Journal of Alloys and Compounds 572 (2013) 17–24

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Journal of Alloys and Compounds

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Microstructural characterization of laser melting depositedTi–5Al-5Mo–5V–1Cr–1Fe near b titanium alloy

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.03.243

⇑ Corresponding author at: Laboratory of Laser Materials Processing and Manu-facturing, Beihang University, 37 Xueyuan Road, Beijing 100191, China. Tel./fax: +8610 8233 9691.

E-mail address: tianxj@buaa.edu.cn (X.J. Tian).

C.M. Liu a, X.J. Tian a,b,⇑, H.B. Tang a,b, H.M. Wang a,b

a Laboratory of Laser Materials Processing and Manufacturing, Beihang University, 37 Xueyuan Road, Beijing 100191, Chinab Key Laboratory of Aerospace Materials of the Ministry of Education, Beihang University, 37 Xueyuan Road, Beijing 100191, China

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

Article history:Received 29 January 2013Received in revised form 22 March 2013Accepted 26 March 2013Available online 3 April 2013

Keywords:Metals and alloysLaser processingMicrostructure

Laser melting deposition (LMD) is characterized by high temperature gradients, high cooling rates andreheating treatment effects. These features of LMD generally lead to an unique microstructure. In thisstudy, the macrostructure, microstructure and hardness of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe near b titanium alloy have been investigated. The macrostructure is characteristic of novel bgrain morphology and layer bands. The b grains exhibit successive alternation of large columnar andbamboo-like grain morphology, which is associated with the LMD processing parameters. Layer bandsexhibit duplex microstructure and low hardness, while ultra-fine basket-weave microstructure, micro-segregation and high hardness are observed between layer bands. Microstructure and hardness of thealloy are non-uniform due to the existence of the layer bands. Thus, special attention is given to the for-mation mechanism of layer bands. It is found that layer bands are heat-affected zones which resultingfrom the sub-melting-point reheating treatment when a new layer deposits.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction about the microstructure characteristics of laser melting deposited

Ti–5Al–5Mo–5V–1Cr–1Fe is a high strength near b titaniumalloy. It is widely used in aerospace industries as key structuralcomponents due to its high strength-to-density ratio, good harde-nability and excellent fatigue crack-propagation behavior [1,2].However, it is costly and time-consuming to fabricate large tita-nium alloy components by traditional wrought-based processes.Laser melting deposition (LMD) is a powder-based additive manu-facturing process that combines laser cladding and advanced rapidprototyping. It can be single-step and waste-free to fabricate nearnet shape components using metal powders. Thus, LMD is particu-larly attractive for the manufacture of aerospace titanium alloycomponents [3–11].

Many studies have focused on the microstructures of lasermelting deposited near a or a + b titanium alloys, including Ti–6Al–4V, Ti–6Al–2Zr–1Mo–1V, Ti–4Al–1.5Mn, etc. [12–21]. Gener-ally, three typical microstructure characteristics can be observed[12,16,20]: (i) coarse columnar prior b grains, which arise due tohigh temperature gradients, distribute approximately parallel tobuilding direction [12,22]; (ii) fine Widmanstätten microstructuresas a result of high cooling rates; (iii) regularly spaced layer bandsrelated to the reheating treatment [16]. However, little information

near b titanium alloys has been reported. Since near b titanium al-loys contain high amount of b-phase stabilizing elements, theirmicrostructures are generally different from those of near a ora + b titanium alloys [23].

