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Welding of aluminum alloys through thermite like reactions in Al–CuO–Ni system
Materials Chemistry and Physics 133 (2012) 757–763
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Welding of aluminum alloys through thermite like reactions in Al–CuO–Ni system Ehsan Bahrami Motlagh ∗ , Jalil Vahdati Khaki, Mohsen Haddad Sabzevar Department of Materials Science and Engineering, Engineering Faculty, Ferdowsi University of Mashhad, P.O. Box 9177948944, Mashhad, Iran
a r t i c l e
i n f o
Article history: Received 4 April 2011 Received in revised form 21 January 2012 Accepted 23 January 2012 Keywords: A. Composite materials B. Welding C. Mechanical testing D. Hardness
a b s t r a c t In this work, first, a metastable composite powder of “14Al–3CuO–Ni” with a decreased ignition temperature was obtained via Arrested Reactive Milling (ARM), then this exothermic blend was used for welding of 1100 Aluminum alloy. The reactive media and the weld zones were investigated using scanning electron microscope. X-ray diffraction experiment and morphological investigations accompanied with the EDS analyses were carried out in order to evaluate the reactions’ products. Vickers microhardness profile across the joint and the shear strength of the joints were determined. The weld zone thickness in each of the parent alloys was measured to be 750 m, approximately. Results showed that different reactions occurring during the process lead to the in situ formation of different intermetallic compounds such as Al3 Ni2 and Al7 Cu4 Ni as well as Al2 O3 nanoparticles at the interface. Thus, this area has the maximum hardness (80–90 VHN) and the minimum hardness of 35 VHN belongs to the parent alloys. The mean shear strength of the obtained joints was 27 MPa. © 2012 Published by Elsevier B.V.
1. Introduction Thermite reactions have been utilized in materials joining applications for more than a century. In rail welding, for example, powders of Fe2 O3 and Al are combined and after thermal ignition, they react to form molten Fe and molten Al2 O3 [1]. Since combustion synthesis welding can harvest the released heat from the highly exothermic chemical reactions, recently, it has been widely used to join different refractory materials such as nickel-based and TiAl intermetallic compounds [2,3], carbon–carbon composites [4] and different combinations of ceramics and metals [5,6]. Liu and Naka [7] have reported that Ni-based composite joints reinforced with different percentages of Al2 O3 have been produced using combustion synthesis reactions. Also, Li and his co-workers [5] have utilized this process in order to achieve a functionally graded joint between SiC ceramic and Ni-based superalloy. Another possible application in repairing gas turbine components has been introduced by Pascal et al. [8]. Joining of bulk metallic glasses using combustion synthesis reactions has been reported by Swiston et al. [9], in which no recrystallization has been observed due to the rapid heating and cooling rates. Joining of dissimilar materials could be another category of applications [10]. Various methods which have implemented combustion synthesis reactions’ heat for welding purposes have been classified well by Mukasyan and White [11].
∗ Corresponding author. Tel.: +98 511 8763305; fax: +98 511 8763305. E-mail address: [email protected] (E. Bahrami Motlagh). 0254-0584/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2012.01.086
However, the drawbacks for utilization of combustion synthesis reactions in welding of non-refractory materials such as aluminum and its alloys have been, first, the porosity in the products of the combustion synthesis reactions which are inevitable and may cause defected bonds. The high ignition temperatures of these reactions which are typically higher than the melting points of these kinds of materials are the second problem. The first problem can be avoided using a brazing agent [3]. Also, some methods have been devised recently to decrease the ignition temperatures of the combustion synthesis reactions such as high energy ball milling (HEBM) [12]. Several types of reactive nano composites have been synthesized by arrested reactive milling (ARM) [13,14]. Among them, Al–CuO thermites are of particular interest, because the reaction is highly exothermic [12,15], and its temperature can be adjusted to produce either molten or vapor-phase copper [12]. More recently, the combination of mechanical activation (high energy ball milling) and thermal activation (combustion synthesis) of chemical reactions has widened the possible applications for both methods [16]. In this procedure, mechanical activation of reactions is used as an intermediate step to enhance the kinetics of a reaction during subsequent thermal treatment. For instance, Essl and his co-workers [17] have prepared a precursor powder mixture, based on aluminum, for reaction bonding of alumina. It is observed that the mechanical activation induced by a HEBM can substantially reduce the reaction temperature required for completing the reactions. Also, the potential limitations of HEBM can be minimized and its productivity can be increased sufficiently by reducing the activation time during the HEBM. This is possible when the mechano-composites are used as precursors [18]. As a result, all the
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advantages of thermo-chemical approaches are conserved while the limitations may be reduced significantly. This procedure has already been implemented in coating of metals successfully [19], and production of bimetals would be another potential application. In this study, the mechanical–thermal activation of chemical reactions in 14Al + Ni + 3CuO system is used for welding of aluminum 1100 alloys. Thus, the applications of thermite reactions have been extended for welding of non-refractory materials. The advantages of this process would be considerable from two points of view: first, elimination of the need for energy sources for welding, and second, using metal oxides as a part of starting materials. Fig. 1. Schematic of the designed fixture for shear test measurement.
