Isothermal Multidirectional Forging Research Articles

Achieving high strength-ductility synergy in TiBw/near α-Ti composites by ultrafine grains and nanosilicides via low-temperature severe plastic deformation

The common problem of low strength-ductility matching prevails in near-α high-temperature titanium matrix composites (TMCs). In this work, the design strategy of ultrafine grains and dispersed (Ti, Zr)6Si3 nanoprecipitates in the microstructure of TiBw/near α-Ti composites via low-temperature isothermal multidirectional forging (IMDF), is expected to break the trade-off dilemma between strength and ductility. The results show that with the decrease in the temperature of IMDF, the grain scale decreased from 0.98 to 0.59 μm, and the location of silicide precipitation shifted from phase boundaries and grain boundaries to α-grain boundaries and intracrystalline regions. The experiments confirm that the local segregation of Si and the temperature of thermomechanical deformation are the key factors affecting the precipitation behavior of silicides. With the decrease of the deformation temperature, the precipitation mechanism of silicides changes from a single diffusion-controlled precipitation to the coupling of two mechanisms, namely, elemental diffusion and dislocation-assisted nucleation, which facilitates the successive precipitation of nanometer-sized silicides at the grain boundaries and in the inner regions. The ultimate tensile strength (UTS) and elongation of the composites were substantially increased after IMDF at 950 and 800 °C, especially the excellent performance at 800 °C, where the strength reached 1320.3 MPa and the elongation was 5.8 %. The room and high-temperature strengthening and failure mechanisms of the composites are analyzed and discussed, and the yield strength (YS) increments provided by various strengthening mechanisms at room temperature are quantified, aiming to provide a potential preparation strategy for the synergistic strengthening of near-α TMCs with ultrafine grains and nanoparticles.

Microstructure Evolution and Constitutive Modelling of Deformation Behavior for Al-Mg-Si-Cu-Sc-Zr Alloy Processed with Isothermal Multidirectional Forging

This research is devoted to the microstructure evolution and deformation behavior of the Al-1.2Mg-0.7Si-1.0Cu-0.1Sc-0.2Zr alloy during the isothermal multidirectional forging (MDF) in a large cumulative strain and temperature range. The structure investigation of the studied alloy revealed several phases precipitated during solidification, among which θ(Al2Cu), Q(Al5Cu2Mg8Si6), Mg2Si, Sc-bearing W(AlScCu) and V(AlSi2Sc2) phases were observed. The MDF at 150–350 °C and a maximum cumulative strain of 14.4 significantly refined grain structure providing a mean grain size of 1.2–2.1 µm. The L12 structured Al3(Sc,Zr) dispersoids with a mean size of 10 ± 1 nm were formed during two-step homogenization annealing. Due to Zener pinning of the nanoscale dispersoids and fine-grained structure, the alloy exhibited near-superplastic behavior in a temperature range of 460–500 °C and strain rate range of 2 × 10−3–1 × 10−2 s−1 with the maximum elongation to failure of ~300%. After a strengthening heat treatment, the forged alloy exhibited the yield strength of 326 ± 5 MPa, ultimate tensile strength of 366 ± 5 MPa, and elongation of 10 ± 3%. The hot deformation behavior was described using the Arrhenius type model. The developed model demonstrated high predictability accuracy with a maximum average absolute relative error of 6.6%.

Particle Stimulated Nucleation Effect for Al-Mg-Zr-Sc Alloys with Ni Addition during Multidirectional Forging

The study aims to investigate the influence of fraction of coarse undeformed particles on the microstructure evolution and mechanical properties of alloys processed by isothermal multidirectional forging (MDF). For this purpose, Al-Mg-Ni-Sc-Zr-based alloys with different Ni concentrations and a fraction of Al3Ni particles of solidification origin phase were subjected to MDF at 350 °C. Precipitates of the L12-structured Al3(Sc,Zr) phase retained their structure, morphology, and size after MDF and were coherent with the aluminum matrix. The Al3Ni phase particles stimulated the nucleation of recrystallized grains and contributed significantly to the formation of an ultrafine-grained structure. The uniformity of the grain structure increased, and the average grain size decreased with an increase in the fraction of Al3Ni particles. A fine-grained structure with a mean grain size of 2.4–3.4 µm was observed after MDF with a cumulative strain of 12. The results demonstrate that a bimodal particles size distribution with a volume fraction of nanoscale f~0.1% and microscale f~8% particles provided for the formation of a homogenous fine-grained structure after MDF and improved the mechanical properties.

