Microstructure Evolution of Laser Repair Welded

Rene 77 Nickel-Based Superalloy Cast

Huei-Sen Wang1;*, Chih-Ying Huang1, Kuen-Sen Ho2 and Sian-Jhih Deng1

1Department of Materials Science and Engineering, I-Shou University, Kaohsiung, 84001, Taiwan, R. O. China2Department of Mold and Die Engineering, National Kaohsiung University of Applied Sciences,Kaohsiung, 80711, Taiwan, R. O. China

This study performed repair-welding tests on Rene 77 Nickel-based superalloy by adopting a Nd:YAG pulsed laser technique, coupledwith Rene 41 filler wire and various welding parameters. After welding, the microstructure evolution, crack behavior and mechanical propertiesof the welds were determined by a combination of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS),transmission electron microscopy (TEM), and micro-hardness test. The experimental results showed that, due to suffering from different thermalhistories during the conduction of the multi-pass welding process, the microstructures within the welds had split into six zones. In terms ofcracking behavior, no cracks formed in the weld fusion zone (WFZ) or the heat-affected zone (HAZ) under appropriate welding parameters;however, setting the laser energy too high resulted in weld centerline cracking or intergranular cracking in the weld fusion zone. Moreover,various types of liquation cracks may occur at the interface between the WFZ and the HAZ. [doi:10.2320/matertrans.M2011264]

(Received August 29, 2011; Accepted September 27, 2011; Published November 16, 2011)

Keywords: super alloy, welding, microstructure, crack

1. Introduction

Rene 77 is a precipitation hardening (� 0 strengtheningphase-Ni3(Al, Ti)) and solid solution strengthening (Cr, Mo,Fe, Co elements) Ni-based superalloy,1) with a designedservice temperature of between 730�C�900�C, often appliedto hot gas turbine engine parts, such as vanes and blades, dueto its remarkable hot corrosion resistance and excellent hightemperature strength. Similar to every Ni-based superalloycast component, some non-critical defects such as surfacecracks may occur during the investment casting or afterservice. These cracks may substantially result in a capital lossor reduction in service life. For economic considerations,these non-critical defects may be scraped and repaired by thewelding techniques.

Among the welding techniques, the Gas Tungsten ArcWelding (GTAW or TIG) welding process is widely used forsurface crack repairs of the precipitation hardening super-alloy cast, due to its lower facility cost. Earlier attempts wereconducted2,3) to investigate the heat-affected zone (HAZ) orinterface liquidation crack after the GTAW or TIG weldingprocess. For a superalloy containing high Al and Ticoncentrations (� 0 formers), due to the presence of thestrengthening phase, ordered Ll2 Ni3(Al, Ti) � 0 phase, therepaired cast tends to be susceptible to hot cracking in theweld metal and liquation cracking in the HAZ during thetraditional GTAW or TIG repair welding process. Suscepti-bility to cracking was attributed mostly to large shrinkagestresses caused by the higher energy input of the TIG weldingtechniques, as a result of large amounts of rapid � 0

precipitation during the cooling process. Recently, a fewstudies,4–6) indicated that a laser welding process, a lowerenergy input, high energy density, and a partial heatingprocess, might eliminate the abovementioned problems andcreate a porosity-free and crack-free weldment, described

with good metallurgical bonding, low dilution, and a narrowHAZ.

However, most of those studies focused on the crackingcharacterization at the interface of the weld fusion zone(WFZ) and the HAZ. Very few papers have systematicallydiscussed the microstructure evolution in the WFZ and HAZof the laser welds. For instance, as a multi-pass laser weldingprocess is conducted, due to the different portions of the WFZor HAZ experiencing different thermal histories during thelaser repair welding, they may display specific differences intheir microstructures. The WFZ and HAZ may be furthersub-divided into multiple regions. Therefore, the repairedRene 77 cast under pre-selected Nd:YAG (Neodymium-doped Yttrium Aluminum Garnet) laser welding parameterswas conducted in this study. After welding, the micro-structure development, crack behavior, and mechanicalproperties of the various regions in WFZ and HAZ wereinvestigated.

