Post-process Machining Research Articles

Interfacial Heterogeneous Microstructure and Subsequent Machinability of 316L/CuSn10 Bimetallic Structure Manufactured by Laser Powder Bed Fusion

Additively manufactured bimetallic structures combine the advantages of dissimilar materials and can achieve localized properties through a customized composition distribution. However, additively manufactured parts may still lack the dimensional accuracy and surface integrity essential for precision mechanical assemblies that the post-machining process can address. Therefore, this study aims to systematically investigate the microstructure and machinability of 316L/CuSn10 bimetallic structures fabricated using laser powder bed fusion. The results show that the fusion zone of the bimetallic structure had refined grains of microscale size owing to the mixture of the primary elements of the bimetals, which resulted in the highest microhardness of 3.4 GPa. The difference in microstructure and microhardness between the single-material and fusion zones also causes significant differences in the cutting response during the ultraprecision process. The 316L stainless steel side exhibited the highest cutting force and more severe material accumulation in the chips. The cutting force drops when cutting through the fusion zone, with an observable fracture in the chips and separation of dissimilar materials on the machined grooves, indicating that the heterogeneous properties of additively manufactured 316L/CuSn10 bimetallic structures pose challenges to the improvement of surface quality. The simulation results also showed that stress accumulation occurred in the tool path through the fusion zone owing to the higher yield strength and hardness of stainless steel, indicating that lower cutting speeds and depths of cut are favorable for reducing cutting force and improving surface quality. This study provides deep insight into the microstructure evolution mechanism and a theoretical basis for improving the surface quality of additively manufactured bimetallic structures using an ultraprecision machining process.

Machinability Characterization of 3D Printed PEEK

The 3D printing technology is being applied more and more every day, this is a consequence of its applicability and low waste generation, becoming one of the best options to obtain good quality pieces. Sometimes, post machining processes are necessary to fulfil tight tolerances or achieve complex geometries by means of the connection between different pieces printed using this technology. The field of knowledge and studies focused on 3D printing is in constant evolution. There are plenty of materials that can be used to apply 3D printing technology. Among them, PEEK is one of the best options when good mechanical properties are required. Being applied in aeronautic or automobile industry, is also used in biomedical applications, such as prosthesis or mechanical components among others. Within the machining processes, milling, turning, and drilling are the most widespread. Orthogonal cutting is a machining process in which the cutting edge of the tool is perpendicular to the cutting speed, and it is commonly used when a simple and pure study of the mechanism behind a material removal process is required. In this study, tests that analyze the orthogonal cutting on 3D printed PEEK samples using different orientations (0o and ±45o) have been conducted. The influence of cutting speed (30, 60 and 90 m/min) and depth of cut (50, 100 and 150 μm) is studied through the analysis of cutting forces and surface finish quality. As a general approximation, it can be seen that the fiber orientation affects significantly to the forces monitored but unexpectedly, lightly to the surface finish.

Constitutive modelling of Ti6Al4V alloy fabricated by laser powder bed fusion and its application to micro cutting simulation

Although there is a rising interest in applications of metal additive manufacturing (AM) for fabrication of highly complex metallic components, poor surface quality of AM parts is an inherent limitation of this technology. Therefore, post-process machining operations are frequently performed to meet the necessary requirements for the application. When compared to wrought Ti6Al4V, the distinct metallurgical and mechanical characteristics of Ti6Al4V fabricated by laser powder bed fusion (LPBF) can lead to very different mechanical behavior, thus different machinability. Therefore, to properly perform the numerical modelling of the cutting processes, it is important to accurately define material mechanical behavior of LPBF Ti6Al4V. This study presents the experimental determination of the coefficients of the Johnson-Cook (J-C) constitutive model and the numerical simulation of orthogonal micro cutting process of hot isostatic pressed (HIP) LPBF Ti6Al4V. Besides, the results were also compared to the wrought Ti6Al4V. For that purpose, quasi-static and dynamic behaviors of wrought and LPBF Ti6Al4V were investigated by quasi-static tensile tests at several temperatures (0.001 s−1 at 20, 400, 600 °C) and at high strain rates using the split Hopkinson pressure bar (SHPB) tests (∼1000-5000 s−1 at 20, 400 °C). Orthogonal micro cutting simulations were performed using experimentally obtained J-C model of wrought and LPBF Ti6Al4V. A series of orthogonal micro cutting tests at different cutting speeds (75, 100, 150 m/min) and uncut chip thickness values (2.5, 5, 7.5, 10 μm) were performed to compare the measured forces and chip compression ratio (CCR) with those obtained by simulation. Good predictions of main cutting force and maximum CCR were obtained for both wrought and LPBF Ti6Al4V under different cutting conditions. Several critical points to be considered for better prediction of thrust force and chip geometries in future studies were highlighted.

