Materials and Design 105 (2016) 133–141

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Materials and Design

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Investigation of dendritic growth and liquation cracking in laser meltingdeposited Inconel 718 at different laser input angles

Yuan Chen a,b, Fenggui Lu a,b, Ke Zhang a,b, Pulin Nie a,b, Seyed Reza Elmi Hosseini a,b, Kai Feng a,b,c,⁎⁎,Zhuguo Li a,b,⁎, Paul K. Chu c

a Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, Chinab Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai 200240, Chinac Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

** Correspondence to: K. Feng, Department of PhysiUniversity of Hong Kong, Tat Chee Avenue, Kowloon, Hon⁎ Correspondence to: Z. Li, Shanghai Key Laboratory o

Modification, School of Materials Science and EngiUniversity, Shanghai 200240, China.

E-mail addresses: fengkai@sjtu.edu.cn (K. Feng), lizg@

http://dx.doi.org/10.1016/j.matdes.2016.05.0340264-1275/© 2016 Published by Elsevier Ltd.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 February 2016Received in revised form 6 May 2016Accepted 11 May 2016Available online 13 May 2016

Nickel-baresulting in the higher sed Inconel 718 (IN718) superalloy is deposited on a polycrystalline substrate bylaser melting deposition. The effects of the laser input angle on the dendritic microstructure, crystal orientationand heat-affected zone (HAZ) liquation cracking tendency are studied by optical microscopy, scanning electronmicroscopy, and electron backscatter diffraction. Larger laser input angles improve the lateral temperature gra-dient, enhance growth of secondary dendrites, and establish firm interconnections among secondary dendritesin the interdendritic regions, resulting in the formation of a “cross-band”morphology inside the grains, especiallylaser input angles of 10° and 20°. Larger laser input angles also improve the homogeneity of heat dissipation flowduring solidification in laser deposition as well as the resulting in the higher crystal orientation. The enhancedinterdendritic bonding improves the cracking resistance of the crystal inner grain and attributed to the decreasein the grain boundary misorientation, the stability of liquation film at the grain boundaries reduces. Owing tothese two reasons, the susceptibility to HAZ liquation cracking is effectively depressed when using larger laserinput angles in laser deposition.

© 2016 Published by Elsevier Ltd.

Keywords:Dendritic microstructureInterdendritic bondingCross bandsCrystal orientationLiquation cracking

1. Introduction

Dendritic growth patterns have fascinated materials scientists for along time and evolution of dendritic microstructure during solidifica-tion has been studied both experimentally and theoretically [1–3]. Nick-el-based superalloys are commonly used in high-temperature industrialcomponents such as turbine blades and discs. The initial dendritic mi-crostructure plays an essential role in the properties of the materialsas it influences themechanical strength, creep resistance, fatigue behav-ior, aswell as structural stability at an elevated temperature up to 650 °C[4]. Laser melting deposition (LMD), an additive welding process capa-ble of producing unique fine dendrites, is an effective repair techniqueto refurbish the worn or damaged components in a near net shapeway [5]. However, owing to the high cooling rate generated during so-lidification in LMD, large thermal stress can be induced resulting inthe formation of hot-cracking in the coating, especially the

cs and Materials Science, Cityg Kong, China.f Material Laser Processing andneering, Shanghai Jiao Tong

sjtu.edu.cn (Z. Li).

interdendritic liquation cracking that can emerge in the heat-affectedzone (HAZ) [6,7].

