Microstructural design of mechanical properties for laser-fabricated stainless steel parts

Rapid prototyping and manufacturing bring us the novel idea of manufacturing that combines computer-aided design with laser processing technique. As one of the rapid prototyping techniques, laser direct deposition becomes attractive in manufacture industry because metal parts can be manufactured directly by this method without intermediate procedures and equipment, therefore saving much time and expenses. This work addresses the problems of fabricating metal parts with desired mechanical properties by using a laser deposition technique. Such problems are considered as the less information about microstructure and mechanical properties of the deposited parts and how the laser parameters and processing conditions affect the quality of the parts. The following studies are the necessary steps required to investigate the relationship from laser parameters and processing condition to microstructure evolution and then mechanical properties.; The experimental and theoretical study on absorptivity of metal powders were conducted towards understanding energy transfer in laser powder processing. Two models concerning Hagen-Ruben relationship and energy balance show the similar tendency for the absorptivity of powders changing with the absorptivity of sheet metals.; Stainless steel 316 and 304L layers have been coated successfully on a plain carbon steel sheet by prepared layer method and blow powder technique, respectively. The clad geometry is influenced by laser processing parameters. The results of the hardness and wear-resistance tests show that laser cladding could improve the mechanical properties of the substrate so much even exceeding those of the cladding materials processed by conventional methods.; Both microstructure characterization and mechanical property test have been performed for laser fabricated stainless steel parts. There is no crack or porosity formed and the austenite structure with dendritic or cellular form is oriented from the bottom to the top of each layer with slightly toward the laser scanning direction. The tensile test results for stainless steel 304L samples show that the mechanical property of the deposited parts is anisotropic and affected by the microstructure very much.; A mathematical model concerning three-dimensional quasi-steady state heat conduction is developed with phase change taken into account. The method of least squares is used to solve the model and establish the relationship between various welding parameters and the weld pool shape and temperature profiles. Compared to experimental data, the maximum error for weld depth is less than 10% and most of the weld width data agree with theory very well. Another mathematical model is presented to predict microstructures formed due to rapid solidification in laser materials processing. The neutral stability curve is found to largely depend on the solidification rate and partition coefficient. The heat conduction model provides the solidification rate data for microstructure determination. The experimental data from laser welding and cladding of stainless steel compare well with the microstructural growth regimes predicted by this model.