Effects of specimen and grain size on electrically-induced softening behavior in uniaxial micro-tension of AZ31 magnesium alloy

Significance 

Micro-forming has become popular in terms of process, modeling, and material, as a micro-manufacturing method for scale production. Unfortunately, size effect has been identified to result in deviations in material and process performances with miniaturization, such as the increase of forming defects and decrease of forming limit and accuracy. Electrically-assisted forming that is a hybrid system applying electricity to metals during plastic deformation, has technical benefits such as improved formability, improved surface quality, reduced stringback, and enhanced geometric accuracy of parts. For that reason, applying electricity to assist in plastic micro-forming is a suitable approach for eliminating the size effect.

Several research works associated with electrically assisted forming are mainly based on the effect of electric pulse parameters on mechanical changes as well as microstructural evolutions of various metals. However, only a few of these focuse on grain size effect on electrically-induced softening behavior and specimen size effect on the electrically-assisted deformation behavior. In addition, most of these studies were conducted at macro-scale level instead of the meso/micro-scale level.

Researchers from Harbin Institute of Technology of China and Northwestern University of USA jointly investigated the effects of grain size and specimen size on the electrically-induced softening behavior in electrically-assisted micro-forming based on the combination of experimental as well as modeling methods. Their research work is published in journal, Materials & Design.

The authors conducted uniaxial micro-tension tests of AZ31 magnesium alloy samples exposed to varying current densities for a number of grain sizes as well as specimen sizes. They presented an approach to assess the effect of electric current on the decrease of tensile strength in electrically-assisted micro-tension, and this resulted in further characterization of the influence of grain and specimen sizes on the electrically-induced softening behavior of AZ31.

The research team then proposed a semi-empirical model of the electrically-induced softening behavior taking into account the size effect in electrically-assisted micro-tension in a bid to forecast the softening behaviors of about five engineering metals reference to a number of current densities during uniaxial tension.

The authors observed that the increasing rate of the average minimum temperature against the square of current density reduced with miniaturization, but remained unchanged with grain size. Electrically-induced softening parameter and the current density followed an inverse-S-shaped decay curve. The curve shifted to the lower and higher current density regions with the increase in specimen size and grain size, respectively.

The researchers observed that a lower current density was adequate for smaller grain sizes and larger sample sizes in order to realize higher softening effect. This indicated that grain number would be an important aspect affecting electrically-induced softening.

The authors used the size effects on electrically-induced softening to modify a semi-empirical softening function of current density that would be implemented to forecast the electrically-induced softening behaviors of five metals. The authors defined and formulated a current density threshold in electrically-induced tension based on semi-empirical softening function. The semi empirical softening function decreased with electrical resistivity and specimen size, and increased non-linearly with the grain size.

Effects of specimen and grain size on electrically-induced softening behavior in uniaxial micro-tension of AZ31 magnesium alloy: Experiment and modeling. Advances in Engineering

About the author

Dr. Xinwei Wang is a Research Associate in Space Environment Simulation and Research Infrastructure (SESRI) of Harbin Institute of Technology (HIT), received his doctoral degree from HIT in 2016 and was a visiting doctoral student at Northwestern University in 2013-2015. His main research interests are novel micro-forming techniques, material characterization and multi-physics modeling, space environment effect on materials.

About the author

Dr. Jie Xu is an Associate Professor of Academy of Fundamental and Interdisciplinary Sciences and Deputy Director of Key Laboratory of Micro-systems and Micro-structures Manufacturing of Ministry of Education, Harbin Institute of Technology. He is the co-corresponding author of this paper. His main research interests are micro-forming technology, ultrafine-grained materials and non-conventional energy field assisted micro-manufacturing. He has published over 50 papers and 20 patents.

About the author

Dr. Debin Shan is a full professor of School of Materials Science and Engineering and the executive vice director of the National Key Laboratory for Precision Hot Processing of Metals at Harbin Institute of Technology (HIT). He has long been engaged in the basic theory and equipment research for plastic forming of materials and micro-forming of metals. He has published over 300 peer-reviewed papers and over 50 patents. He has been invited to write 2 chapters of English books and 7 chapters of Chinese books.

About the author

Dr. Bin Guo is the Professor of School of Materials Science and Engineering, and Vice President at Harbin Institute of Technology. He also is the Dean of Industrial Technology Research Institute of Heilongjiang Province, Commander-in-chief of Nation-level large-scale Scientific Project and Director of Key Laboratory of Microsystems and Micro-structures Manufacturing of Ministry of Education. He has long been engaged in precision metal forming and micro/nanno manufacturing technology. He published more than 200 journal papers and 50 patents.

About the author

Dr. Jian Cao (MIT’95, MIT’92, SJTU’89) is the Cardiss Collins Professor, Director of Northwestern Initiative for Manufacturing Science and Innovation, and an Associate Vice President for Research (AVPR) at Northwestern University. She is the co-corresponding author of this paper. Her current research on flexible dieless forming, micro-forming, laser ablation processes and additive manufacturing has direct impacts on energy-efficient manufacturing, surface engineering and distributed manufacturing. She has published over 300 technical articles, including over 150 journal articles, 14 book chapters, and over 10 patents. She is a Fellow of ASME, CIRP and SME.

Reference

Xinwei Wang, Jie Xu, Debin Shan, Bin Guo, Jian Cao. Effects of specimen and grain size on electrically-induced softening behavior in uniaxial micro-tension of AZ31 magnesium alloy: Experiment and modeling. Materials & Design, volume 127 (2017), pages 134–143.

 

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