Metal 3D Printing
Metal 3D printing comes in different forms. The most commonly found strategies are: Direct Energy Deposition (DED), and Power-Bed Fusion (PBF), in which the heat source is either a Laser or an electron beam. In DED the power is blown onto the built part, while in PBF, layers of powder are progressively deposited and regions that need to be fused are heated up by scanning with the laser or the electron beam. Following movie shows the interior of the chamber of a laser PBF printer (Aconity 3D, @Stanford) while it prints, then it stops for more powder to be deposited, and then the printing resumes.
One of the promises of 3D printing is the possibility of building hierarchical structures, in which solid parts are replaced by complex lattices at the sub-centimeter scale. This offers the possibility of tailoring the mechanical properties of a metal part, reducing the weight, and potentially tuning electromagnetic, catalytic and/or acoustic properties. The following are some pictures of some typical printed lattices by this printer.
A challenge or advantage of as-printed parts is that they have different microstructures than conventionally cast or forged parts. This is because during printing the melted material solidifies much faster in the presence of a high temperature gradient. This is why it is important to understand the relation between the processing conditions, temperature history, and resulting microstructure.
We approach this challenge with a combination of advanced computational tools and carefully designed experiments. With computation we can evaluate and then predict the type of temperature fields during printing. An important mechanism of heat transfer in the melted material is the motion of the liquid metal (convection). Following movie shows the predicted streamlines of the liquid flow under a moving laser; the colors correspond to temperature values.
The microstructure of the parts can be characterized by cutting, etching, and imaging the sections through optical and electron microscopy. Melt pool boundaries, solidification microstructures, and dislocation patterns can all be observed and compared with the prediction of computational models. The following are some typical optical microscopy images.