Friction Stir Processing of Magnesium Alloys: A Review
Wen Wang1,2,3, Peng Han1,2, Pai Peng1,2, Ting Zhang1,2, Qiang Liu1,2, Sheng-Nan Yuan1,2, Li-Ying Huang1,2, Hai-Liang Yu4, Ke Qiao1,2, Kuai-She Wang1,2
1 School of Metallurgical Engineering Xi’an University of Architecture and Technology, Xi’an 710055, China
2 National and Local Joint Engineering Research Center for Functional Materials Processing, Xi’an 710055, China
3 Experimental Teaching Demonstration Center for Materials Processing Xi’an University of Architecture and Technology, Xi’an 710055, China
4 College of Mechanical and Electrical Engineering Central South University, Changsha 410083, China
Magnesium (Mg) alloys have been extensively used in various fields, such as aerospace, automobile, electronics, and biomedical industries, due to their high specific strength and stiffness, excellent vibration absorption, electromagnetic shielding effect, good machinability, and recyclability. Friction stir processing (FSP) is a severe plastic deformation technique, based on the principle of friction stir welding. In addition to introducing the basic principle and advantages of FSP, this paper reviews the studies of FSP in the modification of the cast structure, superplastic deformation behavior, preparation of fine-grained Mg alloys and Mg-based surface composites, and additive manufacturing. FSP not only refines, homogenizes, and densifies the microstructure, but also eliminates the cast microstructure defects, breaks up the brittle and network-like phases, and prepares fine-grained, ultrafine-, and nano-grained Mg alloys. Indeed, FSP significantly improves the comprehensive mechanical properties of the alloys and achieves low-temperature and/or high strain rate superplasticity. Furthermore, FSP can produce particle- and fiber-reinforced Mg-based surface composites. As a promising additive manufacturing technique of light metals, FSP enables the additive manufacturing of Mg alloys. Finally, we prospect the future research direction and application with friction stir processed Mg alloys.
Key words: Friction stir processing Magnesium alloy Superplasticity Grain refinement Mg-based surface composites Additive manufacturing
Fig.1 Microstructures of a BM, b FSPed, c aged samples, d legend and direction . ND normal direction, PD processing direction, TD transverse direction
Fig.5 Schematic representation of SMMCs fabrication by FSP
Summary and Prospect
FSP, as a severe plastic deformation technique, exhibits great advantages in the modification of the cast structure, superplastic deformation, preparation of fine-grained Mg alloys and Mg-based SMMCs, and additive manufacturing. The summary and prospect in research and application of FSP are as follows:
(1) FSP can modify the cast structure of Mg alloys, leading to the break and rapid dissolution of network-like secondary phases, thus decreasing the composition segregation. Moreover, the duplex process of FSP and post-aging heat treatment is an effective method to improve the mechanical properties of precipitation-strengthened Mg alloys. In addition, FSP is also an effective method combination to repair the microstructure of fusion-welded joints of Mg alloys, therefore improving their strength. In the future, it is important to focus on how to enhance the engineering application of FSP in the surface defect repair of cast Mg alloys.
(2) FSP can be used to prepare fine-, ultrafine-, and even nano-grained Mg alloys. The main reason for grain refinement is the dynamic recrystallization during FSP. The recrystallization mechanism includes continuous dynamic recrystallization, discontinuous dynamic recrystallization, and twinning-induced recrystallization. Fine-grained Mg alloys prepared by FSP show low sensitivity of fine-grain strengthening due to shear texture. Therefore, it is necessary to study the relationship between texture and mechanical, corrosion, and biological properties. In the future, the stability of FSP should be further controlled, while the modification area should be expanded in order to prepare bulk mass fine-grained Mg alloys.
(3) The microcrystalline superplasticity, HSRS, and LTSP of Mg alloys prepared by FSP can be achieved. It is generally believed that the superplastic deformation mechanism of FSPed Mg alloys is the multimechanism dominated by GBS. The coordination mechanism of GBS in FSPed Mg alloys includes grain boundary diffusion, dislocation slip, and creep. Future research efforts should focus on how to accelerate the application of FSP in the whole or selection superplastic forming of Mg alloys.
(4) The Mg-based SMMCs can be successfully prepared by FSP, with the reinforcing particles mainly including CNTs, carbon fiber, SiC, SiO2, Al2O3, B4C, TiC, ZrO2, fly ash, hydroxyapatite, and stainless steel powder. The agglomeration and uniform distribution of reinforcing particles can be improved by increasing the number of processing passes. The surface hardness, tensile strength, and wear resistance of Mg alloys are improved by adding reinforcing particles. Dispersion strengthening and fine-grained strengthening are the main mechanisms for the improvement in mechanical properties of Mg-based SMMCs. The following challenges are worth paying attention to in the future: firstly, to prepare the complex and irregular Mg-based SMMCs; secondly, to accurately control the amount of reinforcing particles; and thirdly, to improve the uniformity of reinforcing particles.
(5) FSP additive manufacturing is a prospective method for fabricating bulk Mg alloys with excellent mechanical properties, such as low residual deformation, homogeneous and dense microstructure. However, the research on FSP additive manufacturing technique on Mg alloys has been just initiated as there are some shortcomings in the design, process, equipment, as well as theoretical considerations. It is necessary to carry out a systematic study on the relations between design, process parameters, microstructure, and properties. Equally important is to develop FSP additive manufacturing equipment and realize its engineering application.
(6) In addition to fabricating high-performance structural materials, FSP can also be used to prepare functional materials, such as gradient functional materials, biomedical materials, and hydrogen-storage materials. Therefore, in the future research efforts, researchers should actively explore new application fields for FSP.