Machinability analysis of carbon fibre reinforced PET-Glycol composites processed by additive manufacturing

Fused filament fabrication (FFF) is one of the most widely used additive manufacturing (AM) techniques, which has enabled a simpler, more flexible, and low-cost processing procedure for obtaining functional fibre reinforced composite products. However, one of the shortcomings of AM processing is its poor geometric and dimensional behaviour that hinders the manufacture of high-quality functional parts. Though this limitation can be overcome with surface post-processing machining, the machining of composites is complex due to their non-homogeneous microstructure producing delamination, splintering, and fractures. In this study, the machinability of short carbon fibre reinforced glycol-modified polyethylene terephthalate (CF-PETG) processed by FFF additive manufacturing was assessed in face milling and edging peripheral milling operations by analysing the AM process parameters layer thickness (Lt) and build orientation (Bo). This machinability analysis was based on energy consumption and geometric properties. The former was assessed by measuring cutting forces, and the latter involved examining dimensional accuracy, flatness and surface roughness measurements, combined with 3D-topography and SEM microscope images. PETG processed by FFF showed good machinability in both machining operations with acceptable energy consumption and surface texture, with no significant effect of Lt or Bo. The addition of carbon fibre reinforcement to PETG improved energy consumption in both operations, and enhanced the surface texture in face milling. However, machinability worsened in terms of geometric properties for peripheral milling with the addition of carbon fibre and increased Lt, exhibiting substantial surface defects such as tearing and burrs.

Machining stability improvement in LPBF printed components through stiffening by crystallographic texture control

The quality requirements of functional parts manufactured by the laser powder bed fusion imply post-processing machining operations. These components include thin walls, hollow and complex shapes that can lead to machining instability. This paper presents a strategy to exploit the inherent crystallographic effect as a tool for stiffening and improving machining stability by considering the orientation of machining bending loads and the crystalline orientation distribution. Three samples were manufactured under the same LPBF parameters but different crystallographic orientations. The results show that the optimal crystalline direction increases static and dynamic stiffness and stability, reducing machining chatter risks.

Numerical modeling and simulation of machining of 3D printed CFRP composite

Carbon Fiber Reinforced Polymer (CFRP) composites are gaining popularity in recent years for their high specific strength and superior performance under extreme conditions. Additive manufacturing (AM) or 3D printing of CFRP composites has added a new dimension to the existing research of CFRP composites. However, one of the caveats of 3D printed CFRP composites is that the dimensional accuracy and the surface finish achieved fail to meet the standard tolerance requirement. To achieve the desired accuracy further post-process machining is required. However, machining an anisotropic material such as 3D printed CFRP composites impose challenges. This study aims at investigating the machining behavior and post-processing capability of 3D printed CFRP using numerical modeling and simulation using ABAQUS/Explicit, a commercially available powerful finite element analysis (FEA) software package. Influence of machining parameters, such as feed and depth of cut (DOC) and 3D printing parameter, such as percentage of overlap between adjacent passes have been studied. The post-processing capability or machining behavior of 3D printed CFRP has been evaluated by dependent parameters, such as, cutting forces, strain, chip and burr formation, and the resultant surface topology. With the use of proper damage parameters, such as, ductile and shear damage and Johnson-Cook criterion for plasticity, a 3D model was developed in the FEA. Eight layers were stacked with the orientation of 45°-135°-45°-135°. The layers were joined using cohesive properties. Damage in the cohesive bonds was also defined using the quadratic damage criterion to model delamination. To capture the changes in elemental level, linear tetrahedral elements C3D4T, were used. A reference point on the bottom layer was used to capture the history of cutting forces. The cutting and thrust forces increased with the increase of DOC and feed. The rigidity, shape of the flutes and print direction influenced chip formation. Finally, the proposed model was used to observe the quality of surface finish.

