Effect of Extrusion Temperatures on Microstructures and Mechanical Properties of Mg-3Zn-0.2Ca-0.5Y Alloy Cheng-Jie Li*, Hong-Fei Sun, Wen-Bin Fang

Available online at www.sciencedirect.com ScienceDirect

Procedia Engineering 81 ( 2014 ) 610 – 615

11th International Conference on Technology of Plasticity, ICTP 2014, 19-24 October 2014, Nagoya Congress Center, Nagoya, Japan Effect of extrusion temperatures on microstructures and mechanical properties of Mg-3Zn-0.2Ca-0.5Y alloy Cheng-jie Li*, Hong-fei Sun, Wen-bin Fang

School of Materials Science and Engineering , Harbin Institute of Technology, Harbin, 150001, China

Abstract

After solution treatment, the Mg-3Zn-0.2Ca-0.5Y alloy was subjected to extrusion at various temperatures and extrusion defects, microstructures and mechanical properties of as-extruded samples were studied in detail. The results show the extrusion temperature has a slight effect on DRXed grain size due to the presence of Y-containing phase with a good thermal-stability. On the contrary, the mechanical properties of as-extruded alloys are remarkably influenced by dynamic recrystallization degrees promoted by extrusion temperatures. A good formability of as-extruded bars with an optimal mechanical properties and fine-grained microstructure was obtained by extrusion at 250 °C and the mechanical performances are significantly superior to those of hot-rolled ZK60 obtained at the same temperature. In this paper, the magnesium alloy with fine-grained (<5 ȝm), high ductility (>30%) and high strength (>250 MPa) was fabricated easily by a simple extrusion process.

© 20120144 ThePublished Authors.Published by Elsevier Ltd. by ElsevierThis is an Ltd open. access article under the CC BY-NC-ND license Selection(http://creativecommons.org/licenses/by-nc-nd/3.0/ and peer-review under responsibility of Nagoya). University and Toyohashi University of Technology. Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University

Keywords: Magnesium; Hot extrusion; Warm forming; Mechanical peoperties

1. Introduction

Compared with the traditional structural materials (steel, aluminium, etc.), magnesium alloys have huge potential to be widely used in aerospace and automobile industries due to obvious advantages such as their low densities, high specific stiffness and high specific strengths (Kainer et al., 2006). But magnesium and its alloys also

* Corresponding author. Tel.:+86-451-8640-3365; E-mail address:[email protected]

1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of the Department of Materials Science and Engineering, Nagoya University doi: 10.1016/j.proeng.2014.10.048 Cheng-jie Li et al. / Procedia Engineering 81 ( 2014 ) 610 – 615 611 show the shortcomings of poor ductility which restricts their widespread application in field of industry (Takuda et al., 1998). The last two decades, a variety of thermo-mechanical treatment methods have been introduced by researchers to improve formability of magnesium alloys by grain refinement, such as hot rolling (Chen et al., 2013), hot extrusion (Homma et al., 2012) and equal channel angular extrusion (Jiang et al., 2010), etc. Among these methods, extrusion is a simple method for producing Mg alloys for its convenient, economical and technical advantages in fabrication of structure components. As well known, the extrusion temperature, ram speed and extrusion ratio are three most critical parameters which directly affect the microstructures, texture and mechanical properties. At present, many researchers have focused on optimizing the process variables to achieve their desired microstructures, texture types and improved mechanical properties (Elsayed et al., 2013). Many investigations have demonstrated that the relationship between extrusion conditions and microstructures as well as mechanical performance in a variety of magnesium alloys such as AZ31, Mg-Sn-Al-Zn (Elasyed et al., 2013), Mg-Zn-(Mn)-Ce/Gd alloys (Park et al., 2014), but most studies focus on the hot extrusion conditions, the influence of extrusion variables especially warm extrusion on Mg-Zn-Ca-Y has rarely reported. Thus, the aim of this paper is to clarify the relationship between extrusion temperatures and microstructure, mechanical performance in a new Mg alloy.

