This study investigates the applicability of a novel laser-arc multi-energy deposition of Ti-6Al-4V with independent control of bead geometry and thermal input. A plasma transferred arc is used to generate an initial melt pool and melt wire feedstock, before controlled lateral elongation of the melt pool via a fiber laser and galvo scanner. The applicability to Ti-6Al-4V was first investigated using deposition parameters previously identified. Once successful bead geometry control was achieved, process parameters more conducive to wire directed energy deposition were investigated. This included investigation of the energy per unit area required to achieve accurate deposition of Ti-6Al-4V with minimal penetration and investigation into scanning strategy. In each case, optical microscopy was conducted and analysis of the bead geometry, penetration and heat-affected zone considered to determine the effect of each parameter change. The results demonstrated that independent control of bead geometry and thermal input could be achieved, allowing deposition of Ti-6Al-4V at a desired scan width and layer height and providing a framework for future multi-energy source directed energy deposition of Ti-6Al-4V.

This study investigates the applicability of a novel laser-arc multi-energy deposition of Ti-6Al-4V with independent control of bead geometry and thermal input. A plasma transferred arc is used to generate an initial melt pool and melt wire feedstock, before controlled lateral elongation of the melt pool via a fiber laser and galvo scanner. The applicability to Ti-6Al-4V was first investigated using deposition parameters previously identified. Once successful bead geometry control was achieved, process parameters more conducive to wire directed energy deposition were investigated. This included investigation of the energy per unit area required to achieve accurate deposition of Ti-6Al-4V with minimal penetration and investigation into scanning strategy. In each case, optical microscopy was conducted and analysis of the bead geometry, penetration and heat-affected zone considered to determine the effect of each parameter change. The results demonstrated that independent control of bead geometry and thermal input could be achieved, allowing deposition of Ti-6Al-4V at a desired scan width and layer height and providing a framework for future multi-energy source directed energy deposition of Ti-6Al-4V.

Achieving excellent mechanical properties in cost-effective additively manufactured Al alloys remains a critical challenge due to their high susceptibility to cracking and epitaxial columnar grain growth, ultimately weakening the material. This study employed wire and pulsed cold metal transfer arc additive manufacturing (CMT-WAAM) to fabricate Al-Cu-Mg alloys incorporating nano-TiC particles (TiCnps). The as-deposited (AD) and post-heattreated (T6) alloys exhibited isotropic and uniform mechanical properties, with ultimate tensile strengths and yield strengths of 328/216 MPa and 500/404 MPa, respectively. Microstructural observation showed textureless, equiaxed grains interspersed with intricate networks of micro- and nano-scale phases. For the AD alloy, theta/S eutectics and D022-Al3Ti/TiC particles were associated with solidification and solution segregation, whilst L12Al3Ti, theta'', and S ' nano-phases were formed under intrinsic heat treatment during deposition. This multi-phase microstructure notably contributes to Orowan and coherency shear strengthening. After T6 treatment, uniformly distributed GP zones, L12, and dual S ' precipitates were developed, leading to enhanced coherency shear strengthening. These findings indicate that the TiCnps reinforced Al-Cu-Mg alloy fabricated via CMT-WAAM achieves outstanding mechanical properties and isotropy, making it a promising candidate for highperformance structural applications with low material cost.

316L austenitic stainless steel manufactured by wire-direct energy deposition (w-DED) typically exhibits coarse columnar solidification microstructures and pronounced anisotropy, which significantly limits its structural performance and engineering applicability. In this work, a cold high-pressure inter-layer rolling (HPR) strategy combined with post-deposition solution heat treatment is proposed as an effective and industrially viable thermomechanical route for microstructure and property control. Cold HPR introduces a large plastic strain at a low and well-controlled inter-pass temperature. This suppresses dynamic recovery during deposition and allows substantial stored strain energy to accumulate. As a result, the as-deposited microstructure evolves from a solidification-dominated state to a deformation-controlled state. The rolled components show pronounced deformation of the gamma-austenite and fragmentation of the delta-ferrite. This deformation is accompanied by an increased fraction of low-angle grain boundaries and a higher dislocation density, which leads to significant strengthening. Subsequent heat treatment activates extensive static recrystallisation in the rolled samples. Fine equiaxed gamma-austenite grains with high fractions of high-angle and Sigma 3 boundaries are formed, together with a strongly weakened crystallographic texture. This rolling + heat treatment synergy enables a broad and continuous mechanical property window and delivers a superior strength-ductility balance. The achieved performance surpasses most reported additively manufactured and wrought counterparts. Overall, the proposed cold HPR + heat treatment route provides a practical and flexible pathway for tailoring the performance of large-scale w-DED 316L components.

