Alexander E. Shapiro and Yury Flom
A standard titanium-based BTi-5 (TiBraze®200) and two new zirconium-based filler metals TiBraze®590 and TiBraze®800 (see Table 1) are now manufactured and supplied in the form of amorphous foils (Fig. 1) having thickness in the range of 37-50 microns (0.0015″-0.0020″) and width up to 75 mm (3 inches). Both zirconium-based filler metals provide brazing below the α→β transus of α- and (α+β)-titanium base alloys, and TiBraze®800 has a melting range even below near-β and β-titanium base alloys. Also, these foils resolve the problem of furnace brazing of titanium to copper, because brazing temperatures are much lower than the titanium-copper eutectic point 885°C (1625°F). These homogeneous and ductile foils have excellent wetting and melting characteristics (Table 2) and good compatibility not only with titanium or zirconium alloys but also with ceramics, graphite, and carbon-carbon composites. When used in a preplaced preform, these brazing materials supply a minimum and accurate amount of filler metal in the joint clearances, resulting in joint mechanical properties unattainable for filler metals in powder or sintered strip forms.
Fig. 1 Amorphous brazing foils TiBraze200 and TiBraze590
The amorphous foils were experimentally evaluated as brazing filler metals in combinations of base materials representing Titanium Grade 2, Titanium Grade 5 (Ti-4Al-4V alloy), stainless steel AISI 304, copper, nickel-plated carbon steel 1018, ceramics (alumina, hexagonal boron nitride, silicon carbide) and graphite. Flow characteristics and the ability to fill the gap at different brazing temperatures, shear strength and microstructure of brazed joints were analyzed and reported in this paper.
Brazed joints of titanium-to-titanium, titanium-to-304SS, and titanium-to-copper are shown in Fig. 2-4, titanium-to-alumina, titanium-to-SiC, titanium-to-hBN, and boron nitride-to-boron nitride are shown in Fig. 5-8, and brazed joint of graphite – in Fig. 9. Brazed samples of titanium honeycomb panel and a fin-plate heat exchanger are shown in Fig. 10-11. Average shear strengths of metal-to-metal brazed joints is presented in Table 3. All three brazing filler metals exhibited good wetting of titanium, graphite, and ceramics base materials, and even flow along ceramic or graphite surfaces (Fig. 5 and 9) that is not typical for traditional active filler metals. Wetting and spreading of the liquid braze alloy occurred even on the surface of hexagonal boron nitride (Fig. 8), which is known as an inert, stop-off material. Overheating and longer holding time when brazing ceramic and graphite (in comparison with metal-to-metal joining) gave better results both in quality and strength consideration of ceramic and graphite brazed joints. Recommended parameters of brazing processes are presented in Table 2.
Fig. 2 Ti-6Al-4V alloy brazed by TiBraze200
Fig. 3 Ti6Al4V alloy brazed to 304SS by TiBraze590
Fig. 4 Copper brazed to CP Titanium by TiBraze800
Fig. 5 Alumina brazed to Ti-6Al-4V alloy by TiBraze200
Fig. 6 Silicon carbide brazed to CP Titanium by TiBraze800
Fig. 7 Hexagonal Boron nitride brazed to Ti-6Al-4V
alloy by TiBraze800
Fig. 8 Hexagonal Boron nitride brazed by TiBraze800.
