This chapter contains sections titled: Introduction δ-ferrite → austenite transformation Secondary and intermetallic phases precipitation during welding processes Welding processes for DSS and SDSS Final remarks References

The so-called Fourier Thermal Analysis (or FTA) is an evolution of the integral thermal analysis, which is actually used as a process control in Aluminium and cast iron foundries. It has been developed since late 80´s in order to investigate nucleation and growing kinetics of the various phases in multi-component alloys. FTA is based on the evaluation of the thermal gradient in one-dimensional thermal field that arises in a cylindrical solidifying specimen. During the last twenty years, both the traditional thermal analysis and FTA have been applied to the experimental determination of the solid fraction during solidification, in order to assess results from numerical simulation. Nevertheless, FTA has not been applied to foundry process control or optimisation. Eutectic modification is extensively used in low-pressure permanent mould processes, in order to improve tensile properties and toughness of Al-Si alloys. The effectiveness of the treatment is subjected to the presence a minimum amount of modifying elements, such as Sr, Na or other elements. Traditional thermal analysis is useful in determining modification level of the alloy, then to control the modification treatment. Aim of this work is to verify the potentiality of gradient-based thermal analysis method, such as FTA, in modification investigation. An A356-type hypoeutectic Al-Si alloy has been modified with metallic sodium at four different modification levels. Two-thermocouple thermal analysis curves have been recorded, in order to perform FTA analysis. Fraction solid versus time (fs(t)) and temperature (fs(T)) have been determined at different modification levels. Microstructural characterization has been made using automatic image analysis. Average values of dimension and roundness of Si have been compared to thermal analysis results. A significant correlation between the so-called eutectic depression and silicon morphology has been observed. More relevant is the behaviour of the fraction solid curves, from which it is possible to note a significant delay in the start of reaction. This delay has been observed not only in time but also in temperature and fraction solid domain.

Thanks to their properties of ultra-lightness and high strength/weight ratio, Mg and Al alloys find increasing employ in aerospace, automotive and biomedical applications. These alloys can be formed using all the conventional technologies used for other materials, like casting and forming. However, the mechanical properties of the final components are significantly influenced by the quality of the starting liquid metal. In fact, the quality of the starting liquid metal has been substantially increased in recent years, thanks to the improvement of cleaning technologies. To this purpose, electromagnetic processing of materials has evolved as an important experimental technique in the fields of material processing, associated with applications such as shape controlling, flow driving online detecting, controlled heat generation, inclusion removing magnetic levitation. In particular; electromagnetic levitation, as a promising technique, can be helpful to create some new phenomena and discoveries, especially in melting process. This work describes the design, optimization and realization of a cold crucible levitation melting (CCLM) system for light alloys. Electromagnetic models are used and applied in FEM codes to numerical simulate the working range of the CCLM. The simulation results show good agreement with experimental data.

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.

Considering the safety standards required in the automotive industry, dual phase (DP) steels have gained their popularity thanks to their higher tensile strength in conjunction with superior formability if compared to the steel grades of similar yield strength. Such properties are related to their microstructure which consists of soft ductile ferrite matrix, strengthened by hard martensitic phase.It is well known that welding processes play an important role in the automotive industry. While the conventional arc-welding processes are well-established, flexible and easy to automate, high power density processes guarantee low distortions, narrow fusion and heat affected zones.The performance evaluation of welded automotive components made in DP steels, with respect to durability or crashworthiness, involves the quantification of the properties change of the welded joint. This work is aimed at evaluating the effects of three different welding technologies (GTAW, PAW, EBW) on DP600 steel properties. In particular, the soundness, the fusion and heat-affected zone microstructure, and the mechanical properties of the welded joints are analyzed and compared in detail.

