Effect of Hydrogen Gas Content on Generation of Porosity in Al–Li Casting Alloys

Abstract

The relationship between hydrogen gas content of molten metal and the porosity generated during solidification of Al–Li–Mg–Zr–Ti alloys with or without the addition of lithium was investigated. All alloys were cast into stepped-form metal mold in an argon atmosphere by the melting-casting machine. In the Al-1.90%Mg-0.15%Zr-0.11%Ti alloy step-castings, the maximum and minimum porosity were 0.77% and 0.05%, respectively, when the hydrogen gas content of the molten metal at the time of pouring was (0.19−0.60)×10⁻⁷ m³·kg⁻¹. On the other hand, in the Al-2.36%Li-1.76%Mg-0.15%Zr-0.11%Ti alloy castings, the maximum and minimum porosities were 0.35 and 0.09%, respectively, even when the hydrogen gas content of the molten metal was (1.05−1.23)×10⁻⁷ m³·kg⁻¹. In both types of alloys with or without Li, the locations at which porosity was generated were either grain boundaries or dendrite cell gaps. The size of the pores decreased as the hydrogen gas content decreased, and their shape changed from spherical to flake-like. The critical hydrogen gas content in the molten metal at which no porosity was generated, were (0.70−0.98)×10⁻⁷ m³·kg⁻¹ for the 2.36%Li-added alloy and (0.12−0.28)×10⁻⁷ m³·kg⁻¹ for the no-Li-added alloy, respectively. Therefore, the alloying of 2.36%Li increased the critical hydrogen gas content by (0.58−0.70)×10⁻⁷ m³·kg⁻¹.