- Email: [email protected]
The impact of melt conditioning on microstructure, texture and ductility of twin roll cast aluminium alloy strips
Author’s Accepted Manuscript The Impact of Melt Conditioning on Microstructure, Texture and Ductility of Twin Roll Cast Aluminium Alloy Strips N.S. Barekar, S Das, X. Yang, Y. Huang, Omer El Fakir, A.G. Bhagurkar, L. Zhou, Z. Fan www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(15)30538-4 http://dx.doi.org/10.1016/j.msea.2015.10.079 MSA32930
To appear in: Materials Science & Engineering A Received date: 21 September 2015 Revised date: 19 October 2015 Accepted date: 20 October 2015 Cite this article as: N.S. Barekar, S Das, X. Yang, Y. Huang, Omer El Fakir, A.G. Bhagurkar, L. Zhou and Z. Fan, The Impact of Melt Conditioning on Microstructure, Texture and Ductility of Twin Roll Cast Aluminium Alloy S t r i p s , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.10.079 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 galley proof before it is published in its final citable 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.
The Impact of Melt Conditioning on Microstructure, Texture and Ductility of Twin Roll Cast Aluminium Alloy Strips N. S. Barekara,*, S. Dasa, X. Yanga, Y. Huanga, Omer El Fakirb, A. G. Bhagurkara, L. Zhoua, Z. Fana a
The EPSRC Centre ‐ LiME, BCAST,Brunel University,Uxbridge UB8 3PH, UK
b
Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
* Corresponding author: [email protected],
Abstract: Twin roll casting (TRC) is recognised as an effective processing route for producing low cost Al sheet. However, the quality of the Al alloys sheets produced by the TRC process is limited by the formation of centre-line segregation which prevents its application in a wide range of engineering sectors. To improve the quality of the TRC strips, a new technology, melt conditioning twin roll casting (MC-TRC) has been developed from the industrial point of view. Enhanced nucleation by melt conditioning favours the advance of an equiaxed solidification front which results in reduced sump depth and a refined, uniform, defect free microstructure. The results obtained show that the differences in microstructure and in the crystallographic texture have a significant impact on ductility (22 % improvement) of the MC-TRC sheets compared to TRC sheets. Keywords: Twin roll casting, Solidification, Segregation, Rolling, Deformation
Introduction: Wrought aluminium alloys in sheet form are of interest for applications in the automotive industry because of their low density, tensile properties, formability, ease of recycling and high corrosion resistance [1]. These properties requirements are primarily by a number of Al-Mg alloys (5xxx series). Conventionally, metal sheet is produced by direct chill (DC) casting of thick slabs, followed by extensive thermo-mechanical processing, which includes homogenisation, hot rolling and cold rolling, leading to high energy consumption, low material yield and inevitably high cost. Whereas, twin roll casting (TRC) represents a paradigm shift in metal sheet production via minimising the number of processing steps by integrating casting and hot rolling into a single operation for the production of near-to-net-shape alloy strips [2]. Research on the TRC of aluminium alloys has been focused mainly
1
on process - microstructure relationship [3], defects in TRC sheets [4-6], modification of TRC setup [2, 7], texture assessment [8-10], and productivity [11]. However, TRC is still facing great challenges, which includes non-uniform microstructure formation, severe chemical segregation at the centre-line and a limited number of usable alloys (mainly dilute alloys with narrow freezing ranges). Aluminium alloy strip produced by conventional twin-roll casting requires additional processing including homogenization, rolling, and annealing to achieve desired properties required for application in the automotive industry. Due to the thickness constraints associated with twin-roll casting, the possibility of modifying the as-cast microstructures and formability before final shape forming and application is somewhat limited. The centre-line segregation is barely altered by downstream processing [8, 12] and hinder the mechanical performance of the final products. Therefore, grain refinement and defect reduction of as cast strips during twin-roll casting is of paramount importance. In order to minimise the internal defects and to improve the quality of the casting strips, a grain refiner can be added directly into the launder during casting. In industrial practice, Al-Ti-B master alloys are commonly used as grain refiners for the wrought Al alloys. However, only less than 1% of added TiB2 particles are active for the nucleation of α-Al grains [13]. The excess TiB2 particles have tendency to agglomerate and are detrimental in the final microstructure [14], particularly for the foil and thin strip products. Recently, it has been demonstrated that a fine and uniform microstructure can be achieved by the application of physical field such as intensive melt shearing [15]. This was attributed to the enhanced heterogeneous nucleation by naturally occurring oxides in the liquid Al alloys. The concept of melt conditioning by intensive shear prior to TRC has been previously documented [16, 17], however efforts are still required to make it industrially viable. Apart from the solidification features, the impact of melt conditioned twin roll casting (MC-TRC) on crystallographic texture and tensile properties of aluminium alloy sheets has not to date attracted sufficient attention. Furthermore, the behaviour of casting micro-defects, hot deformation aspect of the MC-TRC, the evolution of the texture during subsequent hot or cold rolling and its effect on tensile properties of AlMg alloy MC-TRC sheets are still not understood. Therefore, the present work was undertaken to investigate these aspect and to assist in developing an industrially viable MC-TRC process for aluminium alloys. The investigation was conducted to gain a comprehensive understanding of the principles that govern the microstructural and texture evolution of Al-Mg strips during MC-TRC.
2
Furthermore efforts were also made to analyse the impact of the as-cast microstructures and texture developed during casting and rolling of MC-TRC strips on tensile properties.
Experimental MC-TRC constitutes integration of a melt conditioning unit [18] with TRC as shown in figure 1. In the present process development, a rotor-stator device was employed for intensive shearing of the melt. A more detailed description of fluid flow in the high shear device can be found elsewhere [18]. The main advantages of the high shear device include the enhancement of kinetics for in situ chemical reactions, homogenisation of chemical composition and forced wetting of usually difficult to wet solid particles in the liquid metal. Moreover, the high shear device can be used for physical grain refinement by dispersing naturally occurring oxides [18].
Figure 1. Schematic illustration of the MC-TRC process
Tundish Design To make the MC-TRC process industrially viable, this investigative study has been associated with a technical challenge to design a fit-for-purpose tundish having a separate chamber to facilitate continuous shearing and stable delivery of the required superheated melt to the roll bite for casting the strip. Figure 2 (a) and (b) shows a section view of the tundish with high shear device inserted and top view of the tundish, respectively. The tundish is divided into three chambers namely, pouring, shearing and delivery. A baffle was placed at entry to the shearing chamber to prevent incoming liquid from bypassing the shear zone. Another baffle was positioned at the exit of the shearing chamber to avoid turbulence created by the rotor and to provide enough residence time and the supply of
3
conditioned melt. The optimised conditioned is realized when adequate melt circulation is achieved and stagnant zones are avoided.
Figure 2. The details of the tundish design showing (a) section view of ceramic tundish and the rotor®
stator device (b) top view of the tundish. The tundish was made from N17 machinable ceramic. All the dimensions are in mm.
Melt Conditioned Twin Roll Casting (MC-TRC) In conventional TRC, if the solute content is more than 3 wt.% (theoretically), it is difficult to cast an aluminium alloy strip without segregation occurring. However, in practice the solute content limit is 2.5 wt.% [19]. It has been demonstrated that the melt conditioning prior to TRC increases the limit on solute content of Al-Mg alloys that can be twin roll cast without severe segregation [20]. In the current work, the experiments were designed for 3 wt. % of Mg. 10 kg of binary alloys were melted in clay graphite crucibles in an electric resistance furnace at 710 °C. A series of static experiments with
4
shearing in the ceramic tundish (without integrating with TRC) were performed to determine the pouring temperature of the melt in order to obtain appropriate melt temperature at the entry of the roll bite (5-10 K superheat, Tmelting point ≈ 645°C). A laboratory scale horizontal twin roll caster was used to produce the alloy strip. The water cooled steel rolls are 318 mm in diameter and 350 mm in width. Before each experiment, the rolls were cleaned. The casting speed, the setback and the strip thickness were fixed to 1 m/min, ~ 43 mm and ~ 5 mm, respectively. Once the melt reached the desired pouring temperature (55-60 K superheat), it was poured into the preheated tundish for melt conditioning followed by strip casting. The melt shearing was carried out at 5000 rpm. For comparison, Al-3Mg alloy strips were produced by conventional TRC (without melt conditioning). The twin roll casting process parameters such as roll gap, setback, casting speed, melt temperature at the entry of the roll bite were the same for TRC and MC-TRC experiments.
