Materials and Design 54 (2014) 149–153

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Materials and Design

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Short Communication

Effect of impact force on Ti–10Mo alloy powder compaction by highvelocity compaction technique

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.07.054

⇑ Corresponding author. Tel.: +86 10 82377286.E-mail address: hqyin@ustb.edu.cn (H. Yin).

Dil Faraz Khan a,c, Haiqing Yin a,⇑, He Li a, Zainul Abideen c, Asadullah b,d, Xuanhui Qu a, Mujtaba Ellahi b

a Laboratory of Particulate and Powder Metallurgy, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR Chinab School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR Chinac Department of Physics, University of Science and Technology Bannu, Bannu 28100, Pakistand Department of Mathematics, Karakoram International University Gilgit-Baltistan, Gilgit 15100, Pakistan

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 May 2013Accepted 15 July 2013Available online 25 July 2013

Ti–10Mo alloy powder were compressed by high velocity compaction (HVC) in a cylinderical form ofheight/diameter (h/d) in die 0.56 (sample A) and 0.8 (sample B). Compactions were conducted to deter-mine the effect of impact force per unit area of powder filled in die for densification and mechanical prop-erties of Ti–10Mo samples. The micro structural characterization of samples were performed by scanningelectron microscope (SEM). The mechanical properties of the compressed samples such as Vickers hard-ness, bending strength, and tensile strength were measured. Experimental results showed that the den-sity and mechanical properties of sample A and sample B increased gradually with an increase in impactforce and decreased with an increase in height/diameter ratio. The relative green density for sample Areached up to 90.86% at impact force per unit area 1615 N mm�2. For sample B, it reached 79.71% atimpact force per unit area 1131 N mm�2. The sintered sample A exhibited a maximum relative densityof 99.14%, Vickers hardness of 387 HV, bending strength of 2090.72 MPa, and tensile strength of749.82 MPa. Sample B revealed a maximum relative sintered density of 97.73%, Vickers hardness of376 HV, bending strength 1259.94 MPa and tensile strength 450.25 MPa. The spring back of the samplesdecreased with an increase in impact force.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Over the last 50 years, the metallurgy of titanium and Ti-basealloys has been extensively studied. Titanium has distinct proper-ties, for instance, high strength to weight ratio and good resistanceto numerous corrosion environments. This material can be used ina wide range of temperatures [1]. Molybdenum increases thestrength and biological compatibility of titanium alloys than vana-dium and aluminum, and produce stable phase element of b-Tita-nium. Mo element has no toxic side effects on the human body.Therefore, Ti–Mo alloy maintains good biocompatibility and highmechanical performance in its raw form. Ti–Mo has an extensiveuse as a biomedical material [2]. Pure Ti and Ti-alloys have exten-sively been used as clinical implanting material because of its bet-ter biocompatibility with excellent mechanical properties such ascorrosion resistance [3]. Ti–Mo alloy has a broad use in orthodonticmechanothropy [4]. Ti–Mo alloy, with different Mo contents, isprepared and reported that Ti–10Mo alloy, comparatively, hasthe highest mechanical properties than presented Mo content al-

loys in [5]. A Ti–7.5Mo and series of Ti–7.5 wt.% Mo–xFe alloys,with Fe contents up to 7 wt.% are prepared using a commercialarc-melting vacuum-pressure type casting system [6,7]. The feasi-bility of applying hot explosive compaction (HEC) to Mo–Ti powdermixtures is explored in [8]. Titanium added 5 wt.% Al, Mo or Fe, aremanufactured through spark plasma sintering (SPS) by usingblended elemental process [9]. Ti–7.5 Mo powder has also beencold compressed into cylindrical green compacts of 7 mm in diam-eter under 200 MPa [10].

High velocity compaction of particulate or powder system is acomparatively novel mass fabrication method with an objectiveto improve the mechanical properties as well as expand the appli-cations of powder materials part [11]. HVC provides high densityand the possibility to make large P/M parts up to more than five kg(10 lbs). The powder is compacted in less than 20 ms by high-en-ergy impact. Further densification is made possible by adding mul-tiple impacts as small as 300 ms after each other [12].

