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摘要
This paper presents the preliminary results of an experimental investigation on the effects of three commercial titanium and boron based grain refiners on impact toughness and Brinell hardness of A356 foundry aluminium alloy. A commercial A356 aluminium alloy was used in this study. The molten alloy was modified using several bars of Al-10%Sr master alloy in order to reach 200 ppm of Sr, and then degassed for 10 min by means of argon inert gas. Then, the melt was stirred and grain refined in the holding oven by the addition of three commercial grain refiners in form of can, tab and granulated flux, which were named respectively A1, A2 and A3. After that, the alloys were poured into a steel mould, obtaining a number of 20 castings, n degrees 5 for each experimental condition. HB hardness samples and Charpy impact specimens were subsequently drawn from the castings, and T6 heat treated. %26lt;br%26gt;The impact tests were performed on a CEAST instrumented Charpy pendulum, specifically designed for testing light alloy specimens, according to the ASTM E-23 specification. During impact testing the energy absorption was evaluated through the measurement of the pendulum%26apos;s angle of rise (Cv); total impact energy was also calculated through a dedicated software, connected to a data acquisition system, as the integral of load-displacement curve (Wt), and through the load and displacement data using an Excel datasheet (Wex). HB hardness was measured (according to the UNI EN ISO 6506-1:2006 specification) on the as cast samples, after solution treatment and finally after artificial aging. %26lt;br%26gt;Metallographic samples were cut out perpendicularly to the fracture surface, then embedded in phenolic resin and finally prepared via standard grinding and polishing procedures. Microstructural analyses were performed by optical microscopy (OM), paying specific attention to fracture profiles. SDAS measurements were carried out by means of an image analysis program. The fracture surfaces of the Charpy specimens and the Fe-based intermetallic compounds were observed and analysed by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS). %26lt;br%26gt;From the results of microstructural and mechanical analyses performed on the samples, the following conclusions can be drawn: %26lt;br%26gt;1. the grain refiners addition leads to a minor increase in SDAS mean values. This increase is more evident for the alloys treated with A1 (can) and A2 (tab) grain refiners, while for A3 (granulated flux) the SDAS size is comparable to the Reference; %26lt;br%26gt;2. microstructural analyses of fracture profiles and surfaces reveal a mixed transgranular-intergranular fracture mode for both Reference sample and the refined ones. Fracture involves the alpha-Al/eutectic-Si interface and the silicon particles; also brittle Fe-based intermetallic compounds, like beta-Al5FeSi, create a preferential path for crack propagation; %26lt;br%26gt;3. the grain refiners addition causes a decrease in total absorbed energy (Wt). Probably this event is related to the SDAS increase. The A3 grain refiner is more effective than A1 and A2, maybe because of its granular shape, which leads to a more homogeneous distribution of the grain refiner into the melt during stirring; %26lt;br%26gt;4. total absorbed energy evaluated through the measurement of the pendulum%26apos;s angle of rise (Cv) is different to those calculated from the software (Wt) and the Excel datasheet (Wex). However, the deviation is small and can be taken into account using the linear regression method; %26lt;br%26gt;5. the effect of grain refiners on HB hardness is negligible until the samples are artificially aged. After aging, the refined samples show a small increase in HB hardness with respect to the Reference. The highest hardness raise is obtained with the A3 grain refiner.
- 出版日期2013-4
