Materials Transactions, Vol. 43, No. 10 (2002) pp. 2509 to 2515c©2002 The Japan Institute of Metals

Electron Beam Welding of Zr50Cu30Ni10Al10 Bulk Glassy Alloys

Yoshihiko Yokoyama1, Nobuyuki Abe2, Kenzo Fukaura1,Takeshi Shinohara1 and Akihisa Inoue3

1Faculty of Engineering, Himeji Institute of Technology, Himeji 671-2201, Japan2Joining and Welding Research Institute, Osaka University, Ibaraki 567-0047, Japan3Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

By using a conventional electron beam welding machine, an electron beam welding of Zr50Cu30Ni10Al10 bulk glassy plates has beenachieved in the welding condition leading to the suppression of heat affected zone (HAZ). In order to obtain the cooling rate, which is sufficientfor glass formation, the welding speed was controlled to be higher than 100 mm/s and the beam-radiated area should be limited to be smallerthan 0.8 mm in diameter. Joint strength of the welded bulk glassy plate is about 1400 MPa, which is lower by 15% than the tensile strength(1650 MPa) of the glassy alloy. The fracture of the welded alloy plate occurs along shear slip plane across the welded area, reflecting goodductility in the welded area of the Zr50Cu30Ni10Al10 bulk glassy alloy.

(Received May 30, 2002; Accepted August 2, 2002)

Keywords: electron beam welding, Zr50Cu30Ni10Al10 bulk glassy alloy, heat affected zone, crystallization, joint strength

1. Introduction

Resent development of bulk glassy alloy1) has lead to abreak through in basic science and engineering of metallicmaterials. However, there have been some problems for fur-ther extension in practical use of bulk glassy alloys. As oneof the problems, one can list up a welding (joining) technol-ogy, because the bulk glassy alloys have been expected tobe used as structural materials in various fields. It is wellknown that most of amorphous alloys are embrittled by heataffection leading to structural relaxation, phase separationand crystallization. More recently, several results on weld-ing/joining have been reported for Zr–Al–Cu–Ni bulk glassyalloy and steels.2, 3) The welding between the Zr-based bulkglassy alloy and Zr metal has been reported4) by using theZr41Ti14Cu12Ni10Be23

5) bulk glassy alloy with a critical cool-ing rate of about 2 K/s. This result indicates that the bulkglassy alloy with a critical cooling rate less than 2 K/s canbe welded. We have tried to weld Zr50Cu30Ni10Al10

6) bulkglassy alloy with a critical cooling rate of about 12 K/s bythe electron beam welding technique. The trial is very impor-tant because the Be-containing glass alloy is not always a safematerial for human body and environment atmosphere. How-ever, the use of the latter Zr-based alloy requires improve-ment of the welding technique for the formation of weldedstructure without crystalline phases. It has previously beenreported7) that Zr–Cu–Al–Ni glassy alloys are formed by thecontinuous casting method. The cast structures of continu-ously solidified bulk glassy alloys were analyzed on the basisof the cast structure diagram8) with G versus V axes, wherethe G and V mean the temperature gravity at solid and liq-uid (S/L) interface and velocity of S/L interface, respectively.The difference between the continuous casting and electron(e-) beam welding of bulk glassy alloys is attributed to themolten area of the sample. The cooling medium in the contin-uous casting process is He atmosphere and water jacket cop-per hearth, while that in the e-beam welding process resultsfrom the non-welded region of samples. In the e-beam weld-

ing, the cooling rate of the welded area in the sample dependson the sample shape, and the shape is fixed in a constant sizeof 20 mm × 40 mm × 2.5 mm in this study.

In this study, we tried to weld Zr50Cu30Ni10Al10 bulk glassyplates by the electron beam technique, in which the energyand shape are controlled to restrain the embrittlement of HAZand melt regions. In order to find an optimum welding con-dition of the bulk glassy alloy, we examined the structureand strength of the e-beam welded area obtained in variouse-beam welding conditions.