In addition, although it is assumed that layer bands are associ-ated with the reheating treatment, the detailed formation processis still unclear and seldom studies can be found. Among the avail-able research, Kelly and Kampe [16] proposed that the layer bandsformed in layer n after the deposition of layer n + 3, which agreedwith their experimental observation that layer bands were absentin the last three layers of laser melting deposited Ti–6Al–4V. How-ever, layer bands were observed even in the last single layer of thelaser deposited Ti–6Al–4V in Ref. [7] and laser deposited Ti–5Al–5Mo–5V–1Cr–1Fe alloy in the present study as will be shown be-low. Moreover, according to Kobryn et al. [17,22], layer band wasassumed to be a new heat-affected zone which was caused bythe reheating treatment during a new layer depositing. This workprovided insights into the formation mechanism of layer bands,but the concept of heat-affected zone was plausible. Because theheat-affected zone can be divided into two regions according tothe reheating temperatures, one is below b-transus temperature(Tb) and the other is above Tb. Which region corresponds to layerband still needs to be demonstrated. And this question is funda-mental importance to further understand the formation mecha-nism for layer bands.

Accordingly, in this study, the microstructure characteristics oflaser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe near b titanium

Table 1Heat treatment processes for investigating the grain morphology and the formation of

18 C.M. Liu et al. / Journal of Alloys and Compounds 572 (2013) 17–24

alloy are investigated and the formation mechanism for layerbands is explained.

layer bands.

Number Heat treatment process Application

HT1 885 �C/15 min, FC + 780 �C, WQ Beta grain morphology analysisHT2 870 �C/1 h, WQ Layer bands analysisHT3 900 �C/1 h, WQHT4 950 �C/1 h, WQ

2. Experimental procedure

2.1. Laser melting deposition

A thick plate of Ti–5Al–5Mo–5V–1Cr–1Fe alloy with a geometrical size of350 mm � 250 mm � 20 mm was fabricated by LMD, using a 8 kW continuouswave CO2 laser materials processing system with four-axes computer-numerical-controlled (CNC) work station. Argon purged processing chamber with oxygen con-tent less than 50 ppm was used to prevent the melt-pool from oxidization. The LMDprocessing pattern was schematically illustrated in Fig. 1a. The process was inter-rupted between each layer until the temperature of the previous layer reached lessthan 300 �C. At the optimized parameter, the melt pool width is approximately12 mm. The applied deposit spacing was 8 mm, and then the width of overlap zonewas about 4 mm. The layer thickness is about 2 mm. After LMD, cubic specimens(12 � 12 � 12 mm3) were cut from the plate-like sample by electric discharge wire.These specimens were used for microstructure observation. The b-transus temper-ature (Tb, �C) of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe alloy was880 ± 5 �C, determined by metallographic method.

2.2. Heat treatment

Four types of heat treatment (HT) processes, named as HT1, HT2, HT3 and HT4,were designed as shown in Table 1. HT1 was designed to get a clear vision of bgrains at low magnification, because clear grain morphology was usually requiredas a prerequisite for better understanding the macrostructure. But laser meltingdeposited titanium alloys hardly exhibited clear grain morphology by etchingmethod [20,24]. A possible reason was that a laths were fine and distributedapproximately uniform in b grains. Thus the sample was etched almost uniformly,making it difficult to observe the grain boundary. Accordingly, HT1 was designed asfollows to investigate the grain morphology. First, the specimens were annealed at885 �C (Tb + 5) to form b phase, setting the anneal time as only 15 min (min) to pre-vent grain growth. Afterwards, the specimens underwent furnace cooling (FC) to780 �C followed by water quench (WQ). In the process, a phase was firstly formin the grain boundary during furnace cooling from b phase field [25], then the fur-ther growth was inhibited by water quench. The microstructure of the samples afterHT1 was shown in Fig. 2. As expected, alpha phase mainly distributed in b grainboundary (see Fig. 2a), and the b grain morphology were clearly visible after etching(see Fig. 2b) because a and b phases had different corrosion resistance. More impor-tantly, no remarkable changes of grain size were observed because the heating tem-perature was relatively low and the holding time in b phase was relatively short[26].

The other three types of heat treatment were designed to determine whetherthe layer band corresponded to the heat-affected zone below Tb or above Tb, andinvestigate the character of layer bands after heat treatment. The process of HT2was: annealed at 870 �C (Tb � 10) for 1 h (h) followed by water quench (WQ). Theprocess of HT3 was: annealed at 900 �C (Tb + 20) for 1 h followed by WQ. Theprocess of HT4 was: annealed at 950 �C (Tb + 70) for 1 h followed by WQ. All ofthe heat treatments were performed in an electric-resistance furnace in the ambi-ent atmosphere.

