Wen Wang

Magnesium (Mg) and its alloys have attached more and more attention because of their potential as a new type of biodegradable metal materials. In this work, AZ31/ZrO₂ nanocomposites with good uniformity were prepared successfully by friction stir processing (FSP). The scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to characterize the microstructure of the composites. The mechanical properties, electrochemical corrosion properties and biological properties were evaluated. In addition, the effect of reinforced particles (ZrO₂) on the microstructure and properties of the composite was studied comparing with FSP AZ31 Mg alloy. The results show that compared with the base metal (BM), the AZ31/ZrO₂ composite material achieves homogenization, densification, and grain refinement after FSP. The combination of dynamic recrystallization and ZrO₂ particles leads to grain refinement of Mg alloy, and the average grain size of AZ31/ZrO₂ composites is 3.2 μm. After FSP, the c-axis of grain is deflected under the compression stress of shoulder and the shear stress of pin. The ultimate tensile strength (UTS) and yield strength (YS) of BM were 283 and 137 MPa, respectively, the UTS and YS of AZ31/ZrO₂ composites were 427 and 217 MPa, respectively. The grain refinement and Orowan strengthening are the major strengthening mechanisms. Moreover, the corrosion resistance in simulated body fluid of Mg alloy is improved by grain refinement and the barrier effect of ZrO₂.

Friction stir processing (FSP) and subsequent aging heat treatment were used to modify the microstructure of AZ80 magnesium alloy. The influence of FSP and aging heat treatment on the corrosion behavior was systematically studied by using potentiodynamic polarization, immersion, and slow strain rate tensile tests. The results revealed that FSP led to grain refinement, rapid dissolution of β‐phase, and the deflection of c‐axis from transverse direction (TD) and processing direction (PD) by approximately 55° and 25°, respectively, improving the static corrosion and stress corrosion cracking (SCC) resistance. The aging heat treatment rendered a little influence on the grain size and slightly affected the grain orientation. The content of β‐phase in FSP‐5 and FSP‐24 samples was 8.1 and 21.8 wt.%, respectively. Static corrosion and SCC resistance of FSP‐5 and FSP‐24 samples were lower than those of the FSP samples. Compared with FSP‐5 sample, the amount of β‐phase and the proportion of Al₂O₃ increased in FSP‐24 sample, leading to enhanced static corrosion and SCC resistance. SCC behavior was controlled by anodic dissolution, whereas the presence of hydrogen accelerated the SCC.

In this study, the fine‐grained Mg–Li alloy was prepared by friction stir processing (FSP). The microstructure and mechanical properties of the friction‐stir‐processed (FSPed) Mg–Li alloy were investigated. The result showed that FSP resulted in the grain refinement, and the average grain size of the β‐Li phase was about 7.5 μm. Besides the α‐Mg and β‐Li phases, a small amount of Li ₃ Mg ₇ , Li ₂ MgAl and AlLi phases were obtained. Compared with the base metal (BM), the weakening of the crystallographic texture occurred in the FSPed material, and the c ‐axis of the α‐phase and the crystallographic orientation of the β‐phase were tilted about 45° with respect to the transverse direction (TD). The average microhardness (HV 67.8) of the stir zone was higher than that of the BM (HV 61.5). The yield strength (YS) and the ultimate tensile strength (UTS) of the FSPed material were higher than those of the BM, while the elongation slightly reduced. Grain refinement had more significant effect on strength improvement compared with the texture variation for the FSPed material. The fracture surfaces of the BM and FSPed materials showed dimple characteristics.

Constitutive analysis for elevated temperature flow behavior of BFe10-1-2 alloy was carried out by using experimental stress–strain data from isothermal hot compression tests on a Gleeble-3800 thermo-mechanical simulator, in a wide of temperature range of 1,023–1,273 K, and strain rate range of 0.001–10 s ⁻¹ . A constitutive equation based on double multiple nonlinear regression (DMNR) was proposed considering the independent effects of strain, strain rate, temperature and their interrelation. The predicted flow stress data obtained from the developed equation based on DMNR was compared with the experimental data. Correlation coefficient ( R ), average absolute relative error ( AARE ) and relative errors were introduced to verify the validity of the developed constitutive equation. The results showed that the developed constitutive equation based on DMNR could predict flow stress of BFe10-1-2 alloy with good correlation and generalization.

Submerged friction stir processing (SFSP), that is, the entire process was carried out underwater, was successfully conducted on low carbon steel. The microstructure, electrochemical corrosion, and salt fog corrosion properties of SFSP low carbon steel were investigated. The results indicated that phase transformation and dynamic recrystallization occurred during SFSP and finally resulted in a dual phase fine grain microstructure in the processed zone (PZ), which consisted of ferrite and homogenous martensite phase. The grain size of ferrite was refined to 5.7 µm, and the width of martensite phase was 650 nm, respectively. In addition to changing the microstructure of low carbon steel, SFSP could also affect its corrosion property. SFSP samples exhibited better resistance against electrochemical corrosion and salt fog corrosion than BM samples. The reason was that, grain refinement and martensite formation induced by SFSP were conducive to forming dense protective layer, which effectively hindered the electrochemical corrosion and salt fog corrosion of the SFSP low carbon steel.