2. Experimental The starting materials were Al powder (99% purity, Khorasan Powder Metallurgy Co., Mashhad, Iran) with an average particle size of 30 m, CuO powder (96% purity, <100 m), Ni powder (99.99% purity) with particle size of 20 m, and hexane as the process control agent (PCA) for milling. Three grams of powder mixture was prepared according to the stoichiometry of “14Al + Ni + 3CuO” and milled for 5 h in the presence of 4 ml hexane and in argon atmosphere (purity 99.9%). A large amount of Al powder was utilized in the reactant powder mixture in order to achieve two goals. The first goal was to lower the adiabatic temperature of the combustion synthesis reactions so that no gaseous products could be formed. Otherwise, the evolved gas can lead to defected bonds (in case of air trapping) or no bonding due to the separation of the parent alloys. Secondly, the extra amount of Al can act as a brazing or soldering agent and after its melting, due to the reaction heat, since it will infiltrate into the possible pores in the reaction products and boundary of each powder. As a result, a dense bonding with improved mechanical properties will be obtained. The obtained powder mixture was cold-pressed into a cylindrical compact in a steel mould by applying 276 MPa uniaxial pressure. Aluminum 1100 plates were selected as the joining parts and cut into samples measuring 20 mm × 20 mm × 5 mm. Their largest surfaces were ground manually up to grit 1200 finish, using SiC grinding paper. The chemical composition is given in Table 1. The mentioned compact was placed between two surfaces of Al1100 plates and this stack was pressed under 157 MPa uniaxial pressure. Then, the joining process was undertaken as follows. First, the obtained stack (sample) was fixed in a steel fixture so that throughout the joining process it was experiencing 9 MPa uniaxial pressure. The steel fixture containing a sample was then placed in a conventional electric furnace at 650 ◦ C to induce the combustion synthesis reaction or thermite reaction. The thermal cycle experienced by the steel fixture and the specimen due to the ignition and combustion of the reactive powder compact were obtained using a data acquisition set (Advantech USB 4718). The procedure used to determine the temperature–time profile has been completely explained in our previous work [19]. The shear strength of the samples was determined using the compressive force of a tension machine (Zwick/Z 250) and a designed fixture (Fig. 1) at a loading rate of 0.5 mm min−1 . The fixture was designed in a way that the shear force could be applied to the bonding surface by a half of the fixture and sample movement was prevented by the other half of the fixture. Thus, if the maximum tolerated force by the specimen is divided by the joined surface area, the compressive shear strength will be obtained. This
is a common way to evaluate the shear strength of combustion joined samples [5,20]. Some joined samples were cut perpendicular to their interface to characterize the interfaces by SEM imaging, EDS analyses, and X-ray diffraction experiments. Vickers microhardness test was carried out across the joint interface, using Buehler 1600-6125, at an applied load of 25 g. 3. Result and discussion 3.1. Thermal cycle of the process Fig. 2 shows the temperature–time profile of the reactive powder compact which is placed in a furnace at 650 ◦ C. It is clear that its ignition and combustion temperatures are 274.8 and 1364 ◦ C, respectively. Thus, in order to trigger the reaction and utilize the released heat for welding, a minimum processing temperature of 275 ◦ C is essential. Higher processing temperatures are favorable because at higher temperatures greater diffusion and transient liquid content accelerates the rate of densification. Furthermore, it will contribute
Fig. 2. Temperature–time profile of the reactive powder compact during the ignition and combustion reactions.
Table 1 Chemical composition of the aluminum 1100 plates. Element
Si
Fe
Cu
Mn
Mg
Zn
Ti
Cr
Ni
Sn
Al
Weight percent
0.096
0.266
0.014
0.01
0.007
0.009
0.002
0.005
0.002
0.003
Rest
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Fig. 3. SEM micrographs of the cross section of welded plates at different magnifications: (a) 80×, (b) 200×, (c) 1000×, (d) 1500× and (e) 20,000×.
to the completion of the reactions. On the other hand, the processing temperature should be low enough to preserve the sample from thermal degradation. Thus, a thermal cycle with a maximum of 650 ◦ C and 40 min was considered for the sample-contained fixture. No dwell time is needed because it may cause degradation of the sample, e.g. grain growth. 3.2. Interfacial microstructure and joint analysis 3.2.1. SEM and EDS investigation results Fig. 3(a)–(e) shows the SEM micrographs of the cross section of the Aluminum 1100 plates welded using thermite like combustion
synthesis reaction. Three regions are distinguishable in Fig. 3(a) as they are illustrated by I, II and III, which are parent alloys, weld zones and reactive media (powder compact), respectively. On the left hand side of this picture there are no weld zones which prove that the combustion synthesis reaction has not occurred in this part of sample. It is probably due to the slow oxidation of elemental Ni and Al powders by the available oxygen from the furnace atmosphere during heating. After consumption of the available oxygen the rest of the reactive media reaches to its ignition temperature without oxidation by free oxygen. At this temperature the reaction in powder mixture starts to release energy in the form of heat causing the surface melting of the Aluminum 1100 plates in both
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Fig. 4. SEM image (a), and EDS analyses of I, II, III, and IV phases within the reacted powder mixture which are presented in (b) to (e), respectively.
sides. Thus, a melted and re-solidified region (weld zone, shown by II) is observed in both sides of the reactive media in the parent alloys. The mean thickness of this layer in each side is about 750 m. Therefore, the amount of heat released from the involved reactions can be estimated and will be discussed further in section of thermodynamic calculations (Section 3.3). The reacted powder compact formed strong and sound bonds with its adjacent Al1100 plates (Fig. 3(b)). The interfaces were further confirmed by their high magnification observations as illustrated in Fig. 3(c). A good bonding was formed between the reacted compact and weld zone without forming any interfacial microcracks or microvoids. As expected, this was because the extra amount of elemental Aluminum has been melted, and infiltrated into the possible voids at the interfaces due to its good wettability. As shown in Fig. 3(d), this joint was comprised of an aluminum matrix composite which is reinforced by various intermetallic compounds and Al2 O3 nanoparticles. The gray region in this micrograph is the aluminum matrix, and the white and black phases dispersed within this matrix are intermetallics and aluminum oxide,
respectively. As illustrated in Fig. 3(e), it is notable that the composite has nano-sized Al2 O3 particles in the Al matrix. Their identity will be interpreted in the next paragraphs using the reported morphology for different phases in the literatures, EDS analyses, and X-ray diffraction experiments. The positions of EDS point analyses from the reacted powder mixture and their results are presented in Fig. 4. Four phases are shown in Fig. 4(a), as marked by I, II, III and IV and their corresponding EDS analyses are shown in Fig. 4(b), (c), (d) and (e), respectively. In the EDS analysis of the black phase (I), corresponding peaks to aluminum, oxygen and carbon elements are seen. The weak carbon peak is due to the contamination from the furnace atmosphere and it is not involved in the reactions. Aluminum and oxygen peaks arise from the Al2 O3 phase, which is in the form of particles. It is confirmed by XRD results. Phase II consists of aluminum and nickel elements according to its EDS analysis in Fig. 4(c). A weak peak of iron (Fe) in Fig. 4(c) may be from two sources: one is from the parent alloy (see Table 1), and the other results from the ball milling process. As mentioned before, this joint is an aluminum matrix
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Fig. 5. (a) SEM image of different phases in the weld zones, EDS analyses prove that the bright phases consist of Al, Ni, and Cu elements (b), the less bright phase consists of Al and Ni elements (c), and the matrix is Aluminum (d).
