Room Temperature Rolling Research Articles

Effect of two-step cryorolling and aging on mechanical and electrical properties of a Cu–Cr–Ni–Si alloy for lead frames applications

Abstract High strength and high electrical conductivity Cu–Cr-based alloys are important candidate materials for preparing lead frames, railway contact wires and connects. However, high strength and high electrical conductivity are mutually exclusive in nature. To optimize the mechanical and electrical properties of a Cu–Cr–Ni–Si alloy, the two-step cryorolling and aging (CRA) was carried out in this study. Here, by the analysis of microstructure evolution, precipitation behavior, and corresponding strengthening mechanisms of the alloy at different rolling and aging conditions, we find that the yield strength and electrical conductivity for the two-step CRA sample are improved by 91.3 MPa and 4.9 %IACS (International Annealed Copper Standard) than those of the sample subjected to single-step room temperature rolling and aging (RTRA). The higher yield strength for the two-step CRA sample is mainly determined by the precipitation strengthening contributed from Ni2Si and Cr precipitates, rather than the twin boundary strengthening and dislocation strengthening. In fact, the dislocation density of the two-step CRA sample is lower than that of the single-step RTRA sample because of the faster drop rate of recovery, which weakens the effect of dislocation strengthening. The combination of the first-step aging and second-step CR process for the two-step CRA sample contributes to the formation of more precipitates from the matrix, thereby significantly increasing the precipitation strengthening and decreasing the impurity scattering in the final aging. The results provide a new preparation process and a reliable theoretical approach for optimizing the mechanical and electrical properties of the Cu–Cr alloys.

Microstructures and Mechanical Properties of a Commercial Pure Zirconium during Rolling and Annealing at Different Temperatures

Rolling and annealing are important processes for the fabrication of zirconium alloy sheets. Herein, the microstructure and mechanical properties of a commercial pure zirconium during rolling and annealing at different temperatures are investigated. Compared with rolling at room temperature (RT), liquid nitrogen temperature (LNT) rolling will cause more inhomogeneous deformation and higher dislocation density, which significantly influences the subsequent annealing. For a given rolling reduction, the higher dislocation density in the LNT‐rolled specimen causes faster recovery and recrystallization and leads to larger average grain size in subsequent annealing compared with that in the RT‐rolled specimen. The inhomogeneous grain size may be attributed to the nonuniform deformation in the LNT rolling. Compared with RT rolling, higher yield strength is obtained in the LNT‐rolled specimen, which is caused by the suppression of dynamic recovery in low‐temperature deformation. With higher strength, the elongation of the LNT‐rolled specimen is not lower than that in the RT‐rolled specimen, which may be related to the inhomogeneous microstructure‐induced extra work‐hardening capability.

Microstructure evolution and mechanical properties of Al−3.6Cu−1Li alloy via cryorolling and aging

Abstract An Al−3.6Cu−1Li alloy was subjected to room temperature rolling and cryorolling to investigate their effects on microstructure evolution and mechanical properties. The microstructure and aging characteristics of the room temperature-rolled and the cryorolled alloys with 70% and 90% of thickness reductions were studied by microstructure analysis and mechanical tests. The samples subjected to cryorolling with 90% of thickness reduction have high strength and good toughness. This is mainly due to the inhibition of dynamic recovery and the accumulation of high-density dislocations in cryorolled samples. In addition, the artificial aging reveals that the temperature at which peak hardness is attained is inversely proportional to the deformation amount and directly proportional to the rolling temperature. Moreover, bright field images of cryorolled samples after aging indicate the existence of T1 (Al2CuLi) precipitates. This suggests that the high stored strain energy enhances the aging kinetics of the alloy, which further promotes the nucleation of T1 phases.

Study on the variation of properties of Cu–Cr–Zr alloy by different rolling and aging sequence

The relationship between rolling-aging sequence and the microstructure evolution, mechanical-electrical properties of hot rolled-quenched Cu-0.67Cr-0.27Zr-0.12Ni-0.03Si-0.04Ti alloy was investigated in details and the underlying mechanism of the abnormal decrease in electrical conductivity of the peak-aged alloy by cold rolling was also revealed. The results indicated that a good combination of the tensile strength (585.5 MPa), yield strength (562.1 MPa), Vickers hardness (197.6 HV), and electrical conductivity (67.4 %IACS) was obtained in the Cu–Cr–Zr-based alloy by a sequence of peak aging at 450 °C for 2 h followed by 80% room-temperature rolling and then secondary aging at 400 °C for 1 h. Compared with the rolling-aging process, the strength improvement of the alloy by the aging-rolling-aging process was mainly dominated by dislocation density strengthening. Additionally, a decrease in electrical conductivity of the peak-aged alloy after cold rolling was attributed to the re-dissolution and the refinement of partial coherent precipitates.

