查看更多>>摘要：A method of laser-hardening using an oscillating beam was utilized to locally increase mechanical properties of laser powder bed fusion (LPBF) manufactured thin-walled specimens made of medium manganese steel. Those results showed a martensite microstructure was obtained in the hardened zone over the entire depth of the specimen and indicated its suitability for laser hardening. The average hardness of the fully hardened zone increased from 300 HV to approximately 440 HV. The martensitic transformation caused an increase in the ultimate tensile strength (UTS) and yield strength (YS) of 66 % and 28 %, respectively, and decreased the level of ductility to 5% of elongation. Fracture analysis of the tensile specimens showed shallow dimples in the center and shear lips on the edges and in the laser hardened zones. The crack was initiated by the lack of fusion just below the surface and was characteristic for LPBF material tensile specimens. Based on bending tests, improper localization of the local hardened zones in the component, i.e. in areas of high plastic deformation, led to the formation of a microcrack due to a significant reduction in ductility and microstructural inhomogeneities. The knowledge of the strategy and technological parameters of the hardening process of heat-treated LPBF medium manganese steel will be beneficial to the automotive industry as this material class is gaining increasing attention.
Zhang, RuiqiangShi, ZhushengYardley, Victoria A.Lin, Jianguo...
14页
查看更多>>摘要：Boron steel is the most widely used material in hot stamping applications for forming automotive body panels with complex shapes and ultra-high strength. Due to the high austenitic transformation temperatures and the complex thermal cycle required for hot stamping, however, it is difficult to evaluate the formability of the material using standard punch test methods developed for room-temperature testing. In this study, a high temperature modification of a recently developed biaxial test method has been used to determine, in a single test procedure and for the first time, forming limit curves (FLCs) and fracture forming limit curves (FFLCs) for 22MnB5 boron steel sheet with a thickness of 1.5 mm under thermal conditions that are representative of industrial hot stamping processes. A direct resistance heating strategy has been developed, and a recently proposed cruciform specimen design has been modified for high-temperature use. For tests with target temperatures in the range of 750 to 925 degrees C, the resulting test specimens had the highest temperature at the specimen centre and a temperature difference of less than 45 degrees C in the gauge area and fracture occurred close to the centre of this area under all test conditions investigated. Limit strains at the onset of necking and at fracture for the material have been determined by applying digital image correlation (DIC) to obtain full-field strain measurements, providing an experimental foundation for constructing both FLCs and FFLCs for industrial applications.
查看更多>>摘要：The ability to predict the geometry of the weld bead plays a key role in accurate path planning and determination of welding parameters in wire arc additive manufacturing. However, little attention has been paid to the weld bead geometry and its prediction when the deposition path is not straight. Thus, this work focuses on the 3D reconstruction of the weld bead based on the deposition path. One of the main findings of this paper is that the weld bead shape changes from a symmetrical cross-section in straight portions of the path to an asymmetrical shape in non-straight regions. To predict the 3D geometry of the weld bead, an AI-based architecture called AIBead was developed. A suitable parametrization of the deposition path is proposed that is a key to train the AIBead properly and to outperform currently used parabolic models.
查看更多>>摘要：Fused deposition modeling (FDM)-based 4D printing uses thermoplastics to produce artifacts and requires computational analysis to assist its design processes of complex geometries. Previously, finite element analysis (FEA) has been used to simulate 4D printing deformations, and its accuracy has been computationally and experimentally verified. However, using FEA also leads to several limitations, such as geometric approximation error and the computational time-cost due to the high degrees of freedom. To address these issues, this paper introduces isogeometric analysis (IGA) into the deformation simulations and propounds a composite design by hybridizing FEA and IGA elements to reduce the number of degrees of freedom while maintaining the simulation accuracy. Moreover, since the hybrid IGA-FEA method used for modeling 4D printing structure deformation excludes real-time interactivity, we develop a polycube-based random forest regressor machine learning (ML) model to learn the IGA-FEA-based structural mechanics simulations and provide fast deformation predictions. Given the input actuator block distribution and geometry configurations, our well-trained model can predict the residual stress-induced deformation behaviors of mesh-like thermoplastic composite structures. With an error less than 0.11% and computation speed 20 times faster than hybrid IGA-FEA simulations, our model can create real-time (0.93 s) and truthful (99.89% accuracy) results. The effectiveness of the proposed model is demonstrated with several complex design examples. We believe the presented workflow effectively combines IGA, FEA, ML, and 4D printing to provide a powerful computational tool that enriches the 4D printing design tool box, and brings huge application potentials.
