Advanced Materials Research Vol. 939

Abstract: This paper provides a design technique of support for being applied in fast fabrication on plastic prototype, the plastic part developed through the computer aided design (CAD) can be rapidly fabricated by a CNC machining center without needs of any specific jigs or fixtures, the function provided is similar to a rapid prototyping (RP) machine used for fabricating the sampling part. By the testing result, the provided design technique of support can not only be applied in the part required for a dual-surface (or multiple-surface) processing, but also applicable for fabricating the part having thin housing. As such, this paper provides another economical and efficient option besides the RP.

Abstract: Polymethylmethacrylate (PMMA) material has excellent characteristics, such as being light weight, low cost, ease of machining, and optical quality, which are useful in numerous applications such as backlit LCD display panels, lens optics, and other photoelectric fields. Laser machining of polymerization material results in a superior machining quality, high accuracy, high speed, and high reproducibility, produces a small variety of products, reduces mold costs, and enables the rapid manufacture of products based on complex graphics by processing different depths and widths of the 3D structure. This paper presents the fabrication of symmetrical array microstructures on PMMA material by using a UV laser system. The PMMA material dimensions and thickness were 20 x 20 mm and 1 mm, respectively. Regarding the machining quality, the laser pulse energy, pulse repetition frequency, and fill spacing were adjusted. For the experiments, a semiconductor laser source (635 nm/5 mW/TEM₀₀) and a beam profiler were used to measure the characteristics of a laser beam passing through the microstructures. The microstructure pitches and morphologies also affected the light uniformity. After laser machining, the surface morphology and the light transmittance were measured using a spectrophotometer.

Abstract: Friction is one of the key factors affecting the metal forming process. If the friction effects of the process can be accurately modeled, it is able to improve simulations and help the research and development of the metal forming process. This study used cylindrical brass (JIS C2600) billets with the height and diameter of 1.1 mm for conducting the experiments of the micro combined forward and backward extrusion. The purpose of the study was to investigate the effects of punch surface topography on friction in the process. Four surface topography conditions for 0.8 mm diameter punches were prepared by grinding, polishing, grooving and micro electrical discharge machining processes. By comparing the ratio of the cup height to rod length of the extruded cups with the calibration curves established by simulations, the friction factor was estimated in a range from 0.3 to 0.6. The results showed that the punch surface topography significantly affect the friction in the extrusion process. The predicted loads using the estimated friction factors were in good agreement with the experimental results.

Abstract: Incremental sheet forming (ISF) is a new promising technology due to its flexibility and low-cost tooling properties compared with conventional forming processes. However, a common technical problem encountered in ISF is non-uniform thickness variation of formed parts, particularly excessive thinning on severely sloped regions, which may lead to the part fracture and limit the process formability. Design of multistage deformation paths (intermediate shapes or preforms) before the final part is a desirable and practical way to control the material flow in order to obtain more uniform thickness distribution and avoid forming failure. Based on the shear deformation and the strain compensation idea, an analytical model for designing multistage deformation paths and predicting the thickness strain distribution is proposed. The feasibility of the proposed model is validated by the finite element analysis (FEA) and experimental tests in terms of the comparison of prediction, simulation and experimental results on the thickness strain distribution and the process formability.

Abstract: In the present paper, the finish-blanking method is taken up because it is an easy-to-use method by using a tool with rounded edge and a small clearance between the punch and die. In finish blanking, the process parameters such as the tool shape and clearance are significant. The purpose of the present work is to clarify the effect of tool shape of PW punch in finish blanking. In the present paper, a high-tensile-strength-steel plate with a thickness of 6mm was used. The edge shape of punch was changed, and the clearance was changed within the range of 0.65%t - 12%t. The experimental results show that the edge angle of PW punch should be in the range of 30° - 60°, and the clearance should be less than 2.0%t to obtain the ratio of the burnished surface more than 80%. The edge shape of the punch has a very limited effect on the shear droop, when the clearance is less than 2.0%t. It was also found that the PW punch is more suitable to control the burr depth than Rounded punch.

