Fangyong Niu

Pulsed Laser Welding of Hastelloy C-276: High-Temperature Mechanical Properties and Microstructure

Based on the welding requirement during the coolant pump manufacture in the nuclear industry, the pulsed laser welding of 0.5 mm thickness Hastelloy C-276 was investigated, and the well defect-free weld joint of less than 1 mm width was obtained. According to the using temperature request of welding structure, the tensile test of as-welded samples at the high-temperature (200°C, 300°C, and 400°C) and the scanning electron microscope (SEM) observation of fracture were conducted. It was found that some tensile samples were broken in the base metal and others were broken in the weld joint. The results indicated that the high-temperature yield strength, ultimate tensile strength and elongation of as-welded sample satisfied the demand compared to those of base metal. Also, the plastic deformation behavior of pulsed laser weld joint was not obviously changed, and just the plastic instability in the weld joint was restrained to some extent. In addition, the high-temperature fracture morphology in the weld joint indicated the weld joint fracture type belonged to the ductile fracture. At elevated temperature, the dislocation movement and reinforced element segregation resulted in the larger size of voids at the fracture, and the same voids characteristic between 200°C and 400°C indicated that the influence of the temperature on the dislocation movement and element segregation was not important.

Effect of second-phase doping on laser deposited Al2O3 ceramics

Purpose – The purpose of this paper is to investigate how a second-phase doping may affect Al2O3 ceramic parts deposited by additive manufacturing (AM) with a laser-engineered net shaping system. Direct fabrication of engineering ceramic components by AM is a relatively new method for producing complex mechanical structures. Design/methodology/approach – In this study, ZrO2 and Y2O3 powders are, respectively, doped into Al2O3 powders at the eutectic ratio as second phases to improve the quality of a deposited part. The deposited Al2O3, Al2O3/ZrO2 and Al2O3/YAG (yttrium aluminum garnet) parts are examined for their micro-structures and micro-hardness, as well as defects. Findings – The experimental results show that doping of ZrO2 or Y2O3 as a second phase performs a significant role in suppressing cracks and in refining grains of the laser-deposited parts. The micro-hardness investigation reveals that the second-phase doping does not result in much hardness reduction in Al2O3 and the two eutectic ceramics...

Microstructure and mechanical properties of solid AL2O3-ZRO2 (Y2O3) eutectics prepared by laser engineered net shaping

High dense Al2O3-ZrO2 (Y2O3) eutectics were directly prepared based on the raw material of Al2O3-ZrO2 (Y2O3) ceramic powders by laser engineered net shaping. The microstructure, the microhardness, and the fracture toughness of Al2O3-ZrO2 (Y2O3) eutectics were evaluated. The compact cellular eutectic with the nanosize microstructure and the homogeneous dispersion of zirconia fibers embedded in the continuous Al2O3 matrix were obtained, and the periodic coarsening banded structure perpendicular to the deposition direction produced due to the layer-by-layer shaping style. Testing results showed that the microhardness of eutectic ceramic was 17.15 ± 0.4 GPa, the actual density was 4.407 g/cm3, and the relative destiny reached up to 98.3%. In addition, the fracture toughness value in the coarsening banded region was 4.79 ± 0.3 MPa·m1/2, and the region with the nanosize microstructure eutectic had the fracture toughness value 3.71 ± 0.3 MPa·m1/2. The coarsening banded region had the higher fracture toughness th...

Additive manufacturing of ceramic structures by laser engineered net shaping

Ceramic is an important material with outstanding physical properties whereas impurities and porosities generated by traditional manufacturing methods limits its further industrial applications. In order to solve this problem, direct fabrication of Al2O3 ceramic structures is conducted by laser engineered net shaping system and pure ceramic powders. Grain refinement strengthening method by doping ZrO2 and dispersion strengthening method by doping SiC are proposed to suppress cracks in fabricating Al2O3 structure. Phase compositions, microstructures as well as mechanical properties of fabricated specimens are then analyzed. The results show that the proposed two methods are effective in suppressing cracks and structures of single-bead wall, arc and cylinder ring are successfully deposited. Stable phase of α-Al2O3 and t-ZrO2 are obtained in the fabricated specimens. Micro-hardness higher than 1700 HV are also achieved for both Al2O3 and Al2O3/ZrO2, which are resulted from fine directional crystals generated by the melting-solidification process. Results presented indicate that additive manufacturing is a very attractive technique for the production of high-performance ceramic structures in a single step.