K. MERTOVÁ et al.: MICROSTRUCTURE AND MECHANICAL PROPERTIES OF COLD-DEFORMED PURE TITANIUM ... 585–588 MICROSTRUCTURE AND MECHANICAL PROPERTIES OF COLD-DEFORMED PURE TITANIUM AND TITANIUM GRADE 5 MIKROSTRUKTURA IN MEHANSKE LASTNOSTI HLADNO DEFORMIRANEGA ^ISTEGA TITANA IN TITANA ^ISTOSTI 5 Kateøina Mertová * , Michal Duchek COMTES FHT a.s., Prùmyslová 995, Dobøany, 334 41, Czech Republic Prejem rokopisa – received: 2019-07-16; sprejem za objavo – accepted for publication: 2020-06-11 doi:10.17222/mit.2019.162 The effects of cold deformation on commercially pure titanium Grade 2, titanium Grade 4 and titanium Grade 5 (TiAl6V4) were explored and compared. The possible use of these wrought materials is discussed. They were worked by rotary swaging at ambi- ent temperature. The purpose was to impart high strengthening, while maintaining sufficient ductility for further processing. The two pure titanium grades were cold deformed with a total area reduction of 90 %. Only minor surface damage was observed on the workpieces. However, in titanium Grade 5, the need for in-process annealing between the cold-deformation steps was identi- fied. High strengthening was obtained in all the materials. Titanium Grade 4 and the titanium alloy had ultimate strengths in ex- cess of 1000 MPa and 1400 MPa, respectively. The hardness profile in the transverse direction was measured and discussed. The interior of the wires had higher hardness than the surface. The microstructure investigation helped to reveal structural changes and clarify the material’s properties. Keywords: cold deformation, titanium, pure titanium, titanium grade 5 Avtorji tega prispevka so raziskovali in primerjali vplive hladne deformacije na komercialno ~isti titan, titan ~istosti 2, ~istosti 4 in 5 (TiAl6V4). Raziskovalci so tudi analizirali mo`nosti uporabe teh materialov. Hladno deformacijo so izvajali s postopkom rotacijskega kovanja pri sobni temperaturi. Namen predelave izbranih materialov s tem postopkom je bil doseganje njihove visoke trdnosti, vendar pri tem ohraniti {e zadovoljivo duktilnost za njihovo nadaljnjo obdelavo. Dve vrsti ~istega titana so hladno deformirali z 90 % stopnjo deformacije (zmanj{anjem celotnega preseka). Na preizku{ancih so po hladni deformaciji opazili le zanemarljive povr{inske po{kodbe. Medtem, ko je bilo potrebno titan ~istosti 5 med posameznimi stopnjami hladne deformacije `ariti. Pri vseh izbranih materialih je pri{lo do mo~ne utrditve oziroma povi{anja mehanske trdnosti. Pri titanu ~istosti 4 in titanovi zlitini se je natezna trdnost povi{ala na 1000 MPa oziroma 1400 MPa. Avtorji so izmerili in komentirali {e profil trdote v pre~ni smeri. Notranjost vlaken je imela vi{jo trdoto kot povr{ina. Karakterizacija mikrostrukture je avtorjem pomagala ugotoviti do kak{nih mikrostrukturnih sprememb je pri{lo med hladno deformacijo in posledi~no do sprememb mehanskih lastnosti. Klju~ne besede: hladna deformacija, titan, ~isti titan, titan ~istosti 5 1 INTRODUCTION Materials engineers can choose from four titanium chemistries specified by standards, which are identified by numbers from 1 to 4. With an increasing number, the content of impurities rises, including both substitutional and interstitial elements. The following are those of the greatest interest: iron, carbon, and several gases: oxygen, hydrogen and nitrogen. These elements have the stron- gest impact on the solid-solution strengthening of -phase in titanium. The -phase has a hexagonal close-packed crystal structure. This structure does not of- fer many slip systems for plastic deformation. It has one slip plane, which is the basal plane (0001), with the slip directions [2110]. Hence, a total of three slip systems is available. This structure is expected to deform primarily by twinning, particularly at small strains, a mechanism which is based on partial dislocation glide. 1 Titanium Grade 2, titanium Grade 4 and a titanium Grade 5 (Ti Grade 5) are widely used for human im- plants, thanks their biocompatibility. With its excellent biological and biomechanical properties, commercially pure titanium has become the material of choice for den- tal applications. Yet, it offers poorer mechanical proper- ties than Ti Grade 5. Generally, high strength values are desired, namely for dental implants, but Ti Grade 5 con- tains potentially cytotoxic elements, such as aluminium and vanadium. In order to improve the mechanical prop- erties of pure titanium and bring them closer to those of Ti Grade 5, more effective processing techniques should be developed. Tensile strength and fatigue performance can be improved for instance by SPD (severe plastic de- formation) processes. Those lead to rapid grain refine- ment and thus improve the mechanical properties. 