Nucleation Effect of Ti-6Al-4V Powder on Al–Si Eutectic Alloy

Abstract

In this research nucleation effect of Ti-6Al-4V powder on grain size and tensile strength of Al-12.1% Si eutectic alloy was studied. Metal powder produced from Ti-6Al-4V machining chips by meshes of 50 and 140 and weight percentages of 0.1, 0.15, 0.2, and 0.25 were used. The microstructure by mesh 50 with 0.15 wt% consists of grain with average size of 6 m, tensile strength and elongation equal to 492 MPa and 7.25%, respectively. By mesh 140 with 0.15 wt% the considered values are equal to 10 m, 540 MPa, and 8.82%, respectively. Before refinement the needle-plate-like eutectic silicon is distributed arbitrarily. The morphology of the eutectic silicon is changed from needle-plate to a fibrous form with addition of 0.15 wt% of mesh 50 and finally changes its morphology from a needle-plate to a globular form after adding 0.15 wt% of mesh 140, which is fully modified. SEM/EDX micro-analyzing shows that TiAl3 compound has been formed, which is the major factor in grain size modification and tensile strength increasing. This phase is formed prior to solidification of Al-Si alloy makes it an efficient nucleation site for aluminum grains upon solidification. IntroductionGrain refinement is a common way of achieving a proper, uniform, and fine grain structure in cast aluminum alloys, since metals and alloys usually solidify with a coarse columnar grain structure under normal casting conditions. The most widely used grain refiners are Al-Ti-C-Sr, Al-Ti-C, Al-Ti-B, Al-B, and Al-Sr master alloys. The results showed that the microstructure refined by Al-5Ti-0.25C-2Sr alloy consists of phases of Al, lath-shaped or tiny blocky TiAl3, granular TiC, and blocky or rim AlTiSr. Size of the Al dendrites decreased to 32 7 m or 30 m. The effectiveness of Al-3Ti-0.15C grain refiner in aluminum alloy with 0.102 wt% of Fe can be explained by the fading of TiB2-containing grain refiner and the significant increase in the recalescence magnitude with holding time, found for Al-5Ti-1B grain refiner. Size of the Al dendrites decreased to 94 m and 120 m, respectively [1,2]. When silicon is present, a silicide can form on the TiB2 particles in place of the TiAl3 layer preventing or decreasing the probability of nucleation of aluminum grains on the particles [3]. Combined addition of strontium and boron led to a much greater decrease in the eutectic grain sizes in near eutectic Al-Si alloys than that caused by addition of strontium alone. The mechanism of this refinement in the presence of strontium and boron is related to the combined effect of increased under cooling of the eutectic reaction and the effective number density of eutectic nuclei. It is found that addition of boron to 0.04% Sr-modified alloy leads to a considerable decrease in eutectic grain size, with near 10% and more than 15% reduction as the level of boron addition

Grain Refinement Induced by a Pulsed Magnetic Field and Synchronous Solidification

“…Fine grain structures are frequently required of solidification processes to limit the formation of casting defects and improve the mechanical properties of castings [1][2][3]. Some environment-friendly physical methods, including low-temperature casting, ultrasonic vibration, and mechanical or electromagnetic stirring, have attracted increasing attention compared with chemical methods because the latter requires the addition of a refiner or modifier into the metallic melt [4][5][6].…”

Mechanical Modification of Al-6%Si By Semisolid Process

“…Uniform and fine grain structure in cast aluminum alloys, since metals and alloys usually solidify with a coarse columnar grain structure under normal casting conditions could be achieved by the proper mechanical modification [1]. The percentage of each zone depends on a multitude of parameters including pouring temperature, cooling rate, temperature gradient in the liquid and the mold material which also influences thermal gradient established within the molten alloy [2].…”