Michael Cardew-Hall

Characterising frictional behaviour in sheet metal forming

Abstract A two-level multi-variable design of experiment (DoE) approach was used to investigate the influence and interaction of lubricant type, die surface finish, contact pressure, sheet metal coating and draw speed on friction. For this DoE, the flat face friction test (FFF) and the draw bead simulator (DBS) were used to measure the coefficient of friction. The experiments were run in random order with at least three replicates. The Yates algorithm was employed to determine the significance of the main effects and their interactions. It was found that the Zincanneal coating lowers friction and reduces variation within a specific test condition. The DBS results illustrated an insensitivity of the variables investigated to the friction coefficient. However, the rough DBS die was sensitive to blank coating. The polished surface of the FFF test illustrated a similar insensitivity.

A finite element simulation of micro-mechanical frictional behaviour in metal forming

Friction is a critical factor for sheet metal forming (SMF). The Coulomb friction model is usually used in most finite element (FE) simulation for SMF. However, friction is a function of the local contact deformation conditions, such as local pressure, roughness and relative velocity. Frictional behaviour between contact surfaces can be based on three cases: boundary, hydrodynamic and mixed lubrication. In our microscopic friction model based on the finite element method (FEM), the case of dry contact between sheet and tool has been considered. In the view of microscopic geometry, roughness depends upon amplitude and wavelength of surface asperities of sheet and tool. The mean pressure applied on the surface differs from the pressure over the actual contact area. The effect of roughness (microscopic geometric condition) and relative speed of contact surfaces on friction coefficient was examined in the FE model for the microscopic friction behaviour. The analysis was performed using an explicit FE formulation. In this study, it was found that the roughness of deformable sheet decreases during sliding and the coefficient of friction increases with increasing roughness of contact surfaces. Also, the coefficient of friction increases with the increase of relative velocity and adhesive friction coefficient between contact surfaces.

Comparison of surface strain for stamp formed aluminum and an aluminum-polypropylene laminate

Laminate structures incorporating thin layers of metal and polymer, or polymer composite, can offer significant weight savings for engineering structures, while retaining excellent mechanical and impact performance. Laminates based on thin layers of aluminum and glassfiber/polypropylene thermoplastic have been the subject of recent study [1, 2], and have exhibited excellent specific mechanical properties and superior specific impact behavior compared to monolithic aluminum. Such materials, therefore, have great potential for widespread application in engineering structures. One such potential area is the automotive industry where weight reduction and impact performance are pertinent issues. Lighter vehicles will result in improved fuel efficiency, and greater energy absorption capability may contribute to improved crash performance. However, for the automotive industry it is necessary to produce components using a high-volume manufacturing process such as stamping. Thermoplastic-based materials and sandwich structures are good candidates for stamp forming as they can be heated to conform to the mold, and then rapidly cooled for removal from the mold. Mosse et al. [3, 4] investigated the effects of blankholder force, laminate preheat temperature, tooling temperature, and tool radii on FML formability. It was found that significantly lower levels of springback could be achieved over aluminum, and forming defects could be eliminated by restricting process variables to a given range. In particular, it was found that delamination at the bimaterial interface and within the composite layer was eliminated when the laminate was pre-heated to 160 ◦C then formed in a heated die. This is significant as delamination would adversely affect the mechanical performance of a formed component. Further, Kim and Thomson [5] found that high forming speed increased the transverse stiffness of polymer-metal laminates, in turn reducing the inter-laminar shear and the degree of springback. They also found that laminates forming at elevated temperatures decreased the rigidity but improved the springback characteristics. This letter presents some preliminary results from research into stamp-forming aluminum-thermoplastic sandwich materials. Here, the permanent strain on the surface of a channel-formed aluminum-polypropylene laminate is compared to monolithic aluminum. Characterization of the strain is significant as it provides insight into the behavior of the material during formation and assists in the production of parameters for subsequent formation methodologies. The materials used in this study were 5005-H34 aluminum and a self-reinforced polypropylene (Curv, BP). An aluminum-Curv laminate was made in a 2/1 configuration in a 200 × 200 mm picture frame mold. A 0.9 mm thick layer of Curv was sandwiched between two layers of 0.5 mm thick aluminum cleaned with a solvent (isopropanol). A 50 μm thick layer of a hot-melt polypropylene adhesive (Gluco Ltd., UK) was placed at each bi-material interface. The laminate was consolidated by heating to 160 ◦C in a platen press followed by rapid water cooling under a pressure of approximately 1 MPa. The nominal laminate thickness was 2.2 mm. Samples of 19 mm width were sectioned from the laminate and from a plain sheet of 2 mm thick aluminum. A 3 mm circular grid etched onto the surfaces enabled post-forming major strain measurements, that is in the direction of the sample length, to be made. Channel sections were stamped in an open die. Plain aluminum was stamped cold whereas the aluminumCurv laminates were pre-heated to 160 ◦C then immediately transferred to the die, which was pre-heated to 80 ◦C. This enabled a temperature window of 125– 140 ◦C to be maintained during the stamping operation. The channel sections were stamped in an Enerpac 30 tonne press using two tool radii of 3 and 7 mm. The blank holder force was 3.5 kN. Surface strain measurements were taken from ten grids around the mid-point of the sidewall area of the channel section, shown in Fig. 1, using an optical microscope with a graticule scale of 20 μm resolution. Measurements were taken from the sidewall area as it is likely to undergo significant tensile strain during formation. Microscope examination of the sidewall edge, prior to taking the strain measurements, confirmed the absence of delamination. The average major surface strain for the aluminum and aluminum-Curv samples is plotted in Fig. 2. (The

