Review of Measuring Methods of Residual Stress in Wire Arc Additive Manufacturing Products

1. Introduction

Wire Arc Additive Manufacturing (WAAM) is an advanced manufacturing process that is categorized as additive manufacturing (AM) or 3D printing. It is the imperative metal manufacturing process for large and complex components using wire metals or alloys as feedstock. In addition, due to its fast build-up rates metallurgical characteristics of WAAM fabricated parts are better than those of other AM techniques [1,2,3]. An important aspect that still hinders this technology is standardization and certification within nondestructive testing of the parts, which is discussed in the literature [4]. In this context, there are no standards for WAAM systems and no in-situ observing, and monitoring techniques for any instant formation of defects (residual stress) that can be fixed after its formation, which leads to waste the materials and time for reproduction. WAAM is an innovative and versatile 3D printing technology with an extensive variety of applications and several advantages in manufacturing.

Early investigations suggest that the adoption of AM technologies in construction could potentially lower labor expenses, diminish material waste, and fabricate intricate custom shapes that typically challenge the manufacturer using conventional construction techniques [5,6,7]. However, the WAAM sector is interested in large-scale with high deposition rates of AM for producing components spanning from hundreds of millimeters to meters in size [8]. It harvests substantial attention in industrial manufacturing due to its cost-reliable fabrication of large-scale metal parts at high deposition rates [9]. Besides, Williams et al. [10] suggest that WAAM is a viable contender for substituting the existing manufacturing approach involving solid billets or extensive forgings, particularly for low to medium-complexity components.

Within the context of WAAM products, the presence of RS and deformations due to process parameters becomes a significant issue as they have a substantial impact on the quality, cost, and precision of the printing process [11,12,13]. Additionally, WAAM, which is a subset of AM, is getting significant interest from all researchers because of its various benefits, including its ability to achieve high metal deposition rates and produce near-net shapes, surpassing conventional manufacturing methods with its higher thermal residual stresses (RS) [3,14,15,16]. It is an emerging metal additive manufacturing technique, that is gradually providing a competitive edge over traditional forging and casting methods [17]. Depending on the heat sources, the WAAM process is categorized into three; namely inert gas welding, metal arc welding, and plasma arc [18,19].

The need and purpose of this review article is to investigate measurement techniques of RS in the products of WAAM. Notably, this review is essential to gain a deep comprehension of how RS is distributed in the products of WAAM and how it correlates with the deposition parameters. In this study, methods of examining and measuring RS, the impact of RS on material characteristics of WAAM products, and process parameters based on both experimental and numerical analysis were discussed, while the mitigation of the RS and refining grain structure of created layer parts improved based on the experimental methods was discussed. In this review, the effects of process parameters like wire diameter, scan length with width, height (thickness) of bead, arc current behavior [20], voltage, travel speed, welding sequence (welding position) [21], types of shield gas, and its flow rate on RS are considered. Also, studies on the consequence of interlayer bead surface condition and inter pass time on the deposit shape additively increased height were considered.

This review also primarily focuses on how to measure RS fabricated with WAAM, drawing upon pertinent data from existing literature. The techniques for measuring RS through experimental approaches are mostly neutron diffraction [22,23,24], and X-ray diffraction (XRD) [25,26,27] that are performed within limited layer depth of printed products. Other methods like the hole-drilling strain gauge method, ultrasonic stress measurement (USM), Barkhausen noise analysis, layer removal (deep hole drilling), contour method, and incremental hole-drilling technique have been employed, and some of them have not been used yet. Importantly, this review explicitly delineates those that have been used and those that have not.

The format of the article is as follows: after this introductory section, Section 2 provides details about the materials and methods used in the research. This is followed by Section 3 and Section 4, which respectively present an overview of the products of WAAM and discuss the impact of RS affecting the mechanical attributes of WAAM components. Section 5 focuses on explaining the mitigation strategies for RS and explores practical applications. Section 6 serves as a discussion section, offering a detailed analysis and interpretation of the findings. Lastly, Section 7 summarizes the article by outlining the key points and providing insights into future research directions.

2. Materials and Methods

This review focuses on the scientific literature published within the last ten years, which encompasses open-access sources primarily published between 2015 and 2024. To ensure comprehensive coverage of related research, the investigation targets the defects mainly on the RS of the WAAM products based on the many process and operational parameters. To achieve the aim of this review article, the review concentrates on the latest publications (as indicated based on the data given in Figure 1), with content thematically arranged around specific issues. The search was restricted to sources that have exclusively available articles published in English. It includes articles in the following scientific databases: scientific.net, Scopus, Elsevier, Science Direct, Web of Science, Compendia, Google Scholar, IEEE Xplore, ProQuest, ASM Digital Library, SpringerLink, Mendeley and ResearchGate, semantic scholar, SCISPACE, PubMed, DOAJ, JSTOR, MPDI, BASE, SAGE, Taylor & Francis and others. It employed the English expression to search terms such as AM, WAAM, RS measurement, process parameters analysis, quality assessment in WAAM, WAAM product defects, characterization of microstructure in WAAM products, WAAM quality control, WAAM RS investigation, and frequently utilized additional descriptive terms to align with the review's objectives precisely. To accomplish these statistical combination and analysis of data from multiple studies to draw overall conclusions.

