Microstructure, Morphology, and Nanomechanical Properties Near Fine Holes Produced by Electro- Discharge Machining P.J. Blau, J.Y. Howe, D.W. Coffey, R.M. Trejo, E.D. Kenik, B.C. Jolly, and N. Yang (Submitted August 3, 2011) Fine holes in metal alloys are employed for many important technological purposes, including cooling and the precise atomization of liquids. For example, they play an important role in the metering and delivery of fuel to the combustion chambers in energy-efficient, low-emission diesel engines. Electro-discharge machining (EDM) is one process employed to produce such holes. Since the hole shape and bore mor- phology can affect fluid flow, and holes also represent structural discontinuities in the tips of the spray nozzles, it is important to understand the microstructures adjacent to these holes, the features of the hole walls, and the nanomechanical properties of the material that was in some manner altered by the EDM hole-making process. Several techniques were used to characterize the structure and properties of spray- holes in a commercial injector nozzle. These include scanning electron microscopy, cross sectioning and metallographic etching, bore surface roughness measurements by optical interferometry, scanning electron microscopy, and transmission electron microscopy of recast EDM layers extracted with the help of a focused ion beam. Keywords automotive, carbon/alloy steels, metallography 1. Introduction Fine holes in metal alloys are employed for many important technological purposes, including cooling and the precise atomization of liquids. They play a particularly important role in fuel injection systems in heavy vehicle and other diesel engines because they can affect not only the combustion timing and fuel efficiency, but also the exhaust gas emissions. Over the past 30 or more years, there has been a trend to raise the operating pressure in fuel injectors. These higher pressures increase spray penetration into the combus- tion chamber and promote better fuel burning characteristics (Ref 1); however, higher operating pressures also places additional demands on the alloys of which the nozzles are composed. In fact, injector pressures in excess of 2000 bars (200 MPa or 29,000 psi) are planned for the next generation of fuel efficient diesel engines. Electro-discharge machining (EDM) is widely employed to produce small, 50-300 lm diameter holes in fuel injector tips, called ‘‘sacks.’’ The enormous number of holes to be produced quickly, and with tolerances of 2 lm or less, has led to the development of highly automated EDM hole- making technologies (e.g., Ref 2). While the shape and bore morphology of holes affect air-fuel mixture flow, the holes can, in principle, form localized stress concentrations to initiate fatigue cracks. As the high pressures place additional demands on nozzle materials it becomes increasingly impor- tant to understand the microstructures adjacent to the spray holes, the features of the hole walls, and the nanomechanical properties of the material that has been altered by the EDM hole-making process. These characterizations were the focus of this investigation. 2. Experimental Methods and Results Several complementary methods were employed to charac- terize the holes and near-hole materials in a commercially produced, alloy steel nozzle from a production heavy vehicle diesel engine. The material was a heat-treated alloy steel, and representative parts are shown in Fig. 1. Initial studies included cross sectioning, etching, and microindentation hardness pro- filing of nozzles in the sack area to determine whether there were any gradients in mechanical properties through the thickness of the bulk material in which the holes had been formed. Subsequently, precision machining methods were used to produce axial cross sections of individual holes for scanning electron microscopy (SEM) and measurement of the surface roughness parameters and profiles of the hole bore. Further cross sectioning enabled nanoindentation hardness measure- ments of EDM recast layers. Finally, a focused ion beam (FIB) method enabled slices to be removed from the recast-layer to study the manner in which the fine structure of the alloy steel had been altered by the hole-making process. The methods used and results of the examination follow. P.J. Blau, J.Y. Howe, D.W. Coffey, R.M. Trejo, E.D. Kenik, and B.C. Jolly, Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831; N. Yang, Caterpillar Inc., Peoria, IL. Contact e-mail: blaupj@ornl.gov. JMEPEG (2012) 21:1744–1750 ÓASM International DOI: 10.1007/s11665-011-0070-6 1059-9495/$19.00 1744—Volume 21(8) August 2012 Journal of Materials Engineering and Performance