Fig. 1. (a) Schematic illustration of LMD process for a thick plate of Ti–5Al–5Mo–5V–1Cillustration of the deposited morphology from the front view.

2.3. Microstructural characterization

Metallographic specimens were prepared by standard mechanical polishing andetched with a solution of 1 ml HF, 6 ml HNO3 and 100 ml H2O. The microstructureswere observed by Olympus BX51M optical microscope (OM) and Cambridge-S360scanning electron microscope (SEM). Vickers hardness was measured by a HXZ-1000 semi-automatic Vicker tester with a test load of 200 g and a dwell time of15 s, and 15 points were measured. The chemical compositions were analyzed byJEOL JXA-8100 electro probe micro-analyzer (EPMA). Multiple WDX point scanswere performed to obtain a better statistical representation of the different local-ized compositions. Five WDX scans were performed within the dendrite regionsand five within the interdendritic regions, the spot size was 3 lm. Another fiveWDX scans were performed within layer bands and five between layer bands, thespot size was 10 lm.

3. Results and discussion

This research is aimed at obtaining an insight into the macro-and micro-structure of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe alloy and revealing the formation mechanism of layerbands. Therefore, b grain morphology (macrostructure) and its for-mation process will be discussed first. Then, the microstructuresand related hardness of the as-deposited sample will be investi-gated. Afterwards, microsegregation of alloying elements will beexamined. Finally, the character of layer bands will be studiedand the formation mechanism for layer bands will be furtheranalyzed.

3.1. Beta grain morphology

Fig. 3 shows typical b grain morphology at low magnification.The sample consists of three types of grains: large columnar grains,small columnar grains and equiaxed grains. The larger columnargrains arrange approximately parallel to the deposited buildingdirection, with the width varied from 0.3 to 1 mm and the heightmore than 8 mm. The height is obviously much larger than the

r–1Fe alloy. Three different views (front, side and top) are indicated. (b) Schematic

Fig. 2. Optical microscope micrographs of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe showing the grain size and grain morphology after heat treatment 885 �C/15 min, FC + 780 �C, WQ: (a) high magnification and (b) low magnification.

Fig. 3. Typical b grain morphology of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe, at the center of the plate-like sample.

C.M. Liu et al. / Journal of Alloys and Compounds 572 (2013) 17–24 19

thickness of one layer. Small columnar grains are 100–300 lm inwidth and 0.5–2 mm in height. It can be seen from Fig. 2b thatthese small columnar grains have zigzag pattern, and they aretilted about 20� away from the building direction as indicted bythe arrow dotted lines, If the scanning direction is from left to right,the grains are right-slanted, and if the scanning direction is fromright to left, the grains are left-slanted. The dependent relationshipsuggests that the heat conduction direction is specific during LMD,and it has a strong influence on the orientation of the grains [12].Equiaxed grains have the diameter about 50-300 lm. They locatebetween the small columnar grains in the building direction. Thesmall columnar grains and equiaxed grain present bamboo-likemorphology in side view. The height of the bamboo-like grains isabout 2 mm, which is identical to the layer thickness as shownin Fig. 1b. This implies that the bamboo-like grains may be formedin single layer. In addition, the large columnar grains and bamboo-like grains are arranged alternately parallel to each other in frontand top view.

To investigate the formation mechanism of b grains, the grainmorphology at the top of the plate-like sample needs further

observation, because the last single layer has not suffered thereheating treatment during the LMD process. The correspondinggrain morphology is shown in Fig. 4. The last layer is made up ofmany overlapping tracks. In the single track of the last single layer(above the layer band in Fig. 4a), small columnar grains form at thebottom of the melting pool, while equiaxed grains tend to form onthe top surface. In the overlap zone (Fig. 4b), there are lager colum-nar grains. Combining with Fig. 3, we can summarize that smallcolumnar grains and equiaxed grains generate at the bottom andthe top of the single track respectively, and large columnar grainsform in the overlap zone between adjacent tracks.

