201October 2010

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ELECTRON BEAM WELDING AND LASER-TIG HYBRIDWELDING OF TZM-ALLOY

Santosh KSantosh KSantosh KSantosh KSantosh Kumarumarumarumarumar, Anjan Chatterjee, C.S. Viswanadham,, Anjan Chatterjee, C.S. Viswanadham,, Anjan Chatterjee, C.S. Viswanadham,, Anjan Chatterjee, C.S. Viswanadham,, Anjan Chatterjee, C.S. Viswanadham,K. Bhanumurthy and G.K. DeyK. Bhanumurthy and G.K. DeyK. Bhanumurthy and G.K. DeyK. Bhanumurthy and G.K. DeyK. Bhanumurthy and G.K. Dey

Materials Science Division

This paper was awarded the D&H Secheron Award 2009 for BestThis paper was awarded the D&H Secheron Award 2009 for BestThis paper was awarded the D&H Secheron Award 2009 for BestThis paper was awarded the D&H Secheron Award 2009 for BestThis paper was awarded the D&H Secheron Award 2009 for BestPresentation at the National Welding Seminar held at Mumbai, duringPresentation at the National Welding Seminar held at Mumbai, duringPresentation at the National Welding Seminar held at Mumbai, duringPresentation at the National Welding Seminar held at Mumbai, duringPresentation at the National Welding Seminar held at Mumbai, during

Feb.4-6, 2009Feb.4-6, 2009Feb.4-6, 2009Feb.4-6, 2009Feb.4-6, 2009

AbstractAbstractAbstractAbstractAbstract

Joining of TZM alloy was performed by Electron Beam Welding (EBW) and Laser-TIG hybrid welding. The weld joint was

characterized by optical microscopy, scanning electron microscopy, microhardness measurements and room temperature

tensile test. The fusion zone (FZ) shows coarse solidification microstructure and the heat affected zone (HAZ) shows

coarse recrystallized microstructure against the elongated wrought microstructure of the parent metal (PM). There is

significant drop in the hardness of the FZ and HAZ (~ 200 – 230 VHN) as compared to that of the parent metal (~ 290

– 300 HVN). Room temperature tensile strength of the weld joint was ~ 40 – 45% as compared to that of the PM. The

weld joint shows significant drop in tensile ductility (< 1%) as compared to the PM (~ 8.4% tensile ductility). The

fracture was predominantly intergranular in nature.

KKKKKeywordseywordseywordseywordseywords: TZM, EBW, Laser-TIG Hybrid Welding, Tensile Strength, Microstructure

IntroductionIntroductionIntroductionIntroductionIntroduction

Molybdenum (Mo) is a refractory metal (melting point

2623 oC) and Mo based alloys have excellent high

temperature mechanical properties. Therefore, there are

many potential applications of these alloys in compact

high temperature reactors (CHTR) and fusion reactors. TZM

is a Mo based alloy with small content of Ti (0.50 wt%),

Zr (0.08 wt%) and C (0.04 wt%). Ti and Zr provide solid

solution strengthening [1] and most importantly form fine

carbide precipitates which improve creep resistance of

the material. Mo forms Mo2C which is reported to improve

cohesion of the grain boundaries [2, 3]. Besides, carbon

is reported to decrease segregation of the trace oxygen

on the grain boundaries. The segregation of bulk oxygen

and nitrogen in the intergranular region is one of the

important culprits responsible for poor ductility of this

alloy in recrystallized, coarse grained structure.

Welding of this material is envisaged as heat sink material

in high temperature reactors. However, welding of this

material is a challenging task; considering its high melting

point (more than 2500 oC), high thermal diffusivity and

high reactivity towards oxygen and nitrogen leading to

weld embrittlement [4]. High melting point requires more

heat to be deposited at the joint line for fusion welding;

but high thermal diffusivity causes heat dissipation at a

very rapid rate away from the joint line. Therefore, one

requires high intensity heat sources like electron beam

and laser beam. In case of laser welding of TZM there is

additional difficulty as highly conducting materials have

high reflectivity as well and this makes loss of significant

proportion of incident laser beam energy by reflection.

The material being extremely prone towards embrittlement

by even small amount of oxygen and nitrogen means

welding should be done under either high vacuum or

very good inert gas shielding.

