Numerical modelling of grain refinement around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials by duplex technique

Microstructure evolution around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials have been investigated and discussed in the present work. Conditions leading to grain refinement during co-rolling stage of the duplex processing technique are analysed using the multi-level finite element based numerical model combined with three-dimensional frontal cellular automata. The model was capable to simulate development of grain boundaries and changes of the boundary disorientation angle within the metal structure taking into account crystal plasticity formulation. Appearance of a large number of structural elements, identified as dislocation cells, sub-grains and new grains, has been identified within the metal structure as a result of metal flow disturbance and consequently inhomogeneous deformation around oxide islets at the interfaces during the co-rolling stage. These areas corresponded to the locations of shear bands observed experimentally using SEM-EBSD analysis. The obtained results illustrate a significant potential of the proposed modelling approach for quantitative analysis and optimisation of the highly refined non-homogeneous microstructures formed around the oxidised interfaces during processing of such laminated materials

The influence of copper addition on crack initiation and propagation in an Al–Si–Mg alloy during cyclic testing

The effect of copper (Cu) addition up to 3.2 wt% on crack initiation and propagation in an Al–Si–Mg cast alloy was investigated using in-situ cyclic testing in the as-cast condition. A combination of digital image correlation, electron backscatter diffraction, and scanning electron microscopy was used to investigate crack initiation and propagation behaviour during in-situ cyclic testing. The results showed that Cu-rich intermetallic compounds with the addition of Cu up to 1.5 wt% do not affect the fatigue behaviour of these alloys, and that crack propagation in these cases is trans-granular and trans-dendritic. However, increasing the concentration of the Cu retained in the primary α-Al matrix in solid solution and Cu-containing precipitates delayed crack propagation during cyclic testing. The results showed that strain accumulation was highest at the grain boundaries; however, the crack preferred to propagate along or across primary α-Al dendrites due to the relatively lower mechanical strength of the matrix compared to the eutectic and intermetallic phases. Moreover, the addition of Cu of more than 3.0 wt% to Al-Si-Mg alloys changes the fatigue behaviour that a rapid failure occurs

The complex interaction between microstructural features and crack evolution during cyclic testing in heat-treated Al–Si–Mg–Cu cast alloys

The study aimed to investigate crack initiation and propagation at the micro-scale in heat-treated Al–7Si–Mg cast alloys with different copper (Cu) contents. In-situ cyclic testing in a scanning electron microscope coupled with electron back-scattered diffraction and digital image correlation was used to evaluate the complex interaction between the crack path and the microstructural features. The three-nearest-neighbour distance of secondary particles was a new tool to describe the crack propagation in the alloys. The amount of Cu retained in the α-Al matrix after heat treatment increased with the Cu content in the alloy and enhanced the strength with a slight decrease in elongation. During cyclic testing, the two-dimensional (2D) crack path appeared with a mixed propagation, both trans- and inter-granular, regardless of the Cu content of the alloy. On fracture surfaces, multiple crack initiation points were detected along the thickness of the samples. The debonding of silicon (Si) particles took place during crack propagation in the Cu-free alloy, while cracking of Si particles and intermetallic phases occurred in the alloy with 3.2 wt% Cu. Three-dimensional tomography using focused ion beam revealed that the improved strength of the α-Al matrix changes the number of cracked particles ahead of the propagating crack with Cu concentration above 1.5 wt%

Additive manufacturing of nano-oxide decorated AlSi10Mg composites: A comparative study on Gd2O3 and Er2O3 additions

AlSi10Mg-based nanocomposites were fabricated by laser powder bed fusion (LPBF) additive manufacturing with the addition of 1 wt% Gd2O3 and Er2O3 nanoparticles. The effect of different process parameters and supplementary remelting on the densification of the samples was evaluated. Results showed that remelting the printed layers could improve the densification. According to the microstructural observations, stacking the nanoparticles on uneven surfaces of irregular AlSi10Mg particles beside van der Waals's attractive force between the adjacent particles eventually forms coarsened clusters in printed samples. The XRD patterns disclosed the partial reaction between the nano-oxides and the aluminum matrix and the formation of some interfacial intermetallic layers, which were also validated by SEM characterization. The measurement of grain size and microhardness implied that the addition of Er2O3 played a more effective role in grain refinement and enhanced the hardness more uniformly compared to Gd2O3. Overall, the acquired average hardness for both nano-oxide reinforced specimens was greater than the reported values for LPBF-fabricated AlSi10Mg-matrix composites in the past. EBSD analyses revealed that due to the pinning effect of the nanoparticles, particle-rich zones demonstrated higher KAM and grain orientation spread (GOS) values which were attributed to the formation of more GNDs at the matrix/ particles interfaces

Dry sliding behavior of AlSi10Mg alloy produced by Laser-based Powder Bed Fusion: influence of heat treatment and microstructure