Fig. 5 schematically shows the evolution process of grain mor-phology during LMD. In this process, the sample is built layer bylayer. Each layer is formed by many overlapping tracks. When laserbeam scans in a single track (Fig. 5a), there are high temperaturegradient and rapid cooling due to heat loss through the substrateor previously deposited layers. This promotes the formation ofcolumnar grains at the bottom of melting pool. With the solidifica-tion going on, both cooling rate and temperature gradient will de-crease at the top of melting pool. Then equiaxed grains gradually

Fig. 4. Beta grain morphology of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe, at the top of the plate-like sample: (a) single track and (b) overlap zone between adjacenttracks. Red dashed lines represent the positions of layer bands. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version ofthis article.)

Fig. 5. Schematic illustration of the formation process of b grains during LMD process.

20 C.M. Liu et al. / Journal of Alloys and Compounds 572 (2013) 17–24

form. The small columnar grains at the bottom of melting pool andthe un-remelted equiaxed grains make up the bamboo-like grainmorphology. If there are multi-tracks (Fig. 5b), equiaxed grains inthe overlap zone will remelt, and the temperature gradient andcooling rate in the overlap zone are higher than those at the centerof melting pool because the depth of melting pool in the overlapzone is less than that at the center of melting pool. Thus, thecolumnar grains will continue to grow up in the overlap zone.When a new layer deposits (Fig. 5c), equiaxed grains in the previ-ous layer partially remelt, and then serve as nuclei and promotethe regrowth of the columnar grains. Afterwards, the grain mor-phology of the new layer turns to be similar to the deposited layer.Moreover, the columnar grains in the overlap zone will continue toepitaxial growth. The process extends through many consecutivelayers. As a result, the columnar grains in the overlap zone becomelarge and dominate in many layers.

In the present study, the width of the overlap zone is about4 mm, and the width of the large columnar grains region is about

2–4 mm as shown in Figs. 3 and 4b. This agreement in width sug-gests that the large columnar grains are formed in the overlapzones. Thus it is reasonable to predict that changing the depositspacing can eliminate or enhance the large columnar grains, theninfluence the alternated structure. In addition, by adjusting theLMD processing parameters, all the equiaxed grains at the top sur-face can even be remelted completely when a new layer deposits.Then samples only with large columnar grains may be obtained.These imply that grain morphology is intimately associated withthe LMD processing parameters. Further studies about the influ-ence of the LMD processing parameters on the grain morphologyare still necessary.

3.2. Microstructure and hardness

The as-deposited sample exhibits regularly spaced layer bandsas shown in Fig. 6. Between layer bands, ultra-fine basket-weavemicrostructure is observed in Fig. 7a. And the width of a laths is

Fig. 6. OM micrograph showing layer bands of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe alloy.

C.M. Liu et al. / Journal of Alloys and Compounds 572 (2013) 17–24 21

approximately 0.1 lm, and its length is about 1–3 lm. Within thelayer bands, it is a duplex microstructure (see Fig. 7b) which con-sists of ultra-fine a laths and small amounts of coarse a laths witha width of 0.3 lm. The ultra-fine microstructures are related to thehigh cooling rate during LMD. Moreover, they are much finer thanthe Widmanstätten microstructures of laser melting depositednear a or a + b titanium alloys.

The results of Vickers hardness measurements indicate that thedifferent microstructures between and within the layer bandresults in a slight difference in hardness. The average microhardnesswithin the layer bands (429 ± 16 HV) is approximately 8% lowerthan that between layer bands (465 ± 18 HV). In Ti–5Al–5Mo–5V–1Cr–1Fe alloy, strengthening is attributable to a laths formed on bmatrix phase. Between layer bands, ultra-fine a laths exhibit largedispersion strengthening effects. However, coarse a laths withinlayer bands reduce the strengthening effects. So, the hardness with-in layer bands is lower than that between layer bands.

