Conventional forging combined with subsequent near-β and two-phase field heat treatments (NTH) is an attractive method to obtain a tri-modal microstructure in TA15 Ti-alloy. However formation and evolution of tri-modal microstructure mainly depend on NTH and distortion energy caused by forging. In this paper, influences of NTH parameters on the volume fraction and size of equiaxed αp and lamellar αs in tri-modal microstructure are revealed and the appropriate NTH conditions to obtain tri-modal microstructure with excellent comprehensive mechanical properties are determined.

•Evolution law of the lamellar α phase during two-phase field heat treatment.•Transformation mechanisms of the lamellar α phase during the heating and holding processes.•Branched lamellae formed during air cooling after two-phase field heat treatment.•Cooling mode coupled with insulation time affect the microstructure significantly.Two-phase field heat treatment experiments have been performed to study the evolution of the primary lamellar α phase in TA15 alloy during the whole heat treatment process. Meanwhile both the evolutions of the primary and secondary lamellar α during air cooling with different insulation histories have been examined. Results indicate that the transformation mechanism for the primary lamellar α phase during heating is the transformation of the thin lamellae at lower α/β region and the dissolving of the small lamellae originated from the decomposition of the branched lamellae at higher α/β region. During the holding process, the volume fraction of the primary lamellar α decreased, the average thickness increased and the variation trend of the average length is complex. When the cooling mode after two-phase field heat treatment is coupled with the insulation history, the variation trend of the volume fraction shows a non-monotonous way.

•Formation mechanism of flat, zig-zag and mixed αGB was revealed via experiment.•A unified growth model was proposed by considering critical length Lc of flat αGB.•Lc lies on GB characteristic and energy, αGB VS, and chemical driving force.•Inhomogeneous nucleation on HAGB and deviated growth result in zig-zag αGB.•Nucleation at TJ and β GB simultaneously and competitive growth lead to mixed αGB.The growth mechanisms of the flat, zig-zag and mixed secondary grain boundary α (αGB) phase in TA15 Ti-alloy were investigated through heat treat experiment (cooled from β-phase field at a constant rate combined with interrupted water quenching). A unified growth model was proposed by introducing the relationship between the critical length of flat αGB (Lc) and β grain boundary (GB) length. The Lc is related to the characteristic and energy of the host GB, possible variants and habit plane of αGB, and chemical driving force. αGB preferentially nucleates at a triple junction (TJ) and extends on one side of β GB to form the flat morphology. Deviated growth of the heterogeneously and separately nucleated αGB on a high-angle grain boundary results in zia-zag morphology. The driving force and time available for αGB growth on the undecorated β GB determines its type, connected or unconnected. The flat and zig-zag αGB showed a competitive growth, and if αGB nucleated at the TJs and in the middle of the β GB simultaneously, the mixed αGB would appear.

For TA15 Ti-alloy, conventional forging combined with subsequent heat treatment provides a new method to obtain a tri-modal microstructure (consisting of about 20% equiaxed αp, 50–60% lamellar αs and transformed β matrix) possessing attractive comprehensive properties, and it is expected to solve the problems (such as, deformation temperature control, microstructure coarsening, or special requirement for the original microstructure) caused by using the existing method. In this paper, the forming process and principle of obtaining a tri-modal microstructure under conventional forging combined with given subsequent near-β and two-phase field heat treatments (NTH, i.e. 975 °C/30 min/WQ + 930 °C/100 min/AC) were investigated. Meanwhile, taking the volume fraction and morphology of equiaxed αp and lamellar αs as targets, the tri-modal microstructure evolution under different conventional forging conditions (deformation temperatures, deformation degrees, strain rates, cooling modes) and significance of influencing factors were revealed.

•Reveal action of temperature drop before WQ on formation of tri-modal microstructure.•Get tri-modal microstructure in TA15 via near-β forging (AC + WQ) + HLT.•Toughening temperature determines mainly volume fractions of αp and αs.•Morphology and size of αp and αs lie on both toughening and strengthening treatments.•Get reasonable toughening and strengthening conditions for tri-modal microstructure.For Ti-alloy tri-modal microstructure can be obtained by the near-β forging (water quenching, WQ) + high temperature toughening and low temperature strengthening treatment (HLT) method. However in actual production, it is hard to accomplish water quenching after near-β forging immediately. In this paper a short time of air cooling (AC) after near-β forging was introduced to consider the temperature drop during the forgings transferring in actual production. For TA15 alloy the formation process of tri-modal microstructure in near-β forging (AC + WQ) + HLT was studied. Under the given near-β forging condition, the influences of the subsequent toughening and strengthening treatment on the obtained tri-modal microstructure were revealed as well as on the corresponding mechanical properties. Then the reasonable HLT route for tri-modal microstructure with excellent comprehensive properties was determined.