composite, the gray phase III in Fig. 4(a) is the aluminum matrix, as supported by the EDS analysis in Fig. 4(d). In this figure, in addition to Al peaks some weak peaks corresponding to copper are observed too. It is possible that some of the copper oxide has been reduced to copper during ball milling by aluminum and the copper is dissolved in Al matrix even if the reduction reaction is prevented by hexane (PCA). The white phase named IV in Fig. 4(a) contains Al, Cu, Ni and O elements. From XRD results without showing any presence of Spinel, it is most likely that this phase contains some Al2 O3 nano particles which are responsible for the observation of oxygen peaks in EDS analysis. EDS analysis accompanied with the reported morphologies for different phases also can help to identify the produced phases in the weld zone (Fig. 5). The bright-skeleton like phase observed in this region (e.g. Fig. 3(e) or Fig. 5(a)) is composed of Al, Cu and Ni elements according to its EDS analysis in Fig. 5(b). Al7 Cu4 Ni ternary phase has this morphology [21] and its presence is confirmed by the XRD results in Fig. 6. Therefore, there is no doubt that this phase is Al7 Cu4 Ni intermetallic compound. The copper containing phases are brighter than other phases in the SEM micrographs. In addition to this phase there is a less bright phase which consists of Al and Ni elements (Fig. 5(c)) and with the aid of XRD results it is claimed that it is Al3 Ni2 intermetallic compound. The gray continuous phase is the parent Aluminum alloy (see Fig. 5(d)).
3.2.2. XRD results X-ray diffraction patterns of the cross section of the joined plates (Fig. 6) indicate the presence of Al3 Ni2 and Al2 O3 phases.
They formed due to the following combustion synthesis reactions, respectively [22,1]. 3Al + 2Ni → Al3 Ni2 ,
◦
H298 = −64.5 kJ mol−1 atoms or
− 322.5 kJ mol−1
(I)
◦
3CuO + 2Al → Al2 O3 + 3Cu, H298 = −1212.5 kJ mol−1
(II)
Subsequently, the obtained Al3 Ni2 intermetallic compound reacts with the remaining copper and aluminum elements, which are probably in the liquid state due to the rapid release of reactions’ heat, to form the Al7 Cu4 Ni phase, as detected by XRD. This result is consistent with the previous report that this phase forms at the constant temperature of 590 ◦ C by the following reaction [23]: L + Al3 Ni2 → (Al) + Al7 Cu4 Ni
(III)
Also, the observed strong peaks corresponding to Al were due to its surplus utilization in the reactive power mixture and the parent Al1100 alloys. Simultaneous consideration of the reported morphologies, XRD, and EDS results indicates that each phase which contains Al and Ni elements should be Al3 Ni2 intermetallic compound (e.g. phase II in Fig. 4(a)) and each phase which consists of three elements of Al, Ni and Cu (e.g. phase IV in Fig. 4(a)) should be Al7 Cu4 Ni intermetallic compounds. Suppose that all three moles of CuO is reduced by the abundant available Al element; and also the produced phases are Al3 Ni2 ,
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Fig. 6. X-ray diffraction pattern of the cross section of the joined plates via thermite like combustion synthesis reactions.
Al2 O3 , Al7 Cu4 Ni, and Al, then the overall reaction occurring during the welding process can be written as follow: Ni + 14Al + 3CuO → (1/8)Al3 Ni2 + Al2 O3 + (3/4)Al7 Cu4 Ni + 6.375Al
(IV)
3.3. Thermodynamic calculation results
3.4. Mechanical testing results 3.4.1. Shear strength measurements The maximum tolerated force by the joined samples was divided by the parallel surface area of the joint and the shear strength of the welded samples was determined. The mean compressive shear strength of three samples was about 27 MPa, which is much higher than that of the diffusion bonded plates (8.05 MPa) at the same test conditions. Since both techniques of diffusion bonding and the
As it was mentioned before, the amount of released heat from the reactions can be estimated by measuring the melted thickness of aluminum plates and some thermodynamic calculations. It should be noticed that the simultaneous combustion (SC) mode of the combustion synthesis reactions is involved here. Since the amount of released heat increases with the increment of utilized powder mixture’s mass the melted thickness of each plate increases, too. Here this relation is discussed at a special point of 0.25 g powder mixture. Suppose that, the released heat would increase the temperature of the reaction’s products as well as the base alloys from the test temperature (650 ◦ C) to the melting point of aluminum plates (which needs Q1 = 128 J heat) and then the additional heat (Q2) would melt a surface layer from each aluminum plates. The amount of Q1 can be calculated from thermodynamic constants as follow: 933 Q1 = H547 (Al sheets + reaction product) = 128 J
On the other hand, Q2 can be calculated from the weld zones dimensions and fusion enthalpy of aluminum. The surface area of weld zone is 220 mm2 and its thickness is 750 micrometer in each side, approximately. Thus, 165 mm3 (0.891 g or 0.033 mole) of Al has been melted during the process, which needs 353 J heat as follow: Q2 = nHs→l,Al (Al sheets) = 353 J Finally, the total released heat due to the combustion synthesis reactions in a 0.25 g reactive powder compact is equal to the summation of Q1 and Q2, which is about 481 J. The little difference between the calculated released heat (481 J) and the enthalpies of the aforementioned reactions per 0.25 g of utilized powder mixture (493 J) indicates the accuracy of our suppositions. In such a simple and theoretical way one can establish a relation between the amount of utilized powder mass and the desired melted thickness.