Since liquation cracking is attributed mainly to the liquation ofinterdendritic low melting eutectic phases, much work has been per-formed to control the formation of these eutectic phases. It has beendemonstrated that S and B can promote HAZ microfissuring because Sreduces the solidus temperature of the liquid films and B decreasesthe melting point of the grain boundary materials during solidification[8,9]. Our previous work has shown that by increasing the cooling ratein the liquid nitrogen during laser deposition, the degree of elementsegregation can be reduced and formation of low-melting Laves phasecan be depressed [10]. However, there have been few investigationson the effects of dendritic morphology on the susceptibility to HAZ li-quation cracking. During the laser welding process, the high-powerlaser beam generates a highly directional temperature gradient field inthe molten pool forming a unique epitaxial dendritic microstructure[11]. In the interdendritic region, growth of secondary dendrites is de-pressed due to the formation of low-melting eutectic phases in thelast stage of solidification. The low-melting eutectic phases in the HAZcan be re-melted and transformed into liquation films by the heat gen-erated in subsequent layer deposition. The cracking resistance of the li-quation film is low and therefore, liquation cracking can be easilyinitiated in the interdendritic region. Meanwhile, the near “straight”

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columnar dendrites and insufficient interdendritic bonding providepaths for the propagation of liquation cracking in the interdendritic re-gion. Hence, besides the high thermal stress generated by the highcooling rate of LMD, the highly directional columnar dendrites with in-sufficient interdendritic bonding contribute to the high liquation crack-ing susceptibility of laser-deposited superalloys [12].

To control the dendritic morphology, Yang et al. [13] have fabricatednickel-based single-crystal superalloys oriented along the ⟨001⟩ and⟨011⟩ lattice directions using a bottom seeding technique in directionalsolidification and the morphologies of the ⟨011⟩ dendrites are differentfrom those of the ⟨001⟩ dendrites, i.e., no typical primary dendrites canbe observed and the dendrites present “V” or “W” morphologies in thelongitudinal section. The inclined primary dendrites grow along the ini-tial crystallographic directions and the side-branches grow along ⟨100⟩crystallographic directions, irrespective of heat-extraction direction [14,15]. Dendritic growth from ⟨001⟩ to ⟨011⟩ can also be achieved by in-creasing the deviation angle between the crystal growth direction andheat-extraction direction to 17° during solidification [15]. In order topredict the dendritic growth pattern of LMD process, Gäumann et al.[16] have derived a simplified columnar-to-equiaxed transition (CET)theory, which states that columnar dendrites can be established whenthe thermal gradient G and solidification velocity V satisfy Gn/V N KCET

everywhere in the molten pool during the solidification, where KCET

and n are material-dependent constants. Recently, Liu and Qi [17,18]have studied the dendritic growth pattern in LMD by means of a 3Dtransient transport phenomenamodel and the results show that growthof dendrites in LMDdepends on the crystallographic orientation, aswellas laser welding parameters such as laser power, welding speed andpowder feeding rate. In this work, to control the dendritic microstruc-ture and liquation cracking tendency, multi-layer Inconel 718 (IN718)clads are fabricated at different laser input angles by LMD. The dendriticmorphology, interdendritic bonding and state of crystal orientation areinvestigated and the effect of laser input angle on the susceptibility toHAZ liquation cracking is also studied.

2. Experimental procedures

The coatings were prepared on casting IN718 superalloy plates(Ф80 mm × 10 mm) with spherical IN718 powders (100 to 150 μm)using a LMD system equipped with a 3.5 kW high power diode laserunit (Rofin DL-035Q). The LMD process at the normal laser inputangle (θ=0°) is shown in Fig. 1a. The powderswere fed into themoltenpool through a coaxial nozzle and the 50 mm long IN718 clad was de-posited at a laser power of 2 kW, laser scanning speed of 5 mm/s andpowder feeding rate of 13 g/min. Argon flow was bled continuously toprotect the molten pool from oxidation and contamination. In order tostudy the effects of laser input angle on the dendritic morphology and

Fig. 1. Schematic of the LMD process at la

liquation cracking susceptibility, three samples were fabricated at tiltangles of 10°, 20° and 30° as schematically illustrated in Fig. 1b.

