Materials. A number of common structural metallic alloys were used in this study, including Al 7075-T6, AISI 4340, AISI 304, Monel 400, molybdenum, and Ti6Al4V. These materials are used extensively within the aerospace industry and provide a large distribution in mechanical properties for study. The pertinent physical and mechanical properties of the metals are listed in Table 1 @16#. Only 7075-T6 aluminum alloy specimens of dimensions 52 mm3356 mm in surface area, 6.4 mm thick, were used for high-pressure waterjet peening study.
Procedure. All experiments were performed with a highpressure waterjet with or without abrasive additives. The highpressure pump with control unit was capable of generating waterjet
pressure up to 400 MPa. The pressurized water was controlled and directed through a 0.3-mm sapphire orifice. The nozzle was oriented perpendicular to the material surface and was moved and adjusted to obtain an appropriate nozzle-to-surface distance. The nozzle assembly, which transforms the high-pressure water into a collimated jet, consists of a 1.0-mm-dia carbide focusing nozzle. All machining was conducted with garnet abrasives. Three different process conditions were used in AWJ machining of the six structural metals for a total of 18 samples. Process parameters were chosen to obtain a moderate machined surface quality while incorporating a large variation in the primary cutting parameters. Precursory experiments were conducted to sustain a moderate surface quality over the range of cutting conditions chosen.
Table 2 lists the parametric combinations used in machining each of the six metallic materials. Further details can be found in @17# and @18#. The variation in the levels of pressure and grit size is the most significant. In water peening experiments shown in Fig. 1, the nozzle was set to move parallel to the surface and varied the standoff distances from 13 to 152 mm. Table 3 lists the peening conditions.
The surface texture resulting from the machined and peened samples was measured using contact profilometry with a Surf Analyzer TM 4000 profilometer and 5-mm-dia probe. Profiles of
the machined surface were obtained parallel to the cutting direction, and perpendicular in the case of peening. All measurements were obtained according to ANSI B46.1-1986 using a 0.8-mm cutoff length and 3.5-mm traverse length. Standard roughness parameters, including the arithmetic average roughness (Ra), peak to valley height (Ry), root-mean-square roughness (Rq), and 10-point height (Rz) were calculated from each profile. The distribution in subsurface plastic deformation was determined from Vickers hardness measurements along the cutting depth. Hardness measurements were obtained from a polished surface perpendicular to the WJ and AWJ machined edge according to ASTM E92-82. Measurements were recorded in 20-mm increments from the machined surface over a total length of 200 mm.
All measurements were performed with a 50-g indentation, load applied over a 10-s period. X-ray diffraction, scanning, and optical microscopy were used in examining the surface and subsurface features.