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Effects of twin-roll casting process parameters on the microstructure and sheet metal forming behavior of 7050 aluminum alloy
Accepted Manuscript Title: Effects of twin-roll casting process parameters on the microstructure and sheet metal forming behavior of 7050 aluminum alloy Author: Hongbin Wang Le Zhou Yongwen Zhang Yuanhua Cai Jishan Zhang PII: DOI: Reference:
S0924-0136(16)30049-8 http://dx.doi.org/doi:10.1016/j.jmatprotec.2016.02.016 PROTEC 14730
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
17-8-2015 22-1-2016 23-2-2016
Please cite this article as: Wang, Hongbin, Zhou, Le, Zhang, Yongwen, Cai, Yuanhua, Zhang, Jishan, Effects of twin-roll casting process parameters on the microstructure and sheet metal forming behavior of 7050 aluminum alloy.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2016.02.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effects of twin-roll casting process parameters on the microstructure and sheet
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metal forming behavior of 7050 aluminum alloy
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Hongbin Wang1, Le Zhou1,*, Yongwen Zhang1, Yuanhua Cai2, Jishan Zhang2
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(1. School of Material and Metallurgy, University of Science and Technology
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Liaoning, Anshan 114051, China; 2. State Key Laboratory for Advanced Metals and
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Materials, University of Science and Technology Beijing, Beijing 100083, China）
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* Corresponding author Tel.: +86 412 5929299, +8618241278103
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E-mail address: [email protected] (L. Zhou), [email protected] (H. Wang)
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Postal address: School of Material and Metallurgy, University of Science and
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Technology Liaoning, Qianshan Zhong Road 185, Anshan 114051, Liaoning, China
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Graphical abstaract
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13 14 15
The SEM images and EDS of the 7050 aluminum alloy sheets obtained with the roll gap
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of 1.8mm at different rolling speeds. As seen in Figure 6(a), bulk precipitations were 1
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distributed along the grain boundaries and microns sized (0.5-10μm) particles were
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observed. Most of the precipitated phases overlapped each other and formed skeletons
19
along the grain boundaries. The coarse second phase is characterized as hard, brittle with
20
no malleability/deformability. In the middle and late stages of roll-casting process, the
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coarse second phases hardly deformed together with grain, so the large stress
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concentration appeared around the second phases. Fractures commence from the
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skeleton of second phases which get separated from the grain boundaries. Hence, micro-
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cracks are initiated. The formation of micro-crack is harmful to mechanical properties
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of high-strength aluminum alloys.
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EDS analysis confirmed that the particles in the grain boundaries are mainly Cu-rich.
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The copper content (48.36 wt.%) in Figure 6(a) appeared to be higher than that Figure
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6(d) (21.38 wt.%). The high copper content can reduce the toughness of the alloy and
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increase the crack propagation rate. The Mg content appeared to be 1.68 and 1.61 wt.%
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in Figure 6(b) and (d), respectively. The Cu combined with Mg and Al easily formed
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Al2CuMg and Al2Cu phases.
32 33
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Abstract
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In the present study, strips of 7050 aluminum alloys were cast using twin roll casting
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technology (TRC) where roll casting speed, roll gap, cooling water flow rate and initial
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cooling water temperature were varied. The optimized process parameters and their 2
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effects on TRC of 7050 aluminum alloys strips were investigated. The optimized process
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parameters and properties were obtained to be roll gap of 1.8 mm, roll casting speed of
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11.4m/min (12 r/min), cooling water flow rate of 11.1 m³/h and initial cooling water
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temperature of 20.8oC. The macro morphology, microstructure, hardness and yield stress
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of the strips were investigated using optical microscopy, scanning electron microscopy
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(SEM), energy dispersion spectroscopy (EDS) and Vickers hardness. The homogeneous
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microstructures and improved mechanical properties were obtained by increasing the
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roll casting speed and roll gap thickness.
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Keywords: Twin-roll casting; 7050 Aluminum alloy; Microstructure; Mechanical
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properties; Process parameters
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1. Introduction
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Twin-roll casting (TRC) is an important technology for producing metal and alloy
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sheets, which was first devised by Bessemer as a new steel making process in 1956. The
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TRC technology has several advantages over the conventional casting methods, such as
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low consumption of power and time. It combines casting and hot rolling together into a
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single operation, thus ensuring an efficient process. In a typical TRC process, the molten
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metal is fed into the gap between two internally water-cooled running rolls. Park and co-
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authors (2007a) demonstrated that the cooling rate of the molten metal can reach up to
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102 ºC/s during TRC processing. Hage et al. (2003) devised a twin roll caster for
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aluminum alloys using high thermal conductivity roll and no lubricant, to investigate the
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effects of these on the casting speed and mechanical properties. In their study, heat 3
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transfer between melt and roll was improved using hydrostatic pressure of the melt and
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the lubricant was not used to increase the casting speed. After solidification, the metal
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undergoes a hot deformation to produce the final sheet, as reported by Ju and Hu (2006).
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The process parameters of TRC significantly influence the quality of the sheets, as
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reported by Mino and his co-authors (2006), particularly for the ultra-thin fast cast (Xiao
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and Cai, 1999). It was found that finer microstructures decreased elemental segregation
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(Watari et al. 2007), and higher solid super saturation rate of strips were obtained with
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TRC (Park et al. 2007b). Fine grain (5 μm) sheets of AZ31 magnesium alloy strip were
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obtained by Zhao et al. (2012) using an asymmetric twin-roll cast. Moreover, the quality
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of the sheet is sensitive to the process parameters, such as the roll casting speed. Haga
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and Suzuki (2001) proposed a melted drag twin-roll caster with a speed of 30 m/min for
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producing sheets from AA3003, Al-6Si, and Al-12Si alloys. Haga et al. (2007) later
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demonstrated a high speed twin-roll caster with speed of 60m/min for Al-Fe alloys.
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Recently, Wang and Zhou (2014) examined the effects of surface conditions and
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microstructure of AA1050 alloy strips on the surface quality of the produced sheets.
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However, rarely research reports the TRC processing of 7050 aluminum alloy.
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As one of the super high-strength aluminum alloys, 7050 aluminum alloy satisfies the
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requirements of aerospace applications, which was reported by Heinz and his co-authors
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(2000). Later, Clark and Johnson (2003) used 7050 aluminum alloy sheet as the parts in
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several aircrafts such as the airliner of Boeing-777, military jet fighter F/A-18 Hornet,
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and F-22 Raptor. 4
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However, to date, there are no reports on the optimization of process parameters during
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TRC preparation of aluminum alloys sheets, especially for high-strength 7050 aluminum
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alloy. Thus, the aim of this study was to investigate the effects of TRC process
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parameters on the microstructure and sheet metal forming behavior of the 7050
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aluminum alloy. Moreover, the TRC process parameters were optimized for producing
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high quality aluminum sheets.
