Learning Objectives
By the end of this section, students will be able to:
- Understand all of the post-processing requirements relative to each AM technology
- Grasp the concepts of:
- Powder removal.
- Support removal.
- Curing.
- Sintering and densification.
- Thermal post-processing (stress relief, heat treatment, HIP).
- Machining surface and surface preparation.
Once an AM part has been processed using any of the aforementioned processes, the part will most likely endure subsequent processing steps on its journey to become a quality part. These additional steps are what is known as post-processing. The specific post-processing steps will vary significantly depending on which AM process is being considered. Reference Table 2.1 on which specific AM technology requires which specific post-processing type.
Powder/Support Removal
Once a part is removed from the build chamber of SLS Polymer, Binder Jet machines, the resulting volume of powder, also known as a build cake, is removed from the build chamber and de-powdered. Parts are dug out of a build cake of powder to expose the parts fabricated.
Next, for powder bed electron beam and laser metal systems, part build cakes are removed from the machine and de-powdered. In addition to de-powdering, since these processes require supporting structure, sacrificial support structures are removed from the parts. This is accomplished by a technician using conventional hand tools to chisel, pry and machine off the support structure from the parts.
Finally, large scale polymer material extrusion, stereolithography and liquid deposition requires a physical removal of support structure after fabrication. Small scale polymer material extrusion may also require a physical removal of support structures, however, water soluble support material has been developed that allows for support structure to be dissolved easily from the part.
Some AM processes require curing, or the introduction of additional heat to strengthen the part after the part is built. For example, stereolithography and liquid deposition processes require a subsequent UV curing process that occurs after parts are temporarily cured in the AM machine because they are UV photopolymer based.
For binder jet technology, a binder catalyst temporarily ‘cures’ a structure into a green part during the fabrication process so that it can be moved to a sintering and densification step.
Curing of the binder is necessary to develop sufficient mechanical strength in the bound metal part for handling, in most of the binder jet processes. Curing is typically done as a batch process where the entire build box is fit into an oven and heated to 150-260°C (300-500°F) for a couple of hours.The temperature allows the polymer in the binder to crosslink, increasing its strength and therefore the strength of the printed part.
Sintering and Densification
Once binder jet technology is partially cured, it is sintered to bake out the binder material and then subjected to a densification process to reduce internal porosity.
There are several important subtopics to sintering: Setters and Trays, Furnace Environment and Control, De-Binding and Sintering. Table 2.2 highlights just a few of the topics and impacts.
| Setters and Trays | Furnace Environment and Control | De-Binding | Sintering | |
|---|---|---|---|---|
| Consideration | Temperature and sintering process dependent | Atmosphere, temperature ramp control | Chemical interactions and thermal control | Final de-bind and densification |
| Impact | Slumping, part distortion or material interaction | Excessive time in the furnace to achieve density or de-bind | Trapped binder could be a defect | Failure to achieve density requirements or excessive sintering time |
Choice of atmosphere in the furnace and thermal control is closely related to the binder employed and the material being processed. There is no one single solution to sintering. The atmosphere in the furnace could be a reducing gas, flowing hydrogen, or a vacuum. Control of the furnace thermal profile is important to achieve good results given variable ramp up and hold schemes to “burn out” the binder.
In addition to the temperature and environment, the part will be necessarily shrinking to densify. This shrinkage is influenced by gravity, so some tooling may be required to control slumping. It is also necessary to consider that great care was taken to put the binder in the right place and cured. The binder is removed as the particles transition from being held together by the binder, to being held together by powder particle to powder particle bonding.
Sintering is fairly simple in concept. Where two particles contact, atoms can begin to move from one to the other, and, with time, multiple particles coalesce in 3 stages: 1) Neck growth, 2) Intermediate sintering and 3) Final sintering. Table 2.3 shows the main considerations, and Figure 2.9 Stages of Sintering shows the stages conceptually.
| Stage 1: Neck Growth | Stage 2: Intermediate | Stage 3: Final | |
|---|---|---|---|
| Consideration | Packing density / disruption by the binder impact | Packing density and part densification | Gas egress from part |
| Impact | Particle contact initiates sintering | Higher density speeds sintering, part densification leads to shrinkage | Decrease in part density |
All sintering stages are important, but the packing density of powders in the printed part is important in Stage 3 as a higher density produces more rapid sintering. The powder bed density can be impacted by the binder impacting the surface. The bulk of densification occurs during this stage which drives the entire body to shrink up to 15-20% by volume. This shrinkage results in distortion of the part, because thick features shrink more than thin features. Abrupt changes in section thickness can result in surface cracks due to the differences in shrinkage.
Large openings and overhangs can distort due to gravity during this stage of sintering, and the drag of the shrinking part on the tray can cause the bottom of the part to shrink less than the top.
A typical debind and sintering cycle for an Iron-based alloy is shown in Figure 2.8_Typical Debind and Sinter Cycle. Debinding occurs at the first hold at 300C, and finishes at the 2nd hold at 600c. This temperature is chosen such that stage 1 sintering begins at about the same temperature. Sintering completes stage 3 in this example cycle during the hold at 1400C.
With the exception of sheet lamination, all metallic AM processes typically require one or more processes of stress relief, heat treatment and hot isostatic pressing to achieve full mechanical density.
Machining/Surface Preparation
All metallic AM processes require some form of post-process machining. Some powder based metallic processes may simply require wire EDM or bandsaw removal of parts from the build plate. Complex parts with threaded holes and features that fit up into assemblies will at least need mating surfaces machined, and many tight tolerance features will also need machining or finishing.
However, all metallic wire fed, sheet lamination and cold spray processes will require subsequent machining to reach final feature definition. Also, large scale polymer material extrusion will need full machining. This is due to the fact that these processes produce parts that are near net shape preforms.
SLS polymer and small-scale polymer material extrusion may also require surface treatment of sanding, painting and seal coating to provide a smooth surface. The need for these additional steps will depend on the customer requirements.