查看更多>>摘要：The medium-Mn steels have a great application potential in the engineering fields due to its excellent strength and toughness. However, the presence of coarse martensite and segregation of Mn in the fusion-welded mediumMn steel joint severely deteriorated toughness and ductility. In this work, 6 mm thick medium-Mn steel plates were joined by friction stir welding, and then were intercritically annealed to enhance the toughness and ductility of the joint. The as-welded nugget zone (NZ) contained lath martensite with low proportion of high angle boundaries (HABs), leading to the joint with lower elongation of 22.5% and the impact energy of 73.6 J/cm(2). However, the as-annealed NZ consisted of ferrite and austenite because partial martensite changed to reversed austenite, while other martensite became ferrite during annealing. In addition, the proportion of HABs in the as-annealed NZ increased obviously. In comparison with as-welded joint, the impact energy and elongation of the as-annealed joint was increased by 70.7% and 44.4%, respectively. The abovementioned excellent mechanical properties were attributed to the high proportion of HABs and a large amount of ultra-fine reversed austenite with high thermal and mechanical stability, which provided a significant transformation induced plasticity effect, obviously inhibiting crack initiation and propagation. This systematic work provided a reference for welding similar materials containing retained austenite.
查看更多>>摘要：The effect of building direction and post fabrication aging treatment on the microstructure, crystallographic texture and high temperature mechanical properties of Inconel 718 (IN718) alloy fabricated by selective laser melting (SLM) method was investigated. After aging, arc-shaped structures seen in as-fabricated samples dis-appeared and converted into a mixture of columnar and equiaxed grains. Nano-sized gamma '' and/or gamma' precipitates were formed upon aging; however, MC type carbides and Laves phase encountered in as-fabricated samples were not dissolved completely after aging. Moreover, aging did not alter the texture ((001)//building direction (BD)) of as-fabricated samples. Mechanical properties of the alloys under tension were influenced by the build direction, aging time and test temperature. As-fabricated samples produced in vertical direction exhibited higher room temperature strengths with lower ductility due to orientation of overlapped prior melt pools. Room temperature tensile test results revealed that peak aging caused a significant improvement in ultimate tensile strength (UTS), from 1066.5 MPa and 998.4 MPa to 1408.5 MPa and 1330.4 MPa whereas elongation values decreased from 27.5% and 32.2% to 19.6% and 23.7% in vertically and horizontally built samples, respectively. Peak-aged samples (aged at 700 degrees C for 8 h) tested at 600 degrees C displayed serrated regions in their stress-strain curves due to dynamic strain aging (DSA). Although strength values of the samples displayed an expected decrease by temperature, ductility of the samples reduced to minimum at temperatures around 700-800 degrees C, which was attributed to intermediate temperature embrittlement.
Tong, ChenpengRong, QiYardley, Victoria A.Shi, Zhusheng...
15页
查看更多>>摘要：Medium manganese (MMn) steels are a promising class of advanced high-strength steels (AHSS) with potential for application as vehicle panels in the automotive industry. In this study, the deformation behaviour of a representative cold-rolled MMn steel and its dependence on processing parameters are studied using uniaxial tensile testing under low-temperature hot stamping (LTHS) conditions, covering austenitisation soaking times of 60-600 s, deformation temperatures of 500-700 ? and strain rates of 0.01-5 s(-1). A yield point phenomenon is observed for the first time in the early stage of deformation at such temperatures in this MMn steel, which has a submicron grain size. The extent of the yield point phenomenon is reduced, and strain hardening capability and total elongation are enhanced, with longer austenitisation soaking times, which give coarser-grained microstructures. The yield point phenomenon also tends to be weaker at higher deformation temperatures and lower strain rates; under these conditions, both the flow stress and the degree of strain hardening decrease, while the total elongation is insensitive to differences in deformation conditions. The mechanisms for the deformation behaviour and its dependence on test conditions are discussed. In addition, a set of unified constitutive equations is established and calibrated using the experimental data to predict the deformation behaviour of the MMn steel under uniaxial LTHS conditions, and close agreement with experiment, including the yield point phenomenon, is obtained.
