Guobing Wei

High fiber loadings often cause a performance cliff in short-glass-fiber-reinforced thermoplastics because of fiber agglomeration. Herein, polybutylene terephthalate (PBT) composites reinforced with flat glass fibers (FGFR) and conventional circular glass fibers (CGFR) are systematically compared at loadings of 20–50 wt%. The flat fibers exhibit a specific surface area of 0.50 μm⁻¹, approximately 1.61 times that of the circular fibers (0.31 μm⁻¹). At 50 wt% loading, FGFR achieves a tensile strength of 115.92 ± 1.29 MPa and a notched impact strength of 13.07 ± 2.06 kJ/m², which are 11.4% and 29.7% higher than those of CGFR, respectively (p < 0.001 and p < 0.05, Welch’s t-test). In contrast, CGFR reaches its maximum tensile strength of 107.45 ± 3.84 MPa at 40 wt% and then decreases by 3.2% at 50 wt%; together with a significant quadratic (inverted-U) trend of the CGFR strength–loading curve (p < 0.001) and the significant FGFR–CGFR divergence at 50 wt%, this reversal marks the onset of the performance cliff. At 10 GHz and 50 wt% loading, FGFR exhibits a dielectric constant of 4.30 and a loss tangent of 0.014, which are 17.8% and 22.2% lower than those of CGFR, respectively, owing to suppressed interfacial polarization associated with the preferential layered arrangement of the flat fibers. Rheologically, as the fiber content increases from 20 to 50 wt%, the melt flow rate of FGFR decreases by approximately 35.5%, whereas that of CGFR decreases by approximately 60.5%. The modified Halpin–Tsai, Kelly–Tyson, Maxwell–Garnett, and Krieger–Dougherty models, incorporating fiber geometry and a unified agglomeration correction, semi-quantitatively capture the tensile, impact, dielectric, and rheological properties. These results demonstrate that flat-fiber cross-section engineering is an effective strategy for developing high-performance wave-transparent composites under high filler loadings.

Ti–6Al–4V alloy is widely used in aerospace and other high-performance engineering components, but its service reliability is often constrained by surface-initiated fatigue, fretting damage, wear, and corrosion. Ultrasonic surface rolling process (USRP) couples a static rolling force with high-frequency mechanical impacts to introduce severe plastic deformation while retaining relatively low surface roughness, thereby producing a gradient-strengthened surface layer. This review systematically summarizes advances in USRP strengthening of Ti–6Al–4V alloy within a “process–microstructure–surface integrity–service performance” framework. The effects of static load, ultrasonic amplitude and frequency, feed rate, spindle speed, processing passes, treatment temperature, and lubrication conditions are first compared. Particular attention is then paid to the mechanisms governing dislocation multiplication and rearrangement, grain subdivision, gradient nanostructure formation, the responses of the α and β phases, deformation-induced phase transformation, and the evolution of depth-dependent residual compressive stress. The intrinsic relationships between these mechanisms and surface roughness, hardness, strengthened layer depth, wear and corrosion resistance, fatigue performance, and fretting fatigue performance are subsequently clarified. Control strategies involving electropulsing, laser/temperature assistance, deep cryogenic treatment, and coating combinations are further reviewed, together with methods for contact dynamics analysis, residual stress prediction, and data-driven optimization. The combined evidence indicates that the performance gains from USRP are jointly controlled by surface defects, gradient microstructure, and residual compressive stress; excessive load, processing passes, or heat input may weaken or even reverse the fatigue benefit because of defect accumulation, gradient mismatch, and residual stress relaxation. Current limitations include inconsistent reporting of process parameters, difficulty in quantitatively separating the contributions of different strengthening mechanisms, insufficient investigation of residual stress stability, and limited validation on complex components.

Multi-pass room-temperature rotary swaging offers a practical route for tailoring radial microstructures in lightweight metallic bars, but the associated surface–core evolution and position-dependent mechanical response remain insufficiently understood. Here, an extruded rare-earth-free Mg–3Al–1Li–0.3Si–0.3Mn alloy was subjected to 2, 6 and 10 swaging passes and examined by multiscale microscopy and local tensile testing. Rotary swaging produced pronounced surface–core microstructural partitioning. The surface underwent earlier grain subdivision, greater geometrically necessary dislocation accumulation, more extensive second-phase fragmentation and stronger texture hardening, whereas the core showed delayed but continuous evolution involving dislocation storage, stacking-fault formation and twinning-assisted deformation. From RS2 to RS10, the estimated average GND density increased from 3.31×1014 to 4.75×1014 m−2 at the surface and from 2.90×1014 to 4.22×1014 m−2 in the core. Similar grain sizes and GND densities in the RS2 surface and RS6 core revealed a time-lagged inward evolution of the deformation-induced microstructure. This partitioning produced contrasting tensile responses: surface-derived specimens exhibited higher yield and ultimate tensile strengths but lower elongation, whereas core-derived specimens retained slightly lower strength and greater ductility. After ten passes, the surface-derived specimen achieved a yield strength of 308 MPa, an ultimate tensile strength of 393 MPa and an elongation of 7.7%. The position-dependent response arose from the combined effects of grain refinement, GND accumulation, texture evolution and particle/interface-assisted defect storage. These findings establish a process–microstructure–property relationship for tailoring radially differentiated rare-earth-free Mg-alloy bars by room-temperature rotary swaging.

