When the 3D printed metal part is finished printing, just pull it out of the machine and it's ready to go, right? Not far.
Metal 3D printed (AM) parts are essentially "welded" to the build plate, and we need to use a cutting tool to remove them. In addition to this, 3D printing parts still require a series of post-processing steps before they are ready for use. The following are methods related to metal 3D printing post-processing parts to share with you.
Powder Removal: 3D printed parts are built "down" in a powder bed fusion system as new layers are added on top, meaning the part is buried in powder after completion. After the build is complete and the part/build plate cools, the machine operator must remove all powder from the build table and sieve/filter/recycle for later use. This is the first operation after printing, only labor costs, take some time.
Stress Relief: As the part is built layer by layer, the heating, and cooling of the metal cause internal stresses that must be relieved before the part can be removed from the build plate. Otherwise, parts may warp or even crack. Stress-relieving parts require an oven or furnace large enough to fit the entire build plate. Many people recommend using an oven with an inert environment to minimize oxidation on the surface of the part. In fact, people prefer a vacuum furnace, but it costs a lot more.
Removing parts: Most companies use wire EDM to remove parts from the build plate, which can take several hours, depending on the number and size of the parts. But many machine shops are starting to use bandsaws because it's faster, more efficient, and can often get the job done in minutes. Keep in mind that materials like Inconel strain harden when machined, so it can be difficult to remove them from the build plate with just a bandsaw.
Heat treatment: Heat treatment can effectively improve the microstructure and mechanical properties of the part, and is necessary for almost all 3D printing metal parts. In many cases, this step also requires an environmentally controlled furnace capable of adjusting the temperature and cooling time. Heat treatment can affect the dimensions of the part, so in most cases, parts are heat treated before machining/finishing the part.
Hot isostatic pressing: Many aerospace companies are beginning to use hot isostatic pressing (HIP) instead of heat treatment, which is often used in the foundry industry to improve the fatigue life of castings. HIP systems cost significantly more than furnaces/ovens and have their own safety measures due to the high pressure (100 MPa or more) at which they operate. If the part has been HIPed, it usually does not need to be heat treated.
Machining: The mating surfaces, surfaces, threads, support structures, etc. need to be machined to ensure the dimensional accuracy of the finished parts. Few 3D printed parts meet "expected" specifications, which requires post-processing to ensure the dimensional accuracy of the model. But establishing machining benchmarks can be tricky, especially for complex, organically shaped parts made using 3D printing. Access to internal channels or cooling channels that need to be machined also adds cost and often requires specialized fixtures.
Surface Treatment: Surface treatment may also be required to improve the surface finish/quality of the part, reduce surface roughness, clean internal channels, or remove partially unmelted particles from the part. Surface roughness has the greatest impact on the static mechanical properties of parts, and is related to wear resistance, corrosion resistance, and fatigue strength of parts. Machining, chemical corrosion, and shot peening can be used, but the parts themselves are damaged to a certain extent.
Inspection and Testing: After post-processing, metrology, inspection, and non-destructive testing using white/blue light scanning, dye penetrant testing, ultrasonic testing, computed tomography (CT) scanning, etc. may be required at multiple points in the post-processing process. Destructive testing of sample parts and analysis of coupons (e.g. tensile bars), powder chemistry, material microstructure, etc. may also be required to collect data to aid in process qualification and ultimately part qualification. 3D printed parts with internal channels, lattice structures, and other internal enhancements may also require CT scans to ensure channel patency, internal geometry assessment, etc.
In fact, metal 3D printing is more complex than we think. Compared with resin or plastic printing, it requires more post-processing processes to assist in completing the final ideal finished part.