Fig. 7. (a) Hardness distribution profile across the welded Al1100 plates, and (b) the image showing the corresponding hardness measurement locations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
E. Bahrami Motlagh et al. / Materials Chemistry and Physics 133 (2012) 757–763
welding procedure explained in this work lead to a large dimensional bonding their obtained strengths are compared here. The shear strength of the diffusion bonded plates is affected by the surface preparation conditions considerably because this process requires the solid state diffusion. In contrast, in combustion synthesis welding because of the occurrence of surface melting (due to the reaction heat) surface preparation is not so important and it does not affect the shear strength as in diffusion bonding and makes the process easier. However, the obtained shear strength of 27 MPa is close to the tensile strength of 1100 Aluminum alloy (90 MPa [24]) by 30%. Since the friction stir spot welded tensile strength is closed to 1100 Aluminum alloy by 33.1% [25], thus, the introduced procedure is successful. 3.4.2. Microhardness test results Fig. 7(a) shows the hardness distribution across the joining interface. In order to avoid the interaction between the nearest indentations, the test measurements were made 50 m far from each other in longitudinal direction and 100 m far in latitude, according to Fig. 7(b). Despite the variations of hardness values it is clear that the mean hardness values change continuously from low values in the base alloys to high values in the reacted media (see the red line in Fig. 7(a)). This general trend of hardness variations is reasonable, because the reacted media containing different intermetallic compounds (such as Al3 Ni2 ) and Al2 O3 nanoparticles in an aluminum matrix, show the maximum hardness. The weld zones which contain dispersed intermetallics in their aluminum matrix occupy the second level of hardness. Another reason for higher hardness of these zones may rise from their finer grains compared to that of the parent alloys due to the melting and rapid solidification phenomena. 4. Conclusion Mechanical activated composite powder of 14Al–3CuO–Ni was used for welding of 1100 Aluminum alloy. The exothermic thermite like combustion synthesis reactions melted a thickness of 750 m from each joining parts. Here, an aluminum matrix composite joint reinforced by Al3 Ni2 , Al7 Cu4 Ni and Al2 O3 phases was obtained. The presence of these phases was confirmed by X-ray diffraction experiments, and morphological investigations accompanied with EDS analyses. The mean shear strength of these joints was
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27 MPa, approximately, which is much higher than that of the diffusion bonded plates (8.05 MPa) at the same test conditions. This reveals the potential of the process for welding of non-refractory materials. Acknowledgement The authors wish to thank vice president for research and technology of Ferdowsi University of Mashhad because of financial support (Grant No. 2.16121). References [1] K.J. Blobaum, M.E. Reiss, J.M. Plitzko Lawrence, T.P. Weihs, J. Appl. Phys. 94 (5) (2003) 2915–2922. [2] C. Pascal, R.M. Marin-Ayral, J.C. Tédenac, J. Alloys Compd. 337 (2002) 221–225. [3] J. Cao, J.C. Feng, Z.R. Li, J. Mater. Sci. 41 (2006) 4720–4724. [4] Jeremiah David Edward White, Combustion based technique for synthesis and joining of refractory materials, PhD thesis, University of Notre Dame, Indiana, 2009. [5] S. Li, Y. Zhou, H. Duan, J. Mater. Sci. 38 (2003) 4065–4070. [6] Y. Miyamoto, T. Nakamoto, M. Koizumi, J. Mater. Res. 1 (1) (1986) 7–9. [7] W. Liu, M. Naka, Trans. JWRI 29 (2000) 1–10. [8] C. Pascal, R.M. Marin-Ayral, J.C. Tédenac, C. Merlet, J. Mater. Process. Technol. 135 (2003) 91–100. [9] A.J. Swiston Jr., T.C. Hufnagel, T.P. Weihs, Scr. Mater. 48 (2003) 1575–1580. [10] W. Liu, M. Naka, Scr. Mater. 48 (2003) 1225–1230. [11] A.S. Mukasyan, J.D.E. White, Int. J. Self-Propag. High-Temp Synth. 16 (2007) 154–168. [12] S.M. Umbrajkar, M. Schoenitz, E.L. Dreizin, Thermochim. Acta 451 (2006) 34–43. [13] M. Schoenitz, T. Ward, E.L. Dreizin, Mater. Res. Soc. Symp. Proc. 800 (2004) 261–266. [14] E.L. Dreizin, M. Schoenitz, United States Patent, No. US007524355B2, Apr. 28, 2009. [15] D. Stamatis, Z. Jiang, V.K. Hoffmann, M. Schoenitz, E.L. Dreizin, Combust. Sci. Technol. 181 (2009) 97–116. [16] B.S.B. Reddy, K. Das, S. Das, J. Mater. Sci. 42 (2007) 9366–9378. [17] F. Essl, J. Bruhn, R. Janssen, N. Claussen, Mater. Chem. Phys. 61 (1999) 69–77. [18] P. Vityaz’, T. Grigor’eva, T. Talako, A. Barinova, A. Letsko, Dokl. Chem. Technol. 405 (2005) 255–257. [19] E. Bahrami Motlagh, H. Nasiri, J. Vahdati Khaki, M. Haddad Sabzevar, Surf. Coat. Technol. 205 (2011) 5515–5520. [20] A.K. Jadoon, J. Mater. Sci. 39 (2004) 593–604. [21] ASM Committe, Metallography and microstructures, ASM Int. 9 (2004) 1693. [22] D. Shi, B. Wen, R. Melnik, S. Yao, T. Li, J. Solid State Chem. 182 (2009) 2664–2669. [23] V.S. Zolotorevsky, N.A. Belov, M. Glazoff, Casting Aluminum Alloys, first ed., Linacre House, Jordan Hill, 2007. [24] J.F. Shackelford, W. Alexander, Materials Science and Engineering Handbook, CRC Press, 2001, 1556 pp. [25] M. Awang, I.M. Ahmat, P. Hussain, J. Appl. Sci. 11 (2011) 1959–1965.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Welding of aluminum alloys through thermite like reactions in Al–CuO–Ni system Ehsan Bahrami Motlagh ∗ , Jalil Vahdati Khaki, Mohsen Haddad Sabzevar Department of Materials Science and Engineering, Engineering Faculty, Ferdowsi University of Mashhad, P.O. Box 9177948944, Mashhad, Iran
a r t i c l e
i n f o
Article history: Received 4 April 2011 Received in revised form 21 January 2012 Accepted 23 January 2012 Keywords: A. Composite materials B. Welding C. Mechanical testing D. Hardness
a b s t r a c t In this work, first, a metastable composite powder of “14Al–3CuO–Ni” with a decreased ignition temperature was obtained via Arrested Reactive Milling (ARM), then this exothermic blend was used for welding of 1100 Aluminum alloy. The reactive media and the weld zones were investigated using scanning electron microscope. X-ray diffraction experiment and morphological investigations accompanied with the EDS analyses were carried out in order to evaluate the reactions’ products. Vickers microhardness profile across the joint and the shear strength of the joints were determined. The weld zone thickness in each of the parent alloys was measured to be 750 m, approximately. Results showed that different reactions occurring during the process lead to the in situ formation of different intermetallic compounds such as Al3 Ni2 and Al7 Cu4 Ni as well as Al2 O3 nanoparticles at the interface. Thus, this area has the maximum hardness (80–90 VHN) and the minimum hardness of 35 VHN belongs to the parent alloys. The mean shear strength of the obtained joints was 27 MPa. © 2012 Published by Elsevier B.V.
