The heterogeneity of microstructure and mechanical performance of welding joints were affected by unique material properties such as high resistivity and low thermal conductivity, instantaneous high temperature of spot welding thermal cycle, and uneven local cooling rate. Because of ultra high strength of hot stamping parts, the welding joint is more inclined to fail as the failure mode of interfacial fracture, the traditional critical nugget formula fell down. Through designing orthogonal experiment, the effect of different welding parameters on joint performance was analyzed, the optimized welding process was obtained; on account of the quality evaluation of welding joint, nugget spherical mode was built to separately analyze the stress condition of welding joint in the failure modes of interfacial fracture and nugget pull-out, the new evaluation criterion aimed at high strength steel was established. Two kinds of 1.2 mm and 2.0 mm steels were used to validate the critical weld diameter. Results showed that the critical formula could accurately evaluate failure of the weld joints.
A typical printed circuit boards (PCBs) has been investigated by using thermo-gravimetric analyser to study its pyrolysis characteristics, the results indicate that the maximum weight loss rate occurs at temperature between 320°C and 360°C. A higher heating rate results in higher initial, final, peak temperature, and a longer process of significant weight loss. At the same pyrolysis temperature, heating rate has little effect on the total weight loss. In addition, 1kg PCBs based FR-4 was pyrolyzed in a fixed-bed reactor. The pyrolysis residues are very friable, the organic, glass fiber and metallic fractions can easily be separated, and the electrical components can easily be removed from the remains. Considering energy-saving, better control and design of the pyrolysis process, the optimal pyrolysis parameters were suggested at heating rate 10°C/min, final pyrolysis temperature 500°C and holding time 30 min.
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