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Intermetallics 46 (2014) 111e117

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Intermetallics

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Laser welding of annealed Zr55Cu30Ni5Al10 bulk metallic glass

B. Chen a,b, T.L. Shi a, M. Li c, F. Yang a, F. Yan a, G.L. Liao a,*

a State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, ChinabDepartment of Mechanical and Electrical Engineering, Hubei University of Education, Wuhan 430205, Chinac School of Materials Science Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA

a r t i c l e i n f o

Article history:Received 10 June 2013Received in revised form20 October 2013Accepted 11 November 2013Available online 30 November 2013

Keywords:B. Glasses, metallicC. Heat treatmentC. WeldingC. NanocrystalsD. Microstructure

* Corresponding author. State Key Laboratory of Dment and Technology, Huazhong University of ScienceRoad, Wuhan 430074, China. Tel./fax: þ86 27 877931

E-mail address: guanglan.liao@mail.hust.edu.cn (G

0966-9795/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.intermet.2013.11.008

a b s t r a c t

Laser welding is one of the promising ways for manufacturing metallic glass products with complicatedshape and geometry. In this work we focus on the effect of annealing treatment and welding parameterson laser welding of annealed Zr55Cu30Ni5Al10 bulk metallic glass as intended and unintended heattreatment occurs in the process. We find that laser welding can produce well welded specimen plateswith no obvious welding defects in the joints and high welding speed may lead to better joints. Althoughhigher annealing temperature or longer annealing time leads crystallization, bulk metallic glass materialstill remains largely amorphous in the heat affected zone. Compared with the welded joint withoutannealing, the micro-hardness and bending strength are enhanced due to the presence of the nano-crystals occurred in annealed welding joint. Therefore, appropriate annealing treatment with theannealing temperature near the glass transition temperature and annealing time as long as that in hotembossing of BMG parts may play a beneficial role in laser welding of metallic glasses.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Bulk metallic glasses (BMGs) are metastable materials in whichthe atomic packing is topologically disordered [1]. As compared tocrystalline metals, BMGs have many promising properties such ashigh strength, high hardness, high elastic strain limit, wear-resistance, corrosion-resistance at ambient temperature, etc., [2e4], which make BMGs very desirable in many engineering appli-cations as well as scientific researches [5]. At elevated tempera-ture, or the supercooled liquid region (SCLR) above the glasstransition temperature (Tg), BMGs exhibit superplasticity-likebehavior and behave like Newtonian fluid [6]. This behavior canbe used to carry out net shape and produce precision micro partsusing hot embossing [7,8]. Among many well-known BMG sys-tems, Zr-based systems have good glass-forming ability, wide SCLRand low critical cooling rates of 1e100 K s�1 [9]. Furthermore, theexceptionally high yield stress and high strength to density ratiomake Zr-based BMGs quite promising for many mechanical andstructural applications [10]. Zhang et al. [11] formed fine spur gearsusing Zr41.25Ti13.75Ni10Cu12.5Be22.5. Schroers et al. [12] preparedseveral fine BMGs parts using Zr44Ti11Cu10Ni10Be25. Wang et al.

igital Manufacturing Equip-and Technology, 1037 Luoyu03..L. Liao).

All rights reserved.

