The current state-of-the-art metal additive manufacturing (AM) process still cannot meet the high industry requirements in terms of surface roughness. In addition, there are limited ISO/ASTM standa...

A review on the state-of-the-art of surface finishing processes and related ISO/ASTM standards for metal additive manufactured components

The surface roughness of components built using the laser powder bed fusion (L-PBF) process is poor. Surface finishing the internal channels of L-PBF components is a challenge. We propose a multi-jet hydrodynamic approach to enhance the surface finish quality of the internal channels. We investigate the hydrodynamic finishing on L-PBF Inconel 625 linear, stepped, and non-linear internal channels with diameters 5 to 1 mm and length up to 100 mm (replicating the geometries in rocket injectors, fuel nozzles, and cooling channels). The multi-jet hydrodynamic finishing approach improved the surface quality by 60–90 % (final Ra, Sa ≤ 1 μm and Rz, Sz ≤ 20 μm), using an abrasive concentration of ≤1 % in 15 min. of processing time. Areal surface texture parameters Sdr and roughness ratio r ≈1, evidenced the uniformity of the surface finish with dominant abrasive microcuts, regardless of the initial non-uniform additive manufactured surface. Most of the surface finished channels had excellent dimensional integrity and internal contour circularity. We then discussed the advancements required in metal additive manufacturing and internal surface finishing—to safely deploy L-PBF components with micro internal channels in fuel injection and fluid transfer applications.

Surface finishing of additively manufactured Inconel 625 complex internal channels: A case study using a multi-jet hydrodynamic approach

Poor surface quality represents an important issue that needs improvement for metal Additively Manufactured parts. Both short or long wavelength lasers have been applied for surface polishing in order to improve the surface finish. In this work, the possible use of a CO2 laser for the surface polishing of AlSi10Mg parts made by Laser-based Powder Bed Fusion (L-PBF) was explored. The high surface roughness in the as-built condition can lead to increased laser energy absorption. In order to assess the effects of the main laser-related process parameters, the experiments were carried out on L-PBF samples built vertically with respect to the build platform. Their effect on the surface were evaluated by means of the surface arithmetical mean height (Sa), surface skewness (Ssk) and primary profile root mean square slope (Pdq), obtained via confocal microscopy. Microstructure evolution was also investigated by means of SEM and EDS analysis. The results showed a large reduction in surface roughness, ranging from the 67% to the 85% of the starting value. Microstructure of the re-molten layer revealed an increased grain size and an increased Si content that led to an overall hardness increase from 85 to 121 HV.

CO2 laser polishing of laser-powder bed fusion produced AlSi10Mg parts

The strengthening that results from the low-temperature formation of strain-induced martensite in austenitic stainless steel was studied. Specifically, the work hardening behaviour was characterized, as well as the spatial distribution of the martensite as a function of prior strain. Neutron diffraction measurements revealed the degree of elastic strain partitioning between the austenite and martensite. It was found that a sufficiently high initial dislocation density leads to a localization of the martensite transformation in the form of a Luders front. The martensite acts as an elastic reinforcing phase as it supports a higher stress than the austenite tensile loading, even though the martensite co-deforms plastically with the austenite. A model was developed that predicts the volume fraction of martensite formed as a function of plastic strain. © 2004 Elsevier B.V. All rights reserved.

The Formation of Strain-induced Martensite in Stainless Steels

Improving the surface characteristics of additively manufactured parts: A review

Additive Manufacturing (AM) can be considered today as a real production technology, which allows to realize parts with a complexity degree that, in some cases, is not achievable otherwise. However, one of the most relevant weak spots is the high surface roughness, especially for metal parts, making necessary the adoption of post-process finishing treatments. This paper deals with the preliminary investigations on the Fluidised Bed Machining (FBM) technology, in which a sample is dipped into a fluidized bed, i.e. a two-phase system where an abrasive, kept in motion through a gas, behaves like a fluid performing a huge number of impacts on the considered surface. This peculiarity could guarantee a high degree of homogeneity of the treated surface morphology. The influence of the impact angle has been investigated by conducting experiments with AlSi10Mg plates made by Selective Laser Melting Technology (SLM), and the fluidized bed operated in bubbling fluidization regime. The treatment has been carried out by dipping the samples for a total time of 3 hours and monitoring the results with a step of 30 min for the first 2 hours. The treated surfaces have been characterized by means of Confocal Microscopy and Scanning Electron Microscopy (SEM). Weight loss measurements have been carried out as well for a preliminary evaluation of wear. Results suggest a poor decrease in surface roughness (Sa), as also demonstrated by the poor weight loss, both due to a low impact energy of the abrasives. However, variations of other surface texture parameters (Sz, Ssk, Sku, Rdq), as well as SEM images, suggest that the main surface-abrasive flow interaction phenomena are micro-ploughing and micro-peening.Additive Manufacturing (AM) can be considered today as a real production technology, which allows to realize parts with a complexity degree that, in some cases, is not achievable otherwise. However, one of the most relevant weak spots is the high surface roughness, especially for metal parts, making necessary the adoption of post-process finishing treatments. This paper deals with the preliminary investigations on the Fluidised Bed Machining (FBM) technology, in which a sample is dipped into a fluidized bed, i.e. a two-phase system where an abrasive, kept in motion through a gas, behaves like a fluid performing a huge number of impacts on the considered surface. This peculiarity could guarantee a high degree of homogeneity of the treated surface morphology. The influence of the impact angle has been investigated by conducting experiments with AlSi10Mg plates made by Selective Laser Melting Technology (SLM), and the fluidized bed operated in bubbling fluidization regime. The treatment has been carried out ...

Fluidised bed machining of metal additive manufactured parts

Metal spinning processes represents an interesting alternative to traditional sheet metal forming processes in several industrial contexts, such as automotive and aerospace. In this work, the production of a combustion chamber liner top prototype using AISI 304L stainless steel is proposed, in order to evaluate the process feasibility for the required part geometry. The prototypes production was carried out using a two-stage semiautomatic spinning process. The effects in terms of wall thickness reduction were investigated. Using optical microscopy and Scanning Electron Microscopy (SEM) techniques, the microstructural behavior of the metal subjected to the forming process was investigated, while for an evaluation of the influence on the mechanical properties Vickers micro-indentation tests were performed. The main result of the process, as observed from all the investigation techniques adopted, is the formation of strain induced martensite due to the severe plastic deformation and cold reduction of the material, ranging in this case from 30% to 50%. In some areas of the part section, some rips indicating an excessive tensile stress were also detected.

A. El Hassanin

Papers

Fluidised bed machining of metal additive manufactured parts

On the manufacturing of a gas turbine engine part through metal spinning process