A liquid-nitrogen-cooling friction stir spot welding (C-FSSW) technology was developed for welding AZ31 magnesium alloy sheets. The liquid-nitrogen cooling degraded the deformability of the welded materials such that the width of interfacial cracks increased with increasing cooling time. The grain size of the stirred zone (SZ) and the heat-affected zone (HAZ) of the C-FSSW-welded joints decreased, whereas that of the thermomechanically affected zone (TMAZ) increased with increasing cooling time. The maximum tensile shear load of the C-FSSW-welded joints welded with a cooling time of 5 or 7 s was larger than that of the friction stir spot welding (FSSW)-welded joint, and the tensile shear load decreased with increasing cooling time. The microhardness of the C-FSSW-welded joints was greater than that of the FSSW-welded joint. Moreover, the microhardness of the SZ and the HAZ of the C-FSSW-welded joints increased, whereas that of the TMAZ decreased, with increasing cooling time.

•A novel mixed Cu nanoparticles of 20 nm and 100 nm paste was investigated.•The optimized mass ratio of (20 nm : 100 nm) was proposed.•The joints showed a high bonding strength of over 15 MPa at 250 °C and 4 MPa.In order to enhance the bonding strength of joints by using Cu nanoparticles paste at a relative low bonding temperature and a suitable pressure, a novel mixed Cu nanoparticles of 20 nm and 100 nm (in diameter) paste for Cu-Cu joints was investigated in this study based on the density increment of packing two different sizes of spheres. The optimized mass ratio of 20–100 nm nanoparticles for maximum sintering density was proposed (9/1) by simulation and experiments. The bonding strength of joints over 15 MPa was achieved even at a lower bonding temperature of 250 °C and pressure of 4 MPa.

•Spin-coated technique is used to fabricate uniform and defect-free coating.•The fabrication method is easy to operate and economical.•The corrosion resistance in physiological environment is quite well.•The composite coating is first introduced to study its biocompatibility.A new organic-inorganic composite coating was generated on AZ31 alloy surface with sol-gel method to improve the anti-corrosion performance and biocompatibility of AZ31 alloy in the physiological environment. The results of electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, pH variations in modified-simulated body fluid (m-SBF) demonstrated that the silane/Mg(OH)2 composite coating significantly enhanced the anti-corrosion ability and slowed down the degradation of AZ31 alloy under in vitro condition. Fourier-transform infrared spectroscopy (FTIR) showed that the major composition of in-situ formation layer was Mg(OH)2. The surface morphology, chemical and phase constitutions of the composite coating were detected using scanning electron microscopy (SEM). The results demonstrated that the composite coating was uniform and defect-free. X-ray diffraction analysis (XRD) was also employed to characterize the corrosion products of all specimens immersed in m-SBF after 30 days. MTT and ALP tests indicated that the composite coating greatly enhanced the cell proliferation and osteogenic differentiation of osteoblasts on the AZ31 alloy surface.

•Dopamine is creatively introduced as an intermediate layer for polyether imide (PEI) deposition to construct a robust coating on magnesium alloy surface.•This coating is fabricated in two steps and is facile.•This PDA/PEI hybrid coating shows good long-term corrosion protection properties in 3.5 wt.% NaCl solution (up to 35 days).•This hybrid coating provides potential application in physiological environment due to the biocompatibility of dopamine and PEI.Dopamine (DA), with strong adhesion to metal substrate via self-polymerization reaction, was introduced as a versatile template layer for subsequent polyether imide (PEI) deposition to confer a highly robust coating on AZ31 magnesium alloy. The complicated corrosion behaviour of the coating was disclosed and investigated by employing electrochemical impedance spectroscopy (EIS) coupled with equivalent electrical circuits (EECs) analysis. The results showed that the hybrid PEI/PDA coating exhibited evidently higher retardation performance (up to 35 days) after immersing in 3.5 wt.% NaCl solution compared to PDA or PEI coated Mg.