To screen the retrieved articles from the sources, the advanced text-mining techniques allowed for the finding of significant relations between keywords and the detection of groups of related terms in the gathered publications. For the research explored various RS measuring methods in WAAM components, some articles modeled bead formation during the WAMM process related to process parameters results for different levels of RS. The assessed number of research fields of RS in WAAM is shown in Figure 1 by the statistical analysis that began by examining the yearly count of research publications. Recently, there has been a noticeable increase in research interest in RS in WAAM parts, reflected by the rising number of publications in this domain. A high citation count usually indicates the influential nature of these findings, implying that they establish fundamental knowledge or introduce groundbreaking results that inspire and impact the necessity of the study area.

For the chosen topic, the main strategy used to screen from the retrieved articles is 691, and finally, the total number of articles that passed the screening (included in this article) is 215 as shown in Figure 2. The inclusion criteria utilized were: (1) any WAAM processes and (2) no restriction regarding existing defects in WAAM products for publication years between 2015, and 20224 up to date. The criteria for inclusion and exclusion are outlined as follows: (1) identification through a database, and other sources like libraries; (2) screening for records included and excluded depending on relevant study for the topic with data that is inconsistently extracted, duplicated, or overlaps with other data; (3) eligibility-only papers focused on RS analysis and related; (4) number of articles included with available for RS measurements using experimental and numerical analysis. The flow diagram used for screening and selection studies is depicted in Figure 2.

3. Overview of WAAM of Process and Products

In this technology, large and complex components are made from multiple materials, with a rapid deposition rate, a feat that is challenging to accomplish with traditional AM methods [28,29]. In this case, wire is supplied to the welding torch and melted by an electric arc [30]. Either robotic or computerized numerical control gantries are used to set the motion of the printing process layer by layer up to the aspired appearance of the components obtained [31]. The 3D digital model serves as the blueprint for this WAAM process. Utilizing robotic guidance in WAAM allows a potent blend of automation and design flexibility, coupled with efficient production processes. However, the material characteristics response of WAAMed products is influenced by both automated robots and computerized numerical control to build up the parts and the associated heat input [32].

Basically, the steps used for the printing process in WAAM products encompass many activities in each stage starting from the CAD model. These steps outline the general workflow for producing components using WAAM, ensuring efficient and reliable manufacturing of complex metal parts. The process of WAAM will be divided into six primary stages from CAD file preparation to component separation to post-processing [31]. The procedure planning steps used for WAAM to produce components are shown in Figure 3.

In essence, the RS is inherent in WAAM components due to the various process parameters that can adversely impact the mechanical properties, fatigue life, and structural integrity of the parts. Additionally, assessing RS in WAAM structures is challenging, either due to being laborious and time-intensive or because it lacks the necessary precision [33]. The significance of understanding RS in WAAM products is critical for maintaining structural integrity to address premature failure and ensure the products’ mechanical performance [18,34]. The characteristics of the deposited materials may vary through the WAAM process, affecting the evolution of RS [35], as a result of the heat-affected zone (HAZ) in the vicinity of the weld pool [36]. RS measuring also helps in understanding, predicting, and correcting dimensional inaccuracies caused by distortions, ensuring that WAAM products meet desired tolerances [37,38]. Likewise, accurate knowledge of RS is important to enhance product quality and ensure WAAM components meet their performance. By reducing the risk of defects, failures, rework, and/or remanufacture, it can lower production costs and minimize material waste [37,39,40].

WAAM can be utilized for repairing and restoring damaged or worn components and extending the lifespan of expensive equipment and machinery [41,42]. Furthermore, it is highly capable of building large and/or complex metal structures with cost-effective, primarily as a result of its superior deposition rate [3,7,30,43,44,45,46,47,48]. WAAM is well-suited for creating sizable metallic parts and holds potential for use in the construction industry among various AM methods [20]. The main advantages of these manufacturing methods include their productivity, suitability for industrial applications, and the capacity to decrease material waste compared to other AM techniques [49,50].

3.1. A Robotic System for WAAM

Robotic systems play an important role in WAAM processes, providing versatility and automation that enable precise control over deposition parameters and the capacity to manage intricate geometries [51,52]. These systems incorporate advanced sensors and automation technology to monitor and adjust printing parameters in real-time, ensuring consistent quality and precise deposition of materials. The study reported in [53] concentrated on two WAAM, parameters, namely (1) the speed of wire feed and (2) the robot's movement speed, and investigated their effects on the metallurgical, dimensional stability, and mechanical characteristics of materials. The study involved the production of fine-walled 308L stainless steel parts utilizing the WAAM system, conducted in two steps. Firstly, the effect of welding current, voltage, and travel speed on the shape of individual weld beads was examined, and these factors were fine-tuned for the construction of 308L steel walls. Secondly, a comprehensive analysis was carried out on the influence of the microstructure and mechanical characteristics of the WAAM 308L steel walls, resulting in improved mechanical properties suitable for industrial applications. Robotic components for the printing process with the path of indentation (a), the setup for experimentation (b), and the deposition tool path are illustrated in Figure 4.

Genetic Algorithms (GAs) are used how to enhance the parameter selection process in near-net shape deposition for minimizing voids and surplus substances in the WAAM process [54]. An uninterrupted deposition process is planned by verifying overlapping conditions within robot swap zones to avoid collisions [55,56]. Likewise, a continuous deposition process is being planned carefully by checking areas where robots might overlap or come close to each other in robot swap zones. This careful planning is done to prevent any collisions between the robots, ensuring smooth and safe operations. The robotic guidance will be fixed on the desired welding speed which affects the heat accumulation and results in variations of the RS in the products and the robotic guidance to monitor the process in the WAAM described in Figure 4. Employing path planning was proposed [54,55] to incorporate bead overlap and enhance internal structural stability by minimizing height eventually, it influences the RS in WAAM products.