[13] investigated the forming of micro-gear with Zr65Cu17.5-Ni10Al7.5. Li et al. [14] investigated the thermoplastic forming mapof a Zr-based BMG. Although these BMGs parts can be obtained, itbecomes increasingly difficult to form complex structures becauseof the high flow stress in the mold and the increasing capillaryeffects at small scales. As a result, die casting or hot embossing islimited in net shaping of BMG parts with complex shapes andgeometry. In addition, it is difficult to produce metal molds withsmall and complex structure; and demoulding for such parts is alsoa problem. Silicon mold is widely used in hot embossing of micro/nano manufacturing because it can be etched into different shapesand dissolved in the KOH solution. However, silicon mold canbreak easily during the forming process, which limits its applica-tion. To date, it is still a challenge for superplastic forming of BMGparts with complex shapes with large aspect ratios and fine scales.On the other hand, laser welding offers a better solution. With highpower density, deep penetration, high welding speed, and littledeformation, laser welding is used widely to join metals and hasbeen introduced in BMGs material processing gradually. Kawahitoet al. [15] welded the Zr55Al10Ni5Cu30 BMG using high energydensity fiber-optic laser under the welding speed of 72 m min�1.Wang et al. [16] used pulsed Nd:YAG laser to weld (Zr53Cu30Ni9Al8)Si0.5 BMG sheets. Li et al. [17] used laser welding to join theZr45Cu48Al7 with the welding speed of 8 m min�1 and the outputpower of 1.2 kW. Wang et al. [18] welded Ti40Zr25Ni3Cu12Be20 withlaser welding parameters of 3.5 kW in power and 10 m min�1 inspeed.

B. Chen et al. / Intermetallics 46 (2014) 111e117112

While much attention has been paid to the laser welding ofBMGs materials, few investigations focus on the laser welding ofBMG parts undergoing hot embossing. As known, besidespatterning hot embossing process can also be considered as anannealing treatment. During appropriate annealing treatment,structural relaxation happens and under certain conditions, nano-crystallizationmay occur that can create another channel to controlthe mechanical properties of BMGs [19]. Moreover, BMGs have atency to crystallize under heating during laser welding due to themetastable nature of the amorphous structure. Therefore, it isimportant to investigate annealed BMG materials in laser weldingand then optimize process parameters for laser welding of complexBMG structure.

In this study, we conduct annealing treatment of Zr55Cu30-Ni5Al10 firstly and then joined the annealed BMG plates using laserbeam. The welding bead, microstructure, thermal properties andmechanical properties of welding joints are examined, and theresults are discussed. It is found that laser welding can be used tojoin the annealed BMGs material, and appropriate annealingtreatment is helpful to improve welding quality and mechanicalproperty of BMG material by controlling nanocrystallization.

2. Experimental procedures

As-received Zr55Cu30Ni5Al10 BMG billets were obtained from theBYD Co. Ltd. The BMGs plates were designed in dimensions of3 � 18 � 1.2 mm3 for isothermal annealing treatment and laserwelding experiment. As shown in Fig. 1, the differential scanningcalorimetry (DSC) result indicates the glass transition temperatureTg ¼ 412 �C and the crystallization temperature Tx ¼ 489 �C (at theheating rate of 10 �C min�1) respectively. The supercooled liquidregion, defined as the difference between the glass transitiontemperature Tg and the crystallization temperature Tx(DTx ¼ Tx�Tg), is 77 �C. There are 10 workpieces used in theexperiment. Group I (sample 1e6) has the same annealing treat-ment conditions but different welding parameters, while Group II(sample 7e9) has different annealing conditions but the samewelding parameters. For comparison, the tenth sample was notannealed. The annealing treatment was carried out by the universalmaterial testing machine (Zwick-Z20). Then the workpieces wereremoved from the furnace and air cooled to the room temperature.

Fig. 1. DSC trace of an as-received Zr55Cu30Ni5Al10 BMG and XRD patterns of annealedsamples.

The laser welding experiments of those ten samples were con-ducted by Ytterbium fiber laser (YLR-4000) in continuous wavemode, where the diameter of focal spot is about 0.4 mm. During thewelding process, the welding beads were protected using argonatmosphere to restrain plasma and cool welded samples. Thedetailed parameters of the annealing treatment and laser weldingare shown in Table 1. The oxide layers occurred during annealingtreatment were removed from the welded joints by using me-chanical lapping. We cut the samples into smaller blocks by CNCprecision dicing/cutting machine (Shenyang Kejing Instrument Co.,Ltd, EC-400) and embedded the blocks in epoxy for metallographicsample preparation. The prepared samples were then polishedusing 1200 precision auto lapping/polishing machine (MTI, UNIPOL-1202) for evaluating the bonding quality and structural change atthe joints.

