Analytical and Experimental Investigation of Thermocapillary Flow in Pulsed Laser Micropolishing

Abstract

The objective of this paper is to define and derive a dimensionless number as a function of material properties and process parameters to quantify the extent (magnitude) of thermocapillary flow in pulsed laser micropolishing (PLμP). Experimental work has shown that thermocapillary flow can tremendously reduce surface roughness (smoothing effect) although it inevitably introduces additional surface features (roughening effect) at the same time. Both the smoothing and roughening effects increase as the extent of thermocapillary flow increases. The extent of thermocapillary flow is the bridge from the available information (i.e., initial surface profile, material properties, and process parameters) to the polished surface profile to be predicted. A dimensionless number, called the normalized average displacement of a liquid particle in a single laser pulse, is proposed and derived via analytical heat transfer and fluid flow equations. The calculated normalized displacement is found to be proportional to the measured slope of the introduced features on Ti6Al4V surface polished with various process parameters, which indicates that the dimensionless number successfully describes the extent of thermocapillary flow. The normalized average displacement will be very useful for prediction of polished surface profile and hence parameter selection and process optimization in the future.

Manufacturing and Characterization of NiTi Alloy with Functional Properties by Selective Laser Melting

“…In this region, an increased melting duration resulted in surface capillary waves being more significantly damped [22,23]. However, as the energy increased beyond 42.7 J/mm 3 , the roughness increased with increasing energy density because thermocapillary flows dominated the melt pool [24][25][26]. The increased roughness was due to ripples induced by the thermocapillary flows when the energy density was increased.…”

Surface prediction model for thermocapillary regime pulsed laser micro polishing of metals

“…Assuming that surface tension gradients cause the surface of the melt pool to flow from the center to the perimeter and solidify there, IFS represents a physical measurement of how big of a dimple is left in the surface as the result of a single laser pulse. Ma et al [6] have demonstrated that the introduced feature slope is strongly related to the normalized average displacement (NAD). NAD (Eq.…”

“…NAD represents a numerical estimation of how far a voxel of liquid metal moves in the melt pool. The derivation of this dimensionless number combines heat transfer and fluid flow models that can be found in [6]: Fig. 1 shows that the correlation between NAD and IFS is very linear.…”

“…The current work presents a surface prediction model for thermocapillary regime PLP based on the previous analytical and numerical investigation of thermocapillary flow [5,6,16]. Experimental results for capillary and thermocapillary regime polishing are also compared to this model's predictions in order to address its suitability across both laser polishing regimes.…”

“…Therefore, it is of particular interest to develop a model that can be used to predict the surface profile and roughness resulting from thermocapillary regime PLP. Ma et al [5,6] have studied the mechanisms of melt pool flow and surface profile evolution and proposed a dimensionless number to quantify the extent of thermocapillary flow in PLP (normalized average displacement), which becomes the key to building the surface prediction model for thermocapillary PLP.…”