Waterjet Machining and Peening of Metals

M. Ramulu S. Kunaporn Department of Mechanical Engineering, University of Washington, Box 352600, Seattle, WA 98195 D. Arola University of Maryland, Baltimore County, MD M. Hashish J. Hopkins Flow International, Kent, WA 98032 Waterjet Machining and Peening of Metals An experimental study was conducted to determine the influence of high-pressure waterjet (WJ) peening and abrasive waterjet (AWJ) machining on the surface integrity and texture of metals. A combination of microstructure analysis, microhardness measurements, and profilometry were used in determining the depth of plastic deformation and surface texture that result from the material removal process. The measurement and evaluation of residual stress was conducted with X-ray diffraction. The residual stress fields resulting from treatment were analyzed to further distinguish the influence of material properties on the surface integrity. It was found that waterjet peening induces plastic deformation at the surface layer of metals as good as shot peening. The degree of plastic deformation and the state of material surface were found to be strongly dependent on the peening conditions applied. S0094-9930 00 00801-5 Introduction Contributed by the Pressure Vessels and Piping Division and presented at the Pressure Vessels and Piping Conference Joint with IC PVT, Boston, Massachusetts, August 1 5, 1999, of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS. Manuscript received by the PVP Division, June 25, 1999; revised manuscript received August 31, 1999. Technical Editor: S. Y. Zamrik. The development of high-performance materials, such as composites and advanced ceramics, presents a variety of manufacturing challenges. Many of these materials cannot be effectively or economically machined by conventional methods, and therefore require methods of shaping and/or postmold processing with unique sources of material removal. Various methods are currently used in machining and surface treatment of structural monolithic alloys. Apart from economics, the means for process selection is often based on the machined surface integrity. The high-pressure waterjet with abrasive additives known as abrasive waterjet AWJ is one viable alternative to conventional processing and has been suggested for use in postmold shaping of composite and other hard-to-cut materials 1 4. Omni-directional cutting potential as well as minimal thermal and mechanical loading are just a few of the advantages realized when cutting with a water-driven abrasive slurry. Despite current and a continuing development of interest in this machining process, only a limited understanding of material removal mechanisms in WJ and AWJ cutting exists. Furthermore, the influence of material properties on the mechanisms of material removal and change in mechanisms with cutting depth has not been well understood. Hence, a comprehensive analysis of the surface integrity resulting from high-pressure WJ machining of metals was necessary. Mechanisms of material removal present in AWJ machining are often described with terminology from the studies of solid-particle impact-induced erosion. Hashish 3,5 suggested from a visualization of the cutting process in plexiglas that material removal occurs by cutting wear and deformation wear, terms used by Finnie 6 and Bitter 7,8 in describing abrasive-induced erosion. Cutting wear defines erosion at small angles of particle impact and occurs when the shear strength of the material is exceeded due to abrasive particle shear loading. The cutting wear zone on an AWJ machined surface is regarded as the uppermost portion of the eroded kerf which exhibits high-quality surface texture with limited macroscopic variation. Deformation wear erosion as defined by Bitter 8 is material removal by repeated particle bombardment at large impact angles greater than 20 deg. In this mode of abrasive removal, the parent material is plastically deformed, locally work-hardened with continual bombardment, and eventually removed due to plastic embrittlement. The deformation wear zone in AWJ cutting exists below the cutting wear zone and is typically identified by waviness or striation patterns caused by severe jet deflection. Waviness patterns have been noted not only in AWJ machining, but also in other beam cutting processes like plasma, electron, and lasers. A variety of materials with uniquely different mechanical properties, both ductile and brittle, exhibit these characteristics. Distinction between the cutting wear and deformation wear mechanisms of material removal in AWJ studies are