metals Article
Effects of Cryogenic Treatment after Annealing of Ti-6Al-4V Alloy Sheet on Its Formability at Room Temperature Zhiqing Hu 1,2, *, Huihui Zheng 3 , Guojun Liu 3 and Hongwei Wu 4 1 2 3 4
*
Rolling Forging Research Institute, Jilin University, Changchun 130025, China Key Laboratory of Automobile Materials, Ministry of Education, Jilin University, Changchun 130025, China College of Material Science and Engineering, Jilin University, Changchun 130025, China;
[email protected] (H.Z.);
[email protected] (G.L.) School of Engineering Technology, University of Hertfordshire, Hatfield AL109AB, UK;
[email protected] Correspondence:
[email protected]; Tel.: +86-186-0442-1257
Received: 24 February 2018; Accepted: 22 April 2018; Published: 24 April 2018
Abstract: In this article, the effects of cryogenic treatment after annealing on the formability of Ti-6Al-4V alloy sheet were experimentally studied. The Ti-6Al-4V titanium alloy was treated by cryogenic treatment after annealing (ACT). Tensile tests were carried out using a universal machine at room temperature. The microstructure evolution of Ti-6Al-4V subjected to ACT was also investigated using an optical microscope (OM). Both the shearing performance and drawing formability were analyzed by punch shearing tests and deep drawing tests, respectively. Results showed that after ACT, the tendency of the β phase can be apparently changing into stable β’ and α’ phases. The elastic modulus is lower than that of the untreated material. It was found that both the yield strength and tensile strength are declined slightly, whereas the ductility is increased significantly. The shear strength in punch shearing is decreased at room temperature and cryogenic temperature. The ratio of smooth zone on the section after ACT3 is much larger than the others. The rollover diameters are not obviously greater than those of the untreated. Additionally, the height of the burr shows a decreasing trend after ACT. During deep drawing, drawing depth is deeper than that of the untreated material, the drawing load after ACT is reduced, and the decreasing tendency of the drawing load slows down. It is noted that the micro-cracks occur at the bottom of the sample. Keywords: Ti-6Al-4V titanium alloys; cryogenic treatment; mechanical properties; punch shearing; deep drawing
1. Introduction Ti-6Al-4V titanium alloy is being increasingly used in automation, energy, aerospace industries, health care and automobile due to their excellent characteristics such as low density, high specific intensity, strong corrosion resistance, excellent heat resistance, good performance at high and low temperature, no magnetism, and good damping performance, as well as it has a special shape memory [1] and outstanding biocompatibility. Ti-6Al-4V is a type of titanium alloy of α + β with excellent comprehensive properties. Nowadays Ti-6Al-4V alloy is widely applied and it accounts for approximately 50% of the total world production of titanium alloys [2]. The cryogenic treatment (CT) is a treatment method that can change material properties by the samples being immersed in liquid nitrogen. CT is a supplementary process of traditional heat treatment which has been acknowledged for decades [3]. CT on materials could be firstly reported by Gulyaev in 1937, and CT could improve the cutting properties of tool steels [4]. Afterwards, Kamran et al. [5,6] investigated the effects of CT on the mechanical properties of some selected tool Metals 2018, 8, 295; doi:10.3390/met8050295
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steels, i.e., D2 and 80CrMo125, and it was found that CT can eliminate the retained austenite, which can make a better carbide distribution and a higher carbide amount. An obvious improvement in wear resistance of cryogenically-treated samples was indicated. Gao et al. [7] studied the effects of CT on the microstructure and properties of WC-Fe-Ni cemented carbides. It was stated that CT could promote lath martensite formation, after CT, the hardness and transverse rupture strength of WC-Fe-Ni cemented carbides were higher than that of the untreated, while fracture toughness was decreased. In addition, the wear resistance was improved obviously. Later on, Baldisseri et al. [8] reported that CT can produce a more homogeneous structure which delayed the nucleation of cracks and increased the fatigue life of the tool. During recent years, many efforts have been devoted to the effects of cryogenic treatment on magnesium alloy. Several researchers [9–11] investigated the effects of CT on the microstructure and mechanical properties of AZ31 magnesium alloy. Their results showed that the tensile stress and hardness of the samples treated in cryogenic environment were higher than those of the untreated samples. The CT of other magnesium alloys, such as Mg-1.5Zn-0.15Gd and AZ91, were investigated, and it was found that CT could improve the microstructure and mechanical properties of the alloys [12,13]. Studies also revealed that CT improved the mechanical properties, dimensional stability, wear resistance and thermal conductivity of aluminum alloys, magnesium alloys, and copper alloys [14]. Zhou et al. [15] experimentally investigated the tensile properties and microstructures of the 2024-T351 aluminum alloy subjected to CT. They revealed that the residual tensile stress in 2024-T351 was removed, and slight compressive residual stress was generated after CT, this could contribute to the improvement of the tensile properties of the alloy. In terms of the residual stress, similar conclusions have been obtained for 2024 aluminum alloy. Zhang et al. [16] investigated the tensile behavior of 3104 aluminum alloy with CT. Their results showed that the sample exhibited superior comprehensive mechanical properties, including tensile strength, yield strength, and elongation. The stress-strain curve of the sample exhibited the Portevin-Le Chatelier effect. In this case the CT could accelerate both the precipitation and dispersion of secondary phase particles, which could enhance the dislocation pinning effect of solvent atoms. Moreover, they also conducted scanning electron microscope (SEM) analyses of the fracture surfaces and a good plasticity was achieved. Araghchi et al. [17] carried out microstructure observation, mechanical properties measurement, and residual stresses determination on 2024 aluminum alloy. It was found that CT not only resulted in reduction of residual stresses, but also improved the mechanical properties. Vidyarthi et al. [18] experimentally studied the effects of CT on the microstructure and wear performance of Cr-Mn-Cu white cast iron grinding media copper alloys. Their results showed that the hardness and wear properties were improved significantly. Most recently, Kim et al. [19] investigated the effects of CT on nickel-titanium endodontic instruments, and found that there was a slight increase in microhardness after CT. Gu [20] found that, after CT, wear resistance was increased due to the reduction of β phase. Yumark [21] also found that the content of β phase was decreased after CT, and the energy absorption increased and damaged areas were decreased. Moreover, Gu et al. [14] also investigated the effect of CT on the mechanical properties of Ti-6Al-4V alloy. They found that the microstructure of Ti-6Al-4V alloy consisted of α phase and β phase, with β phases dispersed in the boundaries of equiaxed α grains. They reported that CT reduced the quantities of β phases by transforming the metastable β phases into stable β phases and α phase. This reduction of β phase particles was beneficial to the improvement in plasticity of Ti-6Al-4V alloy. CT increased the ductility and strength of Ti-6Al-4V alloy. Although CT is beneficial to Ti-6Al-4V alloy, the research about the effect of CT on the mechanical properties of Ti-6Al-4V alloy is scarce. There are much literature on the formability of material described by the forming limit diagram (FLD). Veenaas et al. [22] carried out scaled Nakajima tests to determine the FLD for describing the limit of formability under deep drawing stress state conditions. Zhang et al. [23] carried out uniaxial tensile tests, cupping tests and rigid bulging experiment to obtain the forming limit diagram (FLSD), taking the cold-rolling deep drawing sheet SPCEN-SD and DC04 as research object. The FLSD can provide reference for practical production according to the values of hardening exponent (n),
