A 780 MPa galvanised dual phase (DP) steel was two-, three-and four-layer resistance spot welded under different welding parameters and joint configurations. The splash situation, nugget sizes and tensile shear properties of the spot welds were evaluated systematically. The results showed that the splash occurring current increased in the order of two-, three-and four-layer spot welds owing to the increasing metal volume for melting. The joint configuration had a great influence on the tensile shear properties of the spot welds. The three-and four-layer spot welds with proper joint configurations had better weld quality than the two-layer spot weld. Therefore, well designed multilayer spot welds could be used to replace the commonly used two-layer spot welds in actual production.

An efficient and easy sample pretreatment methodology was proposed for the detection of photoinitiator 4-methylbenzophenone from milk before high-performance liquid chromatography. Appropriate conditions for demulsification were studied. The parameters affecting cloud point extraction, such as concentration of Tween-20, electrolyte salt, equilibration temperature, and time, have been investigated. When the spiked level was 200-1000 μg/kg, the average addition standard recovery was 99.14-105.98% with the optimum cloud point extraction conditions (concentration of Tween-20, 138 g/L; mass of anhydrous sodium sulfate, 0.75 g; equilibration temperature, 65°C; equilibration time, 30 min). To decrease the detection limits, further work about the organic solvent, shaking time, and ultrasonic parameters was carried. When the spiked level was 10-100 μg/kg, the average addition standard recovery was 70.40-106.91% with the optimum cloud point extraction and enrichment conditions (optimum cloud point extraction conditions; volume of cyclohexane, 30 mL; shaking time, 20 min; time of ultrasonic, 20 min; temperature of ultrasonic bath, 45°C).

A 1000 MPa galvanised dual phase steel was fibre laser lap welded by different welding parameters. The surface appearance, cross-section macrostructure and tensile shear properties of laser lap welds were evaluated. The different methods for improving the laser lap weldability of galvanised steel were analysed. The results showed that the increasing laser power resulted in the formation of the keyhole, which could be used as the escape route for the vapourised zinc formed at the faying surface. Therefore, the weld defects, such as spatter, cavity and blow hole, were eliminated, and the full penetrated laser lap welds with smooth surface appearances were acquired. A proper joint gap also worked as the escape route for the zinc vapour and led to the formation of sound welds. Adding proper preheating procedures improved the mechanical properties of the welds to some degree, but weld defects still existed owing to the residual zinc.

Subjected to quenching processing, the samples of weld heat affected zone containing microstructures of martensite in high strength pipeline steel were simulated and prepared. Effects of electropulsing treatment on the corrosion resistance of simulated samples were studied through electrochemical detections and immersion corrosion experiments. It was found that the corrosion resistance of pipeline steel decreased sharply due to the high lattice strains/dislocation densities and residual tensile stress developed after martensite transformation by water quenching. Interestingly, treated by electropulsing the corrosion resistance of simulated weld heat affected zone samples was dramatically improved, and even exceeded that of the base metal when the current density achieved 5.2 kA/mm 2. After electropulsing treatment, the dislocation density and residual stress of the investigated samples were reduced largely, and the rust layer generated after corrosion was more compact, so that its corrosion resistance was improved.

No abstract