Abstract
In the laser/water jet hybrid machining (LWJ), a novel process applicable to brittle materials, the laser beam creates a groove through evaporation and causes solid-state phase transition through heating and shock waves while the water jet performs quenching. Both laser and water jet induce stresses and phase changes that lead to cutting via crack formation and controlled fracture mechanism through the rest of the thickness. While LWJ is very well documented by our group for one-dimensional cutting, it remains as a challenge for two-dimensional cutting because of the changing direction of the crack in relation to the laser beam path. In this paper, a focused continuous wave CO2 laser (400 W) was combined with abrasive-free water jet at 0.4-1.4 MPa (60-200 psi) to generate and steer the controlled crack in two directions in polycrystalline cubic boron nitride tool blanks. A 42.33-mm/s (100 in./min) cutting speed was used in all the tests. The direction of the cracks generated by LWJ was changed to 60A degrees, 108A degrees, 120A degrees, 135A degrees and made to follow a curve with 1 mm radius in accordance with the standard tool shapes. The cut quality was investigated at the point of change in the crack direction using optical profilometer, Raman spectroscopy, and scanning electron microscopy. Results indicated that successful cuts were made with obtuse angles (120A degrees and 135A degrees) in contrast to acute angles (60A degrees). The crack propagation in the heat affected zone, where the phase transition from c-BN to a new phase has occurred, is responsible for the inability to obtain good quality cuts in the acute angle cases. A preliminary graphical representation that explains the observed experimental results is formulated.