Longitudinal sections were cut from the four clads, mounted,polished and etched with a solution of 100 ml C2H5OH + 100 mlHCl + 5 g CuCl2. The dendritic microstructure was investigated by opti-cal microscopy (OM, AxioCam MRc5, Carl Zeiss) and scanning electronmicroscopy (SEM, NOVA NanoSEM 230, FEI). The electron backscatterdiffraction (EBSD) samplewas prepared by a standard polishing processfollowed by mechanical and vibration polishing before introducing tothe EBSD system (AZtec HKL Max) operating at an accelerating voltageof 20 kV and step size of 10 μm. To evaluate the liquation cracking sus-ceptibility at different laser input angles, the total crack length (TCL)and total transverse section area (TTSA)were calculated from the trans-verse sections of the deposited IN718 clads using the opticalmicroscopysoftware (AxioVision Rel. 4.8, Carl Zeiss). The TCL and TTSA were calcu-lated from 10 transverse sections of each clad and the section crack rate(TCL/TTSA) was used to evaluate the HAZ liquation crackingsusceptibility.

3. Results and discussion

Fig. 2 depicts the longitudinal micrographs of the as-depositedIN718 samples. During quick solidification of superalloy in the moltenpool in LMD, columnar dendrites grow epitaxially from the substratealong one of the six preferred growth directions of ⟨100⟩ crystallograph-ic orientations of the face-centered cubic (FCC) crystal cells [19]. Fig. 2shows that the longitudinal cross-sectional microstructures consistmostly of columnar dendrites grown epitaxially from the substrate, aswell as a small volume of equiaxed dendrites at the top of these de-posits. The columnar dendritic arrays in Fig. 2b and c show a more reg-ular feature than those in Fig. 2a and d, revealing that the degree ofcrystal orientation increases when the laser input angles are increasedto 10° and 20° during laser deposition. On the top of the deposits, con-tinuation of columnar dendrite growth is broken by the CET transition.It is widely accepted to be caused by increased constitutionalundercooling and enrichment of detached dendrite arms and branchesin the last stage of solidification [20,21]. The CET transition lines arehighlighted by the white dotted lines in Fig. 2 and larger laser input an-gles improve the formation of equiaxed dendrites at the top of the clads.The solidification boundaries of themolten pools indicated by white ar-rows in Fig. 2 show that the sharpness of the solid-liquid interface atlarge laser input angles increases. The molten pool boundary in the op-tical micrograph can be attributed to the liquid flow in the molten poolduring the solidification inducing growth fluctuation of dendrites and insome areas, the metallographical details of molten pool boundaryremain.

ser input angles of (a) 0° and (b) θ°.

Fig. 2. Optical micrographs of the as-deposited IN718 clads fabricated at laser input angles of (a) 0°, (b) 10°, (c) 20°, and (d) 30°. Regions A–D are the EBSD areas. The white dotted linesrepresent the CET transition lines and the white arrows indicate the solidification boundaries of the molten pools.

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Fig. 3 shows the longitudinal microstructure of the as-depositedIN718 clads fabricated at different laser input angles. Owing to thehigh cooling rate of LMD, most of the dendrites produced are verythin, as shown in Fig. 3a.Meanwhile, most of the dendrites are intercon-nected with the neighbouring dendrite arms producing a net-like mor-phology. The magnified details in Fig. 3b show that the interdendriticbonding is attributed mostly to the growth and interconnection of sec-ondary dendrites. Some dendrites, e.g. dendrites in the grain Ga (Fig.3a), show slightly different features. The enlarged details of Ga in Fig.3b reveal that themetallographical difference of Ga is due to insufficientinterdendritic bonding of the secondary dendrites, as highlighted by theblack arrows, where growth of secondary dendrites is highly deficientand interdendritic bonding is thus bare. At a laser input angle of 10°,the columnar dendrites constitute a larger grainGb in Fig. 3c and regularbands are established at an angle of about 38° to the substrate. Fig. 3dpresents the typicalmorphology of the columnar dendrites, i.e., primarydendrites growing vertically to the substrate and secondary dendritesgrowing perpendicularly to the primary dendrite arms [3]. The regularbands in the grains are formed by the cross bridging of secondary den-drites growing from the adjacent primary dendrite arms and we callthese “cross bands”. At a larger laser input angle of 20°, dendrites forma large grain Gc (Fig. 3e) and regular cross bands are established at anangle of about 180° to the substrate. Themagnifiedmetallographical de-tails in Fig. 3f show that the primary dendrites in the region beneath thewhite dotted line grow vertically from the substrate (⟨001⟩ direction)and above thewhite dotted line, the primary dendrites are nearly paral-lel to the substrate (⟨100⟩ direction). At 20°, a large number of primarydendrites grow from the ⟨001⟩ direction to the ⟨100⟩ direction. The den-dritic microstructure at a laser input angle of 30° is shown in Fig. 3g andcross bands can still be observed but not regularly similar to the cladfabricated at 0° (Fig. 3a).

