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Microstructure and properties analysis of laser-welded Ni–Ti and 316l sheets using copper interlayer
Journal of Manufacturing Processes 26 (2017) 355–363
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Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Microstructure and properties analysis of laser-welded Ni–Ti and 316l sheets using copper interlayer A. Shojaei Zoeram a,∗ , A. Rahmani b , S.A.A. Akbari Mousavi a a b
Welding Lab, School of Metallurgy and Materials, College of Engineering, University of Tehran, P. O. Box: 11155-4563, Tehran, Iran Materials Engineering Department, Imam Khomeini International University, Ghazvin, Iran
a r t i c l e
i n f o
Article history: Received 1 October 2016 Received in revised form 25 January 2017 Accepted 5 February 2017 Keywords: Dissimilar laser welding Ni–Ti alloy X-Ray diffraction analysis SEM EDS Fracture surface
a b s t r a c t Dissimilar pulsed laser welding of 1 mm-thick sheets was carried out on Nitinol/S.S joint. In order to modify the chemical composition of weld metal and to restrict the formation of Fe-Ti intermetallic compounds (IMCs) in the weld metal, Copper thin film with different thicknesses, 100 and 150 m, was used as an interlayer. The effects of interlayer thickness on microstructure, microhardness profile and chemical composition of weld-metal was investigated. Investigations showed that the interlayer thickness has a great influence on the chemical composition of the weld metal and, consequently, affects all of joint properties such as microstructure and ultimate tensile strength. According to X-Ray diffraction analysis, it was revealed that the increase of interlayer thickness decreases the concentration of Fe-Ti IMCs in weld metal and this makes weld metal softer to some extent. Moreover, since the Copper is a ductile metal of low melting temperature, added Copper into weld metal can compensate thermal stresses. Therefore, cracks originated from thermal stresses as well as brittleness of weld-metal were eliminated when the thickness of interlayer increased. However, there were some restricting factors as to the increase of the thickness of Copper interlayer, namely, the formation of Ti-Cu IMCs as well as the formation of Cu-rich globules which have relatively weak bonds with their surrounding matrix and this turns their boundaries to a suitable place for nucleation and propagation of a crack. © 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Unique properties of Nitinol have made it to a special alloy which has a great number of potential applications from medical to aerospace industries [1,2]. Due to undeniable role of successful welding in fabrication of complex parts, weldability is among properties which have a great role in successful engineering applications of an alloy. Poor weldability of Nitinol and lack of knowledge about dissimilar welding of this alloy deprive us of wide potential applications and unique properties of this special alloy [3]. Due to considerable role of dissimilar joints in the reduction of raw materials cost and improvement of design conditions, demand for this type of joints has increased over the last two decades at an accelerating pace [4,5]. Dissimilar welding of materials generally suffers from several drawbacks such as formation of intermetallic compounds (IMCs) in the weld metal, thermal mismatch and different chemical properties of base metals [6,7]. The use of an interlayer
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (A. Shojaei Zoeram).
to modify the chemical compositions of the weld zone [8,9] and employment a high thermal conductive substrate as a heat sink to increase the solidification rate are among suggested solutions for the restriction of the formation of IMCs in the weld metal [10]. Another method suggested to restrict the formation of IMCs in the weld metal is to adjust the welding parameters to decrease heat input as much as possible that contributes to higher solidification rate. In the double-sided welding method in which the penetration depth of each side is halved, the heat input is applied in two stages in smaller amounts. The lower the heat input is, the higher the solidification rate of weld metal would be. The increase of solidification rate restricts the formation of IMCs and this improves mechanical properties of joint [10]. The formation of Fe-Ti IMCs in autogenesis fusion welding of Nitinol and Stainless steel reported as a destructive factor which affects joint properties [11,12]. The formation of these phases in the weld metal makes weld metal brittle. The combination of this brittleness and complex residual stresses formed in weld-metal due to mismatch between thermal coefficients of base metals leads to formation of cracks in the weld-metal and this limits the applications of dissimilar joints in many industries [13]. Copper is a soft metal of lower melting point than stainless steel and Nitinol. Hence it can compensate the effect of thermal mis-
http://dx.doi.org/10.1016/j.jmapro.2017.02.005 1526-6125/© 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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match during the solidification progress in dissimilar welding of stainless steel and Nitinol [14]. Moreover, Nickel atoms could be substituted for Copper atoms in Nitinol lattice and the conclusion of this substitution is the formation of Ti (Ni, Cu) shape memory alloy of different austenitic and martensitic transformation temperatures [15]. Moreover, Copper neither reacts with Iron nor forms any brittle intermetallic compounds with Fe in weld metal. Hence, Copper can be used in dissimilar welding of stainless steel and Nitinol as an interlayer. The increase of the interlayer thickness reduces the fraction of base-metals elements in weld metal [16]. Therefore the amount of Fe and Ti elements in the weld metal which are responsible for formation of Fe-Ti IMCs decreases. 2. Experimental procedure In order to produce Ti-rich Nitinol plates with chemical composition of (Ni – 50.7at.%Ti) and the thickness of 1 mm, high purity nickel and titanium with specified weight ratio were cast in a Vacuum Arc Re-melting (VAR) furnace. Caste ingot with a thickness of 1 cm was successfully roll-formed in the sequential hot and cold rolling operation using low-loads. Rolling procedure was designed in a way that 20% of the final thickness reduction was performed by cold forming. To eliminate cold work, sheets were annealed in 450 ◦ C for 30 min and then quenched in water bath. The black oxide layer formed during the above-mentioned procedure was removed with an acid mixture solution of HF: HNO3: H2O = 1:5:10 ratio. The chemical compositions of the materials used in this study are given in Table 1. Copper thin films of 100 m and 150 m thicknesses were employed as interlayer to modify the chemical composition of weld metal and a Copper substrate was used as a heat sink in order to minimize the thermal cycle that originates from welding operation. Prior to welding, the weld pieces were fixed on substrate by vertical screws, then interlayer was inserted between the base metals before the gap between base metals is minimized by fastening the side screw (Fig. 1). The samples were butt welded in double-sided mode according to Fig. 2. Laser welding process was carried out using an SW-1 pulsed Nd:YAG laser machine with a wave length of 1.064 m. Welding parameters are given in Table 2. Prior to metallographic survey, samples were etched by immersion in an acid solution of HF: HNO3: H2O = 1:4:5 ratio for 20 s. Optical microscopy (OM) and scanning electron microscopy (SEM) techniques were utilized to analysis of weld metal microstructure; X-ray diffraction (XRD) analysis was carried out on fracture surfaces to identify the formed phases in the weld metal. In order to investigate the effect of the increase of Copper interlayer thickness on the chemical compositions and distributions of Ti, Fe, and Cu in the weld metal, EDS analysis was carried out on cross sections of the welded joints. Moreover, to evaluate the change of the hardness distribution through the weld metal, according to the dotted line in Fig. 2, microhardness distribution profiles was attained using a load of 200 g Table 1 chemical composition of base metals and inter layer in wt.%.
Stainless Steel NiTi Cu
Fe
Cr
Ni
Mo
Mn
Cu
Ti
Base – –
17.00 – –
12.00 54.39 0.01
2.00 – –
2.00 – –
0.01 – 99.97
– 45.61 –
Fig. 1. Movable fixture with Copper substrate.
Fig. 2. Double sided welding method, hardness and EDS analysis location.
