Thermal Deformation Behavior of Ti-6Mo-5V-3Al-2Fe Alloy

The Gleeble-3800 thermal simulation machine was used to perform hot compression experiments on a new type of β alloy, Ti-6Mo-5V-3Al-2Fe (wt.%), at temperatures of 700–900 °C, strain rates of 5 × 10−1 to 5 × 10−4 s−1, and total strain of 0.7. Transmission and EBSD techniques were used to observe the microstructure. The results show that the deformation activation energy of the alloy was 356.719 KJ/mol, and dynamic recrystallization occurred during the hot deformation. The higher the deformation temperature was, the more obvious the dislocations that occurred and the more sufficient the dynamic recrystallization that occurred, but the effect of strain rate was the opposite. When the deformation temperature was higher than the phase transition point, the recrystallized grains clearly grew up. The calculated strain rate sensitivity index of the alloy was 0.14–0.29. The constitutive equation of hot deformation of Ti-6Mo-5V-3Al-2Fe alloy was established by using the Arrhenius hyperbolic sine equation. The dynamic DMM hot working diagram with the strain of 0.7 was constructed. The relatively good hot working area of the alloy was determined to be the deformation temperature of 700–720 °C and 0.0041–0.0005 s−1.

Rheological Law and Mechanism for Superplastic Deformation of Ti–6Al–4V

The behaviors of and mechanisms acting in Ti–6Al–4V alloy during low-temperature superplastic deformation were systematically studied by using a Gleeble-3800 thermocompression simulation machine. Focusing on the mechanical behaviors and microstructure evolution laws during low-temperature superplastic compression tests, we clarified the changing laws of the strain rate sensitivity index, activation energy of deformation, and grain index at varying strain rates and temperatures. Hot working images based on the dynamic material model and the deformation mechanism maps involving dislocation quantity were plotted on the basis of PRASAD instability criteria. The low-temperature superplastic compression-forming technique zone and the rheological instability zone of Ti–6Al–4V were analyzed by using hot processing theories. The dislocation evolution laws and deformation mechanisms of the grain size with Burgers vector compensation and the rheological stress with modulus compensation during the low-temperature superplastic compression of Ti–6Al–4V were predicted by using deformation mechanism maps.

THE CHARACTERISTIC OF THE STRAIN RATE SENSITIVITY OF STRESS-STRAIN CURVES IN THE LINEAR VISCOELASTICITY THEORY AND ITS INTERRELATION WITH RELAXATION MODULUS

Properties of the stress-strain curves family generated by the Boltzmann-Volterra linear viscoelasticity constitutive equation under uni-axial loadings at constant strain rates are studied analytically. Assuming relaxation modulus is arbitrary, the general expression for strain rate sensitivity index as the function of strain and strain rate is derived and analyzed. It is found out (within the framework of the linear viscoelasticity theory) that the strain rate sensitivity index depends only on the single argument that is the ratio of strain to strain rate. So defined function of one real variable is termed “the strain rate sensitivity function” and it may be regarded as a material function since it is interconvertible with relaxation modulus. It is found out that this function can be increasing or decreasing or non-monotone or can have local maximum or minimum without any complex restrictions imposed on the relaxation modulus. It is proved that the strain rate sensitivity value is confined in the interval from zero to unity (the upper bound of strain rate sensitivity index for pseudoplastic media) whatever strain and strain rate magnitudes are and its values may be close to unity (even for the standard linear solid model). It means that the linear viscoelasticity theory is able to produce high values of strain rate sensitivity index and to provide existence of the strain rate sensitivity index local maximum with respect to strain rate (for any fixed strain). These properties are the most distinctive features of superplastic deformation regime observed in numerous materials tests. The explicit integral expression for relaxation modulus via the strain rate sensitivity function is derived. It enables one to restore relaxation modulus assuming a strain rate sensitivity function is given. The restrictions on the strain rate sensitivity function are obtained to provide decrease and convexity down of the resulting relaxation modulus as a function of time, i.e. to provide necessary properties of a relaxation modulus in the linear viscoelasticity. Thus, the technique is developed to evaluate relaxation modulus using test data for strain rate sensitivity, in particular, using piecewise smooth approximations (by splines, for example) of an experimental strain rate sensitivity function.

THE CHARACTERISTIC OF THE STRAIN RATE SENSITIVITY OF STRESS-STRAIN CURVES IN THE LINEAR VISCOELASTICITY THEORY AND ITS INTERRELATION WITH RELAXATION MODULUS

Properties of the stress-strain curves family generated by the Boltzmann-Volterra linear viscoelasticity constitutive equation under uni-axial loadings at constant strain rates are studied analytically. Assuming relaxation modulus is arbitrary, the general expression for strain rate sensitivity index as the function of strain and strain rate is derived and analyzed. It is found out (within the framework of the linear viscoelasticity theory) that the strain rate sensitivity index depends only on the single argument that is the ratio of strain to strain rate. So defined function of one real variable is termed “the strain rate sensitivity function” and it may be regarded as a material function since it is interconvertible with relaxation modulus. It is found out that this function can be increasing or decreasing or non-monotone or can have local maximum or minimum without any complex restrictions imposed on the relaxation modulus. It is proved that the strain rate sensitivity value is confined in the interval from zero to unity (the upper bound of strain rate sensitivity index for pseudoplastic media) whatever strain and strain rate magnitudes are and its values may be close to unity (even for the standard linear solid model). It means that the linear viscoelasticity theory is able to produce high values of strain rate sensitivity index and to provide existence of the strain rate sensitivity index local maximum with respect to strain rate (for any fixed strain). These properties are the most distinctive features of superplastic deformation regime observed in numerous materials tests. The explicit integral expression for relaxation modulus via the strain rate sensitivity function is derived. It enables one to restore relaxation modulus assuming a strain rate sensitivity function is given. The restrictions on the strain rate sensitivity function are obtained to provide decrease and convexity down of the resulting relaxation modulus as a function of time, i.e. to provide necessary properties of a relaxation modulus in the linear viscoelasticity. Thus, the technique is developed to evaluate relaxation modulus using test data for strain rate sensitivity, in particular, using piecewise smooth approximations (by splines, for example) of an experimental strain rate sensitivity function.

