Structure Formation in Surface Layers of Aluminum and Titanium Alloys during Plasma Cutting

Abstract

This paper explores the structure and changes in the mechanical properties, chemical composition and surface morphology of aluminum alloys AA5056, AA2024 and Grade 2 titanium alloy after high energy impact during plasma cutting. The studies show that plasma cutting causes the formation of a subsurface layer with a dendritic structure typical of cast material and with a partially altered chemical composition. The subsurface layer material is significantly softened when cutting heat treated alloy AA2024, but changes slightly when cutting AA5056 alloy. During plasma cutting of Grade 2 titanium alloy in shielding atmosphere, the presence of even a small amount of atmospheric oxygen leads to the formation of oxides in the layer closest to the surface, which have microhardness values more than 5–7 times higher than the base metal hardness. Below the surface layer with a molten structure, a heat-affected zone is formed where the structure of the base metal is changed as a result of thermal influence. Significant changes in this zone are characteristic only for heat treated alloy AA2024. Metal flow in the cutting zone initiated by the plasma jet and shielding gas flow occurs in both the laminar and vortex modes. Nonuniform metal flow in the cutting zone and nonoptimal process parameters lead to the formation of structural heterogeneities and defects of different structural and scale levels on the surface of the samples.