Infrared Dissimilar Joining of Ti 50 Ni 50 and 316 L Stainless Steel with Copper Barrier Layer in between Two Silver-Based Fillers

Infrared dissimilar joining Ti50Ni50 and 316L stainless steel using Cu foil in between Cusil-ABA and BAg-8 filler metals has been studied. The Cu foil serves as a barrier layer with thicknesses of 70 μm and 50 μm, and it successfully isolates the interfacial reaction between Ti and Fe at the 316L SS (stainless steel) substrate side. In contrast, the Cu foil with 25 μm in thickness is completely dissolved into the braze melt during brazing and fails to be a barrier layer. A layer of (CuxNi1−x)2Ti intermetallic is formed at the Ti50Ni50 substrate side, and the Cu interlayer is dissolved into the Cusil-ABA melt to from a few proeutectic Cu particles for all specimens. For the 316L SS substrate side, no interfacial layer is observed and (Ag, Cu) eutectic dominates the brazed joint for 70 μm/50 μm Cu foil. The average shear strength of the bond with Cu barrier layer is greatly increased compared with that without Cu. The brazed joints with a 50 μm Cu layer demonstrate the highest average shear strengths of 354 MPa and 349 MPa for samples joined at 820 ◦C and 850 ◦C, respectively. Cracks are initiated/propagated in (Ag, Cu) eutectic next to the 316L substrate side featured with ductile dimple fracture. It shows great potential for industrial application.


Introduction
Equal atomic TiNi shape memory alloy (SMA) is one of the best and intensively studied SMAs with excellent shape memory properties [1,2].At present, equiatomic TiNi SMA has many industrial applications, such as mechanical actuators and micro sensors in intelligent material systems [3,4].AISI (American Iron and Steel Institute) 316L stainless steel (316L SS) has the chemical composition in wt % of Fe-0.03C-17Cr-12Ni-2.5Mo and exhibits high oxidation and corrosion resistance [5].Joining of Ti 50 Ni 50 SMA and 316L SS has the opportunity for industrial applications.For instance, shape-memory actuators are used to replace exploding bolts utilized in petrochemical equipment and nuclear power plants [6].The shape-memory actuator is joined with an infrastructure made of stainless steel or nickel-based alloy.In our reported study, dissimilar joining of Ti 50 Ni 50 SMA and 316L SS by infrared brazing using two Ag-based filler metals, Ticusil (Ag-26.7Cu-4.5Ti,wt %) and Cusil-ABA (Ag-35.25Cu-1.75Ti,wt %), was evaluated [6].Test results demonstrated that the maximum joining strength of 237 MPa shear stress can be obtained for Ticusil filler brazed at 950 • C for 60 s, and 66 MPa for Cusil-ABA one joined at 870 • C for 300 s.The formation of Ti-Fe-(Cr) interfacial layer was found to be detrimental to all joint strengths [6].
Dissimilar joining of Ti and many engineering alloys has been extensively studied in recent years [7][8][9].Titanium (Ti) alloys directly bonded to SS will form TiFe/TiFe 2 intermetallics and result in inherent brittleness of the joint [10][11][12].For avoiding the occurrence of brittle intermetallics, an interlayer of diffusion barrier was proposed to add in between the joining substrates [13][14][15].A nickel interlayer with 30 µm in thickness had been used as a diffusion barrier in diffusion bonding of 304 SS and Ti-6Al-4V, so a robust joint was obtained [16].A nickel barrier layer was also used to braze 17-4 PH (precipitation hardened) SS and Ti-6Al-4V successfully [17].However, although the nickel interlayer can avoid the formation of brittle TiFe/TiFe 2 intermetallics in brazing the Ti alloy and SS, but another brittle one, such as TiNi 3 , can occur in the joint.Therefore, the nickel interlayer may be not the best choice for the diffusion barrier in brazing Ti 50 Ni 50 SMA and 316L SS, because Ti 50 Ni 50 substrate can provide Ti and Ni atoms to enhance the brittle Ti-Ni intermetallic(s) to form.
It is well known that the solubility of Fe in Cu is very low [18].In addition, the interfacial reaction of Ti 50 Ni 50 SMA and Cusil-ABA/Ticusil filler metals forms a TiNiCu intermetallic in the Ti 50 Ni 50 substrate side, and this intermetallic is regarded as not so brittle [6,19].Thus, an interlayer of Cu foil can be selected as a diffusion barrier in between Cusil-ABA and BAg-8 filler metals to prevent the interaction between Ti and Fe atoms in brazing Ti 50 Ni 50 SMA and 316L SS.The Cusil-ABA filler metal has good wettability with Ti 50 Ni 50 SMA and thus is chosen to contact with Ti 50 Ni 50 substrate [6].BAg-8 filler metal is selected to neighbor the 316L SS substrate due having no Ti atom to form brittle Ti-Fe intermetallic(s) at the 316L SS substrate side, although the wettability of BAg-8 one with 316L SS is not so good [20].
The alloys' joining brazed by an infrared power source can quickly heat up the specimen with a maximum heating rate of 50 • C/s, and it is much faster than that of ordinary electric furnace [21,22].Under the condition of temperature being precisely controlled, the joining process by infrared brazing is quite suitable to investigate the early-stage evolution of the reaction kinetics exhibited in the brazed joint.Therefore, microstructural evolution, metallurgical reaction, and bonding strength of brazed joints under different brazing variables have been extensively evaluated.