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2. Experimental procedure
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A vertical type twin-roll caster was used in this study. A schematic diagram of the TRC
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setup is shown in Figure 1. Copper rollers with a diameter of 300 mm and equipped with
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an internal water-cooling system were employed. The width of the roll gap can be
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controlled by a hydraulic system. Immediately prior to rolling, the 7050 aluminum alloy
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was superheated at a temperature of approximately 20~30 K which is above its melting
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point. The molten aluminum alloy was then poured onto the cold surfaces of rollers
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through the nozzle. Because of the wider solidification rage of high alloy content
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aluminum alloy, the detailed experimental parameters are shown in Table 1 which
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according a series of experiments results.
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97 98
Figure 1 Schematic diagram of the twin-roll caster Table 1 Experimental conditions for the twin roll casting process Roller parameters
Copper, D=300 mm, W=100 mm, 20~30 K, above the melting point of Al alloy 7.5 and 11.4 m/min (8 and 12 r/min) 1 and 1.8 mm Al-5.89Zn-2.16Mg-2.1Cu (wt.%)
Heating temperature Roll-casting speed Roll gap Aluminum alloy
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In order to optimize casting parameters and to prepare sheets with high quality surfaces,
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different casting speeds and casting roll gaps were evaluated in this study. The roller
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speeds of 7.5m/min (8 r/min) and 11.4m/min ( 12 r/min), and the roll gap widths of 1
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mm and 1.8 mm were selected as the process parameters to investigate their influences
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on the sheet quality. The experimental process parameters used in the study are shown
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in Table 2. Processes A and B were termed as group 1 experiment with the gap of 1 mm,
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while processes C and D were termed as group 2 experiment with the roll gap of 1.8
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mm. At the beginning of each group experiment, the roll speed was 7.5m/min (Process
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A and Process C) for 100s-120s, and then accelerated to 11.4m/min (Process B and
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Process D), which was then maintained until the end of the experiments. The thickness
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of the roll-casting sheet was recorded during the entire experimental process.
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Table 2 Twin roll casting process parameters used in the present study
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Parameter
Roll speed (r/min)
Roll gap (mm)
Cooling water flow (m3/h)
Initial cooling water temperature (oC)
Process A
7.5
1
8.95
18.6
Process B
11.4
1
8.95
18.6
Process C Process D
7.5 11.4
1.8 1.8
11.1 11.1
20.8 20.8
* The cooling water flow of 8.95 m3/h and 11.1 m3/h were attained by experiments. 6
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3. Results and discussion
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3.1 Optimization of TRC process parameters
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Results of two group experiments are shown in Figures 2(a) and (b). The speed of TRC
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was 7.5m/min at the beginning of the experiment and then was accelerated to 11.4m/min
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after steady casting for 100-120s in both experiments. In this study, casting speed was
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changed to compare the thickness variations in roll-casting sheets under the same
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conditions. The sheet thickness was found to be approximately 3-5 mm with TRC speed
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of 7.5m/min (Figure 2(a)), while it was 2-4 mm at the TRC speed of 11.4m/min. It is
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evident that the thickness of the TRC sheet varies with roll-casting speed. As shown in
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Figure 2(b), the thickness of the TRC sheet did not fluctuate much. At the TRC speed of
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7.5m/min and 11.4m/min, the sheet thickness was 3.5-4.5 and 2-3.5 mm, respectively,
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indicating a reasonably uniform thickness.
124 125
Figure 2 Results of TRC experiments (a) for first group experiment (b) for second
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group experiment
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As shown in Figure 2, the sheet thickness decreased as the roll-casting speed increased.
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If the product/multiplication of roll-casting speed and thickness of the sheet remains
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constant, the following relationship is obtained: 7
K v h1
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K ——productivity constant;
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v ——roll-casting speed, mm/min;
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h1 ——sheet thickness, mm.
(1)
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However, the product of the casting speed and sheet thickness was found to be within a
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certain range in the present study. The sheet thickness will be smaller at higher thermal
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conductivity of the roller shell material and roll casting speed. The same conclusion was
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given by Hou et al. (2010), who commented that for producing a higher thickness of the
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roll casting sheet, a lower speed would be more suitable.
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3.2 Observations of macro morphology
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As shown in Figure 3(a), the aluminum sheets had broken into pieces, and surface of the
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sheets was not smooth following first group experiment. On the other hand, it can be
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seen in Figure 3(b) that the sheet from second group experiment remains in a good shape
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without breaking and its surface quality was significantly better than the one after first
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group experiment.
145 146
Figure 3 Macro morphology of sheets obtained after (a) first group experiment (b)
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second group experiment 8
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From the point of view of the process parameters, these experimental observations can
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be explained using several key factors described below:
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1) Roll gap
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The roll gap thickness controls the amount of melted aluminum accommodation between
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the rollers. For the same volume of melted aluminum, a wider roll gap results in less
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depth of the melted cavity. According to Xiao and Cai (1999), the factors that cause
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increased the depth of melt alloy lead to a tendency to crack. This is because the greater
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the melt alloy depth, the larger the contact area will be between melt alloy and roller. A
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large melt alloy depth also leads to a greater rolling deformation on the sheet surface.
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Relative shear motion easily occurs between the interior and surface of the sheet with
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changing in the shear pressure. The cracks begin to form on the surface of sheet when
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relatively large shear motion occurs. The macro morphology of the sheet indicated that
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sheets with good quality surfaces were obtained with wider rolls gaps in the second
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group experiment.
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2) Cooling water flow
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In order to ensure faster cooling, the cooling water flow underneath the roller shell was
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increased leading to lower surface temperature of the roller. When the cooling water
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flow rate was 11.1m³/h, more heat was radiated from rolling region. Thus, it is
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anticipated that the amount of heat transferred with the flow rate of 11.1m³/h was much
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greater than that of 8.95m³/h water flow causes the temperature drop of the molten
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aluminum more quickly. Sheets with smaller grain sizes were obtained with faster 9
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cooling/solidification speed.
