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Materials of a gradient structure have been shown to possess both high strength and high ductility. To date, materials of a gradient structure can only be produced in small quantities. In this paper, we report a novel 'cyclic skin-pass rolling' (CSPR) technique capable of producing sheets of gradient structure in large quantities. Both experimental and analytical/numerical investigations are reported. In the experiments on aluminium sheets, the outer layer was subjected to 40 passes of CSPR with a reduction ratio of 1% per pass. After CSPR, the sample surface shows an ultrafine-grained microstructure with a mean grain size of 206 nm, while the annealed microstructure is retained in the core of the sample. Compared with cold-rolled aluminium sheets fabricated with the same total reduction ratio, CSPR-processed aluminium sheets have the same yield stress but improved uniform elongation (2.4 times). The scanning electron microscopy was used to study the fracture surface, and The transmission electron microscopy to examine the microstructure near the fracture end, in order to analyse the improvement in ductility. In addition, the finite element method was used to simulate the roll-sample contact pressure and strain distribution as well as residual stress on the sheet surface during CSPR, and to better understand the mechanism leading to improvement of ductility of the sheets by the CSPR technique.

The mechanical properties of the Ti-6Al-4V alloy are significantly determined by the process used for manufacture. In this study, the mechanical properties of the Ti-6Al-4V alloy manufactured using cold rolling, asymmetric rolling and asymmetric cryorolling were characterized by subjecting it to the tensile test and the microhardness test. The evolution in the microstructure was studied using scanning electron microscopy, transmission electron microscopy as well as energy dispersive X-ray spectrometry. Results show that the Ti-6Al-4V alloy sheets subjected to asymmetrical cryorolling with low rolling speed ratio between the upper and lower rolls have the highest tensile strength and microhardness. The tensile stress of the alloy samples after cold rolling was found to be 1008 MPa, 1046 MPa after asymmetric rolling, and 1113 MPa after asymmetric cryorolling. The highest Vickers hardness (HV395) is achieved by asymmetric cryorolling. We discuss the mechanical behavior using the criteria of grain size, size of second phase as well as dislocation density.

Grain growth and grain refinement behavior during deformation determine the strength and ductility of ultrafine-grained materials. We used asymmetric cryorolling to fabricate ultrafine-grained copper sheets with an average grain width of 230 nm and having a laminate structure. The sheets show a high-true failure strain of 1.5. Observation of the microstructure at the fracture surface reveals that ultrafine laminate-structured grains were simultaneously transformed into both equiaxed nanograins and coarse grains under tensile deformation at room temperature.

The microstructure evolution and mechanical properties of ultrafine-grained (UFG) Al sheets subjected to accumulative roll bonding (ARB) and subsequent cryorolling was studied. Cryorolling can suppress the dynamic softening of UFG Al sheets subjected to ARB at room temperature. After the third ARB pass, the grains are slightly refined as the number of ARB passes increases. However, the grains are significantly refined further during cryorolling. The grain size of 460 nm achieved after the third ARB pass is reduced to 290 nm after two cryorolling passes with total reduction ratio 80%. Sheets subjected to ARB + cryorolling show improved mechanical properties compared to only ARB-processed sheets due to a change in the fraction of high-angle boundaries and elongated grains. The deformation mechanism for ultrafine grains at room temperature is determined by grain boundary sliding or dislocation-based recovery, while it is governed by dislocation glide at cryogenic temperature.

The carbon capture and storage (CCS) and enhanced oil recovery (EOR) projects entail the possibility of accidental release of carbon dioxide (CO2) into the atmosphere. To quantify the spread of CO2 following such release, the 'Gaussian' dispersion model is often used to estimate the resulting CO2 concentration levels in the surroundings. The Gaussian model enables quick estimates of the concentration levels. However, the traditionally recommended values of the 'dispersion parameters' in the Gaussian model may not be directly applicable to CO2 dispersion. This paper presents an optimisation technique to obtain the dispersion parameters in order to achieve a quick estimation of CO2 concentration levels in the atmosphere following CO2 blowouts. The optimised dispersion parameters enable the Gaussian model to produce quick estimates of CO2 concentration levels, precluding the necessity to set up and run much more complicated models. Computational fluid dynamics (CFD) models were employed to produce reference CO2 dispersion profiles in various atmospheric stability classes (ASC), different 'source strengths' and degrees of ground roughness. The performance of the CFD models was validated against the 'Kit Fox' field measurements, involving dispersion over a flat horizontal terrain, both with low and high roughness regions. An optimisation model employing a genetic algorithm (GA) to determine the best dispersion parameters in the Gaussian plume model was set up. Optimum values of the dispersion parameters for different ASCs that can be used in the Gaussian plume model for predicting CO2 dispersion were obtained.