Materials Used and Experimental Procedure
Equal atomic TiNi SMA used in the experiment was prepared from titanium and nickel pellets (both with 99.9 wt % purity) with six times remelting in the vacuum arc remelter (Series 5 Bell Jar, Centorr Vacuum Ind., Nashua, NH, USA) under pure argon gas.The solution-treated Ti 50 Ni 50 sample was cut by a diamond saw into small templates at a size of 15 mm × 15 mm × 3 mm for metallographical observations, and 15 mm × 7 mm × 4 mm for shear tests.The surface used for brazing was ground and polished by SiC papers and Al 2 O 3 suspension solution, and then cleaned in an ultrasonic acetone container.Foils of Cusil-ABA, BAg-8, and Cu fillers were all purchased from Wesgo Metals (Hayward, CA, USA), and the former two foils' thickness was 50 µm and those of Cu foils were 25 µm, 50 µm, and 75 µm for comparison.Table 1 lists chemical compositions and melting temperatures of three filler foils.
In the experiment, infrared heating was used to braze Ti 50 Ni 50 SMA and 316L SS using a Cu barrier layer in between Cusil-ABA (active braze alloy) and BAg-8 filler metals, as shown in Figure 1.An infrared heating furnace made from ULVAC Company (Tokyo, Japan) with the model of RHL-P816C was utilized as the main component.The furnace was kept at 5 × 10 −5 mbar vacuum and set at 15 • C/s heating rate in the test.Figure 2 shows the schematic diagram of infrared brazing furnace.Graphite holders on both substrates were used to increase the infrared ray absorption on the specimen.The dissimilar brazed sample was preheated at 700 • C for 300 s before infrared brazing in order to equilibrate thermal gradient of the brazed sample.820 • C and 850 • C for 300 s were selected as the brazing condition.To evaluate joint strength of the brazed sample in this study, shear tests were conducted with three specimens being performed in each brazing condition.According to our past experience, a double lap joint of 316L SS/Ti50Ni50/316L SS was used in shear test [21,22].A Shimadzu universal testing machine with AG-10 model was used to conduct shear tests with a strain rate of 0.0167 mm/s.For the microstructural observations, the cross-section of infrared brazed joints were cut and then prepared by standard metallographic procedures, thereafter examined using a scanning electron microscope (SEM) with NOVA NANO 450 model.To evaluate joint strength of the brazed sample in this study, shear tests were conducted with three specimens being performed in each brazing condition.According to our past experience, a double lap joint of 316L SS/Ti50Ni50/316L SS was used in shear test [21,22].A Shimadzu universal testing machine with AG-10 model was used to conduct shear tests with a strain rate of 0.0167 mm/s.For the microstructural observations, the cross-section of infrared brazed joints were cut and then prepared by standard metallographic procedures, thereafter examined using a scanning electron microscope (SEM) with NOVA NANO 450 model.To evaluate joint strength of the brazed sample in this study, shear tests were conducted with three specimens being performed in each brazing condition.According to our past experience, a double lap joint of 316L SS/Ti 50 Ni 50 /316L SS was used in shear test [21,22].A Shimadzu universal testing machine with AG-10 model was used to conduct shear tests with a strain rate of 0.0167 mm/s.For the microstructural observations, the cross-section of infrared brazed joints were cut and then prepared by standard metallographic procedures, thereafter examined using a scanning electron microscope (SEM) with NOVA NANO 450 model.  2 show the cross-sectional results of SEM backscattered election images (BEIs) and energy dispersive spectroscope (EDS) chemical analysis results of Ti 50 Ni 50 /Cusil-ABA/70 µm Cu foil/BAg-8/316L SS specimens brazed by infrared rays at 820 • C for 300 s.In EDS analyses, the electron beam in a point with 1 µm in diameter was used for all single phase analysis, and a larger beam diameter of 5 µm was applied for eutectic analysis as marked by D in Figure 3. Figure 3a illustrates the cross-sectional SEM BEIs results, in which the area on the Ti 50 Ni 50 side and that on the 316L SS side are displayed in Figure 3b,c, respectively.From Figure 3a, the thickness of the Cu interlayer is about 70 µm (labelled by C and F).The layer marked by C in the left side of the Cu interlayer contains 90.5 at % Cu and has some Cu atoms dissolved into Cusil-ABA melt to form the proeutectic Cu (labelled by G in Figure 3b).Figure 3b indicates that a layer of (Cu x Ni 1−x ) 2 Ti intermetallic (labelled by B) is formed at the interface of Cusil-ABA filler and Ti 50 Ni 50 base metal.It is similar to the Ti 50 Ni 50 side of 316L SS/Cusil-ABA/Ti 50 Ni 50 joint brazed by infrared rays at 870 • C and 900 • C for 300 s [6].In contrast, there is no interaction layer formed in between the BAg-8 braze and the 316L SS substrate, as displayed in Figure 3c.The Cu interlayer is dissolved into both molten brazes resulting in irregular interfaces among Cu and two brazes.

