University of Bordeaux (France) & Sirris (Belgium)
Selective laser melting is an additive manufacturing technique which represents a major technological breakthrough that allows us to envisage structures that are not reachable by conventional technologies such as machining or foundry. However, the development of parts with this technology, particularly in the aeronautical industry, requires the control of the quality of the manufactured parts: surface condition, defects such as porosities and cracks, microstructure, presence of interstitial spaces, ...
Because of the interactions between laser and metal powders, additive manufacturing processes involve coupled physical phenomena of thermal (with phase changes related to the melting phenomena of metal powders and the solidification of the liquid metal), metallurgical (diffusion phenomena), mechanical (presence of residual stresses due to temperature gradients during the process) and hydrodynamic (convection in the melting bath). From a metallurgical point of view, the SLM process involves a rapid melting-solidification kinetics of small-sized (<1 mm) melt zones, which leads to the formation of very specific microstructures.
The main objective of the thesis is to study, on the basis of experimental measurements, the interactions between laser and material in order to describe the thermomechanical and metallurgical histories of the material during the SLM process according to operating parameter.
All measurements, observations of the fusion bath by visible rapid camera, thermal acquisitions by infrared camera, thermocouples or diodes, measurements of residual stresses, will be carried out on the equipment available to the various partners: instrumented SLM machines and experimental test benches used to reproduce fusion kinetics observed in additive manufacturing on powder beds. The reference alloy of the study is AlSi7Mg0.6. All the results obtained will be used as a basis for the implementation of material behaviour laws (WP1) and for numerical modelling and simulation of the process (WP2).
The outline of the thesis can be divided into 5 parts:
- A preliminary study, based in particular on previous work, will describe the microstructures obtained for AlSi7Mg0.6 alloy parts developed by SLM, according to the operating parameters.
- Calibration and validation of data acquisition systems (sensors, data reliability, data management) on samples made in AlSi7Mg0.6 will be carried out on the various devices used in the study. Procedures will also need to be put in place to manage data flows.
- Test specimens will be made in order to establish a correlation between the operating parameters of the process and the in situ data from the instruments, in terms of microstructures and mechanical characterization.
- The study data will form the basis for the model developed by WP1. A validation of the model will be carried out by comparing the results of the model with the microstructures observed during this study.
- Measurement campaigns on titanium and steel samples and parts will be carried out in order to validate the methodology on other materials and to form of the basis of the corresponding models.
This thesis will be shared between SIRRIS (18 first months) and University of Bordeaux (18 months) and will be carried out in collaboration with ENIT, IPC and IK4-Lortek.