The influence of Al-based powder additives on the sintering behaviour of Ti–6Al

Abstract

The sintering behaviour of Ti–6Al was studied using DSC, dilatometry and microscopy. Blended elemental (BE) and master alloy (MA) mixtures, each at two powder sizes, were compared. During heating, the elemental Al in the BE mixtures melted, spread and exothermically reacted with the surrounding Ti particles to form an evenly distributed intermetallic layer over these particles. This exothermic reaction, from ∼550 to 700 °C, was associated with an expansion of the material due to a Kirkendall effect. No such reaction occurred in the MA mixtures since their additions were already in intermetallic form. Sintering rates generally increased once Ti transformed to the β phase (above 882 °C). However, in the BE mixtures this increased sintering rate was delayed due to the presence of intermetallic layers that first had to dissolve. When sintering at 1200 °C, the BE mixtures homogenized faster than the MA mixtures as a result of the Al being more evenly distributed in the sample from the BE melting event. MA additions introduced higher levels of O and N as impurities, which broadened the temperature range of the α to β phase transformation. The use of fine scale BE additions for Al, to allow in-situ alloying of Ti, presents an optimal approach when taking into account homogenization, low impurity content and sintering densification.