Introduction

Residual stresses refer to the internal stresses that remain in a manufactured component even without the presence of external forces and thermal gradients, and are self-equilibrating in nature. These stresses are caused by mismatches in the geometry of components from various regions and phases within a part, as well as regional disparities in elastic moduli, thermal properties, and mechanical attributes, and solid-state phase transition shown in Fig. 1 [1]. The presence of RS can result in a multitude of adverse effects on the characteristics of a component, such as a lack of endurance against fatigue, consequential operational failure, diminished chemical resistance, reduced magnetization, decreased deformation resistance, and weakened static and dynamic strength [2]. In addition to leading to the formation of RS, service loading can cause uneven plastic deformation. The aforementioned stresses are present not solely in the end products, but also in the basic materials. There are four primary classifications for RS’s origin [3], i.e., differential plastic flow, rapid cooling and heating rates, phase transformation accompanied by volume changes, and misfits induced by chemical factors.

The majority of MAM methodologies involve the utilization of a laser, plasma, or electron power source to induce rapid heating, which leads to liquefaction and subsequent solidification of the metal, whether it be in powder or wire form. The local structure and properties of components can be modified through repeated cycles of heating and cooling [4]. Due to the high freezing and heating rates, RS is produced, which can lead to deformation, cracking, and a reduction in the performance of additive manufacturing (AM) -produced materials. In addition, a direct consequence of RS is creating a pronounced anisotropic behaviour in these materials [3].

Fig. 1
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Residual stress formation mechanism in MAM

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The RS are not independent occurrences; instead, they appear as complementary and opposing forces (tensile and compressive) or circumstances that arises compressive and tensile RS within a material. Understanding this duality is crucial in engineering and materials science because it can significantly influence the performance and integrity of components and structures. Compressive RS implies the internal forces or contractions experienced by a specimen in the inward direction. It may develop as a result of various manufacturing procedures, including heat treatment. It exhibits a tendency to impede crack propagation and has the potential to augment the material’s resistance to fatigue and tensile loading. Alternatively, tensile RS induces outward forces or expansions within a specimen. It may manifest as a result of various procedures such as machining, surface grinding, or rapid cooling. The application of tensile RS has the potential to increase the part’s susceptibility to the initiation and propagation of cracks, thereby affecting its structural integrity. The presence of both compressive and tensile residual stresses in a material means that they are, in essence, a paired phenomenon. These pairs of stresses must be carefully managed and balanced so that attempt should be to have compressive residual stress at the surface and tensile within the volume to ensure the safe and reliable performance of components and structures.

The Temperature gradient mechanism (TGM), shown in Fig. 2, can explain the origins of RS in MAM. The power source induces local elastoplastic deformations (designated by \(\:{{\varepsilon\:}}_{pl}\), \(\:{{\varepsilon\:}}_{el}\), and \(\:{\sigma\:}_{yield}\)) and additional tensile tension in the irradiated zone upon heating. Thermal contraction induces diminution and the formation of tensile and compressive RS regions (top and bottom layers, respectively) during the cooling of the molten top layers [4, 5]. The temperature in nearby areas where the heating source (laser/electron beam/plasma) interacts with the powder undergoes rapid escalation. However, the expansion of the hot material region is constrained by the cooler regions that surround it. The imposition of this limitation results in the generation of compressive stress within the region of the material that is at an elevated temperature. The cooling and contraction of the hot region are constrained by the surrounding regions, resulting in the development of permanent tensile RS in the surface of the fabricated part (Figs. 1 and 2). Due to the complexity of the MAM process, the TGM model is merely a simplification of how RS is generated [5]. The formation and intensity of RS are influenced by a multitude of factors, such as the material’s properties (e.g., grain size, heat capacity, porosity and phase composition), the specimen’s geometry, the necessary support structure, and the printing parameters (e.g., laser or electron beam power, pre-heating method, scanning strategies and speed, and layer height) [3, 5]. In addition, the estimation of RS is influenced by various factors, including elastic constants, nonlinearity resulting from texture, stress gradient with depth, micro stresses caused by plastic deformation, and grain interactions [6].

