Investigation of convective behaviour of the melt pool for coinage applications of laser surface alloying

In laser surface alloying the distribution of alloying elements is mainly attributable to the convective behaviour of the melt pool which is influenced by a number of factors including laser power density, size of the melt pool, interaction time, beam profile and the material physical properties. Any variation of the treatment conditions can change the flow pattern and the distribution of alloying elements in the melt pool. The laser alloying process reported in this paper includes the steps of predeposition of multiple layers of different metals and subsequent alloying of these layers using a high power CO₂ laser. Visible distinction between alloyed and unalloyed regions or difference in other physical properties is obtained. The work involves a study of the convection behaviour of the melt pool in the laser alloying process and the influence of the thermal properties of the materials. It was found that properties such as the difference of the melting points between the coating layer and the substrate materials and thermal diffusivity (or thermal conductivity) of the substrate materials played an important role in the process. Laser control variables required for localised alloying for different substrates and coatings types were determined. The influence of both thermal properties and laser control variables on alloy types was investigated. A mathematical model was developed to establish an understanding of the process and the proper selection of the materials combinations required to achieve specific alloying types for coinage applications. Initial work on coin validation showed promising results for automatic recognition of laser treated coins.