Method of Multiaxis Three-Dimensional Printing with Intralayer Height Variation for Stairstep Effect Compensation.

The recent developments in multiaxis three-dimensional (3D) printing have got a large potential for expanding the capability of material extrusion based methods. Especially curved and nonplanar methods can improve buildability, surface quality, and mechanical performance. However, the challenges that arise from using them complicate their deployment. In this article, we propose a hybrid planar method, based on varying the layer height and deposition speed in combination with tool reorientation, that allows us to get a lot of the same benefits that come from using nonplanar layers. The goal of the method is to keep the deposition constant regardless of the overhang angle. This is achieved by simultaneous control of layer height, deposition speed, and tool orientation. The method is the most beneficial for large-scale, single-wall 3D printing, such as clay, concrete, and other composites. The main restriction of the method depends on the minimum/maximum ratio between the nozzle diameter and layer height. A description of the method is provided, and sample objects are evaluated either as trajectories or as test prints. The claims are confirmed by microscopy measurement of the contact patch width. The method as presented allows printing of overhangs up to 82.34°, can be applied to complex geometry without difficulties, and further possibilities of limit expansion are discussed. The contact patch width decreases only by 20% at the 80° of overhang.

Tribological behaviour of 3D printed materials for small joint implants: A pilot study.

Additive manufacturing is a progressive method in endoprosthetics enabling customisation of implants. However, the challenge is to design articulating surfaces that are wear resistant in a long term. To tackle this challenge, it is necessary to understand the interaction between the surfaces and the lubricant synovial fluid as well as the mechanism of lubrication film formation. In this study we observed three synovial fluid constituents (albumin, γ-globulin a hyaluronic acid) in the contact area simultaneously with the coefficient of friction (CoF). Two metal alloys, CoCrMo and Ti6Al4V covered by DLC, were selected for the experiments as they are both suitable for the additive technology and commonly used in implants manufacturing. The tests were running on a custom-made pin-on-plate tribometer equipped for optical fluorescence measurements. The test apparatus allowed reciprocating motion and observation of the contact area. Our results showed differences in the pace of the CoF increase between the alloys and differences between the samples manufactured by the conventional and the additive manufacturing method. Both the conventionally and additively manufactured CrCrMo samples showed a stable CoF values from the beginning of the experiments: 0.66 (SD 0.02) for the conventional manufacturing CrCrMo samples and 0.53 (SD 0.01) for the additive manufacturing CrCrMo samples. The Ti6Al4V/DLC samples showed a stable CoF values similar to those of the CoCrMo samples not until the 240 s of experiment. These results are related to the protein formation in the contact areas as suggested by a similar increasing trend of the individual synovial fluid constituents in the contact. Increasing protein amounts in the contact led to CoF increase. There were also differences in the ratios of the individual constituents, where both the CoCrMo and the Ti6Al4V/ DLC samples manufactured additionally showed lower concentrations of γ-globulin and hyaluronic acid. These pilot results, on the one hand, support the potential of the additive manufacturing in the implantology and, on the other hand, demonstrate the application of a method suitable for the analysis of the lubricant behaviour in the contact. The method is limited in using the intensity of the emitted light to observe the behaviour of the lubricant film. Future development of the method will require a direct quantification of film thickness.

Computational Approaches of Quasi-Static Compression Loading of SS316L Lattice Structures Made by Selective Laser Melting.

Additive manufacturing methods (AM) allow the production of complex-shaped lattice structures from a wide range of materials with enhanced mechanical properties, e.g., high strength to relative density ratio. These structures can be modified for various applications considering a transfer of a specific load or to absorb a precise amount of energy with the required deformation pattern. However, the structure design requires knowledge of the relationship between nonlinear material properties and lattice structure geometrical imperfections affected by manufacturing process parameters. A detailed analytical and numerical computational investigation must be done to better understand the behavior of lattice structures under mechanical loading. Different computational methods lead to different levels of result accuracy and reveal various deformational features. Therefore, this study focuses on a comparison of computational approaches using a quasi-static compression experiment of body-centered cubic (BCC) lattice structure manufactured of stainless steel 316L by selective laser melting technology. Models of geometry in numerical simulations are supplemented with geometrical imperfections that occur on the lattice structure's surface during the manufacturing process. They are related to the change of lattice struts cross-section size and actual shape. Results of the models supplemented with geometrical imperfections improved the accuracy of the calculations compared to the nominal geometry.

Interface Behavior and Interface Tensile Strength of a Hardened Concrete Mixture with a Coarse Aggregate for Additive Manufacturing.

