Computational Dynamics of Laser Alloyed Metallic Materials for Improved Corrosion Performance: Computational Dynamics of Laser Alloyed Metallic Materials

Computational Dynamics of Laser Alloyed Metallic Materials for Improved Corrosion Performance: Computational Dynamics of Laser Alloyed Metallic Materials

Olawale Samuel Fatoba (Tshwane University of Technology, South Africa), Abimbola Patricia Idowu Popoola (Tshwane University of Technology, South Africa), Gabriel Ayokunle Farotade (Tshwane University of Technology, South Africa) and Sisa Lesley Pityana (National Laser Centre, South Africa)
Copyright: © 2016 |Pages: 39
DOI: 10.4018/978-1-5225-0329-3.ch008
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Abstract

Laser alloying is a material processing method which utilizes the high power density available from defocused laser beam to melt both metal coatings and a part of the underlying substrate. Since melting occur solitary at the surface, large temperature gradients exist across the boundary between the melted surface region and underlying solid substrate, which results in rapid self-quenching and re-solidifications. Alloyed powders are deposited in a molten pool of the substrate material to improve the corrosion resistance of the substrate by producing corrosion resistant coatings. A 3D mathematical model is developed to obtain insights on the behaviour of laser melted pools subjected to various process parameters. Simulation with 3D model with different values of various significant processing parameters such as laser power, scanning speed and powder feed rate influences the geometry and dynamics of the melt pool, and cooling rates. It is expected that the melt pool flow, thermal and solidification characteristics will have a profound effect on the microstructure of the solidified region.
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1. Introduction

1.1. Laser Phenomenon

The word laser is an acronym that stands for Light Amplification by Stimulated Emission of Radiation. As said in the definition, laser is light but different from the normal light that is used on daily basis in terms of the beams they possess. In laser, the laser beam is much narrower than flash light with only one colour while the normal light is wide with many different colours (Toma, 2005). Majumdar and Manna (2003) refer to a laser as a device that consists of three basic components; an optical system, active medium and pumping source. An optical system or cavity is also referred to as feedback mechanism consisting of two mirrors placed parallel to each other forming an optical oscillator. The active medium which can be atoms, molecules or ions in gaseous state or solid crystal is placed between the mirrors and the chemical species in the gain medium; it determines the wavelength of the input through the process of stimulated emission. The pumping source supplies energy to the gain medium by exciting the laser medium into higher quantum energy levels. When an external energy is supplied to the irradiated atoms, the atoms attain an excited state and spontaneously emit a photon. The photons moving along the optic axis interact with a large number of excited atoms, stimulate them and get amplified. The process occurs repeatedly creating more photons which exit through the partially transmitting mirror as intense laser beam as indicated in Figure 1 Eventually the laser beam is guided to the work piece by the reflective mirrors or optical fibres.

Figure 1.

Schematic diagram illustrating the basic principle of lasers (Majumdar & Manna, 2003)

1.2. Laser Surface Processing

Compared with other methods of surface modification, laser surface processing is characterized by possibility of forming alloys of non-equilibrium compositions, formation of a fine microstructure, development of a strong metallurgical bond between the surface layer and the substrate, a small heat-affected zone and the combination of a controlled minimal dilution of the substrate by the coating material. It has major advantages of high productivity, automation worthiness, non-contact processing, and elimination of finishing operation, reduced processing cost, improved product quality and greater material utilization. These characteristics and advantages have led to increasing demand of laser in material processing (Lo, Cheng & Man, 2003:96-104; Oberlander & Lugscheider, 1992:657-665; Li & Yuan, 1994).

1.3. Laser Surface Treatment

Laser Surface Treatment has a strong impact on classical manufacturing and repair tasks addressing markets such as turbo machinery, aeronautics, automotive, off-shore and mining as well as tool, die, and mould making and life science (Kelbassa, 2011). According to Steen and Mazumdar (2010), laser has some distinctive properties for surface heating. For opaque materials, such as metals, the laser beam electromagnetic radiation is absorbed within the first few atomic layers and there are no associated eddy currents or hot gas jets. Moreover, there is no radiation spillage outside the optically defined beam area. Compared with other methods of surface modification, laser surface engineering is characterized by possibility of forming alloys of non-equilibrium compositions, formation of a fine microstructure, development of a metallurgical bond between the surface layer and the substrate, a small heat-affected zone and the combination of a controlled minimal dilution of the substrate by the coating material, and nevertheless, a very strong fusion bond between them. high productivity, automation worthiness, non-contact processing, elimination of finishing operation, reduced processing cost, improved product quality, greater material utilization and minimum heat affected zone. These characteristics and advantages have led to increasing demand of laser in material processing (Lo et al. 2003; Oberlander and Lugscheider, 1992; Li et al. 2011).

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