1. What is oxidized slag?

Oxidized slag, commonly referred to as "oxide film inclusion" or "Al ₂ O ∝ inclusion" in professional terminology. Macroscopic morphology: On the low magnification test piece of the ingot, it appears as black strip-shaped, flaky or irregular clusters with blurred or clear edges, mostly distributed in the upper part, subcutaneous or near the central segregation tumor of the ingot. Microscopic morphology: Under electron microscopy, Al ₂ O3 inclusions appear as plate-like and flocculent aggregates, with extremely high hardness (Mohs hardness of 9, close to diamond), no metallurgical bonding with the aluminum matrix, and a clear interface.

Subsequent performance:

When rolling aluminum foil, the mixed areas cannot deform and are elongated into linear defects, ultimately leading to fracture and the formation of pinholes; When extruding profiles, inclusions flow with the metal, forming periodic black lines and peeling on the surface; In aerospace forgings, the oxide film is the preferred source of fatigue cracks, leading to unsatisfactory testing.

2. Birth of Oxide Film: Thermodynamics and Dynamics

Aluminum is an extremely reactive metal with a standard electrode potential of -1.66V. In atmospheric conditions, a layer of oxide film will instantly form on the surface of liquid aluminum.

Thermodynamic mechanism:

4Al + 3O2—— 2Al2O3

The formation free energy of Al ₂ O ∝ is extremely low, and the reaction is extremely thorough. This layer of film is dense in the solid state, but its structure is not singular on the surface of liquid aluminum. The dual structure of surface oxide film:

• Outer layer: loose γ - Al ₂ O3, adsorbing water vapor;
• Inner layer: dense α - Al ₂ O ∝.

picture

More importantly, the oxide film on the surface of aluminum liquid has a "self-healing" characteristic - once the film ruptures, fresh aluminum liquid comes into contact with air and immediately generates a new film. This leads to a terrible phenomenon: every agitation, instead of damaging the membrane, creates two layers of membrane.

3. "Folding and Rolling in" Mechanism: How does the slag get in?

This is the core formation logic of oxidized leather slag. The oxide film on the surface of liquid aluminum, although thin (micrometer level), does not melt into aluminum. Its melting point is as high as 2050 ℃, and it always exists in a solid state in the melt. When the melt is in a static state, the oxide film floats on the surface and can be removed by slag scraping. But when the melt flow creates a valley, tragedy occurs:

• Valley folding: During injection, the aluminum liquid forms waves in the flow channel. The peaks collide with each other, and the surface oxide film folds and curls like two sheets of paper, being rolled into the valley.
• Drop impact: From the static furnace to the flow channel, from the flow channel to the degassing box, if the drop exceeds 100mm, the aluminum liquid splashes and the oxide film fragments are directly "nailed" into the deep part of the melt.
• Vortex capture: At the outlet of the online degassing box and inside the crystallizer, if a vortex is formed on the liquid surface (commonly known as a "funnel"), the oxide film at the center of the liquid surface is sucked into the vortex center and directly captured by the solidification front.

picture

This rolled up oxide film is no longer floating slag, but solid foreign matter inside the melt. It is difficult to "float" refined bubbles out because they have a large surface area and a density similar to aluminum (initially carrying bubbles when rolled in, later bubbles overflow and the density increases, sinking or suspending).

4. Secondary oxidation: "continuous pollution" from the furnace to the casting machine

In addition to the initial oxide film during melting, there are also secondary oxide scales: lining peeling: the refractory materials (especially siliceous) lining the melting furnace and static furnace are eroded by aluminum liquid, and after peeling, they mix into the melt and react with aluminum to form aluminosilicate inclusions. Refining reaction products: When chlorine gas is used for refining, salts such as AlCl3 and NaCl are generated. If not effectively discharged, they can also form reactive inclusions. Flow channel and distributor: The rough surface of the refractory material in the flow channel will adsorb aluminum liquid, repeatedly oxidize, form "hanging slag", and be washed down by the new liquid flow into the ingot.