3. Results and discussion

Before laser welding, the four annealed samples with differentannealing conditions (sample 2, 7e9) and the as-received BMGplate were examined using X-ray diffraction (XRD). The results areshown in the inset of Fig. 1. As can be seen there are obvious broadpeaks without any detectable crystalline phases in the XRD pat-terns, indicating the annealed samples still preserving glassy na-ture. Apparently, the annealing conditions in Table 1 are notsufficient to lead to crystallization.

The macro-planar views of the laser welded samples are shownin Fig. 2. The dark region in the cross section belongs to the heataffected zone (HAZ) where the crystallization is most likely to occur[17,18]. There are many differences among those welded samples,depending on the welding speed and the laser power. Both highwelding power and lowwelding speed lead to large deformation ofwelding seams and small inclined angle formed by HAZ, a typicalnail head shape. Generally, the inclined angle is between 30� and80�, determined by the welding power and speed. Thus, appro-priate welding power and speed can provide desired penetration.

To characterize the welding quality and explore the heat effectof annealing and laser beam on the BMG, the welded samples wereexamined by scanning electron microscopy (SEM), XRD, trans-mission electron microscope (TEM), Vickers micro-hardness andthree-point bend test, respectively. The samples’ micro-morphology in cross section containing welding fusion zone(WFZ) is shown in the SEM results (see Fig. 3(a)e(g)). As can beseen, the welded samples exhibit sound joints, at least visually,where fully penetrated welded beads are obtained and no porosityis generated. There is no visible defect such as pores or cracks inboth the base material zone and the WFZ, indicating that the in-terfaces are joined successfully. The structure of the whole crosssection of the welded joint looks homogeneous.

Table 1The parameters used in annealing treatment and laser welding.

Samplenumber

Annealing Laser welding

Temperature(�C)

Time(min)

Power(kW)

Speed(m min�1)

Argonflow( � 103 L min�1)

1 415 10 1.5 8 1.252 415 10 1.5 10 1.253 415 10 1.8 8 1.254 415 10 1.8 10 1.255 415 10 2.1 8 1.256 415 10 2.1 10 1.257 415 30 1.5 10 1.258 430 10 1.5 10 1.259 430 30 1.5 10 1.2510 e e 1.5 10 1.25

Fig. 2. The macro-planar views of the beads surface and cross section taken from welding joints.

B. Chen et al. / Intermetallics 46 (2014) 111e117 113

The XRD patterns of the seven welded joints are illustrated inFig. 3(h). It can be seen that these samples have similar patternswith broad diffuse halos rather than sharp diffraction peaks typicalfor the presence of crystalline phases, confirming that within thedetection limit of XRD, no large size crystals occur in the weldedjoint during laser welding. The angle positions of the amorphousdiffraction peaks do not shift practically among these samples, onlythe height of the peaks increases with annealing treatment,implying that densification caused by relaxation occurs during theannealing treatment [20].

The absorbed energy tomelt thematerials at interface should beproportional to P*(d/v), where P, d and v stand for laser power, laserspot size and welding speed, respectively. Therein, the spot size ofthe laser beam is a constant, a result of our laser equipment. Thewelding power provides energy to melt and penetrates the BMGs.The high welding speed can lead to fast temperature drop of themelt to below crystallization temperature and thus inhibits crys-tallization [18]. Both of these consequences are closely related tothe welding quality and microstructure of the welding joint.