3.2. Residual Stress in WAAMed Products

RS in WAAM refers to the internal stresses that remain locked inside a part (thermal RS) within fabricated components [56]. When a component within this RS is subjected to cyclic loading or stress, it can accelerate the initiation and propagation of the cracks which is the reason for premature failure [57]. This RS may arise from various factors, such as the cycles of heating and cooling associated with the differential expansion during the welding process, contraction of materials during cooling time, and other process parameters [58,59,60,61,62,63]. Ahsan et al. [64] identified that the high heat input is a near-optimal processing condition to improve surface quality, higher ductility, and higher deposition rate due to the induced high RS. In the WAAM process, the component will induce heat tension and thermal distortion as temperatures fluctuate with the advancement of fabrication [65,66]. This adjustment is made by controlling the values of wire feeding rates and travel speed (TS). With higher heat input the lower RS is observed on the surface of the top layer by long-time cooling durations [67]. Welding RS was measured using five experimental methods: X-ray diffraction (XRD), neutron diffraction (ND), incremental deep hole drilling (iDHD), incremental center hole drilling (iCHD), and contour method (CM) [68].

As researchers delve deeper into WAAM, they have discovered significant enhancements inside the grain arrangement and material properties of the produced components [69]. The transformation of large, elongated grains into smaller, uniformly shaped grains in both the inner and intermediate layers is accomplished by adjusting the amount of heat applied [66,67]. The influence of heat input on the RS, macroscopic structure, microscopic composition, and mechanical characteristics of the components fabricated through the WAAM procedure is studied in [70,71]

3.3. Residual Stress Measurement Methods

Generally, there are two RS measuring techniques, namely experimental and numerical methods, which are reviewed and reported in the following subsections.

3.3.1. Experimental Methods for RS Measurement in WAAM Parts

Several review articles across different literature types have investigated the examination of RS in WAAM products using various techniques [72]. Initially, these studies focused on the stress within central lines, conducting a theoretical analysis of stress evolution along these lines without assuming a specific stress distribution, which is considered RS. Subsequently, the research expanded to improving forecast models for warpage, based on general beam theory [73]. Other techniques encompass neutron diffraction (ND) [22,23,24,74], X-ray diffraction (XRD) [25,26,27,75], contour method [13,76,77,78] hole drilling methods [32], thermomechanical coupling model [79], digital image correlation (DIC) [80], deep hole drilling (DHD) [81], Operando synchrotron or synchrotron X-ray diffraction (SXRD) [82,83]. The selection of the methods depends on elements such as material, component size, geometry, and the level of accuracy required for the measurement. Accurate modeling of the process is crucial for understanding and controlling RS. For instance, XRD analysis is utilized to identify the grain size observed in the thin strips and ferrite of bainite created due to thermal cycles while dye penetration tests are used to investigate surface defects and corrosion behaviors of WAAM components [66,67,84,85].

The choice of the appropriate measurement method is governed by factors such as accessibility, resolution, accuracy, and the desired depth of stress penetration. Table 1 lists the methods used for RS measurement on different materials together with the summary of the experimental studies. As can be observed from the table, the most used methods, according to the literature overview are XRD, ND, digital image correlation (DIC), SXRD, layer removal (deep hole drilling), contour method, and incremental hole-drilling method.

The RS measurement techniques listed in Table 1 can generally be classified into three categories: (1) non-destructive; semi-destructive; and (3) destructive are mentioned in Table 1 with short descriptions; while some of the techniques were not performed in the past research. Non-destructive techniques (NDT) like XRD and ND are common high precision and frequently preferred to measure RS in WAAM components. The choice of the methods depends on fundamentals such as material, component size, geometry, and the level of accuracy required for the measurement.

It is clear that from RS measurement techniques, ND has been utilized more extensively by researchers due to its non-destructive nature, which helps maintain the veracity of the parts while X-ray diffraction is the second method utilized. The pie chart shown in Figure 5 represents the hierarchies of techniques used to measure RS in WAAM products.

In principle, the local thermal cycle makes variations of grain structures and finer grain is observed with thin strips of bainite and ferrite which have been confirmed using XRD analysis [84]. Similarly, the elastic strain is calculated from the change of the lattice spacing then RS [23]. In this case, any stress whether external or residual within a material, leads to deformation and changes in lattice spacing. Stress is determined by measuring lattice distance at different tilt angles using Bragg's law, which explains X-ray diffraction in crystal lattice planes. From several NDTs for identifying RS, XRD was mostly selected due to its capability to penetrate approximately 10 μm and provide spatial resolution ranging from 10 μm to 1 mm, rendering it particularly suitable for thin plate applications [26].

3.3.2. Numerical Analysis of RS in WAAM Products

Basically, behind the numerical analysis of RS, there are mathematical models often based on finite element analysis (FEA) or other computational methods, and the same procedures are performed to determine RS in the WAAM process for simulating thermal and mechanical processes occurring during AM. These models incorporate material properties, process parameters [85], and boundary conditions to simulate the deposition, heat measurements techniques mention the highest proportion and/or the lowest proportion.

