The molten metal of high temperature is Injected into a mold cavity in a high-pressure die casting process. And, the filling, cooling, and solidifying occur under high-pressure conditions. This rapid production cycle leads to multiple degrees of porosity.
Porosity may refer to the visible and hidden voids, pores, or holes within a die-cast component. Their presence can lead to compromised performance and eventual failure. Therefore, porosity reduction strategies are crucial for the production of quality parts.
Different Forms of Die Casting Porosity
Porosity defects in die casting primarily take either of the following three forms:
- Blind –Such porosity can’t be observed from outside as they form below the metal surface. However, blind ones don’t usually affect the desired mechanical properties. Instead, these holes lead to corrosion in continuous contact with moist air.
- Through –These pores are more like a leaked pathway in the metal body. It stretches from one surface to opposite surfaces. This can create a failure point, affecting the overall structural integrity. And the metal can’t become fully airtight.
- Fully Enclosed –These are hidden pores embedded within the thickness. Exposure to the post-machining process makes them either blind or through pores. CT scan and X-rays can confirm their presence in metals without exposure.
Major Causes of Porosities in Die Casting
1.Gas Porosity –These pores mostly have a bubble-like yet distorted circular or oval shape. They form over entrapped gas or air while casting. Gas pores form within the die cast metal’s thickness. Pores close to the metal surface can lead to blistering defect.
Common causes of gas porosity:
- Too fast solidification of molten metals
- Compromised shot speed of casting tool
- Errors in venting, lubrication, and gating
- Putting away die casts for a longer time
- Gas emissions from the material
- Presence of gas contaminants in casting
- Air trapped within the mold
- Irregular pressure on liquids for casting
- Excessive lubricant, causing vaporization
2.Shrinkage Porosity –These pores are irregular in size/shape, often occurring in heavier sections. Shrinks occur from thicker sections to thinner sections to form these voids. They are primarily surface defects that can stretch within metals to affect their integrity.
Common causes of shrinkage porosity:
- Differential shrinking during solidification
- Imbalance between section wall thickness
- Disproportion within material properties
- Wrong molds and designed section errors
- Extremely sharp corners of mold sections
- Low and abrupt metal casting temperature
- Improper control of temperature change
The entire die casting process requires careful, observant, and clean implementation. Slight errors or miscalculated processing can instantly cause severe porosity in products.
Sealing Porosity in Die Casting
Severely porous parts are neither expected nor usable for important works. Sealing the pores can effectively make the treated parts workable. And vacuum impregnation is the most cost-effective method for permanent sealing. It introduces filler materials in the pores using a vacuum or pressure.
This method works great on simple, low-level pores without much time or effort. However, you can only treat pores on the cast surface using sealants.
Porosity Reduction Strategies
Adhering to proper die casting design guidelines is essential to keep porosities in casting to a minimum. Some tips to avoid, reduce, or minimize porosity in die casting are as follows:
- High-Quality Material– A dry, clean, balanced material doesn’t usually generate volatile gas or vapor. The same goes for coating and refining agents required for the process.
- Well-Defined Specifications and Tolerances– Always define specifications and tolerance for defects up to a limit. Allowing minimal porosity in ignorable parts can prevent it in critical sections.
- Mold DesignImprovement – Some ways to improve the die design are as follows –
- Changing the direction & thickness of mold gates.
- The overflow chute should be 60% less than the gate area.
- Maintain a uniform wall thickness
- Setting the discharge & overflow groove for gas/air.
- Improving the slag discharging system within the mold.
- Polishing to reduce the sharpness of mold corner/edge.
- Control on Time and Temperature– Differential cooling within a casting can cause shrinkage porosity to occur in casting. Proper control of the temperature and time of cooling can reduce the chances of porosity.
- Proper Shot Speed and Pressure –Deciding on the optimum shot speed and pressure of injection can greatly reduce the turbulence during injection and reduce the chances of the formation of porosity.
Using analytical design software like MAGMA can simulate the result after its casting process. Strategic placement of running gates, watering lines, overflows, vents, and cooling channels can protect components against porosity. Vacuuming the injection process can further improve the casting.
And Sunrise Metal has a well-experienced team for sophisticated control and advanced planning. Make sure to contact us to know professional opinions/suggestions on porosity reduction in die casting.