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Table 1. Mechanical Properties of A356 Alloy Aluminum Cast in Various Methods Property


Sand


Ultimate Tensile Strength MPa (ksi) 228 (33) 262 (38) Yield Strength MPa (ksi) % Elongation


4


of the ablation process would require changes in the gating/risering. T is would be a subject for future testing. T e overall dimensions of the mold


for the conventional cast and ablation component were the same, but the cope portion of the mold for the ablation process included the following modifi cations: 1. Approximately 6 in. (15.24 cm) of sand was removed directly above the casting to increase the rate of ablation in that location;


2. A channel was cut at the edge of the cope to allow water to drain from the mold cavity.


Both types of castings were soluti-


onized at 1,000F (538 C) for 12 hours, followed by hot quenching at 140F (60C), and aged at 310F (155C) for fi ve hours.


Ablation Results Promising Examination of the microstructures


of the conventional casting and ablated component revealed the presence of sili- con carbide and graphite particles at cell or dendrite boundaries, which is evidence that both the particulate reinforcements were pushed by growing aluminum dendrites into the solute-rich fi nal freezing zones. T e apparent increase in the volume percentage of reinforcement in the ablated sample likely was due to the increased solidifi cation rate and decreased fl otation or settling of the rein- forcement (Fig. 2). T e ablated sample’s microstructure was more cellular, mean- ing reinforcement particles were trapped in a tight network of cell boundaries, increasing the local concentrations of reinforcement and reducing the average interparticle spacing. Although the combination of


silicon carbide and graphite particles in the liquid aluminum resulted in a near neutrally buoyant suspension, some settling and fl otation occurred over time when melting a large batch. If the melt is insuffi ciently mixed, the reinforcements can migrate, producing areas that are rich in silicon carbide or graphite. T is can result in large


28 | MODERN CASTING December 2011


179 (26) 207 (30) 3.5


243 (35) 11.0


Permanent Mold Squeeze Cast Ablation 312 (45)


325 (47) 261 (38) 12.5


diff erences in the volume percent- age of reinforcement in the solidi- fi ed microstructure. T e increase in volume percentage in the thin section of the ablated component compared


to the conventional sand casting may have partly contributed to the higher strength of the ablated samples and could be due to the casting process, if metal is ladled from the top, middle or bottom of the original melt charge. T e ablated samples featured an


improved distribution of particles in the casting compared to the conven- tionally cast samples. Additionally,


COMMERCIAL ABLATION CASTINGS


The ablation sand casting process is not reserved to metal matrix composites. The process already has been used successfully in commercial applications, boosting the performance of standard casting alloys, such as A356, and enabling the consistent production of emerging alloys, such as B206.


—by MODERN CASTING Staff Rear Motorcycle Frame


Material: A356 aluminum with T6 heat treat.


Weight: 4.9 lbs. Dimensions: 10 x 8 x 6 in.


• The aluminum part achieved enhanced mechanical properties in thin sections through the ablation process.


• The part saved weight and cost by incorporat- ing the motorcycle’s shock mount location.





Rear Swing Arm Material: B206


aluminum. Weight: 7 lbs. Dimensions: 26 x 10.25 x 6 in.


• The motorcycle casting features 0.079-in. wall thickness and extensive internal coring. Although prone to hot tears and porosity, the B206 alloy in this case achieved grade level 1 x-ray for porosity and inclusions.


• The casting was filled from one end and gated into the wheel axle. No feeders were used in any other area in order to maintain the finish’s cosmetic quality.





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