3D concrete printing technology (3DCP) is a relatively new technology that was first established in the 1990s. The main weakness of the technology is the interface strength between the extruded layers, which are deposited at different time intervals. Consequently, the interface strength is assumed to vary in relation to the time of concrete casting. The proposed experimental study investigated the behavior of a hardened concrete mixture containing coarse aggregates that were up to 8 mm in size, which is rather unusual for 3DCP technology. The resulting direct tensile strength at the layer interface was investigated for various time intervals of deposition from the initial mixing of concrete components. To better understand the material behavior at the layer interface area, computed tomography (CT) scanning was conducted, where the volumetric and area analysis enabled validation of the pore size and count distribution in accordance with the layer deposition process. The analyzed CT data related the macroscopic anisotropy and the resulting crack pattern to the temporal and spatial variability that is inherent to the additive manufacturing process at construction scales while providing additional insights into the porosity formation during the extrusion of the cementitious composite. The observed results contribute to previous investigations in this field by demonstrating the causal relationships, namely, how the interface strength development is determined by time, deposition process, and pore size distribution. Moreover, in regard to the printability of the proposed coarse aggregate mixture, the specific time interval is presented and its interplay with interface roughness and porosity is discussed.

Effect of Laser Parameters on Processing of Biodegradable Magnesium Alloy WE43 via Selective Laser Melting Method.

The global aim of the theme of magnesium alloy processing by the selective laser melting technology is to enable printing of replacements into the human body. By combining the advantages of WE43 magnesium alloy and additive manufacturing, it is possible to print support structures that have very similar properties to human bones. However, printing magnesium alloy parts is very difficult, and the printing strategies are still under development. Knowledge of weld deposit behaviour is needed to design a complex printing strategy and still missing. The main aim of the manuscript is the find a stable process window and identify the dependence of the weld deposit shape and properties on the laser power and scanning speed. The range of the tested parameters was 100-400 W and 100-800 mm/s for laser power and scanning speed. The profilometry and light microscopy were used to verify the continuity and shape evaluation. The microhardness and EDX analysis were used for the detailed view of the weld deposit. The manuscript specifies the weld deposit dimensions, their changes depending on laser power and scanning speed, and the continuity of the weld tracks. The stable weld deposits are made by the energy density of 5.5-12 J/mm2. Thin walls were also created by layering welds to determine the surface roughness scattering (Ra 35-60) for various settings of laser power and scanning speed.

Effect of Process Parameters and High-Temperature Preheating on Residual Stress and Relative Density of Ti6Al4V Processed by Selective Laser Melting.

The aim of this study is to observe the effect of process parameters on residual stresses and relative density of Ti6Al4V samples produced by Selective Laser Melting. The investigated parameters were hatch laser power, hatch laser velocity, border laser velocity, high-temperature preheating and time delay. Residual stresses were evaluated by the bridge curvature method and relative density by the optical method. The effect of the observed process parameters was estimated by the design of experiment and surface response methods. It was found that for an effective residual stress reduction, the high preheating temperature was the most significant parameter. High preheating temperature also increased the relative density but caused changes in the chemical composition of Ti6Al4V unmelted powder. Chemical analysis proved that after one build job with high preheating temperature, oxygen and hydrogen content exceeded the ASTM B348 limits for Grade 5 titanium.

Dynamic Loading of Lattice Structure Made by Selective Laser Melting-Numerical Model with Substitution of Geometrical Imperfections.

Selective laser melting (SLM) is an additive technology that allows for the production of precisely designed complex structures for energy absorbing applications from a wide range of metallic materials. Geometrical imperfections of the SLM fabricated lattice structures, which form one of the many thin struts, can lead to a great difference in prediction of their behavior. This article deals with the prediction of lattice structure mechanical properties under dynamic loading using finite element method (FEA) with inclusion of geometrical imperfections of the SLM process. Such properties are necessary to know especially for the application of SLM fabricated lattice structures in automotive or aerospace industries. Four types of specimens from AlSi10Mg alloy powder material were manufactured using SLM for quasi-static mechanical testing and determination of lattice structure mechanical properties for the FEA material model, for optical measurement of geometrical accuracy, and for low-velocity impact testing using the impact tester with a flat indenter. Geometries of struts with elliptical and circular cross-sections were identified and tested using FEA. The results showed that, in the case of elliptical cross-section, a significantly better match was found (2% error in the Fmax) with the low-velocity impact experiments during the whole deformation process compared to the circular cross-section. The FEA numerical model will be used for future testing of geometry changes and its effect on mechanical properties.

Selective Laser Melting Strategy for Fabrication of Thin Struts Usable in Lattice Structures.

This paper deals with the selective laser melting (SLM) processing strategy for strut-lattice structure production which uses only contour lines and allows the porosity and roughness level to be managed based on combination of the input and linear energy parameters. To evaluate the influence of a laser scanning strategy on material properties and surface roughness a set of experiments was performed. The single welds test was used to find the appropriate processing parameters to achieve continuous welds with known width. Strut samples were used to find a suitable value of weld overlapping and to clarify the influence of input and linear laser energy on the strut porosity and surface roughness. The samples of inclined hollow struts were used to compare the wall thickness with single welds width; the results showed about 25% wider welds in the case of a hollow strut. Using the proposed SLM strategy it is possible to reach a significantly lower porosity and surface roughness of the struts. The best results for struts with an inclination of 35.26° were achieved with 25% track overlapping, input energy in the range from 9 J to 10.5 J and linear energy Elin from 0.25 to 0.4 J/mm; in particular, the relative density of 99.83% and the surface roughness on the side of the strut of Ra 14.6 µm in an as-built state was achieved.

