Isc Dinhrn, Shui Zhng, Goqing Chen, Qingyu Shi,*

aIDM-Joint Lab, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China

bThe State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China

Abstract Poor ductility is the primary concern of magnesium matrix composites (MMCs) inflicte by non-deformable ceramic particle reinforcements.Metal particles which melt at elevated temperature can be used as reinforcement to improve the deformation characteristics.Ti-6Al-4V particles reinforced AZ31 MMCs were produced through friction stir processing (FSP) which was carried out in a traditional vertical milling machine.The microstructural features as well as the response to external tensile load were explored.A homogenous distribution of Ti-6Al-4V was achieved at every part of the stir zone.There was no chemical decomposition of Ti-6Al-4V.Further, Ti-6Al-4V did not react with Al and Zn present in AZ31 alloy to form new compounds.A continuous strong interface was obtained around Ti-6Al-4V particle with the matrix.Ti-6Al-4V particles underwent breakage during processing due to severe plastic strain.There was a remarkable refinemen of grains in the composite caused by dynamic recrystallization in addition to the pinning of smaller size broken particles.Dense dislocations were observed in the matrix because of plastic deformation and the associated strain misfit Ti-6Al-4V particles improved the tensile behavior and assisted to obtain appreciable deformation before fracture.Brittle mode of failure was avoided.

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Keywords: Magnesium matrix composite; Friction stir processing; Ti-6Al-4V; Microstructure; Tensile strength.

1.Introduction

Particulate reinforced magnesium matrix composites(MMCs) have become an interesting metal matrix composite material because it fulfill the requirement for light weight material in several industries producing transport vehicles[1-3].Ceramic particles(SiC,Al2O3,B4C,TiC etc.)are commonly utilized for reinforcing MMCs.Ceramic particles were helpful to improve the tensile strength significantl.Nevertheless, there was a remarkable loss of ductility in the composites because of poor deformation characteristics of ceramic particles [4-7].Ductility can be improved or retained to desirable level by selecting metallic particles as reinforcement for MMCs.Titanium, nickel, tungsten and molybdenum particles offer a choice of reinforcement due to their high hardness and stability at higher temperature [8,9].Among a spectrum of metallic particles, Ti-6Al-4V particle is a good choice of reinforcement because of the following attributes.Ti-6Al-4V particle can undergo plastic deformation during tensile loading.The crystal structure of Ti-6Al-4V is hexagonal close packed (HCP) which is compatible with the magnesium matrix.Ti-6Al-4V has a low density of 4420kg/m3and a higher tensile strength in the range of 950-1050MPa[10,11].

Preparing magnesium alloys-based composites presents lot of challenges because of inflammabilit and heavy shrinkage.In spite of practical issues, some studies were reported on the production of Ti-6Al-4V particle reinforced MMCs via pow-der metallurgy [12,13], vacuum casting [14], gravity casting[15], stir casting [16,17]and pressure infiltratio [18].Various magnesium alloys such as AZ30, AZ91, AM60, ZK51 and MB15 were used as matrix material.Ti-6Al-4V particles were reinforced to make MMCs successfully but microstructural characterization exposed objectionable features including nonuniform distribution[12],agglomeration[16],micro pores[17], poor interfacial bonding [14,16], coarsened grains [15-18]and intermetallic compounds [16,17].The strengthening of the composites was affected by those features.

Friction stir processing (FSP) has been matured into a viable technology for manufacturing MMCs and receiving constant research focus in the last several years [19].The substrate material is plasticized by the generation of frictional heat and dynamic tool movement and made to fl w across the stirred region.Various strategies are used to preplace the reinforcement particle before processing begins.The material fl w and the tool rotation mix the reinforcement particles to form the composite which is subsequently forged as the tool advances.Since the processing temperature is below the melting point of the matrix materials, all kind of undesirable reactions are thwarted.The difference in physical properties of the matrix and the reinforcement do not dictate the FSP process significantl which is advantageous to obtain proper distribution [20-22].