Welding of TZM alloy by Electron Beam Welding (EBW),

Laser Beam Welding (LBW) and Gas Tungsten Arc Welding

(GTAW) have been reported in the literature [2, 5]. In the

present work welding of 1.2 mm thick plates of TZM in

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wrought condition has been carried out using EBW and

Laser-TIG hybrid welding processes. Hybrid welding is a

process in which two heat sources of complementing

characteristics are combined to harness synergistic benefits

of this combination. In Laser-TIG hybrid welding system a

focused laser beam is combined with a wide TIG arc. The

TIG arc produces a wide but shallow melt pool and

therefore, has joint gap bridging capability. Focused laser

beam on the other hand has high penetrating capability,

but requires very high joint fit up. The Laser-TIG hybrid

combination thus combines the strengths and eliminates

the limitations of the individual welding processes. Also,

the TIG arc creates a wide melt pool improving the

coupling of the incident focused laser beam and facilitates

deeper welding. This combination has special significance

in case of TZM which has very high reflectivity for the

incident laser beam and in the absence of the melt pool

created by the TIG arc most of the incident laser energy

will simply get reflected from the surface. Besides, this

combination acts as an economical power source

producing a deep and sound weldment having

characteristics of both laser welding (deep and narrow

weldment) and arc welding (nice weld bead appearance).

Therefore, it is not surprising that this welding process is

attracting the interests of materials community. This is

the first work on Laser-TIG hybrid welding of TZM. This

paper presents the details of the welding processes

employed in this study and the microstructural analysis

and mechanical property evaluation of the resulting weld

joints.

ExperimentalExperimentalExperimentalExperimentalExperimental

TZM plates of 1.2 mm thickness in rolled and stress relieved

were used to for these experiments. The plates were cut

in the direction normal to the rolling direction into pieces

of 80 x 20 mm. These plates were used to produce square-

butt weld joints by Laser-TIG hybrid welding.

The welded samples were prepared for metallographic

examination. A solution of Lactic Acid, Nitric Acid and

HF in 6:2:1 proportion was used as echant.

Metallographic examination was done using optical

microscopy and scanning electron microscopy (SEM).

Tensile testing was done to determine yield strength,

ultimate tensile strength and tensile ductility of the weld

joints in the direction perpendicular to the welding

direction. For this specimen were made using EDM wire

cutting, keeping the weldment in the centre of the

specimen. Tensile testing was done at a strain rate of 8x10-

5 s-1. Microhardness was measured across the parent metal,

HAZ and weldment to examine the variation in the

hardness across these regions.

Results and DiscussionResults and DiscussionResults and DiscussionResults and DiscussionResults and Discussion

MicrostructureMicrostructureMicrostructureMicrostructureMicrostructure

Low magnification macrograph of the Laser – TIG hybrid

weld joint cross-section and high magnification

microstructure of different regions are shown in the

Fig. 1. From this figure one can see a very wide heat

affected zone in this weld joint. Compared to ~ 3.2 mm

width of the weldment there are HAZ of more than 4 mm

width on each side. Such a wide HAZ is due to high

thermal conductivity of TZM and also because a wrought

microstructure is a fertile ground for recrystallization and

grain growth, when sufficient thermal energy is available

for the same. The EB weld joint was much narrower (FZ

~ 1.5 mm and HAZ ~ 2 mm on the either side) due to

higher welding speed (300 mm/min) in case of EBW as

compared to Laser-TIG Hybrid Welding (welding speed –

100 mm/min).

TTTTTable 1: Wable 1: Wable 1: Wable 1: Wable 1: Welding Process Pelding Process Pelding Process Pelding Process Pelding Process Parametersarametersarametersarametersarameters

Laser-TIG Hybrid Welding EBWLaser Power : 1 kW CW (Nd-YAG) Accelerating Voltage : 20 kVWelding Speed : 100 mm/minute Beam Current : 70 mAShielding Gas : Ar at 15 lpm Welding Speed : 300 mm/minArc Current : 100 A Vacuum : < 1.33 MPaArc Voltage : 9V

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The PM shows elongated grains in the rolling direction.

This is a characteristic microstructure of a rolled product.

Recrystallization begins in the HAZ and the grains begin

to lose the directionality. The variation in the extent of

recrystallization is also evident in the above figure. Regions

close to the PM are only partially recrystallized due to

insufficient thermal energy; while those close to the

weldment are fully recrystallized into equiaxed morphology.

There is significant grain growth as well in the HAZ near

the HAZ-Weldment interface. Directionality in the

microstructure again appears as one comes to the

Weldment. The grains in the weldment have grown

epitaxially from the recrystallized HAZ grains towards the

weld centerline and perpendicular to the interface. The

grains in a weld pool generally grow towards the maximum

temperature gradient, which happens to be normal to

the weldment-HAZ interface near the interface. Towards

the weld centerline the maximum temperature gradient is

along the welding direction and the grains grow towards

the same. The microstructure of the weldment is very

coarse is seen very clearly in Fig.1. Nature of the

microstructural variation was identical in EB weld joint

and Laser-TIG Hybrid weld joint with difference in the

scale of the microstructure. EB weld joints showed relatively

finer microstructure as compared to Laser-TIG hybrid weld

joint on the account of relatively lower heat input.