1. Introduction Thermite reactions have been utilized in materials joining applications for more than a century. In rail welding, for example, powders of Fe2 O3 and Al are combined and after thermal ignition, they react to form molten Fe and molten Al2 O3 [1]. Since combustion synthesis welding can harvest the released heat from the highly exothermic chemical reactions, recently, it has been widely used to join different refractory materials such as nickel-based and TiAl intermetallic compounds [2,3], carbon–carbon composites [4] and different combinations of ceramics and metals [5,6]. Liu and Naka [7] have reported that Ni-based composite joints reinforced with different percentages of Al2 O3 have been produced using combustion synthesis reactions. Also, Li and his co-workers [5] have utilized this process in order to achieve a functionally graded joint between SiC ceramic and Ni-based superalloy. Another possible application in repairing gas turbine components has been introduced by Pascal et al. [8]. Joining of bulk metallic glasses using combustion synthesis reactions has been reported by Swiston et al. [9], in which no recrystallization has been observed due to the rapid heating and cooling rates. Joining of dissimilar materials could be another category of applications [10]. Various methods which have implemented combustion synthesis reactions’ heat for welding purposes have been classified well by Mukasyan and White [11].
∗ Corresponding author. Tel.: +98 511 8763305; fax: +98 511 8763305. E-mail address: [email protected] (E. Bahrami Motlagh). 0254-0584/$ – see front matter © 2012 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2012.01.086
However, the drawbacks for utilization of combustion synthesis reactions in welding of non-refractory materials such as aluminum and its alloys have been, first, the porosity in the products of the combustion synthesis reactions which are inevitable and may cause defected bonds. The high ignition temperatures of these reactions which are typically higher than the melting points of these kinds of materials are the second problem. The first problem can be avoided using a brazing agent [3]. Also, some methods have been devised recently to decrease the ignition temperatures of the combustion synthesis reactions such as high energy ball milling (HEBM) [12]. Several types of reactive nano composites have been synthesized by arrested reactive milling (ARM) [13,14]. Among them, Al–CuO thermites are of particular interest, because the reaction is highly exothermic [12,15], and its temperature can be adjusted to produce either molten or vapor-phase copper [12]. More recently, the combination of mechanical activation (high energy ball milling) and thermal activation (combustion synthesis) of chemical reactions has widened the possible applications for both methods [16]. In this procedure, mechanical activation of reactions is used as an intermediate step to enhance the kinetics of a reaction during subsequent thermal treatment. For instance, Essl and his co-workers [17] have prepared a precursor powder mixture, based on aluminum, for reaction bonding of alumina. It is observed that the mechanical activation induced by a HEBM can substantially reduce the reaction temperature required for completing the reactions. Also, the potential limitations of HEBM can be minimized and its productivity can be increased sufficiently by reducing the activation time during the HEBM. This is possible when the mechano-composites are used as precursors [18]. As a result, all the
758
E. Bahrami Motlagh et al. / Materials Chemistry and Physics 133 (2012) 757–763
advantages of thermo-chemical approaches are conserved while the limitations may be reduced significantly. This procedure has already been implemented in coating of metals successfully [19], and production of bimetals would be another potential application. In this study, the mechanical–thermal activation of chemical reactions in 14Al + Ni + 3CuO system is used for welding of aluminum 1100 alloys. Thus, the applications of thermite reactions have been extended for welding of non-refractory materials. The advantages of this process would be considerable from two points of view: first, elimination of the need for energy sources for welding, and second, using metal oxides as a part of starting materials. Fig. 1. Schematic of the designed fixture for shear test measurement.
