Aluminum–Titanium Alloys: A Possible New Frontier for SSM Processing

Metal alloys in the semisolid state are used in pressure die casting primarily because of their higher viscosity than in the liquid state. The turbulence generated by liquid alloys in conventional die casting produces parts with a 1 to 5 pct porosity, while semisolid materials (SSM) processing reduces this porosity to less than 1 pct. However, the Al–Si alloys traditionally used in SSM processing, namely 319, 319S, 355, A356, 357, and A380, have small processing-temperature ranges with correspondingly high sensitivities, i.e., small fluctuations in temperature cause large variations in the amount of Al_FCC solid immersed in Si-rich liquid, in turn generating significant variations in the viscosity of the slurry. Therefore, process control is one of the main obstacles to using SSM technology. This paper presents a new class of metal alloys in which the primary solid phase consists of stable TiAl₃ particles immersed in an Al-rich liquid. Compositions of 5, 7.5, and 10 wt pctTi were tested and are referred to as 1100/5Ti, 1100/7.5, and 1100/10Ti. The titanium additions were sufficient to obtain 14, 21, and 27 pct solid fractions with an average sensitivity of 0.00025 °C⁻¹, well below that observed for Al–Si alloys (~ 0.02 °C⁻¹). The mixtures remain remarkably stable, and there is very little change in the solid fraction in the 654–800 °C temperature range. The final microstructure is formed of small grains of Al_FCC with TiAl₃ particles in the center. Electrical conductivity was 40, 31, and 26 pct IACS against 56 pct IACS for 1100 alloy and 22 pct IACS for A356 alloy. Thermal conductivity was 186, 173, and 138 W/m.K compared with 237 W/m.K for pure aluminum, 211 W/m.K for 1100 alloy, and 117 W/m.K for A356 alloy. These pore-free alloys, particularly 1100/5Ti, hold great potential for use in producing high-quality parts for the electrical and electronics industries, opening up exciting new possibilities in these fields.