Workability Studies on Al + 2 . 5 % TiO 2 + Gr Powder Metallurgy Composites During Cold Upsetting

The powder metallurgy process is replacing traditional metal forming operations because of its low relative energy consumption, high material utilization, and low capital cost1. Also the uniformity in reinforcement distribution can be achieved by the powder metallurgy process to produce porous structures and this improves structural properties and also reproducibility2,3. Among all composite materials, metal matrix materials are widely used in various applications in which Aluminum based Metal Matrix Composites (MMCs) are widely used in structural applications in the aerospace and automotive industries due to their high strength-to-weight ratio4. Furthermore Aluminium Metal Matrix Composites are mostly used for producing porous parts like bearings because of its porous structure and it is suitable to serve as self-lubricated bearings5. Nowadays AMCs are synthesized with various reinforcements such as graphite, SiC, Al2O3, TiC, VC, AlN, B4C, Si3N4, TiB2, AlB2 and MgB2 [6-16]. Even though Titanium dioxide (TiO2) possesses high hardness and modulus with superior corrosion resistance and wear resistance, it is not concentrated much by the recent researchers17. Al-TiO2 composite system is one of the interesting material systems, and it has been produced by vortex casting and squeeze casting process and its properties are reported18. Also Somaye Alamolhoda et al. produced TiAl/Al2O3 composite from Al-TiO2 powder mixtures using high-energy planetary ball mill through mechanical activation mechanism19. It is proved that the addition of hard ceramic reinforcement particles increases wear resistance and hardness. However, the composites containing hard ceramic particles lack a solid lubricant, such as graphite20. Aluminium alloy–graphite particulate composites (Al/Gr MMCs) exhibit improvement in mechanical properties, low friction and wear, reduced temperature rise at the wearing contact surface, excellent anti-seizure effects, improved machinability, low thermal expansion and high damping capacity because of graphite reinforcement21. Hybrid aluminium matrix composites are produced with the addition of solid lubricant particles such as graphite along with hard ceramic particles to improve the tribological and self-lubricating properties of the composites22. The main drawback of the powder metallurgy process is porosity in components after sintering. To decrease the porosity, many secondary processes are carried out to powder metallurgy components, among which powder forging is particularly suitable because of its cost and material saving advantages as well as the high production rates and property enhancement23. Narayanasamy et al.24 defined the term workability as the capacity of a material to withstand the induced internal stresses of forming before the splitting of material occurs. They also reported that, the workability of a material purely depends on the amount of ductile fracture presented in the material24. The workability behavior for various ferrous and nonferrous alloys and composites (Al-3.5%Al2O3, Al–5%SiC, Al-Fe, Fe-4%TiC and Fe-0.35C) have been studied by various researchers and reported under plane stress, uni-axial stress and tri-axial Workability Studies on Al+2.5%TiO2+Gr Powder Metallurgy Composites During Cold Upsetting


