H. Y. Jing

A modified constitutive model for creep of Sn?3.5Ag?0.7Cu solder joints

In this study, the constitutive behaviour for creep performance of 95.8Sn–3.5Ag–0.7Cu lead-free solder joints was investigated. It was observed that the stress exponent (n) can be well defined into two stress regimes: low stress and high stress. A new, improved constitutive model, which considered back stress, was proposed to describe the creep behaviour of SnAgCu solder joints. In this model, the back stress, which is a function of the applied shear stress in the low stress regime (LSR) and a function of the particle size, volume fraction and coarsening of IMC particles in the high stress regime (HSR), was introduced to construct the relationship between the creep strain rate and the shear stress. The creep mechanism in these two stress regimes was studied in detail. In the LSR, dislocations passed through the matrix by climbing over the intermetallic particles, while in the HSR, the dislocations were glide-controlled. According to the different creep mechanisms in both the stress regimes, the back stress was calculated, respectively, and then incorporated into the Arrhenius power-law creep model. It was demonstrated that the predicted strain rate–shear stress behaviour employing the modified creep constitutive model which considered back stress, was in good agreement with the experimental results.

Pressure-assisted low-temperature sintering for paper-based writing electronics

With the aim of preparing paper-based writing electronics, a kind of conductive pen was made with nano-silver ink as the conductive component and a rollerball pen as the writing implement. This was used to direct-write conductive patterns on Epson photo paper. In order to decrease the sintering temperature, pressure was introduced to enhance the driving forces for sintering. Compared with hot sintering without pressure, hot-pressure can effectively improve the conductivity of silver coatings, reduce the sintering time and thus improve productivity. Importantly, pressure can achieve a more uniform and denser microstructure, which increases the connection strength of the silver coating. At the optimum hot-pressure condition (sintering temperature 120 ° C/sintering pressure 25 MPa/sintering time 15 min), a typical measured resistivity value was 1.43 × 10⁻⁷ Ω m, nine greater than that of bulk silver. This heat treatment process is compatible with paper and does not cause any damage to the paper substrates. Even after several thousand bending cycles, the resistivity values of writing tracks by hot-pressure sintering stay almost the same (from 1.43 × 10⁻⁷ to 1.57 × 10⁻⁷ Ω m). The stability and flexibility of the writing circuits are good, which demonstrates the promising future of writing electronics.

Fatigue strength improvement of stainless steel using weld toes dressing with low transformation temperature welding wire

Abstract In the present study, low transformation temperature welding wire (LTTW), which can induce residual compressive stress around 304L welded joints, has been developed to improve the fatigue performance of welded joints. Procedure of design and chemical composition of weld metal and deposited metal of LTTW are introduced. The microstructure of weld metal of LTTW is composed of low carbon martensite and residual austenite. The residual stress distribution of a single welding bead of LTTW was measured and the result shows that compressive residual stress is generated. In addition, fatigue test was also carried out on no-load cruciform welded joints of 304L stainless steel under three conditions: as welded, 308L dressing and LTTW dressing. The result shows that the fatigue life of LTTW dressing joints (2×106 cycles) is improved by 14–23 times and 3–6 times compared with that of as welded joints and 308L dressing joints.

INDENTATION SIZE EFFECT ON THE CREEP BEHAVIOR OF A SnAgCu SOLDER

In the present study, nanoindentation studies of the 95.8Sn-3.5Ag-0.7Cu lead-free solder were conducted over a range of maximum loads from 20 mN to 100 mN, under a constant ramp rate of 0.05 s-1. The indentation scale dependence of creep behavior was investigated. The results revealed that the creep rate, creep strain rate and indentation stress are all dependent on the indentation depth. As the maximum load increased, an increasing trend in the creep rate was observed, while a decreasing trend in creep strain rate and indentation stress were observed. On the contrary, for the case of stress exponent value, no trend was observed and the values were found to range from 6.16 to 7.38. Furthermore, the experimental results also showed that the creep mechanism of the lead-free solder is dominated by dislocation climb.

High strength steel (600–900 MPa) deposited metals: microstructure and mechanical properties

The microstructure and mechanical properties of the deposited metals of five metal cored wires for welding 600–900 MPa ultralow carbon bainitic steels were investigated. It was found that lowering the C content and optimising the Mn, Ni, Mo and Cr contents of the deposited metal significantly improved the strength and toughness. At the lowest strength level (627 MPa), ferrite dominated, mainly acicular ferrite and proeutectoid ferrite. However, at the highest strength level (>800 MPa), bainite dominated, mainly degenerate upper bainite and granular bainite. Good strength toughness match was achieved by a deposited metal having an interlaced multiphase microstructure with degenerate upper bainite and martensite acting as main strengthening phases, and granular bainite and acicular ferrite acting as toughening phases.

