Procedia Engineering 81 ( 2014 ) 1553 – 1558

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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 Universitydoi: 10.1016/j.proeng.2014.10.189

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11th International Conference on Technology of Plasticity, ICTP 2014, 19-24 October 2014, Nagoya Congress Center, Nagoya, Japan

Twin-Roll-Casting and hot rolling of magnesium alloy WE43 Kristina Neh*,a, Madlen Ullmanna, Rudolf Kawallaa,b

aInsitute of Metal Forming, Technische Universität Bergakademie Freiberg, Bernhard-von-Cotta-Straße 4, 09599 Freiberg, Germany bDepartment of Plastic Deformation of Special Alloys, National University of Science and Technology "MISIS", Moscow, Russia

Abstract

Twin-Roll-Casting enables the production of magnesium strip and sheet material with advanced properties. In the course of a junior research group, supported by the Development Bank of Saxony as well as the European social fund, the magnesium alloy WE43 was successfully Twin-Roll-Cast for the first time. For this, a new developed casting system with a maximum width of 250 mm has been used. This system is part of the Twin-Roll-Casting pilot plant at the Institute of Metal Forming, which is used in collaboration with the MgF Magnesium Flachprodukte GmbH. Previously, differential scanning calorimetry was carried out in order to get more information about the solidification range of the WE43 alloy. Subsequent to Twin-Roll-Casting, the WE43 sheet was homogenized at different temperatures, and hot rolled to a final thickness of 2 mm. with a reduction per pass of 10 % to 15 %. After every second reduction stage an intermediate annealing was performed. The influence of temperature and time of the annealing on microstructure and mechanical properties has been investigated. Based on the results, conclusions for Twin-Roll-Casting and hot rolling of a WE43 alloy in industrial scale were made. Finally, a comparison between WE43 and AZ31 has been established. © 2014 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of Nagoya University and Toyohashi University of Technology.

Keywords: WE43; Twin-roll-casting; Hot rolling

1. Introduction

High requirements in safety and comfort as well as increasing fuel costs lead to a rising interest of the automobile industry in lightweight materials. Magnesium with a density of 1.74 g/cm³ has high potential for light

* Corresponding author. Tel.:+49-3731 39 3308; fax:+49-3731 39 3656. E-mail address: Kristina.Neh@imf.tu-freiberg.de

© 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

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weight constructions. Furthermore, magnesium and its alloys have a high specific strength, a good workability, and are nearly indefinitely available [1-3]. Assuming equivalent operational demands components made of magnesium alloys reach weight savings up to 35 % compared to aluminum and more than 75 % in comparison to steel [4, 5].

Applications of magnesium alloys are used in a broad field, but predominantly as cast products [2, 6]. Flat products like sheets and strips have not been established so far due to the high costs of the conventional thin sheet production. However, Twin-Roll-Casting enables the production of magnesium strip in an economic way. This process achieves more importance and becomes part of the actual research. Twin-Roll-Casting persuades in comparison to the conventional sheet production as an economic and energy-efficient process. Magnesium alloys like AZ31 and AM50 have been successfully twin-roll-cast and the strip or sheet properties were characterized fundamentally [5, 7-9]. But the area of application of these alloys is limited by their poor performance at elevated temperatures. Alloys consisting Rare Earth Elements like WE43 offer good high temperature properties. Furthermore, WE43 exhibits good corrosion and high ignition resistance. But unfortunately WE43 is currently only processed by casting or forging [10-14].

In the present research, the production of sheets of WE43 by Twin-Roll-Casting was studied. Therefore, process parameters and their effect on the sheet quality were analyzed. Microstructure and mechanical properties of the twin-roll-cast sheets were investigated. Moreover, the influence of different heat treatments on the microstructure and the material behavior was studied. After heat treatment the WE43 sheets were hot rolled to a final thickness of 2 mm.

2. Experimental procedure

A commercial WE43 alloy was used for the present work. Main alloying elements of this alloy are Y (4.04 wt %), Nd (2.59 wt%), and Zr (0.52 wt%). For Twin-Roll-Casting a system in laboratory scale was used. This system is part of the Twin-Roll-Casting pilot plant at the Institute of Metal Forming (TU Bergakademie Freiberg), which is used in collaboration with the MgF Magnesium Flachprodukte GmbH. The Twin-Roll-Casting system consists of a tilting crucible melting furnace with a maximum melt volume of 150 kg, a casting channel with a small casting nozzle, which transports the melt into the rolling gap and enables the production of a 315 mm wide casting strip. The casting strip was trimmed in several sheets with a length of 400 mm. Samples about 15 mm x 15 mm were cut for microstructure characterization and heat treatment investigations. The heat treatments were carried out in a temperature range between 400 °C and 525 °C for different holding times. After heat treatment hot rolling was performed at a rolling mill in laboratory scale. Reductions of 10 % (pre-rolling) and 35 % finish rolling were chosen to produce sheets with a final thickness of 2 mm. To ensure a constant initial rolling temperature intermediate heat treatment was required. Subsequent, heat treatment was carried out. The analysis of the microstructure was accomplished by using scanning electron and light microscopy. Determination of mechanical properties was carried out with tensile testing.

