The exothermic reactions in Al+Ni system were used as a power source for welding of Al1100 alloy so that the obtained joint was reinforced with in situ produced Al 3 Ni intermetallic compound, simultaneously. Due to the combustion heat, the surfaces of the plates were melted down which, in turn, eventuated in formation of a strong surficial bond. The cross sections of joints were analyzed using optical microscopy, EDS-equipped scanning electron microscopy (SEM), and X-ray diffraction experiments. XRD and EDS results represented the Al 3 Ni intermetallic compound as the reaction product. The compressive shear strength of the joint was 20 MPa. INTRODUCTIONAluminum welding is more challenging rather than welding of other metals and alloys due to its high thermal conductivity and high oxidation rate characteristics. The problem would be overcome if the power source can release a large amount of energy (in the form of heat) in a short time. Combustion synthesis or self-propagating high temperature synthesis (SHS) reactions provides such a necessity. In our previous work. [1] Al alloys were joined using the exothermic reactions in Al-CuO-Ni system in which a pre-stage of mechanical activation was required in order to decrease the ignition temperature of the utilized powder mixture. In this work, Al+Ni powder mixture was used for welding because its ignition temperature is lower than melting point of Al, thus the pre-stage of mechanical activation is not necessary, which in turn makes the process more economical.

We determined the mechanism of mechanochemical synthesis of fluorapatite from CaO, CaF2 and P2O5 by characterisation of the intermediate compounds. We used atomic absorption spectroscopy, X-ray diffraction, field emission scanning electron microscopy, FTIR spectroscopy and transmission electron microscopy to find the transitional compounds. Investigation of the binary and ternary powder mixtures revealed the appearance of H3PO4, Ca(OH)2, Ca2P2O7 and CaCO3 as the intermediate compounds. At early stages of the milling, conversions of P2O5 to H3PO4 and CaO to Ca(OH)2 occurred in the wet atmosphere. Later, a combination of Ca(OH)2 and H3PO4 formed C a2P2O7 while the unreacted CaO was converted to CaCO3 by CO2 of the ambient atmosphere. Spherical crystalline Ca10 (PO4)6F2 particles formed after 48 hours of milling due to the reaction between Ca2P2O7, CaCO3 and CaF2.

During the in-situ nitridation of Si in the presence of CaO, MgO and Al 2 O 3 formation of reaction bonded silicon nitride ceramics (RBSN) having various microstructures indicated the possibility of different nitridation mechanisms operating. Experimental evidence suggested that, whereas the morphology of pores was controlled by nitridation of Si(g) in CaO-RBSN and SiO(g) in Al 2 O 3-RBSN, a combination of these two reactions occurred during the nitridation of the MgO-RBSN. By inhibiting the growth of whiskers and maximizing the α/β ratio, the CaO addition led to the formation of matte grains creating clean spherical cavities with d < 40 µm and flexural strength of 0.8 MPa. In contrast, when using Al 2 O 3 additions, a microstructure with a very low α/β ratio, fine inter-particle pores and 2.4 MPa flexural strength, reinforced with interlocking whiskers, was produced. The highest porosity (85%) and the lowest strength (0.3 MPa) occurred in the MgO-RBSN, which was composed of both matte grains and fine whiskers. Local supersaturation and low content of β-nuclei led to the formation of anisotropic β-grains with a bimodal microstructure in heat-treated CaO-RBSN while a unimodal microstructure was observed in heat-treated MgO-RBSN. No porosity loss or β-grain growth occurred in the heat-treated Al 2 O 3-RBSN.