Wire And Arc Additive Manufacturing Process Research Articles

Phase-field modeling and Experimental investigation for rapid solidification in wire and arc additive manufacturing

The rapid solidification speed and significant temperature gradient observed in the wire and arc additive manufacturing (WAAM) process result in a deviation of the solid-liquid interface from the equilibrium state assumed by the conventional directional solidification phase field (PF) model. This, in turn, limits the ability of the traditional directional solidification PF model to accurately depict the microstructure evolution in additive manufacturing, where non-equilibrium effects dominate. In this work, we employed a 3D finite element (FE) model to simulate the temperature distribution during the wire and arc additive manufacturing (WAAM) process. Additionally, a modified PF model that applies to additive manufacturing is also used to simulate the dendrite morphology and solute segregation behavior in the WAAM process at different scanning speeds. Experimental validation of the simulation results was also conducted. We observed a significant reduction in dendrite growth velocity and solute segregation with increasing scanning speed, attributable to the combined effects of the temperature gradient G(t) and solidification speed Vp. Furthermore, higher scanning speeds were found to increase the primary dendrite arm spacing (PDAS) and average dendrite width. Importantly, our results demonstrated a quantitative agreement between the simulation results using the modified directional solidification PF model and the experimental observations. This approach provides a foundation for selecting and predicting processing parameters and microstructural characteristics under additive manufacturing conditions, facilitating a comprehensive analysis of dendrite growth and solute segregation at different scanning speeds in the WAAM process.

Detection and quantitative evaluation of surface defects in wire and arc additive manufacturing based on 3D point cloud

ABSTRACT Wire and Arc Additive Manufacturing (WAAM) with high efficiency and low-cost is an economical choice for the rapid fabrication of medium-to-large-sized metallic components and has attracted great attention from scholars and entrepreneurs in recent years. However, defects such as porosity, and humps, could occur occasionally after each layer of deposition on weld bead surfaces due to disturbances and process abnormities. Detection and quantitative evaluation of weld bead defects is crucial to ensure successful deposition and the quality of the entire component. In this paper, a novel defect detection and evaluation system was developed for WAAM utilizing machine vision technology. The system incorporated new defect detection algorithms based on analysing the 2D curvature of the weld bead height curve and the 3D curvature of the weld bead point cloud. Furthermore, a defect evaluation algorithm was developed based on reconstructing the normal weld bead contour using geometric features extracted from the accumulated point cloud. This system enables the automatic detection of weld bead morphology during the WAAM process, offering important information about the location, type, and volume of defects for effective interlayer repairs and enhanced part quality.

Blended structural optimization of steel joints for Wire-and-Arc Additive Manufacturing

Metal Additive Manufacturing, and in particular Wire-and-Arc Additive Manufacturing (WAAM) has proved to be a great and efficient alternative to the common subtractive manufacturing processes to realize complex-shape members and connections for construction. With reference to complex spatial structures such as gridshells, the connections play a crucial role both in the design and the construction processes. Novel computational design tools allow designers to conceive optimized joint solutions while minimizing the material use, resulting in difficult geometries to be fabricated with conventional methods. The present work aims at providing an innovative integrated design and fabrication framework to efficiently apply topology optimization algorithms to improve the efficiency of steel joints in complex spatial structures. The framework is based on the so-called “blended” structural optimization approach aimed at incorporating manufacturing constraints, basic principles of conceptual structural design and structural requirements into topology optimization to design ready-to-fabricate complex steel joints. The designs are suitable for fabrication with the WAAM process. The approach is applied to a case study to re-design the joint connections of the world’s renowned British Museum gridshell rooftop structure. From a catalogue of various designs for each unique joint, one selected design is optimized and then checked for structural requirements in terms of strengths and stiffness. Then the optimized joint is validated for fabrication with WAAM process.

