Papers by Author: Jessica Calvo

Abstract: There is a large number of methods for severe plastic deformation (SPD). Multidirectional forging (MDF) is probably one of the most easily scalable for industrial application. In general, two main conditions need to be fulfilled for successful SPD processing: constant sample geometry and application of a quasi-hydrostatic pressure. The first condition is necessary for strain accumulation by repetitive deformation and the second one helps preventing cracking in the specimens with high accumulated strain. However, MDF is not providing quasi-hydrostatic condition in the processed sample. This paper reports a novel method for severe plastic deformation, namely continuous closed die forging (CCDF), which fulfils both requirements for the successful deformation of samples to a very high accumulated strain. Commercially pure aluminum (1050) was processed to a total strain of 24 by CCDF. After processing, the microstructure was refined down to a mean grain size of 0.78 μm. Tensile testing showed good mechanical properties: yield strength and ultimate tensile strength of the ultrafine-grained aluminum were 180 and 226 MPa, respectively. Elongation to rupture was about 18%. The microstructure, microhardness and grain boundary statistics are discussed with regard to the high mechanical properties of the UFG aluminum processed by this novel method.

Abstract: Inconel 718 is a nickel-chromium-iron superalloy which presents excellent mechanical properties at high temperatures, as well as good corrosion resistance and weldability. These characteristics can be optimized with an appropriate control of microstructural features such as grain size and precipitation. Precipitates of different nature can form in these alloys, i.e. γ’’ (a metastable metallic compound Ni₃Nb), γ’ (Ni₃(Ti, Al), carbides and/or δ phase (intermetallic Ni₃Nb). Aging treatments are usually designed to obtain the precipitation required in order to optimize mechanical properties. However, precipitation can also appear induced by deformation and therefore interfere with hot forming operations, such as forging. Under these conditions, precipitation may lead to an increase of the loads required to carry out the process. The aim of the work was the characterization of precipitation kinetics for Inconel 718. With this purpose, stress relaxation tests were carried out at temperatures ranging from 950°C to 800°C. Moreover, different amounts of deformation were applied to the samples, prior to stress relaxation, to evaluate the effect of this variable on inducing precipitation. Some samples were quenched at different relaxation times for metallographic evaluation. The results obtained through mechanical testing, together with a proper characterization of precipitation by Scanning Electron Microscopy, were the basis for obtaining precipitation-time-temperature (PTT) diagrams after different deformation conditions.

Abstract: The unbending operation is a critical stage of steel continuous casting because it is carried out at thermomechanical conditions for which embrittlement mechanisms can appear leading to transverse cracking. The hot tensile test is commonly used to simulate such thermomechanical conditions, at the surface of the slab, and, the reduction in area of the samples tested to fracture is taken as a measure of the susceptibility to cracking of the steel. However, a further metallographic and fractographic evaluation of the samples is required in order to identify the embrittlement mechanisms. These mechanisms are usually related to transformations in the microstructure, such as precipitation or the appearance of deformation induced ferrite, which imply changes in the strength of the material and should therefore be detectable in the flow curves. However, the features of tensile curves are not usually analyzed when evaluating the hot ductility because necking makes the interpretation of the curves complicated. In this work the hot ductility of a C-Mn steel will be discussed by means of hot tensile and compression tests. The embrittlement mechanism identified for this steel is the appearance of a ferrite layer at austenite grain boundaries. The effect of this mechanism on the features of the tensile curves will be discussed. Moreover, these curves will be compared to compression curves obtained under the same testing conditions to see whether transformation induced ferrite can be detected by means of hot compression testing. The possibility of assessing the ductile behavior of different steel grades through hot compression, which requires less material and is easier to control, will be discussed.