3D Printing for Energy Applications. Группа авторов
1.5 Hybrid AM Through Green Body Sintering
The indirect manufacture of metallic and ceramic components via AM employs a process chain by which a blended feedstock is used to build the component. This feedstock contains a powdered filler material and a matrix material that, during additive manufacture, gives the component its as‐built strength. This part is known as the green part and is the origin of a multistage process by which the binder matrix is removed. The powdered filler is sintered and densified to form the final component. The process chain can be enabled by a variety of AM technologies and with a variety of process chains for debinding, sintering, and densification. As such, this section deals with the three most common AM methods and the two most common sintering process chains.
1.5.1 Common AM Technologies for Green Body Manufacturing
The entry‐level technology by which a green body component is commonly manufactured is through filament extrusion [92]. There is a variety of feedstock for extrusion‐based AM systems that have been engineered for this purpose. For high throughput industrial application, binder jetting is the fastest build method of all industrially applicable manufacturing methods and is thus competitive in many cases to laser PBF. Since no melt pool and plasma is generated during manufacture, the general tolerances are superior to those of laser PBF. As the green body part shrinks up to 30% during sintering, tolerances are further enhanced. The final method for green body manufacture using AM is by employing vat photopolymerization, where the photopolymer acts as the matrix material and is blended with the powdered filler. This method has a very low throughput yet is still highly industrially relevant, given that vat photopolymerization exhibits a spatial resolution, one order of magnitude better than that of the former methods [93]. Whereas there are no commercially available systems that allow for functionally graded materials employing hybrid AM/sintering process chains, this technology is highly relevant for manufacturing geometrical gradients in materials. Micro components made from metamaterials with topology optimized unit cells can thus be manufactured employing vat photopolymerization and sees usage ranging from medical components through energy applications.
1.5.2 CAD Design and Shrinkage Compensation
The hybrid process chain, in its entirety, is as illustrated in Figure 1.6. The CAD body of the component is designed specifically for the process chain. This is done in order to compensate for shrinkage during the sintering step. Shrinkage is typically up to 30% but can be higher for specialty feedstocks. The shrinkage behavior of small‐sized parts (<50 × 50 × 50 mm3) is quite uniform and predictable. Larger sized components, however, require accurate multi‐physics simulation models for shrinkage behavior. The shrinkage typically is considered to be isotropic for binder jetting and vat photopolymerization [95]. For filament extrusion, the shrinkage is normally anisotropic even for smaller parts with a larger shrinkage in the normal direction to the layers. This is due to a flow phenomenon where the powdered filler has a tendency to concentrate in the very core of the individual extruded strands of filament, leaving a skin layer with a little amount of particles in the skin layer. As such, the powdered filler concentration in the normal direction to the layering of the part exhibits alternating laminae of high and low particles. During sintering, the low concentration zones see more matrix material removal during debinding and thus higher shrinkage.
1.5.3 Additive Manufacture
For powder‐bed binder jetting and vat photopolymerization, certain industrially available systems are tailored for the hybrid AM process chain [96]. Examples are the Desktop Metal and ExOne (metals) system for binder jetting and the Lithoz system for vat photopolymerization (ceramics/metals). Given that these systems are tailored to the process chain, the workflow from CAD body design, through debinding to sintering, is offered as a bundled solution that minimizes the efforts needed for successful adoption. For experimental binder/matrix materials, the readily available industrial systems are less suited, as these are qualified to operate solely in a palette of materials offered by the systems provider. For exotic materials, a custom system is needed that allows for an open parameter setup in order to optimize the build for the feedstock chosen. Such solutions are not readily available and require an initial effort in setting up a custom AM system by the adopter, raising the barrier, therefore tremendously.