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Innovative casting process of lighter steel components for the transport industry


The first activity was aimed at finding the best polymeric material in order to overcome the defects due to the carbon pick-up which is frequently encountered in the ferrous castings with patterns made of the expandable polystyrene EPS. This problem would be prevented by using the so called low-carbon bead polymers (a mix of EPS and polymethylmethracrylate or PMMA) or even 100% PMMA for the pattern foam material. The work was addressed to try to modify the density during the pre-expansion step in order to investigate blends of EPS and PMMA to find an optimum ratio. The main expected achievements of the project for development of improved quality foam models, foam materials, polystyrene PMMA, glue materials and quality, can be considered satisfactory.
The manufacture of a hollow crankshaft is a very important achievement in relation to the general aim of weight reduction in the automotive industry: Weight savings achievable are in the order of 15-20% depending on the component to be manufactured. Lost foam technique enables both the production of hollow parts and the reduction of machining operations due to the near-net-shape capabilities of the process. These two features have an important incidence in relation to cost savings. In particular, a hollow crankshaft has been produced (hollow crankpins and toes) with oil channels obtained directly in the as-cast part (so there is not need of further machining). Together with cost reduction, the weight savings attained for this component will provide a reduction in rotating masses and therefore a better performance in terms of engine efficiency and comfort. Also, other potential components have been identified for future application: a hollow camshaft (with similar advantages to previous part) and an exhaust manifold (for improvements in weight savings and less fuel consumption). A complete analysis on simple shaped parts produced by Lost-foam process has been carried out. Both mechanical and microstructural characterisation test were done for evaluation of different typology of defects inherent to the process. In particular, carbon pick-up parameter was studied for a better understanding of carbon content variation in the casts. Also, non-destructive analysis was performed on both simple-shaped parts and the firsts clusters of crankshafts. Also, an exhaustive microstructural and mechanical analysis of several series of crankshafts with different casting parameters and configurations has been carried out. A comprehensive analysis on casting defects, related microstructure, non-destructive analysis and fatigue simulation and tests has been performed for the best characterisation of the process development regarding these components. Although some improvements are further needed to get rid of deleterious defects for dynamic applications overall, at the actual progress point of the process several components should be successfully produced.
In this project, Risø has participated in the selection of suitable steel alloys for the two types of components, i.e. aeronautical and automotive component. A list of present and other potentially interesting alloys was compiled, from which the decision was made with regard to the alloys to be cast. On the other hand, Risø has analysed many samples of crankshafts and aeronautical rib parts, cast at Gussstahl Lienen (GSL). All these activities will provide a Risø National Laboratory with a specialised know-how in characterisation of steel parts produced by the Lost-foam technique.
Process simulation of the lost foam process of steels is a decisive tool to support process reliability and cost reduction during the design of new components produced by means of this innovative technology. Numerical simulation of the lost foam process must include the prediction of flow pattern of liquid metal, including the foam degradation. This is controlled by the amount of gases evolved. The core of the result development is the implementation of a mathematical model for the polymer pyrolysis for the lost foam process. The final model has been designed as a two-step model as far as both heat transfer and mass transfer aspects are concerned. The model has been successfully implemented, keeping track of the movement of the pyrolysing front and the liquid metal front by using the volume fractions of the solid (foam), liquid and gaseous (pyrolysis products) phases, and the liquid metal. The mass and energy balance of each control volume is written in terms of these volume fractions. The pressure of the gas phase is calculated by assuming a polytropic process. Liquid products of pyrolysis take part in the mass and heat balance.
