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JOINING NICKEL to TITANIUM C.E. Ells*,G.F. Taylor* and R.C. Mansey

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JOINING NICKEL to TITANIUM C.E. Ells*,G.F. Taylor* and R.C. Mansey JOINING NICKEL TO TITANIUM C.E. Ells*,G.F. Taylor* and R.C. Mansey** * Fuels and Materials Division Chalk River Nuclear Laboratories Atomic Energy of Canada Limited Chalk River, Ontario **Atomic Power Division Westinghouse Canada Limited Hamilton, Ontario Introduction A number of attempts to join nickel to titanium (referenced,below) have been reported before. When the difficulties arising from the oxide surface on titanium have been overcome, then the major metallurgical problem arises from the mechanical properties of the intermetallic phases in the Ti-Ni system. There are three of these phases, TiNi), TiNi and Ti2 Ni. Although the TiNi phase is ductile, even at 78 K (I), the TiNi3 and Ti2 Ni phases are generally considered to be brittle as are the analogous compounds in the Ti-Cu system (2). In tensile tests the TiNi 3 -Ni eutectic has effectively nil ductility at temperatures below 773 K (3), only developing significant reduction in area at fracture when the test temperature is 873 K. Long (4) considers the Ti2 Ni phase more ductile than TiNi) but no direct comparison is evident in the literature. In applications for which some mechanical strength and ductility are required of the Ti-Ni bond, therefore, neither TiNi3 · nor Tii Ni, should be present as a continuous layer at the bond. The metallurgical problem is compounded when a titanium alloy is required in a heat-treated condition. Hood and Schultz (5) have shown that the diffusivity of Ni in both o:- and ~-phases of Ti is remarkably fast, being about 12 2 1 10- m • f at 1073 K. In a 15 minute solution treatment at 1073 K the diffusion distance, y4Dt, is 60 µm, adequate for growth of a layer of intermetallic phase. Since solution treatments of the bonded composite are unacceptable, the bond must be achieved without destroying the metallurgical condition of the already heat-treated component. The current work arose from a need for a component having a thin (about 3 mm) layer of soft nickel on a titanium alloy, the latter to have mechanical properties equivalent to those of the Ti-6Al-4Vt alloy in its standard quenched and aged condition. The logical approach, electroplating t All alloy compositions given in wt%. 2527 2528 C. E. ELLS, G. F. TAYLOR, AND R. C. MANSEY on the Ti-6Al-4V alloy, was found to be difficult and the investigation was extended to welding, brazing and explosive bonding on this and other alloys. Bond Evaluation Metallography was used for much of the bond evaluation. This examination was supplemented by microprobe identification of the phases present in the bond and correlated with tensile and impact tests on some bonds. Impact tests on brazed specimens were done in a miniature Izod 2 machine at room temperature, using specimens with a bond area of about SO mm • The values are not directly comparable with standard Izod specimens, but were rated against good quality titanium to titanium brazed specimens. Electron Beam Welding Specimens of Nickel-200 (Huntington Alloy designation) and Ti-6Al-4V (the latter in its standard quenched and aged condition) were welded in a Sciaky electron beam welder at 22.S kV 1 and 10 mm·s- . Parts of the welds appeared sound. When the space between the nickel and titanium was pressurized all cracks observed were associated with either the weld bead or the weld bead/Ti-6Al-4V interface; none were at the weld bead/nickel interface. The weld bead had a composition of about 60%Ni, assumed to be a eutectic mixture of TiNi and TiNi3 • A band of composition close to Ti 2 Ni was at the weld bead-titanium interface, probably responsible for the fracture consistently associated with this interface. At least two previous attempts (6), (7) to weld nickel directly to titanium, have had little success. It is difficult to envisage a weld procedure in which the range of intermetallic compounds would not be produced. Explosive Bonding The explosive bonding was done at du Pont's Eastern Laboratory. Conventional explosive techniques were used, but there had been no previous report of the bonding of soft nickel to a hard titanium substrate. In each bonding the Nickel-201 used was about 3 mm thick; the titanium alloy was plate 300 x 600 mm in area and about SO mm thick. Three combinations of titanium alloy and metallurgical conditions were used: (a) Ti-6Al-4V, in a mill annealed condition. (b) Ti-4Al-4Mo-2Sn-O.SSi, (IMI SSO), heat-treated by air-cooling from 1173 Kand aging 24 hat 823 K. (c) Ti-6Al-2Sn-4Zr-6Mo, heat-treated by air-cooling from 1143 Kand aging 8 hat 810 K. Superficially, the