Feasibility Study on Laser Microwelding and Laser Shock Peening using Femtosecond Laser Pulses.

Abstract

Ultrafast lasers of sub-picosecond pulse duration have thus far been investigated for ablation, drilling and cutting processes. Ultrafast lasers also have the potential for laser welding of small components of the order of microns, and for laser shock peening to enhance the peening depth. First, the two-temperature model is implemented in a general-purpose commercial FEM package, ABAQUS, to enable broad based application of the two-temperature model in practical engineering problems. The implementation is validated by comparison with linear solutions obtained using separation of variables. It is then used to investigate the potential for microwelding using an ultrafast laser pulse. Next, the two-temperature model is analyzed using ABAQUS to study the feasibility of laser microwelding with ultrafast lasers. A material model is constructed using material properties and the subsurface boiling model for ablation. Laser processing parameters of repetition rate, pulse duration, and focal radius are then investigated, in terms of molten pool generated in the material, and requirements for those parameters are discussed to obtain feasible parameter ranges for laser microwelding using ultrafast lasers. Then, the feasibility of laser shock peening using ultrafast laser pulses was experimentally investigated. A zinc coating was used for the thermo-protective effect, and a water confining layer was considered in the investigation. A high numerical aperture focusing lens was used to avoid optical breakdown of the water layer. Laser fluence and feed rate were selected as experimental parameters. Microhardness measurements were made on the top surface of the shock peened specimen and compared with the original material hardness. Improvement in microhardness obtained after laser shock peening with ultrafast laser pulses was slight, compared to results in the literature. Finally, conditions to achieve feasible laser microwelding and laser shock peening using femtosecond laser pulses are discussed from the numerical and experimental observations.