Titanium alloys, due to their high specific strength and good corrosion resistance, are particularly suitable for special applications.[14] CP-Ti (Commercially Pure Titanium), which is unalloyed, ranges in purity from 99.5 to 99.0 wt pct Ti. Titanium exists in two allotropic crystal structures. There are α, which has the hexagonal close-packed (HCP) structure, and β, which has the body-centered cubic (BCC) crystal structure. Above the β transus temperature, the hexagonal α phase is transformed on heating to the BCC β phase.[3,4] The β transus temperature is strongly affected by the alloying elements in CP-Ti, e.g., Fe and interstitial elements, carbon, oxygen, nitrogen, as well as hydrogen. Table I illustrates the nominal values and the actual β transformation temperature varied with chemical composition of different CP-Ti grades.[1,4]

Table I Chemical Composition of Unalloyed Titanium[1,4]

It has been reported that the Ti alloys are successfully brazed using the Ti-based braze alloys, Ti-Cu-Ni and Ti-Zr-Cu-Ni alloy systems.[57] Besides, Ti-Cu-Ni and Ti-Zr-Cu-Ni alloy systems are considered as the best choices in brazing Ti and its alloys, especially for the joint operating at moderate temperatures and/or in highly corrosive environments. Ti-15Cu-15Ni in wt pct is one of the most popular Ti-based filler metals in brazing Ti alloys and it demonstrates excellent bonding performance after brazing.[8,9] However, the brazing temperature of Ti-15Cu-15Ni filler metal is above 1243 K (970 °C), exceeding the β transus temperature of CP-Ti. High brazing temperature results in coarsening the grain size of the CP-Ti substrate and deteriorating mechanical properties of the joint.

The brazing temperature of the Ti-Cu-Ni fillers is decreased by alloying of Zr into Ti-Cu-Ni filler. For example, the solidus and liquidus temperatures of Ti-20Zr-20Cu-20Ni (wt pct) braze alloy are 1115 K and 1121 K (842 °C and 848 °C), respectively, which are much lower than those of Ti-15Cu-15Ni (wt pct) filler, 1175 K and 1205 K (902 °C and 932 °C),[10] respectively. Accordingly, the brazing temperature of Ti-Zr-Cu-Ni filler is below 1173 K (900 °C) and lower than β transus temperatures of most CP-Ti except for the grade 1 CP-Ti as displayed in Table I.[1,4] Therefore, the coarsening effect of the CP-Ti grains is greatly abated in brazing.

A cold roll-bonding process, which combines Ti, Zr, Cu, and Ni strips into a layered composite, allows a conventional cold rolling process to produce the Ti-Zr-Cu-Ni brazing filler in foil form with low cost.[11] Since the clad foil is free of binder, minimum contamination of the joint is achieved in vacuum brazing titanium. High quality brazed joints are readily available from the vacuum brazing, and it is suitable for industrial application. Infrared brazing is suitable in studying the microstructural evolution of the joint with the advantage of rapid heating rate as high as 50 K/s.[1215] The brazing cycle does less damage to the base metal during infrared brazing, and it was introduced in the experiment.

The purpose of this investigation is concentrated on the brazing grade 2 CP-Ti using the Ti-20Zr-20Cu-20Ni filler foil. Both infrared and traditional furnace brazing were performed in the experiment for the purpose of comparison. Microstructural evolution, phase identification, and interfacial reaction of the brazed joint are extensively assessed in the experiment.

Grade 2 CP-Ti plates measured 10 mm × 7 mm × 3 mm were prepared for the brazing experiment. Clad Ti-20Zr-20Cu-20Ni (wt pct) foil with the thickness of 50 μm was used as the brazing filler metal.[11] Traditional vacuum brazing was performed under the vacuum of 5 × 10–5 mbar. The heating rate was set at 0.5 K/s throughout the experiment. Infrared brazing was performed using the ULVAC SINKO-RIKO RHL-816C furnace with a vacuum of 5 × 10−5 mbar. The heating rate was set at 10 K/s throughout the experiment. All specimens were preheated at 1023 K (750 °C) for 300 s prior to brazing in order to equilibrate temperature profile of the specimen. Table II summarizes various brazing conditions used in the experiment.