Monitoring in wire-based directed energy deposition additive manufacturing (w-DEDAM) is crucial for ensuring part quality and process stability. It aids in optimising process control, enhancing efficiency, and minimising material waste. Additionally, monitoring provides valuable documentation for regulatory compliance and supports maintenance by tracking equipment performance. Effective monitoring and relevant control enhance process reliability, reduce scrap, and ensure the production of high-quality components. However, current monitoring systems in w-DEDAM face significant challenges. Key issues include a lack of standardisation in monitored variables, leading to inconsistent data and interpretation. Many existing monitoring systems often focus on the adjustable “knob” variable measurements rather than critical factors that more accurately represent physical process conditions. This narrow focus can fail to capture essential aspects of process explanation and part quality. Furthermore, there is a deficiency in integrating monitoring data with digital modelling and qualification methods, limiting the ability to use real-time data for comprehensive process optimisation and predictive analysis. Addressing these gaps is crucial for advancing system effectiveness. In this research, key process variables will be defined and explained to enhance process understanding. A knowledge-driven monitoring framework will be proposed to tackle the issues of standardisation and relevance of monitored parameters, ensuring that critical variables are accurately captured and utilised. Additionally, the study will introduce a commercial industry monitoring software based on this framework, which is currently used in the industry. This software integrates the proposed approach, offering a robust solution for real-time monitoring and process control, thus addressing existing limitations and supporting improved process optimisation and digital modelling.

Process control and quality assurance of wire + arc additive manufacturing (WAAM) and automated welding rely heavily on in-process monitoring videos to quantify variables such as melt pool geometry, location and size of droplet transfer, arc characteristics, etc. To enable feedback control based upon this information, an automatic and robust segmentation method for monitoring of videos and images is required. However, video segmentation in WAAM and welding is challenging due to constantly fluctuating arc brightness, which varies with deposition and welding configurations. Additionally, conventional computer vision algorithms based on greyscale value and gradient lack flexibility and robustness in this scenario. Deep learning offers a promising approach to WAAM video segmentation; however, the prohibitive time and cost associated with creating a well-labelled, suitably sized dataset have hindered its widespread adoption. The emergence of large computer vision models, however, has provided new solutions. In this study a semi-automatic annotation tool for WAAM videos was developed based upon the computer vision foundation model SAM and the video object tracking model XMem. The tool can enable annotation of the video frames hundreds of times faster than traditional manual annotation methods, thus making it possible to achieve rapid quantitative analysis of WAAM and welding videos with minimal user intervention. To demonstrate the effectiveness of the tool, three cases are demonstrated: online wire position closed-loop control, droplet transfer behaviour analysis, and assembling a dataset for dedicated deep learning segmentation models. This work provides a broader perspective on how to exploit large models in WAAM and weld deposits.

Increasing deposition rate is essential for higher productivity of additive manufacturing (AM). However, a high deposition rate usually requires high heat input to fully melt the fast-fed material, which could lead to defects due to molten material overflow. This paper presents a thermo-capillary-gravity model for predicting the overflow occurrence based on the analytically calculated reciprocal Bond number, $1/Bo$1/Bo. Comprehensive experiments show that when the $1/Bo$1/Bo is no greater than 0.74, or the bead height is no less than 1.16 times the capillary length, overflow is highly likely to occur. Two different steel wire-based AM processes were employed to validate the model, demonstrating an overall accuracy of 84%-93%. It is found that both energy and material inputs per unit length significantly affect the molten material overflow, and hence they can be adjusted to prevent overflow. The validated analytical modelling approach enables efficient prediction and control of overflow for a high deposition rate wire-based AM process.