Fig. 3. Dimensions of tensile test samples.
for dwelling time of 15 s. Mechanical properties of the joints were evaluated via tensile tests performed at strain rate of 1 mm/min and in room temperature. Dimensions of the tensile test specimen are given in Fig. 3. 3. Results and discussions 3.1. Microstructure Figs. 4 and 5 depict the microstructure of the first-pass of samples welded with different interlayer thicknesses, 100 and 150 m respectively. As it can be seen, the increase of interlayer thickness had a great influence on microstructure of weld metal. Generally, due to a number of reasons such as pulse nature of pulsed Nd:YAG laser that applies mixing force to melted elements in weld metal, high solidification rate in laser welding and finally diversity of elements in the weld metal microstructure is highly complex one [17]. In the welded sample with 100 m-thick interlayer, solidification sequence along the fusion boundary of Nitinol started with a planar solidification mode and it continued with cellular one just a short distance away from the fusion line (Fig. 4a and d). This is as a result of high solidification rate along fusion boundary adjacent to cold base metal backed by a substrate as well as the substitution of Cop-
Table 2 used parameters for welding of samples. Voltage (V)
Defocusing Distance (mm)
Pulse Duration (ms)
Frequency (Hz)
Speed (mm/s)
Beam Diameter (mm)
average laser power (w)
450
−0.5
2
7.7
1.5
0.4
100
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Fig. 4. Microstructure of welded sample with 100 m Interlayer.
Fig. 5. Microstructure of welded sample with 150 m Interlayer.
per for Nickel atoms in the lattice of Nitinol along fusion boundary of Nitinol. This leads to the formation of the Ti (Ni, Cu) ternary shape memory alloys of which transformation temperatures are different. On the other hand, due to negligible solubility of Copper in iron, microstructure morphology of weld metal along fusion boundary of stainless steel did not show any integrity with that of S.S (Fig. 4c and e). As it is depicted in Fig. 5a and d, the increase of the interlayer thickness reduced the tendency for microstructural integrity along fusion boundary of Nitinol. The addition of excessive amounts of Copper as a result of increasing the interlayer thickness to 150 m unbalances stoichiometric ratio which is required for formation of Ti (Ni, Cu) ternary shape memory alloys. This causes the discontinuity of microstructure morphology between base metal and
weld-metal along Nitinol fusion boundary. The formation of spherical shapes with different sizes in all over the microstructure was another microstructural change which resulted from the increase of interlayer thickness to 150 m (Fig. 5). Fig. 6 depicts the SEM image of the spherical shapes formed in weld metal along with their corresponding chemical compositions. Chemical compositions of these globular features indicate that these zones contain a large amount of Copper in their chemical compositions (Fig. 6). Formation of these features in weld metal of sample welded with the 150 mthick interlayer can be justified by chemical composition of weld metal and Fe-Cu binary phase diagram. Chemical compositions of the weld-metals of welded samples with the 100 and 150 m-thick interlayers are given in Table 3. As can be seen from this table,
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Fig. 6. SEM image of Copper-rich globes and their chemical composition. Table 3 Results of EDS analysis on center areas of fusion metals [wt.%]. Elements
Fe
Ti
Ni
Cu
Cr
100 micrometers 150 micrometers
30.15 27.32
16.10 14.61
24.15 21.60
18.38 24.68
7.50 7.37
the increase of interlayer thickness increased the amount of Cu and, adversely, decreased the amounts of Fe and Ti elements in the weld-metal (Fig. 7). According to Fe-Cu phase diagram, maximum solubility of Cu in γ-Fe is about 13 wt.% in 1400 ◦ C that decreases to 8 wt.% in 1100 ◦ C i.e. the melting point of Copper. That is to say, the reduction of the temperature to the melting point of Copper during solidification process reduces the solubility of Copper in ␥-Fe equivalent to 34 percent. Thus, as the solidification progresses and, consequently, the temperature decreases during cooling cycle, the more amounts of Copper becomes supersaturated and is ejected from the γ-Fe solid solution. Ejected Copper from γ-Fe solid solution contributes to nucleattion and growth of Copper-rich globules in the microstructure of the weld-metal. On the other hand, a certain amount of Copper reacts with Titanium and forms Ti-Cu IMCs. Regarding Table 3, the amount of Copper in the weld metal is more than the solubility limit of Copper in Fe. Therefore, excessive Copper that neither dissolves in γ-Fe solid solution nor reacts with other elements in weld-metal forms the spherical shapes in microstructure of weld-metal. As a result, the thicker the interlayer, the higher the formation possibility of the Cupper-rich globules in microstructure would be. Since the spherical shapes have the lowest ratio of surface area to volume, such shapes apply the lowest surface energy to the system. Moreover, small spheres may join together to produce the larger ones and minimize applied surface energy to the system. The Formation of the spherical shapes was also reported in pulsed Nd:YAG laser welding of S.S to Ti-6Al-4V with Copper interlayer [17]. As can be seen in Fig. 6, there are many fine black points within the Copper-rich globules. According to Cu-Fe phase diagram, Fe has a negligible solubility in Copper and these black points could be supersaturated Iron in Fe-Cu solid solution from which were ejected during cooling [17]. As a result of brittle weld metal and stresses (i.e. thermal or residual stresses), cold cracks develop and propagate in the straight path after solidification of the weld-metal. The formation of the brittle IMCs in the weld metal makes weld metal brittle. And different thermal coefficients of base metals in dissimilar welding create complex stresses during cooling, while base metals start
contracting. The combination of the brittleness originated form the formation of IMCs and complex thermal stresses makes dissimilar welds susceptible to cold cracking. As can be seen in Fig. 4b, the welded sample with the 100 m-think interlayer cracked; straight propagation path is among characters of brittle cold cracking. According to Fe-Ti, Ti-Ni and Ti-Cu binary phase diagrams, IMCs such as TiFe2 , TiFe, TiNi3 , TiNi, Ti2 Ni, TiCu, Ti3 Cu4 , Ti2 Cu3 , TiCu4 and Ti2 Cu can form in the weld metal [18]. The formation of such brittle phases and thermal stress stemmed from welding process can cause the formation of cold cracks in weld metal of welded sample with the 100 m-thick interlayer (Fig. 4b). Sound welds were achieved in the investigation of laser welding of NiTi wires to stainless steel without the use of an interlayer [19]; lower thermal stress in the welding of wires welded in single pulse mode without the use of a fixture could be a reason for achieving the sound joints. Considering distribution maps of elements in cross section of weld metal (Fig. 7) as well as chemical compositions of the weld metal from EDS analysis (Table 3), the formation of Fe2 Ti, FeTi, TiNi and Copper-rich Ti-Cu IMCs phases are more probable than other phases. As shown in Fig. 5, the increase of the interlayer thickness to 150 m eliminated cracks from weld metal. According to the distribution maps of elements as well as chemical compositions of weld metal, the increase of the interlayer thickness reduced amount of Ti and Fe in the weld metal. The reduction of the number of atoms of these elements reduces the interaction between the these atoms in high temperatures; from the kinetic point of view, this leads to the reduction of the reaction rate and finally the reduction of the formation of Fe-Ti IMCs in the weld metal. Restricting the formation of the brittle phases in the weld metal reduced the weld-metal embrittlement which is one of the requirements for the prevention of cold cracking. Moreover, Copper is a soft metal of lower melting point than base metals. Hence, thicker Copper interlayer can result in more compensation of the thermal stresses [14]. This matter is another requirement to prevent cold cracking.
