The microstructure of plasma arc cold-hearth melted Ti-48Al-2Mn-2Nb

The purpose of this paper is to show the results of some work at the IRC in Materials for High Performance Applications at the University of Birmingham into the effect of processing and process parameters on the microstructure, macrostructure and chemistry of 100 mm diameter single plasma-melted ingots of a gamma-based titanium aluminide (Ti-48at%Al-2at%Mn-2at%Nb). The microstructure of the aqmelted bars is almost completely lamellar and consists of a chill layer of fine prior alpha grains at the surface and larger columnar grains growing into the centre of the bar. These microstructural features show little variation with processing conditions. The orientations of the alpha grains have been used to determine the effect of plasma brch current and ingot withdrawal rate on melt pool shape under a variety of operating conditions and it has been established that, at typical operating conditions, the ingot withdrawal rate has a more significant effect on melt pool depth than the plasma torch current, especially at faster withdrawal rates. Chemical analyses has shown that there is negligible net loss of any of the major alloying elements, although the degree of as-cast chemical homogeneity needs to be increased in the light of the extreme microstructural sensitivity. Further investigation has shown that one major cause of inhomogeneity is macrosegregation indued by short-term variations in melt pool shape. m he-implications of these results for the processing of titanium aluminides are then discussed. INTRODUCTION The Interdisciplinary Research Centre in Materials for High Performance Applications (IRC in Materials) was established in 1989 at the LJniversities of Birmingham and Swansea; one of the main aims being the development of new materials for high performance applications through the parallel development of novel processing techniques. As part of the portfolio of programmes, a systematic study is being made of the primary processing and resultant structures and properties of the family of intermetallics based on tit'mium and aluminium, principally gamma-based tikznium aluminides (TiAl and its derivatives). The intermetallics primary processing programme at the IRC in Materials falls into three categories: o plasma arc cold-hearth melting, to produce clean melted ingots o gas atomisation, using remelted clean-melted feedstock, to produce clean powder o spray forming, again using clean-melted feedstock, to form sheet and ring prducts. The main emphasis to date has been on ingot melting, but small quantities of powder and spray formed preforms have been produced and evaluated (1-3). Plasma arc melting is a cold-hearth refining process and as such the metal is melted into a water-cooled copper hearth to form a solid 'skull', which then acts as a seconauy hearth of the same composition as the parent metal. This prevents the pick up of melt-related inclusions by excluding harmful refractories from the melting process and allows time for existing inclusions to either sink into the skull or dissolve into the melt before flowing into a water-cooled copper crucible to solidify and for this reason, these processes are also known as 'clean melting' processes. Cold-hearth melting is seen as an alternative to other melting techniques such as vacuum arc remelting (VAR) and electroslag refining (ESR) for reactive metals where metal cleanness is regarded as k ing of great importance (4.5). Two types of heat source are suitable for this type of melting; electron beam and plasma-arc heating. The different characteristics of the two heat sources mean that they have slightly different applications. In particular, the high vacuum 'and consequent loss of volatile elements, such as aluminium, in electron beam melting me'm that plasma arc melting is the more suitable for the production of tiklnium aluminides. Results of detailed microstructural excamination of Ti-48at%Al-2atWNb-2at%Mn ingots produced at the IRC in Materials during the period February Novemher 1991 have been reported elsewhere (h,7). Forging of this early material and Ti48at%A1 has also been investigated within the IRC by Zhang et a1 (8) and the heat treatment of forged billets has been reported by Zhang et al (9). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993758 372 JOURNAL DE PHYSIQUE IV The aim during the second ingot programme ( September 1992 -June 1993 ) has k e n to obtain a more detailed understanding of the effects of process variables on the ingot melt p o l shape and ingot quality. The processing parameters investigated include crucible torch power, ingot withdrawal rate, dither (the ability to reciprocate the ingot within the crucible during caqting) and electromagnetic stirring. Understanding the relationships between processing and ingot quality is of great imprtance for a number of reasons. Firstly, it enables us to know what aspects of the process are most critical in determining the microstructures, macrostructures and chemistries of the as-melted ingots; knowledge which is essential in order to establish processing windows and in the overall control of the process. Secondly, it reveals the extent of the variations in microstructure that can be achieved through controlled variations in the processing conditions. Thirdly, it provides valuable information for the validation of mathematical thermal models of the process. The aspects of bar quality that will be considered in this paper are the micmstructural features present, the macrostructural distributions and orientations of those features and the chemical composition and homogeneity of single melted Ti-48at%Al-ht%Mn-2at%Nb (hereafter referred to as Ti-48-2-2). The processing parameters considered in this paper are the rate at which the material is cast into and withdrawn from the crucible, and the current drawn by the crucible torch, i.e. the torch used to hot top the ingot in the crucible. PLASMA MELTING FURNACE The furnace, a schematic of which is shown in Fig.1, is capable of producing ingots of lOOmm and 150mm diameter, and up to 1.4m in length. It is powered by two lSOkW servo-hydraulic, computer-contmlled transfer arc plasma torches which use helium as the plasma gas. One torch melt.. feedstock into a water-cooled copper hearth to form the skull. Liquid metal then Bows through a pouring notch from the hearth into a water-cooled copper crucible with a retractable base to form a continuous ingot. A computerised data acquisition system is used to monitor and record operating conditions such as torch power, water temperature, gas pressure, as well as the furnace exhaust gases. Before melting commences, the furnace is evacuated to a pressure of <40p bar. Provided the leak up rate is < 3p barlmin, the funlace is then back tilled with argon to a pressure of 1.1 bar, which is maintained throughout the melting process.