2. Experimental The starting materials were Al powder (99% purity, Khorasan Powder Metallurgy Co., Mashhad, Iran) with an average particle size of 30 m, CuO powder (96% purity, <100 m), Ni powder (99.99% purity) with particle size of 20 m, and hexane as the process control agent (PCA) for milling. Three grams of powder mixture was prepared according to the stoichiometry of “14Al + Ni + 3CuO” and milled for 5 h in the presence of 4 ml hexane and in argon atmosphere (purity 99.9%). A large amount of Al powder was utilized in the reactant powder mixture in order to achieve two goals. The first goal was to lower the adiabatic temperature of the combustion synthesis reactions so that no gaseous products could be formed. Otherwise, the evolved gas can lead to defected bonds (in case of air trapping) or no bonding due to the separation of the parent alloys. Secondly, the extra amount of Al can act as a brazing or soldering agent and after its melting, due to the reaction heat, since it will infiltrate into the possible pores in the reaction products and boundary of each powder. As a result, a dense bonding with improved mechanical properties will be obtained. The obtained powder mixture was cold-pressed into a cylindrical compact in a steel mould by applying 276 MPa uniaxial pressure. Aluminum 1100 plates were selected as the joining parts and cut into samples measuring 20 mm × 20 mm × 5 mm. Their largest surfaces were ground manually up to grit 1200 finish, using SiC grinding paper. The chemical composition is given in Table 1. The mentioned compact was placed between two surfaces of Al1100 plates and this stack was pressed under 157 MPa uniaxial pressure. Then, the joining process was undertaken as follows. First, the obtained stack (sample) was fixed in a steel fixture so that throughout the joining process it was experiencing 9 MPa uniaxial pressure. The steel fixture containing a sample was then placed in a conventional electric furnace at 650 ◦ C to induce the combustion synthesis reaction or thermite reaction. The thermal cycle experienced by the steel fixture and the specimen due to the ignition and combustion of the reactive powder compact were obtained using a data acquisition set (Advantech USB 4718). The procedure used to determine the temperature–time profile has been completely explained in our previous work [19]. The shear strength of the samples was determined using the compressive force of a tension machine (Zwick/Z 250) and a designed fixture (Fig. 1) at a loading rate of 0.5 mm min−1 . The fixture was designed in a way that the shear force could be applied to the bonding surface by a half of the fixture and sample movement was prevented by the other half of the fixture. Thus, if the maximum tolerated force by the specimen is divided by the joined surface area, the compressive shear strength will be obtained. This
is a common way to evaluate the shear strength of combustion joined samples [5,20]. Some joined samples were cut perpendicular to their interface to characterize the interfaces by SEM imaging, EDS analyses, and X-ray diffraction experiments. Vickers microhardness test was carried out across the joint interface, using Buehler 1600-6125, at an applied load of 25 g. 3. Result and discussion 3.1. Thermal cycle of the process Fig. 2 shows the temperature–time profile of the reactive powder compact which is placed in a furnace at 650 ◦ C. It is clear that its ignition and combustion temperatures are 274.8 and 1364 ◦ C, respectively. Thus, in order to trigger the reaction and utilize the released heat for welding, a minimum processing temperature of 275 ◦ C is essential. Higher processing temperatures are favorable because at higher temperatures greater diffusion and transient liquid content accelerates the rate of densification. Furthermore, it will contribute
Fig. 2. Temperature–time profile of the reactive powder compact during the ignition and combustion reactions.
Table 1 Chemical composition of the aluminum 1100 plates. Element
Si
Fe
Cu
Mn
Mg
Zn
Ti
Cr
Ni
Sn
Al
Weight percent
0.096
0.266
0.014
0.01
0.007
0.009
0.002
0.005
0.002
0.003
Rest
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Fig. 3. SEM micrographs of the cross section of welded plates at different magnifications: (a) 80×, (b) 200×, (c) 1000×, (d) 1500× and (e) 20,000×.
to the completion of the reactions. On the other hand, the processing temperature should be low enough to preserve the sample from thermal degradation. Thus, a thermal cycle with a maximum of 650 ◦ C and 40 min was considered for the sample-contained fixture. No dwell time is needed because it may cause degradation of the sample, e.g. grain growth. 3.2. Interfacial microstructure and joint analysis 3.2.1. SEM and EDS investigation results Fig. 3(a)–(e) shows the SEM micrographs of the cross section of the Aluminum 1100 plates welded using thermite like combustion
synthesis reaction. Three regions are distinguishable in Fig. 3(a) as they are illustrated by I, II and III, which are parent alloys, weld zones and reactive media (powder compact), respectively. On the left hand side of this picture there are no weld zones which prove that the combustion synthesis reaction has not occurred in this part of sample. It is probably due to the slow oxidation of elemental Ni and Al powders by the available oxygen from the furnace atmosphere during heating. After consumption of the available oxygen the rest of the reactive media reaches to its ignition temperature without oxidation by free oxygen. At this temperature the reaction in powder mixture starts to release energy in the form of heat causing the surface melting of the Aluminum 1100 plates in both
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Fig. 4. SEM image (a), and EDS analyses of I, II, III, and IV phases within the reacted powder mixture which are presented in (b) to (e), respectively.
sides. Thus, a melted and re-solidified region (weld zone, shown by II) is observed in both sides of the reactive media in the parent alloys. The mean thickness of this layer in each side is about 750 m. Therefore, the amount of heat released from the involved reactions can be estimated and will be discussed further in section of thermodynamic calculations (Section 3.3). The reacted powder compact formed strong and sound bonds with its adjacent Al1100 plates (Fig. 3(b)). The interfaces were further confirmed by their high magnification observations as illustrated in Fig. 3(c). A good bonding was formed between the reacted compact and weld zone without forming any interfacial microcracks or microvoids. As expected, this was because the extra amount of elemental Aluminum has been melted, and infiltrated into the possible voids at the interfaces due to its good wettability. As shown in Fig. 3(d), this joint was comprised of an aluminum matrix composite which is reinforced by various intermetallic compounds and Al2 O3 nanoparticles. The gray region in this micrograph is the aluminum matrix, and the white and black phases dispersed within this matrix are intermetallics and aluminum oxide,
respectively. As illustrated in Fig. 3(e), it is notable that the composite has nano-sized Al2 O3 particles in the Al matrix. Their identity will be interpreted in the next paragraphs using the reported morphology for different phases in the literatures, EDS analyses, and X-ray diffraction experiments. The positions of EDS point analyses from the reacted powder mixture and their results are presented in Fig. 4. Four phases are shown in Fig. 4(a), as marked by I, II, III and IV and their corresponding EDS analyses are shown in Fig. 4(b), (c), (d) and (e), respectively. In the EDS analysis of the black phase (I), corresponding peaks to aluminum, oxygen and carbon elements are seen. The weak carbon peak is due to the contamination from the furnace atmosphere and it is not involved in the reactions. Aluminum and oxygen peaks arise from the Al2 O3 phase, which is in the form of particles. It is confirmed by XRD results. Phase II consists of aluminum and nickel elements according to its EDS analysis in Fig. 4(c). A weak peak of iron (Fe) in Fig. 4(c) may be from two sources: one is from the parent alloy (see Table 1), and the other results from the ball milling process. As mentioned before, this joint is an aluminum matrix
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Fig. 5. (a) SEM image of different phases in the weld zones, EDS analyses prove that the bright phases consist of Al, Ni, and Cu elements (b), the less bright phase consists of Al and Ni elements (c), and the matrix is Aluminum (d).