Downstream Processing and Mechanical Testing The Al-3Mg alloy strips produced using the MC-TRC and the conventional TRC process were deformed by hot and cold rolling to give a final thickness of 1.7 ± 0.1 mm prior to tensile testing. In order to compare the effect of hot and cold rolling, the final thickness after a total rolling reduction of 66 % was achieved either by one pass of hot rolling followed by second pass of cold rolling or by two passes of cold rolling. After rolling, the Al-3Mg alloy strips were heat treated at 250 °C for 1 hour to relieve the potential residual stresses generated during processing and followed by water quenching. The dog-bone shaped tensile test specimens were machined from these homogenised strips. The tensile tests were conducted for at least 4 samples on a Gleeble 3800 thermo-mechanical simulator. -1
The strain rate was set to 1s which is representative of typical strain rates in forming processes for aluminium sheets and the tests were conducted at room temperature till failure. Load was measured with a load cell and the strain was obtained using a C Gauge transducer. The dimensions of the tensile test specimen and the details of the tensile testing machine can be found elsewhere [21].
Microstructure and Texture Characterization The specimens for metallography were mechanically polished and anodized at 20 V for 45 seconds in Barker’s reagent to reveal the grain structure. Figure 3 depicts the schematic illustration of sample selection for microstructure and texture analysis. Microstructural analysis was performed with A Zeiss
5
Axio Vision optical microscope on the transverse and longitudinal cross-sections of the MC-TRC strips. The specimens for texture characterization were cut from the middle of the strip along the casting or rolling direction, and all examinations were carried out on the longitudinal plane or TD plane. Electro polishing at 12 V in a solution of 30% V/V nitric acid in ethanol at 243 K (–30 °C) for 60 seconds was used to prepare surfaces for electron backscatter diffraction (EBSD). EBSD was carried out using a Zeiss Supra 35 field-emission gun (FEG) scanning electron microscope operating at 20 kV and equipped with an EBSD facility. The measured EBSD data, from which texture information was also obtained, were analysed using the TEAM and OIM proprietary software.
Figure 3. Schematic illustration of sample selection for microstructure and texture analysis
6
Result and Discussion The As-Cast Microstructures
Figure 4 - Through-thickness microstructures from the transverse cross-sections of as-cast strips produced by (a) TRC showing coarse grain structure and produced by (b) MC-TRC showing uniform grain structure and no centre-line segregation. MC-TRC was carried out under the shearing and casting speed of 5000 rpm and 1 m/min respectively. Typical optical micrographs of the microstructures obtained by TRC and MC-TRC are shown in Figure 4. The polarized-light micrographs which include the full strip thickness show that the microstructure produced by MC-TRC is uniform, significantly refined, and almost equiaxed throughout the cross section. In contrast, the TRC microstructure is dominated by a coarse dendrite structure with large, equiaxed grains. The grain size was measured using polarized-light micrographs with strong orientation contrast. The average grain size in ND-TD plane of the MC-TRC strip was approximately 222 ± 65 μm, compared with 400 ± 150 μm for the TRC strip. The microstructural examination revealed that the MC-TRC strip was free from centreline segregation, whereas segregation was found at most of the centreline of TRC strip. Such channel segregates which are low melting point regions oriented in the casting direction are clearly seen in the longitudinal cross-section of TRC strips (Figure 5). Similar features have been reported by Lockyer et al. [4] and Yun et al. [6] in the twin roll cast aluminium alloys. The through thickness composition variation of Mg in Al-Mg alloy strips produced by
7
TRC and MC-TRC process were examined by EDX analysis, and the results are presented in figure 6. The results demonstrate that the MC-TRC process is capable of producing strips with uniform solute distribution which is in good agreement with the microstructural analysis. In the case of TRC, the grains nucleate near the roll surface and grow towards the centre of the strip. During growth these grains reject the solute atoms towards the centre of the strip, which reduces the liquidus temperature of the melt at the centre of the strip. These solidification and thermal conditions lead to the solute enrichment and channel segregates.
Figure 5. Through-thickness microstructures from the longitudinal cross-sections of as-cast strips produced by TRC showing the centreline segregation. In the conventional TRC process the solute is pushed towards the centre of the strip causing an increase in solute concentration in the last solidifying liquid.
8
Figure 6. Elemental composition of the solute across the strip thickness for Al-3Mg TRC and MC-TRC samples. Solute distribution is more uniform in case of the MC-TRC compared with the TRC sample. In practice, due to the high affinity between oxygen and Al, the oxidation on the surface of Al alloy melts is inevitable when they are exposed to ambient atmosphere. The oxides formed at the surface of the melt are readily entrained into the casting by the turbulence of melt handling such as stirring and pouring [22]. These oxides may exist either in the form of oxide films/ bifilms and/or as discrete oxide particles [23]. The intensive melt shearing breaks the oxide bifilms as well as oxide particle clusters and disperses them uniformly throughout the liquid. It has been documented that the oxide formed in Al–Mg alloys under normal melting conditions is MgAl2O4, which displays an equiaxed and faceted morphology with {111} planes exposed as its natural surfaces. MgAl2O4 and α-Al, both have the same crystal structure and closely matched atomic spacing (lattice misfit of ~ 1.4%) [15]. According to the crystallographic matching criteria for potent nucleation [24], MgAl 2O4 particles can act as potent nucleation substrates for the primary α-Al phase if there is no more potent nucleating particles present in the alloy melt. Large numbers of such potent oxide particles acting as nucleation sites can enhance the heterogeneous nucleation leading to fine, equiaxed grain structure in MC-TRC strip and the solute segregation towards the centre of the strip is insignificant.
9
Figure 7. Optical micrographs of the as-cast microstructures produced by (a) TRC and (b) MC-TRC. TRC strip shows the features of banded structure (inhomogeneity in grain structure). Typical optical micrographs of the microstructures obtained by TRC and MC-TRC are shown in Figure 7. As can be seen from Figure 7 (a), there is a sudden change midway through the sample and the TRC sheet can be divided into inner and outer bands. Small regions of the inner band have a much coarser structure. As mentioned earlier, the solute enrichment reduces the liquidus temperature of the melt at the centre of the TRC strip. It has been suggested that the sharp division between the outer and central band is a result of the thixotropic nature of the semi-solid metal [4]. Once deformed, the semi-solid material in the central band remains soft until it reaches a much lower temperature compared to grains in the outer band. The central region of the strip appears to be pushed back towards the liquid as a result of the deformation process [6]. On the other hand, no such banded structure was observed in the case of MC-TRC strip (Figure 7 (b)). This is attributed to the enhanced heterogeneous nucleation by oxide particles. Figure 8 shows the grain morphologies with grain boundaries for TRC and MC-TRC as-cast strips. Black lines in the figure are high angle boundaries with misorientation angle above 15 and white lines are low angle boundaries with misorientation angle between 2° and 15°. Boundaries below 2° misorientation angle are not shown here. The grains of TRC strips are equiaxed whereas the grains of MC-TRC strips appear to be elongated in the casting direction. A conservative estimate of the mean true strain:
√
can be made from the average grain aspect ratio ( ) (grain size in the casting
direction/ grain size in the normal direction) because the grain elongation in the casting direction can
10
only be caused by the rolling plastic deformation. For the MC-TRC strip, ≈ 0.45; whereas for the TRC strip,
was measured as ~1.2 and
was measured as ~2.5 and
≈ 0.12.