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3) Roll-casting speed
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Slow roll-casting speeds easily result in over-cooling of the aluminum alloy melt. The
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heat of melt aluminum alloy is rapidly dissipated at slow rolling speeds. Thus, the melt
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aluminum alloy will solidify before rolling in the casting region, and the solidified alloy
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will interrupt the casting process. In contrast, when the roll-casting speed is fast, the heat
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of melt aluminum alloy is slowly reduced. The slow cooling will prevent complete
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solidification at the center region of the sheet during rolling the sheet. This phenomenon
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eventually leads to a less smooth surface of the sheet and poor surface qualities. It is also
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easy to generate a "hot zone" which can lead to hot tearing of the sheets. Therefore, the
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speed must be selected within a suitable range, not too fast and too slow.
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4) Initial temperature of cooling water
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In the experiment, if the bigger temperature difference of initial cooling water will casus
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worse the cool condition of roller and lead inhomogeneous distribution of the transverse
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temperature for the strip. The reduction in initial cooling water temperature generates
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the spray on the roller and results in the pores on the strip surface.
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Based on the experimental results, the optimized process parameters were chosen as
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follows (Table 3).
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Table 3 Optimized TRC process parameters Parameter
Roll speed /m/min
Optimum values
11.4
Roll gap Cooling water flow /mm /m3/h 1.8
11.1
10
Initial cooling water temperature /ºC 20.8
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3.3 Microstructure observations
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190 191
Figure 4 Microstructures of sheets obtained using 1 mm roll gap, (a) edge region and
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(b) center region using the speed of 7.5m/min, (c) the edge region and (d) the center
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region using the speed of 11.4m/min
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Figures 4(a) and (b) show the microstructures of center and edge of the 7050 aluminum
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alloy sheet obtained with the cast-rolling parameters of 7.5 m/min, and roll gap of 1 mm.
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It can be seen that the microstructure of the center of surface consists of equiaxed
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dendrites, which is different from the longitudinal microstructure reported by Song et al.
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(2007). No second phase was observed within the center region of the sheets. There was
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no significant difference between the grain sizes of the edge and the central region of
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the sheet. Figures 4(c) and (d) show the microstructures of the rolling surface at different
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locations obtained with cast-rolling speed of 11.4 m/min, and roll gap of 1 mm. The
11
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grains sizes on the longitudinal edge of the sheet in vertical roll-casting direction were
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different. The grain size after different process parameters are listed in Table 4. A large
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number of second phase particles were observed in the interior of the grain and on the
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grain boundary. Small equiaxed grains were distributed in the central part of the sheet.
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Table 4 The grain size after different process parameters Roll casting speed ( m/min)
Roll gap (mm)
the edge region (µm)
the center region (µm)
7.5
1
49.1
46.3
11.4
1
37.9
30.6
7.5
1.8
35.8
34.3
11.4
1.8
32.5
30.6
207
As shown in Figure 4, the grains in the center of the cast-rolled sheet surfaces were small
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and uniformly distributed, while the grain sizes and distribution in the edges were not
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uniform. The discrepancy can be attributed to different cooling rates at different regions
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of the sheet during roll-casting process. The molten alloy at or close to the roll surface
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rapidly solidified. Consequently, the grains in the edges had insufficient time for full
212
growth, thus their size was small. On the other hand, the grains in the middle section of
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the cast-rolled sheet got sufficient time for growth, thus their sizes were considerably
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larger than that of the grains in the edges. Using same rolling speed and different gap
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thickness, the grain sizes were found to be smaller with increasing the rolling speed.
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This can be attributed to the rapid solidification of molten alloy in the roll-casting zone
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at increased rolling speeds. Over cooling was entirely avoided when the rolling speeds
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were matched with the solidifying speed, resulting in smaller and uniformly distributed
12
219
grains in the sheet. Based on the above results, the optimum roll-casting speed was
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chosen as 11.4m/min.
221
222 223
Figure 5 Microstructures of the sheets obtained using the roll gap of 1.8mm, (a) edge
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region and (b) center region using the speed of 7.5m/min, (c) edge region and (d)
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center region using the speed of 11.4m/min
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Figures 5(a) and (b) show the microstructures of center and edge of the 7050 aluminum
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alloy sheet prepared with the roll-casting speed of 7.5m/min and roll gap of 1.8mm. As
228
shown in Figure 5(a), the grains on the outer ring are a combination of small dendrites
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and isometric hybrid grains, with a visible boundary line between them. The central
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region of the surface also showed uniformly distributed small equiaxed grains (Figure
231
5(b)).
232
Figures 5(c) and (d) show the microstructures of center and edge of roll-casting sheet 13
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prepared at the rolling speed of 11.4m/min and roll gap of 1.8mm. Both regions contain
234
equiaxed grains without significant differences. The grain sizes were almost the same in
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the center and the edge, and the precipitation were more in the edge of the sheet than in
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the center.
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It is evident that the grain sizes were generally larger in Figure 4 than that in Figure 5.
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This can be attributed to different water flow cooling rates used in the two experiments.
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The higher the cooling water flow rate, the faster the temperature of the melted liquid
240
dropped. Thus, the faster the molten aluminum alloys solidifies, the smaller the grains
241
obtained. The grains were of homogeneous in the middle of the sheet than at the edge.
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This is due to the fact that more crystal nuclei were present in the middle. Further, the
243
crystallization time was shorter in the center region of the sheet. Hence, it was more
244
difficult for the grains to grow, resulting in small grain sizes in the middle of the sheet.
245
246 247
Figure 6 SEM micrographs and EDS spectrum of sheets obtained using the roll gap of 14
248
1.8 mm. The microstructures of edge region with the speed of (a) V=7.5m/min, (b)
249
V=11.4m/min
250
Figure 6 shows the SEM images and EDS of the 7050 aluminum alloy sheets obtained
251
with the roll gap of 1.8mm at different rolling speeds. As seen in Figure 6(a), bulk
252
precipitations were distributed along the grain boundaries and microns sized (0.5-10μm)
253
particles were observed. Most of the precipitated phases overlapped each other and
254
formed skeletons along the grain boundaries. The coarse second phase is characterized
255
as hard, brittle with no malleability/deformability. In the middle and late stages of roll-
256
casting process, the coarse second phases hardly deformed together with grain, so the
257
large stress concentration appeared around the second phases. Fractures commence from
258
the skeleton of second phases which get separated from the grain boundaries. Hence,
259
micro-cracks are initiated. The formation of micro-crack is harmful to mechanical
260
properties of high-strength aluminum alloys. The quantity and size of the precipitated
261
phases shown in Figure 6(c) are less than that in Figure 6(a). The second phases appeared
262
to be fine and uniformly distributed in grain boundaries.
263
EDS analysis confirmed that the particles in the grain boundaries are mainly Cu-rich.
