No abstract

No abstract

This work explored and contrasted the effect of microstructure on the tensile properties of AlSi10Mg alloys generated by transient directional solidification depending on variations in cooling rate and magnesium (Mg) content (i.e., 0.45 and 1 wt.% Mg), with a focus on understanding the dendritic growth and phases constitution. Optical and scanning electron (SEM) microscopies, CALPHAD, and thermal analysis were used to describe the microstructure, forming phases, and resulting tensile properties. The findings showed that the experimental evolution of the primary dendritic spacing is very similar when both directionally solidified (DS) Al-10 wt.% Si-0.45 wt.% Mg and Al-10 wt.% Si-1 wt.% Mg alloys samples are compared. The secondary dendritic spacing was lower for the alloy with more Mg, especially considering the range of high growth velocities. Moreover, a greater fraction of (Al + Si + Mg2Si) ternary eutectic islands surrounding the α-Al dendritic matrix was noted for the alloy with 1 wt.% Mg. As a result of primary dendritic spacings greater than 180 μm related to lower cooling rates, slightly higher tensile properties were attained for the Al-10 wt.% Si-0.45 wt.% Mg alloy. In contrast, combining dendritic refining (<150 μm) and a larger Mg2Si fraction, fast-solidified DS Al-10 wt.% Si-1 wt.% Mg samples exhibited higher tensile strength and elongation. The control of cooling rate and fineness of the dendritic array provided a new insight related to the addition of Mg in slightly higher levels than conventional ones, capable of achieving a better balance of tensile properties in AlSi10Mg alloys.

Broad ranges of cooling rates () 0.8–30.5 and 0.4–5.0 K s−1 are attained during directional solidification of eutectic Sn–0.2 wt% Ni and hypereutectic Sn–0.5 wt% Ni alloys, respectively. A reverse high cooling rate cell‐to‐dendrite transition occurs for the eutectic composition and a transition from high cooling rate cells to plate like cells for the hypereutectic alloy. High cooling rate β‐Sn cells are associated with cooling rates >5.5 and >2.7 K s−1 for eutectic and hypereutectic compositions, respectively. A processing diagram, relating the ‘–Ni content’ space with the microstructural morphology, is proposed. A combination of plate like cells and plate NiSn4 eutectic phase results in higher ductility.

Studies of Fe‐contaminated Al‐based scraps have drawn attention for years. However, little has been researched about the effects of cobalt (Co) and nickel (Ni) additions to contaminated scraps both under slow solidification (directional solidification, DS) and fast solidification regime (copper mold centrifugal casting, CC). As such, the current study examines a representative low‐alloy Al–Si chemistry from industrial scraps, i.e., the Al–7%Si–0.6%Fe–0.35%Cu–0.25%Zn (% by weight). This alloy is changed independently using Ni and Co. It is observed that a mixture of uneven distributed α and β Fe‐bearing particles prevails for all tested alloys and solidification conditions. The coexistence of both phases agrees with the charts proposed by Gorny's work for the range of secondary dendritic spacing, λ 2, observed. Although the Fe‐containing particles are smaller for fast‐solidified samples, they still maintain the acute morphology and, in some cases, the Chinese script shape. This results in a reduction in ductility of the order of 65% for the nonmodified and Ni‐modified samples considering mean λ 2 varying from 20 μm (DS) to 6.5 μm (CC), despite improvements in mechanical strength. Furthermore, addition of Co results in a higher number of Fe‐based particles within the microstructure. In this case, the largest particle/α‐Al interfacial area results in slightly poor ductility (~6–8%) and higher strength (175–205 MPa), with less ductility loss when DS and CC samples are compared.

Al-based monotectic alloys show an interesting combination of wear resistance and mechanical strength. While self-lubricating bismuth (Bi) guarantees an adequate wear resistance, the modification with copper (Cu) can increase the ability to support load. Tailoring the micromorphology of the α-Al phase as well as the distribution of the inclusions of Bi embedded in the α-Al matrix is of prime importance in order to improve the properties. In this paper, various solidification experiments are performed with the Al-3.2wt.%Bi-3wt.%Cu monotectic alloy. Real-time radiographs could be acquired during solidification by using the laboratory device SFINX (Solidification Furnace with IN situ X-radiography). The processing conditions could be chosen in such a way that both cellular and dendritic morphologies of the α-Al phase could be clearly seen in a single test. Micromorphological transitions from dendrites to cells are recognized for samples solidified from different cooling rates, i.e. 0.025 K/s and 0.01 K/s. Measurements of the α-Al dendrite velocity and growth parameters of the Bi droplets during solidification have also been extensively performed. A comprehensive model for the cellular-to-dendritic (CDT) transition of Al-based monotectic alloys is outlined. The model considers the interaction between α-Al and Bi droplets during solidification and its impact on the microstructure.

The corrosion behavior of Fe-containing directionally solidified (DS) and centrifugally cast (CC) Al-Si-Cu-Zn alloys with either Co or Ni additions has been investigated. Electrochemical and immersion corrosion methods were used to investigate the corrosion behavior in 0.6 M NaCl after short (1-h) and long (30-day) exposure periods. The employed solidification methods allowed the production of samples with a wide range of secondary dendrite arm spacing (SDAS) while preserving Si and Fe-containing phases. The 0.5 wt.% Ni and Co additions led to the growth of the AlFeSi(Ni) and AlFeSi(Co) phases, but no binary AlNi nor AlCo intermetallic particles have been generated. Potentiodynamic polarization studies at early exposure revealed an increase in the corrosion potential as the Ni was added for either fast or slow solidified samples. The electrochemical impedance spectroscopy at early exposure demonstrated that the Ni-modified alloy, on the other hand, was associated with smaller charge transfer resistances, indicating a reduction in the corrosion resistance after a short elapsed time into the electrolyte. However, the 30-day immersion tests revealed much lower corrosion rate of the Ni-modified alloy than the other alloys, while the corrosion rates of the Co-modified and non-modified alloys were similar. In the Ni-containing alloy, a decreased corrosion rate under a long-term corrosion process was attributed to the formation of a thick and dense alumina layer, effectively protecting the surface under such conditions. This work contributes to better knowledge of the corrosion behavior of Ni- and Co-corrected Al industrial scrap compositions.

No abstract