SURFACE STRUCTURE FORMATION IN PLASMA CUTTING OF ALUMINUM AND TITANIUM ALLOYS USING DIRECT CURRENT STRAIGHT AND REVERSE POLARITY

Abstract

The structural features and phase composition were examined in near-surface layers in samples 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 was found that the flows of molten metal carried away by the gas stream from the cut cavity during cutting form molten and heat affected zones, whose structural morphology, phase composition and thickness depend on both the selected material and the cutting mode. The thickness of the molten zone is larger for samples cut using DCRP than for those cut with DCSP. The thickness of the adjacent heat affected zone is also the greatest under conditions that provide a large thickness of the fused layer. Aluminum alloy samples 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 μm in Al-Cu-Mg and up to 200-250 μm in Al-Mg alloy. In titanium alloy, the thickness of the oxide layers does not exceed 100-150 μm when cutting with DCSP and 200-250 μm when cutting with DCRP. A thin brittle layer of TiO and TiO2 oxides is formed on the titanium alloy surface. It was shown that the release of “water mist” around the plasma jet when cutting materials of all types with DCRP leads to more intense oxidation of metal, less thermal effect on the material, and reduced roughness of the cut surface.