Transfer, and subsequent cooling processes. By solving governing equations for heat transfer, numerical simulations predict the evolution of RS within the part. Similarly, the numerical analyses examine how deposition pattern and travel speed affect RS and warpage in WAAM components [95]. Table 2 provides overviews of the numerical analysis techniques, feedstock materials, and their outcomes when utilized in the WAAM process.

Drexler et al. [107] addressed the numerical modeling of RS and distortions occurring in WAAM parts with weaving deposition. Numerically forecasted thermal stresses across various welding layers were depicted for subsequent experimental evaluation conducted in [108]. Similarly, finite element (FE) simulation along with thermal analysis generated appropriate paths by segmenting a 3D surface scan of the intended repair area [42]. As illustrated in the pie chart in Figure 6 (ref. also Table 2), most of the numerical analysis of RS were investigated using ABUQAS.

3.4. Factors Influencing RS in WAAM

Researchers and engineers can characterize and understand the RS behavior in WAAM components by employing experimental methods and Finite Element Analysis (FEA), leading to improved process control, part quality, and product performance. Many parameters that can influence RS in WAAM parts are discussed in [18], while a profile that includes an assessment of the overall height, width, and smoothness of each generated wall was studied and reported in [109]. Deposition parameters [110], heat input,

Interlayer temperature, and the duration of interlayer pauses were significant factors influencing the formation and distribution of pores in aluminum 5183 alloys printed in the WAAM process [111]. Similarly, different welding travel speeds at which defects become noticeable also imply both wire feed speed and travel speed in the welding process [112]. In contrast, substrate (bed or baseplate) thickness was found to have a major influence on the RS distribution along deposit height. Given the substrate, tensile RS as high as the value of the material yield strength was discussed in [113]. The RS distributions in both the longitudinal and transverse directions indicate that longitudinal stress shifts significantly from compression during the deposition to tension in the baseplate [114].

Moreover, a thicker substrate induces greater RS than a thinner substrate. A substrate that is used for molten pool dropped on (printing process) can be made of a 2xxx aluminum alloy plate for aluminum alloy feedstock [115,116]. In the direct 3D printing process, the position of the printed sample matters to the quality of the printed parts [65]. This is because the expansion of isotherms fails to reach the bottom of a thick plate, resulting in increased heat accumulation [117,118]. The technique controlling inherent RS and distortions faces numerous challenges in the WAAM products leading to unpredictable structural integrity in the printed parts. For such reason, some literature identifies that different process parameters like wire feed rate, travel speed, heat input, deposited path length, width, and depth (thickness) of printed components have their effects [47,56,119]. The main defects induced in metal manufacturing components due to these parameters in the WAAM process identified by many investigators are such as RS, deformity, porosity, crack, and distortion. Thus, the most common factors of WAAM products’ defects are the process parameters, which limit the acceptance of this technology [120,121,122,123,124]. In another way, undercut and humping are the common defects found in the WAAM products with the major influence of deposition height, width, and stable layer is produced heat input per length values [112].

The processing conditions effects, such as interlayer temperature, deposit height, and substrate thickness on the distribution and magnitude of RS were studied in the literature [113,125,126]. The majority of substrates revealed the rewarding compressive RS while the deposit beads showed tensile stress. The instrument of absolute distance measurements is incorporated into the WAAM system to deliver monitoring processes of deposit layer height, profile, and bead volume predictions during the processes. Micrometers by hand and laser scanning measurements were also used after each completed deposit [127,128]. WAAM encompasses a multitude of adjustable process parameters, such as wire feed rate, voltage, current, travel speed, and layer height [129]. These parameters govern various aspects of the deposition process, including heat input, cooling rate, and thermal gradients. Understanding how alterations in these parameters influence RS is vital for achieving optimal WAAM products’ performances [112]. Selection of bead geometry parameters in the WAAM process is required to optimize excess materials and minimize the void created in the multiple layers of bead depositions. Process Parameters and other factors influencing RS in WAAM components are summarized and listed in Table 3.

The height of the bead, width, and cross-sectional area of the beads are expressed using the process parameters [145]. Furthermore, the diameter of the wire is expected to significantly influence both the melting efficiency of the wire and the mode of metal transfer, which are crucial factors in enhancing the deposition rate of WAAM. Nonetheless, the comprehensive effects of wire size on process attributes and deposition rate in plasma transfer arc-based WAAM remain inadequately comprehended. The primary objective is to examine the optimal combination of wire diameter with the Wire Feed Speed (WFS) to achieve higher deposition rates and improved bead geometry, while also addressing process control challenges and defect avoidance at elevated deposition rates. All aspects of limitation of deposition rate, bead shape control, keyhole formation, and metal transfer are studied in [43]. The selection of welding parameters in the WAAM is investigated in [146] and the values of the range used are explained in [147]. In the pie chart diagram in Figure 7, the factors that influence the residual stress in WAAM parts are plotted in percentages.

The elevation of the bead grows in a nearly proportional manner with the rise in wire feed rate, while the width of the bead decreases only at the subsequent feed rate. Moving on to the travel speed, it's observed that the width of the bead diminishes as the travel speed increases, whereas the bead height remains relatively constant. Furthermore, the wetting angle decreases as the travel speed increases [148]. The explanation of computing heat input per unit volume is outlined. The research indicated inconsistencies among various standards in interpreting heat input. Furthermore, the practical implementation of the method for calculating the precise heat input into the weld was validated [132]. Naveen et al. [149] conducted research by focusing on process parameters effects on product made of 5356 aluminum alloy on WAAM with three parameters namely; (1) wire feed rate, (2) welding speed, and (3) gas flow rate of value product of weld beads. The technique adopted was the Taguchi method to decrease the trial number for choosing a range of input variables. Successful efforts have been made to optimize welding modes for the WAAM applications in which more grain microstructure is studied [150].

