The major requirement on gamma-TiAl feedstock materials for subsequent forging operations (forging stocks) is a fine grained microstructure which decreases the flow stresses at elevated temperatures. Novel gamma-TiAl based TNM alloys have been developed from a consortium consisting of GfE, Böhler, MTU and University of Leoben in order to increase wrought processing capability by the addition of the bcc structured beta-Ti phase into the well known alpha2/gamma microstructure of Titanium Aluminides. As a side effect, the microstructure remains fine grained due to a different solidification path of the alloy which is characterized by a complete beta-solidification mechanism. It has been demonstrated that the as-cast TNM alloy can be subsequent wrought processed without primary ingot conversion via extrusion technology. Thus, the objective of the materials conversion technology was reduced to the generation of small size parts. There is a substantial interest in improved homogeneity, high reproducibility and low overall production costs including ingot manufacturing and ingot conversion.
In the project, different materials production technologies such as VAR and PAM were being evaluated. The conversion of the resulting ingots to small size forging stocks were performed via VAR skull melting and VIM skull melting, both followed by casting in permanent moulds, HIP and machining. The objective of the project was the development of a low cost casting process for gamma-TiAl based TNM alloys which guarantees to meet all technical specifications of the products.
Novel gamma-TiAl based intermetallic light-weight high temperature structural materials are recently being used as Low Pressure Turbine blades in aircraft engines.
Depending on the LPT performance (low or high speed), the technical requirements on the materials properties are different. Anyhow, there is a basic requirement for chemical and microstructural homogeneity which exceeds corresponding standards of steel or Ni-based superalloys remarkably. Thus, the production of gamma-TiAl parts is very challenging.
The Project leader, MTU Aero Engines GmbH, has decided to introduce forged TiAl TNM blades in the last stage of the Low Pressure Turbine of Pratt & Whitney`s novel PurePower PW1100G aircraft engine family in order to gain efficiency and reduce specific fuel consumption. The novel TNM alloy has been developed in order to improve the poor wrought processing capability of gamma-TiAl by the addition of the bcc structured beta-Ti phase into the well known alpha2/gamma microstructure of Titanium Aluminides. As a side effect, the microstructure remains fine grained due to a different solidification path of the alloy which is characterized by a complete β-solidification mechanism. It has been demonstrated that the as-cast TNM alloy can be subsequent wrought processed without primary ingot conversion via extrusion technology.
The major objective of the project was to develop an industrial production technology for TNM feed stock parts for forging (forging stocks) offering highest product quality and lowest production costs. Different production routes have been evaluated for the metallurgical alloying (Plasma Arc Melting, Vacuum Arc Melting), the homogenization (Induction Skull Melting, Vacuum Arc Skull Melting) and casting technologies.
Based on experimental work and simulations of the casting and solidification mechanisms the following production technology has been identified to meet the requirements with the best compromise:
- Gamma-TiAl ingot manufacturing from Ti-sponge, Al and master alloys via VAR processing
- Ingot conversion and homogenization in a VAR Skull Melter
- Centrifugal casting in permanent mouldsHIP
- Mechanical machining
The technology has been completely developed and industrialised for the production of gamma-TiAl TNM forging stocks.