3.2. X-Ray diffraction analysis X-Ray diffraction analysis was carried out on the fracture surface of both cracked and sound joints. The results of respective X-Ray diffraction analysis were depicted in Fig. 8. As can be seen, different IMCs, Fe-Ti, Ti-Ni and Ti-Cu, with various chemical compositions were formed in the weld metal. Melting points of FeTi2 , FeTi, TiNi, TiCu and TiCu3 are as follows: 1427, 1317, 1310, 982, and 890 ◦ C, respectively. Therefore, considering Fe-Ti, Ti-Ni and TiCu binary phase diagrams, from thermodynamical point of view, the formation probability of these IMCs in descending order is as follows: FeTi2 > FeTi > TiNi > TiCu > TiCu3 . Hence, the very FeTi2 and FeTi were the first IMCs which formed in weld metal during the solidification process. These IMCs are more brittle than other phases [17]. The addition of a third element reduces the collisions between Ti and Fe elements heated to high temperatures as a result of laser/material interaction and this reduces the probability of formation of Fe-Ti IMCs from kinetic point of view. Therefore, since the increase of interlayer thickness to 150 m adds more amounts of Copper atoms, the probability of collisions between Ti and Fe elements and of formation of Fe-Ti IMCs decreases. Consequently, the peak intensity related to FeTi and FeTi2 phases decreased by the increase of interlayer thickness. As can be seen, the peak intensity of the ␥-Fe increased as a result of the increase of the interlayer thickness. Because the Copper is an austenite stabilizer [20], the increase of the interlayer thickness stabilizes more amount of ␥-Fe. Intensification of the peaks intensity related to Copper as a consequence of the increase of the interlayer thickness could be justified by formation of Copper-rich globules in the weld metal (Fig. 6).
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Fig. 7. Distribution of various elements in weld metal of welded samples with different interlayer thickness a) 100 m, b) 150 m.
3.3. Microhardness
Fig. 8. X-Ray Diffraction analysis of welded samples with different interlayer thickness.
Fig. 9 depicts the microhardness distribution profile performed on cross section of the first pass. As it is evident from Fig. 9, the increase in the interlayer thickness reduced microhardness of weld metal. The average Vickers hardness of sample welded with the 100 m-thick interlayer was equal to 525 VH which reduced to 470 VH as a result of the increase of the interlayer thickness to 150 m. With regard to X-Ray diffraction analysis, the increase of the interlayer thicknesses lessened the peaks intensity related to Fe-Ti IMCs which are more brittle compared to other phases. Thus, the reduction of the concentration of the very Fe-Ti IMCs reduced the weld metal microhardness. Meanwhile, the formation of Copper-rich globules formed in microstructure of sample welded with the thicker interlayer reduced the microhardness of weld-metal since Copper is a soft metal. Fig. 9 indicates that areas adjacent to fusion boundaries of S.S were the hardest areas in weldmetal. This signifies that Fe-Ti IMCs were hardest IMCs formed in weld-metal of this joint. The closer the areas to S.S fusion boundary are, the more the possibility of the formation of Fe-Ti IMCs would
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Fig. 9. Hardness profile of weld metal.
be. Fig. 10 indicates the morphology of mirohardness indentations in both samples. As can be seen in Fig. 10a, some rounded cracks formed around of microhardness indents in the sample welded with 100 m-thick interlayer. Taking this fact into account that residual stress affects the shape of microhardness indentations, tensile residual stress makes the sides of microhardness indentations concave [21], the cause of the formation of such cracks would be justified. The use of a fixture creates tensile residual stress at the central parts of weld-metal [22]. Hence, after microhardness indentation, the sides of microhardness indents tend to become concave in shape, but in the present case the produced weld metal filled by various brittle IMCs is not so ductile that endure deformation.
Therefore it cracks, and concave cracks form around sides of microhardness indentations instead of deformation. On the other hand, as it is seen in Fig. 10b, round cracks were not observed around microhardness indentations of sample welded with a 150 m-thick interlayer. Considering this fact that Copper can compensate thermal stresses that create residual stresses, this could be justified. The Copper is a soft metal with low melting point. Therefore it can compensate thermal stresses through two mechanisms. Firstly, due to its lower melting point, added Copper into weld metal remains molten for longer time than base metals during solidification. And molten phase compensates thermal stresses during contraction of base metals whilst the base metals are solid. Secondly, since Copper is a soft metal of low yield strength, added Copper into weld metal can continue its role in the compensation of thermal stresses after solidification to some extent. Therefore, since the increase of the thickness of Copper interlayer adds more amount of Copper into weld metal, its compensation effect increases. That is to say, the more added Copper, the greater compensation effect, and consequently the lower the level of residual stress would be. This eliminates crack formed around microhardness indentations (Fig. 10b) even around ones that are as hard as the hardest points of the sample welded with 100 m-thick interlayer and around which cracks formed(Fig. 10a). Therefore, it is concluded that for round cracks form around microhardness indentation, both brittleness and residual stresses should exist simultaneously. That is to say, when there is residual stress in a weld metal, the microhardness indentations tend to be deformed, but if their surrounding circumferences are not enough ductile to endure the deformation, some cracks around indentations appear (Fig. 10a). 3.4. Fracture surface Fig. 11 depicts the fracture surface of welded sample with the 100 m-thick interlayer. Fracture surface shows many scattered micro-cracks and river marks. These features are among the charac-
Fig. 10. Morphology of microhardness indentations of welded samples with different interlayer thicknesses a) 100 m, b) 150 m.
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Fig. 11. Fracture surface of welded sample with 100 m-thick interlayer.
teristics of the brittle fracture. According to results of microhadness and X-Ray diffraction analysis, due to high temperature reactivity of Titanium, several IMCs with various chemical compositions in the weld metal formed. Considering this fact that IMCs are usually brittle as well as owing to the results of the microhardness analyses (Fig. 9), brittle fracture behavior of weld metal could be justified by the formation of IMCs. Fig. 12 reveals some dispersed spherical shapes on the fracture surface of the welded samples with the 150 m-thick interlayer. According to EDS analysis, these rounded shapes are Copper-rich and on the opposite sides of these globular shapes, there are cavities that contain a large amounts of Iron in their chemical compositions. As can be seen in Figs. 5 and 6, the microstructure of the weld metal consists of many Copper-rich
globules with different sizes. Therefore, it could be suggested that these rounded shapes on fracture surface are the same as Copperrich globules that were formed in the microstructure. The Solubility of iron in Copper is negligible [18]. Therefore, melted rich Copper globules fail to wet their circumference environment which is Fe-rich. As a result, if surrounding matrix of Copper globules is Fe-rich, it would not be wetted by Copper-rich globules and, consequently, no strong bond is formed between Copper-rich globules surrounded by Fe-rich matrix and their surrounding environment. That is, the boundaries of these Copper-rich globules surrounded by Fe-rich matrix are weak and very suitable for nucleation or propagation of a crack; upon the crack tip arrives at weak boundaries of such Copper-rich globules, it can easily propagate through it. And
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Fig. 12. Formation of globule shapes in fracture surface of welded sample with a 150 m-thick interlayer.
the result of such fracture behavior is the appearance of spherical shapes scattered on fracture surfaces of welded samples with thicker interlayer (150 m). Fig. 12a and b were taken from the opposite sides of an area which contained some Copper-rich globules with different sizes. These figures actually depict the failure behavior of globular shapes. As can be seen in the figures, fracture propagated through the boundary of these Copper-rich globules. Since all of Copper-rich globules are not surrounded by the Fe-rich matrix, only some of these globules show such fracture behavior. Therefore, a fewer number of spherical shapes were detected on the fracture surface compared with those are seen in the microstructure of cross section. 3.5. Ultimate tensile strength Due to scattered cracks formed in the weld metal of samples welded with the 100 m-thick interlayers, mechanical properties of these samples were not measured. Maximum Ultimate Tensile Strength and elongation achieved in welded sample with the 150 m-thick interlayer were about 150 MPa and 3% respectively (Fig. 13). Higher ultimate tensile strengths were achieved in dissimilar laser welding of stainless steel and Nitinol wires by a Copper interlayer in which welding process was carried out on wires in single-pulse mode and without the use of a fixture [11]. Thermal stresses originated from time-temperature cycle in pulsed welding
Fig. 13. mechanical behavior of welded sample with a 150 m interlayer.
are much more complicated in welding of sheets compared with that of single-pulse mode used for welding of thin wires. A fixture, moreover, must be used in order to restrict the rotational distortion.