Effective wetting and spreading of ceramic
Fig. 9 Graphite brazed by TB200
Fig. 10 Titanium honeycomb core brazed
Fig. 11 A section of titanium heat exchanger
brazed by TiBraze200
It is believed that significant difference in strength of brazed joints manufactured with a titanium-based filler metal TiBraze200 and zirconium-based filler metals TiBraze590 and TiBraze800 results from a significant difference in microstructures of the braze interlayers formed by these brazing alloys. Structure of the TiBraze200 braze layer is a matrix of ~89Ti-4.5Al-1.6V-1.2Ni-1.7Cu-2.5Zr wt.% alloy with small content of other elements (black phase in Fig. 12) well-reinforced with needle-like crystals containing ~75Ti-3.2Al-5V-7Ni-5.5Cu-4.2Zr wt.% (white needles in Fig. 12). As a reinforced matrix, this structure resists crack propagation during mechanical loading of the brazed joint, and the crack occurring in the fillet propagates to the base metal. Appearance of significant amounts of vanadium and aluminum in the braze layer, on the one hand, and relatively low content of zirconium, copper and nickel in both matrix and needle-like phases, on the other hand, suggests a very active diffusion exchange of elements between base and braze alloys. It is pertinent to note that shear strength of brazed joints manufactured with zirconium-based amorphous foils is comparable with the strength of joints of the widely-used BTi-1 (Ti-15Cu-15Ni) filler metal, while the shear strength of joints made with titanium-based amorphous foil TiBraze200 is higher by ~20%. Also, formation of a Widmastätten-type microstructure with small plate dimensions in the interface zone is advantageous for strength of brazed joints. Earlier, it was shown (Ref. 1) that the amount of copper in the joints should not exceed 10–12% to yield the joints’ optimal microstructures, which are formed at brazing below 900°C and holding time <10 min, followed by relatively rapid cooling at the rate about 35 °C/min or faster. It is very likely, that strength values of brazed joints presented in Table 4 can be further improved by optimization of brazing thermal cycle.
Fig. 12 Microstructure and crack propagation in brazed joint of Ti-6Al-4V alloy made using TiBraze200 amorphous foil as a filler metal, x200
Zirconium-based filler metal TiBraze590 forms a two-phase, non-reinforced microstructure of the brazed joint (Fig. 13). This cast structure is typical for brazed joints and its resistance to crack propagation is low. The matrix phase is represented by large grains of a Zr-Ti-Ni alloy slightly alloyed with aluminum, vanadium and hafnium. Grains of a dark phase are uniformly distributed in the matrix and constitute a high-temperature α-solid solution of Zr-Ti system alloyed with Hf, Ni, and V, which has high solidification temperature of about 1600°C. Crack propagations occurred mostly through the matrix. At a critical tensile load, multiple cracks are initiated and propagated along grain boundaries of the cast-structured matrix phase, between the hard “islands” of the high-temperature phase. Further, it is very likely that crack propagation in the braze layer has negative impact on shear strength of the brazed structure which is lower in comparison with TiBraze200.
Fig. 13 Microstructure of brazed joint of Ti-6Al-4V alloy made using TiBraze590 amorphous foil as a filler metal, x370
Shear strength of graphite-to-titanium brazed joints was not measured, because all the samples failed in graphite body (Fig. 14a), even at the overlap of one or two thicknesses of the graphite part. Shear strength of ceramic-to-titanium brazed joints also was tested using non-standard “bridge-like” samples (Fig. 5-7). Most of alumina-titanium, SiC-titanium, and hBN-titanium joints failed along the ceramic body (Fig. 14b), not at the joint, which confirms that brittleness of ceramic body played crucial role in testing:—this means that the geometrical design of the samples should be changed. However, these test results at least demonstrate that adhesion of the braze materials to ceramics and graphite is sufficiently strong to resist significant shear loads, and titanium-to-ceramics brazed joints can respond to the requirements of reliability at larger overlaps. Mechanical testing of ceramic-to-metals brazed joints manufactured with new amorphous foils as brazing filler metals should continue using modified sample design.
Fig. 14 Brazed joints of titanium-to-graphite (a) and Ti6Al4V-to-alumina (b) failed
in graphite and ceramic bodies. Brazing filler metal is TiBraze200
1. Botstein O. and Rabinkin A. 1994. Brazing of titanium-based alloys with amorphous 25wt.%Ti-25wt.%Zr-50wt.%Cu filler metal, Materials Science & Engineering A, Vol. 188, No. 1-2, 305-315
Alexander E. Shapiro is with Titanium Brazing, Inc., Columbus, OH and Yury Flom is with Goddard Space Flight Center, NASA, Greenbelt, MD