Effetto delle diverse tecnologie di saldatura sulle proprietà meccaniche e metallurgiche di giunti saldati in acciaio DP600 Parole chiave: acciai altoresitenziali avanzati, DP600, saldatura a fascio elettronico, saldatura ad arco, saldatura al plasma, metallurgia, proprietà meccaniche Il mondo dell’industria, e in particolare il settore automotive, è particolarmente sensibile alle attuali esigenze riguardanti la riduzione delle emissioni inquinanti, la riduzione dei consumi e la sicurezza dei veicoli; tali richieste hanno spinto molti produttori e ricercatori a sviluppare nuove leghe metalliche in grado di contribuire a migliorare le prestazioni dei componenti in relazione ai suddetti scopi. Gli acciai altoresistenziali avanzati (Advanced High Strength Steels AHSS) costituiscono una nuova categoria di materiali in grado di far fronte alle nuove suddette necessità; essi si suddividono in diverse famiglie a seconda della microstruttura e dei processi produttivi secondo i quali vengono ottenuti. In particolare, gli acciai Dual Phase (DP), caratterizzati da una matrice ferritica, rafforzata dalla presenza di isole di martensite, rispettano i requisiti specifici dei materiali destinati ad un impiego nell’ambito automotive. A livello microstrutturale infatti, la ferrite garantisce duttilità e quindi ottime capacità di formatura; d’altro canto, la martensite è caratterizzata da elevata durezza la quale conferisce alla lega un’eccellente resistenza meccanica soprattutto in termini di tensione di snervamento e di rottura. Quest’ultima caratteristica degli acciai DP permette una riduzione degli spessori dei componenti strutturali e conseguentemente una riduzione del peso globale dei veicoli, rispetto a materiali tradizionali quali acciai al carbonio e i classici acciai ad elevata resistenza microlegati (HSLA). Infine la buona combinazione fra duttilità e resitenza meccanica rende questi acciai adatti alla produzione di componenti strutturali con elevata capacità di assorbimento di energia all’urto. Come spesso accade nello sviluppo di materiali innovativi però, possono insorgere alcuni problemi tecnologici, legati per esempio alla saldatura dei componenti. Nel caso degli acciai DP, l’operazione di saldatura è particolarmente delicata: da una parte le esigenze di produzione richiedono processi veloci ed automatizzabili, dall’altra le richieste applicative e le specifiche di progetto necessitano del mantenimento delle ottime proprietà meccaniche tipiche di questi acciai, e di conseguenza una contenuta alterazione metallurgica del materiale base durante il procedimento di giunzione. Scopo di questo lavoro è il confronto degli effetti di diversi processi di saldatura su un acciaio altoresistenziale DP600 in termini di alterazione microstrutturale e proprietà meccaniche del giunto finale. I procedimenti di saldatura considerati sono stati sia quelli ad arco elettrico (GTAW Gas Tungsten Arc Welding e PAW – Plasma Arc Welding) sia quelli maggiormente innovativi come, in questo caso, la saldatura a fascio elettronico (EBW – Electron Beam Welding). Mentre i processi più convenzionali ad arco costituiscono oramai delle tecnologie consolidate, sono flessibili e facilmente automatizzabili, i processi ad alta densità di energia

This study deals with the effect of the annealing temperature on the pitting corrosion resistance of UNS S32750 submerged-arc welded joints. In a companion article (Part I), the influence of post-weld annealing temperature on microstructure evolution and chemical composition of austenite and ferrite was analyzed; this study can thus be considered directly connected with the previous one. The pitting corrosion resistance of the heat-treated welded joints was evaluated by using both electrochemical measurements and ASTM G48 standard gravimetric tests; examinations of initiation sites of pitting attack were carried out in order to correlate the experimental data obtained in this study with the predicted pitting corrosion behavior obtained by using the results described in Part I. Generally, the post-weld annealing treatment enhances the pitting corrosion resistance of UNS S32750 welded joints. By using PREN analysis of single phases, a correlation between the chemical composition evolution of ferrite and austenite and the experimental pitting behavior of the welded joints was found, in relation to welding and post-welding heat treatment temperature. In particular, an exponential relationship between PREN of weaker phase and pitting potential in 3.5% NaCl solution at 80 °C for the weld metal was obtained. The most favorable annealing temperature for the analyzed welded joints was found to be 1100 °C.

Welding of austenitic-ferritic stainless steels is a crucial operation and all the materials and parameters used in this process must be optimized in order to obtain the suitable corrosion and mechanical properties. Since a great part of super duplex stainless steels is used in very aggressive environment, their corrosion resistance, referred in particular to pitting and crevice corrosion, is an all-important facet in production and processing of this type of steels. Pitting corrosion resistance of super duplex stainless steels welded joints depends on several aspects: microstructure of the bead, elemental partitioning between ferrite and austenite, and the possible presence of secondary phases. For these reasons, a post-weld annealing is generally performed to homogenize the microstructure. The annealing temperature is the most important parameter to be optimized in this heat treatment. In the present work, a comparison between the as-welded and solution-treated joints is carried out. An effort has been made to correlate the main factors that affect pitting corrosion of the welded joints (microstructure, secondary phases, chemical composition of single phases) with the experimental data obtained from corrosion tests. In this first part of the work the results regarding microstructure and partitioning of elements are presented. The phase balance and the austenite morphology are locally upset during submerged-arc welding of UNS S32750. In the fusion zone, the two phases (ferrite and austenite) result to have approximately the same composition regarding Cr, Mo, and Ni content, while nitrogen is heavily concentrated in austenite. After annealing treatment, the austenite volume fraction increases and the partitioning ratios of elements reach the equilibrium values. The base material results to be less sensitive to annealing treatment than the fusion zone, and the partitioning of elements in the base material is in agreement with previous works reported in the literature.

