Abstract
Friction stir processing (FSP) has been considered as a novel technique to refine the grain size and homogenize the microstructure of metallic materials. In this study, two-pass FSP was conducted under water to enhance the cooling rate during processing, and an AZ61 magnesium alloy with fine-grained and homogeneous microstructure was prepared through this method. Compared to the as-cast material, one-pass FSP resulted in grain refinement and the β-Mg17Al12 phase was broken into small particles. Using a smaller stirring tool and an overlapping ratio of 100%, a finer and more uniform microstructure with an average grain size of 4.6 μm was obtained through two-pass FSP. The two-pass FSP resulted in a significant improvement in elongation of 37.2% ± 4.3%, but a slight decrease in strength compared with one-pass FSP alloy. Besides the microstructure refinement, the texture evolution in the stir zone is also considered responsible for the ductility improvement.
Highlights
Improving the ductility of magnesium alloys through grain refining has drawn great interest, as the application of magnesium alloys is generally limited by their poor formability
Dadashpour et al [10] investigated the effect of pass number on the microstructure and properties of friction stir processing (FSP) AZ91C Mg alloy and attributed the enhancement of mechanical properties to reinforcement of the second phase and homogenization of microstructure
Microstructure and mechanical properties of the AZ61 alloy prepared by one-pass and two-pass SFSP are investigated in the present work
Summary
Improving the ductility of magnesium alloys through grain refining has drawn great interest, as the application of magnesium alloys is generally limited by their poor formability. Fine-grained metallic materials including Al, Mg and Ti alloys prepared by FSP have been studied extensively, and the properties of these materials are generally improved due to microstructure refinement [2,3,4]. FSP is an effective and efficient method of preparing fine-grained magnesium alloys, according to the literatures [1,5,6]. Based on FSP, some modified methods have been developed to further decrease the grain size by: (1) enhancing the cooling rate during FSP through copper backing plate with higher thermal conductivity, water or liquid nitrogen with higher heat absorbility [7,8,9]; and (2) conducting two or more FSP passes on base material (BM), i.e., multi-pass FSP (MFSP) [10,11,12]. MFSP can be used to repair the defects that appear in the previous processing [13]
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