Damping properties of A357 alloys and A357-stainless steel composites fabricated under different conditions
Mazlee, Mohd Noor
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The lab scale gravity casting technique was used to produce alloy specimens of nonsuperheated A357 alloy, superheated A357 alloy and composite specimens of superheated A357-0.5 and 1.0 wt.% stainless steel composites. The primary cast ingot A357 alloy was melted in graphite crucible before pouring into the stainless steel mould at 700ºC for all specimens. Meanwhile, prior to pouring, the preform of 304 stainless steel wires were aligned in stainless steel mould to produce superheated A357-0.5 and 1.0 wt.% stainless steel composites respectively. The main objective of this research is to study the effect of superheating on the microstructures and dynamic mechanical properties, to identify the phases that presents in all the specimens and also to identify the appropriate damping mechanisms in all specimens at lower and elevated temperatures. The superheating had refined the eutectic Si particles and changed the shapes of α-Al dendrites in superheated A357 alloys. The superheating also changed the shapes of π-Al8FeMg3Si6 intermetallic phase and Mg2Si phase in superheated A357 alloys. Superheated A357-0.5 wt.% stainless steel composite showed poor bonding and less intensified of coarser eutectic Si particles around the matrix-reinforcement interface. Superheated A357-1.0 wt.% stainless steel composite showed good bonding and more intensified of finer eutectic Si particles around the matrix-reinforcement interface. However, no interface reaction layer was observed at the matrix-reinforcement interface of the composite specimens. Dynamic mechanical properties such as storage modulus, loss modulus and damping capacity were investigated by dynamic mechanical analyzer (DMA). Superheated A357-1.0 wt.% stainless steel composite showed the highest storage modulus of 66.30 GPa at 50ºC. Superheated A357-0.5 wt.% stainless steel composite showed the highest loss modulus of 4.10 GPa at 380ºC. Superheated A357 alloys showed the highest damping capacity of 0.0842 GPa at 380ºC. Dislocation damping was the mechanism at lower temperatures range (50 to 280ºC) for the alloys and the composites. Meanwhile, grain boundary damping and interfacial damping were the mechanisms at elevated temperatures range (281 to 380ºC) for the composite and only grain boundary damping was the mechanism at elevated temperature range for the alloys. Differential scanning calorimetry (DSC) study at lower temperatures range (100 to 450ºC) of solution treated alloys and composite specimens showed the presence of two exothermic reactions (precipitation of θ″ and θ′) and an endothermic reaction (dissolution of θ″) while at elevated temperatures range (530 to 630ºC) of solution treated alloys and composite specimens showed three endothermic reactions (Al dendrites, Al + Si and Al + Si + Mg2Si + π-Al8FeMg3Si6).