Spring-back characteristics of fiber metal laminate (GLARE) in brake forming process

The brake forming process has been considered as a feasible method for producing fiber metal laminate (GLARE) stringer. However, the spring-back developed during brake forming leads to serious problems in the final dimensional tolerance of the stringer. A series of experiments were performed to examine the effect of tool design and process parameters on the spring-back of GLARE. The parameters studied include punch radius, punch speed, forming load, and forming temperature. This paper shows that both design and process parameters can significantly affect the amount of spring-back. Scanning electron microscopy (SEM) was also carried out for the observation of delamination or cracking in the bent zone.     

Research on the forming process of three-dimensional metal parts fabricated by laser direct metal forming

To improve the forming quality of parts fabricated by laser direct metal forming (LDMF), the forming process of three-dimensional parts under open-loop LDMF system was studied in this paper. The influencing rule of scanning space on the forming quality was studied, and the optimal scanning space was derived. The software of control system about scanning style is also optimized, and thus, the forming quality of parts is improved. During fabricating three-dimensional parts, uneven heat distribution caused by multi-overlapping is one of the main factors affecting the sidewall forming quality. To solve the uneven heat distribution, the strategy of changing scanning speed was put forward. The influence of the standoff distance on the height of single-trace cladding layer was studied, and it was suggested that uneven surface of parts was caused by instability of process parameters, which could be compensated automatically in the condition of suitable standoff distance in the process of LDMF. Thus a so-called self-regulation effect is reached. Typical multi-overlapped parts with good forming quality are fabricated using the above-mentioned methods.     

金属板料成形CAE系统及其应用

总结了国内外有关金属板料成形ＣＡＥ仿真系统的组成及其关键技术，板料成形质量分析方法以及在工业中的应用，指出板料成形ＣＡＤ在工业设计、生产中的积极作用及广泛的应用前景。     

Fracture predicting in bulk metal forming

An important concern in forming is whether the desired deformation can be accomplished without failure of the work material. This paper describes the utilization of ductile fracture criteria in conjunction with the finite element method for predicting failures in cold bulk metal forming. Four previously published ductile fracture criteria are selected, and their relative accuracy for predicting and quantifying fracture initiation sites is investigated. Experiments with ring, cylindrical, tapered and flanged upset samples are performed to investigate the validity of the workability criteria under conditions of stress and strain similar to those usually found in bulk metal forming processes. The implementation of ductile fracture criteria into a rigid—plastic finite element computer program is presented. Local stress and strain distributions throughout the deformation are computed and compared with experimental measurements. A general good agreement is found. However, only two of these workability criteria have successfully predicted the location at which fracture initiates for all the upset tests performed in this work. The paper concludes with a discussion of the importance of the critical damage at fracture to remain independent from the technological processes.     

Modelling drawbeads in sheet metal forming

In this paper, we describe the numerical modelling of drawbead forces required to draw a sheet metal through a bead with a constant cross section. The model is formulated using an elasto-plastic large strains finite element method combined with an improved numerical technique taking into account the contact and friction conditions, based on linear programming techniques and fixed point conditions. The numerical simulations were carried out with various drawbead geometries and the results are expressed in terms of drawing restraining force versus drawing movement, as a function of the drawbead geometry, gap conditions and friction conditions. With this procedure it is also possible to determine the main deformation paths in the drawbead region where the bending strain predominates over the membrane strain. The results obtained are compared with experimental data and the agreement proves to be good. The calculated results can be used as a basis to obtain better approximations of the drawbead constitutive equations to be used in general 3D FE simulation codes in cases where it is impossible to exactly determine the drawbead geometry.     

Influence of powder flow on sidewall quality of solid parts in laser metal direct forming

In order to improve the sidewall quality of solid parts in laser metal direct forming, the paper focused on the difference of powder quantity actually injected into molten pool at the edge and inner positions when forming the solid parts, and the influence of curvature radius of the solid part’s edge on the powder quantity injected into the molten pool at the edge position was also investigated. The result shows that the distance between the cladding point and the solid part’s edge seriously affects the powder quantity injected into the molten pool and that the powder quantity injected into the molten pool at the edge positions is about 1.2 times more than that at the inner positions. However, the change of curvature radius of the solid part’s edge has little effect on the quantity of powder injected into the molten pool. The sidewall roughness of solid parts can be effectively reduced by increasing the powder feeding rate at the edge positions to about 1.2 times more than that at the inner position. Using this powder compensation method, a tip of turbine blade with the sidewall roughness Ra of 16.2–20.8 μm was fabricated.     

