Caliber Rolling Research Articles

Effects of Temporary Alloying and Severe Plastic Deformation on Microstructure Evolution and Mechanical Properties of Ti-Alloys: A Comparative Study

Near-α (VT 20) and α + β (Ti–6Al–4V) Ti alloys having similar initial grain morphology with equiaxed α in transformed β were subjected to microstructure refinement processes by thermohydrogen processing (THP) and caliber rolling (CR), respectively. The microstructure analysis after THP and CR of the alloys revealed the change in morphology and refinement of microstructure. The microstructure analysis after initial constant strain rate tests at 850 °C exhibited needle-like morphology for THP alloy, whereas it was equiaxed for as-received VT 20 and CR 750 alloy. An additional attempt was made on the CR alloy towards determining the magnitudes of strain rate sensitivity index (m) through differential strain rate test conducted over a number of strain rate change cycles at 850 °C and 927 °C. Both THP and CR processes resulted in grain refinement and the change in morphology of microstructure.

Change in dominant deformation mechanism of Mg alloy via grain boundary control

The impact of grain boundary structures on mechanical properties and its deformation behavior was investigated on a wrought processed Mg-0.3 at%Mn binary alloy. The Mg-Mn alloy, which had an average grain size of 1.2 µm and a high density of low-angle grain boundaries, was fabricated via caliber rolling. This alloy exhibited yield strength of more than 300 MPa and had low strain rate dependence in both tension and compression tests at room temperature. The major deformation mechanism was dislocation slip for tension and deformation twinning for compression, respectively, as is well established for conventional wrought processed magnesium alloys. However, an extruded Mg-Mn alloy having an average grain size of 1.8 µm is reported to exhibit a good elongation-to-failure and large strain rate dependence. While the same cast billet was used, the result obtained from caliber rolled alloy is inconsistent with that from the extruded alloy. This is due to the difference in grain boundary structures, i.e., the existence of low-angle grain boundaries in the caliber rolled alloy. Low-angle grain boundaries, which do not have free volume, are difficult for segregation of alloying elements. The mechanical properties and deformation mechanism can be clearly controlled by grain boundary structures through wrought process.

Finite element simulation of relative sliding in hot rolling

In the rolling process, relative sliding happens due to the different velocities between the billet and work rolls. Quantitative evaluations of the relative sliding are essential to the failure analysis and damage prediction of rolls. The velocity and the distance of the relative sliding were evaluated with varying the billet size, the reduction ratio, the friction coefficient, the rolling temperature and speed, and compared between the flat and the caliber rolling. The relative sliding velocity and distance are increased with increasing the billet height, the reduction ratio and the rolling velocity. They are comparable between the flat and the caliber rolling at under-filling condition, and increased significantly in the caliber rolling at over-filling condition.

Control of mille-feuille structure via caliber rolling process

Deformation kink bands, which are induced by wrought process, are well-known to play a role in increasing the mechanical properties of strength and hardness. However, this microstructural feature such as structure and morphology can vary with thermomechanical processes. In this study, we have investigated the possibility to control such microstructures via caliber rolling process using the Mg-Y-Zn alloy. Microstructural observations of the caliber rolled alloys revealed that deformation kink bands formed irrespective for rolling passes and processed temperatures. In addition, the increase in rolling pass brought about a larger bending angle of long periodic sacking ordered structures associated to deformation kink bands. The properties of strength and hardness enhanced up to 4~6 rolling passes and then they tended to be constant. Good mechanical properties of this caliber rolled alloys were due to existence of both accumulated strains and deformation kink bands.

Mechanical Property of Ultrafine Elongated Grain Structure Steel Processed by Warm Tempforming and Its Application to Ultra-High-Strength Bolt

Our strategy to enhance fracture properties of ultra-high-strength low-alloy steel with a yield strength of 1.4 GPa or over is to arrest the propagation of brittle crack in a hierarchical, anisotropic and ultrafine grain structure designed to be fail-safe, in addition to suppressing the crack initiation. The present article reviews strength, ductility, toughness and delayed fracture resistance of ultra-high-strength low-alloy steel with an ultrafine elongated grain structure that was processed by deformation of a tempered martensitic structure at an elevated temperature (warm tempforming). The evolution of heterogenous microstructure during warm tempforming using multi-pass caliber rolling and the microstructural factors controlling the strength and the fracture properties of the warm tempformed steels are discussed. Furthermore, we introduce the application of the warm tempformed steel with an ultrafine elongated grain structure to ultra-high-strength bolt.