Introduction
The powder metallurgy process is replacing traditional metal forming operations because of its low relative energy consumption, high material utilization, and low capital cost 1 .Also the uniformity in reinforcement distribution can be achieved by the powder metallurgy process to produce porous structures and this improves structural properties and also reproducibility 2,3 .Among all composite materials, metal matrix materials are widely used in various applications in which Aluminum based Metal Matrix Composites (MMCs) are widely used in structural applications in the aerospace and automotive industries due to their high strength-to-weight ratio 4 .Furthermore Aluminium Metal Matrix Composites are mostly used for producing porous parts like bearings because of its porous structure and it is suitable to serve as self-lubricated bearings 5 .Nowadays AMCs are synthesized with various reinforcements such as graphite, SiC, Al 2 O 3 , TiC, VC, AlN, B 4 C, Si 3 N 4 , TiB 2 , AlB 2 and MgB 2 [6-16]   .Even though Titanium dioxide (TiO 2 ) possesses high hardness and modulus with superior corrosion resistance and wear resistance, it is not concentrated much by the recent researchers 17 .Al-TiO 2 composite system is one of the interesting material systems, and it has been produced by vortex casting and squeeze casting process and its properties are reported 18 .Also Somaye Alamolhoda et al. produced TiAl/Al 2 O 3 composite from Al-TiO 2 powder mixtures using high-energy planetary ball mill through mechanical activation mechanism 19 .It is proved that the addition of hard ceramic reinforcement particles increases wear resistance and hardness.However, the composites containing hard ceramic particles lack a solid lubricant, such as graphite 20 .Aluminium alloy-graphite particulate composites (Al/Gr MMCs) exhibit improvement in mechanical properties, low friction and wear, reduced temperature rise at the wearing contact surface, excellent anti-seizure effects, improved machinability, low thermal expansion and high damping capacity because of graphite reinforcement 21 .Hybrid aluminium matrix composites are produced with the addition of solid lubricant particles such as graphite along with hard ceramic particles to improve the tribological and self-lubricating properties of the composites 22 .The main drawback of the powder metallurgy process is porosity in components after sintering.To decrease the porosity, many secondary processes are carried out to powder metallurgy components, among which powder forging is particularly suitable because of its cost and material saving advantages as well as the high production rates and property enhancement 23 .Narayanasamy et al. 24 defined the term workability as the capacity of a material to withstand the induced internal stresses of forming before the splitting of material occurs.They also reported that, the workability of a material purely depends on the amount of ductile fracture presented in the material 24 .The workability behavior for various ferrous and non-ferrous alloys and composites (Al-3.5%Al 2 O 3 , Al-5%SiC, Al-Fe, Fe-4%TiC and Fe-0.35C) have been studied by various researchers and reported under plane stress, uni-axial stress and tri-axial stress state conditions during cold as well as hot forging process [25][26][27][28][29][30] .
In the present study, Al matrix composites with 2.5 weight percentage of TiO 2 and 2 & 4 weight percentage of graphite was synthesized through powder metallurgy route.The SEM and XRD analyses were carried out for the sintered powder metallurgy preforms.The as-sintered pure Aluminium, Al+2.5%TiO 2 , Al+2.5%TiO 2 +2%Gr, and Al+2.5%TiO 2 +4%Gr compacts were cold upset forged to study the forming behavior.The various stresses like true axial stress (s z ), true hoop stress (s ө ) and true hydrostatic stress (s m ) induced during cold upsetting have been correlated with true axial strain (e z ).The formability stress index (β) and stress ratio parameters (s z /s eff ), (s m /s eff ) and (s ө /s eff ) were also correlated with the true axial strain (e z ).

Materials
Atomized aluminium (Al) powder of particle size less than 325 μm and the purity 99.7% supplied by Kemphasol, Mumbai, India was used as matrix material.The rutile grade of titanium dioxide (TiO 2 ) powder with an average particle size of 0.334 mm and purity 99.0% and graphite powders of size less than 10 mm supplied by the Acechemie (India) was used as reinforcements for the present work.The morphology of the as received Al powder particles is presented in Figure 1.

Blending, compaction and sintering
Al and TiO 2 powders are accurately weighed and blended in ball mill to yield the different composites namely, Al+2.5%TiO 2 , Al+2.5%TiO 2 +2%Gr, and Al+2.5%TiO 2 +4%Gr.The blending was carried out with balls and powders weight ratio of 5:1 and the speed of the drum was 100 rpm 31 for a period of 10 hours 32 .XRD analysis for Al+2.5%TiO 2 +2%Gr composite powders were carried out using PANalytical X'Pert X-ray diffractometer C u Kα target, (λ=1.5418Å).The powder mixtures were compacted into cylindrical billets of diameter 24 mm and the height 12 mm using hydraulic press of 400 kN capacity.Graphite mixed with lubricant oil was used as lubricant during compaction process.An indigenously developed ceramic coating was applied on the free surfaces of the compacts to avoid oxidation of the Aluminium matrix composites and dried under room temperature for a period of 12 hours 24 .The ceramic coated compacts were sintered in an electric muffle furnace in the temperature range of (590±10) °C for a period of 3 hours and cooled to room temperature in the furnace.The SEM analysis was carried out for the sintered preforms using Quanta 250 FEG and XRD analysis was conducted by XPERT-PRO diiffractometer.