The Microstructure and Mechanical Properties of Al–Mg–Fe–Ni–Zr–Sc Alloy after Isothermal Multidirectional Forging

The Microstructure and Mechanical Properties of Al–Mg–Fe–Ni–Zr–Sc Alloy after Isothermal Multidirectional Forging

THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF THE Al–Mg–Fe–Ni–Zr–Sc ALLOY AFTER MULTIDIRECTIONAL FORGING

The influence of multidirectional isothermal forging on the grain structure and secondary phase particles of solidification origin and dispersoids in Al–4.9Mg–0.9Ni–0.9Fe–0.2Zr–0.1Sc alloy has been studied. The finite element simulation was used to analyze a strain distribution in the sample during forging in a die. A proposed method considered the influence of friction and the changes in a strain rate to recalculate true stress-strain curves for multidirectional forged alloy. An increase in a number of forging cycles at a temperature of 350°C ensured a twice decrease in a mean size of the particles of solidification origin, provided a mean grain size of 1.3 ± 0.2 μm, and insignificantly changed the size of dispersoids. The isothermal multidirectional forging increased the yield strength of the alloy by 60%, tensile strength by 20%.

Refinement mechanism of centimeter-grade coarse grains in as-cast Ti2AlNb-based alloy during multi-directional forging

Centimeter-grade coarse grains commonly exist in heavy ingots of Ti2AlNb-based alloy and severely deteriorate their workability. To investigate the refinement of coarse grains, multi-directional isothermal forging (MDIF) was carried out on samples cut from an ingot weighing about three tons. The results showed that the alloy can be refined by MDIF after 4 passes with 40% reduction per compression at 1200℃, while it cannot be refined even after 10 passes with 30% reduction per compression at 1200℃ or with 40% reduction per compression at 1050℃. The investigation showed that, as an intermetallics with low strain hardening rate, the material is very difficult to be refined by recrystallization if only increasing the total strain. The special refinement mechanism, i.e., grain thinning and shearing (GTS) induced recrystallization, is proposed. The reduction per compression is crucial to refine the coarse grains of this alloy. Coarse grains should be firstly squashed and thinned by large reduction, and then bent and bowed to form shear bands in the following perpendicular compressions. Coarse grains are truncated by repeated shearing, and the truncation increases interfaces within the coarse grains and contributes to the local misorientation accumulation, which in turn promotes the completion of recrystallization.

Grain refinement of Ti2AlNb-based alloy through canned multi-directional forging

Ti2AlNb-based intermetallic alloy has a great potential application in the manufacturing of aeronautical structures. However, the casted ingots of this alloy often consist of very coarse grains even as large as in centimetre level. The refinement of the coarse grain by hot deformation is essential for prompting this material to the engineering applications. In this work, multi-directional isothermal forging (MDIF) was conducted on Ti-20Al-24Nb at 1200°C and 1050°C with 2, 4, 6, 8 and 10 cycles, and a comparative study was emphasized on the grain refinement by two different reductions (30% and 40%) in each compression. The microstructure evolution has been systematically investigated. Results show that dynamic recrystallization (DRX) occurred preferentially near grain boundary, and very large strains were required to refine B2 grains. For 40% reduction per compression, the recrystallization volume fraction after four cycles was more than 95%, and the grain size was evenly refined from centimetre level to ~250μm. However, for 30% reduction per compression, the original coarse grains still exist after 10 cycles, and the recrystallization is far from complete, although the total strain is much greater than the process by using 40% reduction but with fewer cycles. The results demonstrate that the grain refinement of this material depends mainly upon the height reduction per compression and the deformation temperature and the magnitude of deformation or strain is not the critical factor to determine the recrystallization.