264
The copper content (48.36 wt.%) in Figure 6(a) appeared to be higher than that Figure
265
6(d) (21.38 wt.%). The high copper content can reduce the toughness of the alloy and
266
increase the crack propagation rate. The Mg content appeared to be 1.68 and 1.61 wt.%
267
in Figure 6(b) and (d), respectively. The Cu combined with Mg and Al easily formed
268
Al2CuMg and Al2Cu phases. 15
269
3.4 Mechanical properties
270 271
Figure 7 Hardness of sheets obtained using different roll gaps and roll casting speeds
272
Figure 7 shows Vickers hardness of sheets obtained after two group experiments with
273
different roll gaps of 1 mm and 1.8 mm. Specimen A in the figure is for the edge part of
274
the sheet and specimen B is for the central part. It can be seen from Figure 7 that the
275
hardness of the sheet increased with increasing both the roll casting speed and the roll
276
gap widths. The maximum hardness value of 93 HV is obtained in the central parts of
277
the sheet when the roll-casting speed was 11.4m/min and roll gap was 1mm. When roll-
278
casting speed was 11.4/min and roll gap was 1.8mm, the maximum hardness of the
279
central region reached to 100 HV.
280
The results in Figure 7 show that a small roll gap results in higher hardness than a large
281
roll gap. Overall, the hardness results indicate that the performance of the aluminum
282
casting process can be improved by increasing both the roll gap and the roll casting
283
speed. The best mechanical properties in the central part of the studied aluminum sheet
284
were obtained using roll gap of 1.8mm and roll-casting speed of 11.4m/min.
16
285 286
Figure 8 Yield stress of strips obtained using different roll gaps and roll casting speeds
287
The yield stress of TRC strips using different roll casting speed and roll gaps are
288
presented in Figure 8. No remarkable difference was obtained varying with the roll
289
casting speed and roll gaps. However, yield stress increases with increasing the roll
290
casting speed and roll gaps enlargement. Maximum yield stress of 189MPa was obtained
291
for using the roll casting speed of 11.4 m/min (12r/min) and roll gaps of 1.8mm. Coarse
292
second phases and the amount of Cu segregated along the grain boundaries led to the
293
stress concentration and the toughness reduction.
294
4. Conclusions
295
The highlights of the results from this study are detailed below:
296
(1) The optimum process parameters for obtaining the best quality aluminum sheets were
297
found to be: roll gap of 1.8mm, roll-casting speed of 11.4m/min, cooling water flow rate
298
of 11.1m³/h and initial cooling water temperature of 20.8oC.
299
(2) The roll casting speed and roll gap were the main factors which influenced the
300
experimental results. At a constant roll-casting temperature, the grains tended to be
301
smaller and more uniform with increasing roll-casting speed and roll gap wideness. 17
302
(3) The hardness and yield stress of the sheet increased with increasing the roll-casting
303
speed and roll gap wideness. At a constant roll gap and roll casting speed, the hardness
304
of the center of the sheet was greater than that of the edge. The hardness of the central
305
part reached a maximum value of 100 HV using the roll gap of 1.8mm and roll casting
306
speed of 11.4m/min. The maximum yield stress of 189MPa was obtained for using the
307
roll casting speed of 11.4 m/min (12r/min) and roll gaps of 1.8mm.
308
Acknowledgements
309
This work was supported by the Natural Science Foundation of China (No. 51374128,
310
and No. 51404137) and the State Key Laboratory of Advanced Metals and Materials of
311
China (2012-ZD02). The author would like to thank Dr. Zhao HY for his assistance in
312
twin roll casting.
313
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Technology 212, 1670-1675.
355 356 357 358 359 360 361 362 363 20
364
Figure captions
365
Figure 1 Schematic of the twin-roll caster
366
Figure 2 Results of TRC experiments (a) for first group experiment (b) for second
367
group experiment
368
Figure 3 Macro morphology of sheets obtained after (a) first group experiment (b)
369
second group experiment
370
Figure 4 Microstructures of sheets obtained using 1 mm roll gap, (a) edge region and
371
(b) center region using the speed of 7.5m/min, (c) the edge region and (d) the center
372
region using the speed of 11.4m/min
373
Figure 5 Microstructures of the sheets obtained using the roll gap of 1.8mm, (a) edge
374
region and (b) center region using the speed of 7.5m/min, (c) edge region and (d)
375
center region using the speed of 11.4m/min
376
Figure 6 SEM micrographs and EDS spectrum of sheets obtained using the roll gap of
377
1.8 mm. The microstructures of edge region with the speed of (a) V=7.5m/min, (b)
378
V=11.4m/min
379
Figure 7 Hardness of sheets obtained using different roll gaps and roll casting speeds
380
Figure 8 Yield stress of strips obtained using different roll gaps and roll casting speeds
381 382 383 384 21
385
Table captions
386
Table 1 Experimental conditions for the twin roll casting process
387
Table 2 Twin roll casting process parameters used in the present study
388
Table 3 Optimized TRC process parameters
389
Table 4 The grain size after different process parameters
390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 22
406 407
Figure 1 Schematic diagram of the twin-roll caster
408 409
Figure 2 Results of TRC experiments (a) for first group experiment (b) for second
410
group experiment
411 412
Figure 3 Macro morphology of sheets obtained after (a) first group experiment (b)
413
second group experiment
414
23
415
416 417
Figure 4 Microstructures of sheets obtained using 1 mm roll gap, (a) edge region and
418
(b) center region using the speed of 7.5m/min, (c) the edge region and (d) the center
419
region using the speed of 11.4m/min
420 421 422 423 424 425 426
24
427
428 429
Figure 5 Microstructures of the sheets obtained using the roll gap of 1.8mm, (a) edge
430
region and (b) center region using the speed of 7.5m/min, (c) edge region and (d)
431
center region using the speed of 11.4m/min
432 433 434 435 436 437
25
438
439 440
Figure 6 SEM micrographs and EDS spectrum of sheets obtained using the roll gap of
441
1.8 mm. The microstructures of edge region with the speed of (a) V=7.5m/min, (b)
442
V=11.4m/min
443 444
Figure 7 Hardness of sheets obtained using different roll gaps and roll casting speeds
445
26
446 447
Figure 8 Yield stress of strips obtained using different roll gaps and roll casting speeds
448
27
S0924-0136(16)30049-8 http://dx.doi.org/doi:10.1016/j.jmatprotec.2016.02.016 PROTEC 14730
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
17-8-2015 22-1-2016 23-2-2016
Please cite this article as: Wang, Hongbin, Zhou, Le, Zhang, Yongwen, Cai, Yuanhua, Zhang, Jishan, Effects of twin-roll casting process parameters on the microstructure and sheet metal forming behavior of 7050 aluminum alloy.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2016.02.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Effects of twin-roll casting process parameters on the microstructure and sheet
2
metal forming behavior of 7050 aluminum alloy
3
Hongbin Wang1, Le Zhou1,*, Yongwen Zhang1, Yuanhua Cai2, Jishan Zhang2
4
(1. School of Material and Metallurgy, University of Science and Technology
5
Liaoning, Anshan 114051, China; 2. State Key Laboratory for Advanced Metals and
6
Materials, University of Science and Technology Beijing, Beijing 100083, China）
7
* Corresponding author Tel.: +86 412 5929299, +8618241278103
8
E-mail address: [email protected] (L. Zhou), [email protected] (H. Wang)
9
Postal address: School of Material and Metallurgy, University of Science and
10
Technology Liaoning, Qianshan Zhong Road 185, Anshan 114051, Liaoning, China
11
Graphical abstaract
12
13 14 15
The SEM images and EDS of the 7050 aluminum alloy sheets obtained with the roll gap
16
of 1.8mm at different rolling speeds. As seen in Figure 6(a), bulk precipitations were 1
17
distributed along the grain boundaries and microns sized (0.5-10μm) particles were
18
observed. Most of the precipitated phases overlapped each other and formed skeletons
19
along the grain boundaries. The coarse second phase is characterized as hard, brittle with
20
no malleability/deformability. In the middle and late stages of roll-casting process, the
21
coarse second phases hardly deformed together with grain, so the large stress
22
concentration appeared around the second phases. Fractures commence from the
23
skeleton of second phases which get separated from the grain boundaries. Hence, micro-
24
cracks are initiated. The formation of micro-crack is harmful to mechanical properties
25
of high-strength aluminum alloys.
