Surface Structure Formation in Plasma Cutting of Aluminum and Titanium Alloys Using Direct Current Straight and Reverse Polarity

The structural features and phase composition are examined in near-surface layers of specimens of Al-Mg, Al-Cu-Mg alloys and commercially pure titanium obtained by plasma cutting using direct current straight polarity (DCSP) and direct current reverse polarity (DCRP). It is found that the flows of molten metal ejected by the gas stream from the cut cavity during cutting form the fusion and heat-affected zones, whose structural morphology, phase composition, and thickness depend on both the selected material and the cutting mode. The fusion zone is thicker in specimens cut using DCRP than in those cut with DCSP. The thickness of the adjacent heat-affected zone is also the largest in the mode that provides a thicker fused layer. Aluminum alloy specimens cut in ambient air are characterized by the presence of oxygen in the near-surface layers. The lowest degree of oxidation is observed in Al-Mg alloy. Oxygen penetrates into the fused layer to a depth of 350-500 mu m in Al-Cu-Mg and up to 200-250 mu m in Al-Mg alloy. In titanium alloy, the thickness of oxide layers does not exceed 100-150 mu m during straight polarity cutting and 200-250 mu m during reverse polarity cutting. A thin brittle layer of TiO and TiO2 oxides is formed on the titanium alloy surface. It is shown that the generation of "water mist" around the plasma jet when cutting materials of all types with DCRP leads to a more intensive oxidation of metal, less thermal effect on the material, and reduced roughness of the cut face.