Search

Due to the extremely unbalanced temperature gradient at different heights along the deposition direction during additive manufacturing, the microstructure of different areas of the formed components is quite different, which limits the part's service life to a large extent. In this paper, microstructure of the laser-directed energy deposition (LDED) 34CrNiMo6 steel is homogenized by adding Y2O3, and excellent mechanical properties are obtained. The results showed that the homogenization of microstructure at different heights of the as-deposited samples was realized due to addition of Y2O3. The microstructure of the as-deposited samples at different heights contains lath martensite and lower bainite, only retained austenite is present at the top. With the increase of Y2O3content, the size of the austenite grain of the as-deposited LDED 34CrNiMo6 steel showed a tendency of decreasing and then increasing. The austenite grain size and martensite domain size were minimized when the Y2O3content was 2 wt%. The average microhardness showed an increasing and then decreasing trend with the increase of Y2O3content. When the content of Y2O3was 2 wt%, it had the highest average microhardness, which was 597 HV and 640 HV, respectively. The elongation and percentage reduction of area showed a trend of decreasing and then increasing, and tensile strength showed a trend of increasing and then decreasing. When the Y2O3content was 2 wt%, reached the maximum tensile strength of 1501 MPa, an increase of about 56 %, and its elongation and percentage reduction of area was the lowest in the rare-earth-containing samples, and the fracture mechanism is quasi-dissociative fracture.

Inconel 625 superalloy coatings were prepared on 12CrMoV steel plates by CMT process and MIG process respectively. The microstructures, the mechanical properties and oxidation resistance of the coatings were tested and analyzed. The results show that the heat input of CMT process is obviously lower than that of MIG process, and the forming quality of the coating is better, with smooth surface, no splash, no internal porosity and no other defects. The average weld penetration depth of CMT process coating is 0.51 mm and the dilution ratio is 6.8 %, which are much lower than that of the MIG process. Both coatings are mainly composed of coarse columnar crystals, however, the dendrites in the CMT process coating are finer and have better alignment consistency. The microstructure of two process coatings shows significant differences, and a large amount of Laves phase presents in the coating of the MIG process. The presence of Laves phase was not found in the coating of CMT process, but was replaced by element segregation region. In terms of mechanical properties, the CMT process coating has a higher hardness of about 221 HV and it is higher than the MIG process coating which is about 204 HV. Oxidation resistance testing of the coatings shows that the outer Cr2O3film plays key role to the oxidation resistance. More compact and stable outer oxide film is formed in the oxidation process of the coating prepared by CMT process, which resulted in a better oxidation resistance.

In this paper, arc oscillation is applied in the fabrication of 2319 aluminum alloy samples by cold metal transition wire arc additive manufacturing process (CMT-WAAM). The porosity, microstructure and mechanical properties of 2319 aluminum alloy under different arc oscillation patterns were discussed. The results showed that the stirring of arc oscillation can influence the molten pool shape and improve the fluidity, change the thermal distribution and the growth direction of columnar crystal. Arc oscillation has a obvious effect on reducing the grain size and proportion of pores. After using the spiral arc oscillation pattern and asymmetrical trapezoid arc oscillation pattern, the volume fraction of pores reduced from 2.64% to 1.09% and 1.22%, and the size of pores was mainly smaller than 40 μm, respectively. The tensile strength in vertical direction and plasticity were improved with arc oscillation, but its effect on the horizontal tensile strength was not noticeable. After solid solution-aging treatment, the tensile strength of the sample with spiral arc oscillation pattern increased to 277 MPa, with a cross-section reduction of 3.6%.

Additive Manufacturing is an advanced material processing technology emerging in recent years. However, coarse columnar grain structure of additive manufactured metals reduces the performance of the material. Facilitating the columnar to equiaxed transition (CET) has become a research hotspot. In this paper, the equiaxed grain of additive manufactured Inconel 718 was achieved by adding TiB2particles. Simultaneously, the microstructure evolution and precipitation behavior with nano-TiB2particles addition were studied. The results show that the grain morphology of Inconel 718 exhibited coarse columnar grains with epitaxial growth and strong <100> texture. When 3 wt% TiB2was added, the columnar grains became finer, further increased the content of TiB2to 5 wt%, the equiaxed grains were obtained and the texture intensity decreased significantly. In addition, TiB2partially melted under the high laser energy changed the phase precipitation behavior of Inconel 718 from chain-like Laves phase to reticulated (γ + M3B2) eutectics, which lead to the hardness increased from 276 HV to 578 HV when 5 wt% TiB2was added. After heat treatment, the precipitation of high density γ′ and γ″ strengthening phase were promoted the hardness increased by 87% to 516 HV in Inconel 718. However, the hardness of TiB2reinforced samples showed a little change, which could be attributed to the partial dissolution of M3B2boride during the heat treatment.

