Effects of cryogenic temperature and pre-stretching on mechanical properties and deformation characteristics of a peak-aged AA6082 extrusion

Significance Statement

Aluminum 6000 series alloys play a role in light weight structural applications owing to their attractive mechanical properties, superior corrosion resistance, and good weld ability. Among the aluminum alloys in industrial applications, AA6082 alloy is widely used. The material can be heat treated to attain unique degrees of precipitation.

For all structural elements, mechanical properties are the most essential physical properties that dictate the range of practical applications. However, mechanical properties are dependent on material composition, crystal defects, phase structures, and particularly, temperature. Dr. Zebing Xu and Professor Hans Roven from the Norwegian University of Science and Technology in collaboration with Professor Zhihong Jia at Chongqing University in China reported that aluminum alloys had desirable properties at cryogenic temperature that made them candidates for cryogenic engineering applications. In their study, they found that a simultaneous improvement in strength and ductility of the peak-aged AA6082 aluminum alloy occurred at 77K, but not 295K. Their work is published in journal, Materials Science & Engineering A.

With the intention of studying the effect of pre-stretching at 77K and 295K, an AA6082 alloy was heat treated at 185 ᵒC to maximum hardness. The heat treatment was done because the peak-aged state is the most strain localizing condition. Rectangular tensile test pieces were machined from a plate mid-section of the T6-aged material. This implies that the tensile axis was parallel to the extrusion direction while the test piece width was in the normal direction. Three specimens were subjected to tensile tests at each test temperature.

In addition to these peak-aged pieces tensioned to fracture directly, other six samples were pre-strained at 295K and six others at 77K. Then, three specimens from every pre-straining test temperature were stored at room temperature for four to five and finally tensioned to fracture. The remaining pre-strained specimens were annealed and tensile tests done.

The different contrasts in the Electron Backscatter Diffraction maps obtained in the study corresponded to different grain alignment, indicating lamellar grains positioned parallel to extrusion axis. The alloys contained manganese and iron that formed temperature stable dispersoids that prevented recrystallization. Therefore, the hot extruded material formed a deformed fibrous grain matrix.

Parallel to the fractographic analyses, the deformed morphologies close to fracture of tensile samples provided the authors with more information about the deformation mechanisms. The areas observed were exposed to more strain as compared to the specimen gauge length.

This study successfully investigated the cryogenic behavior of an AA6082 aluminum alloy in the T6 temper condition using tensile tests. The associated characterization of texture, and microstructure and various engagement of several deformation studies were related to the characteristic mechanical behavior. Pre-stretching at the two test temperatures and tensile testing indicated a marked yield point at 77K, higher ductility and yield strength were recorded.

“The AlMgSi alloy shows impressive improvement in mechanical behavior at 77K, as compared to room temperature. The improvement is due to higher friction- and back stress of dislocations, which in turn create more homogeneous distribution of strain at the atom- to the micrometer scale. This is a big surprise, as the study was conducted in aging condition T6” Said Professor Hans Roven.

Effects of cryogenic temperature and pre-stretching on mechanical properties and deformation characteristics of a peak-aged AA6082 extrusion 1 - Advances in Engineering

Effects of cryogenic temperature and pre-stretching on mechanical properties and deformation characteristics of a peak-aged AA6082 extrusion 2 - Advances in Engineering

About The Author

Hans J. Roven received his PhD degree in physical metallurgy in 1989 from NTH (now NTNU), Norway. He is professor at Department of Materials Science and Engineering, Norwegian University of Science and Technology, NTNU (since 1993). He served as Qatalum-Hydro Chair professor at Qatar University in 2013-2014. In the period 2008-2013 he was vice-dean and director of the Strategic Area Materials at NTNU. In 2007 Roven was officially appointed Guest Professor to Shanghai Jiao Tong University and in 2015 Honorary Professor to Jiangsu University, China. Roven has more than 160 papers published in international peer-reviewed journals & proceedings, edited 4 book chapters and he has given more than 100 international presentations. He was at the University of Virginia, The Light Metals Centre, as a postdoc in 1988 -89, and a visiting professor at Colorado School of Mines (2001-2002) and Shanghai Jiao Tong University (2009-2010). In 2004-2005 he served in the National Advisory group to the Norwegian Research Council within Engineering Sciences. He is also peer-reviewer to several national research councils in Europe and North America and he serves as reviewer to numerous international scientific journals.

For 2010- 2012, Roven was appointed a role as an International Expert and engaged by the European Research Council (ERC) evaluating the highly prestigious ‘EU Consolidator and Starting Grants’. Since 2016, he has been appointment a ‘111- foreign expert’ status at Chongqing University in China. He is a member of the Norwegian Academy of Science and Technology, NTVA (1999 -), The Royal Norwegian Academy of Sciences and Letters , DKNVS (2009-), the Materials Research Society, MRS, (2007-), TMS (1991-), the American Society for Metals ASM (1991-) and the American Association for Advancement of Science AAAS (2006-). In 2013 he received the highest ranked Price for Internationalization at NTNU.

His research interests include physical metallurgy of light metals (Al, Mg, Ti), fatigue, nano-structuring of metals, especially novel – but also classical forming processes, 3D printing, rapid solidification, solid state recycling, functional properties of metals, and multi-scale structure characterization of light metals. The research has an overall focus on sustainability, innovation and new solutions, national and international societal impacts.

He has supervised 7 postdocs, 11 PhDs (main supervisor), 9 PhDs (co-supervisor) and ~ 110 MSc students as main supervisor. Roven has a long experience with international collaboration to USA, China, UK, France, Italy, Russia, Germany, Hungary, India and Qatar.

About The Author

Dr. Zebing Xu was educated at Chongqing University (China) and Norwegian University of Science and Technology, NTNU, Norway. He got his PhD degree from NTNU in January 2017 and his education background is within materials science and physical metallurgy. His main research fields are microstructure and the related properties of aluminum alloys, plastic deformation mechanisms of light alloys and thermodynamics of magnesium alloys. During his PhD work, he focused on cryogenic temperature deformation behavior of aluminium 6xxx series alloys and he has authored four research papers.

About The Author

Zhihong Jia, professor (since 2010.10) in College of Materials Science and Engineering, Chongqing University (CQU) in Chongqing, China. He got his PhD from Philipps University of Marburg, Germany in 2005, and subsequently worked as postdoc/researcher at Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU) in the period of 2005-2010. His research interests are focused on aluminum materials; materials microstructure and the relationship to properties; advanced electron microscopy.

He has worked on more than 20 projects from China, Germany and Norway in the past years. Jia has 65 publications with 53 SCI indexed and more than 20 invited presentations in international and national conferences. He, as a key person, had organized the 15th International Conference on Aluminum Alloys (ICAA15, 2016), and the First East-Asian Electron Microscopy (EAMC1, 2013). He is a member of Focus Ion Beam (FIB) Committee of China, a member of Chongqing new material expert committee, a member of Chinese Electron Microscopy Society, and the reviewer for many scientific journals.

Journal Reference

Zebing Xu1, Hans J. Roven1, and Zhihong Jia2. Effects of cryogenic temperature and pre-stretching on mechanical properties and deformation characteristics of a peak-aged AA6082 extrusion. Materials Science & Engineering A, volume 679 (2017), pages 379-390.

Show Affiliations
  1. Department of Materials Science and Engineering, NTNU, Norwegian University of Science and Technology, 7491 Trondheim, Norway.
  2. College of Materials Science and Engineering, Chongqing University, 400044, China.

 

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