Effect of Initial Texture on the Evolution of Microstructure and Texture During Rolling of Commercially Pure Titanium at Room and Cryogenic Temperature

The evolution of texture and microstructure of commercially pure titanium during unidirectional rolling at room and cryogenic temperature was studied for two different initial orientations, namely RD (rolling direction)-split basal (BA orientation) and prismatic-pyramidal (PP orientation) till 90 pct rolling reduction. Effect of micro-mechanisms of deformation by slip and twinning on texture evolution and grain fragmentation was studied by employing characterization tools like bulk texture measurement using X-ray diffraction and electron back scatter diffraction. Both orientations yield split TD (transverse direction) texture for 90 pct rolling reduction at both the temperatures. Orientation BA showed severe twinning that saturated at deformation of 50 pct followed by slip dominated deformation, while orientation PP showed slip dominated deformation with marginal twinning throughout the rolling reduction at room temperature. Profuse twinning led to homogeneous grain refinement in orientation BA, while orientation PP showed heterogeneous grain refinement due to absence of twinning in some grains at the final stage of deformation. Rolling at cryogenic temperature enhances grain refinement due to extensive twinning that leads to weakening of texture in BA orientation and increase in intensity of texture for PP orientation compared to room temperature rolling. The evolution of microstructure and texture contributed to similar hardness for 90 pct room and cryogenic temperature rolled BA orientation, while 90 pct room temperature rolled PP orientation showed lower hardness.

Effect of cryorolling on microstructure and property of high strength and high conductivity Cu−0.5wt.%Cr alloy

Abstract Cu−0.5wt.%Cr alloy with high strength and high conductivity was processed by cryorolling (CR) and room temperature rolling (RTR), respectively. The microstructure, mechanical property and electrical conductivity of Cu−0.5Cr alloy after CR/RTR and aging treatment were investigated. The results indicate that obvious dislocation entanglement can be observed in matrix of CR alloy. The Cr particles in the alloy after CR and aging treatment possess finer particle size and exhibit dispersive distribution. The peak hardness of CR alloy is HV 167.4, significantly higher than that of RTR alloy. The optimum mechanical property of CR alloy is obtained after aging at 450 °C for 120 min. The conductivity of CR Cu−0.5Cr alloy reaches 92.5% IACS after aging at 450 °C for 120 min, which is slightly higher than that of RTR alloy.

Grade-4 commercially pure titanium with ultrahigh strength achieved by twinning-induced grain refinement through cryogenic deformation

The yield strength of commercially pure (CP) Ti of ASTM grade 4, the strongest among all the CP-Ti grades, is too low for structural applications that require high-strength materials. Here, we demonstrate the strengthening of grade-4 CP Ti by cryogenic-temperature rolling (CTR), which enables deformation twinning in grade-4 CP Ti to achieve twinning-induced grain refinement. CTR activated 112-2 twinning and 101-2 twinning, which are the most common twinning systems in pure Ti, whereas room-temperature rolling (RTR) did not activate any twinning system. CTR with imposing an area reduction of just 30% significantly increased the yield strength of the CP Ti to 946 MPa, which is not achievable through typical processes performed at or above room temperature and is comparable to that of commercial Ti-6Al-4V. The significant increase in strength was due to microstructural strengthening caused by twinning-induced grain refinement, combined with dislocation accumulation. In contrast to RTR, CTR greatly increased the stress concentration at grain boundaries (GBs), which caused the unusual activation of twinning in the grade-4 CP Ti by facilitating twin nucleation at GBs. The stress concentration increased because CTR activated the <c+a> slip to a lesser extent compared to RTR, thereby reducing the strain compatibility between neighboring grains. These results will contribute to development of ultrahigh-strength CP Ti and may thereby extend its use to structural applications that require high-strength materials.

