Injection Molding of Mechanical Alloyed Ti­Fe­Zr Powder

H. Ozkan Gülsoy1,2,+, Volkan Günay2, Tarık Baykara2 and Randall M. German3

1Metallurgical and Materials Engineering Department, Technology Faculty, Marmara University, Istanbul, 34722, Turkey2TUBITAK MRC, Materials Institute, Gebze ­ Kocaeli, 41470, Turkey3San Diego State University, 5500 Campanile Drive, San Diego, California 92182-1326, USA

This study focuses on the injection molding of mechanical alloyed Ti­Fe­Zr alloys. Injection molded samples were produced usingmechanical alloying based on hydride-dehydride (HDH) titanium and pure iron and zirconium powders. Mechanical alloyed powders weremixed with a polymeric binder and hot injection molded to form standard tensile bars. The critical powder loading for injection molding was50 vol% for feedstock. Molded bars were debound by solvent and then thermal steps, under ultra pure argon. Debound samples were sintered at1300°C for 60min in a high level vacuum (10¹5mbar). After sintering, the performances of the sintered materials was characterized using tensileand hardness testing, optical microcopy (OM) and scanning electron microscopy (SEM). The strengths and weaknesses of the test conditionshave been analyzed from the microstructure and mechanical properties. Theoretical density, ultimate tensile strength, and hardness of injectionmolded Ti powders increased with the additions of 5%Fe and 5%Zr. [doi:10.2320/matertrans.M2012031]

(Received January 30, 2012; Accepted February 13, 2012; Published May 25, 2012)

Keywords: powder injection molding, sintering, mechanical alloying, titanium alloy

1. Introduction

Powder injection molding (PIM) is a manufacturingtechnology for the net shape production of small, intricate,complex, and precise metallic or ceramic components. ThePIM process includes mixing of powders with a polymericbinder to produce a feedstock, injection molding the hotfeedstock into a cold mold to form a green part with thedesired shape using fast filling under high pressure,debinding to remove the polymer, and sintering to near fulldensity. The process overcomes the shape limitation oftraditional powder compaction, the cost of machining, theproductivity limits of isostatic pressing and slip casting,and the defect and tolerance limitations of conventionalcasting.1,2) If it is necessary, secondary operations suchas heat and surface treatments after sintering can also beperformed.1­3)

Titanium is a highly useful material in the production ofcomponents for various applications ranging from biomedicalimplants to automotive fuel injectors. At the same time, thehigh strength to weight ratio and high resistance to corrosionmake titanium and its alloys ideal materials for many of thesesame applications.4­6) One major barrier to a widespread useof titanium and titanium alloys, especially the cost consciousindustry, are the inherent high cost of the materials andcomponent fabrication. Titanium powder metallurgy (PM)approaches can allow cost effective production of near-netshape components. Use of titanium and titanium alloy couldtherefore increase many-fold if they could be produced frompowder at a low cost.7,8)

Earlier investigations on injection molded Ti and Ti alloysfocused on the effect of powder and binder characteristics,sintering conditions, heat treatment,9,10) and level of impurityelements.11­13) Impurities, such as oxygen, can dramaticallyincrease the strength but seriously degrade ductility in Ti PIMproducts.13) Arockiasamy et al. investigated the Fe and Zraddition on injection molded Ti. Their study showed that the

addition of Fe and Zr to Ti powders increased theoreticaldensity and mechanical properties.14) In this case iron andzirconium are used as alloying elements,14­16) as iron has atendency to stabilize the beta phase and form intermetallicswhile zirconium has a tendency to promote chemicalhomogenization. By optimizing the sintering conditions andalloying it is possible to obtain nearly full density titaniumstructures via PIM.14)

This present study was aimed to investigate the sintering,mechanical, and microstructural properties of the injectionmolded mechanical alloyed Ti­Fe­Zr powders. Metallo-graphic techniques were employed to sintered tensile barsto investigate the sintering behaviors and ensuring micro-structure. Tensile and hardness properties of the sinteredproducts were measured. Fracture surfaces of the sinteredsample were analyzed using SEM.

2. Experimental Procedure

In this study, HDH Ti sponge powder provided by PhellyMaterials was used. The Fe and Zr powder was obtained fromAlfa Aesar. Particle size distributions were determined onMalvern Mastersizer equipment and given in Fig. 1.Morphology of the Ti, Fe and Zr powders observed usingSEM is given Fig. 2. All powders are irregular in shape. The

Fig. 1 Cumulative particle size distributions for Ti, Fe and Zr powder.+Corresponding author, E-mail: ogulsoy@marmara.edu.tr

Materials Transactions, Vol. 53, No. 6 (2012) pp. 1100 to 1105©2012 The Japan Institute of Metals

powder characteristics and chemical properties of Ti, Fe, andZr powders used in this study are given Table 1. To protectthe powders from oxidation, all of the materials were storedin an argon atmosphere before milling, mixing, and molding.An experimental flowchart illustrates the manufacturing ofthe Ti­Fe­Zr samples is shown in Fig. 3. It is a fairlyconventional combination of mechanically alloying andmetal powder injection molding. Final compositions ofsamples were adjusted to give the Ti­5Fe­5Zr composition.

