Porosity in die casting process

In the die casting process, porosity is a significant problem. The development of pores, holes, air pockets, or indentations on the surface or within a die-cast object is known as porosity. It is a conditioning phenomenon that weakens a die cast's structural integrity and produces leaks or corrosion in some areas.

The majority of the time, it is okay in non-critical places, however it is unacceptable for essential sections requiring high-strength applications. This article will discuss die casting porosity, including its origins, manifestations, and preventative steps.

Major Forms of Porosity in Die Casting

The presence of pores in die castings can lead to component failure. Therefore, such castings are unsuitable for applications requiring high structural integrity. Casting porosity takes three major forms.

Blind Porosity

Blind pores begin in the die cast surface and end in its body. Under normal circumstances, this does not affect the mechanical strength of the casting. However, it can lead to corrosion as the passage is not continuous, and water can pool up in the formed pores.

Also, it can lead to other die casting defects when secondary surface finishing options such as anodizing, as the chemicals used in such processes can open up the voids. Seal blind pores after casting when such parts must hold pressure.

Through Porosity

These pores stretch from one side of the casting to the other side, i.e., a leak path. This creates a failure point that affects the part’s structural integrity and prevents them from being pressure tight. Such parts are not suitable for holding gases or fluids. However, you can seal them from both sides.

Fully Enclosed Porosity

These pores are fully enclosed within the die cast without showing on the casting surface. They are not much of a problem unless they become exposed to post-machining processes. When exposed, they become blind or through pores. Only a CT scan, X-raying, or opening of the part can confirm the presence of fully enclosed pores.

Major Causes of Porosity in Die Casting

There are two major causes of porosity in casting. Below is an in-depth explanation of both types:

Gas Porosity

Pores formed due to trapped gas will lead to blisters on the part’s surface. During solidification, these pores are formed when gases are trapped in the die or molten material. Trapped gases in the die can occur due to errors in venting, gating, lubrication, or when you keep the die part for a long time. Also, gas pores can occur when there is/are:

Emission of gases by the materials used in the process

Trapped air trapped within the mold tool or entrained air

Presence of liquids or contaminants that can release gas

Irregular pressure and shot speed

Too much lubricant

Shrinkage Porosity

Shrinkage pores are formed when the casting shrinks away from thick to thin sections, forming a void in the center. This occurs when there is an unequal shrink rate of the molten metal during the solidification phase due to the die's wall thickness and the metal's properties.

Shrinkage pores are formed due to the following:

Wrong mold and cast part design

Shrinkage of the material wall thickness

Sharp corners in the die

Low metal temperatures

Stop Porosity with Vacuum Impregnation

Vacuum impregnation involves introducing a material to seal pores in the casting using vacuum or pressure methods. It is a cost-effective method and can control porosity in die casting permanently. It is a three-step process:

Step 1: Place the die cast in a chamber and use a vacuum to remove any trapped air that may be present in the micropores.

Step 2: Impregnate the pores by placing the part in a chamber containing a sealant, e.g., and force the sealant into the micropores using positive air pressure.

Step 3: Remove the part and cure the sealant. You can cure the impregnated die casting using heat.

Vacuum impregnation is the most recommended solution to having pores casting because it is cheap and does not affect the following component’s properties after sealing.

Assembly characteristics such as mating and sealing surfaces' integrity and tapped holes.

Functional characteristics: such as fluids or gases flowability.

Physical characteristics such as color must not be different.