Overhang Surfaces Research Articles

Research on the redesign of precision tools and their manufacturing process based on selective laser melting (SLM)

Purpose – This paper aims to verify that additive manufacturing technology could be used for the redesign and rapid manufacturing of tools and determine whether the mechanical performance of such tools can satisfy the practical operating requirements. Design/methodology/approach – A special key was selected as the research object in this paper. The special key was innovatively redesigned and manufactured directly using selective laser melting (SLM). The function and critical geometries of the special key were first analysed, which was followed by discussions on the geometrical constraints in the manufacturing of typical geometrical features using SLM technology. Next, the special key was redesigned based on the SLM geometrical constraints and the functional requirements. Finally, the key was manufactured using SLM, and the mechanical performance characteristics of the key were determined. Findings – The minimal geometrical feature was 0.2 mm when manufacturing thin walls using SLM. The reliable building angle of an overhanging surface was 40°. The top surface quality of the part could be greatly improved through laser surface re-melting. The volume of the redesigned special key based on the SLM process was only one-third to one-fourth of the original key. The mechanical properties, such as tensile strength and micro-hardness, of the samples manufactured using SLM were able to reach the practical operating requirements. Originality/value – It is completely feasible to redesign and manufacture precision tools based on the innovative approach of SLM. The advantages of the redesigned tools includes the lack of design restrictions that hinder traditional manufacturing methods, material savings, ability to produce tools that cannot be easily copied and rapid production speed for a small number of tools.

Surface quality of the curved overhanging structure manufactured from 316-L stainless steel by SLM

The efficiency of fabricating an overhanging structure by selective laser melting (SLM) is an important indicator of the performance of metallic parts. This is due to the fact that defects such as warpage and adherent dross may occur during fabrication of the curved surfaces of overhanging structures. In order to investigate the optimum conditions for fabrication of the curved surfaces of the overhanging structures, experiments were carried out using 316-L stainless steel powder. Initially, the almost 100 % dense parts were fabricated. Then, a model that has a circular curved surface along the Z axis was designed. For a given fabrication depth of 25 μm, several overhanging structures were produced when the laser scanning energy input ranges from 0.15 to 0.6 J/mm. Results show that the upper surface of the almost 100 % dense cube fluctuates like ripples and that the fabrication quality of the curved surface of the overhanging structure varies greatly depending on the energy input and the obliquity angle. For a given energy input of 0.2 J/mm, the obliquity angle for fabricating a totally overhanging surface is as low as 30°. The warpage and adherent dross grow with an increase in the energy input and a decrease in the obliquity angle. Warpage may accumulate, and the accumulated warpage of many layers significantly exceeds the predetermined thickness of the layer. All the four overhanging structures fabricated using varying energy inputs have the following four zones: no dross surface, dense-sinking transition surface, totally sinking surface, and forming failure surface. In the overhanging structures, fabricated with varying laser energy parameters, the angle corresponding to each region was different. The quality of the overhanging surface can be improved by reducing the laser energy. Additionally, a better overhanging surface can be obtained by increasing the obliquity angle. The variation trend of the roughness Rz was almost the same as that of Ra, but the variation range of Rz was much larger than that of Ra. Finally, a foldable abacus with several curved-surface overhanging structures was fabricated to verify the research results. Fundamental methods for controlling and optimizing the SLM-based direct fabrication of curved surfaces of overhanging structures are proposed in this paper, from the perspectives of crafting and design.

Modeling Pressure Stability and Contact-Angle Hysteresis of Superlyophobic Surfaces Based on Local Contact Line

Superlyophobic surfaces (SLS) are a promising versatile platform for planar microfluidics due to their excellent nonwetting and low flow-resistance performances for almost any liquid; however, for micro fabrication and practical application, the nonwetting performances, especially pressure stability and contact-angle hysteresis (CAH), are vital parameters yet still difficult to predict well. On the basis of the Laplace pressure formulation and the pinning condition on typical overhang surface structures, we obtained a universal expression to calculate the critical transition pressure and hence proposed two dimensionless parameters as criteria for the Cassie–Baxter state. We also propose a new model for the CAH of both water and oil on SLS by introducing contact line fraction. The theoretical predications are well supported by the experimental data on the SLS with microfabricated T-shape microstructures. These achievements on modeling SLS pressure stability and CAH may shed new light for designing SLS with...

Design optimization of supports for overhanging structures in aluminum and titanium alloys by selective laser melting

Selective laser melting (SLM) process allows fabricating strong, lightweight and complex metallic structures. To successfully produce metallic parts by SLM, additional structures are needed to support overhanging surfaces in order to dissipate process heat and to minimize geometrical distortions induced by internal stresses. However, these structures are often massive and require additional post-processing for their removal. A minimization of support structures would therefore significantly reduce manufacturing and finishing efforts and costs. This study investigates the manufacturability of overhanging structures using optimized support parts. An experimental study was performed to identify the optimal self-supporting overhanging structures using Taguchi L36 design. Experimental results revealed that with optimized supports it is possible to build non-assembly mechanism with overhang surfaces. However, it is necessary to correctly orientate the part in the SLM machine in order to build it with a minimal support structure so to obtain the best trade-off between production time, cost, and accuracy.

