Frank Liou (liou@mst.edu, 573-341-4603), Joe Newkirk, Joohyun Choi

This paper is to conduct research on a multi-axis hybrid Laser Aided Manufacturing Process (LAMP) to fabricate aerospace structures.  This system integrates multi-axis laser deposition and material removal processes to produce aerospace structures with machining accuracy.  Automated process planning issues are investigated and developed to automatically generate motion codes for the integrated hybrid process.  Modeling of the laser-material processes are also conducted.   The LAMP system fully utilizes multi-axis capability to minimize the usage of support structures by fabricating non-uniform thickness layers and changes the building direction as needed. 

1. Lijun Han, Frank W.  Liou and Srinnivas Musti, “Thermal Behavior and Geometry Model of Melt Pool in Laser Material Process,�? Transactions of the ASME: Journal of Heat Transfer, Vol. 127, No 9, pp. 1005-1014, September 2005.
Abstract:  Melt pool geometry and thermal behavior control are essential in obtaining consistent building performances, such as geometrical accuracy, microstructure, and residual stress. In this paper, a three dimensional model is developed to predict the thermal behavior and geometry of the melt pool in the laser material interaction process. The evolution of the melt pool and effects of the process parameters are investigated through the simulations with stationary and moving laser beam cases. The roles of the convection and surface deformation on the heat dissipation and melt pool geometry are revealed by dimensionless analysis. The melt pool shape and fluid flow are considerably affected by thermo-capillary force, surface tension and recoil vapor pressure. Quantitative comparison of interfacial forces indicates that recoil vapor pressure is dominant under the melt pool center while thermo-capillary force and surface tension are more important at the periphery of the melt pool. For verification purposes, the CMOS (Complementary Metal Oxide Semiconductor) camera has been utilized to acquire the melt pool image online and the melt pool geometries are measured by cross-sectioning the samples obtained at various process conditions. Comparison of the experimental data and model prediction shows a good agreement.

2.  J. Ruan, K. Eiamsa-ard, and F. Liou, “Automatic Process Planning and Toolpath Generation of a Multi-Axis Hybrid Manufacturing System,�? SME Journal of Manufacturing Processes, Vol 7, No. 1, pp. 57-68, 2005.
Abstract: With the integration of multi-axis layered manufacturing and material removal (machining) processes, a hybrid system has more capability and flexibility to build complicated geometry with a single setup.  Process planning to integrate the two different processes is a key issue. In this paper, an algorithm of adaptive slicing for five-axis Laser Aided Manufacturing Process (LAMP) is summarized which can generate uniform and non-uniform thickness slices. A method to build a non-uniform (thickness) layer which utilizes two processes is presented and an overall algorithm for integration is described. The newly developed algorithm implemented in the process planning helps the hybrid system build parts more efficiently.

3. Heng Pan and Frank Liou, “Numerical Simulation of Metallic Powder Flow in a Coaxial Nozzle for the Laser Aided Deposition Process,�? Journal of Materials Processing Tech., Volume 168, Issue 2  , 30 September 2005, Pages 230-244
Abstract: Laser aided deposition process offers the ability to make a metal component directly from CAD drawings. Analysis of metallic powder flow in the feeding system is of particular significance to researchers in order to optimize this technique. Powder flow simulation holds a critical role in understanding flow phenomenon so as to ensure proper design and sound functionality of the coaxial nozzle. Numerical study of metallic powder flow in the coaxial nozzle for laser aided deposition is, however, barely existent. This is partially because not all of gas-atomized powders and none of water-atomised powders can be considered spheres and an accurate and economic modeling approach in describing non-spherical powder dispersion behavior can rarely be seen. To quickly simulate realistic powder flow and also meet the practical requirement for design optimization of the coaxial nozzle, a stochastic model, which considers particle shape effects, is developed. The validity of the model is demonstrated through comparison with experiment. The application of the model to the evaluation of various nozzle geometrical configurations is shown.

4.  L. Han, F.W.  Liou, and K.M. Phatak, “Modeling of Laser Cladding with Powder Injection,�? Metallurgical and Materials Transactions B, 2005, VOLUME 35B, DECEMBER 2004, pp. 1139-1150.
Abstract: Laser cladding is one of the material additive manufacturing processes via producing a metallurgically bonded deposition layer. To obtain a high quality resulting part, a deep understanding of the underlying mechanisms is required. In this paper, a mathematical model is developed to simulate the coaxial laser cladding process with powder injection, which includes laser-substrate, laser-powder and powder-substrate interactions. The model considers most of the associated phenomena, such as melting, solidification, evaporation, evolution of the free surface and powder injection. The fluid flow in the melt pool, which is mainly driven by Marangoni shear stress as well as particle impinging, together with the energy balances at the liquid-vapor and the solid-liquid interfaces are investigated. Powder heating and laser power attenuation due to the powder cloud are incorporated into the model in the calculation of the temperature distribution. The influences of the powder injection on the melt pool shape, penetration, and flow pattern are predicted through the comparison for the cases with powder injection and without powder injection. Dynamic behavior of the melt pool and the formation of the clad are simulated. The effects of the process parameters on the melt pool dimension and peak temperature are further investigated based on the validated model.

5.  Han L.J. and F. W. Liou, “Numerical Investigation of the Influence of Laser Beam Mode on Melt Pool�?  International Journal of Heat and Mass Transfer, 47, pp. 4385-4402, 2004.
Abstract: During the laser material interaction process, the laser beam mode has significant influence on the formation and development of the melt pool, which directly affects the product quality. In the present study, a mathematical model and related numerical methods have been developed to simulate the process of laser material interaction for different laser beam modes. Three types of symmetric laser beam modes and one nonsymmetrical rectangular mode are considered. Since spatial distribution of the laser beam intensity varies with beam mode, the formation procedure and the shape of the melt pool in these four cases exhibit quite different features. Flow pattern, which is driven mainly by the thermo-capillary force and recoil pressure, together with temperature distribution are investigated and compared. Physical phenomena such as melting, solidification, vaporization and evolution of the free surface are incorporated in the present model.