Advanced Materials Research Vol. 939

Abstract: High-precision near-net shape parts with excellent surface qualities can be produced with the forging process with a minimum of finishing operations thanks to the good formability of aluminium alloys. There has been a rapid increase in the use of aluminium forgings predominantly in the automotive industry, where weight savings for reduced fuel consumption and exhaust emissions is mandated by legislation. Aluminium forgings provide, in addition to low weight, high strength, good corrosion resistance and a fibrous grain structure to improve fatigue resistance. Typical commercial forging stock is the round bars produced by the extrusion of cast billets. An alternative process route that has received increasing attention in recent years is the casting of forging stock by a horizontal direct chill casting technique to make smaller billets without the need for extrusion to reduce their diameter. The anisotropy imparted to the forging stock via extrusion, often regarded as useful for the forging, is certainly missing in the former. However, cast stock has been reported to be more resistant to the formation of coarse surface grains than the extruded counterpart. The present work was undertaken to compare the casting and extrusion routes for the manufacture of 6082 alloy forging stock.

Abstract: To reduce springback in U-bending the present paper proposes a new technique where the bottom of a U-bent part is clamped between a punch and a counter-punch during bending and it is pushed up with the counter punch at the final stage. The effect of counter punch pressure, both in sheet clamping and bottoming, was investigated by performing experiments on dual phase 590MPa and TRIP 780MPa HSS sheets. From the experiment, an appropriate combination of the clamping force and the final pushing-up force was found where springback was reduced to almost zero. To investigate the mechanism of the reduction of springback in the above three-step U-bending process, FE simulation of the bending with PAM-STAMP 2G was also conducted where the advanced kinematic hardening Yoshida-Uemori model was employed.

Abstract: Incremental sheet forming (ISF) is a promising forming process in which complex 3D shapes are formed from a sheet of metal using a simple moving tool. The efficient prediction of contact forces in ISF is desirable to monitor the forming process, prevent failure, and implement on-line control and process optimization. However, traditional Finite Element (FE) simulation used for force prediction is significantly time-consuming for complex products. The purpose of this study is to investigate the ISF force prediction and characteristics under different forming conditions and build a potential efficient model. In the present work, forces during the cone forming process with different wall angles and step down sizes were recorded and compared. Different force trends were identified and discussed with reference to bending and strain hardening mechanics. Influences of different parameters on designated formability were also qualified which should benefit the product design process. An efficient predictive model based on upper-bound approach was applied for force prediction in this case. Predicted tangential forces were then compared with the experimental results showing relatively good agreement. The limits of the proposed model were also identified and the potential of future improvements were suggested.

Abstract: Incremental sheet forming (ISF) is a method to form a sheet metal into desired shape and surface features in a batch production series. This method includes forming a clamped sheet metal in controlled conditions by a CNC milling machine, lathe machine or a robot. In this study, the effects of forming parameters on the amount of stretch in stainless steel sheet using a CNC milling machine have been investigated. A ball-point shaped tool made of a bronze alloy was fabricated and used throughout the experiments. The tool acted as the indenter that formed the stainless steel sheet into a small pyramid-like shape. The results showed that as the spindle speed and feed rate increased, the amount of sheet stretch also increased, up to a point where the sheet could not stretch anymore and the process changed from forming to shear thinning and chipping. In addition, the surface quality of the part was badly affected at higher spindle speed and feed rate settings. The temperature of the lubrication oil was also measured during the process and the maximum temperature recorded was 45°C which remained constant until the end of the process. In conclusion, to obtain a good quality part while increasing the productivity of ISF, the optimized values of the feed rate and spindle speed in this work were found to be at 500 mm/min and 1000 rpm respectively.

Abstract: In the present investigation, the forming behaviour of adhesive bonded sheets with the pre-generated infinite defects in the adhesive layer is studied. The infinite defects are generated with different orientations like longitudinal, transverse and at an angle of 45°. The base sheet materials used are deep drawing quality steel and SS 316L sheets, and two part epoxy adhesive is used for bonding the base sheet materials. The formability is quantified by monitoring the load-extension behaviour, and limit strain, evaluated through tensile tests and in-plane plane strain (IPPS) formability tests. It is observed that there is a significant decrease in formability because of the presence of infinite defects in the adhesive layer. While comparing the formability of adhesive bonded blanks with respect to different orientations, transversely oriented defect shows more reduction than 45° and longitudinal cases. There is not much difference between the transversely oriented and 45° oriented infinite defects in tensile tests, whereas in the IPPS formability test results, there is no considerable difference between 45° and longitudinally oriented defect.