2. Experimental procedure

A nominal chemical composition of as-cast Mg-3.0Zn-0.2Ca-0.5Y (wt%) was used as raw material which was synthesized from pure Mg (99.95 wt%), pure Zn powder (99.7% wt%) Mg-30Ca (30 wt%) and Mg-30Y (30 wt%) master alloys. The method for preparation of cast billets for extrusion can be found everywhere (Park et al., 2014). The cast ingots were heated to 440 °C for 10h to sufficiently homogenize compositions and then water-quenched. The solution treatment billets were heated to a set temperature for 30min prior to extrusion. Then the pre-heated billets were extruded at different temperatures of 150, 250 and 300 °C with an extrusion ratio of 8.3. Finally, the as-extruded Mg alloy bars with a diameter of 18mm were obtained. As shown in Fig. 1, a schematic diagram of the forward extrusion process and the mold with a die angle of 45° was used in present study. During extrusion process, a thermocouple in the die exit was used to control the mold temperature constant.

Fig. 1. Schematic diagram showing forward extrusion process.

An Olympus GX71 optical microscope was used to study the microstructure characterization along extrusion direction and the grain size was measured using Image-ProPlus 6.0 software. The tensile properties of the as- extruded alloy were carried out at room temperature by an INSTRON-5569 standard testing machine equipped with extensometer with an initial strain rate of 1.67×10-3. The specimens for tensile tests were cut parallel to the extrusion direction with a diameter of 3 mm and a gauge of 15 mm.

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3. Results and discussion

3.1. Effect of extrusion temperature on formability of Mg alloy

Magnesium alloy products were obtained at different extrusion temperatures as shown in Fig. 2. Under the compressive stress environment, material's formability is not only related to inherent properties, but also strongly dependent on environment temperature during plastic deformation. In particular, for magnesium alloy, the better formability can be obtained at a higher temperature (Doege et al., 2001). In present experiment, a very good formability of magnesium bars was achieved in a temperature between 250 and 300 °C. However, forming failure occurred at warm extrusion with a relative low temperature (150 °C in this paper), the fracture initiation and growth result from severe work hardening induced by incomplete recrystallization, which will be discussed in the next section. It is seemed that the warm forming was not suit for magnesium with trance rare earth element.

Fig. 2. Products extruded at different temperatures.

3.2. Effect of extrusion temperature on microstructure

Fig.3 shows the optical microstructures examined from the longitudinal section of the as-extruded alloys. The microstructures of as-extruded alloy have been refined remarkably, which indicates that DRX occurs during extrusion at the above mentioned temperatures. However, there are also some differences between each of extrusion parameters. Overall, there are double effects of extrusion temperature on dynamic recrystallization behaviour and coarsening of DRX grain size. The area fraction of recrystallized grains increases with an increase of the processing temperature. When extrusion temperature is lower 250 °C, there exists two obvious partitions in the as-extruded alloy: DRXed and unDRXed zone. Actually the area fraction of recrystallized grains of 80.6% appears in microstructure of the as-extruded alloy extruded at 150 °C as shown in Fig. 3(a). When processing temperature rise from 150 °C to 250 °C, the area fraction of recrystallized grains increase from 80.6% to 88.3%. A complete dynamic recrystallization process was obtained at 300 °C. As we all known, the starting dynamic recrystallization of metal is about 0.4Tm, where Tm is melting point of metallic material. The melting point of pure Mgand dynamic recrystallization are about 650 and 150 °C, respectively (Ding., 2007). According to a simple calculation, the starting dynamic recrystallization of Mg-3.0Zn-0.2Ca-0.5Y is slightly higher than 75 °C. In addition, deformation heat generated by plastic deformation is another source of energy to promote dynamic recrystallization. Therefore, a large number of dynamic recrystallization grains can still be observed even though the alloy extruded at a relatively low deformation temperature, such as warm forming at 150 °C in this paper. In practice, the complete dynamic recrystallization temperature of metallic material is higher as much as about 150 °C than starting dynamic recrystallization temperature (Cui et al.,1996). The maximal degree of dynamic recrystallization is approximately 225 °C according to a theoretical computing. As for Mg-3.0Zn-0.2Ca-0.5Y alloy used in this paper, an ideal dynamic recrystallization temperature should be between 250 and 300 °C with reference to the results of this experiment. The difference between the theoretical and experimental results shows that the addition of Y element helps to improve the recrystallization temperature of the material. Cheng-jie Li et al. / Procedia Engineering 81 ( 2014 ) 610 – 615 613