Characteristics of residual stress distribution in wire-arc additive manufactured layers of low transformation temperature material

High heat input in wire and arc additive manufacturing (WAAM) tends to result in high tensile residual stress, which adversely affects performance and service reliability. However, low-transformation-temperature (LTT) materials are susceptible to compressive residual stresses due to their low phase-transformation point. Here, the LTT material 10Cr10Ni was used as the deposited material to fabricate a pipe structure, and the contour method was applied to measure the residual stress distribution in the cross-section. A thermomechanical coupling analysis model was developed to investigate the residual stress generation mechanism in 10Cr-10Ni(LTT) during the WAAM process. The simulated deformation and residual stress distributions agree well with the measurements. Through simulation analysis of the deposition of different layers, we found the key to the generation of compressive residual stresses in the LTT layers lies in whether there is a transformation from austenite to martensite during the final thermal cycle. When the deposition height exceeded 5.405 mm (approximately four layers), compressive residual stresses were only generated in the last four LTT layers and transformed into tensile residual stresses in the other layers. This results in a concept called the effective region, which can be used as a guide for reducing stress in WAAM using LTT materials.

Prediction of multi-bead profile of robotic wire and arc additive manufactured components recursively using axisymmetric drop shape analysis

ABSTRACT Dimension prediction of robotic wire and arc additive manufacturing (WAAM) part is fundamentally dependent on the modelling accuracy of single-bead profile and its subsequent overlapping ones. Current multi-bead overlapping models are still not capable of describing the flatten valley area of WAAM parts. This paper proposes a new recursive model, based on coordinate transformation and axisymmetric drop shape analysis (ADSA), to predict multi-bead overlapping profiles. First, a single-bead profile model for WAAM is established based on ADSA, followed by detailed description of conventional and proposed modified recursive ADSA profile model. The properties of developed recursive ADSA model are then investigated to reveal the effects of overlapping ratio and single-bead aspect ratio. Finally, multi-bead overlapping deposition experiment is carried out to validate the model feasibility. The results show that the modified recursive ADSA model is more accurate than the conventional one for its better accountability of valley areas. It is also indicated that the modified recursive ADSA model is suitable for the robotic WAAM process. The research outcome is beneficial to improving the forming accuracy of WAAM parts and geometry prediction of other additive manufactured products.

Flaw Detection in Wire and Arc Additive Manufacturing Using In-Situ Wide Frequency Bandwidth Acoustic Pressure

Wire and arc additive manufacturing (WAAM) is an additive manufacturing (AM) process that can produce large metallic components with low material waste and high production rates. However, WAAM’s high deposition rates require high heat input which can result in potential defects such as pores, cracks, lack of fusion or distortion. For practical implementation of the WAAM process in an industrial environment it is necessary to ensure defects-free production. However, the NDT inspection using traditional NDT techniques (ultrasound, eddy currents, x-ray, for example) is a very demanding task, especially during part production. Therefore, reliable online NDT inspection and monitoring techniques are needed for the industrial spread of WAAM. The objective of this work is to detect flaw formation on WAAM produced parts using in-situ acquired acoustic data with a frequency bandwidth from 10 to 1MHz. WAAM parts were processed with deliberately introduced contaminations while its acoustic signal was obtained to correlate different signal characteristics with defects. To identify flaw formation, two distinct types of microphones were employed to acquire data from the same deposition process. The processing of the signal consisted of applying time and frequency domain techniques, namely, Power Spectral Density and Short Time Fourier Transform. The acoustic signatures obtained allowed for the differentiation between flawed and flaw free signals and for the spatial location of the contaminations. The acoustic signal acquired also showed that the data acquired by conventional microphones is not enough to fully characterize the acoustic spectrum emitted by the WAAM process. This work demonstrates the potential of acoustic data and signal processing in the online inspection of WAAM produced parts.

Dynamic control of aluminum morphology by wire and arc additive manufacturing based on image feedback

The stability of the forming layer shape is a critical factor that impacts the final quality of sample morphology in wire and arc additive manufacturing (WAAM). This paper investigates the forming process and control methods to identify ways to optimize the process and improve the quality of the final product. The study aims to enhance the quality and precision of samples produced by the WAAM process by establishing a morphology control method based on image feedback. The focus is on real-time image acquisition using a CCD and simultaneous extraction of forming process parameter characteristics. Using a central composite experimental design, a prediction model is developed to estimate key process parameters and feature sizes of deposits, including deposition height and width. To further analyze the dynamic characteristics of feature sizes of deposited layers, step response identification is conducted using three process parameters as the system input: forming speed, arc current, and wire feeding speed. The experiments are designed to determine the weight of each process parameter and achieve a high level of response speed and precision. The findings indicate that the feature size of the deposited layer is most sensitive to changes in forming speed, followed by wire feeding speed, while arc current has the least impact on feature size. After verification, the monitoring of the deposition height and width was found to be in good agreement with the prediction model, with an accuracy of over 90%. The results of this study can be used for size measurement and optimization of large aviation aluminum alloy components using WAAM technology and to improve the quality of products produced using these processes.