Microstructures of Brazed Joints
Figure 3 and Table 2 show the cross-sectional results of SEM backscattered election images (BEIs) and energy dispersive spectroscope (EDS) chemical analysis results of Ti50Ni50/Cusil-ABA/70 μm Cu foil/BAg-8/316L SS specimens brazed by infrared rays at 820 °C for 300 s.In EDS analyses, the electron beam in a point with 1 μm in diameter was used for all single phase analysis, and a larger beam diameter of 5 μm was applied for eutectic analysis as marked by D in Figure 3. Figure 3a illustrates the cross-sectional SEM BEIs results, in which the area on the Ti50Ni50 side and that on the 316L SS side are displayed in Figure 3b,c, respectively.From Figure 3a, the thickness of the Cu interlayer is about 70 μm (labelled by C and F).The layer marked by C in the left side of the Cu interlayer contains 90.5 at % Cu and has some Cu atoms dissolved into Cusil-ABA melt to form the proeutectic Cu (labelled by G in Figure 3b).Figure 3b indicates that a layer of (CuxNi1−x)2Ti intermetallic (labelled by B) is formed at the interface of Cusil-ABA filler and Ti50Ni50 base metal.It is similar to the Ti50Ni50 side of 316L SS/Cusil-ABA/Ti50Ni50 joint brazed by infrared rays at 870 °C and 900 °C for 300 s [6].In contrast, there is no interaction layer formed in between the BAg-8 braze and the 316L SS substrate, as displayed in Figure 3c.The Cu interlayer is dissolved into both molten brazes resulting in irregular interfaces among Cu and two brazes.The thickness effect of the Cu barrier layer on the infrared brazed joint was studied.Figure 4 displays SEM BEIs cross sections of Ti 50 Ni 50 /Cusil-ABA/Cu/BAg-8/316L SS specimens brazed by infrared rays at 850 • C for 300 s with various thicknesses of the Cu interlayer.From Figure 4a, one can find that the continuous Cu barrier layer disappears and replaced by many huge proeutectic Cu-rich blocks.In contrast, the thickness of Cu barrier layer above 50 µm is enough to isolate the interaction between Ti 50 Ni 50 and 316L SS, as illustrated in Figure 4b,c.The dissolution of Cu barrier layer into Cusil-ABA melt becomes more prominent in Figure 4c than that in Figure 3a due to the higher brazing temperature.Additionally, one can find that the width of BAg-8 melt (in the right side of the Cu interlayer) in Figure 4c is less than that in Figure 3a, since BAg-8 melt flows out of the joint during brazing at a higher temperature.According to Table 1, the BAg-8 eutectic temperature is less than the liquidus temperature of Cusil-ABA braze.The width of BAg-8 filler is significantly decreased with increasing the brazing temperature.The thickness effect of the Cu barrier layer on the infrared brazed joint was studied.Figure 4 displays SEM BEIs cross sections of Ti50Ni50/Cusil-ABA/Cu/BAg-8/316L SS specimens brazed by infrared rays at 850 °C for 300 s with various thicknesses of the Cu interlayer.From Figure 4a, one can find that the continuous Cu barrier layer disappears and replaced by many huge proeutectic Curich blocks.In contrast, the thickness of Cu barrier layer above 50 μm is enough to isolate the interaction between Ti50Ni50 and 316L SS, as illustrated in Figure 4b,c.The dissolution of Cu barrier layer into Cusil-ABA melt becomes more prominent in Figure 4c than that in Figure 3a due to the higher brazing temperature.Additionally, one can find that the width of BAg-8 melt (in the right side of the Cu interlayer) in Figure 4c is less than