Freeform Parts Research Articles

Integrated optimisation of multi-pass cutting parameters and tool path with hierarchical reinforcement learning towards green manufacturing

Five-axis machining, especially flank milling, is popular in machining thin-walled freeform surface parts with high energy consumption. Reducing the machining energy consumption is paramount for advancing green manufacturing. Therefore, this paper proposes an energy-efficient integration optimisation of cutting parameters and tool path with hierarchical reinforcement learning (HRL). Firstly, a novel multi-pass machining energy consumption model is developed with cutting and path parameters, based on which the integrated optimisation problem is modelled considering a dynamic workpiece deformation constraint. Secondly, HRL with a Soft Actor Critic agent (HSAC) decouples the model into two Markov Decision Processes at different timescales. The higher-layer plans cutting parameters for each pass on a macro timescale, while the micro-timescale lower-layer performs multiple tool path expansions with the planned cutting parameters, and provides feedback to the higher layer. By hierarchical optimisation and non-hierarchical interaction, the model is efficiently solved. Moreover, curriculum transfer learning is applied to expedite task completion of the lower layer, enhancing interaction efficiency between the two layers. Experiments show that, compared with two benchmarks, the proposed method improves machining energy consumption by 35.02 % and 30.92 %, and reduces machining time by 38.57 % and 27.17 %, providing a promising paradigm of green practices for thin-walled freeform parts and the broader manufacturing industry.

Thermally controlled shape programming via image-based optimization towards distortion-reduced composite curing

The complex mechanisms involved in thermosetting composite curing process contribute to the cure-induced distortion (CID) that deteriorates load-bearing capacity as well as in-service life of the assembled component. The emergence of novel approaches for flexible thermal control, such as microwave or self-resistance heating devices, enables shape manipulation through customized temperature field programming, which opens up a new avenue for reducing CID. The determination of such temperature field is however a typical inverse problem that conventionally requires accurate modeling of the forward curing process together with an iterative solver, which becomes intractable due to the high-dimensional space required to identify a temperature field. This paper proposed an image-based optimization scheme to deliver a distortion-reduced temperature field. A simply yet effective thermomechanical relationship is primarily established to generate a grayscale image that encodes a nominal temperature field. The adaptive thresholding algorithm based on image processing is then developed to regularize the temperature field, maintaining global features while eliminating high-frequency outliers, in order to comply with applicable thermal control requirements. Experimental results demonstrate noticeable reduction of the CID distribution, where the mean CID could be reduced by 25–50%, as verified in three typical freeform parts using optimized temperature field.

Research on discretization algorithm of free-form surface for robotic polishing

Traditional surface processing measures for free-form surface parts are unable to distinguish the curvature of the surface to be machined, nor can they identify the concave and convex characteristics of different areas of the surface, as well as the time-varying characteristics of the contact between the tool and the surface of the part, making it impossible to tailor the processing parameters to the different mathematical characteristics of each area of the part surface. The result is that different areas of the same surface cannot be machined uniformly. In order to solve the above problems, this study proposes a discrete partition algorithm for free-form surface parts from the perspective of free-form surface parts simplification. The algorithm takes the spatial coordinates of each triangular patch after meshing the 3D model of the free-form surface part as input parameters. The partition of the free-form surface part is realized by the criterion for determining the spatial position of triangular patches, the method for detecting overlapping edges of triangular facets based on the principle of co-linear determination and the strategy for determining the difference of normal vectors between triangular patches. The 3D solid model after the corresponding surface partition is further obtained by reverse modeling. In addition, in order to effectively improve the machining efficiency of the free-form surface parts after partition, this study proposes an optimal machining path planning algorithm based on the principle of genetic algorithm. This algorithm ensures that the processing equipment can finish each sub-surface and plan the polishing path based on the results of surface partition, and then achieving the complete polishing process of the surface in the shortest time. Finally, the simulation software was used to simulate and analyze the surface polishing quality before and after the partition processing, which proved that the proposed surface partition algorithm can effectively improve the machining accuracy of free-form parts.