查看更多>>摘要：A new methodology involving both experimentation and modeling to predict the recrystallization of nickel-based single-crystal superalloy parts after subsequent heat treatment is presented. Anisothermal mechanical tests are used to provide further validation of a thermal-elasto-viscoplastic behavior model, and process-specific values of the thermal expansion coefficient. Critical test specimens are casted, and modeled to monitor the thermalmechanical histories of interesting zones. The proposed model is capable of predicting higher plasticity locations, consistently with the occurrence of recrystallization. Critical plasticity paths for recrystallization are identified, defining three regions: unrecrystallized, transition and recrystallized zones. Phenomenological-based numerical plastic strain and energy criteria for AM1 single-crystal superalloy are built, and validated on an industrial case. The proposed methodology provides a systematic approach for part design and process parameters optimization, enabling recrystallization to be predicted and hence avoided.
查看更多>>摘要：In this paper, a computational fluid dynamics model considering zinc evaporation was innovatively established to investigate the interaction between zinc vapor and laser keyhole in both zero-gap and small-gap conditions. Under the zero-gap condition, it was observed by a high-speed camera that the entrance of zinc vapor gas into the keyhole had the cyclic characteristics, and the occurrence of spattering was closely related to the entrance of zinc vapor. Following the cyclic characteristics obtained from experiment, the zinc vapor with properties of mass and momentum was set to periodically entering the keyhole in the numerical model. The numerically calculated results under the zero-gap condition showed that the zinc vapor entered the keyhole at an initial velocity of 81 m/s, resulting in large deformation of the keyhole channel. The vapor flowed upward along the rear wall with a velocity of 15-20 m/s, pushing large amounts of melt in the molten pool upward and generating spatter. After the spattering event, the velocity of the remaining zinc vapor in the keyhole dropped to lower than 5 m/s, leading to shrinkage of the keyhole channel, and both the keyhole and the molten pool restored to a normal and stable state. Under the small-gap condition, the zinc vapor entered the keyhole continuously with an initial velocity of 33 m/s, which had little impact on keyhole geometry. The velocity of the zinc vapor inside the keyhole was significantly reduced comparing with the zero-gap condition, and the velocity of the upward flowing melt was lower than 0.75 m/s, thus no spatter occurred. The numerical analysis sufficiently explained why spatter was depressed under the gap condition.
查看更多>>摘要：Continuous casting spur gear with uniform microstructure and excellent performance remains challenging due to the intensive radial heat flux and nonuniform circumferential cooling in the traditional cooling mold. In this paper, a novel process for manufacturing spur gear with uniform microstructure and excellent strength-ductility synergy by warm mold continuous casting was proposed. The effects of the warm mold continuous casting process on the microstructure and mechanical properties of the QAl9-4 aluminum bronze spur gear billets were systematically studied by simulations and experiments to evaluate the effectiveness of the process. The induction heated mold reduces the radial temperature gradient while improving the vertical temperature gradient. The cooling rate was uniform at the transvers section of the spur gear and increased with increasing casting speed and mold temperature. Spur gear billets with good surface quality, dimensional precision and uniform microstructures were successfully prepared by warm mold continuous casting, and had excellent tensile stress and elongation of 692 MPa and 41%. The synchronously improved strength and ductility of the continuously cast QAl9-4 aluminum bronze spur gear billets results from a higher volume fraction of micron-sized fine alpha grains and numerous submicron-sized as well as nanosized coherent Fe-rich kappa precipitates. The present process could be beneficial for manufacturing high-quality spur gear more efficiently and may also introduce a promising method in integrated control of shape, microstructure and performance of metallic products by continuous casting.
查看更多>>摘要：Ultrasonic-assisted honing, which is a finishing grinding method, has been considered effective for improving the cutting performance of abrasive particles. However, the effect mechanism of the ultrasonic vibration on the cutting process of abrasive particles has not yet been clarified, which hinders the application and development of this technology. In this study, new kinematics and cutting process models of a single abrasive particle in ultrasonic-assisted external cylindrical honing (UAECH) were established. The parameters and interruption conditions affecting the intermittent cutting process of the abrasive particle were clarified based on the coupling effect of ultrasonic vibration and rotation of workpiece. Finite element simulation analysis has revealed that the increases in ultrasonic frequency and amplitude can reduce the interruption threshold and cutting time pro-portion in the intermittent cutting process of abrasive particles mainly by increasing the relative velocity of abrasive particles. Furthermore, for the cutting depth and rotation speed, their influence on the intermittent cutting process is mainly based on the change of the deformation rate of workpiece material. Moreover, it was found that there was counteraction effect between the tangential forces on the symmetrical surfaces of the abrasive particle in UAECH, which can help reduce the tangential honing force and improve the honing per-formance. The experimental results demonstrated a significant improvement in the average tangential honing force in UAECH and novel dents surface topography, which can provide guidance for the application of ultrasonic vibration in the grinding field and development of forming processes for functional surface topography in honing.