Importance of Build Design Parameters to the Fatigue Strength of Ti6Al4V in Electron Beam Melting Additive Manufacturing.

The fatigue properties of metals resulting from Powder Bed Fusion (PBF) is critically important for safety-critical applications. Here, the fatigue life of Grade 5 Ti6Al4V from Electron Beam PBF was investigated with respect to several build and component design parameters using a design of experiments (DOE). Part size (i.e., diameter), part proximity, and part location within the build envelope were considered. Overall, metal in the as-built condition (i.e., no post-process machining) exhibited a significantly lower fatigue life than the machined surface condition. In both conditions, the fatigue life decreased significantly with the decreasing part diameter and increasing radial distance; height was not a significant effect in the machined condition. Whereas the surface topography served as the origin of failure for the as-built condition, the internal lack of fusion (LOF) defects, exposed surface LOF defects, and rogue defects served as the origins for the machined condition. Porosity parameters including size, location, and morphology were determined by X-ray micro-computed tomography (XCT) and introduced within regression models for fatigue life prediction. The greatest resistance to fatigue failure is obtained when parts are placed near the center of the build plane to minimize the detrimental porosity. Machining can improve the fatigue life, but only if performed to a depth that minimizes the underlying porosity.

High-throughput screening of surface roughness during additive manufacturing

Smooth surfaces in the printed parts are essential for long fatigue life and high dimensional accuracy. They are currently achieved by post-process machining and grinding of external surfaces that add extra costs. Mitigating the surface roughness of internal channels remains a challenge. Here we use a high-throughput screening approach that analyzes the value of a dimensionless index for many experiments and provides a pathway for reducing the surface roughness without the need for post-processing. The index is derived by dimensional analysis of causative variables that affect the roughness of the surface such as heat input, powder diameter, layer thickness, pool aspect ratio, Marangoni force, contact angle, and enthalpy of melting of alloys. Using the results of high-throughput screening, we develop easy-to-use process maps that are consistent with the experimental observations. Among the causative variables, heat input and the contact angle of the molten material with the substrate have the highest and lowest influence on the smoothness of printed surfaces. An aluminum alloy, AlSi10Mg is found to be the best choice for printing smooth surfaces among the four alloys considered here. These findings can improve the surface quality of additively manufactured parts that now significantly hinder their wider industrial adaptation.

An integrated approach to investigate the energy consumption for manufacturing and surface finishing 3D printed Inconel 718 parts

Current scenarios show the need of making manufacturing more sustainable as it plays a crucial role in global sustainability. For this reason, sustainability should be included among the objectives driving the selection of process parameters. As-built parts manufactured by additive manufacturing techniques always require post-processing to meet industrial quality requirements, and machining is one of the solutions often adopted to improve their surface finishing. However, inevitably an additional manufacturing process requires some supplementary both energy and resource consumption by also impact unavoidably on the environment. This experimental study includes both additive manufacturing and post-process machining to evaluate the global specific energy consumption required to manufacture and then finish Inconel 718 parts. This paper aims at understanding how to obtain Inconel 718 parts manufactured by selective laser melting (SLM) and machined by turning by saving energy without sacrificing roughness and density. The printing process parameters of SLM process have been varied in a wide range: 27 different samples have been produced to evaluate the specific energy consumption (SEC) and to highlight the influence of the process parameters on the energy consumption. Density and roughness measurements have been chosen as measured outputs to prove the effectiveness of the process and to attempt a correlation among process parameters, properties of the printed parts and energy consumption. All the as-built samples were post-treated by turning process under the same conditions to achieve a surface finishing with a Ra value close to 4 μm; the energy consumption of this process has been also recorded and analyzed. Finally, the energy consumption of the whole processing path, including both printing and turning, has been discussed and a correlation between processing parameters and energy consumption has been attempted. Results showed that a proper choice of the process parameters leads up to a 54% reduction in specific energy consumption to obtain SLMelted Inconel 718 parts having low surface roughness and high density.

