Elsevier

Journal of Alloys and Compounds

Volume 578, 25 November 2013, Pages 208-214
Journal of Alloys and Compounds

Microstructure and tensile properties of large-size 7055 aluminum billets fabricated by spray forming rapid solidification technology

https://doi.org/10.1016/j.jallcom.2013.05.117Get rights and content

Highlights

  • Large size as-deposited billets of 7055 Aluminum alloy was successfully fabricated by spray forming technology.

  • The microstructure observation showed that there were equiaxed grains with the size of about 50 μm.

  • The detrimental dendrite microstructure was suppressed completely in our as-deposited 7055 alloys.

  • After extruding, the peak-aged 7055 alloys showed high tensile strength (667 MPa) and enhanced elongation (10%).

  • The microscopic fracture morphology of as-extruded specimens was the transgranular dimple fracture.

Abstract

In this study, high-quality large billets of 7055 aluminum alloy were successfully fabricated by spray forming rapid solidification technology. Microstructure observation showed that there were equiaxed grains with the size of about 50 μm in diameter. The dendrite microstructure was suppressed and the undesirable macrosegregation did not exist in the as-deposited 7055 alloys, attributable to rapid solidification effects. The secondary phases were the main η-Mg(Zn,Cu,Al)2 expansion phases and the very small amounts of AlxFe3Cu Iron-bearing phase. As-deposited 7055 billet was subsequently processed by hot extrusion, solid solution treatment at 450 °C/24 h and aging at 120 °C for 12 h. The as-aged alloy was then subjected to tensile testing. The results revealed that yield strength, tensile strength and elongation rate of the alloy reached 608 MPa, 667 MPa and 10%, respectively. After extruding, the observation of the microscopic fracture morphology of as-extruded specimens manifested that fracture mode was the transgranular dimple fracture.

Introduction

For compression dominated airframe structure materials applications, such as the upper wing structure of large commercial aircraft, the requirement of a superior combination of high specific strength, high fracture toughness, improved corrosion resistance and fatigue performance, led to optimization in the composition limits and heat treatment of 7150 Al alloy, and resulted in the design and development of the 7055-T77 aluminum alloy produced by Alcoa in the early 1990’s [1]. The extruded 7055-T77 alloys provided a strength increase of about 10% relative to that of 7150-T6 and 30% higher than that of 7075-T76 [2]. Other than novel T77 temper, the attractive combination of properties of the 7055 alloy was achieved by high solute content and high ratio of Zn/Mg due to the fact that it enhanced the extent of solute super-saturation and promoted precipitation-hardening [3], [4], [5].

However, the fabrication process of the 7055 ingot is very difficult because the high alloying element content (Zn > 8 wt%, Cu > 2 wt%) under the relatively lower cooling speed may cause coarse phases, chemical and microstructure segregation as well as severe crack tendency [6], [7], [8]. In general, a large number of coarse constituents remain in the alloy after conventional cast processing due to the limitation of solid solubility in the Al–Zn–Mg–Cu system. The remnant constituents can degrade the aging hardenability, lead to cracks initiation and propagation, and deterioration of the mechanical properties of the alloy.

Compared with the traditional forming processes, such as ordinary casting, ingot metallurgy and power metallurgy, the spray forming technology has shown great superiority and potential in the preparation of advanced high-solute Al–Zn–Mg–Cu series alloy [9], [10], [11], [12]. Spray forming technology, which was firstly introduced by Singer and developed by Osprey Metals Ltd., combines metal melt atomization and deposition into a single-step operation. Rapid solidification effect inherent in spray deposition provides a substantial chemical and microstructure homogeneity together with refined grains and the extension of solid solubility [13], [14], [15]. However, our understanding of microstructure characteristics and its formation mechanism in spray-deposited materials is still limited. The relationship between spray-forming conditions and microstructure and mechanical behavior of the alloy needs to be further systemically researched.

In this study, the spray deposition technique was employed to produce high-solute 7055 billets, and alloy composition, microstructure features and related mechanical properties were explored in detail. The formation mechanism of equiaxed grains and microporosities in as-deposited alloys were also discussed. In order to eliminate microporosity defects and improve the mechanical properties, the as-deposited 7055 alloys were performed under the hot extrusions treatment at 450 °C with different area reduction ratio. The effects of extrusion on strength and elongation of the alloys were assessed through ambient uniaxial-tensile tests.

Section snippets

Experimental details

The actual composition of the studied 7055 alloy was: 8.31Zn, 2.07Mg, 2.46Cu, 0.12Zr (wt%) and balance aluminum, which was determined by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) measurement.

The spray-deposition experiments were conducted with a SFZD-5000 type environmental chamber which was manufactured by the Haoran Co. Ltd., Jiangsu, China. During spray-deposition process, the molten metal was atomized by N2 at 820 °C, the distance of atomizing deposition was kept

Results and discussions

Fig. 1a showed a schematic diagram of the spray-forming deposition process and principle. Spray forming technology in the Osprey mode involves sequential stages of metal melting, gas atomization, micro-droplets spraying, deposition and semisolid slurry consolidation. Fortunately, this novel technology can produce near-net shape preforms through the integration of the above stages as one single-step operation naturally. Another important characteristic of spray forming technology is high cooling

Summary

High-solute large billets of 7055 aluminum alloy were successfully fabricated by spray forming rapid solidification technology. The surfaces of as-deposited 7055 billets were smooth and compact without surface cracks. The microstructure observation indicated that the as-deposited alloys had equiaxed grains with the size of 30–50 μm which was finer than that of as-cast alloys. The detrimental dendrite microstructure was suppressed and undesirable macrosegregation did not exist in our as-deposited

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (No. 531107040229), the National Natural Science Foundation of China under Grant Nos. 51271203, 21103046 and 11274365, and partly by Jiangsu Hao Ran Spray Forming Co., Ltd. Cooperation project fund.

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