Fig. 4 depicts the SEMmicrographs of the dendriticmicrostructure ofthe clads fabricated at different laser input angles. With larger

concentrations of Nb and Mo than the dendritic matrix, theinterdendritic Laves phase shows a brighter feature and therefore, de-picts themain dendritic morphology of the clads. Fig. 4a shows the typ-ical dendritic microstructure (fabricated at 0°) of the regions withinsufficient interdendritic bonding. Without regular interconnection ofsecondary dendrites, the fine eutectic Laves phase particles in the bareinterdendritic region can easily interconnect and be transformed intolong chain form Laves. Fig. 4b shows the dendritic morphology of theIN718 clad fabricated at a laser input angle of 10°. The vertical thintrunks separated by the Laves particles are primary dendrites and theregularly inclined trunks are cross bands.More detailedmicrostructuresof the cross bands and interdendritic Laves particles are presented in Fig.4c. The secondary dendrite arms are firmly interconnected and theinterdendritic Laves phase particles are largely eliminated from thecross bands. In the regions adjacent to the cross bands, i.e. eutecticbands, Laves phase particles are largely produced. Fig. 4d shows the typ-ical dendritic morphology of the regions containing regular cross bandswith an angle of about 180° to the substrate. Similar to the dendritic mi-crostructure of IN718 clad fabricated at 10°, the enlarged image in Fig.4e reveals that the regularmorphology in Fig. 4d stems from the forma-tion of cross bands and eutectic bands. Fig. 4f shows the dendritic mor-phology of the clad fabricated at 30°. Although the primary dendritesand cross bands are not that clear distinguished, the homogeneouslydistributed Laves phase particles indicate that the interdendritic regionsare regularly bonded.

Fig. 5 shows the morphology of liquation cracking associated withthe low melting phases [22]. On account of minority phases of silicides,carbides and borides, liquation cracking in IN718 is known to be attrib-uted to the constitutional liquation of Laves phase [8,23]. Laves phase isa Nb-rich brittle intermetallic phase, typically represented as (Ni, Cr,Fe)2(Nb, Mo, Ti). It not only depletes the useful alloying elements ofthe dendriticmatrix, but also promotes the formation of liquation crack-ing in the HAZ [24,25].

Fig. 3. Optical micrographs of the dendritic microstructures of the as-deposited IN718 clads fabricated at laser input angles of (a and b) 0°, (c and d) 10°, (e and f) 20° and (g) 30°.

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TCL calculation from the transverse or longitudinal section is widelyused to evaluate the HAZ liquation cracking susceptibility. However, inthe present study, the powder deposition efficiency at different laserinput angles is not the same as revealed by the transverse section area(Fig. 6). That is, the transverse section areas of the clads fabricated at10° and 20° laser input angles are obviously larger than those at 0°and 30°. Therefore, in the present study, we use the section crack rate(TCL/TTSA) to evaluate the HAZ liquation cracking susceptibility andthe results are also shown in Fig. 6. When the laser input angle is in-creased to 10°, the susceptibility to liquation cracking is depressed.While further increasing the laser input angle, the liquation crackingsusceptibility increases somewhat but is still lower than that of 0°. Ingeneral, larger laser input angles suppress HAZ liquation cracking.

