Influence of incremental ECAP on the microstructure and tensile behaviour of commercial purity titanium

Severe plastic deformation (SPD) is an effective method for producing ultrafine grained (UFG) structures in metals. UFG materials are characterized by an average grain size of <1 µm and mostly high angle grain boundaries. These materials exhibit exceptional improvements in strength, superplastic behaviour and in some cases enhanced biocompatibility. Among various SPD methods available, equal channel angular pressing (ECAP) is the most effective method for obtaining bulk UFG billets. Lately, the interest is towards industrialization of the ECAP technique to enable processing of very long or continuous billets. Incremental ECAP (I-ECAP) developed at University of Strathclyde, offers such possibility. The present work details the processing of commercial purity titanium (CP-Ti), using I-ECAP process, with the objective of improving its strength characteristics. CP-Ti billets were successfully processed for up to four passes at 300 °C using an I-ECAP die with a channel angle of 90°. Electron backscatter diffraction (EBSD) technique was used to characterize the microstructure after first and fourth pass of the process. Analysis of the first pass sample revealed heterogeneous structure with a mixture of elongated and refined equi-axed grains. Moreover, existence of {101 ̅2} tensile twinning in the microstructure was also observed. Remarkable refinement was achieved after fourth pass and ultrafine-grain (UFG) structure was successfully achieved. Room temperature tensile tests carried out on unprocessed and UFG material, display the improvement in strength. The yield strength of the processed material was increased from 308 to 671 MPa and the ultimate tensile strength from 549 to 730 MPa. However, strain-hardening ability of the material was greatly reduced because of processing. Consequently, the material suffers loss in ductility, from 31.9% elongation to failure in the unprocessed form to 21.1% in UFG form. Finally, fracture morphology of the unprocessed and processed CP-Ti displays characteristics of ductile failure. It has been shown that I-ECAP is an effective method for improving strength characteristics of CP-Ti

Warm deformation behaviour of UFG CP-Titanium produced by I-ECAP

The objective of the present study is to investigate the deformation behaviour of Ultrafine-grain (UFG) commercial purity Titanium (CP-Ti) at warm temperatures. Firstly, CP-Ti billets were processed through six passes of incremental equal channel angular pressing (I-ECAP) at 300° C using a die channel angle (Φ) of 120°. Uniaxial compression tests were then performed under isothermal conditions on cylindrical samples obtained from the UFG CP-Ti billets. A series of these tests were conducted at different temperatures of 400, 500 and 600 °C and at varying strain rates of 0.01, 0.1 and 1.0 s-1. In each test, the original height of the sample was deformed by ~50% of its original value. The true stress-strain curves obtained, revealed that the flow stress was sensitive to both temperature and strain rate. In general, the flow stress was higher for lower temperatures and higher strain rates. For tests conducted at 400 and 500 °C, the flow stress quickly reaches a peak value, beyond which it exhibits a steady state response where there is no appreciable change in flow stress with increasing strain. The 600 °C tests however shows a strain hardening behaviour. Microstructure of the sample deformed at 600 °C and 0.01 s-1, exhibited significant grain growth

Effect of incremental equal channel angular pressing (I-ECAP) on the microstructural characteristics and mechanical behaviour of commercially pure titanium

Incremental equal channel angular pressing (I-ECAP) is one of the continuous severe plastic deformation (SPD) processes. This paper presents the processing of commercial purity titanium (CP-Ti) using a double billet variant of I-ECAP process. Ultrafine-grain (UFG) structure was successfully achieved after six passes of I-ECAP at 300 °C. Microstructural evolution and texture development were tracked using EBSD. Analysis revealed continuous dynamic recrystallization (CDRX) as one of the grain refinement mechanism during processing. Room temperature tensile tests carried out before and after six passes, shows significant increase in strength with acceptable levels of ductility. The yield strength was increased from 308 to 558 MPa and ultimate tensile strength from 549 to 685 MPa. Compression tests conducted at different strain rates shows considerable increase in strength and enhanced strain rate sensitivity after processing. A distinct three-stage strain hardening was observed during compression. However the processed material displayed a loss in strain hardening ability during tensile as well as in compression tests. Detailed microhardness measurements show the evolution of hardness after subsequent passes with a reasonable level of homogeneity after the sixth pass. It is demonstrated that I-ECAP is an effective method for grain refinement in CP-Ti and subsequently improving its mechanical properties

Effect of channel angle on the material flow and hardness distribution during incremental ECAP of Al-1050 billets

