Large-Scale and Industrialized HIP Equipment for the Densification of Additive Manufactured Parts

Abstract

Additive manufacturing technology has significant advantages in fabricating parts with complex shape, but the internal defects, such as residual stress, pores and microcracks, would result in critical problems under certain circumstances. To meet the requirement of HIP treatment on additive manufactured parts, we studied the thermodynamic behavior of the gas medium under high temperature and high pressure conditions, explored the deformation discipline of the thin-walled parts and the boundary conditions of controlling deformation, and optimized the process of eliminating residual stress. Based on the above work, a series of HIP equipment were specially designed for the treatment on additive manufactured parts, which could provide solid support for the development of additive manufacturing technology. 1. Advantages of additive manufacturing technology Additive manufacturing is an advanced technology widely developed in the world. Based on the material, it contains rapid forming of plastic, wax, ceramic and metal. Among those, rapid forming of wax-based material combines additive manufacturing and casting technology, which has been widely used for the industrial production of castings. Besides, additive manufacturing of metallic material is the most impressive, which provides universal process for the direct fabricating of key parts with complex shape. With no need of mold and the short manufacturing cycle, it has become the best process for preliminary examination and small-batch production. Recently, additive manufacturing of metallic material mainly focus on super alloy, ultra-high strength steel, titanium alloy and aluminum alloy. Due to the high cost, the application areas are limited in aerospace, military and biomedical industries. In the future, with the development of additive manufacturing and reducing of cost, it will play an important role in more fields. 2. Defects of additive manufactured parts and improvement 2.1 Analysis on defects of additive manufactured parts Compared with traditional technology, additive manufacturing has several advantages. However, due to the uniqueness of the forming process, internal defects tend to appear easily [1]: Types of internal defects result from 1) Residual stress: Thermal strain and residual stress generate due to the high temperature gradient; 2) Spherulization effect: When the laser or electron beam is irradiated, metal powders are partially melted to form a molten pool. Under a certain force, the melt tends to be spherical, resulting in poor surface quality, low density and emergence of pores; 3) Cracks: In the forming process, metal powders undergo rapid heating and cooling. There is no enough liquid metal supplementation during solidification, and the solidification part is bound by the cold substrate, resulting crack. 4) Pore formation: Pores may generate from residual gas during rapid solidification, reaction of carbon and oxygen in the melt, reduction of the metal Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 47-57 doi: http://dx.doi.org/10.21741/9781644900031-7 48 oxide by carbon, volatile of solid material, evaporate of moisture and coagulation shrinkage of sintered layer. Although defects could be reduced by optimization of the process parameters, producing large defect-free parts with complex shape through additive manufacturing is still a great challenge. Fig. 1 Effects of SLM parameters on the micro-defects[1] Compared with traditional technology, fabricated additive manufactured parts show lower mechanical performance because of defects. Besides, due to the manufacturing orientation, the performance shows anisotropic character. Rottger [1] studied the mechanical properties of 316L stainless steel prepared by selective laser melting (SLM) process in different forming directions. It was found that the tensile and yield strength of the specimens built in the horizontal direction are usually higher than in the vertical direction (Tensile specimens were built-up horizontal as well as vertical to the building platform). The reason is that the connection of a molten pool to the neighboring areas in the same slice layer is better than the connection to the underlying slice layer. Performance degradation and anisotropic distribution caused by defects will greatly affect the application of key parts, especially those with complex shape and thin walls. Therefore, after additive manufacturing, further eliminating the defects through HIP improving properties becomes a more critical process. 