Key Engineering Materials Vols. 504-506

Abstract: This paper presents an update on a friction benchmark, that was proposed during the 13^th ESAFORM conference. The goal is to compare different friction test set-ups [1–4] by determining the coefficient of friction (CoF) for Twintex® PP. The benchmark instructions are based on the ASTM standard D1894 [5] but also account for different friction velocities, pressures and temperatures. At the time of writing five research groups contributed to the benchmark, each with a custom designed test set-up, differing in size, mechanism, force control and temperature regulation. All tests will be conducted with woven glass reinforced polypropylene, from the same Twintex® batch. Conclusions will be drawn about the comparability of different testing methods by recognizing and analyzing systematically deviating results.

Abstract: The market demand for thicker complex shaped structural composite parts is increasing. Processes of the Liquid Composite Moulding (LCM) family, such as Resin Transfer Moulding (RTM) are considered to manufacture such parts. The first stage of the RTM process concerns the preforming of the part. During the preforming of multi-layered reinforcements, friction between the plies occurs and needs to be characterize and then to be taken into account for the forming simulation. An experimental device designed to analyze the ply/ply, ply/tool and yarn/yarn frictions has been built. Tests carried out on a glass plain weave highlight specific contact behaviour for dry reinforcement fabric in comparison to non-technical textiles. This behaviour exhibits a substantial variation of the contact tangential loads which is directly related to shocks between the yarns. It demonstrated that period and amplitude of the friction response is directly related to the relative positioning of the samples but also to fabric meso-strucrure. In addition, the sample relative orientation has a significant influence on the friction response. The meso-architecture is responsible for the high variations of the friction coefficient as a function of the ply orientations for different fabrics. In addition, the friction response appears to be very sensitive to tests parameters like the relative positioning, orientation of the samples and the cycling. A honing effect classically observed in dry fabric testing has also been pointed out through cyclic experiments. It can be attributed to both fibre material abrasion and fibre reorganisation inside the yarn.

Abstract: Forming processes are highly influenced by all the interface conditions between the tooling and the workpieces. For thermo-mechanical processes like hot forging or cutting processes friction is widely studied for a long time but for composite parts, it is not the case because the problem is not so crucial: forming forces are generally weak enough to allow the part be realized with any forming device; surface quality is not highly affected by the friction conditions; for pre-impregnated fabrics, the viscous or even fluid matrix acts like a lubricant and avoids defects due to sticking between fibre reinforcement and metallic tools. Nevertheless, friction seems to have an important role when precise simulations are expected. Up to now, few studies have been focussed on friction during composite forming processes. The aim of the present study is to make a contribution on that topic for an experimental point of view using an high precision device able to measure small friction forces. The relative fibre orientations can be monitored in order to explore the whole range of geometrical configurations. The final goal is to develop an efficient tool for finite element simulations of dry and pre-impregnated fibre fabrics accounting for the main specificity of fabrics, that is to say their strong anisotropy.

Abstract: Composites forming processes involve mechanical interactions on the ply, tow, and filament level. The deformations that occur during forming processes are governed by friction between tows and tooling material on the mesoscopic level and consequently between filaments within the tows on the microscopic level. A thorough understanding of the frictional properties of individual filaments is essential to understand and to predict the macroscopic deformations of a fabric during forming. This paper provides a global description of the experimental and modelling approaches to explain the contact friction between fibrous tows and metal tooling material, focusing on contact mechanics at the tow and filament scale.

Abstract: In thixo casting and forming processes, the forming alloy is in a semi solid state - a suspension of solid particles in a liquid matrix. As known from suspension rheology, the solid fraction, particle size distribution, particle shape, surface properties, state of agglomeration etc. determine the complex flow behaviour of the material. In thixo forming, the flow behaviour determines the quality of the produced parts, and it is essential to quantify the flow behaviour to optimize the process route. The goal of the present work was to evaluate the effect of particle size and state of agglomeration on the flow properties in a thixo material-analogue suspension, and to establish an easily applicable model to describe the non-Newtonian behaviour of a thixo material. For this purpose, experiments were conducted applying synthetic suspensions (built up of mono-disperse glass spheres in silicone oil) with behaviour analogue to thixo material. The advantage of synthetic suspensions is that parameters such as particle size and shape can be controlled in a simple manner and hence the number of unknown parameters is decreased. The experimental results show influence of shear rate, solid fraction, particle size and liquid viscosity. A model to determine the shear dependent viscosity was set up. This model was based on the Krieger-Dougherty equation for the relation between viscosity and solid fraction, and on the generally accepted theory that the state of agglomerations is influenced by shear. The model could successfully display the degree of shear thinning and takes into account the influence of particle size. However, it was found that the relation of the viscosity of the suspension and the liquid viscosity was non-linear. This does not correspond to the Krieger-Dougherty equation.