Electrochemical Behavior of Corrosion Resistance Alloys

The electrochemical behavior of corrosion resistance alloys (316L SS, 2205 DSS and Inconel 625) was carried out at ambient temperature in 3.5% NaCl solution using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. Some significant characterization parameters such as Ecorr, Eb, Icorr and △E in polarisation curves were analyzed and compared to reveal the corrosion resistance of corrosion resistance alloys. The ZsimpWin software was utilized to discuss the Nyquist plots of various corrosion resistance alloys. The result shows that the corrosion resistance of the corrosion resistance alloys in 3.5% NaCl solution at ambient temperature follows the sequence: Inconel 625> 2205 DSS > 316L SS.

Study on residual stress in socket weld by numerical simulation and experiment

In this paper, welding residual stress in socket weld of 304L stainless steel pipe was investigated using numerical simulation and validated by X-ray stress measurement. From the simulation results, the maximum tensile residual stresses were located at weld root and weld toe on both sides of the weld along pipe, which led to the fatigue failure. Pre-bevelling and low transformation temperature (LTT) dressing could decrease tensile residual stress both in hoop and axial direction at weld root and weld toe. After LTT dressing, compressive residual stress was generated throughout weld toe. Compressive stress can delay fatigue crack initiation and propagation. Therefore, pre-bevelling and LTT dressing can improve the fatigue life of socket weld.

Study on Fracture Properties of High-Density Polyethylene (HDPE) Pipe

Butt-fusion welding is the main technology to join high-density polyethylene (HDPE) plastic pipes, which are widely used in transport the water, gas and corrosive liquid. Investigation shows that one of the failure modes of HDPE pipe is the crack slowly grows across the thick direction and leads to failure at last, so that it is very important to study the resistance to crack initiation of HDPE pipe and its butt-fusion welded joint. In this study, the elastic-plastic fracture mechanics parameter, crack opening displacement (COD) is used to describe the fracture initiation behaviors for the HDPE materials and its butt-fusion welded joints. The resistance to initiation fracture of HDPE pipe materials and butt-fusion welded joints were investigated at different temperature by using multiple specimen resistance curve method and silicon-rubber replica method. The results show that saturation initial crack COD- δis of HDPE pipe materials and butt-fusion welded joints decreases with the decreasing temperature. The δis of butt-fusion welded joints is lower than that of HDPE pipe materials. Investigation also proved that the silicon-rubber replica method is more suitable for HDPE engineering material than the multiple specimen method. At the same time the statistic distribution of the δis of HDPE butt-fusion welded joint was conducted. The results show that the value of the δis has the statistic variance inherently. The optimum fitting distribution of COD is Weibull distribution with three parameters.

Study on Non-Cyanide Silver Electroplating with Copper Substrate

The non-cyanide silver electroplating on copper substrate has been discussed. Based on the surface morphology by using SEM, the smooth and fine silver layer is obtained. The influences of concentration of silver and plating temperature on the thickness of the plating layer are discussed, the thickness of the silver layer is increasing with silver concentration rising and plating temperature rising.

Ageing study of interfacial intermetallic growth in a lead-free solder reinforced with Ni-coated carbon nanotubes

In this study, varying amounts of Ni-CNTs were incorporated into the Sn-Ag-Cu matrix to form the composite solders. The interfacial intermetallic thickness formed on the Ni/Au metallized Cu substrate was determined under the as-soldered and isothermally aged (at 150°C for up to 42 days) conditions. The results of the interfacial intermetallic thickness showed that for the case of the unreinforced solder joint, the intermetallics grew more significantly than that of the composite solder joints, under both as-soldered and isothermally aged conditions. Moreover, the shear test results revealed that the as-soldered and aged composite solder joints exhibited improved ultimate shear strength as compared to that of their unreinforced counterparts. The improvement was observed to be most significant with the addition of 0.05 wt.% of Ni-CNTs, whereby the ultimate shear strength increased by 29% under the as-soldered condition and by 28% after isothermally aged for 42 days. It was concluded that the presence of Ni-CNTs as reinforcements in the Sn-Ag-Cu solder served to inhibit the growth of interfacial intermetallics and also to enhance its mechanical performance.