Effect of heat treatments on the mechanical and microstructural behavior of a hypoeutectic Al alloy obtained by laser power bed fusion

Large gains in strength and ductility are of little significance if the material’s anisotropy is high. Therefore, improving the mechanical properties and reducing the anisotropy of Al alloys obtained by additive manufacturing is a topic of growing interest. This manuscript examines the effect of distinct heat treatments on the mechanical, anisotropic, and microstructural behavior of a hypoeutectic, almost eutectic, AlSi11Cu alloy obtained by laser powder bed fusion (L-PBF). The microstructural characterization revealed an Al matrix surrounded by a Si-rich network, forming a coral-like pattern with a heterogeneous combination of columnar and equiaxed grains. The texture indicated that the columnar grains were preferentially oriented towards the building direction with strong Cube and Goss components. Different strength-ductility ratios were obtained following the annealing and solution heat treatments at different temperatures (200 °C–550 °C) with a holding time of 1 h. In terms of grain size and dislocation density, no significant changes were found in the microstructure, suggesting that grain size and dislocation strengthening mechanisms are not highly affected by the heat treatments. In addition, the Si-enriched network remained interconnected until 300 °C. At higher temperatures, this interconnection was lost, giving rise to large Si particles depleting the Si content in solid solution in the Al matrix. Digital image correlation maps revealed that deformation fields were more homogeneous when the cellular structure disappeared. The visco-plastic self-consistent model showed that when applying the load at 30° in the building direction (BD), the largest tensile strength was generated, whereas the lowest strength was obtained when the load was parallel to the BD. Heat treatments for 1 h holding time were found to be efficient in reducing the Lankford coefficients dispersion, suggesting improvements in formability and reducing the alloy’s planar anisotropy. These results revealed that annealing up to 400 °C or higher temperatures followed by water quenching leads to good strength and ductility ratios while reducing anisotropy.Peer ReviewedPostprint (published version

Effect of the microstructure generated by repetitive corrugation and straightening (RCS) process on the mechanical properties and stress corrosion cracking of Al-7075 alloy

This study discussed the effect of the heterogeneous microstructure generated through the Repetitive Corrugation and Straightening (RCS) process on the mechanical and stress corrosion cracking behavior of the AA7075. As a result of the RCS process, significant grain refinement was obtained. The average grain size ranged from 126 to 59 µm, for the initial condition and 4 RCS passes, respectively. The yield strength and hardness increased 170% and 15% from the initial pass, remaining almost constant afterward. The evaluation of stress corrosion cracking showed a decrement in the number of cracks of 21.6% and 23.5% between the initial condition and fourth RCS passes. The cracking and pitting corrosion were the dominant mechanisms in the tested samples. The mechanical and corrosion results were also discussed in terms of the microstructural features.Peer ReviewedPostprint (published version

Formability of the 5754-aluminum alloy deformed by a modified repetitive corrugation and straightening process

Sheets of 5754-aluminum alloy processed by a modified repetitive corrugation and straightening (RCS) process were tested in order to measure their formability. For this purpose, forming limit curves were derived. They showed that the material forming capacity decreased after being processed by RCS. However, they kept good formability in the initial stages of the RCS process. The formability study was complemented with microstructural analysis (derivation of texture) and mechanical tests to obtain the strain-rate sensitivity. The texture analysis was done by employing X-ray diffraction, obtaining pole figures, and the orientation distribution function. It was noticed that the initial texture was conserved after successive RCS passes, but the intensity dropped. RCS process did not induce ß-fiber, contrary to common deformation process. The strain-rate sensitivity coefficient was measured through tensile tests at different temperatures and strain rates; the coefficient of the samples processed after one and two passes were still relatively high, indicating the capacity to delay necking, in agreement with the good formability observed in the initial passes of the RCS processPeer ReviewedPostprint (published version

Texture and strain rate sensitivity analysis of solid solution and precipitation hardening aluminum alloys processed by repetitive corrugation and straightening

