Cavitation Peening & Waterjet Vector Prepress Guide

1. Principles of Cavitation Peening & Hydrodynamic Surface Treatment

Cavitation Peening (Waterjet Peening) is an advanced non-contact mechanical surface enhancement technology that introduces deep compressive residual stresses into metallic components without the surface damage, micro-pitting, or particle contamination associated with conventional shot peening. It is widely specified in nuclear reactor pressurized water systems (preventing stress corrosion cracking in alloy 600/690 welds), aerospace gas turbine blisks, rocket propulsion impellers, and orthopedic joint implants.

In cavitation peening, a high-pressure water jet ($20 - 150\text{ MPa}$) is discharged into a submerged water chamber or with peripheral water shielding. The shear layer between the high-velocity jet and surrounding fluid causes static pressure to drop below water vapor pressure ($P_v = 2.3\text{ kPa}$ at $20^\circ\text{C}$), generating an unsteady cavitation bubble cloud. As the cloud impinges against the workpiece, the ambient pressure recovery induces symmetric and asymmetric micro-bubble collapse, generating supersonic liquid micro-jets with localized impact shock pressures of $1.5 - 3.5\text{ GPa}$.

2. Hydrodynamic Formulations & Stress Profiles

Predicting the compressive stress distribution $\sigma_R(z)$ as a function of depth requires modeling cavitation cloud shedding dynamics:

• Cavitation Standoff Number ($x/d$):

  \text{Cavitation Intensity } I_{cav} \propto \exp\left( -\left(\frac{x/d - (x/d)_{opt}}{\Delta}\right)^2 \right)

  where optimum standoff ratio $(x/d)_{opt} \approx 35 - 50$ corresponds to the maximum vortex shedding frequency (Strouhal number $St \approx 0.18$).
• Peak Collapse Impact Pressure ($P_{collapse}$):

  P_{collapse} \approx \rho_L \cdot C_L \cdot v_{microjet} \approx 0.08 \cdot P_{pump} \cdot \left(\frac{d}{x}\right)^{0.5}

• Compressive Layer Depth ($z_{comp}$):

  z_{comp} \approx k_m \cdot \sqrt{\frac{P_{pump}}{v_{scan}}} \cdot d_{nozzle}

  Typical compressive stress depths reach $200 - 600\text{ µm}$, more than twice the depth of conventional glass bead shot peening.

3. Vector Prepress & Robot 6-Axis CAM Raster Rules

Converting workpiece CAD geometry into automated 6-axis robotic peening trajectories requires strict prepress vector controls:

1. Constant Normal Standoff Tracking: The robot tool center point (TCP) vector must maintain the calibrated standoff distance $x_{standoff} \pm 1.0\text{ mm}$ normal to curved 3D fillet radii (e.g., turbine blade roots).
2. Hatch Stepover & Overlap Pitch: To achieve $100\%$ saturation coverage without untreated valleys, raster scan lines must overlap by $40\% - 55\%$: Pitch = (1 - \text{Overlap}) \cdot D_{impact} = 0.5 \cdot (0.4 \cdot x_{standoff})
3. Smooth Acceleration Contouring: Lookahead robot controllers must maintain constant surface velocity $v_{scan}$ across complex contours. Toolpath vector files must replace segmented polylines with continuous $G_2$ NURBS splines to avoid dwell-time erosion pits.
4. Edge Masking Vector Offsets: Because waterjet cavitation can produce knife-edge erosion at sharp corners ($< 90^\circ$), prepress files define a $1.5\text{ mm}$ boundary exclusion offset or sacrificial polyoxymethylene (POM) edge baffles.