In order to understand the crystal orientation of the as-depositedIN718 clads, EBSD is conducted on the longitudinal cross-sections ofthe deposits. Since the equiaxed dendrites at the top of the coatingcan be re-melted and eliminated during the multi-layer deposition,the final crystal orientation of the deposit is determined mostly by theorientation of columnar dendrites. Therefore, EBSD is performed onlyon the columnar dendritic regions of A–D with dimensions of6.5 mm × 0.8 mm, as shown in Fig. 2. The orientation image maps(OIM) of the as-deposited IN718 clads fabricated at laser input anglesof 0°, 10°, 20°, and 30° are presented in Fig. 7a, b, c, and d, respectively.The EBSDmaps are analogous to the optical images in Fig. 2. That is, thecrystals are highly oriented and large crystal grains are presented in thedeposits.

Fig. 4. SEM micrographs of the dendritic microstructures of the as-deposited IN718 clads fabricated at laser input angles of (a) 0°, (b and c) 10°, (d and e) 20°, and (f) 30°.

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In the columnar dendrites, one axis of the crystal is orientated nearlyin the same direction, depending on the orientations of the other twoaxes, both single or multiple grains can be constructed [4]. With regardto columnar dendrites in a single crystal or grain, all three crystal axes ofthe dendrites are parallel to the corresponding crystal axes of all theother dendrites. During LMD, the columnar dendrites grow epitaxiallyfrom the un-remelted sites of the substrate and hence, the crystal orien-tation of the substrate has a profound effect on the crystal orientation ofthe final clad. The substrate used in this study is polycrystalline and it isdifficult to fabricate single-crystal clads directly. However, the laserdiode has a near homogeneous distribution of energy within the beamspot and it generates a more homogeneous heat dissipation field inthe molten pool than that a Gaussian intensity laser source like a fiberlaser or CO2 laser. Consequently, crystal grains established at normal0° in this study are very large, as shown in Fig. 7a and the maximumgrain size can reach 1–1.5 mm. By increasing the laser input angle to

Fig. 5.Morphology of liquation cracking in laser deposited IN718.

10°, the grain size of the clad increases (Fig. 7b) and the maximumgrain G12 reaches about 3 mm. At 20°, the large grain texture isretained, as shown in Fig. 7c, except some stray grains, the crystals inthe grains G20–22 are large single grains. At 30°, formation of straygrains increases and the maximum grain size is reduced, as shown inFig. 7d. Depending on the growth direction of the crystal, single crystalcan form three, four or five spots in the (001) pole figure. Despite theformation of some of the small stray crystals in the grains G7–10 andG17–19 are still generated, but most of the crystals in the deposits fab-ricated at laser input angles of 10° and 20° form three obvious spots inFig. 7g and h, respectively, revealing that the crystals in the clads arehighly oriented and the crystal orientation is enhanced when increasesthe laser input to 10° and 20°.

Fig. 8 shows the heat dissipation field of the molten pool during so-lidification of LMD. At the normal laser input angle (0°), the laser irradi-ation is focused to the top of themolten pool and as a result, most of the

Fig. 6. Distribution of section crack rate at different laser input angles with the insetsshowing the optical microstructure of liquation cracking and transverse section area ofclads corresponding to each bar.

Fig. 7. EBSDmaps of the as-deposited IN718 clads showing the inverse polefigure (IPF) colored OIMmaps of (a) region A, (b) region B, (c) region C, (d) regionD; (e) IPF; (100) polefiguresof (f) region A, (g) region B, (h) region C and (i) region D.

