A new method of preform design in hot forging by using electric field theory

The preform design in metal forging plays a key role in improving product quality, such as ensuring defect-free property and proper metal flow. In industry, preforms are generally designed by the iterative trial-and-error approach. This approach, however, leads not only to the increase of significant tool cost but also to the extended down-time of the production equipment. It is thus necessary to reduce time and man power through an effective method of preform design. In this paper, the equi-potential lines designed in the electric field are introduced to find an appropriate preform shape. The equi-potential lines generated between two conductors of different voltages show similar trends for the minimum work paths between the undeformed shape and the deformed shape. Based on this similarity, the equi-potential lines obtained by the arrangement of the initial and final shapes are utilized for the design of the preform, and then the artificial neural network is used to find the range of initial volume and potential value of the electric field.     

摩擦对叶片精锻预成形毛坯放置位置影响规律的研究

利用自行开发的叶片锻造过程三维有限元模拟分析系统对不同预成形毛坯在不同摩擦条件下材料充填模腔的过程进行了模拟分析,获得了叶片精锻过程中摩擦对预成形毛坯在模具中放置位置的影响规律:当摩擦因数较小时,材料首先充满左侧型腔,此时,要获得合格叶片精锻件,需将预成形毛坯在模具中的放置位置向右移动,不同截面右移的距离大约是叶片该型面弦长的5％-10％。随着摩擦因数的增大,材料几乎同时充满模具左右侧型腔,说明在该条件下预成形毛坯在模具中的放置位置是比较合理的。该研究对叶片精锻工艺的优化设计具有重要的指导意义。     

Circular braiding take-up speed generation using inverse kinematics

Circular overbraiding of composite preforms on complex mandrels currently lacks automatic generation of machine control data. To solve this limitation, an inverse kinematics-based procedure was designed and implemented for circular braiding machines with optional guide rings, resulting in a take-up speed profile for a given braid angle distribution on mandrels with complex 3D shapes including non-axisymmetric, optionally eccentric cross-sections that can vary in shape and size along an optionally curved mandrel centerline, allowing a curved machine movement. This procedure reduces the problem size, resulting in a short computation time, fit for CAE process chain integration. Numerical control data was generated for a complex mandrel with a specified braid angle and a triaxial braid. A simulation using this control data yields a braid angle that deviates a few degrees from the specified braid angle. The simulation was validated experimentally, using the generated instructions to control the braiding machine. This showed a deviation from the simulated braid angle of 3 degrees in the centered, non-tapered mandrel regions, up to 10 degrees in tapered regions and an experimental scatter of 7 degrees. The deviation is mainly attributed to the neglect of yarn interaction and guide ring contact friction in the model, leading to an incorrectly modeled convergence zone length.     

结合数值模拟与物理实验的叶片锻造预成形拓扑优化设计研究

利用所提出的拓扑优化方法,对复杂形状叶片锻造的预成形形状进行了优化设计。详细给出了该方法在三维模式下的优化策略、优化目标、单元增删准则、几何模型处理等关键技术。以合理精简毛坯、提高模腔充填性为综合优化目标,利用自行开发的优化程序,经过十余次的优化迭代,获得了理论上的优化结果。研制了叶片锻造模具,对优化后的预成形件进行了锻造成形实验;利用激光测量设备采集了叶片锻件型面的几何数据,并与数值模拟结果进行了比对。对预成形件制备工艺的可行性进行了详细讨论。结果表明:基于拓扑优化方法所获得的叶片预成形设计结果较为理想。     

A Meso–Macro Three Node Finite Element for Draping of Textile Composite Preforms

The composite textile reinforcement draping simulations allows the conditions for a successful process to be determined and, most importantly, the positions of the fibres after forming to be known. This last point is essential for the structural computations of the composite part and for resin injection analyses in the case of LCM processes. Because the textile composite reinforcements are multiscale materials, continuous (macro) approaches and discrete (meso) approaches that model the yarns have been developed. The finite element that is proposed in this paper for textile fabric forming is composed of woven unit cells. The mechanical behaviour of these is analyzed by 3D computations at the mesoscale regarding biaxial tensions and in plane shear. The warp and weft directions of the woven fabric can be in arbitrary direction with respect to the direction of the element side. This is very important in the case of multi-ply deep drawing and when using remeshing. The element is efficient because it is close to the physic of the woven cell while avoiding the very large number of unknowns in the discrete approach. A set of validation tests and forming simulations on single ply and multi-ply are presented and show the efficiency of the approach. In particular the importance of the in-plane shear behaviour is emphasized in the case of a draping on a cube.     

