Selective Laser Melting (SLM) technology is effective for fabricating complex shape parts with superior strength and ductility. However, the high-temperature gradient and rapid solidification rate in ...the SLM process led to unstable material microstructure, which indicated an excellent strength and poor elongation of the SLM specimen. In order to achieve outstanding strength-plasticity matching, the influences of different solution temperatures (1000 °C, 1100 °C, and 1200 °C) on the evolution of phase and microstructure of Fe–22Cr–5Ni-0.26 N duplex stainless steel (DSS) fabricated by SLM were investigated. The results were then used to clarify the corresponding variations in mechanical properties of the specimens under tensile loading. The results revealed that the AS-SLM specimen showed high yield strength and poor elongation which was 1162.52 MPa and 4.97%. Solution treatment at 1000 °C can increase the elongation from 4.97% to 29.21% due to the two-phase structures recovered, grains refined, as well as eliminate dislocation and residual stress through recrystallization. As the temperature rose, the yield strength declined, while elongation remained almost unchanged. Except for elongation, the tensile strength, yield strength, and hardness were all higher than those of forged specimens. Therefore, solution treatment is an efficient strategy to improve the mechanical properties of the SLMFe-22Cr–5Ni-0.26 N DSS with excellent yield strength and plasticity.
Duplex stainless steel was formed through welding wire and arc additive manufacturing (WAAM) using tungsten inert gas. The effects of wire feeding speed (WFS), welding speed (WS), welding current, ...and their interaction on the weld bead width and height were discussed. Back-propagation (BP) neural network algorithm prediction model was established by taking the bead width and height as the output layer, and the network weight and threshold values were optimized using the particle swarm optimization (PSO) algorithm to obtain the prediction model of bead width and height. The predicted results were verified by experiments. Results show that the weld bead width increases with the increase in WFS and the welding current and decreases with WS. The smaller the WFS, the faster the WS, which is beneficial for the generation of equiaxed crystals. The smaller the welding current, the faster the cooling speed of the metal melt, which is conducive to the formation of dendrites. The interaction among WS, wire feed speed, and welding current has a significant effect on the bead width. The weld bead height is positively correlated with the wire feed speed and negatively correlated with the WS and current. The interaction between the wire feed speed and WS is significant. The optimized WAAM process parameters for duplex stainless steel are a wire feed speed of 200 cm/min, WS of 24 cm/min, and welding current of 160 A. The maximum error of the BP neural network in predicting the weld bead width and height is 7.74%, and the maximum error between the predicted and experimental values of the BP-PSO neural network is 4.27%. This finding indicates that the convergence speed is fast, improving the prediction accuracy.
A systematic study on the densification behavior and build quality of 2205 duplex stainless steel fabricated using laser powder bed fusion (LPBF) was performed by experiment and simulation, aiming to ...offer some supplementary work for research on additive manufacturing (AM) of duplex stainless steel. In this study, samples with differing laser powers were prepared, and a highest relative density of 98.87% was obtained. Then, the pore defects and surface morphologies were investigated to unveil densification behaviors during a building process. The relationship between surface morphologies and the formation of pores was discussed. It reveals that the inter-layer printing on these surface defects caused by unreasonable laser power could increase the possibility of inside pore defects and reduce the density of specimens. Particularly, the big spatters could be the cause of lack-of-fusion defects even under sufficient power input. Therefore, adequate intra- and inter-layer bonding under reasonable processing parameters is crucial for densification. The mechanical properties of the specimens prepared with the laser power of 260 W are the highest, and the yield strength, tensile strength, and elongation are 798.68 MPa, 953.63 MPa, and 10.85%, respectively.
Epoxy asphalt (EA) concrete is widely used in constructing long-span steel bridge pavements (SBDPs). This study aims to derive a fatigue damage evolution law, conducting an experimental investigation ...of SBDP. First, a general theoretical form of the fatigue damage evolution law of materials is established based on the thermal motion of atoms. Then, fatigue experiments demonstrate that this evolution law well represents the known damage-life relationships of SBDP. Taking into account the experimental relationships between damage and fatigue life under symmetrical cyclic loadings with different overload amplitudes and temperature variations, a detailed damage evolution law is deduced. Finally, the role of damage accumulation is discussed on the basis of the proposed damage evolution law for the extreme situation of heavy overload and severe environments. The results show that both heavy loading and falling temperatures increase the fatigue damage of SBDP considerably. EA shows a fatigue life two to three times longer than that of modified matrix asphalt (SMA) or guss asphalt (GA). For the same thickness, EA pavement is demonstrated to be more suitable for an anti-fatigue design of large-span SBDP under high traffic flows and low temperatures.