2. Experimental Method

This experiment used a set of nickel-based superalloyRene 77 turbine vanes (Table 1 shows the composition). Theouter shroud was cut off, and then machined into 30 mm�15 mm� 2 mm test specimens for laser repair welding. Thespecimen surface was cleaned with alcohol prior to repairwelding.

For the filler wire selection, matching the chemicalcomposition of the base metal is generally desirable.However, for a highly strengthened nickel alloy cast (e.g.,Rene 77), creating a small diameter laser wire with the samechemical composition as the base metal is difficult to achieve,and is not cost effective. The use of a substitute filler materialfor the nickel-based gas turbine vane repair has becomeacceptable. A general overview of substitute filler materialsfor the nickel-based alloy repair process can be found inGandy et al.7) In this study, based on the suggestion from GE*Corresponding author, E-mail: huei@mail.isu.edu.tw

Materials Transactions, Vol. 52, No. 12 (2011) pp. 2197 to 2204#2011 The Japan Institute of Metals

aviation internal special print report-(No. 1536M51), Rene41 (composition shown in Table 2) �0:5 mm wire wasselected as the filler material due to its similar compositionto Rene 77, and because of better formability in creating asmaller diameter wire for the laser welding.

To optimize the laser pulse shape, various weldingparameters was empirically investigated. Welding ConditionA, B and C shown in Table 3 were pre-selected parameters inthis study. Conditions A and B were selected due to thoseparameters produce crack-free welds. If the energy input isslightly higher than Condition B (e.g. Condition C), the crackmore likely form in the weld. Furthermore, to obtain aconstant welding speed, the welding process was assisted bya semi-automatic trolley. Five welding beads were depositedto conduct the repair welding process. Each welding beadwas overlapped by the successive melt bead. The overlappingratio of the track is approximately 30%. The ratio was chosenby considering the processing efficiency and surface homo-geneity. After the repair welding, the sectioned pieces wereprepared by grinding, polishing, and etching with a 70%phosphoric acid electrolyte solution. The microstructures inthe HAZ and WFZ were then studied by optical microscope(OM), Hitachi scanning electron microscopy (SEM) equip-ped with an energy dispersive X-ray spectroscopy (EDS) andPhilip Tecnai G2 transmission electron microscopy (TEM).Vicker’s micro-hardness in the HAZ and WFZ was measuredusing a micro-hardness tester under a load of 100 g.

3. Results and Discussions

3.1 Microstructure of the ‘‘as-received’’ cast Rene 77As shown in Fig. 1(a), in addition to the � matrix, Rene 77

consists of typical diced cube shape � 0 intermetallic phases(average grain size is 0.3 mm), evenly distributed within thedendrite or interdendrite area. Smaller volume fractions ofcarbides (see Fig. 1(b)) and �-� 0 eutectic phases (seeFig. 1(c)) are also present. The energy dispersive X-rayspectroscopy (EDS) compositional analysis shows thatcarbides are predominantly the MC-type, enriched in Ti

Table 2 Composition of Rene 41 wire.

Element Ni Cr Mo Al Ti Co

Mass% 54.42 18.54 9.33 1.58 3.21 10.56

Element Fe B Si S C Mn

Mass% 1.63 0.005 0.05 <0:0005 0.07 0.03

Table 1 Composition of the ‘‘as received’’ Rene 77 cast.

Element Ni Cr Mo Al Ti Co

Mass% 58.4 14.63 4.2 4.3 3.4 14.5

Element Fe Zr Si S C Mn

Mass% 0.5 0.06 0.2 0.015 0.07 0.15

Table 3 Repair welding parameters adopted in this experiment.

Parameter

Laser

voltage

(V)

Pulse

duration

(ms)

Peak

power

(W)

Frequency

(Hz)

Speed

(m/s)

Spot

size

(mm)

Shielding

gas

Shielding

gas flow

(l/mm)

A 170 4.6 1� 103 14 1� 10�3 0.4Pure

Argon5

B 180 4.6 1:3� 103 14 1� 10�3 0.4Pure

Argon5

C 190 4.6 1:6� 103 14 1� 10�3 0.4Pure

Argon5

Fig. 1 Cast Rene 77 microstructure of (a) � and � 0 phases; (b) TiC carbide;

and (c) eutectic phase.