A Review of Advances in Cold Spray Additive Manufacturing

Cold Spray Additive Manufacturing (CSAM) produces freeform parts by accelerating powder particles at supersonic speed which, impacting against a substrate material, trigger a process to consolidate the CSAM part by bonding mechanisms. The literature has presented scholars’ efforts to improve CSAM materials’ quality, properties, and possibilities of use. This work is a review of the CSAM advances in the last decade, considering new materials, process parameters optimization, post-treatments, and hybrid processing. The literature considered includes articles, books, standards, and patents, which were selected by their relevance to the CSAM theme. In addition, this work contributes to compiling important information from the literature and presents how CSAM has advanced quickly in diverse sectors and applications. Another approach presented is the academic contributions by a bibliometric review, showing the most relevant contributors, authors, institutions, and countries during the last decade for CSAM research. Finally, this work presents a trend for the future of CSAM, its challenges, and barriers to be overcome.

A novel approach to robotic grinding guaranteeing profile accuracy using rigid-flexible coupling force control for free-formed surfaces

This paper proposes a novel approach to robotic grinding guaranteeing profile accuracy for free-formed surfaces. Based on the material removal model and the feed-drive ability of the robotic grinding system, hybrid constraints of material removal, force-amplitude and robot dynamics are determined simultaneously. The feedrate and contact force are planned simultaneously to guarantee maximum efficiency with the hybrid constraints. A novel rigid-flexible coupling force-control actuator is developed to restrict overshoot and improve the control accuracy of the grinding force. Experiments demonstrate that the proposed method significantly improves the profile accuracy, grinding efficiency and surface quality of free-formed parts.

Post-machining allowance optimization of directed energy deposited impeller blades using point cloud registration

Complex free-form parts manufactured by directed energy deposition (DED) have problems of uneven allowance distributions and severe stair-case effects. In order to optimize allowance for post-machining in a hybrid additive and subtractive machine centre, a point cloud registration method is proposed for allowance optimization to match the on-machine measured point cloud of the deposited part with the point cloud of the theoretical model. Firstly, the deposition path is used to construct the inspection path and obtain the maximum and minimum envelops of the deposited part. Secondly, the allowance distribution for the uniform and positive criterion is solved using the pattern search algorithm, after the datum planes are overlapped. Finally, a free-form blade of a centrifugal impeller is selected as a case study for allowance optimization. The proposed method is also compared with an allowance optimization method using genetic algorithm. The results show that the proposed method is more accurate and efficient for rapid allowance optimization for DED complex parts.

Partition-based 3 + 2-axis tool path generation for freeform surface machining using a non-spherical tool

Abstract When machining a complex freeform part, using a non-spherical tool could significantly improve the machining efficiency, as one can adaptively adjust the tool posture to maximize its contact area with the part surface. However, since adjusting the tool posture requires changing the tool orientation, a five-axis machine tool is needed, which is extremely expensive as compared to a conventional three-axis machine tool. Moreover, for a complex freeform surface with high curvature variation, to match its curvature change, the tool axis has to drastically change accordingly, thus inducing high velocity and acceleration on the machine tool’s rotary axes. To address these issues, in this paper we propose a partition-based 3 + 2-axis strategy for machining a general complex freeform surface with a non-spherical tool. As only a finite small number of distinct tool orientations are needed for 3 + 2-axis machining, an indexed three-axis machine tool suffices, thus relieving the need of an expensive five-axis machine tool. In addition, the much-increased rigidity of the three linear axes of the machine tool will greatly improve the kinematics and dynamics of the machine tool and thus enhance the machining accuracy. Experiments in both computer simulation and physical machining are carried out, whose results confirm that, when compared to using a conventional spherical cutter, by using a non-spherical cutter and adaptively adjusting the contacting tool posture and the feed direction, significant improvement in machining efficiency could be achieved, e.g., more than 50% achieved in our experiments.

