**7. Conclusion**

Transient liquid phase diffusion bonding of particle reinforced Al-6061 MMC using Ni-Al2O3 interlayer was successfully achieved using nano-composite Ni-Al2O3 coating. The results obtained from the spectroscopic analyses using WDS and XRD showed that Ni-Al2O3 coating the bonding process can be characterized in four distinct stages: interfacial contact and solid-state diffusion, which resulted in the formation of three reaction layers promoted by the diffusion of nickel into the aluminum base metal. The second stage of the joining process was the formation of an Al-Ni-Si eutectic liquid at the bonding temperature. It is supported that the reaction layers formed within the joint melted to form a liquid phase, followed by dissolution of the base metal (third stage) as the liquid spread between the bonding surfaces through capillary action. The final stage of bonding involved isothermal solidification at the bonding temperature in which the diffusion of Ni into Al results in a change in the composition of the liquid phase leading to solidification.

The joint shear strength was studied as a function of bonding parameters, bonding time, bonding temperature, interlayer thickness and interlayer particle size. The results showed that the joint shear strength increased with increasing bonding time, bonding temperature and interlayer thickness. On the other hand the results showed that bond strength increased when the interlayer particle size was reduced from 500 nm to 50 nm. The increase joint shear strength seen when Ni-Al2O3 coating were used was attributed to the presence of highlydispersed nano-sized reinforcement particles in the joint region act to strengthen the joint region by Orowan bowing mechanism.

The results showed that an optimum joint strength of 144 MPa can be achieved if the following bonding parameters are used: 30 minutes bonding time, 620oC bonding temperature, and 11 μm thick Ni–Al2O3 coating. Within the parameter ranges tested the bonding pressure had the lease effect on the joint shear strength of TLP bonded joints.