A two-stage unsupervised approach for surface anomaly detection in wire and arc additive manufacturing

Wire and arc additive manufacturing (WAAM) has gradually been applied in industrial applications in recent years due to its low cost, high deposition rate, and high material utilization rate. Anomalies in the WAAM process, such as inclusion, porosity, and lack of fusion, can have unpredictable effects on the quality of the final product. While some studies have investigated anomaly detection methods in the WAAM process, they mainly rely on supervised learning methods that require extensive manual labeling, with less attention paid to unsupervised models. Furthermore, most studies focus on significant anomalies that are rare in actual production, limiting their practical application. This paper proposes a two-stage unsupervised defect detection framework based on online melt pool video data. By considering the motion characteristics of the manufacturing process, a revised threshold method is used to detect anomalies during the WAAM process. Combining machine contextual information, the physical spatial location of defects is further identified and displayed through a human-machine interactive interface. The dataset used in this study is derived from real printing processes of WAAM parts. Compared with baseline methods, the proposed approach significantly improves recall and achieves an F1-score of 86.3% on the test set.

Manufacturing time estimator based on kinematic and thermal considerations: application to WAAM process

Metal additive manufacturing has been pointed as the answer to reduce manufacturing time and cost for aeronautic parts with a high buy to fly ratio. The manufacturability of a part by additive manufacturing depends on important indicators that would allow it to be cost effective. One key indicator is the manufacturing time, which is highly dependent on an important factor: the interlayer time. The interlayer time is the time needed by the material to cool down to a chosen temperature, called interlayer temperature, that allows a new deposition of molten material. The interlayer temperature is defined by using time–temperature-transformation (TTT) diagrams, the final goal being to avoid the appearance of detrimental phases that could lead to a decrease in the material’s mechanical properties. The interlayer temperature is intimately correlated with the cooling curve. The difficulty of predicting the cooling time is due to the influence of the part geometry, the deposition strategy, and the dimensions of the substrate. Their correlation needs to be understood in order to minimize the travel time (ttravel) while ensuring an acceptable material quality. This paper presents a methodology to estimate manufacturing time that combines kinematic and thermal criteria for Wire and Arc Additive Manufacturing (WAAM) process. Application is performed for stainless steel 316L. In this first step toward an advanced manufacturing time estimator, only the first layer attached to the building plate is analyzed from a thermal point of view. The thermal analysis is based on an analytical model enabling the evaluation of the preheating temperature (PhT) in a first approach and providing an adequate framework for the evaluation of cooling curves in a second time. The model includes an accurate description of robot kinematics through the consideration of a realistic travel speed variation along the toolpath. It is used to evaluate an indicator that quantifies the thermal influence of a given deposition strategy. The results show the dependency relationship between manufacturing strategy and inherent thermal gradient and its implications on part production time.

Microstructure and Solute Concentration Analysis of Epitaxial Growth during Wire and Arc Additive Manufacturing of Aluminum Alloy

Microstructure and solute distribution have a significant impact on the mechanical properties of wire and arc additive manufacturing (WAAM) deposits. In this study, a multiscale model, consisting of a macroscopic finite element (FE) model and a microscopic phase field (PF) model, was used to predict the 2319 Al alloy microstructure evolution with epitaxial growth. Temperature fields, and the corresponding temperature gradient under the selected process parameters, were calculated by the FE model. Based on the results of macroscopic thermal simulation on the WAAM process, a PF model with a misorientation angle was employed to simulate the microstructure and competitive behaviors under the effect of epitaxial growth of grains. The dendrites with high misorientation angles experienced competitive growth and tended to be eliminated in the solidification process. The inclined dendrites are commonly hindered by other grains in front of the dendrite tip. Moreover, the solute enrichment near the solid/liquid interface reduced the driving force of solidification. The inclined angle of dendrites increased with the misorientation angle, and the solute distributions near the interface had similar patterns, but various concentrations, with different misorientation angles. Finally, metallographic experiments were conducted on the WAAM specimen to validate the morphology and size of the dendrites, and electron backscattered diffraction was used to indicate the preferred orientation of grains near the fusion line, proving the existence of epitaxial growth.

