The application of low heat input and a high cooling rate in the high-speed laser directed energy deposition （HL-DED） process proves advantageous in mitigating the unfavorable residual stresses， deformation， and crystalline grain size issues that are typically induced by conventional laser directed energy deposition （CL-DED）. Nevertheless， a comprehensive understanding of the thermo-mechanical behaviors in both CL-DED and HL-DED processes remains elusive. In this study， a fully-coupled thermo-mechanical model for laser directed energy deposition （L-DED） is developed and experimentally validated. This model takes into account the effects of solid-liquid phase change and the evolution of the gas-liquid interface. A systematic investigation is conducted to explore the influence mechanisms of HL-DED and CL-DED strategies on the geometries of the melt track and heat-affected zone （HAZ）， as well as on temperatures， stresses， and deformations.Under the combined influence of stress relaxation and the melting effect， the residual stresses in the melt track and the substrate phase transformation zone （PTZ） are predominantly determined by thermal contraction during the cooling stage and the constraint imposed by the substrate. The thermal contraction-induced tensile stresses in the melt track and substrate PTZ give rise to a bow-string effect， causing the substrate to warp towards the melt track.Compared with the CL-DED strategy， the temperature field generated by the HL-DED strategy exhibits a lower gradient along the track direction but higher gradients along the transverse and vertical directions. Additionally， the cooling rates during the solidification and phase transformation stages are significantly elevated under the HL-DED strategy. Although the HL-DED strategy leads to higher residual stresses in the melt track and substrate PTZ， along with a more uneven residual stress distribution in the substrate， it results in lower transient stress in the melt track， lower residual stress in the non-PTZ， and significantly reduced substrate deformation.