Impact of Initial State during Calibre Rolling: Investigating Microstructure and Mechanical Properties of AZ80 Magnesium Alloy

In order to investigate the effect of rolling on microstructure and mechanical properties of different initial states, cast and extruded magnesium alloy AZ80 bars were rolled in calibre. The microstructural characterization was done by light microscopy. As a result, the initial grain size of the cast AZ80 (66 μm) clearly differs from the extruded bar (13 μm). After 14 passes of hot rolling in calibre, a significant grain refining effect was achieved resulting in grain sizes of 5 μm for the cast and 3 μm for the extruded material. To investigate the mechanical properties in the initial and rolled state, tensile tests of both conditions were conducted at room temperature. Due to grain refining, the tensile strength (162 MPa) and the elongation (3 %) of cast AZ80 increased remarkably during 14 passes of calibre rolling (360 MPa and 19 %). The strengthening effect was also evident for the rolled extruded AZ80. However, the cast material exhibited cracks during calibre rolling due to its inexpedient microstructure for a high deformation calibre. On the contrary, the extruded AZ80 was easily deformable. This shows the clear impact of initial states on aspired end properties of processed materials. Future investigations will deal with developing a suitable calibration for cast AZ80.

Interfacial characteristic of multi-pass caliber-rolled Mg/Al compound castings

Herein, magnesium/aluminum (Mg/Al) composite rods were prepared by compound casting and multi-pass warm caliber rolling, and the effect of caliber rolling on the evolution of the interfacial microstructure and mechanical properties of the Mg/Al compound castings was investigated. The results reveal a uniform interface, which can be divided into three layers consisting of Al3Mg2, Al12Mg17 and (Al12Mg17 + Mg) eutectic structure, was formed during the compound casting. Afterwards, the brittle reaction layer fragmented during the rolling process, and was effectively eliminated after 12 passes. Meanwhile, the fresh Al and Mg bases squeezed out and bonded together under the rolling force. The interface formed after the final rolling process had no visible intermetallic compounds. Further, an inhomogeneous deformation in the Mg and Al bases was observed during the rolling process. Initially, the Al clad layer accommodated a larger proportion of the plastic deformation. With increasing number of passes, however, the strain in the Mg core layer increased and, after 12 passes, the deformation of both layers tended to be equivalent. The strength of the Mg/Al composite rod increased gradually and the ultimate tensile stress reached a value of 247 MPa after 12 passes.

Manufacturing of nanostructured titanium Grade2 using caliber rolling

In this paper, a new method of manufacturing nanostructured titanium (n-Ti) is presented. An effective way of grain refinement and strengthening a material using conventional plastic working is proposed. Microstructured titanium (m-Ti) Grade2 was processed using caliber rolling technology. In this work, the m-Ti billet in the initial state was in the form of a rod with a diameter of 10 mm. This billet was subjected to multi-pass longitudinal cold caliber rolling, during which the transverse section of the rod was gradually reduced. The rolling was carried out in such a way as to accumulate a relatively large plastic deformation in the workpiece. The rolling process resulted in a refinement of the microstructure into a nanostructure. The average grain size of 74 nm and enhanced mechanical properties were obtained. This single-phase Ti Grade2 with a nanostructure exhibits mechanical properties typical for titanium alloys. The results of the research confirmed that the technology used in this work makes it possible to generate a heavy strain enough for the purpose of grain refinement.

Microstructure and Mechanical Properties of Mg/Al Clad Bars with Ni Interlayer Processed by Compound Castings and Multi-Pass Caliber Rolling

Magnesium/aluminium clad bars were fabricated by compound casting and multi-pass warm caliber rolling. A Ni interlayer prepared using a plasma spraying process was inserted between the parent metals to improve the interfacial characteristics during the casting process, and the effect of caliber rolling on the evolution of the interfacial microstructure and mechanical properties of the Mg/Ni/Al composites was investigated. The results show that the formation of Mg-Al intermetallic phases was impeded effectively by the Ni interlayer and a typical AZ31/Ni/6061 multilayer structure with metallurgical bonding was formed during the compound casting process. In addition, an inhomogeneous strain distribution in the AZ31 and 6061 alloys were characterized during the rolling process. The AZ31 clad layer accommodated a larger proportion of the plastic strain during the initial passes, while the strain in the Mg core layer increased with increasing number of passes. The Ni interlayer fragmented during the rolling process, and transformed into the dispersed particles at the interface. Meanwhile, the fresh AZ31 and 6061 base alloys squeezed out and bonded together under the rolling force, and a well-bonded interface with no visible defects was formed.