The mechanical performance of friction stir welded (FSW) dissimilar Al/Mg joints is generally limited by brittle interfacial intermetallic compounds (IMCs) and deformation incompatibility between the two base metals. Although tool offset is widely used to improve joint quality, the effects of sub-millimeter micro-offset on interfacial deformation mechanisms remain poorly understood. In this study, AA6061/AZ31B butt joints were fabricated at 800 rpm and 50 mm/min under three tool offset conditions, zero offset, 0.2 mm offset toward the Al side, and 0.2 mm offset toward the Mg side. The effects of tool micro-offset on interfacial phase evolution, grain boundary characteristics, Kernel Average Misorientation (KAM), Geometrically Necessary Dislocation (GND) distributions, and tensile properties were systematically investigated. The results show that Mg-side offset suppresses the growth of brittle Mg₂Al₃ (β) and maintains a thin, discontinuous IMC layer, whereas Al-side offset promotes a thick, continuous dual-phase IMC layer composed of Mg₂Al₃ (β) and Mg₁₇Al₁₂ (γ). KAM and GND analyses indicate that the γ phase exhibits relatively uniform strain distribution, while the β phase acts as the dominant strain-localization site due to severe deformation incompatibility with surrounding matrices. However, joint performance is governed not only by strain localization but also by crack connectivity. By limiting β-phase continuity and enhancing matrix plastic accommodation, the Mg-side offset achieves the best mechanical properties, with an ultimate tensile strength of 88 MPa and an elongation of 0.53%. These findings demonstrate that sub-millimeter tool micro-offset is an effective strategy for tailoring interfacial reactions and fracture behavior in dissimilar Al/Mg FSW joints.

The mechanical performance of friction stir welded (FSW) dissimilar Al/Mg joints is generally limited by brittle interfacial intermetallic compounds (IMCs) and deformation incompatibility between the two base metals. While tool offset is widely used to improve joint quality, the effects of sub-millimeter micro-offset on interfacial deformation mechanisms remain poorly understood. In this study, AA6061/AZ31B butt joints were fabricated at a rotational speed of 800 rpm and a welding speed of 50 mm/min under three tool offset conditions, including zero offset, 0.2 mm offset towards the Al side, and 0.2 mm offset towards the Mg side. The effects of tool micro-offset on interfacial phase evolution, grain boundary characteristics, crystallographic texture, Kernel Average Misorientation (KAM), Geometrically Necessary Dislocation (GND) distributions, and tensile properties were systematically investigated. The results show that the Mg-side offset suppresses the excessive growth of the brittle Mg2Al3 (β) phase and maintains a thin and discontinuous IMC layer, whereas the Al-side offset promotes the formation of a thick and continuous dual-phase IMC layer composed of Mg2Al3 (β) and Mg17Al12 (γ). KAM and GND analyses reveal that the γ phase exhibits a relatively uniform local strain distribution, while the β phase acts as the primary strain-localization phase due to the severe deformation incompatibility with the surrounding matrices. Nevertheless, the joint performance is governed not only by strain localization but also by crack connectivity. By restricting the geometrical continuity of the β phase and improving matrix plastic accommodation, the Mg-side offset achieves the best mechanical properties, with an ultimate tensile strength of 88 MPa and an elongation of 0.53%. These findings demonstrate that sub-millimeter tool micro-offset is an effective strategy for tailoring interfacial reactions and fracture behavior in dissimilar Al/Mg FSW joints.

