Stefania Bruschi

Testing and Modelling of Material Response to Deformation in Bulk Metal Forming

Abstract As the virtual prototyping of forming operations and systems becomes a real prospect for industries, reliable and versatile simulation software must be created to efficiently and accurately predict the events products and processes are subjected to. Useful and efficient models and tests able to evaluate the different aspects of the material response to plastic deformation are among the most critical prerequisites. The knowledge of how the material performs during and after deformation has, in fact, a great impact on product design and manufacturing decisions. This paper attempts to critically assess the modelling and testing approaches and techniques for predicting material response that are available today and then to discuss the latest developments in research labs and industrial applications in the fields of modelling and testing. The emphasis is on what is changing and must still change in order to provide industrial process designers with more useful models and efficient tests that can meet the increasing demand for accurate processes and product simulation.

Physical simulation of longitudinal welding in porthole-die extrusion

Abstract In porthole-die extrusion, the metal flowing through the die has to split up around the webs and then rejoin creating longitudinal welds that extend along the whole profile. The formation and quality of these welds depend on the metal flow around the webs and a number of process parameters such as the thermal and mechanical history stored in the material in the welding area and the temperature and pressure in the welding chamber. The paper presents a new laboratory test principle based on physical-simulation experiments on real materials that proves to be particularly suitable for investigating and modelling longitudinal welds in hot extrusion. In the test, the conditions governing the formation of longitudinal welds in the real industrial process are accurately reproduced and the process parameters affecting the quality of the welds can be individually controlled as well. The results achieved in applications of the test to aluminum alloy AA 6060 are presented with a focus on the operating conditions in the welding chamber that determine the transition from partial to complete longitudinal welding.

Real-time prediction of geometrical distortions of hot-rolled steel rings during cooling

The paper deals with the application of neural network modelling to the real-time prediction of the geometrical distortion of hot rolled steel rings during cooling from rolling to room temperature. The neural network model was designed and developed to be part of a new modular system for the in-line monitoring and real-time control of the geometrical quality of rings, even those with a complex profile, during hot and warm ring rolling operations. The data utilised to train the neural network were generated by numerical simulations of the cooling phase. In order to do these simulations, an FE model capable of coupling thermal, mechanical and metalllurgical events was accurately calibrated. The proposed model was then applied to an industrial case that is described in the paper.

Comparison between Wrought and EBM Ti6Al4V Machinability Characteristics

Electron Beam Melting (EBM) is attracting large interest among the manufacturers of surgical implants as a near-net shape technology. Titanium alloy Ti6Al4V is widely used in the biomedical field thanks to its high biocompatibility, corrosion resistance and mechanical properties. The chemistry and microstructural features of EBM Ti6Al4V indicate lower machinability in comparison with wrought Ti6Al4V. Aim of the paper is to present a comparison between the machinability of wrought and EBM Ti6Al4V in semi-finishing external turning, by quantifying the effects of the cutting speed and the feed rate. Tool wear, surface integrity, chip morphology and microstructural analysis have been used to compare and assess the machinability of Ti6Al4V delivered in the two conditions.

Shape measurement system for single point incremental forming (SPIF) manufacts by using trinocular vision and random pattern

Many contemporary works show the interest of the scientific community in measuring the shape of artefacts made by single point incremental forming. In this paper, we will present an algorithm able to detect feature points with a random pattern, check the compatibility of associations exploiting multi-stereo constraints and reject outliers and perform a 3D reconstruction by dense random patterns. The algorithm is suitable for a real-time application, in fact it needs just three images and a synchronous relatively fast processing. The proposed method has been tested on a simple geometry and results have been compared with a coordinate measurement machine acquisition.

A New Constitutive Model for Hot Forging of Steels Taking Into Account the Thermal and Mechanical History

Abstract The paper presents a new constitutive model which accurately describes the Theological behaviour of steels in hot bulk metal forming operations both in single- and multi-step deformation conditions. In the proposed formulation the effects on the flow curve of all the significant phenomena accompanying the deformation process are accommodated and the previous thermo-mechanical history is represented through the average austenitic grain size measured immediately before the deformation. The number of coefficients to be evaluated is reasonably restricted and the testing required to this aim not too complex.

Influence of stacking fault energy in electrically assisted uniaxial tension of FCC metals

Electrically assisted manufacturing is based on the electro-plastic effect induced by electricity on the material flow during deformation and represents an alternative method for forming materials. Several studies have pointed out the real effectiveness of this technique, but no relations among microstructure, electrical resistivity, crystal structure and deformation-mode have been revealed. In the present work, the stacking fault energy (SFE) was taken into account and three FCC materials possessing different SFEs were strained in electrically assisted uniaxial tension under continuous current application. The results showed an advantageous electric contribution only in the highest SFE material, whereas no enhancements in formability were revealed in the investigated low- and intermediate-SFE metals.

Anisotropic and Mechanical Behavior of 22MnB5 in Hot Stamping Operations

The hot stamping of quenchable High Strength Steels offers the possibility of weight reduction in structural components maintaining the safety requirements together with enhanced accuracy and formability of sheets. The proper design of this technology requires a deep understanding of material behavior during the entire process chain, in terms of microstructural evolution and mechanical properties at elevated temperatures, in order to perform reliable FE simulations and obtain the desired characteristic on final parts. In particular, the analysis of technical‐scientific literature shows that accurate data on material rheological behavior are difficult to find; while the lack of knowledge about anisotropic behavior at elevated temperatures is even more evident. To overcome these difficulties, a new experimental set‐up was developed to reproduce the thermo‐mechanical conditions of the industrial process and evaluate the influence of temperature and strain rate on 22MnB5 flow curves through uniaxial tensile t...

Elevated temperature behaviour of Ti6Al4V sheets with thermo-electro-chemical modified surfaces for biomedical applications:

Deformation at elevated temperature of Ti6Al4V sheets may represent a process route alternative to machining and Additive Manufacturing (AM) operations to produce biomedical implants characterized by a high surface-to-thickness-ratio. The paper investigates the mechanical and microstructural behaviour at elevated temperature of Ti6Al4V sheets whose surface was properly modified through thermo-electro-chemical processes to enhance the roughness, and therefore osseointegration, as well as bioactivity, to meet the requirements imposed by the specific biomedical application. After the surface characterization, tensile tests were carried out at different temperatures on samples modified on the surface in order to investigate the ductility, microstructure and micro-hardness after deformation and the effect of the surface treatments on them. The optimal surface treatment and deformation temperature were identified, which assured, at the same time, significant ductility increase compared to room temperature, and preservation of the roughness and bioactivity characteristics.

Experimental analysis on semi-finishing machining of Ti6Al4V additively manufactured by direct melting laser sintering

The Additive Manufacturing (AM) techniques are particularly appealing especially for titanium aerospace and biomedical components because they permit to achieve a strong reduction of the buy-to-fly ratio. However, finishing machining operations are often necessary to reduce the uneven surface roughness and geometrics because of local missing accuracy. This work shows the influence of the cutting parameters, cutting speed and feed rate, on the cutting forces as well as on the thermal field observed in the cutting zone, during a turning operation carried out on bars made of Ti6Al4V obtained by the AM process called Direct Metal Laser Sintering (DMLS). Moreover, the sub-surface microstructure alterations due to the process are also showed and commented.