Thermokinetic Modeling of Phase Transformation in the Laser Powder Deposition Process
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LASER powder deposition (LPD) in the past two decades has been one of the fastest growing manufacturing processes. Various applications such as building components with complex geometry, syntheses of functionally graded materials, surface coating, and alloying, and repairing valued components have made it a unique processing technique in manufacturing. Locally heating and cooling the material during the process has serious effects on the quality of the deposited material. Kaplan and Groboth[1] analytically studied the effect of parameters on clad layer formation and overlapping of tracks. Labudovic et al.[2] studied the effect of process parameters on the formation of residual stress along the height of a single wall deposition. Choi et al.[3,4] had an extensive experimental study on the effect of laser power, laser scanning speed, and powder flow rate on the geometry, microstructure, and defect formation in the process. The studies of the process have shown that the microstructure depends not only on the already mentioned parameters, but also on any parameter that can change the temperature history of each location throughout the buildup. Costa et al. has studied the effect of idle time between layers[5,6] and substrate size[6,7] on the final microstructure and hardness of the deposition of a single wall. Similar work was performed by Wang et al.[8] to investigate the effect of laser power and traverse speed on the phase transformation during the process. Kelly et al.[9] studied the effect of the number of layers in depositing a single wall of a type of titanium alloy on the microstructure evolution. More detailed research has also been performed on defining the effect of process parameters on phase transformation EHSAN FOROOZMEHR, Ph.D. Candidate, and RADOVAN KOVACEVIC, Director, are with the Center for Laser Aided Manufacturing (CLAM), Southern Methodist University, Dallas, TX 75205. Contact e-mail: [email protected] Manuscript submitted October 13, 2008. Article published online June 24, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
during the heat treatment of materials.[10–12] Obviously, reaching a specific microstructure or hardness level in the buildup requires understanding of the temperaturephase transformation relation. A wide range of metal alloys have been used in the LPD process. Depending on the composition of the material, the microstructure transformation may have different responses to the process parameters. In the current work, a type of low-alloy steel, AISI 4140, is used to cover the surface of a substrate with different deposition patterns. Solid-state phase transformation is defined by the temperature and heating or cooling rate. The molten deposited material after solidification at solidus temperature consists of austenite (c). If the cooling rate is high enough, the martensite transformation, which is a nondiffusive transformation, occurs. The minimum required cooling rate for transforming austenite to martensite for AISI 4140 is about 25 C/s.[13] On the other side, the cooling rate in the LP
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