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Effect of pressure in the microstructure of die cast Al-8.5Si-3.5cu alloys
(2014)
Barbés Fernández, Miguel Ángel
;
Verdeja González, José Ignacio
;
Quintana, María José
;
Verdeja González, Luis Felipe
;
Gonzalez-Ojeda, Roberto
The microstructure of Pressure Die Casting of an Al-8.5 Si-3.5 Cu alloy used for clean room tiles shows amount of constituents (eutectic and Si phases) that do not correspond to the ones indicated by the phase diagram. Furthermore, there are differences in amount of constituents between the core and the surface zones of parts produced by this process. The work presents quantitative microstructural analysis of this type of industrially produced part, and as a possible cause for these differences, simulations of the effect of pressure on the eutectic (temperature and chemical composition) in the Al-Si system. As the production rate of these parts is very high, and solidification is very fast, microstructural heterogeneity may be related to local entrapment of gasses unable to escape during casting.
43 2
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Microstructural changes of a construction steel caused by hot deformation
(2013)
Verdeja González, José Ignacio
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Quintana, María José
;
Gonzalez-Ojeda, Roberto
;
Verdeja González, Luis Felipe
A construction steel (shipbuilding strip) obtained by Advanced Thermomechanical Controlled Rolling Processes presents a room temperature banded ferrite-pearlite microstructure, and when superplastically deformed at 800°C with a strain rate of 5.85x10-5 s-1, the bands disappear as there is grain boundary sliding and grain cluster rotation. Nevertheless, the superplastic deformation does not imply a decrease in mechanical properties, as room temperature tests with strain rates of 1.46x10-3 s -1 with the steel previously deformed in superplastic conditions (until a 110% of straining) result in similar mechanical data. If the steel is deformed at 750°C with low strain rates, cooling results in a microstructure formed only by ferrite and carbides (the pearlitic phase disappears). This behavior may be explained, from a thermodynamical point of view, by the effect of negative hydrostatic pressure during the tensile test and the pronounced ferrite- and carbide-former capacity of Ti and Nb microalloying elements. The samples, tensile tested, in both the hot rolled raw state and superplastically deformed and then room temperature tested, show in the fracture surface SEM analysis almost identical features: decohesions surrounding MnS and (C,N)(Ti,Nb) precipitates and between ferrite and pearlite grains, as well as bedding.
6 2
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Superplasticity of ultrafine grained low-carbon HSLA steels
(2012)
Verdeja González, José Ignacio
;
Quintana, María José
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García García, José Ovidio
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Verdeja González, Luis Felipe
;
Gonzalez-Ojeda, Roberto
Steels with ultrafine grained structure may present superplastic behavior at specific temperatures and strain rates that allow the grain boundary sliding mechanisms to be activated. The work presents high temperature tension tests in a low carbon, low alloy steel obtained by advanced thermomechanical controlled rolling processes, showing at 800°C elongations as high as 200%. The microstructure of the steel was analyzed in order to identify ferrite and pearlite grain boundaries, and their interaction after the specimens were deformed, showing intergranular decohesions, restored ferrite grains and elimination of banded structure, which are evidence of superplastic mechanisms in this material which is, in fact, ultrafine grained as demonstrated by quantitative metallographic techniques and grain size distribution analysis.
8 1
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Dual-phase ultrafine grained steels produced by controlled rolling processes
(2011)
Quintana, María José
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Gonzalez-Ojeda, Roberto
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Verdeja González, Luis Felipe
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Verdeja González, José Ignacio
Double-phase steels are an excellent alternative in the production of automotive parts that require high mechanical resistance, high impact strength and elevated elongation. These materials are produced using low-alloy steels as a basis, reducing costs and resulting in a combination of martensite and ferrite structures with ultrafine grain sizes. These characteristics are accomplished through a strict control of rolling conditions: strain rate, cooling rate and direct quenching. This work presents the results of tension testing of two types of double phased steels, along with microstructural characterization, in order to understand the effect of the advanced thermomechanical controlled rolling processes on the formation of the microstructure and the resulting mechanical properties.
7 2
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Superplastic HSLA steels: Deformation and Failure
(2012)
Fernández, S.
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Verdeja González, José Ignacio
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Quintana, María José
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Gonzalez-Ojeda, Roberto
Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.
5 1
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Structural Ultrafine Grained Steels Obtained by Advanced Controlled Rolling
(2013)
García García, José Ovidio
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Barbés Fernández, Miguel Ángel
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Verdeja González, Luis Felipe
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Verdeja González, José Ignacio
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Gonzalez-Ojeda, Roberto
Steels with ultrafine grains (lower than 5 μpa), which usually known as ultrafine ferrite or ultrafine grained materials, are presently the object of intense research, because of the improvement in resistance and fracture toughness they may reach compared to conventional steels (with grain sizes above this value). It is shown that the forenamed steels designated in the Euronorm EN 10149-2, which are manufactured by advanced techniques of controlled rolling and mainly used in automotive industry, have an ultrafine grain size in the range of 2. 5 to 3. 5 μm, and with elastic yield stresses higher than 400 MPa. Based on the Morrison-Miller criterion, it is shown that values of the strain-hardening coefficient lower than 0. 08 would make the industrial application of these steels unfeasible.