Heat Assisted Dieless Drawing Process of Superplastic Metal Microtubes - From Zn22Al to β Titanium Alloys

A heat assisted superplastic dieless drawing process that requires no dies or tools is applied to the drawing of a Zn-22Al and β titanium superplastic alloy for not only circular but also noncircular microtubes such as square, rectangular and noncircular multi core tubes having square inner and rectangular outer cross sections. As a result, the tendency has been to increase the limiting reduction in area with increasing strain rate sensitivity index m value. We successfully fabricate Zn-22Al alloy, AZ31 magnesium, β titanium circular microtubes with outer diameter of 191μm, 890μm and 180μm, respectively. Furthermore, a noncircular micro tube, which has inner square tubes with a 335μm side, and an outer rectangular tube of 533×923μm were fabricated successfully. During the dieless drawing process, the geometrical similarity law in cross section which the tube is drawn while maintaining its initial shape can be satisfied. The smooth surface can be obtained in case of superplastic dieless drawing process without contact situation with dies and tools. Consequently, it is found that the superplastic dieless drawing is effective for the fabrication of circular and noncircular multicore microtubes.

Determination of Superplastic Material Properties for Parent Material and Friction Stir Welded Joint of Al-Alloy AA6061-T6

Superplastic forming (SPF) takes the advantage of the metallurgical phenomenon of superplasticity (SP) to form complex and highly intricate bulk and sheet metal parts. SP refers to the extraordinary formability of certain metals and alloys, ceramics, composites (both metallic- and ceramic-based), dispersion strengthened materials, nanostructured materials and bulk metallic glasses, which allows them to suffer elongations of several hundred percent under the action of tensile forces. The superplastic forming characteristics of materials like aluminium, titanium and magnesium alloys have been clearly identified in order to produce complicated near-net shapes. These materials are used in the aeronautical manufacturing industry and automotive manufacturing industries due to the significant weight (by ∼ 30%) and cost (by ∼ 50%) saving that is possible. Some research work has proved superplastic forming of friction stir welded (FSW) joints also. The FSW joint efficiencies have been characterized by mechanical and metallurgical examination. Studies are also available on the behavior of FSW joints of similar and dissimilar metals. Information on the performance of friction stir welded joints during superplastic forming is rather limited, but it is important to achieve excellent properties in the friction stir welded joints also during superplastic forming. FSP (friction stir processing) – SPF (superplastic forming) is presently being promoted as a very viable near-net shape technology for making very large and complicated sheet metal products. To achieve this superplastic material parameters are much required in industry to develop new shapes. One has to understand the flow rule relationship and mechanics involved during sheet metal forming at high temperature to select the material and forming tool with selected process parameters. This paper deals with the determination of superplastic material properties of non-superplastic aluminum alloy AA6061-T6. The superplastic material properties like strain rate sensitivity index, flow stress and strain rate were determined for both the selected material and friction stir welded sheets at various tool rotation speeds. The superplastic free blow forming experiments were performed for various constant temperatures and pressure for the parent material. Similarly the superplastic free blow forming experiments were performed for the friction stir welded joint for various tool rotation speed at constant temperature. The methods were used to determine the material properties are straight line fit method and polynomial regression method. The superplastic forming height is significantly high in case of the FSW specimens at 2000 rpm, the initial forming rate is faster and the strain rate sensitivity index obtained is also higher when compared to the parent material properties. The strain rate sensitivity index obtained for friction stir welded specimen during superplastic forming was foundto have improved when compared to the parent material.

Determination of Material Parameters during Superplastic Forming of AZ31B Magnesium Alloy at Elevated Temperatures in Uniaxial Tensile Test

Superplasticity is the ability of the material to produce neck free elongations within a material before fracture. For the past three decades superplastic forming has gained a major development in many industries to produce complex shapes. To perform the superplastic forming at elevated temperatures, the material parameters such as strain rate and strain rate sensitivity index has to be determined. These parameters affect the formability in such a way that higher the strain rate during deformation, lesser will be the percentage elongation and which in turn increases the flow stress of the material there by limiting the formability. Similarly, the strain rate sensitivity index is a measure of resistance to neck formation during deformation. Lesser the strain rate sensitivity value, more will be the neck formation thereby limiting the formability. Hence in this work, an experimental setup is designed to perform the uniaxial tensile testing at elevated temperatures to determine the flow stress, percentage elongation, strain rate and strain rate sensitivity. The determination of these parameters will be helpful in executing the forming at certain temperature and pressure to attain maximum formability. Also the SEM photographs of the fractured specimen were analysed to determine at what temperature and strain rate, the cavitation density increases.

Theoretical Deduction of Constitutive Equations with Variational Parameters during Superplastic Tension

In this article, firstly, the strain hardening index and the strain rate sensitivity index were deducted from the general state equation and the mechanical meaning of the two indexes were correspondingly depicted, and then constitutive equations, where both/either of the two indexes appear as constants, were theoretically deducted from the same state equation. Secondly, constitutive equations where both/either of the two indexes present as variables were put forward by numerical simulation. Next, constitutive equations were built, where mechanical variables are replaced by test data obtained on an electronic universal tensile tester with the capacity to carry out a true constant strain rate path. Finally, based on the test data of Zn-5%Al during superplastic tension, it is proved that the theoretical results in this article are valid.