2. Gas and non-metallic inclusions in ingots 2.1 Non-metallic inclusions (slag inclusions)

 

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 presence of non-metallic inclusions can promote the formation of secondary 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 (impact toughness, fatigue strength and fracture toughness) is more serious. In addition, non-metallic inclusions will also reduce the stress corrosion resistance of the alloy.

 

Non-metallic inclusions (slag inclusions) can usually be found when inspecting the transverse low-magnification test piece of the ingot. YS67-1993 stipulates that 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 0.5 mm2, otherwise it will be rejected. Non-metallic inclusions (slag inclusions) are the most frequently occurring waste products in aluminum alloy ingots. According to statistics, ingots scrapped due to slag inclusion account for 10% to 25% of the total amount of waste calculated by quality. Slag inclusions are usually caused by slag, lining fragments and large-scale oxides that fall into the ingot along with the liquid metal during the casting process. The way to prevent slag inclusion is: 1) Carefully refine, ensure the standing time, and use glass cloth to filter in the flow plate or funnel; 2) Shorten the transfer distance as much as possible, establish good transfer conditions, and close the flow channel, flow plate, All the exposed drop in the hopper can prevent the metal level from fluctuating. If possible, it is best to use the same level of casting; 3) Thoroughly bake the transfer tool, appropriately increase the casting temperature, carefully slag during the casting process, and carefully after the end of the casting Clean the stove, flow plate and other tools; 4) Use clean charge.