that in Figure 3a, since BAg-8 melt flows out of the joint during brazing at a higher temperature.According to Table 1, the BAg-8 eutectic temperature is less than the liquidus temperature of Cusil-ABA braze.The width of BAg-8 filler is significantly decreased with increasing the brazing temperature.3 illustrate the cross-sectional results of SEM BEIs and EDS chemical analyses of Ti50Ni50/Cusil-ABA/25 μm Cu foil/BAg-8/316L SS specimen brazed by infrared rays at 850 °C for 300 s. Figure 5a,b indicate the area on the Ti50Ni50 side and that on 316L SS side, respectively.In Figure 5a, a (CuxNi1−x)2Ti intermetallic layer (labelled by B) is formed at the Ti50Ni50 side which is similar to that in Figure 3b [6,20,22].The silver penetrated into the (CuxNi1−x)2Ti layer is also observed.According to Figure 5b, two interfacial layers are formed at the 316L SS side, one is Ti(Fe, Ni) intermetallic layer (labelled by D), and the other is regarded as a layer of (Fe, Cr)-rich phase (labelled by E) [6].The existence of continuous Ti(Fe, Ni) reaction layer will be discussed later.Both D and E layers have silver agglomerations (white spots in Figure 5b) with the layer E having more.In addition, there is a TiCu precipitate in the braze, as labelled by G in Figure 5b [19].The existence of the Ti-Fe interfacial layer in Figure 5b is quite dissimilar to that in Figure 3c.Because the 25 μm Cu barrier layer is completely dissolved into the braze melt, Ti atoms transport into the 316L SS side forming the interfacial Ti(Fe, Ni) intermetallic layer [6].
Figure 6 and Table 4 display the cross-sectional inspection of SEM BEIs and EDS analyses of Ti50Ni50/Cusil-ABA/50 μm Cu foil/BAg-8/316L SS specimen brazed by infrared rays at 850 °C for 300 s.Interfacial reactions shown in Figure 6 are similar to those in Figure 3. Additionally, the experimental results of the infrared brazed Ti50Ni50/Cusil-ABA/Cu foil/BAg-8/316L SS joint with 70 μm thickness of the Cu foil at 850 °C are similar to those of the 50 μm one illustrated in Figure 6, and therefore are not shown here.[6,20,22].The silver penetrated into the (Cu x Ni 1−x ) 2 Ti layer is also observed.According to Figure 5b, two interfacial layers are formed at the 316L SS side, one is Ti(Fe, Ni) intermetallic layer (labelled by D), and the other is regarded as a layer of (Fe, Cr)-rich phase (labelled by E) [6].The existence of continuous Ti(Fe, Ni) reaction layer will be discussed later.Both D and E layers have silver agglomerations (white spots in Figure 5b) with the layer E having more.In addition, there is a TiCu precipitate in the braze, as labelled by G in Figure 5b [19].The existence of the Ti-Fe interfacial layer in Figure 5b is quite dissimilar to that in Figure 3c.Because the 25 µm Cu barrier layer is completely dissolved into the braze melt, Ti atoms transport into the 316L SS side forming the interfacial Ti(Fe, Ni) intermetallic layer [6].
Figure 6 and Table 4 display the cross-sectional inspection of SEM BEIs and EDS analyses of Ti 50 Ni 50 /Cusil-ABA/50 µm Cu foil/BAg-8/316L SS specimen brazed by infrared rays at 850 • C for 300 s.Interfacial reactions shown in Figure 6 are similar to those in Figure 3. Additionally, the experimental results of the infrared brazed Ti 50 Ni 50 /Cusil-ABA/Cu foil/BAg-8/316L SS joint with 70 µm thickness of the Cu foil at 850 • C are similar to those of the 50 µm one illustrated in Figure 6, and therefore are not shown here.

