Porosity

Although porosity is inherent to any casting process, it is more prevalent in die casting due to the method in which metal is injected into the die.

There are two type of porosity, gas & shrink.

a. Gas.

Gas Porosity is a result of the turbulent nature in which metal is injected into a die during the casting process. As the metal flows in the cavity and is atomized, any air in the tooling that has not been vented is mixed in with the melt. As the melt solidifies, these air bubbles get trapped in the metal. Gas porosity is typically controlled through good runner design, and appropriate venting, whether vacuum or passive venting.

b. Shrink.

Shrink porosity is typically a result of thick wall sections, or poor gating. Thicker wall sections (>0.30"), particularly those that are far away from the gate can exhibit gross shrinkage porosity. The relationship between shrink porosity and thick wall sections stems from the fact that aluminum shrinks as it solidifies. Around a thick wall section, for example, the outside skin of the part solidifies before the inside. Since the inside is still molten, it shrinks towards the solidified portion of the casting skin and creates a void in the casting. Shrink porosity is primarily a function of the part geometry, however, certain precautions can be taken to minimize it. These precautions include ensuring that the part has uniform wall sections throughout, and that the part is gated from the thicker into the thinner sections. Furthermore, proper die cooling or some exotic materials such as Anviloy or TZM, can help. These types of materials offer a very high thermal conductivity if compared to steel, and will help chill the metal faster.

The reason that the metal in die-casting is intensified to cavity pressures of 6000 to 8000 PSI is to minimize this shrink. Intensification feeds the shrink porosity by pushing more metal into the voids, and by compressing the gas in the casting, thereby reducing the size of the gas bubble. The amount of intensification that can be applied is primarily limited by the projected area of the casting since the clamping force needed is dictated by them:

Force needed to keep the die shut = Cavity Pressure X Projected Area of the casting.

F = Force (clamping)

P = Intensification Pressure.

A = Projected Area of all of the surfaces that see aluminum.

The more intensification that is applied, the more clamping force is needed to keep the machine shut.

So, in order to calculate the required machine tonnage:

Approximate Machine Tonnage Needed to Keep the die shut = (Force needed to keep the die shut / 2000) X 60% for runner and overflows X 20% for a safety factor.

In otherwords:

Approximate Machine Tonnage Needed to Keep the die shut = (Force needed to keep the die shut / 2000) X 1.6 X 1.2.

The most popular way to detect porosity embedded in a casting is through the use of x-ray equipment, however, the equipment is typically limited to detecting porosity of a certain size since porosity in die casting can range in size from the micro into gross non-fill.

The above picture shows how shrink porosity looks like. The voids are typically irregular in shape, with a dark, rough surface on the inside. They usually manifest themselves in the thickest sections of the part. The section shwon is approximately one inch thick. The smaller pores to the right of the shrink void is a combination of air and shrink porosity. It is very common for both types of porosity to occur in the same area, so when problem solving, a clear understanding of why the porosity is occurring is critical.

Blisters

Blisters are usually bumps on the die casting. They are compressed air bubbles that have expanded in size after ejection due to their proximity to soft die cast skin. Blisters are a particular concern when heat treating die casting since heat treatment typically happens at elevated temperatures that soften the die cast skin.

Cold Flow

a. Staining

Cold flow typically manifests itself when the tool is first started up. It typically looks like dark stains on the parts, it may show up as knit lines, or poor fill. Furthermore, the casting is typically a dark, dingy color. Cold flow is a problem because the die casting can be brittle or full of porosity in cold areas.

b. Knit Lines

Another way in which cold flow can manifest itself is through knit lines. Knit lines occur because the melt from partially solidified flow fronts, and did not fuse completely. The knit line will look like a crack in the casting.

c. Poor Fill

Drag & Solder

Drag on a die casting typically manifests itself as pulled die-cast skin. It appears as a much rougher surface than surrounding surfaces, and can in-fact take the form of gouges in the die casting. Soldering usually occurs around thin sections of die-steel. Thin section of die steel are usually difficult to cool. Since these thin sections operate at elevated temperatures, the aluminum and the iron in the steel undergo inter-metallic chemical reactions. The result is some kind of Al/Fe substance that adheres to the die steel. These deposits can be substantial in size and cause the casting to "drag" out die cast skin as it is being ejected.

Trim Damage

Some of the most common trim defects are also shown.

a. Impression Marks

Impression marks are made when the trim die surface is not blown off sufficiently. This leaves debris that peens the surface of the part during the trim process.

b. Breakout

Breakout occurs when the part is not supported properly by the trim die.