Grain refinement during isothermal multidirectional forging due to β-phase heterogenization in Al-Mg-based alloys

Multidirectional forging (MDF) is an effective severe plastic deformation method that provides grain refinement and improves the mechanical strength of metallic materials. In this study, the microstructural homogeneity was improved in the MDF-treated Al-Mg alloy due to coarse particles of the β(Al3Mg2)-phase precipitated during intermediate heterogenization annealing. The β-phase stimulated recrystallization due to a particle stimulated nucleation mechanism, improved the grain refinement effect during MDF that increased the yield strength of the alloy.

Tuning the microstructure and mechanical properties of TiAl-based alloy through grain boundary engineering

In order to modify the room-temperature plasticity of TiAl-based alloy, grain boundary character distribution (GBCD) in TiAl-based alloy was tuned through multidirectional isothermal forging (MDIF) combined with annealing. The experimental results showed that MDIF provided appropriate driving force for the regeneration of the coincidence site lattice (CSL) boundaries through recovery and recrystallization process. Combination of MDIF and annealing at 1100 °C for 90 min promoted the formation of a large number of annealing twins and increased the fraction of low-ΣCSL boundaries to 65.88% and the ratio of (Σ9+Σ27)/Σ3 to 15.12%, which contributed to the disruption of random boundary networks and enhanced the room-temperature plasticity of the TiAl-based alloy.

Effect of Multidirectional Isothermal Forging on the Formation of Ultrafine-Grained Structure in Alloy 1570C

Effect of Multidirectional Isothermal Forging on the Formation of Ultrafine-Grained Structure in Alloy 1570C

Effect of thermomechanical processing on microstructure characteristics of γ-TiAl-based alloy

In this study, the grain boundary character distribution (GBCD) of the γ-TiAl-based alloy was modified through the thermomechanical processing route. A compression test was conducted to identify the mechanical property improvements based on the grain boundary engineering (GBE) treatment. After multidirectional isothermal forging (MDIF) and annealing, the average grain size was refined to 20 µm. The length ratio of the low Σ (1 ≤ Σ ≤ 29) coincidence site lattice (CSL) boundaries increased to 64.01%, with the alloy exhibiting a higher compressive strength (2913 MPa) and maximum compression ratio (45.3%), owing to the strengthening mechanism of grain boundaries and dislocations. Consequently, the combination of MDIF and annealing could be used for grain refinement and tuning GBCD of γ-TiAl-based alloys.

Effect of Multidirectional Isothermal Forging on Microstructure and Mechanical Properties in Ti-6Al-4V Alloy.

In the present work, the microstructure and mechanical properties of Ti-6Al-4V alloy during multidirectional isothermal forging (MDIF) were systematically investigated. The evolution of the microstructure and texture of Ti-6Al-4V alloy during MDIF was studied using TEM and electron backscattered diffraction (EBSD). The experiment results showed that the grain size decreased with the increase in cumulative strain, especially in the easy deformation zone. After four deformation cycles, a homogeneous equiaxed grained microstructure with an average grain size of 0.14 μm was achieved. The texture changes of the alloy were studied in detail. After one cycle of MDIF, the texture was mainly composed of (0002) [01 10], and the Euler angles were (8°, 30°, 30°). The density of texture decreased with the increase in loading cycle, but the dispersion of texture increased. After four cycles of MDIF, the non-basal texture (1010) <1102> texture was observed, and the Euler angles were (82°, 33°, 0°). The highest achieved mechanical properties for Ti-6Al-4V alloy in the MDIF condition were a yield strength 900 MPa, ultimate tensile strength of 921 MPa, and an elongation of 12.1% at room temperature. The increase in MDIF cycles improved the hardness of the alloy. The significant improvement in mechanical properties was attributed to the ultrafine-grained microstructure.

