2007-Microstructure mechanical properties of laser melting deposited γ-TiAl intermetallic alloys

Materials Science and Engineering A

466 (2007) 187–194

Microstructure and mechanical properties of laser

melting deposited?-TiAl intermetallic alloys

H.P.Qu,H.M.Wang?

Laboratory of Laser Materials Processing and Manufacturing,School of Materials Science and Engineering,

Beihang University,37XueYuan Road,Beijing100083,PR China

Received13November2006;received in revised form11February2007;accepted14February2007

Abstract

Ti–47Al–2.5V–1Cr intermetallic alloy was fabricated by the laser melting deposition(LMD)manufacturing process.The microstructure was characterized by OM,SEM,TEM and XRD.The tensile properties and Vickers microhardness were evaluated on both longitudinal and transverse directions.Results showed that full density directionally solidi?ed(DS)columnar grain with fully lamellar(FL)microstructure consisted of?-TiAl and?2-Ti3Al lamellae was formed in the as-laser deposited samples along the longitudinal direction due to the high ther-mal gradient.The room-temperature tensile strength of the as-deposited alloy is up to approximately650MPa in the longitudinal direction and600MPa in the transverse direction,while the tensile elongation on both directions is approximately0.6%.The tensile fracture surfaces are different for the longitudinal and transverse directions,indicating that the room-temperature tensile properties were affected by the DS microstructure.

? 2007 Published by Elsevier B.V.

Keywords:Laser melting deposition;?-TiAl intermetallic alloys;Microstructure;Mechanical properties

1.Introduction

TiAl-based ordered intermetallic alloys have been continu-ously developed as the promising high temperature candidate structural materials due to the high melting point(>1450?C), low density(up to4g/cm3),high elastic modulus(160–180GPa) and high creep strength(up to900?C)[1–4].One of the major limitations to their structural applications is lack of ductility at ambient temperature.These alloys are also dif?cult to be pro-cessed by conventional manufacturing routes such as forging, rolling and welding[5].The disadvantage of the conventional casting technologies for TiAl components is its coarse as-cast microstructure that leads to the poor room-temperature mechan-ical properties.On the other hand,metallurgical defects such as porosity and shrinkage are inevitable during the conven-tional slow-cooling solidi?cation process.The shape and the dimension of the products were also restricted by the serious thermal stress-induced casting cracking defects due to its low ductility.Although fairly good components could be fabricated ?Corresponding author.Tel.:+861082317102;fax:+861082338131.

E-mail address:wanghm@https://www.360docs.net/doc/c06080081.html,(H.M.Wang).by conventional casting process,it is relatively too costly and time-consuming[6].

Many new manufacturing and processing routes have long been pursued in order to fabricate high quality TiAl intermetallic alloy components conveniently.These include mechanical alloy-ing(MA)and pulse discharge sintering(PDS),which was also named spark plasma sintering(SPS)[6,7],direct laser fabrica-tion(DLF)[12–14],semisolid forming from blended elemental powders[15],a combination of hot isostatic pressing(HIP)and self propagating high-temperature synthesis(SHS)[16],reactive foil metallurgy[17],laser engineered net shaping(LENS)[18], etc.During those powder metallurgical processes,the nitrogen and oxygen contents are inevitably enhanced,further deteriorat-ing ductility of the TiAl alloys.

Laser melting deposition(LMD)is a rapid solidi?cation material additive layered manufacturing technology for build-ing components from a computer-aided design(CAD)model [8–9,11].During the LMD process,the motion of the high power laser beam is controlled by a CNC motion control sys-tem,which was developed from the CAD model of a desired metal component,to continuously melt the metal powders which were injected into the laser focal zone from a powders delivery nozzle.Successive layers are then fusion-stacked to produce

0921-5093/$–see front matter? 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2007.02.073

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466 (2007) 187–194

Fig.1.Schematic showing the laser melting deposition (LMD)process.

the near-net or net-shape components with full density and extremely ?ne rapid solidi?cation microstructure due to the ultra-high solidi?cation cooling rate [8,10].Compared with the conventional manufacturing routes,high performance full

den-

Fig.2.Photograph showing the macro-surface of the laser deposited TiAl wall-like

specimen.

Fig.3.Geometric shape and size of room temperature tensile specimen in

mm.

