Simulation of a pre-structure device for fountain-like magnetorheological elastomer via Finite Element Magnetic Method (FEMM)

The ability of a pre-structure device for curing the fountain-like alignment of CIPs in a magnetorheological elastomer (MRE) is simulated in this study. In order to generate the fountain-like magnetic flux in the device, the device was equipped with an electromagnet coil and a cylindrical permanent magnet to pick-up the pass magnetic flux trough the MRE mould. While the electromagnetic coil is utilised to control the generated magnetic flux density in the device via manipulating induced currents for different magnetic fields. The analysis then was conducted by using Finite Element Magnetic Method (FEMM) software to determine the magnetic flux density in the device as well as the fountain-like shape of magnetic flux lines that flew in the MRE mould. In the simulation, the primary factor in determining the strength of the magnetic field across the mould is the change in current. The simulation has found that the current required to generate around 0.24T is about 1A comprise of electromagnetic coil and permanent magnet

Microstructures and properties of additively manufactured alloys processed by severe plastic deformation

For the first time, the microstructure-property relationship of additively manufactured (AM) alloys processed by severe plastic deformation (SPD) was investigated in this PhD project. In this study, experiments were conducted on single-material 316L stainless steel (316L SS) and multi-material 316L SS/nickel 718 (IN 718) superalloy fabricated by selective laser melting (SLM) and then processed by high-pressure torsion (HPT). These include x-ray computed tomography (XCT), Vickers hardness (HV), x-ray diffraction (XRD), nanoindentation, electrochemical and wear tests, as well as optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) observations. This study aims to investigate the influence of ultrafine grains (UFG) and nano-scale grains (NG) obtained by HPT processing on the porosity, hardness, deformation mechanism, and corrosion and wear performances of SLM-fabricated alloys. 316L SS was initially additively manufactured by SLM and then processed by HPT through 1/4 – 10 revolutions at room temperature under 6 GPa of pressure at 1 rpm to produce UFG and NG microstructures. About 89 – 91% reduction of the spherical gas-induced pores is attained even at minimum torsional strain (1/4 HPT revolution) as determined from OM, due to the combined compressive force and extreme HPT-imposed torsional strain. HV measurements indicate significant hardness increase that saturates throughout the disk after 10 revolutions, suggesting the evolution towards microstructural homogeneity with increasing torsional strain. XRD spectra show that the alloy retains its single γ-austenite FCC structure even after extreme HPT straining. XRD line broadening analysis reveal significant decrease in crystallite size, Dc and considerable increase in dislocation density, ρ with larger HPT straining. TEM and XRD analysis suggest three stages of deformation mechanisms that are related to the microstructural features and the corresponding equivalent strain values, εeq. Primary twins, dislocations, and twin-matrix lamellae are dominant in stage 1 (εeq. = ~ 0 – 10). Shear banding of the twin-matrix lamellae and secondary nanotwins are prevalent in stage 2 (εeq. = ~ 10 – 40), while the equiaxed nanosized grains at stage 3 (εeq. > 40) indicate that an equilibrium or saturation stage is achieved. A physicalbased model was then established to evaluate the contribution of grain boundaries, dislocations, twins, and solid solution on the hardness increase attained by this alloy. The calculated strain rate sensitivity (SRS), m and activation volume, 푉푝 ∗ values from the results of nanoindentation measurements at both constant and varied strain rates were correlated with the microstructural changes from micro- to nanoregime to evaluate the evolution in plasticity and plastic deformation mechanism for all processing conditions. The high m and small 푉푝 ∗ values calculated for the as-received and HPT-processed disks suggest that reasonably high plasticity level is maintained, and grain boundary (GB)-mediated activities play a major role in the evolution of plastic deformation mechanism. Electrochemical tests, SEM observations, and energy dispersive x-ray spectroscopy (EDX) analysis indicate improved overall corrosion performance in 3.5% NaCl solution after HPT processing as evidenced by the consistently lower corrosion rates compared to the as-received AM 316L SS. The enhanced corrosion performance is attributed to the substantial porosity elimination, homogeneous distribution of UFG/NG microstructures, and the absence of martensite formation. Dry sliding wear tests demonstrate improved wear performance after HPT processing, as implied by the steady reduction in coefficient of friction (COF), mass loss, mloss and specific wear rate, kW values with increasing HPT torsional strains compared to the as-received AM 316L SS. The improvement in the overall wear resistance could be attributed to the combined grain refinement-induced high hardness and the formation of iron oxides that act as solid lubricant to lower the friction between the contact surfaces. In addition, SEM and EDX analysis suggest that the wear mechanism transitioned from severe abrasive wear for the as-received AM 316L SS to a combination of mild abrasive, adhesive, and tribo-oxidative wear for all HPT processing conditions. For the multi-material 316L SS/IN 718, about 91% of irregular shaped process-induced pores is eliminated after only 1/4 HPT revolution via the similar pore closure mechanisms from HPT-processed 316L SS as before. HV measurements suggest that hardness saturation is only achieved at the peripheral regions of the disks, as demonstrated by the consistently high HV values at the disk edges compared to the disk centre. XRD analysis shows that the interfacial 316L SS/IN 718 region retains its γ-austenite FCC structure and some (Nb,Ti)C phase throughout all processing conditions. Substantial decrease in Dc and remarkable increase in, ρ at the interfacial region are attained with larger HPT straining. TEM, EDX, and XRD analysis, and the physical-based strengthening model suggest that the hardness increase at the periphery of the interfacial region is the result of grain boundaries, dislocations, solid solution, and precipitates, with the additional contribution of nanotwins after 1 and 10 HPT revolutions. In the future, hip replacement bio-implants and small gas turbine blades are envisaged to be developed by exploiting the advantages of this hybrid AM/SPD approach, particularly the design flexibility of AM and the excellent strength, and superior corrosion and wear performance attained via SPD processing. Finally, the results from the present study show that the primary aim of establishing processmicrostructure-property relationships for SPD-processed AM alloys has been achieved

