Coordinate system setting for post-machining of impeller shape by wire arc DED and evaluation of processing efficiency | Scientific Reports

Scientific Reports volume 14, Article number: 18262 (2024) Cite this article

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Wire arc additive manufacturing (WAAM) is a direct energy deposition (DED) process that uses arc welding. It is a method of stacking beads made by melting metal wires with an arc heat source generated by a short-circuit current. Compared to other metal additive manufacturing methods, this process can be used to quickly produce large and complex-shaped metal parts. However, due to the multi-bead stacking method, the surface is highly curved and the dimensional errors are large; therefore, post-processing of the surface by cutting is required. Impellers, which are widely used in various industries, have complex shapes and high material consumption during cutting; therefore, the WAAM process can improve the manufacturing efficiency. In this study, a manufacturing process for an impeller with a diameter of 160 mm was developed by using the WAAM process. A 6-bladed fan-type impeller used for high-pressure fluid delivery was similarly modeled, and the product was additively manufactured using an Inconel 625 alloy wire. Manufacturing conditions that ensure productivity and quality or the product were determined through experimentation. Considering the post-processing of the WAAM-fabricated structure, the robot and tool paths of the impeller model were designed, and the error in the process coordinate system caused by attaching and detaching the workpiece between the two processes was reduced. Through the post-processing of the WAAM-fabricated structure, the production efficiency and process reliability were verified when the conventional manufacturing method and WAAM process were applied.

High-value-added mechanical parts in various fields, such as the aerospace, marine, and defense industries, have been manufactured according to the 4th industrial revolution. Most of the mechanical parts required in the industry are produced by cutting, which also plays an important role in the production of most other metal products1,2,3. Cutting is the forming of a desired shape by applying a shear force to the surface of a material using a cutting tool, such as an end mill, drill, or insert, and produces a plastic deformation on the surface of the material. This process is suitable for manufacturing parts that require high dimensional accuracy and surface roughness. However, manufacturing a product with a complex shape is disadvantageous because it consumes a lot of material due to chip evacuation and requires a long processing time. Difficult-to-cut materials with excellent mechanical properties, such as stainless steel, Inconel, and titanium can cause high tool wear during machining. To reduce tool wear, the allowable cutting speed and depth of cut must be low during machining, which increases the machining time for product manufacturing. In addition, because these difficult-to-cut metals are usually expensive, the cost of material loss due to chip evacuation is high4,5. Therefore, the manufacturing industry is attempting to replace the manufacturing process of machine parts with additive manufacturing.

Wire arc additive manufacturing (WAAM) is a multi-bead deposition process that melts and stacks metal wires supplied by an arc heat source generated by a short-circuit current. WAAM can easily produce complex-shaped 3D structures and is economical because very little material is wasted in realizing the desired shape6,7,8,9. Compared to the powder bed fusion (PBF) method, the WAAM process has a faster material deposition rate than the PBF method. It uses a metal wire, which is easier and cheaper to produce than metal powder10,11,12. A welding torch is attached to a 6-axis robotic arm. It was installed for the construction of the equipment, and the robot had a high degree of freedom for the drive, so it was easy to produce 3D-free curved surfaces13,14,15,16,17. The WAAM process, which has these advantages, is suitable for the production of products with complex shapes of medium and large size, and impellers are representative.

The impeller is a component of centrifugal pumps used in most industrial fields and sends fluid at high pressure using rotating blades. Impellers are classified according to the shape of the blade into radial-, axial-, and mixed-flow pumps and the type of pump is determined by the type of impeller used. All parts with blade shapes, such as propellers and turbines, belong to the category of impellers18,19. Currently, impellers are manufactured by cutting a workpiece into a volume that contains the shape of the finished product. For casting, machining time, material consumption, and tool wear are relatively low because the surface of the workpiece produced according to the designed sand mold model is subjected to finishing. However, additional processes and production costs are incurred to produce impeller-shaped castings, and defects such as internal pores and cracks often occur during the casting process. Therefore, applications in fields that require reliability testing, such as the aerospace industry, are difficult. In the case of a cylindrical workpiece, a drawn round bar is used; the preparation process is simpler than the casting method, and internal defects can be prevented. However, in the manufacture of an impeller, all parts except the body and blade must be removed by roughening. Generally, when manufacturing an impeller by processing a cylindrical workpiece, more than half of the workpiece volume is discharged as chips, resulting in long processing time, high material consumption, and tool wear.

