Evolution of the manufacturing control and quality prediction system through thermal analysis

A common issue in foundry process is that with the same or very similar chemical composition of the metal, different results can be obtained in terms of parts quality and apparition of defects. The appearance of metallurgical defects such as microcracks, carbide precipitation and/or graphitic malformations can be a consequence of different metallurgical qualities. Hence the need to control the properties of the liquid metal through the use of thermal analysis tools.

By analyzing the shape of the solidification curve  the metallurgical quality can be characterized and we are in a position to parameterize this abstract concept so that it is represented by numerical values ​​in a clear and measurable way.

Opportunities of thermal analysis as a metallurgical control system for each type of foundry

Thermal analysis can be used as a metallurgical process control system. This is how the Thermolan® system was born, developed by the AZTERLAN Metallurgy Research Centre, responding to the need to define metallurgical quality based on different variables obtained from the solidification curve such as its characteristic values, cooling rates and released energies, measuring the amount of graphite and austenite formed at each stage of solidification, in addition to taking into account the contraction-expansion balance.

The first practical application of thermal analysis is to obtain the carbon content for all types of foundries, which allows the control of one of the most important elements of iron foundries quickly and reliably.

For spheroidal cast iron, thermal analysis allows:

• Predict spheroid density.
• Represent the tendency of the alloy to present shrinkage defects, which Thermolan® does through the Self Feeding factor parameter.
• Predict the risk of carbides and reverse quenching.

In addition to these parameters that define metallurgical quality, we also obtain other applications such as:

• Control of nodularity by controlling the free magnesium present in the liquid metal
• Prediction of the microstructure by defining the ferrite/pearlite ratio throughout the solid-solid transformation.

For gray cast iron, the Thermolan system predicts metallurgical quality by measuring the % of type A graphite, the size of the graphites and the K parameter that represents the risk of presenting shrinkage defects. We also obtain other applications such as the prediction of microstructure and mechanical properties.

For CGI (compact graphite iron) foundry, a specific technique has been developed that allows predicting the spheroidization index.

Thermal analysis develops its latest advances towards in-part prediction. By applying the metal solidification curve to a given piece, introducing the thermal modules of its different critical zones into the system, results are obtained that are extrapolated to those determined zones.
From the previous development we can draw the following conclusions:

• Thermal analysis is a valid tool to control liquid metal
• We can obtain various applications:
  – Definition of metallurgical quality using numerical variables
  – Control of nodularity
  – Microstructure control
• Need to take advantage of your full potential
• Possibility of extrapolating the results to the part
• It is a tool in constant evolution and development
• Applicable to different types of foundries