Incremental equal channel angular pressing (I-ECAP) is an extension of the classical ECAP method used to produce ultrafine grained (UFG) metals. This paper investigates the first pass of I-ECAP performed on AA-1050 billets measuring 10x10x60mm and the effects of processing with two different dies with the channel intersection angle ϕ=90° and ϕ=120°. The forces required to produce billets were examined and compared. Micro hardness measurements were performed to create a hardness distribution contour map and to evaluate the strain distribution. Moreover FE simulations were performed to investigate the plastic strain distribution within the billets. It was found that using the ϕ=90° die results in higher deformation forces and also greater uniformity of strain distribution when compared to billets processed with ϕ=120° die. The experimental results correlated well with the findings of the simulations

On the evolution of microstructure and texture in commercial purity titanium during multiple passes of incremental equal channel angular pressing (I-ECAP)

This paper presents an investigation on the evolution of microstructure and deformation characteristics of commercial purity Titanium (CP-Ti) during incremental equal channel angular pressing (I-ECAP). CP-Ti grade 2 was subjected to six passes at 300 °C following route BC, using a die with channel angle of 120°. Electron backscatter diffraction (EBSD) technique was used to characterize the microstructure after first, second, fourth and sixth pass, in the flow and transverse plane of the samples. Texture development through subsequent processing was also investigated using pole figures in both planes. Following first pass, the grain boundary maps across both flow and transverse plane showed a high degree of heterogeneity in grain morphology with the presence of elongated and fine grains. Also, misorientation peaks associated with {10-12} tensile twins and a small fraction of {11-22} compressive twins were observed in the microstructure. After second pass, microstructure was further refined and the twinning activity was greatly reduced with no noticeable activity after the fourth pass. Remarkable grain refinement was achieved after sixth pass with majority of grains in the ultrafine grain (UFG) range and with a relatively homogenous microstructure. Continuous dynamic recrystallization (CDRX) has been observed during subsequent I-ECAP processing. It was seen that twinning alongside CDRX acted as a dominant grain refinement mechanism during the initial passes of I-ECAP process beyond which slip was dominant deformation behaviour

Determination of friction factor by ring compression testing and FE analysis

The goal of this study was to examine performance of various lubricants for aluminium alloy AA5083. Conventional ring compression tests were conducted at 200 °C. Samples were compressed to 50% of the initial height with a constant ram velocity 0.5 mm/s using a servo-controlled hydraulic press. The optimization procedure was implemented in self-developed software to identify friction factors from experiments. The application launches remotely finite element (FE) simulations of ring compression with a changing friction factor until a difference between experiment and numerical prediction of the internal diameter of the sample is smaller than 0.5%. FE simulations were run using Forge3 commercial software. The obtained friction factor quantitatively describes performance of a lubricant and can be used as an input parameter in FE simulation of other processes. It was shown that application of calcium aluminate conversion coating as pre-lubrication surface treatment reduced friction factor from 0.28 to 0.18 for MoS2 paste. It was also revealed that commercially available graphite-based lubricant with an addition of calcium fluoride applied on conversion coating of calcium aluminate had even lower friction factor of 0.1

The origin of fracture in the I-ECAP of AZ31B magnesium alloy

Magnesium alloys are very promising materials for weight-saving structural applications due to their low density, comparing to other metals and alloys currently used. However, they usually suffer from a limited formability at room temperature and low strength. In order to overcome those issues, processes of severe plastic deformation (SPD) can be utilized to improve mechanical properties, but processing parameters need to be selected with care to avoid fracture, very often observed for those alloys during forming. In the current work, the AZ31B magnesium alloy was subjected to SPD by incremental equal-channel angular pressing (I-ECAP) at temperatures varying from 398 K to 525 K (125 °C to 250 °C) to determine the window of allowable processing parameters. The effects of initial grain size and billet rotation scheme on the occurrence of fracture during I-ECAP were investigated. The initial grain size ranged from 1.5 to 40 µm and the I-ECAP routes tested were A, BC, and C. Microstructures of the processed billets were characterized before and after I-ECAP. It was found that a fine-grained and homogenous microstructure was required to avoid fracture at low temperatures. Strain localization arising from a stress relaxation within recrystallized regions, namely twins and fine-grained zones, was shown to be responsible for the generation of microcracks. Based on the I-ECAP experiments and available literature data for ECAP, a power law between the initial grain size and processing conditions, described by a Zener–Hollomon parameter, has been proposed. Finally, processing by various routes at 473 K (200 °C) revealed that route A was less prone to fracture than routes BC and C

3D thermal finite element analysis of single pass girth welded low carbon steel pipe-flange joints