Fig. 7. SEM micrographs showing the typical microstructures of laser melting deposited Tbands.

Table 2Average EPMA chemical compositions (wt.%) for different regions in laser melting deposit

Location Ti Al Mo

Layer bands 84.06 4.99 4.35Between layer bands 83.37 5.06 4.38Dendrite 83.36 4.81 5.32Interdendrite 84.16 5.20 3.83

3.3. Microsegregation

Table 2 lists the chemical compositions for different regions inlaser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe alloy. It is notice-able that although there is microstructural difference between andwithin layer bands, the macrosegregation of alloying elements doesnot exist. But due to the high cooling rate in LMD, the solidificationof the Ti–5Al–5Mo–5V–1Cr–1Fe alloy is nonequilibrium, hence thedendritic solidification structure is formed. It can be seen that thereare fine well-aligned dendritic structure in as-deposited Ti–5Al–5Mo–5V–1Cr–1Fe alloy as shown in Fig. 8a. And the dendritic struc-ture still can be observed clearly after heat treatment HT2 (Tb � 10)as shown in Fig. 8b. It can be seen that the interdendritic regions(vertical white lands) are composed of large amounts of a particles.Table 2 gives the average EPMA chemical compositions in dendriteand interdendritic regions. It can be found that element Mo tends tosegregate towards the dendrite region during solidification, otherelements exhibit uniform distribution. Because Mo is the b-phasestabilizing element, the dendritic region formed less a laths thaninterdendritic region as shown in Fig. 8b.

From the binary alloy phase diagrams [27], we can calculate theequilibrium distribution coefficient (ko) of the solute elements intitanium alloy by ko = CS/CL. Here, CS is solution concentration in so-lid phase; CL is solution concentration in liquid phase. Generally,the weight percent of the solute elements in titanium alloys areless than 10%. According, the values of ko for Al and V are closeto 1, while ko for Mo is approximately 2. When ko is close to 1,the close spacing of the liquidus and solidus lines indicates littletendency toward segregation. When ko is smaller or larger than1, the wide spacing indicates a strongly partitioning alloying ele-ment and a tendency to segregate on solidification [28]. Thus,microsegregation has been seldom reported in laser meltingdeposited near a or a + b titanium alloys (Ti–6Al–4V). Its presencein Ti–5Al–5Mo–5V–1Cr–1Fe near b titanium alloy should be re-lated to the nonequilibrium solidification and the high Mo content.

i–5Al–5Mo–5V–1Cr–1Fe alloy: (a) between the layer bands and (b) within the layer

ed Ti–5Al–5Mo–5V–1Cr–1Fe alloy.

V Fe Cr Application

5.14 1.16 0.95 Macrosegregation analysis5.11 1.14 0.935.09 1.12 1.03 Microsegregation analysis4.99 1.09 0.95

Fig. 8. (a) OM micrograph showing the microsegregation of as-deposited Ti–5Al–5Mo–5V–1Cr–1Fe alloy. (b) SEM micrograph showing the remained microsegregation afterheat treatment 870 �C/1 h, WQ. The white particles are a phase and other regions are b phase.

Fig. 9. (a) OM micrograph showing the presence of layer bands in the last few layers; (b) SEM micrograph showing the remained layer bands after HT2. The white particlesare a phase and other regions are b phase; (c) OM micrograph (differential interference contrast mode) showing the remained layer bands after HT3; (d) OM micrographshowing nearly no observable layer bands after HT4.

