Hot-pack Rolling Research Articles

Achieving enhanced high-temperature strength in Ti-48Al-1Fe alloy sheets by direct hot pack-rolling of powder-sintered billets without cogging

The TiAl alloy is a novel lightweight high-temperature structural material that exhibits exceptional performance. The brittleness and mechanical properties of the material can be enhanced by improving the microstructure via rolling. The Ti-48Al-1Fe alloy with high density was produced using powder compaction and pressure-less sintering. Subsequently, the TiAl alloy sheet was formed via hot pack rolling. This study examined the sheet formability of PM Ti-48Al-1Fe alloy sheets at various temperatures, as well as the microstructure and mechanical properties at varied levels of rolling deformations. The microstructure of the powder metallurgy (PM) TiAl alloy sheet has a unique duplex structure, consisting of α2/γ lamellar colonies and a composite structure. The rolling deformation process generates spherical recrystallized grains, which effectively reduce stress concentration. The enhanced composite structure is mostly localized at the interfaces between grains, creating a robust obstacle for the movement of dislocations at high temperatures. This results in the desired outcome of reinforcing the mechanical properties of the material at high temperatures through grain boundary strengthening. This study demonstrates that the ultimate tensile strength (UTS) of PM TiAl sheet tensile specimens in the rolling direction at room temperature is 443 MPa with 1 % elongation, whereas at 800 °C, the UTS rises to 548 MPa with 2.5 % elongation. This study proposes a novel process for the efficient production of Ti48Al1Fe sheets with good high-temperature mechanical properties. This technique entails the hot rolling of high-density sintered powder metallurgy billets, offering an innovative approach for the economical and swift production of TiAl alloy sheets during practical manufacturing process.

Hot pack rolling the Ti2AlNb alloy prepared by SPS: Testing of the high-temperature properties of the alloy directly without the prior heat treatment

Ti2AlNb alloy holds large research significance as a lightweight material for high-temperature structural applications in the next generation of aero-engines. In this work, pre-alloyed Ti2AlNb powders were spark plasma sintered to fabricate the raw compact, which was then hot rolled at different phase fields to fabricate sheets. The high-temperature properties of the sheets were tested directly without the prior heat treatment. Results show that the Ti2AlNb sheets have good high-temperature strengths even without the heat treatment prior to testing. For the sheet rolled at the single B2 phase region, the maximum tensile strength was up to 931.4 MPa at 650 °C. The sheet mainly shows intergranular fracture mode due to the weak bonding at grain boundaries. The sheets rolled at the O + B2 phase region have low tensile strengths at different testing temperatures. For the sheet rolled at α2+O + B2 phase region, the tensile strength gains its maximum strength of 956.6 MPa at 650 °C, and the sheet shows transcrystalline fracture mode. The fracture mechanisms of the sheets rolled at different phase regions are analyzed in detail and proved by experiment. This work shows the potential of fabricating Ti2AlNb sheet by using the spark plasma sintered compact.

Formation of heterogeneous interfaces in the TiAl/Ti2AlNb layered composite and its effect on the fracture toughness

TiAl/Ti2AlNb intermetallic-intermetallic laminated (IIL) composites featuring brittle/ductile heterogeneous interfaces were fabricated through vacuum hot-pack rolling. The microstructures and the phase transformation behaviors of the interfaces of the IIL composites before and after annealing at 900 °C/6 h were investigated. The heterogeneous interfaces are composed of four distinct regions, individually I (βo+γ+α2), II (βo/B2+ω) (brittle part), III (O lath), and IV (equiaxed O) (ductile part) regions from TiAl to Ti2AlNb side. Notably, after annealing, an equiaxed O band approximately 50 μm wide was observed in region IV of the interface. In addition, a significant microhardness variation was observed between regions II and IV of the interface, where region II exhibited higher hardness compared to the TiAl alloy, and region IV displayed lower hardness than the Ti2AlNb alloy. The enhanced fracture toughness of the IIL composites, three times that of the TiAl base alloy, is attributed to the formation of the brittle/ductile heterogeneous interfaces and the layered design incorporating the Ti2AlNb alloy. The corresponding toughening mechanism was further discussed. The brittle II region plays a role in increasing crack branching, while the ductile IV region inhibits the propagation of microcracks and prevents the formation of main cracks. This work highlights the crucial role of the brittle/ductile heterogeneous interface in the toughening of laminated composites. Furthermore, the discovery of the O band provides novel insights into the design of TiAl/Ti2AlNb heterostructures.