Fig.4.Photograph showing the directionally solidi?ed columnar grains on lon-gitudinal section of the as-deposited TiAl plate.

sity LMD products with rapidly solidi?ed microstructure could be easily fabricated because those casting defects such as solidi-?cation porosity,shrinkage cavity,serious segregation,etc could be effectively improved or avoided during the rapid solidi?ca-tion.Any complicated shapes and the dimension of the near net shape components could be conveniently produced by the LMD additive layered manufacturing manner from the CAD ?les.In the present study,the laser melting deposition (LMD)man-ufacturing process was used to fabricate Ti–47Al–2.5V–1Cr intermetallic alloy.Microstructure and the chemical

composi-

Fig.5.OM imagines of DS columnar grains on longitudinal (a)and transverse (b)sections.

H.P.Qu,H.M.Wang/Materials Science and Engineering A 466 (2007) 187–194

189

Fig.6.SEM micrographs indicating the FL microstructure on transverse(a and b)and longitudinal(c and d)sections.

tion of the laser deposited rapidly solidi?ed TiAl alloys were investigated.Microhardness and the room-temperature tensile properties of the as-deposited wall-like samples in both the lon-gitudinal and transverse sections were https://www.360docs.net/doc/c06080081.html,parison of tensile properties and fracture surfaces along the two direc-tions have also been discussed in the paper aiming at a better understanding of the effect of microstructure on mechanical properties.

2.Experimental procedures

A GS-TFL-8000CO2laser(maximum output power8000W) and a BSF-2powder feeder are used to melt and deliver the TiAl alloy powder material.A HNC-21M CNC unit is used to control the work station and the laser beam movement.The focused laser beam and co-axial powder delivery nozzle move as an integral unit so that the powder is delivered coaxially into the laser melt pool(Fig.1).The laser melting deposition was con-ducted in a speci?cally designed Ar-purged processing chamber where argon atmosphere in constant pressure with oxygen con-tent less than100ppm was kept to avoid oxidation during the laser melting deposition process.The substrate was pre-heated before the laser melting deposition process,the LMD processing parameters are:laser beam power1500W,beam diameter5mm,beam scanning speed5mm/s,single-layer deposition thickness 0.2–0.3mm,powder delivery rate5.5g/min.

The spherical TiAl alloy powders in chemical composi-tion of Ti–47Al–2.5V–1Cr(at.%)with the oxygen content less than0.06–0.1(wt.%)was produced by vacuum non-contacting plasma melting argon atomization process.Particle size of the powders ranges from70to75?m.A hot rolled near-?high tem-perature titanium alloy Ti–6.8Al–2Zr–1Mo–1V was selected as the substrate.The surface of the substrate,onto which the?-TiAl alloy was deposited,was polished before being laser melting deposition.

Geometrical size of the laser deposited Ti–47Al–2.5V–1Cr intermetallic alloy plates was approximately40mm×40mm×6mm and its density was 3.86g/cm3.The surface of the plate is relatively rough where the traces of single deposition layers could be clearly seen as shown in Fig.2.However, the surface is with metallic shinning,indicating that negli-gible oxidation occurred during the laser melting deposition process.

Metallographic specimens prepared by standard mechanical polishing method were etched in a mixed solution of300ml H2O,100ml HNO3and100ml HF.An olympus BX51M opti-cal microscope,a JSM-5800and a Cambridge-S360SEM?tted with League-2000EDX systems and a Dmax-2200pc XRD were

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466 (2007) 187–194

Fig.7.OM micrographs showing full lamellar (FL)microstructure of larger grain size (a and b)and full lamellar (FL)microstructures of smaller grain size (c and d).

used to characterize the microstructure and to identify the phase constitutions.Microhardness of the deposited components was measured by using a HXZ-1000semi-automatic Vicker tester with a test load of 200g and a dwell time of 15s.Tensile proper-ties at room temperature along both longitudinal and

transverse

Fig.8.The XRD pattern of the LMD TiAl specimen.

directions was evaluated on MTS880using plate tensile test specimen (thickness 1–1.5mm).Fig.3indicated the size and section shape of the LMD room temperature tensile specimen which was fabricated by electric discharging cutting from the central wall-like plate and polished before test.Metallic thin foil specimens were prepared by ion bombardment technique and the structure was examined using the JEM-2100transmission electron microscopy.3.Results

