Yinan Li

A novel plasma-pulsed GMAW hybrid welding (plasma-GMAW-P) process is proposed for joining 6061 aluminum and zinc-coated steel. The results show that the change in welding heat input has little effect on the microstructure of the joint and the composition of the intermetallic compounds (IMCs) but only changes the thickness of the reaction layer (increased from 5 μm to 12 μm). when the plasma arc current is 20 A and the MIG current is 80 A, the welded joint obtained has the highest tensile-shear force. With the optimal process parameters, the weld strength obtained by filling ER4043 welding wire is the highest, accounting for 65% of the tensile-shear force of the base material. The effect of the plasma arc acting on the joint properties is studied through the microstructure and a tensile-shearing test. The action position of the plasma arc plays a significant role in the Al/steel interface, which directly influences the strength of the welded joints. Regardless of the plasma-GMAW-P style used to obtain the joints, Fe-Al IMCs appear at the interface. When the plasma arc is in front of the welding direction and the GMAW-P arc is in the rear, the tensile-shear force reaches the maximum of 3322 N.

The fabrication of various 3D tissue engineering tubular scaffolds with fibrous structures, to assist the human body in rapidly repairing a variety of ailments, is receiving more and more attention. Due to the inefficiency of the majority of fibrous preparation techniques, the question of how to rapidly produce the requisite three-dimensional tubular microfiber scaffold structures has become an urgent problem. In this study, an efficient polymer fiber preparation method was developed, using a high-speed airflow drive. Melt blending of polycaprolactone (PCL), polylactic acid (PLA), and tributyl citrate (TBC), was used for the printing material, to achieve the efficient preparation of tubular microfiber scaffolds with different structures. The scaffold diameter was as small as 2 mm, the wall thickness was up to 100 μm, and the fiber injection efficiency reached 15.48 g/h. By utilizing simulations to optimize the printing parameters and by adjusting the printing settings, it was possible to achieve a controlled fiber diameter in the range of 3 μm to 15 μm. In addition, plasma treatment was applied to the microfibers’ surface, to increase their wettability, and the efficiency of the hydrophilic modification was demonstrated. Furthermore, the mechanical property test demonstrated that the fibers have a tensile strength of 1.36 ± 0.16 MPa and a tensile strain of 30.8 ± 3.5%. The radial compressive strain of the tubular scaffold could reach 60% of the original scaffold’s diameter. Finally, the in vitro degradation of the fibers at various pH values was tested. The results showed that, under alkaline conditions, the surface of the fibers would be severely crushed and the rate of deterioration would increase.

Titanium alloys with special macro-micro composite structures of directional hydrophobicity are difficult to prepare due to poor thermal conductivity and good corrosion resistance, inhibiting the wide engineering applications for aerospace, marine engineering, and biomedicine. To prepare macro-micro composite structures on the surface of titanium alloys and achieve directional hydrophobicity, the sub-millimeter structures with an edge width of 150 μm, a groove width of 250 μm, and a depth of 250 μm were fabricated on the titanium alloy by wire electrical discharge machining (WEDM) technology, and high voltage-induced weak electric arc machining (HV-μEAM) was used to fabricate micro-scale feature size micro-structures on the processed macro-structure edges. The influence of process parameters on the morphology of microstructures was studied experimentally. The smooth surface of the titanium alloy is isotropically hydrophilic, and its contact angle is 68°. After processing the macrostructure on the titanium alloy surface, it shows directional hydrophobicity after being modified by low surface energy materials. The macro-micro composite structure formed by HV-μEAM realizes a directional hydrophobic surface with contact angles (CA) of 140° (parallel direction) and 130° (perpendicular direction), respectively. This surface has been modified with low surface energy to achieve contact angles of 154° and 143°. The results of the abrasion resistance test show that under the load of 100 g, it retains directional hydrophobicity at a friction distance of 700 mm with 600# sandpaper. The existence of the sub-millimeter macrostructure is the reason for the directionality of surface hydrophobicity. The microstructure can realize the transformation of the titanium alloy surface from hydrophilic to hydrophobic. Under the combined effects of the macro and micro composite structure, the surface of the titanium alloy shows obvious directional hydrophobicity.

