Although the casting performance of Al Mg casting alloy parts is not as good as that of Al Si alloy, it has better corrosion resistance. Therefore, this kind of material is often used in some weapon components that need certain corrosion resistance. Some ZL301 alloy products cast by a factory have obvious white corrosion spots on the surface several days after the cast parts come out. After quenching and aging in strict accordance with the traditional heat treatment system of the alloy, the corrosion spots become more serious. In order to find out the causes of product corrosion, the manufacturer entrusted ZHY casting to conduct analysis and research.
After careful analysis of the color and distribution of corrosion spots on alloy casting parts, it is found that the corrosion products are not evenly distributed on the whole surface of casting parts, but in some local areas of the surface. Firstly, the morphology and composition of corrosion products were analyzed by SEM and EDS. Hand saw shall be used for sampling along the diameter direction where the surface corrosion of the test bar (specially made test bar for research corrosion, which is similar to that of cast parts) is serious. One part of the sample was directly observed by scanning electron microscope, the other part was made into metallographic sample for metallographic observation, and then the microstructure was observed and micro area composition was analyzed by band energy spectrum scanning electron microscope. The macro composition of the test bar was analyzed by atomic absorption spectrometry and chemical analysis.
(1) The composition and structure of the test bar of alloy casting parts basically meet the design requirements, and the subcutaneous pores and corrosion only occur on the surface of the test bar.
(2) The corrosion products on the surface of casting parts and the main components of inclusions contained in subcutaneous pores contain a large number of elements such as Ca, Cl, s and Si. The high content of S and Cl is the direct cause of surface corrosion of casting parts.
(3) The inclusions in the subcutaneous pores of cast parts come from heterogeneous particles in the melt whose size is close to or less than a certain critical size. The groove nucleation mechanism is the main mechanism of subcutaneous pore nucleation. The shrinkage of alloy melt during solidification provides a driving force for pore nucleation and growth.