Grain refinement progress of pure titanium during laser shock forming (LSF) and mechanical property characterizations with nanoindentation

Abstract

Laser shock forming (LSF) of commercially pure (CP) titanium foil with different levels of laser energies, namely 675mJ,1200mJ,1800mJ, and 2000mJ has been investigated. Even the microformability of Ti is poor due to its hexagonal close-packed (HCP) structure, Ti exhibits greatly enhanced microformability during LSF, which can be attributed to two mechanisms: (1) changes in the constitution of materials due to high strain rate (up to 106/s), and (2) inertial effects. Failure of workpiece was observed under laser energy of 2000mJ, and adiabatic shear bands (ASBs), which leads to crack formation, was proposed to account for this failure. Refined microstructure can be produced in the formed sample after LSF. The refinement mechanisms were identified by TEM observations: (1) the onset of twins, (2) development of DTs (dislocation tangles) in original grains and DCs (dislocation cells) formed by DTs, and (3) evolution of DCs into subgrains and high misoriented grains. In addition to this refinement progress, nanograins formed through breakdown and rotation of the elongated subgrains can also be observed. Mechanical properties including surface hardness and elastic modulus were characterized by nanoindentation, and both increased greatly after LSF. Increased surface hardness indicates improved surface properties of formed samples, and enhanced elastic modulus indicates an increased stiffness of the workpiece, providing an evidence for reduced springback of Ti during LSF. The investigation in this paper is believed to lay a solid foundation for the application of LSF.