Critical Factors in Laser and Electron Beam Glazing of Materials
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CRITICAL FACTORS IN LASER AND ELECTRON BEAM GLAZING OF MATERIALS. P. R. STRUTT, B. G. LEWIS, University of Connecticut Storrs, CT
B. H. KEAR Exxon Research and Engineering Co. Linden, NJ
ABSTRACT The major effects of laser and electron beam glazing on solidification microstructure and melt zone geometry are described. It is shown that under comparable processing conditions, i.e. absorbed power density and interaction time, the glazed microstructures are similar. Some variations in microstructure of laser and electron beam glazed M2 steel have been noted, which seem to be related to fluid flow effects in the melt zone and possible interactions with the environment. INTRODUCTION The nature of the interaction of a laser, or electron beam with a material surface is controlled primarily by two variables - the absorbed power density and the available interaction time. The rapid soldification surface treatment process, known as 'glazing', requires high power density of the order of that utilized for deep penetration welding, but with substantially shorter in eraction times. Under these conditions, energy inputs range from 1-100 J/cm , and the heating effect is concentrated in a very thin region at the material surface. This gives rise to extremely high thermal gradients, which promote rapid solidification of the melt. As a consequence of the extremely rapid solidification, a variety of interesting metallurgical microstructures can be developed, some of which are unique to the glazing process. This article will describe experimental findings on several glazed materials, using both the laser and electron beam glazing techniques. Amongst the specific areas of fundamental interest that will be considered are: (i) influence of solidification parameters on the microstructure developed in the thin rapidly quenched layers; (ii) dependence of melt zone geometry on processing variables, (iii) comparison of laser and electron beam coupling efficiencies; and (iv) influence of fluid flow on solidification microstructure. HEAT TRANSFER ANALYSIS In a basic investigation of directed energy processing an important aspect is the use of appropriate heat flow models to determine the effect of process variables on (i) melt zone geometry and (ii) solidification parameters. Suitable models include the moving point source model of Cline and Anthony (1)which involves an analytical solution of the thermal diffusion equation using Green's functions. Detailed study of the effect of such parameters as the latent heat of melting involve lengthy numerical procedures such as the finite difference analysis of Mehrabian et al (2). A convenient and simpler approach, proposed by Greenwald et al (3), is to model the glazing
486 process on the heating and cooling of a semi-infinite plate produced by exposure and removal of its surface to a uniform radiant energy flux. In summarizing and further developing this approach it is convenient to introduce the parameters a, b, and k and the function p(x,t), where hp(x,t) = b [aAt . exp - (kx/Vt)
a
(4a/7r)
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