查看更多>>摘要：Laser assisted diamond turning is a potential approach to increase the surface finish quality on the hard and brittle materials and to improve diamond tool life. A high effective laser assisted turning (HE-LAT) method is proposed in this article, which guides the laser beam refracts at rake face, cutting edge, and total reflects at flank face. The HE-LAT method possesses effectively improved laser heating efficiency and can be employed to achieve the homogeneous optical surfaces on hard and brittle materials. The theoretical thermomechanical analysis and systematical experimental investigations are conducted to optimize the machining parameters of the proposed HE-LAT method. The nanoscale constitutive model of binderless WC has been obtained based on the high temperature nanoindentation tests, which facilitates the workpiece thermal filed prediction cooperating with the relevant HE-LAT FEA model. The experimental results indicate that the HE-LAT method can help to eliminate the surface fluctuation effectively, thereby achieving better surface finish quality down to 0.92 nm in Sa on binderless WC. The diamond local graphitization can also be prevented owing to the lower essential laser power and suppressed chip adhesion problem. Furthermore, the workpiece residual stress can be greatly decreased owing to the smaller thermal-affected area of HE-LAT method.
Galbusera, FrancescoDemir, Ali GokhanPlatl, JanTurk, Christoph...
17页
查看更多>>摘要：Concerning tooling applications, Laser Powder Bed Fusion (LPBF) enables new features such as internal cooling channels that can be implemented in cutting or shaping tools. Thus, higher cutting speeds are feasible thanks to the more efficient cooling that could not be obtained by channels fabricated with conventional methods. However, the alloys exploited for the cutting tools production usually contain high levels of carbon, which makes their LPBF processability challenging due to their high crack-susceptibility. In this work, an approach based on the use of basic physical/empirical indicators has been employed to map the processability of six novel highalloyed tool steel grades. A large experimental campaign with variable energy densities, single and double passes, as well as different focal points was designed. The results exhibit highly dense but cracked parts. In particular, the LPBF processability deteriorates with increasing carbon content, suggesting that mostly chemistry, rather than process parameters, plays a key role in the determination of the LPBF feasibility. The cooling rate, cooling time between 800 degrees C and 500 degrees C, equivalent carbon content, solidification interval, martensite start temperature and volumetric energy density were employed as indicators to provide a rapid classification of processability. The work demonstrates that the combined use of the indicators can better explain the cracking behaviour of carbon-containing tool steels. At a screening level, this approach based on complementar use of physical/empirical tools, may significantly shorten the experimental effort during the design of new compositions, especially when dealing with crack susceptible alloys like carbon-containing tool steels.
查看更多>>摘要：In the paper, the effect of beam oscillation frequency on zinc-induced spatter behavior was investigated for remote laser welding of thin-gage zinc-coated steel. The experimental study showed that welding with zigzag beam oscillation of 5 cycles/mm exhibited significantly improved weld aesthetics and less spatter-induced mass loss compared to the baseline condition by conventional straight-line welding. In addition, the weld quality varied with the oscillation frequency. To understand the above-mentioned phenomena, numerical simulations of welding processes by straight line and zigzag oscillations were carried out to study process physics including keyhole opening, zinc evaporation rate, molten pool flow and keyhole wall temperature. It was found that with keyhole penetration mode, variation of keyhole opening exhibited consistent frequency of 135 Hz across welding with the straight line and welding with various beam oscillation frequencies. Welding with beam oscillation frequency larger than the keyhole oscillation of 135 Hz was observed to have a relative stable keyhole and reduced process spatter as long as the frequency is not too high to cause reduced laser energy absorption and inadequate recoil pressure to hold keyhole stable. Further analysis showed that beam oscillation led to a wide and less dense laser energy distribution with low energy gradient, and an adequate oscillation frequency resulted in consistent laser energy input along the feed direction, such as 5 cycles/mm. Together they contributed to large molten pool mass, few hot spots in the laser keyhole rear wall, stable zinc evaporation rate and reduced peak magnitudes of recoil pressure. All these are beneficial to reduction of keyhole opening variation amplitude and prevention of spatter. The application of spatter occurrence measures Zout and Zp proposed in our previous work further confirmed the observation that the welding with oscillation frequency higher than the keyhole oscillation frequency has lower possibility of spattering.