2. Experimental Procedure

In order to obtain a sufficient cooling rate in the e-beamwelded area, the beam radius and profile were examined inseveral beam current conditions. The e-beam welding is char-acterized9) to make a beam hole in the center of beam-radiatedarea, and the preparation of the beam hole brings about a nar-row welded area and homogeneous bonding structure. In ac-tual, the preliminary experiment of the e-beam welding of thebulk glassy alloy is necessary to clarify the relationship be-tween the welded area and the cooling rate. In the prelim-inary experiment, we measured a beam profile and its con-trol method. The e-beam welding of conventional materialshas used a defocus e-beam to produce a wide bead mark onthe butt joint. However, the e-beam for the bulk glassy alloymust be focused on the sample surface to prepare the stablebeam hole with low beam energy. This is because the excessbeam energy occurs heat-affected embrittlement due to crys-tallization or structural relaxation in the melt and HAZ. Thebeam profiles were measured by using the Faraday couplingmethods,10) and the shapes were also controlled by chang-ing condenser lens current in the several beam current condi-tions. The beam profile should be a sharp and symmetricalone to restraint a molten area and achieve homogeneity. Fur-thermore, the beam profile of the shoulder point (edge of thebeam) should be drastically increased to restrain the excessheat affect on the non-welded region of the bulk glassy alloy.

2510 Y. Yokoyama et al.

A quaternary Zr50Cu30Ni10Al10 alloy ingot was preparedby arc melting pure Zr, Cu, Al and Ni metals in an argonatmosphere. The alloy ingot was completely remelted, andsqueeze cast into a plate shape with a width of 50 mm, alength of 60 mm and a thickness of 3 mm by a squeeze cast-ing method.11) The bulk glassy plates were welded with ane-beam welding machine of Mitsubishi Electronic Co. Ltd.(30 kW class) under an evacuated atmosphere of about 1 Pa.The e-beam welding conditions were as follows;(1) accelerate voltage: 70 kV,(2) stage moving velocity (welding speed): 16.7 to 250 mm/s,(3) beam current: 5 to 120 mA, and(4) ab value: 1 as just on focus.

Where the ab value is the ratio of focus distance to speci-men surface distance from electron lens. The e-beam weldedstructure was examined by optical microscopy (OM) andscanning electron microscopy (SEM), and the compositionsof several phases in the samples were determined with anelectron probe microanalyzer (EPMA). Tensile strength of thee-beam welded sample was examined at an initial strain rateof 6.7 × 10−3 s−1 by using an Instron tensile testing machineequipped with a wire resistance strain gage meter. Hardnesswas measured by a micro Vickers hardness tester under a loadof 2.9 N for a loading time of 15 s.

3. Results and Discussion

The development of bulk glassy alloys with a low criticalcooling rate is expected to bring about the break through ofsize restriction. It also leads to the possibility of welding.The most important point in the welding of bulk glassy al-loys is the reformation of glassy phase in the welded moltenregion through the suppression of crystallization in the HAZregion. In order to suppress the crystallization in the HAZ, themelted region must be reduced. The radiated area by electronbeam should be smaller than the circle with 1 mm in diame-ter. In this study, the diameter of the electron beam was about0.8 mm at the just focus condition of electron lens. Further-more, the electron beam welding method can have the nar-rower and deeper welded region. This peculiarity causes theformation of an electron beam hole,9) which is formed be-tween the electron beam and molten alloy as shown in Fig. 1.Figure 1 shows a schematic illustration of the e-beam weld-ing methods for ordinary alloys (a) and bulk glassy alloys (b).In this figure (a), the e-beam condition for ordinary alloys isdefocused so as to have the e-beam size of 5 mm in diame-ter, because the radiated area must be overlapped butt joints.However, the e-beam welding for the bulk glassy alloy de-mands the narrowest e-beam radiated area to minimize theinput energy in the HAZ region as shown in Fig. 1(b).

In order to evaluate the crystallization in the e-beam weldedarea for the bulk glassy alloy, a simplified heat flow calcula-tion was performed. Rosenthal calculated12) the cooling ratein the bonding area by eq. (1),

R = −k(T − T0)n. (1)

Here T is temperature of each point, and T0 is the initial tem-perature as 300 K. t is the time and k and n are coefficients.

Fig. 1 Schematic illustrations of e-beam welding mechanisms for the ordi-nary method (a) and for the present study (b).

