Comparative Analysis of the Hot Isostatic Pressing Densification Behavior of Uniform and Non-Uniform Distributed Powder

1. Introduction

Ti6Al4V alloy has been widely used in aviation, aerospace, ships, and other fields due to its characteristics of low density, high specific strength, strong corrosion resistance, and excellent comprehensive properties [ 1 ]. To meet the high-performance requirements of aerospace parts, the structures of various parts are becoming increasingly complex. At present, the traditional casting, forging, diffusion bonding, and 3D printing technologies have their defects and difficulties in realizing the forming of complex high-performance structural components [ 2 5 ]. As a new near-net shaping method, powder metallurgy combined with hot isostatic pressing (HIP) [ 6 ], can form complex parts. The mechanical properties of the formed parts are equivalent to those of the forged alloy. In the process of HIP, the densification process of powder under high temperature and high pressure [ 7 8 ] involves a series of complex processes such as particle translation, turnover, and plastic deformation. In addition, there is also a hot working process involving significant compression and complex deformation [ 9 ]. Moreover, it is challenging to study the HIP process of powder in real time, and these problems can be solved by numerical simulation [ 10 11 ]. With the help of numerical simulation, the test process can be significantly accelerated and the production cost can be reduced. The prediction and analysis of capsule shrinkage deformation by numerical simulation is a hotspot in the research of HIP near-net shape forming technology. With the progress and innovation of technology, titanium alloy powder metallurgy technology has been applied to load-bearing parts, such as the impeller and rotor of the F-107 engine. It is more widely used in aircraft parts. From the titanium alloy support rod and fuselage pillar of the F-14 fighter, the keel nose of TC4 alloy of the F-15 fighter, to the engine fixing support of the F-18 hornet fighter, the powder metallurgy of the HIP process is widely used. The utilization rate of materials increased from 10%~35% of the casting and forging process to 50%~60%, and the cost is generally reduced by more than 25% [ 12 13 ]. Vinci engine on the French Ariane space rocket adopts a liquid hydrogen impeller fabricated of powder metallurgy, which not only reduces the number of parts of the impeller and the production cost, but also improves the propulsion ratio and prolongs the service time of the engine [ 14 15 ]. The multi-type powder TC4/TA15 rudder wing frame products developed by the Institute of Aerospace Materials and Technology, have the advantages of high forming accuracy, good surface quality, and reasonable internal defect control. The material properties fully reach the forging level, and the net size of the whole wing reaches 2200 mm [ 16 ].