Scopus© Citations 3 9 1
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Superplastic HSLA Steels: Microstructure and Failure
(2013)
Fernández, Sara
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García García, José Ovidio
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Verdeja González, Luis Felipe
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Verdeja González, José Ignacio
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Quintana, María José
Certain materials can show superplasticity when traction tested at temperatures higher than 50% of their melting point and with low strain rates ( < 10−2 s−1), showing very high elongations (>100%) without localized necking and mainly intergranular fractures. This behavior requires that the starting grain size is small (<10 μm) so the flow of matter can be non-homogeneous (sliding and rotating of the grain boundaries, accommodated by diffusion). This work presents the superplastic characteristic of shipbuilding steel deformed at 800 °C and a strain rate slower than 10−3 s−1. The fine grain size (5 μm) is obtained when using Nb as a microalloying element and manufactured by controlled rolling processes (three stages). After the superplastic deformation, the steel presents mixed fractures: by decohesion of the hard (pearlite and carbides) and ductile (ferrite) phases and by intergranular sliding of ferrite/ferrite and ferrite/pearlite, just as it happens in stage III of the creep behavior. This is confirmed through the Ashby–Verrall model, according to which the dislocation creep (power-law creep) and diffusion creep (linear-viscous creep) occur simultaneously.
Scopus© Citations 4 7 1
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Mechanical behaviour of thermomechanically produced ultrafine grained dual-phase steels
(2014)
Gonzalez-Ojeda, Roberto
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García García, José Ovidio
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Verdeja González, Luis Felipe
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Quintana, María José
;
Verdeja González, José Ignacio
Dual-phase (DP) steels are an excellent alternative in the production of automotive parts that require high mechanical resistance, high impact strength and elevated elongation. These materials are produced using low alloy steels as a basis, reducing costs and resulting in a combination of martensite and ferrite structures with ultrafine grain size. These characteristics are achieved through strict control of rolling conditions, strain rate, cooling rate and coiling temperature. This work presents the results of tension testing of two types of DP steels, along with microstructural characterisation, in order to understand the effect of the advanced thermomechanical controlled rolling processes on the formation of the microstructure and resulting mechanical properties.
Scopus© Citations 8 6 2
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Microstructures of a pressure die cast Al-8.5%Si-3.5%Cu alloy
(2017)
Barbés Fernández, Miguel Ángel
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Quintana, María José
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Verdeja González, José Ignacio
;
Gonzalez-Ojeda, Roberto
Quantitative microstructural measurements of constituents of an Al-Si-Cu alloy, used to manufacture a part with thin sections by Pressure Die Casting (PDC), do not correspond to phase diagram calculations. A simulation of the liquid velocity when filling the mold was made in order to understand the relation between this parameter and pressure, cooling rate and the eutectic amount and morphology. Also, the microstructures of the same alloy solidified in a ceramic crucible (low cooling rate) and in a metal flask (high cooling rate) were compared to those obtained by PDC to analyze the role of pressure on the displacement of eutectic composition and formation of different eutectic morphologies. The amount of constituents varies with distance from the mold walls, producing higher or lower pressure zones, which may be estimated from the Al-Si phase diagram simulation at different pressures. As these pressures must be very high (∼ 2 GPa), a possible explanation for the displacement of the eutectic point is the combination of high cooling rate of the manufacturing process, entrapment of gases during solidification and influence of the alloying elements.
8 1
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Ultrafine Grained HSLA Steels for Cold Forming
(2010)
García García, José Ovidio
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Barbés Fernández, Miguel Ángel
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Verdeja González, Luis Felipe
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Verdeja González, José Ignacio
;
Gonzalez-Ojeda, Roberto
The industrial level production of ultrafine grained (or ultrafine ferrite) ferrous alloys was investigated through three examples of steels that complied with the EN 10149-2 Euronorm and were produced by advanced controlled hot rolling techniques. The steel samples were tension tested and chemically analyzed, and the microstructure was evaluated through quantitative metallographic techniques to determine parameters such as yield stress, amount of microalloying elements, strain hardening coefficient, grain size, and grain size distribution. These steels were micro-alloyed with Ti, Nb, and Mn with ASTM grain sizes of approximately 13–15. The careful control of chemical composition and deformation during production, giving a specific attention to the deformation sequences, austenite non-recrystallization temperatures and allotropic transformations during cooling, are indispensable to obtain steels with an adequate strain hardening coefficient that allows cold working operations such as bending, stretching or drawing.
Scopus© Citations 14 14 1

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