Thermal Behavior and Microstructure Evolution during Laser Deposition with Laser-Engineered Net Shaping: Part II. Experi
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INTRODUCTION
A two-part investigation is described in the present work aimed at providing insight into the relationship between thermal conditions and microstructural evolution during laser-engineered net shaping (LENS)* deposition. *LENS is a trademark of Sandia National Laboratories, and is commercialized by Optomec, Inc., Albuquerque, NM.
In Part I,[1] we described the numerical framework used to predict the thermal profile that is present during LENS processing. More specifically, an alternate-direction explicit (ADE) finite difference formulation was used to establish the influence of laser output power, travel speed, and initial temperature of the substrate on the resulting thermal profile characteristics within deposited materials, notably the maximum temperature and cooling rate at various locations and times during LENS deposition. It was predicted that the deposition of individual layers leads to thermal fluctuations that appear as periodic pulse waves, and that the amplitude of wave peak temperature tends to dampen as more layers are deposited. High cooling rates of 103 to 104 K/s can be achieved during the initial stages of deposition, and the rapid quenching effect decreases with thickening due to heat accumulation in the deposited B. ZHENG, Postdoctoral Researcher, Y. ZHOU, Associate Researcher, J.M. SCHOENUNG, Professor, and E.J. LAVERNIA, Dean of College of Engineering, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact e-mail: [email protected] J.E. SMUGERESKY, Senior Staff Member, is with Sandia National Laboratories, Livermore, CA 94551-0969. Manuscript submitted October 22, 2007. Article published online June 24, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
materials. In Part II, we describe a series of experiments completed with LENS-deposited 316L, in an effort to provide experimental validation to the numerical results, as well as to provide insight into microstructural evolution. It is well established that cooling rate has a profound effect on the microstructure evolution of stainless steel alloys.[2–6] For example, five modes of solidification and twelve morphologies of Fe-Ni-Cr stainless steels were observed during the different solidification processes with different cooling rates.[3] High cooling rates can be produced by high power density welding, resulting in microstructures that are far from equilibrium. The temperature profile surrounding the melt pool of 316L fabricated parts were measured by Hofmeister et al.,[4] and the cooling rates at the solid-liquid interface ranged from 102 to 103 K/s. In the case of LENS processing, the thermal field is influenced by several interrelated variables, such as laser power, displacement speed, and powder flow rate, and accordingly, the cooling profile is complex and poorly understood. The relationship between dendrite arm spacing (DAS) and cooling rate provides a useful approach to establish the precise effect of thermal conditions on microstructure. The relationship between second dendrite a
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