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The MBJ (Metal Binder Jetting) process is an additive manufacturing technique, one of the advantages of which is to allow the manufacturing of parts in small and medium series. The Paris-PSL Mines Materials Center is carrying out research to design structural parts intended for the aeronautics sector using this process.
Appeared during the 2010s, the MBJ process (Metal Binder Jetting) is an additive manufacturing technique which consists of spreading a metal powder (ceramic or other materials) layer by layer, while projecting a binder onto the surface of each of them. It is qualified as indirect, because the part obtained when leaving the printer must then undergo post-processing steps. For manufacturers, this process allows them to manufacture high-precision parts with fine surface finishes, in small and medium series. The Paris-PSL Mines Materials Center is carrying out research to design structural parts with strong mechanical strength using this process.
“An MBJ machine is an inkjet type printer, except that instead of projecting ink, we project a binder, that is to say a polymer, where the material must be consolidatedexplains Jean-Dominique Bartout, head of the EPROM (Elaboration, Processes and Materials) platform at the Paris-PSL Mines Materials Center. At the exit of the machine, we obtain a so-called green part which must then undergo several stages, with first a heat treatment to polymerize the binder, followed by a depowdering stage which consists of removing the powder on which there is no binder, another debinding stage, and finally sintering which allows the metal grains to tighten together in order to create the final part. These post-treatment processes result in significant material shrinkage and the whole objective is to obtain parts with the correct dimension while ensuring their good material health. »
Preliminary work consists of quantifying the different material removals during each stage. The whole complexity lies in being able to take into account the anisotropic nature of each shrinkage, that is to say the fact that they do not have the same characteristics in the three directions of the volume “xyz”. An important point is to anticipate them from the design of the part and for this, scientists use checks during manufacturing, thanks to the sending of data from sensors placed on the printer.
The use of modelization is also necessary to obtain a “near net shape” part, that is to say one whose shape comes as close as possible to the dimensions required by the end user. The CEMEF (Mines Paris-PSL Materials Shaping Center) is a partner in this research project and has developed multiphysics sintering models, in particular to predict the anisotropic deformations of parts during densification stages.
Check the atmospheric composition at each stage
The good material health of the final parts depends in part on its chemical composition. However, post-processing steps can lead to their modification. This is the case for example, during heat treatment during which the polymer is burned, which results in the formation of residual carbon which can enrich the material, or the presence of oxides. “We work on each step by controlling the atmospheres to make the phenomena of decarbonization and oxidation compatible, whereas they are normally incompatible, add Jean-Dominique Bartout. To do this, we place sensors to measure the atmospheric compositions of each stage. »
The quality of the final part is measured initially by chemical analyzes of the materials obtained, then by metallographic studies using an optical microscope or electronic transmission in order to determine the quality of the microstructures. To evaluate the mechanical properties of parts, several types of mechanical tests are also carried out: monotonic tests, fatigue tests, creep tests, etc.
The parts developed by the Center des Matériaux de Mines Paris-PSL are mainly intended for the aeronautics sector, such as the companies JPB Systems and Safran. “The challenge is that they can be used as structural parts and that they respect the standards of this sector, completes Jean-Dominique Bartout. The MBJ machine market is almost in a monopolistic situation with the risk that this technology becomes more and more closed. Thanks to our work, we provide recommendations to end users so that they maintain control of this process and in particular of all intermediate stages. Our work continues with four theses which should start soon. »
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