TC4电子束熔炼的XRD分析

Evaluating the Effect of Processing Parameters on Porosity in Electron Beam Melted Ti-6Al-4V via Synchrotron X-ray MicrotomographyROSS CUNNINGHAM,1,3SNEHA P.NARRA,2TUGCE OZTURK,1JACK BEUTH,2and A.D.ROLLETT11.—Department of Materials Science and Engineering,Carnegie Mellon University,Pittsburgh,PA,USA.2.—Department of Mechanical Engineering,Carnegie Mellon University,Pittsburgh,PA,USA.3.—e-mail:rwcunnin@Electron beam melting(EBM)is one of the subsets of direct metal additivemanufacturing(AM),an emerging manufacturing method that fabricatesmetallic parts directly from a three-dimensional(3D)computer model by thesuccessive melting of powder layers.This family of technologies has seensignificant growth in recent years due to its potential to manufacture complexcomponents with shorter lead times,reduced material waste and minimalpost-processing as a‘‘near-net-shape’’process,making it of particular interestto the biomedical and aerospace industries.The popular titanium alloy Ti-6Al-4V has been the focus of multiple studies due to its importance to these twoindustries,which can be attributed to its high strength to weight ratio andcorrosion resistance.While previous research has found that most tensileproperties of EBM Ti-6Al-4V meet or exceed conventional manufacturingstandards,fatigue properties have been consistently inferior due to a signifi-cant presence of porosity.Studies have shown that adjusting processingparameters can reduce overall porosity;however,they frequently utilizemethods that give insufficient information to properly characterize theporosity(e.g.,Archimedes’method).A more detailed examination of the resultof process parameter adjustments on the size and spatial distribution of gasporosity was performed utilizing synchrotron-based x-ray microtomographywith a minimum feature resolution of1.5l m.Cross-sectional melt pool areawas varied systematically via process mapping.Increasing melt pool areathrough the speed function variable was observed to significantly reduceporosity in the part.INTRODUCTIONElectron Beam Melting(EBM)Additive Manufacturing(AM)Additive manufacturing(AM)encompasses a wide range of technologies that utilize a layer-wise approach to manufacture near-net-shape parts from a computer generated three-dimensional(3D)model.1One subset of these,the powder bed-based systems,have gener-ated significant interest due to the recent availability of commercial systems,and their potential to manu-facture complex parts with high resolution and dimensional tolerance.1,2These machines utilize an energy beam to selectively melt areas on a uniformly spread powder bed,sequentially building parts layer by layer.These types of systems are distinguished by their energy source,which is either a laser in selective laser melting(SLM)systems,or an electron beam in electron beam melting(EBM)systems.While SLM and EBM systems share many properties and challenges,there are some defining differences between the two:for example,EBM systems operate in a vacuum,whereas SLM systems use an inert atmosphere.This study focuses on the EBM systems,specifically an Arcam A2.JOM,Vol.68,No.3,2016DOI:10.1007/s11837-015-1802-0Ó2016The Minerals,Metals&Materials Society (Published online January19,2016)765Properties of EBM Ti-6Al-4VThe titanium alloy system Ti-6Al-4V is a popular AM material due to its interest within the biomed-ical and aerospace industries.3–5Results have shown that EBM Ti-6Al-4V components can meet or exceed the tensile properties of conventionally manufactured materials,but have significantly worse and highly variable fatigue properties.3,5–9 Fracture surface analysis from surface-finished fatigue samples has determined that the cause of the poor fatigue resistance in the as-built condition is due to the high porosity levels generally observed in additively manufactured components.5This is not surprising,as pores have been known to serve as preferred fatigue crack initiation sites,after manu-facturing defects and surfaceflaws.10The current approach taken to reduce porosity is to apply a hot isostatic press(HIP)treatment,which has been shown to improve fatigue properties,but at the cost of adding another processing step and reducing strength.6,8Therefore,being able to control porosity levels without the need for post-processing would provide significant benefits toward the commercial-ization of this technology.9Porosity in EBM Ti-6Al-4VMultiple types of porosity have been observed in additively manufactured ck of fusion porosity is formed by insufficient melting of a new powder layer to a previously deposited layer,or between adjacent beam passes.These pores are generally large(>100l m),and are identified by their irregular shape(Fig.1a).9Another type of commonly observed porosity is usually referred to as trapped gas porosity,and it will be the focus of this study.It is characterized by a near-spherical mor-phology attributed to its believed source as gas trapped in the melt that forms a bubble(Fig.1b).9,11 Due