EFFECT OF ALUMINUM-TITANIUM-BORON BASED GRAIN REFINERS ON AZ91E MAGNESIUM ALLOY GRAIN SIZE AND MICROSTRUCTURE
A. Elsayed and C. Ravindran Ryerson University, Toronto, Canada
B.S. Murty Indian Institute of Technology - Madras, Chennai, India Copyright © 2011 American Foundry Society 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 740C (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.
Introduction
The use of magnesium (Mg) alloys is limited by their lower yield and tensile strengths and elongations as compared to aluminum (Al) alloys. The improvement of the mechani- cal properties of Mg alloys will increase the development of Mg alloys for automobile and aerospace industries. The densities of Mg and Al are 1.74 and 2.70 g/cm3 0.0975 lb/in3
(0.0629 and ), respectively.1,2 As a result, the replacement of
Al components with Mg will result in lower vehicle weight and increased fuel efficiency.
Grain refinement improves mechanical properties and in- creases feeding while reducing segregation, porosity and hot tearing.3
Proper grain refiner selection for industrial use must be able to provide adequate grain refinement even at low ad- dition levels and this refining effect must be maintained even with long holding times. In addition, the grain refiner must not adversely affect castability. Some characteristics of good grain refiners as outlined by Campbell4
are: International Journal of Metalcasting/Spring 11
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.
Keywords: magnesium alloys, AZ91E, grain refinement, aluminum-titanium-boron
• Borides, carbides and intermetallic particles which form metallic or weak covalent bonds are more suitable as grain refiners rather than metal oxides which are strongly covalently bonded5
• The nucleating solid and the grain refiner should have a good lattice match
• The grain refiner should not dissolve within the melt As noted by Emley,6
zirconium (Zr) is known as an excel-
lent grain refiner for Al free Mg alloys. Zirconium provides heterogeneous nucleating sites as it possesses similar lat- tice parameters as Mg. The lattice parameters of Zr are a = 0.320 nm and c = 0.520 nm, while the lattice parameters of Mg are a = 0.323 nm and c = 0.523 nm.6 Kabirian and Mahmudi,7
As observed by
5.282 nm and c = 8.748 nm). The presence of these par- ticles led to grain coarsening in the as-cast state, but caused no change in grain size during aging because of the thermal stability of Al2
alloy results in the formation of a Al2 Zr. 29
the addition of Zr to AZ91 Mg-Al Zr phase (HCP, a =
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
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