Study of the lateral pass width for conventional and ultrasonic vibrations-assisted ball burnishing on Ti-6Al-4V specimens

Ball burnishing is a technological finishing process based on plastic deformation of the objective surface by means of a hard ball gliding over it. Along with its easiness of application, possible on the same machine where machining was performed [1], burnishing is a comprehensive process able to achieve surface roughness improvements, and compressive residual stresses up to deep layers of the material [2]. Burnishing results have proved to be depending of a proper selection of parameters, which must be correctly controlled during the process. That is the case of burnishing force or the number of passes [3]. Among these parameters, the lateral pass width has proved to be influential on the surface roughness results, due to the behavior that most materials show when being plastically deformed. In effect, the applied force makes the material to flow to the borders of the burnishing imprint, giving way to a pile-up effect. This paper deals with indentation experiments on Ti-6Al-4V to deepen in the burnishing process of this material. Single burnishing imprints are geometrically characterized combining different levels of force, number of passes, and comparing the conventional process with that assisted with vibrations. An optimal lateral pass width is thus determined, and technological recommendations are made for future applications of the process.Peer ReviewedPostprint (author's final draft

Comparison of thermal performance of 3D printer liquefiers through finite element models

Open source 3D printers have experienced an intense expansion during the last years, mainly because of their accessibility and the vast availability of information thanks to user communities. This fact presents researchers with a perfect context for hardware innovation, by improving the overall printing process, also in terms of durability of the printing machine. A 3D printer liquefier must transmit heat to the thermoplastic material in order to extrude it, reaching temperatures above 200 degrees for some materials like ABS on the tip of the nozzle. The design of the heating process must comply with keeping the balance between proper heating of the material and controlling the temperature along the extruding body, so that the printer itself is not harmed for overtemperature. On the other hand, the design must guarantee that the melting front is located in an intermediate point between the nozzle tip and the entrance of the raw material, to minimize pressure drops in the system, and so decreasing the demanding energy to the feeding motors. An alternative design of the heating system, Twist3D, is proposed in this paper.Peer ReviewedPostprint (published version

Surface roughness assessment after different strategy patterns of ultrasonic ball burnishing

Ball burnishing is a comprehensive finishing process consisting on deforming plastically a certain surface by the action of a rolling sphere-shaped indenter. In this work, an AISI 1038 workpiece is burnished in two phases. The first phase covers the burnishing of a 10x10 mm patch, performed along the direction of the previous milling. The second pass is applied on the previous patch, following three different strategies, namely, parallel, perpendicular and at a 45-degrees angle with regards to that first burnishing direction. The comparison between the non-vibration assisted ball burnishing (NVABB) and vibration-assisted ball burnishing (VABB) effects is included in the study. To assess the influence of these strategies on the final roughness profile, an L9 Taguchi orthogonal array is designed, including as factors the vibrations amplitude, the burnishing force, and the lateral pass width. Surface roughness is measured and compared to determine the optimal burnishing orientation. The VABB process proves to be more effective in surface roughness improvement. Technical recommendations are given to select the best process parameters inside the tested levels for each factor. All burnishing strategy prove to have different effects on the surface, and should be selected according to the preferential burnishing direction and in service application of the part.Postprint (published version

Impresión 3D: Del laboratorio a casa. Programa de divulgación científico-tecnológica sobre la tecnología de impresión 3D

La impresión 3D comienza a ser un fenómeno tecnológico en España, dada la gran variedad de modelos a la venta para aquellas personas emprendedoras que deseen disponer de esta tecnología en su entorno doméstico. Sin embargo, existe un gran desconocimiento sobre de qué consiste la tecnología en que se basa, y cuales son sus potencialidades. Los investigadores implicados en este proyecto han emprendido una serie de acciones financiadas por la FECYT que se realizarán durante el 2015, con la intención de divulgar los avances científicos ligados a las impresoras 3D, y, en particular, a la tecnología en que se basa la fabricación de piezas con estos dispositivos: la FDM (fused deposition modeling). Estas acciones pasan por el lanzamiento de una web donde se publicarán programas y documentales de producción propia sobre impresión 3D, una aplicación móvil que facilite la conexión entre dispositivos móviles e impresoras, y la celebración de cursos de formación destinados a público no universitario que les permitan entrar en contacto con estos dispositivos. Se espera que cientos de personas no universitarias se sientan interesadas en incorporar a su ámbito doméstico una impresora 3D como una nueva herramienta de futur

Mechanical properties of 3D-printing polylactic acid parts subjected to bending stress and fatigue testing

This paper aims to analyse the mechanical properties response of polylactic acid (PLA) parts manufactured through fused filament fabrication. The influence of six manufacturing factors (layer height, filament width, fill density, layer orientation, printing velocity, and infill pattern) on the flexural resistance of PLA specimens is studied through an L27 Taguchi experimental array. Different geometries were tested on a four-point bending machine and on a rotating bending machine. From the first experimental phase, an optimal set of parameters deriving in the highest flexural resistance was determined. The results show that layer orientation is the most influential parameter, followed by layer height, filament width, and printing velocity, whereas the fill density and infill pattern show no significant influence. Finally, the fatigue fracture behaviour is evaluated and compared with that of previous studies’ results, in order to present a comprehensive study of the mechanical properties of the material under different kind of solicitations.Peer ReviewedPostprint (published version

A comparative study of the thermal behavior of three different 3D printer liquefiers

