The Effect of Fe Addition on the Mechanical Properties of Ti–6Al–4V Alloys Produced by the Prealloyed Powder Method*1

Osamu Kanou, Nobuo Fukada and Masashi Hayakawa

Toho Titanium Co., Ltd., Chigasaki 253–8510, Japan

The effect of Fe addition on the mechanical properties of Ti–6Al–4V alloys was investigated. Ti–6Al–4V prealloyed powders were prepared via a hydrogenation and dehydrogenation process using turning chips of Ti–6Al–4V alloys as the starting material. Mixed powders of Ti–6Al–4V powders and 3–4 mass% Fe powders were consolidated using a hot extrusion process and subsequently hot rolled. With increasing Fe content, the tensile strength and 0.2% proof stress of the Fe-containing alloys increased by 2%–30% compared to those of the Fe-free alloy in both as-hot-rolled and air-cooled specimens. The results also showed that the elongation in the 3% Fe-containing alloys was at least 10%, regardless of the treatment procedure. The balance between the generated martensitic phase and the β-phase ratio in (α+β)-dual-phase alloys appears to determine both the strength and elongation of the alloys. The Ti alloys obtained in this study have strong potential for application in automobiles and aircraft. [doi:10.2320/matertrans.Y-M2016806]

(Received December 21, 2015; Accepted February 11, 2016; Published April 25, 2016)

Keywords: titanium alloy, prealloyed powder method, hot extrusion, hot rolling, heat treatment

1. Introduction tent is maintained at approximately 1–2 mass%, because these two elements tend to segregate during consolidation. Ti alloys are widely used as corrosion-resistant materials. This restriction is imposed by the ingot melting process and Substantial cost reduction is a prerequisite for Ti alloys to be does not apply in the case of powder metallurgical process- used as structural materials1); thus, near-net shape (NNS) ing. We believe that new Ti alloys produced by powder met- powder metallurgy has attracted considerable attention. For allurgy rather than by ingot melting may have superior prop- example, R. R. Boyer2) discussed the cost reduction in Ti al- erties, because they can contain high concentrations of loy production by powder metallurgy in the keynote speech at β-stabilizing elements, which effectively reinforce Ti alloys. the 12th World Conference on Ti (Ti-2011). C. A. Brice3) re- We have reported that Cu-enriched Ti–6Al–4V alloy can ported at the same conference that the substantial cost reduc- be produced without Cu segregation by powder metallurgical tion resulting from additive manufacturing made more gener- processes, and that the mechanical properties of Cu-rich Ti al application possible. Ti alloys produced by powder alloys are much improved compared to those of Cu-free Ti metallurgy have also recently been reported to be suitable for alloys5). In this study, Fe was used as the β-stabilizing ele- use in airplanes4). These reports demonstrate that Ti alloys ment at a concentration of 3–4 mass%. produced by a powder metallurgical process will soon nd practical use. 2. Experimental Procedure We have been investigating powder metallurgical processes to produce low-cost, high-strength Ti alloys. Sintered Ti The Ti–6Al–4V alloy powder was manufactured by the alloys are manufactured using either the blended elemental HDH process using an ingot turning chip of Ti–6Al–4V alloy method or the prealloyed powder method. Currently, only the as starting material. The hydrogenated Ti alloy was pulver- blended elemental method is used in practice. In this method, ized under an argon atmosphere using a closed-circuit pulver- powders of alloy elements or master alloys are mixed. On the izing device equipped with a Hosokawa Air Classi cation contrary, prealloyed powders with a predetermined alloy Mill (ACM®) pulverizer and a micron separator (MS). The composition are used as raw materials in the prealloyed pow- setup is shown in Fig. 1. Hydrogenated Ti alloy (400 kg) was der method. Therefore, pure Ti powder and alloying elements placed into the device for pulverization and sieving, and or mother alloy powders are not required in the method. Pre- 320 kg of powder sample between 10 and 150 μm was ob- alloyed powders are prepared via a gas atomization process or tained; the yield was 80%. The pulverized Ti–6Al–4V hydro- the hydrogenation and dehydrogenation (HDH) process. In genated powder was then dehydrogenated in a vacuum fur- the HDH process, relatively inexpensive turning chips of Ti nace; nally, Ti–6Al–4V alloy powder was obtained. alloy can be used as a starting material, which is the reason Carbonyl Fe powder (BASF Corp.) with a mean particle for the low cost of the prealloyed powder method. The blend- diameter of 8.0 μm was mixed with the Ti–6Al–4V alloy ed elemental and prealloyed powder methods are compared powder at compositions of 3, 3.5, and 4 mass% using a V-type in Table 1. mixer. The mixed powders were lled into metal containers Many Ti alloys contain alloying elements such as V, Mo, using a special tool to prevent segregation and were subse- Cr, Fe, and Cu (referred to as β-stabilizing elements), and alu- quently hot-extruded. The powder without Fe was also tested minum (an α-stabilizing element). However, Fe and Cu con- for purposes of comparison. The materials were formed into cylinders with φ205 to φ100 mm by hot extrusion. After hot *1This Paper was Originally Published in J. Jpn. Soc. Powder Powder Met- extrusion, four extruded bars were deformed to φ20 mm by allurgy 62 (2015) 365–370. hot-roll rolling under the same conditions; i.e., the samples 682 O. Kanou, N. Fukada and M. Hayakawa