The potential of improving the mechanical strength by the RCS process is evaluated on the 5754, 6061, and 7075 aluminum alloys, which present different hardening mechanisms related to their respective alloying elements. This work compares the evolution of the texture and the mechanical properties of the different alloys through the RCS processing. The mechanical properties were evaluated by micro-hardness measurements, tensile tests at different temperatures, and strain rates to evaluate the strain-rate sensitivity. The results showed that after two RCS passes, the 6061 and 5754 alloys showed a relatively high strain-rate sensitivity at 300°C. In addition, an increment of 27%, 22%, 15% in hardness was obtained for the 5754, 6061 and 7075 alloys, respectively. Showing the potential of improvement in the mechanical resistance due to the different hardening mechanism. Furthermore, the crystallographic texture was characterized by the obtention of pole figures by X-ray diffraction and the calculation of their orientation distribution functions. The results showed the same trend in the three aluminum alloys, i.e., the initial texture components were conserved, but the texturized volume decreased.Peer ReviewedPostprint (published version

Microstructural evolution and mechanical behavior of an Al-6061 alloy processed by repetitive corrugation and straightening

The repetitive corrugation and straightening process is a severe plastic deformation technique that is particularly suited to process metallic sheets. With this technique, it is possible to develop nano/ultrafine-grained structured materials, and therefore, to improve some mechanical properties such as the yield strength, ultimate tensile strength, and fatigue lifetime. In this study, an Al-6061 alloy was subjected to the repetitive corrugation and straightening process. A new corrugation die design was proposed in order to promote a heterogeneous deformation into the metallic sheet. The evolution of the mechanical properties and microstructure obtained by electron backscatter diffraction of the alloy showed a heterogeneous distribution in the grain size at the initial cycles of the repetitive corrugation and straightening process. Uniaxial tensile tests showed a significant increase in yield strength as the number of repetitive corrugation and straightening passes increased. The distribution of the plastic deformation was correlated with the hardness distribution on the surface. The hardness distribution map matched well with the heterogeneous distribution of the plastic deformation obtained by finite element simulation. A maximum average hardness (147 HV) and yield strength (385 MPa) was obtained for two repetitive corrugation and straightening cycles samplePeer ReviewedPostprint (published version

Study on the formability and texture evolution of AA6061 alloy processed by repetitive corrugation and straightening

The enhanced mechanical properties obtained by refining the grain size down to the ultrafine-grained (UFG) regime have attracted considerable attention in recent years. The severe plastic deformation (SPD) techniques allow obtaining ultrafine-grained materials. Different SPD techniques permit processing sheet shape materials such as repetitive corrugation and straightening (RCS) and accumulative roll bonding (ARB). In this study, the formability of an AA 6061-T6 processed by RCS was evaluated. The forming limit diagrams (FLD) were obtained by Nakazima tests of samples in initial condition (T6 state) and after one and two RCS cycles. The FLD curves showed that the forming capacity decreased from the first RCS cycle. Likewise, uniaxial tensile tests at different temperatures and strain rates were conducted to analyze the effect of the RCS process on the strain rate sensitivity. They showed a relatively high strain rate sensitivity coefficient in the samples after one and two RCS cycles, which indicates an improvement of i) the capacity of the material to delay the onset of the necking and ii) the formability at increasing temperatures. Finally, texture analysis was carried out employing X-ray diffraction, calculating the orientation distribution functions (ODFs). The initial texture showed a predominant cube texture component, whereas, for further RCS cycles, a weakening of the cube texture and an increment of the S texture component were observed.Peer ReviewedPostprint (published version

Mechanical, stress corrosion cracking and crystallographic study on flat components processed by two combined severe plastic deformation techniques

