As a disruptive technology, metal 3D printing is widely used in industry, including a series of fields such as aerospace, biomedicine, and automobiles. Through metal additive manufacturing, the freedom of design and the flexibility of manufacturing are enhanced, and complex topography and customized production can be realized, thereby shortening the time to market. If you want to print a perfect product, the process must be checked layer by layer. The editor of Xianji.com will introduce the three factors that affect the yield of metal 3D printing: raw materials, process parameters, and thermal stress residuals.
Raw materials and consumables for the three major elements of metal 3D printing yield
Metal 3D printing takes place in a forming chamber filled with argon, where the oxygen content is less than 100 ppm to ensure that no oxides are produced during laser scanning. Moreover, the metal materials used for 3D printing have strict requirements in terms of purity, sphericity, particle size distribution and oxygen content. Common metal materials on the market now include titanium alloys, stainless steel, cobalt-chromium alloys, nickel-based alloys and aluminum alloys. The material and thickness of the metal substrate also determine the quality and accuracy of the printed product. Increasing the thickness of the substrate and increasing the temperature of the substrate can significantly suppress the warpage of the molded object and improve the dimensional accuracy of the molded object.
The influence of process parameters of the three major elements of metal 3D printing yield on energy density input
Each final part is made by melting layer by layer. With each layer melted, the platform drops, and new powder covers the layer and repeats the above process. The real forming principle is that the laser inputs a certain energy density of energy into the powder layer, so that the powder in the scanned area reaches a molten state. The energy density received by the powder is related to the input of the laser and the parameters controlled during the sintering process, such as scanning Speed, scanning distance, scanning power, laser energy form a molten pool on the surface of the metal powder, and the molten pool affects the forming effect of the surrounding powder.
The laser will scan to the molding area that needs to be melted according to a certain rule and direction, and the scanning path is reasonably normalized according to different materials. Dividing the scanning area into strips, checkerboards, etc., can effectively release the internal stress of the part, and planning each layer of scanning vector can reduce the stress value generated during the melting process.
Then in the process of selective melting, we can improve the performance of the final product through the following aspects. The following is an enlarged view of the same material at different scanning intervals. We can see that as the scanning interval expands to a certain range, very obvious internal defects will appear:
Hatch line (Hatch Spacing: scanning spacing, which controls the distance between two adjacent parallel lines in laser melting.)
Although a large scanning distance can significantly improve the forming efficiency, the range of the molten pool is limited. If the distance is too large, the overlap rate of the cladding width will be too small. If it is serious, the effect shown in Figure 3 will occur, resulting in internal defects in the formed part. . Insufficient line spacing will lead to local heat accumulation and aggravate thermal deformation.
Laser power and scanning speed are also the core parameters that determine energy density. If the energy density input is too small, the metal powder will not be sintered, and the melting will not be sufficient, resulting in residual voids between the sintered layers; if the energy density input is too large, the metal powder will be spattered due to the massive vaporization of the metal powder. , The thermal deformation caused by too high sintering temperature increases the surface spheroidization and makes the surface uneven.
The spot cannot be too large. Under the same energy density, as the diameter of the spot becomes larger, the energy is concentrated on the upper surface. The powder under each layer cannot be effectively melted under the influence of the laser molten pool, which directly affects the quality of the parts. The tensile strength of the part in the vertical direction is reduced, and it is more prone to cracks. In summary, there are many factors to consider in order to obtain the ideal printing effect. Only by constantly exploring more suitable processes can we provide the best metal 3D printing solutions and can further advance the additive manufacturing technology in various advanced manufacturing fields.
The residual thermal stress of the three major elements of metal 3D printing yield:
At present, the metal 3D printing industry is developing very fast, and it has been gradually applied to all walks of life: aerospace, automotive, medical and so on. Its advantage is that it can achieve lightweight and personalized design of parts, and can solve some technical problems that cannot be achieved by traditional processing and manufacturing methods. However, from a technical perspective, there are many problems in the 3D printing process that make our design not as expected. One of the problems with being printed out is the residual stress.
Residual stress is an inevitable product of rapid heating and cooling, which is an inherent characteristic of the laser powder melting process. When the powder is sintered by laser irradiation, the metal powder becomes a molten state, and after the molten pool is formed, it rapidly freezes and solidifies. At this time, the temperature difference between the substrate and the surface of the molded object causes strain on the surface of the molded object. Strain causes the surface of the modeling object to warp and shift, resulting in cracks on the surface of the modeling object. Therefore, in the metal 3D printing process, if you do not operate with special care, the surface of the modeling object is likely to be warped and displaced. The larger the size of the molded object, the greater the residual stress, and the more serious this phenomenon.
Thermal stress generation mechanism diagram
The laser melts the metal on the top of the solid substrate to form a new molten pool (left). The molten pool moves along the scanning vector and melts the powder, and then by transferring heat to the solid metal below, the molten powder begins to cool. After solidification, the cooled metal shrinks, and a shrinkage stress is formed between the metal layer and the next layer (right).
Residual stress is destructive. When we add another processing layer on top of another processing layer, stress is formed and accumulated, which may cause deformation of the part, the edge of which may be rolled up, and then may be separated from the support. The lower surface of the part is larger and fits the substrate. , The edge of the part will be separated from the base plate. In more extreme cases, the stress may exceed the strength of the part, causing destructive cracking of the part or deformation of the substrate.
These situations generally appear in parts with larger cross-sections, because the cross-section is too large and the residual thermal stress is too high, resulting in severe deformation or cracking of the parts.
For this situation, first of all, we should consider the problem of stress when designing, and try to avoid large-area uninterrupted sintering to reduce the degree of deformation of the parts. Choose a thicker substrate to strengthen the structural strength of the stress concentration area to reduce the degree of cracking of the parts.
Another problem is that when the molded object is separated from the substrate, the residual internal stress in the molded object is released, which will cause the molded object to warp greatly. Therefore, heat treatment must be performed to release the internal stress and then separate from the substrate. However, when the volume of the modeling object is large, or the structure of the modeling object is prone to residual stress, the heat treatment method sometimes cannot solve the problem. Regarding the method of solving residual stress, technicians will conduct thermal stress simulation analysis in the future, control the shape of the molded object where the residual stress exists, and implement local cavitation and low-density of the molded object to relieve internal stress.
We can also reduce the residual stress on the part by changing the laser scanning method, and rotate the direction of the scanning vector when moving from one processing layer to the next processing layer, so that the stresses will not all be concentrated on the same plane. Each layer is usually rotated by a corresponding angle to ensure that the scanning direction is completely repeated after processing many layers, and ultimately to ensure uniform stress distribution.
Above, the strain problem caused by internal stress in metal 3D printing and the countermeasures were briefly introduced. Increasing the thickness of the substrate and reducing the temperature of the substrate can significantly suppress the warpage of the molded object and improve the dimensional accuracy of the molded object.
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