Study On Hot Isostatic Pressing Near Net Shaping Technology Of Difficult Processing Materials And Their Mechanical Properties

Many key components used in the aerospace field are made of difficult processingmaterials such as superalloys and titanium alloys. The traditional manufacturing methodsfor these materials mean high cost and material waste. Hot Isosastic Pressing (HIP)combines powder metallurgy technology and modern mold technology, is able to getcompletely densification of the powder materials and in the mean time, generally near netshape the complex parts which performance are up to the level of homogeneity forgingparts in one process. It can not only overcome the problem of difficult-manufacturing, butalso considerably improve the utilization of expensive materials, and accordingly reducethe manufacturing cost. Parts shape control problem，namely the optimization design ofcapsule, has not yet solved perfectly worldwide due to hot isostatic pressing process is acomplicated thermo-mechanical coupling, nonlinear and large deformation process. Thestudy on HIP-net-shape forming technology is much less in China,and capsulemanufacturing process is not yet mature, thus hindered the industrial promotion andapplication of the technology. Therefore, this article focuses on the optimization designmethod of HIP-net-shape capsule and manufacturing process, and solve the commonproblems in capsule design and manufacturing. On this basis, taking the net-shape formingprocess of titanium and nickel alloy typical parts as an example, research hot isostaticpressing densification mechanism of the two kinds of difficult to cut materials,organization and performance of the net-shape forming parts. Through these studies above,lay a good theoretical basis for the promotion and application of hot isostatic pressingnet-shape forming technology.The researches and results are as follow:(1) Based on the thermal elastic-plastic finite element and optimization method, thepaper present an optimization design method of net-shape forming capsule under HIP. Theoptimization design method above using parameterized B-splint curve for capsuleconfiguration. and regarding the minimum difference between simulated shape and theideal shape of the final HIP parts as the optimization objective function. Research andperfect the common process problems and formed a complete set of capsule process technology consisting of capsule parts manufacturing based on machining and RapidPrototyping Technology, gas shielded welding, airtight test with helium mass spectrometer,vibration loading techniques of the power high-temperature degassing, sealing braze.Taking316L stainless disk parts as an example, verify the above capsule design andmanufacturing process. The results show that the optimization method has a guidingsignificance for optimization design and manufacturing of the capsule, and reflectsexcellent practical value.(2) Taking Ti6Al4V alloy space-cross part as an example, research the densificationdiscipline and the forming process of complex parts under hot isostatic pressing, aiming atthe local structure of complex parts. Combined analysis of theoretical calculation anddifferential thermal analysis，the optimal heat preservation temperature of Ti6Al4V underhot isostatic pressing is950℃. When compulsory controling shape with thick capsule forhot isostatic pressing，the power densification is caused by equal plastic forming of thecapsule, and the elastic deformation of capsule can be neglected. Thick capsule may havean “amplified” effect on the densification of local part of the product, and, there is onlycontract in size but no difference in shape between the part before and after HIP.(3) Taking the net-shape of casing parts of Ti6Al4V under HIP as an example,research the densification discipline, microstructure and property of titanium alloy.Through the capsule optimization design, form casing parts with relative densification of99.8%and critical dimension error of1%. The reaserch shows that the main densificationmechanism is plastic deformation mechanism of Ti6Al4V under HIP. Because of powderparticles under static pressure，plastic deformation only occur in the local area of grainboundaries，forming a polygonal large strain zone of basketweave. Recrystallizationhappens during heat preservation and forms basket-distributed equiaxed grain and grainboundaries disappeare. The internal strain of the particles is relatively smaller, exists lessrecrystallization, where mainly exists strip shaped+β phase.The static strength andductility of Ti6Al4V under HIP are excellent due to fine microstructures and uniformcomposition, and its elastic modulus, yield strength, tensile strength and elongation afterfracture amount respectively to120MPa,1043MPa，1119MPa, and18%, which are allhigher than the minimum requirement of forgings of ASTM. (4) Taking the net-shape of turbine disk parts of Inconel625under HIP as example,research the densification discipline, microstructure and property of Ni-BaseSuperalloy.The relative density of integral formed turbine disk approachs to theoreticaldensity with runner shape undistorted and surface roughness comparable with capsule.The interrupted experiment shows the theoretical relative density is get with matrixorganization of austenitic organization and its size is equal with particle size, when theHIP parameters are1100℃,120MPa and3h, carbide precipitation happens during HIPprocess due to the powder compact is under critical temperature range of carbideprecipitation for a long time.Carbide distribute at the powder original grain boundaries inform of chain and inside grain in form of massive and in punctiform. Inconel625partsunder HIP have high strength, low plasticity due to the existence of much carbide. Theyield strength σ0.2and tensile strength σbrespectively amount to499.12MPa and983.38Mpa, respectively16%and7.5%higher than the requirement of ASTM, and theelongation after fracture is a little lower. After solution treatment, the alloy hardnessdecrease slightly and strength maintain equivalence and plastic has improved significantly.For the hipped Inconel625part with sectional dimensions of2mm×5mm, length90mm,the best solution process are1100℃30min/water cooled, compared to the formertreatment one, its hardness decreases by4.9%, no obviously difference in room temperatestrength and elongation increases by25.8%.Through the above research, provide an effective theoretical method and processroute for optimization design and manufacturing of HIP-net-shape capsule.Provides asuccessful example for HIP-net-shape forming of difficultly processing materials.Provides the experimental data for assessing and improving HIP parts of titanium andnickel alloy.