Table 1 Comparison between the blended elemental method and the prealloyed powder method.

Method Blended Elemental Method (BEM) Prealloyed Powder Method (PA) Mixture of pure Ti powder Pre-alloyed powder and alloying elements powder (No need for pure Ti powder) Raw material powder or (No need for alloying elements powder) Mixture of pure Ti powder (No need for mother alloy powder) and mother alloy powder Merit Densi ed sintered material can be obtained Price of pre-alloyed powder is relatively low Demerit Price of raw material powder is relatively high Dif cult to obtain densi ed sintered material Technically established Technically not established Current status However its application is limited No investigation has been reported

Fig. 1 Schematic of the closed-circuit pulverizing device equipped with a Hosokawa ACM pulverizer® and a micron separator. Fig. 2 The manufacturing ow chart of Ti alloy bars.

3. Experimental Results and Discussion were heated at 1123 K and hot rolled for a total of ve passes. The surface temperature of the specimens during hot rolling 3.1 Chemical composition was measured using a thermometer. The material temperature The analytical results for the Ti–6Al–4V alloy powder and was con rmed to not exceed the β-phase transformation tem- extruded bars with 3–4 mass% Fe are given in Table 2. The perature (Tβ) during the hot rolling process; the temperature aerospace material speci cations (AMS) for the Ti–6Al–4V at the last pass end was 1098 K in the case of the hottest sam- alloy bar produced by ingot melting are also shown in this ple. After the hot rolling process, heat treatments were con- table. The properties of the materials obtained in this study ducted for 7.2 ks at 1143 K for the Fe-containing alloys and satis ed the AMS except for their oxygen content (0.27%), at 1223 K for the Fe-free alloys; the specimens were then which exceeded the AMS. The ingot before turning contained cooled under either air or water. The manufacturing ow dia- 0.2% oxygen, which increased during cutting and pulveriz- gram is shown in Fig. 2. ing. Samples were collected and prepared for metallographic observation, electron-probe microanalysis (EPMA), and ten- 3.2 Microstructure sile testing. Test specimens for tensile testing were prepared Optical micrographs of specimens with 3.5 mass% Fe at immediately after hot extrusion (alloys containing 3.5% Fe), various stages are shown in Fig. 3, i.e., after hot extrusion, after hot rolling, and after heat treatment (air-cooled and wa- after soaking at 1143 K for 3.6 ks, and after hot rolling. No ter-cooled samples). Tensile testing was conducted according pores were detected in the extruded materials. This result sug- to standard test method JIS-H4650. The size of each test gests that densi cation reached the theoretical density level specimen was sub-size No. 4, as de ned by JIS-Z2241 during hot extrusion, even though full densi cation by the (φ4 mm × 14 mm long at gage-point part); the strain rate was prealloyed powder method is dif cult8), as described in Ta- 0.5%/min from the starting point to the 0.2% strain point and ble 1. Figure 3 also suggests that hot-extrusion sintering 10%/min after the 0.2% strain point to rupture. The β-phase solves the densi cation problem in prealloyed powders be- content was measured on the basis of the color difference because the materials experience stresses greater than the defor- tween the α-phase and the β-phase in optical microphoto- mation stress9) during hot extrusion. graphs of the heat-treated Ti–6Al–4V alloy specimens; the The microstructure after hot rolling comprises coarse α-phase appears white, and the β-phase appears black6,7). β-phase grains, α-phase grains within β-phase grains, and The Effect of Fe Addition on the Mechanical Properties of Ti–6Al–4V Alloys Produced by the Prealloyed Powder Method 683