2198 H.-S. Wang, C.-Y. Huang, K.-S. Ho and S.-J. Deng

elements. The �-� 0 eutectic phase, typically formed frommicro-segregation during the final solidification of thecasting process, is observed along the interdendritic area.8)

3.2 Microstructure changes after laser repair weldingThis study conducted the repair welding process by

depositing five welding beads. A new incoming melt beadoverlapped a solidified weld bead in succession. After thesecond-pass welding, the experiment repeated the solid-ification mode of the successive weld bead. Figure 2illustrates the microstructure change after a two-pass weldwhich simplifies and describes the multi-pass welding.Figure 2 also shows that due to the different portions of thewelds undergoing a different thermal history during Nd:YAG

laser repair welding, they display specific differences in theirmicrostructures, and thus, are metallurgically divided into sixmajor regions. The first three regions, along with RegionFive, are within the WFZ. Region Four is the HAZ, andRegion Six is the unaffected parent metal (PM). Themicrostructures in relation to each of these regions aredescribed as follows:3.2.1 Region one

The region marked as Region One is the fusion area that isnot reheated, and has a primary microstructure of the WFZ.Due to the low power of Nd:YAG laser welding, the selectedwelding conditions did not produce the significant differencein the morphology of solidification microstructure withinRegion One. Many studies9,10) have indicated that themorphology of solidification microstructure in this area isaffected by the temperature gradients (G) and grain growthrate (R) differences. The ratio of temperature gradient overgrain growth rate (G/R) is called the solidification parameter.If G/R rises, the final solidified pattern tends to be planar/columnar/cellular grains; alternately, lower G/R leads todendrites or equiaxed dendrites. Due to the low power andhigh density of laser welding, higher temperature gradients(G) are present after repair welding, resulting in a higherG/R. Therefore, columnar and cellular grains dominate thesolidified structures within the region. Figure 3 shows themicrostructure solidification morphology within Region Oneas long and narrow, and the parallel line where the columnargrains are primarily growing along the heat flow direction(see Fig. 3(a)). Earlier study indicated,11) as the HAZliquation initiated, that a crack may propagate along theparallel line-shaped grain boundaries into the WFZ. Thelong columnar grain in Region One may be susceptible to

Fig. 2 Microstructure illustration of Rene 77 after Nd:YAG repair

welding. The first three regions, along with Region Five, are within the

weld fusion zone (WFZ). Region Four is the heat affected zone (HAZ), and

Region Six is the unaffected parent metal (PM).

Fig. 3 The microstructure solidification morphology within Region One. (a) At short distance away the weld fusion line, (b) at the center

of Region One, (c) at the surface of Region One, and (d) Fig. 3(b) at a higher magnification of �20;000.

Microstructure Evolution of Laser Repair Welded Rene 77 Nickel-Based Superalloy Cast 2199

liquation cracking or intergranular cracking, and is highlysensitive to heat input via the laser parameters; thisphenomenon was observed when welding Condition C wasused, as described in Section 3.3.1. In addition to columnargrains, cellular grains are present in the center or at thebottom of Region One (see Fig. 3(b)).

The product of the temperature gradient (G) and graingrowth rate (R) is shown as G� R,10) and is the cooling ratethat governs the size of the final solidified grain. In RegionOne, a rapid cooling rate causes the spacing of the grainsto constitute approximately 2–5 mm, less than that of theoriginal superalloy substrate (approximately 300–400 mm)11)

or the conventional TIG welding process (approximately 30–50 mm). Further examination of the Region One surfaceshows that the surface grains are even finer (about 2 mm) dueto the fast cooling rate (see Fig. 3(c)). Figure 3(d) shows ahigher magnification of Fig. 3(b). The � 0 phase is not clearlyapparent within the grain area, even at �20;000. Region Onemight mainly consist of the � phase, supersaturated with � 0

phase elements (e.g., Al and Ti). To confirm this inference,this study conducted more precise transmission electronmicroscopy (TEM) examinations. Figures 4(a) to 4(d) showthe TEM bright field images (BFIs), and the selected areadiffraction patterns (SADs) of Region One when weldingCondition A (the lower energy input) and welding Condition

C (the higher energy input) were employed. TEM resultsconfirm the inference of the SEM that the region consists ofthe majority of the � austenite phase. Thus, from the Vicker’smicro-hardness test results (see Fig. 5), hardness in thisregion is softer in comparison to the HAZ and PM, which isattributed to the disappearance of the � 0 phase in Region One.