As can be seen in Fig. 2, all BMG plates were fully penetrated. Inthis case, we will center on the welding speed instead of thewelding power. Samples 1e2 and 7e10 were examined using TEMto investigate the microstructural change. As the material in thebase material zone has remained in glass state after laser welding,crystallization is most likely to occur in the HAZ [17,18]. Thus, weonly focus on the HAZ. Fig. 4 indicates that themicrostructure in theHAZ displays a distinct difference. In Fig. 4(a) and (b), differentwelding speeds obviously affect the glassy structure of the samplesunder the same annealing parameters and welding power. Forsample 1 with 8 m min�1 welding speed, the high-resolution TEM(HRTEM) image (see Fig. 4(a)) displays orderly periodic arrange-ment of atoms, indicating that crystallization indeed occurs in theHAZ. The magnified image in the inset for the marked area A showsthat only very little amorphous phase is embedded in the crystal-line substrate. Moreover, the selected area electron diffraction(SAED) pattern shows the matrix with inconspicuous diffractionrings, further revealing that crystallization occurs after the laser

welding. The inset of the bright field image (BFI) also shows thefeather-like shape of crystals. These are consistent with the HRTEMresult. Fig. 4(b) shows the microstructure of sample 2 with thewelding speed of 10 m min�1. The HRTEM image shows that atomsin most areas still remain disordered except in the marked area B.The SAED pattern shows a typical amorphous diffraction ringtogether with several bright specks. The BFI gives the typicalmorphology of these particles during crystal nucleation withflower-like shape and the size about 50 nm. According to thedescription mentioned above, the BMGs maintain amorphousstructure plus a very few crystals present in BMG substrate, con-firming the high welding speed is apparently useful to obtaindesired microstructure. For samples 2 and 7e9 that have the samewelding parameters and different annealing parameters, the mi-crostructures are different, as shown in Fig. 4(b)e(e). The resultssuggest that the annealing temperature and time affect theweldingquality. Both sample 7 (Fig. 4(c)) and sample 9 (Fig. 4(e)) havecrystallized more seriously as compared with sample 8 (Fig. 4(d)).The HRTEM graphs show more ordered atomic packing and thecorresponding SAED patterns show less obvious amorphousdiffraction ring with regular specks. The observations confirm thatpretreatment with higher annealing temperature or longerannealing time facilitate crystallization during welding process.Even so, part of BMG material in the HAZ retains amorphousstructure, as marked in B, C, D and E.

The microstructure of the welded sample 10 (without anneal-ing) under the welding speed of 10 mmin�1 is shown in Fig. 4(f). Inthe HAZ, the BMG has completely crystallized, and the average sizeof the crystals is larger than 100 nm. The white and black phasescoexisting in WFZ are eutectic structures. The HRTEM image of theHAZ is quite different from those annealed welding samples. Firstly,the amorphous phase exists in all annealed welding samples ratherthan complete crystallization in sample 10 without annealing.Secondly, the crystalline phase and geometrical shape is visiblydifferent, e.g., the crystals are “polygon-like” in sample 10 but“flower-like” in sample 2. Thirdly, the grain size of the crystals insample 10, mostly about 100e200 nm, is much bigger than that in

Fig. 3. SEM images of the welding joints. (a)e(g) corresponding to annealed samples 1e6 and 10, respectively, and (h) The XRD patterns of 7 welded samples.

B. Chen et al. / Intermetallics 46 (2014) 111e117114

sample 2. Quite a few nanoparticles appear after annealing andlaser welding in sample 2. This is because the annealing treatmentinduces structure relaxation and rearrangement of atoms, and thenthe BMG after annealing and laser welding can be transformed intoa glass-matrix composite containing nanoparticles.

To evaluate themechanical property of the welded samples, theVickers hardness of thewelded samples 1, 2,10 and the as-receivedBMG billet were measured using a micro-hardness tester (WilsonHardness 432SVD) under the force of 1 kgf and Dwell time of 15 s.

There are 8 points in the welded cross section being randomlyselected, including 4 points in the welding zone and 4 points in thebase material zone. As can be seen in Fig. 5(a), the hardness in thebase material zone of welded samples 1, 2 and 10 basically equalsto the as-received BMG material. Nevertheless, the hardnessvalues of these samples in the welding zone have changed obvi-ously after annealing and laser welding. The hardness values ofsample 2 are larger than others. By comparing the results ofsample 1 and sample 2, we find that the higher welding speed for

Fig. 4. TEM images of the welding joints under different processing conditions. (a)e(f) the HRTEM image, SAED pattern, BFI results in the HAZ corresponding to samples 1e2 and 7e10, respectively.

