Horizontally Solidified Al-3 wt%Cu-(0.5 wt%Mg) Alloys: Tailoring Thermal Parameters, Microstructure, Microhardness, and Corrosion Behavior

doi:10.1007/s40195-018-0852-z

Abstract

Abstract:

In the present experimental investigation, Al-3 wt%Cu and Al-3 wt%Cu-0.5 wt%Mg alloys castings are produced by a horizontal solidification technique with a view to examining the interrelationship among growth rate (G_R), cooling rate (C_R), secondary dendrite arm spacing (λ₂), Vickers microhardness (HV), and corrosion behavior in a 0.5 M NaCl solution. The intermetallic phases of the as-solidified microstructures, that is, θ-Al₂Cu, S-Al₂CuMg, and ω-Al₇Cu₂Fe, are subjected to a comprehensive characterization by using calculations provided by computational thermodynamics software, optical microscopy, and scanning electron microscopy/energy-dispersive spectroscopy. Moreover, electrochemical impedance spectroscopy and potentiodynamic polarization tests have been applied to analyze the corrosion performance of samples of both alloys castings. Hall-Petch-type equations are proposed to represent the HV dependence on λ₂. It is shown that the addition of Mg to the Al-Cu alloy has led to a considerable increase in HV; however, the Al-Cu binary alloy is shown to have lower corrosion current density (i_corr) as well as higher polarization resistance as compared to the corresponding results of the Al-Cu-Mg ternary alloy.

Key words: Aluminum, alloys, Solidification, Dendritic, microstructure, Intermetallic, phases, Microhardness, Corrosion, resistance

Barros André, Cruz Clarissa, P. Silva Adrina, Cheung Noé, Garcia Amauri, Rocha Otávio, Moreira Antonio. Horizontally Solidified Al-3 wt%Cu-(0.5 wt%Mg) Alloys: Tailoring Thermal Parameters, Microstructure, Microhardness, and Corrosion Behavior[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(6): 695-709.

Figures/Tables 15

Table 1 Chemical compositions ( wt%) of metals used and prepared alloys

Metals	Al	Fe	Ni	Cu	Mg	Si	Cr	Zn
Al	Balance	0.175	0.0148	0.024	0.0011	0.103	-	-
Cu	-	-	-	Balance	-	0.09	0.27	-
Mg-Al	6.981	0.001	0.001	0.115	Balance	0.246	-	0.631
Al-3 wt%Cu	Balance	0.22	-	3.14	-	0.04	0.02	-
Al-3 wt%Cu-0.5 wt%Mg	Balance	0.13	-	3.69	0.42	0.05	0.007	-

Fig. 1 Representation of both positioning of thermocouples and removal of the cast ingot samples

Fig. 2 Schematic representation of λ2 measurements at longitudinal sections, in which n is the number of secondary dendritic arms

Fig. 3 Schematic diagram of the electrochemical cell applied for corrosion tests

Fig. 4 Cooling curves for a Al-3 wt%Cu and b Al-3 wt%Cu-0.5 wt%Mg alloys at different positions from the cooled surface of the ingots, c plots of position from the cooled interface of the ingots as a function of time corresponding to the passage of the liquidus front by each thermocouple, dGR and CR as a function of position from the cooled interface for the two analyzed alloys ingots. R2 is the coefficient of determination

Fig. 5 Horizontally solidified macrostructure and as-cast dendritic microstructures of longitudinal sections of: a Al-3 wt%Cu, b Al-3 wt%Cu-0.5 wt%Mg alloys ingots

Fig. 6 Variation of λ2 with: aP, bGR and CR values for both Al-3 wt%Cu and Al-3 wt%Cu-0.5 wt%Mg alloys

Fig. 7 Scheil-Gulliver non-equilibrium solidification path calculated by the Thermo-Calc software for a Al-3 wt%Cu, b Al-3 wt%Cu-0.5 wt%Mg alloys with 0.15 wt%Fe

Fig. 8 EDS element mapping and point microanalysis detailing the phases formed during the solidification process of: a Al-3 wt%Cu, b Al-3 wt%Cu-0.5 wt%Mg alloys

Fig. 9 Evolution of HV as a function of: aP, bGR, cCR, dλ2 for Al-3 wt%Cu and Al-3 wt%Cu-0.5 wt%Mg alloys. R2 is the coefficient of determination

Fig. 10 Bode and Bode-phase diagrams for: a Al-3 wt%Cu and b Al-3 wt%Cu-0.5 wt%Mg alloys. Nyquist plots for: c Al-3 wt%Cu, d Al-3 wt%Cu-0.5 wt%Mg alloys samples in a 0.5 M NaCl solution at 25 °C

Fig. 11 Experimental potentiodynamic polarization curves of investigated alloys in a 0.5 M NaCl solution at 25 °C for samples extracted from aP?=?10 mm, bP?=?50 mm. c Corrosion potential (Ecorr) and corrosion current density (icorr) calculated from potentiodynamic polarization curves

Table 2 Electrochemical parameters of the examined alloys samples in a 0.5 M NaCl solution

Alloy	Sample, P (mm)	E_corr (mV)	I_corr (μA/cm²)	R_pol (Ω cm²)	θ_max (deg.)
Al-3 wt%Cu	10	-?726	0.341	13,139	80.73
Al-3 wt%Cu	50	-?697	0.103	16,494	80.33
Al-3 wt%Cu-0.5 wt%Mg	10	-?622	1.590	9147	78.38
Al-3 wt%Cu-0.5 wt%Mg	50	-?630	1.193	12,612	77.34

Fig. 12 Optical micrographs of a Al-3 wt%Cu, b Al-3 wt%Cu-0.5 wt%Mg alloys samples after the corrosion test in 0.5 M NaCl solution at room temperature

Fig. 13 Typical SEM micrographs with EDS elemental mapping of a Al-3 wt%Cu, b Al-3 wt%Cu-0.5 wt%Mg alloys after the corrosion test in 0.5 M NaCl solution at room temperature

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