In this study, NSSC 2120 economical duplex stainless steel was prepared and the effects of aging temperatures on its intermetallic phase morphology, tensile strength, elongation, corrosion ...resistance, and antimicrobial properties were investigated. The results revealed that after aging the sample at 650 °C, it exhibits better pitting corrosion resistance and higher tensile strength. Upon increasing the aging temperature up to 750 °C, the pitting corrosion resistance and tensile strength of the samples were decreased due to the precipitation of the ε-Cu phase in the matrix. Moreover, with the further increase in the aging temperature to 850 °C, oxides containing Mn and Cr (CrMn
O
) and sulfides (MnS) precipitated from the samples, further decreasing their pitting corrosion resistance and tensile strength. Upon aging the samples at 950 °C, no second phase was observed and the corrosion resistance was less than that of the sample after aging at 650 °C, but the tensile strength was greater than that of the sample after aging at 650 °C. Antibacterial test results revealed that the sample after aging at 750 °C exhibited a good antibacterial effect due to the precipitation of the rod-shaped ε-Cu phase.
Wire feeding speeds of 3.5, 4.5, and 5.5 m/min are operated to arc-braze TC4 titanium alloy to 304 L stainless steel with the cold metal transfer method and CuNi filler wire. The microstructure and ...fracture behavior of TC4−304Ldissimilar joints were characterized by transition electron microscopy (TEM), selected area electron diffraction (SAED) patterns and X-ray microscope (XRM). The results show that the TC4/seam transition zone consists of an inner Ti-rich layer and an outer Ti-poor layer based on the line scanning result, which contains complex eutectic microstructures such as (Cu, Ni)Ti3, CuNiTi2, CuNiTi, (CuxNi1-x)Ti, and (Cu) solid solutions. The microstructure of the 304 L/seam interface consists of CuNi2Ti, (Cu, Ni) and (Fe, Ni) solid solutions. With increasing heat input, the thickness of the TC4/seam transition zone and the size of (Fe, Ni) dendrite grains increase, the ultimate tensile strength decreases from 350.8 to 230.5 MPa, and all samples fracture near TC4 side based on tensile samples and XRM results. The crack propagation is due to both the thickness of TC4/seam transition zone and a large amount of brittle CuNiTi equiaxial dendrite grains with a high hardness of 10.4 GPa.
The effect of quench polish quench (QPQ) nitriding temperature on the microstructure and wear resistance of SAF2906 duplex stainless steel was investigated. Results showed the surface of the nitrided ...samples was composed of an oxidized layer, a loose compound layer, a compact compound layer, and a diffusion layer. The oxidized layer was composed of Fe3O4. The main phases of the loose compound layer were CrN, αN, Fe2–3N, and Fe3O4. The compact compound layer was composed of CrN, αN, and Fe2–3N. In the diffusion layer, CrN and expanded austenite (S) were the main phases. The nitrided layer thickness increased from 20 to 41 μm with an increasing temperature of 570 to 610 °C. When the nitriding temperature was above 590 °C, the precipitates in the diffusion layer became coarsened, and their morphologies gradually changed from spherical particulate to rod-like and flocculent-like. Tribotests showed the cumulative mass loss of QPQ-treated samples was much lower than that of the substrate. The cumulative mass loss of the samples nitrided at 610 °C was higher than that at 570 °C during the first 29 h. When the test time was over 29 h, the former was lower than the latter.
In the field of automotive lightweight manufacturing, the advanced high-strength steel (AHSS) sheet gets a pivotal position owing to a good match between the plasticity and superior strength. ...However, in the complex multi-step loadings, the twist springback phenomenon generated by irregular and curved stampings has arisen great concerns. In this article, the material DP500 is adopted for the twist springback analysis in a two-step loading experiment. First, the large bone-shaped specimen is deformed to 4% strain along the rolling direction. Subsequently, the forming blank, cut from the large specimen at different directions, is stamped in a die to form a 3D curved channel. Meanwhile, the simulation analysis of the two-step loading procedure is built for twist springback prediction. An advanced distortional hardening model incorporated with an attenuated modulus model is applied for exactly capturing the deformation features under mutative strain path. A deep analysis with regard to the residual stress and elastic modulus distribution is conducted to evaluate the pre-strain's influence on the twist springback of this curved part. The result shows that mutative strain path induced by the change of the pre-strain axis has a significant impact on the twist springback.
In this letter, we maximize the multicast capacity of a cellular system with dedicated decode-and-forward (DF) relays, by jointly optimizing transmission data rates of the base station (BS) and the ...relays subject to an average outage constraint in Rayleigh fading channels. The optimal transmission rates of the BS and relays and the optimal relay locations are obtained based on an outage probability analysis for the relay-assisted multicast. Numerical results show that relay-aided multicast significantly outperforms direct multicast, and the best transmission strategy is for the BS and dedicated relays to transmit at the same rate.
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