Showing papers by "Denis P. Dowling published in 2021"

In-situ sensing, process monitoring and machine control in Laser Powder Bed Fusion: A review

Abstract: Process monitoring and sensing is widely used across many industries for quality assurance, and for increasing machine uptime and reliability. Though still in the emergent stages, process monitoring is beginning to see strong adoption in the additive manufacturing community through the use of process sensors recording a wide range of optical, acoustic and thermal signals. The ability to acquire these signals in a holistic manner, coupled with intelligence-based machine control has the potential to make additive manufacturing a robust and competitive alternative to conventional fabrication techniques. This paper presents an overview of the state-of the art of in-situ process monitoring in laser powder bed fusion processes and highlights some current limitations and areas for advancement. Also presented is an overview of real-time process control requirements, which when combined with the emergent process monitoring tools, will eventually allow for in-depth process control of the powder bed fusion process, which is essential for wide-scale industrial credibility and adoption of this technology.

Investigating the fatigue and mechanical behaviour of 3D printed woven and nonwoven continuous carbon fibre reinforced polymer (CFRP) composites

Abstract: This paper evaluates the mechanical properties of woven continuous carbon fibre composites printed by additive manufacturing (AM) Comparison mechanical test studies (tensile, flexural and fatigue) were carried out with two nonwoven AM printed composites (unidirectional and multidirectional fibres), along with those of both a woven composite, as well as a composite reinforced using chopped carbon fibres Compared with the 17 MPa tensile strength obtained for the chopped fibre composite, the average strength of unidirectional (nonwoven), multidirectional (nonwoven) and woven fibre composites were 39, 13 and 19-fold higher, respectively The tensile strength of the woven composites was 52% lower than that attained by the unidirectional (nonwoven) fibre composites; and 38% higher than the multidirectional (nonwoven) fibre composites A comparison was also made between the flexural and fatigue performance of the unidirectional (nonwoven) and woven fibre composites The flexural strength of the latter was approximately 39% lower than the nonwoven composites, however, the load bearing capacity of woven fibre was superior This performance difference was supported by the fatigue testing results At 70% of maximum tensile load capacity after 2 × 105 cycles, the nonwoven composites failed, while the woven composites continued to perform until a level of 85% of maximum load capacity was reached The superior fatigue strength of the AM fabricated woven carbon fibre composites, demonstrates their potential for use in high cyclic load applications

In-situ XRD study on the effects of stress relaxation and phase transformation heat treatments on mechanical and microstructural behaviour of additively manufactured Ti-6Al-4V

Abstract: Additively Manufactured (AM) titanium (Ti) components are routinely post-thermal heat treated (HT), to reduce internal stresses, as well as to obtain more desirable microstructural features, yielding improved mechanical performance. Currently, there is no consensus on the optimum HT method for AM Ti-6Al-4V, as the mechanism for the main phase transformation ( α ′ (martensite) → α + β (equilibrium)) is still ambiguous. In this study, stress relaxation and phase transformation in the alloy are investigated in detail, via isothermal heat treatments and in situ high temperature X-ray Diffraction (XRD). The latter was carried out at heating rates of 5 and 200 °C/min. The relationship between crystallographic evolution during isothermal treatments and mechanical behaviour was determined. Isothermal holding at 400 °C resulted in an increase in ultimate tensile strength (UTS) and yield strength (YS) by 3.4% and 2.1%, respectively, due to the relief of tensile microstrain. It was found that isothermal treatment conducted between 550 and 700 °C promotes martensitic decomposition, resulting in the formation of a transitional - α t r phase, which has an asymmetrical hexagonal crystal lattice. The formation of this α t r phase was determined to be the main factor contributing to a major decrease in ductility.

Adhesion Improvement of Thermoplastics-Based Composites by Atmospheric Plasma and UV Treatments

Abstract: The present work is concerned with adhesive bonding of thermoplastic composites used in general aerospace applications, including polyphenylene sulfide (PPS), polyetherimide (PEI) and polyetheretherketone (PEEK) carbon fibre composites. Three different surface treatments have been applied to the PEEK, PPS and PEI-based composites in order to enhance the adhesion: atmospheric plasma, ultraviolet radiation (UV) and isopropanol wiping as a control. Water contact angles and free surface energies were measured following the standard experimental procedure based on the employment of three different liquid droplets. Infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) were subsequently performed to characterize the surface chemistry of the samples after treatment. The single lap joints were manufactured and bonded by an Aerospace grade epoxy-based film adhesive originally developed for use on metals but with the ability to bond treated thermoplastics to good strength (supplied by Henkel Ireland). Quasi-static (QS) tests were conducted. The lap shear strength was evaluated, and the failure mechanisms of the different joints were examined for the range of surface treatments considered. It was found that the performances of the PEEK and PPS joints were considerably improved by the plasma and UV treatments resulting in cohesive and delamination failures, while PEI was unaffected by the plasma and UV treatments and performed very well throughout.