26
EDS analysis confirmed that the particles in the grain boundaries are mainly Cu-rich.
27
The copper content (48.36 wt.%) in Figure 6(a) appeared to be higher than that Figure
28
6(d) (21.38 wt.%). The high copper content can reduce the toughness of the alloy and
29
increase the crack propagation rate. The Mg content appeared to be 1.68 and 1.61 wt.%
30
in Figure 6(b) and (d), respectively. The Cu combined with Mg and Al easily formed
31
Al2CuMg and Al2Cu phases.
32 33
34
Abstract
35
In the present study, strips of 7050 aluminum alloys were cast using twin roll casting
36
technology (TRC) where roll casting speed, roll gap, cooling water flow rate and initial
37
cooling water temperature were varied. The optimized process parameters and their 2
38
effects on TRC of 7050 aluminum alloys strips were investigated. The optimized process
39
parameters and properties were obtained to be roll gap of 1.8 mm, roll casting speed of
40
11.4m/min (12 r/min), cooling water flow rate of 11.1 m³/h and initial cooling water
41
temperature of 20.8oC. The macro morphology, microstructure, hardness and yield stress
42
of the strips were investigated using optical microscopy, scanning electron microscopy
43
(SEM), energy dispersion spectroscopy (EDS) and Vickers hardness. The homogeneous
44
microstructures and improved mechanical properties were obtained by increasing the
45
roll casting speed and roll gap thickness.
46
Keywords: Twin-roll casting; 7050 Aluminum alloy; Microstructure; Mechanical
47
properties; Process parameters
48
1. Introduction
49
Twin-roll casting (TRC) is an important technology for producing metal and alloy
50
sheets, which was first devised by Bessemer as a new steel making process in 1956. The
51
TRC technology has several advantages over the conventional casting methods, such as
52
low consumption of power and time. It combines casting and hot rolling together into a
53
single operation, thus ensuring an efficient process. In a typical TRC process, the molten
54
metal is fed into the gap between two internally water-cooled running rolls. Park and co-
55
authors (2007a) demonstrated that the cooling rate of the molten metal can reach up to
56
102 ºC/s during TRC processing. Hage et al. (2003) devised a twin roll caster for
57
aluminum alloys using high thermal conductivity roll and no lubricant, to investigate the
58
effects of these on the casting speed and mechanical properties. In their study, heat 3
59
transfer between melt and roll was improved using hydrostatic pressure of the melt and
60
the lubricant was not used to increase the casting speed. After solidification, the metal
61
undergoes a hot deformation to produce the final sheet, as reported by Ju and Hu (2006).
62
The process parameters of TRC significantly influence the quality of the sheets, as
63
reported by Mino and his co-authors (2006), particularly for the ultra-thin fast cast (Xiao
64
and Cai, 1999). It was found that finer microstructures decreased elemental segregation
65
(Watari et al. 2007), and higher solid super saturation rate of strips were obtained with
66
TRC (Park et al. 2007b). Fine grain (5 μm) sheets of AZ31 magnesium alloy strip were
67
obtained by Zhao et al. (2012) using an asymmetric twin-roll cast. Moreover, the quality
68
of the sheet is sensitive to the process parameters, such as the roll casting speed. Haga
69
and Suzuki (2001) proposed a melted drag twin-roll caster with a speed of 30 m/min for
70
producing sheets from AA3003, Al-6Si, and Al-12Si alloys. Haga et al. (2007) later
71
demonstrated a high speed twin-roll caster with speed of 60m/min for Al-Fe alloys.
72
Recently, Wang and Zhou (2014) examined the effects of surface conditions and
73
microstructure of AA1050 alloy strips on the surface quality of the produced sheets.
74
However, rarely research reports the TRC processing of 7050 aluminum alloy.
75
As one of the super high-strength aluminum alloys, 7050 aluminum alloy satisfies the
76
requirements of aerospace applications, which was reported by Heinz and his co-authors
77
(2000). Later, Clark and Johnson (2003) used 7050 aluminum alloy sheet as the parts in
78
several aircrafts such as the airliner of Boeing-777, military jet fighter F/A-18 Hornet,
79
and F-22 Raptor. 4
80
However, to date, there are no reports on the optimization of process parameters during
81
TRC preparation of aluminum alloys sheets, especially for high-strength 7050 aluminum
82
alloy. Thus, the aim of this study was to investigate the effects of TRC process
83
parameters on the microstructure and sheet metal forming behavior of the 7050
84
aluminum alloy. Moreover, the TRC process parameters were optimized for producing
85
high quality aluminum sheets.
