Differential Thermal Analysis Assessment of Beta Phase Precipitation in Al-6.5Si-1Fe Alloy

Abstract

Iron-bearing intermetallic phases formed during solidification of Al-Si casting alloys are known for having detrimental effect on their mechanical properties. This is particularly the case of the b-Al 5 FeSi phase which precipitates as thin and extended plates. Many researchers already studied the factors that could influence the formation of this phase and in most cases it has been concluded that low-level additives (e.g. manganese) may lead to the replacement of the beta phase with other intermetallics that are less harmful because of being more compact.In this preliminary work, differential thermal analysis (DTA) was used to study the effect of cooling rate (0.2-40°C/min) on beta phase formation in Al-6.5Si-1Fe alloy. The effect of cooling rate on the characteristic temperature for phase changes was described and compared to literature information. The beta phase was then characterized using scanning electron microscopy and X-ray tomography. Metallographic analysis showed the phase morphology was heavily affected by the cooling rate, and parameters to quantify this have been selected and measured. Moreover, observation also revealed that some alpha phase precipitates at low cooling rate.

Effect of stirring induced by rotating magnetic field onβ-Al5FeSi intermetallic phases during directional solidification in AlSi alloys

“…Considering the above mentioned compositions following from segregation (in centre and in the outer ring) and analysing the solidification path on the basis of ternary Al-Fe-Si phase diagram, 36 we can distinguish predominant group of alloys and areas where first primary a-Al forms, then follows b-Al 5 FeSi phases and finally eutectics. For example, for AlSi5Fe1, 36 a-Al forms first at the 628uC (LRa-AlzL), then follows to concentration 7%Si and 1?5%Fe and reaches eutectic reaction at 615uC.…”

“…In addition to the predominant group, the second group are the samples of AlSi5/7/9Fe1 alloys solidifying in the specimen centre, where the compositions AlSi8?5Fe1?8 (for AlSi5Fe1 alloy) and AlSi11Fe1?8 (for AlSi7/9Fe1 alloys) have been reached (three alloys). Based on the ternary Al-Fe-Si phase diagram, 36 in the second mentioned group, as first solidifying phase is b-Al 5 FeSi. For example, for AlSi7/9Fe1 alloys, first b at 628uC starts with reaction LRb-Al 5 FeSizL, until it reaches eutectic groove LRa-Alzb-Al 5 FeSizL, and finally eutectic reaction LRa-Alzb-Al 5 FeSizSi occurs.…”

“…This gives an almost constant length of b on the specimen cross-section in the case of fluid flow. Segregation causing changes in local composition, modifying the solidification path (according to phase diagram 36 ) and increasing Si and Fe content, for chosen three alloys, leads to composition at the centre favouring growth of b needles as first phases instead of a-Al dendrites. This could be an additional factor leading to higher density of b at the centre for solidification with stirring.…”

Chinese Script vs Plate-Like Precipitation of Beta-Al9Fe2Si2 Phase in an Al-6.5Si-1Fe Alloy

“…This agrees with the solidification path expected for the investigated alloy, namely primary deposition of (Al), followed by eutectic growth of (Al) and beta, and solidification completion at the invariant ternary (Al)-beta-Si eutectic. [19] but overall was less than 100 lm in length in 2D section, and in fact quite similar to the length of the silicon lamellae. The appearance of script precipitates is dispatched in interdendritic regions and can be observed in the center as well as outer part of the sample.…”

“…Previous studies [19,20] on the effect of cooling rate on the morphology of beta phase in a high-purity Al-6.5Si-1Fe alloy showed Chinese script precipitates appearing in samples cooled at 0.2 K/min. Similar Chinese script precipitates were also reported by Mikolajczak and Ratke [21] on an Al-7Si-1Fe alloy.…”