In the casting workshop, the defects that quality engineers face every day can be mainly divided into six categories:

1. Pores and pinholes
   Hydrogen in the melt precipitates during solidification, forming circular or elliptical pores. It manifests as rolling bubbles on the sheet, surface peeling on the extruded material, and pinholes on the aluminum foil.
2. Oxidized leather slag
   The oxide film (Al ₂ O ∝) on the surface of aluminum melt or the refractory material peeled off from the furnace is rolled into the interior of the melt and solidifies to form hard inclusions. It is the primary cause of aluminum foil perforation, black lines on the surface of high-speed train profiles, and unqualified inspection of aviation thick plates.
3. Loose
   During the solidification process, insufficient shrinkage leads to the formation of micro pores, which are mostly distributed in the center or hot nodes of the ingot.
4. Cracks
   Including hot cracks (the liquid film at the grain boundary is pulled apart at the end of solidification) and cold cracks (excessive cooling stress). Round ingots often have center cracks and surface cracks, while flat ingots often have small surface cracks.
5. Segregation
   Uneven chemical composition. Including intragranular segregation and inverse segregation (surface copper/zinc enrichment). Often related to improper control of casting speed and cooling intensity.
6. Coarse grain size and feather crystals
   Insufficient or ineffective addition of grain refining agents, or excessively high casting temperatures, can lead to the development of columnar crystals and coarse grains, reducing the strength and toughness of the material.

Among the above defects, porosity and oxide inclusion are the two most common and directly related core issues to the purity of the melt. Oxidized leather slag, due to its strong concealment, difficult detection, and great harm, is the most difficult bone to crack in high-end aluminum production.

Quality is not 'inspected', but 'leaked'. The introduction of defects runs through every step of the casting process:

1. Raw material preparation stage
   Ungraded waste mixed with paint, plastic, and high moisture chips; The intermediate alloy becomes damp, and the furnace charge carries in moisture and organic matter; The oxide scale and iron impurities in the raw materials directly enter the furnace.
2. Smelting process (the "source" of defects)
   The flame of the flame reflector furnace directly contacts the aluminum material, causing local overheating and accelerating oxidation; When adding materials, smash them into the melt pool and stir vigorously, causing the surface oxide film to be pressed into the melt; The melting time is too long, the melt is exposed to high temperatures for a long time, and the oxide film continues to thicken.
3. Refining process (the "window" for incomplete purification)
   Insufficient purity or unstable ratio of refined gas, incomplete hydrogen removal; Insufficient depth of injection into the rotor, large bubble diameter, and low flotation efficiency; The refined slag was not removed in a timely manner and mixed back into the melt; The chlorine gas system is unstable and lacks the ability to remove alkali metals and inclusions (referring to the accuracy of the argon chlorine mixing device, see below).
4. Transfer and static stage (in heavily affected areas of secondary oxidation)
   The flow channel is exposed, and there is no covering agent on the surface of the melt or the covering agent has failed; The flow channel has a large drop, causing the melt to fall from a high place, resulting in turbulent flow and splashing, and severe gas and slag entrainment; The liquid level in the static furnace fluctuates greatly, and the surface oxide skin folds.
5. Casting process (last chance)
   The liquid level in the crystallizer fluctuates greatly, and the alumina film is captured by the solidification interface; The design of the distribution bag is unreasonable, and the liquid flow directly impacts the liquid surface, breaking the integrity of the oxide film; The mismatch between casting speed and cooling water leads to tearing or curling of the solidified shell.