However, the information about compaction of Ti–10Mo alloypowder by high velocity compaction technique has not yet beenreported in literature. In this study, Ti–10Mo alloy powder hasbeen compacted by HVC technique to evaluate and investigatethe effect of increasing impact force of the compressed sample cor-responding to its density and mechanical properties.

150 D.F. Khan et al. / Materials and Design 54 (2014) 149–153

2. Experimental technique

Pure hydride de-hydride (HDH) titanium powder and molybde-num powder were used to produce Ti–10Mo wt.% bulk alloy. Ti andMo powder were supplied by Beijing Xing Rong Yuan TechnologyCo. Ltd. China. The pure Ti and Mo powders were mixed with aball-mill at a rotation speed of 20 rpm for 10 h. The SEM micro-graph of the Ti–10Mo powder is shown in Fig. 1. The titanium pow-der exhibited a polygonal shape while molybdenum powderrevealed rounded shape. The powder characteristics and particlesize are given in Table 1.

Ti–10Mo powder was pressed using Hyp 35-2 high velocitycompaction machine. Details of the machine can be seen in [13].Die wall was lubricated before filling the powder with a zinc stea-rate dissolved in acetone to facilitate ejection of samples. Greensamples of powder filled in die at height 14 mm (sample A) and20 mm (sample B) of 25 mm diameter were pressed by a singlecompaction method as shown in Fig. 2.

During high velocity compaction, impact energy is proportionalto the stroke length of the hammer. The impact energy can be cal-culated based on following equation:

E ¼ F � h ð1Þ

where E is the impact energy, J; F is the hydraulic system force ap-plied on the hammer, and h is the stroke length (the distance be-tween starting position and impact position). The hammervelocity can be designed based on the following equation:

V ¼ffiffiffiffiffiffi2Em

rð2Þ

where V is the impact velocity, m s�1; E is the impact energy, J; andm is the weight of hammer. The impact force on the powder bed canbe regulated by compaction tools with variable hammer velocitythrough the following equation:

F ¼ V �Mt

ð3Þ

where F is the impact force, N; V is the hammer velocity, m s�1; M isthe mass of hammer and upper punch, kg and t = 0.2 s, is the time totransfer hammer velocity into powder bed [14]. A new quantity r, isthe ratio between impact force F and per unit area of powder filledin die A as given for comparison through the following equation:

r ¼ FA

ð4Þ

where r is the impact force applied on the powder bed per unitarea, N mm�2; F is the impact force, N; and A = (2 � p � r � h), areaof powder filled in die, mm2. The compressed samples were sin-

Fig. 1. SEM micrograph of Ti–10Mo alloy powder.

tered in Argon (Ar) gas atmosphere up to 1300 �C with a holdingtime 2 h and cooled in the furnace. Archemides method was usedto measure specimen’s density and its relative density was calcu-lated upon theoretical density 4.77 g cm�3 of Ti–10Mo powder.The polished samples were deeply etched with Kroll’s reagent(3 mL HF: 6 mL HNO3: 100 mL H2O). The powder morphology andsamples microstructural evolution were analyzed using SEM. TheVickers hardness was calculated under 0.2 kg load with an indenttime of 15 s from 5 individual measurements on polished sectionsof samples by using SCTMC apparatus.

A computer controlled universal material testing machine wasused to determine the bending and tensile strength. Bendingstrength of the sintered compacts was measured based on thestandard used in [15]. Tensile strength measurement was per-formed on flat dog bone style tensile specimens in accordance withtest standard GB/T 228.1-2010 [16]. The resulting stress versusstrain curves provided values for 0.2% offset yield stress.