Abstract: Forming limits of several high-strength steel (HSS) sheets under non-proportional deformation paths were examined experimentally and predicted analytically. Forming limit curves (FLCs) for 590MPa, 780MPa and 980MPa grade HSS sheets were obtained by performing stretch forming tests under proportional deformation and two types of non-proportional deformation. The experimental results showed strong path-dependent characteristics of FLCs of HSS sheets. Forming limits of equi-biaxially prestrained HSS sheets became markedly lower compared to the original FLCs under proportional deformation, while forming limits of uniaxially prestrained HSS sheets became partially higher than the original FLCs. It was confirmed that Marciniak-Kuczyński type analysis gave reasonably good predictions of forming limits under non-proportional deformation paths. Especially forming limit predictions of equi-biaxially-prestrained sheets showed good agreement with the corresponding experimental results.

Abstract: Stiffness improvement of stamping die without weight increase 780MPa grade high strength steel sheet was achieved by using Finite Element (FE) simulation combined with topology optimization. In the first step of the procedure, FE simulation of a stamping process was conducted in order to evaluate the contact force on a die surface at the end of stamping. Subsequently, topology optimization was carried out subject to the prescribed weight constraint, in which the stiffest die structure for the given force and displacement boundary conditions was obtained. Maximum displacements during stamping between thus optimized die and the one designed by the experienced engineer were compared. It was found that the optimized die showed less deformation than the other even if the total weights of both dies were almost the same.

Abstract: Incremental Sheet Forming (ISF) is a new-emerging sheet forming process well suited for small batch production or prototyping because it does not need any dedicated dies or punches. In this forming process, sheet metal parts are formed by a smooth-end tool in a stepwise way, during which plastic deformation is highly localized around the tool end. The part geometric accuracy obtained in the current ISF process, however, has not met the industry specification for precise part fabrication. This paper deals with a study on step depth, a critical parameter in ISF, for improving the geometric accuracy, surface quality and formability. Two sets of experiments were conducted to investigate the influence of step depth on part quality. Dimensional accuracy, surface morphology and material fracture of deformed parts were compared and analysed. An optimum value of step depth was suggested for forming a truncated cone. The present work provided significant fundamental information for the development of an advanced ISF control system on tool path control and optimization.

Abstract: Bulging deformation during cylinder compression forming results in geometrical imprecision and flow stress prediction error. Consequently, the present study proposes a hybrid technique comprising the Upper Bound Method (UBM) and geometrical fitting method. It is noted that a volume conservation constraint and an elliptical curve-fitting method with UBM for predicting both the compression force and the bulge profile. It is shown that the predicted results for the compression force based on UBM under certain reduction ratios of 30%, 40%, 50% and 60% are in good agreement with those obtained by experiment, Finite Element Method (FEM), and Slab Method (SM). In addition, it is revealed that the use of an elliptical curve-fitting technique provides a reasonable approximation of the bulge profile obtained by experiment and FEM simulations. Generally, the proposed method effectively provides a low-cost, computationally straightforward means of estimating the compression force and bulge profile in cylinder compression forming. Therefore it can be utilized as the widespread applications in the metal forming field.

Abstract: A combined process of hole flanging and flange upsetting was proposed. Both elastic-plastic FEM and experiments were employed to analyze the process. A 2mm thick 08AL sheet with 120mm outer diameter and 24.6mm center hole diameter was used as the blank. The effect of the R_f and the R_c values on the flanging quality were analyzed, where R_f was defined as the ratio of the die fillet size to the workpiece thickness t and R_c was defined as the ratio of the clearance C to the workpiece thickness t. Also the effect of R_f and R_c on upsetting ratio R_u which was defined as the ratio of thickness before and after upsetting were studied. The finite element results were validated by experimental results. Also a 2mm thick flange without thinning and defects was gained in a reasonable range of R_c and a certain value of R_f.