ALLEGRO project seeks to boost lightweight-materials durability
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Numerical illustration of a friction stir-welded lap joint on a test specimen. (Fraunhofer LBF) 

ALLEGRO project seeks to boost lightweight-materials durability

Using resource-optimized processes, researchers at the Fraunhofer Institute LBF are working to enhance durability for lightweight materials.

First define the lightweight material that best complements electric vehicle (EV) and automated-vehicle (AV) range efficiency – then optimize it for increased durability via the use of “resource-optimized” process technologies. That’s the aim of researchers at the Fraunhofer Institute for Structural Durability and System Reliability (LBF) and its partners in a comprehensive materials R&D program scheduled to run until December 2021.

Andreas Maciolek, a member of the research team, told SAE International’s Automotive Engineering: “The process-technologies project ALLEGRO started in January 2018 after a planning phase of approximately two years. Since then, we have been working on adaptions of local evaluation concepts for components with graded material properties and for welded joints with small geometric notches, taking into account the material’s history along the process chain. For aluminum alloys in tempers with large cyclic hardening or softening, it is also expected that the cyclic transient behavior has a major influence on the evaluation results.” 

Maciolek stressed that the experimental determination of cyclic properties for the application of local evaluation concepts on high-quality weld seams – e.g. those made by laser welding, friction stir welding or magnetic pulse welding – still posed what he termed “a certain challenge,” adding: “This is to be mastered by the taking of small-scale specimens and the use of optical measurement technology to gain information about the local material behavior. With these more precise assessments, improvements can be achieved in the fatigue-life approach of components according to the material.” 

UHS aluminum 
The team’s work rests on the combination of ultra-high-strength (UHS) aluminum alloys and new forming processes, demonstrating that local component properties can be adjusted as required. The development of the ALLEGRO project is part of LOEWE, the German Federal State Offensive for the Development of Scientifically Economic Excellence, which is funded by the Hesse Federal State. To make optimum use of the lightweight construction potential of aluminum alloys, the geometric and micro-structural complexity of products must be increased, said Maciolek, who at the recent LightMAT (International Conference on Light Materials 2019) conference in the U.K, gave a presentation of the results achieved of the team work so far.

He said that previous evaluation concepts could only reproduce local material properties to what he terms “a limited extent.” So his research project is developing what is described as “new evaluation concepts” for process-related structural durability, taking into account cyclic transient material behavior.  

“They are an important prerequisite for the industrial application of the new technologies and enable a more sustainable product design in lightweight construction,” he stated. The major target is to use process-related properties of the UHS aluminum to improve the service life of components. 

Fraunhofer LBF is working with the Institute for Production Technology and Forming Machines at the Technical University of Darmstadt, as well as the Department for Separating and Joining Manufacturing Processes at the University of Kassel, to progress ecologically and economically the implementation of efficient forming processes via integrated heat treatment. The processes would be used to produce semi-finished products with the required locally adapted properties calibrated to fit the function and loads of specific applications. 

The Fraunhofer LBF specialists are using an e-bicycle frame to examine in detail the entire process chain to ensure advantage is taken of local process-related lightweight properties. “Our aim is to demonstrate the results of our research with a simple example – the e-bicycle frame. Despite that apparently simple application, in the future, he said, “The processes and concepts can be applied for automotive applications.” 

Cyclically stressed 
The microstructure and the material properties are influenced by various process steps along the chain, which in turn determine the component behavior. “If these local material properties can be determined and taken into account in the design and evaluation of cyclically stressed components and structures, the potential in lightweight construction could be exploited in a targeted manner,” Maciolek claimed.

Joinability of graded tubes also will be investigated using the e-bicycle frame. The team’s challenge is to maintain property gradation of the semi-finished product – or even to improve it by joining. Although laser welding, friction stir welding and magnetic pulse welding all are regarded as suitable, but efficacy is to be investigated further by the project’s team. Other future work will include exploitation of strain-life curves of small-scale specimens to higher fatigue lives by a piezo-driven test device. Investigations also will examine the influence of pre-strain and heat treatment (tool temperature for quenching, artificial aging) on W-temper formed high strength aluminum rolled plate (EN AW-7075) sheet material and also on the cyclic material properties of thermo-mechanical formed EN AW-7075 sheets.

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