This paper presents a detailed computational procedure for predicting the complete thermal history including transient temperature distribution during girth welding and subsequent post weld cooling of low carbon steel pipe flange joints. Using the FE code ABAQUS, 3-dimensional non-linear heat transfer analysis is carried out to simulate gas metal arc welding (GMAW) process. ANSI Class #300 flange is used with a 6 mm thick, 200 mm long and 100 mm nominal diameter pipe. Joint type is a single ‘V-groove’ butt joint with a 1.2 mm root opening. FORTRAN subroutine is utilized for the application of volumetric heat flux from the weld torch using Goldak’s double ellipsoidal heat source model, which is based on Gaussian power density distribution. Temperature dependent thermal properties as well as phase change effects have also been accounted. Apart from comprehensive discussion on the thermal history, in-depth analysis of the axial temperature profile at four different sections on both sides of the weld joint is presented. The simulated results showed that the temperature distribution around the implemented heat source model is steady when the weld torch moves around the circumferential joint. The present simulation model can be used as a proper tool to investigate the effect of different GMAW process parameters

Effect of incremental equal channel angular pressing (I-ECAP) on the microstructural characteristics and mechanical behaviour of commercially pure titanium

Owing to its high specific strength, low density, outstanding corrosion resistance and excellent bio-compatibility titanium and its alloys are a material of choice in many aerospace, military, chemical and biomedical applications. Ti-6Al-4V is the most widely used alloy for medical device applications such as in total replacement implants, where higher strength characteristics are generally a requirement. However, research has suggested that alloying elements such as aluminium and vanadium present in that alloy can be toxic in the long term and are therefore undesirable for full bio-integration. Commercially pure titanium (CP-Ti) has superior biocompatibility but it lacks the strength required for most load bearing implants. One viable solution is to abandon the use of alloying elements and to improve the mechanical strength and performance of CP-Ti by nano-structuring or grain refinement.;Severe plastic deformation (SPD) is an established method for introducing extreme grain refinement in metals. The technique imparts high plastic strain to the material without significantly changing the sample dimensions and is capable of achieving ultrafine grain (UFG) structure in metals. UFG materials are characterized by an average grain size of <1 μm and with mostly high angle grain boundaries. These materials exhibit exceptional improvements in strength, superplastic behaviour and in case for titanium, improved corrosion resistance and enhanced biocompatibility. Among the various available SPD methods, equal channel angular pressing (ECAP) is the most widely used method for obtaining bulk UFG materials. However, ECAP (in its classical form) suffers from low productivity and is not a practical option for commercialization.;Therefore, lately the interest is in the development of continuous SPD techniques, capable of processing very long or continuous billets for use in commercial applications. Incremental ECAP (I-ECAP) developed at the University of Strathclyde, offers such possibility. This promising technique has a strong potential of obtaining the much-needed high strength CP-Ti for biomedical implants on an industrial scale. The aim of the present research work is to investigate the feasibility of the I-ECAP process in improving the mechanical performance of CP-Ti by refining its grain structure.;However, before processing CP-Ti on the I-ECAP experimental rig, it was necessary to eliminate the some existing limitations of the rig and improve the overall process efficiency. Major upgrades and enhancements were implemented as part of the present work. These include: automation of material feeding system, elevated temperature capability, press controller upgrade, data acquisition and process control during experiments. Moreover, finite element analysis was performed to optimize the tooling geometry by studying the billet deformation behaviour and subsequently new I-ECAP dies were designed and manufactured suitable for processing CP-Ti billets.;Using the considerably improved I-ECAP experimental facility, CP-Ti billets were subjected to multiple passes of the I-ECAP process at elevated temperatures. To investigate the effect of different levels of induced shear strain per pass, billets were processed using two separate dies with channel intersection angles of 120 and 90°. Microstructural evolution and textural development in the material was tracked and examined using high-resolution electron backscatter diffraction (EBSD) technique. Twinning and continuous dynamic recrystallization (CDRX) have been observed to act as a grain refinement mechanism during subsequent passes of I-ECAP. Analysis of the microstructure shows that UFG structure was successfully obtained with mostly high angle grain boundaries (HAGB) in the processed billets using the two dies.;Room temperature tensile tests carried out before and after processing show significant increase in strength with some loss in ductility in the processed material. The yield strength and ultimate tensile strength (UTS) of the material after I-ECAP processing using the die angle of 120° was increased by 81% and 25%, respectively. The material processed using the die angle of 90° exhibits an even higher increase in yield strength and UTS i.e. 118% and 33%, respectively.;Compression tests conducted at different strain rates at room temperature show increase in strength with a three stage hardening behaviour, though the severely deformed UFG material suffers a loss in its strain hardening ability. Detailed microhardness measurements also show the increase in hardness after processing with a reasonable level of homogeneity. Finally, workability characteristics of UFG titanium is determined by compression testing at room and