SERS-based ssDNA composition analysis with inhomogeneous peak broadening and reservoir computing

Surface-enhanced Raman spectroscopy employed in conjunction with post-processing machine learning methods is a promising technique for effective data analysis, allowing one to enhance the molecular and chemical composition analysis of information rich DNA molecules. In this work, we report on a room temperature inhomogeneous broadening as a function of the increased adenine concentration and employ this feature to develop one-dimensional and two dimensional chemical composition classification models of 200 long single stranded DNA sequences. Afterwards, we develop a reservoir computing chemical composition classification scheme of the same molecules and demonstrate enhanced performance that does not rely on manual feature identification.

Flat diamond sliding burnishing surface roughness investigation

The sliding diamond burnishing process is one among many burnishing post-machining processes applied for surface integrity enhancement purposes. This experimental study aims to investigate the effect of burnishing parameters on the surface roughness of the product. Taguchi design was used, and the machining parameters considered were burnishing force, feed rate, and the number of passes with three levels each. C45 steel alloy workpieces were face milled with the same machining variables and their surface roughness was measured before and after the diamond sliding burnishing process. Enhanced surface roughness was achieved with increasing burnishing force and minimum feed rate and higher number of passes.

Comparative assessments of machining forces in 3D printed polymer composite during milling operation using two coated carbide end mills

The composites of carbon fiber reinforced polymer (CFRP) are rapidly turning into the most extensively used material in aerospace and automotive applications due to their superior corrosion-resistant exterior and ratio of strength-to-weight. The flexibility to fabricate complex shapes and alter the fiber orientation of CFRP composites via three-dimensional (3D) printing or additive manufacturing is gaining prominence. However, because of their anisotropic and heterogeneous structure these composites were regarded as the difficult-to-cut materials. The parts made out of these composites are often made to near-net shape in astronautic and aerospace industry. However, part finishing typically necessitates post-process machining such as milling, in order to satisfy dimensional accuracy prior to assembly. The aim of this thesis is to explore the machinability aspects in terms of machining forces 3D printed CFRP composites by milling CFRP employing TiAlCrN and hard carbon coated solid carbide end mills in dry cutting environments. A 3D printed multidirectional CFRP workpiece was fabricated using a Mark forged industrial printer and milling operations were carried on additively manufactured workpiece with two distinct coated tools according to the experimental design created on Taguchi’s L16 orthogonal array in which three variables namely feed rate, depth of cut, spindle speed were altered at four distinct levels. With TiAlCrN coated carbide tool, least machining forces observed compared to hard carbon coated carbide tool.

Pure molybdenum manufactured by Laser Powder Bed Fusion: Thermal and mechanical characterization at room and high temperature

As a refractory material, molybdenum is regarded with high interest for high-temperature applications. The concurrent use of powder bed Additive Manufacturing (AM) technology can provide significant design and production advantages. In this work, the Laser Powder Bed Fusion (LPBF) process parameters tuning lead to produce almost fully-dense AM Mo blocks (density of 99.5 ± 0.5%). Fine-tuning of the parameters, also involved the Single Scan Tracks analysis, are aiming at the continuous and homogeneous melt-pools production. High-density Mo specimens have been characterized at room- and high-temperature in terms of thermal and mechanical properties, then compared with conventionally manufactured Mo samples. The thermal diffusivity measurement at room temperature allowed to verify that the thermal conductivity value of AM Mo is approximately half of standard Mo. Stress relieving heat treatment improves the thermal conductivity approximately by 13%. The estimation of emissivity and thermal conductivity carried out in the 600÷1600 °C temperature range led to a similar result. The Vickers microhardness measured on fully dense specimens (212 ± 18 HV 0.15 ) is similar to commercially available Mo. Tensile tests have been performed at both room temperature and 600 °C. The effect of building direction and post-processing machining of AM specimens have been also investigated for tests at room temperature. Although the AM samples exhibited a very similar density to standard Mo, the AM Mo mechanical properties resulted generally lower.