The Taguchi technique along with ANOVA was employed to analyze how travel speed, wire feed rate, current, and argon flow rate impact various outcomes such as bead shape and size, bead roughness, oxidation levels, melt-through depth, and microstructural characteristics [148]. Based on the energy dissipation hypothesis and associated equations, various additive manufacturing processes utilizing energy beams can be compared more thoroughly when subjected to equal energy input. This comparison serves as a foundation for determining the initial process parameters Top of Formand the dynamic adjustment of main parameters [151]. The amount of heat input generated in the welding arc per unit length of the weld is expressed in kilo Joules per millimeter length of the weld (kJ/mm) as shown in Eq. (1).

$$\mathrm{Arc} \mathrm{energy} (\mathrm{kJ}/\mathrm{mm})=\mathrm{Volts} \mathrm{x} \mathrm{Amps}=V\times I$$

(1)

where $V$ is in volts, and $I$ is in ampere (A). The amount of heat input generated in the welding arc per unit length of the welding process is represented as in Eq. (2) and in different ways as per the ASME IX QW-409.1 standard from the American Society of Mechanical Engineers [132].

$$\mathrm{Heat} \mathrm{Input}\left(\mathrm{J}/\mathrm{mm}\right)=\frac{\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{t}\times \mathrm{A}\mathrm{m}\mathrm{p}\mathrm{s}\times 60}{\mathrm{T}\mathrm{r}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{l} \mathrm{S}\mathrm{p}\mathrm{e}\mathrm{e}\mathrm{d}(mm/min)}=\frac{\mathrm{V}\times \mathrm{A}}{\mathrm{v}}$$

(2)

where v is the travel speed. Vora et al. [152] investigated that voltage had the greatest influence on bead width, followed by travel speed and the ratio of gas mixture.

The deposited layer during the fabrication process for bead formation is assumed to be semi-circular. The deposition rate, R, which is influenced by wire diameter, wire feed speed, and wire density is expressed as Eq. (3).

$$\mathrm{R}=\frac{\mathsf{\pi }{\mathrm{d}}^2\mathrm{v}\mathsf{\rho }}{4}=\mathsf{\rho }\mathrm{v}{\mathrm{A}}_{\mathrm{w}\mathrm{w}}$$

(3)

where d is the wire diameter, $\mathrm{v}$, is the wire feed speed (WFS), $\rho$ is the wire density and welding wire cross-section area.

4. Impact of RS on Mechanical Properties in WAAM Components

Although WAAM has shown its capability to produce medium-to-large-sized components from aluminum for automotive and related industries, it cannot yet serve as a comprehensive production method due to practical challenges such as unmatched mechanical properties and significant RS [153]. RS significantly affects the materials and mechanical characteristics of the manufactured component. For instance, it can affect the mechanical properties, such as strength, hardness, and ductility of products, by making them different from the material's intrinsic properties. High heat supply in the WAAM process results in reduced tensile stress within the component and can lead to undesirable mechanical properties and microstructure [154,155].

Microstructural characteristics have an impact on the material properties of parts. Due to the existence of RS in WAAM products caused by the rise in either welding speed or the heat distribution parameter, material properties can be significantly varied [108,156]. The resultant microstructure and RS level in WAAM are directly correlated with the ultimate mechanical properties. Extensive research has been conducted on WAAM due to significant internal porosity in beads formed and its capability to produce large-scale components demonstrates inadequate tensile and fatigue properties [157]. This porosity is a result of RS by affecting the solidification process and the ability of gas to escape during deposition [158,159]. Similarly, RS can influence various mechanical properties of the manufactured parts in several ways by changing strength, toughness, microstructure, fatigue life, cracking, and delamination of the WAAM products [160]. RS formed during the fabrication process markedly impacts mechanical performance and may result in detachment from support structures, undesired shape deformation, and premature crack initiation [77]. The input parameters, resultant microstructure, post-manufacturing treatments, and RS have significant impacts on the ultimate mechanical characteristics of WAAM products [1,161,162]. The accuracy of the formation of the deposition layer is influenced not only by the precision of the arc torch's positioning but also by the stability of the forming process [163].

Figure 3 depicts the challenges and inherent RS in WAAM parts where reheated and re-melted parts are the known phenomena in high technology during the manufacturing process. To achieve good grain structure in each layer, the minimum penetration depth of plastic strain should exceed the sum of the re-melting depth and the height of the newly deposited layer. To enhance the microstructure, the minimum depth of plastic strain must exceed the combined depth of re-melting and the height of the newly deposited layer [33].

The experiments for the investigation of RS were conducted on the WAAM-based products by using cladding of single beads using welding voltage and current parameters as variables. The dependence of the formation of beads on the number of short circuits per unit length is explained in [50]. Even though the WAAM technology is an energy-efficient manufacturing method for metal production, heat accumulations during deposition, metallurgical, and associated mechanical properties results can change the RS profiles across the cross-section of the fabricated components [11,13,164]. The impact of build orientation and heat treatment schedule on the microstructure and mechanical properties of thin-walled components was examined in [165]. The distributions of RS within the component along the deposition path, both internally and externally are equal to zero [166,167].