composite, the gray phase III in Fig. 4(a) is the aluminum matrix, as supported by the EDS analysis in Fig. 4(d). In this figure, in addition to Al peaks some weak peaks corresponding to copper are observed too. It is possible that some of the copper oxide has been reduced to copper during ball milling by aluminum and the copper is dissolved in Al matrix even if the reduction reaction is prevented by hexane (PCA). The white phase named IV in Fig. 4(a) contains Al, Cu, Ni and O elements. From XRD results without showing any presence of Spinel, it is most likely that this phase contains some Al2 O3 nano particles which are responsible for the observation of oxygen peaks in EDS analysis. EDS analysis accompanied with the reported morphologies for different phases also can help to identify the produced phases in the weld zone (Fig. 5). The bright-skeleton like phase observed in this region (e.g. Fig. 3(e) or Fig. 5(a)) is composed of Al, Cu and Ni elements according to its EDS analysis in Fig. 5(b). Al7 Cu4 Ni ternary phase has this morphology [21] and its presence is confirmed by the XRD results in Fig. 6. Therefore, there is no doubt that this phase is Al7 Cu4 Ni intermetallic compound. The copper containing phases are brighter than other phases in the SEM micrographs. In addition to this phase there is a less bright phase which consists of Al and Ni elements (Fig. 5(c)) and with the aid of XRD results it is claimed that it is Al3 Ni2 intermetallic compound. The gray continuous phase is the parent Aluminum alloy (see Fig. 5(d)).
3.2.2. XRD results X-ray diffraction patterns of the cross section of the joined plates (Fig. 6) indicate the presence of Al3 Ni2 and Al2 O3 phases.
They formed due to the following combustion synthesis reactions, respectively [22,1]. 3Al + 2Ni → Al3 Ni2 ,
◦
H298 = −64.5 kJ mol−1 atoms or
− 322.5 kJ mol−1
(I)
◦
3CuO + 2Al → Al2 O3 + 3Cu, H298 = −1212.5 kJ mol−1
(II)
Subsequently, the obtained Al3 Ni2 intermetallic compound reacts with the remaining copper and aluminum elements, which are probably in the liquid state due to the rapid release of reactions’ heat, to form the Al7 Cu4 Ni phase, as detected by XRD. This result is consistent with the previous report that this phase forms at the constant temperature of 590 ◦ C by the following reaction [23]: L + Al3 Ni2 → (Al) + Al7 Cu4 Ni
(III)
Also, the observed strong peaks corresponding to Al were due to its surplus utilization in the reactive power mixture and the parent Al1100 alloys. Simultaneous consideration of the reported morphologies, XRD, and EDS results indicates that each phase which contains Al and Ni elements should be Al3 Ni2 intermetallic compound (e.g. phase II in Fig. 4(a)) and each phase which consists of three elements of Al, Ni and Cu (e.g. phase IV in Fig. 4(a)) should be Al7 Cu4 Ni intermetallic compounds. Suppose that all three moles of CuO is reduced by the abundant available Al element; and also the produced phases are Al3 Ni2 ,
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Fig. 6. X-ray diffraction pattern of the cross section of the joined plates via thermite like combustion synthesis reactions.
Al2 O3 , Al7 Cu4 Ni, and Al, then the overall reaction occurring during the welding process can be written as follow: Ni + 14Al + 3CuO → (1/8)Al3 Ni2 + Al2 O3 + (3/4)Al7 Cu4 Ni + 6.375Al
(IV)
3.3. Thermodynamic calculation results
3.4. Mechanical testing results 3.4.1. Shear strength measurements The maximum tolerated force by the joined samples was divided by the parallel surface area of the joint and the shear strength of the welded samples was determined. The mean compressive shear strength of three samples was about 27 MPa, which is much higher than that of the diffusion bonded plates (8.05 MPa) at the same test conditions. Since both techniques of diffusion bonding and the
As it was mentioned before, the amount of released heat from the reactions can be estimated by measuring the melted thickness of aluminum plates and some thermodynamic calculations. It should be noticed that the simultaneous combustion (SC) mode of the combustion synthesis reactions is involved here. Since the amount of released heat increases with the increment of utilized powder mixture’s mass the melted thickness of each plate increases, too. Here this relation is discussed at a special point of 0.25 g powder mixture. Suppose that, the released heat would increase the temperature of the reaction’s products as well as the base alloys from the test temperature (650 ◦ C) to the melting point of aluminum plates (which needs Q1 = 128 J heat) and then the additional heat (Q2) would melt a surface layer from each aluminum plates. The amount of Q1 can be calculated from thermodynamic constants as follow: 933 Q1 = H547 (Al sheets + reaction product) = 128 J
On the other hand, Q2 can be calculated from the weld zones dimensions and fusion enthalpy of aluminum. The surface area of weld zone is 220 mm2 and its thickness is 750 micrometer in each side, approximately. Thus, 165 mm3 (0.891 g or 0.033 mole) of Al has been melted during the process, which needs 353 J heat as follow: Q2 = nHs→l,Al (Al sheets) = 353 J Finally, the total released heat due to the combustion synthesis reactions in a 0.25 g reactive powder compact is equal to the summation of Q1 and Q2, which is about 481 J. The little difference between the calculated released heat (481 J) and the enthalpies of the aforementioned reactions per 0.25 g of utilized powder mixture (493 J) indicates the accuracy of our suppositions. In such a simple and theoretical way one can establish a relation between the amount of utilized powder mass and the desired melted thickness.