Solidification microstructure evolution and its correlations with mechanical properties and damping capacities of Mg-Al-based alloy fabricated using wire and arc additive manufacturing

Magnesium (Mg) alloys, as the lightest metal structural material with good damping capacities, have important application prospects in realizing structural lightweight and vibration reduction. However, their engineering application is greatly limited by poor plastic formability. Wire and arc additive manufacturing (WAAM) provides a potential approach for fabricating large-scale Mg alloy components with high manufacturing flexibility. In this study, the evolution of the solidification microstructure of a WAAM-processed Mg-Al-based alloy was quantitatively analyzed based on the analytical models; then, the correlations between the solidification microstructure and mechanical properties/damping capacities were investigated. The results revealed that the WAAM-processed Mg-Al-based alloy with an equiaxed-grain-dominated microstructure displayed a simultaneous enhancement in mechanical properties and damping capacities compared to those of the cast Mg-Al-based alloy. The good combination of mechanical properties and damping capacities are mainly attributed to the weakened basal texture with a relatively high Schmid factor for basal <a> slip, the twinning-induced plasticity (TWIP) effect associated with the profuse {10-12} tensile twinning, and the relatively high dislocation density caused by the thermal stress during the WAAM process.

Microstructure and Mechanical Properties of High-Strength, Low-Alloy Steel Thin-Wall Fabricated with Wire and Arc Additive Manufacturing

High-strength, low-alloy (HSLA) steel has attracted much attention in the manufacturing industry because of its good combination of high strength and toughness, low cost, and good formability. Wire and arc additive manufacturing (WAAM) technology can realize the rapid prototyping of HSLA steel parts. This study investigated a 26-layer HSLA steel component fabricated with the WAAM technique. The microstructure of the deposited wall of ER120S-G is mainly acicular ferrite, and there are longitudinal, preferentially growing dendrites along the deposition direction. With the deposition height accumulation, the top sample’s interlayer temperature increases and the amount of acicular ferrite in the microstructure decreases, while the amount of quasi-polygonal ferrite, Widmanstatten ferrite increases. The changes in microhardness were consistent with the corresponding microstructure gradients: the microhardness of the top sample showed a decreasing trend along the deposition direction, while the microhardness of the middle sample was uniform and stable. The present work shows that the mechanical properties of HSLA steel parts deposited using WAAM technology have good strength and toughness. The microstructure gradient of the sample along the deposition direction did not lead to a significant difference in the tensile strength of the sample at different heights. On the contrary, the ductility of the longitudinal sample is slightly lower than that of the transverse sample, indicating some anisotropy in the deposited sample, which is related to the directional growth of grains along the direction of heat flow. From the current work, the thin wall of HSLA steel prepared with the WAAM process has good mechanical properties, which indicates that it is feasible to replace the traditional processing method with the WAAM process to rapidly manufacture an HSLA steel structure meeting the performance requirements.

Investigation into the influence of the interlayer temperature on machinability and microstructure of additively manufactured Ti-6Al-4V

The capability of wire and arc additive manufacturing (WAAM) to produce large, near-net-shaped parts with inexpensive equipment has led to the process being considered as one of the options to significantly decrease the buy-to-fly ratio in aircraft manufacture. Even so, there are several challenges associated with the process: achieving mechanical properties and microstructure similar to wrought material, as well as the low surface quality of the parts. The low surface quality is usually improved by milling. As with the microstructure, here too the question arises as to whether the process is comparable to milling wrought material. A significant factor influencing the microstructure according to literature is the interlayer temperature during the WAAM process. Therefore, the objective of this research was to study the influence of the interlayer temperature on the machinability and the microstructure of wire and arc additively manufactured Ti-6Al-4V. Consequently, the machinability was first determined for Ti-Al6-V4-parts manufactured with three different interlayer temperatures. Then, the macro- and microstructures were analyzed and, finally, the mechanical properties were determined. Contrary to expectations based on the state of the art, the machinability was not influenced by the interlayer temperature. This aligns with the mechanical properties and the macro- and microstructures, which are only slightly affected by the interlayer temperature.

