Isothermal Multidirectional Forging Research Articles

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.

Influence of strain rate on grain refinement in the Al-Mg-Sc-Zr alloy during high-temperature multidirectional isothermal forging

The strain rate effect on ultrafine-grain structure development in the cast Al-Mg-Sc-Zr based alloy during multidirectional isothermal forging was studied. The alloy with the average grain size of 25 μm and coherent Al3(Sc,Zr) precipitates of 25–50 nm in diameter was deformed to a total strain of about 8.4 at 325 °C with strain rates of 10−4 and 10−2 s−1. In both cases development of new crystallites started at the initial grain boundaries via the formation and interaction of deformation/microshear bands. The volume fraction of these crystallites and their boundary misorientation rose with strain increasing; that resulted in formation of fine-grained microstructure through continuous dynamic recrystallization. The kinetics of grain refinement was practically independent of strain rate, leading to similar microstructures, characterized by nearly same misorientation parameters. This indicated that the fine-grain structure formation had an athermal origin. On the other hand, the size of (sub)grains after forging at 10−2 s−1 was smaller than that at 10−4 s−1, suggesting the importance of thermally activated processes in grain refinement being controlled by the rate of dynamic recovery. It was concluded that grain refinement of the studied alloy via continuous dynamic recrystallization was conditioned by both a series of athermal processes and a thermal activated process owing to dynamic recovery.

Microstructural Evolution and Refinement Mechanism of a Beta–Gamma TiAl-Based Alloy during Multidirectional Isothermal Forging

Multidirectional isothermal forging (MDIF) was used on a Ti-44Al-4Nb-1.5Cr-0.5Mo-0.2B (at. %) alloy to obtain a crack-free pancake. The microstructural evolution, such as dynamic recovery and recrystallization behavior, were investigated using electron backscattered diffraction and transmission electron microscopy methods. The MDIF broke down the initial near-lamellar microstructure and produced a refined and homogeneous duplex microstructure. γ grains were effectively refined from 3.6 μm to 1.6 μm after the second step of isothermal forging. The ultimate tensile strength at ambient temperature and the elongation at 800 °C increased significantly after isothermal forging. β/B2→α2 transition occurred during intermediate annealing, and α2 + γ→β/B2 transition occurred during the second step of isothermal forging. The refinement mechanism of the first-step isothermal forging process involved the conversion of the lamellar structure and discontinuous dynamic recrystallization (DDRX) of γ grains in the original mixture-phase region. The lamellar conversion included continuous dynamic recrystallization and DDRX of the γ laths and bugling of the γ phase. DDRX behavior of γ grains dominated the refinement mechanism of the second step of isothermal forging.

Effect of heat treatment on microstructure and tensile properties of 2 vol.% TiCp/near-β Ti composite processed by isothermal multidirectional forging

In this work, effect of heat treatment on microstructure and tensile properties of 2 vol%TiCp/near-β Ti composite processed by isothermal multidirectional forging was investigated. The microstructure observation shows that multi-directional α/β forged composite owns large number of substructures within β grains that present strong texture {100}. After the solution treatment, TiCp and adjacent β grain respect the burgers orientation relationship {111}TiC//{111}β, and there are still some substructures within the β matrix. Same as other alloys, aging temperature has significant effects on the morphology and distribution of needle-like secondary α phase. Owing to the addition of TiCp, the more and refined secondary α phase are precipitated by compared with matrix alloy. The results of the tensile tests show that isothermal multidirectional forged composite possess a reliable strength (~990.6 MPa) and elongation (~10.2%). The elongation of composite is improved by solution treatment, which was attributed to the tailoring of β matrix. After aging treatment, the composite owns the high strength (~1555.7 MPa) and reliable ductility (~2.7%) because of the precipitation of various α phases.