2.2 Research progress of HIP post-treatment on additive manufactured parts Hot isostatic pressing is the best process to improve the performance of additive manufactured parts. Many researchers studied the effect of HIP treatment on the defects, microstructure and mechanical properties of additive manufactured 316L stainless steel [2], titanium alloy Ti-6Al4V [3-7] and super alloy [8, 9, 10]. Results show that the effect of HIP on the elimination of residual pores was related to the initial state of powder material and the atmosphere during additive manufacturing process. For the pores formed inside the original powder particles during atomization process, residual argon was trapped in the pores and could not disperse to the surface at high temperature because of large atom size. Therefore, under the high pressure of HIP, the size of the pore was gradually reduced. While the internal pressure was equal to the HIP working pressure, the pore size reached the limit. Furthermore, if under subsequent high-temperature heat Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 47-57 doi: http://dx.doi.org/10.21741/9781644900031-7 49 treatment, these types of pores are likely to re-grow. A related study found and confirmed this result (seen in Fig. 2[6]). However, for the pores and microcracks formed by incomplete fusion of powders during additive manufacturing process, the effect of HIP on the elimination of residual pores was related to the process atmosphere. SLM process is carried out under argon atmosphere, so part of argon will be blocked in the residual pores or microcracks, resulting in incomplete closing of defects during HIP treatment. However, the argon pressure during SLM process is much lower than that during atomization, the closing effect of HIP on pores and microcracks from additive manufacturing will be better than that formed from atomization. Electron beam melting (EBM) process works under vacuum, and thus the effect of HIP on the elimination of residual pores and microcracks could achieve the best results because there is no residual gas inside the defects. Fig. 2 3D visualisation of the porosity (red) imaged by CT scans of the same cylindrical sample (build direction vertical) (a) as-built; (b) following HIPing; (c) 10min at 1035 °C; (d) 10 h at 1035 °C; and (e) 10 min at 1200 °C. [6] Despite the fact that the process atmosphere affects the densification effect of HIP on additive manufactured parts, HIP process still contributes to the amount and size reduction of residual pores and microcracks, even for SLM parts manufactured under argon atmosphere. Therefore, HIP has an excellent effect of eliminating the residual pores and microcracks in additive manufactured parts and improving the mechanical properties (seen in Fig. 3 and Fig. 4 [4]), ensuring applications under various conditions. Fig. 3 Tensile properties for three conditions: As-Built, stress relieved and HIPed.[4] Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 47-57 doi: http://dx.doi.org/10.21741/9781644900031-7 50 Fig. 4 S–N curve fatigue results for all three conditions: As-built, stress relieved and HIPed. [4] Treatment of residual stress is different from that of pores and microcracks. On the one hand, the residual stress could be reduced by decreasing the temperature gradient through preheating of the powder bed (as for EBM process). On the other hand, the residual stress could be relieved by subsequent annealing process (The stress relief parameters for additive manufactured Ti-6Al-4V: 650 °C, 5h). Besides, the stress relief process can be carried out in a conventional annealing furnace and there is no need to apply such treatment in HIP equipment. Therefore, for additive manufactured parts, stress relief annealing will be firstly carried out, and then HIP treatment will be applied to reduce or eliminate pores and microcracks. Finally, heat treatment is utilized to achieve the required mechanical properties. 3. Characteristics of CISRI-HIP for industrial application of additive manufactured parts 3.1 Characteristics and analysis of CISRI-HIP 3.1.1 Characteristics of CISRI-HIP With the development of additive manufacturing technology, key parts tend to be large and complex. In response to this trend and the urgent need for hot isostatic pressing to accommodate these complex parts, CISRI fully upgraded the existing hot isostatic pressing equipment and developed specialized ultra-large HIP equipment for large-scale production of additive manufactured parts. The specific characteristics of this equipment are as follows: 1) CISRI has developed extra large HIP with diameter > 2100 mm and height > 4500 mm, to meet the HIP post-treatment demands of additive manufactured parts with larger size, more complex shape and more internal defects. 2) CISRI-HIP has the function of preventing the deformation of the additive manufactured parts from happening again, to meet the demands of stability for additive manufactured parts during charging and under high pressure condition. For this, CISRI developed deformation prevention devices to improve the stability during charging and under high-pressure air flow condition. 3) CISRI-HIP has the function of ensuring the temperature uniformity for single large-size parts or high-volume products with small size dur