Microstructures of the Brazed Joints
Phase identification using the EDS chemical analysis results was assisted by related phase diagrams and citing references.From Figures 3, 5

Microstructures of the Brazed Joints
Phase identification using the EDS chemical analysis results was assisted by related phase diagrams and citing references.From Figures 3, 5

Microstructures of the Brazed Joints
Phase identification using the EDS chemical analysis results was assisted by related phase diagrams and citing references.From Figure 3, Figure 5, Figure 6 and Tables 2-4, a continuous layer of the (Cu x Ni 1−x ) 2 Ti intermetallic, as labelled by B, is formed at the interface between Ti 50 Ni 50 substrate and Cusil-ABA filler.Based on Ti-Cu-Ni ternary phase diagram isothermally sectionalized at 870 • C, the stoichiometric composition of this intermetallic layer is near the TiCuNi compound [19].It is a nonstoichiometric compound, and can be indicated by (Cu x Ni 1−x ) 2 Ti with x ranging from 0.23 to 0.75 [19].The EDS chemical analyses of layer B indicated in Tables 2-4 are in accordance with Ti-Cu-Ni diagram [18].
From Figure 5b, there are two continuous layers observed at the interface between 316L SS and the braze.One is the Ti(Fe, Ni) intermetallic layer labelled by D, and the other is the (Fe, Cr)-rich layer indicated by E. Figure 7 illustrates the Fe-Ni-Ti phase diagram isothermally sectionalized at 900 • C [19,23].The Ti 50 Ni 50 substrate dissolved into the braze melt causes high Ni and Ti contents in it.The complete dissolution of 25 µm Cu barrier layer makes the Ti and Ni ingredients readily react with the 316L SS substrate in brazing, and yields a continuous reaction layer of Ti(Fe, Ni) intermetallic.According to Figure 7, the composition of layer D is regarded as the TiFe intermetallic compound alloyed with Ni.The layer E shown in Figure 5b, which is next to the 316L SS substrate, contains about 56% Fe, 22% Cr, 3% Ni, 9% Ag, and 10% Ti (in at %).The transport of Ti from the Ti 50 Ni 50 substrate and Cusil-ABA melt now acts as an active brazing element to react with the surface of 316L SS substrate.Thereafter, Ti, Ag, and Ni atoms interact with 316L SS substrate to form a (Fe, Cr)-rich layer solid-solution alloyed with Ag, Ti, and Ni.In addition, some Ag atoms are not alloyed into (Fe, Cr)-rich phase, and form white Ag-rich precipitates in layer E (Figure 5b).stoichiometric composition of this intermetallic layer is near the TiCuNi compound [19].It is a nonstoichiometric compound, and can be indicated by (CuxNi1−x)2Ti with x ranging from 0.23 to 0.75 [19].The EDS chemical analyses of layer B indicated in Tables 2-4 are in accordance with Ti-Cu-Ni diagram [18].
From Figure 5b, there are two continuous layers observed at the interface between 316L SS and the braze.One is the Ti(Fe, Ni) intermetallic layer labelled by D, and the other is the (Fe, Cr)-rich layer indicated by E. Figure 7 illustrates the Fe-Ni-Ti phase diagram isothermally sectionalized at 900 °C [19,23].The Ti50Ni50 substrate dissolved into the braze melt causes high Ni and Ti contents in it.The complete dissolution of 25 μm Cu barrier layer makes the Ti and Ni ingredients readily react with the 316L SS substrate in brazing, and yields a continuous reaction layer of Ti(Fe, Ni) intermetallic.According to Figure 7, the composition of layer D is regarded as the TiFe intermetallic compound alloyed with Ni.The layer E shown in Figure 5b, which is next to the 316L SS substrate, contains about 56% Fe, 22% Cr, 3% Ni, 9% Ag, and 10% Ti (in at %).The transport of Ti from the Ti50Ni50 substrate and Cusil-ABA melt now acts as an active brazing element to react with the surface of 316L SS substrate.Thereafter, Ti, Ag, and Ni atoms interact with 316L SS substrate to form a (Fe, Cr)-rich layer solid-solution alloyed with Ag, Ti, and Ni.In addition, some Ag atoms are not alloyed into (Fe, Cr)-rich phase, and form white Ag-rich precipitates in layer E (Figure 5b).

The Shear Strengths of Brazed Joints
Table 5 indicates average shear strengths of the Ti50Ni50/Cusil-ABA/Cu foil/BAg-8/316L SS joints with different Cu barrier layers brazed by infrared rays under different brazing conditions.For comparison, the shear strengths of Ti50Ni50/Cusil-ABA or Ticusil/316L SS joint without the Cu barrier layer infrared brazed at 870 °C for 300 s or at 950 °C for 60 s, respectively, are also tabulated [6].From Table 5, average shear strengths of the specimens with 50 μm Cu foil can reach 354 MPa for brazing at 820 °C for 300 s and 349 MPa for brazing at 850 °C for 300 s.These values are significantly increased as compared with those without the Cu barrier layer, as indicated in Table 5.