The L-PBF AlSi10Mg alloy is widely used in the production of structural parts in the transportation sector. However, the high stresses caused by the severe operating conditions require an optimal combination of mechanical and tribological properties. This paper reports on the effect of optimized T5 heat treatment (direct artificial aging: 4 h at 160 °C) and novel T6 heat treatment (rapid solution: 10 min at 510 °C, followed by artificial aging: 6 h at 160 °C) on the tribological behavior of the L-PBF AlSi10Mg alloy. Dry sliding tests (ball-on-disk) were carried out using AlSi10Mg samples as rotating disks against an Al2O3 stationary ball. The optimized T5 led to the formation of a stable protective oxide layer well adherent on the worn surface, increasing the wear resistance of the alloy. In addition, the novel T6 improved wear resistance compared to conventional T6 due to microstructural refinement induced by shorter solutionizing. The sub-surface analysis of the wear tracks highlighted the higher cohesion between the more homogeneous and finer Si particles and the α-Al matrix, as well as the improved load-bearing support compared to the coarser microstructure induced by conventional T6. Therefore, the new T6 could be the optimal solution for high-performance components

Progressive microforming process : towards the mass production of micro-parts using sheet metal

Although there is considerable published literature on micro-metal forming processes, there is still a lack of research towards implementing these processes commercially. Some of the challenges are handling of micro-parts and process intermittency. This work demonstrates the feasibility of producing symmetric micro-parts using a progressive forming set-up. Such a progressive forming process alleviates the challenges in handling and removal of micro-parts. Micro-pins with diameters of 0.3, 0.5, and 0.8 mm were successfully manufactured without defects. Experimental observations together with process simulation results showed that this process has three main stages: (1) indentation at the very beginning, (2) upsetting, and (3) extrusion predominantly occurring at the very end stage of the stroke. The bulk of the pin forming occurs at the end stroke of the process (extrusion stage). The effects of punch/pin diameter ratio on the pin aspect ratio and the maximum forming load were also investigated. In addition, the finite element results also revealed that a hybrid friction model was required to be implemented for better fit with experimental results as compared to the shear and Coulomb friction models

On the fatigue damage micromechanisms in Si-solution–strengthened spheroidal graphite cast iron

Graphite nodules in Spheroidal graphite cast iron (SGI) play a vital role in fatigue crack initiation and propagation. The graphite nodule growth morphology can go through transitions to form degenerated graphite nodules other than spheroidal graphite nodules in SGI microstructure. These graphite nodules significantly influence damage micromechanisms on SGI and could act differently. Most of the damage mechanism studies on SGI were focused on the role of spheroidal graphite nodules on the stable crack propagation region. The roles of degenerated graphite nodules on SGI damage mechanisms were not frequently studied. In this work, fatigue crack initiation and propagation tests were conducted on EN-GJS-500-14 and observed under SEM to understand damage mechanisms of different graphite forms. Crack initiation tests showed dominant influence of degenerated graphite nodules where early cracks initiated in the microstructure. Most of the spheroidal graphite nodules were unaffected at the early crack initiation stage; some of them showed decohesion from the ferrite matrix and internal cracking. At the crack propagation region, graphite-ferrite matrix decohesion was the frequent damage mechanism observed with noticeable crack branching around graphite nodules and the crack passing through degenerated graphite nodules. Finally, graphite nodules after decohesion acted like voids which grew and coalesced to form microcracks eventually causing rapid fracture of the remaining section.MOE (Min. of Education, S’pore)Accepted versio

Design of a hot deformation processing map for a Ni-free, N-bearing austenitic stainless steel

Abstract The hot deformation characteristics of a FeCrMnN austenitic stainless steel containing 0.28 wt.% nitrogen (N) was investigated by hot compression tests using a Gleeble simulator in the temperature range of 800−1200 °C and at constant true strain rates of 0.01–10 s−1 with all specimens deformed to ∼0.9 true strain. The influence of deformation conditions on microstructural mechanisms and phase transformations was characterized. A processing map based on dynamic materials modelling (DMM) was designed and interpreted for predicting the domain of stable flow for safe, defect-free hot deformation. The results revealed the occurrence of dynamic recrystallization (DRX) in a domain extending over the temperature and strain rate ranges of 1100−1200 °C and 0.1–1 s−1, respectively, with the efficiency of power dissipation (η) of 45–55 %. Decreasing temperature and increasing strain rate led to a reduction in DRX grain size following microstructural reconstitution. Another small deterministic domain of 820−1000 °C and 0.01−0.05 s−1 was identified showing occurrence of partial DRX in shear bands leading to formation of a mixed microstructure. The instability criteria delineated the regime of unstable flow covering a large part of the processing map extending over low temperatures (800−950 °C) and high strain rates (0.1–10 s−1) that must be avoided during processing