This work examined effects of hard surface polishing grits and activation on electroless-nickel (EN) plating on cast aluminium alloy substrates in sodium hypophosphite baths. As-received aluminium alloy sample sourced from automobile hydraulic brake master cylinder piston was melted in electric furnace and sand cast into rod. The cast samples were polished using different grits (60 μm–1200 μm) before plating. The effects on adhesion, appearance, and quantity of EN deposits on substrates were studied. Observation shows that the quantity of EN deposit is partly dependent on the alloy type and roughness of the surface of the substrates, whereas the adhesion and brightness are not solely controlled by the degree of surface polishing. The best yield in terms of adhesion and appearance was obtained from the activation in zincate and palladium chloride solutions. Higher plating rates (g/mm2/min) of3.01E-05,2.41E-05, and2.90E-05were obtained from chromate, zincate, and chloride than8.49E-06,8.86E-06, and1.69E-05as obtained from HCl etched, NaOH, and H2O activated surfaces, respectively.
Effects of moulding sand permeabilities prepared from the combinations of four proportions of coarse and fine particle size mixtures and pouring temperatures varied from 700, 750, and 800 (±10 ∘ C) were studied on the hardness, porosity, strength, and microstructure of cast aluminium pistons used in hydraulic brake master cylinder. Three sand moulds were prepared from each of the 80 : 20, 60 : 40, 40 : 60, and 20 : 80 ratios. The surfaces and microstructures of cast samples were examined using high resolution microscopic camera, metallurgical microscope with digital camera, and scanning electron microscope with EDX facilities. The best of the metallurgical properties were obtained from the combination of 80 : 20 coarse-fine sand ratio and 750 ± 10 ∘ C pouring temperature using as MgFeSi inoculant. An 8 : 25 ratio of coarse to fine grained eutectic aluminium alloy was obtained with enhanced metallographic properties. The cast alloy poured at 750 ± 1 ∘ C has a large number of fine grain formations assuming broom-resembling structures as shown in the 100 m size SEM image.
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