Active stressing and the control of macro-scale damage in unsupported ceramics during laser machining

Lasers are particularly useful for the precise machining of ceramics by virtue of their low force signature and cost effectiveness relative to traditional methods. However, the full potential of lasers has yet to be realized because of the fracture problems inherent to the brittle nature of ceramics and the associated higher manufacturing and environmental costs. One of the more vexing problems currently faced when machining unsupported ceramics is the premature fractures that occur as the cut is almost completed and the remaining section is unable to support the component weight. While simple supports on the underside of the component can offset this problem, they are not always practical and/or economical in a manufacturing setting. As a result, premature fractures, separation burrs, and chips can occur along with micro-cracks. Unfortunately, such damage can often prove to be costly since the entire component may have to be scrapped. To help avoid this problem, research funded by the National Science Foundation is investigating new laser-machining methods that hold the promise of improving quality and throughput by delaying fractures and potentially healing micro-crack damage. Since an understanding of the conditions leading up to fracture is essential, the research has modeled the complex relationships between the thermal-transients caused by single- and dual-laser machining and the thermomechanical stresses that result. Given the nature of the laser machining process, an important aspect of this modeling effort involves an understanding of the relationship between the interactions of laser energy and the ceramic and the resulting ablation process. Unfortunately, the numerical procedures for modeling ablation are generally not well developed or available through commercial codes. Moreover, the dual laser configuration under study produces a complex ablation front and moving boundaries that are difficult to model using standard packages. For the complex moving boundaries associated with laser machining and ablation, a finite-element algorithm was developed in collaboration with researchers at Washington State University. The new modeling procedure is based on a fixed grid method and works as follows: after each time step, the averaged temperature is calculated with any elements exhibiting values higher than the materials melting-point deactivated. All relevant nodes and boundary conditions (imposed and natural) associated with the deactivated elements are then assigned to adjacent (and still active) elements. The nodes are then floated once all the elements sharing the node are deactivated resulting in the evolving thermal and ablation front. The thermoelastic stresses can then be quasi-statically calculated directly from the transient thermal-states. While the resulting stress-states indicate where and when fracture is likely to occur, they do not directly indicate the likelihood or stochastic nature of fracture for any given instant during the cutting process. To account for the variability of fracture and damage that is inherent to ceramics, a probabilistic fracture model has been integrated with the thermal-stress calculations. Using a Weibull approach where the effects of the tensile stresses are weighted against the extent of area and volume under their influence, the likelihood, time, and location of fracture can be quantified. This valuable information can in turn, be used to help optimize a dual-beam machining approach designed to reduce any damage from fracture when it does inevitably occur. Because the ultimate goal of the research is the reduction of premature fractures and the related damage, the modeling efforts are being combined with a new process of simultaneously scoring the surface to help control the path and surface of the final fracture. While the use of dual lasers is certainly a new innovation, creating a shallow score-line along a desired fracture path is a relatively easy and economical way of ensuring the quality and direction of a cut as demonstrated by stone and tile masons for centuries. Another intriguing and related method under study involves the use of a dual-laser system to "actively stress" the fracture prone region. In this method, the lead laser will be used to heat the material ahead of the main cutting laser in a controlled fashion. Because the remainder of the ceramic will be cooler, the expansion of the heated section will be constrained and a localized state of thermoelastic compression will be created. The compressive stresses in-turn help offset the tensile bending-stresses that are increasing as the cut progresses. For both pre-score and active stressing techniques, existing systems can be readily adapted by using commercially available beam-splitters. Hence, the quality of the laser cut can be improved without added manufacturing steps or costs.