The factor k is also defined as follows,

k = 2πκρC

(Vws

q

)2

n = 3 (thin)

(2)

k = 2πκVw

qn = 2 (thick)

(3)

where π is the ratio of the circumference of a circle to itsdiameter, κ is thermal conductivity as 7.23 × 10−3 × T(J/s·m·K),13) Vw (m/s) is speed of welding, q is heat input, C

Electron Beam Welding of Zr50Cu30Ni10Al10 Bulk Glassy Alloys 2511

is specific heat as 10 (J/mol·K).1) ρ is density 6850 (kg/m3),and s is thickness of sample. The value of q is calculated bythe product of acceleration voltage of 70 (kV) and beam cur-rent of Ib (mA) as q = 70 × Ib (w). The value of specificheat is underestimated, because the specific heat values of theliquid and supercooled liquid should be lower than that of theglassy solid. In this study, the shape of the bulk glassy alloycorresponds to the thin sample condition in eq. (2). Equation(1) representing the cooling rate can be modified as follows,

R = 3.38 × 10−5

[Vw

Ib

]2

T (T − T0)3. (4)

This equation means that the high cooling rate can be obtainedby the high welding speed and small input heat. It is ordinarycondition to avoid the heat affect. However, the heating andcooling rates in the bonding region tightly depend on the heatflow capacity of the sample. That is, the large welding speedand small input heat do not always bring about high coolingrate. Because the high welding speed causes the wide meltregion along welding direction under the constant heat flowcapacity condition of the sample. Consequently, the e-beamwelding demands the balance between the welding speed andinput heat to obtain the satisfactory welded structure. By us-ing eq. (4), cooling rates were calculated as shown in Fig. 2,under the following different conditions,(a) Ib = 10 mA, Vw = 16 mm/s(b) Ib = 50 mA, Vw = 100 mm/s(c) Ib = 70 mA, Vw = 200 mm/s(d) Ib = 100 mA, Vw = 250 mm/sThese conditions were determined on the basis of the experi-mental data described later. This figure indicates the possibil-ity of the e-beam welding for the Zr–Cu–Ni–Al bulk glassyalloy. The cooling curve in these CCT diagrams14) meansthe solidification condition in the welded region. Where Tl

values (Tl = 1100, 1300, 1500, 1700, and 1900 K) are thestart temperatures (t = 0) of several cooling curves in thewelded region. In the figure (c) the fastest cooling rate, whichcan avoid the nose of CCT diagram, has the highest possi-bility to obtain the satisfactory bonding structure. Besides,the ordinary e-beam welding condition (a) in Fig. 2 meansthe crystallization in the welded region. Consequently, thebest condition between Ib and Vw was selected to evaluate thebonding structure. As shown in Fig. 1(b), the best weldingcondition brings about the similar bead shapes at the top andbottom sides, because of the e-beam bore through the sam-ple to make a beam hole. To determine the Ib value at eachVw value, the shape of the bead is the most important fac-tor to evaluate the stability of the molten region. Figure 3shows bead images on the top and bottom sides at severalwelding conditions. This figure shows the optimum structureof welded region at several welding speeds. In order to createthe narrow melted alloy region leading to satisfactory coolingrate, the stable beam hole formation is the most important fac-tor, which can be estimated by the similarity of the bead widthbetween top and bottom side. Consequently, the welding con-ditions of (a) Ib = 10 mA, Vw = 16 mm/s, (b) Ib = 50 mA,Vw = 100 mm/s, (c) Ib = 70 mA, Vw = 200 mm/s and (d)Ib = 100 mA, Vw = 250 mm/s, were determined from theshapes of beads in Fig. 3. The characteristic points of thebead shape leading to the achievement of the optimum weld-

Fig. 2 Relationships between the calculated cooling rate and the CCTcurve at the welding conditions, Ib = 10 mA, Vw = 16 mm/s (a),Ib = 50 mA, Vw = 100 mm/s (b), Ib = 70 mA, Vw = 200 mm/s (c)and Ib = 100 mA, Vw = 250 mm/s (d).

ing condition are determined as the difference of bead widthon the top and bottom side and the fractal of bead width alongwelding direction on bottom side.