Figure 8. Inverse pole figures EBSD maps showing the grain structure of (a) TRC and (b) MC-TRC as-cast strips. The grains of MC-TRC strip are elongated in the casting direction.
Figure 9. Schematic illustration of solidification process during TRC. Figure 9 shows the typical illustration of the solidification process during TRC. As discussed earlier, in the case of MC-TRC sample heterogeneous nucleation by oxide particles dominates the solidification mechanism. This results in the advance of an equiaxed solidification front from the roll surface to the centre of the strip. As the solute rejection towards the centre of the strip is insignificant, the sump depth is reduced in the case of MC-TRC compared to the TRC process [21]. This effectively reduced
11
sump depth of the solidification zone is considered to be the key reason for the elimination of severe centreline segregation. As the sump depth decreases the deformation region increases. Therefore, MC-TRC strips undergo higher degree of deformation during the casting process in comparison to TRC strips. This is an important feature of the MC-TRC process.
Mechanical Testing
Figure 10. Comparison of tensile flow stress curves of Al-3Mg TRC and MC-TRC samples at room temperature and strain rate of 1/s. The as-cast strips were cold rolled (CR), hot rolled (HR) and homogenised before tensile testing.
Tensile testing of the hot plus cold rolled and cold rolled strips was carried out to examine their mechanical properties and to provide an estimate of their formability. Figure 10 shows typical tensile stress–strain curves of both MC-TRC and TRC samples. It can be observed that the MC-TRC sample had a remarkably high tensile elongation in comparison with the TRC strip. The TRC sample had apparent strain instability in early stages of plastic deformation. In contrast, the MC-TRC sample exhibited a smooth and gradual start of plastic flow without any apparent strain instability. The elongation of the MC-TRC samples is approximately 22% higher than that of TRC samples. It should be noted that the MC-TRC cold rolled sample demonstrated better tensile behaviour than the TRC hot and cold rolled sample. A microstructural examination showed that the significantly improved tensile properties of the MC-TRC strip at final gauge compared with the TRC strip at the same gauge were
12
mainly caused by two reasons: (1) the refined and uniform microstructure and (2) the elimination of severe centreline segregation and banded structure.
Texture Evolution during casting and rolling It is important to control the texture of aluminium alloy sheets during processing so as to optimise their formability [25]. Typical texture components usually observed in rolled aluminium alloy sheets are the deformation textures of Cu, Brass and S, and the recrystallization textures of Cube and Goss [26] . A strong Goss texture is considered to be detrimental to formability [27], whereas a weak cube texture is beneficial [28]. However, for automotive sheet used for exterior panels, the spatial distribution of texture is generally considered to be more important than the overall texture [29]. In this investigation, textures were determined by EBSD, which provides direct texture-microstructure correlation and better understanding of the deformation behaviour of the material than XRD. ESBD mapping for texture measurement was performed on the longitudinal plane (TD plane) over an area no less than 2000×1500µm for the as-cast specimens and 1500×1000µm for the cast and rolled specimens. Figure 11 shows typical (111) pole figures for the as-cast sheets and those processed after casting by cold rolling and hot rolling plus cold rolling.
13
Figure 11. (111) pole figures obtained from EBSD data, showing characteristic features of textures developed under different processing schemes: a) as-cast TRC; b) as-cast MC-TRC; c) cold rolled TRC; d) cold rolled MC-TRC; e) hot + cold rolled TRC and f) hot + cold rolled MC-TRC. The figures indicate pole density times random. Statistical texture results obtained by averaging data form the EBSD maps are given in Table 1, including the maximum pole density (times random), total texture volume fraction and the volume fraction of individual texture components under various processing conditions. Textures of the as-cast TRC sheet were random, no particular preferred orientations developed in the material (Figure 11(a)). For the as-cast MC-TRC sheet, a typical rolling texture just began to form with a certain amount of S and Brass components (Figure 11(b)). It is clear from Table 1 that the as-cast MC-TRC sheet is of a higher texture intensity and volume fraction than the TRC specimens. This suggests that more plastic deformation occurred during MC-TRC than TRC. Detailed analysis revealed that the plastic
14
deformation in MC-TRC is more uniform through the sheet thickness and the distribution of texture is therefore rather more homogeneous. As shown in Figure 12(a), grains of preferred orientations that form the textures for the material (~10%) are essentially in the middle of the sheet for the TRC specimen, indicating that the plastic deformation was largely in the central region of the sheet. This is probably because the solidification during TRC was dominated by the surface nucleation and the temperature in the centre was higher in association with a deeper mushy zone. As a result, the temperature in the sheet centre was higher than the outer part and the material was softer and easier to deform. These solidification and thermal conditions and associated concentrated plastic flows may involve the enrolment of solute rich semisolid slurry at the bottom of the mushy zone, giving rise to the formation of central line segregations. The preferred orientation of the grains in the MC-TRC material, on the other hand, are widely spread as shown in Figure 12(b) and the deformation structure has developed evenly through the sheet thickness, illustrating the advantages of MC-TRC over TRC in the development of a defect free microstructure and texture.
Volume faction of texture components Copper {
}〈
̅〉
S {
}〈
Brass ̅〉
{
}〈 ̅ 〉
Goss {
}〈
Cube 〉
{
}〈
Total Volume
Maximum
fraction
density
〉
as-cast TRC
0.006
0.028
0.036
0.021
0
0.091
2.67
as-cast MCTRC
0.023
0.068
0.066
0.07
0.001
0.228
3.6
cold roll TRC
0.055
0.054
0.043
0.012
0.003
0.167
4.12
cold roll MCTRC
0.095
0.121
0.047
0.017
0.002
0.282
5.67
hot + cold TRC
0.057
0.085
0.049
0.021
0.003
0.215
4.58
hot + cold MCTRC
0.221
0.157
0.04
0.01
0.005
0.433
6.72
Table 1. the maximum pole density (times random), total texture volume fraction and the volume fraction of individual texture components for the Al-3Mg alloy sheet processed under different schemes.
15
Figure 12. Band quality EBSD maps overlapped with colour coded texture component distribution in relation to individual grains for a) TRC sheet and b) MC-TRC sheet. The orientations defined by the colour code are at the bottom of each map. With regard to the two rolling schemes after twin roll casting, the texture analysis has suggested that the combination of hot rolling and cold rolling is better in terms of the spatial distribution of textures. The cold roll MC-TRC Al-3Mg sheet showed good ductility and the texture developed was measured to be weaker than that obtained after the combination of hot rolling and cold rolling, which is reasonable as the total strain applied to the material is less. However, the texture developed in the sheet processed by cold rolling alone after casting was crystallographically distorted in reference to the sheet geometry, suggesting a non-uniform material flow during deformation. Its spatial distribution was also heterogeneous compared to the sheet processed by the combination of hot rolling and cold rolling. This can be seen from the EBSD maps in Figure 13, which show the spatial distribution of individual texture components by coloured grains. It may be seen from Figure 13 that the three major components of Copper {112}〈11 ̅ 〉, S {123}〈63 ̅ 〉 and Brass {110}〈1 ̅ 〉 for the hot rolling and cold rolling combined MC-TRC sheet are more evenly distributed in space than for the cold rolling alone sheet. It should be noted that the combination of hot rolling and cold rolling resulted in an increased texture density. However, the increment was limited and the texture has got much improved spatial distribution and crystallographic symmetry, which is important for formability, particularly for automotive sheet forming [29]. Another point worth mentioning is that the texture volume increase
16
after hot rolling and cold rolling was mainly due to the increase in the volume fraction of copper and S components without introducing any extra Goss and Cube textures. This is important as the latter two components are to be avoided as reported earlier [27, 28]. Overall, it is clear that hot rolling plus cold rolling is a better scheme for microstructure uniformity and texture control, in addition to its defect healing effect.