Chang et al.[23]prepared nano ZrO2reinforced AZ31 MMCs by FSP.They explored various strengthening mechanisms which caused property improvement.Asadi et al.[24]fabricated micro as well as nano SiC particle reinforced AZ91 MMCs and applied multiple passes to improve distribution.The increase in main processing parameters namely rotating and traversing rate had opposite effect in deciding the size of grains.Azizieh et al.[25]prepared AZ31/Al2O3nano MMCs and analyzed the role of process parameters.Higher rotational speed, threaded pin profil and multiple passes provided better dispersion.Jiang et al.[26]prepared AZ31/SiO2nano MMCs and showed remarkable enhancement in hardness by grain refinemen and fin dispersion.Ratna Sunil et al.[27]fabricated AZ31/hydroxyapatite nano MMCs and observed an improvement in corrosion resistance under asimulated body fluid Navazani and Dehghani [28]developed AZ31/TiC MMCs and observed an enhancement in hardness.Mertens et al.[29]reinforced short carbon fibre into AZ31B and AZ91D and showed grain refinemen and improvement in yield strength.Alavi Nia and Nourbakhsh [30]fabricated AZ21/CNTs MMCs and explained the difficult in dispersing CNTs in the composite during processing.Dinaharan and Akinlabi [31]produced AZ31/fl ash MMCs and realized an enhancement in hardness by proper dispersion and grain refinement

Fig.1.(a) SEM micrograph, (b) particle size distribution and (c) XRD pattern of as received Ti-6Al-4V powder.

Fig.2.XRD patterns of AZ31/Ti-6Al-4V MMCs containing Ti-6Al-4V particles; (a) 0vol.% (b) 7vol.%, (c) 14vol.%, and (d) 21vol.%.

It is inferred from the literature that many ceramic particles and other kind of reinforcements were successfully utilized to synthesize MMCs.Nevertheless, there appears a lack of literature on metallic particulate reinforced MMCs.Therefore, the reported work is oriented to apply FSP route to develop AZ31/Ti-6Al-4V MMCs and analyze the evolved microstructure using various kinds of microscopes.The factors which contributed to the improvement in tensile strength are explained and correlated to the observed microstructural features.

2.Materials and methods

In this study, AZ31 magnesium plates of dimensions 150mm × 100mm × 8mm were used as base material to synthesize the composite material.The composition of the purchased plates was tested by XRF and the results are documented in Table 1.The reinforcement particles were titanium alloy Ti-6Al-4V having a mean size of ～29μm (standard deviation of mean size and percentage of particles was 12.3 and 4.05 respectively).A SEM micrograph revealing the particle morphology and the size distribution map are plotted inFig.1.A rectangular recess was created on the surface of the plates by machining for the purpose of packing the particles.Commonly used two stage procedure (processing initially by a tool without pin and later by a tool with a rigid pin) was adopted to make the composites as presented in an earlier work [32].FSP was accomplished (M/s BYJC X5032) using a traditional milling machine.Table 2 lists out the variables and the actual values employed.The effect of process parameters was investigated in a previous work and an optimized condition was chosen for processing.The processed plates were machined in a direction perpendicular to tool traverse to extract specimens for various microscopic observation.Routine metallographic procedure was adopted to grind and polish the specimens.A chemical etchant comprising equal parts of oxalic acid, nitric acid and acetic acid in predetermined quantity of water was applied to expose the microstructural features.Micrographs were captured using an optical microscope (OLYMPUS BX51M), a scanning electron microscope (FEI Quanta 200) and a transmission electron microscope.EBSD was carried out in a FEI Quanta FEG SEM equipped with TSL-OIM software.TEM samples were firs mechanically polished to a thickness of about 50μm.They were sliced by focused ion-beam milling in a Gatan 691 precision ion polishing system.The thin foils were observed usingFE transmission electron microscopy(TEM,Tecnai F20,FEI).Sub sized tensile specimens having 25mm gage length; 4mm width and 4mm thickness were machined from the processed zone.Tensile specimens were pulled using a computerized tensile tester (MTS Exceed E45) until fracture at a strain rate of 0.5mm/min.Fractured tensile specimens were observed using SEM to capture the morphology of the fractured surface to identify the mode of failure.

Table 1The chemical composition of magnesium alloy AZ31B.

Table 2FSP conditions.

Fig.3.SEM micrographs of AZ31/Ti-6Al-4V MMCs containing Ti-6Al-4V particles; (a) 0vol.% (b) 7vol.%, (c) 14vol.%, and (d) 21vol.%.