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During the welding of sheets, rotational distortion either increases or decreases the weld seam. The former leads to escapement of laser beam which of a quite narrow diameter (∼micro scale), and this decreases suffusion melting and, consequently, process efficiency. Therefore, in welding of sheets, the use of a proper fixture is necessary, especially in welding of base metals of higher thermal expansion coefficients such as stainless steels. The constraining forces originated from interaction of fixture and base metals during the contraction of base metals applies tensile thermal stresses to weld metal [22]. As a result of such forces as well as brittleness of weld-metal and above all inadequate added Copper in weld metal to compensate thermal stresses, tiny (micro-scale) and large cold cracks formed in weld-metal of sample welded with the thinner interlayer (Figs. 4 and 11). That is, as a result of mentioned conditions, at initial stages of contraction tiny cracks form (Fig. 11) and as contraction continues cracks become larger (Fig. 4). The formation of the rounded cracks around microhardness indentations in weld metal of samples welded by a 100 m-thick interlayer signifies the presence of the tensile residual stress in weld metal [21]. Therefore, complexities attributed to thermal stresses in welding of sheets make dissimilar welding of stainless steel and Nitinol sheets impossible with a 100 m Copper interlayer, while S.S/Cu/Nitinol wire joint could withstand an approximately 500 MPa tensile force [11]. Comparing to the results of tensile test of Ti-6Al-4V/Cu/Niti joints [16], the tensile strength of S.S 316L/Cu/Niti joints was much lower (316L/Cu/Niti joints welded with the 100 m-thick interlayer cracked). Since the welding conditions, the fixture; interlayer thickness and welding parameters, were the same as well as this fact that the maximum harness of weld-metals was nearly equivalent in both joints, the cause of lower mechanical strength in S.S 316L/Cu/Niti joint must be the different physical properties of S.S 316 l and Ti-6Al-4V. The linear thermal expansion coefficients for S.S 316 l and Ti-6Al-4V are 17.5 and 9.7 m/m-◦ C respectively. Higher thermal expansion coefficient of S.S 316L means higher thermal stress originated from contraction. And this forms cracks in weld metal (Fig. 4 and 11). Fig. 13 illustrates mechanical behavior of welded samples with a 150 m-thick Copper interlayer. As can be seen, stress–strain curves of welded joints show a small diagonal stress plateaus. This can be justified by serial arrangement of Nitinol with a horizontal plateau and S.S of a different Young’s modulus [14,22]. The decrease in the hardness of weld-metal (Fig. 9) as well as the compensation of the thermal stresses that plays a decreasing role in residual stress that resides in weld metal (Fig. 10) turned a cracked joint (Fig. 4) to a sound joint that can withstand a 150 MPa tensile force. Nevertheless, in addition to the formation of the brittle phases such as Fe2 Ti, FeTi and Ti-Cu IMCs due to high temperature reactivity of Ti which makes weld metal brittle, the formation of Cupper-rich globules that in some occasions do not have strong bonds with their surrounding matrix (Fig. 12) and could act as crack initiation sites degrades tensile strength of samples welded with a 150 m-thick interlayer. 4. Conclusion Dissimilar laser welding of Nitinol and stainless steel was carried out using a pulsed Nd:YAG laser beam and the Copper interlayer with different thicknesses of 100 and 150 m, and following results
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were obtained. 1. As a result of the increase of the interlayer thickness, chemical composition of weld metal changed: the concentration of some more brittle IMCs such as Fe2 Ti and FeTi decreased; Cu-Ti IMCs with lower hardness formed; and the hardness of weld metal decreased partly. 2. The increase of the interlayer thickness compensated more thermal stress since Copper is a ductile metal of lower melting pint than base metal. This reduced the remained residual stress in weld metal. 3. The combination of lower hardness and lower residual stresses level decreased the susceptibility of welded sample with a thicker interlayer (150 m) to cold cracking. As a result, cold cracks were eliminated from the weld metal and sound joints were achieved. 4. Some Copper-rich globules formed in weld metal as a result of the ejection of Copper from ␥-Cu-Fe solid solution supersaturated by Cu during cooling of weld metal from the high temperatures to melting point of Copper. 5. As molten Copper fail to wet Fe well, circumstances in which Copper-rich globules were surrounded by the Fe-rich matrixes (lower Ti content) no strong bond between the globules and surrounding Fe-rich matrix were formed. This turned the boundaries of Copper-rich globules/Fe-rich matrix to a suitable place and path for nucleation and propagation of crack. And this acted as a limiting factor toward the more increase of interlayer thickness. References [1] Wolfgang P, Adam K, Bjorn B. Mater Sci Eng A 2008;481–482:598–601. [2] Lagoudas D. Shape Memory Alloys: Modeling and Engineering Applications. New York: Springer; 2008. [3] Akselsen OM. Joining of Shape Memory Alloys. Norway: Sciyopublication; 2010. [4] Chen HC, Pinkerton AJ, Li L. Int J Adv Manuf Technol 2011;52:977–87. [5] Meco S, Pardal G, Ganguly S, Williams S, McPherson N. Opt Laser Eng 2015;67:22–30. [6] Miranda RM. Int J Adv Manuf Technol 2012;61:205–12. [7] Pardal G, Meco S, Ganguly S, Williams S, Prangnel P. Int J Adv Manuf Technol 2014;73:365–73. [8] Tomashchuk I, Grevey D, Sallamand P. Mater Sci Eng A 2015;622:37–45. [9] Oliveira JP, Panton B, Zengb Z, Andrei CM, Zhou Y, Miranda RM, et al. Acta Mater 2016;105:10–5. [10] Borrisutthekul R, Yachi T, Miyashita Y, Mutoh Y. Mater Sci Eng A 2007;467:108–13. [11] Hongmei L, Daqian S, Xiaoyan G, Peng D, Zhanping Lv. Mater Des 2013;50:342–50. [12] Pouquet J, Miranda RM, Quintino L, Williams S. Int J Adv Manuf Technol 2012;61:205–12. [13] Eijk VD, Fostervoll CH, Sallom Z, Akselsen OM.International Conference on Advanced Metallic Materials and Their Joining. 2004. October 25–27. [14] Shojaei Zoeram A, Akbari Mousavi SAA. Mater Des 2014;61:185–90. [15] Bricknell RH, Melton KN, Mercier O. Metall Trans A 1979;10:693–7. [16] Shojaei Zoeram A, Akbari Mousavi SAA. Mater Lett 2014;133:5–8. [17] Tomashchuk I, Sallamand P, Andrzejewski H, Grevey D. Intermetallics 2011;19:1466–73. [18] Alloys phase diagrams. ASM Handbook, ASM Specialty Handbook, Volume 3. Metals Park, OH: ASM International; 1992. p. 327. [19] Gugel H, Schuermann A, Theisen W. Mater Sci Eng A 2008;481–482:668–71. [20] Roberts A, Kennedy R, Krauss G. Tool Steels. 5th edition Materials Park, OH: ASM, International; 1998. p. 50. [21] Tosha K.2ndP Asia-Pacific Forum on Precision Surface Finishing and Deburring Technology. 2002. July. [22] Masubuchi K. Analysis of Welded Structures. London: Pergamon Press; 1980. ISBN 0-08-022714-7.
