Metal 3D printing has been in full swing in recent years and has been gradually applied to all walks of life: aerospace , automotive, medical and so on. The advantage is that it can achieve lightweight and individualized design of parts, which can solve some technical problems that cannot be achieved by traditional processing and manufacturing methods. However, technically, many problems in the process of 3D printing make our design not follow the expectations. It is printed for the following reasons: residual stress, part placement, support, part optimization, etc. So today we will detail the effects of residual stress on the metal 3D printing process and how we should avoid it!
Residual stress: Residual stress is an inevitable product of rapid heating and cooling, which is an inherent characteristic of the laser powder bed melting process.
Each new processing layer is constructed by moving a focused laser on a powder bed, melting the top layer of the powder and fusing it with a processing layer below. The heat in the hot melt pool is transferred to the solid metal below, so that the molten metal cools and solidifies. This process is very fast, only a few microseconds. The new metal layer shrinks when it solidifies and cools on the upper surface of the underlying metal, but due to the limitations of the underlying solid structure, its shrinkage causes shear forces to form between the layers.
The laser melts the metal on top of the solid substrate to form a new weld bead (left). The laser moves along the scan vector and melts the powder, which is then cooled by transferring the heat to the underlying solid metal. After solidification, the cooling metal shrinks, and a shear force (right) residual stress is formed between the metal layer and the next layer to be destructive. When we add another processing layer on top of one processing layer, the stress is formed and accumulated, which may cause the part to deform, the edge of which is rolled up, and then may be separated from the support, and the lower surface of the part is large and the substrate is bonded. Next, the edge of the part will be off the substrate:
In extreme cases, the stress may exceed the strength of the part, causing destructive cracking of the component or deformation of the substrate: these conditions generally occur in parts with larger cross-sections, and the shear force acts longer due to excessive interface. This causes the part to deform or the substrate to deform.
Optimized design
In this case, we should first consider the stress problem in the design, that is, optimize the design and try to avoid large-area uninterrupted sintering. Try to choose a thicker substrate. Substrate heating can also alleviate such problems. For example, most manufacturers selling 316L stainless steel have a heating temperature of 80 degrees Celsius.
Change the scanning method
When we use a laser to sinter the metal powder above the substrate, the laser fills the part along a certain path and geometry. Usually this process moves the laser back and forth, a process called scanning. There are many scanning methods, such as stripe scanning, internal and external helical scanning, and oblique partition scanning.
Taking oblique partition scanning as an example, such scanning is also the way to scan most parts when printing. We can reduce the residual stress on the part by changing the scanning mode of the laser. The direction of the scanning vector is rotated when moving from one processing layer to the next, so that the stresses are not concentrated on the same plane. The layers are typically rotated 67 degrees to ensure that the scan direction is completely repeated after many layers have been processed.
These are some of the commonly used methods for improving stress residuals, but these methods are only suitable for improvement during the printing process, and the best solution is to avoid stress residual problems during the design process.About the goods
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