Fig.4.Optical photomicrograph of AZ31/Ti-6Al-4V MMCs at various locations within the stir zone: (a) and (b) top portion, (c) middle portion, (d) interface at the retreading side, (e) and (f) bottom portion.

3.Results and discussions

3.1.X-ray diffraction analysis of AZ31/Ti-6Al-4V MMCs

The XRD patterns of the synthesized composite material having varying content of Ti-6Al-4V particles are plotted in Fig.2.The XRD patterns clearly record the elemental peaks of the composite ingredients i.e.matrix and reinforcement(αandβ).The altitude counts ofαandβpeaks of Ti increases with respect to the increase in Ti-6Al-4V content which agrees to the laws of physical metallurgy.Peaks of any other elements or compounds are not found in Fig.2.This suggests that Ti-6Al-4V were effectively reinforced in the AZ31 matrix without considerable reaction or diffusion.The solubility of Ti in magnesium is poor even at molten state[33].Therefore, they do not form any other solid solution or inferior compounds.However, the primary alloying element in AZ31 alloy is Al which is inclined to chemically react with Ti to form brittle intermetallic phases such as Al3Ti [16,17].Such reactions occur at very high temperature and longer exposure duration.There are no peaks of Al3Ti in detectable quantity in Fig.2.This can be ascribed to the lower processing temperature of FSP and shorter exposure duration.Unlike other processes, the bulk composite material is not subjected to continuous thermal exposure.This helps to prevent the formation of undesirable phases.However, there could be traces of Al3Ti at the interface which can only be confirme by TEM observation.

Fig.5.(a) SEM micrograph of AZ31/21vol.% Ti-6Al-4V MMC and distribution of elements; (b) Ti, (c) Al, and (d) Mg.

3.2.Microstructure of AZ31/Ti-6Al-4V MMCs

Typical stir zone SEM micrographs at different content of Ti-6Al-4V particle are presented in Fig.3.Fig.3a belongs to unreinforced but friction stir processed stir zone of magnesium alloy AZ31.Small amount of secondary eutectic particle (Mg17Al12) distribution is observed.They manifest in white color and nodular in shape.This phase is unstable at elevated temperature and dissolves into the magnesium matrix.Those particles which did not dissolve are seen in the micrograph.The successful incorporation and the kind of Ti-6Al-4V particle distribution are observed (Fig.3b-d).Ti-6Al-4V particles which were primarily constrained within the machined recess were forced to occupy in all the regions of the stir zone.They increase in quantity and concentration as the volume content is increased.The particles are distributed at nearly constant interparticle distance which indicates a homogenous distribution.The micrographs do not contain any empty spaces or aggression of particles.FSP has provided a homogenous distribution which is beneficia to attain higher mechanical properties.FSP has many processing parameters.A proper selection of process parameters yields a desirable distribution.Improper distribution of particles was reported in some literature [24,26,28].Applying multiple passes is a crucial factor to avoid aggression and particle free regions.FSP produces composites by plasticizing the substrate ma-terial subjected to intense frictional heating and mechanical action.Transportation of plastic state material takes place from advancing side to retreading side.The rotary motion of the non-consumable tool pushes the compacted particles inside the plasticized material and extrudes in the annular gap between the pin surface and the unaffected base metal.The traverse motion causes consolidation of the composite.Casting processes always cause the particles to be segregated along the grain boundaries.No such evidence of particle segregation is found in the micrographs.This observation suggests that many smaller particles are positioned within the grain boundary area i.e.intragranular distribution.The change in volume content did not change the kind of distribution under the applied experimental conditions.

Fig.6.(a) and (b) SEM micrographs of AZ31/21vol.% Ti-6Al-4V MMC and (c) line EDAX across a particle.

