Self Propagating High Temperature Synthesis Of Al-ti-c Master Alloy Structure And Performance

Al-Ti-C master alloys used as grain refiners of the aluminum and its alloys have been prepared by self-propagating high-temperature synthesis (SHS) from Al, Ti and graphite powder. The effect of some processing parameters, such as green-compact pressure, pellet diameter, preheating temperature, igniting current density, size of Al and graphite powder and igniting mode on combustion processing and microstructure of the products, was investigated. The mechanism of structure formation of Al-Ti-C master alloys via SHS was studied preliminarily by means of quenching Al-Ti-C samples. Al-Ti-C grain refiner was prepared through dilution treatment of Al-Ti-C master alloys, its grain refining performance on industry pure aluminum was evaluated qualitatively, at the same time, compared with that of Al-5Ti-B grain refiners made in China and LSM, respectively.In the synthesis of Al-Ti-C master alloy, the igniting time steadily delayed and starting reaction temperature kept rising in company with the increase of green-compact pressure, however, combustion temperature increased firstly and decreased subsequently reaching a maximum when green-compact pressure was lOOMPa, in the process of changing green-compact pressure, the distribution of grains in Al maxi transformed from the uniformity and separation of each other to the non-uniformity and the appearance of grain agglomeration; Being with the increase of size of the Al powder and graphite powder, combustion temperature and igniting time kept the trend of fall, but the starting reaction temperature decreased at the beginning and mounted up subsequently reaching a minimum when powder size was between 320 mesh and 450 mesh. The graph of powder size-temperature showed the shape of "V"; Being with .the increase of preheating temperature, combustion temperature continually ascended and igniting time kept delaying, however, when preheating temperature was very high, there was the growth and the agglomeration of grain due to the product's longly staying in high temperature; Companing with the increase of igniting current density, combustion temperature dropped and igniting time delayed, but this change had less effect on microstructure of the products; Two igniting modes, SHS mode and thermal- explosion mode, were used to trigger combustion reaction of Al-Ti-C system, the results showed that starting reaction temperature and combustion temperature in SHS mode was lower than those in thermal-explosion mode, however, in the latter the over-high combustion temperature and the products' longly stay in liquid resulted in the agglomeration and the melt of AbTi and TiC grain;According to the CFQ experiment, in the synthesis of Al-Ti-C master alloy, Al powder firstly melted then flowed in the gap of green-compact closing round and separating Ti powder and graphite, subsequently it reacted with Ti powder and formed Al3Ti phase giving off a great deal of heat, which can sharply raise temperature of thereactant. When temperature was high enough to melt Ti powder, liquid Ti will react with graphite, which has highly diffused ability in high temperature, to formed TiC particle. These reactions gave out so much heat that Al3Ti particle could partly melt, as a result liquid Al3Ti contacted with graphite directly and reacted to form a lot of submicron size TiC particles which froze out from liquid Al in the course of cooling.Grain refining experiment showed that Al-Ti-C grain refiner prepared by SHS can block the appearance of coarse columnar grain in the course of solidification of liquid Al and efficiently refine grain of Al, in addition, its grain refining effectiveness could be maintained for a long time; Al-Ti-C grain refiner prepared by the shs mode had more efficient grain refining effect on industry pure aluminum. Compared with Al-5Ti-B grain refiner made in china and LSM company, the performace of Al-Ti-C grain refiner on pure aluminum was more efficient.