Effects Of Rare Earth Elements Er And Er-Sc On Microstructures,properties And Aging Precipitation Of Al-Si-Mg Alloys

Rare earth elements have been sufficiently studied on the modification behavior and grain refinement mechanism of eutectic Si in cast Al-Si-Mg alloys.But there are few reports on the effects of synergistic Er-Sc addition and aging characteristics of rare earth modified casting Al-Si-Mg alloy.In our studies,the most widely used Al-7Si-0.3Mg(A356)alloy in 4000 series Al-Si alloys is selected as the research object.Polarizing microscope,hardness test,tensile test,scanning electron microscope(SEM),transmission electron microscope(TEM),high-resolution transmission electron microscopy(HRTEM)and other testing techniques have been used to study the effects of single addition of Er and synergistic addition of Er-Sc on the microstructure,aging precipitation behavior and mechanical properties of Al-7Si-0.3Mg alloys under different heat treatment states.According to the experimental results,the following conclusions can be drawn:(1)The effects of rare earth element additions on the as-cast microstructure and mechanical properties of A356 alloy were obviously different.Single addition of Er can refine the eutectic Si from coarse plate to coral shape and improve the elongation,but Er-Sc synergistic addition can best modify the eutectic Si from plate-like to fiber and make the grain size smaller.Therefore,the mechanical properties of A356-Er-Sc are the best.(2)After the single Er addition to A356 alloy,due to the increase of intermetallic phases,severe galvanic corrosion is caused and the passivation area in the polarization curve disappears.Therefore,the corrosion resistance of A356-Er alloy is reduced.However,the passivation area of A356-Er-Sc expands and the self-corrosion potential shifts positively compared with A356-Er alloy,so its corrosion resistance is better than that of A356-Er alloy.(3)A356,A356-0.2Er,and A356-0.2Er-0.3Sc alloy have different aging precipitation behaviors during artificial aging at 180?.The main strengthening phase of these three alloys is ? ".The addition of rare earth elements will not change the aging precipitation sequence of A356 alloy,but it can effectively promote the precipitation of ?",making the size distribution of ? " more localized and the average length of ?" smaller.Therefore,the thermal stability of the A356 alloy containing rare earth elements is improved,and other mechanical properties such as peak hardness are also improved.(4)During artificial aging at 250?,the time to reach the peak hardness of A356,A356-0.2Er and A356-0.2Er-0.3Sc alloys becomes shorter,and the aging precipitation sequence of the alloy is different from 180?.During over-aged state,there are a large number of release strengthening precipitations,such as the U1/U2/B'/?' phases,so the thermal stability of A356-Er and A356-Er-Sc alloy is reduced.However,the addition of rare earth elements still promotes the finer and more uniform precipitation of the ?"phase.In addition,the synergistic addition of Er-Sc inhibits the precipitation process of the release strengthening phase to a certain extent.(5)Natural aging has a negative effect on the artificial aging of A356-Er and A356-Er-Sc alloys at 180?.With the extension of natural aging,the peak hardness and thermal stability of the alloy decrease.A356,A356-0.2Er,and A356-0.2Er-0.3Sc age hardening curves tend to be consistent with the extension of natural aging time.It was found by TEM that after applying natural aging prior to artificial aging,A356,A356-0.2Er,and A356-0.2Er-0.3Sc alloys have the similar average length and distribution of ?" precipitates.