Figure 4 shows outer appearance of the welded bulk glassytensile specimen (a) and the relationship between the tensilestrength and beam current at different welding speed con-ditions of 100 mm/s, 200 mm/s and 250 mm/s, respectively.The eq. (4) means that the cooling rate increases with an in-crease of welding speed. However, the increasing of coolingrate depends on the heat flow capacity of the welded sam-ple, whereas eq. (1) is assumed the specimen size as infin-ity. The large welding speed demands an excess beam cur-rent to form the stable beam hole. The excess beam cur-rent brings about wide molten alloy region along weldingdirection, because of the limited heat capacity of the speci-

2512 Y. Yokoyama et al.

Fig. 3 OM images of the e-beam welded bead at up and down sides at different welding conditions.

men. Consequently, the optimum e-beam welding conditionsof welding speed and beam current can be determined bythe size of specimen. In this study, the specimen size was20 mm × 40 mm × 2.5 mm. Furthermore, the change in thestrength with beam current has a maximum point at severalwelding speeds. The welded structures, their micro X-raydiffraction patterns at the melt, the HAZ and matrix regionsat these maximum strength conditions are shown in Fig. 5. Inthe figure, the optimum structure of the welded region of the

Zr50Cu30Ni10Al10 bulk glassy alloy is obtained at the weldingcondition of Ib = 70 mA and Vw = 200 mm/s. The weld-ing condition is also leading to the highest tensile strength asshown in Fig. 4. In the other welding conditions, the weldedstructure of the bulk glassy alloys contain crystalline parti-cles with an equiaxed dendrite shape in the vicinity of the in-terface between the melt and HAZ. The crystalline particleswere identified as τ3-phase from the X-ray diffraction patternsshown in Figs. 5(b) and (f). These OM images show a side

Electron Beam Welding of Zr50Cu30Ni10Al10 Bulk Glassy Alloys 2513

Fig. 4 Outer appearance of e-beam welded tensile specimen (a) and the relationship between the tensile strength of the e-beam weldedregion and the beam current at different welding speeds of 100, 200 and 250 mm/s.

Fig. 5 SEM images and small area X-ray diffraction patterns taken from the melt, the HAZ and matrix regions obtained at e-beamwelding conditions Ib = 50 mA, Vw = 100 mm/s (a) (b), Ib = 70 mA, Vw = 200 mm/s (c) (d) and Ib = 100 mA, Vw = 250 mm/s (e)(f).

2514 Y. Yokoyama et al.

Fig. 6 Fracture surface of the welded bulk glassy alloys obtained at e-beam welding conditions Ib = 50 mA, Vw = 100 mm/s (a)–(e),Ib = 70 mA, Vw = 200 mm/s (f)–(j) and Ib = 100 mA, Vw = 250 mm/s (k)–(o).

view of the tensile fractured specimens. The fracture mor-phology in the Zr50Cu30Ni10Al10 bulk glassy alloy welded atthe condition of Ib = 70 mA and Vw = 200 mm/s is com-posed of one shear slipped surface along a maximum shearstress plane. Besides, the fractured surfaces in the other weld-ing conditions are characterized by brittle flat fracture mode,in which the fracture occurs along the plane perpendicular tothe tensile direction. Figure 6 shows fractured surface SEMimages of the e-beam welded Zr50Cu30Ni10Al10 bulk glassyalloys obtained by welding at the conditions of Ib = 50 mA,Vw = 100 mm/s (a-e), Ib = 70 mA, Vw = 200 mm/s (f)–(j)and Ib = 100 mA, Vw = 250 mm/s (k)–(o). In this figure,brittle fractured surfaces (a) and (k) show the flat plane per-pendicular to the tensile direction. The brittle fractured sur-face shows the crystalline brittle fracture mode as shown in(b)–(e) and (l)–(o). The brittle fractured surfaces were formed

along the crystallized region, which was located at the inter-face between the melt and HAZ. Besides, at the welding con-dition of Ib = 70 mA and Vw = 200 mm/s, the good ductilityof the e-beam welded region can be understood by the frac-tured direction and fracture surface mode. In the fracture sur-face, fracture initiation is identified in the deposit region nearsmall crystalline particles as shown in Fig. 6(g). We cannotconclude that the origin of crystalline particle is due to theheat effect by e-beam welding, because the specimens wereprepared by the arc squeeze casting process, and have thepossibility of containing crystalline inclusions in the cast sam-ple.15) However, almost all the fracture surface was formed bythe slip deformation mode that is indicated by the well-grownvein patterns on the fractured surface shown in Figs. 6(h)–(j). Consequently, the best welding condition of Ib = 70 mAand Vw = 200 mm/s is not always perfect to obtain a single