Because the WAAM process stacks beads produced by welding, the surface of the WAAM-fabricated structure has millimeter-level roughness and large roughness variations. In addition, it is difficult to predict the results of additive manufacturing because the volume of the bead produced varies depending on welding conditions and significant dimensional errors. Therefore, post-processing of the surface by cutting is essential for the application and commercialization of the additive manufacturing processes. The cutting process consists of a three-dimensional feed system in the x-, y-, and z-axes based on the absolute coordinate origin of the machine tool. It is a process based on a computer numerical control (CNC) system that receives a command from the designed NC code (tool path) and processes it by transferring machine coordinates and tool rotations. Therefore, the machining origin between the workpiece and tool is defined during operation, and a reference surface is required that can define the origin coordinates20,21,22,23,24. However, due to the irregular surface of WAAM-manufactured structures it is difficult to determine the origin coordinates. In addition, it is difficult to accurately predict the shape of the workpiece when designing a machining path using computer-aided manufacturing (CAM). Various methods and studies have been conducted for setting the workpiece and machine coordinate system in machine tools, and in particular, precise coordinate system settings for products with a rotating body shape such as impellers remain an issue to this day. However, in a situation where additive manufacturing and machining are separated, robot coordinates that take machining into account are added to the current setting method25,26,27,28,29.

In previous studies, a 6-blade impeller with a diameter of 80 mm was additively manufactured through the Hybrid-WAAM process. The framework of the product was designed using the CAD/CAM system. A strategy for fabrication was planned based on the CAD model designed in the designed framework. Tool paths for deposition and machining were designed using the working coordinate system. However, as the entire impeller was additively manufactured on a square mat to fix the coordinate system, the processing time increased, and there was a limit to the process design that did not consider post-processing30. In another study, a WAAM fabrication structure in the form of an impeller blade was fabricated by applying the gas metal arc welding (GMAW) cold metal transfer (CMT) method. Through slicing software, the shape of the structure was predicted according to the welding conditions, and a stable structural shape was produced by changing the conditions. However, post-processing research on the product completion stage has not been conducted31. Currently, process systems and physical property evaluations using the WAAM are continuously conducted. However, no research has been conducted on fabrication methods for the commercialization and post-processing of WAAM-fabricated structures. WAAM post-processing requires a manufacturing process that is different from machining for cylindrical shape workpieces.

The impeller has a rotating body that occupies a large volume and contains several thin blades. The conventional manufacturing process for impellers involves a roughing operation to produce the blade shape, which removes more than half of the material. Therefore, there is significant material consumption and tool wear, which reduces production efficiency. When applying the WAAM process, additional preparation time and equipment are required to fabricate the product. However, the amount of processing can be significantly reduced, and accordingly, all production cost factors required for the process, such as the processing time and tool cost, are reduced. Therefore, it is expected that production efficiency will be significantly improved when material cost and time loss are taken into account. Figure 1 compares the material consumption rates of the cylindrical workpiece machining process and the WAAM process during impeller machining and shows the impeller manufacturing process with WAAM.

Impeller manufacturing process and application of WAAM process (a) Machining of cylindrical workpiece (conventional manufacturing method) (b) WAAM-fabricated structure machining (WAAM method) (c) Wire DED and post-processing strategy.

