Effects of hot dipping temperature on microstructure and mechanical properties of Pb40Sn60 alloy coating on copper wires

Hot dip plating was used to prepare Pb40Sn60 alloy coating on copper wires to improve weldability and corrosion resistance. The microstructure and phase composition of Pb40Sn60 alloy coatings at different hot dipping temperatures were examined by scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD), and the mechanical properties of copper wires after hot dip plated Pb40Sn60 alloy coatings at different temperatures were investigated by the tensile test and hardness test. The results showed that the microstructure of Pb40Sn60 alloy coatings consisted of primary α phase and (α+β) eutectic structure, the phase were composed of α phase and β phase, and β phase was more than α phase in relative amount. With increase of hot dipping temperature from 210 °C to 250 °C, the thickness of Pb40Sn60 alloy coating on copper wire was decreased from 39.4 mm to 20.5 mm, the proportion of α phase in relative amount increased from 33.2% to 40.5%, the crystal morphology of primary α phase changed from lamellar or equiaxial to dendritic, and (α+β) eutectic structure changed from small lamellar to granular. The work hardening of copper wires can be eliminated during hot dip plating. With increase of hot dipping temperature from 210 °C to 250 °C, the strength of copper wire after hot dip plated Pb40Sn60 alloy coating decreased from 234 MPa to 213 MPa, the elongation was not obvious change, and the hardness of copper wire substrate decreased from 87 HV0.2 to 69.9 HV0.2, the hardness of coating increased from 10.1 HV0.2 to 13.4 HV0.2. It has been found that increase in the hot dipping temperature was beneficial to eliminate work hardening of copper wire substrate, while resulted in decreasing in deformation resistance, and also the decrease of coating thickness. On the contrary, decrease in the hot dipping temperature was beneficial to increase thickness of Pb40Sn60 coating, but the effects of eliminating on work hardening of copper wire were weakened.