22 C.M. Liu et al. / Journal of Alloys and Compounds 572 (2013) 17–24

3.4. Layer bands

Layer bands are macroscopic bands, which can be observed ineach layer as shown in Fig. 9a, which implies that layer bandscan be formed locally every time the laser passes. In addition, wefind that the observation of layer bands is related to a suddenchange of the microstructure [22], because no significant boundary

can be observed if the gradient of the change is small. In as-depos-ited samples, layer bands originate from the sudden change in thesize of a particles in the microstructure (see Fig. 7), and the thick-ness is approximately 75 lm. For the layer bands in the sampleafter HT2 (Tb � 10), it results from the sudden change in thenumber of a particles. It can be seen from Fig. 9b that there aremore a particles within layer bands than between layer bands.

Table 3A summarize of experimental observations about layer bands.

Material condition Phase Direct reasons Essential reason Thickness (lm)

As-deposited b + a Size of a particlesa Extent of homogenizationa 75HT2 (Tb � 10) b + a Number of a particlesa 60HT3 (Tb + 20) b Corrosion resistancea 30HT4 (Tb + 70) b 0

a A sudden change in.

Fig. 10. (a) Schematic illustration of the formation process of layer bands during LMD process in the front view. (b), (c) Schematic illustration of the temperature andhomogenization distribution in heat-affected zone (HAZ) during reheating treatment.

Fig. 11. Schematic illustration for the decrease of the layer bands thickness duringthe heat treatments. The increase of color intensity represents the increase of theextent of homogenization.

C.M. Liu et al. / Journal of Alloys and Compounds 572 (2013) 17–24 23

And the thickness of layer bands is approximately 60 lm. AfterHT3 (Tb + 20), the sample is made up of only b phase, but differen-tial interference optical microscopy in Fig. 9c indicates that layerbands remain with the thickness approximately 30 lm, whichdemonstrates that the formation of layer bands is associated withreheating above 900 �C. Besides, layer bands exhibit convexity,implying that the b phase within layer bands has better corrosionresistance. After HT4 (Tb + 70), layer bands have almost disap-peared (see Fig. 9d). These experimental observations are summa-rized in Table 3. Although the direct reasons why layer bands canbe observed in different samples are different, the underlyingessential reason should be unified. Next, the essential reason willbe tentatively interpreted.

In single b phase, the chemical compositions distribute morehomogeneous, the regions should have better corrosion resistance.Thus, according to the results after HT3, we can infer that layerbands should have high extent of homogenization. And this infer-ence can also interpret the experimental observations of the sam-ple after HT2 and as deposited sample. (i) In the sample after HT2,there is no macrosegregation of alloying elements (see Table 2),then why there are more a particles within layer bands than be-tween layer bands? According to our inference, layer bands havehigh extent of homogenization. This means composition fluctua-tions within layer bands are very small. It is well known thatcomposition fluctuations are necessary conditions for nucle-ation during phase transformation. During heat treatment, thephase transformation refers to a ? b. Thus, the new b phase is dif-ficult to nucleate and the parent a phase is more stable withinlayer bands. (ii) In as-deposited sample, why coarse a particlesare present within layer bands? The solid phase transformationduring cooling is b ? a. According to the discussion above, it isdifficult for a phase to nucleate within layer bands. So the numberof primary a particles within layer bands will be much smallerthan that between layer bands. And because there is no macroseg-regation, the volume fraction of a phase within layer bands will beidentical to that between layer bands. Consequently, as cooling, thesmall amount of primary a particles within layer bands will growto large size, while the large amount of primary a between layerbands will have small size.

High extent of homogenization is a key characteristic of layerbands, but it is only one of the necessary conditions for the layerbands formation. The other necessary condition should be the sud-den change in the extent of homogenization between and withinlayer bands, because it corresponds to the sudden change of thecorrosion resistance and microstructures listed in Table 3. In thefollowing, the origins of these two conditions will be discussedrespectively. (i) The extent of homogenization depends on element