Effect of duplex aging on microstructure and mechanical properties of near-β titanium alloy processed by isothermal multidirectional forging

The effects of sub-transus (α+β) annealing treatment (ST), followed by single aging (SA) or duplex aging (DA) on the microstructural evolution and mechanical properties of near-β Ti–4Al–1Sn–2Zr–5Mo–8V–2.5Cr (mass fraction, %) alloy were investigated using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results show that the finer secondary α phase precipitates in the alloy after DA than SA (e.g., 149 nm for SA and 69 nm for DA, both after ST at 720 °C). The main reason is that the pre-aging step (300 °C) in the DA process leads to the formation of intermediate ω phase nanoparticles, which assist in the nucleation of the acicular secondary α phase precipitates. In addition, the strength of the alloy after DA is higher than that of SA at the specific ST temperature. A good combination is achieved in the alloy subjected to ST at 750 °C, followed by DA (UTS: 1450 MPa, EL: 3.87%), which is due to the precipitation of nanoscale secondary α phase by DA. In conclusion, DA is a feasible process for this new near-β titanium alloy.

Structure, strength and superplasticity of ultrafine-grained 1570C aluminum alloy subjected to different thermomechanical processing routes based on severe plastic deformation

Abstract A comparative study of the structure and mechanical behavior of an Al−5Mg−0.18Mn−0.2Sc−0.08Zr− 0.01Fe−0.01Si (wt.%) alloy ingot subjected to multidirectional isothermal forging (MIF) to a strain of 12 or equal-channel angular pressing (ECAP) to a strain of 10 at 325 °C, and subsequent warm and cold rolling (WR and CR) at 325 and 20 °C, was performed. The results showed that the MIF process of ultrafine-grained structure with a (sub)grain size dUFG=2 µm resulted in enhanced room-temperature ductility and superplastic elongation up to 2800%. Further grain refinement under WR as well as development of a heavily-deformed microstructure with high dislocation density by subsequent CR resulted in a yield/ultimate tensile strength increase from 235/360 MPa after MIF to 315/460 and 400/515 MPa after WR and CR, respectively. Simultaneously, WR led to improved superplastic elongation up to 4000%, while after CR the elongation remained sufficiently high (up to 1500%). Compared with MIF, ECAP resulted in more profound grain refinement (dUFG=1 µm), which promoted higher strength and superplastic properties. However, this effect smoothed down upon WR, ensuring equal properties of the processed sheets. CR of the ECAPed alloy, in contrast, led to higher strengthening and slightly better superplastic behavior than those after CR following MIF.

Effect of Severe Forging and Rolling on the Microstructure and Mechanical Properties of Al–Mg–Sc–Zr Alloy

The effect of thermomechanical processing combining one- or two-step isothermal multidirectional forging (MF) and sheet rolling on the structure and mechanical properties of alloy 1570C (Al–5Mg–0.18Mn–0.2Sc–0.02Zr (wt %)) was studied. One-step MF was performed at a temperature of 325°C to a true strain (e) of about 12. In two-step MF, deformation was continued at 250°C to e ≈ 6 (total strain of about 18). Subsequent warm rolling with a total strain e ≈ 1.6 (relative strain of about 80%) was carried out at the corresponding temperatures of each MF step, and cold rolling was performed at room temperature. In both MF steps, relatively uniform (ultra)fine-grained structures with a grain size of 2.2 and 1.9 μm, respectively, were obtained. Subsequent warm rolling led to further grain refinement, whereas cold rolling ensured the formation of highly work-hardened (ultra)fine-grained structures with high densities of lattice dislocations. The best combination of technological and service properties, including strength, plasticity, superplasticity, and thermal stability, was reached in sheets after one-step (higher temperature) MF. Lowering the deformation temperature in the second MF step ensured further alloy strengthening, but impaired its room-temperature rollability and led to an essentially complete degradation of the superplasticity of the obtained sheets.

Comparative analysis of fine-grained structure formation in an Al-Zn-Mg-Cu alloy during multidirectional isothermal forging and uniaxial compression

A comparative study of the mechanical behavior and microstructural evolution in a cast coarse-grained 7475 Al alloy during uniaxial compression and multidirectional forging at 490°C and 10–4 s–1 was performed. It was shown that high-temperature deformation of the alloy led to grain refinement accompanied by strain softening. At similar processing conditions, the deformation path provided by multidirectional forging was found to be more favorable for grain refinement and strain softening than that of uniaxial compression.

