Papers by Keyword: Thermomechanical Process

Abstract: Mechanical properties of a Ti2AlNb-based intermetallic alloy both at room and elevated temperatures were considerably improved due to formation of a homogeneous microstructure with the average grain size of about 300 nm. At room temperature, elongations up to 25% were obtained and the ultimate strength reached 1400 MPa. The alloy exhibited superplastic behavior in the temperature range of 850-1000°C. The maximum elongation of 930% and steady state flow stress 50 of about 125 MPa were obtained at 900°C and strain rate of 4.210-3 s-1. The nanostructured material was used for production of intermetallic sheets and multilayer composite plates consisting of alternating layers of orthorhombic intermetallic and commercial high temperature titanium alloy. Ti2AlNb-based sheets and composites exhibited improved mechanical properties.

Abstract: Homogeneous nanocrystalline structure with the average grain size of about 300 nm was produced in Ti2AlNb-based intermetallic alloy by a thermomechanical processing which included multistep isothermal forging at temperatures below the β-transus and intermediate annealings. Nanostructured material possessed excellent mechanical properties. At room temperature, elongations up to 25% were obtained and the ultimate strength reached 1400 MPa. The alloy exhibited superplastic behavior in the temperature range of 850-1000°C. The maximum elongation of 930% and steady state flow stress σ50 of about 125 MPa were obtained at 900°C and strain rate of 4.2×10-3 s-1. The rolling temperatures of nanostructured alloy were defined from analysis of its mechanical behavior at a typical rolling strain rate of about 10-1 s-1 and intermetallic sheets with improved mechanical properties were produced.

Abstract: Hybrid Rotary Friction Welding is a modified type of common rotary friction welding processes. In this welding method, parameters such as pressure, angular velocity and time of welding control temperature, stress, strain and their variations which play an important rule in defining optimum process parameter combinations in order to improve the design and manufacturing of welding machines and quality of welded parts. Thermo-mechanical simulation of friction welding has been carried out. It has been shown that simulation is an important tool for prediction of generated heat and strain at the weld interface and can be used for prediction of microstructure and evaluation of quality of welds. For simulation of Hybrid Rotary Friction Welding a commercial finite element program has been used and the effects of pressure on temperature and strain variations have been investigated.

Abstract: An assessment of the inhomogeneity of microstructure generated within plane strain compression test specimens has been performed using the nickel based superalloy, Waspaloy. Two variables were investigated: the effect of strain rate and the effect of friction at the tool/specimen interface. Tests were performed at 1040°C at nominal strain rates of 0.01 and 1 s-1 with and without a glass based lubricant. At the low strain rate the microstructure was relatively homogeneous regardless of the friction conditions. At the high strain rate there was significant microstructure variation from surface to mid plane which was further exaggerated by increased friction. Quantification of the inhomogeneity, however, is non-trivial in this alloy due to the complicated recrystallisation behaviour it exhibits and difficulty in differentiating between recrystallised and non-recrystallised grains.

Abstract: A fine grained and recovered structure with densely formed fine particles has been created by means of a thermomechanical process (TMP) in Fe3Al (bcc derivative structure)-based alloys. The TMP consists of the following two parts; the first part is grain refinement and the second the creation of recovered structure stabilized by fine particles. It was found in the first part that coarse particles are required in the process of deformation to refine grain size by particle stimulated nucleation (PSN) mechanism. Fine M2C type carbide particles were densely precipitated in the second part. These particles were observed to inhibit the growth of subgrains formed around coarse particles during annealing at 700°C. This result suggests that if the fine particle density remains high, the recovered structure can be maintained at 700°C.

Abstract: Microstructure and texture of Ti-Nb-Si based alloys, prepared by water quenching from β-phase field, cold rolling and recrystallization heat treatment followed by water quenching, were investigated in terms of optical microstructure and analysis of X-ray pole figure result. Optical microstructure observation and X-ray diffraction analysis revealed that the microstructure of as-quenched sample appeared to mixture appearance consisting of mostly bcc-structured β phase and small amount of orthorhombic-structured α″ phase. After cold rolling elongated structure parallel to the rolling direction was observed, and equiaxed structure with the average grain size of about 20~30μm was developed for the sample after recrystallization heat treatment. In as-cold rolled sample we have found well-developed α-fiber texture components which are frequently observed in bcc-structured metals and alloys. In recrystallized sample, rotated cube texture component was weakly detected. The variation of elastic modulus values was interpreted in terms of changes in texture components depending on thermomechanical processing.

Abstract: With the objective of creating a simulation model for the lifetime calculation of forged aerospace components it is necessary to clarify the damage mechanisms in the materials used. This has been researched for the Ni-base alloy Inconel 718 by varying the forging parameter effective plastic strain rate, which is realised by using three types of equipment: hydraulic press, screw press and hammer. Specimens processed at the screw press show the highest lifetime by keeping all other forging parameters unvaried. Micro structural investigations show that the amount and morphology of dominant as-large-as grains play a important role. This methodology is currently investigated for Ti-6Al-4V. Lifetime tests show that besides effective strain and anisotropy the influence of morphology is important. As soon as the model status allows lifetime analyses the thermo-mechanical process (forging and heat treatment) can be developed depending on the desired lifetime specifications in order to realise an interdisciplinary lifetime optimisation of forgings. A further aim is the use of basic coherences of safe-life and fail-safe approaches in the low and high cycle fatigue region in order to reasonable handle with flaws and defects at the edge layer.

Abstract: The paper deals with the effects of thermo-mechanical processing parameters on the properties of two Al-Fe-Si foil alloys. Two twin roll cast alloys with high Fe/Si ratios (Fe/Si≈6) were processed under different combinations of homogenization, deformation and annealing treatments. The influence of small additions of manganese on the mechanical behavior and plastic anisotropy of sheets in hardening and annealing conditions are described. The corrosion resistance of tested sheets in chloride ambience is also presented, because this type of Al foils is dominantly used in the packaging (food) industry and heat exchangers.

Abstract: The effect of interpass time during thermomechanical processing of AA61111 on flow behaviour and microstructure evolution has been investigated. This was achieved using plane strain compression testing undertaken on the Sheffield thermomechanical compression (TMC) facility, using the hit-hold-hit-quench approach. Following solution treatment at 560°C for 1200s, samples were water mist quenched to 320°C and deformed at a constant strain rate of 85s-1 to an initial strain of 0.5, unloaded and held for delay times of 0.019, 6, 60, 600 and 6000s and then given a second deformation for a further strain of 0.5, followed by a water quench to room temperature. Hardening of the alloy was observed, the extent of which was dependent on the hold time. The microstructure of the samples was quantified by TEM in order to determine the extent of strain induced precipitation. TEM identified precipitation, predominantly β and Q phases, on dislocation lines, the size and volume fraction of which were a function of the hold time. The coarsening rate during the hold period of the precipitates was considerably faster than for coarsening following a conventional precipitation treatment. The size of the microband structure at the end of the double deformation was a function of the hold time, suggesting that coarsening of the precipitates during the hold had altered the Zener pinning potential. The implication of these observations on the thermomechanical processing of 6xxx alloys is discussed.