Directed Light Fabrication of Iron-Based Materials
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ABSTRACT Directed light fabrication (DL`) is a rapid fabrication process that fuses gas delivered metal powders within a focal zone of a laser beam to produce fully dense, near-net shape, 3dimensional metal components from a computer generated solid model. This study used ironbased alloys to evaluate the microstructural development in the DLF process. Continuous microstructural features are evident, implying a continuous liquid/solid interface during processing. In addition, solidification cooling rates have been determined based upon secondary dendrite arm spacings in Fe-25wt.%Ni and 316 stainless steel. Cooling rates vary from 10'-10' K s , and the solidification behavior has been simulated using macroscopic heat transfer analyses. INTRODUCTION Directed light fabrication (DLF) is a rapid prototyping process that fuses gas-delivered metal powders within a focal zone of a laser beam to produce 3-dimensional metal components [1, 2]. The focal zone of the laser beam is programmed to move along or across a part cross-section, and coupled with a multi-axis sample stage, produces complex metal geometries in a single processing step. Candidate alloy systems for rapid prototyping include iron-based materials. In order to define and optimize the processed materials, an examination of the microstructural development is required. The microstructural development in steels affect the mechanical properties of the material, and in particular, the thermal history of fabricated components dictates the potential application. The intent of this study is to address the solidification behavior with a specific focus upon the cooling rates experienced during DLF processing of simple geometries. PROCEDURE Experimental The DLF process consists of generating tool paths from computer generated 3-dimensional solid models. The tool paths continuously move the focal zone of the laser systematically along areas of the part to fuse metal powder particles that are gas-delivered to the focal zone. A schematic diagram of the process is shown in Figure 1. Three Nd-YAG pulsed lasers (1 KW), connected in series to simulate a continuous wave (CW) laser beam, are delivered via fiber optics to a sealed boom that holds the laser focusing head and is attached to the "z" (vertical) axis. The focused laser beam enters the chamber through a quartz window in a nozzle that also delivers the metal powder to the focal zone. The entire process takes place in an inert gas box connected to a dry train that reduces the oxygen content to < 5 ppm. The powder feeder entrains the powder in an argon stream that delivers the powder to the laser focus nozzle and then to the focal zone. A positioning controller drives the "x", "y", and "z" tables, switches the laser shutter and powder feeder on and off, and controls the gas flow. For the purpose of evaluating the solidification behavior in DLF, 1-dimensional and 2341 Mat. Res. Soc. Symp. Proc. Vol. 397 ©1996 Materials Research Society
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