Fig. 2
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(a) Heating and (b) cooling effect on stress/strain in the irradiated zone

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Non-destructive, semi-destructive, and destructive measurement techniques are the three fundamental RS measurement categories. Several types of each category are shown in Fig. 3. The digital photographs of different RS measurement setups are depicted in Fig. 4. In non-destructive methods (NDT), the crystal lattice strain is measured. Then, the corresponding values of RS are calculated using elastic constants under the assumption that the crystal lattice deforms linearly elastically [7, 8]. NDT techniques have a variety of advantages and drawbacks, summarized in Table 1.

Fig. 3
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Residual Stress measurement techniques

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Destructive techniques include eliminating certain pieces of a sample to alleviate RS. The resulting deformations and their associated stresses are then formed. These techniques often demand Finite Element Method (FEM) tools to evaluate deformation behaviour and determine RS values [9]. As a middle scenario, fewer property changes are brought about by semi-destructive procedures than by destructive ones. The most popular of them is hole drilling (HD). This technique is predicated upon the fundamental idea that the introduction of a tiny aperture into a material serves to alleviate a portion of the RS present in the immediate area of such aperture. The application of stress relief induces a localized deformation inside the material, and the subsequent strain is determined as the hole is progressively deeper. The strain measurements are often obtained by observing changes in strain components, particularly normal strains, as a function of depth. The alterations in strain are correlated with the variations in residual stresses, and these measurements are used in future computations. The contour approach is one of the destructive methods that facilitates the evaluation of a two-dimensional RS map on a specific plane of interest. The contour approach offers enhanced spatial resolution, but the sectioning technique is characterized by its ease of use, requiring minimal computations. The contour approach is founded upon the principle of dividing a material or component into slender parts, sometimes referred to as slices, and quantifying the deformation experienced by each segment upon its separation from the adjacent material. The presence of RS inside the material induces deformations in the sliced parts, resulting in either an opening or closing effect upon removal. These deformations are then used to deduce the distribution of RS. The deformation data that has been gathered is analyzed in order to determine the distribution of RS inside the material. Computational approaches, such as the FEM or analytical models, are often used for this purpose. The contour plots depict the distribution of RS that has been determined by calculation. These plots visually represent the varying stress levels at various spatial positions inside the material. The contour technique is extensively used across several sectors, such as aerospace, automotive, and manufacturing, for the purpose of evaluating the RS condition in crucial components. These components may include weld joints, turbine blades, forged materials, quenched and impacted thick plates, as well as structural materials. Table 1 summarizes the comparative analysis among the different categories of RS measurement regarding advantages and drawbacks. Because of their accessibility, the abundance of recognized standards (ASTM, ISO, etc.), and ease of measurement, HD, Ultrasonic wave, and X-ray diffraction (XRD) are now the most used methods for RS measurement in MAM [3].

Fig. 4
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RS measurement setup: (a) XRD stress measurement system [10], (b) Neutron diffractometer [11], (c) MTS 3000-hole drilling system [12], (d) Ultrasonic system [13], (e) Contour RS system [14], (f) Barkhausen Noise evaluation [15]. Reproduced with permission from Elsevier

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Considerable research about RS in MAM has revealed several significant correlations. However, there is a lack of literature reviews in this field. Although review papers play a crucial role in elucidating the RS of MAM, there is currently a dearth of a comprehensive and systematic literature that encompasses the current state of the formation mechanism, parameter dependence, prediction, and adjustment approach of RS in MAM. Additionally, this review has focused on LPBF techniques due to its widespread use in producing high-precision, auxetic behaviour [16], complex metal components, light weight components, which aligns closely with the objectives of this research. This literature work aims to address the lack of comprehensive information on RS studies in MAM. It aims to systematically summarize the significant findings in this field, including measurement and characterization techniques, formation mechanisms, printing parameter dependencies, defect incorporation, mechanical properties degradation, and control techniques of RS in MAM.

This review paper first examines several techniques for measuring residual stress in the AM process, in order to build a basis for comprehending the progress made in analyzing RS in MAM. Then the literature examines the most recent research trends regarding the influence of process parameters on RS, the effect of RS on mechanical properties, RS analysis, and the impact of in-process and post-process techniques on RS modification. It emphasizes key findings and identifies gaps in the existing literature. Finally, this literature survey concludes by providing a concise overview of the important observations obtained from the review and puts up suggestions for furthering the area.