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.

The influence of microstructure and process history on mechanical behaviour of cast Al-Si alloys is reported. In the present work, the EN-AC 46000 and 46100 aluminium alloys have been gravity cast using a stepbar permanent mould, with a range of thickness going from 5 to 20 mm. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural parameters of the ?-Al phase and eutectic Silicon. Microstructure has been also correlated with the results coming from the numerical simulation of the casting process. The results show that SDAS and length of eutectic silicon particles increase with section thickness, and consequently mechanical properties decrease.

DECEMBER 2008, VOL. 87 -s 298 ABSTRACT. The duplex stainless steels are well known for their excellent combination of strength and corrosion resistance, which is strictly related to control of the composition and the microstructural balance. When duplex stainless steels are welded, the thermal cycles and rapid cooling caused by the welding process may alter the original microstrucure, thereby affecting the above-mentioned properties of the base material. However, if welding is accomplished by using specific filler metals for duplex steels, the application of a postweld heat treatment on duplex stainless steels is usually not needed. Nevertheless, for certain applications it is prescribed by technical standards to submit the workpiece to solution heat treatment or stress-relieving annealing before use. The heat treatment of duplex stainless steels requires very accurate control of both time and temperature. In this work, the influence of postweld heat treatments on the corrosion resistance of a duplex stainless steel (SAF 2205, alias UNS 31803) has been analyzed. Different results may be obtained if furnace heat treatment is used instead of an induction one. Thus, the study was specifically aimed at a detailed investigation of the corrosion behavior of welded components after induction postweld heat treatment and furnace postweld heat treatment. It was found that pitting corrosion resistance is affected by the presence of secondary austenite and its morphology. Such morphology depends on time and temperature parameters so that if postweld induction heat treatment is used, the temperature gradient across the thickness of the joint has to be taken into account. Introduction

The duplex stainless steels are well known for their excellent combination of strength and corrosion resistance, which is strictly related to control of the composition and the microstructural balance. When duplex stainless steels are welded, the thermal cycles and rapid cooling caused by the welding process may alter the original microstrucure, thereby affecting the above-mentioned properties of the base material. However, if welding is accomplished by using specific filler metals for duplex steels, the application of a postweld heat treatment on duplex stainless steels is usually not needed. Nevertheless, for certain applications it is prescribed by technical standards to submit the workpiece to solution heat treatment or stress-relieving annealing before use. The heat treatment of duplex stainless steels requires very accurate control of both time and temperature.In this work, the influence of postweld heat treatments on the corrosion resistance of a duplex stainless steel (SAF 2205, alias UNS 31803) has been analyzed. Different results may be obtained if furnace heat treatment is used instead of an induction one. Thus, the study was specifically aimed at a detailed investigation of the corrosion behavior of welded components after induction postweld heat treatment and furnace postweld heat treatment. It was found that pitting corrosion resistance is affected by the presence of secondary austenite and its morphology. Such morphology depends on time and temperature parameters so that if postweld induction heat treatment is used, the temperature gradient across the thickness of the joint has to be taken into account.

An experimental and numerical study of the induction heat treatment applied to ISO C45 steel was carried out. Both normalised and annealed samples were considered. The process parameters were implemented in a numerical code (Sysweld 2000®) with an aim of predicting the thermal and metallurgical history of the material. The aim of this work was to create a thermo-metallurgical model of the induction heat treatment validated by experimental results. The experimental results (microstructure and micro-hardness profiles) were compared to the numerical values. A satisfactory agreement was found.

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 it 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 (R).

Melt run trials were carried out on Cu–Ni bars using a CO2 laser source in order to analyse the effects of welding parameters (i.e. laser power, welding speed) on geometrical characteristics and on the microstructure of the bead. Experimental results were then used to determine the source parameters to be employed in a finite element model (FEM) of the welding process, with particular attention paid to the thermal field induced by the laser beam. A specific procedure, named ‘automatic remeshing technique’, was used in order to minimise the computation time. The aim was to create a reliable numerical model, suitable for the optimisation, in practical cases, of welding processes of these kinds of materials. A good correlation, in terms of predicted cooling rates, with the values calculated from SDAS measurements, was observed.