Elsevier

Scripta Materialia

Volume 51, Issue 6, September 2004, Pages 509-514
Scripta Materialia

Relationship between grain size and Zener–Holloman parameter during friction stir processing in AZ31 Mg alloys

https://doi.org/10.1016/j.scriptamat.2004.05.043Get rights and content

Abstract

The relationship between the resulting grain size and the applied working strain rate and temperature for the friction stir processing in AZ31 Mg is systemically examined. The Zener–Holloman parameter is utilized in rationalizing the relationship. The grain orientation distribution is also studied using the X-ray diffraction.

Introduction

After the success and gradually wider applications of the friction stir welding (FSW) technique initially developed by The Welding Institute (TWI) in UK [1] in joining aerospace aluminum alloys, recent modifications into the friction stir processing (FSP) by Mishra et al. [2], [3] also attract attention. FSP has been demonstrated to be an effective means of refining grain size of cast or wrought aluminum based alloys via dynamic recrystallization. A fine grain size in the typical range of 0.5–5 μm in the dynamically recrystallized zone of the FSP aluminum alloys has been widely reported [4], [5], [6]. Recently, extrafine grain sizes in the range of 30–180 nm have also been demonstrated [7].

Several grain size refinement processing means have been developed for light metals. The refined alloys are proven to be effective in enhancing room temperature mechanical properties, as well as low temperature or high strain rate superplasticity (LTSP or HSRSP). From the industrial point of view, hot rolling or extrusion processes are usually considered to be more economical and feasible.

The low formability nature of the Mg based alloys at room temperature with the hexagonal close-packed (HCP) crystal structure particularly requires grain refinement. It has been shown that the magnesium alloys with fine-grained structures become much more workable at slightly elevated temperatures such as 150–250 °C [7], [8], [9], [10], [11]. Such LTSP and/or HSRSP characteristics are expected to be applied in the 3C (computer, communication and consumer electronic) or automobile industry for the forming of complex components via pressing forming or press forging techniques.

The FSP technique may be powerful in refining the grain size and homogenize the microstructure (such as precipitates and dispersoids, or even the local solute compositions) in particular positions that need special attention. These can be the material locations needed to possess the highest forming limit or the greatest bending curvature. The FSP can also be only applied to selectively desired parts of a large work piece after previous secondary processing at room or elevated temperatures. It can also be used as a repair tool for sensitive parts.

The working temperature and strain rate, combined and expressed by the Zener–Holloman parameter, Z=ε˙exp(Q/RT), where ε˙ is the strain rate, R the gas constant, T the temperature, and Q the related activation energy, has been shown to impose influence on the resulting grain size in extruded Mg based alloys [11], [12]. This parameter appears to be useful in predicting the resulting grain size and controlling the hot extrusion working parameters. In this paper, the assessment in using the Zener–Holloman parameter in FSP of the AZ31 Mg alloys is presented.

Section snippets

Experimental methods

The AZ31B billets used in this study were purchased from the CDN Company, Deltabc, Canada. The chemical composition in mass percent is Mg–3.02%Al–1.01%Zn–0.30%Mn. This alloy is a solution hardened alloy with minimum precipitation. The as-received alloy was fabricated through semi-continuous casting and has the form of extruded billet measuring 178 mm in diameter and 300 mm in length. The billet possessed nearly equiaxed grains around 75 μm (all grain size hereafter was measured based on the

Grain sizes

Microstructure characterization in this study was focused on the dynamically recrystallized nugget zone. Fig. 2 shows the typical grain structures of the billet and extruded specimens after FSP. The grain shape observed from different cross-sectional planes is consistently of the equiaxed fully recrystallized type. The grain sizes near the bottom region of the dynamically recrystallized zone are typically smaller. For example, the grain sizes in the top, middle and bottom regions of the FSP

Conclusion

The relationship between the resulting grain size and the applied working strain rate and temperature for the friction stir processing in the AZ31 Mg alloy is systemically examined. The Zener–Holloman parameter is utilized in rationalizing the relationship, and it was found that the relationship of ln d = 9.0−0.27lnZ is followed. The temperature rise during FSP is traced, and the maximum temperature can reach 250–450 °C, depending on the FSP pin rotation speed. X-ray diffraction results show

Acknowledgment

The authors are gratefully acknowledge the sponsorship by National Science Council of Taiwan, ROC, under the project no. NSC 92-2216-E-110-020.

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