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heat dissipates vertically to the substrate. Owing to the lateral heat dis-sipation, the total heat dissipation flow is not exactly vertical to the sub-strate but at an inclination of φ. Since the thermal conductivity of IN718is much greater than that of air, more heat is dissipated into the side ofpre-deposited layer (lateral heat-dissipation 2) and the total heat dissi-pation flow is therefore inclined into the molten pool, as shown in Fig.8a. By increasing the laser input angle, more laser energy is focusedonto the lateral surface of the molten pool and the lateral temperaturegradient increases. In turn, the lateral heat dissipation to the pre-depos-ited layer (lateral heat-dissipation 2) increases whereas the verticalheat-dissipation diminishes. Consequently, the deviation angle (φ) ofthe total heat- dissipation flow increases, as shown in Fig. 8b.

Fig. 8. Schematic of heat-extractionfields and relationship between theheat-extraction directionvelocity Vhkl generated at laser input angles of (a) 0° and (b) θ°.

During solidification in laser cladding, the solidification velocity ofV100 is nearly equal to the laser scanning speed Vb. Since most of thedendrites grow along ⟨001⟩ direction, the dendrite growth velocityV001 satisfies the following equation:

V001 ¼ V100 � cot α¼Vb � cot α: ð1ÞAccording to the CET theory developed by Gäumann et al. [16], co-

lumnar dendrites growth can be maintained only when the solidifica-tion condition satisfies:

G3:4

VNKCET; ð2Þ

anddendritic growth direction andmagnitudes of thermal gradientGhkl and solidification

139Y. Chen et al. / Materials and Design 105 (2016) 133–141

where G is the thermal gradient, V is the solidification velocity and KCET

is a materials-dependent constant. By increasing the laser input angle,the laser is focused onto the front surface of the molten pool thus in-creasing the steepness of the solidification boundary. Hence, the devia-tion angle α is reduced and the dendrite growth velocity V001 increased.At the same time, the increase of laser input angle reduces the verticalthermal gradient G001 and the columnar dendrite growth conditionexpressed in Eq. (2) can easily be broken. Consequently, more equiaxeddendrites are generated at the top region of the clads.

Fig. 9 shows the schematic growth patterns of the cross bands andthe developed secondary dendrite armsbridgingwith those arms grow-ing from the adjacent primary dendrites and forming the cross bands.With the growth of cross bands, elements with K b 1, for example, Nb,Mo, Si, Ti, etc., diffuse to the adjacent interdendritic regions. Owing tothe enriched alloying elements in these regions, growth of secondarydendrites is depressed whereas the Laves/γ eutectic reaction is en-hanced. As a result, the cross bands are transformed into eutecticbands. Since the Laves/γ eutectic reaction can deplete most of the Nb,Mo, Si, Ti, etc. and when the concentrations of these elements returnto the normal ones proper for the growth of secondary dendrites,cross bands can again be generated. It can be concluded that formationof cross bands and eutectic bands is attributed to the competitivegrowth of secondary dendrites and Laves/γ eutectic particles. On ac-count of larger lateral heat dissipation at larger laser input angles, thedeviation (φ) between the total heat-dissipation flow and primary den-dritic growth increases and the growth of primary dendrites is de-pressed. The secondary dendrite grows much closer to the heat-dissipation flow and its growth is improved resulting in more obviouscross bands formation in the coatings, as shown in Fig. 3c and d.

If the deviation (φ) is increased further, growth of primary dendritescannot be maintained and in order to reduce this deviation φ, the den-drites can grow from ⟨001⟩ to ⟨100⟩, as schematically illustrated in Fig.10. This phenomenon canalso be explainedby theCET theory. As illustrat-ed in Fig. 8, the solidification conditions suitable for the growth of ⟨001⟩and ⟨100⟩ dendrites can be derived fromEqs. (3) and (4) in the following:

G001j j3:4V001j j ¼ Gnj j � sinαð Þ3:4

Vbj j � cot αNKCET: ð3Þ

G100j j3:4V100j j ¼ Gnj j � cosαð Þ3:4

Vbj j NKCET: ð4Þ

Fig. 9. Schematic illustration of the dendrit

As discussed above, the deviation angle α is reduced by increasinglaser input angles. Therefore, |G001 |3.4/V001 decreases and |G100 |3.4/V100 increases and when |G100 |3.4/V100 N |G001 |3.4/V001, the dendriticgrowth transition from ⟨001⟩ to ⟨100⟩ results.