Table 1 Comparative analysis among the residual stress measurement techniques
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Impact of printing parameter on RS

Figure 5 compiles the process variables related to powder bed fusion (PBF) that impact RS. The fabrication process allows for adjusting beam/laser parameters and scan strategy. Conversely, certain process conditions, including factors such as part geometry and supports, are not readily amenable to in-situ modification. Among the variables, one of the crucial factors is the pre-heating of the powder bed, as it significantly reduces thermal gradients, minimizes part distortion, enhances dimensional accuracy and density, and improves mechanical characteristics [44, 45]. Maintaining an equilibrium between this parameter and other variables is imperative to prevent unfavourable outcomes such as excessive grain growth, recrystallization, and precipitation [46]. The mitigation of RS is significantly influenced by laser power and scan speed, irrespective of the material under consideration. Usually, elevated laser powers tend to generate increased RS levels due to the application of more incredible energy to the material, leading to a larger heat-affected zone (HAZ) and more pronounced thermal gradients. The phenomenon mentioned above has the potential to induce substantial deformation and modifications in the microstructural composition of the material, ultimately resulting in heightened RS. Determining the optimal laser power for minimizing RS is contingent upon the material under consideration and the intended final material properties, similar to the scanning speed. In certain instances, it may be imperative to utilize elevated laser powers to attain the intended degree of material modification, despite the accompanying escalation in RS [47,48,49]. Typically, the higher scanning speed reduces the amount of heat input into the build parts during the fabrication process because of very less heat exposure time. Less heat can lead to steeper thermal gradients, a smaller temperature differential between the melted and un-melted regions, and diminished size of the HAZ which can result in lower RS. The reduction in time available for thermal diffusion can impede the relaxation of the material and the formation of RS [47,48,49,50,51,52].

Fig. 5
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Residual stress influencing process variable

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The scan strategy and its associated parameters (Fig. 5) have a multifaceted impact on stress, encompassing its distribution, direction, and magnitude [46]. Kruth et al. [4] investigated that utilizing the island scanning technique, which involves depositing material in a “chessboard” pattern shown in Fig. 6, would reduce distortion compared to alternative scanning strategies. The impact of island size on RS was investigated by Lu et al. [53], and a comparable pattern was detected in the findings of [54]. The research determined that the island size of 2 × 2 mm2 yielded the lowest RS. However, the sample constructed with this island size also exhibited significant cracking. The optimal strategy was to consider an island with dimensions of 5 × 5 mm2 due to its higher density, superior mechanical properties, and comparatively lower RS. Sun et al. [55] investigated that S-pattern exhibits the most favourable outcomes in terms of the minimum values of equivalent RS and maximum principal RS when compared to other patterns like that zig-zag, raster, alternate line, Hilbert, in-out spiral, and out-in spiral (Fig. 7). The S-pattern deposition is regarded as the most optimal among the six available patterns for deposition and shows excellent potential for MAM. The RS exhibited by the AlSi10Mg sample was comparatively lower when deposited using the parallel between layer (where the movements of the printing or laser beam are oriented in a way that is parallel to the plane of the current layer being built) scanning strategy as opposed to the rotate within the layering (where the orientation of the scanning or printing path changes within a single layer of the 3D printed object) approach shown in Fig. 7 [56].

It should be noted that the impacts, as mentioned above, exhibit dissimilarities when considering singular versus multiple layers (specifically, RS in-plane anisotropy), as well as the direction of construction (which results in variations in strain/stress) [57]. When the energy density is too low, the material may not fuse properly, leading to weak inter-layer bonding and RS in part. On the other hand, when the energy density is too high, the material may become overheated, leading to excessive RS and distortion. In general, higher energy densities tend to result in higher RS in MAM parts. Higher energy densities can lead to more significant thermal gradients and faster cooling rates, which can cause differential cooling and solidification across the part, leading to RS [58,59,60,61].

Fig. 6
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Island Scanning Strategy

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Fig. 7
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Infill strategy: (a) zig-zag; (b) raster; (c) alternate-line; (d) out-in spiral; (e) in-out spiral; (f) Hilbert; (g) S-pattern one layer; (h) S-pattern multi-layer; (i) parallel between layer; (j) rotate within the layer

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The layer thickness in additive manufacturing can significantly affect the RS of the manufactured parts. In general, thinner layers tend to produce less RS than thicker layers. This is because thicker layers require more material to be deposited at once, which can lead to more thermal energy being applied to the part. This can cause the material to cool at a different rate, leading to RS within the part. Thinner layers, on the other hand, allow for more gradual cooling and can help to reduce the likelihood of thermal gradients forming in part.