Ti–6Al–4V microstructural functionally graded material by additive manufacturing: Experiment and computational modelling

Abstract: In this study, a functionally graded material (FGM) was fabricated using a powder bed fusion additive manufacturing technique. The FGM evaluated was Ti–6Al–4V, due to its importance in the medical device and aerospace sectors. The microstructure at selected build locations was analysed using both scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques, where a gradient was observed both in terms of grain morphology and texture intensity. In addition, nanoindentation on the FGM sample shows a near quadratic shaped smooth gradient elastic modulus profile, with peak values at the midpoint of the gauge length. A tensile test to failure was conducted on the FGM sample with the aid of digital image correlation for surface strain analysis. Results show a gradient of local maximum principal strain in the highly {0001} textured section, while the reference sample in the control group shows a near-uniform strain distribution throughout the whole gauge length. A crystal plasticity finite element (CPFE) model was developed to explain the effect of texture on the mechanical properties adopting the microscopy-informed texture intensity gradient. Despite exhibiting higher elastic modulus (in the transverse direction, as measured via nanoindentation) the mid-section of the gauge length had a higher concentration of strain when loaded in the axial tensile direction, corresponding to build direction. The microscopy and computational modelling show that this apparent contradiction was explained via a high intensity of {0001} texture in the mid-section, leading to favourable conditions for axial strain accumulation.

NiO/ZrO2 nanocomposites as photocathodes of tandem DSCs with higher photoconversion efficiency with respect to parent single-photoelectrode p-DSCs

Abstract: The nanocomposites of nickel oxide (NiO) and zirconia (ZrO2) (NZNCs) are particularly effective photocathodic materials in p-type dye-sensitized solar cells (p-DSCs) and tandem DSCs (t-DSCs). The t-DSCs obtained from P1-sensitized NZNC as photocathode and nanostructured titania (TiO2) sensitized with squaraine VG10-C8 as photoanode display overall efficiencies of ca. 2% at their best and, more importantly, produced photocurrents that surpassed systematically the values obtained from the parent devices having one photoelectrochemical interface. Such a finding is a consequence of the diminished resistance of the electrolyte the thickness of which is systematically smaller in t-DSCs with respect to parent DSCs with a single photoelectrochemical junction and same interelectrodic separation. The results here reported demonstrate that a careful combination of photoelectroactive electrodes can lead to an increase in current density of more than 15% in the t-DSC with respect to single-junction DSCs employing the same photoelectrodes provided that the whole thickness of the t-DSC is the same as in the single photoelectrode DSC and the photoelectrodes in the t-DSC do not incur in short-circuit phenomena through the electrolyte. For the successful realization of t-DSCs another important aspect is the complementarity of the absorption properties of the chosen colorants with the sensitized electrodes having similar absorbance in their respective ranges of optical absorption. The latter condition in t-DSCs makes possible the achievement of photoactivity spectra with a uniform efficiency of conversion in the whole visible range. For the attainment of efficient t-DSCs the two different photoelectrodes from parent DSCs (i.e. the devices at a single photoeletrochemical interface), should generate anodic and cathodic photocurrent densities with very similar values. Such a matching of photocurrents requires a careful selection of the thickness values for the photoelectrodes especially in case of materials with considerably different characteristics of charge injection. The approach here considered is a promising one for the assembly of quasi-transparent photoelectrochemical tandem devices operating as smart windows that convert light into electrical power.

Synergistic toughening and electrical functionalization of an epoxy using MWCNTs and silane‐ /plasma‐activated basalt fibers

Abstract: This work studied the effects of adding short basalt fibers (BFs) and multi-walled carbon nanotubes (MWCNTs), both separately and in combination, on the mechanical properties, fracture toughness, and electrical conductivity of an epoxy polymer. The surfaces of the short BFs were either treated using a silane coupling agent or further functionalized by atmospheric plasma to enhance the adhesion between the BFs and the epoxy. The results of a single fiber fragmentation test demonstrated a significantly improved BF/epoxy adhesion upon applying the plasma treatment to the BFs. This resulted in better mechanical properties and fracture toughness of the composites containing the plasma-activated BFs. The improved BF/epoxy adhesion also affected the hybrid toughening performance of the BFs and MWCNTs. In particular, synergistic toughening effects were observed when the plasma-activated BFs/MWCNTs hybrid modifiers were used, while only additive toughening effects occurred for the silane-sized BFs/MWCNTs hybrid modifiers. This work demonstrated a potential to develop strong, tough, and electrically conductive epoxy composites by adding hybrid BF/MWCNT modifiers.

Concentric Annular Liquid-Liquid Phase Separation for Flow Chemistry and Continuous Processing

Abstract: A low-cost, modular, robust, and easily customisable continuous liquid–liquid phase separator has been developed that uses a tubular membrane and annular channels to allow high fluidic throughputs while maintaining rapid, surface wetting dominated, phase separation. The system is constructed from standard fluidic tube fittings and allows leak tight connections to be made without the need for adhesives, or O-rings. The units tested in this work have been shown to operate at flow rates of 0.1–300 mL min−1, with equivalent residence times from 80 to 4 seconds, demonstrating the simplicity of scale-up with these units. Further scale-up to litre per minute scales of operation for single units and tens of litres per minute through limited numbering up should allow these low cost concentric annular tubular membrane separators to be used at continuous production scales for pharmaceutical applications for many solvent systems. In principle this approach may be sufficiently scalable to be utilized in-line, in batch pharmaceutical manufacturing also, through further scale-up and numbering up of units. Several solvent systems with varying interfacial tensions have been investigated, and the critical process parameters affecting successful separation have been identified. An additively manufactured diaphragm based back pressure regulator was also developed and printed in PEEK, allowing highly accurate, adjustable, and chemically compatible pressure control to be accessed at low cost.