Antonio Segatori

Grain size evolution simulation in aluminium alloys AA 6082 and AA 7020 during hot forward extrusion process

Abstract The present paper investigates the grain size evolution in aluminium alloys AA 6082 and AA 7020 during hot forward extrusion process. The aim of the present work is the definition and implementation of a predictive algorithm that is able to compute the evolution of the grain shape during the process within the finite element method code Deform. Extrusion experiments were performed at two levels: at reduced scale for investigating and identifying the predictive equations and at industrial scale for validating the developed algorithm. At small scale extrusion, a complete factorial plan was performed for two alloys at three different temperatures, three extrusion ratios and two ram speeds: the discards and extrudates from the experiments were quenched immediately in order to avoid any potential recrystallisation, hence allowing measurements of transitional processing steps. At the industrial scale, instead, the 7020 alloy was extruded with two different die designs, thus producing a 20 mm diameter round bar under different extrusion ratios and strain paths. Finite element simulations were initially validated over visioplastic investigations in order to establish an accurate computation of the material flow, then experimental and numerical results were coupled, thus allowing the definition of the grain evolution model that was successfully integrated and validated on industrial scale trials.

Extrusion Benchmark 2011: Evaluation of Different Design Strategies on Process Conditions, Die Deflection and Seam Weld Quality in Hollow Profiles

In the paper experimental investigations aimed at allowing a detailed and accurate comparison of different FEM codes were presented and discussed. Two hollow profiles within the same die were characterized by different thicknesses within the profile, two welding chambers and critical tongues (one fully supported and one partially supported). The material flow balance was performed by means of feeder size and position on a profile and by means of bearings on the other one. Accurate monitoring of process parameters was carried out by using a self-calibrating pyrometer for profile temperature, six thermocouples for die thermal monitoring, a laser velocitymeter for profile speed and two laser sensors for die deflection on critical tongues. AA6082 alloy was used as deforming material, while H-11 hot-work tool steel was selected for the die material. The experiments were repeated at least three times under the same conditions in order to provide a nearly steady state statistical distribution of the acquired data. These are used as a reference for the 2011 edition of the extrusion benchmark.

Prediction of Fibrous and Recrystallized Structures in 6xxx Alloy Extruded Profiles

The final microstructure of extruded profiles is of great importance for final mechanical properties and, consequentially, the ability to control and predict it is of extreme interest for Academic and Industrial researchers. In the paper a combined model, able to discern recrystallized areas respect to fibrous structures within the same profile, is initially proposed then validated through FEM implementation on an experimental campaign performed by Parson [1]. The model was tested under different die geometries and process conditions and a qualitative comparison with final microstructure obtained in the extrusion of a simple aluminum rod was performed.

Effect of Process Parameters on Seam Weld Quality of ZM21 Tubes

The increasing attention to magnesium alloys in extruded profiles, especially in the transportation industry, is related to their low density associated with good mechanical properties and complete recyclability. This allows to push towards both increasing efficiency and pollution restrictions. However, these advantages are negatively balanced by the production rates drop in relation to dangerous profile temperatures increasing that force to keep low velocities. In this context, a novel porthole die has been purposely designed for magnesium alloys allowing an increasing of the process velocity up to four times with respect to past solutions. The mandrel consisted of three ports made by 120° bridges that created an equal number of seam welds. The extruded tubes, made in ZM21, were 50 mm in diameter and 2 mm in thickness and were tested under different process conditions. In the present work, the quality of the seam welds has been investigated in relation to each process condition by means of the rubber plug testing method that allowed to applied an hydrostatic tensile state.

Effect of Liquid Nitrogen Die Cooling on Extrusion Process Conditions

In the paper, a die for the production of a complex hollow profile made by AA6060 alloy on an industrial 2500 ton press has been manufactured and tested under strict monitored conditions. In particular 5 thermocouples were placed in proximity of interesting positions inside the die: 3 next to the bearings and two near the welding chambers. A self-calibrated pyrometer was used for the temperature monitoring of profile. Press loads, ram speeds and container temperatures were continuously recorded directly from the press system. Six billets were initially extruded in order to reach a steady state condition being the last three used as industrial benchmark for the 2011 edition of the ICEB conference. Then the nitrogen was completely and partially opened and the evolution of the temperature in the die and in the profile recorded together with the process load. The effect on bearing temperature was extreme, in particular in proximity of nitrogen inlet, while almost no change in welding chamber thermocouples and in the process load was revealed.

Comparison of Bulge Test vs. Conical Expansion Test for Hollow Extruded Profile Characterization

An experimental campaign on a tubular hollow profile, extruded in industrial environment at two ram speeds, was performed to compare two testing methodologies used for the assessment of the seam welds strength: the cone expansion and the bulge tests. In the former, a cone-shaped punch is driven into the tube causing the expansion till the specimen fracture; in the latter, an internal rubber plug is used to expand the specimen allowing to apply an hydrostatic tensile state. Results and repeatability of the two tests were analyzed in terms of loads and tube radius elongations at fracture; location and morphology of the fracture were also inspected. In each condition, and for both tests, ductile fractures appeared at seam weld location. The bulge test showed a significant reduced data scattering if compared to the cone test and provided more conservative outcomes in terms of elongation at fracture; in addition, it marked more prominently the effect of the increased ram speed that promoted a weld strength decay.

Investigation And Prediction of Grain Texture Evolution in AA6082

Extrusion applications require a strict control of the mechanical proprieties of the extrudates, in particular when undergoing severe loading conditions like in the transportation sector. Profile mechanical properties directly depend on its microstructure and texture, which are the result of multiple mechanisms based on precipitation mechanism or on grain shape evolution (grain refinement, recrystallizations, recovery and grain growth). In this direction, predicting the final profile microstructure under specific process parameters in the die design stage is of great relevance. The present study involved experimental activity on grain size measurements of profile and butt during interrupted direct extrusion of an AA6082 round profile. The grain size measurements were coupled with simulation results in order to regress analytical models based on effective strain, strain rate and temperature. Finally, the developed model was implemented in the numerical code by means of a subroutine that can be used as micr...

Finite Element Modelling of the Charge Welds Evolution in a Porthole Die

Although the extrusion process is regarded as a continuous process, in practice billets are discretely loaded into the press. The joining of two consecutive billets is guaranteed by the high hydrostatic pressure of the process. Nevertheless, mechanical proprieties of the profile in the welded region are lower than those of the adjacent area thus leading to the discarding of the welded segment. Extension of the segment is mainly affected by the interaction of seam and charge welding phenomena and, nowadays, its prediction is still a tricky task involving rough empirical relations or labour intensive analyses. Aim of this work was to identify the starting and exhausting points of the charge welds evolution inside an industrial multi-profiles die, in order to determine the exact position and the minimal length to be scraped. The extrusion of four AA6060 hollow profiles through a multi-hole die were preliminary monitored in an industrial press by accurate recording of the process loads, temperatures and speeds during the process. The four profiles were sectioned starting from the ‘stop mark’ then grinded and etched in order to investigate the location and the dimension of the zone to be ridded. While the experimental activity is detailed elsewhere, this paper is focused on the finite element modelling of the process. The numerical simulation was performed by means of the Altair HyperXtrude code and predictions compared to experimental data with a particular focus on the computation of the charge evolution. It was found a good agreement both in terms of general trend and prediction of the exhausting points so thus proving the code to be a reliable tool for an accurate determination of the scraps.