To manufacture impellers using WAAM, a fabricated structure must have a volume that can accommodate the product. The size of WAAM-fabricated structures varies depending on the manufacturing conditions, and various design methods can be applied depending on the welding conditions and routes. The production losses of the process vary depending on the design method32. In terms of the stability of the product manufacturing process, large-scale deposition of structures can reduce surface defects on the product after processing; however, the resulting material and processing time losses increase. Conversely, if the structure is scaled down to reduce processing losses, there is a risk of defective products during processing. Therefore, when applying the wire DED, it is important to create optimal conditions for creation by considering both conservative (sound) and economical designs.

The WAAM-fabricated structure was fixed to the jig clamp of the MCT processing machine for post-processing. The workpiece must be clamped to match the tool path of the CAM. In general, the origin of a machine tool can be set with an error within 1/1000 mm if an indicator is used through the reference plane of a fixed workpiece before machining. However, for WAAM-fabricated structures, it is difficult to determine the machining origin because of the irregular shape of the surface. Even if the size of the workpiece satisfies the product size, the tool path for actually machining the workpiece will be distorted if the coordinates of the machining origin are not accurately determined will be distorted. Unlike conventional machining methods, post-processing in wire DED requires selective processing of the surface of the workpiece. The tool path that is twisted on the basis of the workpiece, deviates from the machine coordinate path for machining the surface of the WAAM-fabricated structure, and tool overcuts and non-machined paths occur during machining. In the case of overcuts, tool wear and damage due to overcuts can occur quickly. For materials that are difficult-to-cut, such as high-hardness steel or heat-resistant alloys, this is an even more sensitive issue because the stable depth of cut allowed for the tool is very small. In addition, the surface of the product is not processed because of the non-processing path, which ultimately, leads to product defects.

Post-processing is critical to optimizing the manufacturing process with WAAM, and it is necessary to set the machine coordinate system to prevent machining path errors during post-processing. For two operations to run smoothly, the robot torch path for WAAM and the tool path for post-processing were designed based on the absolute coordinates of the modeling. During that two the processing, the base plate was arranged in the same manner as the path of the robot torch, according to the absolute coordinates of the model. During post-processing of the WAAM-fabricated structures, the workpieces were clamped in the same way as the CAM software. The coordinate system was set based on the absolute coordinate plane of the workpiece placed on the jig. Figure 2 shows the process and method of setting the coordinate system for the post-process manufacturing process.

Method for optimizing origin coordinates between processes for WAAM post-processing.

In this study, a process for manufacturing impellers using the wire arc DED method was planned and designed. A vertical impeller fan with a size of 160 mm, used in the actual industry, was simulated and modeled, and the manufacturing process was planned. A baseline experiment was conducted to determine the optimal fabricating conditions and method for creation of the impeller, and the product defects that occurred during the process were identified. Based on the fabricating conditions selected by the experiment, the modeled impeller shape was additively manufactured, and the finished product was manufactured through post-processing. For the processing of the WAAM-fabricated structures, CAM designs for the machining paths and machine tool clamping were presented to optimize the origin coordinate system between the two processes. Based on the processed product, we analyzed the production efficiency compared with the conventional process and verified the reliability of the process to highlight the advantages of the WAAM method and confirm whether it can be used practically.

The modeling was designed for fabricating of the impeller shape using the WAAM process. The designed model was a flat impeller-shaped fan with six main blades and six secondary blades. The thickness of the blades was up to 6.07 mm, taking into account the thickness of the bead formed when the GMAW process was performed in a single-line path. The diameter of the impeller was 160 mm, and a SUS 316 L rolled round bar disk with a diameter of 170 mm and thickness of 25 mm was used as the base plate for making the products. An experiment was conducted based on a single-blade shape to select the conditions for manufacturing the part. Figure 3 shows the modeling of the impeller and blade shape designed in this study, as well as the basic experimental progress using single-blade fabrication. Considering the shape characteristics of the impeller, a base plate was used for the body, and only a small volume of the blade was stacked. This strategy can significantly reduce the amount of material and time required to manufacture the impeller and increase production efficiency. An experiment was planned to fabricate a product by post-processing the surface of the stacked blade and base plate. During welding, the welding directions of each layer were crossed and stacked, taking into account the end deflection caused by current reduction at the arc beginning and endpoints. A Fronius Mig welding equipment (TPS-500I) and ABB's 200 kg industrial 6-axis robot were used to perform the experiment. An IR camera from FLIR was used to measure the interlayer temperature of the multi beads structure in real time during deposition.

