Melt-solid interactions in laser cladding and laser casting
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I. INTRODUCTION
THE process of laser cladding has been the subject of scientific research and commercial applications since the late 1970s,[1–6] in particular, investigating the track cross section, absorption, substrate dilution, and mechanical properties. Moreover, crack and pore formation are essential phenomena to be avoided, but highly depend on the metallurgical properties of the different materials and alloys applied. The motivation of laser cladding is usually to create a wear- or corrosion-resistant surface. Thus, many investigations also include wear tests such as pin-on-disc. There are two basic methods of using a laser to clad one metal with another: the preplaced powder method and the blown powder method. These two techniques are illustrated in Figure 1. This article investigates the thermo and fluid dynamics of the preplaced powder cladding method and of a more recently developed technique called “laser casting.” Laser casting was developed at the Luleå University of Technology[7] and involves the deposition of a laser-melted metal into a mold (Figure 2). The process parameters are deliberately chosen to prevent the “clad” deposit forming a bond with the substrate, and thus, cast objects can be produced. Laser casting can be described as laser cladding, which has no substrate–clad layer bond. This article is an investigation into the process by which such substrate clad layer bonds are created or avoided. This work uses experimental results to support two theoretical models. The first, which is an extension of earlier work by Powell et al.,[1,2] investigates the macroscopic aspects of the melt-solid interactions during preplaced H. GEDDA, Postdoctoral Student, and A. KAPLAN, Professor, are with the Division of Manufacturing Systems Engineering, Luleå University of Technology, S-971 87 Luleå, Sweden. Contact e-mail: [email protected] J. POWELL, Visiting Professor, Division of Manufacturing Systems Engineering, Luleå University of Technology, is with Laser Expertise Ltd., Acorn Park Industrial Estate, Nottingham NG7 2TR, United Kingdom. Manuscript submitted July 30, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
powder laser cladding. The second model, which builds on earlier work by Kaplan et al.[8,9] looks at the microscopic effects of the interaction of the melt with individual powder particles. II. EXPERIMENTAL PROCEDURE For the purposes of this study, a number of clad tracks were produced over a wide range of process speeds. The experimental details are provided in Table I. In the present case, the average beam power density ranges from 8102 to 3.2104 W/cm2, which is a typical range for laser surface treatment, as melting of the surface takes place at 103 to 104 W/cm2. An upper limit is of the order of 106 W/cm2 due to the onset of intense evaporation. The power density in conjunction with the processing speed and other conditions such as the interface powder grain size determines the operating windows for cladding and for casting. The absorptivity correspondingly alters this range. For
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