commonly differentiated by waviness patterns on the kerf. Based on our extensive experimental work at the University of Washington for the past ten years, we found the formation of waviness patterns are a function of the energy of the impinging jet 9 11, not the mechanisms of material removal 12 14. An understanding of the mechanisms of material removal in high-pressure WJ and AWJ machining process is extremely important due to their effect on structural performance. External forces applied to create a new surface through mechanical work can result in a near sub-surface stress field. Burnham and Kim 15 found that the cutting force during AWJ machining of alumina and steel increased with jet penetration depth. An increase in cutting force may influence the size and distribution of the residual stress field near the surface in a material susceptible to work hardening. For materials where plastic deformation or brittle fracture occurs, surface properties of the workpiece material may change during material removal. In this study, we will review our investigations on common ductile metallic materials, which were treated and machined with WJ and AWJ. Machining tests were conducted with numerous parametric combinations to promote varying degrees of cutting wear and deformation wear zones on the machined surfaces. A thorough visual analysis was carried out with scanning electron microscopy to distinguish and differentiate mechanisms of material removal as a function of cutting depth and material properties. Material removal is discussed with regard to machine parameters, material properties, and as a function of the standoff distance in WJ impacting and depth of cut in AWJ machining, respectively. Experimental Procedures Materials. A number of common structural metallic alloys were used in this study, including Al 7075-T6, AISI 4340, AISI 90 Õ Vol. 122, FEBRUARY 2000 Copyright 2000 by ASME Transactions of the ASME

Table 1 The physical and mechanical properties of the test materials Fig. 1 Table 3 Waterjet peening experimental setup Test conditions for water peening study 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 mm 356 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- m-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 (R a ), peak to valley height (R y ), root-mean-square roughness (R q ), and 10- point height (R z ) 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- m increments from the machined surface over a total length of 200 m. 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. Table 2 AWJ machining test matrix Results and Discussion Waterjet Peening. Figure 2 shows the typical erosion surface photomicrographs of water-peened specimens with different jet conditions. The erosion region resulting from the jet obviously varied with respect to the standoff distance SOD, with it being narrowest and deepest at the minimum SOD. Removal characteristics within the impact zone were found to be predominantly dependent on the standoff distance. Characteristics of this region consist of extensive plastic deformation and local work hardening due to the waterjet impacting process. The detectable region is uniformly wider and shallower, until reaching the point at which the surface appears similar to the base material i.e., the point of zero mass removal. The erosion region is deeper, larger, and more severe in the specimens subjected to the higher pressure. The depth of erosion is greater at the edge than at the center of the erosion region because the high velocity results in much higher radial velocity along the specimen surface. Using the erosion surface features, the relation of the erosion depth with respect to SOD and the zero mass loss point of each condition were obtained. Based on experimental data, the relation between the jet pressure and the standoff distance associated with Journal of Pressure Vessel Technology FEBRUARY 2000, Vol. 122 Õ 91

Fig. 4 Microhardness distribution of specimens Fig. 2 Graphic representation of the eroded surface specimens of a ML-1, b ML-2, c ML-3, d ML-4, e ML-5, f ML-6, and g ML-7 the zero mass loss SOD 0 was developed. A linear relation was found to exist between SOD 0 and the peening pressure and is independent of jet velocity. For a given jet exposure time, the linear relation with a correlative coefficient at R 2 0.9618 was SOD 0 0.269 P 3.261. Therefore, the equation was used to calculate the zero mass loss point for any applied jet pressures of aluminum alloy 7075-T6 for water peening experiments. The performance of water-peened specimens was analyzed by employing an x-ray integration method