In addition to the � phase matrix, several white and nano-size particles (see Fig. 3(d)) are mainly distributed along

Fig. 4 The TEM observation in Region One: (a) bright field image (BFI) and (b) the corresponding selected area diffraction (SAD) pattern

of � phase in the [00�11] zone axis obtained from welding Condition A; (c) bright field image (BFI) and (d) the corresponding selected area

diffraction (SAD) pattern of � phase in the [�11�110] zone axis obtained from welding Condition C.

Fig. 5 Hardness profiles in Rene 77 welds.

2200 H.-S. Wang, C.-Y. Huang, K.-S. Ho and S.-J. Deng

the grain boundaries, suggested by earlier study11) that theparticles are MC-type carbides. This type of carbide typicallyforms at the end of solidification by eutectic-type reactionswith the � matrix, and is typically distributed along the grainboundary region.12,13)

3.2.2 Region two and region threeThe region marked as Region Two is the unmixed zone

(UZ) located in the outer extremities of Region One.Figures 6(a)–6(c) show the UZs of three welding conditions.As shown in the figures, the higher energy inputs were used,the wider UZs were obtained.

The UZ in Region Two is a boundary layer12) which sharesa similar composition with PM. Figure 6(d) shows EDS linescans (Mo composition profiles) across the fusion boundaryof a Rene 77 welded with Rene 41 filler wire, suggestingstagnant melted Rene 77 based metal unmixed with Rene 41filler wire.

The formation of the UZ can be attributed to the lowerenergy power of the Nd:YAG laser welding process. Duringthe Nd:YAG welding process, the filler wire may preferen-tially absorb the majority of input energy. The remainder ofthe energy is sufficient to melt parts of the PM, but too weakfor thorough mixing with the melted filler wire.12)

In the UZ, the phase-to-cellular grains transition, withbarely visible side arms, occurs near the weld fusion line.The grains grow epitaxially from the fusion line, along theeasy-growth direction to the next fusion line.

In cases of multi-pass welding, the second pass weld beadwould remelt the ratio of the first pass weld bead, and repeatthe first pass solidification mode. As shown in Fig. 6(d), theUZ solidification mode occurs in the second pass (RegionThree in Fig. 2) remelt area. Like Region Two, the selectedwelding conditions did not exhibit significant difference inthe morphology of solidification microstructure in RegionThree; however, if the higher energy inputs were used, thewider Region Three resulted.

3.2.3 Region four and region fiveThe area marked as ‘‘Region Four’’ is the heat-affected

zone (HAZ). For a conventional TIG welding process, whichhas high power, a slow cooling rate, and a wide heat exposurearea, a wide HAZ can form. The HAZ in the TIG weld isfurther divided into several zones, such as the grain coarsenof � 0, the partial reversion of � 0, and the full reversion of � 0.10)

An Nd:YAG laser welding process creates a high temperaturegradient, and therefore, the HAZ is not as evident undercommon OM; however, � 0 grain changes in the HAZ can beobserved under the higher magnification of the SEM. Asdemonstrated in Figs. 7(a)–7(c), the � 0 grains around theWFZ produced by the selected welding conditions, are onlyslightly coarsened; this slightly affects the hardness in thisarea (see Fig. 5).

When joined by a multi-pass welding, the successive passoverlap results in a HAZ in the previous weld pass, whichis marked, ‘‘Region Five’’. Similar to Region Four, RegionFive from the selected Nd:YAG laser welding conditions isnot clear, with only slightly coarsened grains. Figure 7(d)shows an example of Region Four obtained from weldingCondition C.