warm temperature conditions (400 to 600 °C). The work has successfully demonstrated that I-ECAP process is effective in improving the mechanical performance of titanium and has a potential for commercialization.Owing to its high specific strength, low density, outstanding corrosion resistance and excellent bio-compatibility titanium and its alloys are a material of choice in many aerospace, military, chemical and biomedical applications. Ti-6Al-4V is the most widely used alloy for medical device applications such as in total replacement implants, where higher strength characteristics are generally a requirement. However, research has suggested that alloying elements such as aluminium and vanadium present in that alloy can be toxic in the long term and are therefore undesirable for full bio-integration. Commercially pure titanium (CP-Ti) has superior biocompatibility but it lacks the strength required for most load bearing implants. One viable solution is to abandon the use of alloying elements and to improve the mechanical strength and performance of CP-Ti by nano-structuring or grain refinement.;Severe plastic deformation (SPD) is an established method for introducing extreme grain refinement in metals. The technique imparts high plastic strain to the material without significantly changing the sample dimensions and is capable of achieving ultrafine grain (UFG) structure in metals. UFG materials are characterized by an average grain size of <1 μm and with mostly high angle grain boundaries. These materials exhibit exceptional improvements in strength, superplastic behaviour and in case for titanium, improved corrosion resistance and enhanced biocompatibility. Among the various available SPD methods, equal channel angular pressing (ECAP) is the most widely used method for obtaining bulk UFG materials. However, ECAP (in its classical form) suffers from low productivity and is not a practical option for commercialization.;Therefore, lately the interest is in the development of continuous SPD techniques, capable of processing very long or continuous billets for use in commercial applications. Incremental ECAP (I-ECAP) developed at the University of Strathclyde, offers such possibility. This promising technique has a strong potential of obtaining the much-needed high strength CP-Ti for biomedical implants on an industrial scale. The aim of the present research work is to investigate the feasibility of the I-ECAP process in improving the mechanical performance of CP-Ti by refining its grain structure.;However, before processing CP-Ti on the I-ECAP experimental rig, it was necessary to eliminate the some existing limitations of the rig and improve the overall process efficiency. Major upgrades and enhancements were implemented as part of the present work. These include: automation of material feeding system, elevated temperature capability, press controller upgrade, data acquisition and process control during experiments. Moreover, finite element analysis was performed to optimize the tooling geometry by studying the billet deformation behaviour and subsequently new I-ECAP dies were designed and manufactured suitable for processing CP-Ti billets.;Using the considerably improved I-ECAP experimental facility, CP-Ti billets were subjected to multiple passes of the I-ECAP process at elevated temperatures. To investigate the effect of different levels of induced shear strain per pass, billets were processed using two separate dies with channel intersection angles of 120 and 90°. Microstructural evolution and textural development in the material was tracked and examined using high-resolution electron backscatter diffraction (EBSD) technique. Twinning and continuous dynamic recrystallization (CDRX) have been observed to act as a grain refinement mechanism during subsequent passes of I-ECAP. Analysis of the microstructure shows that UFG structure was successfully obtained with mostly high angle grain boundaries (HAGB) in the processed billets using the two dies.;Room temperature tensile tests carried out before and after processing show significant increase in strength with some loss in ductility in the processed material. The yield strength and ultimate tensile strength (UTS) of the material after I-ECAP processing using the die angle of 120° was increased by 81% and 25%, respectively. The material processed using the die angle of 90° exhibits an even higher increase in yield strength and UTS i.e. 118% and 33%, respectively.;Compression tests conducted at different strain rates at room temperature show increase in strength with a three stage hardening behaviour, though the severely deformed UFG material suffers a loss in its strain hardening ability. Detailed microhardness measurements also show the increase in hardness after processing with a reasonable level of homogeneity. Finally, workability characteristics of UFG titanium is determined by compression testing at room and warm temperature conditions (400 to 600 °C). The work has successfully demonstrated that I-ECAP process is effective in improving the mechanical performance of titanium and has a potential for commercialization

Numerical study on the effect of the thermo-mechanical properties of alloys on the behaviour of super plastic forming tools

The paper describes a numerical study to investigate the effect of the thermo-mechanical properties of heat resisting Nickel and Chromium alloys, used for super plastic forming (SPF) tools, on the tool service behaviour. The purpose of the paper is to rank the relative importance of each property studied for the heat resisting class of cast nickel and chromium alloys, subjected to repeated thermal cycles under typical industrial SPF conditions. A finite element model of a tool block within an industrial press furnace was developed to simulate the typical thermal cycles of an SPF tool, and predict the resulting mechanical performance. Important thermal and mechanical properties were identified for the cast nickel and chromium class of alloys studied in this paper and suitable ranges for the properties were determined for numerical simulations. The results include a quantitative analysis of the effect of the properties studied