In this study, round-robin experiments were carried out to evaluate the effectiveness of inter-pass peening as an in-process residual stress (RS) mitigation technique for Wire Arc Additive Manufacturing (WAAM) of Ti-6Al-4V components. A newly developed method, Phased Array Ultrasonics for Residual Stress Measurement (PAURS), was evaluated for its capability to accurately characterise residual stress distributions. To validate PAURS, two established techniques, X-ray Diffraction (XRD), a non-destructive method, and the Contour Method (CM), a destructive technique, were employed. PAURS, particularly when implemented with Full Matrix Capture (FMC), demonstrated enhanced accuracy through improved data redundancy and outlier rejection. Results from PAURS and XRD showed good qualitative agreement with finite element (FE) models derived from CM data, despite some quantitative differences. Inter-pass peening was found to significantly reduce tensile RS in the heat-affected zone, with performance comparable to post-process peening. These findings support the use of inter-pass peening as an effective in-process RS mitigation strategy and highlight the potential of PAURS as a reliable, non-destructive RS measurement method for WAAM Ti-6Al-4V components.

The thermal characteristics of a plasma transferred arc (PTA) and its mathematical representation are primary considerations when designing and modelling PTA-based wire arc additive manufacturing (WAAM). However, most of the currently used PTA thermal characteristics are derived from welding processes, which are not directly applicable to WAAM. In this study, the power density distribution, arc diameter and arc efficiency of PTA in the WAAM process were measured using the split anode calorimetry (SAC) method. The effects of key process parameters, including current intensity, plasma gas composition, plasma gas flow rate, and arc length, on the PTA power profile were systematically examined. The results show that for a typical PTA used in WAAM, the arc diameter ranged from 9.6 mm to 10.8 mm, with an arc efficiency of approximately 60 % within the tested parameter range. The PTA power becomes more concentrated as power density increases with higher current intensity and plasma gas flow rates. Additionally, a softer plasma was achieved by increasing helium content in the plasma gas or by using a longer nozzle-to-workpiece standoff distance, both of which are beneficial for avoiding keyhole defects. To accurately represent PTA power distribution, a binomial Gaussian heat source model was proposed, which captures the details of the arc power profile with a high accuracy of over 99.94 %, outperforming the conventional monomial Gaussian heat source model. The PTA calorimetry characterisation and the proposed binomial Gaussian model can be useful in establishing a better understanding of the PTA power profile and enhancing process control for high-precision WAAM.

Hybrid wire-arc directed energy deposition (WDED), in which complex features are deposited onto a forged base, offers a cost-effective solution for manufacturing geometrically complex ultra-high-strength steel components, particularly for aerospace applications. However, cracking at the base forging/build interface during post-build heat treatment limits its widespread application. This study investigates the underlying causes of interfacial cracking, highlighting microstructural inhomogeneity, elemental segregation and transformation stresses as likely key contributing factors. A modified three-step post-build heat treatment incorporating a normalisation step was developed to mitigate some of these issues. The optimised process successfully suppressed cracking by refining prior-austenite grains before the application of a conventional quenching step. This enhanced tensile performance beyond AMS6419K standards, supporting the industrial implementation of hybrid WDED in aerospace structures.

Wire arc additive manufacturing (WAAM) is suitable for building large-scale engineering structures with high deposition rates and relatively low costs. However, in a typical plasma transferred arc (PTA)–based WAAM process using an inclined wire and vertical torch, keyhole defects can occur due to the high arc pressure, and the process is sensitive to the wire-feeding position with respect to the workpiece. Therefore, in this study, a PTA-based WAAM process with a new configuration employing a vertical wire and an inclined plasma torch was investigated for the potential of mitigation of keyhole formation and improvement of process tolerance. In particular, detailed investigations were carried out on the metal transfer mechanisms and bead formation characteristics under various processing conditions. The results show that the new configuration significantly reduces the likelihood of keyhole formation compared with the conventional approach due to the changes in arc pressure and heat distribution. Systematic analysis reveals that process parameters, including wire feed speed, arc current, and plasma gas flow rate, strongly influence droplet transfer stability, melt pool dynamics, and final bead morphology, which offer guidance for future process optimisation.

