Mixed mode ablation used for improvements in laser machining

As the average power for short pulsed lasers (femto-, pico- and nano-second pulse durations) increases, it may be economical to transition from micro to macro laser machining applications. Removal rates have usually been driven by parameters that meet a given surface finish and/or dimension requirement that can be achieved with a given power laser. The present approach to laser machining is to use one set of parameters, while low in removal rates, to achieve the dimensional and finish requirements. Using this approach the only way to increase removal rates is to simply increase the average power of the laser. An alternative approach is to combine higher removal rates but less control on dimension and finish with lower removal rate parameters that do achieve the finish and dimensions required. This paper reviews efforts accomplished at the Connecticut Center for Advanced Technology (CCAT) in this area. Several machining lasers were examined to determine if increases in removal rates could be achieved while resulting in acceptable surface quality, including the; DMG Lasertec 80 PowerShape with a Rofin-Sinar RSY 100 DP Q-Switched laser, Lumera Super Rapid laser, and Trumpf TruMicro 7050. The trials were accomplished on 304L stainless steel and plasma sprayed ceramic coating on Inconel 625. A parameter study was conducted to optimize removal rates and surface quality through an evaluation of: average power, pulse frequency, linear speed, overlap, layer orientation, focus position, and number of machined layers. The evaluation was based on 3D microscopy that measured the volume removed and surface roughness of a standard shape. A contact profilometer was used to calibrate the 3D microscope surface roughness measurements. A mixed mode process of high and low removal rates was determined to be an effective method to decrease the processing time to produce a standard shape with a set surface quality. A reduction of up to 55% in processing time was demonstrated. At the same time the combination resulted in a surface improvement of between 40% and 70% over simply using the “low rate removal” parameters. It is believed that this technique used for complex shapes and geometries will result in considerable reduction in processing time and/or surface quality and therefore increase the use of laser machining in new industrial applications.