RIM 19 to calculate the stress tensor. The resulting analysis of the stress field was assumed to be biaxial since the measurement took place at the surface of the specimen. The surface residual stresses were evaluated by using the equivalent stress and normalized with the base material, plotted in Fig. 3. Note that an increase in jet pressure and a decrease in SOD increased the compressive residual stresses. It is interesting to see that the magnitude of peened surface residual stress is about three times the base material residual stress. The microhardness distribution as a function of SOD for water peening is plotted in Fig. 4. The results clearly show that the amount of compressive residual stresses induced at a subsurface depth of 50 microns is about 15 percent higher than the base material hardness. However, the degree of subsurface hardening Fig. 3 Residual stresses versus standoff distance was extended to about 200 microns. In comparison to the shotpeened hardening effect reported on 7075-T6 20 values, the water peening process was found to yield similar results. From our ongoing experiments, improving the peening conditions was found to give high surface hardening and was better than shot peening. Typical surface profiles of the peened surface at different standoff distances and only the average surface finish (R a ) are presented in Fig. 5. The surface roughness resulting from the peening process was increased when SOD decreased. Figure 6 shows the scanning electron micrographs of the peened and unpeened surface. Water-peened surfaces were found to have pitlike surfaces with shear lips, demonstrating the high plastic deformation on the surface. This confirms our speculation that extensive plastic deformation contributed to the high residual strength and rapid degradation of this hardening behavior as the depth of subsurface increased. Abrasive Waterjet Cutting. The AWJ machined surface of the metals was distinguished by the presence of three distinct macroscopic regions, including the initial damage region IDR, smooth cutting region SCR, and rough cutting region RCR. An example of these three regions on the AWJ machined surface of 7075-T6 is shown in Fig. 7. The IDR near the point of jet entry has been previously ignored in the literature, and was not considered as an individual portion of the machined surface. Traditionally, the machined surface is divided into, at most, only two regions, the division distinguished by the presence of waviness patterns. From this point forward, the surface integrity resulting from AWJ machining will be addressed in each of the three regions based on the acute difference in the microscopic surface features, as evident in Fig. 7. Note that the only difference in microscopic features resulting from material removal in the SCR and RCR resulted from the increase in jet deflection. The deflection angle increases with depth as a result of the reduction in cutting energy and the jet s capacity for material removal. However, the IDR exhibits considerable deformation due to the nearly normal repeated impact of abrasives on the jet periphery. Indeed, the mechanisms of material removal are unique from that of the SCR and RCR. SEM micrographs Figs. 7 b and c revealed that the material removal mechanism was predominantly shear deformation and is evident from the microscopic features. Material removal occurred through a combination of lip formation at the abrasive forefront and ploughed adjacent to the abrasive path. The microscopic features within the IDR of all six metals were far different than those within the two other characteristic regions. Surface deformation and the visibility of wear tracks on the machined surfaces were much more significant for the Monel 400. However, the low-ductility materials appeared to have undergone far less deformation. In comparison, with less extensive plough tracks and more evidence of abrasive rubbing, occasionally abra- 92 Õ Vol. 122, FEBRUARY 2000 Transactions of the ASME