The Effect of Isothermal Multi-Directional Forging on the Grain Structure, Superplasticity, and Mechanical Properties of the Conventional Al–Mg-Based Alloy

The current study observed a grain structure evolution in the central part and periphery of the sample of an Al–Mg–Mn-based alloy during isothermal multidirectional forging (IMF) at 350 °C with a cumulative strain of 2.1–6.3 and a strain per pass of 0.7. A bimodal grain size distribution with areas of fine and coarse grains was observed after IMF and subsequent annealing. The grain structure, mechanical properties, and superplastic behavior of the samples subjected to IMF with a cumulative strain of 6.3 and the samples exposed to IMF with subsequent cold rolling were compared to the samples exposed to a simple thermo-mechanical treatment. The micro-shear bands were formed inside original grains after the first three passes. The fraction of recrystallized grains increased and the mean size decreased with an increasing cumulative strain from 2.1 to 6.3. Significant improvements of mechanical properties and superplasticity were observed due to the formation of a homogenous fine grain structure 4.8 µm in size after treatment including IMF and subsequent cold rolling.

Effect of Multidirectional Forging on the Grain Structure and Mechanical Properties of the Al⁻Mg⁻Mn Alloy.

The effect of isothermal multidirectional forging (IMF) on the microstructure evolution of a conventional Al–Mg-based alloy was studied in the strain range of 1.5 to 6.0, and in the temperature range of 200 to 500 °C. A mean grain size in the near-surface layer decreased with increasing cumulative strain after IMF at 400 °C and 500 °C; the grain structure was inhomogeneous, and consisted of coarse and fine recrystallized grains. There was no evidence of recrystallization when the micro-shear bands were observed after IMF at 200 and 300 °C. Thermomechanical treatment, including IMF followed by 50% cold rolling and annealing at 450 °C for 30 min, produced a homogeneous equiaxed grain structure with a mean grain size of 5 µm. As a result, the fine-grained sheets exhibited a yield strength and an elongation to failure 30% higher than that of the sheets processed with simple thermomechanical treatment. The IMF technique can be successfully used to produce fine-grained materials with improved mechanical properties.

Structure, strength and superplasticity of the bulk 1570C alloy subjected to high-temperature multidirectional isothermal forging

The structure and mechanical properties of the cast non-age-hardenable complex-alloyed 1570C alloy (Al-5Mg-0.18Mn-0.2Sc-0.08Zr, wt%) were studied after multi-directional isothermal forging at 325 °C up to accumulative strain e ≈ 12 and subsequent annealing in the temperature interval from 325 to 500 °C. It is shown that even amid the absence of substantial alloy hardening, high-temperature multi-directional forging is an effective method for processing bulk billets, resulting in a stable and homogeneous (ultra)fine-grained structure with a grain size of about 2 mm, which provided improved ductility and extraordinary high superplastic properties.

Microstructural mechanisms during multidirectional isothermal forging of as-cast Ti-6Al-4V alloy with an initial lamellar microstructure

Microstructural evolution and tensile properties of Ti-6Al-4V alloy with an initial lamellar microstructure during the multidirectional isothermal forging (MDIF) were investigated. After three steps isothermal forging, a homogeneous equiaxed grained microstructure with an average grain size of 1.9 μm was achieved. The grain refinement mechanism included both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). The necklaces of new DDRX grains with high angle grain boundaries (HAGBs) were formed along the initial β grain boundaries. Grain subdivision was through CDRX. The fraction of recrystallization and the homogeneity of microstructure were improved with the isothermal forging steps increasing, the fractions of recrystallization increased from 43% to 63% and the fractions of HAGBs increased from 48% to 71%, respectively. The tensile properties of as-cast Ti-6Al-4V alloy were significantly improved at both room temperature and 400 °C, respectively. The yield strength, ultimate tensile strength and elongation increased 21%, 23%, and 210% at room temperature, respectively. And at 400 °C, the yield strength, ultimate tensile strength and elongation increased 46%, 48%, and 21%, respectively. The fracture mechanism changed from brittle fracture to ductile fracture after the MDIF process.

