Materials Science Forum Vols. 838-839

Abstract: The AA5083 alloy is already being used in applications that require lightweight construction and moderate strengths. In order to carry out accurate simulations of the superplastic forming of this alloy, the used constitutive models should be able to predict the deformation and thinning behavior during the forming process. In this paper, we compare the dome height and pole thickness evolution during gas bulge forming using different AA5083 constitutive material models. The models considered have different levels of complexity and are fitted using either tensile or biaxial experimental data. The simulation results are also compared with experimental data from literature. In addition, recommendations are made for developing accurate material models for the considered AA5083 alloy.

Abstract: Superplastic behavior of a Zn 22 mass % Al eutectoid alloy (SPZ) with small addition of Sn (SPZSn) was investigated. Granular grain size of about 0.3 μm was obtained by water quench after annealing SPZ and SPZ05Sn (addition of 0.05 mass % Sn into the SPZ) at 653 K for 2 h. The fundamental microstructure of the SPZ05Sn was similar to that of the SPZ, but, microstructure observation by STEM showed additive Sn was present at the α’ grain boundary in the SPZ05Sn. Excellent high strain rate superplasticity was achieved in the SPZ05Sn, with elongation of more than 1300 % at 523 K at strain rate of 10^-1 S^-1. Furthermore, large elongation of about 1100 % was recorded at 473 K at strain rate of 10^-1 S^-1. The large elongation and high strain rate sensitivity value of the SPZ05Sn tend to shift to higher strain rate region as compared to those of the SPZ. It was considered that the small addition of Sn into the SPZ effectively suppressed the grain growth of α and β phase during the superplastic deformation, because granular grains less than 2 μm is maintained after superplastic deformation at 523 K.

Abstract: The work is aimed at processing highly efficient one-piece units of gas turbine aircraft engines discs with blades (blisks) to exclude heavy locking joints and to reduce the weight of the units while improving their reliability. Intermetallic γ ́+γ alloys based on γ ́-Ni₃Al phase of the VKNA type selected as blade material and Ni-based high alloy EP975 selected for discs have similar initial melting temperatures (~1340^оС and~1300^оC, respectively). Solid state joints (SSJ) EP975 //VKNA-mono are obtained at the homologous temperatures of 0.86-0.91 Tm (K).It is shown that the most efficient method for producing solid joints between the Ni₃Al based intermetallic alloys and the EP975-type high-temperature nickel alloys is a solid state pressure welding in the conditions of high-temperature superplasticity; this technique has been developed in the IMSP RAS (grant RFBR No 130812200).

Abstract: The study was carried out to understand the effect of inhomogeneous microstructure on thickness variation in superplastically formed bulge. Friction stir processing was performed at rotational and traverse speeds of 720rpm and 155mm/min respectively on a 6mm sheet maintaining 50% overlap on the retreating side. Different probe dimensions were selected to obtain different proportions of fine grained stir zone in thickness direction. The proportions of the fine grained stir zone were 25%, 50%, 72% and, 100%. The sheets containing inhomogeneous microstructure were subjected to superplastic bulge forming under constant gas pressure up to a bulge height of 23.5mm. The sheet which was processed with 72% fine grains showed lower thickness variation from edge to apex and the bulge shape in this condition was close to the ideal spherical profile.

Abstract: In this work, magnesium alloy sheets of non-superplastic grade AZ31 were successfully formed by a proposed hybrid superplastic forming at 400 °C within 22 min. During the forming process, hot drawing first formed the part partially from the starting metal sheet within a few seconds, and then followed by a designed gas forming process to achieve the desired conical shape by high gas pressure at a targeted strain rate. The maximum thinning of 59 % was found to occur at the first contact area between the material and the punch. The thickness distribution and superplastic deformation behavior during the hybrid superplastic forming were investigated. In addition, the microstructure evolutions of AZ31 at different forming stages were examined by electron backscatter diffraction. Superplastic forming capability of the non-superplastic grade magnesium alloy was achieved. Furthermore, the part formed by this superplastic-like forming was done faster and attained a more even material distribution than conventional superplastic forming.

Abstract: Superplasticity is the ability of a polycrystalline material to exhibit, very large elongations without necking prior to failure in generally isotropic manner. Elongations in excess of 400% are usually referred to as Superplasticity. As the limitations of sheet metal fabrication are most often determined by the tensile ductility, superplasticity in sheet metals offers advantages for the forming of complex shapes easily as a regular production process.The focus of the present work is to arrive at the optimum process parameters in the superplastic forming in re-entrant shape of 7075 aluminium alloy, so as to achieve minimum thinning, lesser forming time and reduction in micro cavities.

Abstract: The substitution of Si with Al in 0.2%C-1.5%Si-1.25%Mn-0.2%Cr ultrahigh strength transformation-induced plasticity (TRIP)-aided martensitic (TM) sheet steel improves galvanization. The effect of Al content on the microstructure and formabilities of the TM steel was therefore investigated. Replacement of Si with Al maintained the high volume fraction of the retained austenite and the high stretch-formability and stretch-flangeability, whereas it decreased the tensile strength. Complex addition of Si and Al yielded the best formabilities with 1.5 GPa tensile strength grade. The superior formabilities of Si-Al bearing TM steel were attributed to the strain-induced transformation of the metastable retained austenite and the relatively soft lath-martensite structure matrix. The former leads to plastic relaxation of the localized stress concentrations, thus suppressing void formation.

Abstract: Determination of material constants describing its behavior during superplastic gas forming is the main subject of this study. The main feature of free bulging tests is the stress-strain conditions which are very similar to ones occurring in the most of gas forming processes. On the other hand, the interpretation of the results of such tests is a complicated procedure. The paper presents a simple technique for the characterization of materials superplasticity by free bulging tests, which is based on inverse analysis. The main idea of this technique is a semianalytical solution of the direct problem instead of finite element simulation which allows one to reduce the calculation time significantly. At the same time the results this simplified solution are accurate enough to obtain realistic material constants.

Abstract: Complex shaped, ultra thin-walled parts can be manufactured using superplastic forming. Hot working temperature for the production of fine-grained billets (d=5-15 μm) out of ATI Allvac 718Plus^® superalloy is in the range of 982-1038°C. An ultrafine-grained structure (d=0.3 μm) was produced by multi-axial forging with a gradual decrease of the forging temperature from 950 to 700°C. Superplastic properties of the alloy were carried out in the temperature interval of 700-950°C. It has been revealed that the fine-grained alloy provided superplastic elongations about 300% at 950°C and strain rate of 10^-4 s^-1. The highest elongation of ultrafine-grained alloy was about 1450% and very low flow stresses were reached at 900°C and strain rate of 3×10^-4 s^-1. The ultrafine-grained alloy showed superplastic properties also at 700°C (0.62T_m). The microstructure and superplastic properties of the alloys 718 and 718Plus are compared.

Abstract: ACB (France) and its sister company Cyril Bath (USA) have a long experience in the fields of hydraulic presses and metal forming for the aerospace industry. This experience is particularly focused on the manufacturing of structural and engines parts. The purpose of this presentation is to show how the combination of both activities in the fields of Hot Forming/Sizing and Superplastic Forming results in continual progress and how recent evolution open new fields of applications. First, both processes will be shortly introduced. The advantages of Hot Forming, Superplastic forming and Diffusion Bonding technologies will be demonstrated regarding current customer’s requirements. To conclude an overview of on-going research programs will be made to present strong advantages of dual presses combining Hot Forming and Superplastic processes.