Methodology of Technological Adaptation Applied to Powder Rolling

Powder rolling is used for manufacturing long-length strip. For obtaining the product with high green density it is necessary to ensure shear strain in the deformation zone. Based on the principles of technologic adaptation the dual roll closed caliber with adaptively changed rigidity was constructed. It consists of upper bandage with shoulder, bottom bandage with groove in which the set of three rings (two aside and one central) is located. The pass is arranged by aside rings and outside surface of the central ring forming closed caliber while interacting with the shoulder of the upper bandage. The caliber output is equal to zero and the broadening at rolling is fully excluded. Such construction of the tool makes it possible to achieve high level of hydrostatic stress of tensor simultaneously with intensification of shear strains resulting in practically nonporous rolled strip. Taking into consideration peculiarities of calibre rolling the new criterion was proposed. This criterion enables to characterize roll system for each material, incompact materials in particular, considering retraction ability, to assess and identify the final square of the rolled material at different caliber configuration. Dependence of maximum value of powdered rolled strip thickness on dual roll closed caliber retraction surface value at different rolled strip width is presented.

Ultrahigh-strength CoCrFeMnNi high-entropy alloy wire rod with excellent resistance to hydrogen embrittlement

An ultrahigh-strength CoCrFeMnNi high-entropy alloy wire rod was produced using cryogenic temperature caliber rolling. Because of the highly increased twinning activity caused by lowering the temperature to 77 K, significant twinning-induced grain refinement occurred; thus, an ultrafine (< 100 nm) grain structure could be achieved in the processed material. The processed material showed a remarkably high tensile strength of ~ 1.7 GPa, and also had excellent resistance to hydrogen embrittlement (HE), in contrast to the typical trade-off relationship between these two properties. The exceptionally high resistance to HE was attributed to the combined effects of (1) difficulties in accumulating hydrogen owing to the sluggish hydrogen diffusion caused by the face-centered cubic crystal structure and the severe lattice distortion, (2) the high hydrogen threshold required for HE at the dominant cracking sites of twin boundaries, and (3) absence of martensite transformation.

Development of an Innovative and Quality-Focused Production Technology for Magnesium Wire

In order to meet customer requirements, technology developments need to stay focused at process rationalization and quality improvements. Combining the twin-roll casting technology and caliber rolling technology for magnesium wire, customer demands concerning cost-efficiency and high quality can be satisfied. The resulting long products can be applied in the automotive industry and aircraft sector as joining and welding elements or in biomedicine as screws. For this purpose, the paper analyzes potential application fields and their induced quality properties. Special requirements of illustrative magnesium long products are shown by adapting existing requirements of aluminum wire. Finally, the innovative magnesium wire production technology will be explained and the advantages, in particular the resource efficiency, of the developed technology will be discussed.

Ultrafine-grained CoCrFeMnNi high-entropy alloy produced by cryogenic multi-pass caliber rolling

The ultrafine-grained (UFG) structure (∼100 nm) of CoCrFeMnNi high-entropy alloy was achieved using cryogenic multi-pass caliber rolling at 77 K to impose less deformation (ε = 1) compared to severe plastic deformation (SPD, ε > 4). Highly increased twinning activity at 77 K caused the grain interiors to fragment significantly, so UFG structure could be induced without imposing SPD. Intersected twin morphology and secondary twins played a key role in twinning-induced grain refinement to achieve UFG structure. The processed material showed an exceptional high yield strength of ∼1.5 GPa with a reasonable fracture elongation of ∼10% at room temperature.

