| Titanium alloys have been used in many industrial applications due to their excellent,comprehensive properties.To acquire fine microstructures and satisfactory properties,a welldesigned thermo-mechanical processing(TMP)route is necessary in the industrial production of titanium products.Titanium-based alloys are broadly used in the medical sector,such as implants for replacing damaged human tissues.Among titanium alloys,Ti6A14V alloy is the most applicable due to its excellent mechanical properties and improved corrosion resistance.By adding copper to the Ti6A14V alloy,the Ti6A14V-xCu alloys have been fabricated.The addition of the copper element provides importance by enhancing the properties of the Ti6A14V-xCu alloy,such as plasticity and tensile properties.The exploration for titanium alloys that are capable of affording optimal deformation processing has been continued,and that could be founded only on a profound understanding of the alloys properties such as grain structure arrangement,crystalline orientation,size,shape,and their spatial distribution.It also requires proper understanding of the various intermetallic precipitant formation,growth,and nature of transformation via the various thermo-mechanical processing conditions,which could be appropriate for particular engineering,structural,and safety applications.The effectiveness of a manufacturing process is determined by the proper utilization of materials,since the cost-benefit advantage of a particular firm over the global competitive environment is limited by its capability to afford items with minimum cost and longer service life.Based on this justification,processing alloys under optimized processing conditions of temperature,strain rate,and strain is more economical and desirable,which provides material reduction and reduces undesirable product failure.Hot deformation in various practical industrial processes has utilized hot forging or rolling mechanisms for the manufacturing of semi-finished and finished products of titanium alloys.Most products fabricated by hot processing equipment possess complex set-ups,tedious processes,and high production costs.The processing of the alloy is initiated solely in a single or dual phase region,or both,depending on the desired goal of imparting a set of properties to a particular alloy.Most of the hot deformation behavior involves investigating whether there are apparent differences among the alloy flow stress curves acquired in the dual(α+β)or single(β)phase regions by analyzing the interrelationship among flow stress,temperature,strain rate,and microstructural evolutions.During the hot deformation process at a high strain rate above 1 s-1 and low temperatures,most of the titanium alloys had some microcracks,inhomogeneous deformation,and a non-uniform microstructure.One of the purposes of the study involves investigating the hot deformation behavior of ascast Ti6Al4V-5Cu alloy,aimed at reducing the energy barriers to facilitate alloy hot workability through studying the microstructure evolution and deformation mechanisms by means of the Gleebel 3800 thermo-mechanical simulation system in the temperature range of 790-1040℃with a 50℃ interval at the strain rate interval of 10-3-10 s-1 with a total strain of 0.4,and finally generated a processing window by establishing relationships among deformation variables.In the single-phase region,power dissipation efficiency(η)reached 89%.At high strain rates,such as above 1 s-1,intense flow localization occurs,which causes cracking.During the Ti6A14V alloy,the peak flow stress increased with the increased strain rates.However,in the Ti6A14V-5Cu alloy,due to the addition of the copper element,the effect of the change in peak flow stress with strain rate was found to be different in the dual(α+β)and single(β)phase regions.In the temperature range of 790-890℃,the peak flow stress decreased with the increase in temperature,whereas at temperatures above 890℃,the flow stress does not have a constant relationship with the change in temperature.Thus,the peak flow stress of the Ti6Al4V-5Cu alloy was temperature and strain rate sensitive.Such differences in flow stresses occurred because of the effect of high-temperature deformation under various processing conditions.The degree of instability in the dual(α+β)phase decreased with an increase in temperature and a decrease in strain rate.The alloy activation energies(Q)in the dual(α+β)phase and single(β)phase regions were 175.43 kJ mol-1 and 159.03 kJ mol-1,respectively.The composition and distribution of copper have played a significant role in improving the flow stress tendency in Ti6A14V alloy to enhance its overall plasticity behavior.The peak flow stress variations under different processing conditions were obviously related to the occurrence of slightly different copper content for a shorter duration when the alloy compression reached the plastic zone.In the temperature range of 890-1040℃,the acquired peak flow stress had a direct relationship with the change in copper content,especially in the β phase morphology.The change of strain rate sensitivity with respect to strain rate showed a zigzag pattern.The Ti6Al4V-5Cu alloy showed different strain rate sensitivity effects with regard to change in strain rate,especially at lower rates in the order of magnitude between 10-3 and 10-1 s-1.The hot-deformation behavior of Ti-5Cu alloy was studied in the temperature range of 550800℃ at a strain rate interval of 10-3-10 s-1.Based on the developed microstructure,true strainstress curves,and the generated processing window,a relationship among various deformation variables and,finally,a constitutive equation have been established.Based on the results of the experiments,annealing of the hot forged Ti-5Cu alloy above β transus temperature promoted the formation of fully recrystallized grains,which were developed by the α to β phase transformation mechanism.The fraction of recrystallized grains and their relative size distribution were enhanced by temperature.The relative area fraction of recrystallized grains as a result of annealing was also proportional to the grain size(i.e.,the higher area fraction(%)corresponded to a bigger grain size and vice versa).The occurrence of the dynamic recrystallization process(DRX)began on the bulging boundaries that were the dominant deformation mechanism,especially during the hot deformation process of titanium alloys.The formation of DRX grains contributed to the weakening of the texture of the Ti-5Cu alloy.Such weakening of texture arises from the rotation of DRX grains towards the preferred grain orientation of the higher misorientation angle.The preferred orientations of DRX grains were supported by β twins,which