86
2. Experimental procedure
87
A vertical type twin-roll caster was used in this study. A schematic diagram of the TRC
88
setup is shown in Figure 1. Copper rollers with a diameter of 300 mm and equipped with
89
an internal water-cooling system were employed. The width of the roll gap can be
90
controlled by a hydraulic system. Immediately prior to rolling, the 7050 aluminum alloy
91
was superheated at a temperature of approximately 20~30 K which is above its melting
92
point. The molten aluminum alloy was then poured onto the cold surfaces of rollers
93
through the nozzle. Because of the wider solidification rage of high alloy content
94
aluminum alloy, the detailed experimental parameters are shown in Table 1 which
95
according a series of experiments results.
96 5
97 98
Figure 1 Schematic diagram of the twin-roll caster Table 1 Experimental conditions for the twin roll casting process Roller parameters
Copper, D=300 mm, W=100 mm, 20~30 K, above the melting point of Al alloy 7.5 and 11.4 m/min (8 and 12 r/min) 1 and 1.8 mm Al-5.89Zn-2.16Mg-2.1Cu (wt.%)
Heating temperature Roll-casting speed Roll gap Aluminum alloy
99
In order to optimize casting parameters and to prepare sheets with high quality surfaces,
100
different casting speeds and casting roll gaps were evaluated in this study. The roller
101
speeds of 7.5m/min (8 r/min) and 11.4m/min ( 12 r/min), and the roll gap widths of 1
102
mm and 1.8 mm were selected as the process parameters to investigate their influences
103
on the sheet quality. The experimental process parameters used in the study are shown
104
in Table 2. Processes A and B were termed as group 1 experiment with the gap of 1 mm,
105
while processes C and D were termed as group 2 experiment with the roll gap of 1.8
106
mm. At the beginning of each group experiment, the roll speed was 7.5m/min (Process
107
A and Process C) for 100s-120s, and then accelerated to 11.4m/min (Process B and
108
Process D), which was then maintained until the end of the experiments. The thickness
109
of the roll-casting sheet was recorded during the entire experimental process.
110
Table 2 Twin roll casting process parameters used in the present study
111
Parameter
Roll speed (r/min)
Roll gap (mm)
Cooling water flow (m3/h)
Initial cooling water temperature (oC)
Process A
7.5
1
8.95
18.6
Process B
11.4
1
8.95
18.6
Process C Process D
7.5 11.4
1.8 1.8
11.1 11.1
20.8 20.8
* The cooling water flow of 8.95 m3/h and 11.1 m3/h were attained by experiments. 6
112
3. Results and discussion
113
3.1 Optimization of TRC process parameters
114
Results of two group experiments are shown in Figures 2(a) and (b). The speed of TRC
115
was 7.5m/min at the beginning of the experiment and then was accelerated to 11.4m/min
116
after steady casting for 100-120s in both experiments. In this study, casting speed was
117
changed to compare the thickness variations in roll-casting sheets under the same
118
conditions. The sheet thickness was found to be approximately 3-5 mm with TRC speed
119
of 7.5m/min (Figure 2(a)), while it was 2-4 mm at the TRC speed of 11.4m/min. It is
120
evident that the thickness of the TRC sheet varies with roll-casting speed. As shown in
121
Figure 2(b), the thickness of the TRC sheet did not fluctuate much. At the TRC speed of
122
7.5m/min and 11.4m/min, the sheet thickness was 3.5-4.5 and 2-3.5 mm, respectively,
123
indicating a reasonably uniform thickness.
124 125
Figure 2 Results of TRC experiments (a) for first group experiment (b) for second
126
group experiment
127
As shown in Figure 2, the sheet thickness decreased as the roll-casting speed increased.
128
If the product/multiplication of roll-casting speed and thickness of the sheet remains
129
constant, the following relationship is obtained: 7
K v h1
130 131
K ——productivity constant;
132
v ——roll-casting speed, mm/min;
133
h1 ——sheet thickness, mm.
(1)
134
However, the product of the casting speed and sheet thickness was found to be within a
135
certain range in the present study. The sheet thickness will be smaller at higher thermal
136
conductivity of the roller shell material and roll casting speed. The same conclusion was
137
given by Hou et al. (2010), who commented that for producing a higher thickness of the
138
roll casting sheet, a lower speed would be more suitable.
139
3.2 Observations of macro morphology
140
As shown in Figure 3(a), the aluminum sheets had broken into pieces, and surface of the
141
sheets was not smooth following first group experiment. On the other hand, it can be
142
seen in Figure 3(b) that the sheet from second group experiment remains in a good shape
143
without breaking and its surface quality was significantly better than the one after first
144
group experiment.
145 146
Figure 3 Macro morphology of sheets obtained after (a) first group experiment (b)
147
second group experiment 8
148
From the point of view of the process parameters, these experimental observations can
149
be explained using several key factors described below:
150
1) Roll gap
151
The roll gap thickness controls the amount of melted aluminum accommodation between
152
the rollers. For the same volume of melted aluminum, a wider roll gap results in less
153
depth of the melted cavity. According to Xiao and Cai (1999), the factors that cause
154
increased the depth of melt alloy lead to a tendency to crack. This is because the greater
155
the melt alloy depth, the larger the contact area will be between melt alloy and roller. A
156
large melt alloy depth also leads to a greater rolling deformation on the sheet surface.
157
Relative shear motion easily occurs between the interior and surface of the sheet with
158
changing in the shear pressure. The cracks begin to form on the surface of sheet when
159
relatively large shear motion occurs. The macro morphology of the sheet indicated that
160
sheets with good quality surfaces were obtained with wider rolls gaps in the second
161
group experiment.
162
2) Cooling water flow
163
In order to ensure faster cooling, the cooling water flow underneath the roller shell was
164
increased leading to lower surface temperature of the roller. When the cooling water
165
flow rate was 11.1m³/h, more heat was radiated from rolling region. Thus, it is
166
anticipated that the amount of heat transferred with the flow rate of 11.1m³/h was much
167
greater than that of 8.95m³/h water flow causes the temperature drop of the molten
168
aluminum more quickly. Sheets with smaller grain sizes were obtained with faster 9
169
cooling/solidification speed.
170
3) Roll-casting speed
171
Slow roll-casting speeds easily result in over-cooling of the aluminum alloy melt. The
172
heat of melt aluminum alloy is rapidly dissipated at slow rolling speeds. Thus, the melt
173
aluminum alloy will solidify before rolling in the casting region, and the solidified alloy
174
will interrupt the casting process. In contrast, when the roll-casting speed is fast, the heat
175
of melt aluminum alloy is slowly reduced. The slow cooling will prevent complete
176
solidification at the center region of the sheet during rolling the sheet. This phenomenon
177
eventually leads to a less smooth surface of the sheet and poor surface qualities. It is also
178
easy to generate a "hot zone" which can lead to hot tearing of the sheets. Therefore, the
179
speed must be selected within a suitable range, not too fast and too slow.