Development and Characterization of C/C-SiC Brake Disc

A new process was developed to produce full-size carbon/carbon-SiC brake discs consisting of two friction layers and a structural layer. Different lengths of chopped carbon fibers were used for the friction layers and structural layer. A preform of each layer was produced by hot-pressing a mixture of resin and carbon fibers. After pyrolyzing the preforms, the layers were joined by hot-pressing. Finally, liquid Si infiltration was performed to obtain a C/C-SiC brake disc. The tensile strength, compressive strength, and bending strength were 40, 46, and 61 MPa, respectively. The density of the disc was 2.1 g/Cm³. The heat transfer coefficient in the vertical direction was 16.5 W/m-°C, and it was 45.9 in the horizontal direction. The friction coefficients obtained under various braking conditions showed stable and suitable values, 0.2–0.6.     

Defects in woven preforms: Formation mechanisms and the effects of laminate design and layup protocol

Defects, such as in-plane waviness and out-of-plane tow wrinkles, cause significant reductions in the mechanical performance of RTM-manufactured composite parts based on woven preforms. To avoid this problem and achieve a greater acceptance rate in industrial processes, the mechanisms behind these defects must be understood. This paper presents a mechanism for the formation of these defects, which is supported through layup trials of woven preforms. Laminate design and layup protocol were found to be significant drivers behind the mechanism. Defect severity can be controlled through intelligent stacking sequence design and reducing ply bridging by manual forming actions and ply–ply adhesion during layup.     

Influence of drapability by using stitching technology to reduce fabric deformation and shear during thermoforming

Forming of woven fabric into a complex shape involves shear deformation of the fabric elements (warp and weft). Various researchers investigated the shear deformation during the manufacturing of thermoplastic composites (TPC) based on the hemispherical geometry, where the highly deformed edges and corners are considered as a waste. In many applications the complete formed geometry including areas around the curvature has to be used. A new idea to influence the deformation behavior of woven fabric by incorporated seams at localized sections is presented in this paper. The study is based on the photographic analysis of dry sewn preforms and thermoplastic matrix (PA12) impregnated laminates, before and after the deformation. The seams applied at the highly deformable and predefined zones help to reduce fabric shearing or to transfer the shear forces into the unsheared areas during the thermoforming process. The intensity of reduction in the fabric shear depends on the type of the sewn geometry. The results obtained can be applied in the part design, the preform engineering, and the positioning of stitches.     

Design optimization of extruded preform for hydroforming processes based on ideal forming design theory

The conventional practice to predict preform shapes in hydroformimg processes based on finite-element analysis and/or experiment is an iterative procedure and requires many trials. In this paper, a computationally efficient direct design method, which effectively improves the design procedure, was introduced. The direct design method based on ideal forming theory, which was successfully applied for the design of flat blanks for stamping processes, was extended for the design of non-flat preform for tube hydroforming processes. A preform optimization methodology for non-flat blank solutions was proposed based on the penalty constraint method for the cross-sectional shape and length of a tube. The hybrid membrane/shell method was employed to capture thickness effect while maintaining membrane formulation in the ideal forming theory. Several classes of examples were analyzed to verify the current formulation.     

Inter-ply stitching optimisation of highly drapeable multi-ply preforms

An efficient finite element model has been developed in Abaqus/Explicit to solve highly non-linear fabric forming problems, using a non-orthogonal constitutive relation and membrane elements to model bi-axial fabrics. 1D cable-spring elements have been defined to model localised inter-ply stitch-bonds, introduced to facilitate automated handling of multi-ply preforms. Forming simulation results indicate that stitch placement cannot be optimised intuitively to avoid forming defects. A genetic algorithm has been developed to optimise the stitch pattern, minimising shear deformation in multi-ply stitched preforms. The quality of the shear angle distribution has been assessed using a maximum value criterion (MAXVC) and a Weibull distribution quantile criterion (WBLQC). Both criteria are suitable for local stitch optimisation, producing acceptable solutions towards the global optimum. The convergence rate is higher for MAXVC, while WBLQC is more effective for finding a solution closer to the global optimum. The derived solutions show that optimised patterns of through-thickness stitches can improve the formability of multi-ply preforms compared with an unstitched reference case, as strain re-distribution homogenises the shear angles in each ply.