Shrinkage porosity is the most common solidification defect for metalcastings, but it's a particular problem in ferrous alloys because solidification can be complicated if dissolving carbon partly precipitates as graphite with a lower density than the bas
|An illustration correlates a simulation and an actual part in terms of ferrite formation|
|A correlation between a simulation and an actual part in terms of the graphite nodule radius.|
|Isolated pocket of liquid in an X-ray view.|
|A cut-away view of the hot spots.|
|A cut-away view of shrinkage porosity showing casting density, due to accurate modeling of the graphite expansion. |
Shrinkage porosity is the most common solidification defect for metalcastings. It happens in almost all alloys as they contract during cooling from the pouring temperature to the solidus. When casting ferrous alloys, the solidification process can be complicated by the fact the dissolving carbon partly precipitates as graphite with a lower density than the base iron. Thus, graphite formation is associated with a volume increase.
Under certain circumstances this expansion may compensate for the contraction of the metal to reduce or even avoid shrinkage. Therefore, the generation of shrinkage cavities in cast irons is linked closely to the local density change during solidification. The expansion and shrinkage behavior is affected by alloy composition, cooling rate, and other process conditions leading to the microstructure.
Now, advanced simulation can be used to understand and control such a complex process. ESI’s (www.esi-group.com) ProCAST is a casting process simulation software based on finite element analysis (FEA) that’s used to reduce manufacturing costs and shorten lead times for mold design and to improve overall casting process quality.
In ProCAST, ESI supplies a comprehensive micromodel that can accurately profiles microstructural conditions as well as mechanical properties, including yield strength, tensile strength, elongation, and hardness. The micromodel, along with an extensive thermodynamic database, is coupled with the porosity model, resulting in accurate shrinkage prediction capabilities that take into account the complex phenomenon of graphite expansion.
Microstructure formation during solidification is important for controlling the properties and quality of cast products. This is especially important in large ferrous castings, such as the hubs, shafts, frames, blade adapters, housings for reduction units, caps, and brackets that make up wind turbines — a notable component of Spain’s industrial sector.
It was this capability that drew the attention of Tecnalia, a multi-disciplinary Spanish industrial research group. Tecnalia, which specializes in metalcasting R&D, undertook the research with the backing of Fundiciones Urbina S.A. and TS Fundiciones, two Spanish ferrous foundries, and Federacin Espaola de Asociaciones de Fundidores (FEAF), the Spanish foundry trade association.
Microstructure and mechanical properties
To gather microstructure predictions, ProCAST couples thermodynamic calculations (from CompuTherm® databases) with micromodels and macro-scale thermal and fluid-flow calculations. Tecnalia applied this methodology to a 12-metric ton wind turbine frame in high-tenacity ductile iron, EN-GJS-400-18LT. The presence of magnesium in the composition causes nodular graphite precipitation during solidification.
The pouring temperature was 1,360-1,370°C for a filling time of about 80 seconds. The mold was made of a high-resistance resin-bound silica sand. The proportion of ferrite and pearlite as well as nodule count provided insight into the mechanical state of the as-cast part, and the simulation results were compared with actual results.
Next, Tecnalia used microstructure calculations to predict the final mechanical properties. In cast iron, the type, amount and morphology of the eutectic will determine the effective mechanical properties. The structure of the matrix is essentially determined by the cooling rate through the eutectoid temperature range. Slow cooling rates promote the transformation of ferrite, and thus lower tensile strength as demonstrated in the Tecnalia research project.
The final integrity of a casting, including mechanical properties and surface finish, is greatly influenced by the presence of porosity.
|A comparison of the predicted mechanical properties for ProCAST and measurements. |
As explained, the graphite expansion exhibited by cast iron can be significant, and very difficult to model. Indeed, it is necessary to consider microstructure, process conditions, material properties, inoculation, fading, density variation and mechanical properties of the mold to accurately predict shrinkage porosity in cast iron. Reliable control over the metallurgical quality and the graphite expansion means that there is no shrinkage, either in simulation or in the actual part, despite the presence of an important isolated pocket of liquid as shown on the following computer illustrations.
Tecnalia compared the predictions of microstructure mechanical and porosity with experimental results and found these to be in good agreement. “ProCAST’s recent developments prove to be excellent for predicting the microstructure and the basic mechanical properties of casting materials,” summarized Tecnalia metallurgist Dr. Antton Melndez Arranz. “In 13addition ProCAST solves one of the main complex phenomenon in cast iron solidification, i.e. graphite expansion.
“Using the microstructure module, the simulation of local graphite expansion is possible with a sensitively higher accuracy for shrinkage defects prediction,” Melndez Arranz continued. “The microstructure module opens a new line of possibilities and makes other types of analysis possible, particularly related to the adjustment of the metallurgical quality level using the inoculation parameters in simulation.”