The analysis of flow behavior of Ti-6Al-2Sn-4Zr-6Mo alloy based on the processing maps
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The paper presents the analysis of hot deformation behavior of Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) alloy using the theory of dynamic material modeling (DMM) based on hot compression tests performed to a total true strain of 1 at the strain rates from 0.01 to 100 s−1 and at the temperatures within the range between 800 and 1100 °C. The processing maps according to the Prasad’s criterion were developed. The analysis of the processing maps allowed for the placement of domains describing the areas of potentially favorable combinations of hot deformation parameters. The microstructure observations of the investigated alloy specimens after hot deformation in stability and instability areas were conducted. The optimal processing parameters for numerical modeling of Ti-6246 alloy forging were selected based on processing maps. After complex analysis of the obtained results, microstructural observations and numerical modeling of forging of selected part, the forging tests of Ti-6246 alloy were realized. The obtained product quality assessment was carried out by computed tomography non-destructive testing.
KeywordsTi-6246 alloy Processing maps Microstructure Forging Computed tomography
The Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) alloy belongs to the group of α + β alloys characterized by long-term high strength properties at elevated temperatures and susceptibility to heat treatment such as aging and solution treatment. It is used for the production of the gas turbine parts working at intermediate temperature such as compressor disks and blades [1, 2, 3].
Hot die forging is commonly used for producing good quality Ti-6246 components having good mechanical properties and therefore appropriate for many applications. The microstructure of Ti-6246 alloy includes primary αp and secondary αs phase with β phase matrix [4, 5]. The temperature of β transus is about 950 °C [6, 7]. For controlling the microstructure and final properties of Ti-6246 alloy, it is very important to determine the precise deformation conditions. The occurrence of the α phase in the microstructure reduces the growth of β phase grains during plastic deformation, and increasing deformation temperature increases an advancement of the recrystallization proces of β phase . The authors of work  noted the effect of breaking up and partially dissolving primary α-lamellae and α precipitations within β matrix as a result of heating up to high temperature ranges in α + β region. It was also noticed, that lower strain rates lead to almost complete dissolution of α plates in β matrix. Moreover, the amount and distribution of untransformed α phase and β phase result from thermal treatments like solution treatment and aging . The study of evolution of the microstructure and subsequent phase transformations at hot deformation temperatures below β transus  showed the influence of this temperature on the morphology of the resulting phases and β to α phase transformation kinetics. It was also noticed, that during slow cooling, the transformation kinetics of deformed material was accelerated as compared with the transformation kinetics of undeformed structure. As mentioned in , a constant load practically has no influence on fatigue life of Ti-6246 alloy.
The instability maps are the variations of ξ with the temperature and strain rate. They are indicative of the flow instability in deformation at different conditions and can be used to optimize the process parameters during thermomechanical processing of metallic materials.
Chemical composition of the investigated Ti-6246 alloy
Content (at %)
Based on the results from experimental part of the work, numerical simulation of forging of component made of studied alloy was performed. QForm 2D/3D commercial software was used in this study for the numerical analysis of forging of selected Ti-6246 alloy part. The flow stress curves obtained from isothermal hot compression tests were used for the description of the material behavior during FE simulations. Load-displacement data recorded during Gleeble compression tests were an input for the inverse analysis. After correction performed using custom FE code the stress-strain data were entered into the program. Specific heat and thermal conductivity curves for Ti-6246 alloy were obtained experimentally in Differential Scanning Calorimetry (DCS) and Laser Flash Apparatus (LFA) tests. The friction coefficient was determined in ring compression tests. Calculation of friction in QForm program is based on Levanov’s first friction law with combination of Tresca’s and Columb’s laws. The boundary conditions assumed the use of two forging stages performed on 25 kJ hammer and glass lubricant with friction coefficient μ of 0.06. The selection of the optimal deformation temperature was made according to the processing maps obtained on the basis of the flow stress curves. These flow stress curves were also used for the description of the material behavior during numerical modeling. The assumed temperature of tools used in the forging process modeling was 250 °C. 71 mm in height and 50 mm in diameter billet was used as initial material for processing. The time of billet transportation and cooling in a die was also taken into account and was 2 s and 1 s respectively. Verification of the obtained results was carried out on the basis of the forging tests of Ti-6246 alloy realized at ATI ZKM Forging Company, Stalowa Wola, Poland. For the analysis of the quality of the obtained part the computed tomography (CT), as one of the non-destructive testing methods, was used. CT testing was performed at the Institute of Lightweight Engineering and Polymer Technology (ILK) of Technische Universität Dresden. The high resolution CT-system V|tome|x-L 450 (GE Phoenix X-ray) was used for non-destructive testing. The maximal resolution of the CT-system is 1 μm. The resolution applied in performed CT testing of forged part was 50 μm.