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Microstructure and properties analysis of laser-welded Ni–Ti and 316l sheets using copper interlayer A. Shojaei Zoeram a,∗ , A. Rahmani b , S.A.A. Akbari Mousavi a a b
Welding Lab, School of Metallurgy and Materials, College of Engineering, University of Tehran, P. O. Box: 11155-4563, Tehran, Iran Materials Engineering Department, Imam Khomeini International University, Ghazvin, Iran
a r t i c l e
i n f o
Article history: Received 1 October 2016 Received in revised form 25 January 2017 Accepted 5 February 2017 Keywords: Dissimilar laser welding Ni–Ti alloy X-Ray diffraction analysis SEM EDS Fracture surface
a b s t r a c t Dissimilar pulsed laser welding of 1 mm-thick sheets was carried out on Nitinol/S.S joint. In order to modify the chemical composition of weld metal and to restrict the formation of Fe-Ti intermetallic compounds (IMCs) in the weld metal, Copper thin film with different thicknesses, 100 and 150 m, was used as an interlayer. The effects of interlayer thickness on microstructure, microhardness profile and chemical composition of weld-metal was investigated. Investigations showed that the interlayer thickness has a great influence on the chemical composition of the weld metal and, consequently, affects all of joint properties such as microstructure and ultimate tensile strength. According to X-Ray diffraction analysis, it was revealed that the increase of interlayer thickness decreases the concentration of Fe-Ti IMCs in weld metal and this makes weld metal softer to some extent. Moreover, since the Copper is a ductile metal of low melting temperature, added Copper into weld metal can compensate thermal stresses. Therefore, cracks originated from thermal stresses as well as brittleness of weld-metal were eliminated when the thickness of interlayer increased. However, there were some restricting factors as to the increase of the thickness of Copper interlayer, namely, the formation of Ti-Cu IMCs as well as the formation of Cu-rich globules which have relatively weak bonds with their surrounding matrix and this turns their boundaries to a suitable place for nucleation and propagation of a crack. © 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Unique properties of Nitinol have made it to a special alloy which has a great number of potential applications from medical to aerospace industries [1,2]. Due to undeniable role of successful welding in fabrication of complex parts, weldability is among properties which have a great role in successful engineering applications of an alloy. Poor weldability of Nitinol and lack of knowledge about dissimilar welding of this alloy deprive us of wide potential applications and unique properties of this special alloy [3]. Due to considerable role of dissimilar joints in the reduction of raw materials cost and improvement of design conditions, demand for this type of joints has increased over the last two decades at an accelerating pace [4,5]. Dissimilar welding of materials generally suffers from several drawbacks such as formation of intermetallic compounds (IMCs) in the weld metal, thermal mismatch and different chemical properties of base metals [6,7]. The use of an interlayer
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (A. Shojaei Zoeram).
to modify the chemical compositions of the weld zone [8,9] and employment a high thermal conductive substrate as a heat sink to increase the solidification rate are among suggested solutions for the restriction of the formation of IMCs in the weld metal [10]. Another method suggested to restrict the formation of IMCs in the weld metal is to adjust the welding parameters to decrease heat input as much as possible that contributes to higher solidification rate. In the double-sided welding method in which the penetration depth of each side is halved, the heat input is applied in two stages in smaller amounts. The lower the heat input is, the higher the solidification rate of weld metal would be. The increase of solidification rate restricts the formation of IMCs and this improves mechanical properties of joint [10]. The formation of Fe-Ti IMCs in autogenesis fusion welding of Nitinol and Stainless steel reported as a destructive factor which affects joint properties [11,12]. The formation of these phases in the weld metal makes weld metal brittle. The combination of this brittleness and complex residual stresses formed in weld-metal due to mismatch between thermal coefficients of base metals leads to formation of cracks in the weld-metal and this limits the applications of dissimilar joints in many industries [13]. Copper is a soft metal of lower melting point than stainless steel and Nitinol. Hence it can compensate the effect of thermal mis-
http://dx.doi.org/10.1016/j.jmapro.2017.02.005 1526-6125/© 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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match during the solidification progress in dissimilar welding of stainless steel and Nitinol [14]. Moreover, Nickel atoms could be substituted for Copper atoms in Nitinol lattice and the conclusion of this substitution is the formation of Ti (Ni, Cu) shape memory alloy of different austenitic and martensitic transformation temperatures [15]. Moreover, Copper neither reacts with Iron nor forms any brittle intermetallic compounds with Fe in weld metal. Hence, Copper can be used in dissimilar welding of stainless steel and Nitinol as an interlayer. The increase of the interlayer thickness reduces the fraction of base-metals elements in weld metal [16]. Therefore the amount of Fe and Ti elements in the weld metal which are responsible for formation of Fe-Ti IMCs decreases. 2. Experimental procedure In order to produce Ti-rich Nitinol plates with chemical composition of (Ni – 50.7at.%Ti) and the thickness of 1 mm, high purity nickel and titanium with specified weight ratio were cast in a Vacuum Arc Re-melting (VAR) furnace. Caste ingot with a thickness of 1 cm was successfully roll-formed in the sequential hot and cold rolling operation using low-loads. Rolling procedure was designed in a way that 20% of the final thickness reduction was performed by cold forming. To eliminate cold work, sheets were annealed in 450 ◦ C for 30 min and then quenched in water bath. The black oxide layer formed during the above-mentioned procedure was removed with an acid mixture solution of HF: HNO3: H2O = 1:5:10 ratio. The chemical compositions of the materials used in this study are given in Table 1. Copper thin films of 100 m and 150 m thicknesses were employed as interlayer to modify the chemical composition of weld metal and a Copper substrate was used as a heat sink in order to minimize the thermal cycle that originates from welding operation. Prior to welding, the weld pieces were fixed on substrate by vertical screws, then interlayer was inserted between the base metals before the gap between base metals is minimized by fastening the side screw (Fig. 1). The samples were butt welded in double-sided mode according to Fig. 2. Laser welding process was carried out using an SW-1 pulsed Nd:YAG laser machine with a wave length of 1.064 m. Welding parameters are given in Table 2. Prior to metallographic survey, samples were etched by immersion in an acid solution of HF: HNO3: H2O = 1:4:5 ratio for 20 s. Optical microscopy (OM) and scanning electron microscopy (SEM) techniques were utilized to analysis of weld metal microstructure; X-ray diffraction (XRD) analysis was carried out on fracture surfaces to identify the formed phases in the weld metal. In order to investigate the effect of the increase of Copper interlayer thickness on the chemical compositions and distributions of Ti, Fe, and Cu in the weld metal, EDS analysis was carried out on cross sections of the welded joints. Moreover, to evaluate the change of the hardness distribution through the weld metal, according to the dotted line in Fig. 2, microhardness distribution profiles was attained using a load of 200 g Table 1 chemical composition of base metals and inter layer in wt.%.
Stainless Steel NiTi Cu
Fe
Cr
Ni
Mo
Mn
Cu
Ti
Base – –
17.00 – –
12.00 54.39 0.01
2.00 – –
2.00 – –
0.01 – 99.97
– 45.61 –
Fig. 1. Movable fixture with Copper substrate.
Fig. 2. Double sided welding method, hardness and EDS analysis location.