Figure 13. EBSD band quality map a) cold rolled alone MC-TRC sheet and b) hot+cold rolled MCTRC sheet. Grains of different colour represent different texture components as defined by the colour code at the bottom of each map, showing the spatial distribution of textures.
Conclusions: The application of intensive shearing prior to twin roll casting of an Al-3Mg alloy resulted in a refined, uniform microstructure and the elimination of centre-line segregation. The present study has demonstrated that by using liquid intensive shearing, it is possible to control the solidification during TRC, which can reduce the sump depth. The MC-TRC strip at the final gauge exhibited 22% higher elongation compared with the TRC strip. The texture analysis revealed that more and uniform plastic deformation has occurred during MC-TRC than TRC. The results of the texture evolution suggest that the combination of hot and cold rolling is better in terms of the spatial distribution of textures.
17
Acknowledgement: The financial support from Siemens Metals Technologies, Sheffield, UK for MC-TRC process development is acknowledged with gratitude. Authors would like to thank Department of Mechanical Engineering, Imperial College London, UK for allowing us to use their laboratory facilities for materials testing. Special thanks to Mr. Steve Cook, Mr. Peter Lloyd, Mr. Graham Mitchell, Mr. Carmelo for their technical support for MC-TRC equipment and process at BCAST, Brunel University London. Authors would like to thank Dr. Brian McKay for stimulating discussions.
References: [1] M. Ferry, Direct strip casting of metals and alloys – processing, microstructure and properties; Woodhead Publishing Ltd.: Cambridge, England (2006) pp.101–194. [2] N. S. Barekar, B. K. Dhindaw, Twin roll casting of Al alloys – An overview, Mater. Manuf. Process 29 (2014) 65–1661. [3] S. Das, N. S. Lim,J. B. Seol, H. W. Kim, C. G. Park, Effect of the rolling speed on microstructural and mechanical properties of aluminium–magnesium alloys prepared by twin roll casting, Mater.Des. 31 (2010) 1633–1638. [4] S. A. Lockyer, M. Yun, J. D. Hunt, D.V. Edmonds, Micro- and macro defects in thin sheet twinrollcast Aluminium alloys, Mater. Char. 37 (1996) 301–310. [5] Ch. Gras, M. Meredith, J.D. Hunt, Microdefects formation during the twin-roll casting of Al–Mg–Mn aluminium alloys, J. Mater. Process. Technol. 167 (2005) 62–72 [6] M. Yun, S. Lokyer, J.D. Hunt, Twin roll casting of aluminium alloys, Mater. Sci. Eng. A 280 (2000) 116–123. [7] M. McBrien, J. M. Allwood, N. S. Barekar, Tailor Blank Casting—Control of sheet width using an electromagnetic edge dam in aluminium twin roll casting, J. Mater. Process. Technol. 224 (2015) 60– 72. [8] Ch. Gras, M. Meredith, J. D. Hunt, Microstructure and texture evolution after twin roll casting and subsequent cold rolling of Al–Mg–Mn aluminium alloys, J. Mater. Process. Technol. 169 (2005) 156– 163 [9] J. P. Martins, A. L. M. Carvalho, A. F. Padilha, Microstructure and texture assessment of Al-Mn-FeSi (3003) aluminium alloy produced by continuous and semicontinuous casting processes, J. Mater. Sci. 44 (2009) 2966–2976.
18
[10] M. Slamova, M. Karlik, F. Robaut, P. Slama, M. Veron, Differences in microstructure and texture of Al-Mg sheets produced by twin-roll continuous casting and by direct-chill casting, Mater. Char. 49 (2003) 231–240. [11] M. Yun, D. J. Monagham, X. Yang, J. Jang, D. V. Edmonds, J. D. Hunt, R. Cook, P. M. Thomas, An experimental investigation of the effect of strip thickness, metallostatic head and tip setback on the productivity of a twin-roll caster, Cast Met, 4-2 (1991) 108–111. [12] R. G. Kamat, AA3104 can-body stock ingot: Characterization and homogenization, JOM, 48(6) (1996) 34–38. [13] A. L. Greer, Control of grain size in solidification, in: B. Cantor, K. A. Q. O’Reilly (Eds.), Solidification and Casting, Institute of Physics Publishing, Bristol and Philadelphia (2003) pp. 214. [14] B. S. Murty, S. A. Kori, M. Chakraborty, Grain refinement of aluminium and its alloys by heterogeneous nucleation and alloying, Inter. Mater. Rev. 47 (1) (2002) 3–29. [15] H.-T. Li, Y. Wang, Z. Fan, Mechanisms of enhanced heterogeneous nucleation during solidification in binary al–mg alloys, Acta. Mater. 60 (2012) 1528–1537. [16] S. Kumar, N. Hari Babu, G. M. Scamans, Z. Fan, Microstructural evaluation of melt conditioned twin roll cast Al–Mg alloy, Mater. Sci. Technol. 27(12) (2011) 1833–1839. [17] N. S. Barekar, S. Das, Z. Fan, Melt conditioned twin roll casting (MC-TRC) for aluminium alloys, Mater. Sci. Forum 794-796 (2014) 1115–1120. [18] J. Patel, Y. Zuo, Z. Fan, Liquid metal engineering by application of intensive melt shearing, in M. J. M. Krane, A. Jardy, R. L. Williamson, J. J. Beaman (Eds.), Proceedings of the 2013 International Symposium on Liquid Metal Processing and Casting, TMS, Austin, Texas (2013) 291–299. [19] R. E. Sanders, Jr. Technology innovation in aluminium products, JOM 53(2) (2001) 21–25. [20] N. S. Barekar, S. Das, Z. Fan, R. Cindery, N. Champion, Microstructural evaluation during melt conditioned twin roll casting (MC-TRC) of Al–Mg binary alloys, Mater. Sci. Forum 790–791 (2014) 285–290. [21] S. Das, N. S. Barekar, Omer El Fakir, Liliang Wang, A. K. Prasada Rao, J. B. Patel, H. R. Kotadia, A. Bhagurkar, John P. Dear, Z. Fan, Effect of melt conditioning on heat treatment and mechanical properties of AZ31 alloys produced by twin roll casting, Mater. Sci. Eng. A (2014) 620 (2015) 223–232. [22] J. Campbell, An overview of the effects of bifilms on the structure and properties of cast alloys, Metall. Mater. Trans. B, 37(6), 2006, 857–863. [23] Y. Wang, H.-T. Li, Z. Fan, Oxidation of aluminium alloy melt and inoculation by oxide particles, Trans. Indian Inst. Met. 65(6) (2012) 653–661.
19
[24] M.X. Zhang, P.M. Kelly, M. Qian, J.A. Taylor, Crystallography of grain refinement in Mg–Al based alloys, Acta Mater.53, 2005, 3261–3270. [25] P. D. Wu, K. W. Neale, E. Van Der Giessen, M. Jain, A. Makinde, S. R. Macewen, Crystal plasticity forming limit diagram analysis of rolled aluminum sheets, Metall. Mater. Trans. A 29 (1998) 527–535. [26] F. Barlat, Prediction of tricomponent plane stress yield surfaces and associated flow and failure behavior of strongly textured F.C.C. polycrystalline sheets, Mater. Sci. Eng. 95 (1987) 15–29. [27] K. Yoshida, T. Ishizaka, M. Kuroda, S. Ikawa, The effects of texture on formability of aluminum alloy sheets, Acta Mater. 55 (2007) 4499–4506. [28] P. D. Wu, S. R. MacEwen, D. J. Lloyd, K. W. Neale, Effect of cube texture on sheet metal formability, Mater. Sci. Eng. A 364 (2004) 182–187. [29] P. D. Wu, D. J. Lloyd, A. Bosland, H. Jin, S. R. MacEwen, Analysis of roping in AA6111 automotive sheet, Acta Mater. 51 (2003) 1945–1957.
