Dario Ripamonti

Microstructure Evolution and Aging Kinetics of Al-Mg-Si and Al-Mg-Si-Sc Alloys Processed by ECAP

A study was carried out on a ECAP processed Sc-containing Al-Mg-Si alloy and on a reference 6082 alloy to investigated grain structure evolution during severe plastic deformation and post-ECAP aging behaviour. The results showed that the mechanism of ultrafine structure development was substantially unchanged with respect to a reference Sc-free alloy. Also the aging sequence and precipitation kinetics of the two alloys revealed to be comparable. The ECAP processed samples of the 6082 reference alloy showed a clear recrystallization peak at temperatures in the range 315-360°C, depending on the amount of strain experienced, whereas the Sc-containing alloy retained its ultrafine structure up to temperatures well exceeding 450°C, under the conditions reproduced in a DSC temperature scan.

Microstructural and Mechanical Properties of UFG Silver Subjected to Severe Plastic Deformation by ECAP

The present contribution is aimed at investigating the microstructure evolution of commercially pure silver under severe plastic deformation conditions. ECAP billets have been produced by using a die with channels intersecting at 90° and straining the samples at room temperature. The evolution of the microstructure as a function of imparted strain was evaluated by scanning electron microscopy as well as X-ray diffractometry. Furthermore, tensile properties were measured from ECAP billets in order to evaluate the strengthening and work hardening behaviour of silver as a function of structure evolution. Comparison in terms of grain structure and corresponding properties are also drawn by considering published data about Al-Mg-Si alloy samples ECAP-processed by identical routes and parameters.

Mechanical Behaviour of Materials during Creep with Changing Loads

A component in service experiences stress conditions that change continuously with time. Since service conditions are usually difficult and expensive to reproduce in laboratory, the creep behaviour of alloys in service has to be extrapolated from a limited number of creep tests at constant loads and temperatures. Empirical rules have been proposed to forecast the effects of variable load and temperature both on the time to rupture, as the life fraction rule (LFR), and on the accumulation of creep strain with time, as the strain hardening rule (SHR). Two directionally solidified (DS) nickel based superalloys have been investigated with creep tests at constant and variable loads and constant temperature. Nickel based superalloys, for the typical stresses experienced in service, are often characterised by a small negligible primary, a minimum of strain rate with no secondary state, and a dominant accelerating creep caused by dislocation multiplication. The damage mechanisms causing the final rupture appear only in the very last percentage of life. In the present work, simulation results are reported to show that the physical-sounded model used to describe the accelerating creep due to dislocation multiplication can be employed to better predict the times to rupture and the creep curves of the two DS nickel based super-alloys with step-like variable stress than the empirical LF and SH rules.

Simulation of Thermo-Mechanical Controlled Rolling and Continuous Cooling of Wire Rods

In this study the effect of thermo-mechanical controlled rolling and continuous cooling of different grades of steel wire rod (e.g. high-carbon for cold drawing applications, medium-carbon micro-alloyed for cold forming) has been analysed through the application of a set of integrated mathematical models simulating hot rolling and continuous cooling, and a laboratory work involving hot rolling simulation on a pilot plant and heat treatments on a laboratory scale. The samples have been characterised by means of instrumented tensile tests, metallographic analyses including determination of pearlite interlamellar spacing, and controlled compression tests. The results show that: - The mechanical strength of high-carbon steel is essentially related to interlamellar pearlite spacing, and can be enhanced through the control of continuous cooling. Improvements in cold drawability can be obtained by means of prior austenitic grain size (PAGS) reduction, through the application of thermo-mechanically controlled hot deformation processes. - In the case of medium-carbon micro-alloyed steels for cold forming, the reduced PAGS achieved by means of thermo-mechanically controlled process reflects on a closer control of as-rolled mechanical properties, avoiding hardness hot spots asking for annealing treatments before cold forming. Moreover, the finer ferrite grain size could affect the forces needed during forming at the same deformation level.

Modelling Tensile Curves of AISI 316L at High Temperatures Starting from Strain Hardening Analysis

The tensile curves of AISI 316L deformed at temperatures between 700 and 1000 °C in the strain rates range 10-5-10-2 s-1 are modelled with the Voce equation, starting from strain hardening analysis. The parameters, needed to draw the Voce equation, are the saturation stress σV, the critical strain εC and the stress σo, that respectively define the height of the flow curve, the velocity to achieve σV and namely the back-extrapolated flow stress to zero strain. A two-parameter model of strain hardening recently proposed [ is used to analyze the strain hardening rate, dσ/dε, vs. the flow stress, σ. Through this analysis, σV, εC and the thermal activation of plastic flow s are obtained. In fact, the two-parameter model assumes that s and the total dislocation density ρ are the only two parameters needed to describe strain hardening. It has been reported [ that the parameter s can be parameterised in terms of strain rate and temperature and, furthermore, relationships between σV, σo, εC and s can be established. At this stage, the Voce equation can reproduce the experimental tensile curves at the explored temperatures and strain rates. However, the obtained Voce equations can well describe the tensile curves at large strains, while significant discrepancy occurs at small strains [. Preliminary results of an improved model based on two coupled differential equations with physical meaning are reported to correct this discrepancy at low strains.

Heat Treatment Analysis of Large Al-Cu-Mg-Si Alloy Extrusion

Heat treatable aluminium alloys show their best properties when properly heat treated. Most of the high-strength alloys are usually serviced in the so-called T6 temper consisting of ageing at moderate temperature after a solution treatment and a subsequent quenching. A large extrusion was investigated in this paper. The part was solution annealed at 505°C, water quenched and aged at 160°C for 16 hours. Each stage of the heat treatment is analyzed in this paper in the light of the properties achieved, by experimental investigations and several kinds of numerical simulations. In particular, a thermodynamic calculation (Pandat® software) provides solidification temperatures and equilibrium phases, in order to check whether the solution temperature is adequate. A finite element analysis (performed with COMSOL Multiphysics® software) supplies a simulation of the temperature field during water quenching. The cooling curves obtained are drawn on CCT (Continuous Cooling Transformation) curves, calculated by means of JMatPro® software, to verify if any undesired high temperature precipitation could occur during quenching. Finally, calculated TTT (Time Temperature Transformation) curves can be related to the ageing treatment.Thermal analyses, microstructural investigations and microhardness profile measurements on extrusion sections are also performed to validate calculated results.