A new mechanism of lamellar kinking produced by α twins

This study investigated the hot pack rolling of Ti-43Al-1.5Cr-0.5Re (at. %) alloy sheets at 1250 ℃ with a total deformation of 80 %. The results revealed a novel type of lamellar kinking, distinct from conventional lamellar kinking caused by mechanical stress. The kinking interface exhibits a serrated morphology and conformed to the {11-22} <11-2-3> contraction twin relationship. A new lamellar kinking mechanism induced by α twins was proposed, providing a new perspective for understanding the kinking mechanism in TiAl alloys and other anisotropic metallic materials.

Interface characteristics and mechanical properties of a Ti–TiAl laminated composite

In this study, a laminated composite (Ti–6Al–4V (wt%)/Ti–44Al–8Nb–0.2 W–0.2B–0.03Y (at.%) (Ti–44Al–8Nb)) was successfully fabricated via a hot-pack rolling process. The microstructure and mechanical properties of the composite were investigated. No defects were observed at the interfacial region, thereby indicating a high bonding strength between Ti–44Al–8Nb and Ti–6Al–4V alloys. Electron back-scattered diffraction analysis exhibited a few dislocations, sub-structures, and residual stress in the interfacial region. The flexural strength of the composite in the arrester orientation (AO) was 2001.7 MPa, which was 53% higher than that of the Ti–44Al–8Nb alloy. The fracture toughness of the composite in AO (44.5 MPa·m1/2) was 3.4 times to that of Ti–44Al–8Nb alloy. Tensile test at 25 °C indicated higher ultimate tensile strength (802 MPa), yield strength (688 MPa), and tensile plastic strain of the composite (0.94%) than those of the monolithic Ti–44Al–8Nb alloy. The toughening, strengthening, and fracture mechanisms of the composite were discussed in detail.

Design a Ti6Al4V–Ti43Al9V laminated composite with high strength and fracture toughness

A novel Ti6Al4V–Ti43Al9V metal-intermetallic-laminated (MIL) composite was successfully fabricated using hot-pack rolling. The microstructure and mechanical properties of the composites were also investigated. The microstructure observation showed that the composite exhibited a low extent of dynamic recrystallization (DRX), the acicular α2 phase was precipitated in the interfacial region and the γ phase was elongated along the rolling direction. This unique microstructure enables the composite to simultaneously achieve high strength and fracture toughness. The enhanced tensile strength was attributed to the defect-free interfacial region and the low extent of DRX in the composite. The composite was toughened by crack bridging, crack deflection, the acicular α2 phase precipitated in the interfacial region, and the elongated γ phase.

Vacuum melting of compressed powders and hot rolling of the as-cast Ti-48Al-2Nb-0.7Cr-0.3Si intermetallic alloy: Mechanical properties and microstructural analysis

The amalgamated effect of hot-rolling, rapid cooling and heat treatment on the grain refinement and the resultant mechanical properties of an intermetallic sheet of nominal composition Ti-48Al-2Nb-0.7Cr-0.3Si was investigated. The sheet of 4 mm thickness was fabricated by hot-pack rolling from the vacuum arc remelted (VAR) ingot using a conventional two-high rolling mill. After the final rolling pass, the canned specimen was rapidly cooled in the air to room temperature followed by heat treatment in the α + γ region. The scanning electron microscopy (SEM) micrographs of the as-rolled sheet revealed slightly elongated grains of a typical “duplex (DP)” microstructure with a mean grain size of about 3 μm which grew to about 4 μm and became more equiaxed and homogeneous after heat treatment. Moreover, the EBSD micro-texture indicated a weak cube texture in the rolled + heat-treated sheet. Furthermore, the results from the profilometry-based indentation plastometry of the rolled + heat treated specimen illustrated the balanced and improved mechanical properties compared to the as-cast and as-rolled samples, and also those of similar alloy systems found in the literature.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of a Ti-TiAl Laminate Composite