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plasticratio strain(r)ratio and strength coefficient (K). However, it should be noted that notenough enoughto to strain and(r)strength coefficient (K). However, it should be noted that it it is isnot evaluate the formability sheet only by by tensile tests. Punch shearing teststests and deep evaluate formability of ofthe thetitanium titaniumalloy alloy sheet only tensile tests. Punch shearing and drawing tests are employed to study the formability. In addition, the punching depth indepth deep drawing deep drawing tests are employed to study the formability. In addition, the punching in deep is also a material characteristic value forvalue evaluation. Therefore, punch shearing tests, deep drawing, drawing is also a material characteristic for evaluation. Therefore, punch shearing tests, deep and tensile tests were employed in this paper to investigate the effects of CT of Ti-6Al-4V titanium drawing, and tensile tests were employed in this paper to investigate the effects of CT of Ti-6Al-4V alloys onalloys its formability at room temperature. titanium on its formability at room temperature. 2.Experimental ExperimentalProcedure Procedure 2. 2.1.Material Material 2.1. Incurrent currentwork, work,the thecommercial commercialTi-6Al-4V Ti-6Al-4Valloy alloysheet sheetwas wasselected selectedand andTable Table11 list list is is chemical chemical In compositions.The The tensile strength, yield strength (σ), ), and ductility were 930 MPa, 0.2and compositions. tensile strength, yield strength ( 0.2 ductility were 10701070 MPa,MPa, 930 MPa, and and 11% respectively. 11% respectively. Table 1. Chemical compositions of Ti-6Al-4V titanium alloy (wt%). Table 1. Chemical compositions of Ti-6Al-4V titanium alloy (wt%). Ti
TiAl
89.1755 89.1755 6.14
Fe Al V V Fe 6.144.074.07 0.075 0.075
CC N N H HO 0.016 0.1 0.016 0.022 0.0220.0015 0.0015
O Others 0.4 0.1
Others 0.4
2.2. 2.2.Samples Samples The ◦ Cfor Thesamples sampleswere wereannealed annealedat at760 760°C for90 90min min[24], [24],then thenthey theywere were cooled cooledin in the the QTF-1200X QTF-1200X vacuum tube furnace with the temperature controlling accuracy is ±1 °C, as shown in Figure 1a. The ◦ vacuum tube furnace with the temperature controlling accuracy is ±1 C, as shown in Figure 1a. maximum heating rate and cooling rate are 20 °C /min. During annealing the Argon gas ◦ The maximum heating rate and cooling rate are 20 C/min. During annealing the Argon gas was was continuously into the thetube. tube.After Afterannealing annealingthe the samples (see Figure different CT continuously injected injected into samples (see Figure 1b) 1b) withwith different CT time time were stored in a liquid nitrogen. All the samples were divided into seven groups according to were stored in a liquid nitrogen. All the samples were divided into seven groups according to CT time. CT time. The details of groups are shown in Table 2. The details of groups are shown in Table 2.
(a)
(b)
Figure vacuum tube tube furnace; furnace; and Figure 1. 1. (a) (a) QTF-1200X QTF-1200X vacuum and (b) (b) the the annealed annealed samples samples for forpunch punchshearing shearingtests. tests. Table Table2. 2.Treatment Treatment groups. groups.
Group Index Group Index AT AT ACT1 ACT1 ACT2 ACT2 ACT3 ACT3 ACT4 ACT4 ACT5 ACT5 ACT6 ACT6
Treatment Method Treatment Method Annealed Annealed Annealed Annealed Annealed Annealed Annealed Annealed Annealed Annealed Annealed Annealed Annealed Annealed
CT Time CT Time −196 -× 4 h −196 × 4 h −196 −196 ××88hh −196 −196××12 12hh −196××16 16hh −196 −196××20 20hh −196 −196 × 24 h −196 × 24 h
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2.3. Metallographic Observation
2.3. Metallographic Observation After treatments, the surface of the samples was initially grinded by abrasive paper and then polished. Afterwards, the sample surface was corroded by chemical reagents with 88 vol% H2O, 2 After treatments, the surface of the samples was initially grinded by abrasive paper and then vol% HF, and 10 vol% HNO3. Finally, the sample of each group was detected by optical microscopy polished. Afterwards, the sample surface was corroded by chemical reagents with 88 vol% H2 O, (ZEISS microscope; Axio.Imager.A2m, manufactured by Carl Zeiss Microlmaging GmbH,Gottingen, 2 vol% HF, and 10 vol% HNO3 . Finally, the sample of each group was detected by optical microscopy Germany). (ZEISS microscope; Axio.Imager.A2m, manufactured by Carl Zeiss Microlmaging GmbH, Gottingen, Germany). 2.4. Vickers Hardness 2.4. Vickers Hardness Vickers hardness values of the samples were obtained using Vickers hardness tester under different treatment process. tests, were the load is 100using N and the duration is tester 5 s with the Vickers hardness valuesDuring of the the samples obtained Vickers hardness under magnification is 400process. times. InDuring the current study,the theload sample surface o roughness is Ra0.2. For each different treatment the tests, is 100 N and the duration is 5 s with the group sample, five points were tested for hardness, and the average values were calculated magnification is 400 times. In the current study, the sample surface o roughness is Ra0.2. For each accordingly. group sample, five points were tested for hardness, and the average values were calculated accordingly. 2.5. Tensile Tensile Tests Tests 2.5. 2, the the tensile tensile tests tests were carried carried out at The details of the sample dimensions are shown in Figure 2, room temperature temperature using using aa universal universal testing testing machine machineat ataatensile tensilerate rateof of22mm/min. mm/min. Three samples in room each group groupwere were selected the test. The maximum force ( Fbsample ) of the the total each selected for for the test. The maximum force (F andsample the totaland deformation b ) of the deformation of thesection gaugeafter length section afterof tensile fracture of obtained. the sample can be obtained. of the gauge length tensile fracture the sample can be Engineering stress σb b and strain and strain δ can be calculated by theengineering stress-strain Engineering stress can be calculated by theformula. engineering stress-strain formula.
(b) 1200 1100
(a)
Engineering stress/MPa
1000 900
AT ACT1 ACT2 ACT3 ACT4 ACT5 ACT6
800 700 600 500 400 300 200 100 0
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18
Engineering strain/%
Figure 2. 2. Tensile Tensile sample sample and and stress stress and and strain strain curves Figure curves under under different different treatment treatment (a) (a) the the tensile tensile sample; sample and (b) stress-strain curves under different treatment. (unit: mm); and (b) stress-strain curves under different treatment.
