Paolo Ferro

The influence of phase transformations on residual stresses induced by the welding process—3D and 2D numerical models

In this work, a numerical study of laser beam welding of steel was performed. In particular, phase transformation effects were considered, which consist mainly of volume change and transformation plasticity. Thanks to the possibilities of numerical modelling, additional analyses were performed (a) without taking into account phase transformations and (b) considering only the transformation plasticity phenomenon.The aim of this study was to examine the influence of phase transformation on the residual stress induced by the welding process, by comparing the results obtained with the described differences in the analyses. Finally, the residual stress field computed by the three-dimensional (3D) model was compared with the one computed by a two-dimensional (2D) model in order to estimate the grade of reliability of the more efficient 2D analyses, also in the presence of phase transformations. It was found that both volume changes due to phase transformations and transformation plasticity have a great influence on the residual stress induced by the welding process. 2D numerical models can be used with good accuracy instead of 3D models, if the in-plane stresses are of primary interest. All analyses in this investigation were performed with the finite element code SYSWELD®.

A Semiempirical Model for Sigma-Phase Precipitation in Duplex and Superduplex Stainless Steels

Sigma phase is known to reduce the mechanical properties and corrosion resistance of duplex and superduplex stainless steels. Therefore, heat treatments and welding must be carefully performed so as to avoid the appearance of such a detrimental phase, and clearly, models suitable to faithfully predict σ-phase precipitation are very useful tools. Most fully analytical models are based on thermodynamic calculations whose agreement with experimental results is not always good, so that such models should be used for qualitative purposes only. Alternatively, it is possible to exploit semiempirical models, where time-temperature-transformation (TTT) diagrams are empirically determined for a given alloy and the continuous-cooling-transformation (CCT) diagram is calculated from the TTT diagram. In this work, a semiempirical model for σ-phase precipitation in duplex and superduplex stainless steels, under both isothermal and unisothermal conditions, is proposed. Model parameters are calculated from empirical data and CCT diagrams are obtained by means of the additivity rule, whereas experimental measurements for model validation are taken from the literature. This model gives a satisfactory estimation of σ-phase precipitates during both isothermal aging and the continuous cooling process.

Methodologies and experimental validations of welding process numerical simulation

The development of numerical codes for the simulation of metallurgical processes, such as welding, assumes a great importance in the industrial and research field. The prediction of residual stresses, deformations, phases proportion and temperature can be a fundamental step for a good planning of a welding operation. Due to the complexity of the problem, such numerical simulations require specialised tools and qualifications. The aim of this work is to summarise the methodologies currently used in the numerical simulation of welding process, with a particular attention paid to the strategies used to reduce the computational time. Some examples, mainly concerning high power source welding process will be described; numerical simulations, performed with the code SYSWELD and experimental results will be compared and discussed.

Metallurgical and mechanical characterization of electron beam welded DP600 steel joints

Several new commercial advanced high-strength steels exhibit high strength and enhanced formability. These materials have the potential to affect cost and weight saving while improving performance. However, welding, by modifying the microstructure of the steel, has in general a detrimental effect on the mechanical properties of structural components. If high power density technologies are used, the result is that the mechanical properties of such kind of joints can be improved. This article presents a metallurgical and mechanical characterization of electron beam welded joints in advanced high-strength steel DP600. The experimental analysis was supported by a thermal numerical model obtained through the Sysweld® code. Results show that mechanical properties of the electron beam welded joints are comparable with those of parent metal both in terms of static strength and ductility.

Fatigue Behavior of Porous Ti-6Al-4V Made by Laser-Engineered Net Shaping

The fatigue behavior and fracture mechanisms of additively manufactured Ti-6Al-4V specimens are investigated in this study. Three sets of testing samples were fabricated for the assessment of fatigue life. The first batch of samples was built by using Laser-Engineered Net Shaping (LENS) technology, a Direct Energy Deposition (DED) method. Internal voids and defects were induced in a second batch of samples by changing LENS machine processing parameters. Fatigue performance of these samples is compared to the wrought Ti-6Al-4V samples. The effects of machine-induced porosity are assessed on mechanical properties and results are presented in the form of SN curves for the three sets of samples. Fracture mechanisms are examined by using Scanning Electron Microscopy (SEM) to characterize the morphological characteristics of the failure surface. Different fracture surface morphologies are observed for porous and non-porous specimens due to the combination of head write speed and laser power. Formation of defects such as pores, unmelted regions, and gas entrapments affect the failure mechanisms in porous specimens. Non-porous specimens exhibit fatigue properties comparable with that of the wrought specimens, but porous specimens are found to show a tremendous reduced fatigue strength.

