Jose Manuel Prado

Universitat Politècnica de Catalunya | UPC · Department of Materials Science and Metallurgy (CMEM)

Artificial neural network (ANN) and dynamic material model (DMM) are considered to be powerful methods to characterize the flow behaviour of metallic materials. The aim of this study is to analyze the performance of these two methods in the characterization of flow behaviour and deformation stability of medium-carbon microalloyed steel. Flow curves...

En la industria abundan procesos térmicos o termo-mecánicos durante los cuales se producen distorsiones difíciles de predecir y controlar. Las diferentes velocidades de enfriamiento ocasionadas a lo la rgo de la pieza durante el proceso producen una distribución de fases con volumen específico y propiedades mecánicas diferentes, que generan tension...

Fracture toughness of advanced high strength steels (AHSS), can be used to optimize crash behavior of structural components. However it cannot be readily measured in metal sheet because of the sheet thickness. In this work, the Essential Work of Fracture (EWF) methodology is proposed to evaluate the fracture toughness of metal sheets. It has been s...

The automotive industry has increased significantly the use of advanced high strength steels (AHSS) in many structural components of vehicles, with the aim to reduce weight and also increase safety. The high resistance of these steels is accompanied by a decrease in formability, which makes simulation a requirement to adequately foresee the formabi...

The knowledge of the flow behavior of metallic alloys subjected to hot forming operations is of particular interest for designers and engineers in the practice of industrial forming processes simulations (i.e. rolling mill). Nowadays dynamic recrystallization (DRX) is recognized as one of the most relevant and meaningful mechanisms available for th...

Advanced high strength steels (AHSS) are increasingly being applied in safety related automotive components. Thus, a detailed knowledge of their fracture properties, as fracture toughness, is needed to properly design high performance components. Fracture toughness (KIC) cannot be readily measured in metal sheet. It is because the small thickness (...

The strain induced effect on the precipitation kinetics of a high strength low alloy (HSLA) steel microalloyed with Nb-V was investigated. The stress relaxation method was employed to determine the dependence of the precipitation -time - temperature curves (PTT) on previous strain. Relaxation tests were performed over a temperature range of 850 to...

Hot working processes often induce high levels of strain at high strain rates, and impose very complex multi-axial modes of solicitation. These processes are essentially limited by the apparition and development of plastic instabilities. These may be the direct cause of rapid crack propagation, which lead to a possible final rupture. The complexity...

In the study of hot working processes, the knowledge of interaction between microstructural behaviour and control process parameters such as temperature, strain rate, and strain is very important. In the last decades, processing maps have been developed to design, control, and optimize the hot strain of various metallic materials. In this work, to...

Experimental and numerical studies of powder flow during the die filling stage in powder metallurgy cold compaction processes are presented. An experimental setting consisting of a horizontal pneumatically activated shoe, a vertical die and high-speed video system has been designed. The experiments show the existence of three flow regimes: continuo...

En el sector automovilístico se ha incrementado el uso de aceros de alta y muy alta resistencia para conformar piezas de carrocería. La utilización de estos nuevos aceros lleva asociada una problemática específica en la construcción de los útiles de conformado. El elevado límite elástico de estos aceros obliga a utilizar elevadas presiones de confo...

This work carries out a research about the elastic behaviour of metallic powder compacts. Cylindrical specimens with different levels of density have been submitted to uniaxial compression tests with loading and unloading cycles. The analysis of the elastic loadings shows a non-linear elasticity, which can be mathematically represented by means of...

In the study of hot forming processes the knowledge of interaction between microstructural behaviour and control process parameters such as temperature, strain rate and strain is very important. In the last decades, processing maps have been developed to design, control and optimize the hot deformation of various metallic materials. In this work, i...

Hot working processes often induce high levels of deformation at high strain rates, and impose very complex multiaxial modes of solicitation. These processes are essentially limited by apparition and development of plastic instabilities. These may be the direct cause of rapid crack propagation, which lead to a possible final rupture. The complexity...

Experimental and numerical studies of powder flow during the die filling stage in powder metallurgy cold compaction processes are presented. An experimental setting consisting of a horizontal pneumatically activated shoe, a vertical die and high-speed video system has been designed. The experiments show the existence of three flow regimes: continuo...

This paper reviews the ductility of nanostructured and ultrafine iron obtained using a variety of methods. Mechanical milling of powder and subsequent hot consolidation, one of the most popular methods offer high mechanical strength but poor ductility. Improvements made in the consolidation processes and the introduction of final heat treatments, i...