Experimental equipment and method for fabricating impellers via wire DED (a) Experimental equipment and environment (b) Impeller modeling shape and dimensional information (c) Real-time temperature monitoring during deposition (d) Additively manufactured impeller blade geometry.

The experiments used an Inconel 625 alloy wire. Inconel 625 alloy is extremely resistant to corrosion and oxidation. It has high high-temperature strength, oxidation resistance, and a long lifespan in high-temperature and high-pressure environments33,34. However, these heat-resistant alloys have excellent thermal conductivity, hardness, and toughness. Therefore, they are classified as difficult-to-cut materials that cause severe tool wear during cutting and processing is very difficult. In addition, products with complex shapes and high material consumption rates, such as impellers, are in the spotlight as high-value-added products because they are more expensive than other metals. Therefore, wire DED using Inconel 625 alloy with these characteristics can significantly increase the manufacturing efficiency of impellers.

The volume, such as the thickness and height, of the additively manufactured structure produced by the WAAM process was determined as a function of the shape of the bead and the welding conditions selected. The shape of the bead is affected by various welding conditions, such as welding current/voltage, torch travel speed, and wire feed speed, and the amount of heat input is determined accordingly. In general welding machines, the wire feed speed is subordinate to the optimum value of welding conditions depending on welding current and voltage, and a stable single bead can be produced based on welding current and torch travel speed. In this experiment, the shape and size of the structure manufactured using the welding current and torch travel speed as variable factors were analyzed, and the optimum welding conditions for manufacturing the designed blade were selected during post-processing. As shown in Table 1, the welding conditions ranging from 140 to 170 A welding current and 3.5 to 4.5 mm/s torch travel speed were selected based on general welding conditions of Inconel 625 alloy from previous studies.

Based on the blade modeling, the torch path was generated and a total of 12 layers were deposited according to the welding conditions to produce a structure with a height of 25 mm. The thicknesses and heights of the single-blade WAAM-fabricated structures fabricated under different welding conditions were measured. The measurement results show that the thickness and height of the structure are proportional to the welding current and voltage, and inversely proportional to the torch travel speed. The volume of the structure varies according to the wire feeding speed, which depends on the welding current. As the volume of the WAAM-fabricated structure increased, the chip emission increased during product processing. Therefore, the volume of the WAAM-fabricated structure created using the wire feeding speed, torch travel speed, welding path length, and generated chip evacuation rate were theoretically confirmed. In addition, the heat input generated per unit length was confirmed using the welding current, voltage, and welding speed. Table 2 lists the shape dimensions, chip evacuation rate, and heat input of the single-blade WAAM-fabricated structure based on the welding conditions.

Surface defects were confirmed by visual inspection of a single post-processed blade. By classifying specimens by welding current, product defects were confirmed at welding currents of 140–160 A. The reason for product defects is that surface processing is not performed in an area where the additively manufactured structure adequately covers the volume of the designed model. Millimeter- and micrometer-sized defects were observed and could be confirmed with the naked eye. In general, the blade-side defects occurred because the additively manufactured structure did not satisfy the modeling thickness under welding current conditions of 140 and 150 A. The upper part of the blade defect caused by insufficient height was confirmed at a torch feed speed of 4.5 mm/s. At welding current levels of 160 and 170 A, no defects were observed with the naked eye, but several micrometer-sized defects were observed and confirmed with a metallographic microscope. As the unit heat input increases due to the welding current, torch-moving speed, and wire feeding speed, the width of the bead increases and the height decreases, so it can be assumed that it is affected by the shape of the entire stacked product. Under the condition of high heat input that can sufficiently contain the modeling volume, no defects were found due to insufficient volume; however, several micrometer-sized microscopic defects were found on the surface of the processed part. It was suspected that this was due to the development of porosity due to overheating rather than a lack of volume. Figure 4 shows the defect types as a function of welding conditions. Finally, a welding current of 170 A and a welding speed of 4 mm/s were selected considering the chip removal rate, heat input, and defects according to the welding conditions.