Fig. 6 Micrographs of water-peened surfaces using nozzle B under different standoff distances with high pressure: a SODÄ36 mm, b SODÄ53 mm, c SODÄ76 mm, d SODÄ102 mm, and e unpeened surface Fig. 5 Surface profile and average roughness resulting from WJ peening of nozzle B sives were found lodged in the free surface. However, abrasive sticking was not prominent for any of the metals. The subsurface Vickers hardness of the metals was measured from a sectioned plane normal to the AWJ machined surface in increments of 20 mm. The distribution in normalized microhardness of the six metals machined with condition AWJ A is shown in Fig. 8. Although all the metals exhibited some degree of hardening within the IDR, Monel 400 underwent the most extensive depth of hardening. Monel 400 and Al 7075-T6 both underwent strain hardening below the machined surface, with the maximum for both metals near 70 m from the free surface. No increase in surface hardness was noted within the SCR of the Ti6Al4V, which complies with the reports from previous investigations 13,14. A comparison of the hardness measurements obtained from the SCR and RCR of the metals revealed that the depth and magnitude of subsurface hardness was not cutting depth dependent below the IDR. The distribution in the arithmetic average roughness (R a ) with Fig. 7 Microscopic feature of three regions on the AWJmachined surface of 7075-T6 Journal of Pressure Vessel Technology FEBRUARY 2000, Vol. 122 Õ 93

Fig. 10 Depth of subsurface deformation from Vickers hardness measurements, cutting conditions AWJ A: a IDR and b SCR Fig. 8 Typical surface profiles of the AWJ-machined metals Fig. 9 Average roughness resulting from AWJ machining: a AWJ A, b AWJ B, and c AWJ C depth of cut is shown in Fig. 9 for the three AWJ conditions A, B, and C. In general, the roughness of the nickel and aluminum alloys exceeded that for the remaining materials in conditions A and B. In contrast, AWJ machining of Ti6Al4V and molybdenum with these two conditions resulted in the lowest machined roughness over the cutting depth. Larger abrasives serve to maintain the jet energy with cutting depth as indicated by the minimal changes in R a and no. 50 mesh abrasives. However, the increase in R a with development of large wavelength surface fluctuations is much more acute for condition C no. 100 mesh. Jet energy decreased most readily in machining AISI 304 and Ti6Al4V due to some particular aspect of the mechanical properties. The machined surface skewness was calculated from the height profiles to emphasize the depth of removal volume per abrasive for different metals. Surface skewness defines the nature of an asymmetrical surface distribution with respect to a purely symmetric or Gaussian spread. A negative skewness is ideal for bearing surfaces that require large effective contact areas and lubrication reservoirs within the valleys of the lay. Positive skew is more effective in minimizing fatigue failures through free abrasive erosion; metals with greater resistance to abrasive penetration would be prone to exhibit a negatively skewed surface. Materials responsive to abrasive wear are more likely to exhibit a positively skewed surface due to the reduction in wear-resistant surface stress concentrations 18. Despite some irregularity over the depth of cut, the AISI 304, Monel 400 and Al 7075-T6 surfaces resulting from condition A are the highest positively skewed. Ti6Al4V and molybdenum possessed the most predominant negatively skewed surface, implying that these two materials are the most resistant to abrasive wear from AWJ machining. The surface skewness resulting from AWJ machining with the smaller abrasives remains constant over the depth of cut and appears to show a material property dependence. Similar to the surface structure that resulted when using the larger abrasives, the Al 7075-T6, Monel 400, and AISI 304 exhibited the highest positive surface skew in the order of increasing magnitude. In addition, the Ti6Al4V and the molybdenum surfaces were both negatively skewed. The largest depth of deformation in AWJ machining was noted in the IDR and results from the large angles of abrasive impingement near the beginning of cutting; see Fig. 10. Unfortunately, a residual stress analysis confined to the IDR was impossible due to the size and surface features within this region. However, the stress gradients along the direction of jet penetration between the SCR and RCR were negligible ( 5 percent). Indeed, this agrees with the absence of subsurface deformation changes with cutting depth. Though differences in depth of deformation with cutting condition were not noted directly from microhardness measurements, the residual stress fields do exhibit a mild parametric dependence as reported in our early investigations 13,14. The magnitude of the in-plane residual stress components from condition C were consistently lower than those from conditions A and B utilizing larger abrasives no. 50 Garnet. 94 Õ Vol. 122, FEBRUARY 2000 Transactions of the ASME

