Simulation And Experiment Of Near Net Shaping Ti6A14V Leaf Disc Under Hot Isostatic Pressing

Hot isostatic pressing(HIP) is an important powder metallurgy manufacturing technology that can meet the performance requirements of the next generation of aero-engine. It has the advantages of saving materials and good mechanical performance close to the forgins. It is very suitable for forming aircraft-grade titanium, nickel alloy parts. The density and size of HIPped parts are decided by the design of capsule and core mold and the parameters of HIP process. Numerical simulation can predict the final shape of HIPped parts and be used as a guide to optimize the design of capsule and core mold.In this paper, Ti6Al4 V alloy powder was chosen as the forming materials because of its versatility. A leaf disc with blade tips shroud was designed as well as the corresponding capsule and core mold. Then HIP experiment was carried out and good shape leaf disc was made. In this paper, the entire process of numerical simulation was discussed including picking up powder yield criteria, choosing constitutive equations, giving finite element formulation and deciding boundary conditions. The numerical simulation results were compared to the experimental results. What?s more, HIP experiments under process of in-phase increase of temperature and pressure(process 1) and process of firstly increasing temperature and secondly increasing pressure(process 2) were carried out. The microstructure, tensile properties and fracture morphology, fatigue property and fatigue fracture morphology, room temperature impact toughness of specimen under both process 1 and 2 were observed. The tensile fracture mechanism and fatigue failure mechanism were discussed. To solve the problem that it is difficult to make the parts with complex structure dense, this study proposes a two-step hot isostatic pressing method: at a low temperature and pressure shaping original parts without connected pores from alloy powder, then to dense the low density regions at an appropriate temperature and pressure after removing capsule and shape-control mold, which can guarantee uniform performance. Two-step HIP parameters are determined by combining finite element simulation and experimental tests. The main research contents and results are as follows:(1) The modified Von Mises yield criterion with two parameters based on the porous structure was adopted as the powder material yield criterion. Perzyna constitutive model was picked to describe Ti6Al4 V alloy powder?s viscoplasticity deformation behavior during HIP. The simulation deformation result is a little bigger than that of experiment in both X and Y direction. The maximum error is 4% in the radial direction and 2.27% in the axial direction. The error is controlled within a satisfactory range and the simulation method can predict the final HIPped parts shape well.(2) High resolution XPS spectra result showed that Ti6Al4 V powder surface has a certain thickness of the oxide layer that has a negative influence on the HIPped parts. The Microstructure of two different HIPed parts were observed.The alloy phase consists primarily of grey(black) ? phase and white ? phase and ? phase distributes along the boundary of ? phase. Microstructure of a process 1 can be observed obvious equiaxed zone that was continuous space network structure,which was not found in parts shaped by process 2. Microstructure of both two processes exist the cluster of ? phase of different length and the maximum length of the cluster is about 200?m. The cluster of process 1 is almost straight which of process 2 is short, coarse and twisted. The corresponding tensile strength ?b is 970 Mpa and 980 Mpa and the yield strength?0.2 is 876 MPa and 880 MPa. The elongation ? is respectively 9.1% and 10%. Room temperature tensile properties of process 2 were almost equivalent with that of process 1. Performance of process 2 is slightly higher than process 1. Fracture analysis shows: fracture of specimen under process 1 and process 2 is typical of quasi-cleavage fracture type.(3)The specimen fabricated under process 2 has better fatigue performance with the fatigue limit sress of 450 Mpa than specimen under process 1 with the fatigue limit stress of 350 Mpa. However, the fatigue performance of specimen under both processed is worse than that of aero-grade forgings(539Mpa, f = 130 Hz, bars). The fracture shows: fatigue crack initiation from prior particle boundaries(PPB) leads to fatigue failure under cyclic stress. Oxygen content of the source region is higher than the crack extension area, indicating that the oxygen content of particulate impurities affect the metallurgical bonding and hi gher oxygen content of the area tend to fatigue defect initiation. Optimized process 2 can promote fatigue performance by reducing the impact of oxide impurities.(4)The Charpy impact toughness at room temperature of specimen under both processes was tested. The specimen fabricated under process 2 has better impact toughness performance with the impact toughness of 55.1 J / cm2 than specimen under process 1 with the impact toughness of 44.2 J / cm2.(5) Two-step HIP process can guarantee good shape-control performance and high density of region that hard to be dense when forming superalloy parts. The uniform and high performance contributes to long life of parts. The method to determine two-step HIP process parameters is carried out. Finite element simulations and experiments are combined to determine the most suitable parameters according to specific material. Among all the simulation tests with different parameters combinations, the lowest temperature and pressure combination is picked for the first-step HIP if it can guarantee no connected pores remaining in any region of compacts. For the second-step HIP process, the parameters combination leading to the best mechanical properties is picked. For the example part discussed in this study, the parameters combination(850 ?, 100 Mpa, 3h / 910 ?, 120 Mpa, 3h) is the most suitable for it. The fracture morphology of specimens cut from part formed with appropriate two-step HIP process doesn?t remain PPB, and no near spherical pit is found. The tensile property of part formed by suitable two-step HIP process is equal to that of forging parts with the same size scale and slightly better than that of parts with the same size scale formed with conventional HIP process.