3. Results

3.1. Twin-Roll-Casting

Twin-Roll-Casting of an unknown alloy requires information about the solidification range. For the identification of liquidus and solidus temperatures of the WE43 alloy differential scanning calorimetry was conducted. Different heating and cooling rates between 5 K/min and 20 K/min were used. The liquidus temperature is 638 °C and the solidus temperature is 592 °C. The solidification range of the WE43 alloy is 46 K, which is higher compared to AZ31 ( T = 39 K), where parameters of the Twin-Roll-Casting are well known. Consequently, temperature, casting speed, and cooling of the rolls have to be adjusted. Twin-Roll-Casting of the WE43 alloy was carried out at a furnace temperature of 735 °C and a casting temperature of about 720 °C. Lower temperatures cause poor strip quality due to the formation of cracks at the edges and the surface of the strip.

The WE43 alloy was melted in a crucible melting furnace. After that the liquid melt was tilted into a holding tank, which is connected to the casting channel. To transfer the melt from the holding tank into the casting channel a tamping control is used, which regulates the flow rate of the melt and thereby the melt level into the casting

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channel. The casting channel transports the melt over a casting nozzle into the rolling gap, where the material solidifies quickly and is deformed partially. The casting speed was 1.4 m/min. The Twin-Roll-Cast strip with dimensions of 5 mm x 315 mm was cut into small sheets for characterization of microstructure and mechanical properties.

In as-received condition (directly after Twin-Roll-Casting) the microstructure of the WE43 alloy does not show the typical dendritic microstructure like AZ- or AM-alloys. For example Twin-Roll-Cast AZ31 strip mainly consists of large dendritic columnar grains (Fig. 1a and 1b). Small grains appear only near the surface and in the mid-thickness of the strip. Eutectic and intermetallic compounds are observed in interdendritic areas [7, 15]. The Twin-Roll-Cast WE43 strip shows a microstructure, which bears resemblance to a grain structure. But the grains of this structure offer irregular, non-globular grains with partially serrated grain boundaries (Fig. 1c and 1d). After the solidification of the primary -magnesium eutectic solidification occurs. The eutectic -phase is mainly located along the grain boundaries. Due to the different solidification conditions (local cooling rate of the mid-thickness is much higher than at the surface) segregation occurs in the mid-thickness.

Fig. 1. Microstructure of twin-roll-cast strip in the as received condition (a) AZ31 overview, (b) AZ31 detail (c) WE43 overview and (d) WE43 detail.

Point Mg Y Nd 1 68,80 12,07 19,13 2 70,00 11,86 18,14 3 69,46 12,46 18,08 4 93,05 5,68 1,27

Fig. 2. SEM micrograph of the microstructure of the Twin-Roll-Cast WE43 strip with the measuring points of the EDX-analysis (points 1 to 3: eutectic, point 4: magnesium matrix) and the regarding chemical composition (mass fraction in wt%).

Fig. 2 illustrates the SEM microstructure of the WE43 alloy in the as-received condition. It can be found that the microstructure consists of a grey matrix and different morphology of the eutectic -phase, which exhibits a lamellar profile. Scanning electron microscopy in combination with energy dispersive X-ray spectroscopy (EDX) enables the determination of the chemical composition of the different structural constituents. The measuring points of the EDX-analysis are marked in Fig. 3. Points 1 to 3 belong to the eutectic -phase, point 4 to the matrix. The eutectic predominantly consists of magnesium (Mg), yttrium (Y), and neodymium (Nd). Based on the Nd/Y ratio it is assumed that the eutectic -phase conforms to the ternary Mg14Nd2Y phase [16]. The amount of Y and Nd in the matrix (point 4) is lower compared to the eutectic. Only 5.7 wt% Y and 1.3 wt% Nd are solved in the magnesium matrix.

The WE43 strip offers very good mechanical properties in the as-received condition, but with a broad distribution. Maximum ultimate tensile strength and yield strength, that can be reached, are 410 MPa (UTS) and 376 MPa (YS). The achievable elongation alternates from 1.0 % to 3.8 %. In comparison to AZ31 higher strength values are possible. Average ultimate tensile strength of AZ31 in TRC condition is 284 MPa, yield strength of

1

2

3

4

(a) (b) (c) (d)

10μm

50 μm 10 μm 50 μm 10 μm

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about 250 MPa. In comparison to a cast WE43 alloy and an extruded bar of Elektron 43 produced by Magnesium Elektron the WE43 sheet in TRC-condition offer good mechanical properties with higher strength values (Fig.3). To increase the lower elongation heat treatment was carried out.

Fig. 3. Mechanical properties of WE43 strip in TRC-condition compared to a cast WE43 alloy and an extruded bar of Elektron 43 (2Magnesium Elektron UK, Magnesium Technology Centre, Rake Lane, Manchester).