Understanding the behavior of laser surface remelting after directed energy deposition additive manufacturing through comparing the use of iron and Inconel powders

Laser directed energy deposition with powder (pDED-L) is one of the most popular techniques for additive manufacturing. However, for some applications, the resulting surface finish requires post-processing machining. Laser remelting processing has been cited as a means of decreasing machining needs in other applications. The objective of this study was to assess the performance and gain a better physical understanding of the laser remelting process as an alternative to the conventional post-processing techniques applied after pDED-L. pDED-L and remelting were performed using a 10 kW fiber laser source and iron and Inconel 625 powders as feedstock to compare the surface roughness and waviness under different processing conditions (three heat input levels). Through 2D and 3D surface measurements, as well as scanning electron microscopy (SEM), surface features were qualified and quantified. SEM and optical microscopy (OM) were also employed for metallurgical characterization of the roughness. The surface micro-notch effect was assessed through the stress concentration factor (kt). The results indicated attenuation of the average roughness (Ra) by around 30% for iron and 70% for Inconel when laser remelting was employed. In addition, kt presented a 31% reduction for iron and 29% reduction for Inconel. The performance varied according to the type of material used and was mainly related to differences in thermal diffusivity and electrical resistivity. It was concluded that laser remelting is a promising technology for coupling with pDED-L aimed at producing 3D metal components with superior quality while allowing a faster production rate in comparison to current practices.

Characterization of Inconel 718® superalloy fabricated by wire Arc Additive Manufacturing: effect on mechanical properties and machinability

Wire and Arc Additive Manufacturing has the potential to become an appropriate technique to produce large complex-shaped metallic parts. However, a post-processing machining operation is necessary to reach the final geometry. In this work, Inconel 718 walls were manufactured in a monitored environment and their microstructure and mechanical properties were characterised. Then, slot milling operations were performed to investigate the influence of cutting speed and machining direction. The conclusions drawn from this article can be used as a guide for a correct definition of strategies and milling parameters. It was observed that at higher cutting speeds a better surface quality and lower torques are obtained. Moreover, the main novelty of this work is that is shows the influence of the anisotropy of WAAM-Inconel 718 on its machinability. Milling along the torch's travel direction offers better dimensional tolerance values with lower cutting torques, being more favourable than machining in the building direction.

Integration of a multi-directional wire arc additive manufacturing system with an automated process planning algorithm

With the increasing demand for high productivity and low cost, the advanced manufacturing system has become more complex. It is challenging to develop an advanced manufacturing system by engineers or researchers from manufacturing engineering disciplines. To combine knowledge and skills from two or more disciplines, interdisciplinary engineering (IDE), from industrial information integration, was carried out as a promising subject for further developing advanced manufacturing systems. Recently, Wire arc additive manufacturing (WAAM) technology has attracted attention from industrial sectors due to its capability to fabricate medium-to-large scale components with low capital investment and high productivity. In addition, a variant of WAAM, called multi-directional WAAM techniques, has been developed for the direct fabrication of parts with overhanging features. The multi-directional approach can reduce the need for additional supporting structures, reducing (amongst other things) material costs, manufacturing time, and post-process machining requirements. Although the multi-directional WAAM has a great potential for practical industrial use, the process planning also becomes more complex for developing an automated system for industrial use. This paper presents a novel automated processing planning algorithm and integrates with the automated robot offline programming (AOLP) engine and WAAM hardware. The proposed system aims at planning the multi-directional WAAM process automatically. The process planning algorithm consists of four key modules relating to (a) robot motion planning, (b) initial collision processing, (c) layer sequence optimisation, and (d) weld torch pose adjustment. In the first stage, the required robot motions to deposit each layer are obtained through an AOLP engine. Then a collision matrix is generated to guide later steps of the planning process from these robot motions. After this, layer sequence optimisation is performed to eliminate collision between the robotic system and the part. If collision still exists, adjustments to the torch pose are made to avoid the remaining collision. The final chapter of this paper demonstrates the effectiveness of the proposed system via a real-world case study, where a workpiece with overhanging features was fabricated.