Abstract: A method for modifying the properties of adhesives is by the addition of metallic wire strand in fibre form to improve the thermal conductivity for use in electrical applications. Clearly any advantage gained as far as thermal conductivity is concerned should not be counteracted by deficiencies in mechanical properties. In this work, the influence of copper wire reinforcements on the tensile behaviour of adhesive bonded steel sheets will be presented. The adhesive used is a two-part epoxy resin with a mix ratio of 1:1 parts by weight of resin to hardener. Wire strands used are copper wires having a diameter of 0.3 mm. The base materials are made of Stainless Steel. The stress-strain behaviour, and limit strain are evaluated through tensile tests and in-plane plane-strain (IPPS) formability tests. It is found that the ductility and limit strain of adhesive bonded sheets with wire reinforcement are better than that of the case without reinforcement. The ductility and limit strain of wire reinforced adhesive bonded sheets increases till three wires in the case of tensile tests, and four wires in the case of IPPS formability tests, after which it decreases.

Abstract: Developing the interior structures of polymer films is essential for creating novel functions of such films. This study investigates the effects of extension parameters and processing procedures on the super-structure of a polypropylene (PP) film. An biaxial extension device is also designed and fabricated to analyze the effects of annealing, extension temperature, extension rate and other parameters on the super-structures of PP films. Based on super-structure analyses, the relationships between extension parameters and film structures are determined, and an extension process that involves a second time stretching procedure is suggested. Experimental results indicate that void structures can be generated by biaxial extension at the interface between the crystalline and amorphous regions in a film. The mechanical strength of the film is also affected by stretching and become highly oriented. Although a low extension temperature can increase the number of void structures generated at a low extension ratio, structure sizes are dispersed. Additionally, voids with uniform size can be obtained when the film is stretched at a high temperature; however, it needs a high extension ratio to promote the formation of void structures. As for annealing treatment, when the annealing temperature increased, the size and quantity of the spherulites that were formed in films by annealing also increased, but these films tend to have similar sizes and numbers of void structures. Based on the experimental results, this study also describes how parameters influence the void structures of a PP film created during the extension process. Analysis results further demonstrate how extension parameters and processing procedures can be adjusted to improve the fabrication of PP films.

Abstract: Glass micro lens arrays (GMLAs) have several advantages such as a high transmission rate, anti-environment, and can be used for special wavelength applications. Precision glass molding (PGM) has been used to mass produce high-accuracy aspherical glass lenses. Ultra-precision diamond grinding (UPDG) is a fundamental part of the precision glass molding process. Using UPDG, grinding spherical and aspherical to sub-micrometer form and nanometer surface roughness is simple. However, asymmetrical surface MLAs are difficult to generate using the UPDG process. UPDG, together with the wheel-forming method and a strategy used to separate the entire surface generation process into several grinding loops, were studied and developed to generate high filling factor MLAs on the mold surface. The GMLA material used was K-CSK120, made by Sumita Inc., Japan. Finally, GMLAs with an approximately 100% filling factor were generated using PGM with form accuracy and surface roughness that were respectively less than 0.3 μm and 10 nm. The tolerance of each single micro lens figure was greater than 95%.

Abstract: The viscous pressure forming (VPF) was applied to form an automotive aluminum alloy panel in this paper. The formability of the panel was investigated. Further, the springback characteristics of the panel were analyzed. The results show that the viscous medium is helpful to improve the flow of the sheet blank. The positive and negative springback occur for the panel formed by VPF. It is different from the panel formed by rigid punch forming, in which only the positive springback occurs. Furthermore, the springback value of the panel formed by VPF is smaller than the latter.

Abstract: Aluminum alloy sheet metals have been widely utilized for a light weight construction of automobile. However, Aluminum sheet metals still remain one of the difficult materials to predict the accurate final shapes after press forming processes, because of several mechanical weak features such as lower Youngs modulus, strong plastic anisotropy of yield stress, Lankford values, and so on. In order to solve the problems, the present author has developed a new constitutive model called Modified Yoshida-Uemori model. The present model can describe accurate non-proportional hardening behaviors of Aluminum alloy sheet metals. In the present research, several experimental procedures were carried out to reveal the mechanical properties of Aluminum alloy sheet metals. From the comparison between experimental data and the corresponding calculated results by our constitutive model, the performance of our model was evaluated. In addition to the above mentioned research, the evaluation of some springback analyses were also carried out. The calculated results show good agreements with the corresponding experimental data.