TiAl intermetallics offer great potential application on aerospace structural and automobile components because of their high specific modulus, high creep resistant property, high oxidation resistant property and high specific yield stress at high temperatures. As for TiAl alloys with different microstructure, full lamellar microstructures with refined colony size and lamellar spacing have the best balanced mechanical properties. It can be obtained by the breakdown of as-cast microstructure with canned extrusion, which can prevent oxidation of the preform in air and insulate the core from die chilling. The main disadvantage with the technique involves nonuniform deformation during the extrusion process and removal of the sleeve after extrusion.
It is all along a big question to obtain uniform temperature distribution and uniform deformation during canned extrusion of γ-TiAl. Non-uniformity of temperature distribution leads to inhomogeneous microstructure and different mechanical properties of the extruded bar, while non-uniform deformation of the workpiece can result in cracks in the billet, and sometimes fractures of the sleeve and the core. So it is necessary to control temperature distribution and flow pattern of the workpiece during the extrusion process. Finite element simulation is carried out to investigate the factors, which influence temperature distribution and flow uniformity, such as process parameters, workpiece design and lubrication condition.
During the modeling process, simplified models were set up to calculate temperature distribution and deformation characteristics of the workpiece during the extrusion process. Finite element analysis is carried out to investigate the effect of selection and design of sleeve and insulating layers on the uniformity of temperature and deformation of the workpiece during the extrusion process. The simulation results show that silica fabric is better than ZrO2 powder as insulating material, and the geometry design of the sleeve and the insulator is modified to optimize temperature distribution of the workpiece. To obtain uniform deformation, thickness of the sleeve should be larger than 5mm and flow stress ratio between the sleeve and the core should be in the range of 1/2-1/3.
Extrusion temperature, extrusion speed, extrusion ratio and controlled-dwell time are analyzed and optimized for better temperature distribution and uniform deformation of the workpiece. The simulation results show that extrusion temperature, extrusion speed and extrusion ratio have great effect on temperature increase because of heat generation of deformation, but have little effect on temperature decrease of TiAl billet. As for deformation aspect, extrusion temperature has little influence on flow uniformity, but low extrusion speed and appropriate extrusion ratio are critical to the successful extrudate. The improvement of preheating temperature of die tooling and interface gap between the container and the workpiece is beneficial to protect the workpiece from die chilling.
Glass coating is proved to be effective on heat insulation of the workpiece. In good lubrication condition, no fracture of the extrudates occurs and the flow pattern is quite uniform. For ill-lubricated condition with friction coefficient of 0.3 and 0.5, fractures of the sleeve happen at different locations, and it agrees well with the experimental results.
For the sleeve of stainless steel, the deformation process is quite steady and uniform if the transfer time and the extrusion time are minimized and the lubrication condition is improved. The finite element simulation results agree well with the experimental one at different extrusion temperature, which manifests that finite element method is a useful tool for process control and defect prediction, and the simulation results are meaningful to the design and control of the extrusion process for γ-TiAl.
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