The effect of an innovative two-step heat-treatment process on the microstructure and mechanical properties of a Ti-TiAl laminate composite fabricated by hot-pack rolling was studied in this paper. After heat treatment, the fracture toughness of the composite was enhanced and the elongation of the composite was almost twice that of the initial one. These changes were due to the dislocations and substructures stored in the Ti-43Al-9V alloy being decreased, the microstructure of the DsTi700 alloy turning to a duplex structure, the acicular α2 phase being precipitated at the interfacial region and the residual stresses stored in interfacial region being eliminated. The precipitation of dual-scale silicides was the main reason for the slightly reduced strength. Compared with the initial composite, the tensile strength of the heat-treated composite at 25° and 700° only reduced by 2.7% and 4%, respectively. The primary annealing temperature had a huge impact on the mechanical properties of the composite. However, with the change in secondary annealing temperature, the mechanical properties of the composite were not changed significantly. After heat treatment at 940–960 °C/2 h/AC + 725–750 °C/6 h/AC, the composite might possess high, comprehensive mechanical properties.

Microstructure evolution and properties of powder metallurgy Ti43Al9V0.3Y alloy sheets at different rolling temperatures

TiAl alloy is a new type of lightweight high temperature structural material with excellent properties. Its room temperature brittleness can be improved by refining the microstructure and adjusting the composition. In this study, the pre-alloyed powder was sintered and densified by hot isostatic pressing, and TiAl alloy sheets were prepared using hot pack rolling. The present work establishes a fundamental understanding of the microstructure and properties during hot rolling. An ultrahigh ductility TiAl alloy was prepared by a combination of introduction of twins, purifying matrix and high sphericity grains. The room temperature elongation of the TiAl sheet is 3.0% and the elongation reaches 65% at 700 °C. The high uniform malleability might be due to several reasons: (i) Some twins are formed during the rolling deformation, in the process of dislocation slip, twins can adjust the crystal orientation to facilitate the continued dislocation slip. In this way, twinning slightly improves the room temperature plasticity of TiAl alloys; (ii) Y purifies matrix of the TiAl alloy; (iii) the grain has the high sphericity, and the spherical grain could minimize the stress concentration.

Microstructure evolution and recrystallization of Ti-44.5Al-3.8Nb-1Mo-0.2B alloy with different initial microstructure during hot pack rolling

Ti-44.5Al-3.8Nb-1Mo-0.2B alloy (TNM) sheets were prepared by hot rolling, and the effect of the initial microstructure on the process was discussed in this work. The as-received TNM alloy billets with nearly and fully lamellar structures were prepared by forging and casting processes (named M1 and M2), respectively. For the as-rolled sheets from M1, the fraction of recrystallization increased gradually with the decrease of reduction per pass based on GOS analysis. Compared with M1, M2 required a larger deformation to achieve complete recrystallization. Meanwhile, the as-rolled sheets from M2 possessed a smaller grain size than that from M1. In addition, the γ-DRX behavior during the multi-pass hot rolling was discussed, and the DDRX was found to play the dominant role in this process. The phase transformation of β/β0 → α occurred in the 1200 °C rolling process. It was presumed that this transformation was promoted by increasing the rolling temperature, therefore, the volume fraction of the β0 phase decreased. The α2 lath decomposed gradually with the increasing of rolling temperature from 1200 °C to 1250 °C, simultaneously, the combination of the adjacent γ laths resulted in the coarsening of γ lath.