Fig. 3. Dimensions of tensile test samples.
for dwelling time of 15 s. Mechanical properties of the joints were evaluated via tensile tests performed at strain rate of 1 mm/min and in room temperature. Dimensions of the tensile test specimen are given in Fig. 3. 3. Results and discussions 3.1. Microstructure Figs. 4 and 5 depict the microstructure of the first-pass of samples welded with different interlayer thicknesses, 100 and 150 m respectively. As it can be seen, the increase of interlayer thickness had a great influence on microstructure of weld metal. Generally, due to a number of reasons such as pulse nature of pulsed Nd:YAG laser that applies mixing force to melted elements in weld metal, high solidification rate in laser welding and finally diversity of elements in the weld metal microstructure is highly complex one [17]. In the welded sample with 100 m-thick interlayer, solidification sequence along the fusion boundary of Nitinol started with a planar solidification mode and it continued with cellular one just a short distance away from the fusion line (Fig. 4a and d). This is as a result of high solidification rate along fusion boundary adjacent to cold base metal backed by a substrate as well as the substitution of Cop-
Table 2 used parameters for welding of samples. Voltage (V)
Defocusing Distance (mm)
Pulse Duration (ms)
Frequency (Hz)
Speed (mm/s)
Beam Diameter (mm)
average laser power (w)
450
−0.5
2
7.7
1.5
0.4
100
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Fig. 4. Microstructure of welded sample with 100 m Interlayer.
Fig. 5. Microstructure of welded sample with 150 m Interlayer.
per for Nickel atoms in the lattice of Nitinol along fusion boundary of Nitinol. This leads to the formation of the Ti (Ni, Cu) ternary shape memory alloys of which transformation temperatures are different. On the other hand, due to negligible solubility of Copper in iron, microstructure morphology of weld metal along fusion boundary of stainless steel did not show any integrity with that of S.S (Fig. 4c and e). As it is depicted in Fig. 5a and d, the increase of the interlayer thickness reduced the tendency for microstructural integrity along fusion boundary of Nitinol. The addition of excessive amounts of Copper as a result of increasing the interlayer thickness to 150 m unbalances stoichiometric ratio which is required for formation of Ti (Ni, Cu) ternary shape memory alloys. This causes the discontinuity of microstructure morphology between base metal and
weld-metal along Nitinol fusion boundary. The formation of spherical shapes with different sizes in all over the microstructure was another microstructural change which resulted from the increase of interlayer thickness to 150 m (Fig. 5). Fig. 6 depicts the SEM image of the spherical shapes formed in weld metal along with their corresponding chemical compositions. Chemical compositions of these globular features indicate that these zones contain a large amount of Copper in their chemical compositions (Fig. 6). Formation of these features in weld metal of sample welded with the 150 mthick interlayer can be justified by chemical composition of weld metal and Fe-Cu binary phase diagram. Chemical compositions of the weld-metals of welded samples with the 100 and 150 m-thick interlayers are given in Table 3. As can be seen from this table,
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Fig. 6. SEM image of Copper-rich globes and their chemical composition. Table 3 Results of EDS analysis on center areas of fusion metals [wt.%]. Elements
Fe
Ti
Ni
Cu
Cr
100 micrometers 150 micrometers
30.15 27.32
16.10 14.61
24.15 21.60
18.38 24.68
7.50 7.37
the increase of interlayer thickness increased the amount of Cu and, adversely, decreased the amounts of Fe and Ti elements in the weld-metal (Fig. 7). According to Fe-Cu phase diagram, maximum solubility of Cu in γ-Fe is about 13 wt.% in 1400 ◦ C that decreases to 8 wt.% in 1100 ◦ C i.e. the melting point of Copper. That is to say, the reduction of the temperature to the melting point of Copper during solidification process reduces the solubility of Copper in ␥-Fe equivalent to 34 percent. Thus, as the solidification progresses and, consequently, the temperature decreases during cooling cycle, the more amounts of Copper becomes supersaturated and is ejected from the γ-Fe solid solution. Ejected Copper from γ-Fe solid solution contributes to nucleattion and growth of Copper-rich globules in the microstructure of the weld-metal. On the other hand, a certain amount of Copper reacts with Titanium and forms Ti-Cu IMCs. Regarding Table 3, the amount of Copper in the weld metal is more than the solubility limit of Copper in Fe. Therefore, excessive Copper that neither dissolves in γ-Fe solid solution nor reacts with other elements in weld-metal forms the spherical shapes in microstructure of weld-metal. As a result, the thicker the interlayer, the higher the formation possibility of the Cupper-rich globules in microstructure would be. Since the spherical shapes have the lowest ratio of surface area to volume, such shapes apply the lowest surface energy to the system. Moreover, small spheres may join together to produce the larger ones and minimize applied surface energy to the system. The Formation of the spherical shapes was also reported in pulsed Nd:YAG laser welding of S.S to Ti-6Al-4V with Copper interlayer [17]. As can be seen in Fig. 6, there are many fine black points within the Copper-rich globules. According to Cu-Fe phase diagram, Fe has a negligible solubility in Copper and these black points could be supersaturated Iron in Fe-Cu solid solution from which were ejected during cooling [17]. As a result of brittle weld metal and stresses (i.e. thermal or residual stresses), cold cracks develop and propagate in the straight path after solidification of the weld-metal. The formation of the brittle IMCs in the weld metal makes weld metal brittle. And different thermal coefficients of base metals in dissimilar welding create complex stresses during cooling, while base metals start
contracting. The combination of the brittleness originated form the formation of IMCs and complex thermal stresses makes dissimilar welds susceptible to cold cracking. As can be seen in Fig. 4b, the welded sample with the 100 m-think interlayer cracked; straight propagation path is among characters of brittle cold cracking. According to Fe-Ti, Ti-Ni and Ti-Cu binary phase diagrams, IMCs such as TiFe2 , TiFe, TiNi3 , TiNi, Ti2 Ni, TiCu, Ti3 Cu4 , Ti2 Cu3 , TiCu4 and Ti2 Cu can form in the weld metal [18]. The formation of such brittle phases and thermal stress stemmed from welding process can cause the formation of cold cracks in weld metal of welded sample with the 100 m-thick interlayer (Fig. 4b). Sound welds were achieved in the investigation of laser welding of NiTi wires to stainless steel without the use of an interlayer [19]; lower thermal stress in the welding of wires welded in single pulse mode without the use of a fixture could be a reason for achieving the sound joints. Considering distribution maps of elements in cross section of weld metal (Fig. 7) as well as chemical compositions of the weld metal from EDS analysis (Table 3), the formation of Fe2 Ti, FeTi, TiNi and Copper-rich Ti-Cu IMCs phases are more probable than other phases. As shown in Fig. 5, the increase of the interlayer thickness to 150 m eliminated cracks from weld metal. According to the distribution maps of elements as well as chemical compositions of weld metal, the increase of the interlayer thickness reduced amount of Ti and Fe in the weld metal. The reduction of the number of atoms of these elements reduces the interaction between the these atoms in high temperatures; from the kinetic point of view, this leads to the reduction of the reaction rate and finally the reduction of the formation of Fe-Ti IMCs in the weld metal. Restricting the formation of the brittle phases in the weld metal reduced the weld-metal embrittlement which is one of the requirements for the prevention of cold cracking. Moreover, Copper is a soft metal of lower melting point than base metals. Hence, thicker Copper interlayer can result in more compensation of the thermal stresses [14]. This matter is another requirement to prevent cold cracking.