Many scientists have researched the numerical simulation of the HIP technology. You et al. [ 17 ] used the Shima model to simulate and compare the HIP forming process of milled and atomized Ti6Al4V powders. The maximum error between the experimental and simulated results is 2.66%. Numerical simulation is accurate and effective in predicting experiments. Xu et al. [ 18 ] studied the microstructure and formation mechanism of TC11 powder titanium alloy. The HIP temperature is 930 °C and the holding time is 2 h. The finite element simulation is carried out with Deform3D software. Procopio et al. [ 19 ] used the multi particle finite element model (MPFEM) to explore the densification yield surface of a two-dimensional single-size particle set. A single particle discretized by finite element mesh can fully describe contact mechanics and local and global particle kinematics. The hydrostatic pressure and powder’s densification process under different friction levels between particles were studied. The isodensity curve in the stress space during densification shows the equivalent shape of the cap cone model, and the slope of the shear failure line is a function of the friction between particles. MPFEM provides the necessary degrees of freedom to allow local non-uniform contact deformation while capturing the statistical characteristics of the powder. Therefore, the model can deal with corners, non-affine motion, particle rotation, and large deformation under high relative density. The corresponding DEM of the discrete element method is only suitable for the case of relative density <0.85. Kohar et al. [ 10 ] used the finite element simulation (FEM) method to simulate the HIP process to avoid trial and error methods in product and process development. The finite element simulation of the HIP process requires that the constitutive model can capture various deformation mechanisms in powder densification, such as plasticity and creep. Since the HIP process may last for several hours, the realization of these values needs to be accurate and efficient. They use the finite element software LS-DYNA to simulate the HIP process of the 304/316L stainless steel jacket. Yuan et al. [ 20 ] of the University of Birmingham in the United Kingdom assumed the powder was a compressible porous medium. They used the plastic theory finite element model to describe the powder material. The complex casing product was analyzed by numerically simulating the near-net forming process of the titanium alloy TC4 powder. The simulation results were compared with the test results for the deformation and relative density distribution. The errors in each direction of the simulated dimensions are all less than 2%. Lu et al. [ 21 ] of Huazhong University of Science and Technology used the modified Robin hyperbolic sine power rate creep model to predict the plastic deformation behavior of Inconel625 alloy powder. The turbine disc parts with complex shapes were formed and combined with the SLS investment mold to manufacture the shape control core. The simulation error of deformation is 3.27%, and the density error is 2.37%. Xu et al. [ 22 ] of Beijing University of Aeronautics and Astronautics used MSC.MARC software to study the numerical simulation process of HIP of thin-walled aluminum alloy complex parts, and the numerical simulation results of the characteristic structure error is about 5%. Baccino et al. [ 23 ] proposed the ISOPREC forming process system called HIP near-net shape forming technology. Their products, including blade discs and shaft parts, can be controlled with a forming accuracy of 0.1 mm. Chung et al. [ 24 ] introduced the finite element simulation and optimization method into the field of HIP capsule design and proposed the capsule optimization design prototype system. They used Abouaf’s constitutive model and his colleagues, adopted the optimization design technology based on the design sensitivity evaluation scheme, and obtained the required cladding structure based on the rectangular target parts through reverse iterative optimization calculation. Wang et al. [ 25 ] studied the influence of different thicknesses of capsule on HIP. The shielding effect of the sheath on the force in the process of HIP is noted. The smaller the thickness of the package, the greater the pressure on the powder body. The molten liquid phase inside the powder is extruded and filled into the gap between the powder particles. The irregular solid phases are extruded and self-locking with each other. The liquid phase filled at the junction of the powder particles promotes the bonding of the particles. It improves the welding degree between the particles and the densification degree of the material. Therefore, the tensile strength, yield strength, and elongation after fracture of powder parts can be improved.

The present research shows that it is feasible to predict the powder deformation process in the HIP process and optimize the design of the capsule and die through numerical simulation. However, most previous studies did not consider the effect of initial powder-filling density in the numerical simulation. This parameter is generally set to 65% (100% represents that the material is entirely dense or the theoretical density). Moreover, the set temperature is evenly distributed in the container, which needs to attract more attention, resulting in inevitable errors in predicting the near final forming results [ 26 ]. This phenomenon may be related to the non-uniform distribution of density after vibration and the temperature gradient of powder particles in the capsule. To reveal the possible causes of this phenomenon, numerical simulation can be carried out by the finite element method before HIP to study the influence of the alloy powder’s initial relative density distribution on its densification process. Numerical simulation by finite element method can not only visually analyze the densification process and stress field distribution of powder but also monitor the deformation of powder in the simulation process and optimize the capsule structure through its deformation [ 6 27 ]. In most of the research, the target structure is relatively simple or has a small size. This parameter has little effect on the deformation law of powder in the HIP process. It has little effect on its dimensional accuracy and is often classified as an error. However, for complex part structures or large-size parts, this problem has a significant impact on practical production and applications.

In this paper, the HIP process of Ti6Al4V powder is numerically simulated and experimentally studied. The improved Shima model [ 28 29 ] describes the powder and studies the thermal-mechanical coupling deformation. The material parameters of Ti6Al4V powder are determined through relevant physical experiments, and the HIP process parameters are determined based on previous research. By comparing numerical simulation with experiments, the powder’s densification process and stress distribution of the capsule are analyzed for uniform and non-uniform initial powder distributions. Finally, the feasibility of the simulation method is verified through the sampling analysis of relative density.