Key Engineering Materials Vol. 344

Abstract: In new vehicles development weight reduction is one of the most important driving force often conflicting with other missions which would require additional mass (crashworthiness, comfort, etc). The use of HSS, especially new grades with very high strength and good formability is the most competitive way to reduce weight. Nevertheless some limitations come from manufacturing HSS: springback, residual stresses, weldability should be taken into account in product development in order to avoid an expensive set up phase in the press shop and in the body shop. This paper focus on the recent experience achieved by Fiat in introducing HSS up to reach a share higher than 60% in weight applying and developing new methodologies to solve in the design phase any criticality arising from the use of this material.

Abstract: Within the last years in sheet metal forming a trend towards forming at elevated temperatures as well as temperature assisted forming technologies can be observed. This development is caused by the increasing need on light and high strength materials in order to fulfill the demands of light weight structures. The decision which kind of temperature assistance is the most useful in order to improve the formability of the material depends on a hugh number of process influencing parameters, like e.g. the material itself, the geometry of the component, the number of forming operations etc.. In this paper the general possibility to separate different temperature assisted forming processes with regard to the used materials will be introduced. The different forming procedures will be explained and discussed. Examples with an industrial relevance are shown.

Abstract: A 3-D solid finite element simulation of sheet forming processes is briefly discussed. Examples of cold or warm deep-drawing and sheet hydro forming are presented. Sheet work-pieces can be assembled to produce complex components by using different techniques: such as welding or mechanical fastening. They must also be simulated in order to evaluate and optimise the quality of the parts; examples of hemming and of self piercing riveting are described. Structural computation allows us to evaluate the strength of a component and especially the strength of the joining. In the future, more precise optimization of the components will be possible by the transfer of data from the previous stages of sheet forming and joining, to the structural computation code. This input data will be firstly the distribution of residual stresses, the evolution of local properties such as elastic limit, damage and anisotropy. An example of structural computation on a system of two sheets assembled by a single rivet is presented.

Abstract: The formability of AZ31 magnesium alloy sheets has been investigated in the temperature range varying from 200 to 300°C. Forming limit diagrams have been obtained by performing Nakazima-based tests. The different straining conditions have been investigated using sheet blanks with several length to width ratios. The forming limit curves have been related to the microstructural evolution occurring during deformation. The forming limit diagrams have shown a remarkable increase in formability with temperature that could be related to the occurrence of full dynamic recrystallization at 300°C.

Abstract: In the present work the definition of a test procedure for evaluating the formability of Mg alloy thin sheets was investigated taking into account both temperature and strain rate. A numericalexperimental approach was adopted by the authors: numerical simulations were run with the aim of: (i) defining the punch geometry of the formability test equipment in order to have a uniform, fast and constant temperature distribution on the specimen; (ii) setting the test operating conditions in order to force the specimen failure in a region where temperature and strain can be easily acquired. Some formability tests were performed and strain fields were measured using an optical measurement system.

Abstract: Optimum variable strain rate forming paths based on two multiscale deformation-based stability criteria are developed. The first criterion is based on Hart’s linear stability analysis while in the second criterion; we introduce a modified one dimensional nonlinear long wavelength analysis introduced by Hutchinson and Neale [7] based on the well known 2-D Marciniak-Kuczynski criterion. The stability criteria are calibrated for the AZ31 Mg alloy at 400 °C yielding two different variable strain rate forming paths. These paths show that the nonlinear wavelength analysis is more sensitive to strain rate sensitivity and results in larger attainable uniform strains than Hart’s approach especially at low strain rates. This result is demonstrated through finite element simulations of a deep rectangular box using pressure profiles derived from the two variable strain rate forming paths. The FE results clearly illustrate that Hart’s approach underestimates the amount of uniform deformation and therefore prolongs the forming time to prevent failure compared to the nonlinear analysis.

Abstract: Interstitial-free (IF) sheet steels are largely used in automotive deep-drawing applications, as they offer both good formability and adequate strength for auto bodies. In order to improve the corrosion resistance they are usually coated, mostly by hot-dip galvanizing. In the present work two IF steels purposely designed for a specific continuous hot-dip galvanizing line are investigated. The employed methodology, based on physical simulation, is presented and discussed in the paper. Its reliability is demonstrated by comparing some experimental and industrial results. The simulated specimens are tensile tested and the effects of annealing temperature and line speed on their mechanical properties are evaluated.