Microstructural Evolution and Mechanical Properties of Ultrafine‐Grained Ti Fabricated by Cryorolling and Subsequent Annealing

Ultrafine‐grained (UFG) commercial purity titanium (CP Ti) has a significant potential for use in medical implants and aerospace structural parts. Herein, UFG CP Ti sheets are processed by cryorolling and room‐temperature rolling (RTR), respectively, followed by annealing for 1 h at temperatures from 250 to 350 °C. The grain size is reduced from ≈75 μm to ≈85 and ≈220 nm after cryorolling and RTR, respectively. The results show that the curves of tensile stress versus engineering failure strain for samples subjected to cryorolling and subsequent annealing are above those for samples subjected to RTR and subsequent annealing. In addition, the curves of ultimate tensile stress × fracture elongation versus grain size after cryorolling and annealing are above those for RTR and annealing. It is demonstrated that a combination of cryorolling and annealing leads to improved toughness compared with the processing by RTR and annealing.

Effect of rolling temperatures on mechanical and fracture behavior of AA 3003 alloy and pure Cu

Ultrafine-grained materials have an enormous scope of structural and functional applications owing to their higher mechanical properties compared to that of coarse-grained materials. The influence of room temperature and cryogenic temperature rolling of AA 3003 alloy and pure Cu on its mechanical properties and fracture behavior has been investigated in the present study. Both the materials are rolled to a thickness of 2 mm and 1 mm with a true plastic strain of 0.40 and 1.09, respectively. The deformed samples are characterized by tensile test, hardness test, and X-ray diffraction analysis. It has been observed that tensile strength and hardness of cryorolled (CYR) samples are higher than cold rolled (CR) samples and it is due to the fact that rolling at an extremely low temperature (− 196 °C) suppresses the dynamic recovery and hence mobility of dislocations becomes difficult as a result dislocation density increases. In CYR, low stacking fault energy material (Cu) has higher tendency to increase strength and work hardened compared to higher stacking fault energy materials (AA 3003) without much loss of ductility. SEM pictorial view of fractography revealed that CYR samples have mix mode of failure compared to CR samples. The dimple size is gradually decreased with increasing plastic strain and that is due to severe strain hardening and grain refinement.

Effect of artificial ageing on strength and ductility of an Al-Cu-Mg-Mn alloy subjected to solutionizing and room-temperature rolling

In order to expand the appilication of age-hardenable aluminum alloys, simultaneous improvement of strength and ductility is always desired. In this work, the effect of ageing at 140 °C on tensile properties, microstructures and fractographies of an Al-Cu-Mg-Mn (AA 2024) alloy subjected to solutionizing at 490 °C followed by water quenching and room-temperature rolling with 80% thickness reduction was studied. Tensile results show that both the strength and elongation of the alloy increase first and then decrease during ageing, which leads to the simultaneous improvement of strength and ductility. Microstructural analyses indicate that both the strength and ductility change with the precipitation of S″ and S precipitates and decrease in dislocation density, but not with the grain and texture characteristics. The fractographies occupied by the dimples with the decreasing diameters are due to the removal of precipitation free zones (PFZs) and the increasing S precipitates during ageing.

Microstructural characteristics of AZ31 alloys rolled at room and cryogenic temperatures and their variation during annealing

• AZ31 alloy was rolled at room temperature (RT) and cryogenic temperature (CT). • Cryorolling forms more side cracks owing to the reduced workability at CT. • Cryorolling induces a lower dislocation density and forms less twins than RT rolling. • SRX during subsequent annealing is less pronounced in the CT-rolled material. • The annealed CT-rolled material exhibits an inhomogeneous bimodal structure. This study investigates the microstructural characteristics of AZ31 Mg alloys rolled at room temperature (RT) and cryogenic temperature (CT) and the variation in their microstructure and hardness during subsequent annealing. Cryorolling induces the formation of more side cracks than does RT rolling, because of the reduction in the ability of the material to accommodate deformation at CT. Numerous {10–11} contraction and {10–11}-{10–12} double twins are formed in both the material rolled at RT and that rolled at CT, because the grains of the initial material are favorably oriented for {10–11} twinning under rolling. The RT-rolled material has a higher dislocation density than the cryorolled material, and more twins are uniformly distributed throughout the former material. As a result, static recrystallization during subsequent annealing is more pronounced in the RT-rolled material, which results in the formation of a highly recrystallized homogeneous microstructure after annealing. In contrast, the formed twins are predominantly present along the shear bands in the cryorolled material, as a result of which this material has an inhomogeneous bimodal structure containing a large amount of coarse unrecrystallized grains after annealing. The hardness of the annealed RT-rolled material is higher than that of the annealed cryorolled material owing to the finer grain structure of the former.