Cold upsetting
The sintered preforms were cleaned and measurements such as initial height (H o ), diameter (D o ) and mass (m) were taken.The cold upsetting was carried out between two flat high speed steel dies with graphite lubricant at both of die contact surfaces, to ensure minimum friction.The incremental compressive load was applied on the cold upset specimen in steps of 10 kN, until fine cracks appeared on its free surface.Figure (2-4) shows the preforms before and after deformation.After each interval of loading dimensional changes in the specimen such as height after deformation (H D ), top contact diameter (D TC ), bottom contact diameter (D BC ), bulged diameter (D B ) and density    of the preform (ρ f ) were measured.Archimedes principle was used to find the actual densities of the preforms after each step of deformation.The measured values were used to calculate the true axial stresses (s z ), true hoop stress (s ө ), true hydrostatic stress (s m ), true effective stress (s eff ), formability stress index (β) and true axial strain (e z ).These workability parameters under tri-axial stress state conditions are calculated by using following expressions 24,25 .
The true hoop strain (e θ ) can be determined by the following expression, D o , initial dia ; D B , bulge dia ; Dc, average surface contact dia The true hydrostatic stress (s m ) is given by, The true hoop stress (s ө ) can be expressed by, ( (2 ) Where α is Poisson's ratio The true effective stress (s eff ) is calculated by, ) Where R is Relative Density of the preforms The formability index (β) is given by, Hardness tests have been conducted for sintered and cold forged pure Al, Al+2.5%TiO 2 , Al+2.5%TiO 2 +2%Gr and Al+2.5%TiO 2 +4%Gr powder metallurgy preforms in Rockwell hardness testing machine, with a load of 100 kg using 1/16 inch ball indenter.The mean values of at least five measurements conducted on different areas of each preforms was considered.As can be seen, fine grain boundaries are formed in all composite preforms during sintering and pores also observed.From Figure 5(b-d), it can be observed that, the TiO 2 particles are embedded within the Al matrix.The graphite particles also distributed evenly in the Al matrix as well as around the TiO 2 particles as shown in Figure 5(c  & d).Both the TiO 2 and Graphite reinforcements are acted as a pore closing agent and contributed to the decrease in porosity.It is very important that all particles should be homogeneously distributed in the mixture in order to obtain a good microstructure 2 .Here the selection of ball milling parameters such as milling speed, milling time and ball to powder ratio are contributed to produce the compacts with uniform distribution of reinforcements.

XRD analysis
The XRD pattern of Al+2.5%TiO 2 +2%Gr milled composite powders is presented in Figure 6.The XRD results confirmed the presence of TiO 2 and graphite particles in Aluminium matrix (JCPDS No. 80-1501).Figure 7 shows the XRD patterns of Pure Al, Al+2.5%TiO 2 , Al+2.5%TiO 2 +2%Gr and Al+2.5%TiO 2 +4%Gr powder metallurgy preforms sintered at 590 °C.The peaks of Al, TiO 2 (87-0920) and Gr (75-2078) were observed for the pure Al, Al+2.5%TiO 2 composite and hybrid composite preforms.The Al peaks in (111), ( 200), ( 220    addition of TiO 2 and Gr reinforcements.Also the increased peak broadening was observed for sintered preforms.The reason could be, during sintering the Al matrix becomes solid solution structure.Also it has been observed from the XRD patterns, no intermetallic compounds were formed in as-milled and as-sintered conditions.

Stress analysis on powder metallurgy preforms
The true axial stress (s z ) versus true axial strain (e z ) curve of pure Al, Al+2.5%TiO 2 , Al+2.5%TiO 2 +2%Gr and Al+2.5%TiO 2 +4%Gr powder metallurgy compacts are presented in Figure 8.The true axial stress (s z ) increases for any given true axial strain (e z ) while increasing the reinforcement contents (TiO 2 & Gr) in the compacts.The addition of 2.5 weight percentage of TiO 2 in the pure Al matrix shows the increase in true axial stress (s z ), also the addition of 2% and 4% of Graphite to the Al+2.5%TiO 2 composite further increases the axial applied load for the same level of true axial strain (ε z ).The increase in true axial stress (s z ) is because of addition of both TiO 2 and graphite reinforcements in Al matrix and it also contributes to the decrease in porosity.Narayanasamy et al. studied the workability behavior of Al-SiC composites and reported that, the SiC particulates obstruct the motion of dislocations and hence the stress required for further plastic deformation increases and the same author reported that fine graphite particles are contributed for the pore closing in powder metallurgy steel preforms 33,34 .In the present study both TiO 2 and graphite reinforcement contributed for the pore closing and it increases the true axial stress (s z ).The same behavior also observed for the graph drawn between the other stresses like true hoop stress (s ө ) and true hydrostatic stress (s m ) with true axial strain (e z ) as shown in Figure 9 & 10.Addition of 2.5 weight percentage of TiO 2 to the Al matrix and addition of 2 & 4 weight percentage of graphite to the Al+2.5%TiO 2 composite increases true hoop stress (s ө ) and true hydrostatic stress (s m ) under tri-axial stress state conditions.Also higher true axial strain (better deformation) was observed for the pure Aluminium preforms with the lower applied stress, however for Al+2.5%TiO 2 composite and Al+2.5%TiO 2 +2%Gr & Al+2.5%TiO 2 +4%Gr hybrid composite preforms shows lower true axial strain (e z ) with the higher applied stress because of obstacles created by the reinforcements for dislocation during plastic deformation.