Twin-roll strip casting is an effective technology to produce magnesium alloy strips economically. The aim of this work is to propose suitable (optimized) process parameters for manufacturing AZ61 strips using the twin-roll strip casting technology. Experiments on twin-roll strip casting of an AZ61 magnesium alloy were carried out. Temperature fields, fluid flow fields, and stress fields accompanying the process were simulated using thermal-fluid and thermal-mechanical finite element methods. The effects of casting speed and pouring temperature on temperature fields, fluid flow fields, and stress fields during the process were analyzed. It was found that the optimum pouring temperature should be in the range 690 similar to 715 degrees C and the optimum casting speed in the range 2.3 and 2.5 m/min.

The behavior of internal crack healing in low-carbon steel samples undergoing hot plastic deformation was investigated using the MMS 200 thermo-mechanical simulator. The characterization of cracks after plastic deformation was analyzed using scanning electron microscopy under different heating temperatures, reduction ratios, numbers of deformation passes, strain rates, and holding time durations. It was found that the degree of crack healing increases with increasing heating temperature, reduction ratio, and holding time duration, and with decreasing number of deformation passes and strain rate.

The development of CO2 pipelines for Carbon Capture and Storage (CCS) raises new questions regarding the control of ductile fracture propagation and fracture arrest toughness criteria. The decompression behaviour in the fluid must be determined accurately in order to estimate the proper pipe toughness. However, anthropogenic CO2 may contain impurities that can modify the fluid decompression characteristics quite significantly. To determine the decompression wave speed in CO2 mixtures, the thermodynamic properties of these mixtures must be determined by using an accurate equation of state. In this paper we present a new decompression model developed using the Computational Fluid Dynamics (CFD) package ANSYS Fluent. The GERG-2008 Equation of State (EOS) was implemented into this model through User Defined Functions (UDF) to predict the thermodynamic properties of CO2 mixtures. The model predictions were in good agreement with the experimental data of two 'shock tube' tests. A range of representative CO2 mixtures was examined in terms of the changes in fluid properties from the initial conditions, with time and distance, immediately after a sudden pipeline opening at one end. Phase changes that may occur within the fluid due to condensation of 'impurities' in the fluid were also investigated. Simulations were also conducted to examine how the initial temperature and impurities would affect the decompression wave speed.

This paper describes two separate field experiments involving the venting of natural gas pipelines. The pipelines were 32 km and 60 km in length respectively. The studies were carried out as part of an investigation of the phenomenon of Low Temperature Excursions (LTE) under the aegis of the Energy Pipelines Cooperative Research Centre (EPCRC), Australia. When a highly compressed gas is allowed to escape from a pipeline, the large drop in pressure is accompanied by a significant drop in the temperature of the gas. The general impression is that the pipeline material is also cooled to a comparable extent. This has often led to over-specification of the properties of the pipeline material. Theoretical and CFD studies have shown that although the gas undergoing severe decompression can indeed attain very low temperatures, the adjacent pipe wall does not experience cooling to a comparable extent. This appears to be in part due to the short time scales involved, and due to frictional effects at the gas-pipe wall interface [1]. In the field experiments described, in-situ measurements of gas pressure and pipe wall temperature were carried out. One of the field experiments also involved thermal imaging of the vent pipe surface. The measurements showed that even the most susceptible parts of the vent pipe were not excessively cooled during the event, i.e. much less than the cooling experienced by the gas at the corresponding location. CFD simulations of the highly transient flow in one of the field studies suggest that this may be due to a combination of the time scales involved, and frictional dissipation in severe pipeline decompression, i.e. rapid decompression from an initially high pressure level. Based on the above findings, work is in progress to improve the temperature calculations in computer applications.

The bond strength in bimetallic materials is an important material characteristic. In this study, 0.1-mm thick bimetallic foils (AA1050/AA6061) were produced using one pass of accumulative roll bonding followed by three passes of asymmetric rolling (AR). The AR passes were carried out at roll speed ratios of 1.0, 1.1, 1.2, 1.3, and 1.4 separately. Finite element simulation was used to model the deformation of the bimetallic foils for the various experimental conditions. Particular attention was focused on the bonding of the interface between AA1050 and AA6061 layers in the simulation. The optimization of the roll speed ratio was obtained for improvement of the bond strength of the interface of AA1050/AA6061 bimetallic foils during AR process. In the simulation, the mean equivalent strain at the interface zone between the AA1050 and AA6061 layers was seen to reach a peak value at a roll speed ratio of about 1.2 to 1.3, which also corresponded to a high quality bond at the interface as observed experimentally.