Influence of Scanning Strategies on Processing of Aluminum Alloy EN AW 2618 Using Selective Laser Melting.

This paper deals with various selective laser melting (SLM) processing strategies for aluminum 2618 powder in order to get material densities and properties close to conventionally-produced, high-strength 2618 alloy. To evaluate the influence of laser scanning strategies on the resulting porosity and mechanical properties a row of experiments was done. Three types of samples were used: single-track welds, bulk samples and samples for tensile testing. Single-track welds were used to find the appropriate processing parameters for achieving continuous and well-shaped welds. The bulk samples were built with different scanning strategies with the aim of reaching a low relative porosity of the material. The combination of the chessboard strategy with a 2 × 2 mm field size fabricated with an out-in spiral order was found to eliminate a major lack of fusion defects. However, small cracks in the material structure were found over the complete range of tested parameters. The decisive criteria was the elimination of small cracks that drastically reduced mechanical properties. Reduction of the thermal gradient using support structures or fabrication under elevated temperatures shows a promising approach to eliminating the cracks. Mechanical properties of samples produced by SLM were compared with the properties of extruded material. The results showed that the SLM-processed 2618 alloy could only reach one half of the yield strength and tensile strength of extruded material. This is mainly due to the occurrence of small cracks in the structure of the built material.

Use of digital technologies for nasal prosthesis manufacturing.

BACKGROUND AND AIM: Digital technology is becoming more accessible for common use in medical applications; however, their expansion in prosthetic and orthotic laboratories is not large because of the persistent image of difficult applicability to real patients. This article aims to offer real example in the area of human facial prostheses. TECHNIQUE: This article describes the utilization of optical digitization, computational modelling, rapid prototyping, mould fabrication and manufacturing of a nasal silicone prosthesis. This technical note defines the key points of the methodology and aspires to contribute to the introduction of a certified manufacturing procedure. DISCUSSION: The results show that the used technologies reduce the manufacturing time, reflect patient's requirements and allow the manufacture of high-quality prostheses for missing facial asymmetric parts. The methodology provides a good position for further development issues and is usable for clinical practice. Clinical relevance Utilization of digital technologies in facial prosthesis manufacturing process can be a good contribution for higher patient comfort and higher production efficiency but with higher initial investment and demands for experience with software tools.

Finite element analysis for the evaluation of the structural behaviour, of a prosthesis for trans-tibial amputees.

The finite element analysis (FEA) has been identified as a useful tool for the stress and strain behaviour determination in lower limb prosthetics. The residual limb and prosthetic socket interface was the main subject of interest in previous studies. This paper focuses on the finite element analysis for the evaluation of structural behaviour of the Sure-flex™ prosthetic foot and other load-bearing components. A prosthetic socket was not included in the FEA. An approach for the finite element modelling including foot analysis, reverse engineering and material property testing was used. The foot analysis incorporated ground reaction forces measurement, motion analysis and strain gauge analysis. For the material model determination, non-destructive laboratory testing and its FE simulation was used. A new, realistic way of load application is presented along with a detailed investigation of stress distribution in the load-bearing components of the prosthesis. A novel approach for numerical and experimental agreement determination was introduced. This showed differences in the strain on the pylon between the experimental and the numerical model within 30% for the anteroposterior bending and up to 25% for the compression. The highest von Mises stresses were found on the foot-pylon connecting component at toe off. Peak stress of 216MPa occurred on the posterior adjusting screw and maximum stress of 156MPa was found at the neck of the male pyramid.

A biomechanical study of a suture between the deltoid muscle and a free tendon graft for reconstruction of the elbow extension.

AIMS: It is possible to reconstruct the elbow motion in tetraplegic patients using the posterior portion of the deltoid muscle. In this surgery however, it is a problem to achieve a firm suture between the deltoid muscle and the tendon graft which extends the muscle and is sewn in order to compensate for the plegic musculus triceps brachii function. This study assesses two methods of attachment between muscle and free tendon graft from the biomechanical point of view. METHODS: The assessment was made on 7 fresh-frozen cadaveric samples where the rear portion of the deltoid muscle was sewn with the strip of fascia lata (A1-A7) and 7 samples (B1-B7) where the free tendon graft was attached with a strengthened part of deltoid fascia. The character of the attachment defect was evaluated as strength and elongation parameters using the device Zwick Z020-TND. RESULTS: The ANOVA showed a statistically significant greater suture solidity connecting the muscle and tendon for group B (B1-B7) than group A. The deformation of the actual suture location was smaller in group B than the deformation of attachment surroundings. CONCLUSION: From the biomechanical solidity point of view, it is more efficient to use the strengthened fascia of the deltoid muscle on its inner side for the suture with the tendon graft for reconstruction of the elbow extension in tetraplegic patients.