Strain localization and damage development during bending of Al alloys for automotive

Key words: 6xxx aluminium alloys, Automotive outer panels, Instrumented bending test, Digital image correlation, Crystallographic texture, EBSD, Interferometry

Context:

Due to its low density, aluminium (Al) is a key material for sustainability as it allows lightweighting. In order to decrease the CO2 emissions associated with its production, it is required to massively use recycled aluminium, meaning remelting Al scrap to make new ingots. However, switching from primary synthesis (bauxite reduction) to secondary synthesis (scrap melting) often leads to increasing impurities, the main one being iron in aluminium. Such contaminants may impact the performance of the current alloys in terms of ductility/formability, especially on sharp edges. It is the case of automotive outer panels that are bent around the inner part after stamping. The internship project is linked to microstructure optimization, and especially crystallographic texture (meaning grain orientations and their spatial arrangement), in order to mitigate the detrimental effects of impurities. Thus, it is key to identify how grain microstructure could be tuned to maintain current product performance, while increasing recycled content. Constellium C-TEC (Voreppe, France) and SIMaP laboratory (Grenoble, France) work together on a PhD project to relate damage nucleation and growth to intermetallic particles-related features (size, shape, spatial arrangement…) thanks to in-situ nano-tomography data acquired at the synchrotron. As part of this collaboration, the present internship will be focused on the quantification of strain localization and its relationship with damage development and final failure thanks to several experimental techniques.

Objectives:

The objective of the project is to better quantify the relation between strain localization and crystallographic texture and its influence on the bending performance of 6xxx aluminium alloys for automotive applications.

The project has several components:

Literature review to identify failure mechanisms in bending and analyze existing results
Mechanical testing instrumented by DIC (Digital Image Correlation) for different materials in different loading directions to quantify strain localization patterns at the scale of grain aggregates
Quantification of roughness development on the outer surface of the bent specimen by interferometry and development of an experimental protocol to relate hemming performance to roughness-based indicators
EBSD (Electron Backscatter Diffraction) on interrupted bending test to identify crystallographic orientations responsible for premature strain localization and those that should be promoted
Damage quantification by post-mortem sample SEM (Scanning Electron Microscopy) analysis
Identification of relevant indicators to quantify strain localization and its relation to microstructural features
Analysis and synthesis of the results, report writing and presentation to the team

At C-TEC, the intern will be part of a R&D team of engineers with expertise in metallurgy and mechanics. In addition, the candidate will spend some time with the PhD student at the academic partner (SIMaP laboratory, GPM2 group consisting in 15 researchers and several PhD and post-doctoral students) for part of the experimental work (mainly DIC-instrumented bending tests). Interactions with another C-TEC intern working on the finite-element simulation of strain localization in bending will be possible to compare modeling results to experimental data.

Education level: Engineering school or Master of Science

Competencies (technical & soft skills requirements):

Good knowledge in Physical Metallurgy and Mechanics
Motivation for experimental work, including image analysis
Strong capacity to post-treat a significant amount of data from the experiments
Serious, autonomy, critical mind and creativity
Candidate interested in research and development (possibility to pursue with a PhD)
Good English level, both written and spoken