Table 2 Chemical analysis of the Ti powder and extruded bars. The AMS for Ti-6Al-4V alloy bars manufactured from ingots are also shown for reference purposes.

Element Al V Fe O C N H Ti Ti-6Al-4V Alloy Powder 6.0 4.2 0.2 0.27 0.006 0.017 0.008 Bal. Ti-6Al-4V-3Fe Alloy Bar 5.9 4.1 3 0.26 0.006 0.02 0.006 Bal. Ti-6Al-4V-3.5Fe Alloy Bar 5.9 4.1 3.5 0.27 0.006 0.018 0.006 Bal. Ti-6Al-4V-4Fe Alloy Bar 5.9 4.1 4.1 0.27 0.006 0.017 0.006 Bal. min 5.5 3.5 - - - - - Reference AMS-4928 Bal. max 6.75 4.5 0.3 0.2 0.08 0.05 0.0125

Fig. 3 Optical micrographs of Ti-6Al-4V-3.5Fe alloy bars: (a) after hot extrusion, (b) after heat treatment of extruded bar, and (c) after hot rolling.

Fig. 5 EPMA analysis results for Ti-6Al-4V-3.5Fe alloy. (a) backscattered electron image, (b) Fe X-ray image, (c) Al X-ray image, and (d) V X-ray image.

