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To improve the rigidity of large composite fuselage panels and reduce the deformation caused by factors such as gravity during the assembly locating process, this paper presents a multi-objective optimisation method of the discrete fixturing layout based on the integration of a back-propagation neural network (BPNN) with the elitist nondominated sorting genetic algorithm (NSGA-II), which considers the characteristics of the composite fuselage panel, such as its complex structure and high tendency to undergo damage when subjected to force, in addition to the control the fixturing system cost. This method can simultaneously determine the optimal number and positions of tooling fixturing points. First, a binary integer variable is proposed to represent the fixturing layout, to realise the coupling representation of the number and positions of the fixturing points. To improve the optimisation efficiency, after comprehensively comparing the prediction accuracies of several commonly used surrogate models, a BPNN prediction model is constructed to describe the nonlinear mapping relationship between the fixturing layout and the maximum deformation (maximum Mises stress) of the composite fuselage panel based on the limited samples obtained by finite element analysis (FEA) and minimax Latin hypercube sampling (MLHS). Thereafter, the allowable deformation of the composite fuselage panel is considered the constraint; the number of fixturing points and the stress of the panel are used as the optimisation objectives. The Pareto solution set of the fixturing layout is determined using NSGA-II. Finally, a composite front fuselage panel case study of a wide-body airliner is considered to demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Titanium alloy extruded profiles with complex cross-section are usually used to manufacture special-shaped structural parts in the aviation industry. In this paper, a Ti-6Al-4 V profile with asymmetric Y shape was manufactured by a direct extrusion process. A comparative study was conducted at the different lubrication conditions, in which the different process deformation mechanisms, resulting microstructures, and mechanical properties of profiles have been investigated by finite element simulation and a series of experiments, i.e., scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). It was revealed that the metal flow is blocked at the narrow die orifice due to the deterioration of lubrication, resulting in the continuous shrinkage defect. The metal deformation coordination mechanism becomes gradually significant due to poor friction contact, which promotes the internal stress in the cross-sectional center to transfer outward to balance the friction at the edge of the orifice, resulting in the reduction of the effective strain in the center. The heat generated by friction weakens the heat exchange behavior between the metal and the die, which helps the profile retain a more uniform temperature field. In the sectional center of the profiles made from the two different lubrications, extrusion with good lubrication results in greater grain/colony refinement due to greater effective strains, and a slightly greater dislocation density was found due to the uniform deformation. The tensile tests revealed that compared with poor lubrication, due to the improvement of Hall–Petch strengthening and dislocation strengthening in the cross-sectional center with good lubrication, the variation range of yield strength and tensile strength is reduced to 11 MPa and 7 MPa, respectively, and the variation range of microhardness is reduced to 44 HV and 32 HV, respectively. The improvement of lubrication not only helps to reduce forming defects but also can obtain uniform mechanical properties. [ABSTRACT FROM AUTHOR]

This investigation applies acoustic emission (AE) to study the microstructural features of additively manufactured (AM) Ti-6AL-4V parts fabricated through direct energy deposition (DED). Microstructural features including anisotropy, porosity, and grain size were tailored by processing parameters such as laser power, powder feed rate, and scan speed. Additionally, rolled and annealed Ti-6Al-4V specimens were used as a baseline. AE features were measured for specimens subjected to cyclic loading via three-point bending tests in two orientations. Higher levels of AE counts and energy were observed in the specimens fabricated with higher laser power while lower levels of AE counts and energy were observed in the specimens fabricated with lower laser power. For AM specimens fabricated with low laser power, the AE features were similar in two different testing orientations where microstructures were columnar for both orientations. However, for AM specimens fabricated with high laser power, the AE were different along different testing orientations, with higher indications being along the growth direction. This may be due to the microstructural anisotropy where AM specimens exhibited columnar grains aligned along the growth direction and a mixed columnar-equiaxed grains in a direction perpendicular to the growth direction. The AE features in the baseline rolled and annealed Ti-6Al-4V were considerably higher than those in AM samples. This may be related to fine recrystallized (α + β) microstructure for rolled and annealed Ti-6Al-4V compared to coarse microstructure in AM samples. The AE observations were further studied by AE waveform analysis. [ABSTRACT FROM AUTHOR]