Post-processing result of a single blade (a) Defect due to lack of thickness (sample 1, 2, 3, 4) (b) Defect due to lack of height (sample 3, 4, 5, 6) (c) Millimeter size defects that can be confirmed visually (sample 4, 5, 6) (d) Micrometer-sized microdefects (sample 7, 8, 9, 10, 12) (e) Post-processed additive manufacturing single blade surface micro-defect magnification 50 × (f) Post-processed additive manufacturing Single blade surface micro-defect 100 × magnification (g) Surface micro-defect boundary observation.

Wire DED is disadvantageous because its mechanical properties are inferior to those of the raw materials. Moreover, the mechanical strength of each layer varies in the direction of the stack height, depending on the welding and interlayer cooling conditions. This is because the metal structure of each layer grows differently due to the repeated cooling and reheating of the molten metal, and it is a fundamental problem that wire DED has not yet been commercialized. If the metal structure of a single structure is different and there is a deviation in rigidity, it is impossible to select the optimal cutting conditions during post-processing, and tool wear may occur significantly. Therefore, the optimal post-processing conditions were selected by measuring the cutting force as a function of the interlayer temperature. As shown in Fig. 5, the additively manufactured structure with a size of 50 × 30 × 9 was produced by stacking 12 single straight layers. The temperature of the stacked beads was measured in real time during specimen production using an IR camera. Since thermal equilibrium is achieved throughout the structure when the selected interlayer temperature is reached, the average value of the previously deposited bead surface is defined as the interlayer temperature. Deposition was performed on an SUS 316 L base plate material, and the welding current and speed were applied at 150 A and 4 mm/s, respectively. For this, the CMT method was applied under the condition of an 12.2 mm bead thickness with reference to a previous study on the WAAM process with Inconel 625 alloy. The specimens were prepared by applying an interlayer temperature of 100, 150, and 200 ˚C at which the next layer is stacked as interlayer cooling conditions and an interlayer cooling time of 120 s at 20 ˚C room temperature. Ar 99.9% was used as the protective gas, and the recommended preheating temperature of 150˚C for Inconel 625 alloy was considered. Table 3 shows the stacking temperature of the specimens as a function of the interlayer cooling conditions.

Fabrication of specimens for cutting force measurement according to interlayer temperature and real-time temperature monitoring using IR camera (a) Definition of experimental variables, (b) WAAM specimens in the shape of a wall for cutting force measurement according to interlayer temperature, (c) Temperature distribution during deposition, (d) Temperature distribution when cooling starts after arc is turned off, (e) Temperature distribution when the selected interlayer temperature is reached.

The time and temperature at which welding and cooling occurred under each condition were determined. For the sample (Sample 1) where the interlayer cooling time between stacked layers was kept constant at 120 s, the bead surface temperature on which the next layer was stacked increased as the layers were stacked. This is because the cooling time corresponding to the welding heat input is insufficient because the beads are stacked for each layer. This means that the interlayer temperature was not constant during deposition. For the samples (Samples 2–4), where the interlayer temperature was kept constant at 100, 150, and 200 °C, the subsequent layer stacking continued when the selected bead surface temperature was reached after sufficient air cooling, and the welding conditions between the layers were the same. When the interlayer temperature is kept constant, the lower the selected interlayer temperature, the higher the cooling time for each layer.