•PVDF films (45–130 nm) are fabricated by the developed HSCA-LBD method.•Their dielectric constant is up to 15 with loss less than 0.1.•Their breakdown field is up to 171 MV/m with the energy density up to 2 J/cm3.Nanoscale polymer dielectric films are critical demanded by continuously integration and miniaturization of energy electronic devices. In this paper, PVDF films are fabricated by present developed HSCA-LBD method, and thus, the dielectric properties of PVDF films with nanoscale thickness are investigated in detail for the first time. The results show that the films with thickness lower than 100 nm present excellent dielectric performances, the effective dielectric constant is up to 15 with loss less than 0.1 in frequency range of 1K - 2 MHz. The breakdown field of them is up to 171 MV/m with the energy density up to 2 J/cm3, which are in same level with those of reported PVDF films with thickness higher than 10 μm. Moreover, the mechanisms of dielectric behaviors are also discussed in detail.Download high-res image (217KB)Download full-size image

The effects of graphene nanoplates (GNPs) on the microstructures and mechanical properties of nanoparticles strengthening activating tungsten inert gas arc welding (NSA-TIG) welded AZ31 magnesium alloy joints were investigated. It was found that compared with those of activating TIG (A-TIG), and obvious refinement of α-Mg grains was achieved and the finest α-Mg grains of fusion zone of NSA-TIG joints were obtained in the welded joints with TiO2+GNPs flux coating. In addition, the penetrations of joints coated by TiO2+GNPs flux were similar to those coated by the TiO2+SiCp flux. However, the welded joints with TiO2+GNPs flux coating showed better mechanical properties (i.e., ultimate tensile strength and microhardness) than those with TiO2+SiCp flux coating. Moreover, the generation of necking only occurred in the welded joints with TiO2+GNPs flux.

The effects of heat treatment on the microstructures and mechanical properties of tungsten inert gas-welded AZ31 magnesium alloy joints with Cr2O3 flux coating were investigated by microstructural observations, microhardness, and tensile tests. The results showed that the activating flux of Cr2O3 improved the weld penetration and the depth/width ratio of the tungsten inert gas-welded AZ31 magnesium alloy joints markedly. Heat treatment (1) eliminated the coarsened network-like β-Mg17(Al, Zn)12 particles which formed in activating tungsten inert gas and (2) resulted in more uniform grain size distribution of partially melted zone. Apart from that, either the microhardness of the seams, the ultimate tensile strength, or the elongation of the welded joints was improved by aging treatment, while too high aging temperature would give rise to a sharply decreased ultimate tensile strength.

The FeCrNi alloy powders were used on the dovetail groove of FV520B steel to fabricate the multilayer laser cladding layers. The effects of heat treatment on the microstructure and mechanical properties of FeCrNi layers were investigated. The results showed that the heat treatment at the temperature ranged from 1073 to 1273 K refined the grains of the substrate materials and removed the soft zone of hardness between the fused zone (FZ) and base material (BM) effectively mainly due to a secondary quench of heat treatment. When the temperature of heat treatment was 1073 K, the maximum ultimate tensile strength (UTS) values of the laser cladding component were obtained. However, the heat treatment at high temperature had a bad effect on wear resistance of coatings at some extent.

In this work, a new water bath friction stir spot welding (WB-FSSW) process was developed. The effects of dwell time on the microstructures and mechanical properties of the WB-FSSW-welded AZ31 magnesium alloy joints were investigated by microstructural observation, tensile tests, and microhardness tests. It was found that compared with those of ordinary FSSW welds, the WB-FSSW welds have poorer tensile shear force, almost equal tensile shear strength per unit area τ and higher microhardness mainly due to the narrower bonded zone width of the hooks and microstructures in the welded joints, respectively. Moreover, with the increase in the dwell time, the microhardness and the tensile shear strengthen per unit area τ of the WB-FSSW-welded joints increased firstly and then decreased because of the fluctuant evolution of grain size of α-Mg and the volume fraction of the β-Mg17All2. The optimal dwell time for the WB-FSSW welding was from 10 to 15 s whereby the finest grains and the highest mechanical properties of the welded joints can be achieved.

•Two different Cu–Zn intermetallic compounds were detected in Sn–40Bi–2Zn–0.1Cu solder instead of Cu–Sn intermetallic compounds.•Both the microhardness and the ultimate tensile strength were improved with the Cu addition, and further enhanced by Zn.•The elongation decreased because of the formation of Zn-rich phase.The effects of Cu and Zn additions on microstructures, thermal and mechanical properties of Sn–Bi-based solder alloy were investigated. Thermal analysis indicated that Cu addition decreased both melting point and paste region of Sn–Bi-based solder while Zn played a reverse effect. Alloying Cu into binary solder resulted in an increase in both ultimate tensile strength and ductility. The improved strength of the Sn–40Bi–0.1Cu solder was attributed to the microstructural refinement and uniform distribution of the Cu6Sn5 intermetallic particles. The addition of Zn further depressed the precipitation of Bi, formed uniform globular CuZn2 particles as well as flat blocky Cu5Zn8 phase. The enhanced strength of Zn-containing solder was ascribed to the presence of the globular CuZn2 particles and structural refinement. Needle-like Zn with high aspect ratio forms at the position around the Bi-rich phase and leads a significant decrease of the elongation of Sn–40Bi–2Zn–0.1Cu solder. Fracture surface analysis indicated that the addition of Cu and Zn in Sn–Bi-based solder alloy did not affect the mode of fracture, and all tested solder exhibited brittle fracture with a pattern mixing with tongue and cleavage on the fracture surface.

