Thermo-mechanical material response and hot sheet metal forming of Ti-6242

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Abstract

The thermo-mechanical response of a Ti-6242 alloy has been studied in elevated temperature compression tests (CT) together with cold and hot sheet metal forming tests (FT) to evaluate the suitability of different cold and hot sheet metal forming processes. The CT are designed to function as input for the estimation of material model parameters such as the parameters of constitutive equations. Furthermore, results from the FT will be used in correlation of finite element (FE) models for the prediction of sheet metal forming. Experiments were performed in a broad range of temperatures and strain rates. In CT at 400–900 °C and strain rates 0.05–1 s−1. In FT at 20–1000 °C in both isothermal and non-isothermal forming, at forming velocities of 5 and 10 mm/s. The microstructures of as-received material and deformed specimens were examined using optical microscopy. Experimental results of the CT show that initial material hardening was followed by specimen failure where cracks have formed in deformation bands or by flow softening, depending on the temperature. Compressive logarithmic strains of 10–50% were achieved. The FT reveals that optimal forming conditions are a combination of forming velocity, temperature and holding time. Hence increasing forming temperatures alone does not necessary imply better forming characteristics. A change in spring-back characteristics occurred at elevated temperatures. It can be concluded that, under the current conditions in this study, Ti-6242 is suitable to be formed by hot sheet metal forming.

Introduction

Titanium alloys are extensively used in aerospace applications such as turbine engines, airframe applications and space shuttles, mainly because of their superior strength to weight ratio. Ti–6Al–2Sn–4Zr–2Mo–0.08Si (hereinafter referred to as Ti-6242) was developed in the late 1960s and is extensively used in turbine-engine applications, mostly for gas turbine components such as compressor blades, disks and impellers but also in form of sheet metal parts, e.g. various “hot” airframe skin applications and in engine afterburner structures. Ti-6242 is a titanium alloy with high temperature stability used for long-term applications and is one of the most creep-resistant titanium alloy, often used when the temperature range do not permit usage of the most widely used α–β titanium alloy Ti–6Al–4V. Ti-6242, sometimes categorized as a near-alpha alloy, has structures that are typically fully transformed or have an equiaxed α in a transformed β matrix. The α-phase has a hexagonal close-packed (hcp) crystal structure and the β-phase has a body-centred cubic (bcc) crystal structure. The fraction of primary α in the structure of sheet products is often of about 80–90%, which tends to be greater than in forgings. The size of the equiaxed α grains in sheet products also tend to be smaller compared to those in forged products. As for other titanium alloys, e.g. Ti–6Al–4V, the microstructure is strongly influenced by the processing and heat treatment history and the mechanical properties are mainly determined by the initial microstructure, the thermo-mechanical loading history and the present impurities together with alloy concentration [1], [2], [3], [4].

Titanium alloys are often considered more difficult to form and generally have less predictable forming characteristics than other metallic alloys such as steel and aluminum. This can partly be explained by their high yield stress, σy, and low elastic modulus, E, which in combination yields a high degree of spring-back when formed by cold- or hot-forming. The hexagonal crystal structure of the α-phase also possesses anisotropic characteristics that affect its elastic properties. However, if conventional sheet metal forming is performed under favourable circumstances titanium alloys can be successfully formed into complex parts. If forming is followed by a so called hot sizing operation, in which the part is allowed to creep into the desired shape, the difficulties with spring-back can be reduced or even overcome. At the present, components manufactured by super plastic forming (SPF) are available, though it is often reserved for components with a high degree of complexity or when a substantial degree of material stretching is necessary [1], [2], [5], [6], [7], [8]. SPF is an area in which great research efforts have been made. Currently, very limited research is published in the field of direct cold- and hot-forming of Ti-6242. However, research efforts have been made in sheet metal forming of, e.g. Ti–6Al–4V, revealing many interesting forming characteristics of this particular alloy including forming limit diagrams (FLD) [9], [10], [11], [12], [13], [14]. For example, Thomas et al. [9] deals with material behaviour models to establish the proper conditions for fabricating titanium alloys, more specifically Ti–6Al–4V, by conventional sheet forming processes. They calculated FLDs and the results indicate that the most critical formability index is the strain rate sensitivity of the material. The forming limits increase with temperature for a given punch speed and with decreasing punch speed for a given temperature. The punch speed was found to be particular important at the investigated temperatures 538 and 677 °C predominantly due to the changes in the strain rate sensitivity of the alloy.

An effort to standardize the description of titanium sheet formability has also been made where a dimensionless index called the minimum bend radius TR is defined as the ratio between the die radius R and the sheet thickness H, such that TR = R/H. Furthermore, extensive experimental results on the compressive deformation behaviour (mostly in the hot forging range) for a broad range of strain rates and temperatures of Ti–6Al–4V are available [3], [4], [15], [16], [17], [18], [19], revealing many different characteristics of this particular alloy. Semiatin and co-workers [20], [21], [22], [23], [24] studied the effect on the properties of Ti-6242 in isothermal and non-isothermal hot forging (816–1010 °C). Flow stress behaviour, occurrence of shear bands, chill zones, deformation-induced microstructures and shear cracks were also observed and discussed. For example, it was established that the flow behaviour to a large extent is determined by the starting microstructure. In hot compression it is stated that, for the alloy with an equiaxed α structure, the deformation is stable with a decreasing flow stress with straining due to adiabatic heating. With a transformed β structure, unstable flow was observed and the cause of flow softening was prescribed microstructural modification together with a small adiabatic temperature increase. Criteria for the occurrence of shear bands were established.