20
PII: DOI: Reference:
S0921-5093(15)30538-4 http://dx.doi.org/10.1016/j.msea.2015.10.079 MSA32930
To appear in: Materials Science & Engineering A Received date: 21 September 2015 Revised date: 19 October 2015 Accepted date: 20 October 2015 Cite this article as: N.S. Barekar, S Das, X. Yang, Y. Huang, Omer El Fakir, A.G. Bhagurkar, L. Zhou and Z. Fan, The Impact of Melt Conditioning on Microstructure, Texture and Ductility of Twin Roll Cast Aluminium Alloy S t r i p s , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.10.079 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 galley proof before it is published in its final citable 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.
The Impact of Melt Conditioning on Microstructure, Texture and Ductility of Twin Roll Cast Aluminium Alloy Strips N. S. Barekara,*, S. Dasa, X. Yanga, Y. Huanga, Omer El Fakirb, A. G. Bhagurkara, L. Zhoua, Z. Fana a
The EPSRC Centre ‐ LiME, BCAST,Brunel University,Uxbridge UB8 3PH, UK
b
Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
* Corresponding author: [email protected],
Abstract: Twin roll casting (TRC) is recognised as an effective processing route for producing low cost Al sheet. However, the quality of the Al alloys sheets produced by the TRC process is limited by the formation of centre-line segregation which prevents its application in a wide range of engineering sectors. To improve the quality of the TRC strips, a new technology, melt conditioning twin roll casting (MC-TRC) has been developed from the industrial point of view. Enhanced nucleation by melt conditioning favours the advance of an equiaxed solidification front which results in reduced sump depth and a refined, uniform, defect free microstructure. The results obtained show that the differences in microstructure and in the crystallographic texture have a significant impact on ductility (22 % improvement) of the MC-TRC sheets compared to TRC sheets. Keywords: Twin roll casting, Solidification, Segregation, Rolling, Deformation
Introduction: Wrought aluminium alloys in sheet form are of interest for applications in the automotive industry because of their low density, tensile properties, formability, ease of recycling and high corrosion resistance [1]. These properties requirements are primarily by a number of Al-Mg alloys (5xxx series). Conventionally, metal sheet is produced by direct chill (DC) casting of thick slabs, followed by extensive thermo-mechanical processing, which includes homogenisation, hot rolling and cold rolling, leading to high energy consumption, low material yield and inevitably high cost. Whereas, twin roll casting (TRC) represents a paradigm shift in metal sheet production via minimising the number of processing steps by integrating casting and hot rolling into a single operation for the production of near-to-net-shape alloy strips [2]. Research on the TRC of aluminium alloys has been focused mainly
1
on process - microstructure relationship [3], defects in TRC sheets [4-6], modification of TRC setup [2, 7], texture assessment [8-10], and productivity [11]. However, TRC is still facing great challenges, which includes non-uniform microstructure formation, severe chemical segregation at the centre-line and a limited number of usable alloys (mainly dilute alloys with narrow freezing ranges). Aluminium alloy strip produced by conventional twin-roll casting requires additional processing including homogenization, rolling, and annealing to achieve desired properties required for application in the automotive industry. Due to the thickness constraints associated with twin-roll casting, the possibility of modifying the as-cast microstructures and formability before final shape forming and application is somewhat limited. The centre-line segregation is barely altered by downstream processing [8, 12] and hinder the mechanical performance of the final products. Therefore, grain refinement and defect reduction of as cast strips during twin-roll casting is of paramount importance. In order to minimise the internal defects and to improve the quality of the casting strips, a grain refiner can be added directly into the launder during casting. In industrial practice, Al-Ti-B master alloys are commonly used as grain refiners for the wrought Al alloys. However, only less than 1% of added TiB2 particles are active for the nucleation of α-Al grains [13]. The excess TiB2 particles have tendency to agglomerate and are detrimental in the final microstructure [14], particularly for the foil and thin strip products. Recently, it has been demonstrated that a fine and uniform microstructure can be achieved by the application of physical field such as intensive melt shearing [15]. This was attributed to the enhanced heterogeneous nucleation by naturally occurring oxides in the liquid Al alloys. The concept of melt conditioning by intensive shear prior to TRC has been previously documented [16, 17], however efforts are still required to make it industrially viable. Apart from the solidification features, the impact of melt conditioned twin roll casting (MC-TRC) on crystallographic texture and tensile properties of aluminium alloy sheets has not to date attracted sufficient attention. Furthermore, the behaviour of casting micro-defects, hot deformation aspect of the MC-TRC, the evolution of the texture during subsequent hot or cold rolling and its effect on tensile properties of AlMg alloy MC-TRC sheets are still not understood. Therefore, the present work was undertaken to investigate these aspect and to assist in developing an industrially viable MC-TRC process for aluminium alloys. The investigation was conducted to gain a comprehensive understanding of the principles that govern the microstructural and texture evolution of Al-Mg strips during MC-TRC.
2
Furthermore efforts were also made to analyse the impact of the as-cast microstructures and texture developed during casting and rolling of MC-TRC strips on tensile properties.
Experimental MC-TRC constitutes integration of a melt conditioning unit [18] with TRC as shown in figure 1. In the present process development, a rotor-stator device was employed for intensive shearing of the melt. A more detailed description of fluid flow in the high shear device can be found elsewhere [18]. The main advantages of the high shear device include the enhancement of kinetics for in situ chemical reactions, homogenisation of chemical composition and forced wetting of usually difficult to wet solid particles in the liquid metal. Moreover, the high shear device can be used for physical grain refinement by dispersing naturally occurring oxides [18].
Figure 1. Schematic illustration of the MC-TRC process
Tundish Design To make the MC-TRC process industrially viable, this investigative study has been associated with a technical challenge to design a fit-for-purpose tundish having a separate chamber to facilitate continuous shearing and stable delivery of the required superheated melt to the roll bite for casting the strip. Figure 2 (a) and (b) shows a section view of the tundish with high shear device inserted and top view of the tundish, respectively. The tundish is divided into three chambers namely, pouring, shearing and delivery. A baffle was placed at entry to the shearing chamber to prevent incoming liquid from bypassing the shear zone. Another baffle was positioned at the exit of the shearing chamber to avoid turbulence created by the rotor and to provide enough residence time and the supply of
3
conditioned melt. The optimised conditioned is realized when adequate melt circulation is achieved and stagnant zones are avoided.
Figure 2. The details of the tundish design showing (a) section view of ceramic tundish and the rotor®
stator device (b) top view of the tundish. The tundish was made from N17 machinable ceramic. All the dimensions are in mm.
Melt Conditioned Twin Roll Casting (MC-TRC) In conventional TRC, if the solute content is more than 3 wt.% (theoretically), it is difficult to cast an aluminium alloy strip without segregation occurring. However, in practice the solute content limit is 2.5 wt.% [19]. It has been demonstrated that the melt conditioning prior to TRC increases the limit on solute content of Al-Mg alloys that can be twin roll cast without severe segregation [20]. In the current work, the experiments were designed for 3 wt. % of Mg. 10 kg of binary alloys were melted in clay graphite crucibles in an electric resistance furnace at 710 °C. A series of static experiments with
4
shearing in the ceramic tundish (without integrating with TRC) were performed to determine the pouring temperature of the melt in order to obtain appropriate melt temperature at the entry of the roll bite (5-10 K superheat, Tmelting point ≈ 645°C). A laboratory scale horizontal twin roll caster was used to produce the alloy strip. The water cooled steel rolls are 318 mm in diameter and 350 mm in width. Before each experiment, the rolls were cleaned. The casting speed, the setback and the strip thickness were fixed to 1 m/min, ~ 43 mm and ~ 5 mm, respectively. Once the melt reached the desired pouring temperature (55-60 K superheat), it was poured into the preheated tundish for melt conditioning followed by strip casting. The melt shearing was carried out at 5000 rpm. For comparison, Al-3Mg alloy strips were produced by conventional TRC (without melt conditioning). The twin roll casting process parameters such as roll gap, setback, casting speed, melt temperature at the entry of the roll bite were the same for TRC and MC-TRC experiments.
