's surface penetrates and forms tumor forms of segregation.
Prevention methods: 1. Properly lower the pouring temperature. 2. Appropriately improve the cooling ability of the mold. 3. Extend the opening time.
9. Super poor compositions
Features: Chemical elements are higher than the maximum number or lower than the lower limit, and impurity elements exceed the allowable upper limit.
Analysis of causes: 1. Compositions of intermediate alloy or prealloy are not uniform or errors of composition analysis are too great. 2. Calculations of furnace burdens or weights of burdens are wrong. 3. The smelting operation is improper, and elements which are easily oxidized burn too much. 4. Stirs of smelting are uneven, and easily segregated elements distribute evenly.
Prevention methods: 1. Properly adjust the composition when compositions of alloy are unqualified. 2. Consult with the design department when the final test is unqualified.
10. Cold shuts
Features: The molten metal is separated by the oxide skin, which causes the metal not completely fuse, resulting in forming cold shuts. If the situation is serious, metal shortages will happen. Cold shuts often appear on top walls of die casts, thin horizontal and vertical surfaces as well as parts between thin and thick parts, and they appear to be lines.
Analysis of causes: 1. Improper running gate systems make flow paths of alloy liquids too long or alloy liquids converge at very thin parts. 2. The discharge of the mold is poor, which makes alloy liquids blocked, oxidize and not fuse well. 3. The pouring temperature is too low. 4. Structures of die casts are improper. For example, outer corners are too small, which will lead to poor flows of alloys.
Prevention methods: 1. Improve designs of running gate systems; avoid flow paths of alloy liquids being too long and prevent alloy liquids converging at very thin parts. 2.Tilt the pouring to make the liquid flow smoothly. 3. Improve the discharge capability of the mold and strictly control the moisture of the molding sand. 4. Appropriately increase the pouring temperature and pouring speed. 5. Improve the structural design of die casts.
Features: The chemical composition at the bottom and top of the crucible is uneven during the smelting process. The die casting part which solidifies first and the die casting part that solidifies later have uneven chemical composition.
Analysis of causes: The precipitated phase and the liquid phase contain different solute concentration during the solidification of alloy. In most cases solute of the liquid phase gathers and there is no enough time for it to spread, which will lead to the die casting part which solidifies first and the die casting part that solidifies later have uneven chemical composition. The chemical composition at the bottom part of the crucible is uneven in a certain temperature range during the smelting process, because compounds which are formed by some element have different density from mother liquor.
Prevention methods: 1. Stir the metal liquid more and put it aside appropriately during the smelting process. 2. Appropriately increase speeds of solidification and cooling.
8. Tumor forms of segregation
Features: The bump on the surface of the die cast has the shape of tumors.
Analysis of causes: When the central part of the die cast has not solidified, the liquid phase which doesn't solidify in the shrinkage center of the die cast's surface penetrates and forms tumor forms of segregation.
Prevention methods: 1. Properly lower the pouring temperature. 2. Appropriately improve the cooling ability of the mold. 3. Extend the opening time.
9. Super poor compositions
Features: Chemical elements are higher than the maximum number or lower than the lower limit, and impurity elements exceed the allowable upper limit.
Analysis of causes: 1. Compositions of intermediate alloy or prealloy are not uniform or errors of composition analysis are too great. 2. Calculations of furnace burdens or weights of burdens are wrong. 3. The smelting operation is improper, and elements which are easily oxidized burn too much. 4. Stirs of smelting are uneven, and easily segregated elements distribute evenly.
Prevention methods: 1. Properly adjust the composition when compositions of alloy are unqualified. 2. Consult with the design department when the final test is unqualified.
10. Cold shuts
Features: The molten metal is separated by the oxide skin, which causes the metal not completely fuse, resulting in forming cold shuts. If the situation is serious, metal shortages will happen. Cold shuts often appear on top walls of die casts, thin horizontal and vertical surfaces as well as parts between thin and thick parts, and they appear to be lines.
Analysis of causes: 1. Improper running gate systems make flow paths of alloy liquids too long or alloy liquids converge at very thin parts. 2. The discharge of the mold is poor, which makes alloy liquids blocked, oxidize and not fuse well. 3. The pouring temperature is too low. 4. Structures of die casts are improper. For example, outer corners are too small, which will lead to poor flows of alloys.
Prevention methods: 1. Improve designs of running gate systems; avoid flow paths of alloy liquids being too long and prevent alloy liquids converging at very thin parts. 2.Tilt the pouring to make the liquid flow smoothly. 3. Improve the discharge capability of the mold and strictly control the moisture of the molding sand. 4. Appropriately increase the pouring temperature and pouring speed. 5. Improve the structural design of die casts.
