Study On The Laser-ignited Self-propagating Sintering And Products Of Al-C-Ti System Powder Materials

With the entrance to 21st century, the resource and environment problems have become the principle problems in the sustainable development of human beings. Therefore in the long term, the trends of the industrial development all over the world can be reduced to the saving of energy and resource, and what is more, the pursuit of natural production. Al/TiC metal ceramic is kind of the engineering materials, which have many excellent and unique properties, such as good corrosion resistant and wearability, high specific strength, and therefore, increasing attention has been paid to Al/TiC metal ceramic in automotive, communication apparatus and aerospace applications. Consequently, it is considered that the Al/TiC composite material is the most worthy and exploitative"natural engineering"with greatly potential application in this century.Presently, the composite material is maily fabricated by several processing routes such as fusion casting, vacuum liquid metal infiltrating and powder metallurgy etc.. However, in all these methods, their great application is limited, owing to the limitation in the complexity and high cost of fabrication processing, and long production period.Aiming at the drawbacks of above methods, recently, a new processing method i.e. the laser ignited self-propagating high-temperature synthesis (LISHS) has been developed to fabricate the composite material. Compared with the conventional methods, the laser induced self-propagating high-temperature synthesis (LISHS) exhibits the following advantages:(1) the irradiation source can be positioned at a considerable distance away from the point of ignition, thus eliminating distortion due to wall or intrusive sources; (2) large power of flux levels can be utilized and focused to a point, sheet, or core, thus offering the ability to direct the energy input to specific modes or configuration of the ignition; and (3) the process is safer and easier to control without any pollution. Consequently, the laser induced self-propagating high-temperature synthesis (LISHS) is one of the best selections and the optimum point of arrest of preparation of the composite material from a long-term interest of industry applications.Aiming at the inherent feature of Al-C-Ti system, the Al/TiC composites in the present study are successfully fabricated for the first time based on the experiment research. In consideration of the point, a low cost starting material system of Al-C-Ti is used to fabricate the Al/TiC composites. The thermodynamics, reaction mechanisms of the system and the effect of Al content, C/Ti atomic ratio, particulate size, compacted density, laser power as well as the addition of Sn, Ni and Cu on LISHS synthesis products of Al-C-Ti system have been investigated detailedly. On the basis of the above work, the solution and precipitation mechanism is put forward, a schematic representation of the mechanisms for the phase formation and the reaction kinetics is presented. The major research efforts of the present study are as follows:(1) Study of the reaction behavior, densification behavior and microstructure of synthesis products of Al-C-Ti system by LISHSIt is found by the experiment that the Al content has significantly affected on the reaction behavior, densification behavior and microstructure of the SHS products of Al-C-Ti system. With the increasing of Al content, the combustion temperature and porosity of the Al-C-Ti system greatly decreases and the densificantion increases, when the Al content is 10~50wt.%, the SHS products consist of TiC and Al, when the Al content increases to 60wt.%, in addition to TiC and Al, Al3Ti phase is also found in the final products; Furthermore, the size of synthesized TiC gradually decreases with the increasing of Al content.It is found by the experiment that the C/Ti atomic ratio has significantly affected on the densification behavior and microstructure of the SHS products of Al-C-Ti system. With the increasing of C/Ti atomic ratio, the porosity of the Al-C-Ti system greatly increases and the densificantion decreases. When the C/Ti atomic ratio is 0.6, in addition to TiC, the dendritic Al3Ti phase is also found in the final products; when the C/Ti atomic ratio is above 0.6, the amount of dendritic Al3Ti phase decreased and that of TiC increased, Al3Ti phase gradually transforms from the dendritic shape to the strip shape.It is found by the experiment that the laser power and relative green density have significantly affected on the reaction behavior, densification behavior and microstructure of the SHS products of Al-C-Ti system. When laser power and relative green density increase, the combustion temperature of the former increases and that of the latter decreases, the porosity of the SHS products between them greatly increases and the densificantion decreases. Furthermore, when laser power increases, the TiC particulates gradually transform from the multiangular shape to the spherical shape; when relative green density increases, the sizes of TiC decreases.It is found by the experiment that the particulate sizes of powder have significantly affected on the microstructure of the SHS products of Al-C-Ti system. The particle sizes of Al have hardly effect on the combustion temperature and microstructure. However, the particle sizes of C and Ti have a very large effect on the combustion process and microstructure, the combustion process decreases and the particle sizes of TiC decrease with the increasing of the particle sizes of C, the combustion process increases and the particle sizes of TiC increase with the increasing of the particle sizes of Ti.(2) Thermodynamic and lattice parameter calculation of TiCx produced from Al-Ti-C powders by LISHSIt is found by the calculation that the adiabatic combustion temperature, Tad, decreases almost linearly with the increasing of Al content. When the Al content is over 60wt. %, the Tad drops below 1800K, and the combustion synthesis reaction of the system can not self-maintain. Therefore, there is significant effect of Al content on the LISHS reaction, the preferred ranges of them are suggested as follows: 10wt.%-50wt.%.It is found by the calculation that the change of the formation enthalpy of Al3Ti and TiC with the reaction temperature is a critical temperature, when the reaction temperature is greater than the critical temperature, TiC is more thermodynamically stable than Al3Ti, TiC might be formed in the first step and then Al3Ti begin to form. Furthermore, the critical temperature increases with the increasing of Al content.It is found by the calculation that the lattice parameter of TiCx for a given ratio of C/Ti increases with increasing Al content, but the increase of Al content has little effect on the lattice parameter of TiCx when C/Ti ratio is above 0.8. The lattice parameters of TiCx increase with increasing C/Ti ratio at four different Al contents. The differences in the lattice parameters of TiCx decrease with increasing C/Ti ratio at four different Al contents.It is found by the calculation that the effective thermal conductivity firstly decreases and then increases with the increasing of porosity; and the combustion rate of system decreases with the increasing of Al content and particulate sizes.(3) Study of LISHS reaction mechanism of Al-C-Ti systemThe reaction kinetics and the microstructural evolution of the Al-C-Ti system compacts during the LISHS are thoroughly investigated using combustion front quenching technique. The solution and precipitation mechanism of the formation of Al/TiC product phases is put forward and the reaction kinetics model corresponding to the dissolution–precipitation mechanism is set up based on the experimental results. It is found that in addition to the Al-Ti reaction among Al, Ti and C to form metastable Al3Ti phase which further reacts with C to form TiC, the direct reaction between Ti and C can also perform to form TiC.(4) Effect of the third phase low melting alloy element on the densification behavior and microstructure of synthesis products of Al-C-Ti system by LISHSThe effects of the third phase low melting alloy element (Sn, Ni, Cu) on the densification and microstructure of synthesis products of Al-C-Ti system are discussed. As far as Sn is concerned, with the increasing of Sn content, it is shown that not only the combustion temperature decreases and the densification increases, but also the particulate size and shape of TiC are significantly changed; As for Ni, the density of the products varies with y value (3Ni+Al), and a maximum value of density appears at 50wt.% y value, the TiC particle size decreases with the increasing of y value, however, the decreasing of grain size is unobvious when y value is over 60wt.%; For Cu, the addition of Cu not only decreases the combustion temperature and increases the densification, but also the TiC particulate size is significantly decreased.In a word, the research of the present study lies not only in the further investigation and broadening on the present SHS technique, but also in the more embedded comprehension of the traditional SHS technique and the fabrication processing of composites as well as the correlation theories. At the same time, it provides the referencable ideas for the studies of metal ceramic composites which are more prevailed internationally. Therefore, the work in this paper is of rather available values in theory and practical application.