The Forming Limit Diagrams (FLD) are widely used in the formability analysis of sheet metal to determine the maximum strain, which gives the Forming Limit Curve (FLC). It is well known that these curves depend on the strain path during forming and hence on the test method used to calculate them. In this paper, different stretching tests such as the...

The microstructural control of rolled products is based on managing the austenite phase transformations during and after hot deformation to attain the desired microstructure after the cooling step. Therefore, it is very important an appropriate description of the kinetics of the hardening and softening phenomena taking place during the deformation...

Low alloy transformation-induced plasticity aided (TRIP) steels have attracted much interest over the last years. TRIP steels were initially developed for automotive applications as they offer an excellent combination of strength and ductility at reasonable costs. These excellent mechanical properties mainly arise from a complex multiphase microstr...

High dense bulk specimens of 0.55% C steel with ferrite grain sizes in the nanocrystalline and ultrafine regimes have been produced by a warm consolidation process from ball-milled iron powder and subsequent heat treatments. The resulting microstructure was investigated and related to the mechanical behavior studied by uniaxial compression tests. I...

Several authors have noticed that relatively pure coppers with different oxygen levels present a higher hot flow stress as the oxygen content increases. This work attempts to demonstrate that a large contribution to the observed increase of stress is due to precipitation of fine Cu2O particles during hot working. Three fire-refined 99.9% pure coppe...

Due to the demand for lighter structures AHSS are becoming the steels of choice on structural automotive applications. Although their sheet forming behavior has long since been under study, not much work has been done on their fracture resistance properties. It is especially true in toughness measurements since the thickness of the metal sheet does...

High dense bulk specimens of 0.55% C steel with ferritic grain sizes within the nanocrystalline and ultrafine regimes have been produced from ball-milled iron powder through the warm consolidation process. The microstructure, the mechanical properties and the strain rate sensitivity parameter (m) were investigated for the different grain sizes obta...

This work is focused on the evaluation of plastic deformation of near fully dense nanostructured and ultrafine grained bulk samples of carbon steel (0.55%C). The specimens were obtained by warm static pressing from mechanically milled powder. Compaction parameters and annealing treatments were selected to promote a relatively wide range of the grai...

The plastic deformation of nanostructured and ultra-fine grained Fe-1%Cr of bulk specimens obtained from mechanically milled iron powder has been investigated by means of compression and tensile tests. The resulting microstructure was related to the mechanical behaviour. It was found that nanocrystalline grained specimens presented strain hardening...

Due to the differences in the coefficients of thermal expansion between the components of electronic assemblies (substrate-solder-circuit) the variations of temperature (either by heating during the current passage or by environmental conditions) promote cyclical deformation in the component and in the solder alloys. Bench checks of the assemblies...

RESUMEN En el presente trabajo se evalúan las propiedades mecánicas de compactos de acero de bajo contenido en carbono (0,05%C) con tamaños de grano ultrafino y nanométrico. Las muestras se obtuvieron a partir de consolidación de polvo de hierro severamente deformado mediante molienda mecánica con un proceso de consolidación en dos pasos. Posterior...

Initial work on compaction modelling assumed that the density of the powder bed in a die at the start of compaction is uniform. This has been challenged from two directions. Firstly, measurements of the density distribution in compacted components did not always agree with predictions and nonuniform fill-density has been used as one explanatory fac...

La introducción en la industria del automóvil de nuevos aceros de alta resistencia, ha permitido la disminución del peso estructural aumentando la resistencia a impacto de los vehículos. Sin embargo, las dificultades encontradas en la conformación de estos aceros –desgaste y rotura de matrices, springback elevado- han forzado la búsqueda de nuevos...

Semi-solid materials (SSM) in the thixotropic state behave like liquids, i.e they show low or null shear resistance and, at the same time, they behave like solids as do not fall to pieces under applied forces. At present, the potential advantages and industrial applications of these materials are well recognized, in particular for the production of...

In this study, the hardness and fracture toughness of different primary carbides present in tool steels have been evaluated by nanoindentation. Toughness was evaluated by the indentation method using Laugier’s equation, after experimentally assessing its applicability. The results obtained allow different types of carbides to be discerned, in terms...

Modelling hot flow stress during grain refinement operations of fee metals has largely included the use of an Avrami type equation to describe the decrease in stress due to Dynamic Recrystallization (DRX). However when refining large-grained copper, the processing temperatures and strain rates often produce a multi peak behaviour, which is not pred...

