Materials Science Forum Vols. 838-839

Abstract: Titanium alloy bellows has advantages of light weight, excellent mechanical property, good heat resistance and corrosion resistance, etc. But the cold formability of titanium alloy is poor, so it is difficult to manufacture titanium alloy bellows by traditional mechanical forming and hydroforming. In this paper, current assisted superplastic forming technology was used to process titanium alloy bellows, which could overcome some shortcomings of traditional superplastic forming effectively, such as slow heating rate, high energy loss and low production efficiency. And titanium alloy bellows formed by this technology is of good quality with uniform wall thickness, light oxidation.

Abstract: It is generally accepted that fine-grained and equiaxed microstructure enables superplastic deformation of two-phase titanium alloys. Appropriate microstructure is usually developed in the thermomechanical processing with careful selection of the parameters of plastic deformation and heat treatment. Based on results of own research in this area increased superplasticity was found in Ti6Al4V alloy having microstructure containing highly deformed and elongated α-grains – considerably different from equiaxed ones. It was found that during heating up and first stage of superplastic deformation fragmentation of elongated α-phase grains occurred, followed by formation and growth of globular grains of that phase. Particular role of quenching of the Ti6Al4V alloy from the stable β-phase temperature range in thermomechanical processing was identified. It leads to increase of elongation coefficient of α-phase grains after plastic deformation but also restrains nucleation of the precipitates of secondary α-phase in further stages of thermomechanical processing. It was established that developed phase morphology of the alloy determined its hot plasticity – especially in the range of low strain rates typical for superplastic deformation.

Abstract: The ultrafine grained (UFG) structure with an average size of ∼0.8 μm was produced in an Al-Li-Mg-Sc alloy by equal-channel angular extrusion (ECAE) at 325oC with a total strain of ~16. Superplastic behavior was examined in the temperature range 150-250oC at strain rates ranging from 10^-5 to 10^-2 s^-1. A maximum elongation-to-failure of 440% was recorded at 175oC (~0.5 T_m, where T_m is the melting point) and a strain rate of 2.8×10^-5 s^-1 with the corresponded strain rate sensitivity coefficient of 0.32. Mechanisms of low-temperature superplasticity (LTSP) and high-strain-rate superplasticity (HTSP) are essentially the same. The difference between superplastic behaviors at low and high temperatures is attributed to applied stress.

Abstract: Mg alloy AZ31B is of interest for hot forming because it can achieve a superplastic response at high temperatures and slow strain rates. As temperature decreases and forming rate increases, its strain-rate sensitivity decreases and significant plastic anisotropy can arise. These effects are the result of a transition in deformation mechanisms from grain-boundary-sliding (GBS) to dislocation-climb (DC) creep. However, sheet production using warm rolling can produce a material with a smaller grain size and weaker basal texture. These microstructural changes promote GBS creep and decrease the degree of anisotropy under DC creep. Microstructural and tensile data are presented to show these effects at 350 and 450C through comparisons to a similar material having a more usual microstructure.

Abstract: Boron carbide-based ceramics will be probably the most promising materials during the next decades due to their excellent mechanical properties combined with its chemical stability and low-density. Boron carbide itself is a very challenging system because of its complicated but highly-symmetrical crystallographic structure. Room-temperature mechanical properties of pure boron carbide have deserved considerable attention due to its remarkable hardness and resistance to shock-impact; however, its high-temperature plasticity with unsual ductility remains unexplored. The high-temperature creep of pure B₄C polycrystals and the microsturctural observation were performed to find the mechanism of deformation in this material.

Abstract: The SPF experiments were conducted on two sheet materials: Ti-6Al-4V Fine Grain Sheet (Ti-64 FGS) and Ti-6Al-4V Standard (Ti-64 STD) to investigate the influence of SPF conditions on the reduction of yield strength (YS) and change in microstructure for those two products. Results show that a) Ti-64 FGS has better formability at high testing strain rate than Ti-64 STD, b) initial YS values of Ti-64 FGS is 10% higher than Ti-64 STD, c) reduction of YS in Ti-64 FGS is 15-27%, which is much higher than 4-9% YS reduction for Ti-64 STD, d) the most influential factor on YS reduction is the forming temperature for Ti-64 FGS, but is the strain rate for Ti-64 STD, and e) microstructure pictures of initial Ti-64 FGS before forming is finer and more isotropic than Ti-64 STD, but Ti64 FGS shows more grain growth after SPF, which results in greater drop of yield strength.

Abstract: It’s well known that the microstructure dramatically affects the strain behaviour of superplastic materials. Virtually, each batch should be characterized ex novo: optimal ranges of temperature and strain rate as well as material constants have to be defined. An accurate and simple characterization methodology based on a strain condition close enough to the real forming process is of great industrial interest. In this work, a characterization methodology based on an experimental and numerical approach is proposed. Experimental free inflation tests with a pressure jump were carried out on a titanium alloy. Results were used as reference data for an inverse analysis based on the height evolution of the dome. Material constants were calculated by means of a genetic algorithm. The approach was verified with further experimental results and a good correlation was found.

Abstract: The present investigation aims at evaluating and understanding the thermo-mechanical be-haviour of a titanium alloy under hot forming conditions. In this work, several considerations are ad-dressed. First, Scanning Electron Microscopy observations are performed to assess the evolutions of(α − β) phases, grain size, defects regarding the thermo-mechanical loadings from different static anddynamic tests (various temperatures and strain rates). Hence, the relationships between mechanicalproperties and micro-structure evolutions in such conditions allow a first assessment of the deforma-tion mechanisms in link with the macroscopic stress-strain curves. Afterwards, a behaviour modelformulation associated to an identification procedure of the parameters of the constitutive equationsis proposed. Finally, several tests performed under hot forming conditions and conducted on an in-dustrial press are compared to Finite Element calculations. Results are compared and provide someinteresting improvement ways in order to investigate the influence of the process parameters on thefinal shape of the part.

Abstract: TIMETAL^®54M (Ti-5Al-4V-0.75Mo-0.5Fe) or Ti-54M is an alpha/beta titanium alloy developed by TIMET. Earlier technical publications have confirmed that laboratory scale Ti-54M sheets exhibit superior SPF capabilities compared to Ti-6Al-4V (Ti-64), in terms of flow stress, temperature and strain rate. Various gauges of these sheets, ranging from 0.64mm to 4.57mm, were produced on a production scale using TIMET patented process. Coupon tests were performed in accordance to ASTM E2448 and the results confirmed that the SPF capability of these sheets at a temperature as low as 774°C, which is a significant cost benefit in the SPF fabrications. The present paper discussed basic properties of Ti-54M sheets along with SPF coupon tests results showing lower temperature superplasticity. The low temperature superplasticity of Ti-54M is due to the combined effect of alloying elements and finer primary alpha particle size.

Abstract: In AIRBUS, most of the complex shaped titanium fairing parts of pylon and air inlets are produced by superplastic forming (SPF). These parts are cooled down after forming to ease their extraction and increase the production rate, but AIRBUS wastes a lot of time to go back over the geometric defects generated by the cooling step. This paper investigates the simulations of the SPF, cooling and clipping operations of a part on Abaqus^® Finite element software. The different steps of the global process impact the final distortions. SPF impacts the thickness and the microstructure/behavior of material, cooling impacts also the microstructure/behavior of material and promotes distortions through thermal stresses and finally, clipping relaxes the residual stresses of the cut part. An elastic-viscoplastic power law is used to model material behavior during SPF and a temperature dependent elastic perfectly plastic model for the cooling and clipping operations.