Enhanced strength-ductility synergy in a wire and arc additively manufactured Mg alloy via tuning interlayer dwell time

Strength-ductility trade-off is a common issue in Mg alloys. This work proposed that a synergistic enhancement of strength and ductility could be achieved through tuning interlayer dwell time (IDT) in the wire and arc additive manufacturing (WAAM) process of Mg alloy. The thermal couples were used to monitor the thermal history during the WAAM process. Additionally, the effect of different IDTs on the microstructure characteristics and resultant mechanical properties of WAAM-processed Mg alloy thin-wall were investigated. The results showed that the stable temperature of the thin-wall component could reach 290 °C at IDT=0s, indicating that the thermal accumulation effect was remarkable. Consequently, unimodal coarse grains with an average size of 39.6 µm were generated, and the resultant room-temperature tensile property was poor. With the IDT extended to 60s, the thermal input and thermal dissipation reached a balance, and the stable temperature was only 170 °C, closing to the initial temperature of the substrate. A refined grain structure with bimodal size distribution was obtained. The remelting zone had fine grains with the size of 15.2 µm, while the arc zone owned coarse grains with the size of 24.5 µm. The alternatively distributed coarse and fine grains lead to the elimination of strength-ductility trade-off. The ultimate tensile strength and elongation of the samples at IDT=60s are increased by 20.6 and 75.0% of those samples at IDT=0s, respectively. The findings will facilitate the development of additive manufacturing processes for advanced Mg alloys.

Grain refinement and mechanism of steel in ultrasound assisted wire and arc additive manufacturing

The small volume molten pool of wire and arc additive manufacturing (WAAM) undergoes complex non-equilibrium solidification process, resulting in the formation of columnar grains that severely limits the mechanical properties of the fabricated components. For refining the microstructure of deposition layers, the optimized ultrasonic equipment and loading methods are installed on a deposition platform, and the ultrasonic energy is coupled synchronously with the WAAM process using ER70S-6 welding wire as the filling material. Experimental results show that high-intensity ultrasound can transform coarse columnar grains into fine equiaxed grains, the area and degree of grain refinement increase with the ultrasonic amplitude. A computational fluid dynamics model of ultrasound assisted WAAM was developed to study the pressure field distribution in the molten pool. The region of acoustic cavitation in molten pools is highly consistent with the region of grain refining, showing that acoustic cavitation is the key factor in changing the grain structure. The collapse of cavitation bubbles generates cyclic alternating local high pressure, resulting in high velocity melt flow near the grain, which causes fatigue fracture of growing columnar grains exceeding the yield strength. A theoretical model is proposed to calculate the maximum length of grain growth under acoustic cavitation, which is in agreement with the experimental statistics of grain size. This work reveals the effect area and mechanism of ultrasound on the rapidly solidified molten pool, and provides a theoretical basis for grain refinement in ultrasound assisted WAAM.

A Nonautoregressive Dynamic Model Based Welding Parameter Planning Method for Varying Geometry Beads in WAAM

Wire and arc additive manufacturing (WAAM) is a promising method for directly manufacturing parts with complex shapes. However, the accuracy of the existing welding parameter planning methods would dramatically decrease when the bead geometry changes dynamically due to the long-term dependence, strong coupling, and hysteresis properties of the WAAM process. To this end, a nonautoregressive dynamic model is proposed to predict the bead geometry, and an adaptive model predictive control (aMPC) method is proposed to plan welding parameters to achieve high manufacturing accuracy. First, in the proposed dynamic model, the long-term dynamic characteristics of the WAAM process are modeled by a resample long short-term memory network by considering the fluidity of the welding pool, which is the crucial factor of dynamic characteristics of the welding process. Second, in the proposed aMPC method, the strong coupling is addressed by high multiobjective performance, and the hysteresis is considered by a long control horizon. Thus, the aMPC method can reduce the control latency and improve the manufacturing accuracy for varying geometry beads. The proposed methods are validated by experiments, which indicate that the proposed methods are effective.

