Gas porosity is the formation of rounded air pockets or voids on the surface or within the casting. It is one of the most prevalent types of die-casting defects. Porosity analysis has shown that a combination of both gas porosity and shrinkage porosity forms together in a casting.
While minor levels of porosity may be acceptable for non-critical parts, it is undesirable for critical parts with high-strength applications. Therefore, manufacturers stick to stringent casting porosity standards as gas entrapment can compromise the quality and integrity of a die-cast product.
In this article, we have discussed the types, causes, effects and porosity prevention measures necessary during a die-casting process.
Different Forms of Gas Porosity
Gas porosity can be present in a casting in different forms, such as,
1. Blind Porosity:
This type of porosity form on the surface of the casting and extend into its interior. It usually doesn’t affect the mechanical strength but can lead to corrosion. Blind porosity is often exposed during surface finishing.
2. Fully Enclosed Porosity:
Such porosity are internal defect formed within the casting. Therefore, it may go unnoticed unless specialized porosity detection methods like ultrasonic or X-ray are used.
3. Through Porosity:
The pores run through the wall of the casting, weakening its structural integrity. Such a form of gas porosity defect makes the casting unsuitable for automotive or marine casting applications, where the pressure tightness of parts is usually of prime importance.
Primary Sources of Gas Porosity
Some of the primary sources of gas porosity defect include,
1. Entrapped Air:
Gas entrapment within a casting can happen due to various reasons such as,
- Plunger system
- Runner system
- Vents and vacuum
A lot of air may be trapped as molten metal is turbulently pushed into the cavity. The high speed of the plunger is a prerequisite for high-pressure die casting. So, other features such as runners, venting systems, and vacuums are required to reduce the amount of entrapped air within the system. If they are not designed properly, air porosity will be expected.
2. Lubricants:
Die lubricants or plunger lubricants can vaporize or burn during casting. The die lubricant vaporizes inside the mold cavity upon coming in contact with the hot metal and creates gas pores. It is usually the result of excessive use of die lubricants.
3. Steam:
Steam is generated during a casting process when the molten metal encounters any water. The steam is generated through an explosion and occupies drastically more space than the water.
Water sources that contribute to this defect may come from,
- Water-based lubricants
- Leaking water lines
- Cracks in the die connected to water lines
- Dripping sprayers, etc.
Causes of Gas Porosity
The following factors and process parameters are typically responsible for occurrences of gas porosity in casting:
1. Poor Casting Design:
Poor design choices can affect the casting, such as,
- Thick sections
- Abrupt changes in wall thickness
- Sharp edges, etc.
These can result in increased turbulence, uneven cooling, and solidification. These increases the likelihood for air porosity to form.
2. Low-quality Material:
The mold and melt material can both affect the quality of a casting. If impurities or moisture is present in the material, gas-forming reactions may take place resulting in porosity defect.
3. Inadequate Venting System:
Insufficient or poor venting system design can prevent the expulsion of gas during the casting process.
4. Inconsistent Temperature:
Inconsistent temperature control can result in the formation of hotspots in a casting where gas can be entrapped leading to porosity defect.
5. Unsuitable Process Parameters:
Variables such as injection speed and pressure play a crucial role in gas porosity. Excessive speed and pressure can increase the turbulence and cause air entrapment.
Effects of Gas Holes
Gas porosity can negatively affect the functional and aesthetic requirements of a casting, which include,
1. Diminished Aesthetics:
Gas holes can ruin the visual appearance of a casting. In industries like consumer electronics, the aesthetics of the product is very important.
2. Reduced Strength:
Gaspores formed within a casting can negatively affect the structural integrity, making it more susceptible to failure under stress.
3. Surface Imperfections:
Porosity is often formed on the surface of a casting resulting in roughness or pitting. It may necessitate additional machining operations which would increase the production cost.
4. Leakage and Sealing Issues:
Gas porosity formed in thin walls can compromise the sealing properties of parts. It is critical for components used in hydraulic systems and automotive engines.
Controlling Gas Hole
Minimizing gas holes would require careful porosity analysis, adjustment, and monitoring of several factors:
1. Material Selection:
Choosing a low gas-forming alloy could serve as a preventive measure to deal with porosity. Also, materials should be dried properly to remove any moisture.
2. Process Parameters:
Through testing, trial, and error, the process parameters such as pressure, temperature, and injection speed should be fine-tuned to minimize the formation of gas porosity.
3. Cooling Optimization:
Uniform cooling can help reduce thermal gradients and prevent hotspots from forming. Gas hole is more likely to occur in such areas. Avoiding thicker sections, and variable wall thickness can reduce hotspots in casting.
4. Die Lubricants:
High-quality die lubricants with low moisture contents should be used if the defects seem to originate from the lubricants.
Incorporate Vents and Vacuum: During mold design, proper venting systems should be provided to allow gas to escape during casting. Vacuum die casting is also a great way to minimize porosity in casting.
The aforementioned approaches are porosity prevention measures to reduce/minimize the chances of gas porosity occurring during casting. However, strict quality control measures are also essential to detect and address gas porosity in casting. Non-destructive methods like ultrasonic and x-ray inspection could be used to check whether the parts comply with the casting porosity standards.