Improvement of strength, toughness and ductility properties in carbon steel

The relationship among strength, ductility, toughness and microstructure was studied in order to find out microstructure image of stronger and tougher steel. Initial samples with two different microstructures, ferrite-pearlite and martensite (and/or bainite), were prepared and then caliber rolling was conducted at warm working temperature. Two kinds of low carbon steel bars with ultrafine elongated grained (UFEG) structure in transverse grain size of 1.0 μm and 1.3 μm, respectively, were produced. For comparison, conventionally quenched and tempered 0.29%C steel and 1.03%C steel with a martensitic structure and low-carbon steel with ferrite (grain sizes, 10μm and 18μm)/pearlite structure were also prepared. The Charpy impact and static tensile tests were conducted at a temperature range from 200°C to －196°C. The reduction in area and the plastic deformation limit were used as a universal parameter of ductility. In the Charpy impact test, only the UFEG steels fractured with delamination crack, the delamination remarkably appeared near energy transition temperature and the impact energy becomes larger than all other steels. As a result, the UFEG steel with transverse grain size of 1.0 μm was best balance in correlation between strength and ductility and between strength and toughness.

Microstructure and Mechanical Properties of CP-Titanium Processed by ECAP Followed by Warm Caliber Rolling

In this study, microstructural evolution and mechanical properties of commercial purity titanium after a combined equal channel angular pressing (ECAP) and warm caliber rolling (WCR) was investigated. The ECAP process was applied to enhance the hardness and strength of the specimen by decreasing the grain size and producing UFG microstructure. WCR was applied to reduce cross-section and increase the ductility of the ECAPed specimens. Results show that WCR reduces the work-hardening rate by increasing grain size and also increases elongation and workability while it reduces the yield and ultimate tensile strength. It has been shown that the strength ratio ( $${{\sigma_{UTS} } \mathord{\left/ {\vphantom {{\sigma_{UTS} } {\sigma_{y} }}} \right. \kern-0pt} {\sigma_{y} }}$$ ) and strain ratio ( $${{\varepsilon_{UTS} } \mathord{\left/ {\vphantom {{\varepsilon_{UTS} } {\varepsilon_{t} }}} \right. \kern-0pt} {\varepsilon_{t} }}$$ ) of the processed samples are comparatively larger than all previously post processed ECAPed materials at lower temperatures.

Cryogenic Mechanical Properties of Warm Multi-Pass Caliber-Rolled Fine-Grained Titanium Alloys: Ti-6Al-4V (Normal and ELI Grades) and VT14

The effect of microstructural refinement and the β phase fraction, V β, on the mechanical properties at cryogenic temperatures (up to 20 K) of two commercially important aerospace titanium alloys: Ti-6Al-4V (normal as well as extra low interstitial grades) and VT14 was examined. Multi-pass caliber rolling in the temperature range of 973 K to 1223 K (700 °C to 950 °C) was employed to refine the microstructure, as V β was found to increase nonlinearly with the rolling temperature. Detailed microstructural characterization of the alloys after caliber rolling was carried out using optical microscopy (OM), scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), and transmission electron microscopy (TEM). Complete spheroidization of the primary α laths along with formation of bimodal microstructure occurred when the alloys are rolled at temperatures above 1123 K (850 °C). For rolling temperatures less than 1123 K (850 °C), complete fragmentation of the β phase with limited spheroidization of α laths was observed. The microstructural refinement due to caliber rolling was found to significantly enhance the strength with no penalty on ductility both at room and cryogenic temperatures. This was attributed to a complex interplay between microstructural refinement and reduced transformed β phase fraction. TEM suggests that the serrated stress–strain responses observed in the post-yield deformation regime of specimens tested at 20 K were due to the activation of $$ \left\{ {10\bar{1}2} \right\} $$ tensile twins.

Effects of caliber rolling on microstructure and mechanical properties in twinning-induced plasticity (TWIP) steel

The effect of caliber rolling on microstructure and mechanical properties of Fe-Mn-Al-C twinning-induced plasticity (TWIP) steel has been investigated to find alternative methods of wire drawing process using the numerical simulation, electron backscatter diffraction (EBSD) techniques, transmission electron microscopy (TEM), and hardness test. Behavior of twinning, texture, and effective strain was different with areas of rolled wire due to the difference in stress state and strain. The center area had maximum twin density, low angle boundary (LAB), effective strain, and hardness; whereas the surface area had minimum values. In comparison with wire drawing process, caliber rolling process imposed higher stain with slightly uniform distribution on wire, indicating that caliber rolling can manufacture high strength wires more effectively. For instance, after the area reduction of 22% by caliber rolling, the tensile strength was 47% higher and hardness inhomogeneity factor was 15% lower as compared to samples processed by wire drawing. In other words, caliber rolling process was suitable to make high strength materials deformed and hardened by twinning mechanism such as TWIP steels due to the characteristics of imposing severe strain with multi-direction, multi-pass with different shape at each pass, and alternating the loading direction between passes. Therefore, caliber rolling process can be a strong candidate in replacement of wire drawing process, especially TWIP steels.

