Effects of strontium-modification, iron-based intermetallics and aging conditions on the impact toughness of Al-(6--11)%Si alloys

The main intention of the current study is to investigate the effects of Sr-modification, Fe-based intermetallic phases and aging conditions on the impact toughness of commercially used 356 and 319 alloys. A Full understanding of such parameters with respect to these alloys would also serve as a reference base for characterizing the impact properties of the newly developed 396 Al-Si alloy, currently being studied as a part of an ongoing research program on alloy development. The results obtained from the current study were analyzed in term of the effects of this range of parameters on the total absorbed energy (ET), crack initiation energy (Ej) and crack propagation energy (Ep). The fracture occurring in non-modified and Sr-modified 356, 319, and 396 alloys containing different levels of additions was also investigated. The total absorbed energy was measured using a computer-aided instrumented Instron Charpy impact testing machine, model SI-1D3 from SATEC Systems Inc. This testing was conducted on unnotched samples in order to emphasize the role of metallurgical parameters on both crack initiation and crack propagation processes. The dimensions of the sample according to ASTM-E23 standard are 10 x 10 x 55 mm. The results of microstructure examination reveal that the as-cast non-modified 356 alloys display large acicular eutectic Si particles. The pi-iron phase was found to precipitate either in close association with the beta-iron phase platelets or else in the form of independent script-like particles in the ascast 356 alloys. Both beta-iron needles and pi-iron script-like particles seem to be segregated away from the modified eutectic Si colonies in the Sr-modified 356 alloys. Solution heat treatment results in dissolving the small-sized particles of the pi-phase completely in the matrix, particularly in the 356 alloys containing low levels of iron. The large-size particles of pi-phase appear to partially dissolve into the matrix after solution treatment due to insufficient solution time to produce complete dissolution. It will also be observed that there are a number of thin beta-needles undergo necking and ultimately leads to fragmentation into small needles. The results obtained from the Charpy impact test revealed that increasing the level of iron additions diminishes the impact energy values of both 356 and 319 alloys to a noticeable degree.;Introducing 200 ppm Sr to both as-cast 356 and 319 alloys was found to slightly improve the impact energy values. The application of solution heat treatment in conjunction with Sr-modification significantly was found to improve the overall impact energy of both as-cast 356 and 319 alloys, particularly at low levels of additions. The Sr-modified alloys show higher impact energy values compared to non-modified alloys in the same conditions, regardless of the level of additions. Artificial aging of non-modified and Sr-modified 356 alloys at a peak-aging temperature of 180°C diminishes the impact energy values with an increase in the aging times up to 8 hrs compared to those obtained under the solution heat-treated alloys. On the other hand, aging at an over-aging temperature of 220°C gradually increases the impact energy values with further aging time up to 12 hrs. A significant improvement in impact values of about 20 J and 18 J was observed for both Sr-modified 356 alloy containing 0.15 wt% Fe and combined addition of 0.22 wt% Fe and 0.14 wr'1o Mn, respectively. The impact behavior of iron-containing 319 alloys exhibit the similar behavior at both 180°C and 220°C aging temperatures, however, the impact values obtained after aging cycle of 220°C for various aging times show slightly lower impact values than those obtained after 180°C aging cycle. The Sr-modified and non-modified 319 alloys containing 0.18 wt% Fe aged at 180°C for 12 hrs display the highest impact energy values of ∼ 12 J among all the 319 alloys involved. A similar variation in the impact energy values of 319 alloys containing combined additions of Fe-Mn and Fe-Mg is observed after aging at both 180°C and 220°C temperatures for different aging time. A slight increase in the impact values of both non-modified and Sr-modified 319 alloys may be observed after aging at 180°C either for 2 hrs or 12 hrs compared to the impact values obtained for as-cast alloys. The Sr-modified 319 alloys containing 0.3 wt% Fe-0.09 wt% Mn-0.09 wr'1o Mg was observed to exhibit higher levels of impact values for all the aging time at 220°C than those obtained for the same alloys under solution heat-treated conditions. The impact energy values of newly-developed 396 alloys exhibit a similar improvement to that observed in hypoeutectic 356 and 319 alloys with the application of Sr-modification and solution heat treatment. The 396 alloys, however, display lower impact values than those obtained for 356and 319 alloys. The near-eutectic Si content of ∼11 wt% together with the high volume fraction of intermetallics including Cu- and Fe-containing phases provide a wide number of crack initiation sites and thus reduces the impact energy values of these 396 alloys. (Abstract shortened by UMI.)...