Table II Summary of Brazing Conditions Using Ti-20Zr-20Cu-20Ni Filler Foil

All brazed specimens were sectioned and prepared using a standard metallographic procedure prior to microstructural examination. Cross sections of joints after brazing were cut by a low-speed diamond saw and subsequently examined using a JEOL 8600SX electron probe microanalyzer (EPMA) equipped with the wavelength dispersive spectroscope (WDS). The operation voltage was 15 kV, and the minimum spot size was 1 μm. For the detailed microstructural observation, transmission electron microscope (TEM) specimens were sectioned in thin slices within brazed zones of the joint. Thin foils were prepared by a standard twin jet polisher using an electrolyte of 10 pct HClO4, 90 pct C2H5OH at −40 °C. The operation voltage of twin jet polisher was kept at 30 V, and its current was 35-40 mA. Thin foil specimens were examined using a Philips TECNAI G2 TEM operated at 200 kV. It was equipped with an energy dispersive spectroscope (EDS) for chemical analysis of specific location in the joint.

Figure 1 shows microstructural observations and EPMA quantitative chemical analysis results of the traditional furnace brazed CP-Ti/Ti-20Zr-20Cu-20Ni/CP-Ti joint at 1143 K (870 °C) for 1800 s. Figures 1(b) and (c) are enlargements of area I and II, respectively, in Figure 1(a). According to Figure 1(b), there are at least four phases readily identified from the brazed zone, including white (Ti,Zr)2Ni as marked by A, gray blocky Ti2Cu as marked by B, black blocky α-Ti as marked by C, and eutectoid as marked by D. The presence of these phases will be proven by following TEM examinations. Figure 1(c) displays the enlargement of area II in Figure 1(a); lamellar eutectoid is widely observed from the entire brazed joint. TEM analyses are necessary in order to unveil microstructures of the brazed joint in greater depth.

Fig. 1
figure 1

Microstructural observations and EPMA chemical analysis results of the traditional furnace brazed CP-Ti/Ti-20Zr-20Cu-20Ni/CP-Ti joint at 1143 K (870 °C) for 1800 s: (a) cross-sectional overview of the joint, (b) higher magnification of area I in (a), (c) higher magnification of area II in (a)

Figure 2 shows TEM micrographs of the eutectoid in a traditional furnace brazed CP-Ti/Ti-20Zr-20Cu-20Ni/CP-Ti joint at 1143 K (870 °C) for 1800 s. Eutectoid Ti2Cu (marked by E), Ti2Ni (marked by I), (Ti,Zr)2Ni (marked by G), and α-Ti (marked by F, H, J) are identified from the TEM analyses. According to Figure 1(c), the coarser eutectoid in area III is mainly comprised of Ti2Cu and α-Ti, and the finer eutectoid in area IV primarily consists of (Ti,Zr)2Ni and α-Ti. The α-Ti is primarily alloyed with a few at. pct Zr as marked by F, H, and J in the figure. Based on experimental observation, Ti2Cu dissolves 15.7 at. pct Ni and Ti2Ni dissolves only 4.3 at. pct Cu. The amount of Ni dissolved in Ti2Cu is much greater than that of Cu in Ti2Ni. It is consistent with the Cu-Ni-Ti ternary phase diagram.[1618] Ti2Cu dissolves Ni up to 16.9 wt pct, and the maximum solubility of Cu in Ti2Ni is below 8 at. pct.

Fig. 2
figure 2

TEM micrographs of the eutectoid in traditional furnace brazed CP-Ti/Ti-20Zr -20Cu-20Ni/CP-Ti joint at 1143 K (870 °C) for 1800 s: (a) BF image of central Ti2Cu, (b) DF image of Ti2Cu using \( \left( {10\bar{1}} \right) \) spot with the zone axis of \( \left[ {1\bar{1}1} \right] \), (c) BF image of (Ti,Zr)2Ni, (d) DF image of (Ti,Zr)2Ni using spot \( \left( {0\bar{1}1\bar{3}} \right) \) with the zone axis of \( \left[ {0\bar{3}32} \right] \), (e) BF image of central Ti2Ni, (f) DF image of Ti2Ni using \( \left( {1\bar{3}\bar{1}} \right) \) spot with the zone axis of \( \left[ {714} \right] \)

Figure 3 illustrates the isothermal section of Ni-Ti-Zr ternary alloy phase diagram at 973 K (700 °C) in at. pct.[16] According to the figure, (Ti,Zr)2Ni is a non-stoichiometric compound. It can be expressed as (Ti1−x Zr x )2Ni where x equals 0.21–0.30.[16] The structure type of (Ti,Zr)2Ni is MgZn2. It has a hexagonal Laves structure with lattice constants of a = 0.5191 nm and c = 0.8520 nm. The Zr content in (Ti1−x Zr x )2Ni intermetallic compound is between 14 and 20 at. pct, and it is very different from the Zr content in Ti2Ni, which is between 0 and 10 at. pct.

