Breakdown of porous metals by laser radiation

Breakdown of a powder metal by laser radiation is analyzed, considering first the fundamental characteristics of melting as a quasi-steady process under a constant localized energy flux. Calculations for the typical 100,000 - 10,000,000 7 W/sq cm range of laser radiation density reveal that the trends of the porosity dependence of both the melting depth and the critical radiation flux density for initiation of boiling is determined by two thermophysical properties of the metal powder, namely its latent heat of evaporation and thermal conductivity, as well as by the laser-pulse duration. The melting depth is maximum within the 24-32% range of porosity, the peak becoming sharper and shifting toward lower porosity as the incident radiation flux density increases. Pore kinetics and formation of a crater under high-intensity laser radiation are considered, with the free surface energy taken into account, calculations revealing the role of pores as heat boosters. An approximate relation for the crater depth is derived for process design purposes.