In this study, a new solid-state technique of friction stir additive manufacturing (FSAM) based on friction stir welding (FSW) principle was used to build successfully a multilayered stack of an ...Al-based component. The results show that a hook stretches into the nugget zone on advancing side, while it moves upwards to the periphery on retreating side for a single-level welding. With manufacturing the second layer, the hooks bend outward significantly attributing to the extrusion of above plastic material, which can avoid the hook to stretch into the stirred zone. A transition zone (TZ) is also formed near the interface between two layers. In addition, fine equiaxed grains are observed due to the dynamic recrystallization in the whole. However, a difference in grain size still exists through the build direction and in the TZ is forming coarse band grains. A similar change occurs in the precipitate morphology, size, and distribution. Form the top to the bottom, the microhardness changes dramatically, and a maximum 115 HV at the top is obtained. The tensile strength of all the slices increases and the elongation decreases slightly in comparison of Al substrate, and the slice top has the highest mechanical properties, which is attributed to fine grains and desirable precipitate characterization.
► MWCNTs/Al composite can be fabricated by FSP method. ► Perfect dispersive distribution of MWCNTs and the ultrafine grains in friction stir zone. ► Tensile strength and microhardness increased with ...the increase of amount of MWCNTs addition.
Aluminum matrix composites reinforced by different contents of multi-walled carbon nanotubes (MWCNTs) were fabricated by friction stir processing (FSP). The microstructure of nano-composites and the interface between aluminum matrix and MWCNTs were examined using optical microscopy (OM) and transmission electron microscopy (TEM). It was indicated that MWCNTs were well dispersed in the aluminum matrix throughout the FSP. Tensile tests and microhardness measurement showed that, with the increase of MWCNT content, the tensile strength and microhardness of MWCNTs/Al composites gradually increased, but on the contrary, the elongation decreased. The maximum ultimate tensile strength reached up to 190.2MPa when 6vol.% MWCNTs were added, and this value was two times more of that of aluminum matrix. Appearances and fracture surface micrographs of failed composite samples indicated that the composites become more and more brittle with the increase of the MWCNT content.
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 δ aging treatment + solution treatment. The microstructure analysis shows that the fully precipitation of δ phase in LDED Inconel 718 superalloy can “cut” the long strip Laves phase into small pieces. According with the δ phase solution treated at 1020 °C for 30 min, only tiny particle Laves phase was remained. The equilibrium volume fraction of Laves phase after δ phase 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 δ aging + δ solution + 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.
▶ Laser scanning path patterns affect the microstructure of LSF Inconel 718 alloy. ▶ CDRS sample has more uniform grain structure than SDRS sample. ▶ CDRS sample possesses higher ductility but lower ...yield stress and microhardness. ▶ Grain size inhomogeneity results in worse ductility of SDRS sample.
Two kinds of laser scanning paths, i.e. single direction raster scanning (SDRS) and cross direction raster scanning (CDRS), were used to prepare Inconel 718 alloy parts by laser solid form (LSF) technology. The microstructures and mechanical properties of LSF Inconel 718 samples were investigated. It is shown that the as-deposited microstructure of SDRS sample is composed of columnar dendrites which grow epitaxially along the deposition direction; but in the CDRS sample, the continuous directional growth of columnar grains is inhibited and the orientation deviation of dendrites in two adjacent layers increases. The as-deposited grains of the CDRS sample are finer than those of the SDRS sample. After heat treatment, recrystallization occurs and grains of both samples are refined. However, the recrystallized grains in SDRS sample are not as uniform as that of the CDRS sample. Tensile testing at room temperature indicates that the ultimate tensile strength of these two samples is similar; however, the ductility of the CDRS sample is much better than that of the SDRS sample. Fracture surface examination presents a transgranular mode of crack propagation for the SDRS sample and a mixture of transgranular and intergranular modes of crack propagation for the CDRS sample. The inhomogeneity of grain size is considered to be the prime reason for the worse ductility of the SDRS sample.
•AA7075 of 10mm is friction stir welded with tools of different pin eccentricities.•The nugget area increases first and then decreases with increasing the eccentricity.•The structure of nugget is the ...smallest and most uniform with 0.2mm eccentricity.•Joints FSWed by the pin with an eccentricity of 0.2mm have the highest UTS and EL.
Four different tools with the pin eccentricity of 0.1mm, 0.2mm, 0.3mm and 0.4mm were designed to friction stir weld 10mm thick AA7075-O plate. The effect of pin eccentricity on microstructure, secondary phase particles transformation and mechanical properties of the joints was investigated. The results show that the nugget area (ANZ) increases firstly and then decreases with increasing the pin eccentricity. When the pin with 0.2mm eccentricity is applied, the ANZ is the largest; meanwhile the grains size is the smallest which is about 3μm and secondary phase particles are the most dispersive in nugget zone compared with other tools. While the grains are coarsened to 7–11μm as the eccentricity is more than 0.4mm, some coarse hardening particles get to cluster in the thermo-mechanically affected zone. The joints produced by the pin with 0.2mm eccentricity perform the highest tensile strength and elongation, which is attributed to better interfaces, finer grains and more dispersive secondary phase particles.
