| Ti2AlC is a typical representative of 211-typal MAX phase, which has ceramic and metallic excellent properties, such as thermal shock resistance, thermal and electrical conductivity because of its unique laminated structure. The excellent properties make Ti2AlC a potential material for project application values. Researching on the bonding theory and technology of Ti2AlC will greatly expand the application field of this ceramic and will realize the low cost manufacturing technology foundation for Ti2AlC. Aluminum based filler was firstly used to braze Ti2AlC in this thesis, the effect of brazing parameters and the composition of the filler on micro structure and properties of the joints were investigated. The forming mechanism of joint and the effect of Al and Si elements on the structural stability of Ti2AlC were revealed. We also analyzed the shear fracture behavior of joint.The Al based filler was introduced into joining the Ti2AlC. The effect of chemical, brazing temperature, holding time, filler thickness and brazing pressure on the structure, shear strength and conductivity of the joints were explored. The typical micro structure of Ti2AlC/Ti2AlC joint was as follows:Ti2AlC/interaction area/reaction layer/intermetallic compound/Al based filler. When we brazed Ti2AlC by pure Al layer, only TiAl3 compound was formed in the joints. Brazing temperature and holding time mainly affects the morphology, quantity and distribution of TiAl3. With the increase of brazing temperature and heat time, TiAl3 grew up from granular to the rod. The shear strength of the joint first increased and then decreased. The maximum joint shear strength(94MPa) was obtained while Ti2AlC ceramic was brazed with a thickness of 150μm pure Al filler at 850℃ for 10 min. And the conductivity of the joint was 3.87×106s/m. On the one hand, Si was mainly functioned as solution strengthening elements. On the other hand, the addition of Si in the Al filler would reduce the solder melting point, thereby reducing the brazing temperature. We have investigated the structure and properties of the joints brazed by hypoeutectic, eutectic and hypereutectic Al-Si alloy. The structure of the joint is:Ti2AlC/ Ti2Al(Si)C+Ti-Al-Si compounds/reaction layer/Ti(AlSi)3/Al(s,s). The shear strength of the joint brazed by the Al-6Si alloy was higher than that of pure Al solder. The maximum shear strength is 108 MPa, increased by 15%, while the brazing temperature is 710℃. Electrical conductivity is about 92% of the parent ceramic.The joining mechanism of Ti2AlC-Ti2AlC was investigated. With the increase of brazing temperature, Al-Si alloy melt and spread on both sides of the Ti2AlC matrix. The diffusion path of Al and Si are divided into two kinds: intergranular penetration and intracrystalline penetration. It was confirmed Si element was formed solid solution with the parent and Ti-Al intermetallic from SEM, EDS and XRD analysis methods. Alloy layer react with parent metal at the interface between ceramic and solder. In the high brazing temperature condition, the reaction of T12AIC with Al occurred and generated TiC+TiAl3. The solution of Si made the reaction between Ti2AlC and Al-Si, and produced Al4C3+Ti(AlSi)3. Between the interface of Ti2AlC matrix and alloy filler formed a reaction layer and continuous intermetallic layer. When brazing Ti2AlC matrix with eutectic and hypereutectic Al-Si alloy, we found that the Ti(AlSi)3 phase and acicular Ti(AlSi)2 phase were generated. Si element diffused along the grain boundary. The formation of "drip" Ti-Al-Si compounds between the grain boundaries weakened the parent ceramic grain boundary strength. The intracrystalline penetration of Si in Ti2AlC formed Ti2Al(Si)C solid solution. Thus the interaction zone was formed, and with the increase of Si content, the width of this area was broadened.Our research shows that the strength of the Ti2AlC-Ti2AlC joint is closely related with its structure. It indicated that brazing process condition and solder composition affect the coupling relationship among interaction zone, reaction layer and residual solder layer of the Ti2AlC-Ti2AlC joint. Though Si element strengthens the brazing layer and the interface bonding, it weakens the strength of the parent ceramic grain boundary. Therefore, we should not only make certain reactions occur at the interfaces, but also ensure the stability of the ceramic structure during the brazing process. |