Microstructure evolution of TiBw/Ti composites during severe plastic deformation: Spheroidization behavior

In this study, 2.5 vol% TiBw /Ti composites were subjected to isothermal multi-directional forging (IMDF) at 1020 °C with various strain rates, aimed to investigate the spheroidization behavior of TiBw /Ti composites with initial widmanstatten microstructure. It is found that strain rate has a significant effect on the spheroidization of microstructure. Dynamic recovery (DRV) of α phase occurs at high strain rate while dynamic recrystallization (DRX) dominates microstructure spheroidization with the decreasing of strain rate. It is worth noting that α lamella and α colony exhibit different spheroidization behavior, which leads to the heterogeneity of microstructure. Moreover, the relationship between crystallographic orientation and imposed strain play an important role in the spheroidization process. The schmid factor calculated by EBSD was used to identify and predict heterogeneous deformation phenomena of α colony. The spheroidization process can be promoted by TiB whiskers due to the introduction of large deformation in the matrix.

Development of Ultrafine Grain Structure in an Al–Mg–Mn–Sc–Zr Alloy During High-Temperature Multidirectional Isothermal Forging

Mechanisms of grain refinement under multidirectional isothermal forging (MIF) at 325 °C (~ 0.65 Tm) and the strain rate 10− 4 s− 1 of the Al–Mg-based alloy with complex additions of transition metals were investigated. The starting alloy had an equiaxed grain structure with grain size 25 µm and a uniform distribution of coherent Al3(Sc,Zr) dispersoids of 20–50 nm. A distinguished structural feature in the early MIF stage was the formation of high strain- and misorientation gradients, followed by deformation banding. Due to the sequential changes of the loading axis, such bands were developed in various directions and fragmented the original grains. The number of bands and misorientation of their boundaries gradually rose with strain, resulting in formation of (ultra)fine grain structure with the grain size 2 µm. New grain formation was concluded to occur via continuous dynamic recrystallization and controlled by the nanosized precipitates, which preferably remained stable and coherent with the surrounding matrix.

Effect of forging steps on microstructure evolution and mechanical properties of Ti-6Al-4V alloy during multidirectional isothermal forging

In this study, the multidirectional isothermal forging (MDIF) was used to modify the β transformed microstructure into an equiaxed α+β structure in Ti-6Al-4V alloy. After two steps forging, the initial β grains were nearly disappeared. α colonies with different orientations became kinked and globularized due to the change of loading direction. The microhardness of the sample was 323 HV. The yield strength and ultimate tensile strength were 859 MPa and 907 MPa, respectively. After three steps forging, the lamellar α structures were globularized completely, a homogeneous microstructure was formed. The microhardness of the sample increased to 335 HV. The yield strength and ultimate tensile strength increased up to 900 MPa and 921 MPa, respectively. The homogeneity of microstructure and mechanical properties were improved with the isothermal forging steps increasing.

Microstructural manipulation and improved mechanical properties of a near α titanium alloy

In this study, a newly developed near α titanium alloy with a typical Widmannstätten as-cast microstructure was subjected to isothermal multidirectional forging (IMDF) at 15 °C below β transus temperature. Microstructural observations reveal that the prior β grain boundaries disappeared and α laths transformed into spheroids gradually with increasing the IMDF pass. Ultrafine α grains with an average grain size of 1.57 μm was achieved after four-pass IMDF. Microstructural homogeneity was enhanced and the fraction of recrystallized grains increased remarkably with increasing IMDF pass. The spheroidization process of α laths is a result of the recrystallization during IMDF, including the dissociation of α laths and the migration of subgrains. Compared to the as-cast titanium alloy, both the tensile strength and elongation to failure at room temperature were improved, which is primarily attributed to the microstructure refinement and spheroidization of α laths. In addition to grain refinement, dislocation hardening and substructure strengthening are also the critical mechanisms for the increased strength observed in the samples after IMDF.