The cutting load generated when end milling of the additively manufactured specimen under the interlayer cooling conditions was measured. The cutting force was measured using an arbor-type tool dynamometer from Promicron and confirmed by the bending moment generated in the tool during machining. The cutting force was measured using the end mill corner milling method, which is a processing method for additively manufactured blade workpieces as shown in (Fig. 6), the cutting force generated when cutting the upper, middle, and lower parts of the surface of the specimen in a flat state after removing the surface curvature of the additively manufactured structure was compared. The tool used was YG1's Titanox D10 end mill, and optimum cutting conditions were applied to the heat-resistant alloy processing of the tool. Table 4 lists the cutting conditions that were used to measure the cutting force.

Method for measuring the cutting force of specimens by the interlayer temperature.

From the experimental results, it can be confirmed that there was a significant deviation in the cutting force between the lower and upper parts of the specimens when the interlayer temperature was not constant. The strength increased as the stacking on the bead layer progressed at high temperatures. As confirmed in a previous study, it is predicted that the difference in thermal curing is according to the preheating temperature of the Inconel 625 alloy. Assume that a cutting force deviation occurs for each part during the post-processing of the WAAM structure. In this case, the optimal cutting conditions cannot be maintained constantly during product processing, which affects the tool life and product quality, depending on the product production cycle. In addition, the strength of the product is reduced, and weak parts may be produced. For the specimens where the interlayer temperature was kept constant, the cutting force was kept constant in the range of 10% depending on the interlayer height. Depending on interlayer temperature maintained, there is a difference in the average cutting force, and the lowest cutting load is measured at the 100 ˚C condition. However, the lowest deviation was confirmed at the 200 ˚C condition. The 200 ˚C interlayer temperature condition was selected considering product production time and production efficiency as a function of the interlayer cooling time. Figure 7 shows the cutting force values for the upper, middle, and lower parts of the additively manufactured specimen as a function of the interlayer cooling conditions.

Cutting force results for each part depending on cooling conditions between layers.

It is necessary to set the post-processing coordinate system, for which the pretreatment of the base plate was performed before starting deposition35,36. As for the shape of the impeller, the blades on the circular base plate were rotationally symmetric about the central axis. Therefore, the torch path of the robot and the tool path for post-processing must have exact origin coordinates in the x-, y-, and z-axis directions, and the z-axis rotational coordinates of the workpiece before and after deposition must match. The impeller in this study had a rotational structure, and it was possible to set the origin of the vertical transfer in the x-, y-, and z-axes by interlocking chuck clamping. However, because the origin of the rotational coordinates along the z-axis was not fixed, an additional reference point was needed. To match the designed robot path with the tool path for post-machining, sketching was made with a 90° chamfer mill in the same path based on the z-axis rotational coordinates. In addition, rotation coordinates origin grooving was performed at the edge of the circular specimen. This allows the rotational coordinates of the blade shape to be corrected during the attachment and detachment of the workpiece after the deposition. Based on the deposit conditions according to the previous experimental results, the impeller blade was stacked on the base plate where the sketching was completed. Before the deposition, the robot was operated along the same sketch path as the robot path, and the base plate was positioned to match the robot path. Figure 8 shows the pre-processing of the base plate and the arrangement of the robot coordinate system before the process. To remove moisture and preheat the surface of the base plate after the coordinate arrangement along the robot path, a preheating torch was used to preheat at 150 ˚C, and the wire DED was completed.

Setting the base plate coordinate system before deposition for post-processing (a) Defining the base plate coordinate system and machining the groove path (b) Setting the torch path of the base plate with the coordinate system defined.

The path of the robot was such that the six main blades were rotated and stacked in sequence, and after all the main blades were stacked, the sub-blades were stacked in the same sequence. This is most efficient considering the high-temperature heat generated by heat input during deposition and the cooling cycle of the beads. For the depositing conditions, the optimal requirements were based on previous experiments, and a welding current and torch speed of 4 mm/s at 170 A for the main blade and 4.5 mm/s at 150 A for the sub-blade were applied. During the process, the maximum temperature of the structure was monitored using a real-time IR camera. All additively manufactured structures in the shape dimension measurements satisfied the thickness and height requirements of the modeled impeller.