Abstract: Metal matrix nanocomposites (MMNCs) could be strong candidates for use in the automotive and aerospace industry, where the mechanical performance is a crucial factor. This study aims to determine the feasibility of Al/SiC nanocomposite fabrication by the ultrasonic technique for the production of thixoforming feedstock material. Aluminium (A356) matrix composites were fabricated with the addition of 0.2 wt.% SiC nanoparticles using an ultrasonic method. Two different particle feeding mechanisms; the Al foil double capsulate method and the crucible placement approach, were employed. Better results were obtained for the double capsulate method. Also, Ti and Nb probe tips were tried to provide ultrasonic vibration in the melt. It has been shown that Nb is a promising ultrasonic probe tip material to produce MMNCs above 650 °C as it is thermodynamically stable in comparison with Ti.

Abstract: The deformation behavior of semi-solid aluminum alloy is strongly dependent on the microstructure. This paper illustrates several experimental research works concerning thixoextrusion of 7075 aluminum alloy which was carried out at “Arts et Métiers ParisTech” of Metz. Inductive re-heating of the aluminum billet is the method used in order to obtain the target liquid fraction for thixoextrusion. To minimize the heat losses, a sample obtained from a direct extruded bar is inserted in a die for reheating in semisolid state and thixoextrusion. During the experimental re-heating process, the temperature was directly controlled by using thermocouples for temperature measurements in the sample and also in the die. The influence of different working ram speeds and reheating temperature on the microstructure evolution was studied by optical microscopy. The experimental results on extrusion load and microstructure evolution of the component are reported.

Abstract: In this study, the effects of laser parameters on the properties of glazed die steel were investigated. A Rofin DC-015 diffusion-cooled CO2 slab laser with 10.6 µm wavelength was used. Die steel sample surfaces were prepared with a 3 µm roughness and chemically treated to improve CO2 laser wavelength absorbance. One set of processing parameters were processed through the thermal simulation program and correlated with physical results determined from actual test samples. Set processing parameters were 1138 W peak power, 2900 Hz PRF, 24% duty cycle and 261 mm/s traverse speed. Scanning Electron Microscopy (SEM) micrographs and micro-hardness properties of the affected surface were measured. An analytical mathematical model of the heat field generated in the laser glazing process was used to predict the nominal temperature distribution in the surface and dimensions of melt pool. A thermal model using point source surface energy inputs was used to predict the thermal profiles in the die steel. This allowed estimation of the depths of microstructurally altered regions. For higher energy absorbance, the depth of the glazed surface increased from 20 μm to 40 μm. At high nominal heating to cooling rate ratios, high micro-hardness values were recorded.

Abstract: The present paper is focused on the thixo lateral forging of a nearly 3 Kg commercial automotive spindle. A thixoforming cell has been implemented in the forming lab consisting on an induction unit, six-axes industrial robot for the handling tasks and a servo-mechanical forming press. A special tool has also been designed in order to use all the press capacity during the forming stage. First trials have been made using LTT45 steel. The usual material for this spindle fabrication is a CrMo alloyed steel (W-Nr 1.7225) employed in automotive components with high requirements on toughness. The selected thixoforming steel has been modified to have a wider solidification range and lower solidus temperature.

Abstract: This paper presents an analysis of an advanced squeeze casting process suitable for the manufacturing of high performance industrial components more quickly and cheaply. After a short description of the process, some produced components are considered. The components, in A380 alloys, have been T6 heat treated and their soundness has been certified by non destructive tests. All considered components have been designed for advanced application in the automotive field, in particular for quite important sport cars. Standard samples for tensile and impact tests have been machined directly from the previous components. After the execution of the tests the fracture surface of samples has been observed by SEM in order to analyze details and to evaluate the influence of the process and of the alloy on the fracture behavior. On polished transverse sections of samples the microstructure of the alloy has been observed, highlighting a mainly globular shaped microstructure as expression of the attained semi-solid conditions during the processes. A critical analysis has been developed to evaluate the real potential and to present some criticism of the process.