Effect of Grain Structure on Machinability of LPBF Inconel 718: A Critical Review

Laser-powder bed fusion LPBF techniques can be used to manufacture complex-shaped, thin-walled, hollow, or slender parts. Although the dimensions of the generated components are close to the final measurements, additional machining processes are required to obtain the desired surface finish and dimensional tolerance. The melt pool dynamic during the LPBF operation results in directional gain structures in alloys. The resulting mechanical properties are strongly dependent on the component build orientation, which can affect the machinability of the produced part. This review paper provides knowledge on the role of microstructure in the machinability of LPBF-produced IN718. The effect of grain shape and distribution, grain boundary density on the surface integrity, and resulting cutting forces are investigated

Effect of Powder Bed Fusion Laser Sintering on Dimensional Accuracy and Tensile Properties of Reused Polyamide 11

Polyamide 11 (PA11) is a plant-based nylon made from castor beans. Powder bed fusion laser sintering (PBF-LS) is an additive manufacturing process used for PA11 which allows for the reuse of the unsintered powder. The unsintered powder is mixed with virgin powders at different refresh rates, a process which has been studied extensively for most semi-crystalline polyamides. However, there is lack of information on the effect of using 100% reused PA11 powder and the effect of the number of times it is reused on its own, during powder bed fusion laser sintering. This paper investigates the effect of reusing PA11 powder in PBF-LS and the effect of the number of times it is reused on the dimensional accuracy, density and thermal and tensile properties. From the 100% virgin powder to the third reuse of the powder, there is a decrease in powder wastage, crystallinity and tensile strength. These are associated with the polymerisation and cross-linking process of polymer chains, upon exposure to high temperatures. This results in a higher molecular weight and, hence, a higher density. From the fourth reuse to the tenth reuse, the opposite is observed, which is associated with an increase in high-viscosity unmolten particles, resulting in defects in the PBF-LS parts

Multilayered composite coatings of titanium dioxide nanotubes decorated with zinc oxide and hydroxyapatite nanoparticles: Controlled release of Zn and antimicrobial properties against staphylococcus aureus

This study aimed to decorate the surface of TiO2 nanotubes (TiO2 NTs) grown on medical grade Ti-6Al-4V alloy with an antimicrobial layer of nano zinc oxide particles (nZnO) and then determine if the antimicrobial properties were maintained with a final layer of nano-hydroxyapatite (HA) on the composite. The additions of nZnO were attempted at three different annealing temperatures: 350, 450 and 550 °C. Of these temperatures, 350°C provided the most uniform and nanoporous coating and was selected for antimicrobial testing. The LIVE/DEAD assay showed that ZnCl2 and nZnO alone were >90% biocidal to the attached bacteria, and nZnO as a coating on the nanotubes resulted in around 70% biocidal activity. The lactate production assay agreed with the LIVE/DEAD assay. The concentrations of lactate produced by the attached bacteria on the surface of nZnO-coated TiO2 NTs and ZnO/HA-coated TiO2 NTs were 0.13±0.03 mM and 0.37±0.1 mM, respectively, which was significantly lower than that produced by the bacteria on TiO2 NTs alone, 1.09±0.30 mM (Kruskal–Wallis, P<0.05, n=6). These biochemical measurements were correlated with electron micrographs of cell morphology and cell coverage on the coatings. nZnO on TiO2 NTs was a stable and antimicrobial coating, and most of the biocidal properties remained in the presence of nano-HA on the coating.Plymouth University; University of Derb

On the Lubricity and Comparative Life Cycle of Biobased Synthetic and Mineral Oil Emulsions in Machining Titanium Ti-6Al-4V at Low Cutting Speed

The paper discusses an instrumented tapping test method using a CNC machine tool to compare the lubricity of MWFs by cutting threads in a Ti-6Al-4V alloy at low speed. The method uses a spiral flute tap size typical of industrial practice. A soft synchronous tap holder and spindle mounted dynamometer were incorporated on the machine to measure torque and thrust force. The tapping test method was demonstrated on three groups of MWFs that were commercially available and classified by ASTM E2523-13:2018. The method developed stable results free of chip clogging in tool flutes which could otherwise mask their comparative lubricity. The fully synthetic (FS) group displayed the best lubricity and within this group the FS from renewables (FS-bio) was the best overall. The method was shown to be effective in mitigating biasing effects on lubricity performance due to the generous tool chamfer angle tolerance and was practical and economical to implement. The significance of the results is discussed enabling an understanding of friction effects in tapping using a soft synchronous tap holder. A life cycle assessment of each MWF group found total Greenhouse Gas emitted from the FS group was 17% of the hydrocarbon group whilst FS-bio emitted just 7%

Microstructure Evolution in AM Produced Superalloy Thin Struts at Low Plastic Strain

This paper discusses the microstructures and textures that develop in thin struts fabricated in Inconel 718 using laser melting powder bed fusion. Thin struts 1 mm wide were fabricated in the vertical (ZX) and horizontal build (XY) orientations with respect to the build plate. Three distinct regions of well-defined grain structures with centerline symmetry from the outer border to the center of the inner volume were present, which was attributed to the laser scan strategy, consisting of a fill hatch, inner and outer border scan on each 30 m layer. Using ex situ micro-tensile testing and enhanced Back-Scattered Electrons (BSE) and Electron Back Scattered Diffraction (EBSD), the microstructure evolution to a plastic strain near 1%, in both the ZX and XY build direction was studied in the initial undeformed and final deformed states. For the ZX build direction, grain rotation as well as grain boundary migration was observed and was more significant than in the samples fabricated in XY. This paper discusses the evolution of the microstructure offering valuable insights into the relationship between microstructure and mechanical properties in complex AM produced microstructures