Fig. 7. (a) Hardness distribution profile across the welded Al1100 plates, and (b) the image showing the corresponding hardness measurement locations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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welding procedure explained in this work lead to a large dimensional bonding their obtained strengths are compared here. The shear strength of the diffusion bonded plates is affected by the surface preparation conditions considerably because this process requires the solid state diffusion. In contrast, in combustion synthesis welding because of the occurrence of surface melting (due to the reaction heat) surface preparation is not so important and it does not affect the shear strength as in diffusion bonding and makes the process easier. However, the obtained shear strength of 27 MPa is close to the tensile strength of 1100 Aluminum alloy (90 MPa [24]) by 30%. Since the friction stir spot welded tensile strength is closed to 1100 Aluminum alloy by 33.1% [25], thus, the introduced procedure is successful. 3.4.2. Microhardness test results Fig. 7(a) shows the hardness distribution across the joining interface. In order to avoid the interaction between the nearest indentations, the test measurements were made 50 m far from each other in longitudinal direction and 100 m far in latitude, according to Fig. 7(b). Despite the variations of hardness values it is clear that the mean hardness values change continuously from low values in the base alloys to high values in the reacted media (see the red line in Fig. 7(a)). This general trend of hardness variations is reasonable, because the reacted media containing different intermetallic compounds (such as Al3 Ni2 ) and Al2 O3 nanoparticles in an aluminum matrix, show the maximum hardness. The weld zones which contain dispersed intermetallics in their aluminum matrix occupy the second level of hardness. Another reason for higher hardness of these zones may rise from their finer grains compared to that of the parent alloys due to the melting and rapid solidification phenomena. 4. Conclusion Mechanical activated composite powder of 14Al–3CuO–Ni was used for welding of 1100 Aluminum alloy. The exothermic thermite like combustion synthesis reactions melted a thickness of 750 m from each joining parts. Here, an aluminum matrix composite joint reinforced by Al3 Ni2 , Al7 Cu4 Ni and Al2 O3 phases was obtained. The presence of these phases was confirmed by X-ray diffraction experiments, and morphological investigations accompanied with EDS analyses. The mean shear strength of these joints was
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27 MPa, approximately, which is much higher than that of the diffusion bonded plates (8.05 MPa) at the same test conditions. This reveals the potential of the process for welding of non-refractory materials. Acknowledgement The authors wish to thank vice president for research and technology of Ferdowsi University of Mashhad because of financial support (Grant No. 2.16121). References [1] K.J. Blobaum, M.E. Reiss, J.M. Plitzko Lawrence, T.P. Weihs, J. Appl. Phys. 94 (5) (2003) 2915–2922. [2] C. Pascal, R.M. Marin-Ayral, J.C. Tédenac, J. Alloys Compd. 337 (2002) 221–225. [3] J. Cao, J.C. Feng, Z.R. Li, J. Mater. Sci. 41 (2006) 4720–4724. [4] Jeremiah David Edward White, Combustion based technique for synthesis and joining of refractory materials, PhD thesis, University of Notre Dame, Indiana, 2009. [5] S. Li, Y. Zhou, H. Duan, J. Mater. Sci. 38 (2003) 4065–4070. [6] Y. Miyamoto, T. Nakamoto, M. Koizumi, J. Mater. Res. 1 (1) (1986) 7–9. [7] W. Liu, M. Naka, Trans. JWRI 29 (2000) 1–10. [8] C. Pascal, R.M. Marin-Ayral, J.C. Tédenac, C. Merlet, J. Mater. Process. Technol. 135 (2003) 91–100. [9] A.J. Swiston Jr., T.C. Hufnagel, T.P. Weihs, Scr. Mater. 48 (2003) 1575–1580. [10] W. Liu, M. Naka, Scr. Mater. 48 (2003) 1225–1230. [11] A.S. Mukasyan, J.D.E. White, Int. J. Self-Propag. High-Temp Synth. 16 (2007) 154–168. [12] S.M. Umbrajkar, M. Schoenitz, E.L. Dreizin, Thermochim. Acta 451 (2006) 34–43. [13] M. Schoenitz, T. Ward, E.L. Dreizin, Mater. Res. Soc. Symp. Proc. 800 (2004) 261–266. [14] E.L. Dreizin, M. Schoenitz, United States Patent, No. US007524355B2, Apr. 28, 2009. [15] D. Stamatis, Z. Jiang, V.K. Hoffmann, M. Schoenitz, E.L. Dreizin, Combust. Sci. Technol. 181 (2009) 97–116. [16] B.S.B. Reddy, K. Das, S. Das, J. Mater. Sci. 42 (2007) 9366–9378. [17] F. Essl, J. Bruhn, R. Janssen, N. Claussen, Mater. Chem. Phys. 61 (1999) 69–77. [18] P. Vityaz’, T. Grigor’eva, T. Talako, A. Barinova, A. Letsko, Dokl. Chem. Technol. 405 (2005) 255–257. [19] E. Bahrami Motlagh, H. Nasiri, J. Vahdati Khaki, M. Haddad Sabzevar, Surf. Coat. Technol. 205 (2011) 5515–5520. [20] A.K. Jadoon, J. Mater. Sci. 39 (2004) 593–604. [21] ASM Committe, Metallography and microstructures, ASM Int. 9 (2004) 1693. [22] D. Shi, B. Wen, R. Melnik, S. Yao, T. Li, J. Solid State Chem. 182 (2009) 2664–2669. [23] V.S. Zolotorevsky, N.A. Belov, M. Glazoff, Casting Aluminum Alloys, first ed., Linacre House, Jordan Hill, 2007. [24] J.F. Shackelford, W. Alexander, Materials Science and Engineering Handbook, CRC Press, 2001, 1556 pp. [25] M. Awang, I.M. Ahmat, P. Hussain, J. Appl. Sci. 11 (2011) 1959–1965.