Downstream Processing and Mechanical Testing The Al-3Mg alloy strips produced using the MC-TRC and the conventional TRC process were deformed by hot and cold rolling to give a final thickness of 1.7 ± 0.1 mm prior to tensile testing. In order to compare the effect of hot and cold rolling, the final thickness after a total rolling reduction of 66 % was achieved either by one pass of hot rolling followed by second pass of cold rolling or by two passes of cold rolling. After rolling, the Al-3Mg alloy strips were heat treated at 250 °C for 1 hour to relieve the potential residual stresses generated during processing and followed by water quenching. The dog-bone shaped tensile test specimens were machined from these homogenised strips. The tensile tests were conducted for at least 4 samples on a Gleeble 3800 thermo-mechanical simulator. -1
The strain rate was set to 1s which is representative of typical strain rates in forming processes for aluminium sheets and the tests were conducted at room temperature till failure. Load was measured with a load cell and the strain was obtained using a C Gauge transducer. The dimensions of the tensile test specimen and the details of the tensile testing machine can be found elsewhere [21].
Microstructure and Texture Characterization The specimens for metallography were mechanically polished and anodized at 20 V for 45 seconds in Barker’s reagent to reveal the grain structure. Figure 3 depicts the schematic illustration of sample selection for microstructure and texture analysis. Microstructural analysis was performed with A Zeiss
5
Axio Vision optical microscope on the transverse and longitudinal cross-sections of the MC-TRC strips. The specimens for texture characterization were cut from the middle of the strip along the casting or rolling direction, and all examinations were carried out on the longitudinal plane or TD plane. Electro polishing at 12 V in a solution of 30% V/V nitric acid in ethanol at 243 K (–30 °C) for 60 seconds was used to prepare surfaces for electron backscatter diffraction (EBSD). EBSD was carried out using a Zeiss Supra 35 field-emission gun (FEG) scanning electron microscope operating at 20 kV and equipped with an EBSD facility. The measured EBSD data, from which texture information was also obtained, were analysed using the TEAM and OIM proprietary software.
Figure 3. Schematic illustration of sample selection for microstructure and texture analysis
6
Result and Discussion The As-Cast Microstructures
Figure 4 - Through-thickness microstructures from the transverse cross-sections of as-cast strips produced by (a) TRC showing coarse grain structure and produced by (b) MC-TRC showing uniform grain structure and no centre-line segregation. MC-TRC was carried out under the shearing and casting speed of 5000 rpm and 1 m/min respectively. Typical optical micrographs of the microstructures obtained by TRC and MC-TRC are shown in Figure 4. The polarized-light micrographs which include the full strip thickness show that the microstructure produced by MC-TRC is uniform, significantly refined, and almost equiaxed throughout the cross section. In contrast, the TRC microstructure is dominated by a coarse dendrite structure with large, equiaxed grains. The grain size was measured using polarized-light micrographs with strong orientation contrast. The average grain size in ND-TD plane of the MC-TRC strip was approximately 222 ± 65 μm, compared with 400 ± 150 μm for the TRC strip. The microstructural examination revealed that the MC-TRC strip was free from centreline segregation, whereas segregation was found at most of the centreline of TRC strip. Such channel segregates which are low melting point regions oriented in the casting direction are clearly seen in the longitudinal cross-section of TRC strips (Figure 5). Similar features have been reported by Lockyer et al. [4] and Yun et al. [6] in the twin roll cast aluminium alloys. The through thickness composition variation of Mg in Al-Mg alloy strips produced by
7
TRC and MC-TRC process were examined by EDX analysis, and the results are presented in figure 6. The results demonstrate that the MC-TRC process is capable of producing strips with uniform solute distribution which is in good agreement with the microstructural analysis. In the case of TRC, the grains nucleate near the roll surface and grow towards the centre of the strip. During growth these grains reject the solute atoms towards the centre of the strip, which reduces the liquidus temperature of the melt at the centre of the strip. These solidification and thermal conditions lead to the solute enrichment and channel segregates.
Figure 5. Through-thickness microstructures from the longitudinal cross-sections of as-cast strips produced by TRC showing the centreline segregation. In the conventional TRC process the solute is pushed towards the centre of the strip causing an increase in solute concentration in the last solidifying liquid.
8
Figure 6. Elemental composition of the solute across the strip thickness for Al-3Mg TRC and MC-TRC samples. Solute distribution is more uniform in case of the MC-TRC compared with the TRC sample. In practice, due to the high affinity between oxygen and Al, the oxidation on the surface of Al alloy melts is inevitable when they are exposed to ambient atmosphere. The oxides formed at the surface of the melt are readily entrained into the casting by the turbulence of melt handling such as stirring and pouring [22]. These oxides may exist either in the form of oxide films/ bifilms and/or as discrete oxide particles [23]. The intensive melt shearing breaks the oxide bifilms as well as oxide particle clusters and disperses them uniformly throughout the liquid. It has been documented that the oxide formed in Al–Mg alloys under normal melting conditions is MgAl2O4, which displays an equiaxed and faceted morphology with {111} planes exposed as its natural surfaces. MgAl2O4 and α-Al, both have the same crystal structure and closely matched atomic spacing (lattice misfit of ~ 1.4%) [15]. According to the crystallographic matching criteria for potent nucleation [24], MgAl 2O4 particles can act as potent nucleation substrates for the primary α-Al phase if there is no more potent nucleating particles present in the alloy melt. Large numbers of such potent oxide particles acting as nucleation sites can enhance the heterogeneous nucleation leading to fine, equiaxed grain structure in MC-TRC strip and the solute segregation towards the centre of the strip is insignificant.
9
Figure 7. Optical micrographs of the as-cast microstructures produced by (a) TRC and (b) MC-TRC. TRC strip shows the features of banded structure (inhomogeneity in grain structure). Typical optical micrographs of the microstructures obtained by TRC and MC-TRC are shown in Figure 7. As can be seen from Figure 7 (a), there is a sudden change midway through the sample and the TRC sheet can be divided into inner and outer bands. Small regions of the inner band have a much coarser structure. As mentioned earlier, the solute enrichment reduces the liquidus temperature of the melt at the centre of the TRC strip. It has been suggested that the sharp division between the outer and central band is a result of the thixotropic nature of the semi-solid metal [4]. Once deformed, the semi-solid material in the central band remains soft until it reaches a much lower temperature compared to grains in the outer band. The central region of the strip appears to be pushed back towards the liquid as a result of the deformation process [6]. On the other hand, no such banded structure was observed in the case of MC-TRC strip (Figure 7 (b)). This is attributed to the enhanced heterogeneous nucleation by oxide particles. Figure 8 shows the grain morphologies with grain boundaries for TRC and MC-TRC as-cast strips. Black lines in the figure are high angle boundaries with misorientation angle above 15 and white lines are low angle boundaries with misorientation angle between 2° and 15°. Boundaries below 2° misorientation angle are not shown here. The grains of TRC strips are equiaxed whereas the grains of MC-TRC strips appear to be elongated in the casting direction. A conservative estimate of the mean true strain:
√
can be made from the average grain aspect ratio ( ) (grain size in the casting
direction/ grain size in the normal direction) because the grain elongation in the casting direction can
10
only be caused by the rolling plastic deformation. For the MC-TRC strip, ≈ 0.45; whereas for the TRC strip,
was measured as ~1.2 and
was measured as ~2.5 and
≈ 0.12.