Modelling of the carburizing and quenching process applied to caterpillar track bushings

The carburizing–quenching process applied to caterpillar track bushings was studied by means of experimental and numerical analyses. The numerical model was developed on the basis of the real cycle. The purpose of this work is to predict the carbon profiles, microstructural phase changes, hardness and residual stress that occur during quenching using finite element techniques. Good agreement was obtained between the experimental and numerical results in terms of carbon diffusion and hardness profiles. The Sysweld® numerical code was used to perform the simulations.

Strain evolution in cold-warm forged steel components studied by means of EBSD technique

Electron BackScatter Diffraction (EBSD) in conjunction with Field-Emission Environmental Scanning Electron Microscopy (FEG-ESEM) has been used to evaluate the microstructural and local plastic strain evolution in different alloys (AISI 1005, AISI 304L and Duplex 2205) deformed by a single-stage cold and warm forging process. The present work is aimed to describe the different behavior of the austenite and ferrite during plastic deformation as a function of different forging temperatures. Several topological EBSD maps have been measured on the deformed and undeformed states. Then, image quality factor, distributions of the grain size and misorientation have been analyzed in detail. In the austenitic stainless steel, the γ-phase has been found to harden more easily, then α-phase and γ-phase in AISI 1005 and in duplex stainless steel, sequentially. Compared to the high fraction of continuous dynamic recrystallized austenitic zones observed in stainless steels samples forged at low temperatures, the austenitic microstructure of samples forged at higher temperatures, 600–700 °C, has been found to be mainly characterized by large and elongated grains with some colonies of fine nearly-equiaxed grains attributed to discontinuous dynamic recrystallization.

Non-isothermal Dissolution Modelling of Sigma Phase in Duplex Stainless Steels

In this work, the non-isothermal dissolution kinetics of the sigma phase in duplex stainless steels has been studied and modelled. A semi-empirical model is proposed to describe the kinetics of sigma phase precipitation/dissolution during continuous heating starting from the isothermal transformation kinetics. The proposed model, which presumes validity of the additivity rule, is validated by means of experimental investigations. A good agreement is found between experimental and analytical results.

Experimental Damage Criterion for Static and Fatigue Life Assessment of Commercial Aluminum Alloy Die Castings

Defects, particularly porosity and oxides, in high-pressure die casting can seriously compromise the in-service behavior and durability of products subjected to static or cyclic loadings. In this study, the influence of dimension, orientation, and position of casting defects on the mechanical properties of an AlSi12(b) (EN-AC 44100) aluminum alloy commercial component has been studied. A finite element model has been carried out in order to calculate the stress distribution induced by service loads and identify the crack initiation zones. Castings were qualitatively classified on the basis of porosities distribution detected by X-ray technique and oxides observed on fracture surfaces of specimens coming from fatigue and tensile tests. A damage criterion has been formulated which considers the influence of defects position and orientation on the mechanical strength of the components. Using the proposed damage criterion, it was possible to describe the mechanical behavior of the castings with good accuracy.

Molten Pool in Welding Processes: Phenomenological vs Fluid-Dynamic Numerical Simulation Approach

The metallurgical and mechanical properties of fusion welded joints are influenced, among others phenomena, by the weld pool dimension and shape. Weld pool shape is important in the development of grain structure and dendrite growth selection process as well as in the development of residual stresses. For these reasons, significant advances have been made in recent years to understand, in greater detail, the dynamics of the heat and fluid flow in the weld and the subsequent development of the pool shape. In numerical simulation of welding processes, there are two different approaches used to model the fusion zone. If the prediction of distortions and residual stresses is the primary objective of the simulation (computational weld mechanics simulation), the phenomenological approach is the most suitable method used to model the fusion zone. Otherwise, when the weld pool shape has to be predicted, the fluid-dynamic equations must be solved at the expense of a significant ‘computational load’ increase. In this work, after a brief description of weld pool characteristics, such two different approaches are described and compared.