An investigation on microstructure, texture and formability of AZ31 sheet processed by asymmetric porthole die extrusion

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Abstract

This paper provided an effective plastic deformation technique, asymmetric porthole die extrusion, for fabricating AZ31 magnesium alloy sheets. Three kinds of asymmetric porthole extrusion dies were designed and entitled as APE-45, APE-60 and APE-90 die in terms of asymmetric porthole die angle, respectively. The effect of different APE processes on the microstructures, texture evolutions and mechanical properties of AZ31 sheets was investigated at room temperature. For comparison, conventional extrusion (CE) and symmetric porthole die extrusion (PE) were also conducted on processing AZ31 sheets. Shear deformation induced by APE declined the grain size and promoted a broad angular distribution of basal planes in the APE sheets compared with the CE and PE sheets. Especially, the APE-90 sheet obtained finest grain size of 5.2 µm and made basal planes tilted towards the extrusion direction by ~ 21° rotation in the sheet plane. With increasing asymmetric porthole die angle, the volume fraction of recrystallized grains gradually increased, resulting in the decrease of basal pole intensity. Due to the increased activity of basal <a> slip, APE sheets exhibited the decrease in yield strength and r-value and increase in elongation to failure, especially for the APE-90 sheet. The improved formability of the APE sheets was attributed mainly to texture weakening. The APE-90 sheet exhibited the highest index Erichsen value and improved by ~ 74% and ~ 94% compared to the CE and PE sheets, respectively. Consequently, microstructure-texture control induced by APE could enhance the room-temperature stretch formability of AZ31 sheets.

Introduction

Magnesium (Mg) alloys, because of their low density, high specific strength and ability to be recycled, have received great attention in the automotive application and aircraft industries [1], [2], [3]. Mg alloys are classified into two types (cast and wrought Mg alloys, respectively). Although the wrought Mg alloys exhibit better mechanical property than the cast ones, structural applications of them are still very limited owing to a poor formability at ambient temperature. Such the poor ambient formability during a secondary process is attributed to a strong basal texture formed by a primary process [4], [5], [6]. Therefore, achieving a weak basal texture, by altering the primary process, is an effective approach to ameliorate the ambient formability of Mg alloys.

In recent decades, many improvement methods were proposed for room temperature formability, essentially, stretch formability. Controls of the plastic deformation approaches enabled the creation of a tilted or split texture; as a result, these alloys showed good stretch formability. The extrusion process is an industrially available and effective method to improve the mechanical properties of alloys, such as traditional equal channel angular pressing (ECAP) [7], accumulative back extrusion (ABE) [8] and shear Assisted Processing and Extrusion (ShAPE) [9]. The wrought Mg alloys processed by those methods weaken or broaden the basal texture and grain refinement due to its recrystallization mechanism. However, those approaches are only applicable for bulk products, and thus limit the broad application promising of Mg alloys. To overcome this disadvantage, some novel extrusion approaches are developed to fabricate thin sheets with high performance, such as large strain extrusion machining (LSEM) [10], novel equal channel angular pressing (ECAP) [11] and asymmetric hot extrusion [12], [13], [14]. LSEM is an effective severe deformation process for making enhanced workability sheet, strip and foil by controlling shear strain paths. The report of Sagapuram et al. [10] demonstrated that basal pole inclination of AZ31 sheet processed LSEM towards to the extrusion direction (ED) from the normal direction (ND) could be obtained in the range of 32–53° by controlling sheet thickness ratio of 0.5–1.4. Besides LSEM process, ECAP technology also could achieve thin AZ31 sheet (sheet thickness of 2 mm) with a weak broad distributed texture and high cold formability by processing route C [11]. Recently, asymmetric hot extrusion process provided an attractive thought that achieving grain refinement and texture weakening via modification of extrusion die. Chang et al. [13] pointed out that asymmetric hot extrusion process could introduce great shear strain. Such great shear strain contributed to a significant gradient grain refinement and weak texture with an inclination of ~ 90° from the ND to the transverse direction (TD). Yang et al. [14] discussed the effect of horizontal flow passage lengths of the asymmetric extrusion die on the microstructure and mechanical properties of AZ31 sheet. They argued that the inclination of basal pole towards the ED was gradually obvious with increasing flow passage lengths, leading to the decline of yield strength and the enhancement of uniform elongation. As mentioned above, introducing shear deformation is a significant approach to achieve grain refinement and texture weakening during extrusion process, further ameliorating the mechanical properties of Mg alloy sheets.

