José de Jesús Cruz-Rivera

The present research focuses on enhancing the mechanical properties and the electrical conductivity of alloys corresponding to the Al-Mg-Zn (two different compositions) and Al-Mg-Si systems, compared with the commercial 6201-T8 and 1350-H16 alloys, by using a novel approach based on T8 tempering (solution heat treated, cold worked, then artificially aged). After T8 tempering, the Al-Mg-Zn and Al-Mg-Si alloy systems show a better combination of electrical and mechanical properties, with an enhancement of the electrical conductivity by about 2.8% compared to that of 1350 alloy and 13% higher than for the 6201 alloy series. All studied alloys exhibit better mechanical properties than 1350-H16 and are similar to those of 6201-T8.

The microstructural, mechanical, and tribological evaluation was carried out through an abrasion test of low alloy steel: 0,21%C, 1,22%Mn, and 2,16%Si with/without heat treatment. The steel was manufactured in an electric induction furnace from AISI-1018 steel with additions of FeMn and FeSi. Later it was hot rolled and heat treated. The microstructure of steel without heat treatment is ferritic-pearlitic with σ _u = 714 MPa, S = 8%, and a hardness of 21 HRC. The microstructure of the heat-treated steel corresponds to a Plasticity Induced Transformation (TRIP) steel with σ _u = 890 MPa, S = 30%, and hardness of 22 HRC. Both types of steel were subjected to abrasive wear (ASTM G-65) with silica sand particles and loads of 45, 66,5, and 130 N. The TRIP effect (transformation of retained austenite into martensite during plastic deformation) was evaluated before and after the tensile test by x-ray diffraction analysis. The results of the abrasion tests showed that the TRIP steel did not present high resistance characteristics, however, it shows better performance than steels with high carbon and/or alloy contents. The results show that with 45 N the TRIP steel is less resistant than the ferritic-pearlitic steel, however, with 66,5 and 130 N, its abrasive wear performance improved by 17% concerning the ferritic-pearlitic steel, which is attributed to the deformation capacity of TRIP steel. The main wear mechanisms present in TRIP steel are: ploughing, fractures, plastic deformation, pits, cracks, cutting, and micro-cutting. On the other hand, the surface hardness in the abrasion test footprint did not show any change that could be related to the TRIP effect. The microstructure analysis reveals that the material’s detachment occurs at the grain boundaries of the ferrite, in which the cracks that cause the detachment of the material were generated and propagated.

Computational simulation has become more important in the design of thermomechanical processing since it allows the optimization of associated parameters such as temperature, stresses, strains and phase transformations. This work presents the results of the three-dimensional Finite Element Method (FEM) simulation of the hot rolling process of a medium Mn steel using DEFORM-3D software. Temperature and effective strain distribution in the surface and center of the sheet were analyzed for different rolling passes; also the change in damage factor was evaluated. According to the hot rolling simulation results, experimental hot rolling parameters were established in order to obtain the desired microstructure avoiding the presence of ferrite precipitation during the process. The microstructural characterization of the hot rolled steel was carried out using optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that the phases present in the steel after hot rolling are austenite and α′-martensite. Additionally, to understand the mechanical behavior, tensile tests were performed and concluded that this new steel can be catalogued in the third automotive generation.

A novel medium-Mn steel microstructure with 0.1 wt.% Nb was designed using Thermo-Calc and JMatPro thermodynamic simulation software. The pseudo-binary equilibrium phase diagram and time–temperature transformation (TTT) and continuous cooling transformation (CCT) diagrams were simulated in order to analyze the evolution of equilibrium phases during solidification and homogenization heat treatment. Subsequently, the steel was cast in a vacuum induction furnace with the composition selected from simulations. The specimens were heat-treated at 1200 °C and water-quenched. The results of the simulations were compared to the experimental results. The microstructure was characterized using optical microscopy (OM) and scanning electron microscopy (SEM). We found that the as-cast microstructure consisted mainly of a mixture of martensite, ferrite, and a low amount of austenite, while the microstructure in the homogenization condition corresponded to martensite and retained austenite, which was verified by X-ray diffraction tests. In order to design further production stages of the steel, the homogenized samples were subjected to hot compression testing to determine their plastic flow behavior, employing deformation rates of 0.083 and 0.83 s⁻¹, and temperatures of 800 and 950 °C.