Development of graded-boundary materials by laser beam

A method to develop graded-boundary materials (GBM) by laser beam is suggested, and its properties and characteristics are discussed. In this method, the differences of laser beam absorptance, thermal expansion coefficient, thermal conductivity, and solid solubility cause difficulties in making ceramic/metal GBM. Meanwhile, the presence of a stepwise compositional profile, the formation of root porosity, and an oblique crack near the boundary region are important in the making of metal/metal GBM by laser beam, and some ways to remove these defects are suggested. As an example, the experimental procedures and results of graded-boundary Alloy 690/steel alloy material produced by laser beam were presented. In this work, the laser surface-alloying method was applied to steel in order to develop a compositional gradient Cr,Ni/steel material. An Alloy 690 sheet was placed on a steel substrate, and then a series of CO2 laser beams were irradiated on that surface in order to produce a homogeneous alloyed layer. Simultaneous compositional gradients of two different kinds of alloying elements (Cr and Ni) were investigated. Sequential repetition of laser surface-alloying treatments up to four times resulted in a fair compositional gradient region with the thickness of about 3 mm from 21% to 0% for Cr, from 40% to 0% for Ni, and from 39% to 99% for Fe in this material.