Ti-6Al-4V is widely utilized to manufacture airframe component structures with curvature, because of its excellent strength to weight ratio, outstanding resistance to corrosion, and inherent thermal and electrical compatibility with carbon fiber composite. Hot stretch bending (HSB) is an effective technology to manufacture these kinds of structures. When comparing the thin-walled extrusion with the thick-walled one, however, it is more difficult to form. The reason is that the local temperature of extrusion decreases more because of the heat transfer between extrusion and die. In this study, the material properties of Ti-6Al-4V were measured experimentally, such as the tensile property within the temperatures from 873 to 1023 K and the strain rates from 0.0005 to 0.005 s, the stress relaxation behavior in a wide range of temperatures (773-973 K) and prestrains (0.7-10%), as well as the heat transfer rule between Ti-6Al-4V (extrusion material) and asbestos cement (die material) under different pressures (8-25 MPa). The heat transfer coefficients (HTCs) were determined by an inverse analysis procedure, which was based on the comparison between measured and calculated temperature. Then, the coupled thermomechanical finite element (FE) model considering the effect of heat transfer was established. The influence of preheating temperature of die, initial temperature of extrusion, and dwell time on spring-back was researched based on orthogonal array testing strategy (OATS). The optimized parameters were verified by process test. It was showed that the established FE model could be used to predict spring-back within a relative deviation of 8.05%. [ABSTRACT FROM AUTHOR]

Although the material microstructure attributes play an important role in the mechanical properties of the additively manufactured (AM) part, their impact is largely ignored in the mechanics modeling of AM processes. A physics-based mechanical threshold stress (MTS) model is proposed to consider the material internal attributes, such as grain size and dislocation to dislocation barrier during simulation of the material flow stress and deformation. This model is based on the dislocation dynamics and thermal activation theory with a combination of three main components of athermal stress, thermal stress, and threshold stress. The MTS flow stress model is embedded into a fully coupled thermo-mechanical incremental plasticity model to predict the residual stress during AM processes from the prediction of temperature field and associated thermal stresses. The experimental measurements of residual stress are conducted on additively manufactured Ti-6Al-4 V samples for the model validation. The proposed model provides insight into the impact of microstructure on residual stress prediction in the additive manufacturing process modeling. [ABSTRACT FROM AUTHOR]

Although the material microstructure attributes play an important role in the mechanical properties of the additively manufactured (AM) part, their impact is largely ignored in the mechanics modeling of AM processes. A physics-based mechanical threshold stress (MTS) model is proposed to consider the material internal attributes, such as grain size and dislocation to dislocation barrier during simulation of the material flow stress and deformation. This model is based on the dislocation dynamics and thermal activation theory with a combination of three main components of athermal stress, thermal stress, and threshold stress. The MTS flow stress model is embedded into a fully coupled thermo-mechanical incremental plasticity model to predict the residual stress during AM processes from the prediction of temperature field and associated thermal stresses. The experimental measurements of residual stress are conducted on additively manufactured Ti-6Al-4 V samples for the model validation. The proposed model provides insight into the impact of microstructure on residual stress prediction in the additive manufacturing process modeling. [ABSTRACT FROM AUTHOR]

Heat exchangers (HXs) are crucial engineering components for thermal management. The broad range of applications and the current limits of traditional manufacturing techniques have directed designers and researchers to utilize additive manufacturing (AM) technologies to open a new chapter in HX design and development. AM has created a world of opportunity to redesign HXs in pioneering forms, shapes, and sizes. The outstanding advantages of AM-enabled HXs include optimized geometries with enhanced surfaces, controlled surface roughness, fully controlled and organized porous structures, and eliminating the need for weld or braze. On the other hand, there are certain challenges to overcome in order to fully grasp the benefits of AM technologies. This thorough review study intends to highlight and elaborate the challenges and opportunities in design and manufacturing of HXs. In addition, part of this study is dedicated to investigate the experimental characterization of additively manufactured HXs. [ABSTRACT FROM AUTHOR]

Excessive sheet thickness thinning is the main shortcoming in incremental sheet metal forming (ISF). The focus of the previous studies was to improve the sheet thickness variation in vertical wall components having no inner feature. This study investigates a complex part containing convex, concave, and redundant features due to the frequent customized requirements of the aeronautic industries. In ISF, a redundant area is usually added to the target area to ensure a continuous toolpath. The redundant area effect on sheet thickness variation is the least investigated area. The best toolpath strategy from the literature was employed to see its effectiveness for the complex part investigated in this work. Two profiles in the redundant area were trialed with P-TPIF (positive) and N-TPIF (negative) based on numerical analyses and were verified experimentally. The redundant region profile significantly affects the sheet thickness distribution in the target area in both N-TPIF and P-TPIF. The previously best toolpath strategies, mainly for vertical wall components, do not work effectively for the complex component. [ABSTRACT FROM AUTHOR]