Fig.4 visualizes optical micrographs of AZ31/21vol.% Ti-6Al-4V composites snapped at various regions inside the stir zone.A variation in particle distribution at different spots is negligible.The distribution is nearly identical across thestir zone.Ti-6Al-4V particles are driven to every corner of the stir zone.No region lacks particle reinforcement.It is preferable to have the same kind of distribution throughout the bulk composite.Obtaining a constant distribution through conventionally used casting methods is difficult Particles do not stay at one location after mixing within the melt due to density gradient and movement of the solidificatio front.The composite is manufactured in solid state using FSP sans whole melting of the substrate.This arrests any free movement of particles due to the variation in physical properties during consolidation and cooling.The movement of particles is only governed and influence by the mechanical action of the whirling tool.The kind of distribution obtained during consolidation is retained until the temperature drops down to atmospheric temperature.But, FSP process is not exempted of changes in the distribution all over the stir zone.Some investigators observed wide variation in the distribution under their experimental conditions [23,24,28].Selection of appropriate combination of process parameters is required to promote optimum material fl w for obtaining a constant distribution.Optical micrographs recorded at the bottom of the stir zone (Fig.4e and f) do not showcase the pattern of onion rings.The lameller arrangement of particles in reducing and increasing concentration in circular pattern is known as onion ring structure in composites produced by FSP [25].Absence of onion ring pattern could be due to wide variation in temperature gradient along the thickness direction which did not generate the material fl w to favor the formation of onion rings.FSP was done up to 6mm in the substrate plate.There is a remaining plate of about 2mm thickness before the actual backup plate.This portion expedites heat transfer and causes large variation in the vertical heat fl w.

Fig.7.EBSD (IPF+grain boundary) images of AZ31/Ti-6Al-4V MMCs containing Ti-6Al-4V particles; (a) 0vol.% (b) 7vol.%, (c) 14vol.%, and (d) 21vol.%.

Fig.5 shows the distribution of elements in AZ31/21vol.%Ti-6Al-4V MMC.The distribution of Ti and Al elements all over the micrograph ensures the uniform distribution of Ti-6Al-4V particles within the composite.Magnifie view of AZ31/21vol.% Ti-6Al-4V composite is presented in Fig.6a and b.These micrographs help to understand the nature of interface established between Ti-6Al-4V particle and the AZ31 magnesium alloy.There is no break up or interruption in the interface around each particle.No other compounds or particles are found in the interface.The matrix alloy appears to be adhering perfectly with Ti-6Al-4V particles on every side.Pores or voids are not present at the interface.This can be attributed to sufficien amount of plasticization of AZ31 alloy and unobstructed fl w over the particles subsequently.The morphology of the Ti-6Al-4V particles did not divert the plasticized fl w.Pores or voids occur at the interface if the particle shape is irregular and have many sharp edges[21].Moreover, the intense stirring action of the tool is ca-pable of removing any oxide layers existing on the particle surface and makes atomic level contact with the matrix for proper bonding.Fig.6c represents a line EDAX which was observed across a Ti-6Al-4V particle covering the AZ31 matrix on both the sides.The elemental spectrums of Mg and Ti are intersecting sharply at the interface.Neither a reduction in Ti spectrum nor an increase in Al spectrum is observed at the interface.This observation rules out any sort of chemical reaction among the primary alloying elements i.e.Al and Ti resulting in the formation of undesirable compounds which might have been accumulated near the interface area.An able interface is a key feature to attain improved mechanical properties by transferring the applied loads efficientl.Presence of pores and voids make the interface vulnerable to premature fracture.Such an interface is preferred apart from homogenous distribution for improved performance from the composites.

Fig.8.Misorientation distribution of AZ31/Ti-6Al-4V MMCs containing Ti-6Al-4V particles; (a) 0vol.% (b) 7vol.%, (c) 14vol.%, and (d) 21vol.%.

Variation in the size and the shape of reinforcement particles was reported by several researchers [20,21].FSP generates large amount of strain to that of other severe plastic deformation techniques.The reinforcement particles are broken by the excessive strain in many cases.In the present study,a change in size and shape of Ti-6Al-4V particles can be visualized comparing Figs.3 and 6 with Fig.1.Large size particles are broken into small sizes.Numerous submicron level debris are seen in Fig.6.Ceramic particle reinforcements usually break due to their brittle nature.But,Ti-6Al-4V particles possess sufficien ductility.It can absorb more strain to that of a ceramic particle.The breakage of Ti-6Al-4V particle can be related to their larger size.A large size particle does not fl w freely along the plasticized material and act as an obstruction.The material fl w simply collides with the particle multiple times which causes breakage.The fractured particles and the debris are carried away by the material fl w and the stirring action of the tool does not allow them to cluster in any part of the stir zone.The initial morphology shows some rough corners.Many round corners are visible in Fig.4 and 6 which can be ascribed to the abrasive action of the tool.

