Laser Melting and Surface Alloying
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LASER MELTING AND SURFACE ALLOYING
B. L. MORDIKE and H. W. BERGMANN Institut fdr Werkstoffkunde und Werkstofftechnik, Technische UniversitAt Clausthal, D-3392 Clausthal-Zellerfeld,
FRG
ABSTRACT The metallurgical aspects of laser melted and laser alloyed surfaces are discussed. It is shown how varying the laser parameters can influence the microstructure and properties.
INTRODUCTION High power lasers enable the surface of a component to be rapidly heated without the bulk material being effected. Consequently they can be used for heat treatment or surface alloying when different surface properties from the bulk properties can be obtained, e.g.,improvements in wear resistance, fatigue life or corrosion resistance. The surface alloying treatment thus enables relatively cheap substrate materials to be used and hence offers economic advantages over many other methods. Lasers are already established tools for cutting, welding and also hardening without melting [1]. In this paper we shall restrict discussion to the new and promising areas of surface melting or alloying. A chronological literature survey has been undertaken by Draper [2]. Table I shows what the effects of surface melting or alloying can be, divided into the effects of high quenching rates, high solidification rates, high heating rates and composition. TABLE I.
Effects of laser surface melting and surface alloying. Cause
Effect
high quenching rates
undercooling of the melt and hence changes in nucleation rates, grain refinement, formation of metastable phases, change in transformation kinetics
high solidification rate
reduction in time for diffusion in the liquid state, high supersaturation resulting in alloying hardening and changed transformation kinetics
high heating rates
change in the number of heterogeneous nuclei, incomplete solution of high melting point phases
changes in composition (surface layer)
changed alloy thermodynamics and hence different precipitation and grain refinement, dispersion hardening or martensite transformation
Not every material is automatically suitable for laser treatment. Of importance are parameters such as reflectivity, heat conduction, specific heat, melting and evaporation temperature, heat of fusion, thermodynamic properties and phase equilibria. In the examples we have chosen, the imporMat. Res.
Soc.
Symp.
Proc. Vol.
28 (1984)
)Elsevier Science Publishing Co.,
Inc.
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In addition to the matetance of these parameters will become apparent. rial properties there are many possibilities of modifying the effect by controlling the laser parameters.
INTERPLAY BETWEEN LASER PARAMETERS AND MATERIAL PROPERTIES In Table II the parameters which are important in and alloying are listed: TABLE II.
Important parameters in
surface melting
laser surface melting and alloying.
Determined by Laser Variables
Determined solely by Composition
power density P/A, effective rate of energy input
initial
composition
C*
temperature of equal enthalTJ(C*) pies in solid and liquid 0 T(*/ states
traverse speed v
For
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