Early Stage Melt Ejection in Laser Percussion Drilling

T. Eppes[1]
[1]University of Hartford, Hartford, CT, USA

Laser percussion drilling is widely used in the aerospace industry to produce cooling holes in jet engine components. This process is a thermal, contact-free process which involves firing a sequence of focused optical pulses onto a target material [1-4]. During each optical pulse, the central portion of the target area heats to a liquid then vapor state where the expanding gas produces a recoil pressure that forces the liquid material to move outward and upward in a conical fashion. This paper presents a 2-D, time-dependent analysis of laser percussion drilling that focuses on the early stage of melt formation and ejection. A non-isothermal laminar flow (nitf) model was developed in COMSOL 4.2 that includes the effects of angle of incidence and optical intensity profile (Gaussian and flat-top). The target material is iron with temperature dependent material properties to enable the phase transitions. The primary force component is provided by the expanding iron vapor that first appears at the surface of the melt pool. Figure 1 shows the velocity field 2ms after the optical pulse begins and soon after the vaporization point is reached for a Gaussian pulse and normal incidence. A convective plume begins to develop from the surface rising upward into the air. Figure 2 and Figure 3 show the early stage of movement at a depth of 0.01mm below the surface in the x-direction and y-direction, respectively. Note that the flow moves outward and then rapidly upward. After a short time, the vapor region expands horizontally and begins to affect a larger portion of the melt pool. Figure 4 is a particle trace showing the flow history at a point 0.5mm beneath the surface. The paper includes results for a range of incidence angles as well as a similar study for a flat-top pulse shape. Lastly, the width of the temperature transition region (liquid to vapor) explores the impact on melt ejection velocities and removal efficiencies.