Process simulation
CAE methodologies are gaining an ever-increasing role in the design phases both of components and of processes. Currently, various separated codes are used to support the different phases of the production chain of a component. The proposed research is focused on aluminium casting and is aimed at integrating the use of the these codes. The use of structural analysis codes by component designers has been a feature for many years, while the use of casting simulation codes is spreading in the industry. Crash behaviour is an important aspect in the completion of the chain. Up to now all crash simulations have been done with homogeneous distributions of mechanical properties, while the project will focus also on this. Communication between all these kinds of codes is just making its first steps, but it remains completely absent when it comes to aluminium.
Microstructure-Properties correlation
From a general viewpoint, the links between processing, microstructure and final properties are known from a qualitative point of view. Some correlations are available between cooling rates (or local solidification time) and secondary dendrite arm spacing (SDAS), as well as between SDAS and mechanical properties. However, when industrially relevant processes and alloys are specifically considered, the coefficients to be inserted into such correlations are not available, or are not sufficiently validated. The dendrite arm spacing generally indicates the cooling rate. The most important microstructure features are size, shape and number of phases and defects (oxide films and porosities). The ultimate properties of a specific microstructure are lowered by the defects and quality mapping can give the relation of these to the casting process.
Quality mapping
One of the main targets of the numerical simulation of casting processes is the ability to predict the characteristics and location of defects. This aspect, as regards gravity casting processes, can be approached by means of micromodelling techniques. More complex is the situation for die-casting processes, whose role is growing in the production of automotive parts. The key point is certainly the definition, from empirical approaches and rules, of criteria for correctly predicting the various kinds of defects that can arise during the filling of the die cavity, which is the most critical stage in the die-casting processes. In this project, quality mapping is the interface between microstructure simulation and the prediction of properties and therefore leads to quantitative extended casting simulation.
Stress analysis
Every casting process, during the solidification and the subsequent cooling of the cast product, creates residual stress fields. These can be determined by means of numerical simulation codes. However there is a lack in the set up of reliable procedures and codes which would allow for the comprehensive evaluation of residual stresses in the castings, when comparing them to design requirements, as well as to the distribution of the castings' mechanical properties, directly resulting from the process itself.
Post-casting treatments
The quality of casting can be significantly improved by means of suitable treatments. Among these, hot isostatitic pressing is a densification method which has been demonstrated to completely eliminate shrinkage and hydrogen porosity from aluminium castings. The main consequence of this is a moderate improvement in static tensile properties, a dramatic improvement in fatigue properties and a marked reduction in the variability of tensile properties over the component geometry, as internal voids are brought to the surface, where major casting defects are easily detectable by visual inspection. The first three points have a great impact on the design, and in particular on the design of safety components, while the fourth is of major concern in process economics, since it eliminates costly tests (X-rays).
On the other hand, traditional pressing under inert gas has major disadvantages, which have relegated its use to niche applications:
a) safety is a major concern in high pressure gas systems, since the failure of the pressure vessel can result in catastrophic explosions (500 °C/1200 bar are typical operating parameters for Al alloys). This involves high plant and safety systems costs.
b) cycle time is at least several hours per load, due to the high compressibility of the gas being pumped in the pressure vessel.
c) the HIP treatment cannot be efficiently coupled with current heat treatments. Parts have to be loaded and unloaded cold.
Conventional heat treatments also result in significant improvement of properties of aluminium alloys. While, on one hand, the theoretics of the ageing processes are well known, they are not currently considered in process simulation codes. The modelling of such treatments, which are typically used for gravity cast aluminium alloys, will allow a complete evaluation of the final mechanical characteristics of this family of casting. They also make for more accurate selection of time-temperature parameters.
Process optimisation
Optimisation is a relatively new product on the software market and some structural optimisation code is now commercially available. General purpose optimisation codes are also now appearing on the scene, but their programming and interfacing with the codes they drive is neither easy nor fast, and much time is needed for the selection and modelling of optimisation criteria and constraints.
Process optimisation, as well as being a useful tool in designing castings, needs to be performed by developing modules dedicated to specific foundry processes that have to work by being linked to and integrated with the process simulation code.
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