Autonomous Robotic Sensing for Simultaneous Geometric and Volumetric Inspection of Free-Form Parts

Robotic sensing is used in many sectors to improve the inspection of large and/or complex parts, enhancing data acquisition speed, part coverage and inspection reliability. Several automated or semi-automated solutions have been proposed to enable the automated deployment of specific types of sensors. The trajectory to be followed by a robotic manipulator is typically obtained through the offline programmed tool paths for the inspection of a part. This method is acceptable for a part with known geometry in a well-structured and controlled environment. The part undergoing assessment needs to be precisely registered with respect to the robot reference system. It implies the need for a setup preparation phase for each new part, which can be very laborious and reliant on the human experience. This work combines real-time robot control and live sensor data to confer full autonomy to robotic sensing applications. It presents a novel framework that enables fully autonomous single-pass geometric and volumetric inspection of complex parts using one single robotised sensor. A practical and robust robot control sequence allows the autonomous correction of the sensor orientation and position to maximise the sensor signal amplitude. It is accompanied by an autonomous in-process path planning method, capable of keeping the inspection resolution uniform throughout the full extension of the free-form parts. Last but not least, a by-product of the framework is the progressive construction of the digital model of the part surface throughout the inspection process. The introduced framework is scalable and applicable to widely different fields.

The Effect of Laser Focus Shift on the Morphology of Melt Pool and Microstructure in Selective Laser Melting AlSi10Mg

Selective Laser Melting (SLM) enables to fabricate complex freeform geometries parts from computer models. In this paper, the effect of laser focus shift on the morphology, surface quality, grain size and texture of the SLMed AlSi10Mg was studied. Deep melt pool was found in 0 mm laser focus shift, while shallow and wide melt pool were found in-1.5 mm and +1.5 mm laser focus shift. The proportion of high angle grain boundaries in 0mm laser focus shift sample is the biggest han that in-1.5mm and +1.5mm. Height has a maximum value of 338 μm in +1.5 mm laser focus shift. The maximum Arithmetic average height is 33.5 μm in +1.5 mm laser focus shift. When the laser focus shift is 0 mm, the average grain diameter has a minimum grain diameter of 4.9 μm. {001}<100> cubic texture was found in sample fabricated by +1.5 mm laser focus shift rather than 0 mm and-1.5 mm laser focus shift.

Side surface topography generation during laser powder bed fusion of AlSi10Mg

Additive manufacturing (AM) is a direct manufacturing process that makes it possible to fabricate “near net shape” freeform parts. Among the many metal AM techniques, laser powder bed fusion (LPBF) is the most effective at obtaining complex structures with internal cavities, such as tortuous heat exchangers or lightweight lattice structures. AM technologies have therefore attracted considerable attention, which has led to research and development in many industries. Nevertheless, the surface topographies obtained by current AM techniques are still limiting industrial implementation for parts with high requirements. In this work, surface generation during LPBF was studied and optimized. The main aims were i) to optimize both side surface roughness and material density by studying the influence of the primary process parameters and ii) to investigate the effect of process options on side surface roughness generation for optimization purposes. The roughness dispersion and process reproducibility were also monitored and evaluated. A relationship between top and side surface roughness and material density was established. As a result, both optimizations could be performed in parallel. Analysis of the process reproducibility revealed an important roughness dispersion, especially from one side to the other. Consequently, recommendations on surface measurements were proposed. Compensations and contour settings are key parameters that can help reduce the side surface roughness. Indeed, geometrical positioning of the different weld tracks is also an important issue that must be addressed to reduce surface roughness. Based on the findings of this study, it is possible to reduce the areal average roughness Sa from 40 to 10 µm.

Fused deposition modelling process environmental performance through the carbon footprint evaluation

Additive manufacturing (AM) is a free-form build process in which an object is created adding multiple layers. AM process is characterized by on-demand freeform part manufacturing with multiple materials and not for big production batches. There are different types of additive technologies that are capable of building pieces in diverse materials. For this study, various types of materials were tested in a fused deposition modeling (FDM) process using a standard workpiece and the build process was simulated in CURA software. The aim is to show the variability of the energy and the CO2 equivalent emissions produced in FDM manufacturing technology depending on the material used, specimen orientation, and the production batches. Variability in the material choice and the orientation of the piece was found for the environmental performance.

