The initial stage of liquid phase sintering for an Al–14Si–2.5Cu–0.5Mg (wt%) P/M alloy

Abstract

The present paper focuses on the initial stage of the liquid phase sintering (LPS) of a commercially available P/M Al–Si alloy, with nominal composition Al–14Si–2.5Cu–0.5Mg (in wt%). The microstructural examination of the as-received powder showed that it is constituted by pure Al particles and master alloy particles with composition Al–28Si–5Cu–1Mg (in wt%). Its compressibility is lower than for the conventional elemental P/M aluminium premixes, but it is still better than for prealloyed Al powders and other P/M powders. Thermogravimetry experiments showed that the elimination of the organic lubricant takes place between 275 and 490 °C during heating to the sintering temperature. The phase transformations leading to the formation of the liquid phase were studied by differential scanning calorimetry (DSC). The dimensional changes associated with the generation of the liquid were measured by dilatometry. Samples quenched into water from different temperatures (between 450 and 575 °C) and times (between 0 and 30 min) were studied to reveal the microstructural evolution of the alloy. The first liquid is formed inside the master alloy particles at around 505 °C. This liquid spreads across the compact, enhancing the chemical homogenization of the material. The alloying elements diffuse from the liquid inside the originally pure Al particles, reducing their melting temperature. This alloying process is almost concluded at around 535 °C. When the temperature is increased the liquid starts to be formed also in the originally pure Al particles. The melting of the FCC Al-rich phase finishes around 575–590 °C. The full melting of the alloy occurs at about 635–645 °C. After analyzing the different possible causes, it is concluded that the main swelling mechanism is the volume change associated with the melting of a fraction of the material when the temperature is increased. The phases detected by X-ray diffraction (XRD) in the as-received powder and in the sintered compact are FCC Al-rich solid solution, Si, θ-phase (CuAl 2), and Q-phase (Cu 2Mg 8Si 6Al 5).