| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 86 === The infrared joining of titanium-aluminides, including Ti50Al50, Ti-48Al-2Nb-2
Cr, Ti60Al40 and Ti70Al30, using Ti-15Cu-15Ni foil as brazing filler-metal was
investigated in this study. Experimental results show that all the joint inte
rfaces show distinct multilayered structures which were mainly formed by isoth
ermal solidifcation and following solid-state interdiffusion during joining. T
he high-temperature multilayered structures are formed prior to the room-tempe
rature ones which are cooling phase-transformed from the former ones. The diff
usion of Al atoms from the base-metal into the joint zones at the joining temp
erature (Tw)is the main controlling factor pertaining to the microstructural e
volution of the joint interface. Due to this Al diffusion, several new solid-l
ayers are gradually formed in the joint, and then the high-temperature multila
yered microstructure appears. Seven characteristic zones can be distinguished
in the room-temperature joint. The corresponding high-temperature phases of th
ese seven zones are γ-TiAl (Ⅰ+Ⅱ), α+β two-phase mixed (Ⅲ), high Al% α-p
hase (Ⅳ), α2-Ti3Al (Ⅴ), β-Ti (Ⅵ) and residual liquid filler-metal (Ⅶ), r
espectively. The observed joint microstructures are obtained from the cooling
phase transformation of these well-established high-temperature phases through
rapid cooling to the room temperature. Because of the sensitive thermal respo
nses of infrared joining, the microstructural evolution of joint interfaces ca
n be effectively realized. In this study, a five-step microstructural evoluti
on mechanism at 1500℃ joining temperature is proposed. These time-dependent e
volution steps include (a) β-Ti formation, (b) columnar α+β two-phase zone
formation, (c) α2-phase nucleation, (d) high Al% α-phase formation and (e)
α2-phase integration. These steps are in good agreement with the observed mic
rostructures and are consistent with the multiphase diffusion theories in soli
d-state systems. The different joining temperature (Tw) effects, which have gr
eat influences on the microstructural evolution, are also investigated in this
study. Based on the proposed evolution mechanism, the different Tw mainly aff
ects the kinds of stable phases and the stability of individual phases in each
joining condition. Because different Tw have different corresponding ternary
isotherms and stable phases, small variation of Tw can result in significant c
hanges of the microstructural morphologies. The compressive tests of the Ti-48
Al-2Nb-2Cr joint interfaces show that the infrared joint interfaces have relat
ive good joint strength. The real joint strength is about 319-322 MPa. The mic
rostructure examinations indicate that the microstructural morphologies of joi
nt interfaces of Ti-48Al-2Nb-2Cr, Ti60Al40 and Ti70Al30 intermetallics are sim
ilar with those of Ti50Al50 interfaces, except the base-metal zone and the bas
e-metal interface zone which are near the base-metal. All these differences ca
n be reasonablely explained by the effects of different base-metal composition
s. Therefore, based on the proposed evolution mechanism, all the observed join
t interfaces of four different titanium-aluminides used in this study can be e
ffectively realized.
|
|---|