Thermal modelling of the laser-assisted thermoplastic tape placement process

Thermoplastic tape placement opens the possibility of a fully automated composite production. The resulting quality is highly dependent on the thermal history during consolidation. This article focuses on the thermal modelling of a tape placement system employing a near-infrared laser. A nonlinear two-dimensional finite element model is presented for a carbon fibre reinforced thermoplastic (AS4/PEEK) composite placement process using a conformable roller. The relative influence of roller geometry, roller temperature and thermal contact resistance was studied. Temperature measurements were performed using thermocouples welded to the substrate. The model predictions show good correlation in terms of timing of the irradiation, shadow and consolidation regions. The roller temperature was found to have the most significant impact on the bond line temperature distribution.

Effect of membrane stress on surface roughness changes in sheet forming

Friction plays an important role in sheet metal forming (SMF) and the roughness of the surface of the sheet is a major factor that influences friction. In finite element method (FEM) models of metal forming, the roughness has usually been assumed to be constant; even though it is commonly observed that sheet drawn under tension over a tool radius results in the surface becoming shiny, indicating a major change in surface morphology. An elastic–plastic FEM model for micro-contact between a flat surface and a single roughness peak has been developed. The model was used to investigate the effect of the membrane stress in the sheet on the deformation of an artificial roughness peak. From the simulation results, the change in asperity, or deformation of the local peak, for a given nominal tool contact stress is significantly influenced by the local substrate stress. The height of the asperity decreases with increasing substrate stress and the local pressure is much higher than the nominal pressure. In addition, the local contact stress decreases with an increase in the substrate stress levels.

Identifying variation in sheet metal stamping

This work looks at two different “Design of Experiments”(DoE) methods for defining an operating window in the sheet metal stamping process. The first involves the use of replicates at the different experimental points, while the second is a nonreplicated method. The two methods are compared by looking at the relationship results produced and the indication of variation in the process. It is found that the results from both the methods are very similar. However, the replicated method provides a greater level of confidence in the results. In the stamping process, where performing large numbers of replicates is expensive in both time and money, the nonreplicated method provides a cost effective way of understanding the process.

A Hybrid Draw Die Optimization Technique for Sheet Metal Forming

Recent years have seen considerable advances in the use of Finite Element (FE) modeling techniques, to the point where they can be used confidently to predict the output of the sheet metal forming system. The limiting factor in the use of FE analysis in the optimization process is now shifting from the accuracy of simulations, to the time required to optimize the system. This paper proposes a new approach aimed at reducing the time to optimize a draw die design, through a combination of Finite Element Modeling, semi-analytical models, and a knowledge based expert system.

Geometric shape errors in forging: developing a metric and an inverse model

Abstract The complexity of the forging process ensures that there is inherent variability in the geometric shape of a forged part. While knowledge of shape error, comparing the desired versus the measured shape, is significant in measuring part quality the question of more interest is what can this error suggest about the forging process set-up? The first contribution of this paper is to develop a shape error metric which identifies geometric shape differences that occur from a desired forged part. This metric is based on the point distribution deformable model developed in pattern recognition research. The second contribution of this paper is to propose an inverse model that identifies changes in process set-up parameter values by analysing the proposed shape error metric. The metric and inverse models are developed using two sets of simulated hot-forged parts created using two different die pairs (simple and ‘M’-shaped die pairs). A neural network is used to classify the shape data into three arbitrarily chosen levels for each parameter and it is accurate to at least 77 per cent in the worst case for the simple die pair data and has an average accuracy of approximately 80 per cent when classifying the more complex ‘M’-shaped die pair data.

A Study of the Effect of Process Variables on the Stamp Forming of Rectangular Cups Using Fibre-Metal Laminate Systems

The quality of the part and the robustness of the process in stamp-forming of sheet materials are determined by a number of variables. This study looks at the application of the stamping process to a Fibre-Metal Laminate (FML) material system and the effect of the process variables on the formability characteristics of these material systems. The effect of pre-heating temperature on the splitting and wrinkling behaviour has been investigated for two different FML systems. It has been found that different FML systems exhibit different failure modes.

Design, implementation and use of a knowledge acquisition tool for sheet metal forming

Knowledge Management systems utilising a number of different techniques have been developed for manufacturing processes including sheet metal forming. These are designed to overcome knowledge loss and allow an organisation to more effectively leverage its corporate experience base. Most systems focus on design activities only, they do not capture and integrate the experience gained on the shop floor during testing and production. In order to be used in these areas, knowledge capture must be as simple and time efficient as possible, even if this is at the expense of later potential reasoning. The design, implementation and results from the use of a knowledge acquisition system of this kind for the automotive stamping industry are described. Despite perceived conceptions of data captured, it is concluded that underlying relationships can be extracted from the information entered, allowing significant reuse of captured experience, whilst maintaining timely knowledge acquisition.Copyright