POWDER CHARACTERISTICS BLENDING AND MICROSTRUCTURAL ANALYSIS OF A HOT-PACK ROLLED VACUUM ARC-MELTED gamma-TIAL-BASED SHEET

In the quest for cost-effective fabrication processes capable of producing sound gamma-TiAl products, the microstructure and mechanical properties of a modified second-generation hot-rolled gamma-TiAl-based alloy with nominal composition Ti-48Al-2Nb-0.7Cr-0.3Si were investigated in this work. The alloy was fabricated using a processing route that involved uniaxial cold-pressing of powders and vacuum arc re-melting. Prior to the cold pressing, the elemental powder characteristics, such as particle sizes and morphologies, were blended to minimise porosity in the compact that might be inherited by the final ingot. A hot-pack rolling process was carried out directly from the melted button-ingot using a conventional two-high rolling mill to produce a 4 mm-thick sheet. The relative density results of both as-compacted and as-melted alloy parts showed a significant reduction of porosity in the alloy. In addition, both the optical and the scanning electron microscopy micrographs of the rolled sheet revealed a typical ‘duplex’ microstructure with a mean grain size of about 9 µm. Moreover, the results from a room-temperature indentation plastometry test of the hot-rolled sheet indicated good mechanical properties with recorded yield strength of about 600 MPa, an ultimate tensile strength of about 850 MPa, and a true plastic strain of about 3%.

Hot pack rolling of spark-plasma-sintered pre-alloyed powders to fabricate Ti2AlNb sheets

In this work, spark-plasma-sintered Ti-20.3Al-24.7Nb (at.-%) compact was pack rolled in different phase fields to fabricate Ti2AlNb sheets with high strength and ductility without post-heat treatment. The nano O phases in the B2 matrix contributed to the high tensile strength and ductility in a single B2 phase field. In the O + B2 phase field, the O phases remarkably increased in size and resulted in high tensile strength of 1407.1 MPa. Maintaining the O phases to nano sizes and cooperation with micro-sized α 2 phases resulted in extremely high elongation of 13.91% in the α 2+O + B2 phase field. The , , , and strengthening effects of the α 2 and O phases under different conditions were analysed in detail. Highlights Spark-plasma-sintered pre-alloyed powders are used as the raw compact before hot pack rolled Ti2AlNb sheet. Ti2AlNb sheets with good mechanical properties are obtained without any post-heat treatment. This ‘SPS pre-alloyed powder + hot pack rolling’ method has great potential in breaking the limit in sheet properties. The strengthening effects of different factors are analysed.

Oxidation resistance of TiAl alloy improved by hot-pack rolling and cyclic heat treatment

The isothermal oxidation behavior of two TiAl alloys (as-HIP and as-RHT) were compared to explain the effect of microstructure on the oxidation resistance of TiAl alloy. After hot-pack rolling and cyclic heat treatment, the size of lamellar colonies was refined from 35.4 μm to 21.5 μm, and the β/B2 phase was effectively removed. It is concluded that the as-RHT TiAl alloy has better oxidation resistance than the as-HIP TiAl alloy. The main reason is due to refinement of lamellar colony size, elimination of β/B2 phase, uniform distribution of Nb and Mo, and the crushing of Y compounds.

Effect of residual stresses on the mechanical properties of Ti-TiAl laminate composites fabricated by hot-pack rolling

The Ti-TiAl laminate composites with different titanium alloy volume fraction were successfully designed and fabricated by hot-pack rolling. Due to the mismatch of coefficient of thermal expansion (CTE), the composites show pronounced residual stresses. The TiAl alloy possesses residual tensile stress while the titanium alloy possesses residual compressive stress. With the increase of titanium alloy volume fraction, the residual compressive stress is decreased but the residual tensile stress is opposite. The residual compressive stress is beneficial for improving the mechanical properties of Ti-TiAl laminate composites, which is because it enhances the strength and postpones the rupture of composites during tensile test. The higher residual compressive stress also improves the fracture toughness of composites by extending crack propagation. The composite with 42 vol% titanium alloy shows the optimal comprehensive mechanical properties, which is higher than most of other Ti-Al based metal-intermetallic laminate composites.

