Dry friction during sliding of AA1050 on AA2024 at elevated temperature

The friction development between Aluminium AA1050 and Dural AA2024 is studied. Continuous full surface rotational sliding experiments of AA1050 over AA2024 were performed at temperatures between 150 and 350 °C and at contact pressures between 2 and 15 MPa. The required torque was measured as function of rotation angle. A model is set up for the evolution of the friction coefficient, which takes into account temperature, normal pressure and sliding distance. Validation of the model is done by implementation in a finite element program and reproduction of observed behaviour

Drying strategies to reduce the formation of hydrogen porosity in Al alloys produced by Additive Manufacturing

Laser powder bed fusion (L-PBF) is an additive manufacturing (AM) technology using a high-power laser to selectively melt metal powders for building complex parts. Coping with contamination in the powder is usually a challenge for this technology. Gases entrapped in the material in contact with the laser create a plasma of impurities that can lead to porosity. This contamination is usually coming from moisture, organics and adsorbed gases from the atmosphere (i.e. oxygen and nitrogen). In this work, two drying strategies to reduce the formation of porosity will be investigated

In-situ mechanical and microstructural characterization of miniaturized Al-Mg-Sc-Zr and AlSi10Mg specimens processed by laser powder-bed fusion (PBF-LB)

Manufacturing through powder-bed fusion laser-beam (PBF-LB) enables innovative part design strategies, facilitating weight reduction, and capitalizing on the metallurgical conditions developed during the manufacturing of designed alloys. Consequently, Al-based light alloys hold enormous potential for reducing fuel consumption in the transport industry. Fabricating such small features has a significant impact on heat dissipation, thereby affecting microstructure, porosity, and, consequently, mechanical properties. This study proposesthe use of near-net shape miniaturized tensile specimens in both horizontal and vertical orientations to characterize Al-Mg-Sc-Zr, commercially known as Scalmalloy®, and AlSi10Mg, two aluminum alloys typicallyemployed in PBF-LB. The size and distribution of both grains and pores were analyzed and compared, with Al-Mg-Sc-Zr exhibiting a more competitive set of properties compared to AlSi10Mg. This difference also influences mechanical properties. Al-Mg-Sc-Zr demonstrated double the Ultimate Tensile Strength (UTS) of AlSi10Mg (450MPa versus 225 MPa) and higher hardness values (142 HV30 versus 75 HV30), with similar elongation in both alloys (approximately 12–16%), owing to its fine microstructure and low porosity of the near-net shape miniaturized tensile specimens. Neither material exhibited any form of anisotropy. In-situ SEM tensile tests were conducted to monitor damage evolution, allowing continuous observation of crack nucleation and propagation through imperfections typically encountered in PBF-LB. Despite differences in static strength, the fracture sur-faces of the samples displayed a ductile behavior in both materials

New process optimization framework for laser assisted tape winding of composite pressure vessels: Controlling the unsteady bonding temperature

This paper presents an effective process optimization methodology for laser assisted tape winding (LATW) of complex part geometries by means of a numerical optical-thermal model. A winding path on the cylindrical and ellipsoidal (dome) part of a pressure vessel is considered with varying tooling curvature. First, the process model output is verified with the literature data based on the laser intensity distribution. Then, the transient laser irradiation and temperature distributions on the tape and substrate are described comprehensively. It is shown that the maximum laser intensity increases approximately by 80% and the process (bonding) temperature changes by 80 °C at the intersection of the cylindrical and dome section of the pressure vessel. In order to keep the transient process temperature constant, a robust optimization scheme is utilized by means of a genetic algorithm. The design variable is determined as the total laser power and temperature constraints are defined. The proposed optimization methodology regulates the temperature within 1.5 °C variation with respect to the desired value. In order to compensate the transient local curvature effects on the process temperature, the total laser power varies approximately between 30% and 175% of the reference (non-optimized) case

The influence of physical ageing on yielding and failure of unplasticised poly(vinyl chloride) pipes

Physical ageing can cause significant changes of the mechanical properties of unplasticised poly(vinyl chloride) pipes during service life. A model is presented which incorporates these changes and can predict long-term failure times quantitatively, based on short term test

3D Numerical modeling of laser assisted tape winding process of composite pressure vessels and pipes-effect of winding angle, mandrel curvature and tape width

