Time:2023-12-13 Browse: 30
Causes and Solutions for Surface Bubbles in Zinc Alloy Die Castings
Solutions to Die Casting Porosity First identify the root cause of the porosity, then implement corresponding measures:
1. Use dry and clean alloy materials.
2. Control melting temperature to avoid overheating and perform degassing treatment.
3. Select reasonable die casting process parameters, especially shot speed, and adjust the high-speed switchover point.
4. Sequential filling facilitates gas evacuation from the cavity. Ensure the sprue and runner have sufficient length (>50mm) to allow stable flow of molten alloy and effective gas discharge. Modify gate thickness/direction, and install overflow grooves or exhaust slots at porosity-prone locations. The total cross-sectional area of overflow grooves must be no less than 60% of the total cross-sectional area of inner gates; otherwise, slag removal efficiency will be poor.
5. Choose high-performance coatings and control the spray amount.
Analysis of Die Casting Porosity
Porosity is the most common defect in die castings:
Characteristics of Porosity
- Smooth surface; may appear on the casting surface, as subsurface pinholes, or internally.
- Internal porosity (within casting walls) is generally round or oval with a smooth, often bright oxide film (sometimes oil-yellow).
- Surface bubbles can be detected by sandblasting; internal porosity is identifiable via X-ray inspection or machining.
- Porosity appears as black spots on X-ray films.
<<A>> Gas Sources
1. Gas evolved from molten alloy:
a. Related to raw materials
b. Related to melting process
2. Gas entrapped during die casting:
a. Related to die casting process parameters
b. Related to mold structure
3. Gas generated by release agent decomposition:
a. Related to coating properties
b. Related to spraying process
<<B>> Analysis of Gas Generated from Raw Materials and Melting Process
Hydrogen accounts for approximately 85% of total gases in molten aluminum (relevant to zinc alloys due to similar melting principles).
Solubility of hydrogen in molten metal increases with temperature, but is extremely low in solid metal. Thus, hydrogen precipitates during solidification to form porosity. Sources of hydrogen:
1. Water vapor in the atmosphere (molten metal absorbs hydrogen from humid air).
2. Hydrogen content in raw materials (moisture on alloy ingot surfaces, contaminated return scrap, oil stains).
3. Moisture in tools or fluxes.
<<C>> Analysis of Gas Generated During Die Casting
The shot sleeve, gating system, and cavity are all exposed to the atmosphere. Molten metal fills the cavity at high pressure and speed; if flow is not orderly and stable, eddy currents will form, entrapping gas. Key considerations for process design:
1. Can molten metal flow cleanly and stably in the gating system without separation or eddy currents?
2. Are there sharp corners or dead zones?
3. Does the gating system have cross-sectional area changes?
4. Are exhaust/overflow slots properly positioned, sufficiently sized, and unobstructed? Can gas be discharged effectively and smoothly?
Computer simulation of the filling process is used to analyze these phenomena and select optimal process parameters.
<<D>> Analysis of Gas Generated from Coatings
- Coating performance: High gas evolution directly increases casting porosity.
- Spraying process: Excessive coating application leads to high volatile gas emissions; excessive or burnt plunger lubricant also contributes to gas generation.
<<E>> Solutions to Die Casting Porosity
First identify the root cause of the porosity, then implement corresponding measures:
1. Use dry and clean alloy materials.
2. Control melting temperature to avoid overheating and perform degassing treatment.
3. Select reasonable die casting process parameters, especially shot speed, and adjust the high-speed switchover point.
4. Sequential filling facilitates gas evacuation from the cavity. Ensure the sprue and runner have sufficient length (>50mm) to allow stable flow of molten alloy and effective gas discharge. Modify gate thickness/direction, and install overflow grooves or exhaust slots at porosity-prone locations. The total cross-sectional area of overflow grooves must be no less than 60% of the total cross-sectional area of inner gates; otherwise, slag removal efficiency will be poor.
5. Choose high-performance coatings and control the spray amount.
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