Figure 2b plots the stress and strain curves of the samples. From Figure 2b, in the process of Figure 2b plots the stress and strain curves of the samples. From Figure 2b, in the process of deformation, elastic deformation occurs firstly, and the stress is proportional to strain. When the deformation, elastic deformation occurs firstly, and the stress is proportional to strain. When the stress exceeds e ( e is the elastic limit), and the material starts to yield, it can also be noted that stress exceeds σe (σe is the elastic limit), and the material starts to yield, it can also be noted that there there is not apparent yield platform in the curves. Under the condition, the stress value that produces is not apparent yield platform in the curves. Under the condition, the stress value that produces 0.2% residual deformation is defined as its yield limit ( s ). When the stress exceeds the yield limit, 0.2% residual deformation is defined as its yield limit (σs ). When the stress exceeds the yield limit, plastic deformation occurs. As the cross-sectional area shrinks, the maximum force ( Fb ) when the plastic deformation occurs. As the cross-sectional area shrinks, the maximum force (Fb ) when the sample is broken is divided by the original cross-sectional area ( S 0 ), the result is called tensile sample is broken is divided by the original cross-sectional area (S0 ), the result is called tensile strength strength ( b ). When the stress reaches b , the stage of uniform plastic deformation has ended. The (σb ). When the stress reaches σb , the stage of uniform plastic deformation has ended. The sample starts sample starts to produce uneven plastic deformation and form necking, followed by the stress to produce uneven plastic deformation and form necking, followed by the stress decreases. When the decreases. When the final stress reaches K (Fracture strength), the specimen is found to be broken. final stress reaches σK (Fracture strength), the specimen is found to be broken. 2.6. Punch Punch Shearing Shearing Tests Tests 2.6. In the the current current study, study, punch punch shearing shearing was was employed employed to to investigate investigate the the effects effects of of ACT ACT on on the the In properties of Ti-6Al-4V titanium alloy. The principle of punch shearing and the die are shown in properties of Ti-6Al-4V titanium alloy. The principle of punch shearing and the die are shown in Figure 3a,b,d, respectively. The diameter of the punch is 3 mm and the die is 3.07 mm. A slot is
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Figure 3a,b,d, respectively. The diameter of the punch is 3 mm and the die is 3.07 mm. A slot is machined in the die to clamp the sample. In order to obtain better section quality, the punching speed machined in the die to clamp the sample. In order to obtain better section quality, the punching speed is set to be 5 mm/s 24. After ACT, the samples with the length of 30 mm, the width of 19 mm and the is set to be 5 mm/s 24. After ACT, the samples with the length of 30 mm, the width of 19 mm and height of 0.2 mm will be punched and divided into two groups. A group of the samples will be the height of 0.2 mm will be punched and divided into two groups. A group of the samples will immersed into the liquid nitrogen and punched as shown in Figure 3c. Another group of the samples be immersed into the liquid nitrogen and punched as shown in Figure 3c. Another group of the will be punched at room temperature. Fracture surface was detected by optical microscopy (ZEISS samples will be punched at room temperature. Fracture surface was detected by optical microscopy microscope; Axio.Imager.A2m). Table 3 lists the test numbers of the samples under different (ZEISS microscope; Axio.Imager.A2m). Table 3 lists the test numbers of the samples under different conditions. During punch shearing, the relationship between the maximum deformation resistance conditions. During punch shearing, the relationship between the maximum deformation resistance P max and shear strength is calculated by the following formula: Pmax and shear strength τ is calculated by the following formula:
= P max 2πravgt
(1) (1)
τ = Pmax / 2πravg t
where t is the thickness of the plate, ravg rp rd 2 , rp is the radius of the punch, and rd is where t is the thickness of the plate, ravg = rp + rd /2, rp is the radius of the punch, and rd is the the radius of the radius of the die.die. Servo press (b) Punch Guide sleeve and claming
(a)
Liquid nitrogen Liquid nitrogen container
Sheet
R=1.515mm
(d)
Punching
h=8mm
(c)
Die
R=1.535mm Die
h=3.5mm Punch
Slot liquid nitrogen tank Guide sleeve and clamping
R=8.95mm
Figure 3. (a) (a) Principle Principle of of punch shearing; (b) the punching process; (c) at cryogenic temperature punching; and (d) the punch and die. Table tests. Table 3. Test samples for punch shearing tests. Group Index Group Index Number at the Untreated AT AT the Untreated Diff. Temp. Number at Diff. Temp. Number atroom roomtemp. temp. Number at 3 3 33 Number online cryogenic 3 Number online cryogenic 3 33
ACT1 ACT1
ACT2 ACT2
ACT3 ACT3
ACT4 ACT4
ACT5 ACT5
ACT6 ACT6
33 33
33 33
33 33
33 33
33 33
33 33
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2.7. Deep Drawing 2.7. Deep Drawing In the current work, deep drawing tests were also conducted using a servo press to study the In the current work, deep drawing tests were also conducted using a servo press to study the effect of ACT on the formability of Ti-6Al-4V titanium alloy. The size of the sample for the deep effect of ACT on the formability of Ti-6Al-4V titanium alloy. The size of the sample for the deep drawing test is 29 mm × 29 mm × 0.2 mm. Table 4 listed the test cases. The principle of deep drawing, drawing test is 29 × 29 × 0.2 mm. Table 4 listed the test cases. The principle of deep drawing, the the schematic diagram of the die and punch are shown in Figure 4. Additionally, the diameter of the schematic diagram of the die and punch are shown in Figure 4. Additionally, the diameter of the punch with sphere is 4 mm, and the die is 5.4 mm. During deep drawing, the punch continuously punch with sphere is 4 mm, and the die is 5.4 mm. During deep drawing, the punch continuously descends down until the sample happens to crack. At this time, the displacement value of the punch is descends down until the sample happens to crack. At this time, the displacement value of the punch the limit depth of deep drawing. is the limit depth of deep drawing. Table 4. Test samples for deep drawing tests. Table 4. Test samples for deep drawing tests. AT ACT1 ACT1 ACT2 ACT2 ACT3 ACT3 AT 33 33 33 3
Servo press Punch Guide sleeve and claming
(a)
(b)
ACT4 ACT4 ACT5 ACT5 ACT6 ACT6 33
33
33
R=2mm h=10mm
GroupIndex Index the theUntreated Untreated Group Number 33 Number
Punching
R=2.7mm Die
Liquid nitrogen Liquid nitrogen container
Die
Sheet
R=8.95mm
Figure Figure4. 4. (a) (a) The The principle principle of of deep deep drawing; drawing;and and(b) (b)the thepunch punchand anddie. die.