sults are shown in Fig. 5 for 3.5% Fe-containing alloy. In the backscattered electron images, dark (black) and bright (gray) regions are observed and correspond to the α-phase and the β-phase, respectively. Because the contrast is reversed between the optical micrograph and backscattered electron images, dark regions correspond to the α-phase and bright regions correspond to β-phase in the case of backscattered electron images. Fe condenses in the β-phase and is present in Fig. 4 Optical micrographs of Fe-enriched hot-rolled Ti alloy bars after low concentration in the -phase. Low-Fe regions ( -phase) heat treatment. The upper and lower rows show water-quenched and air- α α cooled specimens, respectively. (a)Ti-6Al-4V-3Fe alloy, (b) Ti-6Al-4V- and high-Fe regions (β-phase) exist periodically in intervals 3.5Fe alloy, and (c) Ti-6Al-4V-4Fe alloy. of approximately 10 μm. Segregation of Fe in Ti alloy is usu- ally detected as a continuous structure of high-Fe regions and low-Fe regions in millimeter-order intervals. Thus, the EPMA plate-like α-phase grains along the β-phase grain boundaries data indicate that Fe was not segregated in the investigated (Fig. 3 (a)). Metals with such microstructures are not desir- alloy. able for use in aircraft because their elongation is insuf cient. Equiaxed α-phase and β-phase grains without primary 3.3 Tensile test results α-phase grains are desired; however, we observed no im- The results of tensile tests are listed in Table 3. The test provement in the microstructure even after the specimen was results for the Fe-free specimens are included for reference. soaked at 1143 K for 3.6 ks (Fig. 3 (b)). Both prior β-phase The specimens after hot extrusion exhibited high tensile and plate-like α-phase grains disappeared and were replaced strengths and high 0.2% proof stresses; however, their elon- by ne grains after hot rolling from φ100 to φ20 mm, and the gation and reduction in area were small, presumably because deformation ratio was 96%. However, no recrystallization of of their microstructure. The specimens after hot extrusion the microstructure was observed after hot rolling; heat treat- comprise coarse prior β-phase and primary α-phase grains. ment appears to be required for recrystallization. Figure 4 This microstructure clearly falls short of AMS *2 because of shows optical micrographs of the heat-treated specimens. The water-cooled specimens had partially recrystallized grains, whereas the air-cooled specimens had fully recrystallized *2 The speci cation of AMS 2380F. equiaxed grains. Both Fe-free and Fe-enriched Ti–6Al–4V 3.2.2 Microstructure: Shall be that structure resulting from processing within the alpha-beta phase eld. Microstructure shall conform to 3.2.2.1 alloys are dual-phase alloys, and the α-phase and the β-phase or 3.2.2.2. can be distinguished by the difference in the shades of the 3.2.2.1 Equiaxed or equiaxed and elongated alpha in a transformed beta grains, as previously described. The β-phase ratio increased matrix with no continuous network of alpha at prior beta grain boundar- with increasing Fe content and was higher in the air-cooled ies. 3.2.2.2 Essentially complete eld of equiaxed and /or elongated alpha specimens than in the water-cooled specimens. with or without intergranular beta and with no continuous network of al- EPMA was performed to check for Fe segregation. The re- pha at prior beta grain boundaries. 684 O. Kanou, N. Fukada and M. Hayakawa

Table 3 Tensile test results of the specimens.

