Thermal cycles and microstructures in laser welding of carbon steel

Abstract

In laser materials processing of metals and alloys, rapid heating and cooling can be easily attained and a non-equilibrium metallic phase or homogeneous phase with some fine precipitates are obtained. For instance, the thermal cycle in laser welding at a high travel speed is different from that of arc welding at a low travel speed. In the case of laser welding, the thermal cycles can be controlled easily by laser power, pulse duration, pulse frequency, oscillation technique, travel speed and so on, so that a quite rapid heating and cooling of metals and alloys occur. A measurement of thermal cycles was carried out using a Pt-PtRh thermocouple of 0.1 mm in diameter and a high memory recorder. 2 kW fast axial flow type CO2 laser facility (Daihen Co.) and 300 W pulse YAG laser (Toshiba Corp.) were used to weld carbon steel plates of 4 mm and 8 mm in thickness. The welded specimens were color etched by Klemm and Beraha etching reagents for observation of microstructures of laser welds.As the results a measured heating rate from 500 C to 1200 C in CO2 laser welding was more than 450,000 C/s. It was about 890,000 C/s in pulse YAG laser welding. A cooling rate from 800 C to 500 C was about 18,000 C/s and 62,000 in CO2 and YAG laser in welding respectively. The microstructures in heat affected zones were ferrite and martensite unmixed structures.In laser materials processing of metals and alloys, rapid heating and cooling can be easily attained and a non-equilibrium metallic phase or homogeneous phase with some fine precipitates are obtained. For instance, the thermal cycle in laser welding at a high travel speed is different from that of arc welding at a low travel speed. In the case of laser welding, the thermal cycles can be controlled easily by laser power, pulse duration, pulse frequency, oscillation technique, travel speed and so on, so that a quite rapid heating and cooling of metals and alloys occur. A measurement of thermal cycles was carried out using a Pt-PtRh thermocouple of 0.1 mm in diameter and a high memory recorder. 2 kW fast axial flow type CO2 laser facility (Daihen Co.) and 300 W pulse YAG laser (Toshiba Corp.) were used to weld carbon steel plates of 4 mm and 8 mm in thickness. The welded specimens were color etched by Klemm and Beraha etching reagents for observation of microstructures of laser welds.As the results a measure...