3.3 Weld-cracking examination after laser repairwelding

The repair welding results show that no defects occur afterrepair welding in welding Conditions of A and B; however,in Welding Condition C, the use of higher power causescracking in both the WFZ and the HAZ of Welding ConditionC. Observations of cracks in the WFZ and the HAZ aredetailed below:3.3.1 Cracks at interface of the HAZ and the WFZ

In this study, the HAZ cracks involve various liquationcracks derived from the intergranular � 0 phase, MC-typecarbide, and �-� 0 eutectic phases, and usually occur at theinterface of the WFZ and HAZ.

Fig. 6 The UZs located in the outer extremities of the Region one obtained from the welding (a) Condition A, (b) Condition B, and (c)

Condition C. (d) An example of EDS line scan (Mo composition profile from welding Condition C) across the fusion boundary of a Rene 77

welded with Rene 41 filler wire; (e) an example of the UZ observed in the successive pass remelt area obtained from welding Condition C.

Microstructure Evolution of Laser Repair Welded Rene 77 Nickel-Based Superalloy Cast 2201

High-power density characteristics of the Nd:YAG laserwelding heat or cool the alloy more rapidly compared to othertraditional welding processes (e.g., the TIG welding process);non-equilibrium is obtained. It is well known11) that rapidheating may increase the liquidus of an alloy to several tensor even hundreds of degrees. Especially if a higher energyinput is used, this promotes even more severe localizedmelting of the intergranular � 0 phase and low temperature �-� 0 eutectics or dissolution of carbides in the HAZ. Duringsubsequent rapid cooling, the higher heat input furtherinduces the greater thermal strain and higher shrinkage stressby rapid solidification. As a result of these factors, the cracksoriginating from the intergranular � 0 phase (see Fig. 8(a)),�-� 0 eutectics (see Fig. 8(b)), or carbides (see Fig. 8(c))2,3,14)

located on the weld fusion line and propagating along theparallel line-shaped grain boundaries into the WFZ.3.3.2 Examination of WFZ cracking defects

This study examined two types of WFZ solidificationcracking: (1) intergranular cracking at the interface of theoverlapping joint in multi-pass welding (see Fig. 9(a)); and(2) weld centerline cracking at the weld bead (see Fig. 9(b)).Research failed to discover evident liquation in eithercracking patterns, and both cracks occurred at a slight higherwelding power (Parameter C).

As stated in 3.2, when a new incoming welding passoverlaps a solidified weld bead, an UZ and a HAZ appear atthe interface of the overlapping joint. Once solidificationstress and welding thermal stress in these zones exceed thematerial strength, intergranular cracking occurs easily at theoverlapping joint.

Weld centerline cracking at the weld bead is primarilylocated where the weld metal finally solidifies, as the meltingpoint and strength decrease easily,12,13) due to lower melting

point solute buildup in this zone, as well as greater thermalstress derived from the higher welding power, which initiatescracking at the weld centerline area.

In summation, the WFZ and HAZ cracking formationsare mainly the compounded results of material strength andcrack driving force in the zone (including stress or strain).Therefore, less liquation, less stress buildup, or less strainduring welding should reduce occurrences of cracking.Cautious choices of laser welding parameters become asignificant factor. In this study, the use of Conditions A and Bobtained crack-free WFZs and HAZs (see Fig. 10), thoughthe intergranular � 0 phase, or carbides, were present on theweld fusion lines of both laser welded samples.

4. Conclusions

This study performed repair welding on the Rene 77 byNd:YAG laser welding under various parameters, coupledwith a Rene 41 filler wire, and upon completion, examinedthe microstructure and cracking behavior. The conclusionsare as follows:(1) As the multi-pass welding process is conducted, differ-

ent portions of Rene 77 welds undergo diverse thermalexposures during Nd:YAG laser repair welding. Thewelds have specific dissimilarities in their microstruc-tures, and thus, are divided into six zones.

(2) As Nd:YAG laser welding causes higher temperaturegradients, the primary microstructure morphologieswithin Region One (the area in a solidified weld thatis not reheated by the successive melt bead) arecolumnar or cellular. Furthermore, a faster cooling rateleads to the finer microstructures in this region afterNd:YAG laser welding.

