The mechanism governing cutting of hard materials with hybrid Laser/Waterjet system through controlled fracture

Ceramics are generally difficult to machine due to their inherent natures of high hardness and low toughness. The present dissertation investigated the mechanism governing cutting of hard ceramic materials with a novel hybrid CO2-Laser/Waterjet (LWJ) system through controlled fracture propagation. It has been proved that the hybrid machining method is more efficient and is capable of providing high quality cutting that overcomes the major drawbacks with current machining techniques including EDM and Nd:YAG laser. The LWJ method implements a high power laser heating followed by low pressure waterjet quenching which subsequently dictates fracture initiation and propagation along the cutting path. The driving force of fracture can be categorized into two groups based on the specific material’s thermal properties and phase transformation capabilities. For materials with low thermal conductivity such as Alumina and Zirconia, laser heating and waterjet quenching cooperatively induce large thermal stresses, which drive the crack and hence achieves material separation. For materials with high thermal conductivity such as Aluminum Nitride, Polycrystalline Cubic Boron Nitride and Polycrystalline Diamond, the stresses that drive the crack propagation result from phase transformation induced volume expansion, rather than thermal stresses. Based on the two mechanisms, the material merit indices governing thermal shock induced fracture and transformation induced fracture can be derived respectively for material selection. The cutting mechanism of high-conductivity materials including PCBN and PCD was studied through combined experimental and numerical methods. Surface deformation, morphology, and phase compositions were characterized on the cut sample using profilometry, scanning electron microscopy, and Raman spectroscopy in order to identify the mechanistic origin underlying the material separation. A finite element model was developed to predict the surface deformation, which was compared with surface profiling measurements by optical profilometry in order to estimate the expansion strain and dimensions of the phase transformed region. Fracture mechanics analysis based on the obtained expansion strain and transformed zone was performed to predict crack configurations and to validate experimental cutting results. Based on comparison between experimental observations and numerical predictions, a “score and snap” mechanism is identified: (1) The laser beam results in scoring the sample through localized laser heating and subsequent waterjet quenching transform PCBN near the top surface from sp3-bonded phases into sp2-bonded phases (cBN transforms to hBN, diamond transforms to graphite); (2) The phase transition leads to volumetric expansion that induces tensile stresses in the surrounding material and drives the crack through the specimen leading to material separation. Cutting results indicate that as line energy of the laser was increased the sample response transitioned from scribing to through cutting. Good agreement between simulation and experimental observation was achieved. The results show that transformation induced crack propagation is the feasible mechanism for cutting during CO2-LWJ machining