3. Results Results and and Discussion Discussion 3.
3.1. 3.1. Microstructural Microstructural Characteristics Characteristics Figure Figure 55 shows shows the the microstructures microstructures of of the the samples samples after after AT AT and and ACT. ACT. After After recrystallization recrystallization annealing, annealing, the the microstructure microstructure is is composed composed of of αα and and ββ phases, phases, and and the the ββ phase phase is is dispersed dispersed in in the the boundary of equiaxed α grains. In Figure 5 the bright regions are α phases, and the black grains are β boundary of equiaxed α grains. In Figure 5 the bright regions are α phases, and the black grains are phases. The distribution ofof ββ phases β phases. The distribution phasesisisshown shownininFigure Figure5a,b, 5a,b,and andititcan canbe beseen seenclearly clearlythat thatββphases phases disperse of ααgrain grainafter afterAT. AT.However, However,after afterACT1, ACT1,new newphases phasesincluding including and disperse in in the boundaries of α’α’ and β’ β’ are precipitated [26]. The α’, β’ and untransformed β phase disperse uniformly in boundaries are precipitated [26]. The α’, β’ and untransformed β phase disperse uniformly in boundaries of α of α phases, as shown in Figure 5c,d. ACT1 From ACT1 to ACT2, the change in microstructure the phases, as shown in Figure 5c,d. From to ACT2, the change in microstructure with thewith increase increase the CT time, isinshown Figure 5e,f. Itbeshould noted that the precipitates of the CToftime, is shown Figurein5e,f. It should noted be that the precipitates includingincluding α’, β’, andα’,β β’, and β remarkably increase remarkably and disperse uniformly. When time arrives 12 quantities h, the quantities increase and disperse uniformly. When the CT the timeCT arrives at 12 h,atthe of the of the precipitates are increased, sizes become larger, shownininFigure Figure5g,h. 5g,h. When When the time precipitates are increased, and and the the sizes become larger, as as shown time increases from ACT4 to ACT6, the change in the microstructure is exhibited in Figure 5i–n. It needs to increases from ACT4 to ACT6, the change in the microstructure is exhibited in Figure 5i–n. It needs be that that the both and sizes further, which that ACT can to observed be observed the quantities both quantities andincrease sizes increase further,indicates which indicates thattransform ACT can the metastable β phase into phase and phase, there isand no obvious change in αchange phase. transform the metastable β stable phase β’ into stable β’ α’ phase andand α’ phase, there is no obvious The as above areas in above consistence those obtained in the literature 24.literature 24. in αresults phase.achieved The results achieved are in with consistence with those obtained in the ItIt can can be be seen seen from from all all the the figures figures that that both both the the quantities quantities and and sizes sizes of of the the precipitates precipitates are are apparently apparently increased increased after after ACT. ACT. The The reason reason could could be be that, that, the the α-Ti α-Ti isis aa hexagonal hexagonal close-packed close-packed structure, structure,whereas whereasthe theβ-Ti β-Tiisisthe the body body centered centered cubic cubic structure, structure,and and the the diffusion diffusionability abilityof of hexagonal hexagonal close-packed farfar below thethe body centered cubiccubic structure owingowing to hexagonal close-packed close-packedstructure structureis is below body centered structure to hexagonal closestructure is in close array. Gu et al. [26] stated that the contraction velocity of the β phase is faster packed structure is in close array. Gu et al. [26] stated that the contraction velocity of the β phase is faster than that of α phase at low temperature, which can produce enough driving force to promote the precipitate of the α titanium particle s depending on the matrix. Moreover, at low temperature,
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than that of α phase at low temperature, which can produce enough driving force to promote the Metals 2018, 8, x FOR PEER REVIEW 7 of 16 precipitate of the α titanium particle s depending on the matrix. Moreover, at low temperature, the solubility of vanadium drops significantly, resulting in β phase is in metastable state and transformed the solubility of vanadium drops significantly, resulting in β phase is in metastable state and to stable β’ phase. ACT can transform the metastable β phase into stable β’ phase and α’ phase. Due transformed to stable β’ phase. ACT can transform the metastable β phase into stable β’ phase and α’ to the precipitation of precipitates, the bonding force between grain boundary and grain boundary is phase. Due to the precipitation of precipitates, the bonding force between grain boundary and grain small, this will lower the bond strength at strength the crystal boundary, resulting in the decrease of tensile boundary is small, this will lower the bond at the crystal boundary, resulting in the decrease strength. On the other hand, the effect of the cold compression in ACT process could make themake gaps of tensile strength. On the other hand, the effect of the cold compression in ACT process could inthe thegaps material Especially at the grain boundary, the number of the pores due to in thelarger. material larger. Especially at the grain boundary, the number of the increases pores increases weak bonding force between the precipitates and the grain boundary. Thus, the unrecoverable plastic due to weak bonding force between the precipitates and the grain boundary. Thus, the unrecoverable deformation is easier to occur, the elastic modulus and tensile strength decrease. plastic deformation is easier to occur, the elastic modulus and tensile strength decrease.
Figure 5. Cont.
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Figure 5. Microstructure of Ti-6Al-4V with different the treatment: (a) AT, 500×; (b) AT, 1000×; (c) Figure 5. Microstructure of Ti-6Al-4V with different the treatment: (a) AT, 500×; (b) AT, 1000×; ACT1, 500×; (d) ACT1, 1000×; (e) ACT2, 500×; (f) ACT2, 1000×; (g) ACT3, 500×; (h) ACT3, 1000×; (i) (c) ACT1, 500×; (d) ACT1, 1000×; (e) ACT2, 500×; (f) ACT2, 1000×; (g) ACT3, 500×; (h) ACT3, 1000×; ACT4, 500×; (j) ACT4, 1000×; (k) ACT5, 500×; (l) ACT5, 1000×; (m) ACT6, 500×; (n) ACT6, 1000×. (i) ACT4, 500×; (j) ACT4, 1000×; (k) ACT5, 500×; (l) ACT5, 1000×; (m) ACT6, 500×; (n) ACT6, 1000×.