This paper reveals the influence mechanism of friction stir processing on the microstructure, mechanical properties and hydrogen storage properties of duplex Mg-8Li alloy. Under severe stirring, the coarse α-Mg and β-Li in Mg-8Li are significantly refined, and extensive dislocations are formed inside the Mg grains. Grain refinement strengthening and dislocation strengthening increase the ultimate tensile strength of Mg-8Li to 165 MPa. The shift of preferred orientation maintains the elongation at 23.1%. These mechanical properties changes further improve the morphology of the hydrogen storage powder. The increased serrated morphology and reduced thickness of the powder accelerate the hydrogenation process of H atom diffusion control. Since LiH can act as the heterogeneous nucleation site of Mg, the fine and uniformly distributed β-Li facilitates the dehydrogenation process controlled by the Mg nucleation and growth, thereby enabling the dehydrogenation rate of the processed sample to be nearly 4 times that of the as-cast sample.

The low absolute strength and insufficient room-temperature ductility remain key bottlenecks that restrict the engineering application of magnesium alloys in high-end industrial fields. In the present study, 1 vol.% carbon nanotubes (CNTs)-reinforced Mg-xAl (x = 0, 1, and 1.5 wt.%) composites were synthesized via a powder metallurgy route coupled with hot extrusion–rolling processing to realize a simultaneous improvement in mechanical properties. The hot extrusion–rolling processed 1 vol.% CNTs/Mg-1Al composite exhibits an ultimate tensile strength of 300 MPa and an elongation to failure of 9%, showing an excellent strength–ductility synergy. Microstructural characterization reveals a well-bonded interface between CNTs and the Mg matrix. Deformation incompatibility between CNTs and the magnesium matrix during hot extrusion–rolling induces a high density of dislocations, providing an important strengthening contribution. Moreover, an increased proportion of low-angle grain boundaries and the development of a bimodal texture promote significant grain refinement and effectively activate non-basal slip systems, thereby alleviating plastic deformation constraints. The synergistic effects of interfacial strengthening, dislocation strengthening, grain boundary strengthening, and texture regulation together contribute to the simultaneous improvement of strength and ductility in CNTs-reinforced Mg-Al composites.

To achieve an optimal balance of mechanical properties in low-cost alloy systems, this study tailored a heterogeneous bimodal structure in dilute Mg-0.4Al-0.3Ca-0.2Mn-xSn alloys (x = 0, 0.1 wt.%) and systematically investigated the critical role of trace Sn microalloying during hot extrusion. Mg-0.4Al-0.3Ca-0.2Mn-xSn alloys were fabricated via melting, homogenization, and subsequent hot extrusion at 320 °C. Trace Sn addition induced the formation of uniformly distributed CaMgSn phases within the homogenized matrix, facilitating a synergistic enhancement of strength and ductility. Specifically, the extruded alloys exhibited a characteristic bimodal grain structure consisting of coarse un-dynamic recrystallized (unDRXed) grains and fine dynamic recrystallized (DRXed) grains. Sn microalloying effectively refined the DRXed grains from 2.66 μm to 2.11 μm and significantly boosted the elongation (EL) from 12.9% to 26.3% while maintaining an Ultimate Tensile Strength (UTS) of 274 MPa. The Sn-containing secondary phases served as potent sites for particle-stimulated nucleation (PSN), thereby promoting the DRX process and reducing the texture intensity from 20.89 to 9.99. Overall, the superior strength-ductility synergy is primarily governed by the formation of the heterogeneous bimodal structure, where trace Sn facilitates grain refinement and texture weakening through PSN mechanisms, providing a robust strategy for the design of high-performance dilute magnesium alloys.

In this study, the rolling process of the warm-rolled duplex-phase Mg–8.3Li–3.1Al–1.09Si alloy and the strengthening mechanism of as-rolled Mg–Li alloy were investigated. The highest ultimate tensile strength (UTS, 323.66 ± 19.89 MPa) could be obtained using a three-pass rolling process with a 30% thickness reduction for each pass at 553 K. The strength of the as-rolled LAS831 alloy is determined by a combination of second-phase strengthening, grain refinement strengthening, dislocation strengthening, and load-transfer reinforcement. Of these factors, dislocation strengthening, which is caused by strain hardening of the α-Mg phase, can produce a good strengthening effect but also cause a decrease in plasticity. The Mg₂Si phase is broken up into particles or strips during the rolling process. After three passes, the AlLi particles were transformed into an AlLi phase, and the Mg₂Si particles and nanosized AlLi particles strengthened the second phase to form a hard phase. The average size of the DRXed β-Li grains decreased with each successive rolling pass, and the average size of recrystallized grains in the three-pass-rolled LAS831 alloy became as low as 0.27 μm. The interface between the strip-like Mg₂Si phase and the α-Mg phase is characterized by semicoherent bonding, which can promote the transfer of tensile and shear forces from the matrix to the strip-like Mg₂Si phase, thereby improving the strength of the matrix and thus strengthening the LAS831 alloy.