Influence of energy density on metallurgy and properties in metal additive manufacturing

Fabricating metallic components for highly specialised industries such as automotive and aerospace has become the main focus of additive manufacturing (AM) due to its many advantages over traditional processes. This review initially outlines current AM techniques for processing metallic components, particularly on ‘powder bed fusion’ and ‘directed energy deposition’ categories. Various solidification and metallurgical aspects, microstructure and properties of fabricated parts are described in subsequent sections. In addition, the influence of energy density on metallurgy, microstructure and mechanical properties is addressed. The need to establish processing maps for various materials and techniques, and the challenges currently faced in metal AM are then highlighted. The final section provides an outlook for the future of research in AM of metals

Review: The impact of metal additive manufacturing on the aerospace industry

Metal additive manufacturing (AM) has matured from its infancy in the research stage to the fabrication of a wide range of commercial functional applications. In particular, at present, metal AM is now popular in the aerospace industry to build and repair various components for commercial and military aircraft, as well as outer space vehicles. Firstly, this review describes the categories of AM technologies that are commonly used to fabricate metallic parts. Then, the evolution of metal AM used in the aerospace industry from just prototyping to the manufacturing of propulsion systems and structural components is also highlighted. In addition, current outstanding issues that prevent metal AM from entering mass production in the aerospace industry are discussed, including the development of standards and qualifications, sustainability, and supply chain development

Comparison between virgin and recycled 316L SS and AlSi10Mg powders used for laser powder bed fusion additive manufacturing

In this study, the comparison of properties between fresh (virgin) and used (recycled) 316L stainless steel (316L SS) and AlSi10Mg powders for the laser powder bed fusion additive manufacturing (L-PBF AM) process has been investigated in detail. Scanning electron microscopy (SEM), electron-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) techniques are used to determine and evaluate the evolution of morphology, particle size distribution (PSD), circularity, chemical composition, and phase (crystal structure) in the virgin and recycled powders of both materials. The results indicate that both recycled powders increase the average particle sizes and shift the PSD to higher values, compared with their virgin powders. The recycled 316L SS powder particles largely retain their spherical and near-spherical morphologies, whereas more irregularly shaped morphologies are observed for the recycled AlSi10Mg counterpart. The average circularity of recycled 316L SS powder only reduces by~ 2%, but decreases~ 17% for the recycled AlSi10Mg powder. EDX analysis confirms that both recycled powders retain their alloy-specific chemical compositions, but with increased oxygen content. XRD spectra peak analysis suggests that there are no phase change and no presence of any undesired precipitates in both recycled powders. Based on qualitative comparative analysis between the current results and from various available literature, the reuse of both recycled powders is acceptable up to 30 times, but re-evaluation through physical and chemical characterizations of the powders is advised, if they are to be subjected for further reuse

Effect of sample orientation on the microstructure and microhardness of additively manufactured AlSi10Mg processed by high-pressure torsion