It is interesting to imagine what would happen when a mixture of soft-boiled eggs and stones is deformed together. A foil made of pure Ti is stronger than that made of Cu. When a composite Cu/Ti foil deforms, the harder Ti will penetrate into the softer Cu in the convex shapes according to previously reported results. In this paper, we describe the fabrication of multilayer Cu/Ti foils by the roll bonding technique and report our observations. The experimental results lead us to propose a new deformation mechanism for a hard metal surrounded by a soft metal during rolling of a laminated foil, particularly when the thickness of hard metal foil (Ti, 25 mm) is much less than that of the soft metal foil (Cu, 300 mm). Transmission Electron Microscope (TEM) imaging results show that the hard metal penetrates into the soft metal in the form of concave protrusions. Finite element simulations of the rolling process of a Cu/Ti/Cu composite foil are described. Finally, we focus on an analysis of the deformation mechanism of Ti foils and its effects on grain refinement, and propose a grain refinement mechanism from the inside to the outside of the laminates during rolling.

An asymmetric cryorolling technique was used to reduce the thickness of an Al-Mg-Si alloy sheet from 1.5 mm to 0.19 mm. The samples were subsequently aged for 48 h at 100 degrees celcius. The hardness and tensile strength of both rolled and aged sheets increased with the number of passes up to the sixth pass, but the tensile stress decreased after the seventh pass. Investigation of the microstructure of the sheets showed that the grain size after seven passes was about 235 nm and revealed the presence of Fe-Cr-Mn-Si particles in the samples. The deformation of Fe-Cr-Mn-Si particles and sheet thickness affects the ductility when the sheet thickness is less than 0.4 mm, and the strength when the thickness is less than 0.2 mm.

This paper reports a new technique that combines the features of Accumulative Roll Bonding (ARB) and Asymmetric Rolling (AR). This technique has been developed to enable production of ultra-thin bimetallic foils. Initially, 1.5 mm thick AA1050 and AA6061 foils were roll-bonded using ARB at 2006C, with 50% reduction. The resulting 1.5 mm bimetallic foil was subsequently thinned to 0.04 mm through four AR passes at room temperature. The speed ratio between the upper and lower AR rolls was 151.3. The tensile strength of the bimetallic foil was seen to increase with reduction in thickness. The ductility of the foil was seen to reduce upon decreasing the foil thickness from 1.5 mm to 0.14 mm, but increase upon further reduction in thickness from 0.14 mm to 0.04 mm. The grain size was about 140 nm for the AA6061 layer and 235 nm for the AA1050 layer, after the third AR pass.

ONE OF THE key issues in the design and construction of any gas pipeline is the prevention of material fracture. Since the gas is generally transported under high operating pressures, it must be ensured that the gas pipeline is sufficiently tough to arrest the propagation of any potential fracture. For several decades, control of gas pipeline fracture propagation has been under scrutiny due to economic considerations and ecological and safety hazards related to pressurised pipe failure. The Battelle Two Curve approach has been widely used to determine the minimum material arrest toughness by comparing the gas decompression wave velocity with the fracture velocity, both as functions of the local gas pressure. Sufficient knowledge of the gas decompression behavior following the rupture is therefore crucial in determining running fracture arrest toughness levels. The decompression behaviour is influenced by the operating conditions, fluid composition and the material properties of the pipeline itself. This paper describes a two-dimensional decompression model developed using the commercial Computational Fluid Dynamics (CFD) software ANSYS Fluent. The GERG-2008 Equation of State has been implemented into this model to simulate the rapid decompression of common natural gas mixtures. The evolution of the decompression wave speed and phase changes under arbitrary initial conditions is reported. Comparison with experimental results obtained from shock tube tests showed good agreement between simulation and experiment.

Globally, steel pipelines are widely used to transport energy in the form of liquid petroleum and natural gas. The steel used in the manufacture of these pipelines must have high strength and toughness, and high resistance to fracture. The Drop Weight Tear Test (DWTT) is the most widely used test to assess brittle fracture characteristics in steel. The zones of ductile and brittle fracture during DWTT characterize the quality of pipeline steels. In this paper, the Gurson-Tvergaard-Needleman (GTN) fracture models are coupled in a Finite Element model. The ductile and brittle fracture zones in the samples are analyzed under different conditions. The results show that the change in fracture mode during the DWTT is from the brittle to the ductile, then again to the brittle. The calculated absorbed energies during DWTT compare well with experimental findings. Finally, we present an analysis of the transition from ductile to brittle fracture under different conditions.