Fully dense TiAl alloys plates were fabricated by the LMD process (Fig.4).Directionally solidi?ed columnar grains were formed in the deposition direction at the center of the plate due to the ultra-high temperature gradient generated by the high melt-pool temperature and heat conduction along the longitu-dinal direction.The diameter of the DS columnar grain was up to 0.25mm.Negligible changes in chemical composition of the Ti–47Al–2.5V–1Cr intermetallic alloy after laser melting deposition were detected in comparison to the original pow-

H.P .Qu,H.M.Wang /Materials Science and Engineering A 466 (2007) 187–194

191

Fig.9.Microhardness pro?les of the LMD TiAl plate on transverse and longi-tudinal sections.

ders (Table 1).This is attributed to the highly protective argon environment in the processing chamber during laser melting deposition process.

Finer microstructure was observed at the upper part of the wall-like components,indicating that the top part solidi?ed

at

Fig.10.Ti–Al binary phase diagram [19].

a higher solidi?cation rate.Full lamellar (FL)microstructure of different grain size (Fig.5)inside the component was also observed.The heat radiation to the protection atmosphere asso-ciated with the repeated heating and cooling generated by the back-and-forth scanning movement of the laser beam during the whole process is the reason for the formation of the re?ned grains at the surface of the thin wall-like component.The re?ned grain size was 50–300?m and the ultra-?ne inter-lamellar

spacing

Fig.11.The indexed SAD patterns (a)and TEM BF micrograph (b)showing a typical lamellar structure consisting of ?/?2within the laser deposited specimen,the

Z (zone axis)is [ˉ1

10]?.

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Table 1

Chemical compositions (at.%)of Ti–47Al–2.5V–1Cr intermetallic alloy before and after laser melting deposition

Element Ti

Al V Cr Original TiAl powder 48.0147.32 2.51LMD TiAl specimen

48.67

47.17

2.27

0.89

(Fig.6)was approximately 0.5?m (Fig.7).The XRD pattern (Fig.8)of the LMD TiAl components indicates that the main constituent phases are ?-TiAl and ?2-Ti 3Al.

Fig.9reveals that the microhardness of the laser deposited Ti–47Al–2.5V–1Cr intermetallic alloy gradually changed between approximately 300Vickers hardness number (VHN)at surface and approximately 380VHN at the center of the component.It is interesting that the hardness on longitudinal section is slightly lower than that on transverse section due to the directionally solidi?ed anisotropic structure.Generally,it can be concluded that the difference of the microhardness along different orientations is the consequence of the variation of the texture within the LMD TiAl specimen.The hardness in the

near-surface zone is also slightly lower than that in the central zone because of the different microstructure of the two zones.In the central zone,directionally solidi?ed structure with texture was formed during the LMD process,while misoriented equaxed grains in the near-surface zone were formed due to the lower ther-mal gradient and lower melt temperature.Ultimately,different structures in the two zones lead to their different microhardness.4.Discussion

The lamellar microstructure consisting of ?-TiAl and ?2-Ti 3Al is the result of solid state phase transformation.According to the Ti–Al binary alloy phase diagram (Fig.10),TiAl alloy which containing aluminum of 46–49%(at.%)under-goes a solid-state phase transformation [20]?→?+?2at about 1125?C.During the laser melting deposition process,the pre-vious deposited layers beneath the depositing melt pool were heated by the laser beam scanning to very high temperature (>1500?C)[21].Solid state phase transformation mentioned above inevitably reacted in the heat affected zone around the melt pool generated by the laser beam and ultimately lead to the formation of the fully lamellar microstructures of laser

melting-

Fig.12.SEM fractographs of the laser deposited alloy along the longitudinal (a and b)and transverse (c and d)sections.

