Preparation of Al–Ti–C–Sr master alloys and their refining efficiency on A356 alloy

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Abstract

Al–Ti–C–Sr master alloys with various amounts of Sr were prepared through a method of liquid solidification reactions. The as-prepared Al–Ti–C–Sr master alloys were then used as grain refiners to modify A356 alloy. The microstructures of the Al–5Ti–0.25C–2Sr, Al–5Ti–0.25C–8Sr alloys and modified A356 alloy were investigated. The results showed that the Al–5Ti–0.25C–2Sr alloy consisted of phases of α-Al, lath-shaped or tiny blocky TiAl3, granular TiC, and blocky or rim AlTiSr, while the Al–5Ti–0.25C–8Sr alloy contained an irregular blocky Al4Sr phase besides the above-mentioned phases. Satisfactory grain refining and modifying effects were obtained by the addition of Al–Ti–C–Sr alloys (0.5 wt.%) to the A356 alloy. Meanwhile, the sizes of the α-Al dendrites / SDAS(40 µm) decreased to 32.7 µm (or 30 µm).The morphology of eutectic silicon was changed from needle-/platelike form to fibrous/globular form. The grain refinement and modification effects of Al–Ti–C–Sr alloys on A356 alloys were mutually promoted. Compared with the Al–5Ti–0.25C–2Sr alloy, the Al–5Ti–0.25C–8Sr alloy possessed higher efficiency in grain refinement and modification of the A356 alloys.

Introduction

Al–Ti–B and Al–Ti–C alloys (as refiners) and Al–Sr master alloys (as modifiers) are extensively used in aluminium alloys now. Compared with the Al–Ti–B alloy, the Al–Ti–C alloy has some advantages, such as the low agglomeration tendency and the ability to resist the poisoning effects of Zr, Cr, Mn and V element on grain refinement. There are many reports on the grain refinement and modification of Al–Si casting alloys. by simultaneous addition of gain refiners and modifiers [1], [2], [3]. However, the smelting processes are complicated in the cases of simultaneously adding gain refiners and modifiers. It is, hence, necessary to search for novel master alloys with high grain refining and modifying efficiency. Recently, Sagstad [4] and Wang [5] synthesized a new Al–Ti–B–Sr master alloys, which combined grain refiners and modifiers together. It has also been reported that B and Sr have an adverse effect on the grain refinement and modification because of the formation of a SrB6 compound, resulting in a mutual poisoning effect. This study, therefore, aims to prepare a novel Al–Ti–C–Sr master alloy by a liquid–solidification-reaction method, avoiding the mutual poisoning effect.

A356 alloys are the major cast components in the aeronautic, automotive and marine industries. However, A356 alloys have coarse grains and large needle-/platelike eutectic silicon phases, which led to low mechanical properties. Recently, grain refinement and modification has been concentratively studied to improve the properties of A356 alloys [6], [7], [8]. In this study, the A356 alloys are treated by the Al–Ti–C–Sr master alloys, and the microstructures of Al–Ti–C–Sr master alloys and their effects on the microstructures of A356 alloys were investigated by the techniques of optical microscopy, scanning electron microscopy and X-ray energy-dispersive spectroscopy. The grain refinement and modification mechanisms of Al–Ti–C–Sr master alloy on A356 alloy are also discussed.

Section snippets

Experimental Details

Firstly, commercial pure aluminium was melted in medium frequency furnace. The pretreated potassium fluotitanate and graphite powders were added to the superheated aluminum melt at 800–900 °C. Then the covering flux was sprinkled to the surface of the melt. After 20 min, the temperature was increased to 1200–1300 °C, which was kept for 10 min. Then the temperature was decreased to 800–900 °C, and pure strontium was added to the melt when the slag was eliminated completely. After treated at the

Microstructures of Al–Ti–C–Sr Alloys

Fig. 1, Fig. 2 present the optical micrographs and SEM photomicrograph of RM1 alloy, respectively. In Fig. 1, the grayish regions are α-Al matrix, and the parallelly distributed lath-shaped phase and the tiny blocky phase are TiAl3. The white particles shown in Fig. 2 are the TiC phase.

Fig. 3a shows a SEM image of the blocky phase in RM1 alloy. Fig. 3b shows the X-ray energy-dispersive spectrum, which indicates that the blocky phase containing elements of Al, Ti and Sr is a compound with an

Conclusions

The as-prepared Al–5Ti–0.25C–2Sr alloy consisted of α-Al matrix, lath-shaped or tiny blocky TiAl3, granular TiC and blocky or rim AlTiSr, while the Al–5Ti–0.25C–8Sr alloy contained Al4Sr besides the above-mentioned phases. Good grain refinement and modification effects were obtained by adding 0.5 wt.% Al–Ti–C–Sr alloys to A356 alloy. Al–5Ti–0.25C–8Sr possessed a better grain refining and modifying performance than Al–5Ti–0.25C–2Sr.

When the Al–Ti–C–Sr alloy was added to the melt, TiAl3

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