Analysis of the reasons for the blackening of the surface of 4032 alloy bars: A certain aluminum company reported that a batch of 4032 alloy had blackened surface and coarse grains, with a quantity of about 6 tons. After on-site observation of the samples and understanding of the workshop process, as well as detailed consultation with well-known casting experts in China, the following opinions have been formed:

4032 alloy chemical composition

1. The blackening of the rod is due to insufficient refinement of the primary Si, and the grain size of the primary Si is generally controlled below 0.08mm. If the eutectic Al Si alloy is not modified on the primary Si, it is inevitable to produce coarse primary Si structure during melting and casting without proper process measures. The coarse primary Si has high fiber hardness, which not only wears down the tool but also has structural defects that affect the mechanical properties of the product, and can cause the phenomenon of surface blackening of the product. Domestic factories producing 4 series products may encounter this problem to some extent. Through observation of the provided samples, the surface blackened products also have coarse primary Si grains. Therefore, it can be confirmed that the 4032 product with surface blackening is caused by insufficient modification treatment of Si.  

2. There are generally several modification methods used for eutectic Al Si alloys, including the following

Strontium is commonly used for modification in industry, and a dosage of 0.01-0.05% can achieve good modification effect. However, it should be noted that a incubation period of about 30 minutes is required for strontium modification. After adding strontium modifier, it should be cast after 30 minutes to achieve uniform and good results.  

3 Suggestions: ① Maintain an appropriate melting temperature, control the melting temperature higher, and try to melt the silicon completely to prevent residual primary silicon.

② Control the range of Si content to prevent the occurrence of primary silicon structure, and generally control it between 11.2-12.5% in factories. If the composition is uneven, it will result in areas with high silicon content, leading to the formation of primary silicon.

③ It must undergo spoilage treatment. And attention should be paid to controlling the deterioration process. Only the modified 4032 alloy can obtain a fine fibrous structure and eliminate product defects caused by primary silicon, such as decreased mechanical properties, reduced lifespan, and surface blackening.