Microstructure, bonding evolution and mechanical properties of continuously extruded magnesium AZ31 sheet

Significance 

Magnesium alloys are potential candidates for lightweight applications owing to their remarkable mechanical properties. Several methods for producing magnesium sheets, such as twin-roll casting and traditional rolling, have been developed. Unfortunately, due to the difficulty in deforming magnesium sheets at low temperatures, the use of these methods is expensive for large-scale industrial production. As a promising alternative, the continuous extrusion extending forming (CEEF) technique has been proposed to produce magnesium sheets at relatively low costs. This technique enables effective control of the microstructure to enhance the properties of the resulting magnesium alloy sheets. Nevertheless, despite the extensive research on magnesium alloy production, research on the application of the CEEF technique is limited.

Previous findings revealed that continuous extrusion is a potential energy-saving manufacturing method as it depends on friction rather than an external heat source. It also changes the basal texture by refining grains, thereby improving the product’s ductility and strength. Continuous extrusion with double feedstock exhibits superior performance than that with single feedstock as far as metal flow optimization and distribution of extrusion temperature are concerned. This is extremely beneficial to the detailed investigation of the microstructural evolution and properties of the extruded magnesium sheets. However, studies on CEEF with double rods as well as the associated properties and microstructure of magnesium alloy sheets, are sparse.

Despite the extensive research work, developing cost-effective and highly efficient sheet forming approaches has remained a great challenge. This can be attributed to the poor formability of magnesium that often leads to severe edge cracking. To address this problem, Dalian Jiaotong University researchers: Dr. Lili Guo, Ms. Jingru Yuan, Dr. Jiuyang Pei and Dr. Ying Zhao proposed a continuous extrusion extending forming method for cost-effective production of magnesium alloy sheets via solid-state extrusion welding of two rods. Dr. Lilli Guo, Ms. Jingru Yuan, together with Mr. Kai Zhang and Dr. Jun Jiang from Imperial College London investigated the bonding evolution, microstructural evolution and mechanical properties of an extruded magnesium AZ31 sheet during the CEEF process. The research paper is currently published in the journal, Materials Science and Engineering A.

In their approach, the novel CEEF technique was used to produce an 8 mm thick and 160 mm wide AZ31 sheet. The experimental work was conducted using a TLJ400 continuous extrusion machine and involved feeding the two rods into the groove to extrude the sheets. The CEEF evolution process as well as the microstructural evolution and properties of the extruded sheets, were thoroughly studied via a combination of experimental and finite element simulation.

The findings demonstrated successful weld extrusion of the AZ31 sheets with remarkably good welding quality, as indicated by the high temperature in the welding region (450 °C) and high deformation strain greater than 200%. A huge disparity in the velocity, temperature and strain was observed between the welding regions and feeding rods at the chamber entrance, but it nearly diminished towards the exit allowing for the formation of sheets with high tensile strength. Additionally, several tensile twins and banded microstructures, induced by the rotational dynamic crystallization and non-basal slip, were noted in the welding region. Elongation in the extrusion and transverse directions was reported to be 18.6% and 9.1%, respectively.

In a nutshell, the study reported a novel CEEF method for producing AZ31 sheets through extrusion welding of double rods. The microstructural properties and bonding evolution around the welding interface areas were thoroughly investigated and discussed. The high weld quality was beneficial in suppressing cracks in the weld region to produce AZ31 sheets with improved properties. By addressing some of the inherent issues associated with the convention sheet forming processes, the authors, in a statement to Advances in Engineering, explained the presented new technique is viable, robust, cost-effective, and would transform the future production of magnesium sheet alloys.

Study of the microstructure, bonding evolution and mechanical properties of continuously extruded magnesium AZ31 sheet - Advances in Engineering
Figure (a) Schematic of continuous extrusion extending forming process with double rods as feedstock, (b) simulation results of strain distribution inside the extrusion cavity, (c, (d), (e), (f) IPF of the welding regions inside (c) the chamber, (d, e) extending die and (f) extrusion die of the ED-ND plane during continuous extrusion of AZ31 Mg alloys.

About the author

Dr. Lili GUO is currently an Associate Professor in Materials Science and Engineering School, Continuous Extrusion Engineering Research Center, Ministry of Education, in Dalian Jiaotong University, China. She earned her Ph.D. in Materials Science and Engineering from Tohoku University, Japan, in 2011.

Her research interests are concentrated on light-weight metals, such as Magnesium, Aluminium alloys as well as composite materials, and the microstructure, texture and mechanical properties, finite element simulation of plastic deformation. She is also a regular reviewer for certain international journals.

About the author

Dr. Ying Zhao is currently an Associate Professor in Materials Science and Engineering School, Continuous Extrusion Engineering Research Center, Ministry of Education, in Dalian Jiaotong University, China. She earned her Ph.D. in Materials Science and Engineering from Dalian Jiaotong University, in 2016.

Her research interests are light-weight metals continuous extrusion process, numerical simulation and product properties.

Reference

Guo, L., Yuan, J., Pei, J., Zhao, Y., Zhang, K., & Jiang, J. (2021). Study of the microstructure, bonding evolution and mechanical properties of continuously extruded magnesium AZ31 sheet. Materials Science and Engineering: A,819, 141456.

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