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Fig.13.SEM micrographs showing the sections perpendicular to the fracture surface of the longitudinal (a and b)and transverse (c and d)tensile specimens.

deposited Ti–47Al–2.5V–1Cr intermetallic alloy.It is known [22,23]that the ?/?2interphase interface has an orientation rela-tionship of (0001)?2||(111)?and [11ˉ2

0]?2||[1ˉ10]?0.In this study,the TiAl and Ti 3Al lamelle formed after the eutectic reaction at 1125?C also have a certain orientation relationship according to the selected area electron diffraction (Fig.11a).The tensile fracture surface (Fig.12)and subsurface (Fig.13)of the laser deposited TiAl alloy for the longitudinal and trans-verse directions were also examined.The fracture surfaces for the two directions were ?at and simple.In particular,the tenacity pit of the deposition direction specimens indicates the charac-teristics of the lamellar microstructure (Fig.12b),indicating that the initial crack microscopically generated within a single grain and grew along the lamellar interfaces within FL grains (Fig.13b).On the other hand,both lamellar microstructure and grain boundary were observed at the sub-surface for the trans-verse direction specimen and the crack tip propagated mainly along the grain boundary (Fig.13d).

Signi?cantly,tensile ductility of the deposited TiAl alloy on longitudinal and transverse directions was very low (0.6%)which was much lower in comparisons to TiAl intermetallic alloys prepared by conventionally casting process [24,25].The tensile fracture surface appears to be in an inter-granular manner for the longitudinal specimens and a combination of inter-granular manner and trans-granular manner for the transverse specimens,indicating that the room-temperature tensile property was affected by the rapidly solidi?ed DS microstructure.

In this paper,the laser deposited Ti–47Al–2.5V–1Cr inter-metallic alloy components along both the longitudinal and the transverse directions has outstanding room-temperature mechanical properties.Microstructure with colony size 50–100?m and lamellar spacing of 0.3–0.5?m exhibited a room temperature tensile strength of 600–650MPa for the longitudinal direction and 550–600MPa for the transverse direc-tion.The ductility was about 0.6%for the both directions.Table 2shows the comparison about the ultimate tensile strength,microstructure,grain size and ductility of the specimen pro-cessed by LMD and by conventional routes,indicating that microstructure of the specimen with smaller grain size has higher UTS.But ductility of the LMD specimen was also very low due to the FL microstructure.Many studies show that yield strength and tensile ductility of TiAl alloy with lamellar microstructures at room temperature depend mainly on the colony size and inter-lamellar spacing [2,26].Control of microstructures and grain re?ning by various solid-state thermal and mechanical process-ing was intensively carried out because the sensitive dependence of mechanical properties and high temperature behavior on microstructure [27–29].Further studies will be focused on solid-

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Table2

Comparison of the mechanical properties of TiAl alloys fabricated by LMD and conventionally processes

Processing route Microstructure Grain size(?m)Ultimate tensile strength(MPa)Ductility(%) Forging,1390?C,1h900?C,6h FL13004280.8 Forging,1370?C,2h900?C,8h FL10503760.7 Forging,1370?C,0.5h900?C,24h FL600487 1.1 Forging,1350?C,1h1000?C,5h FL300588 1.1 Forging,1330?C,0.5h900?C,6h NL140702 2.8 Forging,1280?C,3h900?C,4h DP30557 3.8 Forging,1280?C,2h900?C,6h DP25601 2.5 Forging,1000?C,24h NG5100662 2.9

Laser melting deposition FL50–300650(LD),600(TD)0.6

state phase transformation behavior and their effect on the microstructure and room temperature mechanical properties of laser melting deposited Ti–47Al–2.5V–1Cr intermetallic alloy.

5.Conclusion

(1)Ti–47Al–2.5V–1Cr intermetallic alloy with full density

and rapidly solidi?ed columnar grain microstructure was successfully fabricated by the laser melting deposited man-ufacturing process.

(2)The initial?grain size of the laser deposited TiAl alloy

is approximately50?m and the inter-lamellar spacing of the full-lamellar structure is approximately0.5?m.The laser deposited TiAl intermetallic alloy exhibited outstand-ing room temperature tensile properties and with some anistropy.

(3)The tensile strength is up to600–650MPa in the longitudi-

nal direction and550–600MPa in the transverse direction, while the ductility was approximately0.6%for both direc-tions.

(4)The?/?2interphase interface orientation relationship is

(0001)?2||(11ˉ1)?,[11ˉ20]?2||[1ˉ10]?0. Acknowledgements

The research was?nancially supported by the National Nat-ural Science Foundation of China(Grant Nos.50331010and 50625413).The authors would like to thank Mr.S.Q.Zhang and Mr.A.Li for his help and suggestions during the laser melting deposition.

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