Overall, understanding and controlling RS are essential for ensuring the mechanical reliability, dimensional accuracy, and functional performance of WAAM products in various applications. Many research efforts focused on characterizing RS, developing predictive models, and implementing advanced manufacturing techniques, which are critical for advancing the field of WAAM and unlocking its full potential in industrial applications [103]. Comprehending the distribution of heat input and its implications, including transient temperature distribution, material transformation, stress accumulation, and distortion, holds significant relevance for applications involving high-strength steels. Numerical simulation can offer valuable insights for assessing these factors [41].

Figure 8. Diagram illustrating the re-melting phenomenon occurring during the deposition process [33]. Copyright 2019, in accordance with CC BY license, Open access.

Figure 8. Diagram illustrating the re-melting phenomenon occurring during the deposition process [33]. Copyright 2019, in accordance with CC BY license, Open access.

The RS and deformation significantly impact the performance of the components [102]. As a result of repetitive cycles, RS becomes confined within the components, causing an array of defects such as cracks, deformations, warping, and diminished component lifespan [168].

5. Mitigation Strategies for RS in WAAM and Practical Applications

The generation of RS and distortion in parts impedes its broad adoption due to the intricate thermal build-up histories characteristic of WAAM components [169]. However, mitigation strategies for RS in various manufacturing processes in WAAM, involve several techniques to reduce or manage the RS that can impact the reliability and functionality of manufactured components. A hybrid process of WAAM and high-pressure rolling able to build large-scale components produces low detrimental RS and distortion [170,171]. In the same way, numerical methods can offer an additional understanding of how rolling can be effectively utilized to diminish RS and generate the necessary plastic strains to enhance microstructural properties [33]. Combining the WAAM process with other fabrication processes, including machining or subtractive techniques, can assist in removing excess material. This hybrid manufacturing is used in the medical device industry to produce low-stress WAAM parts with complex geometries or add complementary features to reduce RS [172].

Controlling RS is vital for ensuring the durability and reliability of these critical components [135]. A compilation of different methods for mitigating RS and distortion in WAAM has been assembled to offer a blueprint for future advancements [173]. Finite element process simulation offers an effective method to explore strategies to mitigate these distortions and RS [174]. Some methods provided techniques to handle RS, enhance mechanical properties, and eliminate defects like porosity [31,175]. Some common mitigation strategies and practical applications are listed and explained with practical application in Table 4.

In this study, we explore how various factors such as process parameters, substrate heating, and cooling influence bead appearance formed during WAAM of Inconel 625 on EN 8 steel. Additionally, the track of temperature history of the molten pool during deposition using an IR pyrometer and examine how these factors affect the cooling rate, which in turn impacts the bead geometry [142,187]. For the morphological aspect of the beads, it is essential to respect the volume of material supplied and the size of the part [188]. The graphs in Figure 8 illustrate the distribution of published work on approaches used to mitigate residual stress and indicate the frequency of their adoption in WAAM applications. This graphs present different mitigation methods of RS in WAAM process versus the utilized number of methods published in articles.

Figure 8. Mitigation method of RS vs publication in percentage.

Figure 8. Mitigation method of RS vs publication in percentage.

The graph illustrates the hierarchy of various RS mitigation techniques on successively deposited layers, while other methods are the treatments after production is finished. For instance, inter-pass rolling is mostly applied during the printing process to refine the grain structure, bonding, and adhesion between the consecutive layers of deposit materials. Heat treatment and shot peening are used after fabrication is completed, whereas, process monitoring and control are the mitigation methods used during the printing process.

An approach was developed to analyze the shape of the constructed wall, focusing on its inherent symmetry to ensure efficient material utilization. Additionally, it employs thermography techniques to monitor the symmetry of the melting pool [18]. This volume deposition geometry V (m³) can easily be deduced from the path (more specifically the travel length of the welding path l_scan (m)), the speed of the wire v_w (m s⁻¹ ), the diameter of the wire d_w (m), and the welding speed v_t (m s⁻¹ ) [26,148] as expressed in Eq.(4).

$$V=\pi d_w^2\times v_w\times \frac{l_{scan}}{v_t}$$

(4)

For a given bead width (w), layer thickness (h), and penetration depth (p) of the bead from the molten pool during the WAAM process, the shape of the bead shape is shown in Figure 9. These parameters are important because wider beads offer improved material coverage, leading to a smoother surface finish, while maintaining a controlled bead height minimizes surface irregularities for a smoother finish. Therefore, modeling of the bead profile is required to establish the relationship among different process parameters. Some studies consider the shapes of beads formed in WAAM process as ellipses [189,190]. The WAAM process constructs intricate parts by layering weld beads, it's vital to model both individual weld beads and the overlapping of multiple beads to ensure superior surface quality and dimensional precision in the fabricated parts. This begins by developing models for single weld beads using diverse curve fitting techniques [191,192].