Milling was performed under high purity argon in avertical attrition mill (Szegvari attritor-Union Process), using

a hardened stainless steel vessel and balls. Other millingconditions, shown in Table 2, were chosen according to theexperimental results from preliminary studies.17) The pow-ders produced after different stages of milling were examinedusing a laser particle size analyzer and SEM.

A multiple-component binder system consisting of paraffinwax (PW), polypropylene (PP), carnauba wax (CW), andstearic acid (SA) was used. Feedstock was prepared at 175°Cwith the binder melted first and then powder blend addedincrementally under vacuum. The powder loading in thismixture was 50 vol%. After cooling, the feedstock waspelletized by hand. These feedstocks were injected usinga 12.5MPa specially made injection-molding machine toproduce standard tensile (MPIF 50) samples. The melttemperature was 150°C, the mold temperature was kept at 35to 40°C and the molding cycle time was 12 s.

Debinding was conducted in a two-step solvent/thermaloperation. Green parts were solvent debound at 70°C for 7 h

(a)

(b)

(c)

Fig. 2 SEM micrograph of Ti (a), Fe (b) and Zr (c) powder.

Table 1 Powder characteristic of Ti, Fe and Zr powders.

Item Ti Fe Zr

Vendor Phelly Mater. Alfa Aesar Alfa Aesar

Shape Irregular Irregular Irregular

Particle size

D10 16.05 4.81 1.95

D50 32.69 11.50 6.97

D90 57.15 26.84 26.72

Pycn. Density, g/cm3 4.51 7.88 6.53

Purity, % Commercial Purity 99.9 99.9

Fig. 3 Experimental flow chart of mechanical alloyed Ti­Fe­Zr alloy.

Table 2 Milling conditions for Ti­Fe­Zr powder.

Operating parameters

Vessel volume 1400 cm3

Balls : powder ratio 50 : 1

Balls diameter 5mm

PCA Wax

PCA content 1%

Rotor speed 750 rpm

Cooling Water flow

Milling time 0­45min

Atmosphere High purity Ar

Injection Molding of Mechanical Alloyed Ti­Fe­Zr Powder 1101

in heptane, followed by thermal debinding using a heatingramp of 1°C/min to 700°C for 1 h in high purity argonatmosphere. The sintering cycle applied to the samples wasas follows; samples were heated to 1300°C sinteringtemperatures at a rate of 5°C/min and they were held at1300°C for 1 h under vacuum (10¹5mbar).

The pre-sintered samples were cut into smaller testcoupons, approximately 3mm by 3mm in cross-sectionalsurface area and used for dilatometry studies. Dilatometrywas conducted in a vertical push rod dilatometer to quantifythe dimensional changes and identify any phase changes inthe material as it is sintered. The dilatometer cycle ramped at10°C/min to 1300°C and was held for 1 h in high purityargon.

The densities of the sintered samples were measured bymeans of the Archimedes water-immersion method. Formetallographic examination, samples were cut from thecenter of the each sintered tensile test bar. Different reagentwas used to etch the samples for optical metallographicexamination. The structure of the prepared alloys wasanalyzed by X-ray powder diffraction using Cu K¡ radiation.Step-scanning has been carried out from 0 to 70°. Thechemical compositions of the samples were analyzed byenergy dispersive X-ray (EDX) spectroscopy and SEM. Alltensile tests were performed using Zwick 2010 mechanicaltester at a constant crosshead speed of 1mm/min (25mmgauge length). The hardness tests were performed usingan Instron-Wolpert Dia Testor 7551 at HRB scale. At leastthree specimens were tested under the same conditions toguarantee the reliability of the results. The powder morphol-ogies and fracture surfaces of the molded and sinteredsamples were examined using a SEM (JEOL-JSM 6335F).

3. Results and Discussion

Figure 4 shows the X-ray diffraction patterns obtained forthe Ti­Fe­Zr alloys milled with increasing milling times. Themajority of the X-ray diffraction peaks correspond to the Tielement. As can be observed the elemental metals went intosolution with increased milling time and with 30min ofmilling the alloy appear as a solid solution. The diffractionpatterns show a disappearance of Zr peaks with milling timewhich may be a result of the deformation induced by theprocess, the refining of the microstructure and of the possibleformation of a supersaturated solid solution in Ti matrix. Inaddition, Fig. 5 shows the energy dispersive X-ray mappingfrom the center of mechanical alloyed powder. The matrix iscomposed titanium and elemental Fe and Zr homogeneouslydistributed.

