Thermal Analysis for aluminum

The SFTA system offers a complete thermal analysis solution for the aluminum melt.  Use SFTA to measure solidification properties like grain refinement and eutectic modification in aluminum alloys on your shop floor within 7 minutes.

Increase productivity and diagnose defects with a controlled melt from casting to casting. For example, here’s how the operator knows if his grain refinement or strontium modification treatments are still good or if melt properties needs to be corrected. Maybe that aluminum ingot batch was offset on the low/high side for silicon, strontium, etc?

Your R&D and quality manager will also get an interface for advanced applications like fraction solid curve, hot tearing and custom criterions. Fine tune your solidification simulation parameters with the true alloy properties and cooling curve.

SFTA Operator’s interface : this 356.1 thermal analysis sample indicates the melt in the furnace needs grain refinement and strontium modification treatment.
SFTA overview video
Showing the SFTA sampling stand for aluminum melt and the converter box. The flexibility of the system allows for mobile or fixed installation. View the results on a remote screen for the operator and/or directly on your laptop as the quality manager ( laptop not included ).

Applications

Grain refinement

SFTA provides a value between 1 to 5 for grain refinement level. A result of 1 indicates the melt needs to be treated in order to reach the target set for the casting job.

The goal of grain refinement in aluminum alloys is to increase the number of nucleation sites. Titanium can also sediment the oxydes and clean the melt if it’s allowed to settle. The result is a uniformly fine, equiaxed as-cast microstructure, typically resulting in less shrinkage and porosity defects. A common practice in foundries is to introduce master alloys or inoculant particles of TiB2 to the melt before casting.  However, the effects of the inoculant fades over time. Thermal analysis allows the foundry to determine the proper amount of grain refiner to add to the melt and what is the fading time or effectiveness of the chosen grain refining treatment.

Melt operator taking a sample. SFTA is a shop floor melt measurement tool to help the operator take decisions within 5-7 minutes.

Eutectic Modification

SFTA provides a value between 1 to 6 for eutectic modification level. A result of 1 indicates the melt needs to be modified in order to reach the target set for the casting job.

Strontium modifies the eutectic structure of Al-Si alloys from acicular to fibrous. Many aluminum foundries buy pre-modified ingots. The main drawback of this practice is the level of strontium varies from ingot batches and suppliers. Therefore, the mechanical properties and the level of porosities will be affected.

Recently, a foundry stopped buying pre-modified ingots and started to add strontium to the melt by themselves. This was achieved using thermal analysis to control the modifification level. This change in practice improved the elongation by 30%, the yield strength by 15% and they are now less dependent on the ingot supplier for metallurgical consistency.

Solid fraction curve in simulation software

The solid fraction obtained through thermal analysis can be used in simulation software in order to better design the gating of the mold. For example, thermal analysis samples can be taken at different states. Then, the engineer can use the solid fraction curves and simulate the same part under different conditions. Therefore, the engineer can determine what are the optimal conditions to produce the part. The main advantage is that the simulated solidification of the part will be significatively more representative of the shop floor. This is particularly the case when casting proprietary alloys that are not documented in the simulation software database.   An example of a solid fraction curve is shown below. SFTA results can be used as a control loop on the shop floor in order to know if the simulated melt properties are within the range of the real shop floor melt properties.

Typical temperature versus solid fraction curve – A356 sample

Terminal Freezing Range & Hot Tearing Susceptibility

Certain aluminum alloys such as Al-Cu, Al-Zn, Al-Mg are prone to hot tearing. A complete dendritic solidification tends to have the adverse effect of extending the time period the alloy is exposed to the hot tearing susceptible zone, which causes more solidification defects in comparison to eutectic alloy (Al-Si). 

The solid fraction and the temperature curves are used in order to the hot tearing susceptibility of aluminum alloys.  For example, the temperature range between 90% and 99% solid gives an indication of how at risk the alloy is to hot tearing before casting. The casting design needs to account for the hot tearing tendency of the alloy, the liquid metal condition should be within the acceptable hot tearing susceptibility index range. If not, the melt should be treated before casting.

The SFTA system can be used control parameters that are known to reduce the hot tearing susceptibility of the alloy such as grain refinement and gating design.

The Zero Curve

The zero curve, also known as the baseline curve, is the cooling rate curve assuming there is no phase transformation. The Newtonian Method is used in order to determine the zero curve. The figure 3 presents an example of a typical cooling curve obtained with the SFTA system.

This is a typical curve for an aluminum A356 SFTA sample.   The blue plot is the temperature, the orange plot is the cooling rate, and the pink plot is the zero curve.

The integrated area between the zero curve and the cooling rate indicates the energy released during each phase of the solidification. Different phases will appear based on the chemical composition of the aluminum alloy and degree of oxydation of the elements. Are the elements metallurgically active for the phase transformation or not ?

It is also possible to visualize up to the fifth derivative to detect subtle metallurgical reactions such as intermetallic phases and other thermal events as R&D works progress.

SF Thermal Analysis measures the energy of each phase forming during solidification of the sample. This is the integration between the zero energy curve and the cooling rate. The chemical element can show on the spectrometer reading but it might not be available to participate in the phase formation. For example in aluminum 356, all available magnesium will show on the spectrometer reading. But how much of it is oxydized ? SFTA will measure the Mg2Si phase potential of the melt before you fill the mold so you can be confident on the heat treatment.

Contact your SF metallurgists today and let’s measure your melt :