180
4) Initial temperature of cooling water
181
In the experiment, if the bigger temperature difference of initial cooling water will casus
182
worse the cool condition of roller and lead inhomogeneous distribution of the transverse
183
temperature for the strip. The reduction in initial cooling water temperature generates
184
the spray on the roller and results in the pores on the strip surface.
185
Based on the experimental results, the optimized process parameters were chosen as
186
follows (Table 3).
187
Table 3 Optimized TRC process parameters Parameter
Roll speed /m/min
Optimum values
11.4
Roll gap Cooling water flow /mm /m3/h 1.8
11.1
10
Initial cooling water temperature /ºC 20.8
188
3.3 Microstructure observations
189
190 191
Figure 4 Microstructures of sheets obtained using 1 mm roll gap, (a) edge region and
192
(b) center region using the speed of 7.5m/min, (c) the edge region and (d) the center
193
region using the speed of 11.4m/min
194
Figures 4(a) and (b) show the microstructures of center and edge of the 7050 aluminum
195
alloy sheet obtained with the cast-rolling parameters of 7.5 m/min, and roll gap of 1 mm.
196
It can be seen that the microstructure of the center of surface consists of equiaxed
197
dendrites, which is different from the longitudinal microstructure reported by Song et al.
198
(2007). No second phase was observed within the center region of the sheets. There was
199
no significant difference between the grain sizes of the edge and the central region of
200
the sheet. Figures 4(c) and (d) show the microstructures of the rolling surface at different
201
locations obtained with cast-rolling speed of 11.4 m/min, and roll gap of 1 mm. The
11
202
grains sizes on the longitudinal edge of the sheet in vertical roll-casting direction were
203
different. The grain size after different process parameters are listed in Table 4. A large
204
number of second phase particles were observed in the interior of the grain and on the
205
grain boundary. Small equiaxed grains were distributed in the central part of the sheet.
206
Table 4 The grain size after different process parameters Roll casting speed ( m/min)
Roll gap (mm)
the edge region (µm)
the center region (µm)
7.5
1
49.1
46.3
11.4
1
37.9
30.6
7.5
1.8
35.8
34.3
11.4
1.8
32.5
30.6
207
As shown in Figure 4, the grains in the center of the cast-rolled sheet surfaces were small
208
and uniformly distributed, while the grain sizes and distribution in the edges were not
209
uniform. The discrepancy can be attributed to different cooling rates at different regions
210
of the sheet during roll-casting process. The molten alloy at or close to the roll surface
211
rapidly solidified. Consequently, the grains in the edges had insufficient time for full
212
growth, thus their size was small. On the other hand, the grains in the middle section of
213
the cast-rolled sheet got sufficient time for growth, thus their sizes were considerably
214
larger than that of the grains in the edges. Using same rolling speed and different gap
215
thickness, the grain sizes were found to be smaller with increasing the rolling speed.
216
This can be attributed to the rapid solidification of molten alloy in the roll-casting zone
217
at increased rolling speeds. Over cooling was entirely avoided when the rolling speeds
218
were matched with the solidifying speed, resulting in smaller and uniformly distributed
12
219
grains in the sheet. Based on the above results, the optimum roll-casting speed was
220
chosen as 11.4m/min.
221
222 223
Figure 5 Microstructures of the sheets obtained using the roll gap of 1.8mm, (a) edge
224
region and (b) center region using the speed of 7.5m/min, (c) edge region and (d)
225
center region using the speed of 11.4m/min
226
Figures 5(a) and (b) show the microstructures of center and edge of the 7050 aluminum
227
alloy sheet prepared with the roll-casting speed of 7.5m/min and roll gap of 1.8mm. As
228
shown in Figure 5(a), the grains on the outer ring are a combination of small dendrites
229
and isometric hybrid grains, with a visible boundary line between them. The central
230
region of the surface also showed uniformly distributed small equiaxed grains (Figure
231
5(b)).
232
Figures 5(c) and (d) show the microstructures of center and edge of roll-casting sheet 13
233
prepared at the rolling speed of 11.4m/min and roll gap of 1.8mm. Both regions contain
234
equiaxed grains without significant differences. The grain sizes were almost the same in
235
the center and the edge, and the precipitation were more in the edge of the sheet than in
236
the center.
237
It is evident that the grain sizes were generally larger in Figure 4 than that in Figure 5.
238
This can be attributed to different water flow cooling rates used in the two experiments.
239
The higher the cooling water flow rate, the faster the temperature of the melted liquid
240
dropped. Thus, the faster the molten aluminum alloys solidifies, the smaller the grains
241
obtained. The grains were of homogeneous in the middle of the sheet than at the edge.
242
This is due to the fact that more crystal nuclei were present in the middle. Further, the
243
crystallization time was shorter in the center region of the sheet. Hence, it was more
244
difficult for the grains to grow, resulting in small grain sizes in the middle of the sheet.
245
246 247
Figure 6 SEM micrographs and EDS spectrum of sheets obtained using the roll gap of 14
248
1.8 mm. The microstructures of edge region with the speed of (a) V=7.5m/min, (b)
249
V=11.4m/min
250
Figure 6 shows the SEM images and EDS of the 7050 aluminum alloy sheets obtained
251
with the roll gap of 1.8mm at different rolling speeds. As seen in Figure 6(a), bulk
252
precipitations were distributed along the grain boundaries and microns sized (0.5-10μm)
253
particles were observed. Most of the precipitated phases overlapped each other and
254
formed skeletons along the grain boundaries. The coarse second phase is characterized
255
as hard, brittle with no malleability/deformability. In the middle and late stages of roll-
256
casting process, the coarse second phases hardly deformed together with grain, so the
257
large stress concentration appeared around the second phases. Fractures commence from
258
the skeleton of second phases which get separated from the grain boundaries. Hence,
259
micro-cracks are initiated. The formation of micro-crack is harmful to mechanical
260
properties of high-strength aluminum alloys. The quantity and size of the precipitated
261
phases shown in Figure 6(c) are less than that in Figure 6(a). The second phases appeared
262
to be fine and uniformly distributed in grain boundaries.
263
EDS analysis confirmed that the particles in the grain boundaries are mainly Cu-rich.
