Plasma and mechanical properties and process parameter selection criteria for laser rapid manufacturing

Laser-based techniques to fabricate parts from non-polymer material have been developed in the past years. These processes are referred to as Solid Freeform Fabrication, Rapid Manufacturing, Positive Shaping, Rapid Tooling, and Near-Net Shape Fabrication. Data concerning the energy transfer from the processing laser beam to the material powder, such as the metal vapor-plasma plume temperature and plume absorption coefficient, the efficiency of laser energy transfer and mathematical analysis for the thermal and dimensional process characteristics are unavailable. Furthermore, guidelines to select process parameters (power, scanning speed, and powder feed rate) have not been investigated.; The above-mentioned part fabrication techniques are not fully useful without predictive capability to calculate the geometries of the fabricated parts or the expected yield and ultimate strengths. Similarly, any operator needs a set of process parameter selection rules to identify stable operating conditions that result in a continuous powder deposition. Furthermore, it is difficult to determine for any material or alloy the operating conditions that result in a stable powder deposition process. Therefore, it is highly desirable to have a set of selection rules to determine the stable operating regime for any material based on the known stable process parameters of a given material.; A one-dimensional model to calculate the thermal and dimensional process characteristics is developed. The model accounts for the transmission of the laser beam through the plume, energy transfer in the molten phase and the Stefan conditions at the solid-liquid and liquid-vapor interfaces. The model is used to determine the amount of energy transferred from the laser beam to the powder. A set of dimensionless numbers characterizing the powder deposition process is identified using Buckingham's pi-Theorem. These numbers are used to represent the stable and unstable operating regimes and to apply the stable operating conditions of a known material to other materials.; Mechanical properties such as yield and ultimate strengths of laser-fabricated stainless steel (SS 304) parts have been measured and correlated to the operating conditions through the dimensionless similarity parameters. A model, accounting for directionally preferred solidification, to calculate the residual stresses generated in the part during solidification is developed.