B. Chen et al. / Intermetallics 46 (2014) 111e117 115

sample 2 results in quickly drop of temperature during laserwelding. There is not enough time for the crystals to grow up, andthe heat effect of annealing increases the nucleation rate andnanocrystallization. The nanocrystal can improve the material

properties, including hardness, strength, plasticity, etc [21,22].Thus, the hardness of sample 2 is enhanced. However, the largegrain size with 100e200 nm leads to a slightly decrease of hard-ness in sample 10.

Fig. 5. The Vickers hardness and stressestrain curve from a three-point bend test forselected welding samples.

B. Chen et al. / Intermetallics 46 (2014) 111e117116

In addition, we performed three-point bend test at an initialstrain rate of 1 � 10�3 s�1 to measure the bending strength forsamples and the joints. We did it for sample 2, 7e10 and as-received BMG material in the universal material testing machine(Zwick-Z20). The results are shown in Fig. 5(b). The base materialhas the maximum bending strength up to 2940 MPa. Bendingstrength for sample 2 and sample 10 is 2667 MPa and 2568 MPa.They are 90.7% and 87.3% as high as the maximum bendingstrength of the as-received billets, respectively. The results areconsistent with other works [18]. Because of the existing nano-particles, the bending strength in sample 2 is slightly higher thanthat of sample 10. For samples 7e9, the higher annealing tem-perature and longer annealing time result in more severe crys-tallization with larger crystalline particles, thus causing thedecrease in the strength.

4. Conclusions

The annealing treatment and laser butt welding for Zr55Cu30-Ni5Al10 BMG plates were conducted. The welded joints wereinspected by Keyence photos, SEM, XRD, TEM, Vickers hardness,three-point bend test, respectively. There is no incomplete pene-tration or visible defect observed, demonstrating that the BMGplates are joined together successfully. Different welding speed andannealing treatment conditions can lead to sub-micron scale

crystals and nano-scale particles in the HAZ and also differentcrystals phases.

For complete penetration, we found that the high welding speedcan lead to a quick temperature drop, which is beneficial to obtain abetter joint, including small nanoparticle formation and bettermechanical property at the joints. Under the same welding condi-tions, however, annealing affects both the microstructure andwelding quality significantly. We found that part of material in theHAZwith annealing treatment still maintains amorphous structure.Especially, the welding sample with the annealing temperature of415 �C and the annealing time of 10min before laser process retainsalmost full amorphous structure with very few nanoparticles withthe average size of 50 nm. Due to the existence of nanocrystallinephase, the welded joint with appropriate annealing treatment ex-hibits larger micro-hardness and bending strength than the un-annealed samples and the BMG billets. However, too higherannealing temperature and longer annealing time is not beneficialfor BMGs.

Therefore, we conclude that if the penetration bead is obtainedin the laser welding process, higher speed need to be selected.Besides, appropriate isothermal annealing can suppress crystalgrowth and hence leads to nanocrystallization, which improves thewelding quality and mechanical property of the welding joints.Specifically, for Zr55Cu3Ni50Al10 BMG plates, appropriate annealingtemperature should be near the glass transition temperature andthe annealing time can be selected as long as that in hot embossingof BMG parts. This work shows that the laser welding of annealedBMG material is a promising technology for the welding of BMGmaterials and the manufacturing of the complex BMG structure.

Acknowledgments

This research is financially supported by the National NatureScience Foundation of China (Grant Nos. 51222508, 51175210 and51175211) and Science and Technology Research Projects of HubeiMinistry of Education (Grant No. Q20133004). The authors wouldlike to thank Dr. ChunmingWang for his assistance during the laserwelding experiment.

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