Microstructure and Mechanical Properties of 5052 Al Alloy by Cryogenic Rolling

The microstructure, mechanical properties, texture evolution and microstructure-property relationship of 5052 Al alloy by cryogenic-rolling (CR) and room-temperature rolling (RTR) were investigated. The results show that CR can effectively refine the grain size and optimize the comprehensive mechanical properties of the material. At the same time, the maximum strengthening effect of CR can be achieved when the deformation is 50%. In addition, the temperature benefit of CR can reduce stacking fault energy.

Highly Ordered and Dense Thermally Conductive Graphitic Films from a Graphene Oxide/Reduced Graphene Oxide Mixture

Summary We report a new approach to making highly dense, oriented, and crystalline graphite films from heat-treated and pressed graphene oxide (G-O). By introducing small-diameter reduced graphene oxide (rG-O) flakes into the graphene oxide starting material, we found that after heat treatment at 3,000°C, the sample density and atomic order substantially improved over a film composed, at the outset, only of pure G-O flakes. A subsequent mechanical press increased the density but reduced the atomic order. A second 3,000°C heat treatment restored the graphitic structure with graphitization metrics exceeding even those of the first heat treatment. The optimized graphitic film with an original concentration of 15 wt % reduced G-O in G-O gave well-oriented graphitic films with a density of 2.1 g cm−3, cross-plane thermal conductivity of 5.65 W m−1 K−1, and in-plane thermal conductivity of 2,025 ± 25 W m−1 K−1.

Precipitate phase transformation behavior, microstructure, and properties of Cu–Cr–Co–Si alloy

Abstract

Effect of pre-ageing treatment on second nucleating of GPII zones and precipitation kinetics in an ultrafine grained 7075 aluminum alloy

To clarify the influence of a pre-ageing treatment on precipitation behaviors in the AA7075 aluminum alloys with ultrafine grain (UFG) structure, the alloys were pre-aged prior to a room temperature rolling process (PA + RTR). The precipitation kinetics in PA + RTR alloys during isothermal heat treatment at 100 °C were systematically investigated by transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and Vickers hardness measurements compared to its counterpart without pre-ageing treatment (i.e. RTR). Results reveal the pre-ageing treatment can lead to a higher dislocation density (22.8 × 1014 m−2) and a larger concentration of Mg (~12.85 at.%) and Zn (~8.33 at.%) atoms in the vicinity of dislocation, which causes a considerable increase in the size and volume fraction of the precipitates in PA + RTR alloy. Specially, our study shows, for the first time, that many GPII zones come from second nucleating in PA + RTR alloy at 108 h, which may ascribe to the pre-ageing treatment. More importantly, the pre-ageing treatment not only makes both two hardness peaks of PA + RTR alloy appeared ahead of but also higher than those of RTR alloy, by virtue of the enhanced precipitation kinetics in PA + RTR alloy.

Simultaneous Achievement of High Strength and Superior Ductility in an As-Rolled Cu-30Zn Brass

Strength and ductility are the two most essential properties for copper alloys which are commonly used as structural materials. However, these properties are often mutually exclusive and seldom improved simultaneously in the samples fabricated through plastic deformation. For example, room temperature rolling (RTR) samples with a large deformation amount usually exhibit high strength but poor ductility. The present study provides an effective strategy for obtaining satisfactory mechanical properties in a Cu-30Zn brass. Compared with RTR sample with a large deformation amount (90%), a desired and outstanding combination of high strength and superior ductility can be achieved in a low deformation (70%) sample fabricated through ultra-low temperature rolling (UTR). The average tensile strength of 90% deformation RTR sample is 641.7 MPa, whereas that of 70% deformation UTR sample is 695.4 MPa. The average elongation of the 90% deformation RTR sample is only 3.7%. By contrast, the average elongation of 70% deformation UTR sample is 7.2%. In the meantime, the average yield strength of 70% deformation UTR sample is higher than that of 90% deformation RTR sample, which are 598.2 and 561.7 MPa, respectively. Large proportions of ultrafine grains and nanometer-scale deformation twins are responsible for the excellent mechanical properties of 70% deformation UTR sample. This effective method is vital for industry production to simultaneously obtain high strength and superior ductility in Cu-30Zn brass and may be also suitable for other copper alloys.