Joining sheets perpendicular to one other by sheet-bulk metal forming

This paper presents a variant of the traditional ‘mortise-and-tenon’ joint, which has been used for thousands of years by carpenters and blacksmiths to connect wood or metal parts. The new proposed joint is utilized to fix longitudinally in position two metal sheets (or plates) perpendicular to one other by sheet-bulk metal forming, at room temperature. The development is performed by means of a combined finite element and experimental investigation focused on the identification of the major process parameters and on the understanding of their influence on the overall joining feasibility. Destructive testing is carried out to characterize the performance of the new proposed joint, and an analytical expression is provided to determine the maximum tensile force that the joint can safely withstand.     

Optimization for process plans in sheet metal forming

Determining a process plan in the early phase of the sheet metal forming process is a mandatory task for a process planner. The objective of a process planner is to find a feasible and cost optimal process plan which, in particular, optimizes the assignment of processing elements to processing steps of the production process. We propose to find such an assignment in an automatic way for all the hole features by splitting the entire task into three subsequent steps. At each step, the combinatorial optimization problem is modeled as a bin packing problem with conflicts, and heuristically solved by a specifically designed ant colony optimizer. It is ensured that, at each step, the process plan is feasible while minimizing the tooling costs. In our computational results, we compare our approach to the existing greedy heuristic when computing a process plan for five different practice-relevant sheet metal parts, and show that we can save up to 50 % of the entire tooling costs.     

Electromagnetic blank restrainer in sheet metal forming processes

Electromagnetic blank restrainer (EMBR) is a new technology that was recently developed to control material movement in sheet metal forming processes. Magnetic attraction on the ferrous sheet metal is the intrinsic property of EMBR. Such magnetic force is quantified using Maxwell's stress tensor to assess the feasibility of EMBR in the sheet metal forming process. The 3D finite element analysis (FEA) of an electromagnetic system is conducted to determine the distribution of magnetic flux density on contacting surfaces of the sheet metal. The distribution is then used to estimate the magnetic force. Experiments have been conducted to measure the magnetic force and compare with results from the FEA. Biaxial-loading apparatus has been built to measure restraining forces on the sheet metal with blankholder, drawbead, and EMBR. All the restraining forces are put together in a chart to see where each method stands with respect to one another. In order to evaluate the quality of forming with each method, an experimental die has been built. The die forms a channel in a single stroke and provides a direct indication of how each restraining method controls blank movement in the die. The real advantage of EMBR lies in the effectiveness of force control and its flexible location in a sheet metal forming die. To prove this, a prototype has been built in a tryout die where house appliance panel is formed with blankholder and EMBR. EMBRs are locally installed in the die and actively controlled during the forming process. The part formed with EMBR shows a significant improvement in the forming quality. At the end of this paper, two immediate impacts that EMBR can bring to the sheet metal forming industry are also discussed.     

Use of crystal plasticity in metal forming simulations

Some recent work on the implementation of crystal plasticity constitutive relations in the simulation of metal deformation processes is discussed. The rate-dependent crystal plasticity theory employed is first briefly presented. We then consider some special cases of fcc metals with certain “ideal textures”, for which closed-form analytical solutions are given for rolling and biaxial sheet stretching modes. Finally, the incorporation of the crystal plasticity constitutive relations into the finite element analysis of large-strain boundary-value problems is illustrated. The example treated is the behaviour of axially constrained and freely elongating solid circular bars subject to finite strain torsion.     

Comparisons of friction models in bulk metal forming

A friction model is one of the key input boundary conditions in finite element simulations. It is said that the friction model plays an important role in controlling the accuracy of necessary output results predicted. Among the various friction models, which one is of higher accuracy is still unknown and controversial. In this paper, finite element analyses applying five different friction models to experiments of upsetting of AA 6082 lubricated with four lubricants are presented. Frictional parameter values are determined by fitness of data of friction area ratio from finite element analyses to experimental results. It is found that calibration curves of the friction area ratio for all of the five chosen friction models used in the finite element simulations do fit the experimental results. Usually, calibration curves of the friction area ratio are more sensitive to friction at the tool/workpiece interface than those of the normal pressure.     