Warm tempforming effect on the hydrogen embrittlement of 1.8-GPa-class ultra-high-strength low-alloy steel

Hydrogen embrittlement properties were investigated for 1.8-GPa-class ultra-high strength low-alloy steels by means of slow-strain-rate test of the pre-hydrogen-charged notched specimens, accelerated atmospheric corrosion test, and thermal desorption spectrometry. A Mo-bearing steel with a chemical composition of Fe-0.4C-2Si-1Cr-1Mo (mass%) was quenched and tempered at 773K for 1h and then deformed by multi-pass caliber rolling with a cumulative rolling reduction of 76% at 773K to create an ultrafine elongated grain structure with a strong <110>//rolling direction fiber texture. The warm tempformed (TF) sample was subsequently annealed for 1h to clarify the hydrogen trapping effect of nanoscale carbides relative to additive Mo. When the TF sample was annealed at 843K (TFA sample), the hydrogen absorption capacity was enhanced significantly through the formation of nanoscale Mo-rich precipitates in the matrix of ultrafine elongated grains. A high potential for hydrogen embrittlement resistance in an atmospheric corrosion environment was demonstrated in both the TF and TFA samples with an ultra-high tensile strength of 1.8GPa. The TF and TFA samples were much less susceptible to hydrogen embrittlement as compared to the tempered martensitic samples at an ultra-high tensile strength of 1.8GPa. The hydrogen trapping states and the high resistance to hydrogen embrittlement in the TF and TFA samples are discussed in association with the anisotropic, ultrafine grained structures with the nanoscale Mo-rich precipitates.

Microstructure and texture evolution in AZ31 magnesium alloy during caliber rolling at different temperatures

Magnesium alloy AZ31 was caliber rolled at different temperatures viz. 523K, 573K, 623K, 673K and 723K imposing a total cumulative reduction of 92% and a cumulative strain of 2.6. The associated microstructure and texture evolution were studied using Electron Back Scattered Diffraction (EBSD) and X-ray macro-texture observations respectively. Grain refinement was observed in all the caliber rolled bars indicating the occurrence of dynamic recrystallization. Asymmetry parameter and texture index were used to study the evolution of texture. Near doubling of the yield strength at room temperature was observed by caliber rolling at a temperature of 573K. The improvement in mechanical properties was explained on the basis of the fine grain microstructures and suitable development of crystallographic texture. Based on the present study, warm caliber rolling can be used as a good production method for obtaining long bars of high strength magnesium alloy AZ31.

A thermomechanical process to achieve mechanical properties comparable to those of quenched-tempered medium-C steel

In quenching-tempering (QT), limiting factors such as hardenability, temper embrittlement, quenchant pollution, and quenching distortion and cracks have to be considered. In order to avoid these limitations, we propose a thermomechanical process to fabricate spheroidized cementite in ferrite matrix in a medium-C steel with mechanical properties that are comparable to QT, which is based on multi-pass caliber rolling at start rolling temperature (SRT) in two-phase α+γ region. In this study, the effect of the SRT (in the range from 720°C to 820°C) on microstructures, mechanical properties, and impact fracture behavior were investigated by using 45 steel as experimental material. Results indicate that the rolling can result in a fibrous “ferrite + pearlite” microstructure. If the SRT is at 740°C to 760°C, then the multi-sized ferrite grains with intensive α-fiber texture (<101>//RD) and the completely spheroidized cementite distributed in the fine ferrite grain region can be formed. The spheroidization of cementite is attributed to the deformation-induced divorced eutectoid transformation. The mechanical properties significantly exceeded the requirements of the Chinese National Standard GB/T 699−1999 for quenched-tempered 45 steel. When the SRT decreased from 780°C to 820°C to 720°C to 760°C, the Charpy impact absorbed energy (AKV) increased from approximately 50J to 100J, and the impact fracture behavior changes from dimple along with some splits into quasi-cleavage. This thermomechanical process is a potential alternative to QT of medium-C steels.