Surface tension derivation from laser-generated keyholes

Abstract

Surface tension is an essential material property that defines many aspects of thermal processes involving liquids. Metal materials have high melting temperatures, and surface tension could often be measured around melting temperature and is, therefore, known for many pure materials and simple material systems. However, high-energy input during laser, electron beam, or plasma processes is known to increase the material temperatures far above the melting point. To build theoretical models, simulate processes, and increase process understanding, surface tension values at those high temperatures would be beneficial to know. However, it can be difficult to create stable circumstances and measure surface tension in those conditions. Therefore, it is suggested in this work to indirectly derive surface tension values from the pressure balance inside keyholes created during laser deep penetration processing. A variety of different keyhole shapes were created using dynamic beam shaping by means of coherent beam combining. From the observed keyhole shapes using inline x-ray observations, temperature distributions on the keyhole walls were calculated using ray tracing. The temperature defines the local recoil pressure that counteracts the surface tension pressure, which contains the surface tension value as the only unknown variable. At increasing temperatures above the boiling point, an increasing surface tension was observed.