查看更多>>摘要：In the conical-section profiled ring rolling (CSPRR), due to the asymmetrical geometric features and uncoordinated deformation of the ring along the axial direction, multiple geometric defects may occur, such as diameter error (i.e., the diameter of big and small ends cannot simultaneously reach the target values), roundness error and tilting. In this work, a growth velocity model, describing the ring diameter growing behavior at different axial positions, is developed through the theoretical analysis. Based on the growth velocity model, the largest growth velocity difference along the axial direction is expressed by a mathematical function which relates to the ring geometry parameters (including inner slope angle alpha(1), outer slope angle alpha(2), ring height H) and the mandrel feed rate v(f). Further, combined with a mass of FE simulations, it is found that the growth velocity difference along the axial direction is the formation mechanism of multiple geometric defects in CSPRR process. By adjusting alpha(1) or alpha(2), reducing H or reducing v(f), the largest growth velocity difference along the axial direction can be made close to zero, which is beneficial to improving geometric accuracy. Following the above understanding, three methods to avoid geometric defects are proposed, including the improved design of target rolled ring (forging drawing), the reduction of v(f) and the improved design of ring blank, which are applied in the process design for a complex conical-section casing ring. The corresponding simulations and industrial experiment indicate that the proposed methods work well to avoid multiple geometric defects.
查看更多>>摘要：An Al alloy (A5052) sheet was joined with a carbon fiber-reinforced thermoplastic (CFRTP) sheet by hot pressing without melting the CFRTP sheet (that is, solid-state joining). A micro-structure was manufactured via laser irradiation of Al-Ti-C powders bedded on the A5052 sheet. The laser irradiation to Al-Ti-C powders generates numerous particle-shaped protrusions additively on the A5052 sheet, which facilitate the infiltration of CFRTP and mechanically interlock with the CFRTP sheet. To determine the optimal conditions for joining A5052 and CFRTP consisting of a polyamide 6 (PA6) matrix, the effects of joining temperature during hot pressing on the interfacial structure and joint strength were investigated. The maximum joint strength was attained at the joining temperature of 210 degrees C. At lower temperatures, PA6 was not sufficiently softened to infiltrate the micro-structure, resulting in lower joint strengths. At higher temperatures, carbon fibers (CFs) were exposed on the CFRTP surface and inhibited the penetration of PA6 and CFs into the micro-structure, thereby lowering the joint strength. To infiltrate PA6 and CFs into the micro-structure, hot pressing was performed in a gradient temperature field. The proposed process enabled appropriate infiltration of CFs and PA6 into the micro-structure and led to high and stable joint strength. It is expected that using the additively manufactured micro-structure approach proposed herein, the operating windows of various cutting-edge joining processes can be expanded to more moderate and lower energy conditions.
查看更多>>摘要：In this study, polycrystalline Ni50.8Ti49.2 and Ni(53)Ti(47 )single-walled components were prepared by dual-wire-arc additive manufacturing (in-situ alloying) using TA1 and ER-Ni wires as feedstocks. The microstructural evolution in the building direction and the mechanical properties of the two components were also investigated. The phases of Ni50.8Ti49.2 and Ni(53)Ti(47 )change from NiTi + NiTi2 to NiTi + NiTi2 + Ni4Ti3 and from NiTi + NiTi2 through NiTi + Ni3Ti to NiTi + Ni3Ti + Ni3Ti2 + Ni4Ti3, respectively, with increasing distance from the substrate. The presence and absence of the NiTi(R) phase in the Ni50.8Ti49.2 and Ni(53)Ti(47 )components, respectively, were confirmed using differential scanning calorimetry. The two components have similar fracture stress values (i.e., 2.45 GPa); however, the fracture strain of Ni(53)Ti(47 )is 51.7% less than that of Ni50.8Ti49.2. The unrecoverable strain of Ni50.8Ti49.2 gradually approaches 4.5%, while that of Ni(53)Ti(47 )increases linearly during a cyclic compression test (15 load/unload cycles). This study reveals, for the first time, the phase evolution and mechanical properties of Ni(53)Ti(47 )components prepared using dual-wire-arc additive manufacturing and proposes a novel method for the fabrication of a NiTi shape memory alloy with a flexible composition that meets the needs of aerospace applications.
NSTL
Elsevier