•The thickness of IMC at the cathode increased with aging time.•The thickness of IMC at anode sides grew faster, compared with no-current case.•The equation needs to be modified in case of low current density stress.This study investigated the effects of isothermal aging and low density current on intermetallic compound (IMC) growth rate and microstructural evolution of lead-free solder interface at a temperature of 398 K. The results showed that the morphology of IMC layers under high temperature aging and current stressing was basically same. The growth rate of IMC at the anode was the fastest. That was because chemical diffusion force and electronic wind acted together to drive the growth of IMC at the anode. The current density was not high enough for obvious polarization effects and crack along the electron flow direction to be observed. Next the mean-time-to-failure analysis was used to calculate the lifetime of ball grid array solder joints stressed electrically. However, the calculated value was much shorter than the true value. Indicating that perhaps the equation needs to be modified when applied to Cu interconnects and flip chip solder joints.

In this paper, the effects of Mn powder on fusion property of Sn3.0Ag0.5Cu solder alloy and microstructures as well as tensile property of the solder joints of Sn3.0Ag0.5Cu/Cu were investigated by differential scanning calorimetry analysis, scanning electron microscopy and tensile tests. The results showed that the addition of Mn dramatically suppressed under cooling of SnAgCu solder alloy. Mn addition contributed to the growth of Cu6Sn5 intermetallic compound layers since it provided nucleation sites for Cu6Sn5 at the solder joints. Moreover, Mn addition increased the hardness of the solder alloys and reduced the tensile strength of SnAgCu/Cu solder joints. During aging, the growth of IMC layers of SnAgCuMn/Cu solder joints was slower than that of SnAgCu/Cu solder joints, and the tensile strength of all the solder joints increased after aging.

Aging treatment and various heat input conditions were adopted to investigate the microstructural evolution and mechanical properties of TIG welded 6061-T6 alloy joints by microstructural observations, microhardness tests, and tensile tests. With an increase in heat input, the width of the heat-affected zone (HAZ) increases and grains in the fusion zone (FZ) coarsen. Moreover, the hardness of the HAZ decreases, whereas that of the FZ decreases initially and then increases with an increase in heat input. Low heat input results in the low ultimate tensile strength of the welded joints due to the presence of partial penetrations and pores in the welded joints. After a simple artificial aging treatment at 175°C for 8 h, the microstructure of the welded joints changes slightly. The mechanical properties of the welded joints enhance significantly after the aging process as few precipitates distribute in the welded seam.

In this paper, the effects of welding speed on the microstructures and mechanical properties of laser welded AZ61 magnesium alloy plates were investigated by microstructural observations, microhardness tests and tensile tests. The results show that the microstructure in the fusion zone consisted of fine α-Mg equiaxed dendrite crystals and dispersed β-Mg17Al12 particles. With an increase in welding speed, the sizes of α-Mg grains and β-Mg17Al12 particles in the fusion zone decreased and the volume fraction of β-Mg17Al12 particles increased. The ultimate tensile strength, yield strength and elongation of welded joint increased when the welding speed increased from 1800 mm min−1 to 2800 mm min−1. In addition, the average hardness value of fusion zone and heat-affected zone increased with the increase in welding speed.

The influence of minor POSS (polyhedral oligomeric silsesquioxanes) molecules additions on the microstructure and hardness of SnAgCu–xPOSS (x = 1, 3 and 5) was investigated. A mechanical mixture method was adopted by adding POSS molecules as dispersoids into SnAgCu solder to fabricate SnAgCu–xPOSS composite solders. The microstructural evolution and the hardness of the solders were investigated in details by microstructural observations, Vickers hardness tester and nanoindentation tests. The results showed that the effects of the dispersed POSS molecules in eutectic SnAgCu structure and the refined Ag3Sn IMC particles increased the hardness of eutectic SnAgCu phase and then increased the hardness of composite solder. In addition, because of the agglomeration of POSS molecule, a coarse lath-shaped structure (composed of POSS molecules, Ag3Sn phase and minor Cu6Sn5 phase) formed in SnAgCu–5POSS solder matrix, which reduced the hardness of SnAgCu–5POSS solder.