The toolpath based on the support tool adjustment to the local normal of the master tool has a substantial effect on the final component quality in double-sided incremental forming (DSIF). Two novel toolpaths based on two new support tool positions to the master tool were investigated for their effects on forming quality. These toolpaths were based on adjusting and maintaining the support tool at a specific gap (ϕ = 7.75° and 15.5°) from the local normal of the master tools in the meridional orientation in the direction of the already formed part without pneumatic-assisted support tool. The adjustment of the support tool at ϕ = 7.75° deforms the sheet based on the sheet compression and reverse bending, whereas support tool adjustment at ϕ = 15.5° deforms the sheet based on reverse bending. The two new toolpaths were compared to each other and to a normal DSIF toolpath based on experimental results for geometric precision, surface finish, processing time, and component sheet thickness variation. Simulation results for these toolpaths were also evaluated to support the experimental results and understand the process mechanism. The three main challenges posed by the DSIF process, geometric precision, sheet thickness distribution, and surface roughness, were improved up to 30%, 1.07%, and 21.81%, respectively, using the new toolpaths without compromising the forming time. The combined reverse bending and compression improved the forming quality effectively compared to only reverse bending or compression. These toolpath benefits are the no additional requirements for additional equipment, reducing capital and operational cost. [ABSTRACT FROM AUTHOR]

The main objective of this research is to investigate the tradeoffs between design complexity and thermal performance in additively manufactured metallic heat exchangers. Such heat exchangers have become a favorite topic for investigation in various fields ranging from air conditioning to aircraft gas turbine engines. In particular, high manufacturing cost has emerged as the major drawback in broad applications of manifold-microchannel heat exchanger (MMHXs). The possibility of cost-effective manufacturing has generated wide interest in applying additive manufacturing (AM) for fabricating MMHXs. Furthermore, AM technologies will provide an opportunity for enhanced design and superior heat transfer performance. In this study, innovative MMHXs are designed and fabricated through selective laser melting. An experimental setup was designed to measure pressure drop and heat transfer for the developed MMHXs with a wide range of Reynolds numbers. The coefficient of performance was calculated based on the heat flow rate, air pressure drop, and air flow rate. Measurement results demonstrated that in some scenarios adding more internal features, i.e., increasing design complexity, does not necessarily improve the MMHX performance. [ABSTRACT FROM AUTHOR]

Incremental sheet forming (ISF) has received increasing attention with its advantages of economy and flexibility for small batch sheet metal parts. However, the excessive thinning may be resulted at large forming angles for complex parts, which cannot meet the design requirements and seriously impedes the industrial application of ISF. Although the multistage incremental sheet forming (MISF) has been proposed to overcome the dilemma of excessive thinning in ISF, the design of the optimal strategy which can control material flow and improve thickness distribution is a critical issue. In this paper, three promising multistage two point incremental sheet forming (MTPIF) path strategies, i.e., parallel line strategy, variable angle strategy, and stretch-bend-assisted strategy, are selected to investigate the influence of tool paths on thickness distributions, material flow, and geometric error. Firstly, experimental thickness distributions of truncated pyramids formed with three path strategies and single-stage TPIF strategy are compared. Meanwhile, the theoretical models are introduced to illustrate the difference of thickness distributions both at the wall and the corner. Compared with single-stage TPIF strategy, MTPIF path strategies studied in this paper are effective to improve the thickness distribution of truncated pyramids at both the component wall and the corner. Through the theoretical analysis, it is found that the thickness distribution at the wall can be controlled by adjusting intermediate forming angles. Meanwhile, the intermediate shape in circumferential direction has significant influence on the thickness distribution for complex parts. Then, the material flows of three path strategies at different forming stages are analyzed. It is found that the materials at the flange as well as transition arcs are deformed into wall to increase the thickness distribution. Finally, considering both thickness distributions and geometric errors, the variable angle strategy is found as the effective MTPIF strategy for the studied process. [ABSTRACT FROM AUTHOR]