The use of a Zn/Ag–Cu–Zn/Zn multi-interlayer was observed to avoid the formation of Mg–Al binary intermetallic compounds (IMCs), which cause embrittlement and low strength of bonding when dissimilar metals such as Mg/Al are joined using ultrasound-assisted transient liquid phase bonding (U-TLP). The change in the microstructure and mechanical properties of the AZ31B/LY12 joints at 410, 440, and 460 °C with prolonging ultrasonic treatment (UST) time was investigated. The results showed that the diffusion of Ag and Cu was faster into the brazing seam on the LY12 side than that on the AZ31B side with increasing UST and temperature. The IMCs on both sides of the joints were transformed with the diffusion of Ag and Cu. The transformation made the fracture path shift from the AZ31B side (410, 440 °C) to the LY12 side (460 °C), and the maximum shear strength of the joints from 43.3 (410 °C) to 65.7 (440 °C) to 84.7 MPa (460 °C). The IMCs on the surface of the fracture path corresponding to the joints with optimal mechanical properties changed from Mg₇Zn₃+MgZn₂+α-Mg (410 °C) to MgZnCu+Mg₇Zn₃ (440 °C) to Al₂Cu (460 °C).

An electric-field-driven (EFD), μ-3D printed, fused polymer technique has been developed for the fabrication of large-area microscale prototype molds using typical polymer materials, including microcrystalline wax (MC-wax), polycaprolactone (PCL), and polymathic methacrylate (PMMA). This work proposes an alternative for large area microscale modes and overcomes the limitation of high cost in the traditional mold manufacturing industry. The EFD principle enables printing of fused polymers materials more than one order of magnitude lower than the nozzle diameter, contributing to the necking effect of the Taylor cone jet, which is the key factor to achieve the microscale manufacturing. Numerical simulation of electric field distribution between the meniscus and substrate was carried out to elucidate the dependence of electric field distribution on the meniscus condition of three types of polymers under printable voltage, and the electrical field parameters for the EFD μ-3D printing were determined. A number of experiments were printed successfully using a large range of viscosity materials, ranging from tens of mPa·s to hundreds of thousands of mPa·s of PCL and PMMA. The differences in parameters of different materials, such as viscosity, tensile properties, and surface energy, were studied to assess their use in different fields. Using proper process parameters and a nozzle with an inner diameter of 200 μm, three different application cases were completed, including a Wax microarray and microchannel with a minimum dot diameter of 20 μm, a PCL mesh structure with a minimum line width of 5 μm, and a PMMA large-area mold with a maximum aspect ratio of 0.8. Results show that the EFD μ-3D printing has the outstanding advantages of high printing resolution and polymer material universality.

Ultrasound-assisted transient liquid phase bonding (U-TLP) has been regarded as a promising brazing process to join magnesium alloys with a Sn and Zn interlayer; however, the formation of brittle magnesium intermetallic compounds (Mg₂Sn, MgZn, and MgZn₂) compromises the mechanical properties of the joints. In this study, Mg alloy U-TLP joints with a Ni interlayer were evaluated based on shear strength and hardness measurement. Microstructural evolution along with ultrasonic duration time and intermetallic compound formation were characterized using X-ray diffraction and electron microscopy methods. The results show that incremental ultrasonic durations of up to 30 s lead to the microstructural evolution from the Mg₂Ni layer, eutectic compounds (Mg₂Ni and α-Mg) to α-Mg (Ni), accompanied by shear strength increases. The maximum value of the shear strength is 107 MPa. The role that ultrasound vibration played in brazing was evaluated, and showed that the MgO film was broken by the acoustic softening effect when the interlayer and base metal were solid. As the MgO and Mg substrate have different stress reduction τ, this plastic mismatch helps to break the oxide film. Additionally, the diffusion between the solid Mg substrate and Ni interlayer is accelerated greatly by the acoustic pressure based on the DICTRA dynamic calculation.