Summary and Conclusions An experimental study was conducted to determine the influence of material properties on the surface integrity and texture in WJ and AWJ machining of metals. Based on experiment, it can be concluded that waterjet peening with high-pressure jet is capable of inducing surface/subsurface work hardening. The resulting compressive residual stresses were comparable to those introduced by shot peening. Both hardening and residual stresses were functions of jet pressure and standoff distance. High-pressure water in some conditions was found to create pitlike surfaces, resulting in an increase in the surface roughness and possible subsurface damage. Mechanisms of material removal below the initial damage zone in AWJ machining of both ductile and brittle materials do not change with cutting depth of cutting parameters, despite the distinct macrofeatures observed. AWJ cutting parameters influence the macrofeatures of the machined surface only due to their effect on jet energy. Subsurface deformation and strain hardening occurred in AWJ machining of the metals. The extent of deformation was found to depend on the metal strain-hardening behavior and the abrasive attack angle. The largest degree of deformation occurred within the IDR due to the large abrasive attack angles at jet impingement. Below the initial damage region, only minimal subsurface deformation was noted from hardness measurements and microstructural analysis. The lack of deformation within the SCR and RCR results from the shallow abrasive attack angles within these regions. No differences in subsurface hardening were apparent between the SCR and RCR. Acknowledgments The authors are grateful to the National Science Foundation and Washington Technology Center for financial support. References 1 Hamatani, G., and Ramulu, M., 1990, Machinability of High Temperature Composites by Abrasive Waterjet, ASME J. Eng. Mat. Technol., 112, pp. 381 386. 2 Ramulu, M., and Arola, D., 1993, Waterjet and Abrasive Waterjet Cutting of Unidirectional Graphite/Epoxy Composite, Composites, 24, No. 4, pp. 299 308. 3 Hashish, M., 1984, A Modeling Study of Metal Cutting with Abrasive Waterjets, ASME J. Eng. Mater. Technol., 106, pp. 88 100. 4 Hashish, M., 1989, Machining of Advanced Composites with Abrasive Waterjets, Manufac. Rev., 2, No. 2, pp. 142 150. 5 Hashish, M., 1991, Characteristics of Surfaces Machined with Abrasive Waterjets, ASME J. Eng. Mater. Technol., 113, pp. 354 362. 6 Finnie, I., 1958, The Mechanism of Erosion of Ductile Metals, Proceedings, Third National Congress of Applied Mechanics, ASME, New York, pp. 527 532. 7 Bitter, J. G. A., 1963, A Study of Erosion Phenomenon: Part I, Wear, 6, pp. 5 21. 8 Bitter, J. G. A., 1963, A Study of Erosion Phenomenon: Part II, Wear, 6, pp. 169 190. 9 Arola, D., and Ramulu, M., 1993, Mechanism of Material Removal in Abrasive Waterjet Machining in two Commonly used Aerospace Material, Proceedings, 7th American Water Jet Conference, WJTA, St. Louis, MO, 1, pp. 43 64. 10 Arola, D., and Ramulu, M., 1993, Micro-Mechanisms of Material Removal in Abrasive Waterjet Machining, Processing of Advanced Materials, 4, pp. 37 47. 11 Arola, D., and Ramulu, M., 1995, Abrasive Waterjet Machining of Titanium Alloy, Proceedings, 8th American Waterjet Conference, WJTA, St. Louis, MO, 1, pp. 389 408. 12 Arola, D., and Ramulu, M., 1996, A Residual Stress Analysis of Metals Machined with the Abrasive Waterjet, Proceedings, Symposium on Jetting Technology, BHRA Group, UK, pp. 269 290. 13 Arola, D., and Ramulu, M., 1997, Material Removal in Abrasive Waterjet Machining of Metals, Surface Integrity and Texture, Wear, 210, No. 2, pp. 50 58. 14 Arola, D., and Ramulu, M., 1997, Material Removal in Abrasive Waterjet Machining of Metals, A Residual Stress Analysis, Wear, 211, No. 2, pp. 302 310. 15 Burnham, C. D., and Kim, T. J., 1989, Statistical Characterization of Surface Finish Produced by a High Pressure Abrasive Waterjet, Proceedings, 5th American Waterjet Conference, WJTA, pp. 165 175. 16 Ross, R. B., 1980, Metallic Materials Specification Handbook, 3rd Edition, Chapman and Hall Publ. Ltd., UK. 17 Metals Handbook, 1972, 8th Edition, Atlas of Microstructures of Industrial Alloys, Vol. 7, ASM, Columbus, OH. 18 Kruszynski, B. W., and Van Luttervelt, K. A., 1989, The Influence of Manufacturing Processes on Surface Properties, Adv. Manufact. Eng., 1, pp. 30 38. 19 Noyan, I. C., and Conen, J. B., 1987, Residual Stress: Measurement by Diffraction and Interpretation, Springer, Germany. 20 Was, G. S., Pelloux, R. M., and Frabolot, M. C., 1981, Effect of Shot Peening Methods on The Fatigue Behavior of Alloy 7075-T6, Proceedings, First International Conference on Shot Peening, Pagamon Press Ltd., pp. 445 451. Journal of Pressure Vessel Technology FEBRUARY 2000, Vol. 122 Õ 95