Although the current field of application of Al–alloy 7075 (AA7075) is vast, it is still limited due to some drawbacks, especially due to its susceptibility to stress corrosion cracking (SCC). This work aims to evaluate the microstructural, mechanical, and stress corrosion cracking (SCC) behaviors on an AA7075 in flat format deformed by a combination of repetitive corrugation and straightening (RCS) and accumulative roll bonding (ARB) techniques. Four different deformation routes were applied, namely: ARB (A), RCS (R), RCS + ARB (RA) and ARB + RCS (AR). As expected, the efficiency for grain refinement depends on the applied route, in terms of average grain size regarding the initial condition IC): AR > A > RA > R. All conditions resulted in unimodal and widened grain size distributions of micro-, submicro- and nano-metric dimensions. The study of crystallographic orientations showed that route R did not generate any new texture, whereas different preferred orientations were obtained for routes A, RA, and AR. The hardness and three-point bending tests showed an improvement of mechanical strength in the following order: AR > RA > A > R. The cracks per cm2 obtained in the corrosion study indicated that the best SCC resistance was R > A > AR > RA. Based on the above, the best combination of microstructural, mechanical, and SCC properties until one deformation pass was obtained by the single route of the ARB process.Peer ReviewedPostprint (published version

Repetitive corrugation and straightening effect on the microstructure, crystallographic texture and electrochemical behavior for the Al-7075 alloy

Anti-corrosion susceptibility is one of the top criteria for selecting metallic materials for several industrial applications. This work studies the corrosion performance on an Al-7075 alloy obtained by Repetitive Corrugation and Straightening (RCS). This processing method generated a microstructure formed by randomly distributed micro-, submicro-, and nano- metric grain sizes. The samples exhibited a drop in corrosion resistance for a longer duration in the electrolyte and higher deformation. However, the samples processed by RCS showed better electrochemical stability in comparison with the non-deformed condition. The improvement of electrochemical stability could be associated with the particular microstructure generated during the RCS process.Postprint (published version

Robocasting of dense 8Y zirconia parts: Rheology, printing, and mechanical properties

Advanced ceramics with complex geometry have become indispensable in engineering applications. Due to limitations of traditional ceramic fabrication processes, additive manufacturing represents a revolution for shaping and consolidation because of its unique capabilities for increasing shape complexity and reducing waste material. Among the additive manufacturing techniques, robocasting is often considered to yield fine and dense ceramic structures with geometrically complex morphology and high strength. Within this context, it is the objective to attain dense 8 mol% yttria-stabilized zirconia (8Y-ZrO2) by evaluating the influence of solid loading and filament orientation on the physical and mechanical properties of sintered parts. In doing so, a printable ink was developed using an inverse-thermoresponsive hydrogel. Results revealed that ceramic charges of 67.5 and 70 wt% achieved the best balance regarding density, hardness, and compression strength. Furthermore, rectilinear geometry with a filament orientation at 45º displayed higher mechanical response than 0/90º and cylindrical ones.Peer ReviewedPostprint (published version

The effect of preheating temperature on the forming limit diagram of AA1050/AA7050 Al multilayered sheets produced by accumulative roll bonding (ARB)

Multilayered sheets of AA1050 and AA7050 Al alloys produced by hot accumulative roll bonding (ARB) are tested to estimate their forming limit diagrams (FLDs). Sheets processed with preheating at 450 and 500¿°C for up to six ARB cycles are submitted to Nakazima tests with an in situ digital image correlation system and tensile tests to analyze the mechanical behavior. X-ray measurements and electron backscatter diffraction are performed to obtain the crystallographic texture and the mesotexture, respectively, and thus characterize the heterogeneous microstructure. A bimodal grain size distribution is observed in these sheets since AA7050 layers present elongated and fine grain size with nanometric precipitates and AA1050 layers present coarser and equiaxed grains. In addition, texture analysis indicates both rolling and shear components in the sheets. The FLDs show that the forming properties did not follow the expected rule of mixtures when compared with monolithic alloy sheets. Overall, the multilayered AA1050/AA7050 processed by means of ARB at 500¿°C presents better formability than that at 450¿°C, due to the combination of lower anisotropy measured by the Lankford coefficients values close to 1 and low normal anisotropy ¿r values.Peer ReviewedPostprint (author's final draft