The Shear Strengths of Brazed Joints
Table 5 indicates average shear strengths of the Ti 50 Ni 50 /Cusil-ABA/Cu foil/BAg-8/316L SS joints with different Cu barrier layers brazed by infrared rays under different brazing conditions.For comparison, the shear strengths of Ti 50 Ni 50 /Cusil-ABA or Ticusil/316L SS joint without the Cu barrier layer infrared brazed at 870 • C for 300 s or at 950 • C for 60 s, respectively, are also tabulated [6].From Table 5, average shear strengths of the specimens with 50 µm Cu foil can reach 354 MPa for brazing at 820 • C for 300 s and 349 MPa for brazing at 850 • C for 300 s.These values are significantly increased as compared with those without the Cu barrier layer, as indicated in Table 5.  [6] in which the thickness of the filler metal is 50 µm.
All brazed joints failed along the interface between BAg-8 braze and 316L substrate.8a, the fracture appearing in the brazed specimen is along the (Ag, Cu) eutectic (labelled by D in Figure 6b) near the 316L SS substrate side.The fracture of fine (Ag, Cu) eutectic, as shown in Figure 6b, results in many miniature dimples in Figure 8b.For the specimen with 70 µm Cu foil, the brazed joint is fractured along the (Ag, Cu) eutectic next to the 316L SS substrate side as shown in Figure 8c.A few solidification voids are observed from the fractured surface.All brazed joints failed along the interface between BAg-8 braze and 316L substrate.Figure 8a illustrates the cross-sectional results of SEM BEIs and the SEI fractograph of the fractured Ti50Ni50/Cusil-ABA/25 μm Cu foil/BAg-8/316L SS joined at 850 °C for 300 s.The brazed specimen failed along the interfacial reaction layer of Ti(Fe, Ni) (labelled by D in Figure 5b) next to the 316L SS substrate side.The SEM SEI fractograph shown in Figure 8a is features brittle fracture, and the EDS analysis result is consistent with that of layer D in Figure 5b. Figure 8b illustrates the cross-sectional results of SEM BEIs and the SEI fractograph of the fractured Ti50Ni50/Cusil-ABA/50 μm Cu foil/BAg-8/316L SS joined at 850 °C for 300 s.Different from Figure 8a, the fracture appearing in the brazed specimen is along the (Ag, Cu) eutectic (labelled by D in Figure 6b) near the 316L SS substrate side.The fracture of fine (Ag, Cu) eutectic, as shown in Figure 6b, results in many miniature dimples in Figure 8b.For the specimen with 70 μm Cu foil, the brazed joint is fractured along the (Ag, Cu) eutectic next to the 316L SS substrate side as shown in Figure 8c.A few solidification voids are observed from the fractured surface.Shear strengths revealed in Table 5 indicate that the optimal thickness of the Cu barrier layer is 50 µm.The Cu barrier foil 25 µm in thickness is dissolved into the melt during brazing, and a continuous Ti(Fe, Ni) interfacial layer next to the 316L SS substrate side is formed which is the fracture location in shear test.In addition, the BAg-8 filler metal brazed at 820 • C is not high enough to sufficiently wet the 316L SS substrate.These reasons cause the brazed joint with 25 µm Cu foil to have a lower shear strength than that with 50 µm foil, especially the joint brazed at 820 • C. Shear strength of the joint with 50 µm Cu foil demonstrates the highest joint strength due to the presence of very fine (Ag, Cu) eutectic and free of interfacial Ti(Fe, Ni) reaction layer.The shear strength of the joint with 70 µm Cu foil is lower than that of 50 µm one.The BAg-8 melt is easier to flow out of the joint during brazing than the Cusil-ABA one.The width of BAg-8 braze melt in 70 µm Cu foil is much less than that in 50 µm one, as compared between Figure 8b,c.Because solidification shrinkage voids are prone to be formed in the joint with the 70 µm Cu layer, the joint's shear strength is deteriorated.Shear test is the first stage in evaluating bonding strength of the brazed joint.Dynamic tests, such as fatigue testing, of such a brazed joint will be performed in the future in order to evaluate the effect of interfacial intermetallics on the reliability of the brazed joint.

Conclusions
Shear strength and microstructural evolution of Ti 50 Ni 50 SMA/Cusil-ABA/Cu foil/BAg-8/316L SS joints brazed by infrared rays at 820 • C and 850 • C for 300 s with the Cu foil as the diffusion barrier have been investigated.The introduction of the Cu barrier layer shows great potential in joining Ti 50 Ni 50 SMA and 316L SS for industrial application.Important conclusions are as follows: 1.
The Cu foil serves as a barrier layer at thicknesses of 70 µm and 50 µm, and it successfully isolates the interfacial reaction between Ti and Fe at the 316L SS substrate side.In contrast, the Cu foil with 25 µm in thickness is completely dissolved into the braze melt during brazing and fails to be a barrier layer.

