Effect of aluminum-titanium-boron based grain refiners on AZ91E magnesium alloy grain size and microstructure

Abstract

Vehicle weight in the aerospace and automotive industries directly impacts carbon emissions and fuel efficiency. An increase in the use of magnesium alloys in structural applications to replace aluminum alloys will result in lighter vehicles. However, the strength of magnesium alloys is lower than that of aluminum alloys. Grain refinement can significantly improve the mechanical properties of alloys. This study investigates the refining potential and fading of Al-5Ti-1B and Al-1Ti-3B grain refiners in AZ91E magnesium alloy. The grain refiners were added at 0.1, 0.2, 0.5 and 1.0 wt.% levels. These alloys were added to the molten AZ91E magnesium alloy at 74^oC (1364F) and stirred for 30 seconds. To examine fading of the grain refiners, they were allowed to settle for 5, 10 and 20 minutes in the molten magnesium alloy prior to pouring. The prototype castings were characterized using optical microscopy, SEM and TEM. The average grain size of the base alloy, AZ91E decreased significantly with the addition of the grain refiners. Minimum average grain sizes were obtained using 0.1 wt.% Al-5Ti-1B and 1.0 wt.% Al-1Ti-3B. For both refiners, grain refinement was attributed to TiB₂ particles providing nucleating sites and grain growth restriction. In addition, AlB₂ particles provided nucleating sites in the case of Al-1Ti-3B refiner. Proper choice of the type and quantity of grain refiner additions for grain refinement with an understanding of the fading effects will enable improvement of mechanical properties at minimal cost. The improved mechanical properties of these refined magnesium alloys will promote their use for more structurally demanding applications.