Results and discussion
Besides, the high peak of power dissipation efficiency is often considered as evidence of safe deformation behavior such as DRX, DRV and superplasticity [12, 27, 28, 29]. The DRX can be attributed to processes that increase the hot workability of alloy. The value of power dissipation efficiency for occurrence of DRX is considered to be within 30–50% range, while the values over 60% are often associated with superplasticity. The smaller values of η (20–30%) are often associated with DRV .
Figure 4 shows the domain with the maximum efficiency of power dissipation of 50% at the temperature of 900 °C and strain rate of 0.01 s−1, what is typical for DRX process. With the increasing true strain another domain is formed, having a higher value of power dissipation efficiency and covering a wider range of temperatures and strain rates. Figure 4(b-e) shows the domains with the same location of the temperature range (1075 °C–1100 °C) and strain rates (0.01 s−1-0.02 s−1) with a peak of power dissipation efficiency in the range of 48–52%. These domains are interpreted as representing DRX of β phase. It should be noted, that in the case of these domains, the phase transformation may affect the visibility of the DRX process in the microstructure. The values of power dissipation efficiency η within a range of 20–30% is typical for domains corresponding to the temperatures of 900 °C and 950 °C, and strain rate of 100 s−1. This can be observed in almost all processing maps shown in Fig. 4. This deformation behavior of the material can be associated with DRV.
The authors of work  noted, that the superplasticity of Ti-6246 alloy can be observed at the temperature of 750 °C and at the strain rate of 0.01 s−1. The maps shown in Fig. 4(b-e) have similar stability domains at the strain rates ranging from 0.01 to 0.03 s−1 and at the temperature range of 800–950 °C, with the maximum value of η higher than 54%. Moreover, in the case of maps shown in Fig. 4(c) and Fig. 4(d) this value is greater than 60%. Taking into account the distribution of isoclines in these domains, a possible occurrence of superplasticity can be indicated in this region, what corresponds to the previously discussed results.
The optimization and precise control of hot processing parameters for Ti-6246 alloy is possible by the analysis based on the processing maps elaborated for this alloy in conjunction with the numerical simulation of hot forging processes. This type of modeling of forging of selected structural parts was used by Wojtaszek and Śleboda  for describing the distribution of the temperature, state of stress and strain of P/M Ti6Al4V alloy. The optimal processing parameters used for the numerical modeling of forging Ti-6246 alloy part were chosen based on the processing map (Fig. 5) described in processing window I. Taking into account an increase of the forged part temperature during processing, the temperature of the billet was assumed as 900 °C.
For representing different parameters of forming process (strain, stress, temperature, filling of dies and so on) updated Lagrangian method was used in Qform 3D. In this program remeshing algorithm such as finite element mesh generation and remeshing during simulation are carried out automatically. The Automatic Mesh Generator produces optimized mesh and can modify the mesh when the acceptable criteria are violated. This system based on flow formulation and flow stress depends on deformation temperature, strain and strain rate. In the performed simulations elastic deformation was neglected and material was accepted as isotropic and incompressible. In the performed FE simulations, the mesh for the workpiece had a form of triangles. During the simulation process mesh density distribution changed depending on the shape of the forged part.
Prasad criterion was used in this work for proper understanding of the hot deformation behavior of Ti-6246 alloy. The hot compression tests were conducted on Gleeble 3800 thermomechanical simulator at the temperature range of 800 °C to 1100 °C, and strain rate range of 0.01 to 100 s−1. The flow stress curves were used to elaborate processing maps for the investigated alloy. The instability domain of this alloy can be observed at the strain rates ranging from 0.04 to 4 s−1 and temperature range of 800 °C – 950 °C, and also at the highest strain rates and temperatures, which should to be avoided in hot forming of Ti-6246 alloy. The analysis of the processing maps enables describing the areas of potentially favorable combinations of hot working parameters. The microstructure analysis supports the determination of Ti-6246 alloy behavior in a wide range of temperatures and strain rates. For optimization of hot deformation processing parameters, numerical modeling of forging selected part of the investigated alloy was conducted. Based on the complex analysis of the obtained results describing deformation behavior of Ti-6246 alloy, microstructural observations and numerical modeling of forging selected structural part, the industrial forging tests were realized. CT results did not reveal defects in the volume of the forged part and allowed to confirm the expediency of complex analysis performed for the optimization of hot processing of this alloy.
Financial assistance of the National Science Centre Poland project No. UMO-2015/19/B/ST8/01079 is acknowledged.
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