Research On The Preparation Of Al-5Ti-0.2C Master Alloy Via An Improved SHS Approach And Its Refinement Performance

Master alloys of Al-Ti-C system are an attractive and developing area in the refiners sector of Al industry.These refiners can find wide application for variety of Al-based alloys,in particular,for the high-strength A1 alloys containing Zr and Cr(no poisoning),and for Al-Si alloys with high Si content.The small size of TiC particles is also favorable for the thin products and high precision applications.However,high cost and complexity of production process has always led to the restriction of Al-Ti-C refiners usage scope.From the production point of view,the Self-propagating High-temperature Synthesis(SHS)is considered the most promising in terms of energy consumption and cost reduction.Nevertheless,despite the engineering and scientific achievements in this field,the manufacturing temperature for Al-Ti-C remains above 1250 ?.In this regard,developing an improved method to produce Al-Ti-C refiners is of great importance for the A1 industry.Another important point of interest regarding Al-Ti-C refiners is the disputable refinement mechanism.In particular,the role of excess Ti on the growth and inoculation ability of TiC particles has not yet been clearly explained.Profound research on the master alloy microstructure and refinement performance is required to deeper understand relevant processes.In present study,research on following aspects of Al-5Ti-0.2C master alloy production and mechanism of Al-grain refinement was conducted:1.Investigation on the key parameters of the conventional SHS approach(SHSconv)for Al-5Ti-0.2C production.An attempt to decrease the temperature or duration of SHSconv.was made by analyzing how A1 content in a powder preform,production temperature and production duration influence the combustion reaction,microstructure and performance of Al-5Ti-0.2C.Low TiC concentration and large volume of A1 base in Al-5Ti-0.2C master alloy are suggested to impede utilization of benefits provided by combustion phenomena during the SHS.The heat released at the onset of reaction dissipates fast within the large volume of comparatively cool base melt without significant contribution to the melt temperature.In this regard,positive effect of higher A1 powder content in a green preform,which should increase the intensity of combustion reaction,levels soon after the ignition.The reduction of production temperature lower than 1250? becomes unadaptable.The formation of carbide phase is controlled by the diffusion processes in the melt during holding after the combustion;therefore,long holding(30 min)after the combustion is required to obtain the master alloy with desirable properties.Results obtained at this stage developed our understanding of the factors that degrade the positive contribution from combustion phenomena during the production of Al-5Ti-0.2C via the SHSconv.2.Optimization of the SHS method for Al-5Ti-0.2C production.The thermochemical calculations were used as a basis for designing an improved SHS method(SHSimpr.).The thermochemical calculations helped to predict the heat release and melt temperature increase upon the combustion reaction.The results suggested that the amount of heat release increases significantly with the increase of TiC content in a base Al melt.The separate preparation of Al-10TiC and Al-4.7Ti followed by their intermixing to obtain Al-5Ti-0.2C master alloy was then proposed as simple way to improve the SHS method.The microstructure and refinement performance of Al-5Ti-0.2C master alloy produced via SHSconv.and SHSimpr.were analyzed and compared.The separation of raw materials led to more than 200%stronger heat release(from?97 kJ to?318 kJ)during the combustion as compared to SHSconv.Consequently,the overall melt temperature(AT,?)at 1050 ? increased by 225%as compared to SHSconv.(from 38? to 123 ?).In addition,the smaller volume of A1 base melt in Al-10TiC implied shorter diffusion paths for C and Ti interaction,i.e.faster TiC formation.By this method,the production temperature for Al-5Ti-0.2C was decreased from 1250 ? to 1050 ?,while maintaining high refinement performance.Different holding parameters-?1(TiC formation)and ?2(TiC and excess Ti interaction)were experimentally assessed,and the optimal combination at 1050 ? is ?1/?2 ?10 min/20 min.Obvious refinement effect was observed when using Al-5Ti-0.2C for four types Al alloys:commercial purity A1(CPAl,grain size decreased by 86.5%),6061 alloy(decreased by 45.7%),7075 alloy(decreased by 73.6%)and A356 alloy(decreased by 57.9%).3.Study on the effect of excess Ti on the growth of TiC in Al-5Ti-0.2C and on its ability to inoculate Al grains.Alloys of Al-Ti-C system,with different Ti and C content(Al-5Ti-0.2C,Al-0.8Ti-0.2C,Al-8Ti-2C,Al-18Ti-2C and Al-10Ti)were prepared and used for the investigation.The results of microstructure analysis were interrelated to the results of CPAl refinement test.The presence of excess Ti was found to be essential at the stage of the master alloy production.In the presence of excess Ti,carbide particles grew faster and to higher extent and distributed more evenly within the master alloy structure.Moreover,TiC that formed and grew without excess Ti,failed to refine CPAl even when the excess Ti was added at the stage of refinement.These results suggested that TiC particles and excess Ti have to undergo certain interaction at the stage of master alloy production to form Ti-rich layer.The formation of such layer plays crucial role for the master alloy performance.Further SEM and TEM analyses evidenced the presence of a thin nanometric layer(10-35 nm)at the surface of TiC particles after its interaction with excess Ti.