For post-processing of the additively manufactured structure, the workpiece was clamped in an interlocking chuck. The machining origins of the x- and y-axes of the machine tool were determined during the sketching process. In the preprocessing, the rotational origin of the additively manufactured structure was fixed based on the sketched groove processing part to determine the coordinates of the rotational origin. After the workpiece was fixed to the jig and the tool coordinate system was determined, the post-processing of the additively manufactured structure was performed. A Doosan’s Vertical 3 Axis MCT Machine was used for the machining. The machining path was designed using the CATIA CAM program, and the NC codes were extracted and applied according to the tool path. The workpiece was designed considering the shape dimensions of the stacked blades, which was confirmed by previous experiments, and the helix path was applied to prevent overcutting considering the cutting conditions. Before processing, the process was verified by a simulation. The tool used for machining was an end mill dedicated to heat-resistant alloys, such as titanium and Inconel, and machining was performed considering the optimum cutting conditions for the tool. Table 5 lists the processing conditions for the roughing and finishing processes during post-machining. Figure 9 shows the final product manufacturing results through Wire DED and post-processing.

Wire DED and post-processing process and impeller manufacturing results (a) Impeller blade manufacturing process through wire DED (b) Deposition of the first layer of an impeller blade and verification of the rotational coordinate system (c) Real-time monitoring of deposition temperature through IR camera (Interlayer temperature maintenance range 150~250 °C) (d) Post-processing according to the coordinate system of the deposited product (e) Additively manufactured impeller with post-processing completed.

Impeller manufacturing efficiency was compared between the additive and machining-only processes. Machining simulation was performed based on the cutting conditions and the designed tool path. CIMCO software was used for the machining simulation of the machine tools, and the actual machining time was compared based on the tool path of the NC code designed according to the tool feed and cutting depth. Figure 10 shows the simulation according to the cutting conditions for each process. Under the same cutting conditions, the machining of the circular material required 48 h 54 min 24 s, and the machining of the WAAM structure, including the wire DED, required 18 h 3 min 54 s, confirming that the final production time was reduced to 30 h 50 min 30 s. This is the processing time required after the wire DED and does not include the time required to detach the additively manufactured workpiece from the jig and adjust the coordinate system of the machine tool. Even taking into account the additional working time required, the processing time could be significantly reduced, and it is predicted that improvements can still be made with the development of auxiliary equipment and technical methods to optimize the two processes. The modeling volume was calculated, and the material removal rate was confirmed by CATIA software. The machining of the circular workpiece decreased by 17.7 to 49.4%, and the machining of the additively manufactured workpiece processing decreased by 31.7%. Tool wear and the number of tools used were not quantitatively confirmed, but are expected to decrease in proportion to the amount of material removed. Table 6 shows the production efficiency using the machining-only and WAAM processes.

Toolpath and machining simulation for impeller manufacturing (a) Machining of cylindrical workpieces with 1mm margin compared to the product (machining-only method) (b) Machining of workpieces manufactured by wire DED.

