Influence of melting conditions on structure and quality of plasma melted titanium aluminides

This paper presents some of the current results of continuing research into the structure and chemistry of plasma melted ingots of Ti-48Al-2Mn-2Nb [1]. Cold-hearth melting of titanium-based alloys and other advanced materials is continuing to arouse interest as a means of producing high quality materials, particularly for the aerospace industry. The metallurgical structures of the as-melted bars are characterised by a chill layer of fine prior alpha grains at the surface and larger columnar grains growing inwards and upwards along the direction of heat extraction. The prior alpha grains act as colonies for alpha-2 and gamma lamellae. These general features are common to all ingots and are relatively insensitive to plasma processing processing conditions. On a more detailed scale, the pattern of growth of the columnar grains and the fine-scale orientation and substructure of the colonies are indeed sensitive to the plasma operating conditions; these variations, and their importance, are described in this paper. The orientations of the alpha grains and the lamellae have been used to determine the shapes of the melt pool and the lamellae transformation temperature isotherm, respectively, in order to establish the effect of torch current and withdrawal rate on heat distributions in the ingot. It will be shown that at typical operating conditions the effect of variations in the torch current are far smaller than variations in the withdrawal rate,. The implications of these observations in the light of further processing and process control will then be highlighted. The chemical homogeneity and control, with reference to the Al, Mn and Nb contents, are described and assessed in the light of the microstructural implications. Work which is in progress to establish links between chemical homogeneity and processing conditions (in order to optimise chemical control) is reported. The surface quality of the as-melted bars is also assessed and the effect of withdrawal dither on surface quality is described. The use of a large dither leads to the formation of a more uneven appearance compared with bars processed with a smaller dither; a mechanism is proposed for this. Finally, the process control requirements and strategy are outlined in the light of the preceding results.