to the vacuum environment required for EBM, any source of gas must be from the supplied metal. One commonly attributed source comes from argon trapped in the powder during atomization that is transferred to the melt.9,11,12It is unlikely that other types of gas will precipitate out as pores due to their high solubility in solid titanium.9,13,14The alloy may also vaporize under high energy input conditions,forming voids in the material known as keyhole pores(Fig.1c).11,15Keyhole pores tend to be large and rounded,though not as spherical as the trapped gas porosity,and are frequently accompa-nied by a very narrow and deep melt pool.15 However,this is believed to be more of an issue with SLM technologies than EBM due to the process parameter controls put in place on EBM machines that prevent excessive energy input.13Prior studies on porosity in AM parts heavily relied on conventional methods such as the Archi-medes’method or metallographic analysis.15How-ever,density-based methods do not give sufficient information on pore morphology or size and spatial distribution.This makes it difficult to determine the type of porosity in the part or their potential impact on properties.Some recent studies have startedto Fig.1.Different morphologies of characteristic porosity seen in EBM Ti-6Al-4V.(a)Irregularly shaped lack of fusion porosity,(b)backscat-tered SEM image of spherical porosity generally attributed to gas trap-ped in starting powder,and(c)keyhole porosity observed in a longitudinal cross section of an SLM Ti-6Al-4V melt pool on rolled plate.Cunningham,Narra,Ozturk,Beuth,and Rollett766utilize x-ray computed tomography(XCT)to over-come this issue.9,16,17XCT is an x-ray absorption-based method that generates a3D model of a material’s external and internal structure from a series of two-dimensional radiographs of a rotating sample.This method allows for pore size,morphol-ogy and spatial distribution to be determined, allowing for trends to be much more definitively observed compared to traditional methods.XCT is being strongly considered as an ideal non-destructive industrial inspection method for AM parts due to its ability to inspect the complex internal structures AM is capable of producing,as well asflaws such as pores and cracks.18However, due to the high x-ray attenuation of metals,the resolution of full-part scans is limited by the sample size,material,and equipment used(25–70l m in prior studies).9,16,17X-ray microtomography(l XCT) is limited to a small volume of material,thus requiring sectioning of most parts,but offers signif-icantly better resolution( 5l m for laboratory-scale instruments).Few studies to date have utilized l XCT to analyze porosity in AM components at high resolution.7This study utilizes synchrotron-based l XCT,which offers a number of advantages over laboratory-scale instruments,as the high-energy x-rays with accompanying high brilliance provided by the synchrotron source allow rapid acquisition times with resolutions on the order of1l m.18–20 Some studies have show that processing param-eters can have an effect on resulting porosity,but results are not definitive.Studies using the Archi-medes’method have shown that increasing energy by reducing speed or increasing power will reduce porosity,and is generally attributed to a reduction in lack of fusion porosity by providing sufficient melting between subsequent layers by having larger melt pools.9,15However,the effect of processing parameters on trapped gas porosity has not been the subject of significant study in the EBM process. Some results have shown that parameters resulting in larger melt pools may reduce trapped gas poros-ity,but the mechanism is not entirely understood, and few data are available with the required resolution to make conclusive claims on this effect.9,15EXPERIMENTAL METHODSample PreparationTest blocks for this study were fabricated on an Arcam A2machine at North Carolina State Univer-sity.Processing parameters were designed at Carnegie Mellon to vary the melt pool cross-sec-tional area(normal to the direction of beam move-ment)in a systematic manner.As shown in Table I, melt pool cross-sectional areas of1,2,4,½,and¼times the nominal value were prescribed through changes in the Arcam speed function.The process mapping approach developed by Beuth et al.has demonstrated that process outputs such as melt pool geometry can be related to primary process variables including beam power and travel velocity for direct metal AM.21Using the process mapping approach,Fox has shown that a constant melt pool cross-sectional area can be maintained throughout the Arcam EBM process space by adjusting the beam power and beam travel velocity.22Further,based on the process maps of constant melt pool cross-sectional area for the Arcam EBM process,Narra et al.observed that, during a build,for any specified beam current,beam travel velocity is adjusted such that melt pool cross-sectional area is constant throughout the process. These adjustments are made