As the substrate is polycrystalline, the original crystalmisorientationcan induce the polycrystalline texture in the final clad, as shown in Fig.7a. However, crystal orientation is improved at larger laser input anglesdue to the better homogeneity of heat-dissipation since only those crys-tals that have one of the six ⟨100⟩ directions that are nearly parallel tothe heat-dissipation flow can grow rapidly during solidification of FCCcrystals. Therefore, the increase of heat-dissipation homogeneity uni-forms the growth of dendrites resulting to an improved crystal orienta-tion. Owing to the cooling effects of pre-deposited alloy and ambient airas shown in Fig. 8, the lateral heat-dissipation tends to proceed on thetwo sides (lateral heat-dissipation flows 1 and 2). Therefore, the localheat flow field at a laser input angle of 0° is not very uni-directional(Fig. 8a). Increasing the laser input angle increases laser radiation ontothe front of the molten pool and only heat that is dissipated to thepre-deposited alloy side (lateral heat-dissipation 2) can have an effecton the heat dissipation of the molten pool. Consequently, the homoge-neity of heat dissipation is improved (Fig. 8b) and the crystal orientationand grain size are enhanced by increasing the laser input angles.

As shown in Fig. 5, liquation cracks in laser deposited IN718 coatingare generated mostly along the long chain form Laves phase. Duringcooling of LMD, the IN718 alloy solidifies as follows [26]:L → L + γ → L + Nb/γ → L + γ → Laves/γ. Owing to the temperaturedifference between the matrix γ phase (~1350 °C [12]) and eutecticLaves/γ compound (~1000 °C [12]), the dendrites in the HAZ remainin solid state whereas the interdendritic Laves/γ re-melt transforminginto an interdendritic liquation film. According to the strain localizationtheory [27], stress can be transmitted by the solid but not liquid. Whenthe interdendritic region is bonded by the secondary dendrite arms,stress can mostly be borne by the dendrite arms. The stress concentrat-ed in the liquated Laves/γ film is minimal and liquation cracking canhardly be generated. When growth of the secondary dendrites in theinterdendritic region is bare, the Laves particles are interconnectedtransforming into the long chain form Laves phase particles, as shownin Fig. 4a. Without the interdendritic bonding, a large amount of stresswould be localized at the liquated long chain form Laves/γ film andwhen the stress exceeds the tensile limit, the liquation film will bepulled apart and liquation cracking is generated. In this study, largerlaser input angles improve the lateral heat dissipation. Growth of the

ic growth patterns of the cross bands.

Fig. 10. Schematic view of the dendritic growth transition from the ⟨001⟩ to⟨100⟩ directions.

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secondary dendrites in the interdendritic region is enhanced therebyimproving the interdendritic bonding (especially due to the formationof “cross bands”).The enhanced interdendritic bonding also preventsthe formation of the long chain form Laves phase particles. Consequent-ly, the inherent cracking resistance of the clads increases and the HAZliquation cracking tendency decreases.

Interdendritic bonding affects mostly the cracking resistance of thedendrites in the grains and as shown in Fig. 3, most of the “crossbands” are established in the grains but not at the grain boundaries.The grain boundaries are weak sites where liquation cracking can begenerated due to the insufficient bonding. Fig. 7 shows that the quantityand misorientation angle of the grain boundary at laser input angles of10° and20° are larger than those at 0° and 30°. Corresponding to thedis-tribution of section crack rate in Fig. 6, the susceptibility to liquationcracking depends on the grain boundary misorientation. That is, the li-quation cracking susceptibility increases with increased grain boundarymisorientation as consistent with our previous findings [12].