3.2. Annealing

Heat treatment was carried out in a temperature range from 400 °C to 525 °C. The holding times were varied between 0.25 and 12 h. Size and amount of the eutectic -phase decrease significantly with increasing temperature and holding time. At once precipitation occurs. These precipitations seem to be Mg24(Y,Nd)5 or Mg41(Nd,Y)5 [17]. Only temperatures above 500 °C enable a nearly complete solution of the eutectic -phase and the precipitations. Even increasing holding times of about 12 h cannot help to advance their solution. Furthermore, the recrystallization of the twin-roll-cast microstructure can be reached at these temperatures already with short holding times of about 15 minutes. After heat treatment of 500 °C and 15 min the average grain size is 22 μm. As shown in Fig. 4, higher holding times at the same temperature lead to grain growth. Thus the average grain size after a holding time of 2 h is 47 μm. In comparison to AZ31, where an average grain size of 13 μm can be reached after a heat treatment of 430 °C and 4 h (Fig. 4a), the grain size of WE43 strip in the annealed condition is higher.

Fig. 4. Microstructure of (a) AZ31 Twin-Roll-Cast strip after heat treatment of 430 °C, 1h, (b) WE43 Twin-Roll-Cast strip after heat treatment 500 °C, 0,25 h and (c) 500 °C, 2 h.

After heat treatment the strength values decrease dramatically in comparison to as-received condition. Increasing temperatures lead to a decrease of the yield strength to values below 150 MPa. The ultimate tensile strength keeps a constant level, but compared to the Twin-Roll-Cast condition the values decrease of about 234 MPa. The elongation increases from ~ 2.0 % to 7 – 11 % due to annealing (Fig. 6a). Holding times of 0.25 h at a temperature of 500 °C effect a slightly increase of the yield strength compared to 2 h holding time. The mechanical properties of the annealed condition are non-satisfactory results. The low elongations as well as broad distribution of the mechanical properties are due to inhomogeneity of the microstructure. The analysis of the fractured surface offers areas of different structural constituents. The curve progression of the stress-elongation curve shows a premature rupture of the tensile specimen.

0510152025303540

050

100150200250300350

cast WE43² extruded barElektron 43²

TRC WE43

A [%

]

Rp0

,2 a

nd R

m [M

Pa]

Rm

Rp0,2

A

(a) (b) (c)

50 μm 50 μm 50 μm

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3.3. Hot Rolling

After heat treatment the material was hot rolled at a rolling mill in laboratory scale. The working rolls were pre-heated using heating mats. Several passes were used to produce sheets with a thickness of 1.8 mm. Pre-rolling was carried out with a maximum reduction per pass of 10 %. Higher reductions lead to the formation of cracks as well as dents and waves. At finish rolling higher reductions per pass of 35 % are realized. During hot rolling intermediate heat treatments were required to adjust rolling temperature and to effect recrystallization of the WE43 alloy. Finally, the hot rolled sheets were heat treated and cooled in air to room temperature. The microstructure of the hot rolled and heat treated sheets offers a recrystallized grain structure with uniform grain size (Fig. 5b).

Fig. 5. (a) Comparison of mechanical properties of the WE43 sheets from own work with WE43 products in several conditions from the literature and (b) Microstructure of WE43 sheet after hot rolling and heat treatment.

The mechanical properties of the hot rolled and heat treated WE43 sheets are compared to WE43 products in several conditions from the literature (Fig. 5a). With an ultimate tensile strength of about 350 MPa and yield strength of 310 MPa the WE43 sheet of this work offers good strength values compared to the literature. However the elongation only achieves the average range, and thus requires an improvement. Improving mechanical properties, especially the elongation, optimization of the intermediate heat treatment as well as of the hot rolling parameters, particularly reduction per pass and temperature control, are required. Therefore further investigations have to be carried out.

4. Conclusions

(1) The investigations show the feasibility of Twin-Roll-Casting of WE43 in laboratory scale. For the first time WE43 was successfully processed with Twin-Roll-Casting.

(2) The microstructure of twin-roll-cast WE43 strip bears resemblance to a grain structure and does not show the typical dendritic structure like AZ- or AM-alloys. But the grains of this structure offer irregular, non-

-phase appears mainly located along the grain boundaries.

(3) Compared to AZ31 strips in TRC condition, WE43 strips possess higher strength values, but with a broad distribution. The distribution is due to the inhomogeneity of the microstructure. The analysis of the fractured surface offers areas of different structural constituents.

(4) Annealing in a temperature range from 400 °C to 525 °C lead to a dramatically decrease of the strength -phase decrease significantly and precipitation occurs with

increasing temperature and holding time. Only temperatures above 500 °C enable a nearly complete solution of the eutectic and the precipitations. Extension of holding times leads to undesirable grain

(a) (b)

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growth.

(5) With an ultimate tensile strength of about 350 MPa and yield strength of 310 MPa the hot rolled WE43 sheet of this work offers good strength values in the direct comparison the evaluation of the literature. However the elongation only achieves the average range, and is thus not satisfactory.

(6) The results show that the WE43 has a high potential for applications made of strip and sheet. For a transfer in industrial scale further investigations to optimize the material properties of the TRC-condition as well as the parameters of heat treatment, hot rolling and intermediate heat treatment are essential.

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