In industrial applications it is often of great importance to know the thermo-mechanical properties of the material and also to be able to predict and detect changes in these properties. Further need for accurate material property data descends from the industrial requirement to perform numerical analyses in the product development process to obtain short lead times and efficient manufacturing techniques resulting in high quality components. The computational capacity of today makes precise analyses, such as finite element (FE) analysis, possible. The knowledge of performing numerical analyses to develop and improve industrial processes such as forming is becoming more common. In order to perform high quality FE analyses, accurate model input is crucial. Available material data is often not in an appropriate format and traditional methods of obtaining data are expensive and time consuming.

In the present work we seek to examine experimentally the thermo-mechanical properties of Ti-6242 by elevated temperature compression tests. Sheet metal forming tests are performed in order to determine suitable hot and cold sheet metal forming processes for the alloy. Effects of the temperature, the strain rate and the initial material state on the mechanical properties are studied, spring-back characteristics are detected and metallographic studies are performed. The CT are also designed to function as input for estimation of material model parameters such as the parameters of constitutive equations. A method for identification of material model parameters is inverse modelling. This method, in which the raw data from compression tests are used as experimental reference, has shown to be an effective method requiring a reduced number of experiments with improved accuracy of model parameters compared to traditional methods [25], [26], [27], [28], [29]. The experimental data consists of the compression force and diametric displacement, the numerical reference are generated by an FE-analysis of the actual compression test. The improved accuracy in material model parameters can mainly be prescribed (a) that boundary effects can be included in the evaluation by means of FE-analyses, i.e. no assumption of homogeneous stress/strain distribution in the test specimen which the estimated flow curve as direct reference data would imply, (b) compression tests facilitate an easier evaluation at higher strain levels compared to evaluation of tensile tests, after the occurrence of necking, no extrapolation of tensile flow curves for strain levels after necking are needed and (c) compression tests under continuous cooling generate parameters accurate in a desired temperature interval which means that the introduced errors in interpolating between temperatures can be avoided.

Furthermore, the resulting parameter values obtained from the CT will be used in FE simulation for predicting sheet metal forming in future work. The FT will functions as validation tests.

Section snippets

Materials

Specimens for the elevated temperature compression tests (CT) and sheet specimens for the forming tests (FT) were extracted from two different duplex annealed sheet metal plates of Ti-6242. In the compression tests cylindrical specimens with 5 mm diameter and 7 mm height are used. One exception is made for the specimen extracted in thickness direction of the sheet where the height is 5.86 mm. The specimens were machined from a documented alpha case free sheet with thickness 5.86 mm purchased from

Material response observations, compression tests

In Fig. 3, true stress–logarithmic strain relations are presented at the indicated strain rates and temperatures. Fig. 3(a) displays the isothermal stress–strain curves for the strain rate 0.05 s−1 at various temperatures. Compression tests with prior heat treatment (HT) for 20 min at equilibrium temperature 950 °C are also presented for comparison. Initial material hardening was observed followed by specimen cracking at the lower temperatures 400–600 °C. Tests at 700–900 °C indicate material

Discussion and conclusions

In the present work, the thermo-mechanical response of Ti-6242 was studied in a set of tests with a broad range of temperatures and at different strain rates. By elevated temperature compression tests and sheet metal forming experiments cold and hot sheet metal forming processes for the alloy were evaluated. In future work the experimental data will be used to obtain material model parameters such as those of constitutive equations. Furthermore, results from the hot sheet metal forming tests

Acknowledgements

The research founding by VINNOVA, grant P22122-1A, the collaboration with Prof. Pentti Karjalainen and Ph.D. Mahesh Somani at University of Oulu, Finland, and research assistant Mr. Jan Granström at Luleå University of Technology, Sweden, are all gratefully acknowledged.

References (32)

  • S.L. Semiatin et al.

    Mater. Sci. Eng. A

    (1998)
  • R. Ding et al.

    Mater. Sci. Eng. A

    (2002)
  • B.P. Bewlay et al.

    Mater. Des.

    (2000)
  • N. Alberti et al.

    CIRP Ann. Manuf. Technol.

    (1998)
  • A. El-Domiaty

    J. Mater. Process. Technol.

    (1992)
  • J. Satoh et al.

    J. Mater. Process. Technol.

    (2003)
  • W.-S. Lee et al.

    J. Mater. Process. Technol.

    (1997)
  • R. Boyer et al.

    Materials Properties Handbook: Titanium Alloys

    (1994)
  • G. Lütjering et al.

    Titanium

    (2003)
  • R. Boyer, G. Welsch, E.W. Collings, Materials Properties Handbook: Titanium Alloys, Technical note 5A, Superplastic...
  • E.J. Tuegel et al.

    Adv. Mater. Process.

    (1989)
  • N. Peter

    J. Mater. Eng. Perform.

    (2004)
  • J.F. Thomas et al.
  • M.H. Shipton et al.

    Mater. Sci. Technol.

    (1991)
  • K.S. Chan et al.

    Metall. Trans. A

    (1983)
  • A.J. Wagoner et al.

    Metall. Mater. Trans. A

    (2003)
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