Fig. 3. Optical microstructure of the alloy extruded at different temperatures: (a) 150ć (b) 250ć(b) 300ć

Furthermore, investigation has been proved that the size of DRXed grains strongly depends on the strain rate and deformation temperature (Yu et al., 2013). DRXed grain size is closely related to a famous Zener-Hollomon parameter (Z parameter) (Watanabe et al., 2001):

m Zd A , (1) §·Q Z H exp ¨¸, (2) ©¹RT where A is constant, H is the strain rate, Q is the active energy for dominant diffusion (135kJ/mol for Mg alloys), T is absolute temperature, and R is the gas constant. For the extrusion process, the equivalent strain rate can be calculated by the following equation (Park et al., 2014):

6lnDV2 E BR , (3) H 33 DDBE

W\where VR is the extrusion speed, E is the extrusion ratio, and DB and DE are initial ingot and extrudate diameters, respectively. In this article, the extrusion speed VR, the diameters of initial ingot and extrudate DB and

DE are constantˈthat is to say the equivalent strain rate H is constant. In other words, all of extrusion parameters, the only variable is temperature. According to Eq. (2), the Zener-Hollomon parameter Z is proportional to exp(1/T). Therefore, Eqs. (1) and (2) can be rewritten as:

A 1 d v, (4) ZZ §·1 Z v exp¨¸. (5) ©¹T Put Eq. (5) into Eq. (4), then another expression is obtained: §·1 d v 1/exp¨¸. (6) ©¹T With reference to Eq. (6), the DRX grain size increases as the deformation temperature increases. This conclusion is proved in this paper as shown in Fig.3. When the deformation temperature increases from 150 °C to 300 °C, the average grain size increases from 2 μm to 4.7 μm. There is a competing relationship between grain nucleation and growth. When the alloy is extruded below 250 °C, the grain nucleation rate is greater than growth rate. The former is predominant which benefits refinement of grain size. However, some elongated grains appear in 614 Cheng-jie Li et al. / Procedia Engineering 81 ( 2014 ) 610 – 615

the unDRXed region due to decreasing of dynamic recrystallization degree at low temperature, and their orientation approximately parallel to the extrusion direction. The incomplete dynamic recrystallization microstructure brings an adverse effect on formability and mechanical properties of materials at room temperature. When processing temperature rises to 300 °C, the evolution of dynamic recrystallization degree results in the improvement of material's formability and mechanical properties at room temperature, furthermore, the grain growth rate is greater than nucleation rate, which leads to coarsening of grain size during hot extrusion process. As can be seen from the Fig. 3(c), only small amount of coarsening grains appear in the microstructure, and their grain size are still less than 10 ȝP, which owing to thermal-stability of Y-containing phase.

