Influence of High-Productivity Process Parameters on the Surface Quality and Residual Stress State of AISI 316L Components Produced by Directed Energy Deposition

AbstractThe production of large components is one of the most powerful applications of laser powder-directed energy deposition (LP-DED) processes. High productivity could be achieved, when focusing on industrial applications, by selecting the proper process parameters. However, it is of crucial importance to understand the strategies that are necessary to increase productivity while maintaining the overall part quality and minimizing the need for post-processing. In this paper, an analysis of the dimensional deviations, surface roughness and subsurface residual stresses of samples produced by LP-DED is described as a function of the applied energy input. The aim of this work is to analyze the effects of high-productivity process parameters on the surface quality and the mechanical characteristics of the samples. The obtained results show that the analyzed process parameters affect the dimensional deviations and the residual stresses, but have a very little influence on surface roughness, which is instead dominated by the presence of unmelted particles

Experimental analysis of residual stresses on AlSi10Mg parts produced by means of Selective Laser Melting (SLM)

Abstract During the Selective Laser Melting (SLM) process, the scanned layers are subjected to rapid thermal cycles. Steep temperature gradients generate residual stresses. Residual stresses can be detrimental to the proper functioning and the structural integrity of built parts. In this paper, the semi-destructive hole-drilling method has been used to measure the residual stresses on AISi10Mg parts after building, stress relieving and shot-peening, respectively. The outcomes have shown the presence, on the as-built components, of high tensile stresses that the usual post-processing operations are not able to minimize. The adopted method has proved to be a suitable tool to identify optimal process parameters for each step of the production cycle

Microstructure and Residual Stress Evolution of Laser Powder Bed Fused Inconel 718 under Heat Treatments

AbstractThe current work aimed to study the influence of various heat treatments on the microstructure, hardness, and residual stresses of Inconel 718 processed by laser powder bed fusion process. The reduction in residual stresses is crucial to avoid the deformation of the component during its removal from the building platform. Among the different heat treatments, 800 °C kept almost unaltered the original microstructure, reducing the residual stresses. Heat treatments at 900, 980, and 1065 °C gradually triggered the melt pool and dendritic structures dissolution, drastically reducing the residual stresses. Heat treatments at 900 and 980 °C involved the formation of δ phases, whereas 1065 °C generated carbides. These heat treatments were also performed on components with narrow internal channels revealing that heat treatments up to 900 °C did not trigger sintering mechanisms allowing to remove the powder from the inner channels

laser powder bed fusion l pbf additive manufacturing on the correlation between design choices and process sustainability

Abstract The specific energy consumption of Additive Manufacturing (AM) unit processes for the production of metal parts could be much higher than that of more traditional manufacturing routes, such as machining. However, AM, due to its intrinsic process peculiarities, including the flexible realization of (almost) any kind of complex shape, has a great potential for improving the material use efficiency, with positive environmental impact benefits from the material production to the product use and disposal at the end of first life. Aim of this paper is to assess the role of the design choices on the environmental AM process sustainability. An integrated design methodology (accounting for the product re-design via topological optimization, the design of support structures, and the design of allowances and features for post-AM finishing operations) for components produced by means of laser powder bed fusion processes is considered. One resource (the cumulated energy demand) and one emission (carbon dioxide) are assumed as metrics for the impact assessment across the product life cycle. The results demonstrate the importance of a proper design for AM to improve the overall energy and emission saving potential

On the Effect of Part Orientation on Stress Distribution in AlSi10Mg Specimens Fabricated by Laser Powder Bed Fusion (L-PBF)

Abstract The freedom of design of AM products suffers from some limitations in case of powder bed metal processes, because AM part's integrity is affected by the residual stress state that is a consequence of the thermal history during part fabrication. Aim of this work is to evaluate the effect of part orientation on stress distribution. Thus, flat samples of AlSi10Mg alloy built along different orientations are produced by means of laser powder bed fusion (L-PBF) process, also known as Selective Laser Melting (SLM). Then, the semi-destructive hole-drilling method is used to evaluate residual stresses beneath the surfaces of samples. The outcomes of the study can be exploited to define design rules in order to both minimize support structures and optimize the orientation of the part in the building volume

An investigation on the effect of deposition pattern on the microstructure, mechanical properties and residual stress of 316L produced by Directed Energy Deposition