The geometric representation of different overlapping beads, combined with an algorithm, determines the process conditions required for both materials to achieve consistent bead layer heights [193]. A geometrically flawed segment of a bead, or geometric defect, is a flaw resulting in voids within the final printed part because of incomplete fusion between two irregularly shaped overlapping bead segments [194]. It is preferable to write the same parabola in terms of the process parameters $v_w,$ wire feed speed, and $v_t$- torch speed. The geometric configuration of the weld bead created through WAAM differs from that of conventional welding methods [195,196]. The bead profile on the substrate can be considered as a symmetric parabola shape which is given in Figure 9 is presented in Eq. (5).

$$y=a+c{x}^2$$

(5)

To solve for a and c in Eq (5), the thickness (height) and width of the beads is substituted into the parabola equation to find the constants a and c. The turning points or vertex is at (0, a) for this form of the equation. The parabola is concave downwards because the result of $c<0$ as depicted in Figure 9 and the bead profile expressed as parabolic eqaution can be solved as Eq. (6).

$$a=h; c=-\frac{4h}{w^2}$$

$$y=h\left[1-{\left(\frac{2hx}{w}\right)}^2\right]$$

(6)

The area of a bead (area of the parabolic bead) relative to its geometric parametric is obtained from Eq. (7).

$$A=\frac{2hw}{3}$$

(7)

The volume flow rate is expressed in Eq. (8),

$$Q=\frac{2}{3}hw\times v_t$$

(8)

For the wire, when the molten pool flows, the volume flow rate is given by Eq. (9).

$$Q=\frac{\pi }{4}{d}_w^2\times v_w$$

(9)

where d_w is the wire diameter and $v_w$ is the speed of the wire. Equating Eqs (8) and (9), the width-w of the bead is determined from Eq. (10).

$$w=\frac{3\pi d_w^2{v}_w}{8h{v}_t}$$

(10)

From Eq. (10), replacing for w in the Eq. (6) will give Equation (11).

$$y=h\left[1-{\left(\frac{16h{v}_{t}x}{3\pi v_w{d}_w^2}\right)}^2\right]$$

(11)

For the different wire diameters, the beads formed vary in thickness and width. However, the shape of the deposited bead is greatly influenced by many welding parameters, including torch angle, wire travel speed, filler feed rate, and cooling conditions [197]. For a given layer thickness (h), bead width (w), and torch speed ($v_t$), the volumetric flow rate (Q) required for complete filling is given by Eq. (12).

$$Q=v_{t}wh$$

(12)

Setting optimum parametric in the WAAM process would be advantageous for the gradual deposition of weld beads layer by layer in the AM of components [198,199]. Koli et al. [133] studied the WAAM products made of wire of SS308L with a 1.2 mm diameter used to manufacture the samples. The parameters such as welding speed, arc current, and shielding gas flow rate were the most influential in affecting the multiple responses of WAAM products.

The practical applications of components printed by WAAM processes are mainly applicable in the automotive [160], aerospace [200], naval, and defense [201,202] industries’ using materials like aluminum due to their high quality and strength with less mass. Generally, WAAM is highly demanded across industries such as automotive, aerospace, chemical plants, maritime, nuclear, shipbuilding, and energy, where it is utilized for fabricating components like aircraft parts, automotive chassis, and marine propellers [8,65,88,153,156,203,204,205,206,207,208,209]. The WAAM process in these industries widely used aluminum alloy series such as 2xxx (Al-Cu), 4xxx (Al-Si), and 5xxx (Al-Mg) [204,210,211]. Aerospace components produced in the WAAM process such as turbine blades, airfoils, and structural elements, need to be carefully controlled RS to prevent premature failure and maintain high-performance standards [86,135,212,213]. WAAM is also used for producing engine parts like cylinder heads and pistons [135].

6. Discussion

Besides the measurement of RS mentioned earlier, certain welding-related defects may potentially occur in WAAMed components. These measurements include many factors that influence the magnitudes of the RS WAAM which are described in Table 1 with several references. The inclusions primarily consist of three classifications of RS measuring methods which are discussed in Section 3.4. From these three, non-destructive measuring techniques are mostly applicable. However, in large samples, the strain of certain components may be difficult to determine or cannot be precisely identified due to variations of dissimilar RS in the penetration depth beam light through the components [214]. Still further research is required to thoroughly examine the RS present in WAAM products resulting from different process parameters. Post-fabrication heat treatment processes can significantly impact the elimination of defects. Though many defects could be eliminated in WAAMed products, RS was only minimized to the greatest extent possible through heat treatment [136] and other processes mentioned in Table 4.

The quality of metal components fabricated in WAAM relies seriously on key parameters such as travel speed, heat input, wire feed rate, deposition direction, material properties, etc., and others are mentioned in Table 1. While high heat input is necessary to achieve a rapid deposition rate in WAAM, it also presents challenges like RS and distortions. Therefore, managing heat input becomes crucial, especially when fabricating large metal components using WAAM, as it directly impacts both deposition rate and structural integrity [1,47].

6.1. Challenges and Limitations

Like any manufacturing technique, WAAM comes with its own set of challenges and limitations in the measurement and control of RS. It can be used to fabricate complex geometries which makes it difficult to predict and measure RS accurately, especially in intricate structures. WAAM products often exhibit heterogeneous microstructures and material properties due to the fast solidification and thermal cycles experienced during deposition. The variations such as material composition, grain structure, and phase transformations can influence the magnitude and distribution of RS, complicating measurement and interpretation. Measuring RS in the products of WAAM poses several challenges due to the unique characteristics of the process of manufacturing and the resulting material properties. For example, non-destructive testing techniques such as ultrasonic testing or X-ray diffraction are promising for measuring RS in WAAM products, adapting these techniques to the unique characteristics of additive manufacturing materials and geometries presents technical challenges. The common challenge during the test is the capacity of beam light penetration depth dimensions through the printed products. WAAM involves the successive deposition of layers of material, which can result in anisotropic RS distributions. Measuring and characterizing these complex stress patterns accurately requires advanced techniques capable of analyzing stress variations in different directions and depths within the material [215].

