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 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 R250.9618 was ~SOD!050.269 P23.261. 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 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 (Ra) 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 abrasives 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 mm 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 (Ra) with 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 Ra and no. 50 mesh abrasives. However, the increase in Ra 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 (Ds,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!.