The slag mixed into the ingot or other non-metallic impurities falling into the ingot are called non-metallic inclusions. The fracture features are black strips or flakes, and the microstructure features are mostly black linear, massive, and flocculent disordered tissues, with obvious color difference from the matrix. Non-metallic inclusions are an important cause of delamination and many surface defects in aluminum processed products. In the heat treatment and heating process, the existence of non-metallic inclusions can promote the formation of sub-porosity and bubbles. In terms of mechanical properties, non-metallic inclusions are places where stress is concentrated, which reduces the strength limit and elongation of the alloy. In particular, the reduction in transverse elongation and dynamic mechanical properties (φ impact toughness, fatigue strength and fracture toughness) is more serious. In addition, non-metallic inclusions will reduce the stress corrosion resistance of the alloy.
Non-metallic inclusions (slag inclusions) can usually be detected when inspecting the transverse low-magnification test piece of the ingot. For all alloy products, there should be no more than two slag inclusions on the ingot test piece, and the single area is less than 0.5mm^2, otherwise it will be rejected. Non-metallic inclusions (slag inclusions) are the most frequent waste products in aluminum alloy ingots. According to statistics, calculated by quality, ingots scrapped due to slag inclusion account for 10% to 25% of the total waste.
Slag inclusions are usually caused by slag, lining fragments, and larger oxides that fall into the ingot along with the liquid metal during the casting process.
The way to prevent slag inclusion
1) Refining carefully, ensuring the standing time, and filtering with glass cloth in the flow plate or funnel.
2) Shorten the transfer launder distance as much as possible, establish good transfer conditions, close all the exposed drops in the launder, flow plate, and funnel to prevent metal liquid level fluctuations. It is best to use the same level casting when possible.
3) Bake the transfer tools thoroughly, raise the casting temperature appropriately, slag carefully during the casting process, and carefully treat the furnace pan and other tools after casting.
4) Use clean charge.