Research On Building And Application Of Fracture Criterion In Hot Deformation Based On Accumulated Plastic Energy

Metal hot forming technology plays an important role in the manufacturing industry.It has the merits of high productivity, stable quality and effective utilization of rawmaterial. The mechanical properties of the metal are also improved in the formingprocesses. In metal forming, workability or formability of material is one of thefocusing subjects, which is mainly controlled by ductile fracture.With the developmentof computer and its correlation technique as well as the continuous improvement ofdamage fracture mechanics, it has been one of the frontier researches to introducedamage fracture mechanics into finite element (FE) analysis. However, it is limited toapply existing ductile fracture criteria to predict the hot deformation fracture due to bebuilt based on room temperature, and it is difficult to determine the critical damageparameters at the high temperature and high strain rate. This makes the establishing ofhot deformation fracture criteria and how the accuracy prediction of hot deformationfracture can be achieved become the challenging problem that needs to be solvedurgently for development of metal hot forming technology. Therefore, based on as-castTi60titanium alloy as the research object, this paper studied the hot deformationbehavior of the alloy, revealed the fracture behavior of the alloy and its mechanism,established the hot deformation fracture criterion. Combined with the finite elementsimulation analysis technology, the process parameters for open die forging of large sizeTi60alloy could be optimized and verified. The main research contents and results areas follows:Based on the analysis of the stress-strain behavior, inverse proportion between themagnitude of the discontinuous flow stress reduction and the average grain size wasrevealed, and the strain compensated constitutive mode was developed. Based on theDynamic Material Model and the Prasad’s instability criterion, the processing maps forthe alloy were constructed at strains of0.4and0.7. The stable and unstable regimeswere verified, and the better hot-working region of950~1050°C/0.01~0.1s-1wasbotained, which was ideal hot-working region for achieving recrystallization anddecreasing fracture.Fracture behavior and mechanism of as-cast Ti60titanium alloy were investigatedsystematically based on observing of hot compression deformation specimen. The mainfracture modes consist of45°shear fracture and longitudinal fracture on free-surface. Moreover, the cracking degree increased with the decreasing of deformationtemperature, the increasing of strain rate and height reduction. The cracks nucleationsites were observed at β grain boundary, α/β phase boundary and alpha-case of thespecimens, and the cracks tended to expand along β grain boundaries or α/β phaseboundary.A novel high-speed photography was introduced to determine the critical fracturestrain of Ti60titanium ally during hot compression deformation. This methodeffectively addressed the problems of capturing the initial crack due to the randomnessof crack position, and dark as well as unstable observation field by two high-speedcameras fixed on opposite sides of the specimen, strong fill lights and Professionalphoto frames. It improved the accuracy of location and time for capturing the initialcrack.Based on the theoretical analysis of the existing typical crack criterion and predictionof the crack location and the critical damage value, it can be conluded that Frudenthal,could predicte the center cracking, C&L, O&K, Brozzo, McClintock, Oyane and R&Tcriteria could predicte the free-surface cracking, which was in a good agreement withexperimental result. Further, The O&K criterion could synthetically consider stressfactors, which need to be considered when a fracture criterion is established, and to acertain extent, was equivalent to the Kuhn experience criterion and McClintock emptycombined model. Based on tensile stress caused by the cracking for the alloys, the O&Kcriterion in accumulated plastic energy models was chosen to be modified forestablishing the hot deformation fracture criterion.Based on O&K criterion in accumulated plastic energy models, a new hotdeformation fracture criterion considering strain rate and temperature effects was builtand could be modeled as the function of Zener-Holloman parameter. By combining theDEFORM-3D simulation software and FROTRAN language, the prediction model hadbeen implanted in the FE software, and the coupled FE model of deformation-heattransmission-damage was established. Comparison between the experimental resultsindicated that the proposed model was useful in precisely predicting the fractureinitiation in the upsetting of Ti60alloy ingot.In terms of the built fracture criterion for hot deformation, with the goal of fullybreaking the coarsed as-cast organization and avoiding fracture, the process parametersfor open die forging of large size Ti60alloy ingot, focusing on different anvils (flatanvil,120oV-shaped anvil, and U-shaped anvil), height reduction and speed as well asaxial feed in single pass, and different process scheme (stretch forging, stretch forging with upsetting) in multi-passes, could be optimized by quantifying the damage valuepredicted by the proposed fracture criterion.The result shows that the cracking on surface was severe and defect called “end-faceindentation” was observed when stretch forging using flat anvil, and the flat anvil wasnot changed during cogging process. On the other hand, slight cracking surfaces andlarge strain in center-part were obtained when stretch forging using120oV-shaped anviland U-shaped anvil. However, in practical open die forging, the size of U-shaped anvilwill change with that of workpiece, which results in a low efficiency.Optimized height reduction and speed as well as axial feed in single pass were30%,10-15mm/s, and130mm respectively. Based on optimization of process parameters insingle pass, both slight cracking surfaces and large center strain were acquired after fourpasses stretch forging or stretch forging with upsetting. Moreover, it can be found thatthere was much more homogeneous and larger the strain at the hexagonal cross-sectionof the billet after stretch forging though rotating45°than that at the square cross-sectionof the billet after stretch forging though rotating90°. Finally, based on the optimalcogging process parameters, experimental results for the cogging of large-size Ti60titanium alloy ingot investigate that the fracture of can be avoided.