Particle size distribution of Ti­Fe­Zr milled under vacuumatmosphere for 10, 20 and 30min, can be seen in Fig. 6. Itshows that particle size of Ti­Fe­Zr was decreased and thenincreased with milling times. Morphology of the mechanicalalloyed Ti­Fe­Zr powder observed using SEM is givenFig. 7. When SEM images of the base Ti powder (Fig. 2(a))are compared to the mechanical alloyed Ti­Fe­Zr powders(Fig. 7), coarsening of particles is very evident.

Figure 8 shows photographs of the molded and sinteredsamples. There are no defects evident such as blistering orslumping during molding, debinding, or sintering stages. A

set of sintering experiment was conducted at 1300°C for60min to study the effect of Fe and Zr addition on thetheoretical density. As expected, Ti samples without Fe andZr addition were produced a maximum theoretical density ofonly 95.5%. The sintered density increases with the additionsof Fe and Zr. When samples were admixed with 5mass% Feand 5mass% Zr, a theoretical density of 98.5% was achieved.Fe and Zr activate the sintering process of the Ti powders bythe formation of the liquid phase, produced by the eutecticreaction which occurs at 980­1065°C.14,15) Favorablediffusion and solubility change promote densification in thepresence of liquid phase. The liquid phase remains as analmost continuous network between solid grains, favoring theclassical phenomenon of the liquid phase sintering. If theamount of Fe, Zr and the sintering temperature are correctlychosen, near full density may be obtained.15) Figure 9 showsthe variation in the sintering shrinkage with time. The base Tisample when sintered to 1300°C for 60min in high purityargon atmosphere exhibited shrinkage of approximately 15,25% and was found to be 95.5% dense. The density ofmechanical alloyed Ti­Fe­Zr powder was found to be 98.5%with shrinkage of 19.13%. Figure 10 shows the micro-structures of the samples with and without 5mass% Fe and5mass% Zr. The additive free samples exhibit the porositycharacteristic of low theoretical density as shown Fig. 10(a).For comparison, Fig. 10(b) shows the microstructures of the5Fe­5Zr samples. With 5Fe­5Zr addition, the amount ofeutectic liquid is not sufficient for full densification and

Fig. 4 XRD patterns of the mechanical alloyed mixtures for differentmilling times.

H. O. Gülsoy, V. Günay, T. Baykara and R. M. German1102

sintered density is 98.5% theoretical. On the other hand, ahigher level of Fe and Zr addition results in formation ofa eutectic phase at the grain boundaries.15) As a result,theoretical densities increase with Fe and Zr additions.

Table 3 shows the mechanical properties of PIM Ti andTi­Fe­Zr alloy. Ti samples without Fe and Zr additionsresulted in a maximum theoretical density of 95.5% whereasthe addition of Fe and Zr in 5mass% and 5mass% increasedthe theoretical density to 98.5%. The increase in theoreticaldensity is accounted for by the formation of liquid phase inthe temperature range of 980 to 1065°C which enhances thedensification of the alloy due to the diffusion in the liquidphase along the grain boundaries.14­16) PIM Ti samplespresent average mechanical properties. The tensile strength

Fig. 5 EDS-Xray mapping of center of mechanical alloyed Ti­Fe­Zr particles.

Fig. 6 Cumulative particle size distributions for Ti and mechanical alloyedTi­Fe­Zr powder with different milling times.

Fig. 7 SEM micrograph of mechanical alloyed Ti­Fe­Zr powder.

Fig. 8 Photographs of molded and sintered Ti­Fe­Zr samples.

Injection Molding of Mechanical Alloyed Ti­Fe­Zr Powder 1103

was increased with amount of alloying. Tensile strengthincreased with Fe and Zr additions at 5% and 5%, due to thepresence of brittle intermetallic along the grain boundaries.Ductility decline is even more drastic with Fe and Zr additionsamples. The maximum tensile strength and elongation of929.8MPa and 3.7% were measured for Ti with 5Fe and 5Zradditions. The effect of Fe and Zr additions on the hardnessof Ti­Fe­Zr alloy is shown in Table 3. The results ofhardness measurements exhibited an increasing trend similarto that of theoretical density. The maximum hardness of 86.6HRB was reached with 5Fe and 5Zr additions.