The thinner layers may allow for more uniform heating and cooling of the part, which can further reduce RS [48, 58, 59, 62,63,64]. The RS in a MAM part can be significantly influenced by the geometry of the component and the utilization of support structures. The part’s geometry has the potential to impact the cooling rate and the extent of thermal expansion and contraction, ultimately resulting in RS. The RS in a tall and slender structure may be more significant than a short and stubby structure owing to the higher temperature gradients and cooling rates experienced by the former. Utilizing auxiliary structures can also affect the remaining stresses in additive manufacturing components. Using support structures is a common practice in fabrication processes to secure the part and mitigate any potential distortion or bending. Incorporating supplementary structures may generate concentrated stress and thermal entrapment regions, resulting in further RS [58, 65].

Various methods can be employed to alleviate RS in MAM components. The utilization of optimized printing parameters, such as laser power, layer height, energy density, scan strategy and speed, can aid in the reduction of the cooling rate and mitigate RS. Furthermore, implementing in-processing and post-processing methodologies detailed in Sect. 5, such as heat treatment or shot peening, can alleviate RS and enhance the mechanical characteristics of the component.

Residual stress impact on mechanical properties

The mechanical properties, microstructure, corrosion resistance, fracture initiation and propagation of MAM components are substantially influenced by RS [66, 67]. Tensile RS can reduce the load-carrying capacity of a material and can lead to premature failure due to cracking or fracture. In some cases, RS can enhance the strength of fabricated parts. The compressive RS can improve the fatigue life of build parts by reducing the propagation of cracks and the resistance of a material to bending or buckling. When a material is subjected to RS, it can create areas of localized strain that can alter the microstructure of the build parts, creating regions that are more susceptible to corrosion.

The RS can lead to the formation of microcracks and other defects in the MAM, which can provide sites for corrosion and crack, initiation, and propagation. In some cases, RS may indirectly influence the susceptibility of a fabricated parts to chemical reactions or corrosion. The presence of tensile RS can enhance the probability of stress corrosion cracking when subjected to corrosive environments. Even so, the predominant factor influencing alterations in chemical composition is the exposure to distinct chemical agents or circumstances, as opposed to the mere existence of RS [68]. Regarding fracture initiation, RS can create areas of high-stress concentration within the printed sample, increasing the likelihood of crack formation. These areas of high-stress concentration can be caused by residual tensile stresses or by changes in the microstructure of the material due to RS. Further, it can affect the rate at which cracks propagate through the material during fracture propagation. Residual tensile stresses can reduce the load-carrying capacity of the printed material, making it more susceptible to crack propagation.

The RS can contribute to the formation of intergranular cracks, particularly in materials that are susceptible to stress corrosion cracking [69]. RS can cause changes in the microstructure of printed material, particularly in areas where the stress is concentrated. The residual tensile stresses can cause elongation of the material’s grains, while residual compressive stresses can cause compression of the grains. These changes in the microstructure can affect the material’s mechanical properties, such as its strength, flexibility, and toughness. In addition to changes in the grain structure, RS can also cause changes in the chemical composition of the material. It can cause segregation of alloying elements, leading to changes in the material’s corrosion resistance. RS can also cause changes in the concentration of defects and impurities within the MAM, which can affect its mechanical properties [70, 71].

The presence of RS may not necessarily have a detrimental effect on the material’s performance. Shot peening is a surface treatment technique aimed at enhancing the fatigue behaviour of a material specimen through the induction of residual compressive stress [72]. RS is considered a crucial factor that influences the future implementation of MAM technology. This is due to its substantial effect on the deterioration of MAM product quality and the possible risk to product performance.

Residual stress analysis

This section has explored the defect incorporated due to RS in MAM process and developed a numerical model for the prediction of RS. The numerical model has developed to enable the incorporation of current developments into the existing body of research. The section not only consolidates current literature but also illustrates how emergent results add to the continuing discussion in the field. This approach enriches the review by providing a more comprehensive perspective and highlighting the relevance and impact of the proposed research in addressing existing gaps or advancing the field.

Defect incorporated into residual stress

The process of MAM continues to encounter obstacles in maintaining optimal product quality control. One such challenge is the occurrence of part distortion in terms of cracking, delamination, wrapping, and part deformation, as depicted in Figs. 8 and 9. These issues are attributed to the presence of RS [73, 74].