Research on the fabricating quality optimization of the overhanging surface in SLM process

Overhanging surface is inherent geometric restraint during selective laser melting (SLM), which is suitable for various complex parts fabrication. In order to improve the fabricating quality of overhanging surface, a series of experiments were designed to investigate the effects of inclined angle, scanning speed, laser power, accumulated residual stress, and scanning vector length on overhanging surface fabrication. Analysis found that overhanging surface would warp easier when the inclined angle and the scanning speed became smaller and the warping trend will be larger as the laser power became larger. The relationships of laser power, scanning speed, and the critical inclined angle were mutual restraint, that is, larger inclined angle will be designed when the laser power becomes larger and scanning speed gets smaller, or vice versa: the selection of the fabricating parameters will be determined by established inclined angle of the overhanging surface. More serious warp would happen as the processing layers increased as a result of residual stress accumulation, and it was found that longer scanning vector were more helpful to stress accumulation, leading to more serious warp than shorter vector. At last, two effective methods were adopted to optimize overhanging surface fabrication, including adjusting part orientation to improve the inclined angle at the key position, and controlling regional parameters to reduce energy input. Above two ways were adopted to manufacture complex parts with typical overhanging surface, the results proved that adjusting part orientation and controlling regional parameters were effective ways to improve the fabricating quality of overhanging surface. In this study, the basis for building overhanging surface by SLM was provided from the view of process and design, and the preliminary solutions were proposed to manufacture complex metal parts with lower risk.

Wind-induced forces on eaves of low buildings

Wind standards and codes of practice specify design pressure coefficients for eaves of low buildings with gabled roofs of angles smaller than 10°. The paper presents the results of a recent experimental study on the effects of wind on eaves of gabled-roof buildings with higher roof slopes, namely 4:12 and 12:12. The experiments were carried out in the boundary layer wind tunnel of the Building Aerodynamics Laboratory of the Centre for Building Studies under conditions of simulated open country exposure. Both local and area-averaged pressures were measured for 36 azimuths. Simultaneous measurements of each pair of upper and lower surface taps were also carried out in order to evaluate the critical forces acting on various overhang zones. Area-averaged pressures were measured as well. The study has found that the 4:12-sloped overhang is the most severely loaded, particularly as far as suction on the upper surface near the gable is concerned. Positive pressures on the upper surface are higher for the 12:12 slope, as expected from wind flow considerations. However, lower eave surfaces are subjected to higher pressures and suctions for the 4:12 slope. The effect of building height on the pressure coefficients measured on either overhang surface is negligible, as also found previously for the case of eaves of almost flat roofs. Pressures measured on lower eave surfaces compare generally well with those measured on the walls, although some positive peaks are significantly higher under the eaves. Comparisons between actual uplift force coefficients and the presently used values show the significant overestimation that occurs in some cases when the current wind standards are used. The paper makes appropriate recommendations for design provisions.

The development of an Early Ordovician hard ground community in response to rapid sea‐floor calcite precipitation

The Kanosh Shale (Upper Arenig, Lower Ordovician) of west-central Utah. USA. contains abundant carbonate hardgrounds and one of the earliest diverse hardground communities. The hardgrounds were formed through a combination of processes including the development of early digenetic nodules in clay sediments which were exhumed and concentrated as lags by storms. These cobble deposits. together with plentiful biogenic metrical. were cemented by inorganically precipitated calcite on the sea floor. forming intraformational conglomerate hardgrounds. Echinoderms may have -played a critical role in the development of hardground faunas since their disarticulated calcite ossicles were rapidly cemented by syntaxial overgrowths. forming additional cobbles and hardgrounds. The echinoderms thus may have taphonomically facilitated the development of some of the hard substrates they required. A significant portion of the hardground cements may have been derived from the early dissolution of aragonitic mollusk shells. Kanosh hardground species include the earliest bryozoans recorded on hardgrounds and large numbers of stemmed echinoderms. primarily rhipidocystid cocrinoids. Bryozoans and echinoderms covered nearly equal areas of the hardground surfaces. and there was a distinct polarization between species which preferred the upper. exposed portions of the hardgrounds and others which were most common on undercut. overhang surfaces. The Kanosh Shale hardground fossils combine elements of Late Cambrian assemblages and Middle Ordovician faunas, thus confirming predicted trends in hardground community evolution. especially the replacement of cocrinoids by bryozoans and. to a lesser extent, by other stemmed echinoderms, especially crinoids. The Kanosh community marks the transition from the Cambrian Fauna to The Paleozoic Fauna in The hardground ecosystem. *Carbonate hardgrounds, aragonite dissolution, calcite cement, Echinodermara, Trepostomata, Nicholsonclla. Dianulites. Porifpra. taphonomic facilitation, Utah. Pogonip Group, Kanosh Shale. Ordovician.

Detection and Avoidance of a Cuprous Oxide Antifouling Paint by Bivalve and Gastropod Larvae (Mollusca)

The reactions of the larvae of seven species of molluscs to a cuprous oxide antifouling paint were investigated in the laboratory. These included larvae of the bivalves Mytilus planulatus Lamarck, Lasaea australis Lamarck, one other unidentified bivalve species; and four other unidentified gastropods. Brief descriptions are given of the unidentified species. By means of currents formed by the foot or tentacle cilia, crawling larvae were able to detect the antifouling paint before they reached it. The behavioural responses included agitated foot or tentacle movements, temporary retraction of larvae into their shells, and turning movements. In six of the species this was followed eventually by more permanent retraction of the larvae into their shells, where they remained stationary for at least 5 min. It is suggested that if similar reactions occurred on a vertical or overhanging surface, such as a ship's hull, the larvae would fall away passively since in the experiments their attaching organs (feet) were automatically withdrawn once the larvae retracted into their shells. This might account for the relative absences of molluscs on ships docked in Sydney Harbour shortly after their antifouling paints have failed, although other groups of fouling organisms have already begun to colonize them.