0.2% PS Sample Status TS (MPa) EL (%) RA (%) (MPa) As Extruded - - - - Ti-6Al-4V-3Fe As Hot Rolled 1325 1257 13.6 27.0 Alloy Bar After HT (AC) 1301 1218 8.9 15.9 After HT (WQ) 1131 1035 15.0 26.0 As Extruded 1238 1134 4.6 7.1 Ti-6Al-4V-3.5Fe As Hot Rolled 1357 1292 5.0 7.8 Alloy Bar After HT (AC) 1325 1237 5.7 8.1 After HT (WQ) 1130 1048 11.1 19.0 As Extruded - - - - Ti-6Al-4V-4Fe As Hot Rolled 1380 1313 2.5 3.0 Alloy Bar After HT (AC) 1356 1269 2.1 2.7 After HT (WQ) 1132 1062 12.5 22.7 Fig. 6 Relationship between the 0.2% proof stress, the elongation and the β-phase ratio in 3 titanium alloys with different iron contents and different As Extruded - - - - heat treatment conditions. (Reference) As Hot Rolled 1293 1230 4.3 5.4 Ti-6Al-4V Alloy Bar After HT (AC) 1091 960 15.4 26.0 After HT (WQ) 1216 1079 11.8 29.6 tility is very common in metallic materials. The tensile properties of the water-cooled specimens can- not be explained solely on the basis of the solid-solution hard- its associated lack of ductility. The microstructure improved ening effect of Fe. The Fe-free alloy exhibited the highest after hot rolling in the temperature range of the (α+β)-phase. tensile strength and 0.2% proof stress among the four investi- We here discuss the tensile properties of the hot-rolled and gated specimens. All four specimens exhibited elongation heat-treated specimens. Both the tensile strength and 0.2% values >10%, and the 3%Fe-containing alloy exhibited the proof stress of the specimens after hot rolling increased with highest value of 15%. In the water-cooled specimens, marten- increasing Fe content. This behavior appears to simply be an sitic transformation occurred; the tensile properties were ap- effect of solid-solution hardening by the alloying Fe atoms. parently inuenced by the generated martensitic structure. In However the differences in tensile strength and 0.2% proof cases where the cooling rate is the same, the generated mar- stress between Fe-containing and Fe-free alloys were small, tensitic structure depends on the quenching temperature and in the order of 2%–6%. Ouchi10) has reported that the me- the Mo equivalent11). The Fe-free alloy was quenched from chanical properties of Ti alloys after heat treatment are 1223 K, and the Fe-containing alloys were quenched from strongly inuenced by the thermomechanical treatment con- 1143 K. The Mo equivalents of the specimens were calculat- ditions. The strength and ductility of Ti–6Al–4V alloy have ed as 3.3 in the case of the Fe-free alloy and 10.2, 11.5, and also been reported to improve with decreasing hot-rolling 12.7 in the case of the Fe-containing alloys, (3, 3.5, and 4% temperature10). These improvements are a consequence of the Fe, respectively12,13). Given these Mo equivalents, the α′ mar- metallographic structure; i.e., a lower hot-rolling temperature tensitic phase was generated in the Fe-free alloy, and the α′′ results in ne, uniform α-grains and α′ martensite grains10). martensitic phase was generated in the Fe-containing al- We interpreted our data in view of Ouchi’s report. In this loys11). The high strength of the Fe-free alloy is possibly study, both Fe-containing and Fe-free alloys were hot rolled caused by its α′ martensitic phase enhancing its strength to a under the same conditions in the (α+β) temperature region greater degree than the α′′ martensitic phase enhances the (i.e., the heating temperature was 1123 K for all four speci- strength of the Fe-containing alloys. The abovementioned mens). In addition, the nal temperatures after hot rolling martensitic phase transformation in both (α+β) Ti alloy and were approximately the same for all four specimens, as previ- near-β Ti alloy has been previously reported14). ously described. The Tβ of the Fe-containing alloy and the The effect of heat-treatment on the 0.2% proof strength can Fe-free alloy differ by approximately 70 K. This difference in be well explained using the β-phase ratio. Figure 6 shows the Tβ means that the Fe-free alloy was hot rolled under the con- 0.2% proof stress and the elongation as a function of the dition of a higher volume fraction of α-phase compared to the β-phase ratio. The 0.2% proof stress decreases with increas- α-phase content of the Fe-containing alloy. Thus, the Fe-free ing β-phase ratio, whereas the elongation increases. The ten- alloy could accumulate greater strain than the Fe-containing sile properties of the Fe-containing alloy appear to be gov- alloy, resulting in greater tensile strength and greater 0.2% erned by a balance between the generated martensitic proof stress for the Fe-free alloy. structure and the β-phase ratio, as previously discussed. No- The tensile strength and 0.2% proof stress of the air-cooled tably, high strength and high ductility were concurrently specimens were 20%–30% higher in the case of the Fe-con- achieved in the 3% Fe-containing alloy in its as-hot-rolled, taining alloys than in the case of the Fe-free alloy; however, air-cooled, and water-cooled conditions. We speculate that the elongation and reduction in area of the Fe-containing al- the balance of martensitic structure and β-phase ratio was op- loys were low, which became noticeable with increasing Fe timized in the case of the 3% Fe-containing alloy. This opti- content. This effect is regarded as a solid-solution hardening mized balance suggests that a Ti alloy with a high concentra- effect of Fe addition. This relation between strength and duc- tion of Fe is possible. The tensile strengths of the Ti–6Al–4V The Effect of Fe Addition on the Mechanical Properties of Ti–6Al–4V Alloys Produced by the Prealloyed Powder Method 685