Prediction of interfacial phase formation and mechanical properties of Ti6Al4V–Ti43Al9V laminate composites

Ti6Al4V–Ti43Al9V laminate composites were successfully fabricated by hot-pack rolling using as-forged Ti43Al9V alloy and as-rolled Ti6Al4V alloy. The microstructure evolution of composites was investigated by electron probe microanalysis (EPMA) equipped with energy dispersive X-ray spectrometry (EDS) and the electron back scattered diffraction (EBSD). In order to better control the properties of laminate composites, two models are established for better control the thickness and phase formation of interfacial region, respectively. The mechanical properties of composites show a negative correlation with rolling temperature. The composite fabricated at 1125 °C exhibits the highest tensile strength 805 MPa and fracture toughness 49.2 MPa•m1/2 which are much higher than Ti43Al9V alloy and other Ti–Al based laminate composites. The fracture features reveal that the increase of tensile strength is attributed to high density of dislocations and sub-structures stored in TiAl alloy and the crack propagation features show that the enhancement of toughness is associated with the aggravation of misorientation of α2 phase and the increase of the amount of γ/B2 boundaries.

The Effects of Hot-Pack Coating Materials on the Pack Rolling Process and Microstructural Characteristics during Ti-46Al-8Nb Sheet Fabrication.

The effects of the package materials on the hot workability and stress-strain characteristics of high-Nb TiAl alloy with a nominal composition of Ti-46Al-8Nb (in at.%) were systematically studied via “sandwich structure” hot compression. TiAl sheet fabrication was conducted by hot pack rolling, and the microstructural characteristics and deformation mechanisms were investigated. Based on the analysis of compressed samples and stress-strain curves, the stainless steel/TiAl structure showed better deformation compatibility with homogeneous deformation and decreasing resistance. However, severe interfacial reactions were inevitable. Meanwhile, for the titanium alloy/TiAl structure, few interfacial reactions happened, but wavy deformation and high resistance complicated the compression process. Finally, a package structure with an outer stainless steel isolation layer and inner titanium alloy was determined for the pack rolling process. A TiAl sheet with no crack defects was obtained with 80% reduction. The pack-rolled TiAl sheet took on alternate microstructure of the grain-boundary Al-enriched ribbons and elongated lamellar colonies ribbons. The grain-boundary recrystallized α2 phase, lumpy γ phase, and massive α2/γ lamellae could be observed, which led to the scatter microstructure. The microstructural characteristics mainly resulted from the solute segregations of as-cast Ti-46Al-8Nb alloys, which triggered the local flow softening and deformation incompatibility during hot pack rolling.

Enhanced tensile strength and fracture toughness of a Ti-TiAl metal-intermetallic laminate (MIL) composite

High temperature titanium alloy/Ti-43Al-9V metal-intermetallic laminate composite was successfully fabricated by hot-pack rolling. The titanium alloy exhibits a lamellar structure and the TiAl alloy consists of B2 phase and elongated γ phase. Due to the low rolling and annealing temperature, the TiAl alloy shows a low extent of dynamic recrystallization (69.3%) and a high dislocations density (17%). The composite shows an excellent combination of the enhanced tensile strength and fracture toughness. The tensile strength of composite at 25 °C and 700 °C are 845 MPa and 730 MPa respectively. The fracture toughness of composite are 29.4 MPa∙m1/2 and 26.6 MPa∙m1/2 in arrester orientation and divider orientation respectively, which are much higher than monolithic Ti-43Al-9V alloy. The strengthening of composite is mainly attributed to high density dislocations and sub-structures stored in composite and the toughening mechanisms of composite are crack deflection, secondary cracks and crack tip convolution.

Microstructure-Property Relationships in Cold Rollable, High-Strength α/β Alloys

For decades Ti-6Al-4V has been the workhorse alloy for aerospace sheet applications due to its good balance of properties and the known ability to hot roll it with relative ease. Sheet of Ti-6Al-4V is made by hot pack rolling, which is a costly and time consuming process, due to the alloy having insufficient room-temperature workability to support significant cold reduction or forming. Consequentially, Ti-6Al-4V is not typically offered in foil gauges, since the direct product of hot pack rolling contains an undesirable surface finish and insufficient gauge control. Hot pack rolling also limits the maximum sheet size and annual capacity. As a world leader in advanced sheet alloys of titanium, nickel, cobalt, and specialty stainless steels, ATI is developing new titanium alloys with improved strength compared to Ti-6Al-4V that take advantage of a recent understanding of cold workability in high-strength alpha-beta titanium. These α+β alloys exceed Ti-6Al-4V strength while being highly cold formable. Cold rolling via coil processing also enables longer lengths of sheet with significantly improved gauge control and surface finish. Results from pilot scale ingots will be presented upon, including final properties of these unique alloys and microstructure-property correlations developed through modelling.

