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High-temperature tensile properties of unmodified A356 alloy with and without the addition of Ni or V in traces (600 and 1000 ppm of Ni and V, respectively) were investigated by analyzing samples obtained from sand and permanent mold castings in the as-cast and T6 heat-treated conditions. Tensile tests were performed at 508 K (235°C) at a crosshead speed of 1 mm/min. In addition, samples were subjected to artificial aging at 508 K (235°C) for different times, and corresponding hardness curves were plotted. Microstructures and fracture surfaces, analyzed by FEG-SEM equipped with energy dispersive X-ray spectroscopy, showed that neither Ni nor V addition had a detrimental effect on high-temperature tensile properties. Aging curves showed a strong loss of hardness affecting the T6 class between 30-min and 1-h exposure time. After 6-h aging, no evidence of aging treatment persisted on hardness of the tested material. Hardness values did not reveal any significant difference between the reference alloy and the Ni-and V-containing alloys in both casting conditions, in complete analogy with the tensile properties. Unmodified eutectic silicon particles provided inhomogeneity in the a-Al matrix and acted as the principal source of stress concentration leading to fracture.

In this study, the effect of aging on the mechanical properties of unmodified A356 aluminium casting alloy with trace additions of Ni or V was investigated. Trace elements were added in concentrations of 600 and 1000 ppm of Ni and V, respectively. Samples from sand and permanent mould castings in as cast and T6 heat-treated conditions were tested. Tensile tests were performed at both room and high temperature (235 °C). Taking into account the results from both testing conditions, Vickers hardness was measured in order to endorse the hypothesis of artificial aging occurring during high temperature tensile tests. In order to study this effect, a series of specimens was aged at 235 °C for different aging times, and aging curves were plotted. The occurrence of static and dynamic aging was evaluated by comparing hardness values of tensile specimens and aged samples, particularly in the range of 5-20 min, as this range corresponds to the time necessary for pre-heating and testing of the tensile samples. A basic correlation between tensile strength and hardness is also given.