bonds on the Ti-6Al-4V and IMI-SSO were satisfactory. Some cracking occurred in the plate of Ti-6Al-2Sn-4Zr-6Mo, but du Pont personnel are confident that a slight modification in the technique would eliminate this feature. Metallography revealed the characteristic undulations of explosively bonded metals, Figure 1. Shear tests at room temperature 2 on the as-bonded Ni/Ti-6Al-4V gave a shear strength of 276 MN·m- , the failure occurring in the nickel away from the bond. Heating this bond for 30 min at 1123 K, equivalent to the solution treatment of the standard quenched and aged condition of the Ti-6Al-4V, resulted in very uniform diffusion zones, Figure l(b), confirming the good metallurgical contact achieved. When these intermetallic layers were present the shear strength of the composite dropped to 138 MN·m-2 at room temperature and fracture occurred along the bond. JOINING NICKEL TO TITANIUM 2529 Table I. Tensile Properties at Room Temperature of the Titanium Alloy in Bonded Sheets after Annealing 30 min at 823 K Direction* 0.2% U.T.S. Unif. Total Reduction in Sheet Y.S. Elong. Elong. In Area Alloy MN·m- 2 MN.m-2 % % % Ti-6Al-2Sn Long 1069 1179 7 12 31 -4Zr-6Mo Trans. 1151 1200 4 9 30 Ti-4Al-4Mo Long. 976 1071 6 12 28 -2Sn-0.5Si (IMl-550) Trans. 1070 1200 7 11 32 * denotes length and width directions in the sheet received from fabricators After bonding, some sections contained areas of intermetallic phase, identified as TiNi in the IMl 550, and TiNi3 in the Ti-6Al-2Sn4Zr-6Mo. Heating the composites for 30 min at 873 K to soften the nickel did not increase the amount of intermetallic phase present, but resulted in nickel diffusion into the titanium to a depth of about 5 µ.m. No nickel concentrations as high as TiiNi were observed in this band. Hardness of the as-bonded nickel was about 195 VPN. Heating the composites at 823 K for 30 min softened the Nickel-201 down to about 140 VPN, while leaving the IMI 550 and Ti-6Al-2Sn4Zr-6Mo in nearly a fully heat-treated condition, Table I. From these results it is concluded that explosive bonding is a satisfactory technique for applying a layer of nickel on titanium alloys and little further development should be necessary in any specific application. Brazing The brazing of titanium alloy to titanium alloy has been of commercial importance for two decades, and by 1956 McQuillan and McQuillan (2) were able to present general methods, including the necessity to avoid intermetallic phase formation at the bond. Heap and Riley (8) investigated nickel foil as a filler in brazing commercially pure titanium. They showed that the bond strength equalled that of the titanium only when the time/temperature cycle was severe enough to diffuse the nickel into the titanium sufficiently to remove all intermetallic phases from the bond. Afanas'ev (7) attempted a direct diffusion weld of titanium to nickel, and could not- find a time/temperature treatment which gave a ratio of bond strength to strength of titanium base alloy any higher than "=' 0.35. These results were not encouraging for the success of a nickel-titanium braze. 2 In the current experiments the braze specimens had an area of about 100 mm , formed into a sandwich with the filler material, and heated in a vacuum induction furnace. The results using various filler materials and with either or both of Ti-6Al4V and Ti-6Al-2Sn4Zr-6Mo as the titanium alloy were as follows: 2530 C. E. ELLS, G. F. TAYLOR, AND R. C. MANSEY Table II. Typical Izod Impact Strengths of the Brazed Bonds Between Nickel and Ti-6Al-2Sn-4Zr-6Mo Alloy Filler Braze Cycle Impact Strength Material Temperature Time At j K Temperature s Cu 1148 30 0.8 - 1.1 Cµ 1148 60 0.7 - 0.8 Cu 1173 60 0.7 Cu 1273 60 0.1 Al 973 60 < 0.1 Al 1073 60 1.2 Al 1273 60 < 0.1 (a) Copper: (0.025 mm foil). The maximum impact strength was obtained after short braze times at temperatures close to the Ti-Cu eutectic temperature of 1143 K. However, even this bond was weak, (Table II), compared to impact strengths of up to 22 j obtained from brazed specimens of titanium to titanium. In this "optimum" braze cycle with copper, 30 sat 1198 K, the braze zone was dendritic with an infilling of eutectic, Figure 2. The principal embrittling component in this braze appeared to be Th Cu. However, a variety of eutectic/dendritic structures were obtained (9) with different braze cycles. With increasing brazing temperature or time at temperature the impact strength deteriorated, Table II, attributed in some specimens to gross cracking in the braze zone. (b) Aluminum: (0.025 mm foil). Using 3003 aluminum, to make use of the low melting temperatures, a series of brazes with times of 1 - 30 min at temperatures from 973 - 1273 K were attempted.
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