The role of the line energy density of electron beam remelting in repairing damaged 34CrNiMo6 parts was investigated. Remelting technology can be used to rapidly repair defects with superior performance, but the value of the heat input during operation was difficult to control, which has a substantial impact on the microstructure and mechanical properties of the parts. The line energy density Qwas introduced to calculate the experimental parameters. The microstructure was observed at different regions of the sample, and its mechanical properties were analyzed. From the results, the following conclusion was drawn. An appropriate increase of line energy density could increase the depth and width of remelting to improve the remelting efficiency. Under a low line energy density (Q1), the mainly thermal cycle on 34CrNiMo6 steel was tempering rather than the preheating. With the increase of energy density, the preheating effect of ex-remelting came into sight, and the martensite of repaired zone with energy density Q2and Q3became coarse. As to the enhanced diffusion of carbon, the amount and size of carbides precipitated from ferrite increased. The microhardness decreased with the increase of energy density, the max value was about 660 HV (Q1) and the min was 330 HV (Q3). Tensile properties in different Qwere similar, the max tensile strength σbwas 845 MPa by remelting. The samples after quenching and tempering (QT) exceeded the forging standard even for the lowest strength value (1013 MPa). The fracture mode of remelted samples was brittle fracture in low energy density (Q1), and changed to quasi-cleavage fracture at energy density Q2and Q3. All the modes of the QT samples were ductile fractures.

34CrNiMo6 steel was fabricated by laser solid forming technology. The effect of quenching and tempering treatment on the microstructure and mechanical properties was studied. The results showed that the microstructure of as-deposited samples consisted of martensite and bainite and tempered martensite. After quenching and tempering, the microstructure of the deposited layer was finer and became uniform, which was mainly tempered sorbite. When the sample was quenched at 830 °C and tempered at 460 °C, the microstructure was the tempered martensite. With the increase of tempering temperature, the content of carbide precipitated in martensite gradually increased. With the increase of quenching temperature, hardness of the samples increased firstly and then decreased. The hardness of the deposited layer gradually decreased with increasing the tempering temperature. The best quenching and tempering process for the laser solid forming 34CrNiMo6 steel was 830 °C quenching +540 °C tempering, which differed from the tempering treatment process for forged parts. The tensile strength increased from 980 MPa under the standard quenching and tempering conditions to 1230 MPa (increased by 25.5%) and the yield strength increased from 916 MPa under the standard quenching and tempering conditions to 1123 MPa (increased by 22.6%). But the elongation and percentage reduction of area obtained by the two quenching and tempering conditions were basically the same. The mechanical properties of LSFed 34CrNiMo6 steel after the optimal tempering treatment process had reaches the forging standard. The fracture mode of heat-treated LSFed sample was ductile fracture.

In this study, the effect of Al addition on the microstructure and microhardness of Inconel 718 superalloy fabricated by laser additive manufacturing (LAM) was studied. The addition of Al had little effect on the grain morphology and orientation, but the segregation of Nb element intensified with the increase of Al content, which promoted the formation of Laves phase in the inter-dendritic region. After HSDA (Homogenization + Solution + Double aging) heat treatment, the precipitation of δ phase at grain boundaries changed from cross needle-like to the rod-like, and at the same time, the more content of Al was added, the more rod-like δ phase formed inside the grains. Besides, the addition of Al can promote the precipitation of γ′ phase, and inhibit the precipitation of γ" phase through promoting the segregation of Nb element. When 5% wt.% Al was added, only γ′ phase was precipitated in the matrix. With the increase of Al content, the microhardness of as-deposited LAMed Inconel 718 increased and reached 325 HV when 5% wt.% Al was added. For microhardness of heat-treated samples, it showed the similar evolution trend with the increase of Al content.

•The microstructure of as-deposited LDED 300 M steel is different from that of forgings.•After isothermal heat treatment, the microstructure of LDED 300 M steel is refined.•The ductility of LDED 300 M steel with isothermal heat treatment exceeds the forged standard.•The work hardening behavior of LDED 300 M steel is analyzed by true stress-strain curves.

Laser remelting can be used not only to improve the metal surface structure, but also to repair metal surface defects. 34CrNiMo6 belongs to European standard high strength alloy structural steel. It has been widely used in the manufacture of large-size shaft parts which are prone to defects such as cracks and wear during service. This study proposed a repair method for these defects by laser remelting the filled block and then performed heat treatment. The microstructure, m9icrohardness and tensile strength of the repaired sample and heat-treated sample were analyzed using scanning electron microscope (SEM), Vickers hardness tester and universal tensile tester. The final results showed that the metallurgical bonding between the substrate and the filler was good. The microstructure of repaired sample could be divided into three regions: I, II, and III. The main microstructure in region I and II was tempered martensite, and in region III was bainite and martensite. Different regions of repaired sample presented uneven hardness distribution, and the fracture mode was quasi-cleavage fracture. However, after heat treatment, the microstructure of the sample was tempered martensite. Its hardness became uniform, and the fracture mode of the heat-treated sample was ductile fracture. The average hardness value of the heat-treated sample was 296.4HV0.2, which was slightly higher than the substrate. The tensile strength value of the heat-treated sample reached 972.91MPa, which was about 99.3 % of substrate, and its elongation δwas 14.25 %. Overall, the mechanical properties were close to those of the forging.

