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.

Friction Stir Processing of Magnesium Alloys: A Review

With the increasing demand for bone implant therapy, titanium alloy has been widely used in the biomedical field. However, various potential applications of titanium alloy implants are easily hampered by their biological inertia. In fact, the interaction of the implant with tissue is critical to the success of the implant. Thus, the implant surface is modified before implantation frequently, which can not only improve the mechanical properties of the implant, but also polish up bioactivity and osseoconductivity on a cellular level. This paper aims at reviewing titanium surface modification techniques for biomedical applications. Additionally, several other significant aspects are described in detail in this article, for example, micromorphology, microstructure evolution that determines mechanical properties, as well as a number of issues concerning about practical application of biomedical implants.

Surface Modification of Biomedical Titanium Alloy: Micromorphology, Microstructure Evolution and Biomedical Applications

Friction-based welding processes are considered as very efficient solid-state metal joining processes due to soundness of the welded joint with remarkably less energy consumption and environmentall

Friction-based welding processes: friction welding and friction stir welding

In-vitro bio-corrosion behavior of friction stir additively manufactured AZ31B magnesium alloy-hydroxyapatite composites.

Friction stir technique played a vital role in recent industries as it has been utilized in welding and processing of metallic materials. Friction stir welding (FSW) is applied for joining the poorly weldable materials and enhancing the microstructure and the mechanical properties of the welded joints. Friction stir spot welding (FSSW), underwater friction stir welding (UFSW) and vertical compensation friction stir welding (VCFSW) are variants of FSW process. On the other hand, friction stir processing (FSP) is another method, whose basic principal originated from friction stirring technique, which can be utilized for manipulating the base materials by performing dynamic recrystallization on grains resulting in superior properties of the processed material. Friction stir alloying (FSA) is analogous to FSP with implanted reinforcement particles, producing surface composites. Like other fusion welding techniques, FSW process has its own defects which especially characterize the friction stir welded joints. Tunnels, voids, flash, lack of penetration, kissing bond and surface grooving are the common defects of FSW method. Since friction stirring action generates both thermal and mechanical loads beside the higher plastic deformation, finite element modeling (FEM) has been used in model and simulation this of the process. A few research gaps are pin pointed and some research recommendations are included.

Welding and processing of metallic materials by using friction stir technique: A review

Manufacture of biodegradable magnesium alloy by high speed friction stir processing

Microstructural characterization and mechanical properties of micro friction stir welded dissimilar Al/Cu ultra-thin sheets

Material flow during friction stir welding of ultra-thin 2024 aluminium alloy plates was investigated by using copper powder as marker material. Based on the visualised experiment and numerical computation of material flow produced in the research, the effect of rotational speed on material flow was revealed. In high rotational speed FSW process, flow distance along welding direction of plastic metal on both sides and transverse flow distance to the weld centre line on the retreating side were shorter; however, a larger flow velocity was shown. The flow velocity of plastic metal decreased with the increasing of distance from the weld centre, while it increased with the increasing of rotational speed under a constant rotational speed/welding speed ratio. Maximum and minimum values of the flow velocity of plastic metal were located at the intersection of triple spiral groove of shoulder and both sides of its outer diameter edge, respectively.

Visualisation and numerical simulation of material flow behaviour during high-speed FSW process of 2024 aluminium alloy thin plate

0.5-mm-thick 7075-T6 aluminum alloy sheets were butt joined successfully by high-speed micro friction stir welding (μFSW) as well as conventional μFSW using a pinless tool. The influences of welding parameters on the axial force, residual stresses, and deformations were studied through experiments. Good surface morphology and defect-free joints were obtained by high-speed μFSW as well as conventional μFSW in a fixed rotational speed/welding speed of 6.67 rad/mm. The axial force during μFSW increased with the increment in welding speed and decreased with the increment in rotational speed. When the ratio of rotational speed/welding speed was 6.67 rad/mm, compared with conventional μFSW (2000/300), the lower axial force of 1460 N was obtained by using high-speed μFSW (10,000/1500). Longitudinal tensile residual stress and transverse compressive residual stress occurred around the weld zone for all μFSW joints. The residual stresses of joint fabricated by a parameter of 10,000/1500 were slightly less than that fabricated by 2000/300. Both longitudinal maximum bending deformation Zmax and transverse angular deformation α increased with the increment in rotational speed and decreased with the increment in welding speed. As a comparison to 2000/300, the welded sheet with a parameter of 10,000/1500 exhibited slightly lower Zmax and α.

Influences of welding parameters on axial force and deformations of micro pinless friction stir welding

Aiming at the difficulty of feeding filler wire in the dual laser-beam synchronous welding (DLSW) of Ti–6Al–4V titanium alloys T-joints, the self-fusion DLSW process based on prefabricated welding materials on the stringer was developed. Prefabricated bosses on both sides of the stringer were used as welding materials. In this paper, the influence of boss size and laser power on the weld formation was studied. The influence of microstructure transformation on mechanical properties was demonstrated. It is found that T-joints with desirable weld formation can be obtained when the boss size of 0.8 × 0.8 mm2 was applied. The temperature distribution of the T-joint during laser welding process was obtained by finite element simulation method. It indicates that the cooling rate of fusion zone is 140 °C/s, indicating that massive transformation occurs in the fusion zone. The microstructure of fusion zone characterized by acicular martensite plates α′ and a small amount of intergranular α phase meets with the cooling rate of FZ. Due to multiple strengthening effects, the average microhardness of fusion zone is 351.6 HV, which is higher than the average microhardness of base material. The pull-out strength of T-joint is not related to the weld penetration, and its average pull-out strength is 1035.5 MPa. The fracture occurred in the weld toe of the pull-out specimen, and fracture surface presents quasi-cleavage fracture characteristics. 

Dingqiang Qin

Papers

Manufacture of biodegradable magnesium alloy by high speed friction stir processing

Microstructural characterization and mechanical properties of micro friction stir welded dissimilar Al/Cu ultra-thin sheets

Visualisation and numerical simulation of material flow behaviour during high-speed FSW process of 2024 aluminium alloy thin plate

Influences of welding parameters on axial force and deformations of micro pinless friction stir welding

Dual laser-beam synchronous self-fusion welding of Ti-6Al-4V titanium alloys T-joints based on prefabricated welding materials