Modeling of laser cladding with powder injection
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I. INTRODUCTION
INDUSTRIAL applications require parts with special surface properties such as good corrosion resistance, wear resistance, and hardness. One of the techniques to improve surface properties is laser cladding, which now has been integrated into the industrial manufacturing lines to create a quality surface. Unlike conventional methods, laser cladding is fast and precise; at the same time, it has a small heat-affected zone (HAZ). The laser cladding process produces metallurgically well-bonded coatings, which may have a variety of materials of intermediate thickness from 0.1 to 1.0 mm. In order to obtain a strong fusion bond, the process requires the formation of the melt pool on the substrate. For a better coating, the depth of the melt pool must be as small as possible; hence, a minimal dilution in the substrate.[1] Since the dilution and clad thickness will vary with different process parameters, it is essential to understand the relationship of the process parameters to the optimal product quality. The complexity of the interaction of the many process variables makes this optimization goal the main challenge.[2] This will require a quantitative understanding of the influence of the process parameters such as laser power, scanning speed, and powder feeding rate on the surface quality of the clad and microstructure of the resulting part. The laser cladding process is usually carried out based on the single step powder delivery, which is also called blown powder laser cladding technology. This process is shown schematically in Figure 1. In our current study, the laser beam is coaxial with the nozzle and forms a beam spot on the surface of the substrate. The beam spot usually has a diameter ranging from 0.5 to 1.2 mm, and the beam energy
L. HAN and K.M. PHATAK, Graduate Students, and F.W. LIOU, Professor, are with the Department of Mechanical Engineering, University of Missouri–Rolla, Rolla, MO, 65409-1350. Contact e-mail: [email protected] Manuscript submitted February 26, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
is concentrated in the spot to create a melt pool by fusing the substrate surface. The powder particles are transported through the nozzle into the melt pool by a carrier gas and are heated during the flight. Some of them are melted during travel, but more of them start to melt when they strike the melt pool surface and are then trapped in the melt pool. In the coaxial system, when the nozzle goes forward together with the laser beam or the substrate moves in the opposite direction, the powder entering the melt pool will stick up during the solidification, thus generating a clad layer track. The injection of the powder into the melt pool distinguishes the cladding process from other laser processes, such as surface hardening or laser remelting. The powder injection influences the melt pool dynamics, and it is critical to the product quality for the following reasons: (1) laser power is attenuated by the powder cloud, so the threshold power that forms the melt pool on the substrate will va
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