Abstract In this work, 316L cubes were produced by Directed Energy Deposition (DED) process. To evaluate the effect of deposition patterns on the microstructure, mechanical performance and residual stress of 316L samples, two different deposition strategies are selected (67° and 90°). The general microstructure is revealed, and then the effect of deposition pattern on the microstructure of 316L alloy is evaluated through the Primary Cellular Arm Spacing (PCAS) analysis. The cooling rate in each sample is estimated according to the PCAS values. Interestingly, it is found that by increasing the rotation angle per layer, the PCAS value decreases as a consequence of increment in the cooling rate. On the other hand, in both cases, by increasing the distance from the substrate, due to the changes in cooling mechanisms, the cooling rate at first decreases and then at the last layers increases again. The phase composition analysis of 316L samples confirms the predictions that suggested the presence of residual δ-ferrite in the final microstructure. In fact, the final microstructure of samples is characterized by austenitic dendrites together with some residual δ-ferrite in the interdendritic regions. Moreover, the microstructural evaluations exhibit that during the DED process, some metallic inclusions are formed within the 316L samples that consequently deteriorates their mechanical properties. Tensile results show that the samples with 90° rotation per layer have a better mechanical performance such as slightly higher ultimate tensile strength and almost 35% higher elongation to fracture, mainly owing to their finer microstructure and slightly less oxide content. However, in both cases, the elongation of the 316L samples is lower than the typical elongation of this material produced via DED. This discrepancy is found to be as a result of higher inclusions contents in the samples produced in this work with respect to those of literature. Lastly, it is found that the residual stresses on the top surfaces are similar for both deposition patterns, although higher stress values are observed on the lateral surfaces of the cubes produce using the 90° rotation per layer

Enhancing the dimensional accuracy of a low-cost 3D printer

3D printing is widely used in the entertainment industry by filmmakers, effect studios and game designers to easily and fast fabricate characters or objects that are first virtually modelled through Computer Graphics. There are several commercial proposals in the field of low-cost 3D printers, with prices starting from a few hundred euros for kits that the users should assemble by themselves. However, their performances in terms of part accuracy are quite limited and are basically the consequence of a lack of optimization both in mechanical terms as in software. Starting from these considerations, an optimization project was assigned to the students of the Specializing Master in Industrial Automation of the Politecnico di Torino. The Master is developed in collaboration with COMAU S.p.a., a company worldwide leader in automation expecially for the automotive sector. The task of enhancing the performances of the 3D printer Prusa i3, that is supplied in the assembly box, was assigned to sixteen engineers attending the Master who were divided into 4 groups. The activities have led to the birth of four new 3D printers: Fluo, Ghost, Metallica and Print-Doh. In order to assess and validate the improvements, a benchmarking activity was carried out to evaluate the dimensional accuracy of the four printers. The benchmarking was based on the manufacturing of an innovative reference artifact whose geometrical features are designed to fit within different ISO basic sizes. Each group printed a replica of the reference part with their own new printer and then the replicas were measured by means of a coordinate measuring machine (CMM). Measures were used to compare the performances of the four printers and the results of the benchmarking considers the dimensional accuracy of the replicas in terms of ISO IT grades, but also the form errors of the geometrical features through GD&T tolerances

Dental Failure Analysis: The Need of a Comprehensive Failure Classification

For more than thirty percent of patients with implant-supported fixed dental prosthesis, various complications can be observed over five-years of function. In some cases, failure can be ascribed to mechanical reasons such as loosening of the retaining screws or fracture of the implant components. The paper evaluates three different failures of implant-supported prostheses. All cases were analyzed by optical and SEM microscopy to identify the failure modes and the possible failure causes. Improper design or errors in finishing operations or in assembly are identified as dental failure causes. A matrix classification is proposed to collect rupture cases of implant-supported prostheses

The Use of Self-replicated Parts for Improving the Design and the Accuracy of a Low-cost 3D Printer

Abstract Low-cost entry-level 3D printers suffer from reduced optimization, that is a consequence of development cost savings. A student challenge was used to modify four Prusa i3 machines with the aim of enhancing the design and performances by means of self-replicated parts. The challenge results were assessed through benchmarking of the four modified 3D printers, whose dimensional accuracy was evaluated by means of CMM measurements of 3D printed replicas of a reference part. The ISO IT grades related to the dimensional quality of the replicas were considered in the analysis of the CMM measures for the challenge assessment