查看更多>>摘要：This paper proposed a novel method of laser welding-brazing aluminum to titanium assisted by titanium mesh interlayer to further improve load capacity of Ti/Al joints. Experimental design method was used to study the effect of the titanium mesh on microstructure and mechanical properties of Ti/Al joints, and the specific process of Ti/Al interface reaction was revealed. The titanium mesh promoted spreading and wetting abilities of the molten aluminum on the titanium surface. Titanium dissolved into the molten aluminum and Al3Ti intermetallic compounds layer nucleated heterogeneously on the titanium surface during Ti/Al interface reaction. Compared with joints without titanium mesh, higher Ti element content and more Al3Ti particles were detected in the aluminum weld at joints with titanium mesh, which increased microhardness of the aluminum weld. High-density dislocations and low-angle grain boundaries were detected in the aluminum adjacent to the Al3Ti layer, and cracks tended to initiate there. The second phase strengthening of Al3Ti particles and larger connection area increased load capacity of the Ti/Al joints with titanium mesh. The optimal average tensile shear property of joints increased from 3.2 kN to 4.05 kN assisted by the titanium mesh, increasing by 26.6 %.
查看更多>>摘要：Metallurgical bonding was achieved between carbide-dispersion-strengthened Cu (CDS-Cu) and oxide dispersion-strengthened W (ODS-W) after diffusion bonding via an interface tailoring method of anodization and deoxidation of ODS-W combined with the introduction of Cu wetting layer. The shear strength of the CDSCu/ODS-W joints fabricated by the interface-tailoring bonding is 124 MPa, which is higher than that of 99 MPa of the joint prepared by direct bonding. The increase in strength is due to the achievement of the metallurgical bonding between CDS-Cu/ODS-W, the increase of the debonding energy and the increase of propagation distance of the cracks. After metallurgical bonding, the second phase particles of WC in CDS-Cu and Y2O3 in ODS-W were dispersed at the phase boundary to reduce the lattice strain energy. This work paves the way for the preparation of advanced plasma-facing components comprised of high-performance CDS-Cu and ODS-W for China Fusion Engineering Test Reactor (CFETR).
查看更多>>摘要：To minimize the adjustments to the initial roll speed setting in tube manufacturing, both the tube profile and the tube stress distribution during the rolling are needed to be predicted before the actual rolling. While predicting the tube profile is to make the rolling reach to the metal flow balance state efficiently, predicting the tube stress distribution is to decrease the potential formation of the rolled defects caused by the non-ideal stress distribution in tube at each pass. This requires the rolling model to combine the prediction of tube profile with the prediction of stress distribution of tube in its formulation. A new analytical model which can meet the above requirements is presented in this paper. The mechanics of rolling tube in three-roll continuous retained mandrel rolling has been analyzed in the model. Validity of the model has been examined by hot rolling experiments at the plant. According to the experimental results, the predicted tube profiles are in good agreement with the measured, with the prediction errors of cross-sectional areas being in the range of 0.8 similar to 2.5 %. The theoretical initial roll speed setting calculated based on the proposed model can be applied directly to tube manufacturing, with further adjustments at the mill being less than 4 %. The predicted roll forces calculated based on the proposed model have a maximum deviation from the measured less than 15 %. In general, the hot rolling experimental results at the plant have proved the validity of the proposed model.
NSTL
Elsevier