2–4 Rotary swaging is a forming technology for reducing the cross-section of tubes, rods and wires. It is widely used for the sequential reduction of products of circular cross-section. Its main advantages include a short time cycle, good final surface and dimensional tolerances. The total reduction is controlled by selecting the forging die and the size of the feedstock. The workpiece is de- formed locally at high frequency and favourable strain Materiali in tehnologije / Materials and technology 54 (2020) 5, 585–588 585 UDK 67.017:620.1:669.018.254:669.295 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(5)585(2020) *Corresponding author's e-mail: katerina.mertova@comtesfht.cz (Kateøina Mertová) states can be achieved. Rotary swaging is considered by many researchers as one of the severe plastic deforma- tion (SPD) processes. 5–7 In this study, the influence of the cold deformation of commercially pure titanium Grade 2 and Grade 4 and the commonly known titanium alloy titanium Grade 5 is dis- cussed. 2 EXPERIMENTAL PART The composition of the feedstock was measured us- ing a Bruker Q4 Tasman optical emission spectrometer. The values are given in Table 1. The wires were rotary swaged at ambient temperature, achieving a total area re- duction of 90 %. The cross-sectional area was reduced by 20 % in each pass. The initial and final diameters of the feedstock were 10 mm and 3.28 mm, respectively. The frequency of the forging was 24 Hz and the wire was fed at approximately 3 m/min. After grinding and polishing, the metallographic specimens were etched with Kroll’s reagent. The optical microscope Carl Zeiss – Observer.Z1m and bright-field illumination were used for the microstructure observa- tion. The tensile testing was carried out in an electrome- chanical testing machine. The tests followed the EN ISO 6892-1 standard. Extension was measured with a me- chanical extensometer. Yield stress at 0.2 % deformation (OYS), ultimate stress (UTS), elongation (A 5 ) and reduc- K. MERTOVÁ et al.: MICROSTRUCTURE AND MECHANICAL PROPERTIES OF COLD-DEFORMED PURE TITANIUM ... 586 Materiali in tehnologije / Materials and technology 54 (2020) 5, 585–588 Table 1: Chemical composition of the materials, in mass fractions (w/%) Material Fe O C H N Al V Ti Ti Grade 2 0.046 0.12 0.023 0.0026 0.0076 - - bal. Ti Grade 4 0.500 0.40 0.010 0.0125 0.0050 - - bal. Ti Grade 5 0.190 0.11 0.080 < 0.006 0.0131 6.22 4.23 bal. Figure 2: Micrographs of the materials after 90 % area reduction: a) Ti Grade 2 – longitudinal section, b) Ti Grade 2 – transverse section, c) Ti Grade 4 – longitudinal section, d) Ti Grade 4 – transverse section, e) Ti Grade 5 – longitudinal section, and f) Ti Grade 5 – transverse section Figure 1: Feedstock microstructures on transverse metallographic sections: a) Ti Grade 2, b) Ti Grade 4 and c) Ti Grade 5 tion of area (RA) were calculated. The average values from three valid measurements are plotted in the results section. The Vickers microhardness HV0.3 was mea- sured by means of Durascan 50 automatic hardness tester across the cross-section of the products. 3 RESULTS The microstructure of the feedstocks is shown in Fig- ure 1. Micrographs of specimens after the final (90 %) area reduction are presented in Figure 2. On longitudinal metallographic sections, the gradual extension of the grains with increasing deformation was observed in each material, accompanied by a reduction of the grain cross section. Significant grain refinement was found on trans- verse sections with the mean grain size estimated to be under 1 μm in all the materials. The surface of the pure titanium contains no cracks or large defects. Ti Grade 5 contains cracks in the surface after 90 % area reduction, (Figure 2f). No signs of recovery were found. A detail of the microstructure of Ti Grade 2 after cold deformation showing deformation twins is presented in Figure 3. The deformation twins are characteristic of pure titanium at a smaller amount of cold deformation. With each reduction, the ultimate strength and yield strength increased thanks to structure refinement, but the ductility decreased, as seen in the plot in Figure 4.T i Grade 4 and Ti Grade 5 had post-forged ultimate