Experimental Characterization and Defect Mapping of Coaxial Wire Laser Directed Energy Deposition of AISI 304

Abstract Wire laser-directed energy deposition (WLDED) is a specially designed wire deposition process using a high-intensity laser beam as a source for the fabrication of bulk and free-form components. In this process, a laser beam strikes the substrate, creating the melt pool simultaneously; the wire is fed, which melts and integrates with the substrate. This process finds wide applications in biomedical, aerospace, marine, energy, and automotive sectors for component development and repair. WLDED is classified into two subparts: lateral feeding and coaxial WLDED. Coaxial WLDED is highly flexible for free-form deposition with uniform energy distribution across the deposits, diminishing microstructural, geometrical, and mechanical defects. Despite the listed advantages, defects like stubbing, burning, and dripping are challenging to avoid owing to the wire feed variability. Therefore, this research focuses on developing an autonomous, flexible, and state-of-the-art wire feeding setup and its integration with a coaxial wire laser deposition head. In addition, this work also focuses on deposition characterization, defect determination, and defect mitigation. The wire feeder setup is developed in-house, utilizing a highly optimized and stable roller engagement mechanism. To validate the component fabrication, single track deposition of AISI 304 stainless steel wire (1.2 mm diameter) on SS304 has been carried out at varying laser power (1200 W to 2000 W with the increment of 400 W) and scan speed (16 mm/s to 20 mm/s with the increment of 2 mm/s). The geometrical analysis at varying parameters is conducted by tracking the variation in deposition width, deposition height, dilution depth, contact angle, and deposit area. The dilution depth, substrate area, dilution %, and deposition width increase and decrease with laser power and scan speed, respectively. Acute contact angle (< 90°) values are obtained for all deposition parameters. Dilution %, dilution depth, and substrate area increases with increase in laser energy density due to higher laser energy reaching the substrate. Interestingly, stubbing, dripping, and burning are tracked by abnormal variations in deposition height, deposition width, and deposition area at extreme laser powers of 1200 W-2000 W and scan speed of 20 mm/s-16 mm/s. Therefore, the detrimental defects can be avoided by operating at medium scanning speed and laser power. All these results indicate successful integration of in-house developed coaxial WLDED setup with sound deposits. Microstructure variation obtained using electron backscattered diffraction (EBSD) results depicting directional solidification of columnar grain for all process parameters. Grain structure variation shows fine to coarse columnar grains transition from low to high energy density due to reduction in cooling rate. The linear energy density has insignificant effects on the microhardness value of the deposits. A small increase in microhardness (192.5 HV to 211.1 HV) is observed if laser energy density is reduced (111 J/mm to 66.7 J/mm). However, due to fine and twined equiaxed microstructure, the substrate displayed higher microhardness (260 ± 10 HV) than the deposits (190 ± 20 HV). Geometrical, microstructural, and microhardness results show 1600 W and 18 mm/s as the best-suited deposition parameter. The most exciting part of this work is that significantly fewer defects than earlier presented works have been obtained. Hence, we successfully integrated an in-house developed highly flexible wire feeding setup with a coaxial wire deposition head capable of producing sound deposits for the required applications. In addition, we also detected and listed the conditions of the defect’s formation.