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3.2. Tensile and Hardness Properties 3.2. Tensile and Hardness Properties Figure 6 plots the average values of elastic modulus, tensile strength, yield strength, and the FigureIt 6can plots the average values of elastic modulus, tensile strength, yield strength, and the ductility. be seen from Figure 6a that, after AT, the elastic modulus is 121.6 MPa, whereas after ACT1, theItaverage of theFigure elastic6a modulus is decreased to 119.2modulus MPa. From Figure 6a, itwhereas is also ductility. can be value seen from that, after AT, the elastic is 121.6 MPa, observed that overallvalue average values of the elastic modulus are decreased slowly withFigure the increase after ACT1, thethe average of the elastic modulus is decreased to 119.2 MPa. From 6a, it is of CT time, reaching minimum value values of 115.8ofMPa ACT4. Withare thedecreased increase ofslowly CT time, from also observed that thea overall average the after elastic modulus with the ACT5 toofACT6, thereaching average avalue of thevalue elastic slightly increase from MPa increase CT time, minimum ofmodulus 115.8 MPa after ACT4. With the117.5 increase ofto CT118.7 time, MPa.ACT5 The correlation equation is established and it canmodulus be concluded thatincrease the elastic modulus of allto from to ACT6, the average value of the elastic slightly from 117.5 MPa the samples with AT or ACT is decreased, whereas theitelastic is increased after modulus ACT4, butof 118.7 MPa. The correlation equation is established and can bemodulus concluded that the elastic thethe overall values of AT the elastic is lower than that the untreated samples. Figureafter 6b shows all samples with or ACTmodulus is decreased, whereas theofelastic modulus is increased ACT4, tensile strength, and it can be seen that the values obtained by three tests approach each other both but the overall values of the elastic modulus is lower than that of the untreated samples. Figure 6b after AT and strength, ACT. Moreover, with the increase CT time, the tensile strength decreaseseach andother the shows tensile and it can be seen that the of values obtained by three tests approach both after ATtrend and ACT. Moreover, withwith the increase of CT time, the tensile k the decreasing follows accordance the equation as follows: , whereand is b strength ktADCTdecreases β decreasing trend follows accordance with the equation as follows: σ = kt , where k is constant, ADCT b constant, and calculated as 1144.38 MPa, and is constant, and calculated as −0.03. Figure 6c shows and calculated as 1144.38 MPa, and β is constant, and calculated as −0.03. Figure 6c shows the yield the yield strength it is clearly observed that with the increase of the CT time, the values of the yield strength it is clearly observed that with the increase of the CT time, the values of the yield strength strength decreases, and the equation of declining trend is in accordance with that of the tensile decreases, and the equation of declining trend is in accordance with that of the tensile strength as strength as mentioned above, where k is calculated as 1028.74 MPa, and is calculated as −0.027. mentioned above, where k is calculated as 1028.74 MPa, and β is calculated as −0.027. Figure 6d Figure 6d demonstrates the ductility, and it can be seen that the ductility is increased gradually with demonstrates the ductility, and it can be seen that the ductility is increased gradually with treatment treatment methods from AT to ACT. Furthermore, with the increase of the CT time, the ductility is methods from AT to ACT. Furthermore, with the increase of the CT time, the ductility is increased increased remarkably until, the peak value is achieved after ACT3, which is increased by 17.5% remarkably until, the peak value is achieved after ACT3, which is increased by 17.5% compared to the compared to the values after AT. Afterwards, the values decrease from ACT4 to ACT6. The values after AT. Afterwards, the values decrease from ACT42 to ACT6. The correlation equation can,
-(0.18t-2)
2
−(0.18t−2) δ=15.93+1.67e thus, be δ = 15.93 + 1.67e improvement of mechanical has obvious influence correlation equation can, thus, be. The . Theproperties improvement of mechanical on the formability of Ti-6Al-4V sheet. After ACT, plastic deformation is more likely to occur due to the properties has obvious influence on the formability of Ti-6Al-4V sheet. After ACT, plastic decrease of yield strength and modulus. ductility is clearly increased, which results in the deformation is more likely to elastic occur due to the The decrease of yield strength and elastic modulus. The late occurrence of cracks during deformation. proportion of smooth zone in punch shearingThe and ductility is clearly increased, which results in The the late occurrence of cracks during deformation. the depth in of deep drawing proportion smooth zoneincrease. in punch shearing and the depth in deep drawing increase.
(a)
1200
123
Elastic modulus/GPa
121 120
119.2
119
118.3
118
118.7 118.1
117.5
117 115.8
116 115 114 113 112
(b)
1160 121.6
AT
ADCT1
ADCT2 ADCT3
Tensile strength σb/MPa
122
1120 1080 1040
σb=1144.38t-0.030
1000 960 920 880
ADCT4 ADCT5 ADCT6
AT
ACT1
1080
17.7
ACT4
ACT5
ACT6
(d)
17.4
1020
17.1
1000
16.8
Ductility δ/%
Yield strength σs/MPa
(c)
1040
980 960 940
16.5 16.2 15.9
δ=15.93+1.67e -(0.18t-2)
15.6
σs=1028.74t-0.027
2
15.3
900 880
ACT3
18.0
1060
920
ACT2
Different treatment
Different treatment
15.0
AT
ACT1
ACT2
ACT3
ACT4
Different treatment
ACT5
ACT6
AT
ACT1
ACT2
ACT3
ACT4
Different treatment
ACT5
ACT6
Figure6.6.Changes Changes tensile (a) elastic modulus; (b) tensile strength; (c)strength; yield strength; (d) Figure of of tensile test:test: (a) elastic modulus; (b) tensile strength; (c) yield (d) ductility. ductility.
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Five points distributed uniformly on each sample were marked by the Vickers hardness tester. Variation Vickers hardnessuniformly with different treatment shown in Figure 7. Vickers The average values of Five of points distributed on each sampleiswere marked by the hardness tester. 0.0058 Variation hardness different in Figure The average five valuesof inVickers each group were with calculated andtreatment the curve is of shown the variation is as7.follows: HV values vt of 0.0058 five values groupand were calculated and the variation is as follows: = vtfrom v is in t isthe where theeach constant calculated as 388.4 HV,curve and of variable of CT time. It can HV be seen where 7v that is thethe constant andvalues calculated as 388.4 t isofvariable of CT time.can It can be seen Figure hardness distribute on HV, bothand sides the curve. It also be seen thatfrom the Figure 7 that the hardness values distribute on both sides of the curve. It also can be seen that harness values are increased with the increase of CT time, and the increasing trend of values is the in harnesswith values increased with the increase of CTof time, the increasing of values is in accord accord theare equation. Although the hardness the and sample after ACTtrend is overall increased, the with thein equation. Although the obvious hardnessbecause of the sample after ACT is overallbetween increased, changes changes the hardness are not the maximum difference thethe maximum in the notHV. obvious the maximum between the on maximum and the and thehardness minimumare is 4.5 It canbecause be concluded that ACTdifference has insignificant effect the hardness of minimum the alloy. is 4.5 HV. It can be concluded that ACT has insignificant effect on the hardness of the alloy. 397
Vickers hardness/HV
396
The plots of hardness values Fitting curve
395 394 393
HV=388.4t0.0058
392 391 AT
ACT1
ACT2 ACT3 ACT4 Different treatment
ACT5
ACT6
Figure Figure7.7.Variation VariationofofVickers Vickershardness hardnesswith withdifferent different treatment. treatment.