strengths above 1000 MPa and 1400 MPa, respectively. The elongation decreased significantly during the first two deformation steps and then plateaued below 10 % for both Ti Grade 4 and Ti Grade 5. Cold-worked Ti Grade 2 wires had a higher elongation, in the range of 10 to 15 %, (Figure 5). Figure 6 shows a hardness profile on the cross-section of wires with 0 % (feedstock) and 90 % area reductions. The hardness of the feedstocks is higher near the sur- face than in the centre. In the cold-deformed ro- tary-swaged products, the higher hardness is in the inte- rior. 4 DISCUSSION Cold reduction tends to increase the strength and re- duce the ductility of materials. In Ti Grade 2, the incre- ment of strength due to the cold deformation was larger than in Ti Grade 4. Its lower content of interstitial ele- K. MERTOVÁ et al.: MICROSTRUCTURE AND MECHANICAL PROPERTIES OF COLD-DEFORMED PURE TITANIUM ... Materiali in tehnologije / Materials and technology 54 (2020) 5, 585–588 587 Figure 4: Offset yield strength and ultimate yield strength (OYS, UTS) vs. area reduction Figure 3: Detail micrograph of Ti Grade 2 after 20 % area reduction Figure 5: Elongation and reduction of area (A 5 , RA) vs. area reduc- tion by forming Figure 6: Vickers hardness across the cross-section ments appears to allow for a greater accumulation of dis- locations in the solid solution. However, higher strength was found in Ti Grade 4 in which strengthening is mainly due to interstitial elements, which are more abun- dant in Ti Grade 4 than in Ti Grade 2. In pure titanium, the dominating deformation mechanism was twinning. The largest strengthening occurred in Ti Grade 5. The main reason is its larger content of alloying elements. In addition, Ti Grade 5 showed the smallest decrease in elongation. This titanium Grade 5 contains both alpha and beta phases. The beta phase possesses good forma- bility thanks to which the material retains ductility better than pure titanium. However, since the initial elongation in Ti Grade 5 is approx. 13 %, Ti Grade 5 loses the ca- pacity for plastic deformation earlier than pure titanium. The loss of ductility in a material that was only worked by rotary swaging can lead to poorer fatigue properties. In these rotary-swaged materials, the grains became elongated in the longitudinal direction, and therefore they did not reach nano size. In general, ultrafine grains cannot be attained in rotary-swaged materials. Large cracks formed in the surface of the mechanically worked Ti Grade 5, which calls for in-process annealing between the individual cold deformation steps. The hardness of the feedstocks is higher near the surface than in the cen- tre. This is due to its processing history, as the wire was made by rolling. Rolling leads to greater work hardening near the surface. 9 The strain magnitude is largest in the centre of the cold-worked wire’s cross-section. With in- creasing amount of strain, the distribution of hardness on the cross-section becomes more uniform because of in- tensive work-hardening. Considering these drawbacks, it would be useful to combine one additional process and rotary swaging to work these materials. This could produce a material with high strength and good ductility suitable for further pro- cessing. A handful of earlier studies focused on using an SPD (severe plastic deformation) process and rotary swaging for forming commercially pure titanium. 10–12 5 CONCLUSIONS Pure titanium of two Grades 2 and 4 was successfully cold deformed with a total area reduction of 90 %. Only minor damage was found on the surface of these workpieces. With titanium Grade 5, there proved to be a need for in-process annealing between the cold-deforma- tion steps. In all the materials, the grains became elon- gated in the longitudinal direction and refined in the transverse direction. Based on the present results, the ro- tary swaging of titanium does qualify as an SPD process, although several other reports suggest so. In order to ob- tain a titanium material for an implant with high strength and good ductility suitable for further processing, it ap- pears desirable to combine one additional process with rotary swaging. Acknowledgment This paper was created under the project TH03010354 – Graded and functionally-structured long-life coxal implant (Gradientní funk~nì struktu- rovaný ky~elní implantát s vysokou `ivotností.) 6 REFERENCES 1 M. Duchek, J. Palán, T. 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