Figure 8. Inverse pole figures EBSD maps showing the grain structure of (a) TRC and (b) MC-TRC as-cast strips. The grains of MC-TRC strip are elongated in the casting direction.
Figure 9. Schematic illustration of solidification process during TRC. Figure 9 shows the typical illustration of the solidification process during TRC. As discussed earlier, in the case of MC-TRC sample heterogeneous nucleation by oxide particles dominates the solidification mechanism. This results in the advance of an equiaxed solidification front from the roll surface to the centre of the strip. As the solute rejection towards the centre of the strip is insignificant, the sump depth is reduced in the case of MC-TRC compared to the TRC process [21]. This effectively reduced
11
sump depth of the solidification zone is considered to be the key reason for the elimination of severe centreline segregation. As the sump depth decreases the deformation region increases. Therefore, MC-TRC strips undergo higher degree of deformation during the casting process in comparison to TRC strips. This is an important feature of the MC-TRC process.
Mechanical Testing
Figure 10. Comparison of tensile flow stress curves of Al-3Mg TRC and MC-TRC samples at room temperature and strain rate of 1/s. The as-cast strips were cold rolled (CR), hot rolled (HR) and homogenised before tensile testing.
Tensile testing of the hot plus cold rolled and cold rolled strips was carried out to examine their mechanical properties and to provide an estimate of their formability. Figure 10 shows typical tensile stress–strain curves of both MC-TRC and TRC samples. It can be observed that the MC-TRC sample had a remarkably high tensile elongation in comparison with the TRC strip. The TRC sample had apparent strain instability in early stages of plastic deformation. In contrast, the MC-TRC sample exhibited a smooth and gradual start of plastic flow without any apparent strain instability. The elongation of the MC-TRC samples is approximately 22% higher than that of TRC samples. It should be noted that the MC-TRC cold rolled sample demonstrated better tensile behaviour than the TRC hot and cold rolled sample. A microstructural examination showed that the significantly improved tensile properties of the MC-TRC strip at final gauge compared with the TRC strip at the same gauge were
12
mainly caused by two reasons: (1) the refined and uniform microstructure and (2) the elimination of severe centreline segregation and banded structure.
Texture Evolution during casting and rolling It is important to control the texture of aluminium alloy sheets during processing so as to optimise their formability [25]. Typical texture components usually observed in rolled aluminium alloy sheets are the deformation textures of Cu, Brass and S, and the recrystallization textures of Cube and Goss [26] . A strong Goss texture is considered to be detrimental to formability [27], whereas a weak cube texture is beneficial [28]. However, for automotive sheet used for exterior panels, the spatial distribution of texture is generally considered to be more important than the overall texture [29]. In this investigation, textures were determined by EBSD, which provides direct texture-microstructure correlation and better understanding of the deformation behaviour of the material than XRD. ESBD mapping for texture measurement was performed on the longitudinal plane (TD plane) over an area no less than 2000×1500µm for the as-cast specimens and 1500×1000µm for the cast and rolled specimens. Figure 11 shows typical (111) pole figures for the as-cast sheets and those processed after casting by cold rolling and hot rolling plus cold rolling.
13
Figure 11. (111) pole figures obtained from EBSD data, showing characteristic features of textures developed under different processing schemes: a) as-cast TRC; b) as-cast MC-TRC; c) cold rolled TRC; d) cold rolled MC-TRC; e) hot + cold rolled TRC and f) hot + cold rolled MC-TRC. The figures indicate pole density times random. Statistical texture results obtained by averaging data form the EBSD maps are given in Table 1, including the maximum pole density (times random), total texture volume fraction and the volume fraction of individual texture components under various processing conditions. Textures of the as-cast TRC sheet were random, no particular preferred orientations developed in the material (Figure 11(a)). For the as-cast MC-TRC sheet, a typical rolling texture just began to form with a certain amount of S and Brass components (Figure 11(b)). It is clear from Table 1 that the as-cast MC-TRC sheet is of a higher texture intensity and volume fraction than the TRC specimens. This suggests that more plastic deformation occurred during MC-TRC than TRC. Detailed analysis revealed that the plastic
14
deformation in MC-TRC is more uniform through the sheet thickness and the distribution of texture is therefore rather more homogeneous. As shown in Figure 12(a), grains of preferred orientations that form the textures for the material (~10%) are essentially in the middle of the sheet for the TRC specimen, indicating that the plastic deformation was largely in the central region of the sheet. This is probably because the solidification during TRC was dominated by the surface nucleation and the temperature in the centre was higher in association with a deeper mushy zone. As a result, the temperature in the sheet centre was higher than the outer part and the material was softer and easier to deform. These solidification and thermal conditions and associated concentrated plastic flows may involve the enrolment of solute rich semisolid slurry at the bottom of the mushy zone, giving rise to the formation of central line segregations. The preferred orientation of the grains in the MC-TRC material, on the other hand, are widely spread as shown in Figure 12(b) and the deformation structure has developed evenly through the sheet thickness, illustrating the advantages of MC-TRC over TRC in the development of a defect free microstructure and texture.
Volume faction of texture components Copper {
}〈
̅〉
S {
}〈
Brass ̅〉
{
}〈 ̅ 〉
Goss {
}〈
Cube 〉
{
}〈
Total Volume
Maximum
fraction
density
〉
as-cast TRC
0.006
0.028
0.036
0.021
0
0.091
2.67
as-cast MCTRC
0.023
0.068
0.066
0.07
0.001
0.228
3.6
cold roll TRC
0.055
0.054
0.043
0.012
0.003
0.167
4.12
cold roll MCTRC
0.095
0.121
0.047
0.017
0.002
0.282
5.67
hot + cold TRC
0.057
0.085
0.049
0.021
0.003
0.215
4.58
hot + cold MCTRC
0.221
0.157
0.04
0.01
0.005
0.433
6.72
Table 1. the maximum pole density (times random), total texture volume fraction and the volume fraction of individual texture components for the Al-3Mg alloy sheet processed under different schemes.
15
Figure 12. Band quality EBSD maps overlapped with colour coded texture component distribution in relation to individual grains for a) TRC sheet and b) MC-TRC sheet. The orientations defined by the colour code are at the bottom of each map. With regard to the two rolling schemes after twin roll casting, the texture analysis has suggested that the combination of hot rolling and cold rolling is better in terms of the spatial distribution of textures. The cold roll MC-TRC Al-3Mg sheet showed good ductility and the texture developed was measured to be weaker than that obtained after the combination of hot rolling and cold rolling, which is reasonable as the total strain applied to the material is less. However, the texture developed in the sheet processed by cold rolling alone after casting was crystallographically distorted in reference to the sheet geometry, suggesting a non-uniform material flow during deformation. Its spatial distribution was also heterogeneous compared to the sheet processed by the combination of hot rolling and cold rolling. This can be seen from the EBSD maps in Figure 13, which show the spatial distribution of individual texture components by coloured grains. It may be seen from Figure 13 that the three major components of Copper {112}〈11 ̅ 〉, S {123}〈63 ̅ 〉 and Brass {110}〈1 ̅ 〉 for the hot rolling and cold rolling combined MC-TRC sheet are more evenly distributed in space than for the cold rolling alone sheet. It should be noted that the combination of hot rolling and cold rolling resulted in an increased texture density. However, the increment was limited and the texture has got much improved spatial distribution and crystallographic symmetry, which is important for formability, particularly for automotive sheet forming [29]. Another point worth mentioning is that the texture volume increase
16
after hot rolling and cold rolling was mainly due to the increase in the volume fraction of copper and S components without introducing any extra Goss and Cube textures. This is important as the latter two components are to be avoided as reported earlier [27, 28]. Overall, it is clear that hot rolling plus cold rolling is a better scheme for microstructure uniformity and texture control, in addition to its defect healing effect.
