Journal of Materials Processing Technology 211 (2011) 1247–1259 Contents lists available at ScienceDirect Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec An experimentally based thermo-kinetic hardening model for high power direct diode laser cladding Soundarapandian Santhanakrishnan, Fanrong Kong, Radovan Kovacevic ∗ Research Center for Advanced Manufacturing, Southern Methodist University, Dallas, TX, USA article info Article history: Received 18 August 2010 Received in revised form 21 January 2011 Accepted 14 February 2011 Available online 19 February 2011 Keywords: High power direct diode laser (HPDDL) Laser cladding Tool steel H13 AISI 4140 steel Finite element (FE) thermal model Thermo-kinetic (TK) model abstract High power direct diode laser (HPDDL) based cladding is found to be an economical process for repair- ing or building valued components and tools that are used in the automotive, aerospace, nuclear and defense industries. In this study, a 2-kW HPDDL of 808 nm in wavelength, rectangular-shaped laser spot of 12 mm × 1 mm with uniform distribution (top-hat) of laser power is used to carry out the experiments. An off-axis powder injection system is used to deposit tool steel H13 on the AISI 4140 steel substrate. A number of experiments are carried out by changing the laser power and scanning speeds while keeping a constant powder feed rate to produce different sizes of clad. An experimentally based finite element (FE) thermal model is developed to predict the cross-sectional temperature history of the cladding process. The temperature-dependent material properties and phase change kinetics are taken into account in this model. As-used experimental boundary conditions are adopted in this model. The acquired temperature history from the FE model is used to predict the temperature gradient, rates of heating and cooling cycles, and the solidification of the clad to the substrate. The FE thermal model results are coupled with thermo- kinetic (TK) equations to predict the hardness of the clad to the substrate. Metallurgical characterization and hardness measurements are performed to quantify the effect of processing parameters on the varia- tion of clad geometry, microstructure, and the change of hardness of the clad to the substrate. The results show that a good metallurgically bonded clad of hardness uniformity is achieved. © 2011 Elsevier B.V. All rights reserved. 1. Introduction For a number of years, lasers have been used in the materials processing industry in broader applications such as heat treat- ment, melting, alloying, and cladding. However, today the one-step cladding by using a high power direct diode laser (HPDDL) com- pared to a CO 2 or a Nd:YAG laser is found to be a cost-effective process. The HPDDL-based cladding is used in the automotive, aerospace, nuclear, and defense industries to repair or build the valued components and tools. Kinkade (2006) briefly reported the advantages of HPDDL compared to other high power lasers (CO 2 , Nd:YAG), related to its application for laser cladding. A focused laser beam with a rectangular spot (12 mm × 1 mm) and a shorter wavelength (808 nm) has better absorption by metals than CO 2 or Nd:YAG lasers. A higher wall-plug efficiency (∼30%) makes this type of laser more economical. A more uniform distribution (top- hat) of laser power across the length of the laser spot (12 mm) pro- vides more uniform cladding with a small heat-affected zone (HAZ). ∗ Corresponding author at: Research Center for Advanced Manufacturing and Cen- ter for Laser Aided Manufacturing, Southern Methodist University, 3101 Dyer Street, Dallas, TX 75205, USA. Tel.: +1 214 768 4865; fax: +1 214 768 0812. E-mail address: kovacevi@lyle.smu.edu (R. Kovacevic). In the laser cladding process, the same or different chemical composition of clad material with respect to the substrate is used to produce a new component or a good metallurgical bond struc- ture on the existing component. During the cladding process, a high power laser is used to melt the blown or preplaced powder particles or wire-feed at the substrate. Cook et al. (2000) demon- strated that a denser microstructure laser clad with stable bond to the substrate is generated at the high temperature. The metal- lurgically transformed clad structure on the valued components’ enhances/improves the surface properties such as wear-resistant, corrosion-resistant, and heat-resistant. Several methods are used to achieve a thin layer of clad in the laser cladding process, such as (1) injecting the powder particles into the molten pool generated by the high power density laser beam; (2) feeding the wire into the focal spot of the laser beam; and (3) a thin layer of powder is pre-placed on the substrate in order to expose it to the high density laser beam. In the past, the laser cladding process by powder injection has proved to produce uniform, defect-free, and a good metallurgically bonded clad on the substrate. Syed et al. (2005) studied the influence of the powder feeding direction (front side and rear side) with respect to the laser scanning direction. The angle and position of the powder feeding nozzle greatly affect the clad geometry. A small variation in the powder feed rate could significantly generate larger variations in 0924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2011.02.006