Previous research works assert that the observed increase in hot flow stress of commercially pure copper is attributed to the interactions between solute atoms and dislocations, specifically by interstitial oxygen. This work shows TEM images of the formation of Cu2O precipitates after warm working temperatures that in part help explain the increase...

A mixture of a liquid Sn-Pb alloy reinforced with solid Cu particles has been found to show thixotropic and pseudoplastic behaviour. The presence of an intermetallic compound (IMC) between the Cu particles and the molten matrix has some very important consequences in the rheological behaviour of the slurry. The semisolid material is obtained mixing...

The mechanical properties of bulk specimens of nanocrystalline 0.55% C steel with a grain size of 30nm and a relative density higher than 97% have been determined. Samples were obtained by cold compaction and warm sintering at 425°C of nanocrystalline powders obtained by mechanical attrition in a planetary ball mill. In both processes an Ar protect...

The results of monotonic and cyclic uniaxial compression tests, in which the deviatoric component of the stress is predominant, carried out on green and recrystallized iron compacts with different levels of density are presented and discussed in order to analyse the macro and micromechanisms governing the mechanical behaviour of non-sintered PM mat...

It might seem that elastic behaviour plays a secondary role during powder compaction. This is true only during the initial densification of the powder in the die in which very inelastic mechanisms, i.e. particle sliding and rearrangement, are acting. However, as densification proceeds plastic deformation of particles, mainly in metals, becomes the...

This work is focused on the evaluation of macroscopic mechanical properties of near fully dense nanostructured and ultrafine grained bulk samples of low and medium carbon steel (0.04 to 0.60%C). The specimens were obtained by warm static pressing from mechanically milled powder at temperatures between 400 °C and 500 °C. Subsequently, some samples w...

TiN/W and TiN/TaN multilayer coatings were deposited on stainless steel by Chemical Vapor Deposition in a Fluidized Bed Reactor at Atmospheric Pressure (AP/FBR-CVD). First, the conditions for the deposition of TiN single layers were investigated, both from the experiment and thermochemical estimations. TiN was deposited from TiCl4 and NH3 at temper...

Powder metallurgical industry is very interested in spreading its applications and improve the quality of the PM products. Therefore, a detailed knowledge of their processes, in order to control them, is necessary. Many different types of models have been used to represent the mechanical behaviour of these materials; however, nowadays, most of the...

Powder metallurgical industry is very interested in spreading its applications and improve the quality of the PM products. Therefore, a detailed knowledge of their processes, in order to control them, is necessary. Many different types of models have been used to represent the mechanical behaviour of these materials; however, nowadays, most of the...

Several technological applications demand materials able to have good mechanical performance at relatively high temperatures (as high as 650°C). This performance must be kept constant during long periods at these high temperatures. Superalloys, and particularly Ni–Cr–Fe alloys (Inconel series) appear to be candidates to accomplish such requirements...

En el sector automovilístico se están dando importantes cambios en cuanto a las exigencias mecánicas de los materiales, dando lugar a la aplicación de los aceros de alta resistencia mecánica, con el fin de reducir los espesores en piezas de la carrocería. No obstante, la utilización de estos nuevos aceros, lleva asociada una problemática específica...

The main aim of the present investigation was to study the flow behaviour of two medium carbon microalloyed steels under hot forming conditions, and to analyse their microstructural evolution. The dependence of recrystallized grain size (D_rec) on the Zener-Hollomon parameter Z shows a bimodal behaviour with transition from single to cycl...

The main aim of the present investigation was to study the flow behaviour of two medium carbon microalloyed steels under hot forming conditions, and to analyse their microstructural evolution. The dependence of recrystallized grain size (Drec) on the Zener-Hollomon parameter Z shows a bimodal behaviour with transition from single to cyclic dynamic...

The hot ductility of a structural steel produced from scrap recycling has been studied to determine the origin of the transverse cracks in the corners that appeared in some billets. Samples extracted both from a billet with transverse cracks and from a billet with no external damage were tested. To evaluate the influence of residual elements and in...

The hot ductility of a structural steel produced from scrap recycling has been studied to determine the origin of the transverse cracks in the corners that appeared in some billets. Samples extracted both from a billet with transverse cracks and from a billet with no external damage were tested. To evaluate the influence of residual elements and in...

A study has been carried out on the grain size distribution of cylindrical compacts obtained by consolidation of iron powder severely deformed by mechanical milling. Consolidation has been performed in two consecutive steps: cold and hot conditions. The hot one was done at two temperatures, namely 425 and 475ºC. After milling, the iron powder has a...

