Dongjiang Wu

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.

Microstructure and performance optimisation of stainless steel formed by laser additive manufacturing

In the present study, laser engineered net shaping technology was successfully utilised to fabricate 316L stainless steel bulk specimens using unidirectional scanning path and weaving scanning path. Influence of scanning path and post-heat treatment on microstructural and mechanical properties of the as-deposited builds has been investigated. The results show that scanning paths have a significant impact on the grain morphology evolution. Consequently, the as-made samples by different scanning strategies show a great difference in the mechanical properties. Furthermore, the experimental results also demonstrate that post-heat treatment is an essential step in further optimising microstructure and improving mechanical properties.

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...

Effect of Mechanical Anisotropy on Grinding of CdZnTe Wafers

In this study, nanoindentation and nanoscratching were employed to research on the mechanical anisotropy of CdZnTe (110) and (111) planes, and their effects on the grinding of CdZnTe wafers were studied. The results suggest that the primary slip system is responsible for the anisotropy of hardness and frictional coefficient. It is easy to slip along directions on (110) plane and directions on (111) plane during scratching, hence the frictional coefficient is the lowest compared to that of other directions, and high surface quality could be obtained during grinding along these directions. The problem of embedded abrasives, which is due to the soft nature of CdZnTe crystal, could be solved when using the grinding process instead of the lapping process.

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.

Anisotropic Damage Mechanism during Grinding of CdZnTe Wafers

The grinding wheels with grit size of #600 and #2000 are employed to grind CdZnTe (110) and (111) planes, and ground surface and subsurface damages were investigated. The experimental results show the damage types are affected by the crystallographic orientation and grit size. When grinding CdZnTe wafers with grit size of #600, due to different angle between the cleavage plane and ground surface, the surface morphology and directions of cracks on the (111) plane are different from those on the (110) plane. When grinding CdZnTe wafers with grit size of #2000, the (111) plane has less plastic deformation than the (110) plane, and there is obvious direction of stacking faults on subsurface.

Effect of constant temperature substrate on microstructure and hardness of Al 2 O 3 -based eutectic ceramics

Al2O3-YAG eutectic ceramic has become an ideal alternative to high-temperature alloys because of its excellent high temperature strength, high oxidation resistance and high temperature structural stability. The technology of laser engineered net shaping was used to prepare the Al2O3-YAG eutectic ceramic thin-wall samples. These samples were prepared on a common substrate and a water-cooled constant temperature substrate respectively. Their microstructure and microhardness were compared. The results show that the microstructure of thin-wall sample prepared on the common substrate is three-dimensional network structure with an average eutectic spacing of 0.96 μm. And the microstructure of top part of the sample prepared on the water-cooled constant temperature substrate is colony structure, while the microstructure of bottom part is dendrite structure which grows in the reverse direction of the heat flow. The average eutectic spacing of samples prepared on the water-cooled substrate has reduced to 0.21 μm. Compared with the microhardness of two kinds of thin-wall samples prepared on the different substrates, it is found that the microhardness of the thin–wall samples prepared on the water-cooled constant temperature substrate is increased by about 10%.

Pulsed laser net-shaped welding of thin hastelloy C-276

Based on the welding request during the coolant pump manufacture in the nuclear industry, the pulsed laser welding of 0.5 mm Hastelloy C-276 was investigated and the weld joint of less than 1mm was obtained. Then, the analysis of weld joints shape, the tensile test and XRD test of fracture were conducted. The results indicated that the thin Hastelloy C-276 could be joined by the pulsed laser with the flat weld joint which was related to the convection effect in the melting pool. The yield strength of as-welded sample was the same as that of base metal, and the tensile strength was nearly 90% of that of base metal. The same yield strength was due to the similar effect of solution strength and refining grain, and the reduced tensile strength of welded sample was attributed to the favorable formation of micro-fracture which stemmed from the abundant voids induced by the rapid solidification of laser welding. In addition, there were no enrichment of brittle phase, and the single fcc austenite was still the main phase in the weld joint.Based on the welding request during the coolant pump manufacture in the nuclear industry, the pulsed laser welding of 0.5 mm Hastelloy C-276 was investigated and the weld joint of less than 1mm was obtained. Then, the analysis of weld joints shape, the tensile test and XRD test of fracture were conducted. The results indicated that the thin Hastelloy C-276 could be joined by the pulsed laser with the flat weld joint which was related to the convection effect in the melting pool. The yield strength of as-welded sample was the same as that of base metal, and the tensile strength was nearly 90% of that of base metal. The same yield strength was due to the similar effect of solution strength and refining grain, and the reduced tensile strength of welded sample was attributed to the favorable formation of micro-fracture which stemmed from the abundant voids induced by the rapid solidification of laser welding. In addition, there were no enrichment of brittle phase, and the single fcc austenite was still the ma...

Millisecond pulse laser bending of silicon

Experiment of bending silicon with Nd:YAG laser of millisecond pulse width is implemented. By the experiment, the influence of frequency and pulse width on the bending angle is investigated and degradation of angle is not occurred. And then the bending silicon surface properties such as the profile and crystal phase are detected by optical microscope and Raman spectrum and so on. The results indicated that the irradiated surface is distributed to fringe region, transition region and main irradiated region. With analyzed results, the surface of transition region appeared the massive stacking fault and main irradiated region is changed obviously, and the tiny transformation of Si-III is found in Raman spectrum of main irradiated region compared to the un-irradiated region. Meanwhile slip line appears in fringe region. Finally, temperature distribution of silicon during process of laser bending is analyzed. The calculated results indicated that the sharp temperature shake is existent during process of scanning. In the later process of scannings, Temperature Gradient Mechanism (TGM) and Buckling Mechanism (BM) are proved to co-exist as well as the temperature increment is decreasing as the increase of scanning numbers and finally the temperature distribution is hardly changing, and the dislocation could influence on the process of bending.Experiment of bending silicon with Nd:YAG laser of millisecond pulse width is implemented. By the experiment, the influence of frequency and pulse width on the bending angle is investigated and degradation of angle is not occurred. And then the bending silicon surface properties such as the profile and crystal phase are detected by optical microscope and Raman spectrum and so on. The results indicated that the irradiated surface is distributed to fringe region, transition region and main irradiated region. With analyzed results, the surface of transition region appeared the massive stacking fault and main irradiated region is changed obviously, and the tiny transformation of Si-III is found in Raman spectrum of main irradiated region compared to the un-irradiated region. Meanwhile slip line appears in fringe region. Finally, temperature distribution of silicon during process of laser bending is analyzed. The calculated results indicated that the sharp temperature shake is existent during process of scanni...