Effect of alloying additions on work hardening, dynamic recrystallization, and mechanical properties of Ti–44Al–5Nb–1Mo alloys during direct hot-pack rolling

There is significant gap in the understanding of the effect of alloying elements, particularly V and B, on the dynamic recrystallization (DRX) behavior, and the corresponding microstructure evolution in Ti–44Al–5Nb–1Mo-(V, B) alloys during uniaxial hot compression. In this regard, we have studied here the relationship of work hardening rate (θ) and critical strain of dynamic recrystallization (εc) on strain, deformation parameters. The θ values showed typical dynamic recrystallization characteristics, and were sensitive to temperature and strain rate. Deformation mechanism involving twinning-dislocation interactions and phase transformation contributed to characteristics of θ. DRX occurred in Ti–44Al–5Nb–1Mo–2V-0.2B alloys even at higher rolling speed and at ambient temperatures over a wide range, which implied a wider processing window during the direct hot-pack rolling process. Crack-free sheets were directly obtained from the as-HIPed alloys at 1250 oC with rolling speeds of 0.3 m/s, and 10%–20% reduction per pass. Their microstructure, high-temperature tensile properties and fracture mechanisms are discussed. As rolling progressed, Ti–44Al–5Nb–1Mo alloy was characterized by residual lamellae with locally dissolved α2 laths, broadening γ laths, and fine sub-grains. Dislocation-induced DRX and phase transformation of α2/γ lamellae were the primary deformation modes. In contrast, on the addition of V and B, deformation twinning was activated. Multi-phase microstructure was characterized by equiaxed grains of ~ 5–20 μm. The as-rolled sheets of both the alloys exhibited superior performance at 800 oC. Ti–44Al–5Nb–1Mo alloy showed a higher ultimate tensile stress of ~689 MPa, while superior ductility of greater than 75% was obtained for Ti–44Al–5Nb–1Mo–2V-0.2B alloy. The microstructure with different degree of dynamic recrystallization contributed to various aspects of high-temperature tensile properties.

Effect of Pack Rolling and Heat Treatment on Microstructure and Mechanical Properties of B4CP/6061Al Composite Prepared by Powder Metallurgy

This study aims to investigate the effect of hot pack rolling and T6 heat treatment on microstructure and mechanical properties of 25 vol% B4CP/6061Al composite fabricated by powder metallurgy. Results show that hot pack rolling process could break the particle agglomeration, refine grain size, decrease porosity and improve the interface bonding quality. Almost 15%, 21% and 328% improvement in ultimate tensile strength, yield strength and elongation for B4CP/6061Al composite were obtained after hot pack rolling. It was also found that the average grain size increased due to the static recrystallization during the T6 heat treatment process. XRD patterns revealed that Mg2Si precipitation hardening phase was formed in the matrix. Compared with the rolled B4CP/6061Al composite, T6 heat treatment resulted in improving the ultimate tensile strength, yield strength and elongation by 7%, 11% and 50%. The strengthening mechanisms and fractured surfaces under different stages were also discussed. The hot pack rolling process could break the particle agglomeration, refine grain size, decrease porosity and improve the interface bonding quality. The properties test indicates that almost 15%, 21% and 328% improvement in ultimate tensile strength, yield strength and elongation for B4CP/6061Al composite were obtained after hot pack rolling. Compared with the roll-ed B4CP/6061Al composite, T6 heat treatment resulted in improving the ultimate tensile strength, yield strength and elongation by 7%, 11% and 50%.