Grain Size Control Method for Al-Cu-Mg-Ag Alloy

A recent article published in the Journal of Materials Research and Technology proposed a grain size control method to improve the durability of Al-Cu-Mg-Ag alloy at high temperatures. A relationship between grain size, holding time, and annealing temperature was established through theoretical analysis.

Grain Size Control Method for Al-Cu-Mg-Ag Alloy

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Background

Increasing the density of grain boundaries can enhance the room-temperature strength of polycrystalline alloys. However, at high temperatures, the diffusion of grain boundaries decreases their resistance to dislocations, reducing the alloy's service life.

Larger grain sizes (reducing grain boundary density) achieved through heat treatment are expected to enhance the high-temperature durability of Al-Cu-Mg-Ag alloys. However, conventional trial-and-error methods for identifying appropriate annealing conditions are labor-intensive, time-consuming, and expensive.

This study proposed a novel method to rapidly determine annealing conditions for appropriate grain sizes in Al-Cu-Mg-Ag alloys. Recrystallization kinetics parameters were derived from temperature characteristic points on the recrystallization curves.

Methods

Using a semi-continuous casting method, pure Al, Mg, and Mn metals (purities >99.9 %) and Al-50Cu, Al-10Ag, and Al-10Zr alloys were used to prepare the alloy with a chemical composition of Al-4.45Cu-0.37Mg-0.41Ag-0.25Mn-0.09Zr (wt.%).

Rectangular samples (70×10×1 mm³) were cut for recrystallization curve testing, and block-shaped samples (15×10×1 mm³) for microstructure observation using a field emission scanning electron microscope (FESEM).

Dog-bone-shaped samples with a gauge distance of 15 mm and a cross-section of 20×1 mm² were used for high-temperature durability tests performed on a micro-controlled electronic creep testing machine.

Tensile tests were conducted using dog-bone-shaped specimens with a gauge distance of 35 mm and a cross-section of 2.9×1 mm².

The recrystallization curve was established using a self-developed three-point bending stress relaxation experimental setup. Continuous temperature-load data were recorded by exposing three-point bending specimens to U-shaped support with a quartz pressure head. Consequently, two recrystallization curves were obtained at different heating rates to determine the recrystallization starting and finishing temperatures.

The appropriate annealing temperature range and temperature at which the alloy achieves maximum grain size were rapidly calculated based on the heating rate and holding time conditions. Solution-aging (T6) treatments were then performed to obtain high-temperature durable alloys with suitable types, morphologies, and distributions of precipitates.

Results and Discussion

Using the annealing parameters from theoretical analysis, multiple samples were tested for enhanced durability. With increasing annealing temperature, the grain size of both the annealed and annealed+T6 samples initially increased, then decreased, and subsequently increased again, with peaks/valleys around 325 °C and 425 °C. This variation was due to changes in the alloy’s recrystallization nucleation rate and grain boundary mobility.

An appropriate annealing temperature of 325 °C was determined for the 75 % cold-deformed Al-Cu-Mg-Ag alloy, with an annealing rate of 10°C/min and a holding time of six hours. Annealing at 325 °C for six hours, followed by T6 treatment, increased the alloy's grain size by over 60 %.

At 210 °C and 190 MPa, the duration of the 325 °C annealed+T6 sample was 61.8 % longer than the T6 sample, with only a 2.5 % decrease in room-temperature strength.

At high temperatures, grain boundaries become weak points in a material. Therefore, enhancing the high-temperature strength of materials can be effectively achieved by reducing grain boundary density through grain coarsening, mitigating the weakening effect of grain boundaries.

During high-temperature deformation processes, grain boundaries were more prone to fracture than the interior of grains. Thus, reducing grain boundary density (achieving larger grain sizes) through appropriate annealing followed by T6 treatment resulted in an enhanced service life of the alloy at elevated temperatures.

Conclusion

This study demonstrated a grain size-controlling method based on isothermal/non-isothermal recrystallization kinetics to enhance the high-temperature durability of Al-Cu-Mg-Ag alloy—the proposed method required only two to three samples to determine suitable annealing conditions.

Recrystallization kinetics parameters were obtained through three-point bending thermal relaxation experiments under different heating rates, determining the annealing temperature for maximum grain size.

Solution-aging (T6) treatment increased the alloy's grain size by over 60 % compared to direct solution-aging, enhancing durability by over 60 % at 210 °C/190 MPa without significantly reducing room-temperature strength.

Compared to traditional methods, like hardness testing and metallographic analysis, this method was more efficient, resource-saving, and easier to operate.

Journal Reference

Zhao, Z., Haochen, X., Honglei, L., Fu, H., Zhang, Z., Namin, X. (2024). Grain size control method for enhancing high-temperature durability of Al-Cu-Mg-Ag alloy. Journal of Materials Research and Technology. doi.org/10.1016/j.jmrt.2024.06.017

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Nidhi Dhull

Written by

Nidhi Dhull

Nidhi Dhull is a freelance scientific writer, editor, and reviewer with a PhD in Physics. Nidhi has an extensive research experience in material sciences. Her research has been mainly focused on biosensing applications of thin films. During her Ph.D., she developed a noninvasive immunosensor for cortisol hormone and a paper-based biosensor for E. coli bacteria. Her works have been published in reputed journals of publishers like Elsevier and Taylor & Francis. She has also made a significant contribution to some pending patents.  

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