by the Arcam propri-etary beam speed function parameter.23A previous study made similar observations,namely that the speed function maintains a constant melt pool depth irrespective of the beam current.24Figure2pro-vides an experimentally derived plot of the variation of melt pool cross-sectional area withspeed Fig.2.Experimentally derived plot demonstrating the relationship between melt pool cross-sectional area and speed function.18 Table I.Speed functions used and resulting change in approximate melt pool cross-sectional areaApprox.relative melt poolarea(area/nominal area)aSpeed function(raster)1X362X204X101/2X721/4X152a Cross-sectional area of melt pool perpendicular to travel direc-tion.Evaluating the Effect of Processing Parameters on Porosity in Electron Beam MeltedTi-6Al-4V via Synchrotron X-ray Microtomography767function.Accordingly,this relationship between speed function and melt pool cross-sectional area was used to vary the speed function for the test blocks given in Table I.Ti-6Al-4V test blocks(cylinders)with dimensions of3cm(diameter)91.5cm(height)were fabri-cated from standard Arcam-supplied Ti-6Al-4V yer thickness,hatch spacing and start plate temperature were kept at the nominal values of70l m,200l m and750°C,respectively.The hatching direction was rotated by90°after each layer was deposited.The nominal beam spot size available on the machine was used for all specimens (which depends somewhat on the particular machine used and on the initial machine calibra-tion).Tomography specimens were then machined from the centers of each sample with dimensions of 1mm91mm915mm with the long axis parallel to the build direction.Synchrotron X-ray Microtomography Synchrotron x-ray microtomography was per-formed at the Advanced Photon Source at Argonne National Lab.The2-BM beamline operating in60-kV white beam mode was used in order to get sufficient contrast and penetration through the samples.A total of1500projections were taken over180°with a50-ms exposure time resulting in approximately a2-min scan time.A0.65-l m voxel size(edge length)was obtained.The radiographs were reconstructed andfiltered using TomoPy,a software package supplied by APS. TomoPy is a python-based package that provides preprocessing in the form of artifact removal,as well as image reconstruction through the Fourier-based GridRec method.20Avizo9software was used for segmentation and generating the3D reconstruc-tions from the reconstructed images.Analysis was performed in order to determine pore shape,vol-ume,and spatial distribution.RESULTSSynchrotron X-ray TomographyTwo scans were analyzed per sample,encompass-ing the regions from0–1.5mm to6.5–8mm from thefinal deposited layer surface.Morphology was determined using the‘anisotropy’function in Avizo 9with a value of0.7being the cutoff for gas porosity.Any porosity found outside of this range was considered lack of fusion porosity,with only a few exceptions noted below.Examples of each type are shown in Fig.3.Projections of these reconstruc-tions,displayed in Fig.4,show a significant differ-ence in porosity between samples of different melt pool area,as well as between regions of individual samples.Figures4show spherical porosity observed in each region highlighted in blue.For the½X and¼X samples,it was observed that a few pores with a higher anisotropy(>0.7)were spherical pores beginning to merge(Fig.3a),and were included in the data,but only accounted for a very small volume fraction.Due to the high resolution possible with synchrotron XCT,a minimum feature size of 1.5 l m diameter was achieved.A minimum of8face-connected voxels were required in order to be considered a valid feature.While there may be pores below this limit,they represent a very small volume fraction,and are likely inconsequential to mechanical properties.Statistical data on the observed porosity are given in Table II.DISCUSSIONThe approach described above uses synchrotron-based3D XCT to analyze porosity in EBM Ti-6Al-4V samples fabricated over a range of processing conditions at a resolution that,to our knowledge, has not been achieved before.The average diameter of spherical pores was foundto be below10l m.The nearly spherical morphology of this porosity suggests that it is caused by gas trapped during solidification.9,11 Because this process is carried out in a vacuum environment,the sources of gas that can be cap-tured in the melt are limited.A significant source of gas is believed to be argon trapped in the powder particles during atomization.Evidence of porosity in partially melted powder was observed in this study (Fig.3c).Tammas-Willams et al.showed evidence of porosity in Arcam Ti-6Al-4V powder of a similar size to that observed in these samples.9Our preliminary results from a follow-up experiment at the2-BM beamline appear to confirm theirfindings,as shown in Fig.5,and will be examined in more detail in a later paper.However,this does not entirelyexplain Fig.3.Examples of porosity from the½X sample.