Fig. 8
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Defects in MAM parts caused by RS: (a)-(d) Cracking [75,76,77]; (e) Distortion [78, 79]; (f) Base plate delamination [28]; (g), (h) Delamination between layers [80]. Reproduced with permission from Elsevier

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According to the evaluations’ findings, most parts constructed through MAM displayed a consistent distortion pattern, whereby the farthest edges of the part were distorted in the direction of the build [9, 81,82,83,84,85]. Figure 9a and b demonstrate that the prism specimens constructed by selective laser melting (SLM) exhibited comparable distortion patterns in both the horizontal and vertical orientations, wherein the remote ends of the specimens peeled upwards along the build direction. Figure 9c and d indicate that the corners of the substrate exhibited the highest degree of wrapping following the deposition of six blocks via electron beam selective melting (EBSM). Figure 9e and f demonstrate that the occurrence of thermal stress during laser melting deposition (LMD) can result in the simultaneous generation of cracks and distortion.

Fig. 9
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Defect incorporated to metal additive manufactured parts: (a, b) Distortion in vertical and horizontal orientation [86]; (c, d) Wrapping in specimens [83]; (e, f) FEM-based distortion model with experimental validation [84]. Reproduced with permission from Elsevier

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The MAM process involves the quick heating of the material to its melting point, followed by rapid cooling and solidification at an exceptionally high cooling rate. The material is subjected to multiple cycles of the deposition process. Consequently, the MAM process is accompanied by an imbalance in thermo-mechanical-metallurgical factors. This disparity leads to a significant level of RS in the produced components, as evidenced by previous studies [86, 87]. Various methods for measuring part distortion are available, such as the coordinate measuring machine (CMM) [88, 89], laser displacement sensor (LDS) [74, 90,91,92,93], and digital image correlation (DIC) [9, 94].

Prediction model for distortion in MAM

The primary objective of this research is to calibrate a sequential thermo-mechanical FEA model for accurately predicting the inconsistent shrinkage behaviour of rectangular blocks manufactured by the laser powder bed fusion (L-PBF) process. ANSYS software was used for RS analysis due to its extensive usage and validation in both industry and academic research.

A sequential thermo-mechanical FEA was performed using the L-PBF AM modeler plugin integrated into the ANSYS finite element solver. To get a comprehensive understanding of the additive manufacturing simulation capabilities in ANSYS, it is recommended that the reader consults the ANSYS 2023 user handbook [95]. The fundamental measurements of the rectangular block under examination are shown in Fig. 10a. The dimensions of the block were as follows: it had a length of 15 mm, a width of 10 mm, and a thickness of 2 mm. The experimental and model setups were identical, with the rectangular block positioned centrally on the build plate as depicted in Fig. 10a. The build plate, often referred to as the base plate, has dimensions of 30 mm x 15 mm x 5 mm. The calibration component was manufactured via a L-PBF technique as illustrated in Fig. 10b, with the machine’s process parameters configured as shown in the subsequent Table 2. Certain processing parameters were used as inputs in the simulation. The component was fabricated using Ti-6Al-4 V material, directly deposited onto a build plate without the use of any auxiliary support structures. After the fabrication process, a produced rectangular block was scanned using an ATOS Triple Scan (Capture 3D, Inc., Santa Ana, United States), which had a volumetric accuracy of 8 μm.

Fig. 10
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(a) Rectangular block dimension; (b) L-PBF fabricated parts

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The Cartesian mesh was used to discretize the block, with an average element size of e = 0.5 mm, in order to strike a balance between precision and computing efficiency as shown in Fig. 11a. Conversely, the mesh size of the build plate was increased to a maximum element size of e = 1 mm. The construction of the block took place inside the yz-plane. The heat transfer study and subsequent structural analysis used an identical mesh.

Fig. 11
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(a) Model assembly highlighting the mesh to the model and build plate; (b) reference line of nodes

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The material Ti64 was designated for both base plate and rectangular block. The temperature-dependent physical and mechanical characteristics of Ti-6Al-4 V were included into the study using the data from the material database [96] (Fig. 12). The inelastic behaviour was characterized by a temperature independent plasticity model using bilinear-isotropic strain hardening behaviour, with a yield strength of \(\:{\sigma\:}_{y}\) = 1098 MPa and a tangent modulus of \(\:{E}_{T}\) = 1.332 GPa. Temperature-independent plasticity is a widely used technique in the modeling of part-scale L-PBF processes simulation [97,98,99]. The bilinear isotropic hardening material properties were applied to the model and triggered nonlinear effects for both the rectangular block and the base plate. The machine parameters were adjusted to maintain the deposition thickness at 0.06 mm, the hatching spacing at 0.12 mm and the laser speed at 1250 mm/sec.