In order to achieve the microstructure homogeneity at a lower temperature and improve the mechanical properties, a novel post heat treatment was introduced to Inconel 718 fabricated by laser direct energy deposition (LDED), by means of δaging treatment + solution treatment. The microstructure analysis shows that the fully precipitation of δphase in LDED Inconel 718 superalloy can “cut” the long strip Laves phase into small pieces. According with the δphase solution treated at 1020 °C for 30 min, only tiny particle Laves phase was remained. The equilibrium volume fraction of Laves phase after δphase solution treatment is about 1%, which is much lower than that of the as-deposited sample or the sample only solution treated at 1020 °C. Due to the solution strengthening and precipitation of γ″ phase during aging treatment, the tensile strength of δaging + δsolution + double aging treated samples was 10.27% higher than that of directed solution + double aging treated samples, as well as a 31.38% and 52.71% increases in elongation and the reduction of area during tensile tests.

Abstract Because of the excellent mechanical properties of 34CrNiMo6 steel, it is widely used in high-value components. Many conventional approaches to strengthening-steels typically involve the loss of useful ductility. In this study, 34CrNiMo6 Steel having high strength and ductility is produced by laser solid forming (LSF) with a quenching-tempering (QT) treatment. Tempering of bainite is mainly by solid phase transformation in the previous LSF layers during the LSF process. The stable microstructure of LSF consists of ferrite and fine carbides. The microstructure transfers to tempered sorbite after heat-treatment. The tensile properties of the LSF steel meet those of the wrought standard. The UTS and elongation of LSF sample at 858 MPa, 19.2%, respectively, are greater than those of the wrought. The QT treatment enhanced the ultimate tensile strength and yield strength of the LSF sample. The ultimate tensile strength, yield strength, reduction in area, and elongation of the LSF+QT sample at 980 MPa, 916 MPa, 58.9%, and 13.9%, respectively, are greater than those of the wrought standard. The yield strength of the LSF+QT sample is approximately 1.27 times that of the wrought. The LSF samples failed in a ductile fracture mode, while the LSF+QT samples showed mixed-mode failure. The defects have only a small effect on the tensile properties owing to the excellent ductility of the LSF sample. [ABSTRACT FROM AUTHOR]

ABSTRACTDissimilar Al–Li alloys (2198 and 2060) were laser welded without the addition of filler material. The effects of welding parameters on the formation of the welded joints, microstructure evolution, solute segregation, porosity and their relationships with the mechanical properties of the joint were investigated. It was found that reducing the weld heat input can effectively prevent grain coarsening, while decreasing porosity and reducing the tendency for hot cracking, thereby enhancing the properties of the joint. The dissolution of precipitates such as the T1 phase and θ′ phase in the base metal, and the variations in Cu and Mg content between grain interiors and boundaries, resulted from solute segregation during welding, leading to reductions in the strength of the weld.

Dissimilar Al–Li alloys (2198 and 2060) were laser welded without the addition of filler material. The effects of welding parameters on the formation of the welded joints, microstructure evolution, solute segregation, porosity and their relationships with the mechanical properties of the joint were investigated. It was found that reducing the weld heat input can effectively prevent grain coarsening, while decreasing porosity and reducing the tendency for hot cracking, thereby enhancing the properties of the joint. The dissolution of precipitates such as the T1 phase and ?' phase in the base metal, and the variations in Cu and Mg content between grain interiors and boundaries, resulted from solute segregation during welding, leading to reductions in the strength of the weld.

In this study, 20 mm thick AA7075-T6 alloy plates were joined by friction stir welding. The microstructure and mechanical properties of the nugget zone along the thickness direction from the top to the bottom was investigated. The results showed that the microstructure including the grain size, the degree of dynamic recrystallization, the misorientation angle distribution and the precipitation phase containing its size, type and content exhibited a gradient distribution along the thickness direction. The testing results of mechanical properties of the slices showed that the nugget was gradually weakened along the depth from the top to the bottom. The maximum ultimate tensile strength, yield strength and elongation of the slice in the nugget top-middle are obtained, which are 415 MPa, 255 MPa and 8.1%, respectively. [ABSTRACT FROM AUTHOR]

Laser cladding repair technology is valued for its ability to repair of damaged parts with high performance. However, there are so many parameters affect the repair quality that it takes a lot of work to determine the optimal parameters. In this article, a variety of parameters were combined as dimensionless heat input (Q*), and the effect of Q* on the microstructure and mechanical properties of 300M steel repaired samples were studied. Results showed that with the increase of Q*, microstructure of heat affected zone transformed from tempered martensite to tempered martensite and low bainite. The microstructure of the repaired zone transformed from untempered martensite to untempered martensite and a small amount of retained austenite. With increasing of Q*, residence time above the austenitizing temperature was extended, and more uniform microstructure were obtained in the repaired zone. What’s more, at Q* = 5.1, the repair efficiency increases twice than Q* = 1. The tensile strength of the repaired samples reached 1017.6 MPa and the elongation reached 6.9 %, which were far lower than that of forged samples. The heat treatment was carried out on the repaired parts, which eliminated the microstructure difference due to different Q*. Compared with the repaired samples without heat treatment, the tensile strength increased by 75 % to 1783.8 MPa and the elongation increased by 86 % to 10.4 %.