The Eckert number emerges as an important non-dimensional parameter, in addition to the Reynolds number and the Prandtl number, in problems involving heat transfer in compressible flows. The Eckert number is considered to represent a ratio of the flow kinetic energy at the wall and the specific enthalpy difference between the wall and the fluid, and is important when viscous dissipation is significant. This paper investigates the role played by the Eckert number during rapid decompression of high pressure gas pipelines. During such processes, the gas temperature attains very low values corresponding to sonic flow at the vent location, and it is often assumed that the pipeline material is also cooled to a comparable degree ("Low Temperature Excursions"). This has often led to over-specification of the properties required of the pipeline material. In this paper, it is shown using Computational Fluid Dynamics (CFD) simulations that the highly complex flow during rapid decompression of high-pressure gas pipelines leads to significant frictional dissipation (heating) adjacent to the pipe wall. This prevents the pipeline from attaining excessively low temperatures. It is shown that frictional heating may sometimes lead to a heat transfer reversal. This finding may help pipeline designers in making appropriate recommendations regarding the properties required of the pipeline materials. The paper also describes a preliminary experiment designed carried out to validate the CFD simulations. More detailed experiments are under way.

The shock tube experimental results have shown clearly that the decompression wave was slowed down in a pipe with a rough inner surface relative to that in a smooth pipe under comparable conditions. In the present paper a one-dimensional dynamic simulation model, named EPDECOM, was developed to investigate the effects of pipe wall roughness and pipe diameter on the decompression wave speed. Comparison with experimental results showed that the inclusion of frictional effects led to a better prediction than that of the widely used model implemented in GASDECOM. EPDECOM simulation results showed that the effect of roughness on the decompression wave speed is significant for pipe diameters less than 250 mm. However the decompression wave speed is nearly independent of the roughness for diameters above 250 mm as the frictional effect becomes negligible at such diameters.

This paper describes an investigation of the flow field created by the Gas Metal Arc Welding (GMAW) process using numerical and physical modelling. The aim was to model the dispersion of fume in the immediate vicinity of the welding nozzle and to optimize the design of a novel on-torch fume extraction system. The numerical modelling was carried out using Computational Fluid Dynamics (CFD), and the physical modelling used a combination of the saline solution scale modelling technique and the LIF (Light-Induced Fluorescence) visualization technique. Important details of the structure of GMAW-induced flows were discovered and used to develop a novel on-torch welding fume extraction system, which has been patented. Prototypes were fabricated and tested in full scale welding experiments to validate the design concept and gravimetric results are presented here to demonstrate the quantitative fume extraction efficiency of the full-scale device.

This paper describes an investigation of the flow field created by the Gas Metal Arc Welding (GMAW) process using numerical and physical modelling. The aim was to model the dispersion of fume in the immediate vicinity of the welding nozzle and to optimize the design of a novel on-torch fume extraction system. The numerical modelling was carried out using Computational Fluid Dynamics (CFD), and the physical modelling used a combination of the saline solution scale modelling technique and the LIF (Light-Induced Fluorescence) visualization technique. Important details of the structure of GMAW-induced flows were discovered and used to develop a novel on-torch welding fume extraction system, which has been patented. Prototypes were fabricated and tested in full scale welding experiments to validate the design concept and gravimetric results are presented here to demonstrate the quantitative fume extraction efficiency of the full-scale device.

Computational Fluid Dynamics (CFD) has been used to model the local shielding gas flow and fume dispersion in Gas Metal Arc Welding (GMAW). Experiments using scale models and salt water have been used to validate the numerical models. These techniques allow a quantitative analysis of GMAW-induced flow patterns and were used to study the dispersion of gas around the weld zone. The numerical and physical modelling show good correlation with real wending conditions, where direct flow visualization is more difficult. The paper describes the approach and its application to the understanding of shielding effectiveness and local fume extraction efficiency. The secondary effects of poor shielding seen in practice are consistent with the numerical modelling results.

A non-intrusive cross-correlation method to measure the particle velocities in dilute phase pneumatic conveying is described. The cross-correlation function generated gives information about the time it takes for a particle to travel between two optimally placed measurement planes. Experiments and CFD simulations are used to estimate an optimal inter-plane distance for various flow conditions.

This paper reports on the application of Computational Fluid Dynamics (CFD) techniques to the analysis of gas and air flow in Gas Metal Arc Welding (GMAW), so as to estimate the efficiencies of on-torch welding fume extraction system configurations. It is shown that the flow field induced by the GMAW peocess is a combination of an 'impinging fountain' flow and a 'buoyant wall jet' flow. Previously these two flows have been studied individually, but not together in conjunction with GMSW-induced flow fields. CFD simulations for a free jet flow and an extraction flow are presented, and compared to simulations of impinging fountains. The simulations are in excellent agreement with documented experimental evidence. CFD simulations of on-torch fume extraction systems are then presented. It is shown that extraction through concentric sleeves fails to affect the gas-fume flow in the wall jet in any significant way. This appears to be part of the reason for the poor fume extraction efficiancy of on-torch extraction sleeves to date.