Fig. 7 The heat-affected coarsened � 0 phase in Region Four obtained from the welding (a) Condition A, (b) Condition B, and

(c) Condition C, (d) a slightly coarsened grains observed in Region Five obtained from welding Condition C.

2202 H.-S. Wang, C.-Y. Huang, K.-S. Ho and S.-J. Deng

(3) The SEM and TEM observations indicate that RegionOne consists of the majority of the � austenite phase,and � 0 precipitate appears almost absent in this zone.Thus, hardness in this zone is softer in comparison tothe HAZ and PM areas.

(4) Due to the low power of the Nd:YAG laser, the HAZ isnarrow, and has only a slight coarsening of the � 0 phaseor grains in this zone.

(5) When Nd:YAG laser welding power is improperlycontrolled, the intergranular � 0 phase, carbide, or(� þ � 0) eutectic phase, as derived from liquationcracking, can easily occur at the interface of the WFZand HAZ.

(6) Two major WFZ cracks are in this study: the first is anintergranular cracking at the interface of the over-lapping joint in the multi-pass welding. The secondis a weld centerline cracking at the weld bead. Theformation of the cracks in this zone and welding power,cracking region composition, and welding stress orstrain are primarily closely related.

Fig. 9 WFZ solidification cracking (a) intergranular cracking at the

interface of the overlapping joint in multi-pass welding; and (b) weld

centerline cracking at the weld bead.

Fig. 8 Various HAZ liquation cracking derived from (a) � 0 phase; (b) TiC

carbide; (c) � þ � 0 eutectic phase.

Fig. 10 The use of the welding (a) Conditions A and (b) Condition B

obtained crack-free WFZs and HAZs, though the intergranular � 0 phase,

or carbides, were present on the weld fusion lines of both laser welded

samples.

Microstructure Evolution of Laser Repair Welded Rene 77 Nickel-Based Superalloy Cast 2203

REFERENCES

1) K. S. Ho and W. S. Hwang: J. Cast. Metal. Res. 15 (2002) 331–336.

2) O. A. Ojo and M. C. Chaturvedi: Mater. Sci. Eng. A 403 (2005) 77–87.

3) O. A. Ojo, N. L. Richards and M. C. Chaturvedi: Scr. Mater. 50 (2004)

641–646.

4) R. G. Ding, O. A. Ojo and M. C. Chaturvedi: Intermetallics 15 (2007)

1504–1510.

5) M. Pang, G. Yu, H. H. Wang and C. Y. Zheng: J. Mater. Process.

Technol. 207 (2008) 271–275.

6) M. F. Chiang and C. Chen: Mater. Chem. Phys. 114 (2008) 415–419.

7) D. W. Gandy, G. Frederick, J. T. Stover and R. Viswanathan: ASM

Materials Solutions Conference & Exposition, Energy & Utilities

Program, (ASM International, St. Louis, 2000).

8) D. C. Madeleine: The Microstructure of Superalloys, (Gordon and

Breach Science Phblishers, Amsterdam, 1997) pp. 33–37.

9) J. F. Lancaster: Metallurgy of welding, 6th ed., (Abington Publishing

Ltd., Cambridge, 1999) pp. 193–195.

10) S. Kou: Welding Metallurgy, 2nd ed., (John Wiley & Sons, Inc., New

Jersey, 2003) pp. 76–387.

11) M. Zhong, H. Sun, W. Liu, X. Zhu and J. He: Scr. Mater. 53 (2005)

159–164.

12) C. T. Sims and W. C. Hagel: The Superalloys, (Wiley-Interscience,

New York, 1972) pp. 513–514.

13) O. A. Idowu, O. A. Ojo and M. C. Chaturvedi: Mater. Sci. Eng. A 454–

455 (2007) 389–397.

14) J. C. Lippold, S. D. Kiser and J. N. Dupont: Welding Metallurgy and

Weldability of Nickel-Base Alloys, (John Wiley and Sons, Inc., New

Jersey, 2009) p. 127.

2204 H.-S. Wang, C.-Y. Huang, K.-S. Ho and S.-J. Deng