were found in a straight sub-grain boundary component inside the DRX grains.As a result,the concurrent formation of DRX grains and sub-structure twins along[111]direction was attributed to the formation of weak texture in Ti-5Cu alloy.At various temperatures and strain rates,different features of the microstructure were developed.The flow stress increased with the rise in strain rate and the fall in temperature.The work-hardening effect was pronounced at a lower strain rate and lower temperature.However,at higher strain rates,particularly at strain rates of 1 s-1,the work hardening effect dominated.At higher temperatures and lower strain rates(i.e.,at 800℃ and a strain rate of 10-3 s-1),the curve also showed a relative trade-off between work hardening and flow softening behavior.The deformation of the alloy was highly temperature and strain-rate sensitive.An optimum processing route was found at higher temperatures and a slower strain rate.The power dissipation efficiency of the alloy reached 42%.At a higher strain rate,such as 1 s-1,the transformation of the β to α phase was the dominant deformation mechanism in the fully recrystallized Ti-5Cu alloy.The activation of the basal{0001} textured planes paralleling the CD supported the restraint of the deformation of coarse grains and promoted the prior recrystallized grains to remain un-deformed.Setting basal activated planes paralleling to the CD with a 90-degree spread orientation possibly contributed to the formation of elongated α grains.Generally,for a particular intended use in service,the deformation characteristics of the hot-forged Ti-5Cu alloy could be varied,which can be governed by controlling the change in process variables such as temperature,strain rate,strain,material constants,microstructure,texture evolution mechanisms,etc.The effects of a lamella on as-cast Ti6Al4V-5Cu alloy microstructure features and fracture toughness were not investigated.As a result,the influence of annealing temperature on its microstructure evolution,tensile properties,hardness,and fracture toughness behavior were studied.The fracture toughness testing was performed to search for an optimum processing route that provided the highest fracture resistance ability of the Ti6Al4V-5Cu alloy for isothermal deformation processes and to meet the requirements for the safety and damage tolerance design of the alloys as a structural medium.Following annealing treatment in the range of 740-940℃,the phase thicknesses in the alloy were changed.The fracture toughness testing was performed for searching an optimum processing route that provided highest fracture resistance ability of the Ti6Al4V-5Cu alloy for isothermal deformation process and to meet the requirements for the safety and damage tolerance design of the alloys as a structural medium.Following annealing treatment in the range of 740-940℃,the phase thicknesses in the alloy were changed.The αlamellar thickness was found in a range of 1.28-7 μm and β phase thickness in a range of 0.352.3μm.For higher ductility and good fracture toughness,higher annealing temperatures were desirable.The a lamellar thickness was an important microstructural feature influencing the fracture toughness.Lowering the thickening of a platelet/lamellar structure could enhance the fracture toughness of the as-cast Ti6Al4V-5Cu alloy.At various annealing temperatures,the Ti6Al4V-5Cu alloy exhibited varying degrees of deformation resistance.The elongation percentage of the alloy increased with annealing temperature.The effect of the change in annealing temperature on the hardness(HV)of the alloy exhibited a zig-zag profile.The crack path tortuosity and nature of plastic deformation,particularly on the α/β interface structure,were also important influencing factors in fracture toughness.Good plasticity and a regularly tortuous crack propagation path were both favorable for the fracture toughness of the alloy.The influence of hot deformation on the microstructure,internal crystallographic orientation,and texture evolution of the Ti6Al4V-5Cu alloy deformed in various thickness reduction ratios of 15%,58%,and 73%using a rolling process was also investigated.A differential scanning calorimetry investigation also provided insight into the relationship between heat flow capacity and temperature,which aids in the identification of a viable process variable aimed at imparting a specific mechanical performance to Ti6Al4V-5Cu alloy.It was found that the basal<a>and pyramidal<c+a>type slip planes could be activated in the a phase,which dominated the deformation behavior of Ti6Al4V-5Cu alloy.Under various deformation conditions,the alloy revealed different microstructure features.On the 15%deformed alloy,the deformation was performed by the breakdown of prior β grain boundaries(GBβ),which was attributed to the formation of coarse α grains,rotated nearly 45° with respect to the transversal and rolling directions.The presence of different sub-structure geometries made the interior grain size distribution heterogeneous.At room temperature,the percentage elongation,El,of the alloy reached 23.15%on the 58%hot-rolled alloy.On the 73%deformed alloy,refined and randomly oriented characteristics of grains were obtained due to higher thickness reduction,which resulted from the segregation of very fine granules.The influence of grain rotation during hot-rolling revealed that the α/β texture fiber separation angle to maintain the Burger orientation relationship(BOR)of ｛0001}α//{110}β planes decreased with increase of the thickness reduction ratio when the Ti6A14V-5Cu alloy was severely deformed using a hot-rolling mechanism.Activation of tensile {1012}<1011>and compressive {1122}<1123>twins on the deformation of the alloy were also studied.The innovation of the study involves the deformation mechanisms and processing routes of Ti6Al4V-5Cu and Ti-5Cu alloys,which were suggested by considering the influence of copper.The deformation mechanisms and processing routes of these two alloys are still not well described.Thus,Gleebel experiments were performed to obtain the deformation characteristics and processing maps that guide the processing of these two Cu-bearing titanium alloys.Through lowering the thickness of the lamellar by means of appropriate annealing treatments,the alloy’s internal structure has been modified.As a result,the fracture toughness has improved significantly,and the service safety of the alloys has also improved.Different microstructure features have been developed during the hot-rolling of the Ti6Al4V-5Cu alloy with various thickness reduction ratios,and the microstructures have been characterized systematically to obtain comprehensive mechanical properties. |
PDF Full Text Request