The properties and behavior of different materials can vary significantly in different manners and make it challenging to establish a universal approach for RS measurement and control. Management of RS is a critical aspect in the WAAM to ensure the quality and performance of the manufactured components. There were limited industry standards for measuring and controlling RS in the WAAM. The absence of standardized procedures can hinder the adoption of this technology in critical applications. Many investigations can’t give a specific magnitude of RS, rather they indicate numerical results and show the character throughout the fabrication operations of WAAM by modeling.

Quantifying this RS is not assigned for all process and welding parameters. For instance, from the input parameters like wire diameter, art welding directions, length, width, thickness, and volume of the products there is a comparison of the printed components. Likewise, from parameters related to welding, such as speed of welding, welding gas, gas flow rate, volts, peak currents, and shielding gas [1,4]. The techniques used in the past investigations for measuring RS in the WAAMed products are already listed in Table 2.

In the other case, shielding gas, types of shielding gas, and its flow rate affect the distribution and amount of RS in products of WAAM. Similarly, the shielding gases types with their flow rate such as argon and helium, and reactive gases like carbon dioxide and oxygen can have significant impacts on RS formation in WAAM products [3,137,148]. However, researchers didn’t identify the optimum amount of the shielding gas and types of shielding gas with other processes and input parameters during the WAAM process. Additionally, several reasons that make RS in WAAM are rapid heating and cooling, multi-layer deposition, process parameters, material phase transformations, and part geometry.

The literature review, in general indicates that continuous investigation and improvement in these areas are essential for advancing the understanding and control of RS within WAAM product applications.

6.2. Future Directions

Challenges and limitations for controlling RS in products of WAAM are discussed in Section 6.1. Based on the measurements, mitigations, and limitations found in the past research, which can be mentioned and recommended more studies in future investigations could focus on optimizing the parameters of the WAAM process. By elucidating the intricate relationship between process parameters in WAAM and RS, this review contributes and leads to optimizing the process of parameters, using RS measurement techniques and the assurance of product reliability [85] Likewise, some of the RS measuring methods which are not been performed yet in the products of WAAM in the past research are:

(1) Non-destructive methods	▪ Ultrasonic Waves ▪ Magnetic Barkhausen
(2) Semi-destructive methods	▪ Ring Core ▪ Deep Hole method
(3) Destructive methods	▪ Bridge Curvature ▪ Sectioning Techniques ▪ Slitting or Crack Compliance

In another way, the filler wire diameter directly affects the deposition rate and heat generated in WAAM. Larger wire diameters typically result in higher deposition rates but also lead to increased thermal input rate per unit length. This increased heat input can influence the cooling gradients and heat input gradients within the deposited material, affecting the spread and extent of RS. The influence of wire diameter over the molten pool deposition to form beads is described in Section 6 using formulas to present the relationships between height and width throughout the WAAM process. For the significant impact of wire size, the optimum size of wire diameters on the RS in the products of WAAM has not been identified in past investigations. Also, among the categories of measuring RS, the appropriate techniques and limitations with formed-bead geometry are not studied.

Since the past researches are not inclusive regarding materials dependence, methodology modifications, application area, and related research issues of WAAM in existing literature, there is ample opportunity for upcoming research to explore various possibilities, including:

• Most researchers used frequently non-destructive methods like XRd, ND, and some other semi-destructive and fully destructive techniques of measuring RS in components’ WAAM. Thus, future studies can perform those techniques listed above.
• In the future, researchers should determine the most suitable quantity and varieties of shielding gas, in addition to other process and input parameters, throughout the WAAM processes.
• As explained in Section 5, the mathematical formula provides a particular formation of bead profile and the relationship of wire diameter with the width and thickness (height) of beads. The result of variation in wire diameter, the thickness, and width of beads can vary consequently heat distribution of the process results in a variation of RS in WAAM parts. Therefore, the future researcher can focus on a variety of wire diameters to reduce RS in WAAM parts with less diameter of wire diameter.
• Materials weld-ability depends on their physical properties that influence the accumulation of RS in products of WAAM. In the future, further research endeavors should aim to investigate these physical properties of materials which are listed in Table 3, and other robot adjustable effects to RS fabricated components through WAAM.

7. Conclusions

In conclusion, this literature review has delved deeply into the extensive research surrounding the investigation and measurement of RS in WAAM products, with specific emphasis on the influence of fabrication parameters. The significance of RS in determining the material characteristics and structural integrity of WAAM components has been underscored throughout the review work. Moreover, the study has provided an all-inclusive summary of key findings, measurement methods, problems, and future directions in the WAAM dynamic field.

The review highlights the critical role of RS in shaping the dimensional constancy of WAAM products, emphasizing the necessity of precise measurement and control for effective utilization. Synthesizing diverse research studies, techniques, and methodologies, the review work has offered a valuable understanding of the present research status in this domain. Furthermore, this review serves as a guide for future researchers to use the other RS measurement techniques those that have not been performed yet and compare previous techniques used in the literature. To measure RS WAAMed products, experimental methods including semi-destructive (only hole-drilling), destructive (contour method), and non-destructive techniques (such as neutron crystallography and X-ray diffraction, digital image correlation, and Synchrotron X-ray diffraction) have been reported through literature.

Overall, this review is inclusive of examining and measuring RS techniques in WAAM products based on input parameters and other influences of RS products. Through meticulous evaluation and coordination of existing literature, this review underscores the importance of understanding the factors influencing and mitigation methods of RS in products of WAAM.