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A Study on Peripheral Grain Structure Evolution of an AA7050 Aluminum Alloy with a Laboratory-Scale Extrusion Setup

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Abstract

A laboratory-scale hot extrusion setup was designed to investigate recrystallization and grain growth behavior of an AA7050 alloy during extrusion and subsequent heat treatments. Compared with industrial extrusion, the laboratory-scale process enabled rapid water quenching of extrudate with less delay so that the dynamic grain structure development was captured. After extrusion, static microstructure evolution in the extrudates was studied using salt bath annealing for 5 and 15 s at 490 °C and solutionization treatment for 1 h at 490 °C. The salt bath annealing was a simulation of the delay of press quenching in typical industrial extrusion practices. In the as-quenched extrudates, the peripheral region mainly exhibited continuous dynamic recrystallization and geometric dynamic recrystallization, whereas in the core region discontinuous dynamic recrystallization dominated. A <100> and <111> double fiber texture was identified in extrudates, and recrystallization behavior was found to be orientation dependent. The <100> oriented grains contained more sub-grain boundaries and better-defined sub-grains and had a higher tendency to fragment via continuous recrystallization, while the <111> oriented grains produced less sub-grain boundaries and did not recrystallize. Subsequent heat treatments resulted in static recrystallization and abnormal growth of the continuously recrystallized grains. Additionally, the effects of extrusion temperature (440, 480 and 520 °C) and punch speed (0.7, 1.4 and 2.1 mm/s) on grain structure were discussed. A revised grain structure evolution mechanism based on the observation of 7050 extrusion was proposed.

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Acknowledgment

The authors gratefully acknowledge financial support on this research from Shandong Nanshan Aluminum Co. and Beijing Nanshan Institute of Aeronautical Materials. Materials for the project donated by Arconic Lafayette Operations are also acknowledged.

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  1. Yiwei Sun

    Present address: Department of Mechanical Engineering, Bourns Hall, University of California Riverside, Riverside, CA, 92521-0419, USA

Authors and Affiliations

  1. School of Materials Engineering, Purdue University, 701 W Stadium Ave, West Lafayette, IN, 47907-2045, USA

    Yiwei Sun, Xiaolong Bai, Daniel Klenosky, Kevin Trumble & David Johnson

  2. Purdue Center for Metal Casting Research, Purdue University, 701 W Stadium Ave, West Lafayette, IN, 47907-2045, USA

    Yiwei Sun, Xiaolong Bai, Daniel Klenosky, Kevin Trumble & David Johnson

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  1. Yiwei Sun
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Correspondence to Yiwei Sun.

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Appendices

Appendix 1

Shown in Fig. 8 is the optical micrograph of the water-quenched billet discard from the 480 °C, 1.4 mm/s extrusion run. A typical direct extrusion flow pattern and deformation zones can be seen, including the inflow zone (IFZ), shear intensive zone (SIZ) and dead metal zone (DMZ).

Fig. 8
figure 8

Water-quenched extrusion billet discard showing the flow pattern and deformation zones of inflow zone (IFZ), shear intensive zone (SIZ) and dead metal zone (DMZ)

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Appendix 2

The preheating and extrusion finishing temperatures for all the extrusion runs conducted are listed in Table 2 as measured by the thermocouple in the extrusion container. The location of the thermocouple is indicated in Fig. 1(a). The temperature changes shown in Table 2 are much smaller than those in typical industrial extrusion (Ref 25). For the laboratory-scale setup, when extrusion started, the cold push rod came into contact with the punch, extracting heat from the hot parts (Fig. 1(a)). Also, the extrudate directly went into the water bath below the die exit during extrusion. Therefore, more heat loss was expected for the small-scale setup compared with industrial extrusion, and less heat was generated because of the smaller dimensions of the billet.

Table 2 Preheating temperature, punch speed and extrusion finishing temperature
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Sun, Y., Bai, X., Klenosky, D. et al. A Study on Peripheral Grain Structure Evolution of an AA7050 Aluminum Alloy with a Laboratory-Scale Extrusion Setup. J. of Materi Eng and Perform 28, 5156–5164 (2019). https://doi.org/10.1007/s11665-019-04208-7

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  • DOI: https://doi.org/10.1007/s11665-019-04208-7

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