Workability behavior of P/M Al-TiO2 composite preform
Figure 11 illustrates the graph between the formability stress index (β) and true axial strain (e z ) for the pure Al, Al+2.5%TiO 2 , Al+2.5%TiO 2 +2%Gr and Al+2.5%TiO 2 +4%Gr powder metallurgy preforms.It has been observed from the graph that, the formability stress index (β) increases with increase in true axial strain (e z ) for all the preforms tested.Further the addition of 2.5 weight percentage of TiO 2 to the Al matrix and 2 & 4 weight percentage of graphite to the Al+2.5%TiO 2 composite increases the formability stress index (β).The reason for the increase in formability stress index (β) is due to the increase in relative density and true hydrostatic stress (s m ) during cold upsetting of sintered composite preforms.These results are well agreed with findings of 4%TiC-Steel composites tested under tri-axial stress state condition 29 .
Figure 12-14 shows the graph between the stress ratio parameters [(s z /s eff ), (s m /s eff ) & (s ө /s eff )] and the true axial    strain (e z ) for all the preforms and it has been observed that the stress ratio parameter also increases with increase in true axial strain (e z ).The increase in weight percentage of reinforcements also increases the stress ratio parameters.The reason for the increase in all the stress ratio parameters could be the amount of porosity.The presence of pores in pure Aluminium preform was bigger and more than the composite preforms.The decrease in porosity of composite preforms increases the stress ratio parameters [(s z /s eff ), (s m /s eff ) & (s ө /s eff )].The similar kind of trend was observed for all the three stress ratio parameters drawn against true axial strain (e z ).

Hardness of the powder metallurgy preforms
Figure 15 shows the hardness of sintered and cold forged preforms.The hardness increases with the addition of 2.5 weight percentage of TiO 2 to the soft aluminium matrix.The brittle phase of TiO 2 contributed to the increase in hardness value.Zhangwei Wang et al. reported for Al-SiC composites that, the addition of SiC particles also increases the hardness of the composite 34 .The addition of 2 weight percentage of graphite to the Al+2.5%TiO 2 composite increases the hardness.However, addition of 4 weight percentage of graphite decreases the hardness value of the composite.The hardness decreases with increasing graphite content, because of soft nature of graphite 35 .Also the increased amount of graphite particles together with the increased tendency of crack initiation an propagation at the graphite-metal interface are the responsible for the decrease in hardness 36 .The lower hardness values were obtained for sintered specimens and higher values are observed for the cold forged specimens, since the pores are closed and occupied by the reinforcements during cold upset forging.The maximum hardness values were obtained for the cold forged preforms containing 2.5 weight percentages of TiO 2 particles.The pore closing mechanism and presence of hard phase in the composite contributed to the increase in hardness after cold upsetting.

Figure 1 .
Figure 1.SEM images of as received Aluminium powders.

Figure 5 (
Figure 5(a-d) illustrates the SEM image of pure A l , A l + 2 .5 % Ti O 2 , A l + 2 .5 % Ti O 2 + 2 % G r a n d Al+2.5%TiO 2 +4%Gr powder metallurgy sintered preforms.As can be seen, fine grain boundaries are formed in all composite preforms during sintering and pores also observed.From Figure5(b-d), it can be observed that, the TiO 2 particles are embedded within the Al matrix.The graphite particles also distributed evenly in the Al matrix as well as around the TiO 2 particles as shown in Figure5(c & d).Both the TiO 2 and Graphite reinforcements are acted
) & (311) plane, TiO 2 peaks in (101) & (200) plane and graphite peaks in (111) & (222) plane were observed.From the XRD pattern it is also observed that the peak broadening of Al increases with the

•
Al+2.5%TiO 2 +Gr hybrid composite was synthesized through powder metallurgy technique and its workability behavior was studied during cold upsetting test under tri-axial stress condition.• The sintered preforms were characterized by SEM and XRD analyses and it reveals that the uniform distribution of reinforcements (TiO 2 & Gr).The true axial stress (s z ), true hoop stress (s ө ) and true hydrostatic stress (s m ) increases with the addition of TiO 2 and Gr particles in pure Al matrix during cold upsetting.• The formability stress index (β) increases with the addition of 2.5 weight percentage of TiO 2 and 2 & 4 weight percentage of Gr reinforcements in pure Al matrix during cold upsetting.• The stress ratio parameters [(s z /s eff ), (s θ /s eff ) & (s m /s eff )] also increases with the addition of TiO 2 and increase in weight percentage of graphite.It is also found to be higher for the preform containing 2.5 weight percentage of TiO 2 and 4 weight percentages of graphite and lower for the pure Al preform.• The addition of 2.5 weight percentage of TiO 2 increases the hardness of the preforms and addition of 4 weight percentage of graphite to the Al+2.5%TiO 2 composite decreases the hardness.

Figure 15 .
Figure 15.Hardness of as-sintered and as-cold forged Aluminium P/M preforms.