Laser-Assisted Ultrasonic Surface Rolling of Ti-6Al-4V: Coupled Diffraction, Residual-Stress, Microhardness and Areal Topography Responses with Calibration and Leave-One-Condition-Out Validation of a MATLAB Response Model

Laser-assisted ultrasonic surface rolling (LA-USR) combines localized laser heating with high-frequency mechanical rolling to modify the near-surface state of Ti-6Al-4V. This study evaluated nine processing conditions formed by laser powers of 150, 200 and 250 W and nominal rolling-depth settings of 30, 50 and 70 µm. X-ray diffraction, sin²ψ residual-stress measurements, three-dimensional areal profilometry, HV0.2 microhardness testing and an experimentally calibrated MATLAB response-surface model were used to assess the coupled response. The lowest measured surface-topography values occurred at 150W-50D, with Sa = 0.406 µm, Sq = 0.524 µm, Sdq = 0.0754 and Sdr = 0.281%. The 200W-50D condition was also smooth, whereas 250W-70D produced the most severe roughening (Sa = 3.974 µm, Sq = 9.146 µm and Sdr = 4.616%). Microhardness reached a maximum of approximately 818 HV0.2 at 250W-50D, demonstrating that maximum hardening and minimum roughness did not coincide. Residual stress was predominantly compressive but strongly reflection-dependent; for example, 200W-70D gave -164.0 and +132.2 MPa for the two evaluated reflections. The MATLAB model reproduced Sa and Sq well in calibration (R² = 0.977 and 0.974), but leave-one-condition-out performance was weaker, particularly for residual stress (LOOCV R² = -0.879). The results identify a non-monotonic thermo-mechanical process window and show that LA-USR optimization requires simultaneous consideration of surface topography, hardness, crystallographic response and residual stress.