•Growth models for grain boundary and intragranular Widmanstatten α were proposed.•Clustered lamellar αWGB grows via adjacent lamellae merging and subsequent coarsening.•Holes in thick lamellae expand and further fracture result in forked αWI.•Merger of residual thick αWI with thin tertiary parallel αWI results in double forked αWI.The morphology evolution and growth mechanism of secondary Widmanstatten α phase in the TA15 Ti-alloy, including the clustered grain boundary Widmanstatten α (αWGB), and forked intragranular Widmanstatten α (αWI), were investigated and the growth model were proposed. Results indicate that the clustered lamellar αWGB grows via the merger of adjacent thin lamellae and subsequent coarsening. Two types of forked αWI exist: a forked αWI with short and thick branches, and a forked αWI with thin and long branches. They results of the fracture of thick lamellar αWI and merger of residual thick αWI with the newly precipitated thin lamellar αWI, which showed a clustered distribution and paralleled the residual thick αWI, respectively.

For TA15 Ti-alloy two types of tri-modal microstructures with excellent comprehensive mechanical properties were obtained via a new approach, near-β forging combined with the solution and aging treatments (SAT), regardless of whether there was a short time of air cooling (AC) during forgings transferring in practice before water quenching. Performance of tri-modal microstructure, with different morphologies and contents of constituent phases (primary equiaxed αp and secondary lamellar αs), was investigated. More and larger αp contributes to higher plasticity, while more and thicker αs benefits for room and high temperature strengths and impact toughness. The formation of tri-modal microstructure via near-β forging+SAT was investigated. AC may change the formation process of lamellar αs and the stored distortion energy in as-forged specimen, and further results in more and larger αp and fewer and thinner αs in final tri-modal microstructure, leading to lower strength but higher plasticity and fracture toughness. The dependences of near-β forging conditions on the obtained tri-modal microstructure via near-β forging and SAT were revealed. The tri-modal microstructure and its performance for service need can be adjusted via control of air-cooling time before water quenching combined with near-β forging conditions selection.

The microstructure evolution and mechanical properties in different loading regions during isothermal near-β local forging were investigated using three TA15 Ti-alloy workpieces of different sizes, namely, a cylindrical sample, a quadrate billet, and a bulkhead component. Reasons for differences in the microstructure and mechanical properties were determined. Fine primary equiaxed α and clustered acicular secondary α were obtained for the small cylindrical sample, and the different loading regions exhibited good uniformity in microstructural morphology and size. Significant differences in microstructure existed between the first and second loading regions for the quadrate billet and bulkhead component, especially in the secondary lamellar α, which resulted in different mechanical properties. Specifically, coarse primary equiaxed α and straight and thin clustered lamellar α existed in the first loading region, and less coarse primary equiaxed α and thickened and globularized secondary lamellar α with short rod-like or equiaxed shape existed in the second loading region. The holding time and different cooling rates caused by the workpiece sizes resulted in differences in their primary equiaxed α and secondary lamellar α size. The combined effects of strain history, holding time, and cooling rate led to microstructural differences in different regions for the quadrate billet and bulkhead component, whereas the effect of strain history was reduced for the cylindrical sample because of the short holding time and fast cooling rate, and little difference existed between the first and second loading regions.

For TA15 Ti-alloy in near-β forging and subsequent heat treatment, the evolution of equiaxed α is complex and difficult to control, but a tri-modal microstructure has strict requirements of the volume fraction, size and so on for equiaxed α. In this paper, a prediction model based on improved BP neural network was adopted to investigate quantitative evolution laws of the volume fraction, average grain size, and average aspect ratio of equiaxed α under different deformation temperatures, degrees and strain rates in near-β forging and subsequent high and low temperature double heat treatments (HLT, 950 °C/100 min/WQ+800 °C/8 h/AC). Then, taking the tri-modal microstructure as target, the control of equiaxed α was realized and a reasonable processing parameters match of near-β forging under HLT treatment was determined. Finally the reliability of prediction model and results were verified through experiments, and the tri-modal microstructure with excellent mechanical properties was obtained. The results provide a guide for obtaining a tri-modal microstructure of Ti-alloy through the near-β forging technology.

In triple valve forming process by multi-way loading severely nonuniform deformation and temperature distributions are prone to occur, which may lead to poor forming quality and macro-micro defects. A 3D coupled thermo-mechanical rigid-viscoplastic finite element (FE) model for multi-way loading forming of AISI-5140 steel equal diameter triple valve was developed based on DEFORM-3D. Through comprehensive simulation and analysis, the influences of main process parameters on the forming process and nonuniformity of deformation and temperature were studied. The results showed that: (1) the degree of deformation nonuniformity decreased with the increase of the punch loading speed, initial temperature of billet, or the decrease of the friction factor; (2) the average temperature of forming body increased as the punch loading speed, initial temperature of billet and the friction increased, while the degree of temperature nonuniformity decreased with the increase of punch loading speed or decrease of initial billet temperature.