3.3. 3.3.Effect Effecton onPunch PunchShearing ShearingProperties Properties 3.3.1. 3.3.1.Shear ShearStrength Strength Figure Figure8a,b 8a,bshow showthe theshear shearstrength strengthof ofthe thesamples sampleswith withdifferent differenttreatments treatmentsat atboth bothroom roomand and cryogenic can be be seen from Figure 8a that whether afterafter AT or the cryogenictemperature, temperature,respectively. respectively.It It can seen from Figure 8a that whether ATACT, or ACT, shear strength decreases in comparison to the untreated material. It It should the shear strength decreases in comparison to the untreated material. shouldbebenoted notedthat, that,after afterthe the treatments strength gradually gradually decreases decreasesfrom from768 768MPa MPatoto749 749MPa, MPa,then thena treatments from from AT AT to ACT2, the shear strength aquick quickincrease increase ACT3, which is 754 MPa, afterwards it decreases toMPa 744 MPa at ACT4. atat ACT3, which is 754 MPa, andand afterwards it decreases to 744 at ACT4. From From ACT4 ACT4 to ACT6, the shear strength is increased from 744to MPa 758 MPa. In order to study the effects to ACT6, the shear strength is increased from 744 MPa 758to MPa. In order to study the effects of CT of the punch shearing, the punch shearing was performed at cryogenic temperature, andshear the onCT theon punch shearing, the punch shearing was performed at cryogenic temperature, and the shear strength is shown in Figure 8b. It can be seen clearly from Figure 8b that the change in shear strength is shown in Figure 8b. It can be seen clearly from Figure 8b that the change in shear strength strength of samples AT isthan smaller than untreated ones. From ACT1 to shear decreases strength of samples after AT after is smaller untreated ones. From ACT1 to ACT4, theACT4, shear the strength decreases fromto831 807 MPa, buttendencies decreasing slowed. At ACT5, thereaches shear from 831 MPa 807MPa MPa,tobut decreasing aretendencies slowed. Atare ACT5, the shear strength strength reaches the minimum value of 801 MPa, and then it increases to 823 MPa at ACT6. The the minimum value of 801 MPa, and then it increases to 823 MPa at ACT6. The comparison of shear comparison of shear strength atand room temperature and cryogenic temperature 8c. strength at room temperature cryogenic temperature is shown in Figure is 8c.shown It canin beFigure observed Itfrom can Figure be observed 8c atthat shear strength at room than temperature is lower than that at 8c that from shearFigure strength room temperature is lower that at cryogenic temperature. cryogenic temperature. Meanwhile, the change of the shear strengthisatmuch cryogenic is Meanwhile, the change of the shear strength at cryogenic temperature gentlertemperature than the shear much gentler thantemperature. the shear strength at room temperature. strength at room
Metals 2018, 8, 295 Metals 2018, 8, x FOR PEER REVIEW (a) 810
800 780
768
760
760
754
749
758
752
744
Shear strength/MPa
Shear strength/MPa
(b)
Punching at room temperature
840 820
11 of 16 11 of 16
740 720 700 680 660
the untreated
AT
ACT1
ACT2
ACT3
ACT4
ACT5
ACT6
Different treatment
860 850 840 830 820 810 800 790 780 770 760
Online cryogenic shear process 837
750 740 730 720
the untreated
834
831
826
823
820 807 801
AT
ACT1
ACT2
ACT3
ACT4
ACT5
ACT6
Different treatment
Shear strength/Mpa
(c) 880 860 840 820 800 780 760 740 720 700 680 660 640 620 600
the AT untreated
Punching at Punching room attemperature room temperature Online cryogenic shearshear process Online cryogenic process
ACT1
ACT2
ACT3 ACT4 ACT5
Different treatment processes Different treatment
ACT6
Figure Figure 8. 8. Shear Shear strength strength at at room room and and cryogenic cryogenic temperature: temperature: (a) (a) averaged averaged shear shear strength strength at at room room temperature; (b) the averaged shear strength at cryogenic temperature; and (c) the comparison temperature; (b) the averaged shear strength at cryogenic temperature; and (c) the comparison of of shear shear strength. strength.
3.3.2. 3.3.2. Characteristics Characteristicsof ofPunch PunchShearing Shearing ItIt is is recognized recognized that that the the smooth smooth zone zone isis employed employed to to evaluate evaluatethe thequality qualityof of the the cross-section, cross-section, and the smooth zone of the cross-section by punch shearing on the different positions and the smooth zone of the cross-section by punch shearing on the different positionsare are shown shownin in Figure 9. It can be seen from Figure 9a–e that the heights of smooth zone are increased from 97 µ m Figure 9. It can be seen from Figure 9a–e that the heights of smooth zone are increased from 97 µm to m after to 153 153 µµm after the the treatments treatments from from AT AT to to ACT3. ACT3. ItIt is is noted noted that that with with the the increase increase of of CT CT time time from from ACT3 to ACT6, the heights of the smooth zone decreases from 153 µ m to 121 µ m. This implies ACT3 to ACT6, the heights of the smooth zone decreases from 153 µm to 121 µm. This impliesthat, that, after ACT3, the smooth zone is better than the others. after ACT3, the smooth zone is better than the others. Since Since the the rollover rollover is is also also one one of of the the characteristics characteristics to to evaluate evaluate the the quality quality of of punch punch shearing, shearing, itit is is essential to be investigated. Figure 10 shows the change in the rollover. It can be seen from essential to be investigated. Figure 10 shows the change in the rollover. It can be seen fromFigure Figure 10 10 that that the the rollover rollover diameters diameters are are increased increased with with the the increase increase of of CT CT time. time. When When the the CT CT time time reaches reaches ACT3 (12 h), the peak value of the rollover diameter reaches 148 µ m, and then the rollover ACT3 (12 h), the peak value of the rollover diameter reaches 148 µm, and then the rolloverdiameters diameters start startto todecrease decreasefrom fromACT3 ACT3to toACT6. ACT6.Overall, Overall,the therollover rolloverdiameters diametersare aregreater greaterthan thanthose thoseuntreated. untreated. The main reason could be that the strength of the Ti-6Al-4V alloy was decreased and the ductility The main reason could be that the strength of the Ti-6Al-4V alloy was decreased and the ductility was was increased increased afterafter ACT.ACT. The burrisisalso also of important the important characteristics punch shearing. order to The burr oneone of the characteristics during during punch shearing. In order toInunderstand understand the effect of ACT on punch shearing in detail, the changes in the burrs are measured, as the effect of ACT on punch shearing in detail, the changes in the burrs are measured, as are shown are shown11. in The Figure 11. The results measured results of the heights burrs in are shown in Figure 11a,b, in Figure measured of the heights of burrs areof shown Figure 11a,b, whereas the whereas the measured positions are shown in Figure 11c and the changes in the burrs are shown in measured positions are shown in Figure 11c and the changes in the burrs are shown in Figure 11d. Figure 11d. It can be seen from Figure 11d that the heights of burrs are increased after AT and ACT It can be seen from Figure 11d that the heights of burrs are increased after AT and ACT comparing to comparing to the untreated. ACT1 to ACT3, heights of burrs increase continuously, the the untreated. From ACT1 toFrom ACT3, the heights ofthe burrs increase continuously, and the wholeand heights whole heights of burrs after ACT3 are maximum. Afterward, the heights of burrs are decreased of burrs after ACT3 are maximum. Afterward, the heights of burrs are decreased gradually from gradually from ACT3 to ACT6. that It is both concluded both AT disadvantages and ACT showfor disadvantages for but punch ACT3 to ACT6. It is concluded AT andthat ACT show punch shearing, the shearing, but the difference of the height of the burr is small, and the effect of ACT on punch shearing difference of the height of the burr is small, and the effect of ACT on punch shearing is very small. is very small.
