Influence of Process Parameters on the Phase Transformation and Consequent Hardness Induced by the Lens Process

A three-dimensional finite element model was developed to predict the temperature distribution and phase transformation in deposited stainless steel 410 (SS410) during the Laser Engineered Net Shaping (LENS) rapid fabrication process. The development of the model was carried out using the SYSWELD software package. The model calculates the evolution of temperature in the part during the fabrication of a SS410 plate. The metallurgical transformations are taken into account using the temperature dependent material properties and the continuous cooling transformation diagram. The ferritic and martensitic transformation, as well as austenitization and tempering of martensite are considered. The influence of processing parameters such as laser power and traverse speed on the phase transformation and the consequent hardness are analyzed. The potential presence of porosity due to lack of fusion is also discussed. The results show that the temperature distribution, the microstructure and properties in the final part depend significantly on the processing parameters. Introduction The Laser Engineering Net Shaping (LENS) process is a laser-assisted, direct metal manufacturing process for rapid fabrication [1-3]. Figure 1 shows the schematic of the LENS fabrication process. The LENS fabricated part is built on a solid substrate usually made of the metal to be deposited or similar material. Initially, the laser beam is focused on the substrate to create a molten pool, where metallic powder particles are simultaneously injected by a set of converging nozzles. A single layer of material is built up when the substrate moves beneath the laser beam in the x-y plane under computer guidance. After deposition of a single layer, the laser beam and the powder delivery nozzle assembly is incremented in the positive z-direction to begin the deposition of subsequent layers. Accordingly, the process is repeated and a threedimensional part is built up in a layer-by-layer manner. The feasibility of depositing different metals including stainless steel, tool steel, nickel-based superalloys and titanium into near-net shape parts in a single processing step has been illustrated