Inclusions

Inclusions refer to contaminating unwanted particles/compositions/products in die casting. Depending on the contaminants, there are several forms of inclusions in metal casting. It’s a serious defect that affects the mechanical properties of cast parts.

Inclusions shouldn’t be confused with porosity, as the latter occurs due to gas entrapment. On the contrary, only solid products, compositions, or particles initiate inclusions. An X-ray can detect both defects where inclusions appear as jagged, dark, and asymmetric shapes.

It’s impossible to achieve the desired outcome without addressing this troublesome issue. This article briefly sheds light on inclusion types, effects, and countermeasures in die casting.

Different Types of Inclusions

Inclusions found in die casting are primarily categorized into –

• Dross Inclusions (Solid Impurity)
• Slag Inclusions (Liquid Impurity)

But there are variations in defects between the two. And the most common ones are discussed right below with their probable cause.

1. Oxides

Oxide inclusions are the most troublesome problem concerning aluminum alloy casting. Some oxides always form during the process, no matter what. But it becomes an issue with negligent metal handling and poor furnace cleanup. Being a highly active metal, aluminum immediately bonds with airborne oxygen to form alumina (Al₂O₃).

Aluminum casts without the oxide deliver a visibly shiny appearance during the process. But the oxide remains floating in a softer state (a.k.a. gamma form). It’s incredibly difficult to re-melt the Al2O3 into aluminum because it is an entirely different chemical product. But it may take a closer look to understand the presence of floating oxide.

Polymorphic aluminum oxide can change its crystal properties drastically under specific conditions. Highly heated gamma Al₂O₃ tends to become alpha Al₂O₃, making things more problematic. It may eventually form corundum, weighing more than original alloys. The solid impurity floating over the molten metal is often called dross.

2. Refractory Particles

A small fraction of the inclusions come from refractory particles or hard chemical compounds. Much like corundum, these rock-hard crystals can damage metal-made machining tools. Silicon Carbide (SiC) is the most noteworthy refractory particle in aluminum die casting.

3. Sludge

This is a heavy liquid-like impurity that forms within the molten alloys. Complex inter-metallic bonds among the compounds of heavy elements (like Silicon, Iron, Magnesium, or Chromium) form sludge. This sugary or sandy impurity weighs way more than aluminum alloys to hit the furnace’s bottom.

However, due to poor temperature control, the crystalline sludge often turns into straight-side pentagons. There’s an empirical formula to set a minimal value to reduce sludge formation, as follows –

[{3 X (%Cr)} + {2 X (%Mn)} + (%Fe)] < 1.75

Cr = Chromium, Mn = Manganese, Fe = Iron

Increasing the value of 1.75 by a slight margin can hold minimal sludge based on the alloy. In fact, 1.80 is the maximum allowable value based on certain casting requirements.

4. Hard Spots and Comet Tails

Zinc casting alloys also suffer from solid impurities from time to time. When the aluminum bonds with the pot iron, iron-aluminum inter-metallic (FeAl₃) is formed. This inclusion starts to accelerate rapidly at high temperatures. Poor temperature control can worsen the formation of hard spots in zinc alloy casts.

They have more aluminum, which can aggravate the combination with pot iron. Also, due to this complex compound’s activity, comet tails are negligible yet defective. It refers to the random scratch marks on cast surfaces while buffing. But the lighter dross mostly ends up floating on top.

5. Sand Inclusions

The flow of molten metal triggers mold surface abrasions with considerable thermomechanical stress. And the combined force of compression and shear can break sand grains called erosions. Also, larger mold sections can be torn off, termed a scab defect.

Insufficient mold material compaction and non-uniform sand core/mold hardening abrade the particles. It often occurs on the thicker zones like the gate and remote casting sections. Also, CO (Carbon Monoxide) bubbles and oxides may accompany this inclusion.

6. Slag Inclusions

This liquid defect occurs in almost all kinds of die-casting metals without depending on the molding process. Gas bubbles may accompany this issue to cause defective surfaces. Thus, the non-metallic inclusions remain visible to the naked eye.

Their irregularities primarily reflect the composition responsible (carbide, melt, treatment slags) for the defect. High-viscosity slags appear within solidified casts, whereas low-viscosity slags hit the surfaces. Rapid oxidization of the molten metal while cooling creates a favorable environment for slags.

Although the slag holds its liquidity on cast iron and steel alloys, it quickly turns to dross on aluminum and magnesium alloys. On the contrary, subsequent reactions between the mold material and steel/iron slags remain difficult to avoid.

Effect of Inclusions

Inclusions don’t lead to any immediate mechanical damage. But it heavily accelerates the die-casts’ vulnerability to catching other defects. Some of the direct effects of inclusions include –

• Rapid erosion of the die-casts’ surface finishing.
• Formation of stringers to weaken the casting.
• Automated drifting of oxide to affect other spots.
• Formation of leak paths on pressurized casts.
• Accelerated wearing of metallic machining tools.
• Metallic protuberance may occur on the parts.

Preventing Die Casting Inclusions

Implementing a proper die-casting process throughout the cycle is the ultimate countermeasure. Still, the following precautions are advised to minimize the defect.

• Having a quality temperature controlfor the furnace.
• Skimming out the floating dross (oxides or hard spots) from the cast surface.
• Proper reviewing of the fluxing and cleaning process.
• Waiting at least 30 minutes before tapping the furnace following its cleanup.
• Using trained furnace tenders with appropriate tools.
• Maintaining proper temperature range (1350ºF – 1425ºF or 732ºC – 773ºC).
• Regular checking on the tools required for the casting.
• Installing a subtle filter system sufficiently adjacent to the ladle dip if possible.
• Ensuring a clean workspace with satisfactory venting.

It’s nearly impossible to discard every bit of inclusions from die-casting parts. And most of the defective segments are barely noticeable to the naked eye. However, modern casting processes are exact to keep this defect within minimal limits.