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Figure 9. Diagrams of blanking fracture surface: (a) untreated; (b) AT; (c) ACT1; (d) ACT2; (e) ACT3;
Figure 9. Diagrams of blanking fracture (a)untreated; untreated;(b)(b) AT; ACT1; ACT2; (e) ACT3; Figure 9. Diagrams blanking fracture surface: surface: (a) AT; (c)(c) ACT1; (d) (d) ACT2; (e) ACT3; (f) ACT4; (g) ACT5;ofand (h) ACT6. (f) ACT4; (g) ACT5; and (h) ACT6. (f) ACT4; (g) ACT5; and (h) ACT6. (a) (a)
Rollover Diameter(Dr)
Rollover Diameter(Dr) Rollover
(b) (b)
D1=3152.3μm D2=3023.8μm D1=3152.3μm D1 D2=3023.8μmD2
Rollover Smooth zone Smooth zone Fracture Burr Fracture
Burr
(c) (c)
D1=3171.4μm D12=3171.4μm =3023.8μm D D1
D2=3023.8μm D1
D1
D2 D1 is Rollover diameter D2 is Circular hole D1 is Rollover diameter diameter D2 is Circular hole diameter
D2
D2
The size of filleted corner(D1-D2)/μm The size of filleted corner(D1-D2)/μm
(d) 150 (d) 150 147 147 144 144 141 141 138 138 135 135 132 132 129 129 126 126
the AT untreated the AT untreated
ACT1
ACT2
ACT3
ACT4
ACT5
ACT6
ACT4
ACT5
ACT6
Different treatment ACT1
ACT2
ACT3
Different treatment Figure 10. Rollover diameter of the specimens: (a) untreated; (b,c) example of measured results; and (d) changes in rollover diameters. Figure 10. Rollover diameter of the specimens: (a) untreated; (b,c) example of measured results; and
Figure 10. Rollover diameter of the specimens: (a) untreated; (b,c) example of measured results; rollover diameters. and(d) (d)changes changesinin rollover diameters.
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(a)
13.11μm
6.56μm
(b)
20
(c)
Raw material AT ADCT1 ADCT2 ADCT3 ADCT4 ADCT5 ADCT6
° 30
0°
180° 33
240 °
0°
270°
° 300
21
0° 0°
The height of burrs /μm
90°
60
° 120
15
°
18 16 14 12 10 8 6 4 2 0
0°
30° 60° 90° 120° 150° 180° 210° 240° 270° 300° 330°
Viewing position
Figure 11. Change Changeofofthethe burrs: (a,b) example of measured results; (c) measured and Figure 11. burrs: (a,b) example of measured results; (c) measured positionposition and changes changes in the heights of burrs. in the heights of burrs.
3.4. Deep Drawing 3.4. Deep Drawing In the current work, in order to further investigate the effects of ACT on the material, deep In the current work, in order to further investigate the effects of ACT on the material, deep drawing drawing was carried out at room temperature. Figure 12 demonstrates the drawing depth and drawing was carried out at room temperature. Figure 12 demonstrates the drawing depth and drawing load at load at room temperature. As shown in Figure 12a, the drawing depth of the untreated material is room temperature. As shown in Figure 12a, the drawing depth of the untreated material is 0.573 mm, 0.573 mm, while the drawing depth increases to 0.614 mm after AT. The drawing depth gradually while the drawing depth increases to 0.614 mm after AT. The drawing depth gradually increases increases and then the peak value of 0.638 mm is achieved at ACT3 (12 h). The drawing depth then and then the peak value of 0.638 mm is achieved at ACT3 (12 h). The drawing depth then begins to begins to decrease from ACT3 to ACT6, but the drawing depth values are all greater than those of decrease from ACT3 to ACT6, but the drawing depth values are all greater than those of untreated untreated material. It can also be seen from Figure 12b that the drawing load after AT or ACT material. It can also be seen from Figure 12b that the drawing load after AT or ACT decrease compared decrease compared to untreated material. However, with the increase of CT time, the decreasing to untreated material. However, with the increase of CT time, the decreasing trend of the drawing load trend of the drawing load slows down. This can be explained by the fact that both the yield strength slows down. This can be explained by the fact that both the yield strength and tensile strength decrease and tensile strength decrease after ACT as described above, which result in the drawing load after ACT as described above, which result in the drawing load decreased. Meanwhile, after ACT, decreased. Meanwhile, after ACT, the values of ductility are increased, which result in the drawing the values of ductility are increased, which result in the drawing depth is increased. The images of depth is increased. The images of the samples are shown in Figure 13. Some samples with microthe samples are shown in Figure 13. Some samples with micro-cracks are shown in Figure 14. It can cracks are shown in Figure 14. It can be seen from Figure 14 that the micro-cracks occur at the bottom be seen from Figure 14 that the micro-cracks occur at the bottom of the samples. For those untreated of the samples. For those untreated samples, the micro-cracks occur when the displacement of the samples, the micro-cracks occur when the displacement of the punch is 0.573 mm and the load is 355 N. punch is 0.573 mm and the load is 355 N. After ACT3, the displacement increases to 0.638 mm, while After ACT3, the displacement increases to 0.638 mm, while the load is reduced to 310 N compared to the load is reduced to 310 N compared to the untreated. the untreated. In the present work, the β phases (in the untreated material) were changed into new α‘ and β’ In the present work, the β phases (in the untreated material) were changed into new α‘ and phases through ACT, resulting in the decrease of the amount of β phase in the microstructure, which β’ phases through ACT, resulting in the decrease of the amount of β phase in the microstructure, played an important role in the formability of Ti-6Al-4V. A lower amount of β phases indicates a which played an important role in the formability of Ti-6Al-4V. A lower amount of β phases indicates better plastic performance of Ti-6Al-4V alloy. However, deeper insight into the effect of the detailed a better plastic performance of Ti-6Al-4V alloy. However, deeper insight into the effect of the detailed changes of the β phase on formability requires further study. changes of the β phase on formability requires further study.