Advanced thermoplastic composites manufacturing using laser assisted tape placement or winding (LATP/LATW) is a challenging task as monitoring and predicting nip point (bonding) temperature are difficult especially on curved surfaces. A comprehensive numerical analysis of the heat flux and temperature distribution near the nip point is carried out in this paper for helical winding of fiber reinforced thermoplastic tapes on a cylindrically shaped mandrel. An optical ray-tracing technique is coupled with a numerical heat transfer model in the process simulation tool. The developed optical-thermal model predictions were compared with experimental data available in literature to validate its effectiveness. The influences of winding/placement angle, mandrel curvature and tape width on the incident angles, the laser absorbed intensity, and the process temperature distribution are studied extensively using the validated model. Winding/placement angle has a considerable effect on the temperature distribution. Increase in winding angle results in a higher temperature for tape due to more reflections coming from the substrate. On the other hand, substrate temperature decreases as the winding angle increases due to a decrease in the laser incident angles based on the local surface curvature. An increase in mandrel curvature results in higher nip point temperatures for substrate and lower one for tape. Different mandrel sizes for 90° placement path do not have a strong effect on the substrate process temperature as for other winding angles because of less curvature change of the corresponding irradiated area. Tape width causes local temperature variations at the edges of the tape/substrate. In order to obtain the desired process temeprature during LATW or LATP processes, the laser intensity distribution on the tape and substrate surfaces should be regulated

Optical characterization of fiber-reinforced thermoplastic tapes for laser-based composite manufacturing

The optical properties of unidirectional (UD) fiber reinforced thermoplastic (FRTP) tapes were characterized to enable a better description of the heating phase in laser-based manufacturing process of FRTP composites. The tapes included PP-GF (glass-fiber) 45% fiber volume content (FVC), PVDF-CF (carbon-fiber) 45% FVC, PVDF-CF 60% FVC and PA12-CF 60% FVC. The transmittance of the tapes was found to be 0.00–0.2% whereas the reflectance was 9.8–11.8% corresponding to a refractive index of 1.91–2.05. The anisotropic reflectance measurements, as obtained through a gonioreflectometry, were used to fit the bidirectional reflectance distribution function (BRDF) for the first time. The obtained BRDF parameters σt and σf had a range of 0.1–0.18 and 0.006–0.015, respectively, for different tapes. Employing the new BRDF parameters empowers a more accurate prediction and optimization of the process settings of laser-based composite manufacturing. Laser-assisted tape placement/winding (LATP/LATW), anisotropic reflection, unidirectional tape (UD), bidirectional reflection modelling

Measuring the spreadability of pre-treated and moisturized powders for laser powder bed fusion

For AM processes—specifically Laser Powder Bed Fusion (L-PBF) processes—powder flowability is essential for the product quality, as these processes are based on a thin layer spreading mechanism. However, the available techniques to measure this flowability do not accurately represent the spreading mechanism. Hence, this paper presents two novel applicator tools specifically designed to test the spreadability of L-PBF powders in thin layer application. The results were checked by running standard tests to analyze the powder morphology, moisture content, chemical composition and flowability using the Hall-flowmeter. For this study, four common L-PBF metal powders were selected: Inconel 718, Ti6Al4V, AlSi10Mg and Scalmalloy. From the as-received state, drying (vacuum and air) and moisturizing treatments were applied to compare four humidity states and investigate the feasibility of pre-treating the powders to remove moisture, which is known to cause problems with flowability, porosity formation and enhanced oxidation. The tests reveal that AlSi10Mg is the most susceptible alloy to moisture and oxygen pick-up, considerably decreasing the spreadability and relative density on the build platform. However, the results also reveal how challenging the direct measurement of moisture levels in metal powders is

Printing of complex free-standing microstructures via laser-induced forward transfer (LIFT) of pure metal thin films

A combined approach of laser-induced forward transfer (LIFT) and chemical etching of pure metal films is studied to fabricate complex, free-standing, 3-dimensional gold structures on the few micron scale. A picosecond pulsed laser source with 515 nm central wavelength is used to deposit metal droplets of copper and gold in a sequential fashion. After transfer, chemical etching in ferric chloride completely removes the mechanical Cu support leaving a final free-standing gold structure. Unprecedented feature sizes of smaller than 10 μm are achieved with surface roughness of 0.3 to 0.7 μm. Formation of interfacial mixing volumes between the two metals is not found confirming the viability of the approach

Phase-separated mixed-macromer hydrogel networks and scaffolds prepared by stereolithography

Mixed-macromer networks prepared by using biodegradable functionalized oligomers and a combinatorial chemistry approach have shown to be networks with high water uptake, excellent mechanical properties, and good cell adhesion. This may be due to phase separation within these networks, although this has not been investigated to date. Here we show that these networks are indeed phase-separated. In differential scanning calorimetry experiments, Tg values of each constituent material used in the networks were observed. Furthermore, atomic force microscopy and X-ray diffraction experiments showed phase separation of the crystalline and amorphous phases of the networks in the dry state. In the hydrated state, however, the crystalline phase was not visible. Subsequently, we prepared designed porous 3D structures of the mixed-macromer networks by using stereolithography. We have shown that such structures have excellent mechanical properties with compression moduli of 20–170 kPa. Unlike conventional synthetic hydrogels such as poly(ethylene glycol) hydrogels, these structures do not fail under compression and return to their original dimensions after re-equilibration in water. This make these materials excellent candidates for soft tissue engineering of tissues such as menisci or intervertebral discs. Finally, a designed porous meniscus implant was prepare