The purpose of the present study was to improve the
impact toughness, tensile properties and hardness of Al–7Si
and Al–7Si–2.5Cu cast alloys using grain refiner and/or
modifier and combined addition of both, and to discuss the
mechanism of the improvements.
2. Experimental Details
Al–7Si alloy was prepared by melting commercially pure
aluminum (99.7%) along with Al–20%Si master alloy in clay
graphite crucible in a pit type resistance furnace under a cover
flux (45% NaCl+45% KCl+10% NaF) and the melt was held at
720 °C. After degassing with 1% solid hexachloroethane,
master alloy chips duly packed in aluminum foil were added
to the melt for grain refinement. For modification Al–10%Sr
master alloy was used with addition level being kept constant
at 0.02 wt.% Sr [3]. The melt was stirred for 30 s with zirconia
coated iron rod after the addition of grain refiner and/or
modifier. Melts were held for 5 min and poured into a
cylindrical graphite mould (25 mm diameter and 150 mm
height) surrounded by fireclay brick. The details of the alloys
and details of grain refinement and modification treatment
are given in Table 1. The chemical compositions of the cast
alloys and master alloys (Table 2) were assessed using atomic
emission spectroscopy. The microstructures of the samples
that had been cut from the longitudinal section of the billets
were studied. Grain size analysis was carried out by the linear
intercept method after etching the polished surface with
Keller's reagent (2.5% HNO3, 1.5% HCl, 1% HF and 95% H2O).
Samples for optical microscopy have been electropolished
using an electrolytic bath comprising of 80% methanol and
20% HNO3 by volume. Selected grain refined and modified
samples were characterized using SEM and XRD.