A practical fabrication strategy for wire arc additive manufacturing of metallic parts with wire structures

Wire arc additive manufacturing (WAAM) is well suited for the manufacture of sizeable metallic workpieces featuring medium-to-high geometrical complexity due to its high deposition rate, low processing conditions limit, and environmental friendliness. To enhance the current capability of the WAAM process for fabricating structures with complex geometry, this paper proposes a robot-based WAAM strategy adapted specifically for fabricating freeform parts with wire structures composed of multiple struts. Contributions in this work include: (i) The study of bead modelling, which establishes optimal welding parameter selection for the process; (ii) the novel manufacturing strategy, including the adaptive slicing methodology and height control system for accurately depositing every single strut; and (iii) detailed manufacturing procedures for multi-strut branch intersections as well as the collision-free path planning to control the overall fabrication process. To verify the effectiveness of this proposed WAAM approach, two complex wire structures were fabricated successfully, indicating the feasibility of the proposed fabrication strategy.

New mechanistic model to predict machining time for milling free form geometries using 4-axis milling

Predicting the required time to machine a component is very important for planning the production of a factory. However, in milling of free form parts, commercial CAM software are not able to estimate the time precisely. This is because the CAM does not consider individual machine limitations. This work presents a mechanistic method developed to estimate more accurate the time to mill a free form geometry using any 4-axis milling machine. The maximum error observed on experimental validation of the method was less than 6%, 200 times more accurate than commercial CAM.

Multi-Axis Support-Free Printing of Freeform Parts with Lattice Infill Structures

In additive manufacturing, infill structures are commonly used to reduce the weight and cost of a solid part. Currently, most infill structure generation methods are based on the conventional 2.5-axis printing configuration, which, although able to satisfy the self-supporting condition on the infills, suffer from the well-known stair-case effect on the finished surface and the need of extensive support for overhang features. In this paper, based on the emerging continuous multi-axis printing configuration, we present a new lattice infill structure generation algorithm, which is able to achieve the self-supporting condition for both the infills and the boundary surface of the part. The algorithm critically relies on the use of three mutually orthogonal geodesic distance fields that are embedded in the tetrahedral mesh of the solid model. The intersection between the iso-geodesic distance surfaces of these three geodesic distance fields naturally forms the desired lattice of infill structure, while the density of the infills can be conveniently controlled by adjusting the iso-values. The lattice infill pattern in each curved slicing layer is trimmed to conform to an Eulerian graph so to generate a continuous printing path, which can effectively reduce the retractions of the nozzle during the printing process. In addition, to cater to the collision-free requirement and to improve the printing efficiency, we also propose a printing sequence optimization algorithm for determining a collision-free order of printing of the connected lattice infills, which seeks to reduce the air-move length of the nozzle. Ample experiments in both computer simulation and physical printing are performed, and the results give a preliminary confirmation of the advantages of our methodology.

Correction to: A skeleton-based process planning framework for support-free 3 + 2-axis printing of multi-branch freeform parts

Correction to: A skeleton-based process planning framework for support-free 3 + 2-axis printing of multi-branch freeform parts

A skeleton-based process planning framework for support-free 3+2-axis printing of multi-branch freeform parts

Continuous 3+2-axis additive manufacturing (AM) is an emerging AM technology that overcomes the signature problems of the need of support structures and the staircase effect of the traditional 3-axis AM. This paper presents a novel process planning framework for automatically generating a multi-axis support-free printing path for continuous 3+2-axis AM of an arbitrary freeform part. The framework is based on the geometric processing of skeletonization and decomposition and is particularly suitable for a part with distinct multiple trunk-branch structures. The physical printing experimental results carried out by the authors indicate that the proposed framework has performed well and fabricated some challenging models with large overhangs and twisty spatial topologies.