264
The copper content (48.36 wt.%) in Figure 6(a) appeared to be higher than that Figure
265
6(d) (21.38 wt.%). The high copper content can reduce the toughness of the alloy and
266
increase the crack propagation rate. The Mg content appeared to be 1.68 and 1.61 wt.%
267
in Figure 6(b) and (d), respectively. The Cu combined with Mg and Al easily formed
268
Al2CuMg and Al2Cu phases. 15
269
3.4 Mechanical properties
270 271
Figure 7 Hardness of sheets obtained using different roll gaps and roll casting speeds
272
Figure 7 shows Vickers hardness of sheets obtained after two group experiments with
273
different roll gaps of 1 mm and 1.8 mm. Specimen A in the figure is for the edge part of
274
the sheet and specimen B is for the central part. It can be seen from Figure 7 that the
275
hardness of the sheet increased with increasing both the roll casting speed and the roll
276
gap widths. The maximum hardness value of 93 HV is obtained in the central parts of
277
the sheet when the roll-casting speed was 11.4m/min and roll gap was 1mm. When roll-
278
casting speed was 11.4/min and roll gap was 1.8mm, the maximum hardness of the
279
central region reached to 100 HV.
280
The results in Figure 7 show that a small roll gap results in higher hardness than a large
281
roll gap. Overall, the hardness results indicate that the performance of the aluminum
282
casting process can be improved by increasing both the roll gap and the roll casting
283
speed. The best mechanical properties in the central part of the studied aluminum sheet
284
were obtained using roll gap of 1.8mm and roll-casting speed of 11.4m/min.
16
285 286
Figure 8 Yield stress of strips obtained using different roll gaps and roll casting speeds
287
The yield stress of TRC strips using different roll casting speed and roll gaps are
288
presented in Figure 8. No remarkable difference was obtained varying with the roll
289
casting speed and roll gaps. However, yield stress increases with increasing the roll
290
casting speed and roll gaps enlargement. Maximum yield stress of 189MPa was obtained
291
for using the roll casting speed of 11.4 m/min (12r/min) and roll gaps of 1.8mm. Coarse
292
second phases and the amount of Cu segregated along the grain boundaries led to the
293
stress concentration and the toughness reduction.
294
4. Conclusions
295
The highlights of the results from this study are detailed below:
296
(1) The optimum process parameters for obtaining the best quality aluminum sheets were
297
found to be: roll gap of 1.8mm, roll-casting speed of 11.4m/min, cooling water flow rate
298
of 11.1m³/h and initial cooling water temperature of 20.8oC.
299
(2) The roll casting speed and roll gap were the main factors which influenced the
300
experimental results. At a constant roll-casting temperature, the grains tended to be
301
smaller and more uniform with increasing roll-casting speed and roll gap wideness. 17
302
(3) The hardness and yield stress of the sheet increased with increasing the roll-casting
303
speed and roll gap wideness. At a constant roll gap and roll casting speed, the hardness
304
of the center of the sheet was greater than that of the edge. The hardness of the central
305
part reached a maximum value of 100 HV using the roll gap of 1.8mm and roll casting
306
speed of 11.4m/min. The maximum yield stress of 189MPa was obtained for using the
307
roll casting speed of 11.4 m/min (12r/min) and roll gaps of 1.8mm.
308
Acknowledgements
309
This work was supported by the Natural Science Foundation of China (No. 51374128,
310
and No. 51404137) and the State Key Laboratory of Advanced Metals and Materials of
311
China (2012-ZD02). The author would like to thank Dr. Zhao HY for his assistance in
312
twin roll casting.
313
5. References
314
Clark, D.A., Johnson, W.S., 2003. Temperature effects on fatigue performance of cold
315
expanded holes in 7050-T7451 aluminum alloy. International Journal of Fatigue 25, 159-
316
165.
317
Haga, T., Ikawa, M., Watari, H., Kumai, S., 2007. High speed twin roll casting of
318
recycled Al-3Si-0.6Mg strip. Journal of Achievements in Materials and Manufacturing
319
Engineering 21, 7-12.
320
Haga, T., Suzuki, S., 2001. A high speed twin roll caster for aluminum alloy strip. Journal
321
of Materials Processing Technology 113, 291-295. 18
322
Haga, T., Takahashi, K., Ikawa, M., Watari, H., 2003. A vertical-type twin roll caster for
323
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Figure captions
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Figure 1 Schematic of the twin-roll caster
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Figure 2 Results of TRC experiments (a) for first group experiment (b) for second
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group experiment
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Figure 3 Macro morphology of sheets obtained after (a) first group experiment (b)
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second group experiment
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Figure 4 Microstructures of sheets obtained using 1 mm roll gap, (a) edge region and
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(b) center region using the speed of 7.5m/min, (c) the edge region and (d) the center
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region using the speed of 11.4m/min
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Figure 5 Microstructures of the sheets obtained using the roll gap of 1.8mm, (a) edge
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region and (b) center region using the speed of 7.5m/min, (c) edge region and (d)
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center region using the speed of 11.4m/min
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Figure 6 SEM micrographs and EDS spectrum of sheets obtained using the roll gap of
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1.8 mm. The microstructures of edge region with the speed of (a) V=7.5m/min, (b)
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V=11.4m/min
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Figure 7 Hardness of sheets obtained using different roll gaps and roll casting speeds
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Figure 8 Yield stress of strips obtained using different roll gaps and roll casting speeds
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Table captions
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Table 1 Experimental conditions for the twin roll casting process
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Table 2 Twin roll casting process parameters used in the present study
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Table 3 Optimized TRC process parameters
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Table 4 The grain size after different process parameters
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406 407
Figure 1 Schematic diagram of the twin-roll caster
408 409
Figure 2 Results of TRC experiments (a) for first group experiment (b) for second
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group experiment
411 412
Figure 3 Macro morphology of sheets obtained after (a) first group experiment (b)
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second group experiment
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23
415
416 417
Figure 4 Microstructures of sheets obtained using 1 mm roll gap, (a) edge region and
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(b) center region using the speed of 7.5m/min, (c) the edge region and (d) the center
419
region using the speed of 11.4m/min
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24
427
428 429
Figure 5 Microstructures of the sheets obtained using the roll gap of 1.8mm, (a) edge
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region and (b) center region using the speed of 7.5m/min, (c) edge region and (d)
431
center region using the speed of 11.4m/min
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25
438
439 440
Figure 6 SEM micrographs and EDS spectrum of sheets obtained using the roll gap of
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1.8 mm. The microstructures of edge region with the speed of (a) V=7.5m/min, (b)
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V=11.4m/min
443 444
Figure 7 Hardness of sheets obtained using different roll gaps and roll casting speeds
445
26
446 447
Figure 8 Yield stress of strips obtained using different roll gaps and roll casting speeds
448
27