(3–4%) Fe alloys are 2%–30% greater than those of the Fe- the Ministry of Economy, Trade and Industry in scal year free alloys in the cases of both the hot-rolled and the air- 2014. Hot extrusion was carried out at the Sanyo Special cooled specimens. The decrease in elongation observed in Steel Co., Ltd; the authors are thankful to Sanyo Special Steel this work is very common with increasing strength in various Co., Ltd. for their cooperation. The authors would like to metallic alloys. However, the results obtained for the 3% thank Enago (www.enago.jp) for the English language re- Fe-containing alloy strongly suggest that high-strength Ti al- view. loys with an elongation of 10% are possible if the β-phase ratio is properly controlled. REFERENCES The β-phase ratio affects the 0.2% proof stress and the elongation, as has already been reported for other (α+β)-type 1) W. Peter and Y. Yamamoto: ORNL TM-2012 (2013) 1–98. th alloys15). Because the 0.2% proof stress and elongation are 2) R. Boyer, and J. Williams: Proc. 12 World Conference on Titanium, ed. by L. Zhou, H. Chang, Y. Lu and D. Xu, (The Nonferrous Metals strongly inuenced by the β-phase ratio in (α+β)-dual-phase Society of China, 2011) pp.10–19. alloys, desirable properties can be obtained by controlling the 3) C. Brice: Proc. 12th World Conference on Titanium, ed. by L. Zhou, H. processing parameters, such as the solution treatment tem- Chang, Y. Lu and D. Xu, (The Nonferrous Metals Society of China, perature and the cooling rate. An aging treatment might be 2011) pp.1697–1703. nd useful but was not tested in this study. 4) P. Whittaker: IMPD net 2 May, 2012. 5) S. Sugawara, O. Kanou, H. Takatori, H. Ikeda, Y. Saimen and N. Yama- na: Proc. 12th World Conference on Titanium, ed. by L. Zhou, H. 4. Conclusion Chang, Y. Lu and D. Xu, (The Nonferrous Metals Society of China, 2011) pp.1759–1762. The effect of Fe addition on the mechanical properties of 6) T. Morita, K. Shinoda, K. Kawasaki and Y. Misaki: J. Soc. Mater. Sci., Ti–6Al–4V alloys produced by prealloyed powders was in- Japan 56 (2007) 345–351. 7) L.M. Gammon, R.D. Briggs, J.M. Packard, K.W. Baston, R. Boyer and vestigated. With increasing Fe content, the tensile strength C.W. Domby: ASM Handbook Volume 9: Metallography and Micro- and 0.2% proof stress of the Fe-containing alloys increased structures, (2004) pp.899–917. by 2%–30% compared to those of the Fe-free alloy in as-hot- 8) M. Quian: Int. J. Powder Metallurg 46 (2010) 29–44. rolled and air-cooled specimens. In addition, the elongation 9) H. Suzuki, T. Ashina, K. Aoyagi, H. Fujii, and K. Tanabe: Tetsu to Ha- in all of the 3% Fe-containing alloy specimens approached gane (J. Jpn Iron and Steel Inst.) 86 (1986) 587–594. 10) C. Ouchi: J. Jpn. Inst. Metals 25 (1986) 672–679. 10%. The balance between the generated martensitic phase 11) Y. Murakami: Tetsu to Hagane (J. Jpn Iron and Steel Inst.) 73 (1987) and the β-phase ratio in (α+β)-dual-phase alloys appears to 420–426. determine both strength and elongation. The Ti alloys ob- 12) A. Ogawa, and H. IIzumi: JPN Patent, 2010-70833. tained in this study have strong potential for applications in 13) Z. Guo, S. Malinov and W. Sha: Comput. Mater. Sci. 32 (2005) 1–12. automobiles and aircraft. 14) T. Kunieda, K. Mori, K. Takahashi, and H. Fujii: Nippon Steel & Sum- itomo Metal Technical Report. No.106 (2014) 47–52. 15) T. Kunieda, K. Mori, K. Takahashi and H. Fujii: Nippon Steel & Sum- Acknowledgements itomo Metal Technical Report. 396 (2013) 50–55.

This research was performed with nancial support from