Thermal Analysis of Laser Chemical Machining: Part I: Static Irradiation

Abstract

The laser chemical machining is a non-conventional substractive processing method. It is based on the laser-activation of a material dissolution of metals in electrolyte ambient via local-induced temperature gradients and allows a gentle and smooth processing of especially temperature-sensitive metals. However, the material removal is characterized by a narrow process window and is restricted by occurring disturbances, which are supposed to be related to the localized electrolyte boiling. In order to control the removal quality and avoid disturbances, the correlation between the laser-induced temperatures and the resulting removal geometry has to be better understood. In this work an analytical modeling of the laser-induced temperatures at the surface of titanium based on a Green-function approach is presented. The main influencing factors (laser, electrolyte, material) as well as possible heat transfer into the electrolyte are included and discussed. To verify the calculated temperatures, single spot experiments are performed and characterized for titanium in phosphoric acid solution within laser irradiation of 1 s. The correlation between the temperature distribution and the resulting removal geometry is investigated based on a spatial superposition. Thereby, the bottom limit temperature is found to range between 63˚C and 70˚C whereas the upper limit is related to the nucleate boiling regime. Based on the performed correlation an indicator is identified to predict the ruling removal regime and thereby to reduce the experimental expenditure.

Dynamic Process Behavior in Laser Chemical Micro Machining of Metals

“…This effect intensified with increasing laser power. Similar results were demonstrated for the case of static irradiation of titanium in a 5-molar phosphoric acid and can be explained by the adherence of boiling related gas bubbles [17]. On the one hand, these gas bubbles interrupted the dissolution reaction at the interaction zone because of the deposition of metal salts and oxides.…”

“…However, the increase in laser power and/or feed velocity enhances the dynamic behavior in LCM and has a direct influence on the resulting removal quality. As demonstrated in previous works [16,17] a disturbance-free removal is limited to a process window of only some 100 mW. Once the removal is disturbed, the removal cross section changes from a bell-like shape to a W-like one.…”

“…Therefore, a characteristic threshold temperature should be achieved. As shown in [17], the removal rate is increasing continuously within a laser power window of some 100 mW. This regime is referred to as the disturbance-free regime.…”

“…Once a gas bubble with a size of some 10 µm is adhering to the interaction zone, the incident laser beam will be significantly affected. The formation of these bubbles could be related to electrolyte boiling due to high heat impact at the workpiece surface [17] or to the agglomeration of smaller bubbles during their scent at certain conditions, depending on the density and the viscosity of the electrolyte and on the surface tension [21]. The influence on the resulting removal geometry in dependence of the bubble size is shown in Figure 15b,c.…”

“…Energy dispersive X-ray analysis has shown that those disturbances are related to the deposition of metallic salts (phosphor/sulphur peaks) and oxides (oxygen peaks) layers [18]. Up to now, the occurrence of LCM removal disturbances is assumed to be the consequence of formed gas bubbles that are caused by thermochemical influence and electrolyte boiling [16,17]. However, interlinked evidence between experiments and models is still missing.…”