RESEARCHERS’ FINDINGS
ablation appeared to refine the den- dritic cell size, which is in contrast to early reports in a monolithic A356 alloy. Te particulate reinforce- ment within the liquid melt during solidification may have restricted the dendrite cell size while it was pushed by the solidifying aluminum. In ablation, the impingement of solvent on the solidifying surface increases cooling rates, entrapping the particles more rapidly without allowing time to substantially migrate, leading to increased branching of the dendrites and finer overall cell size. In the study, the samples produced via both processes showed relatively high levels of porosity. In the ablated component, gas porosity occurred— likely due to poor gating design and a filling system that was not optimized for ablation. Te pouring technique and mold geometry should be modi- fied significantly for ablation in the future since the time delay to remove the mold was significant. Despite the
Table 2. Properties of the Ablated Component and Conventional Casting
# 1
2 3 4 5 6 7 8 9
10 Mean
Conventional Casting Tensile 30.6
30.7 30.3 31.2 31.2 34.1 32.5 33.2 30
31.2 31.5
Standard Deviation 1.34
# 1 2 3 4 5 6 7 8
Mean
% Elongation 0.35
0.35 0.25 0.4
0.45 0.45 0.4 0.4
0.35 0.4
0.38% 0.06
Ablated Component
Tensile % Elongation 38.4 41
31.6 37
35.7 36
33.2 35.6 36.1
Standard Deviation 2.91
0.35 0.4 0.3
0.25 0.4 0.3 0.3 0.3
0.33% 0.05
December 2011 MODERN CASTING | 29
Researchers concluded the following after comparing conventionally sand cast and ablated aluminum metal matrix composites. 1. Ablation increases the yield strength of hybrid aluminum silicon carbide- graphite composite by 20% over conventional, heavily-chilled sand casting. Further improvements are possible through higher cooling rates and a reduc- tion in porosity achieved with solidification modeling optimized for ablation. The percent elongation of the castings produced in both processes was similar.
2. The average particle size of the silicon carbide was 0.00059 in. (15 µm), and graphite platelet size was 0.0033 in. (84 µm). In addition to the reinforcements, intermetallic phases and porosity were observed in the composites made by each process.
3. The dispersion of the reinforcements was more uniform in the ablated samples than in the conventionally cast samples.
4. The dendrite arm spacing/cell size of the ablated sample was 0.00198 in. (50.3 µm) compared to 0.00246 in. (62.5 µm) for the conventionally cast sample.
5. The fracture surface of each sample exhibited predominantly brittle characteristics, with evidence of graphite flakes, shrinkage pores and gas pores at the surface.
porosity, the strength of the ablated sample was higher than that of the conventionally cast sample and further improvement in mechanical properties may be realized by improving the pour- ing technique to reduce turbulence in the melt (Table 2).
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