The lost foam casting process is one of the future processes that will be used in the foundries worldwide. The technical and ecological advantages of the process are fundamental, although there are still some technical difficulties to overcome for the casting of steel parts. The aim was to develop the lost foam process for low alloy steels (e.g. automotive crankshaft) and stainless steels (e.g. aeronautical pylon rib). Both steel grades showed major problems when cast in the lost foam process caused by carbon pick-up phenomena and defects from the pattern degradation products. The casting technique has been developed for both steel grades to overcome part of those problems by optimising the pouring parameters, the gating technique and the pattern material. So far the production of crankshafts has been successful achieved by using a dedicated gating system, pouring parameters and a special pattern polymer. The surface and volume defects have been minimised with this technique, although further improvements should be made to the complete optimisation of the component. The development of the aeronautical part has been less problematic than the previous one, with quite successful results regarding complete filling of the part (even in very thin areas) and avoidance of defects as hot tearing and internal shrinkage by the correct use of reinforcement ribs and risers. The results gained by the project will be used to open the market potentials for near net shape steel parts in low alloy and stainless steel grades.
For the aircraft industry it is essential to lower costs. The application of Lost foam technology to the production of steel parts for the aeronautical industry has a great potential in terms of cost savings. It is foreseen that Lost-foam castings would substitute at a much lower price components currently produced by very costly forging and machining operations. Firstly, this technology enables the reduction of machining operations due to its near-net-shape capabilities. Secondly, the capability of lost foam process to integrate more than one part in a single component will also help to reduce the manufacturing costs. At the same time this process will permit to introduce new technologies on structural parts. The casting trials for the production of the aeronautical rig carried out inside de projects showed very promising results for the production of these kinds of parts, although some minor problems regarding process optimisation should still be solved. EADS will not only use it for the components identified and manufactured but also intends to introduce this innovative casting technology in other suitable parts where its application could be beneficial.
One of our main objectives was to develop an experimental arrangement to simulate the Lost-Foam casting process in laboratory scale to substitute casting trials. The intensive efforts to develop a dynamic arrangement to simulate the movement of the melt failed because of the insoluble problems with the ceramic components of the heated part of the system. The arrangement was changed to a static process with the possibility to decompose defined portions of foam material and to determine the heat and pressure development. Additionally a real casting program was designed to get information especially about the dimension of the gas gap at real castings by thermal monitoring of the form filling process. To get information about the parameters that influence the form filling behaviour of Lost Foam steel castings especially with regard to the implementation of a software tool for numerical simulation by Magma Soft, experiments were created and carried out. The gas-permeability of the coating was determined at different thickness and under various temperature conditions. Pressure course measurements pointed out the different removal conditions dependent on the vertical location in the sand filled flask especially for gaseous degradation components evolved in the reaction zone melt-foam. Real casting trials with simple plate geometry were performed to get information about the form filling and pressure development in the gas gap. The experiments were done with variations of parameters to point out their influence on the form filling. One of the main results is that the coating thickness and herewith the gas permeability of the coating is the most influencing parameter on filling time and pressure in the gas gap. The environmental investigations were realised by different analysis of the degradation products. Gas samples from degradation of different foam materials were taken by a real casting trial and the degradation device. The samples were qualitatively analysed by GCMS measurements. The most important result of these trials is the knowledge that there could be reached a considerable decrease of low volatile degradation products by using pure PMMA instead of PS/PMMA copolymer. With regard to the environmental impact these results are important because a high amount of steady volatile decomposition products is much better post-processable by catalytic afterburning of the waste gas stream. An analysis of the emissions after the post-processing at GSL by the TÜV Sachsen-Anhalt brought the result that the content of high toxic polycyclic aromats like Benzene could be considerably reduced by the used catalytic waste gas cleaning. That makes clear that the Lost-Foam steel casting process could be applied in an ecologically acceptable way if an optimal process control is guaranteed. The comparison of the results of all degradation and form filling experiments that were carried out during the project brought the knowledge, that a much higher amount of liquid and low volatile degradation components evolve in the reaction zone between melt and foam material than it could be estimated from the high degradation temperatures caused by the steel melt. This fact stands in direct relation to the quality of castings because especially liquid degradation products can cause significant defects both as defects caused by carbon pick up and as internal defects as result from an absorb of liquid fractions into the steel melt. The research program of the project showed some promise to reduce the difficulties by an optimisation of process influencing parameters like pattern material, gating system, coating thickness and pouring temperature. But nevertheless follow up work is needed for process optimisation especially on the field of pattern material is a great potential of reducing casting defects by better degradation behaviour.