An upper-bound analysis of friction in metal forming

In the analysis of metal forming processes, a knowledge of friction is important, especially when the microstructure evolution and criteria for limiting phenomena are predicted by numerical simulation. The friction wave model has been studied by several researchers. Their analyses are mainly based on the assumption that there is no plastic deformation of the bulk material. However, it is necessary to clarify the influence of bulk material deformation on the surface asperity deformation. This paper deals with the development of a friction wave model by considering the influence of bulk material on the surface asperity deformation. The situation of rough tool—smooth workpiece (RT—SW) contact during forming process has been investigated. Based on this condition, an admissible velocity field is constructed for the upper bound analysis. The relationship between the normal pressure and the sliding resistance is established over a large range of pressure. The role of surface roughness, bulk displacement and bulk strain on metal forming friction is analysed.     

Real area of contact and boundary friction in metal forming

Upper-bound models for asperity flattening on a workpiece surface undergoing bulk plastic deformation are developed. It is found that the effective hardness of the surface can be greatly reduced by the presence of underlying plastic flow. Theoretical predictions of the variation of real area of contact with strain show excellent agreement with experiments using model asperities in rolling. Friction models which allow for the reduction in effective hardness are developed for cases in which roughness is concentrated on either the workpiece or tooling.     

Optimum blank design in sheet metal forming by the deformation path iteration method

Optimum blank design methods have been introduced by many researchers to reduce development cost and time in the sheet metal-forming process. Direct inverse design method such as Ideal Forming (Chang and Richmond, Int J Mech Sci 1992; 34(7) and (8): 575–91 and 617–33) [7, 8] for optimum blank shape could play an important role to give a basic idea to designer at the initial die design stage of the sheet metal-forming process. However, it is difficult to predict an exact optimum blank without fracture and wrinkling using only the design code because of the insufficient accuracy. Therefore, the combination of a design code and an analysis code enables the accurate blank design. In this paper, a new blank design method has been suggested as an effective tool combining the ideal forming theory with a deformation path iteration method based on FE analysis. The method consists of two stages: the initial blank design stage and the optimization stage of blank design. The first stage generated a trial blank from the ideal forming theory. Then, an optimum blank of the target shape is obtained with the aid of the deformation path iteration method which has been newly proposed to minimize the shape errors at the optimization stage. In order to verify the proposed method, a square cup example was investigated.     

Experimental research on ductile fracture criterion in metal forming

Ductile fracture criterion is key limitation parameter in material forming. Accuracy predicting surface and internal failure in plastic deformation process affects on the technology design of workpiece and die greatly. Tension, compression, torsion and shearing test on 45# steel are utilized for providing the experimental values of the critical values at fracture, and 11 widely used ductile fracture criterion are selected to simulate the physical experiments and their relative accuracy for predicting and quantifying fracture initiation sites are investigated. The comparing results show that metal forming process under high triaxiality can be estimated successively using both Normalized Cockcroft-latham and the Brozzo ductile fracture criteria, but the Ayada and general Rice-Tracey model work very well for the low triaxiality cases.     

汽车覆盖件冲压成形仿真研究进展

汽车覆盖件冲压成形仿真技术的发展,突破了原有汽车冲压件模具及工艺设计的设计方法,对保证工件质量、减少材料消耗、缩短产品开发周期、降低制造成本具有重要意义.概述了目前汽车覆盖件冲压成形仿真所涉及到的热点领域,如摩擦与接触、回弹分析、模具系统和工艺参数、材料屈服模型和板料形状设计,讨论了这些领域的研究进展和进一步研究的发展方向.     

Contact simulation for predicting surface topography in metal forming

A surface contact model that takes account of flattening, roughening and tool elastic microwedge effects on workpiece surface is developed. The model can be implemented in FEM codes to predict the final surface qualities of the products in metal forming. As an example, the proposed model has been combined with a membrane finite element code of sheet metal forming process to predict the contact area ratio, surface roughness and mean asperity spacing. Numerical results showed good agreement with the experiments.     

Tooling design in sheet metal forming using springback calculations

A method to design tooling in sheet metal forming using springback calculations is presented. The designed tooling produces a part which matches the desired shape, thereby compensating for springback. To design the appropriate tooling, traction distributions on the sheet in the fully loaded deformed state are computed using the finite element method. The calculated tractions are then used to numerically reproduce springback of the desired part shape by elastic unloading of the part in a reverse manner. The method is examined for materials covering a range of steel strength and hardening, and is found to produce parts with negligible shape error. The success of the tooling design algorithm leads to a proposal for an experimental method to design tooling based on traction distribution measurements.