Proposal for enhancing the compressive strength of alkali-activated materials-based binder jetting 3D printed outputs

Additive manufacturing (AM), also known as 3D printing, is a fabrication method used to create geometric 3D objects. Particularly, binder jetting 3D printing (BJ3DP) can be used to produce complex geometries. In this study, we have enhanced the compressive strength of alkali-activated materials (AAM)-based BJ3DP outputs by optimizing the powder mixture, post-processing machine, and post-processing storage solution. These optimization measures present a compressive strength of 22.56 MPa at 7 days (20 mm ×20 mm ×20 mm sized printed output). Consequently, we have confirmed the possibility of the application to construction by enhancing the compressive strength of BJ3DP.

SURFACE ROUGHNESS CONSIDERATIONS IN DESIGN FOR ADDITIVE MANUFACTURING - A LITERATURE REVIEW

AbstractOne often-cited benefit of using metal additive manufacturing (AM) is the possibility to design and produce complex geometries that suit the required function and performance of end-use parts. In this context, laser powder bed fusion (LPBF) is one suitable AM process. Due to accessibility issues and cost-reduction potentials, such ‘complex’ LPBF parts should utilise net-shape manufacturing with minimal use of post-process machining. The inherent surface roughness of LPBF could, however, impede part performance, especially from a structural perspective and in particular regarding fatigue. Engineers must therefore understand the influence of surface roughness on part performance and how to consider it during design. This paper presents a systematic literature review of research related to LPBF surface roughness. In general, research focuses on the relationship between surface roughness and LPBF build parameters, material properties, or post-processing. Research on design support on how to consider surface roughness during design for AM is however scarce. Future research on such supports is therefore important given the effects of surface roughness highlighted in other research fields.

Geometry and tool motion planning for curvature adapted CNC machining

CNC machining is the leading subtractive manufacturing technology. Although it is in use since decades, it is far from fully solved and still a rich source for challenging problems in geometric computing. We demonstrate this at hand of 5-axis machining of freeform surfaces, where the degrees of freedom in selecting and moving the cutting tool allow one to adapt the tool motion optimally to the surface to be produced. We aim at a high-quality surface finish, thereby reducing the need for hard-to-control post-machining processes such as grinding and polishing. Our work is based on a careful geometric analysis of curvature-adapted machining via so-called second order line contact between tool and target surface. On the geometric side, this leads to a new continuous transition between "dual" classical results in surface theory concerning osculating circles of surface curves and osculating cones of tangentially circumscribed developable surfaces. Practically, it serves as an effective basis for tool motion planning. Unlike previous approaches to curvature-adapted machining, we solve locally optimal tool positioning and motion planning within a single optimization framework and achieve curvature adaptation even for convex surfaces. This is possible with a toroidal cutter that contains a negatively curved cutting area. The effectiveness of our approach is verified at hand of digital models, simulations and machined parts, including a comparison to results generated with commercial software.

High productivity fluence based control of Directed Energy Deposition (DED) part geometry

Successful and efficient hybrid manufacturing consisting of Directed Energy Deposition (DED) followed by milling as a postprocess requires that material deposited during the DED step be geometrically consistent and slightly larger than the desired final, machined dimensions. DED clad width is directly correlated to meltpool energy, while powder fluence directly correlated to DED clad cross-sectional area. Due to heat accumulation into the DED part during deposition, meltpool energy does not remain constant without modification of DED process parameters over the deposition cycle. Meltpool energy is typically managed by decreasing the laser power to reduce the energy fluence into the meltpool – though this technique limits DED productivity. If the working distance and laser optics within the deposition head are static, energy fluence reduction can also be achieved by increasing deposition head feedrate. Modifying powder mass flowrate through the deposition nozzle to maintain powder fluence into the meltpool allows for both the clad geometry to be maintained while also increasing the productivity of the DED process. Numerical simulation of the heat flow in a DED thin wall part was used to derive laser energy fluence values to maintain a constant meltpool energy. Derived values were tested using both the laser power and feedrate to moderate energy fluence into the meltpool. Results are compared to a thin wall deposited without meltpool energy control. Clad geometry variation and cycle time are both shown to reduce when using a feedrate-based meltpool energy control method as compared to laser power-based meltpool energy control or no meltpool energy control, increasing the geometric accuracy for postprocess machining and productivity.