3.3. Effect of extrusion temperature on mechanical properties

The engineering stress-strain curves obtained from tensile tests are shown in Fig. 4, and the ultimate tensile strengths (ıb), yield strength (ıs), elongation to failure (į), uniform elongation (İU), yield ratio (ıs/ıb) and grain size are summarized in Table.1. In general, the ultimate tensile strength (ıb), yield strength (ıs) and yield ratio (ıs/ıb) of the as-extruded alloys decrease with the temperature increases mainly due to the refinement of grain size, which is consistent with the Hall-Petch relationship. While the elongation to failure (į), uniform elongation (İU) of the as-extruded alloys increase as the temperature increases owing to the effect of dynamic recrystallization. Here, the highest į of 36.7% and İU of 23.0% with the lowest ıb of 244 MPa and ıs of 160 MPa is obtained by extrusion at 300 °C. The dynamic recrystallization process is completed in the sample extruded at 300 °C, which benefits the enhancement of ductility. In addition, the basal texture is weakened and the non-basal slip system is improved due to the addition of rare element Y, both of which are helpful to the evolution of ductility (Sandlöbes et al., 2011). The highest ıb of 318.3 MPa with a lowest į of 11.3% is obtained by extrusion at 150 °C. The strength mechanism of Mg alloy is mainly related to grain boundary strengthening and deformation strengthening in this paper. As shown in Fig. 4(a), the smallest grain size (2 ȝP) and the largest non-recrystallized regions (19.4%) (mainly composed of high density of dislocations and sub-grain) occur in sample extrusion at 150 °C.But the ductility is greatly reduced due to lack of work hardening denoted by a high value of ıs/ıb.A more excellent mechanical properties (į = 32.9% and ıs = 279.2 MPa) is obtained by extrusion at 250 °C, which results from the good balance between strengthening and softening mechanism. Compared with ZK60 hot rolled at same temperature, the as- extruded alloy has the similar ultimate tensile strength, but the higher values of yield strength, uniform elongation, elongation to failure, and the finer grain size (The yield strength, ultimate tensile strength, uniform elongation, elongation to failure and grain size in hot rolled ZK60 at 250 °C are 206.1 and 321.6MPa, and 15.6%, 20.4% and 4.8μm, respectively.) (Chen et al., 2013).

Fig. 4. Stress-strain curves of as-extruded bars at different temperatures.

Cheng-jie Li et al. / Procedia Engineering 81 ( 2014 ) 610 – 615 615

Table 1. Grain size and mechanical properties of Mg-3.0Zn-0.2Ca-0.5Y alloy extrusion at various temperatures.

Temperature (ć) Grain size ȝP ıୠ (MPa) ıୗ (MPa) į (%) İ୙ (%) ıୗ/ıୠ 150 2.0 318.3 308.9 11.3 5.3 0.97 250 2.5 304.7 279.2 32.9 17.5 0.92 300 4.7 244.2 160.0 36.7 23.0 0.65

4. Conclusions

A nominal chemical composition of as-cast Mg-3.0Zn-0.2Ca-0.5Y alloy was subjected to solution treatment before extrusion at different temperatures. To summary the effects of extrusion temperatures on the microstructures and mechanical performances of Mg-3.0Zn-0.2Ca-0.5Y alloy, the following main conclusions are drawn from this investigation: (1) A good formability can be obtained by extrusion above 250 °C, while, the fracture occurred in the as-extruded bar by warm extrusion at 150 °Cdue to severe work-hardening. (2) The extrusion temperature has a slight effect on DRXed grain size. With the extrusion temperature increases from 150 °Cto 300 °C the grain size only increases from 2 ȝP to 4.7 ȝP , which was due to the growth of DRXed grains inhibited by Y-containing compounds. (3) The extrusion temperature has an obvious effect on mechanical performances of the as-extruded alloy. The ıb, andıs of as-extruded alloy decrease remarkably with increasing the extrusion temperature, on the contrary, the ductility increase greatly due to dynamic recrystallization. An optimal mechanical properties was obtained by extrusion at 250 °C because of the good balance between work-hardening and dynamic recrystallization, and it's mechanical properties significantly superior to those of hot-rolled ZK60 obtained at the same temperature. (4) The difference between theory and experiment shows that the recrystallization temperature is enhanced by the addition of Y element.

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