Microhardness Profile and TMicrohardness Profile and TMicrohardness Profile and TMicrohardness Profile and TMicrohardness Profile and Tensile Propertiesensile Propertiesensile Propertiesensile Propertiesensile Properties

Microhardness profile across the weld joint is shown in

Fig. 2. There is considerable drop in the hardness in the

FZ and the HAZ region as compared to that in the PM on

the account of grain coarsening and carbide dissolution.

This has resulted in significant weakening of the weld

joint as evident in the lower tensile strength of the weld

joint (Table 2). The tensile strength of the weld joint is

approximately 40% - 45% of that for the parent metal.

The values of tensile strength and tensile ductility of the

parent metal shows agreement with the values reported

in the literature [2,

5]. The lower value of

the tensile strength of

the weld joint is due

to coarsening of the

microstructure as

well as weakening

along the grain

boundaries of the

Fig. 1: Low magnification Macrograph (above) and High Magnification Microstructures of Different RegionsFig. 1: Low magnification Macrograph (above) and High Magnification Microstructures of Different RegionsFig. 1: Low magnification Macrograph (above) and High Magnification Microstructures of Different RegionsFig. 1: Low magnification Macrograph (above) and High Magnification Microstructures of Different RegionsFig. 1: Low magnification Macrograph (above) and High Magnification Microstructures of Different Regions(below) of Laser – TIG Hybrid Weld Joint in TZM plates(below) of Laser – TIG Hybrid Weld Joint in TZM plates(below) of Laser – TIG Hybrid Weld Joint in TZM plates(below) of Laser – TIG Hybrid Weld Joint in TZM plates(below) of Laser – TIG Hybrid Weld Joint in TZM plates

Material Yield Strength Ultimate Tensile Tensile Ductility(MPa) Strength (MPa) (% Elongation)

Parent Metal 830 855 8.4EB Welds 390 390 < 1Laser-TIG Hybrid Welds 340 340 < 1

TTTTTable 2: Table 2: Table 2: Table 2: Table 2: Tensile properties of the parent metal and the weldsensile properties of the parent metal and the weldsensile properties of the parent metal and the weldsensile properties of the parent metal and the weldsensile properties of the parent metal and the welds

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solidified grains. It is reported in the literature that trace

impurities like O and N segregate along the grain

boundaries leading to weakening along the grain

boundaries. This argument find are also supported by our

observations that fracture was in either in the weldment

or along the weldment-HAZ interface and the mode of

fracture was predominantly intergranular brittle fracture

as seen in the fractograph in Fig. 3.

ConclusionsConclusionsConclusionsConclusionsConclusions

Sound Weld joints were produced in TZM by EBW and

Laser-TIG hybrid welding process. The major achievement

of this work was that Laser-TIG hybrid welding of TZM

Fig. 2: Microhardness Profi le across WeldFig. 2: Microhardness Profi le across WeldFig. 2: Microhardness Profi le across WeldFig. 2: Microhardness Profi le across WeldFig. 2: Microhardness Profi le across WeldJoint in TZMJoint in TZMJoint in TZMJoint in TZMJoint in TZM

Fig. 3: Fractograph Showing PredominantlyFig. 3: Fractograph Showing PredominantlyFig. 3: Fractograph Showing PredominantlyFig. 3: Fractograph Showing PredominantlyFig. 3: Fractograph Showing PredominantlyIntergranular Britt le Fracture of Weld Joint inIntergranular Britt le Fracture of Weld Joint inIntergranular Britt le Fracture of Weld Joint inIntergranular Britt le Fracture of Weld Joint inIntergranular Britt le Fracture of Weld Joint in

TZM during Room TTZM during Room TTZM during Room TTZM during Room TTZM during Room Temperature Temperature Temperature Temperature Temperature Tensi le Tensi le Tensi le Tensi le Tensi le Testestestestest

was carried out first time ever and that too in open

atmosphere under flowing argon shielding. Detailed

microstructural characterization, microhardness profile

measurement and room temperature tensile test of these

weld joints were done and defect free joints could be

obtained with both of these processes.

ReferencesReferencesReferencesReferencesReferences

1. Fan J. et al. “Effect of alloying element Ti, Zr on

property and microstructure of molybdenum”. Int.

Journal of Refractory Metal & Hard Materials, 27

(2009) 78-82.

2. Morito F. “Tensile properties and microstructure of

electron beam welded molybdenum and TZM”.

Journal of Less-Common Metals, 146 (1989) 337-

346.

3. Morito F. “Effect of heat treatment on microstructure

and mechanical behavior of TZM alloy”. Journal of

Nuclear Materials, 212-215 (1994) 1608-1612.

4. TZM Technical Datasheet. http://

www.thyssenduro.com/ internet /content /

objectpiro.do/ID~1026220. Retrieved on December

24, 2008.

5. Wadsworth J., Morse G. R. and Chewey P. M. “The

microstructure and mechanical properties of a welded

Mo alloy”. Materials Science and Engineering, 59

(1983) 257-273.