Fig. 3
figure 3

Isothermal section of Ni-Ti-Zr ternary alloy phase diagram at 973 K (700 °C) in at. pct[17]

Dissolution of CP-Ti substrate into the braze melt results in isothermal solidification of the molten braze and forms primary β-Ti during brazing. The residual melt is solidified via eutectic reaction upon the cooling cycle of brazing. The eutectic consists of (Ti,Zr)2Ni, Ti2Cu, and α-Ti as displayed in Figure 1(b). According to related binary alloy phase diagrams, the β-Ti is completely soluble with Zr.[19] Maximum solubilities of Cu and Ni in the β-Ti are 13.5 and 10 at. pct, respectively. In contrast, Cu and Ni are dissolved in α-Ti up to 1.6 and 0.2 at. pct, respectively, which are significantly lower than those in β-Ti. Both Cu and Ni belong to β stabilizers of titanium. The β phase can transform to α plus intermetallic compound(s) upon cooling to room temperature.[4,19] Accordingly, decomposition of the β-Ti proceeded via eutectoid solid-state transformation upon the cooling cycle of brazing. The eutectoid of Ti2Cu, (Ti,Zr)2Ni, Ti2Ni, and α-Ti is observed in the prior β-Ti grains of the brazed joint at room temperature.

Figure 4 shows microstructural evolution of infrared brazed CP-Ti/Ti-20Zr-20Cu-20Ni/CP-Ti joints at different brazing temperatures for 1800 s. Increasing the brazing temperature greatly enhances the depletion rate of Cu, Ni, and Zr driven by concentration gradient from the braze melt into CP-Ti substrate during brazing. For specimens brazed at lower temperature [1123 K (850 °C)], most Cu, Ni, and Zr are preserved in the brazed zone and form eutectic of (Ti,Zr)2Ni, Ti2Cu, and α-Ti after brazing as illustrated in Figure 4(a). For specimens brazed above 1163 K (890 °C), Cu, Ni, and Zr are depleted from the brazed zone into the CP-Ti substrate, and coarse eutectic disappeared from the joint as displayed in Figures 4(b) and (c).

Fig. 4
figure 4

Microstructural evolution of infrared brazed CP-Ti/Ti-20Zr-20Cu-20Ni/CP-Ti joints at (a) 1123 K (850 °C), (b) 1163 K (890 °C), (c) 1183 K (910 °C) for 1800 s

Higher brazing temperature results in higher volume fraction of the β-Ti in the joint, and greatly enhances the depletion rates of Cu, Ni, and Zr from the braze melt into CP-Ti substrate during brazing. Maximum solubilities of Cu and Ni in β-Ti are up to 13.5 and 10 at. pct, respectively.[19] Because Cu, Ni, and Zr are all dissolved in the β-Ti, isothermal solidification of braze melt during brazing results in forming only β-Ti in the joint. Eutectic of (Ti,Zr)2Ni, Ti2Cu, and α-Ti disappeared from the brazed zone. The β-Ti alloyed with Cu, Ni, and Zr is transformed to fine eutectoid of (Ti,Zr)2Ni, Ti2Cu, Ti2Ni, and α-Ti upon the subsequent cooling cycle of brazing. It is expected that the disappearance of coarse eutectic intermetallic compounds from the brazed zone is beneficial to bonding strength of the joint.

In summary, microstructural evolution of the clad Ti-20Zr-20Cu-20Ni foil brazed CP-Ti alloy has been investigated. For the specimen furnace brazed at 1143 K (870 °C) for 1800 s, the joint is dominated by primary β-Ti and coarse eutectic of (Ti,Zr)2Ni, Ti2Cu, and α-Ti during brazing. The primary β-Ti is transformed to fine lamellar eutectoid of (Ti,Zr)2Ni, Ti2Cu, Ti2Ni, and α-Ti upon the subsequent cooling cycle of brazing. Increasing the brazing temperature above 1163 K (890 °C) results in higher volume fraction of β-Ti in the joint, and it greatly enhances the depletion rates of Cu, Ni, and Zr from the braze melt into CP-Ti substrate during brazing. The loss of Cu, Ni, and Zr from the braze melt results in isothermal solidification of molten braze, and the β-Ti grains dominate the entire joint at the brazing temperature. The β-Ti alloyed with Cu, Ni, and Zr is subsequently transformed to fine eutectoid of (Ti,Zr)2Ni, Ti2Cu, Ti2Ni, and α-Ti. The clad Ti-20Zr-20Cu-20Ni foil shows potential in brazing CP-Ti alloy for industrial applications.