The metal 3D printing process has not yet reached the point where manufacturing is flawless, and today's best equipment vendors are struggling to fully address the problems of evaporation, oxidation, spheroidization, and thermal deformation that exist during the printing process. Although the density, strength and surface quality of parts can reach a relatively ideal level, but for metal printing, especially SLM process parts are still prone to retain pores and stress cracking problems, directly manufactured parts still need to be processed with the help of traditional means, including densification and, stress relief and so on.
Among these post-processing methods, hot isostatic pressing is an extremely important step that cannot be omitted in both aerospace and medical applications. Thermal isostatic pressing (HIP for short) is to place the product into a closed container, apply equal pressure in all directions to the product, and at the same time apply high temperature, under the action of high temperature and high pressure, the product can be sintered and densification. So what does the hot isostatic pressing process entail?
1. Densification eliminates defects
In the high temperature metal material strength is very low, excellent plasticity, there are holes in the region of the metal by the role of external gas pressure plastic deformation, holes in the region of the metal in contact with each other metallurgical bonding occurs so that the holes disappeared, all parts of the densification can be completed.
2.Improvement of organization and morphology
The high temperature heating of hot isostatic pressing process is equivalent to annealing, which can eliminate the overcooling organization or sub-stable organization formed due to the fast cooling speed in the SLM process, and transform the organization to form the high-temperature annealed form.
3. Improvement of mechanical properties
The mechanical properties of the material before and after hot isostatic pressing also change significantly. Whether SLM or EBM, hot isostatic pressing after the strength of the material has a tendency to decline, the plasticity will rise, especially for SLM process materials more obvious. The hardness of the material also changes with HIP, and the hardness decreases by 5-10% after HIP. Overall, hot isostatic pressing can improve the toughness and resistance to fatigue crack extension of the material.
It was found that hot isostatic pressing was also effective in reducing the porosity of the lattice structure, with the 4 mm cell size showing a greater response to HIP, with a 40% reduction in the void-to-volume ratio, compared to only a 22% reduction in the void-to-volume ratio for the 2 mm cell size sample. Porosity changes based on the cell size of the samples were also observed. The porosity of the HIP sample with a 4 mm cell was reduced by 57% and the porosity of the sample with a 2 mm cell size was reduced by 44%.
Common applications of hot isostatic pressing include defect repair (pore fusion) of additively manufactured parts, powder sintering, and diffusion bonding of different types of metals or alloys. For aerospace, medical, and marine applications, it is necessary to use this technology to optimize material properties and increase part life. And a single hot isostatic pressing machine can service multiple printers.
The anisotropy of the fracture properties of 3D printed high temperature alloys affects their further application in the aerospace field, which is closely related to the grain morphology of the material, the state of the grain boundaries as well as the morphology and location of the precipitated phases. After the newly designed hot isostatic pressing + solution + aging post-treatment, the room-temperature tensile properties of IN718 high-temperature alloy for both vertical and horizontal specimens manufactured by LPBF are maintained at a high level, and the average high-temperature fracture life at 650°C and 690MPa reaches 173 and 131 hours, respectively, which meets the requirements of the relevant standards.
Although hot isostatic pressing can eliminate internal defects, the use of this technology is still subject to a number of conditions, and is not suitable for any part, the paper reveals only the advantages of hot isostatic pressing. Relying on post-processing means to solve the internal defects of metal printing is only one way, the development of technology should also be adjusted from the internal process, to the direction of no defects.