3.2. X-Ray diffraction analysis X-Ray diffraction analysis was carried out on the fracture surface of both cracked and sound joints. The results of respective X-Ray diffraction analysis were depicted in Fig. 8. As can be seen, different IMCs, Fe-Ti, Ti-Ni and Ti-Cu, with various chemical compositions were formed in the weld metal. Melting points of FeTi2 , FeTi, TiNi, TiCu and TiCu3 are as follows: 1427, 1317, 1310, 982, and 890 ◦ C, respectively. Therefore, considering Fe-Ti, Ti-Ni and TiCu binary phase diagrams, from thermodynamical point of view, the formation probability of these IMCs in descending order is as follows: FeTi2 > FeTi > TiNi > TiCu > TiCu3 . Hence, the very FeTi2 and FeTi were the first IMCs which formed in weld metal during the solidification process. These IMCs are more brittle than other phases [17]. The addition of a third element reduces the collisions between Ti and Fe elements heated to high temperatures as a result of laser/material interaction and this reduces the probability of formation of Fe-Ti IMCs from kinetic point of view. Therefore, since the increase of interlayer thickness to 150 m adds more amounts of Copper atoms, the probability of collisions between Ti and Fe elements and of formation of Fe-Ti IMCs decreases. Consequently, the peak intensity related to FeTi and FeTi2 phases decreased by the increase of interlayer thickness. As can be seen, the peak intensity of the ␥-Fe increased as a result of the increase of the interlayer thickness. Because the Copper is an austenite stabilizer [20], the increase of the interlayer thickness stabilizes more amount of ␥-Fe. Intensification of the peaks intensity related to Copper as a consequence of the increase of the interlayer thickness could be justified by formation of Copper-rich globules in the weld metal (Fig. 6).
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Fig. 7. Distribution of various elements in weld metal of welded samples with different interlayer thickness a) 100 m, b) 150 m.
3.3. Microhardness
Fig. 8. X-Ray Diffraction analysis of welded samples with different interlayer thickness.
Fig. 9 depicts the microhardness distribution profile performed on cross section of the first pass. As it is evident from Fig. 9, the increase in the interlayer thickness reduced microhardness of weld metal. The average Vickers hardness of sample welded with the 100 m-thick interlayer was equal to 525 VH which reduced to 470 VH as a result of the increase of the interlayer thickness to 150 m. With regard to X-Ray diffraction analysis, the increase of the interlayer thicknesses lessened the peaks intensity related to Fe-Ti IMCs which are more brittle compared to other phases. Thus, the reduction of the concentration of the very Fe-Ti IMCs reduced the weld metal microhardness. Meanwhile, the formation of Copper-rich globules formed in microstructure of sample welded with the thicker interlayer reduced the microhardness of weld-metal since Copper is a soft metal. Fig. 9 indicates that areas adjacent to fusion boundaries of S.S were the hardest areas in weldmetal. This signifies that Fe-Ti IMCs were hardest IMCs formed in weld-metal of this joint. The closer the areas to S.S fusion boundary are, the more the possibility of the formation of Fe-Ti IMCs would
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Fig. 9. Hardness profile of weld metal.
be. Fig. 10 indicates the morphology of mirohardness indentations in both samples. As can be seen in Fig. 10a, some rounded cracks formed around of microhardness indents in the sample welded with 100 m-thick interlayer. Taking this fact into account that residual stress affects the shape of microhardness indentations, tensile residual stress makes the sides of microhardness indentations concave [21], the cause of the formation of such cracks would be justified. The use of a fixture creates tensile residual stress at the central parts of weld-metal [22]. Hence, after microhardness indentation, the sides of microhardness indents tend to become concave in shape, but in the present case the produced weld metal filled by various brittle IMCs is not so ductile that endure deformation.
Therefore it cracks, and concave cracks form around sides of microhardness indentations instead of deformation. On the other hand, as it is seen in Fig. 10b, round cracks were not observed around microhardness indentations of sample welded with a 150 m-thick interlayer. Considering this fact that Copper can compensate thermal stresses that create residual stresses, this could be justified. The Copper is a soft metal with low melting point. Therefore it can compensate thermal stresses through two mechanisms. Firstly, due to its lower melting point, added Copper into weld metal remains molten for longer time than base metals during solidification. And molten phase compensates thermal stresses during contraction of base metals whilst the base metals are solid. Secondly, since Copper is a soft metal of low yield strength, added Copper into weld metal can continue its role in the compensation of thermal stresses after solidification to some extent. Therefore, since the increase of the thickness of Copper interlayer adds more amount of Copper into weld metal, its compensation effect increases. That is to say, the more added Copper, the greater compensation effect, and consequently the lower the level of residual stress would be. This eliminates crack formed around microhardness indentations (Fig. 10b) even around ones that are as hard as the hardest points of the sample welded with 100 m-thick interlayer and around which cracks formed(Fig. 10a). Therefore, it is concluded that for round cracks form around microhardness indentation, both brittleness and residual stresses should exist simultaneously. That is to say, when there is residual stress in a weld metal, the microhardness indentations tend to be deformed, but if their surrounding circumferences are not enough ductile to endure the deformation, some cracks around indentations appear (Fig. 10a). 3.4. Fracture surface Fig. 11 depicts the fracture surface of welded sample with the 100 m-thick interlayer. Fracture surface shows many scattered micro-cracks and river marks. These features are among the charac-
Fig. 10. Morphology of microhardness indentations of welded samples with different interlayer thicknesses a) 100 m, b) 150 m.
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Fig. 11. Fracture surface of welded sample with 100 m-thick interlayer.
teristics of the brittle fracture. According to results of microhadness and X-Ray diffraction analysis, due to high temperature reactivity of Titanium, several IMCs with various chemical compositions in the weld metal formed. Considering this fact that IMCs are usually brittle as well as owing to the results of the microhardness analyses (Fig. 9), brittle fracture behavior of weld metal could be justified by the formation of IMCs. Fig. 12 reveals some dispersed spherical shapes on the fracture surface of the welded samples with the 150 m-thick interlayer. According to EDS analysis, these rounded shapes are Copper-rich and on the opposite sides of these globular shapes, there are cavities that contain a large amounts of Iron in their chemical compositions. As can be seen in Figs. 5 and 6, the microstructure of the weld metal consists of many Copper-rich
globules with different sizes. Therefore, it could be suggested that these rounded shapes on fracture surface are the same as Copperrich globules that were formed in the microstructure. The Solubility of iron in Copper is negligible [18]. Therefore, melted rich Copper globules fail to wet their circumference environment which is Fe-rich. As a result, if surrounding matrix of Copper globules is Fe-rich, it would not be wetted by Copper-rich globules and, consequently, no strong bond is formed between Copper-rich globules surrounded by Fe-rich matrix and their surrounding environment. That is, the boundaries of these Copper-rich globules surrounded by Fe-rich matrix are weak and very suitable for nucleation or propagation of a crack; upon the crack tip arrives at weak boundaries of such Copper-rich globules, it can easily propagate through it. And
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Fig. 12. Formation of globule shapes in fracture surface of welded sample with a 150 m-thick interlayer.
the result of such fracture behavior is the appearance of spherical shapes scattered on fracture surfaces of welded samples with thicker interlayer (150 m). Fig. 12a and b were taken from the opposite sides of an area which contained some Copper-rich globules with different sizes. These figures actually depict the failure behavior of globular shapes. As can be seen in the figures, fracture propagated through the boundary of these Copper-rich globules. Since all of Copper-rich globules are not surrounded by the Fe-rich matrix, only some of these globules show such fracture behavior. Therefore, a fewer number of spherical shapes were detected on the fracture surface compared with those are seen in the microstructure of cross section. 3.5. Ultimate tensile strength Due to scattered cracks formed in the weld metal of samples welded with the 100 m-thick interlayers, mechanical properties of these samples were not measured. Maximum Ultimate Tensile Strength and elongation achieved in welded sample with the 150 m-thick interlayer were about 150 MPa and 3% respectively (Fig. 13). Higher ultimate tensile strengths were achieved in dissimilar laser welding of stainless steel and Nitinol wires by a Copper interlayer in which welding process was carried out on wires in single-pulse mode and without the use of a fixture [11]. Thermal stresses originated from time-temperature cycle in pulsed welding
Fig. 13. mechanical behavior of welded sample with a 150 m interlayer.
are much more complicated in welding of sheets compared with that of single-pulse mode used for welding of thin wires. A fixture, moreover, must be used in order to restrict the rotational distortion.