Effect of temperature of isothermal multidirectional forging on microstructure development in the Al-Mg alloy with nano-size aluminides of Sc and Zr

The effect of deformation temperature on structural changes in the homogenized ingot of a complex-alloyed aluminum alloy 1570C (Al-5% Mg-0.2% Sc-0.1% Zr) during hot isothermal multidirectional forging (IMF) was investigated. The alloy with equiaxial grains of 25 μm and coherent precipitates of Al3(Sc, Zr) 5–10 nm in diameter was subjected to IMF at the temperatures of 325° and 450 °C (∼0.6 and 0.8 Tm, respectively) at a strain rate of 10−2 s−1 to a total strain of ε = 8.4 with a pass strain of 0.7. It has been shown that grain refinement takes place during IMF at both temperatures. The formation of new fine (sub)grains surrounded by low- and high-angle boundaries started in the vicinity of the original grain boundaries in the earlier stages of deformation. With increasing strain, the volume fraction and boundary misorientations of such crystallites increased; that results in almost complete replacement of the original structure with a new fine-grained one via continuous dynamic recrystallization. The alloy grain refinement was controlled by the interaction of lattice dislocations and/or grain boundaries with precipitates, effectively inhibiting migration of the boundaries as well as dynamic recovery, leading to rearrangement of lattice dislocations, and their annihilation. Herewith, the kinetics of the fine-grained structure formation was practically independent on the processing temperature, resulting in roughly similar microstructures after the same strains, which characterized by the closed mean misorientation angles and fractions of high-angle boundaries. However, the size of (sub)grains processed at 325 °C was significantly lower than at 450 °C, i.e., 1.2 and 2.5 μm, respectively. This indicated that parameters of the developed structure were thermally activated and controlled by precipitates. The features of microstructure formation and the nature of grain refinement of a complex-alloyed aluminum alloy under high-temperature severe plastic deformation conditions are discussed.

Effect of the Size of Al3(Sc,Zr) Precipitates on the Structure of Multi-Directionally Isothermally Forged Al-Mg-Sc-Zr Alloy

The effect of Al3(Sc,Zr) dispersoids on the evolution of the cast Al-Mg-Sc-Zr alloy structure under multi-directional isothermal forging (MIF) has been investigated. The alloy, which has an equiaxed grain structure with a grain size of ~25 μm and contains dispersoids 5–10 and 20–50 nm in size after onestage (at 360°C for 6 h) and two-stage (360°C for 6 h + 520°C for 1 h) annealing, respectively, was deformed at 325°C (~0.65 Tm) up to cumulative strains of e = 8.4. In the initial stages of MIF, new fine (sub)grains surrounded by low-angle and high-angle boundaries (HABs) were formed near the initial grain boundaries. With increasing strain, the volume fraction and misorientation of these crystallites increased, which led to the replacement of a coarse-grained structure with a fine-grained one with a grain size of ~1.5-2.0 μm. Dynamic recrystallization occurred in accordance to a continuous mechanism and was controlled by the interaction of lattice dislocations and/or (sub)grain boundaries with dispersoids that effectively inhibited the migration of boundaries, as well as the rearrangement of lattice dislocations and their annihilation. The particle size and the density of their distribution significantly affected the parameters of the evolved structure; in an alloy with smaller particles, a structure with a smaller grain size and a larger HAB fraction developed.

Achieving grain refinement and enhanced mechanical properties in Ti–6Al–4V alloy produced by multidirectional isothermal forging

This study investigated the principle of multidirectional isothermal forging (MDIF) and determined the major microstructural evolution features and unique room-temperature mechanical properties of extra-low interstitial-grade Ti–6Al–4V alloy. The grain refinement mechanism, grain boundary characteristics, and phase transformation during MDIF were explored. After three-step MDIF, a homogeneous microstructure with a grain size of about 0.5µm was produced. The ultrafine grained Ti–6Al–4V alloy exhibited high yield strength (1170MPa), high ultimate tensile strength (1190MPa), and good ductility (10.4%). The mechanism of grain refinement during MDIF included continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). Grain subdivision resulted from CDRX or the transformation of cellular dislocation substructures into new ultrafine grains. The necklace of new DDRX grains formed along the initial grain boundaries of the Ti–6Al–4V alloy. The main strengthening mechanisms were grain boundary and dislocation strengthening. The strength and grain size followed the typical Hall–Petch relationship.