La industria pulvimetalúrgica está interesada en extender las aplicaciones y mejorar la calidad de sus productos. Para ello, necesita un conocimiento detallado de sus procesos que permita su control. La representación del comportamiento mecánico de estos materiales se ha realizado utilizando diversos tipos de modelos; en la actualidad, la mayoría d...

The present work is devoted to show the capabilities of this new technique in studying and describing the DRX process. For this purpose various examples are shown.

The thixotropic behavior of a material is associated to the coexistence of a liquid and a solid phase in suitable proportions so that the mixture does not crumble. In these conditions, the slurry is relatively insensible to shear stresses, being able to deform itself very easily. In this work, a thixotropic material formed by an alloy 63%Sn-37%Pb (...

Die filling is the first step in the process of powder compaction. The density distribution depends of the powder characteristics and method of filling. In this work we present the first results obtained with different experimental methods of the filling. Results have been analyzed using a video system which permits the observation of the powder fl...

Several works concerning the hot flow behaviour of steels have been performed up to present. However, one misses a methodological approach in the description of the characteristic parameters that describe the thermomechanical behaviour. Accordingly, present authors are developing a systematic study to describe the high temperature behaviour of stee...

In the present work, rates of work hardening and DRV were evaluated in five alloys. For this purpose, the parameters U and Ω were determined over a wide range of strain rates and temperatures.

The main aim of the present investigation was to study the flow behaviour of two medium carbon microalloyed steels under hot forming conditions, and to analyse its microstructural evolution. The dependence of recrystallized grain size (Drec) on the Zener-Hollomon parameter Z shows a bimodal behaviour with transition from single to cyclic dynamic re...

The aim of this work is to show the potential of the Particle Finite Element Method (PFEM) in the simulation of the powder transfer stage, in powder metallurgy industrial processes. The most innovative aspects of the work are: a) the intensive use of the particle finite element method technology to trace the motion of a representative set of partic...

Die filling is the initial stage in the powder metallurgy process. During this stage heterogeneous density distributions inside the mould can be originated, which can give place to defects in the compacted part. In this work an experimental study of the behaviour of the powder during the filling stage is presented. Three filling regimes have been o...

The influence of initial microstructure on discontinuous dynamic recrystallization (DDRX) has been investigated by using high purity and ultra high purity austenitic stainless steels with various initial grain sizes. After uniaxial compression tests at constant strain rates and various temperatures, the steady state microstructure or the state corr...

The effect of carbon content on material hot flow behaviour of 0·06 and 0·50%C steels has been investigated. Constitutive equations for the flow behaviour, taking into account dynamic recovery and recrystallisation, have been fitted using results of hot compression tests. Validation suggests that the model describes well the stress–strain behaviour...

Fatigue fractures in Al–Si cast alloys are mainly triggered by existing pores, at which cracks may be nucleated and propagated under an applied stress. The variability of pore size produces a noticeable scatter in fatigue life results, which is associated with differences in the size of the fracture-controlling pore. Thus, pore size distribution ac...

The technique of electron back scattering diffraction (EBSD) is considered as a powerful instrument for the study of the microstructural changes during hot forming processes and gives the possibility to present the information in different ways (OIM, misorientation diagram and pole figures). The present work is focused on the observation by EBSD of...

The hot ductility and fracture behaviour of a C-Mn steel susceptible to transversal cracking in the corners is studied. In order to evaluate the influence of residual elements and inclusions, this steel was compared with another one with the same main composition obtained by electro-slag from almost pure iron. Tensile tests at temperatures ranging...

The hot ductility and fracture behaviour of a C-Mn steel susceptible to transversal cracking in the corners is studied. In order to evaluate the influence of residual elements and inclusions, this steel was compared with another one with the same main composition obtained by electro-slag from almost pure iron. Tensile tests at temperatures ranging...

A study of the elastic response before and after tensile plastic strain was undertaken for two commercial low-alloyed TRIP steels. These steels, TRIP 700 (C-Mn-AI alloy) and TRIP 800 (C-Mn-Si) are commercial alloys used in sheet metal stamping. The behaviour of the instantaneous tangent modulus (E T) versus stress during loading and unloading was m...

The thixotropic behavior of a material is associated to the coexistence of a liquid and a solid phase in suitable proportions so that the mixture does not crumble. In these conditions, the slurry is relatively insensible to shear stresses, being able to deform itself very easily. In this work, a thixotropic material formed by an alloy 63%Sn - 37%Pb...