(a)Two spherical pores coming into contact and beginning to merge,(b)a large pore found in melted region,(c)pore located in interior of partially melted powder particle,and(d)flat lack of fusion porosity.Images were taken at the same magnification and merged into a singlefigure.Cunningham,Narra,Ozturk,Beuth,and Rollett768the source of this type of porosity,as the maximum size of porosity seen in the parts has been reported to be larger than any porosity seen in the powder.9It has been suggested that gas bubbles may get larger through expansion due to the vacuum atmo-sphere and low hydrostatic pressure from the small melt pool,or through consolidation of multiple pores in the melt pool.9,25With the exception of the results from the ½X and ¼X samples,the results also show a significant decrease in the number and overall volume fraction of pores as melt pool area increased (Fig.6).The ability of a bubble to escape the melt pool is thought to be affected by a combination of melt pool geometry,fluid flow in the melt pool,and buoyancy forces acting on the bubbles.9,25It has been suggested that larger melt pool depths for a given powder layer thickness will allow more remelting of previous layers,giving more opportu-nity for gas to escape.9The results from the scan of the surface of the 2X and 4X samples would appear to support this,as large pores are only located within one melt pool depth of the surface,where it has not been remelted by subsequent passes.Analysis of the lack of fusion porosity will be discussed in a future paper,but it was observed to significantly increase in volume fraction as melt pool areadecreased.Fig.4.Synchrotron XCT results with lack of fusion highlighted in red and spherical porosity in (a–e).The top 1.5mm region,while (f–j)the 6–8.5mm region.Significant variation in porosity is observed across different samples and between regions of individual samples.Table II.Summary of gas pore statistics determined from XCT analysisSample Location a Volume scanned (mm 3)b Pore count Vol.norm.pore count (#/mm 3)Volume fraction (%)Ave.spherical equivalent diameter (l m)Max.spherical equivalent diameter (l m)1/4X Top 1.323422590.03148.4242.9Middle 1.343942940.03098.2042.91/2X Top 1.09361331.20.04698.0156.2Middle 1.21445367.80.07438.0567.01X Top 0.723164226.80.0347 6.2158.6Middle 0.709293510.0150 4.6052.12X Top 0.9477073.90.009887.2836.7Middle 1.001111110.00146 4.4518.04XTop 0.9744849.30.008998.7055.1Middle0.9823838.70.0002733.8510.4aTop 0–1.5mm from surface;Middle 6.5–8mm from surface.b Lack of fusion porosity not included in volume scanned.Evaluating the Effect of Processing Parameters on Porosity in Electron Beam Melted Ti-6Al-4V via Synchrotron X-ray Microtomography769These results have shown that changing melt pool area through speed function has a profound effect on the resulting porosity.Further,they suggest that reducing speed function from nominal settings can significantly reduce spherical porosity.However,this may not necessarily have significant improve-ments on fatigue resistance,as the clustering of pores at the surface are particularly damaging to fatigue properties.26Larger melt pools have also been observed to impact microstructure and chem-istry,so further characterization is underway to determine to what extent other material properties are affected by these parameter adjustments.9,27–29CONCLUSIONSynchrotron-based XCT was used to characterize the effect of beam velocity on the size and spatial distribution of gas pores in EBM Ti-6Al-4V samples at a resolution higher than any previous work. Volume fraction of spherical porosity was ob-served to decrease with decreasing speed func-tion.Average spherical pore diameter for all samples was less than 10l m.Larger pores on the order of 50l m were found concentrated at the surface of the increased melt pool area samples.Lack of fusion porosity increases dramatically once melt pool area is decreased below nominal settings keeping other parameters constant.ACKNOWLEDGEMENTSThe authors acknowledge America Makes for providing funding for this research under the pro-ject entitled ‘‘A Database Relating Powder Proper-ties to Process Outcomes for Direct Metal AM,’’Award Number FA8650-12-2-7230,and the Na-tional Science Foundation for providing funding under Grant CMMI-1335298.They would also like to thank Dr.Xianghui Xiao and the rest of the 2-BM beamline staff at the Advanced Photon Source for assisting in the acquisition of the synchrotron tomography data,and North Carolina State and Dr.Ron Aman for assisting with the sample fabrication.REFERENCES1.W.E.Frazier,J.Mater.Eng.Perform.23,1917(2014).2.L.E.Murr,S.M.Gaytan, D.A.Ramirez, E.Martinez,J.Hernandez,K.N.Amato,P.W.Shindo,F.R.Medina,and R.B.Wicker,J.Mater.Sci.Technol.28,1(2012).3.L.E.Murr,E.V.Esquivel,S.A.Quinones,S.M.Gaytan,M.I.Lopez, E.Y.Martinez, F.Medina, D.H.Hernandez, E.Martinez,J.L.Martinez,S.W.Stafford, D.K.Brown,T.Hoppe,W.Meyers,U.Lindhe,and R.B.Wicker,Mater.Charact.60,96(2009).4. 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