3. Results and discussion

3.1. Effect of impact force per unit area on green and sintered density

Fig. 3 shows the density and relative density of Ti–10Mo pow-der compacts with an increasing impact force per unit area. Thegreen density of the sample A is higher than the sample B. The rel-ative green density of sample A (h/d 0.56) reaches at 90.86% with agreen density of 4.334 g cm�3 at impact force per unit area1615 N mm�2. The relative green density of the sample B (h/d0.8) is 79.71% of green density 3.802 g cm�3 at impact force perunit area 1131 N mm�2. The Ti-alloys density varies between 69–71% and 93% to 95% by using cold isostatic pressing (CIP) techniqueand sintering [17] respectively. Obviously, the relative density ofcompacts formed by HVC technique is better than CIP process com-pactions. Ti powder compaction is complicated process with con-ventional compaction method due to its hardness and inductileproperties as compare to stiff metal like iron or copper and softmetal like aluminium powder. The hammer velocity, impact forceand impact force per unit area can be adjusted during compactionbased on Eqs. (2)–(4) respectively.

The increase in compacting force leads to higher densification ofthe powder body. The densification is achieved through a force cre-ated by a hydraulically driven hammer. Kinetic energy of hammeris transferred through the upper punch to the powder compact.The densification of the powder is performed by transmissionand reflection of stress waves propagated within the powder body[11].

In HVC technique, hammer velocity and impact force areincreasing positively to act upon a powder body in unit time. Incase of sample A, the impact force per unit area is higher. There-fore, small particles are easily filled into pores among coarser par-ticles. These fillings lead to increase the green density and toreduce the size and number of pores. Under high compressionforce, the powder particles in a compact are forced into contactpores to eliminate the extensive areas of true contact amongformed particles [18]. The required density of the powder dependson its energy per unit mass [19,20]. The applied impact force onboth samples is identical except for the difference of powder filledin die per unit area. Consequently, Sample A has higher relativegreen density than sample B. It can be concluded that the similarrelative green density for both samples can be obtained with theidentical impact force per unit filled area. Therefore, improvementin green density is directly dependent upon impact force per unitarea of powder filled in die. It is productive for designing parame-ters in HVC technique and estimating the maximum density for atype of established HVC machine and certain powder.

Table 1Characteristics and particle size of powder.

Element Mesh size Theoretical density (g cm�3) Apparent density (g cm�3) Purity (%) Particle size (lm)

D10 D50 D90

Ti �500 4.50 1.39 99.99 13.677 22.646 36.175Mo �500 10.22 1.61 99.99 6.061 18.977 41.724

Fig. 2. Ti–10Mo alloy powder compacts (a) sample A and (b) sample B.

Fig. 3. Comparison of green and sintered densities as a function of impact force perunit area.

D.F. Khan et al. / Materials and Design 54 (2014) 149–153 151

Fig. 3 shows the result of sintered density of samples as a func-tion of impact force per unit area. The sintered density of sample Aincreases from 4.684 g cm�3 (relative density 98.19%) to4.729 g cm�3 (relative density 99.14%) according to increase in im-pact force per unit area from 1377 to 1615 N mm�2. The sintereddensity of sample B increases from 4.642 g cm�3 (relative density97.32%) to 4.662 g cm�3 (relative density 97.73%) when impactforce per unit area increases from 964 to 1131 N mm�2 respec-tively. Sintering process bonds the contacting particles of the pow-der at higher temperatures. On a microstructural scale, the bondingbecomes obvious as neck growth between touching particles at aspecific high sintering temperature. The complete dissolution ofthe particles and elimination of porosity results in a relative in-crease in sintered density of compacts which can be observed fromthe SEM micrographs of Fig. 4. During titanium alloy sintering, theexpansion/shrinkage behavior provides important information inorder to obtain high levels of densification [1]. Fig. 3 exhibits thedifference between green and sintered densities of sample A andB. However, the difference is more obvious in case of sample B. Thisdifference reveals higher shrinkage of sample B than sample A dur-ing sintering due to less compression of particles in green form.

3.2. Effect of impact force on radial spring back

Elastic–plastic deformation of the powder particles occur in thecompaction process. In the compact’s particles, elastic inner stressexists due to the applied external force that exhibits in differentdirection after removing the pressure. This relaxation changesthe shape and contact state of particles that causes the expansionof the powder compacts as well as limits the deformation of parti-cles [21]. Fig. 5 shows the spring back of the Ti-alloy decreasesgradually with an increase in impact force. It is in the range of�0.62% to �0.46%. The low impact force applied on the powderbed lead to weak inter-particular bonding during unloading stage.The high impact force gives better compressibility and good inter-locking of particles with reduction in spring back.