2.
A layer of (Cu x Ni 1−x ) 2 Ti intermetallic is formed at the Ti 50 Ni 50 substrate side, and the Cu interlayer is dissolved into the Cusil-ABA melt to from a few proeutectic Cu particles for all specimens.For the 316L SS substrate side, no interfacial layer is observed and (Ag, Cu) eutectic dominates the brazed joint for 70 µm/50 µm Cu foil.However, an interfacial Ti(Fe, Ni) intermetallic layer and (Fe, Cr)-rich layer are formed in the brazed joint with the 25 µm Cu layer.

3.
The joint with the 50 µm Cu barrier layer demonstrates the best average shear strengths of 354 MPa and 349 MPa for samples brazed at 820 • C and 850 • C, respectively.All specimens are fractured along the interface between the BAg-8 braze and 316L SS side.Cracks are initiated/propagated in (Ag, Cu) eutectic for the 70 µm and 50 µm thickness Cu foils and at the interfacial Ti(Fe, Ni) reaction layer for the 25 µm foil.The formation of a brittle Ti(Fe, Ni) intermetallic layer deteriorates the bonding strength of the joint.

Figure 2 .
Figure 2. The schematic diagram of infrared brazing furnace.

Figure 2 .
Figure 2. The schematic diagram of infrared brazing furnace.

Figure 2 .
Figure 2. The schematic diagram of infrared brazing furnace.

Figure 3
Figure 3 and Table2show the cross-sectional results of SEM backscattered election images (BEIs) and energy dispersive spectroscope (EDS) chemical analysis results of Ti 50 Ni 50 /Cusil-ABA/70 µm Cu foil/BAg-8/316L SS specimens brazed by infrared rays at 820 • C for 300 s.In EDS analyses, the electron beam in a point with 1 µm in diameter was used for all single phase analysis, and a larger beam diameter of 5 µm was applied for eutectic analysis as marked by D in Figure3.Figure3aillustrates the cross-sectional SEM BEIs results, in which the area on the Ti 50 Ni 50 side and that on the 316L SS side are displayed in Figure3b,c, respectively.From Figure3a, the thickness of the Cu interlayer is about 70 µm (labelled by C and F).The layer marked by C in the left side of the Cu interlayer contains 90.5 at % Cu and has some Cu atoms dissolved into Cusil-ABA melt to form the proeutectic Cu (labelled by G in Figure3b).Figure3bindicates that a layer of (Cu x Ni 1−x ) 2 Ti intermetallic (labelled by B) is formed at the interface of Cusil-ABA filler and Ti 50 Ni 50 base metal.It is similar to the Ti 50 Ni 50 side of 316L SS/Cusil-ABA/Ti 50 Ni 50 joint brazed by infrared rays at 870 • C and 900 • C for 300 s[6].In contrast, there is no interaction layer formed in between the BAg-8 braze and the 316L SS substrate, as displayed in Figure3c.The Cu interlayer is dissolved into both molten brazes resulting in irregular interfaces among Cu and two brazes.

Figure 5
Figure 5 and Table3illustrate the cross-sectional results of SEM BEIs and EDS chemical analyses of Ti50Ni50/Cusil-ABA/25 μm Cu foil/BAg-8/316L SS specimen brazed by infrared rays at 850 °C for 300 s.Figure5a,b indicate the area on the Ti50Ni50 side and that on 316L SS side, respectively.In Figure5a, a (CuxNi1−x)2Ti intermetallic layer (labelled by B) is formed at the Ti50Ni50 side which is similar to that in Figure3b[6,20,22].The silver penetrated into the (CuxNi1−x)2Ti layer is also observed.According to Figure5b, two interfacial layers are formed at the 316L SS side, one is Ti(Fe, Ni) intermetallic layer (labelled by D), and the other is regarded as a layer of (Fe, Cr)-rich phase (labelled by E)[6].The existence of continuous Ti(Fe, Ni) reaction layer will be discussed later.Both D and E layers have silver agglomerations (white spots in Figure5b) with the layer E having more.In addition, there is a TiCu precipitate in the braze, as labelled by G in Figure 5b[19].The existence of the Ti-Fe interfacial layer in Figure5bis quite dissimilar to that in Figure3c.Because the 25 μm Cu barrier layer is completely dissolved into the braze melt, Ti atoms transport into the 316L SS side forming the interfacial Ti(Fe, Ni) intermetallic layer[6].Figure6and Table4display the cross-sectional inspection of SEM BEIs and EDS analyses of Ti50Ni50/Cusil-ABA/50 μm Cu foil/BAg-8/316L SS specimen brazed by infrared rays at 850 °C for 300 s.Interfacial reactions shown in Figure6are similar to those in Figure3.Additionally, the experimental results of the infrared brazed Ti50Ni50/Cusil-ABA/Cu foil/BAg-8/316L SS joint with 70 μm thickness of the Cu foil at 850 °C are similar to those of the 50 μm one illustrated in Figure6, and therefore are not shown here.

