Following the Heat Treatment of Aluminum Alloys in Real Time

Steinbeis experts and a team from Mubea conduct investigations into innovative automotive components

A family-run business, Mubea is a global market leader in the development and manufacture of complex components used in the automotive and aviation industries. The focus in its business unit responsible for rollbonding products lies in components used in battery housings. To make these housings, Mubea employs a continuous hot rolling process that involves rollbonding a variety of aluminum alloys to produce cooling plates and complexly molded cooling parts. The components are highly versatile in terms of design and adaptable in terms of alloy combinations, allowing customized solutions to be offered for a wide range of installation areas and requirements. To identify efficient processes, it is essential to gain an understanding of the microstructural processes inside materials, especially with high-strength alloys. An important factor in identifying the best possible process is in-situ analysis. This is where the Steinbeis Transfer Center for Thermal Analysis came into play, providing the team at Mubea with the competent support of its experts.

 

With a relatively low density of only 2.7 g/cm³, aluminum alloys are highly suitable for use in lightweight components. Used in electrically powered cars, they can be particularly effective in reducing weight and increasing driving range. An important factor in this is not just their low density, but also their mechanical properties (e.g. strength and stiffness) and the way how aluminum alloys can be processed into finished components throughout the entire process chain.

Their mechanical properties are significantly influenced by the internal structures of materials on both a micrometer and nanometer scale – their so-called micro- and nanostructures. Adapting and adjusting micro- and nanostructures takes place during a number of production steps of the process chain, particularly during heat treatment. This involves subjecting components to a defined temperature and time regime. Key parameters of this process are heating rates, annealing temperatures, annealing times, and cooling rates. Carefully selecting the right parameters for heat treatment determines the resulting structures of a material and thus also material properties. This selection process is usually carried out with the help of so-called ex-situ experiments; components are analyzed in their initial state, i.e. before heat treatment, and in their final state after heat treatment.

 

In-situ analysis – better results thanks to live measurements

 

Using ex-situ experiments delivers little in the way of detailed information, so one gains no real understanding of the processes that take place inside the aluminum alloy during heat treatment. It would be more beneficial to analyze heat treatment in-situ, i.e. live and in real time during heating and cooling. Basically, there are a variety of material science techniques for doing this. These allow characteristic properties to be measured which change depending on the material structure. These techniques include:

• Light and electron microscopy with heating platforms
• X-ray diffraction; analysis of crystal structures
• Dilatometry; analysis of changes in volume or length
• Calorimetry; analysis of heat flow
• Measurement of thermal or electrical conductivity

The criteria for determining the suitable methods of in-situ analysis are the effort and cost of analysis and sample preparation, but also their duration in order to be able to carry out a sufficient number of measurements, even during rapid heating and cooling processes. Two highly insightful methods are in-situ electron microscopy and X-ray diffraction, but these often involve high expenditures when it comes to assessment and sample preparation. These contrast with in-situ dilatometry, in-situ calorimetry, and the in-situ measurement of thermal or electrical conductivity, which are often extremely effective as analysis techniques.

In its search for a competent partner for applying such dilatometric and calorimetric methods to aluminum alloys, the team at Mubea turned to the Steinbeis Transfer Center for Thermal Analysis. The Steinbeis Enterprise has often used these methods of analysis to gain an understanding of and select heat treatment parameters, involving a whole variety of projects with partners in industry. To conduct their projects, the Steinbeis experts joined forces with the Chair of Materials Science at the University of Rostock, offering access to a broad selection of dilatometers and calorimeters. Unique in international terms, these span an extensive range of heating and cooling rates, from approx. 10^-5 K/s to approx. 10⁵ K/s at peak temperatures of up to approx. 1,500 K.

Detailed examination of aluminum alloys

 

Dilatometry is primarily used with metallic materials undergoing phase transitions (i.e. changes in microstructures) during heat treatment, which tend to go hand in hand with pronounced changes in volume or length. This typically includes metallic materials that undergo phase transitions throughout the entire volume. Typical examples of this are steels, where dilatometry is often applied to in order to plot so-called time-temperature-transformation (TTT) diagrams. In the case of aluminum alloys, only a small proportion of the material (typically a few percent) undergoes phase transitions. As a result, the changes in volume or length that occur are significantly smaller, posing a particular challenge in measurement – a problem that the experts from Steinbeis have nevertheless succeeded in solving.

Aluminum alloys are more frequently assessed using calorimetry. All phase transitions, i.e. changes in the material structure, of metallic materials, are associated with the release of transformation heat (with exothermic reactions) or the absorption of transformation heat (endothermic reactions). These transformation heats can be used to characterize the processes within materials in-situ.

“With the right calorimeters (differential scanning calorimetry, or DSC), the degree of transformation heat can be measured extremely accurately, so this method is particularly suited to aluminum alloys, in which only a minor proportion of the material is involved in phase transformation,” summarizes Steinbeis Entrepreneur Professor Dr.-Ing. habil. Olaf Kessler. Measurement samples typically lie somewhere between ⌀6 x 1 mm and ⌀6 x 20 mm, and in many cases it is not difficult to extract samples from the components under investigation.

The diagram above of the DSC heat flow curve of an aluminum-magnesium-silicon alloy shows a complex sequence of exothermic and endothermic reactions. These can be attributed to the so-called precipitation sequence of the alloy. In principle, it is possible to determine start and end temperatures as well as the intensity of individual reactions. In some cases, there are several overlaps between reactions. When these happen, it is particularly difficult to distinguish between overlapping reactions. It is also possible to attribute characteristic stages of the to specific temperature ranges. An example of the practical results produced by such live investigations of heat treatment would be defining the minimum temperature at which observed precipitation and dissolution processes are finished. This could be used, for instance, to determine suitable annealing temperatures.

DSC heating tests are frequently carried out on aluminum alloys, and there is a good understanding of the measurement and evaluation strategies applied to such testing. This is not the case with DSC cooling tests, which are relevant for important stages of heat treatment, such as cooling down from annealing temperatures. “That is when you have to consider special measurement and evaluation strategies, which we have developed at the Steinbeis Transfer Center for Thermal Analysis in collaboration with the Chair of Materials Science at the University of Rostock,” explains Kessler.

The above DSC cooling curve of an aluminum-magnesium-silicon alloy displays a sequence of exclusively exothermic reactions. This is due to the fact that only precipitation reactions (exothermic) occur during cooling and there are no dissolution reactions (endothermic). In principle, it is also possible in this case to determine start and end temperatures, as well as the intensity of individual reactions. In practice, the goal is often to cool materials down so quickly that as many precipitation reactions can be suppressed as possible, meaning no ongoing reactions can be detected in the DSC cooling curve.

Microstructures and their properties

In addition to the live analyses described here, it is also possible to conduct ex-situ experiments on samples to characterize microstructures and properties. To do this, there are methods such as light and electron microscopy or hardness testing. These allow selected peaks in reactions to be attributed to specific stages of phase transition, offering a more concrete understanding of the precipitation sequence. While results from joint investigations with industry partners, including Mubea, remain confidential, the experts from Steinbeis have also shared information on publicly funded projects looking at the transformation behavior of approximately 20 different aluminum alloys during heating and cooling. These findings have been published in a scientific journal with open access [1].

Contact

Univ.-Prof. Dr.-Ing. habil. Olaf Keßler (author)
Steinbeis Entrepreneur
Steinbeis Transfer Center Thermal Analysis (Retschow)

Head of Department
Chair of Materials Science at the University of Rostock (Rostock)

Source
[1] https://www.mdpi.com/1996-1944/12/24/4083

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