Microstructure Effects on the Machinability of AM-Produced Superalloys

This paper discusses the microstructure effects on the machinability of Inconel 718 by conducting machining tests on an additively manufactured (AM) workpiece with a strongly textured grain structure and a wrought workpiece incorporating a finer and more equiaxed grain structure. The AM workpiece was produced as a thin tube using Laser Melting Powder Bed Fusion and optimal processing conditions for this alloy. A lathe was used to conduct instrumented orthogonal machining tests on the two workpiece materials under dry cut and coolant conditions using a semisynthetic emulsion coolant. The process parameters studied were feed from 0.05 to 0.15 mm/rev and cutting speed from 60 to 120 m/min with a cut time of 2 sec duration for each process condition. Measures for each process condition included cutting forces in the feed and main cut direction, and images of chip forms were obtained. The grain structures of the workpiece materials were characterized using Electron Back Scattered Diffraction (EBSD). New findings suggest that grain structures can significantly affect the machinability of the superalloy at a higher feed for all cutting speeds studied, and insights into the cause are discussed. Other important findings comment on the effectiveness of the coolant as a lubricant for reducing friction in machining

Orientation effects on the fracture behaviour of additively manufactured stainless steel 316L subjected to high cyclic fatigue

In this paper, stainless steel 316L (SS316L) bars were additively manufactured (AM) in three orientations (Z – vertical, XY – horizontal, ZX45 – midway between vertical and horizontal) by using the Laser Powder Bed Fusion Melting (LPBF-M) method. The AM specimens were subjected to load control fatigue testing under full tension and compression (R = -1) at stress amplitudes ± 350, ±400 and ± 450 MPa. The XY and ZX45 printing orientations were found to significantly improve service life. Although similar strain response was found for each orientation when the same stress amplitude was applied, slightly different fracture mechanisms were identified during the post-mortem surface observations

Fatigue Damage Evolution in SS316L Produced by Powder Bed Fusion in Diferent Orientations with Reused Powder Feedstock

Background Metal Laser Powder Bed Fusion Melting (LPBF-M) is considered economically viable and environmentally sustainable because of the possibility of reusing the residual powder feedstock leftover in the build chamber after a partbuild is completed. There is however limited information on the fatigue damage development of LPBF-M samples made from reused feedstock. Objective In this paper, the stainless steel 316 L (SS316L) powder feedstock was examined and characterised after 25 reuses, following which the fatigue damage development of material samples made from the reused powder was assessed. Methods The suitability of the powder to LPBF-M technology was evaluated by microstructural observations and measurements of Hall fow, apparent and tapped density as well as Carr’s Index and Hausner ratio. LPBF-M bar samples in three build orientations (Z – vertical, XY – horizontal, ZX – 45° from the build plate) were built for fatigue testing. They were then subjected to fatigue testing under load control using full tension and compression cyclic loading and stress asymmetry coefcient equal to -1 in the range of stress amplitude from ±300 MPa to ±500 MPa. Results Samples made from reused powder (25 times) in the LPBF-M process exhibited similar fatigue performance to fresh unused powder although a lower ductility for vertical samples was observed during tensile testing. Printing in horizontal (XY) and diagonal (ZX) directions, with reused powder, improved the service life of the SS316L alloy in comparison to the vertical (Z). Conclusions Over the 25 reuses of the powder feedstock there was no measurable diference in the fowability between the fresh (Hall Flow: 21.4 s/50 g) and reused powder (Hall Flow: 20.6 s/50 g). This confrms a uniform and stable powder feeding process during LPBF-M for both fresh and reused powder. The analysis of fatigue damage parameter, D, concluded cyclic plasticity and ratcheting to be the main mechanism of damage

Surface engineering of carbon fibre/epoxy composites with woven steel mesh for adhesion strength enhancement

Effects of surface modifications on the adhesive joint of carbon fibre reinforced polymer (CFRP) were investigated. CFRP surfaces were treated with traditional methods, such as acetone cleaning, sanding, grit blasting and peel ply. As a novel type of surface modification, stainless steel 316 wire mesh was co-cured on the CFRP laminate during the manufacturing stage. The surface topography of CFRP adherends was analysed through microscopic imaging. Surface roughness values were measured using a surface roughness tester. Contact angle measurements of probe liquids were performed to calculate surface energy. All adherends were bonded with a low viscosity epoxy adhesive. The single lap shear tests were used to measure the bonding strength of the CFRP joints. The experimental results showed that the highest average shear strength (24.2 MPa) was achieved for metal mesh modified joints. This amounted to a 101.7% increase compared with acetone-cleaned specimens. There was no statistically significant difference in shear strength among joints subjected to traditional surface treatments. Fracture surface images were analysed and correlation between surface treatment and lap shear strength was critically discussed

Characterisation of impact resistance of composite reinforced by hybridised carbon-flax fibres in polyfurfuryl alcohol resin

Research on polyfurfuryl alcohol (PFA) bioresin has increased due to its unique flame-retardant property. This study utilised the hybridisation of carbon and flax fibres in combination with PFA resin to make fibre-reinforced polymer (FRP) composites. The samples were tested for low-velocity impact at an energy of 20 Joules. Results showed that the fibres-hybridised PFA-based sample had higher energy absorption compared to the carbon-fibre only sample. More importantly, the hybridised sample retained almost the same amount of flexural modulus after impact, while carbon fibre only FRP sample lost more than 20% of its flexural modulus. This highlights the benefits of hybridisation in impact resistance and retention of mechanical properties post-impact in the presence of PFA resin, which is consistent with previous reports of epoxy-based FRP composites. In conclusion, the combination of fibre hybridisation and PFA bioresin presents a promising solution to meet strict flame-retardant requirements, impact resistance needs, and reduce environmental carbon footprint for engineering applications