Figure 13. EBSD band quality map a) cold rolled alone MC-TRC sheet and b) hot+cold rolled MCTRC sheet. Grains of different colour represent different texture components as defined by the colour code at the bottom of each map, showing the spatial distribution of textures.
Conclusions: The application of intensive shearing prior to twin roll casting of an Al-3Mg alloy resulted in a refined, uniform microstructure and the elimination of centre-line segregation. The present study has demonstrated that by using liquid intensive shearing, it is possible to control the solidification during TRC, which can reduce the sump depth. The MC-TRC strip at the final gauge exhibited 22% higher elongation compared with the TRC strip. The texture analysis revealed that more and uniform plastic deformation has occurred during MC-TRC than TRC. The results of the texture evolution suggest that the combination of hot and cold rolling is better in terms of the spatial distribution of textures.
17
Acknowledgement: The financial support from Siemens Metals Technologies, Sheffield, UK for MC-TRC process development is acknowledged with gratitude. Authors would like to thank Department of Mechanical Engineering, Imperial College London, UK for allowing us to use their laboratory facilities for materials testing. Special thanks to Mr. Steve Cook, Mr. Peter Lloyd, Mr. Graham Mitchell, Mr. Carmelo for their technical support for MC-TRC equipment and process at BCAST, Brunel University London. Authors would like to thank Dr. Brian McKay for stimulating discussions.
References: [1] M. Ferry, Direct strip casting of metals and alloys – processing, microstructure and properties; Woodhead Publishing Ltd.: Cambridge, England (2006) pp.101–194. [2] N. S. Barekar, B. K. Dhindaw, Twin roll casting of Al alloys – An overview, Mater. Manuf. Process 29 (2014) 65–1661. [3] S. Das, N. S. Lim,J. B. Seol, H. W. Kim, C. G. Park, Effect of the rolling speed on microstructural and mechanical properties of aluminium–magnesium alloys prepared by twin roll casting, Mater.Des. 31 (2010) 1633–1638. [4] S. A. Lockyer, M. Yun, J. D. Hunt, D.V. Edmonds, Micro- and macro defects in thin sheet twinrollcast Aluminium alloys, Mater. Char. 37 (1996) 301–310. [5] Ch. Gras, M. Meredith, J.D. Hunt, Microdefects formation during the twin-roll casting of Al–Mg–Mn aluminium alloys, J. Mater. Process. Technol. 167 (2005) 62–72 [6] M. Yun, S. Lokyer, J.D. Hunt, Twin roll casting of aluminium alloys, Mater. Sci. Eng. A 280 (2000) 116–123. [7] M. McBrien, J. M. Allwood, N. S. Barekar, Tailor Blank Casting—Control of sheet width using an electromagnetic edge dam in aluminium twin roll casting, J. Mater. Process. Technol. 224 (2015) 60– 72. [8] Ch. Gras, M. Meredith, J. D. Hunt, Microstructure and texture evolution after twin roll casting and subsequent cold rolling of Al–Mg–Mn aluminium alloys, J. Mater. Process. Technol. 169 (2005) 156– 163 [9] J. P. Martins, A. L. M. Carvalho, A. F. Padilha, Microstructure and texture assessment of Al-Mn-FeSi (3003) aluminium alloy produced by continuous and semicontinuous casting processes, J. Mater. Sci. 44 (2009) 2966–2976.
18
[10] M. Slamova, M. Karlik, F. Robaut, P. Slama, M. Veron, Differences in microstructure and texture of Al-Mg sheets produced by twin-roll continuous casting and by direct-chill casting, Mater. Char. 49 (2003) 231–240. [11] M. Yun, D. J. Monagham, X. Yang, J. Jang, D. V. Edmonds, J. D. Hunt, R. Cook, P. M. Thomas, An experimental investigation of the effect of strip thickness, metallostatic head and tip setback on the productivity of a twin-roll caster, Cast Met, 4-2 (1991) 108–111. [12] R. G. Kamat, AA3104 can-body stock ingot: Characterization and homogenization, JOM, 48(6) (1996) 34–38. [13] A. L. Greer, Control of grain size in solidification, in: B. Cantor, K. A. Q. O’Reilly (Eds.), Solidification and Casting, Institute of Physics Publishing, Bristol and Philadelphia (2003) pp. 214. [14] B. S. Murty, S. A. Kori, M. Chakraborty, Grain refinement of aluminium and its alloys by heterogeneous nucleation and alloying, Inter. Mater. Rev. 47 (1) (2002) 3–29. [15] H.-T. Li, Y. Wang, Z. Fan, Mechanisms of enhanced heterogeneous nucleation during solidification in binary al–mg alloys, Acta. Mater. 60 (2012) 1528–1537. [16] S. Kumar, N. Hari Babu, G. M. Scamans, Z. Fan, Microstructural evaluation of melt conditioned twin roll cast Al–Mg alloy, Mater. Sci. Technol. 27(12) (2011) 1833–1839. [17] N. S. Barekar, S. Das, Z. Fan, Melt conditioned twin roll casting (MC-TRC) for aluminium alloys, Mater. Sci. Forum 794-796 (2014) 1115–1120. [18] J. Patel, Y. Zuo, Z. Fan, Liquid metal engineering by application of intensive melt shearing, in M. J. M. Krane, A. Jardy, R. L. Williamson, J. J. Beaman (Eds.), Proceedings of the 2013 International Symposium on Liquid Metal Processing and Casting, TMS, Austin, Texas (2013) 291–299. [19] R. E. Sanders, Jr. Technology innovation in aluminium products, JOM 53(2) (2001) 21–25. [20] N. S. Barekar, S. Das, Z. Fan, R. Cindery, N. Champion, Microstructural evaluation during melt conditioned twin roll casting (MC-TRC) of Al–Mg binary alloys, Mater. Sci. Forum 790–791 (2014) 285–290. [21] S. Das, N. S. Barekar, Omer El Fakir, Liliang Wang, A. K. Prasada Rao, J. B. Patel, H. R. Kotadia, A. Bhagurkar, John P. Dear, Z. Fan, Effect of melt conditioning on heat treatment and mechanical properties of AZ31 alloys produced by twin roll casting, Mater. Sci. Eng. A (2014) 620 (2015) 223–232. [22] J. Campbell, An overview of the effects of bifilms on the structure and properties of cast alloys, Metall. Mater. Trans. B, 37(6), 2006, 857–863. [23] Y. Wang, H.-T. Li, Z. Fan, Oxidation of aluminium alloy melt and inoculation by oxide particles, Trans. Indian Inst. Met. 65(6) (2012) 653–661.
19
[24] M.X. Zhang, P.M. Kelly, M. Qian, J.A. Taylor, Crystallography of grain refinement in Mg–Al based alloys, Acta Mater.53, 2005, 3261–3270. [25] P. D. Wu, K. W. Neale, E. Van Der Giessen, M. Jain, A. Makinde, S. R. Macewen, Crystal plasticity forming limit diagram analysis of rolled aluminum sheets, Metall. Mater. Trans. A 29 (1998) 527–535. [26] F. Barlat, Prediction of tricomponent plane stress yield surfaces and associated flow and failure behavior of strongly textured F.C.C. polycrystalline sheets, Mater. Sci. Eng. 95 (1987) 15–29. [27] K. Yoshida, T. Ishizaka, M. Kuroda, S. Ikawa, The effects of texture on formability of aluminum alloy sheets, Acta Mater. 55 (2007) 4499–4506. [28] P. D. Wu, S. R. MacEwen, D. J. Lloyd, K. W. Neale, Effect of cube texture on sheet metal formability, Mater. Sci. Eng. A 364 (2004) 182–187. [29] P. D. Wu, D. J. Lloyd, A. Bosland, H. Jin, S. R. MacEwen, Analysis of roping in AA6111 automotive sheet, Acta Mater. 51 (2003) 1945–1957.
20