The effect of overlap rate on the residual stress, recrystallization nucleation location and the distribution of recrystallized grains of LSFed Inconel 718 superalloy were investigated. It is found ...that the as-deposited microstructures with different overlap rates have the similar characteristics including that columnar grains grow along the deposition direction. The overlap area possesses higher residual stress compared with the inner-pass area, and the position of peak residual stress is closer to previous pass. The increase in the overlap rate will broaden the variation range of the residual stress. During heat treatment, the recrystallization nucleation first occurs in the overlap area with the lower overlap rate, and then expands to both overlap area and inner-pass area paralleling with the increasing of the overlap rate. Although the overlap rate is only one of basic factors in the Laser Solid Forming process, it plays an important role in controlling residual stress distribution, material microstructure and mechanical properties. The higher overlap rate adopted, the finer and the more evenly distributed grains could be obtained through recrystallization for LSFed Inconel 718 superalloy.
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► The overlap rate brings very little change in the as-deposited microstructure. ► Overlap area has higher residual stress than inner-pass area. ► The higher overlap rate results in higher residual stress. ► Recrystallized nucleation location expands along with overlap rate increasing. ► The higher overlap rate could obtain finer and more evenly distributed grains.
Three kinds of cooling conditions, i.e., water cooling, metal cooling, and air cooling, were used to prepare 34CrNiMo6 steel thin-wall component by laser rapid forming (LRF) technology. The ...microstructure and mechanical properties of LRFed thin-wall component were investigated. Results indicate that 34CrNiMo6 steel thin-wall components without internal defect could be fabricated under three kinds of cooling conditions. Cooling condition has a significant effect on the microstructure and mechanical properties of LRFed thin-wall component due to the different tempering effects of subsequent LRFed process. The microstructure along the deposited direction is substantially uniform under the water-cooling condition, which consists of bainite and retained austenite. However, the microstructure of gradient structure changes from bainite to ferrite in thin walls produced under the metal-cooling and air-cooling conditions. The LRFed component under the metal-cooling condition has the best tensile property, of which the maximum tensile strength, yield ratio, and elongation reached to 993 MPa, 0.78, and 10.4 %, respectively. The LRFed component under the water-cooling condition is most brittle, and it is only 4 %. In contrast, the LRFed component under the air-cooling condition has the best plasticity.
Laser forming repair (LFR) technology has considerable potential in high strength steel structure repair. 34CrNiMo6 steel has been widely used in high-value components, and it is imperative to repair ...these damaged components. In this study, two different thicknesses of repaired layers are deposited on the 34CrNiMo6 wrought substrate with five layers and 20 layers via LFR technology. The microstructure, phases, microhardness, and tribological properties are analyzed using optical microscopy, scanning electron microscopy, X-ray diffraction, Vickers hardness testing, and dry sliding wear testing. These results show that the 34CrNiMo6 repaired layers were successfully deposited on the substrate. The microstructure of the laser-repaired layers in the five-layer sample included bainite and retained austenite. For the 20-layer sample, the microstructure in the top of the repaired layers was still bainite and retained austenite, whereas that in the bottom of the repaired layers was transformed into ferrite and cementite. The average coefficients of friction of repaired layers is not significantly different from the substrate. The wear rate of the five LFR layers, 20-layer LFR, and substrate samples were 12.89 × 10
, 15 × 10
, and 23.87 × 10
mm³/N·m, respectively. The laser forming repaired samples had better wear resistance compared to the substrate. The wear mechanism of laser forming repaired samples is abrasive wear; whereas that of the substrate is abrasive wear and fatigue wear.
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.
Objective
Stress urinary incontinence (SUI) is prevalent worldwide, particularly among elderly women. Although electroacupuncture (EA) has been accepted by many patients and physicians in Asia, its ...efficacy for SUI has not been evaluated scientifically and systematically. We aimed to conduct a systematic evaluation of the efficacy and safety of EA treatment for women with SUI.
Methods
We retrieved publications up to February 2019 from seven databases. Randomized controlled trials for women with SUI treated by EA were included. Therapeutic effect, 1-hour urine leakage and International Consultation on Incontinence Questionnaire – Short Form (ICIQ-SF) scores were the primary outcomes. The Cochrane Collection’s RevMan 5.3 software was used to pool data.
Results
The 15 included articles demonstrated that EA for SUI was effective (odds ratio OR, confidence interval CI = 5.64, 4.19–7.59; Ι2 = 22%). ICIQ-SF scores increased (standard mean difference, CI = −0.48, −0.62 to −0.33; Ι2 = 32%) and 1-hour urine leakage decreased (OR, CI = −4.14, −4.96 to −3.33; Ι2 = 78%) in patients undergoing EA compared with those receiving sham EA, physical exercise or medication.
Conclusion
EA for women with SUI exhibited significant efficacy and safety for key outcomes.
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