The aim of this work is to show the potential of the Particle Finite Element Method (PFEM) in the simulation of the powder transfer stage, in powder metallurgy industrial processes. The most innovative aspects of the work are: a) the intensive use of the particle finite element method technology to trace the motion of a representative set of partic...

Die filling is the first step in the process of powder compaction. The density distribution depends of the powder characteristics and method of filling. In this work we present the first results obtained with different experimental methods of the filling. Results have been analyzed using a video system which permits the observation of the powder fl...

The hot ductility and fracture behaviour of a C-Mn steel susceptible to transversal cracking in the corners is studied. In order to evaluate the influence of residual elements and inclusions, this steel was compared with another one with the same main composition obtained by electro-slag from almost pure iron. Tensile tests at temperatures ranging...

Powder transfer inside the compacting die is a frequent and important stage in the elaboration of PM parts of complex shapes. It introduces shear bands inside the uncompressed powder given place to initial unhomogeneities that can produce defects and even failures in the compacted part. In this work, the extent and size of these shear bands are stu...

.In the field of Powder Metallurgy industrial powders are generally mixes of different constituents with complex behaviour. The response of a powder during compaction can be described in terms of a suitable constitutive law, which can be calibrated using a range of experimental tests with different degrees of sophistication. This paper deals with c...

Wear resistance of a new powder metallurgy (P/M) Al–Si alloy was studied at two successive development stages, with marked microstructural differences. Wear tests were conducted by a pin-on-disk tribometer, without lubricant, at different loads. Microstructural observations of the alloy at the first stage of development revealed the existence of a...

The relationship between the initial grain size and the critical Zener-Hollomon parameter value (D0-Zc) defines the conditions for which a material will dynamically recrystallize with a single or with multiple peaks. The relationship between the stable dynamically recrystallized grain and the Zener-Hollomon parameter (Drex-Z) predicts the condition...

The present work shows that clustering of particles promotes deviation in the classical mathematical expressions describing the grain growth control by second-phase particles. On the basis of experimental results and theoretical laws, a semiempirical expression to predict the limiting grain size is presented. The latter expression takes account of...

The hot deformation of metallic materials is the overall balance between two antagonistic mechanisms. On one hand the material undergoes work hardening, and on the other hand, due to the thermal activation, it undergoes softening by dynamic recovery and recrystallization. Both phenomena are usually studied by mechanical tests carried out at constan...

The relationship between the initial grain size and the critical Zener-Hollomon parameter value (D-0-Z(c)) defines the conditions for which a material will dynamically recrystallize with a single or with multiple peaks. The relationship between the stable dynamically recrystallized grain and the Zener-Hollomon parameter (D-rex-Z) predicts the condi...

Duplex stainless steels (DSSs) have become established materials, successfully employed in many industrial applications. Their combination of mechanical properties and corrosion resistance is particularly appreciated in the petrochemical field. Hot deformation of these two-phase materials is still a critical point because the different mechanical r...

This work is divided into two parts. First, a mathematical modeling of the austenite transformation during deformation operations at high temperature is presented. The model introduced by Estrin, Mecking and Bergstrom is employed in predicting the dynamic recovery step while the additional softening by dynamic recrystallization is modeled according...

In this work, a plasticity model for granular materials, based on the Drucker–Prager/CAP model, is applied to represent the behaviour of the metallic powders during their cold die compaction. The elastic deformation has been simulated by means of a non-linear law. These constitutive equations have been implemented in a commercial code to simulate t...

The solder joint failures in printed circuit boards (PCB's) which control most of the functions in modern cars are mainly caused by the different thermal coefficients of expansion of all the components involved in the boards. During validation tests, PCB's are subjected to cycles of severe thermal shocks, which promote complex stress - strain distr...

The hot ductility behavior of a medium carbon steel (0.48%C, 0.742%Mn, 0.183%Si, 0.028%A1) was studied by tensile tests carried out at strains rates varying from 1.10-4 to 1.10-3 s-1 and temperatures ranging from 590°C to 960°C. Before testing, samples were treated at 1200°C for 5 minutes. Then they were cooled down to the experimental testing temp...

Die filling is the first step in the process of powder compaction. The density distribution depends of the powder characteristics and method of filling. In this work we present the first results obtained with different experimental methods of the filling. Results have been analyzed using a video system which permits the observation of the powder fl...