For the first time, high-pressure torsion (HPT) was applied to additively manufactured AlSi10Mg built in two directions (vertical and horizontal) by Selective Laser Melting (SLM) and the influence of extreme torsional strain on the porosity, microstructure, and microhardness of the alloy was investigated. ImageJ analysis indicates that significant porosity reduction is achieved by 1/4 HPT revolution (low strain). Optical microscopy (OM) and scanning electron microscopy (SEM) observations reveal the steady distortion and elongation of the melt pools, the continuous elongation of the cellular-dendritic Al matrix, and breakage of the eutectic Si phase network with increased HPT revolutions. Microhardness measurements indicate that despite the significant increase in hardness attained from HPT processing, hardness saturation and microstructural homogeneity are not achieved even after 10 HPT revolutions. X-ray diffraction (XRD) line broadening analysis demonstrates increased dislocation densities with increased HPT revolutions, which contributes to the considerably higher hardness values compared to as-received samples

Microstructural evolution and strengthening of selective laser melted 316L stainless steel processed by high-pressure torsion

The microstructural evolution and deformation mechanisms of 316L stainless steel fabricated by Selective Laser Melting (SLM) and then processed by high-pressure torsion (HPT) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray diffraction (XRD). TEM observations and XRD line broadening analysis reveal that the deformation in the HPT-processed alloy can be categorised into three deformation stages, related to the microstructural features and the corresponding equivalent strain values, εeq. Twinning, dislocation generation and multiplication, and the formation of twin-matrix lamellae are the main deformation mechanisms in stage 1 (εeq. = ~0–10). Shear banding of the twin-matrix lamellae and formation secondary nanotwins contribute to the deformation process in stage 2 (εeq. = ~10–40), while an equilibrium or saturation stage indicated by the formation of equiaxed nano-sized grains is reached in stage 3 (εeq. > 40). A model based on the linear additive theory was then established to evaluate the contribution of solid solution, dislocation, grain boundary, and twinning to the hardness increase and overall strengthening attained by this alloy

Interfacial characterisation of multi-material 316L stainless steel/Inconel 718 fabricated by laser powder bed fusion

In this study, the interfacial region of multi-material 316L stainless steel/Inconel 718 (316L SS/IN718) fabricated by laser powder bed fusion (L-PBF) is investigated in detail for the first time. The interfacial region consists of a fusion zone (FZ) with intermixed fused Fe and Ni, and individual 316L SS and IN 718 regions. Solid metallurgical bonding is achieved, as evidenced by the low porosity level (~0.27%) and absence of cracks. Microhardness measurements show an average of ~265 HV at the interfacial region, and ~304 HV and ~223 HV at the individual IN 718 and 316L SS regions, respectively

Tribological behaviour of 316L stainless steel additively manufactured by laser powder bed fusion and processed via high-pressure torsion

For the first time, the tribological behaviour of 316 L stainless steel (316 L SS) additively manufactured via laser powder bed fusion (L-PBF) with ultrafine- and nano-grains obtained from high-pressure torsion (HPT) processing has been investigated. The pin-on-disk dry sliding wear test results demonstrate enhancement wear performance after HPT processing, as indicated by the consistently lower coefficient of friction (COF), mass loss, m and specific wear rate, k values than the as-received state. The improvement in overall wear resistance could be attributed to the significantly high hardness obtained due to the nano-scale grain refinement with increasing torsional strains. Microscopy analysis suggests that the wear mechanism transitioned from severe abrasive wear before HPT to a combination of mild abrasive, adhesive, and tribo-oxidative wear after HPT

Microstructure and corrosion performance of 316L stainless steel fabricated by selective laser, melting and processed through high-pressure torsion

For the first time, the novel combination of severe plastic deformation (SPD) and Additive Manufacturing (AM) in a single process sequence was explored. 316L stainless steel (316L SS) alloy was firstly fabricated by Selective Laser Melting (SLM) AM process and subsequently processed by high-pressure torsion (HPT) SPD technique under a constant pressure of 6 GPa for different torsional revolutions. All the processed samples were subjected to electrochemical testing in a 3.5 wt % NaCl aqueous solution using open-circuit potential, potentiodynamic polarisation, and electrochemical impedance spectroscopy techniques, and characterised with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microscopic measurement results revealed that the melt pools and cellular structures obtained via SLM become increasingly refined through increased HPT revolutions, accompanied by significant porosity reduction and significant increase in microhardness. TEM observations revealed a homogeneously distributed nano-scale grains after 10 turns. Moreover, the results demonstrated that HPT processing significantly enhances corrosion performance of the 316L SS alloy in NaCl solution, due to the cellular structure refinement, homogeneous microstructure distribution, and the substantial removal of pores and defects. SEM and energy dispersive x-ray spectroscopy (EDX) analysis were also carried out on the corroded samples to determine the influence of cellular structure refinement on the corrosion characteristics of the 316L SS alloy