Characterization of microstructural effects in a percussion laser-drilled powder metallurgy Ni-based superalloy

In the combustion zone of gas turbine engines for aircraft, it is essential to maintain a large temperature differential between the combustion gases and the Ni superalloy components so that fuel is used efficiently while avoiding degradation of the turbine blades, rotors, casings, etc. This is typically achieved using a combination of thermal barrier coatings (TBCs) and cooling holes [1–3]. In the latter case, air from the compressor section is forced through internal channels in the components and this emerges from arrays of fine (\500 lm in diameter) cooling holes, establishing a thin air layer between the combustion gases and the TBC. Current generation engines can contain in excess of 10 cooling holes and most of these are produced by electrodischarge machining (EDM) or laser drilling. The use of laser drilling is increasing because it enables holes to be drilled through components with TBCs (e.g. [4, 5]); this is a significant advantage over EDM where the holes must be formed in uncoated components and the subsequent application of a TBC can cause cooling-hole blockage. Laser drilling of aerospace components is normally performed in percussion mode whereby the laser and component are stationary and a few short (m each pulse melts and partially vaporizes the alloy, leading to ejection of the molten material and vapor through the irradiated surface [4–6]. Percussion laser drilling is usually preferred over both laser trepanning with a moving beam, which produces very high quality holes but with a longer cycle time, and single-shot drilling, which produces more debris and other artifacts (e.g. [5, 6]). The main metallurgical effects that can occur during percussion laser drilling are the formation of a recast layer and/or a heat-affected zone (HAZ) [5–8]. The recast layer is comprised of molten/vaporized metal that is not ejected from the hole by the vapor generated during the laser pulse but instead resolidifies on the sidewall of the hole [5, 6]. The HAZ consists of a region of the base metal surrounding the hole that has not melted during the drilling but has undergone microstructural changes due to the thermal history in a manner akin to the HAZs formed during laser welding and cladding (e.g. [9–12]). It is known that the recast and/or HAZ could lead to significant fatigue debits for laser-drilled components, and indeed this has been the motivation behind recent work on femtosecond ablative laser drilling in which recast and/or HAZ effects can be avoided [13, 14]. While there have been several parametric studies in which optical and scanning electron microscopy have been used to measure the extent of the recast and HAZ layers for such laser-drilled components (e.g. [15–18]), there has been little high-resolution characterization work to reveal the structures of these layers. In our work, we have used a variety of high-resolution microstructural characterization techniques to investigate the recast and HAZ structures for a variety of different Ni-based superalloys and preliminary data have been presented elsewhere [19]. The present paper describes a more detailed electron microscopy study of these structures for percussion laser-drilled samples of the powder metallurgy (P/M) superalloy IN100, which has a nominal composition of Ni–18.5Co–12.4Cr–5.0Al–4.3Ti– 3.2Mo–0.8V–0.07C–0.06Zr–0.02B (all in wt%). This particular alloy was selected because in the P/M form it has a very uniform chemistry and a well-defined microstructure comprising a hierarchy of L12 c0 precipitates on different J. K. M. Garofano H. L. Marcus M. Aindow (&) Department of Chemical Materials and Biomolecular Engineering, Materials Science and Engineering Program, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269-3136, USA e-mail: m.aindow@uconn.edu