L. C. Chan

Application of the finite-element deformation method in the fine blanking process

Abstract This paper presents the theoretical analysis of the fine blanking process by the application of the rigid-plastic finite-element method. According to the characteristics of the fine blanking process, a mathematical model suitable for the theoretical analysis has been established. At the same time, a computer program with remeshing facility has been developed. From the results of the modeling, it was found that the values of σ ¯ and e ¯ within or near to the narrow clearance zone were much more greater than those at other areas. Deformation was localized and violent in the narrow clearance area, which coincided with the practical results and reflected the particular characteristics of fine blanking. Furthermore, it was also found that the values of σ ¯ and e ¯ around the edge of the punch and the d changed very severely, and that the value of e ¯ was greatest at these places: this is because the material here had been deformed severely during operation. It had also been observed that the value of e ¯ increased continuously when the fine blanking operation proceeded further, i.e., the deeper was the penetration of the punch, the greater was the degree of deformation, which further reflected the actual performance in fine blanking. The results obtained from the analysis have demonstrated that the rigid-plastic finite-element method should be a useful and practical tool to be applied to the fine blanking process.

Formability and Weld Zone Analysis of Tailor-Welded Blanks for Various Thickness Ratios

Cold-rolled steel sheets of thicknesses ranging from 0.5 to 1.0 mm were used to produce tailor-welded blanks (TWBs) with various thickness ratios. In this study, the formability of the TWBs, as well as the mechanical characteristics of the weld zones, were analyzed experimentally under the effects of various thickness ratios of TWBs. The formability of the TWBs was evaluated in terms of three measures-failure mode, forming limit diagram, and minimum major strain, whereas the mechanical characteristics of the weld zones were investigated by tensile testing, metallographic study, and microhardness measurement. In particular, circular TWBs with different radii and cutoff widths were designed where all the welds were located in the center of the blanks and perpendicular to the principal strain direction. Nd:YAG laser butt-welding was used to weld the TWB specimens of different thickness ratios. The experimental findings in this study showed that the higher the thickness ratio of the TWBs, the lower the forming limit curve level, and the lower formability. The minimum major strain was clearly inversely proportional to the thickness ratio of the TWBs. On the other hand, the results of uniaxial tensile tests clearly illustrated that there was no significant difference between the tensile strengths of the TWBs and those of the base metals. The metallographic study demonstrated a difference of grain size in the materials at base metal, heat-affected zones, and fusion zone. The microhardness measurement indicated that the hardness in the fusion zone increased by about 60% of the base metal.

An investigation on the formation and propagation of shear band in fine-blanking process

Abstract An investigation has been made on the formation and propagation of shear band in fine-blanking process. Examinations by optical microscopy and SEM reveal that the highly elongated narrow subgrains extended in the shear direction within the band, while in the other regions, fine equiaxed cell were observed. Both the presence of white-etching trace and the distribution of surface microhardness in the shear band reveal that fine-blanking process involves large deformation and high temperature. Although strain localization is severe in the shear band, especially in the area adjacent to the edges of the punch and die, no cracking has been observed. It is indicated that high hydrostatic pressure, built up by specially designed fine-blanking fixture, plays a significant role to suppress the generation of fracture zones in the sheared surface. On the basis of present findings, the mechanisms of the formation and propagation of shear band in fine-blanking are discussed.

Formability Analysis of Tailor-Welded Blanks of Different Thickness Ratios

This paper presents a formability analysis of tailor-welded blanks (TWBs) made of cold rolled steel sheets with varying thicknesses. Steel sheets ranging between 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, and 1.0 mm in thickness were used to produce TWBs of different thickness combinations. The primary objective of this paper is to characterize the effects of thickness ratios on the forming limit diagram (FLD) for a particular type of TWB. The TWBs chosen for the investigation are designed with the weld line located in the center of the specimens perpendicular to the principal strain direction. Nd:YAG laser butt-welding was used to prepare different tailor-made blank specimens for uniaxial tensile tests and Swift tests. The experimental results of the uniaxial tensile test clearly revealed that there were no significant differences between the tensile strengths of TWBs and those of the base metals. After the Swift tests, the formability of TWBs was analyzed in terms of two measures: The forming limit diagram and minimum major strain. The experimental findings indicated that the higher the thickness ratio, the lower the level of the forming limit curve (FLC) and the lower the formability of the TWBs. The findings also show an inverse proportional relationship between thickness ratios and minimum major strains. TWBs with a thickness ratio of close to I were found to have a minimum major strain closer to those of base metals. The effects of different thickness ratios on TWBs were further analyzed with a finite element code in a computer-aided engineering package, PAM-STAMP, while the failure criteria of the TWBs in the finite element analysis were addressed by the FLCs which were obtained from the experiments. However, the weld of the TWB in the simulation was simply treated as a thickness step, whereas its heat affected zones were sometimes disregarded, so that the effects of the thickness ratio could be significantly disclosed without the presence of weld zones. The results of the simulation should certainly assist to clarify and explain the effects of different thickness ratios on TWBs.