Fig. 12
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Temperature-dependent material properties and plasticity: (a) Density, (b) coefficient of thermal expansion, (c) specific heat at constant pressure, (d) thermal conductivity, (e) elastic modulus and (f) plasticity behaviour

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Table 2 Ti64 processing parameters
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The preheating temperature of Ti-6Al-4 V powder and surrounding gas (argon) were considered as 200℃. The convection from the fabricated part to the build volume gas was included and the radiation was ignored during the fabrication. The gas convection coefficient was considered to 1 × 10-5 W/mm2 ℃. The convection from the fabricated part to the build volume powder was also included. The powder temperature was set to 200℃ and the powder convection coefficient was set as 1 × 10-5 W/mm2 ℃. The cooldown temperature of the fabricated parts was considered as 22℃ (room temperature). On the build plate, the thermal boundary condition and structural boundary condition were applied. The boundary conditions included a consistent preheat temperature during the building process, an ambient cooling temperature during the cooldown phase, and a fixed condition throughout the simulation. A fixed support was applied at the bottom face of the build plate, with a build boundary temperature of 200℃ as shown in Fig. 13.

Fig. 13
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Model subjected to thermo-mechanical boundary condition (a) build boundary condition; (b) fixed boundary condition

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The transient temperature evaluation of the rectangular block during the L-PBF process was determined in the heat transfer analysis. The mechanical response and development of residual strains were determined based on the provided thermal history. The distortion of the L-PBF block was derived in the structural analysis. The block under investigation was anticipated to display significant distortion at the line of nodes located at X = 0 and Y = 1 mm along the Z-direction, as shown in Fig. 11b. A reference location was established for the purpose of calibrating the thermo-mechanical analysis and serving as a reference point for the final outcome. At the designated line of nodes, a target experimental distortion of 0.075 mm was assigned in the X-direction.

Fig. 14
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FEA simulated results for (a) total deformation; (b) calibrated deformation

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Fig. 15
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Plot for optimized TSSF to achieve target objective

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The Fig. 14 shows the result of FEA simulation for total deformation and calibrated deformation. The result revealed that the maximum total deformation and calibrated deformation value were determined to be 0.16201 mm and 0.12667 mm respectively at a default thermal strain scaling factor (TSSF)1. Direct optimization system was used to optimize the calibration through identify the optimum TSSF value that would yield the highest distortion along the measured line, therefore closely aligning with the experimental distortion value. The optimization system performed the thermal-structural simulations in an iterative manner. This was achieved by systematically varying the TSSF values, beginning from the default value of 1. The optimized result revealed that the calibrated TSSF value was determined to be 0.44, leading to a distortion measurement of 0.075073 mm. This is within the acceptable range of 1% tolerance as shown in Fig. 15.

Impact of in-process and post-process techniques in residual stress mitigation

Stresses in MAM are more significant in the vertical direction than in the scanning direction by a factor of 1.5–2.5 [77, 100,101,102], possibly due to asymmetry caused by MAM processing and material characteristics. Strong tensile tensions are introduced at a depth of around 40 μm from the cut surface in processes that necessitate Electrical Discharge Machining (EDM) cutting (particularly in SLM and EBM) [1]. Compared to SLM, parts processed through Electron Beam Melting (EBM) exhibit significantly lower levels of RS. This is attributed to the cooling rate, approximately one order of magnitude lower in EBM. This is evident from the solidification features, such as the dendrite arm spacing, observed in both processes [103,104,105]. Due to the extensive pre-heating of the powder bed and the isolated vacuum chamber, the cooling rate in EBM is substantially lower. The heat takes longer to dissipate and fade away from the EBM component. Preheating the substrate also shows the critical role in RS mitigation in MAM process. The RS decreases on increasing the preheating temperature of substrate/baseplate for SLM and Direct energy deposition (DED) techniques as shown in Fig. 16 [106, 107]. However, in-process and post-processing methods can minimize, shift, or even eradicate RS shown in Table 3. Laser shock peening (LSP), ultrasonic, hammer, shot peening (SP), heat treatment, explosive treatment, machining, and annealing are the post-process techniques used to mitigate the RS [47, 108,109,110].