3.3. Effect of impact force on Vickers hardness

The hardness of the sample A and sample B gradually increaseswith an increase in the impact force as shown in Fig. 6. The hard-ness of sintered sample A varies between 366.4 and 387.22 HV,while the hardness range of sample B is 362.82–375.6 HV. Thehardness of the sample A is higher than the sample B due to itsgood compressibility during compaction. During sintering the en-hanced diffusion bonding and solution hardening by diffusion ofMo atoms into the bulk can be seen in the SEM micrograph of bothtypes of samples as shown in Fig. 4. The density of compacts in-creases with reduced ratio of pores after sintering. Thus, samplesgain strength with better resistance to deformation for appliedforce [13].

3.4. Effect of impact force on bending strength

Fig. 7 shows the bending strength of the sintered compacts as afunction of impact force.

The bending strength values of the samples increases with anincrease in impact force. At the same impact force, bendingstrength of the sample A is higher than the sample B. The bendingstrength of sample A and sample B increases from 1640.98 to2090.72 MPa and from 1209.03 to 1259.94 MPa respectively withan increase in impact force from 1.51 to 1.78 kN. The increased im-

Fig. 4. SEM images of the sintered compacts (a) of sample A, pressed with impact force 1615 N mm�2, and (b) sample B pressed with impact force 1131 N mm�2.

Fig. 5. Spring back of sample A and sample B as a function of impact force.

Fig. 6. Vickers Hardness of the sample A and sample B vs. impact force.

Fig. 7. Bending strength of sample A and sample B as a function of impact force.

Fig. 8. Tensile strength of sample A and sample B vs. impact force.

152 D.F. Khan et al. / Materials and Design 54 (2014) 149–153

pact force per unit area on powder bed improves the capacity ofpowder particles to slide, rearrange and deform which enhancedgreen density and green strength of the compacts. The bondingamong particles is significantly improved with an enhancedstrength of samples during sintering.

3.5. Effect of impact force on tensile strength

As impact force increases, tensile strength of both the samplesincreases gradually. A tensile strength of sample A is higher thanthe sample B as shown in Fig. 8. The tensile strength of the sampleA increases from 597.99 to 749.82 MPa and of sample B increasesfrom 274.75 to 450.25 MPa with an increase in impact force from1.51 to 1.78 kN. The mechanical properties of the sintered body de-

pend on porosity [22]. As the sintering temperature increases, thepores shrinkage ratio also increases. The particles bonded and en-larged the grain size to achieve significant strength with improvedproperties after sintering process [23].

4. Conclusions

In this work, Ti–10Mo alloy powder was compacted by HVCtechnique and the green compacts were sintered up to 1300 �C inArgon (Ar) gas atmosphere with a holding time 2 h followed by fur-nace cooling. The density and mechanical properties of these com-pacts were investigated. The results obtained in this research aresummarized as follows:

D.F. Khan et al. / Materials and Design 54 (2014) 149–153 153

(1) The green and sintered density of Ti–10Mo alloy powderincreased with an increasing impact force and decreasedwith an increase in height/diameter ratio of the compacts.At an identical impact force, the density of sample A ishigher due to higher compaction force applied on per unitarea of powder bed. The relative green density of sample Aand sample B reaches to 90.86% and 79.71% respectively.

(2) The spring back of both type of samples decreased with theincrease in impact force.

(3) With the increment of impact force, the Vickers hardness ofboth type of samples increased. Hardness of sample A ishigher than sample B.

(4) At the same impact force, the bending strength and tensilestrength of sample A is higher than sample B. The higherbending strength and tensile strength for sample A is dueto less per unit area filled in die despite the fact of sameapplied force.

Acknowledgements

This work was financially supported by National Natural Sci-ence Foundation of China (No. 51172018), The National Key Tech-nology R&D Program (2009BAE74B00), National 973 Program(2006CB605207) and Innovative Research Team in University ofChina (I2P407).

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