The Innovative Aluminium filler Wires for Aircraft Structures (IAWAS) project aimed to demonstrate the potential of Wire Arc Additive Manufacture (WAAM) for the production of aluminium lithium components. Preliminary testing demonstrated the possibility of depositing an 2395 aluminium lithium filler wire using a plasma arc heat source and a local shielding device. The deposit had a low porosity level but also low ductility caused by long, vertical, segregated grain boundaries. Both chemical composition and deposition conditions are known to impact the deposit microstructure. In-situ alloying, an efficient technique to develop new material, was implemented using plasma arc as a heat source on aluminium lithium alloys. The results aligned with the literature review on the impact of copper on crack sensitivity and led to the design of a new alloy. Unfortunately, the composition selected yielded challenges during the drawing process, and the filler material quality was poor, leading to a low WAAM deposit quality. Machine hammer peening was implemented on the AA2395 alloy, resulting in a drastic increase in ductility and yield strength of 480 MPa after solution treatment and ageing. This alloy was used to manufacture an aluminium lithium demonstrator to showcase the potential of WAAM to produce real-life components.

The increasing demand for high-efficiency and high-quality wire-based arc directed energy deposition processes presents a significant challenge to the plasma arc, which is greatly influenced by plasma gas composition. This paper introduced mixed gases of helium (He) and argon (Ar) into the plasma gas for the deposition of Ti-6Al-4 V. The effect of plasma gas composition on the deposition characteristics concerning arc electrical characteristics, keyhole formation, metal transfer, bead geometry, and surface waviness were investigated. Results demonstrate that the arc voltage and heat input exhibited a linearly increasing tendency with the increase of He in the plasma gas, attributed to a higher ionization potential and thermal conductivity. The threshold current value for keyhole formation was effectively increased by adding He to the plasma gas. The keyhole formation was completely suppressed as the He content was higher than 50

This paper presents a study on residual stress measurement in wire-arc additively manufactured (WAAM) titanium samples using the non-destructive method of phased array ultrasonics. The contour method (CM) was used for the verification of the phased array ultrasonic results. This allowed for a comparison of measurement methods to understand the effects on the distribution of residual stress (RS) within Ti-6Al-4V samples and the effectiveness of measurement of residual stress using phased array ultrasonics. From the results of the experiments, the phased array ultrasonic data were found to be in good agreement with the CM results and displayed similar residual stress distributions in the samples. The results of the individual elements of the phased array were also compared and an improvement in accuracy was found. From per-element results, anomalies were found and could be mitigated with the ability to average the results by using phased array ultrasonics. Therefore, based on these results, there is a strong case for the benefits of using phased array ultrasonics as a method of residual stress measurement for WAAM Ti-6Al-4V components over other existing residual stress measurement techniques.

Multi-material components featuring high performance and design flexibility have attracted considerable attention, providing solutions to meet the performance demands of high-end equipment components. Achieving material diversity in additive manufacturing (AM) is a fundamental step towards manufacturing multi-material components. Wire arc additive manufacturing (WAAM), an important branch of AM technology, boasts notable advantages in the efficient and customized preparation of large-scale parts due to its high deposition efficiency and unrestricted forming size. However, achieving material diversity in WAAM, constrained by its reliance on wire-form raw materials, has emerged as a compelling challenge. Wire innovation, including multiple, stranded, and cored wires, have furnished solutions to this challenge. To this end, this review provides an overview of the current developments in WAAM via wire innovation and suggests future research directions, aiming to serve as a reference for the further advancement of WAAM. Initially, the article introduces several WAAM printing forms, their manufacturing features, printable materials and inherent manufacturing limitations, and the intermixing of metal constituents of WAAM, prior to highlighting the advantages and necessity of achieving material diversity. Subsequently, the exposition of multi-wire-arc AM demonstrates its utility in the preparation of binary or ternary alloys, inclusive of intermetallic compounds and functionally graded materials, responding adeptly to the deficiencies of conventional WAAM, which is limited to single-material printing. The merits and progression of stranded-wire-arc AM for high-entropy alloy production are synthesized and debated, especially given that creating components with multiple metal elements via multi-wire-arc AM customarily confronts the constraint of necessitating more intricate manufacturing equipment and processes. Further, the review explores the recently developed cored-wire-arc AM technology, which actualizes the manufacturing of composite materials, amalgamating metals and non-metals, to remedy the issues encountered with standard WAAM, incapable of realizing non-metallic material printing. Considering machine tools as an important means to achieve material diversity in WAAM, we expand on the current machine tool architecture and its corresponding design tools. Finally, the current research status on WAAM via wire innovation is summarized and potential future research directions are proposed.

