Effect of Nb addition on the size and morphology of the \u03b2-Fe precipitates in recycled Al-Si alloys

Carlos Narducci,André Silva Antunes,Gabriela Lujan Brollo,Antonio Jorge Abdalla,Rafael Humberto Mota De Siqueira

doi:10.1038/s41598-021-89050-5

Abstract

The recycled Al alloys have a Fe level above the recommended limits, leading to the precipitation of β-Fe intermetallic particles in their microstructure. The brittle β-Fe particles show a rough morphology in the form of highly-faceted platelets, which is detrimental to the alloy's mechanical performance containing these precipitates. This work analyses the possible interactions of the addition of 0–1 wt% of the grain refiner Nb + B to the morphology and size of β-Fe precipitates in alloys melted with Al-(7, 9, 12) wt% Si and 1 wt% Fe. The results indicate that the addition of the Nb + B inoculant revealed a significant change in the precipitates' size and morphology, which have become remarkably refined and spheroidized. Moreover, deepening the study through qualitative and quantitative analyses, it was found that the behaviour of the β-Fe precipitates follows an exponential decay with the increasing addition of Nb + B, a curve analogous to the primary α-Al grain refinement one, revealing a direct correlation between the events. Finally, it was possible to suggest a mechanism that shows how the phenomenon of morphological transformation of the β-Fe precipitates occurs in the material with the addition of the Nb + B inoculant.

Highlights

The recycled Al alloys have a Fe level above the recommended limits, leading to the precipitation of β-Fe intermetallic particles in their microstructure
From the classical theory of heterogeneous nucleation used in the phenomenon of grain refinement in Al-Si alloys, the possibility that one of the effects of this transformation could be the most homogeneous distribution of solute elements arise, in the case Fe, in the solidification fronts during the eutectic phase of the material transformation, avoiding the interconnectivity of the Fe elements dispersed in the liquid metal in the shape of a needle or thick platelet
Clusters of AlB2 and Al3Nb substrates are revealed as crystals that grow along a non-specific direction, without a particular orientation relationship, initiating several nucleation points simultaneously, limiting the size of the α-Al grain dendrites. This phenomenon creates a mechanism that modifying the morphology of 2nd phase precipitates[12], such as the β-Fe one

Summary

Introduction

The recycled Al alloys have a Fe level above the recommended limits, leading to the precipitation of β-Fe intermetallic particles in their microstructure. IV fragmentation theory as the main mechanism for refining the primary β phase, based on experimental evidence[5] In this theory, morphological change of β-phase can be induced by suitable heat-treatment, it is only recommended in the low-Si high-Fe content recyclable Al-Si alloy, in addition to increasing the cost of the final product. Basak proposed another approach, the gravitational segregation, but this is not a good productivity process. The study lacks depth or novelty to characterize the real change in intermetallic precipitates’ morphology (β-Fe)

Methods

Results

Conclusion