Papers by Author: Massimo Tolazzi

Abstract: This paper presents a method for the experimental determination of forming limit diagrams under non linear strain paths. The method consists in pre-forming the sheets under two different strain conditions: uniaxial and biaxial, and then stretching the samples, cut out of the preformed sheets, using a Nakajima testing setup. The optical deformation measurement system used for the process analysis (ARAMIS, Company GOM) allows to record and to analyze the strain distribution very precisely with respect to both time and space. As a reference also the FLDs of the investigated grades (the deep drawing steel DC04, the dual phase steel DP450 and the aluminum alloy AA5754) in as-received conditions were determined. The results show as expected an influence of the pre-forming conditions on the forming limit of the materials, with an increased formability in the case of biaxial stretching after uniaxial pre-forming and a reduced formability for uniaxial load after biaxial stretching if compared to the case of linear strain paths. These effects can be observed for all the investigated materials and can be also described in terms of a shifting of the FLD, which is related to the art and magnitude of the pre-deformation.

Abstract: Tribological aspects strongly influence the deep drawing behaviour of sheet metal, being responsible for both the punch force and the material draw-in from the flange. Recent developments in lubrication technology have shown that dry film lubricants can be a good alternative to conventional fluid lubricants in case of aluminium sheets. Their main advantages are the homogeneous distribution on the blank surface, the good adhesion to the surface and the possibility of applying it already in the rolling mill. In order to introduce this technology in series production also for steel sheets more knowledge concerning the influence of the lubricant amount on the drawing forces is needed. A further important aspect to be considered is the surface quality of the steel grades and its interactions with the dry lubricant. In this paper four steel grades with different mechanical and/or surface properties are considered. After a microscopical and topographical investigation of the surface the materials were tested in a cup drawing test after applying two different amounts of the dry film lubricant Drylube E1. Further process parameters which were varied are the blank holder force FN and the drawing ration b. In this way, information about the influence of the lubricant amount on both the drawing force and on the maximum blank holder force FZ could be obtained for three different values of b. The experimental results have shown how a strong decrease of the lubricant amount (from 1.0 down to 0.5 g/m2) causes, at a given drawing ratio, only a small increase in the maximum drawing force as well as a small increase in the maximum blank holder force.

Abstract: The finite element analysis of sheet forming processes needs precise and reproducible data of the tribological conditions, which influence the material flow during the process. In this work the friction coefficient and its dependency on the hardness and surface topography of the blank are investigated. The selected materials are four steels with different surface qualities and coatings, FeP04, ZStE340, DP450 and TRIP800, which are joined by laser welding. Moreover, each material both with and without weld seam, was tested at three different values of normal contact pressure in a strip drawing test. The topography of both the base material and the weld line was obtained by measuring the surface roughness and implementing the data in a surface analysis software. Micro-hardness profiles along the tailored welded blanks were also determined. The strip drawing test was used to measure the friction coefficient between sheet metal and tool both in the case of tailored welded blanks and base material. The comparison between the results obtained for base materials and tailored welded blanks shows that the presence of the weld seam causes a clear increase of the friction between blank and tool.