Towards design of mechanical part and electronic control of multi-material/multicolor fused deposition modeling 3D printing

Additive manufacturing (AM) is an important, rapidly emerging, manufacturing technology that takes the information from a computer-aided design (CAD) and builds parts in a layer-by-layer style. Thus, since the development of 3D printing, the fused deposition modeling (FDM) is one of the most promising and inexpensive techniques for shaping free-form parts. It used a thermoplastic filament as a starting material to extrude, layer by layer, the suitable 3D object. It is assisted by a large community of developers, searchers, and users and with multiple free hardware and software open sources expanding the scope of its applications. However, in this process, printing objects with multi-materials and multicolors becomes a major challenge for most extrusion system of printing platforms. For example, in case it is required to print multi-material and multicolored parts, a high-quality head extruder should be used. The main objective of this work is to show the importance of proposing a new extruder system of printing parts with multicolors and materials. For this purpose, in the present paper, the different extrusion systems capable of printing parts with multicolors and materials were presented. The operating principle of each extrusion system is well explained through functional diagrams. Then, its advantages and disadvantages are cited. Finally, some prospects for improving these systems are presented and that will be the work of the next paper.

FormPATCH - Forming of Complex Composite Structures using Patching Technologies

Fiber-reinforced plastics parts produced in medium or large quantities can be manufactured by forming flat textile fabrics to a component contour. Due to the stiffness of fibrous semi-finished products, forming of complex geometries with an uncut fabric is not always possible. Various effects in the fabric structure can occur, that affect the function or design of the final component. Therefore, patch placement of cut fabrics is a common way to build complex geometries. Usually, fewer and larger patches lead to a more economical process as handling steps can be reduced.This paper presents an optimization algorithm for large patch geometries. A possibility to simulate fabric forming and resulting shear deformations is the fast, but approximate, kinematic method. We use state of the art kinematic simulation approaches and enhance them for our purposes with integration in an optimization process. The optimization objective is to generate as few patches as possible to cover the entire geometry. For the simulation, different positioned and randomly distributed starting points are generated that lead to different patches. The algorithm uses Open Cascade’s Computer Aided Design (CAD) libraries and a Qt user interface (library for cross-platform graphical user interfaces). After importing CAD parts, users can run the patch optimization algorithm, or alternatively a classic kinematic simulation by selecting a start point, a direction and fabric properties. Each calculated patch is stored locally in a Scalable Vector Graphics (SVG) file. The SVGs contain shear angles at cross-over points of the textile and, furthermore, the surrounding border of a patch. Saving this border in Drawing Interchange Format (DXF) makes it available for CNC cutters. Finally, we demonstrate the proof of concept by the experimental patching of a generic free-form part with three calculated patch geometries.

Model-based temperature offset compensation for additive manufacturing by directed energy deposition

Laser based directed energy deposition (DED), also known as laser metal deposition or laser cladding, is an additive manufacturing technology for building 3D freeform parts. Reliable temperature measurements are of obvious interest and importance for the control of these processes. We propose a model-based method for the correction of temperature measurements from an imperfectly aligned sensor, which is a pyrometer in our process. We show that the proposed method can improve the reliability of the pyrometer-based temperature measurements even if the pyrometer is carefully aligned and calibrated according to industrial standards. We apply the proposed method to a powder-based directed energy deposition process. Due to its simplicity, the proposed method can easily be adapted to other additive manufacturing process types.

Bonding between silicones and thermoplastics using 3D printed mechanical interlocking

Silicones have desirable properties such as skin-safety, high temperature-resistance, and flexibility. Many applications require the presence of a hard body connected to the silicone. Traditionally, it has been difficult to create strong bonding between silicones and hard materials. In this study, a technique is presented to control the bonding strength between silicones and thermoplastics through mechanical interlocking. This is realized through a hybrid fabrication method where silicone is cast onto a 3D-printed mold and interlocking structure. The influence of the structure's design parameters on the bonding strength is explored through theoretical modeling and physical testing, while the manufacturability of the 3D-printed structure is ensured. A CAD tool is developed to automatically apply the interlocking structure to product surfaces. The user interface visualizes the theoretical strength of the cells as the designer adjusts the cell parameters, allowing the designer to iteratively optimize the structure to the product's load case. The bonding strength of the presented mechanical interlocking structure is more than 5.5 times higher than can be achieved with a commercially available primer. The presented technique enables custom digital design and manufacturing of durable free-form parts. This is demonstrated through application of the technique in over-molded products, airtight seals, and soft pneumatic actuators.