The morphologies of fracture surface of the Ti and Ti­5Fe­5Zr alloy samples after sintering at 1300°C for 60min isshown in Fig. 11. Fracture surface of Ti samples exhibitdimpled features and porosities as shown in Fig. 11(a).The morphology of features surface with 5%Fe and 5%Zraddition is shown in Fig. 11(b). This sample exhibit quasi-cleavage fracture and low porosity. In this case the brittlefracture occurred through the intermetallic liquid phasenetwork. The eutectic network improved sintered densityand mechanical properties and decreased porosity.

4. Conclusions

The addition of iron and zirconium to a low cost titaniumpowder provided some benefit in terms of sinteringdensification, strength and hardness, but did not produce

-5

0

5

10

15

20

25

0 100 200 300 400 500

Time, min.

Sh

rin

kag

e, %

0

200

400

600

800

1000

1200

1400

Tem

pet

aru

re, °

C

Base TiTi-Fe-ZrTemperature

Fig. 9 Variation in the shrinkage of base Ti and mechanical alloyed Ti­Fe­Zr alloy powder.

(a)

(b)

Fig. 10 SEM micrographs of sintered Ti (a) and Ti­Fe­Zr (b) sample.

Table 3 Mechanical properties of PIM Ti and Ti-Fe-Zr alloys.

Sintering Temperature: 1300°C ­ Sintering Time: 60min

Ti Ti­5Fe­5Zr

Theoretical Density, g/cm3 4.51 4.68

Sintered Density, g/cm3 4.31 4.61

Relative Density, % 95.5 98.5

Tensile Strength, MPa 551.2 929.8

Elongation, % 5.2 3.7

Hardness, HRB 71.4 86.6

(a)

(b)

Fig. 11 Fracture SEM image for failed Ti (a) and Ti­Fe­Zr (b) sample.

H. O. Gülsoy, V. Günay, T. Baykara and R. M. German1104

significant improvements in ductility. The maximum sintereddensity achieved this study was 98.5% for a Ti­5Fe­5Zralloy. The formation of liquid phase gives some densification,strength and hardness gain. Tensile strength of 929MPa andhardness of 86.6HRB were achieved for Ti containing5mass% Fe and 5mass% Zr.

Acknowledgements

This work was supported by the Scientific ResearchProject Program of Marmara University (Project No. FEN-C-YLP-210311-0055).

REFERENCES

1) R. M. German and A. Bose: Injection Molding of Metals and Ceramics,(MPIF, NJ, 1997) pp. 21­25.

2) R. M. German: Powder Injection Molding, (MPIF, NJ, 1990) pp. 50­52.

3) N. H. Loh, S. B. Tor and K. A. Khor: J. Mater. Process. Technol. 108(2001) 398­402.

4) Y. Xu and H. Nomura: J. Jpn. Soc. Powder Powder Metall. 48 (2001)1089­1093.

5) R. Gerling and F. P. Schimansky: Mater. Sci. Eng. A 329­331 (2002)45­51.

6) S. Terauchi, T. Teraoka, T. Shinkuma and T. Sugimoto: J. Jpn. Soc.Powder Powder Metall. 47 (2000) 1283­1288.

7) G. Adam, D. L. Zhang, J. Liang and I. Macrae: Adv. Mater. Res. 29­30(2007) 147­151.

8) G. Lutjering and J. C. Williams: Titanium, 2nd Edition, (SpringerBerlin Heidelberg, New York, 2007), pp. 89­101.

9) G. Shibo, Q. Xhuanhui, H. Xinbo and D. Bo-hua: Trans. NonferrousMet. 14 (2004) 1055­1060.

10) E. Nyberg, M. Miller, K. Simmons and K. S. Weil: Mater. Sci. Eng. C25 (2005) 336­341.

11) D. Kuroda, M. Niinomi, M. Morinaga, Y. Kato and T. Yashiro: Mater.Sci. Eng. A 243 (1998) 244­249.

12) K. Kato, A. Matsumoto and T. Leki: J. Jpn. Soc. Powder PowderMetall. 44 (1997) 1029­1035.

13) T. Leki, K. Katoh, A. Matsumoto, T. Masui and K. Andoh: J. Jpn. Soc.Powder Powder Metall. 44 (1997) 448­454.

14) A. Arockiasamy, R. M. German, D. F. Heaney, P. T. Wang, R. L. Kingand B. Adcock: Powder Metall. 54 (2011) 420­428.

15) H. E. Kadiri, L. Wang, H. O. Gulsoy, P. Suri, S. J. Park, Y. Hammi andR. M. German: J. Metals 5 (2009) 60­66.

16) S. J. Park, D. F. Heaney and R. M. German: Metall. Mater. Trans. 40A(2009) 215­221.

17) T. Tsuchiyama, S. Hamamoto, K. Nakashima and S. Takaki: Mater. Sci.Eng. A 474 (2008) 120­126.

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