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(b) (b) (b) 400 400
0.65 0.64 0.64 0.64 0.625 0.625 0.627 0.63 0.63 0.625 0.627 0.63 0.62 0.62 0.614 0.614 0.62 0.614 0.61 0.61 0.61 0.6 0.6 0.6 0.59 0.59 0.59 0.58 0.573 0.573 0.58 0.58 0.573 0.57 0.57 0.57 0.56 0.56 0.56 0.55 0.55 0.55 0.54 0.54 the AT ADCT1 ADCT2 0.54 the the AT ADCT1 ADCT2
untreatedAT untreated untreated
0.638 0.637 0.638 0.637 0.637 0.631 0.631 0.630 0.630 0.631 0.630 Load P /N Load Load PP /N/N
(a) (a) (a) 0.65 0.65 Limit Limit depth depth /mm Limit depth/mm /mm
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ADCT3 ADCT3 ADCT4 ADCT4 ADCT5 ADCT5 ADCT6 ADCT6 ADCT1 ADCT2 ADCT3 ADCT4 ADCT5 ADCT6 Different treatment
Different treatment Different treatment
400 380 380 355 380 360 355 360 355 360 340 340 340 320 320 320 300 300 300 280 280 280 260 260 260 240 240 240 220 220 220 200 200 200 180 180 180 160 160 160 140 140 140 120 120 120 100 100 the the 100
335 335 335
the untreated untreated untreated
AT AT AT
318 318 318
302 302 302
310 310 310
286 286 286
281 281 281
277 277 277
ADCT1 ADCT6 ADCT1 ADCT2 ADCT2 ADCT3 ADCT3ADCT4 ADCT4 ADCT5 ADCT5 ADCT6 ADCT1 ADCT2 treatment ADCT3 ADCT4 ADCT5 ADCT6 Different
Different treatment Different treatment
Figure 12. 12. Results Results of of deep Figure deep drawing drawing tests: tests: (a) (a)averaged averagedlimit limitdepth; depth;and and(b) (b)averaged averagedload. load. Figure 12. Results of deep drawing tests: (a) averaged limit depth; and (b) averaged load.
The depth is The depth is The depth is 0.56mm 0.56mm 0.56mm (a)
(a) (a)
The depth is The depth is The depth is 0.625mm 0.625mm 0.625mm (b)
(b) (b)
Figure 13. Drawing deep samples (a) untreated material; and (b) ADCT3. Figure and (b) (b) ADCT3. Figure 13. 13. Drawing Drawing deep deep samples samples (a) (a) untreated untreated material; material; and Figure 13. Drawing deep samples (a) untreated material; and (b) ADCT3. ADCT3.
Figure 14. Micro-cracks on the samples when the crack appears at the bottom of the samples. Figure 14. Micro-cracks on the samples when the crack appears at the bottom of the samples. Figure samples. Figure 14. 14. Micro-cracks Micro-cracks on on the the samples samples when when the the crack crack appears appears at at the the bottom bottom of of the the samples.
4. Conclusions 4. Conclusions 4. Conclusions In this paper, the effects of ACT on the formability of Ti-6Al-4V alloy were experimentally 4. Conclusions In this paper, the effects of the ACT on the formability ofdrawn Ti-6Al-4V alloy were experimentally studied. Major findings based on experimental results are as follows: In this paper, the effects of ACT on the formability of Ti-6Al-4V alloy were experimentally In this paper, the effects of on ACT the formability of Ti-6Al-4V alloy were experimentally studied. studied. Major findings based theon experimental results are drawn as follows: studied. Major findings based on the experimental results are drawn as follows: (1) The ACT has a positive effect on theresults microstructure. AT, the original β phases will be Major findings based on the experimental are drawnAfter as follows: (1) The ACT has a positive effect on the microstructure. After AT, the original β phases changed β’ phases and the refined begin to appear. After will ACT,be (1) The ACTinto has stable a positive effectand on α’ thephase, microstructure. Aftergrains AT, the original β phases will be (1) changed The ACT has a positive effect on the microstructure. After AT, the original β phases will be into stable β’ phases and α’ phase, and the refined grains begin to appear. After ACT, the grains are further refined, and the tendency of the β phase being changed into stable β’ and changed into stable β’ phases and α’ phase, and the refined grains begin to appear. After ACT, changed stable phases and α’ phase, andofthe grains begin to appear. Afterβ’ACT, the grainsinto are furtherβ’refined, the tendency the β phase being changed into stable and α’ phases apparent. This is inand favor increasing therefined plasticity of the Ti-6Al-4V alloy. the grains is are further refined, and theof tendency of the β phase being changed into stable β’ and the grains are further refined, and the tendency of the β phase being changed into stable β’ and α’ phases is apparent. This is in favor of increasing the plasticity of the Ti-6Al-4V alloy. (2) α’ After AT or ACT, the elastic than those of untreated. or ACT has a positive phases is apparent. This ismodulus in favor is oflower increasing the plasticity of theAT Ti-6Al-4V alloy. α’ phases isACT, apparent. This ismodulus inand favor increasing the plasticity of theAT Ti-6Al-4V alloy. (2) After AT or the elastic isoflower than those of untreated. or ACT has a positive effect on the tensile strength yield strength, and the decreasing trend is obvious. The (2) After AT or ACT, the elastic modulus is lower than those of untreated. AT or ACT has a positive on initially theACT, tensile strength and yield strength, and the decreasing trend is has obvious. The (2) effect After AT or the elastic modulus is lower than those of untreated. AT or ACT a positive ductility increases and then decreases, and ACT has little effect on the hardness of effect on the tensile strength and yield strength, and the decreasing trend is obvious. the The ductility initially increases and then decreases, and ACT has little effect on the hardness of the effect on the tensile strength and yield strength, and the decreasing trend is obvious. The ductility alloy. ductility initially increases and then decreases, and ACT has little effect on the hardness of the initially ACT has little on the hardness thecryogenic alloy. (3) alloy. After ATincreases and ACT,and the then sheardecreases, strength inand punch shearing is effect decreased both at roomof and alloy. After AT and ACT, thethe shear strength in punch shearing is decreased boththan at room and cryogenic temperatures. Moreover, the shear strength at room temperature is lower that at cryogenic (3) After AT and ACT, shear strength in punch shearing is decreased both at room and (3) After AT and ACT, the shear strength in punch shearing is decreased both at room and cryogenic temperatures. Moreover, the shear strength at room temperature is lower than that at cryogenic temperature, and the decreasing of the shear at room temperature is more cryogenic temperatures. Moreover, the shear atstrength room temperature is lower that at temperatures. Moreover, the sheartendency strength atstrength room temperature is lower than that atthan cryogenic temperature, and the decreasing tendency of the shear strength at room temperature is more obvious. temperature, and the decreasing tendency of the shear strength at room temperature is more obvious. obvious.
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cryogenic temperature, and the decreasing tendency of the shear strength at room temperature is more obvious. The smooth zone is much better than those of untreated material after AT, and the best result is obtained after ACT3. The rollover diameters are bigger than those of the untreated, and with the increase of CT time, the heights of burrs showed a decreasing trend. However, the variations of the rollover diameters and the heights of burrs are small. The drawing depth is deeper than the untreated material, and the drawing load after AT or ACT is reduced compared to that of the untreated. However, with the increase of CT time, the decreasing trend of the drawing load slows down.
Author Contributions: Z.H. and H.Z. conceived and designed the experiments; H.Z. performed the experiments; Z.H. and H.Z. analyzed the data; G.L. contributed experimental equipment; Z.H. wrote the paper; and H.W. polished the writing and grammar. Funding: This research received no external funding. Acknowledgments: This research is funded by the NSFC (grant numbers 51275201 and 51311130129) and the Jilin Key Scientific and Technological Project (20140204062GX), and the author is grateful to those funds. Conflicts of Interest: The authors declare no conflict of interest.
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