24 C.M. Liu et al. / Journal of Alloys and Compounds 572 (2013) 17–24

diffusion rates and time, which is associated with the reheatingtemperature and time during LMD. During the reheating treatmentin LMD, layer bands homogenize in a short time, but other regionshardly exhibit the same extent of homogenization even after theheat treatment at 900 �C holding for 1 h, according to the resultsafter HT3. This indicates that the temperature of layer bandsformation is much higher than 900 �C, which is expressed as TC.When a new layer deposits during LMD (see Fig. 10a), in theheat-affected zones (HAZ), the closer the position gets into themelting pool, the higher the temperature are, and consequentlythe higher the extent of homogenization are. Hence, in the sub-melting-point HAZ (see Fig. 10a), there is high temperature(T P TC), which leads to the high extent of homogenization. (ii)Temperature gradient can be defined as the derivative of the tem-perature T with respect to its position x in HAZ, kT ¼ dT

dx, and simi-larly, the homogenization gradient kH ¼ dH

dx . Here, H is the extentof homogenization. In HAZ, the nearer the position gets to themelting pool, the higher the temperature gradient is [29] (seeFig. 10b), and consequently the higher the homogenization gradi-ents are (see Fig. 10c). It can be seen that in the sub-melting-pointHAZ (T P TC), there is high temperature gradient (k P kCT), andhence high homogenization gradient (kH P kCH), which means asudden change in the extent of homogenization. Here, kCT andkCH are the critical values which can result in sudden change. Con-sequently, the sub-melting-point HAZ can fulfill the above twoconditions, where can form the layer bands.

The above layer bands formation mechanism can be used totentatively interpret several experimental results. (i) After HT2,HT3 and HT4, the thickness of layer bands decreases gradually withincreasing heating temperature, as shown in Table 3. The reason isillustrated in Fig. 11. Rising the heating temperature increases thediffusion rate, and hence increases the extent of homogenization,but decreases the homogenization gradient (kH). Accordingly,the regions which can fulfill the condition that kH P kCH will re-duce. Then, the thickness of layer bands decreases gradually. WhenkH < kCH in the whole HAZ, the layer bands cannot be observed asshown in Fig. 9d. (ii) Why the observations of layer bands aresensitive to the materials and LMD processes? For example, layerbands were observed in all layers of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe alloy in the present study and laser depos-ited Ti–6Al–4V block in Ref. [7]. But no layer bands are present dur-ing the last three layers in laser melting deposited Ti–6Al–4V platein Ref. [16]. Because the necessary condition for the formation oflayer bands is kH P kCH, i.e. kT P kCT during LMD. The factors whichinfluence the temperature gradients will influence the formation oflayer bands, such as the LMD processing parameters, the shape andsize of deposited components, and the materials thermalconductivity.

4. Conclusion

Laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe near b tita-nium alloy exhibits four microstructure characteristics: novel bgrain morphology, layer bands, ultra-fine basket-weave micro-structure and microsegregation. These characteristics are differentfrom that of laser melting deposited near a or a + b titanium alloys.The main findings from this study can be summarized as follows:

1. The b grains consist of three types, large columnar, smallcolumnar, as well as equiaxed. Large columnar grains distributeat the overlap zone between adjacent tracks, and dominate inmany layers due to epitaxial growth. Small columnar grains

and equiaxed grains generate at the bottom and the top of indi-vidual tracks, respectively. They locate in single layer and formbamboo-like grain morphology.

2. Layer bands have duplex microstructure, which is made up ofultra-fine a laths and small amounts of coarse a laths. Themicrostructure between layer bands exhibit ultra-fine basket-weave morphology. Moreover, the hardness within layer bandsis lower than that between layer bands.

3. There is microsegregation of alloying elements between layerbands. Element Mo segregates preferentially to the dendriticregions.

4. Layer bands are formed in the sub-melting-point HAZ. Their for-mation has two necessary conditions. One is sub-melting-pointreheating treatment, which results in high extent of homogeni-zation within layer band. The other is high temperature gradi-ents during LMD, which results in a sudden change in theextent of homogenization between and within layer bands,and hence leads to macrostructural and microstructuralchanges.

Acknowledgement

The work was financially supported by the State Key BasicResearch Program of China (Grant No. 2010CB731705).

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