Fig.7 displays the EBSD images of the base metal along with the fabricated composites which assist to visualize the grain structure within the material.The base metal showcases a coarse grain structure registering a mean grain size of 66.7μm.The rolled direction of the base metal was per-pendicular to the processing direction exhibiting larger grains.On the other hand, AZ31/Ti-6Al-4V composites comprise of a fin grain structure.There is a remarkable refinemen in the size of grains from 66.7μm to 9.1μm (the vales of standard deviation of grain size are 2.34 (0vol.%), 0.29 (7vol.%),0.23 (14vol.%) and 0.27 (21vol.%)) in AZ31/7vol.% Ti-6Al-4V MMC.The excessive grain refinemen is achieved because of the phenomenon acknowledged as dynamic recrystallization which is well agreed in literature [19].The severe plastic deformation of the material at raised temperature leads to dynamic recrystallization.The increase in the volume content of Ti-6Al-4V particles further reduced the grain size to 4.5μm in AZ31/21vol.%Ti-6Al-4V MMC.This reduction can be ascribed to the pinning effect of broken particles and increased grain nucleation sites.The corresponding misorientation maps in Fig.8 show that more percentage (40-50%) of high angle boundaries is generated compared to base metal.Huang and Shen [34]found a similar behavior and ascribed to adequate dynamic recrystallization.

Fig.9.TEM micrograph of AZ31/21vol.% Ti-6Al-4V MMC showing; (a) fin grains, (b) interface of a single particle, (c) and (d) dislocations.

Fig.10.(a) Stress strain graphs of AZ31/Ti-6Al-4V MMCs, (b) extracted values and (c) comparison of elongation with AZ31/SiC MMCs [39].

Fig.9 shows a sequence of TEM micrographs of AZ31/21vol.% Ti-6Al-4V composite detailing different aspects of the microstructure.A fin grain structure is observed in Fig.9a.No distribution of particles is visible.The larger size particles might have detached during sample preparation for TEM observation.A smaller particle which may be a debris of a broken large particle is found in Fig.9b.Interface appears to be clear and no other compounds or gap is seen.This observation confirm a strong interfacial bond existing between the particle and the matrix.The matrix alloy AZ31 is not plain but fille with dislocations.The generation of dislocation can be related to the following two causes;(a) the matrix experienced a severe plastic deformation and(b) adjusting the strain misfi arising due to different thermal properties of the matrix and the Ti-6Al-4V particles.

