Effect of hydrogen gas content on generation of porosity in Al-Li casting alloys

The relationship between hydrogen gas content of molten metal and the poros ity generated during solidification of Al-Li-Mg-Zr-Ti alloys with or withou t the addition of lithium was investigated. All alloys were cast into stepp ed-form metal mold in an argon atmosphere by the melting-casting machine. I n the Al-1.90%Mg-0.15%Zr-0.11%Ti alloy step-castings, the maximum and minim um porosity were 0.77% and 0.05%, respectively, when the hydrogen gas conte nt of the molten metal at the time of pouring was (0.19-0.60) x 10(-7) m(3) .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%, respecti vely, even when the hydrogen gas content of the molten metal was (1.05 - 1. 23) x 10(-7) m(3).kg(-1). In both types of alloys with or without Li, the l ocations at which porosity was generated were either grain boundaries or de ndrite cell gaps. The size of the pores decreased as the hydrogen gas conte nt decreased, and their shape changed from spherical to flake-like. The cri tical hydrogen gas content in the molten metal at which no porosity was gen erated, were (0.70-0.98)x 10(-7) m(3).kg(-1) for the 2.36%Li-added alloy an d (0.12-0.28) x 10(-7) m(3).kg(-1) for the no-ii-added alloy, respectively. Therefore, the alloying of 2.36%Li increased the critical hydrogen gas con tent by (0.58-0.70) x 10(-7) m(3).kg(-1).