Elsevier

Materials Science and Engineering: A

Volume 652, 15 January 2016, Pages 221-230
Materials Science and Engineering: A

Effect of microstructure on hot tensile deformation behavior of 7075 alloy sheet fabricated by twin roll casting

https://doi.org/10.1016/j.msea.2015.11.079Get rights and content

Abstract

7075 aluminum alloy sheet with a thickness of 1 mm was successfully fabricated by twin roll casting and subsequent rolling process. The hot tensile deformation behavior of 7075 alloy was evaluated and large elongation over 200% was obtained at 450 °C under the high strain rate of 1×10−1 s−1. In order to clarify the reason for the large elongation obtained at high strain rate, the effects of microstructure on hot tensile deformation behavior were investigated. The results show that high solidification rate during TRC casting process induced small particles (~1 μm) in homogeneous distribution. The relatively high fraction of particles over 1 μm in size attributed the homogeneous recrystallized microstructure with fine grains induced by particle simulated nucleation (PSN). This fully recrystallized fine-grained microstructure of TRC alloy sheet contributed to high ductility and formability of 7075 alloy sheet under high strain rate.

Introduction

There has been recently a growing demand for producing lightweight vehicles in order to reduce the energy consumption and CO2 emission. High strength-weight ratio, good formability, good corrosion resistance are considered as general characteristics of aluminum alloys applied to automotive industry [1]. Although, 5XXX and 6XXX series aluminum alloys have been widely used for automobile application, more high strength aluminum alloys are required for light weight of automobile body structure. Nowadays, 7XXX series of alloys which have been developed for aircraft and space applications [2] are considered as a good candidate for the automobile body structure.

Twin roll casting process (TRC) is considered as a cost-efficient process to fabricate aluminum alloy sheets with good mechanical properties, wherein molten metal is fed onto water-cooled rolls and solidifies with a high cooling rate [3]. Due to the high solidification rate achieved in twin roll casting, the microstructure of TRC alloys differs significantly from that of conventional casting alloys. Traditional twin roll casting process is difficult to produce high strength aluminum alloy strip, because of thermal fragmentation and segregation induced by high contents of alloying elements [4]. However, we successfully fabricated 7075 alloy sheets by twin roll casting and subsequent rolling process, the sheets showed good tensile properties for automobile body application [3]. Moreover, it presented large elongation over 200% when deformed at elevated temperature under high strain rate [5]. Small and homogeneous particles were induced during TRC process, which was considered to have a great effect on the alloy formability [6].

It is reported that micro-cracks were easily initiated by the debonding of the interface between matrix and particles [7] or by inner fracture of large particles [8], [9]. Tewari et al. [6] reported the influence of the particle distribution of 5754 aluminum alloys on their localization in uniaxial strain. It is presented that large particles lowered the localization strain.

In this study, the effect of particles on the microstructural evolution was investigated to clarify the reason for large elongation obtained at high strain rate. For comparison of different particle distribution, the alloy sheet fabricated by permanent mold casting (PMC) process was also prepared.

Section snippets

Experimental procedures

The material in the present study is 7075 aluminum alloy. The chemical composition is Al–5.2Zn–2.3Mg–1.5Cu (wt%) shown in Table 1. For comparison of different particle distribution, test samples are produced by two different casting processes; twin roll casting (TRC) and permanent mold casting (PMC).

The TRC sample with 4.5 mm in thickness was fabricated by a horizontal type twin roll caster with water-cooled Cu–Cr rolls with the diameter of 300 mm [3], [5]. At this time, the molten alloy heated

Initial microstructure

Fig. 1 shows the microstructure of TRC and PMC alloys. The grain structures in as-cast alloys (Fig. 1(a) and (c)) were mostly equiaxed. The grain sizes were ~40 μm and ~41 μm, respectively, for as-cast TRC and PMC alloys. Moreover, TRC sample exhibited very fine second dendrite arm spacing (DAS) around 6 μm due to the high cooling rate during casting process. It is seen that cold-rolled TRC and PMC samples had a deformation structure, where band-liked structures were observed along the rolling

Particle's effect on microstructure and ductility

Fig. 8 shows the particle number density per unit area in TRC and PMC alloys. It is obvious that the total number density of particles in PMC alloy was much higher compared to TRC alloy. Especially, the density of particles smaller than 1 μm in PMC alloy was about four times than that in TRC alloy. However, the particles in size over 1 μm in PMC alloy were in much lower density. Moreover, it is noticed that a few of remarkably large particles (>5 μm) was also observed in PMC alloy, while TRC alloy

Conclusions

7075 aluminum alloy strip was successfully fabricated by twin roll casting. Large elongation over 200% was obtained at the temperature of 450 oC under the high strain rate of 1×10−1 s−1. In order to investigate the particle effect on the microstructure and tensile deformation behavior at high temperature, 7075 aluminum alloy strip fabricated by permanent mold casting (PMC) was for comparison in this study.

(1) High solidification rate in TRC alloy induced small particles (~1 μm) in homogeneous

Acknowledgments

The authors are grateful for financial support from the Fundamental Research Program of Korea Institute of Materials Science (KIMS, No. PNK4240). Lei Wang is also grateful for financial support from the China Scholarship Council (CSC, No. 201306220124). Mr. W.J. Kim and Mrs. Y.M. Oh for TEM and EBSD test operation, respectively, are also appreciated.

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