3.3.Tensile behavior of AZ31/Ti-6Al-4V MMCs

Fig.10a and b respectively display the stress strain graphs and the values of the tensile testing parameters of AZ31/Ti-6Al-4V MMCs.The contribution of Ti-6Al-4V to the tensile behavior is well pronounced in these plots.UTS of the composite increased from 226MPa (0vol.%) to 322MPa(21vol.%) due to the reinforcement of Ti-6Al-4V particles.There is also an improvement in the yield strength of the composite.Yield strength was estimated to be 98MPa (0vol.%)to 205MPa (21vol.%).Yielding point moved to a higher level after the addition of Ti-6Al-4V.It can be concluded from these graphs that Ti-6Al-4V is a potential reinforcement to positively improve the tensile behavior of MMCs.The strengthening factors which caused an improvement in tensile behavior are discussed below one by one.The reinforcement chosen for the composite should possess higher properties to that of the metallic matrix in order to improve the strength as per the known rule of mixtures [35].The tensile strength of Ti-6Al-4V particles is several times higher to that of the matrix AZ31 which is the firs cause of strengthening [36].A homogenous distribution of Ti-6Al-4V particles and the dispersion of fractured particles bring Orowan strengthening into operation [37].Dislocations fin it difficul to progress through the dispersed particles due to the resistance which causes bowing around particles.The trajectory of dislocation motion is reversed many times to further movement.The applied tensile load is distributed to individual particles due to homogenous distribution and absence of clustering.The micrographs revealed the existence of good interfacial bonding of Ti-6Al-4V particles in the composites.A good interface allows smooth transfer of the tensile load to the particle.Abundant strain field fille with numerous dislocations are found in the composite.These strain field provide opposition to the motion of the dislocation during tensile loading.Finally, the composite achieved tremendous grain refinemen during processing which contribute to strengthening according to Hall-Petch relationship[38].Assessing the individual effect of each strengthening factor is a difficul task to carry out.Because all factors collectively interact with the motion of dislocations to provide strengthening.The increase in the quantity of Ti-6Al-4V particles amplifie the strengthening effect and enhances the tensile behavior.It is interesting to notice the percentage elongation of the composites which was recorded to be 14.5%at 0vol.% and 9.3% at 21vol.%.The increase in the volume content of Ti-6Al-4V reduces the ductility due to strengthening effect which decreases the deformation of the matrix alloy.However, the ductility of the prepared AZ31/Ti-6Al-4V MMCs is considerably superior to similar composites reinforced with conventionally used ceramic particles.Fig.10c shows a comparison between AZ31/Ti-6Al-4V MMCs and AZ31/SiC MMCs which was prepared by Subramani et al.[39].SiC particles caused significan loss in ductility.Ti-6Al-4V particles did not cause a huge loss of ductility but enabled to maintain the ductility to higher levels.The enhanced ductility can be ascribed to the following causes.Ti-6Al-4V particles are deformable during tensile load compared to nondeformable ceramic particles.Ti-6Al-4V particle allow conduction of heat through them which is not easily done by a ceramic particle.These two factors help to lower the work hardening of the matrix adjacent to the particle and enable to increase the plastic fl w during tensile loading.

Fig.11 reveals the SEM micrographs of fractured tensile specimen surfaces of AZ31/Ti-6Al-4V MMCs.The unreinforced matrix material AZ31 shows a network of elongated dimples (Fig.11a).Two kinds of dimple distribution are observed on the fracture surfaces of the composite.Smaller dimples originate from the deformed matrix while slightly larger dimples represent pulled out Ti-6Al-4V particles.Further, the distribution of Ti-6Al-4V particles is clearly visible.Fig.11e and f show the fracture surface at a higher magnification It is observed that Ti-6Al-4V particles are bonded well with the matrix material.A strong interfacial bonding is visualized which helps to improve the tensile strength.The increase in the content of Ti-6Al-4V particles leads to flatte the fracture surface.However, the overall fracture surface features show that the failure of the composites occurred by ductile fracture.

Fig.11.SEM micrographs of fracture surfaces of AZ31/Ti-6Al-4V MMCs containing Ti-6Al-4V particles; (a) 0vol.% (b) 7vol.%, (c) 14vol.%, (d), (e) and(f) 21vol.%.

4.Conclusions

·Friction stir processing (FSP) was utilized to produce Ti-6Al-4V particles reinforced AZ31 magnesium composite with varying reinforcement content (0 to 21 in steps of 7vol.%) efficientl.

·The integrity of Ti-6Al-4V particles was conserved without any sort of chemical decomposition or undesirable reaction with magnesium and its alloying elements.

·A homogenous distribution of Ti-6Al-4V particles in the composite was achieved at every part of the composite.Ti-6Al-4V particles bonded strongly with the matrix alloy and the interface did not show any presence of a diffusion or reaction layer and pores.However, larger Ti-6Al-4V particles fractured during processing due to the severe plastic strain.

·The grains in the composites showed extreme refinemen because of dynamic recrystallization and pining effect of smaller size and fractured Ti-6Al-4V particles.The severe deformation and the misfi in strain introduced large amount of dislocations in the matrix.

·Ti-6Al-4V particles contributed to an enhancement in UTS from 226MPa (0vol.%) to 322MPa (21vol.%) in this research work.The percentage elongation of the composites which was noted to be 14.5%at 0vol.%and 9.3%at 21.%.Ti-6Al-4V particles not only useful to obtain strengthening of the composite but also to cause adequate plastic fl w before fracture which is limited by conventional ceramic reinforcements.The fracture surfaces demonstrated ductile mode of fracture.

Acknowledgments

We thank ZKKF (Beijing) Science and Technology Co.,Ltd for TEM observation and Sprint Testing Solutions, Mumbai for EBSD observation.