Straining behaviour in blanking process ― fine blanking vs conventional blanking

Abstract This paper is to present a comparative study of the straining behaviour of the material during fine blanking and conventional blanking. It can be demonstrated that if one metal hardens easier than the other, the resulting hardness distribution surrounding the blanked edge should reflect this phenomenon under different blanking actions. In this paper, materials used for this investigation are cold rolled mild steel sheet (SPCC) and aluminium alloy sheet which are both subjected to fine and conventional blanking under the same condition. It can be shown that fine blanking process will be mostly dominated by shearing mechanism rather than a combination of shearing and fracturing mechanisms as in conventional blanking process. This confirms that the metal with a higher n-value (ie., straining factor) will normally exhibit a gradual decrease in the hardness value over a wider area and vice versa. Also, the straining behaviour of the material which will be just the mirror image of the corresponding hardness distribution curve, is found to be heavily dependent on the nature of the blanking process, ie., whether the material is processed by fine blanking or normal conventional blanking.

Optimization for Loading Paths of Tube Hydroforming Using a Hybrid Method

Tube hydroforming (THF) is an advanced technology with the advantages of lightweight and integrity, which can be used to manufacture hollow structural components. The process of THF is influenced by many factors, among which the matching relation between the internal pressure and axial feed, i.e., loading paths, is particularly important. In this article, a hybrid method is proposed to optimize loading paths of THF. Firstly, a three-layer back-propagation artificial neural network (BP-ANN) is built, and 200 samples from finite element (FE) simulations are applied to train and test the artificial neural network (ANN). Then genetic algorithm (GA) is adopted to search the optimal loading paths in the specified bounds of the design variables by using the trained ANN as the solver of the objective function and constraint functions. After 59 iterations, the optimal loading paths are obtained. Finally, the verified experiments are performed on the special hydroforming press. The results show that the proposed method can effectively search the optimal loading paths of THF and remarkably improve the quality of the final formed parts.

Numerical simulation of fine-blanking process using a mixed finite element method

Abstract In order to achieve a more intensive understanding of the forming mechanism of the fine-blanking process, a numerical simulation has been carried out by using a mixed displacement/pressure (u/p) finite element method. According to the special requirement of the fine-blanking technique, the major process attributes, such as the vee-ring, the ejector and the edge radii of tools, have been taken into account in the finite element model. The punch–die clearance was set to 0.5% of the thickness of the workpiece. To verify the effectiveness of the simulation, the equivalent strain on the sheared surface of a SS400 steel specimen has been determined experimentally. The experimental values of the equivalent strain have been estimated by measuring the relative displacements of the local grids pre-etched on the meridian plane of the specimen. The results of the finite element simulation are in proper agreement with the experimental findings. The distributions of the shear stress and the equivalent plastic strain have been computed for discussion. Moreover, a diagram of the blanking force versus the punch penetration has also been constructed. In order to investigate the fracture mechanism in the fine-blanking process, the concept of damage mechanics has been applied. By using a void growth model, the evolution of damage at different stages of the fine-blanking has been evaluated. It has been realized that the compressive hydrostatic stress built up by the fine-blanking fixture plays an important role to suppress the initiation of macrocracks.

A study on the precision modeling of the bars produced in two cross-roll straightening

Abstract This paper studies systematically the straightening process of a bar in a two cross-roll straightener. A mathematical model on the precision of the straightened bars is developed. It considers the straightening process to be a continuous alternating bending and reverse bending process. The solution for the residual curvature of the straightened bars involved is an iterative function in the mathematical treatment. The precision form of the bar is to be virtually produced to its final shape after it has been straightening severely, using a self-developed prediction model. The developed quantitative model is found to coincide excellently with the experimental results and is proven to be effective after successful applications in industrial designs.

Damage-based Formability Analysis for TWBs

This paper describes a damage-based formability analysis for tailor-welded blanks (TWBs) using an anisotropic damage model and damage criterion for localized necking. The damage model and criterion are coupled into finite element code to simulate the stamping process of JIS G3141 steel TWBs. Forming limit strains are computed based on the damage criterion for different values of strain ratio. The computed limit strains are found to be satisfactory as compared with the measured values, thus validating the anisotropic damage model and the failure criterion of TWBs at localized necking.

Experimental and Theoretical Analysis on Formability of Aluminum Tailor-Welded Blanks

This paper presents an investigation on forming limits of tailor-welded blanks (TWBs) made of 5754-O aluminum sheets using both experimental and numerical approaches. TWBs may be composed of two or more welded flat metal sheets of different thicknesses, shapes, or mechanical properties. Due to the existence of weldment and the individual configurations of base blanks, TWBs should be considered as heterogeneous in its structure. The mechanical properties of those base metals and weld metal required for the simulation were measured individually. With the aid of the acquired data, finite element simulations for analyzing the forming process of TWBs were carried out using a general purpose finite element package, LS-DYNA. A localized necking criterion based on the vertex theory was employed to predict forming limit strains and failure locations of the aluminum TWBs. The theoretical predictions were satisfactorily validated with those obtained from the experiments.