A thermal model of laser cladding by powder injection
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INTRODUCTION
THE selection of a material for a given application can be based upon its surface properties when a wear and/or corrosion environment is involved and/or upon its bulk properties when mechanical resistance to traction or creep is required. Modification of the surface layer by laser treatment is therefore an area of much interest to the manufacturing industry, Eu because it makes possible the production of high-quality surfaces while maintaining the mechanical properties of the base material. In the cladding process, a laser beam is used to heat the clad/substrate surface and produce a molten pool (Figure 1). The facing compound is delivered in a powder form into this interaction zone by means of a carrier gas. On impinging on the melt pool surface, the powder is entrained in the melt, exchanging energy, mass, and momentum with the liquid. When melting occurs in both the clad and substrate materials, a chemically homogeneous surface layer is produced with a metallurgical bond between the clad and the base material. It is the high degree of control over the laser energy which allows the optimization of the process. I21 An excess of energy which substantially remelts the substrate material leads to the dilution of the clad composition and, thus, the degradation of the clad properties. On the other hand, where there is insufficient energy, a fusion bond may not be formed, leading to poor adhesive properties and the risk of the clad spalling during service. However, it is precisely the localized nature of the laser energy which makes in situ measurements so difficult. Therefore, computer modeling can yield real insight in the laser cladding process, furthering our comprehension of the underlying physics occurring in the laser interaction zone. At the same time, simulation is a tool for quantifying the effects of the operating parameters, thereby aiding the optimization, scaleup, and control of the process. The injection of the powder into the melt pool distinguishes cladding from other laser or moving heat source A . F . A . HOADLEY, formerly Postdoctoral Fellow, Ecole Polytechnique Frdrrale de Lausanne, is Senior Research Engineer, Comalco, Thomastown, Australia. M. RAPPAZ, Professor, is with the Materials Department, Ecole Polytechnique Frdrrale de Lausanne, MX-G, Ecublens, 1015 Lausanne, Switzerland. Manuscript submitted September 16, 1991. METALLURGICAL TRANSACTIONS B
processes, such as surface hardening or surface remelting. Very few models of cladding have previously been published. Kar and Mazumder t31 considered the dissolution of the powder and the mixing of the clad in a onedimensional study. Weerasinghe and Steen 141 developed a finite difference model of laser cladding by powder injection. In their study, they included the effect of preheating the powder by the laser beam and also the effect of the powder in shadowing part of the laser energy from the surface. However, in assuming that the powder melted instantaneously on the clad surface, they did not allow for mixing within the melt pool.
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