Densification Mechanism for the Precursor of AFS under Different Rolling Temperatures.

The effect of rolling temperature on the precursor of aluminum foam sandwich (AFS) prepared by powder metallurgy through Pack Rolling method is investigated in this work. The cross-section along rolling direction of the precursors was observed. It was found that periodic corrugated morphology with micro-cracks on the composite interface as well as cracks and micro-holes among core powder particles emerged abundantly at room temperature rolling. These defects degraded with increasing rolling temperature and completely disappeared when the rolling temperature reached 400 °C. Combining with foaming ability of these precursors, the densification mechanism of core powders was discussed. Powder particles deformed with difficulty at low rolling temperature; the gap between them cannot be effectively filled through their plastic deformation. Fracture occurred in powder core layer during co-extension with the outer panel and was partly embedded by it, resulting in corrugated composite morphology at the interface. The precursors of high density and excellent bonding interface were prepared at the rolling temperature of 400 °C. A more suitable foaming condition was determined.

Nanocrystallization of pure Zr via room temperature rolling deformation

Bulk nanostructured metallic materials attract the growing interest of material scientists due to their exceptional properties. However, it is difficult to form nanostructures unless using severe plastic deformation strategies, e.g. high-pressure torsion. Here, a traditional symmetric rolling was employed to deform hcp pure Zr at room temperature. Fortunately, the nanostructure with a mean grain size of ∼90 nm was formed in the metal, which can be attributed to the enhanced grain refinement by decreasing its initial (002) texture.

Structure and strength of the 1570C aluminum alloy after complex SPD processing

The effect of severe plastic deformation (SPD), realized via multi-directional forging (MDF) at a constant temperature of 325 °C, as well as via a combination of MDF with subsequent room temperature rolling or high pressure torsion, on the microstructure, static and fatigue strength parameters of an ingot of non-age-hardenable commercial aluminum alloy 1570C (Al-5Mg-0.18Mn-0.2Sc-0.08Zr) was studied. It is found that the most effective treatment to enhance the balance of the alloy properties involves a sequence of hot MDF and cold rolling due to the development of a work hardened ultrafine-grain structure with uniformly distributed nanosized aluminides of transition metals. The role of precipitates in the alloy structure- and property-formation is discussed.

Effect of Room Temperature Rolling Followed by Annealing on Tensile and Fracture Behaviour of Bulk Ultrafine-Grained Al–Li 8090 Alloy

The mechanical properties and microstructural evolution of aluminium–lithium (Al–Li) 8090 alloy subjected to room temperature rolling (RTR) followed by annealing were investigated in the present work. Tensile, hardness test and three-point bend tests were performed to identify the mechanical properties, while optical microscopy, scanning electron microscopy and transmission electron microscopy were used for correlating mechanical properties with microstructural features. Results showed that the tensile strength and hardness increase firstly and then decrease with elevating annealing temperature when the holding time is kept for 1 h that is attributed to the strengthening of grain boundary, solid solution strengthening and precipitation strengthening. An increment in ultimate tensile strength from 265 to 359 MPa (approx. 35% increase) and yield strength from 186 to 234 MPa (approx. 25% increase) was observed substantiating the formation of ultrafine-grained microstructure at room temperature. On annealing in the 100–350 °C temperature range with 50 °C interval, the mechanical properties such as tensile strength and yield strength increase significantly up to 150 °C (UTS-522 MPa and YS-398 MPa); however, there is gradual decrease in the tensile and yield strength observed beyond these temperatures and these values (UTS-124 MPa and YS-109 MPa) become least on annealing at 350 °C. It is observed that a 19% increase in the apparent fracture toughness (KQ) occurred for RTR Al–Li (8090) alloy as compared to solution-treated Al–Li 8090 alloy in three-point bend test.