Figure 5 and
Figure 5 and Table 3 illustrate the cross-sectional results of SEM BEIs and EDS chemical analyses of Ti 50 Ni 50 /Cusil-ABA/25 µm Cu foil/BAg-8/316L SS specimen brazed by infrared rays at 850 • C for 300 s. Figure 5a,b indicate the area on the Ti 50 Ni 50 side and that on 316L SS side, respectively.In Figure5a, a (Cu x Ni 1−x ) 2 Ti intermetallic layer (labelled by B) is formed at the Ti 50 Ni 50 side which is similar to that in Figure3b[6,20,22].The silver penetrated into the (Cu x Ni 1−x ) 2 Ti layer is also observed.According to Figure5b, two interfacial layers are formed at the 316L SS side, one is Ti(Fe, Ni) intermetallic layer (labelled by D), and the other is regarded as a layer of (Fe, Cr)-rich phase (labelled by E)[6].The existence of continuous Ti(Fe, Ni) reaction layer will be discussed later.Both D and E layers have silver agglomerations (white spots in Figure5b) with the layer E having more.In addition, there is a TiCu precipitate in the braze, as labelled by G in Figure 5b[19].The existence of the Ti-Fe interfacial layer in Figure5bis quite dissimilar to that in Figure3c.Because the 25 µm Cu barrier layer is completely dissolved into the braze melt, Ti atoms transport into the 316L SS side forming the interfacial Ti(Fe, Ni) intermetallic layer[6].Figure6and Table4display the cross-sectional inspection of SEM BEIs and EDS analyses of Ti 50 Ni 50 /Cusil-ABA/50 µm Cu foil/BAg-8/316L SS specimen brazed by infrared rays at 850 • C for 300 s.Interfacial reactions shown in Figure6are similar to those in Figure3.Additionally, the experimental results of the infrared brazed Ti 50 Ni 50 /Cusil-ABA/Cu foil/BAg-8/316L SS joint with 70 µm thickness of the Cu foil at 850 • C are similar to those of the 50 µm one illustrated in Figure6, and therefore are not shown here.
, 6 and Tables 2-4, a continuous layer of the (CuxNi1−x)2Ti intermetallic, as labelled by B, is formed at the interface between Ti50Ni50 substrate and Cusil-ABA filler.Based on Ti-Cu-Ni ternary phase diagram isothermally sectionalized at 870 °C, the
, 6 and Tables 2-4, a continuous layer of the (CuxNi1−x)2Ti intermetallic, as labelled by B, is formed at the interface between Ti50Ni50 substrate and Cusil-ABA filler.Based on Ti-Cu-Ni ternary phase diagram isothermally sectionalized at 870 °C, the
Figure 8a illustrates the cross-sectional results of SEM BEIs and the SEI fractograph of the fractured Ti 50 Ni 50 /Cusil-ABA/25 µm Cu foil/BAg-8/316L SS joined at 850 • C for 300 s.The brazed specimen failed along the interfacial reaction layer of Ti(Fe, Ni) (labelled by D in Figure 5b) next to the 316L SS substrate side.The SEM SEI fractograph shown in Figure 8a is features brittle fracture, and the EDS analysis result is consistent with that of layer D in Figure 5b. Figure 8b illustrates the cross-sectional results of SEM BEIs and the SEI fractograph of the fractured Ti 50 Ni 50 /Cusil-ABA/50 µm Cu foil/BAg-8/316L SS joined at 850 • C for 300 s.Different from Figure

Table 1 .
Compositions and melting behaviors of three filler foils.

Table 2 .
EDS chemical analysis results in Figure3.

Table 2 .
EDS chemical analysis results in Figure3.

Table 3 .
EDS chemical analysis results in Figure5.

Table 4 .
EDS chemical analysis results in Figure6.

Table 3 .
EDS chemical analysis results in Figure5.

Table 3 .
EDS chemical analysis results in Figure5.

Table 4 .
EDS chemical analysis results in Figure6.

Table 4 .
EDS chemical analysis results in Figure6.

Table 5 .
Average shear strengths of infrared brazed Ti 50 Ni 50 SMA and 316L SS joints.

Table 5 .
Average shear strengths of infrared brazed Ti50Ni50 SMA and 316L SS joints.