The expansion of 3D printing systems as mechatronic devices able to localize manufacturing activities has attracted the attention in academic and professional labs worldwide. However, little is known from a formal point of view about the actual possibilities of optimization in terms of hardware and software. This paper proposes alternative geometrical designs of RepRap 3D printer liquefiers, and offers an evaluation of their thermal performance by analyzing the temperature profiles derived from their functioning in a steady state regime. For that purpose, finite element calculation and experimental techniques are combined and compared. A detachable model of liquefier, used in numerous RepRap 3D printers, has been taken as a reference to design and manufacture two new variations. As the objective of the liquefier is to heat the plastic material while limiting the temperature at its top, heat conduction must be prevented by means of a refrigerating fan faced at the liquefier body. The effect of that fan has been introduced in the system of study, by evaluating the influence of five different fan velocities on the final temperature profile. All combinations of fan speed and liquefier geometry have been calculated through a finite elements model. Then, they have been reproduced experimentally, measuring discrete temperatures at different heights of the liquefier by a group of thermocouples installed on the surface of the fins composing the liquefier heat sink. Results showed that the defined FEM model reproduces acceptably the final temperature profiles obtained by experimental measures. Furthermore, the new proposed design provided with thick fins shows its good performance when the 3D printing process is performed with a fan speed higher than 10% (defined according to a PWM function programmed at the 3D printer's firmware), while its design presents a much lower manufacturing time with regards to the present available design. It is also concluded that refrigerating the liquefier during a standard 3D printing process with an airflow higher than PWM¿=¿20% is not recommended, due to a fall in efficiency and not remarkable refrigerating effect obtained through it.Postprint (author's final draft

Efectos del bruñido con bola acustoplástico sobre la integridad superficial de Ti6Al4V

Postprint (published version

Influence of peak height prior to milling the resulting surface roughness of the ball burnishing process on convex and concave pieces of aluminum

Molds, counterfoils of conformation, and many other industrial pieces with important presentations, face constant the engineers with the problem of obtaining superficial qualities that minimize the friction, optimize the adjustments, or improve the superficial hardness. Among the many procedures to improve surface finish, highlights the ball burnishing, which have been obtained roughness values recommended in diverse applications. The ball burnishing process is a technological operation which is plastically deformed surface irregularities to improve the surface finish, by the action of the force exerted by a cylinder or ball. This process can be used on cylindrical surfaces, flat front, so, conical, with changes in section and radios, etc., which have been previously machined. The ball burnishing process in question is performed after a machining operation, and the results have influence on it. An important parameter to control is the height of the ridge that remains after the machining, because burnishing process will be able to reduce it in a certain percentage. This study is aimed to conduct an analysis of the influence of peak height prior to milling the resulting surface roughness of the ball burnishing process with concave and convex parts of two different types of aluminum (A92017 & A96351). The milling is done with a ball mill of ϕ8mm, rotating at a cutting speed of 3000min-1, with a cutting depth of 1 mm and three values of progress for three different conditions that leave a ridge height: 0.02 mm, 0.06 mm and 0.10 mm. The main contribution of this study is the recommendations on strategies for burnish these pieces, and measuring the roughness.The work is based on experimental data where through DOE techniques are carried out different experiments in which the surface roughness is measured in directions parallel and perpendicular to the previous milling for various conditions. We compare the results and an analysis of how this affects the height of the ridge left by the previous machining on roughness values obtained. Finally, it has been concluded that the burnish process improvement between 33 and 71% surface roughness of the pieces.Postprint (published version

Fatigue lifespan study of PLA parts obtained by additive manufacturing

Currently, additive manufacturing (AM) is not limited to prototype manufacturing, but is also used to generate parts with final applications. This paper considers this aspect of 3D printing, and aims to characterize fatigue life of parts manufacturied through fused filament fabrication. This is one of the most complex AM technologies, due to the high number of parameters that must be taken into account. The knowledge of the influence of the different manufacturing parameters on the mechanical behavior of the parts has been previously considered for static forces, but so far, dynamic working regimes have not been explored. In this paper, a design of experiments through Taguchi orthogonal arrays is applied to analyze the influence of five factors on fatigue life on PLA specimens. Five fatigue tests are performed for each combination of parameters. Results show that fill density, nozzle diameter and layer height are the most influential factors on fatigue lifespan. Finally, honeycomb proves to be the most beneficial infill pattern with regards to fatigue life.Postprint (published version

Ball-burnishing effect on deep residual stress on AISI 1038 and AA2017-T4

Ball-burnishing induces compressive residual stresses on treated materials by the effect of plastic deformation. The result is an increase in the fatigue life of the treated part, retarding the initiation of cracks on the surface. Compressive residual stresses have been previously measured by X-ray diffraction near the surface, revealing considerably high values at the maximum analyzed depth, in relation to other finishing processes such as shot peening. However, the maximum analyzed depth is very limited by using this technique. In this paper, the incremental hole drilling (IHD) technique is tested to measure residual stresses, being able to reach a 2-mm measuring depth. To that objective, a commercial strain gage is used and calibrated using finite element model simulations. A second Finite Element Model based on material removal rate is developed to obtain the equations to calculate the strain release through IHD. Finally, residual stresses are measured experimentally with that technique on two different materials, confirming that ball-burnishing increases the compressive residual stresses in layers up to 0.5¿mm deep for the testing conditions, which is a good response to industrial needs. The method proves to be suitable, simple and inexpensive way to measure the value of these tensions.Preprin