Although the effects of grain boundary misorientation on the sus-ceptibility to theHAZ liquation cracking are seldom taken into consider-ation, recent study results reveal that grain boundary misorientationmay have a profound effect on liquation cracking. Carter et al. [28]have fabricated a nickel-based superalloy coating by a unique ‘island’scanning strategy in LMD and demonstrated that most of the cracksare generated in the chaotic fine grain regions. Previously, we have in-creased the base cooling effect by imposing the continuous coolingwater on the back of the substrate during LMD process. The resultsshow that the crystal orientation of the clads increases and with re-duced grain boundary misorientation, the susceptibility to HAZ liqua-tion cracking reduces [29]. Wang et al. [30] have studied thesolidification cracking behavior of laser welded single- and bi-crystalswith pure tilt grain boundary angles. Solidification cracks are absentfrom single crystals and low angle bi-crystals but produced beyond acritical grain boundary angle of 13°.

Rappaz et al. [31] have established a theoretical coalescence theoryon the last stage of solidification. The dendritic bonding behavior is in-herently related to the grain boundary energy γgb and solid-liquid inter-facial energy γsl. When γgb N 2γsl, the last remaining liquid remainsstable at a lower temperature. It is referred to as coalescenceundercooling ΔTb and defined as:

ΔTb ¼ γgb−2γsl

Δs f � δ; ð5Þ

where ΔSf is the fusion entropy and δ is the thickness of the isolatedsolid-liquid interface. According to Read-Shockley's theory [32], the

grain boundary energy can be estimated as a function of grain boundaryangle θ:

γgb ¼ Gbθ4π 1−vð Þ 1þ ln

b2πr0

− lnθ� �

; ð6Þ

whereG is the shearmodulus,ν is the Poisson ratio, b is the Burgers vec-tor, and r0 is the core radius that encompasses the energy of the disloca-tion core. According to Eqs. (5) and (6), when the grain boundarymisorientation increases, dendrite coalescence undercooling increasesand the stability of the liquation film is enhanced. Consequently, the li-quation film at large angle grain boundary remains stable at a lowertemperature. Then large amount of tensile tress will localize on the li-quation film and the susceptibility to HAZ liquation film increases. Ourresults demonstrate that larger laser input angle improves the homoge-neity of heat dissipation flow, reduces the grain boundary misorienta-tion, and suppresses HAZ liquation cracking.

4. Conclusion

The laser input angle affects the dendriticmorphology, crystal orien-tation and liquation cracking tendency. Larger laser input angles in-crease the lateral temperature gradient thereby fostering the growthof secondary dendrites and enhancing the interdendritic bonding, espe-cially in “cross bands”. Regular “cross bands” are presented at 38° and180° to the deposits fabricated at laser input angles of 10° and 20°, re-spectively. Dendritic growth from ⟨001⟩ to⟨100⟩ is observed at a laserinput angle of 20°. The crystal orientation increases at large laser inputangle on account of the improved homogeneity of heat dissipationflow. The enhanced secondary dendrite growth increases theinterdendritic bonding in the grains and reduces the stability of the li-quation film at grain boundaries due to decreased grain boundary mis-orientation. Consequently, the susceptibility toHAZ liquation cracking issuppressed by increasing the laser input angle during laser deposition.

Acknowledgements

The authors acknowledge fundings from the National Natural Sci-ence Foundation of China (Grant No. 51171116), City University ofHong Kong Applied Research Grant (ARG) No. 9667104, and HongKong Research Grants Council (RGC) General Research Funds (GRF)No. CityU 11301215 as well as analysis support by Y.F. Gu, L.Z. Hong,and Z.Q. Bao of Instrumental Analysis Center at SJTU.

141Y. Chen et al. / Materials and Design 105 (2016) 133–141

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