Microstructural Characterization and Thermodynamic Simulation of Cast Al–Zn–Mg–Cu Alloys

Abstract

Wedge castings were produced using six Al–Zn–Mg–Cu aluminum alloys with zinc concentrations ranging from 8 to 15 wt%, Zn/Mg ratios ranging from 1.7 to 6.5, and copper ranging from 0.6 to 1 wt%. Thermal analysis and microstructural characterization of the samples in the as-cast, solution-treated, and aged conditions were performed and compared to Calculation of Phase Diagram simulations. The calculated phase stability in the alloys was overall remarkably accurate for both the solidification and equilibrium phase formation. Additional equilibrium calculations were made to determine the range of alloying concentrations that would not contain eutectic T or η phase. The phase stability in a total of 195 chemistries was calculated.

Technical Review and Discussion

Microstructural Characterization and Thermodynamic Simulation of Cast Al-Zn-Mg-Cu Alloys

Maria D. David, Robin D. Foley, John A. Griffin, Charles A. Monroe; The University of Alabama at Birmingham, Birmingham, AL, USA

Reviewer:

Authors can use JMatPro to calculate phase diagram for the base alloy. It would be helpful to add a phase diagram of baseline alloy or the alloy associated with cooling curves they measured in Figures 6 or 7 since the complete description of solidification sequence of the alloys they studied is not included.

Authors:

The phase diagram below is generated from JMatPro Scheil calculations. For this diagram, we used the alloying content of the Low Zn-Low Mg in the paper (see Table 2 for composition) and varied the Mg content. This diagram shows the solidification sequence for all 3 experimental low zinc alloys:

$$ \begin{aligned} & Low \, Zn \, - \, High \, Mg = 5.15wt\% \, Mg \\ & Low \, Zn \, - \, Med \, Mg = 1.91wt\% \, Mg \\ & Low \, Zn \, - \, Low \, Mg = 1.41wt\% \, Mg \\ \end{aligned} $$

For the Low Zn-Medium Mg and Low Zn-Low Mg, the eta phase forms after the alpha. The T phase forms after the alpha for the Low Zn-High Mg. It may not be too clear in the diagram since the area is too small, but for a Mg content range that has both T and eta in its solidified microstructure: between 2.18 and 2.31 wt % Mg, the eta phase forms first, while at 2.31 to 3.5wt%Mg, the T formation precedes eta.

Reviewers:

Can you verify the accuracy of the composition from the EDS scans?

Authors:

We did 4 scans for each phase (we picked relatively coarse particles to avoid inclusion of the Al-matrix in the measurements) and presented the average as the final phase composition (individual measurements were significantly comparable to each other; thus, 4 scans for each phase were considered sufficient).

Reviewers:

How was curve fitting done for Figures 9, 11, 13 and 17?

Authors:

In plots shown in Figures 9 , 11 , 13 , and 17 , simple linear regression (using Excel) with the intercept set to 0 was used to show the correlation between the calculated values and the experimental measurements. The relatively low correlation for Figure 11 (plot of T where metal is fully solid) can be attributed to the sensitivity of the predicted/calculated temperature value to the defined solidification cut-off in the thermodynamic simulations. As we have mentioned in the Results and Discussion section of the paper, this could also be due to the Scheil-Gulliver simplification that diffusion in the solid is negligible.