Electron Beam Welding of Zr50Cu30Ni10Al10 Bulk Glassy Alloys 2515

Fig. 7 Simplified cast structure diagram with V vs. G axes of theZr50Cu30Ni10Al10 bulk glassy alloy. Calculated cooling rates in the rangefrom melting temperature Tm (1140 K) to nose temperature Tn (1000 K) forthe e-beam welded region obtained at different welding conditions werenoted. Estimated critical cooling rate (Rc) line for glass formation wasalso shown in the diagram.

glassy phase in the welded region. The mean value of tensilestrength for the welded Zr50Cu30Ni10Al10 bulk glassy alloyobtained at the condition of Ib = 70 mA and Vw = 200 mm/sis 1400 MPa, which is lower by 15% than that (1650 MPa) ofthe no-welded Zr50Cu30Ni10Al10 bulk glassy alloy.

The solidification in the welding treatment is similar tothe unidirectional solidification of the moving liquid at thesolid and liquid (S/L) interface. The cast structure of theunidirectional solidified Zr60Cu15Ni5Al10Pd5 bulk glassy al-loy was reported in 1995.7) In this report, the critical cool-ing rate for the glassy phase formation at the unidirectionalsolidification S/L interface is about 40 K/s. However, in thecase of a V-shape copper mold equipped with thermocouples,the critical cooling rate with about 104 K/s13) for the forma-tion of the Zr60Cu15Ni5Al10Pd5 bulk glassy alloy. Besides,the critical cooling rate of the arc-melted Zr50Cu30Ni10Al10

alloy was estimated as 12 K/s16) from the cooling rate dataof arc-melted Zr–Cu–Al–Ni alloys measured by a radiationalthermometer. That is, the critical cooling rate changes signif-icantly from 12 to 104 K/s, whereas the alloy compositionsremain almost unchanged. The difference in the critical cool-ing rates is probably due to the difference in the amount ofquenched-in nuclei in the molten alloy. The influence of thequenched-in nuclei on the continuous cooling transformation(CCT) curve was reported for Pd40Cu30Ni10P20 bulk glassyalloy.17) The quenched-in nuclei cause the transition of CCTcurve to shorter time side. The sort of quenched-in nuclei wasconsidered as impurity phosphorous oxide inclusions. How-ever, in the case of unidirectional solidification, there is a pos-sibility of quenched-in nuclei without any impurities in themolten alloy. At the rough S/L interface, as exemplified fordendritic growth, a lot of nuclei are supplied into the melt by

cutting off the arm of dendrite. The roughness of the S/L in-terface can be estimated by the G and V values in the caststructure diagram as shown in Fig. 7. In Fig. 7, the G andV values at the critical cooling rates of 12, 40 and 104 K/sare estimated as a© to c© points in this figure by assumingthe V values as 1, 10 and 100 mm/s, respectively. Conse-quently, the critical cooling rate for glass formation can bechanged by the solidification condition. The relationship be-tween cooling rate R and nose at CCT curves shown in Fig. 2is not appropriate to estimate the optimum welding condition.In Fig. 7, the cooling rates at the melting and nose tempera-tures are also shown at several welding conditions by usingeq. (4). This figure also points out that the glass formationin the welded region becomes difficult with an increase in Vto over 250 mm/s. The increase in G value is more effectiveto obtain a glassy structure in the welded region. As a result,to obtain a glassy structure in the welded region, appropriatecooling methods leading to an increase in the heat capacityand heat flow ability of the specimens should be used.

4. Summary

In order to achieve the joining of the Zr50Cu30Ni10Al10 bulkglassy alloy, the e-beam welding was performed. The struc-ture of the welded region and the bonding strength of the e-beam welded specimens were examined. The results obtainedare summarized as follows.

(1) The welding condition of Ib = 70 mA and Vw =200 mm/s brings about the highest bonding strength of1650 MPa as well as the formation of the glassy phase in thewelded region.

(2) The estimation of the welded structure on the basis ofthe only CCT diagram was not suitable. In the case of unidi-rectional solidification as exemplified welding, one should bepaid attention to that the critical cooling rate to form glassyphase will be changed by the roughness of solid/liquid inter-face.

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