Approach towards a Quality Assurance System for Wire and Arc Additive Manufacturing

Wire and Arc Additive Manufacturing (WAAM) of Ti-6Al-4V is becoming increasingly important in the aerospace industry for the production of large parts. Due to the high welding requirements of the material, high quality demands are placed on the process. To meet these high demands, quality assurance measures are applied to maintain mechanical and geometrical part properties. First, the interlayer temperatures that are applied influence the final geometry. The part must meet geometric accuracies in order to be machined after the WAAM process. Second, Ti-6Al-4V materials have a high affinity to absorb oxygen from the environment at elevated temperatures. This oxygen uptake results in a discoloration of the surface and an embrittlement of the material. Therefore, a defined and monitored oxygen content in the build chamber is crucial. This work presents an approach to determine limitations for the interlayer temperature of the part and the oxygen content in the build chamber. The influence of a temperature deviating from the set interlayer temperature on the layer width was analyzed. By varying the interlayer temperature, the layer width varied by up to 3 mm. It was shown that different restrictions for the oxygen content in the build chamber apply depending on the part size.

On the connection of the heat input to the forming quality in wire-and-arc additive manufacturing of stainless steels

WAAM (wire-and-arc additive manufacturing) is recently gaining much interest for the fabrication of parts with wide size. A crucial issue associated to this technology is how to control the process variables to achieve expected qualities of parts. The aim of this investigation is to explore the relationship of the heat input (Q) with the forming quality of 308L stainless steel. The outcomes demonstrate that the heat input has significant effects on the shape, microstructures, and mechanical properties of the walls. Both the height and width of single weld beads increase with an increment in Q. The walls fabricated with the low heat inputs feature better mechanical properties and lower surface roughness. The tensile strengths (YS0.2% and UTS) and the surface roughness (Sa) of the walls built with the lower heat input (230 J/mm) are 292 ± 4.95 MPa, 523 ± 4.24 MPa, and 106 μm, respectively, whilst YS0.2% = 273 ± 4.34, UTS = 482 ± 5.66 MPa, and Sa = 179 μm for the wall fabricated with the higher heat input (576 J/mm). This investigation contributes to enriching the understanding on the WAAM process of steels and helping operators to select a proper heat input for specific applications.

Underlying Methodology for a Thermal Process Monitoring System for Wire and Arc Additive Manufacturing

The Wire and Arc Additive Manufacturing (WAAM) process has a high potential for industrial applications in aviation. The interlayer temperatures influence the dimensions and geometric deviations of the part. Monitoring the absolute interlayer temperature values is necessary for quantifying these influences. This paper presents an approach for determining the absolute values of the interlayer temperatures during the process using Ti-6Al-4V. The emissivity and transmittance are determined and calibrated, enabling precise thermographic measuring during the WAAM process. The recorded thermographic data are then compared to signals of thermocouples so that the absolute temperature values can be aligned. The methodology is validated by its transfer to measure the interlayer temperature at different regions of interest. The effect of a heat accumulation using Ti-6Al-4V in WAAM was determined. The methodology enables a reproducible and non-tactile measurement of the interlayer temperature during the WAAM process. The results show that with an interlayer temperature of 200 °C, a heat accumulation occurs within a layer. The heat accumulates in the center of the layer because the free ends of the layer cool down faster than the center of the layer.

Location-dependent microstructure analysis and mechanical behavior of inconel 625 using Cold Metal Transfer(CMT) based wire and arc additive manufacturing

Wire and Arc Additive Manufacturing(WAAM) is an emerging economic fabrication method to produce components with complex features. Inconel 625 (IN625) is a nickel-based super alloy with high strength and corrosion resistance at elevated temperatures. A single-walled Inconel 625 part was fabricated using a cold metal transfer(CMT) based WAAM process. This study focuses on location-based microstructure and mechanical analysis of a WAAM-printed wall. Three regions were analyzed, i.e., Top, Bottom, and Middle. The results show that the grain morphology varies with different locations. The texture {001} <001> is identified as preferred grain orientation using Electron Backscatter Diffraction(EBSD) analysis. The X-Ray Diffraction(XRD) analysis depicts the variation in peak intensity for different samples, and the γ-Ni FCC phase is only detected in all samples. The tensile strength is higher for the horizontal sample, and elongation is higher for the vertical sample. The microhardness values decrease with increasing height.