Influence of SiC nanoparticles addition on the microstructural evolution and mechanical properties of AZ91 alloy during isothermal multidirectional forging

In this study, low volume fraction SiCp/AZ91 magnesium matrix nanocomposites billets intended for structural applications were synthesized using semisolid stirring assisted ultrasonic vibration, leading to the dispersion of SiC nanoparticles. Both the AZ91 alloy and nanocomposite billets were then subjected to isothermal multidirectional forging (IMDF). Micrographic observations illustrated that the mean grain size of the developed nanocomposite showed an initial increase after 3 IMDF passes, followed by a decrease after 6 IMDF passes compared to the AZ91 alloy. This indicated that the effect of dispersed SiC nanoparticles on the inhibition of the dynamic recrystallization grains growth was impaired due to the present high IMDF temperature. The improved yield strength of the nanocomposite could be attributed to Orowan strengthening effect related to the dispersed SiC nanoparticles as well as the second phases precipitated far from the dynamic recrystallization grains.

Microstructures and mechanical properties of AZ61 magnesium alloy after isothermal multidirectional forging with increasing strain rate

In this work, AZ61 magnesium alloy was processed using a new strategy for multidirectional forging (MDF) with an increased strain rate to obtain homogeneous and fine microstructures. The effect of the MDF process on the microstructure and mechanical properties of the alloy was investigated. It was revealed that the grain size decreases, and the homogeneity of the microstructure simultaneously increases, with the number of MDF passes. After four MDF passes, a homogeneous and fine microstructure with the average grain size of ~6.1μm was achieved. Additionally, dynamic precipitation of the Mg17Al12 phase occurred during the fifth MDF processing pass. The mechanical properties of the AZ61 alloy increased gradually as the number of passes increased from one to four passes, and the sample undergoing MDF exhibited an excellent combination of mechanical properties after the fourth pass, including the yield strength, ultimate strength and elongation to failure, which reached 241MPa, 303MPa and 13%, respectively. These values are 97%, 66% and 189% higher, respectively, than for the sample that did not undergo MDF. After the fifth MDF processing pass, the mechanical properties of the alloy decrease sharply, which may be attributed to the dynamic precipitation of the Mg17Al12 phase during this pass.

Influence of rolling on the superplastic behavior of an Al-Mg-Sc alloy after ECAP

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.

Necking and Spheroidization of α 2 Plates in Lamellar Microstructure of a Hot-Deformed Two-Phase TiAl Alloy during Annealing

In recent years, the Ti2AlNb-based alloys are selected as potential alloys for elevated temperature applications to replace conventional Ni-based superalloys owing to their good creep resistance and oxidation resistance which are related to the O precipitates. In this paper, the precipitation mechanisms of O phase, phase transformation and microstructure control of Ti2AlNb-based alloys are reviewed. Ti2AlNb-based alloys generally consist of B2/β, α2, and O phase with different morphologies which are derived from the various heat treatment processes, including equiaxed α2/O particles, bimodal microstructure, and Widmannstätten B2/β + O structures etc. As a newly developed strengthening phase, O precipitates can be precipitated from the B2/β matrix or α2 phase directly as well as generated by means of peritectoid reaction of α2 phase and bcc matrix. Microstructural control of the Ti2AlNb-based alloys can be implemented by refining the original B2/β grain size and regulating the O precipitates. Multidirectional isothermal forging (MIF) and powder metallurgy technique are two effective methods to refine the original B2/β grains and the morphology and size of O precipitates can be regulated by adding alloying components and pre-deformation process. Moreover, the phase diagram as well as coarsening behavior of Ti2AlNb-based alloys in ageing process is also reviewed. For the further application of these alloys, more emphasis should be paid on the deep interpolation of microstructure-property relationship and the adoption of advanced manufacturing technology.

Observations on the creep behavior of fully-lamellar polycrystalline TiAl: Identification of critical effects

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.