Recently, we developed a kind of extrusion process entitled as composite extrusion and achieved the AZ31 alloy sheet with good mechanical properties by means of texture weakening attributable to an extra friction shear deformation [15], [16]. Compared with the CE process, the mean yield strength of the sheet processed by the asymmetric composite extrusion (ACE) decreased by ~ 25 MPa and the mean elongation to failure increased from ~ 24% to ~ 28%, and the ACE sheet exhibited low in-plane anisotropy in the tension [16]. However, there was a tedious preparation before the ACE process.

Therefore, exploiting a sample, controllable and effective extrusion method, through altering extrusion die structure to improve mechanical properties of Mg alloys, becomes a research hotpot of material scientists and the main emphasis of our work. In the present study, asymmetric porthole die extrusion (APE) is developed to fabricating AZ31 sheets. The effect of different APE processes on the microstructures, texture evolution and mechanical properties of APE sheets are investigated and compared with those of AZ31 sheets processed by conventional extrusion (CE) and porthole die extrusion (PE).

Section snippets

Material and extrusion process

Cylindrical samples with 80 mm in diameter and 60 mm in height were machined from the as-received commercial AZ31 (Mg-2.78 Al-0.63 Zn-0.21 Mn in wt%) cast billet, homogenized at 673 K for 12 h, and furnace cooled at room temperature. Teflon tape was used as a lubricant to reduce the friction at interfaces between the die and material. Those samples were further extruded by the CE die, porthole die extrusion (PE) and three kinds of asymmetric porthole die extrusion dies (entitled as APE-45,

Microstructures

The result of the optical microscopy observation obtained on the longitudinal sections of the specimens PE, APE-45, APE-60 and APE-90 is shown in Fig. 2. As seen from geometry of two specimens, the AZ31 alloys completely filled the die chambers. Fig. 2(a, e) shows the longitudinal section of the specimen PE. It is apparent that the alloy flow exhibits a strict symmetry taking the ED as axis. Under a very careful observation, there is a symmetrical flow angle of 5°. The same qualitative result

Discussion

In the present study, the microstructures, texture evolutions and mechanical properties at room temperature in the CE, PE, APE-45, APE-60 and APE-90 sheets are investigated. We have shown that: 1) During APE processes, the θ gradually increases with the increase of APE die angle. Simultaneously, significant grain refinement is accompanied by the increase in θ. 2) The decrease in the maximum basal pole intensity and the inclination of basal plane appear in the APE sheet. Especially, the APE-90

Conclusions

The APE processes were applied to fabricate AZ31 Mg alloy sheets, and their microstructures, textures and mechanical properties at room temperature were investigated and compared with those of the CE and PE sheets. The results were concluded as follows:

  • 1.

    During the APE processes, asymmetric flow angle of 12°, 17° and 21° were observed for the APE-45, APE-60 and APE-90 processes, respectively. Shear deformation induced significant grain refinement from 17.2 µm to 5.2 µm. With increasing asymmetric

Acknowledgements

The authors are grateful for the financial supports from The National Natural Science Foundation of China [51531002, 51474043]; and the National Key Research and Development Program of China [2016YFB0301104, 2016YFB0101700]; the Chongqing Science and Technology Commission [cstc2014jcyjjq0041, cstc2014jcyjjq50002, cstc2015zdcy-ztzx50003 and cstc2015yykfc5001]; and the Education Commission of Chongqing Municipality [KJZH14101]; and the Fundamental Research Funds of the Central Universities [

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