Directed energy deposition additive manufacturing (DED-AM) has gained significant interest in producing large-scale metallic structural components. In this paper, a knowledge-based machine learning (ML) approach, combining both physics-based simulation and data-driven modelling, is proposed for a study on thermal variables of DED-AM. This approach enables both forward and backward predictions, which breaks down the barriers between the basic process parameters and key process attributes. Process knowledge plays a critical role to enable the prediction and enhance the accuracy in both prediction directions. The proposed ML approach successfully predicted the thermal variables of wire arc based DED-AM for forward modelling and the process parameters for backward modelling, typically within 7% errors. This approach can be further generalised as a powerful modelling tool for design, control, and evaluation of DED-AM processes regarding build geometry and properties, as well as an essential constituent element in a digital twin of a DED-AM system.

This study utilized double-wire plasma arc direct energy deposition to produce functionally graded materials (FGMs) with two transition designs, abrupt (AT) and gradual (GT), from Er90s steel to Invar. The study systematically compared the transition in chemical composition, microstructure, phase evolution, thermal stress, and mechanical performance. Both FGM types exhibited a band structure in the Er90s section and coarse columnar grains in the Invar section, with the AT deposit showing a 1mm thick, defect-free interface and the GT deposit having an 18mm thick transition region with distinct boundaries. It revealed diverse microstructures across the transition zones, including fine ferrite, martensite with minor retained austenite (RA), coarse columnar austenite with martensite dendrites, and single FCC austenite. The GT sample uniquely featured a microstructure of martensite laths inside prior austenite decorated by RA semicontinuous network, with a crack detected due to dilatational stresses from martensite transformation. Hardness was similar in both FGM types, with higher values at the interfaces, especially in the GT FGM. The GT FGM demonstrated higher strength but lower ductility compared to the AT FGM, with failure occurring in the Invar portion for both. Thermal stress modelling indicated smoother stress transitions in the GT sample but no significant performance differences between Er90s and Invar. This study showcases the effectiveness of double-wire plasma arc DED in producing steel/Invar FGMs with varying composition gradients. It also underscores the importance of selecting the right mixing ratio for Er90s/Invar FGM deposits to avoid cracking and deterioration of properties in the gradient area.

The coarse β-grain structures typically found in titanium alloys like Ti–6Al–4V (wt pct, Ti64) and Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242), produced by high deposition rate additive manufacturing (AM) processes, are detrimental to mechanical performance. Certain modified processing conditions have been shown to lead to a more refined grain structure, which has generally been attributed to a change in the solidification conditions with respect to the experimental Hunt diagram proposed by Semiatin and Kobryn. It is shown that with Wire Arc AM (WAAM) increasing the wire feed speed (WFS) is effective in promoting a columnar-equiaxed transition (CET). Conversely, estimates of the dendrite-tip undercooling using the KGT model suggest that this will be too small for free nucleation without the addition of artificial nucleants, due to the very low solute partitioning in Ti alloys. It is also shown that it is difficult to promote a CET with plasma transferred arc WAAM as computational fluid dynamics (CFD) melt-pool simulations indicate that the solidification parameters remain within the columnar region on the Semiatin-Kobryn Hunt map, within the constraints of a stable process. However, a high fraction of twin boundaries was observed in the refined β-grain structures seen at high WFS. This has been attributed to departure of ⟨001⟩ _β alignment from the direction of maximum thermal gradient, caused by the curvature of the fusion boundary, stimulating dendrite twinning during solidification. In addition, it is shown that increasing the WFS leads to a change in melt-pool geometry and a reduction of remelt depth, which promoted dendrite twinning and grain refinement.

Metal additive manufacturing is rapidly gaining popularity and interest from sectors aiming to produce larger-scale high-value components cost-effectively. To ensure each component is leaving the fabrication cell defect-free, it is highly desirable to inspect each layer or selected volume of the build. This is a significant challenge, given that conventional non-destructive evaluation (NDE) is a post-manufacturing operation. The opportunity exists in the development of novel flexible automated manufacturing systems aiming to merge deposition and inspection. Hence, enabling defect detection at the point of the creation allows subsequent rapid repair or reduction in scrappage. In this work, the authors present research from one such multi-robot cell, where a directed energy deposition process called wire + arc additive manufacture is used to build components while novel in-process ultrasound and eddy-current approaches are deployed to inspect a component with artificially embedded reflectors. The outcome of this work demonstrates a promising ability to merge manufacturing and NDE into a single process and hence, strengthen the overall benefits of metal additive manufacturing fields.