Last modified: 16. 12. 2018
Abstract
Laser beam processing of steel has increasing application in many industries, mainly due to high process speed and good quality of the product as well as a small zone of thermal influence on the material. The concentration of a laser heat energy leads to the generation of high cooling rates in heat affected zone (HAZ). As a result phase transformations in solid state are present, which leads to the formation of various structures in HAZ with differing mechanical properties.
This work concerns numerical modelling and computer simulations of temperature field and phase transformations during Yb:YAG laser heating of sheets made of S355 steel. The distribution of Yb:YAG laser power emitted by Trumpf laser head D70 is used in the analysis. The heat source is modelled on the basis of interpolation algorithms using geostatistical kriging method. Coupled heat transfer and fluid flow in the fusion zone are described respectively by transient heat transfer equation with convective term and Navier-Stokes equation. The kinetics of phase transformations and volumetric fractions of arising phases are obtained on the basis of Johnson-Mehl-Avrami (JMA) and Koistinen-Marburger (KM) models. Continuous Heating Transformation (CHT) diagram is used for heating process and Continuous Cooling Transformation (CCT) diagram is used for heated steel with the decomposition of final volume fractions of phases transformed form austenite dependant on cooling rates.
Examples of computer simulations are presented including temperature field, velocity field of liquid material in the fusion zone and estimated structure composition of elements made of S355 steel, heated by Yb:YAG laser.