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During the welding of sheets, rotational distortion either increases or decreases the weld seam. The former leads to escapement of laser beam which of a quite narrow diameter (∼micro scale), and this decreases suffusion melting and, consequently, process efficiency. Therefore, in welding of sheets, the use of a proper fixture is necessary, especially in welding of base metals of higher thermal expansion coefficients such as stainless steels. The constraining forces originated from interaction of fixture and base metals during the contraction of base metals applies tensile thermal stresses to weld metal [22]. As a result of such forces as well as brittleness of weld-metal and above all inadequate added Copper in weld metal to compensate thermal stresses, tiny (micro-scale) and large cold cracks formed in weld-metal of sample welded with the thinner interlayer (Figs. 4 and 11). That is, as a result of mentioned conditions, at initial stages of contraction tiny cracks form (Fig. 11) and as contraction continues cracks become larger (Fig. 4). The formation of the rounded cracks around microhardness indentations in weld metal of samples welded by a 100 m-thick interlayer signifies the presence of the tensile residual stress in weld metal [21]. Therefore, complexities attributed to thermal stresses in welding of sheets make dissimilar welding of stainless steel and Nitinol sheets impossible with a 100 m Copper interlayer, while S.S/Cu/Nitinol wire joint could withstand an approximately 500 MPa tensile force [11]. Comparing to the results of tensile test of Ti-6Al-4V/Cu/Niti joints [16], the tensile strength of S.S 316L/Cu/Niti joints was much lower (316L/Cu/Niti joints welded with the 100 m-thick interlayer cracked). Since the welding conditions, the fixture; interlayer thickness and welding parameters, were the same as well as this fact that the maximum harness of weld-metals was nearly equivalent in both joints, the cause of lower mechanical strength in S.S 316L/Cu/Niti joint must be the different physical properties of S.S 316 l and Ti-6Al-4V. The linear thermal expansion coefficients for S.S 316 l and Ti-6Al-4V are 17.5 and 9.7 m/m-◦ C respectively. Higher thermal expansion coefficient of S.S 316L means higher thermal stress originated from contraction. And this forms cracks in weld metal (Fig. 4 and 11). Fig. 13 illustrates mechanical behavior of welded samples with a 150 m-thick Copper interlayer. As can be seen, stress–strain curves of welded joints show a small diagonal stress plateaus. This can be justified by serial arrangement of Nitinol with a horizontal plateau and S.S of a different Young’s modulus [14,22]. The decrease in the hardness of weld-metal (Fig. 9) as well as the compensation of the thermal stresses that plays a decreasing role in residual stress that resides in weld metal (Fig. 10) turned a cracked joint (Fig. 4) to a sound joint that can withstand a 150 MPa tensile force. Nevertheless, in addition to the formation of the brittle phases such as Fe2 Ti, FeTi and Ti-Cu IMCs due to high temperature reactivity of Ti which makes weld metal brittle, the formation of Cupper-rich globules that in some occasions do not have strong bonds with their surrounding matrix (Fig. 12) and could act as crack initiation sites degrades tensile strength of samples welded with a 150 m-thick interlayer. 4. Conclusion Dissimilar laser welding of Nitinol and stainless steel was carried out using a pulsed Nd:YAG laser beam and the Copper interlayer with different thicknesses of 100 and 150 m, and following results
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were obtained. 1. As a result of the increase of the interlayer thickness, chemical composition of weld metal changed: the concentration of some more brittle IMCs such as Fe2 Ti and FeTi decreased; Cu-Ti IMCs with lower hardness formed; and the hardness of weld metal decreased partly. 2. The increase of the interlayer thickness compensated more thermal stress since Copper is a ductile metal of lower melting pint than base metal. This reduced the remained residual stress in weld metal. 3. The combination of lower hardness and lower residual stresses level decreased the susceptibility of welded sample with a thicker interlayer (150 m) to cold cracking. As a result, cold cracks were eliminated from the weld metal and sound joints were achieved. 4. Some Copper-rich globules formed in weld metal as a result of the ejection of Copper from ␥-Cu-Fe solid solution supersaturated by Cu during cooling of weld metal from the high temperatures to melting point of Copper. 5. As molten Copper fail to wet Fe well, circumstances in which Copper-rich globules were surrounded by the Fe-rich matrixes (lower Ti content) no strong bond between the globules and surrounding Fe-rich matrix were formed. This turned the boundaries of Copper-rich globules/Fe-rich matrix to a suitable place and path for nucleation and propagation of crack. And this acted as a limiting factor toward the more increase of interlayer thickness. References [1] Wolfgang P, Adam K, Bjorn B. Mater Sci Eng A 2008;481–482:598–601. [2] Lagoudas D. Shape Memory Alloys: Modeling and Engineering Applications. New York: Springer; 2008. [3] Akselsen OM. Joining of Shape Memory Alloys. Norway: Sciyopublication; 2010. [4] Chen HC, Pinkerton AJ, Li L. Int J Adv Manuf Technol 2011;52:977–87. [5] Meco S, Pardal G, Ganguly S, Williams S, McPherson N. Opt Laser Eng 2015;67:22–30. [6] Miranda RM. Int J Adv Manuf Technol 2012;61:205–12. [7] Pardal G, Meco S, Ganguly S, Williams S, Prangnel P. Int J Adv Manuf Technol 2014;73:365–73. [8] Tomashchuk I, Grevey D, Sallamand P. Mater Sci Eng A 2015;622:37–45. [9] Oliveira JP, Panton B, Zengb Z, Andrei CM, Zhou Y, Miranda RM, et al. Acta Mater 2016;105:10–5. [10] Borrisutthekul R, Yachi T, Miyashita Y, Mutoh Y. Mater Sci Eng A 2007;467:108–13. [11] Hongmei L, Daqian S, Xiaoyan G, Peng D, Zhanping Lv. Mater Des 2013;50:342–50. [12] Pouquet J, Miranda RM, Quintino L, Williams S. Int J Adv Manuf Technol 2012;61:205–12. [13] Eijk VD, Fostervoll CH, Sallom Z, Akselsen OM.International Conference on Advanced Metallic Materials and Their Joining. 2004. October 25–27. [14] Shojaei Zoeram A, Akbari Mousavi SAA. Mater Des 2014;61:185–90. [15] Bricknell RH, Melton KN, Mercier O. Metall Trans A 1979;10:693–7. [16] Shojaei Zoeram A, Akbari Mousavi SAA. Mater Lett 2014;133:5–8. [17] Tomashchuk I, Sallamand P, Andrzejewski H, Grevey D. Intermetallics 2011;19:1466–73. [18] Alloys phase diagrams. ASM Handbook, ASM Specialty Handbook, Volume 3. Metals Park, OH: ASM International; 1992. p. 327. [19] Gugel H, Schuermann A, Theisen W. Mater Sci Eng A 2008;481–482:668–71. [20] Roberts A, Kennedy R, Krauss G. Tool Steels. 5th edition Materials Park, OH: ASM, International; 1998. p. 50. [21] Tosha K.2ndP Asia-Pacific Forum on Precision Surface Finishing and Deburring Technology. 2002. July. [22] Masubuchi K. Analysis of Welded Structures. London: Pergamon Press; 1980. ISBN 0-08-022714-7.