Laser surface alloying

Abstract

Laser alloying is a material-processing method which utilizes the high power density available from focused laser sources to melt metal coatings and a portion of the underlying substrate. Since the melting occurs in a very short time and only at the surface, the bulk of the material remains cool, thus serving as an infinite heat sink. Large temperature gradients exist across the boundary between the melted surface region and the underlying solid substrate. This results in rapid self-quenching and resolidification. Quench rates as great as 1011 KS-l and concomitant resolidification velocities of 20m S-l have already been achieved. What makes laser surface alloying both attractive and interesting is the wide variety of chemical and microstructural states that can be retained because of the rapid quench from the liquid phase. These include chemical profiles where the ‘alloyed’ element is highly concentrated near the atomic surface and decreases in concentration over shallow depths (hundreds of nanometres), and uniform profiles where the concentration is the same throughout the entire melted region. The types of microstructures observed include extended solid solutions, metastable crystalline phases, and metallic glasses.