Fig. 16
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Effect of RS on preheating temperature of substrate (a) 316 L stainless steel (b) Ti-6Al-4 V

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Table 3 The significant impact of in-process and post-process techniques on RS modification
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Fig. 17
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Effect of LSP technique on RS generation in the Ni-Ti sample

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Conclusion and future perspectives

The impact of RS on MAM is a critical aspect that significantly influences the mechanical properties, dimensional stability and reliability of the produced parts. This paper summarizes and discusses the typical characteristics of RS in MAM. The mechanism for the formation of RS has been proposed, and its effects have been analysed. This study examines the techniques employed for measuring RS, its correlation with microstructure, and the methods used to control them.

The generation of RS in MAM can be attributed to various formation mechanisms. Uneven plastic strain resulting from the temperature gradient, cool-down phase, and cooling rate gradient mechanisms are identified as critical contributors to RS formation. Heating and cooling rates influence the RS formation mechanism—parts processed through EBM exhibit significantly lower RS levels than SLM due to a lower cooling rate. The most commonly utilized RS measurement methods in MAM include HD, Ultrasonic wave, and XRD. These techniques are favoured due to their accessibility, the abundance of recognized standards such as ASTM and ISO, and ease of measurement. Process parameters significantly influence the formation of RS. To mitigate the impact of RS, one may consider implementing techniques such as the island and S-pattern scanning strategy, employing a thinner layer, reducing laser power, increasing scanning speed, and decreasing energy density. The parallel between the layer scanning strategy reduces the RS compared to the rotation within the layering technique. Scan trajectories can reduce about 55% RS.

It is strongly advised to utilize the pre-heating function of the equipment when conducting the MAM process with metal powder. Pre-heating the Ti64 powder over 570 ℃ reduces the RS [116]. The RS present in a tall and slender structure may be comparatively more significant than those in a short and stubby design.

Tensile RS has been observed to cause significant degradation in the mechanical characteristics of the fabricated parts. The presence of tensile RS in a material can negatively impact its load-carrying capacity, potentially resulting in early failure due to cracking or fracture. Compressive RS, on the other hand, can potentially prolong a material’s fatigue life by decreasing the propagation of cracks. They also can make the material more resistant to buckling and bending. Distortion in damages, part deformation, wrapping, base plate and layers delamination, and dimensional inaccuracy incorporated to tensile RS in the fabricated parts. LSP, ultrasonic, hammer, SP, heat treatment, explosive treatment, and machining are the post-process techniques used to mitigate the RS. LSP reduces RS and induces work hardening, resulting in an increase in microhardness by 20%. Implementing post-heat treatment is a viable method for eliminating RS in a diverse range of materials. But it is essential to select the appropriate heat treatment regime. Heat treatment and machining combinedly improve 22% compressive RS on the surface. They were subjecting the base plate to a temperature of 200 ℃ resulted in a 40% reduction in RS aluminium alloy. In contrast, a temperature of approximately 570 ℃ yields minimal RS for titanium alloy. The annealing heat treatment process decreases RS by up to 70%. Hot isostatic pressing (HIP) reduces porosity (< 0.01%) and releases tensile RS but increases surface roughness.

The sequential thermo-mechanical modeling technique enhances the ability of accurately predicting the quality and performance of a product, providing useful information for optimizing the AM process. The model demonstrates its effectiveness as a resilient technique for minimizing RS and enhancing the overall reliability of AM. Additional investigation and improvement of this model might boost its precision and suitability for a wider variety of materials and geometries in AM. Numerous studies have been conducted in the field of process research, including both experimental and predictive model research. These investigations aim to ascertain the impact of various factors on RS or deformation of components produced using MAM techniques. The findings of these studies serve as valuable guidance in formulating and refining the MAM process. This process is both labour-intensive and costly. Hence, the incorporation of supplementary measurement information feedback and process modification techniques into a closed loop system becomes very advantageous in reducing the possibility of modeling failure resulting from RS. An analysis may be conducted to examine the impact of geometric parameters of the additively manufactured components on the RS.

A specific set of NDT standards for MAM components should be devised. Given the complexity of MAM, this will increase our understanding of how to perform NDT on MAM components with high levels of accuracy and repeatability. More research has been conducted for selecting appropriate heat treatment regimes during the heat treatment process of building parts.