A product evaluation of the manufactured impeller was performed. To date, products manufactured using wire arc additive manufacturing have not been used in practice, so the evaluation of product reliability has not been standardized. Mechanical parts require precise machining of shape defects; accordingly, both robots and machine tools require precise coordinate transfer. However, the wire DED fabrication of a product, the manufacturing space and post-processing space are separated by machining. Even if the work is performed by precise coordinate transfer in each process, an error occurs in the process coordinate system of the product when the workpiece is transferred for the secondary process. In this study, additional reference points were sketched on the workpiece to correct the machining path for defining the coordinate system of the subsequent process. To ensure the reliability of the strategy, the shape errors and compliance with product specifications were checked by 3D precision measurements. The shape error of each blade was measured using the rotational axis of the impeller. The modeling in the x and y directions and the positional error of the actual product were measured. The impeller had a shape tolerance of 0.02 mm, depending on the manufacturing method and the application environment. When the error was measured at the points of the 16 blade surfaces (10 main blades, six sub-blades), an error of up to 0.013 mm was found. It was confirmed that the shape accuracy of the designed model was met by the precise measurement of the manufactured product. In addition, to verify the reliability of the process, 3D scans of additively manufactured structures under the same manufacturing conditions were also performed and compared with the designed impeller model. To obtain a high-quality 3D scan model, the surface of the specimen was painted before scanning to suppress light reflections. An additional straight groove was machined at the center of the additively manufactured specimen to define a reference straight line that could be compared to the model coordinate system. Based on the matched coordinate systems, it was confirmed that the arrangement of the additively manufactured impeller blades and the geometry of the model matched and that the geometry of the model did not exceed the geometry of the scanned additively manufactured structure in all areas. Figure 11 shows the reliability evaluation of the product through 3D precision measurement and 3D scanning of the impeller.

Reliability evaluation of manufactured products (a) Evaluation of impeller machining precision through 3D precision measurement (b) Verification of process reliability through interference analysis of 3D scanned data and modeling geometry.

Wire DED and post-processing were performed to fabricate the impeller-shaped product. A deposition method for achieving the selected model shape and solutions to the problems encountered during the process were presented.

A 160 mm class vertical 6-blade impeller used in industry was modeled, and stacking conditions for manufacturing the product were selected by basic experiments. In general, products made of Inconel 625 alloy less than 6 mm thick can be stacked through a single pass, and the optimal fabrication conditions were selected considering processing time, material loss, and process loss. During deposition, the interlayer temperature must be kept constant, taking into account the mechanical properties of the product.

The impeller fabrication was performed based on the optimal conditions for manufacturing, which were determined by basic experiments. To match the post-processing origin during the WAAM, groove processing, and rotation origin, grooving was performed as a pre-processing step for the base plate. The base plate and robot path were aligned before making the impeller blades. The rotational origin was fixed by the machined groove to match the rotational origin with the machining path during post-processing of the additively manufactured stacked circular impeller structure. A general machining path for impellers was designed using a CAM program and a stock model was used based on the shape and dimensions of the additively manufactured blade structure, which were confirmed by basic experiments. After designing the cutting path, the process was verified by a simulation, and post-processing was performed.

Process and product evaluations of the impeller were conducted. Precise post-processing measurements, 3D scans of structures, and model comparisons ensured the method of setting the coordinate system for wire DED and post-processing, and the reliability of the product.

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

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This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Grant No. 2019R1A5A8083201) and by the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korean government (MOTIE) (141588607, Additive Manufacturing Technology Innovation Alliance for Aerospace/Small modular reactor/Defense industry).

Department of Smart Manufacturing Engineering, Changwon National University, Changwon-si, 51140, Korea

Hwi Jun Son, Bo Wook Seo, Chang Jong Kim, Seok Kim & Young Tae Cho

Department of Mechanical Engineering, Changwon National University, Changwon-si, 51140, Korea

Seok Kim & Young Tae Cho

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Hwi Jun Son: conceptualization, investigation, methodology, writing—original draft. Bo Wook Seo: investigation, software. Chang Jong Kim: investigation, software. Seok Kim: methodology, project administration, writing—reviewing and editing. Young Tae Cho: project administration, supervision, funding acquisition, writing—reviewing and editing.

Correspondence to Young Tae Cho.

The authors declare no competing interests.

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Son, H.J., Seo, B.W., Kim, C.J. et al. Coordinate system setting for post-machining of impeller shape by wire arc DED and evaluation of processing efficiency. Sci Rep 14, 18262 (2024). https://doi.org/10.1038/s41598-024-68723-x

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Received: 21 December 2023

Accepted: 26 July 2024

Published: 06 August 2024

DOI: https://doi.org/10.1038/s41598-024-68723-x

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