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The influence of annealing at various temperatures on the phase stability and microstructure of pulsed-wave laser mode (PW) 316L stainless steel fabricated via Directed Energy Deposition (DED) was systematically investigated. The microstructural alterations resulting from heat treatment were examined to clarify their influence on the mechanical properties of the specimens subjected to tensile loading. The results showed that cell size increased with annealing temperature, with the cellular microstructure disappearing at higher temperatures (T ≥ 1000 °C). A decrease in the mechanical strength of the specimens was observed as annealing temperature increased. Additionally, the influence of different laser pulse frequencies and duty cycles on residual stresses was examined, revealing that moderate laser frequencies and duty cycles effectively reduced residual stress levels. [ABSTRACT FROM AUTHOR]

Continuous-wave laser (CW) and pulsed-wave laser (PW) are the two laser modes in direct energy deposition (DED). This paper mainly reports on a study into the effects of the two laser modes on residual stresses with a given energy input. The contour method (CM) with non-uniform spatial distribution of inspection points was used to capture residual stress distributions in DED of Fe3000 on a substrate made of 316L stainless steel. Residual stresses in the transition zone between the deposit and the substrate were carefully examined to gain an understanding of cracks frequently observed at the connection between the substrate and the deposit. Furthermore, X-ray diffraction, along with successive material removal, was used to reveal residual stresses at various depths in the substrate. The results showed that significant tensile longitudinal stresses developed at the substrate–deposit junction for both CW and PW laser modes. It increased sharply (about 64%) with the increase in energy input for CW mode, while it showed the opposite trend for PW mode; the longitudinal residual stress decreased 13.2% with the increase in energy input. PW, however, introduced lower residual stress than that of CW under the condition of high-energy input; the maximum longitudinal residual stress decreased by about 10.4% compared to CW mode. This was due to stress relaxation at high-energy inputs in PW mode. In addition, residual stresses were found to be higher than the initial yield stress, and yielding occurred in the deposited part. The results determined by the CM and X-ray diffraction depth profiling were found to be consistent. [ABSTRACT FROM AUTHOR]

Directed energy deposition with laser beam (DED-LB) components experience significant residual stress due to rapid heating and cooling cycles. Excessive residual tensile stress can lead to cracking in the deposited sample, resulting in service failure. This study utilized digital image correlation (DIC) and thermal imaging to observe the in situ temporal evolution of strain and temperature gradients across all layers of a deposited 316 L stainless steel thin wall during DED-LB. Both continuous-wave (CW) and pulsed-wave (PW) laser modes were employed. Additionally, the characteristics of thermal cracks and geometric dislocation density were examined. The results reveal that PW mode generates a lower temperature gradient, which in turn reduces thermal strain. In CW mode, the temperature–stress relationship curve of the additive manufacturing sample enters the "brittleness temperature zone", leading to the formation of numerous hot cracks. In contrast, PW mode samples are almost free of cracks, as the metal avoids crack-sensitive regions during solidification, thereby minimizing hot crack formation. Overall, these factors collectively contribute to reduced residual stress and improved mechanical properties through the adjustment of pulsed-wave laser deposition. [ABSTRACT FROM AUTHOR]

This paper mainly analyzes the typical thermodynamic response (thermal history, thermal strain and residual stress) in a conventional continuous-wave (CW) laser during Directed Energy Deposition (DED). The influence of process parameters (laser power and scanning speed) on the temperature gradient in the heat-affected zone, thermal strain and residual stress are studied, and the corresponding relationship are established. The results show that a reduction in residual stress can be obtained by decreasing the temperature gradient. However, the method of reducing the temperature gradient by changing process parameters leads to low forming quality and low density. A pulse-wave laser (PW) is proposed to actively control the residual stress of the deposited sample. This laser mode can reduce not only the temperature gradient in the process of DED but also the in situ release of thermal stress, correspondingly greatly reducing the residual stress. [ABSTRACT FROM AUTHOR]

In this work, both numerical simulations and experimental characterization were used to obtain a broad understanding of the thermo-mechanical history, residual stress, and microstructure of the directed energy deposition (DED) process of austenitic stainless steels. To investigate the effect of process factors on residual stress, the global sensitivity analysis approach based on D-MORPH-HDMR was utilized. The results of the research reveal that the amplified effect of the influence of the three input variables (layer thickness, L; laser power, P; and scanning speed, v) on the transverse residual stress and thickness-direction residual stress is L > P > v; in contrast, the influence of longitudinal residual stress is P > L > v. We also found that general tendencies in local plastic strain accumulation are analogous to the relative distribution of geometrically necessary dislocations (GNDs). Additionally, we investigated post-solidification structures connected to residual stress, such as submicron dislocation cells and dynamic recrystallization (DRX) in austenitic stainless steels during DED. The investigation revealed that the DDRX and CDRX phenomena were caused by the bulging of initial grain boundaries and progressive sub-grain rotation (PSR). The fact that the sample bottom had more thermo-mechanical cycles than the top led to a higher dislocation density and hence more DDRX. This study presents a unique perspective on the link between residual stress and microstructure in additive manufacturing. [ABSTRACT FROM AUTHOR]

• The temperature gradient and thermal strain show different characteristics in continuous-wave (CW) mode and pulsed-wave (PW) mode during laser additive manufacturing. • In the direct laser deposition process, residual stress was reduced by about 61% using PW laser mode. • The reduction of temperature gradient and stress release contributing to the reduction and homogenization of the residual stress in PW laser mode. Components manufactured by direct laser deposition (DED) are prone to produce excessive residual stress due to its rapid heating by laser and cooling. Excessive residual tensile stress leads to cracking of deposited sample, even cause failure to serve. Understanding thermodynamic behavior during processing is critical for controlling residual stress. In this study, in-situ transient strain and temperature gradient evolution of each layer on deposited sample 316L stainless steel thin wall during DED using both continuous-wave (CW) and pulsed-wave (PW) laser modes were emerged by digital image correlation (DIC) method and thermal imager. The longitudinal residual stress of deposited sample was evaluated using two laser modes. The results demonstrated that the PW mode creates a smaller temperature gradient and hence a lesser thermal strain than the CW mode. Furthermore, the cyclic thermal stress characteristics of PW mode was observed. All these factors contribute to the reduction of residual stress. [ABSTRACT FROM AUTHOR]