Research On Brazing Process And Mechanism Of TiB_w/TC4 Composite And Ti60 Alloy

TiBw/TC4 is a newly-developed Ti matrix composite, its matrix is TC4 alloy and is reinforced by TiB whiskers(TiBw). TiBw/TC4 possesses better mechanical properties, wearability, as well as more excellent high-temperature performance than conventional Ti alloys do. Ti60 is a near-α high- temperature Ti alloy, it can be used under the condition of up to 600℃ due to its good high-temperature oxidation resistance and mechanical properties. For improving the brazing technique of Ti alloy using TiZrNiCu, composite filler Ti ZrNiCu+B was developed in this study and the B was able to react with Ti, in situ synthesizing TiBw in the braze seam during brazing, so as to intensify the joint. The wetting ability of TiZrNiCu+B on Ti60 was first studied, and then the effects of B content and brazing parameter on the microstructure and joint mechanical properties were studied as well. Finally, the most appropriate brazing parameter and B content were clarified and the strengthening mechanism of B was analyzed.The typical interfacial microstructure of joint brazed at 940℃ for 10 min using TiZrNiCu was: TiBw/TC4/α-Ti+β-Ti+TiBw/(Ti, Zr)2(Ni, Cu)/α-Ti+β-Ti/Ti60. As brazing temperature increased, the thickness of(Ti, Zr)2(Ni, Cu) decreased. The continuous(Ti, Zr)2(Ni, Cu) disappeared completely when brazing temperature was 1020℃.The fracture surface indicated, that the joint fractured at(Ti, Zr)2(Ni, Cu) area when the brazing temperature was low. Increasing brazing temperature intensified the elements dissolving towards substrates, so that the continuous(Ti, Zr)2(Ni, Cu) vanished. The highest shear strength of 208 MPa was achieved when brazing temperature was 1020℃. The fracture path propagated from one substrate to brazing seam then to the other substrate. Further increasing brazing temperature, the structure of braze seam and Ti60 turned coarse and the shear strength started decreasing.The wetting experiment of TiZrNiCu+B on Ti60 indicated that B had a suppressing effect on the wetting behavior of TiZrNiCu+B. Increasing B content in composite filler worsened the wetting behavior of TiZrNiCu+B. The typical interfacial microstructure of joint brazed at 940℃ for 10 min using TiZrNiCu+B as filler was: TiBw/TC4/α-Ti+β-Ti+(Ti, Zr)2(Ni, Cu)/(Ti, Zr)2(Ni, Cu)+TiBw/α-Ti+β-Ti+(Ti, Zr)2(Ni, Cu)/Ti60. As B content increased, the number of in situ TiB increased, and the(Ti, Zr)2(Ni, Cu) in the braze seam become more scattered, moreover, the microstructure got refined due to the existence of TiB. The highest shear strength was 236 MPa when brazing temperature was 1020℃, holding time was 10 min and B was 0.6 wt.%. The traces of pulled-out TiBw indicated that TiBw bore part of the load during shear test. It could be known that refining strengthening function and load-bearing strengthening function of TiBw improved the braze joint to some extent. Excessive B would lead to forming large amount of TiBw, which resulted in the micro-pores and disconnected area in the joint, and the shear strength decreased inevitably.