1 Introduction

Additive manufacturing (AM) is a rapidly growing technology that has revolutionized the production of metallic alloys with improved properties (Ref 1,2,3,4,5,6,7). One of the critical advantages of AM is its flexible process capacity, its ability to fabricate functionally graded materials, and the freedom to process a variety of materials and geometries (Ref 8,9,10,11). Among the metal AM processes is laser powder bed fusion (LPBF), which fabricates the part layer by layer using a laser beam with a predefined track to sinter and melt powder (Ref 12,13,14,15). In manufacturing, the impact of residual stresses (RS) induced by AM parameters is a critical factor because of its substantial influence on the performance and integrity of a part (Ref 3,4,5,6). RS refers to the internal stress that remains within a material or structure after manufacturing (Ref 16, 17). RS can significantly affect the mechanical properties of materials and structural components, such as fatigue life, dimensional stability, distortion, corrosion resistance, and susceptibility to brittle fracture (Ref 18).

In LPBF, the material is exposed to laser radiation, undergoing thermal expansion during heating and contraction during cooling. However, the materials surrounding the laser track constrain the expansion and contraction of the irradiated materials, which leads to elevated levels of compression and tension (Ref 18). RS typically involves three types of stress: tensile, compressive, and neutral (Ref 19,20,21).

Nickel-based superalloys with a wide range of alloy compositions have been used in various industrial and aerospace applications, particularly after rapid solidification technologies for the development of pre-alloyed powders (Ref 17, 22, 23). The superior mechanical and thermal properties of these materials facilitate the creation of intricate parts designed for specific applications. Applications of nickel-based superalloys have expanded owing to the development of advanced manufacturing techniques, particularly LPBF. Heat treatment (HT), including annealing, normalizing, hardening, and tempering, remains a critical post-processing step used to modify the microstructure and mechanical properties of Ni alloys. By employing HT techniques, RS can be effectively mitigated, the grain structure refined, and the performance properties of Ni alloys enhanced (Ref 24, 25). A traditional method for relieving RS involves heating the sample, which can significantly reduce the RS generated by LPBF (Ref 26,27,28,29,30). Another technique for reducing the RS in AM is to preheat the powder (Ref 31).

The high RS in Ni alloys, especially IN718 produced by LPBF, diminishes the load-bearing capacity of the parts (Ref 32, 33). A few studies have shown that increasing the power decreases the RS in IN718 (Ref 34, 35). When the laser speed in the LPBF process is increased, the resultant RS in the material decreases (Ref 35). As the layer thickness of the fabricated material increases, the RS correspondingly decreases (Ref 36).

Other LPBF processing parameters, such as scanning strategies, can influence the transient thermal history and consequently impact the resulting RS (Ref 37,38,39,40,41). During LPBF builds, preheating the building plate at elevated temperatures can help reduce the RS and allow for better control of cooling, resulting in more consistent mechanical characteristics of the finished material (Ref 42). Some samples have shown compressive RS, which benefits the fatigue performance of the parts (Ref 43). The cooling rate is inversely related to the powder bed temperature, and increasing the bed temperature results in a slower cooling rate. This slower rate leads to parts with higher yield strength, as observed in IN718 at a bed preheating temperature of 570 °C. Compared to samples created at 100 °C, preheating at 370 °C resulted in a 3.2% increase in yield strength for IN718 (Ref 44). RS can cause damage to parts during SLM manufacturing or, subsequently, in service (Ref 45). Park et al. studied RS at various preheating building plate temperatures in IN718, namely 50, 100, and 150 °C. Under the 50 °C preheating condition, the z-axis RS was the highest, averaging 422 MPa. RS was reduced with increasing preheating temperature, reaching a minimum of 332 MPa at 150 °C, reduced by approximately 22% as the preheating temperature increased (Ref 46).

While many studies have explored various scanning strategies and process parameters, a literature review revealed a need for more consensus on optimizing these parameters to minimize RS in additively manufactured IN718, particularly in heat-assisted LPBF. The objective of the present study was to analyze the effects of various LPBF process parameters on the resultant RS in IN718. The LPBF process parameters under investigation are the in situ heated plate temperature, laser scanning strategies, HT procedure, and the relationship between energy density and laser power. The aim is to understand how to increase the energy density by adjusting the power input during the LPBF processes, thereby potentially enhancing the densification of IN 718 material, and managing RS and residual distortion induced by laser beam heating, as shown in Fig. 1.

Fig. 1
figure 1

(a) Strain and stress caused by laser heating and the thermal expansion (εth), strain partially plastic (εpl), (b) simplified thermal stress σtensile and σcompressive and strain formation model in the irradiated zone, (c) LPBF of IN718 in an inert gas environment

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2 Methodology

2.1 Sample Preparation

Gas-atomized IN718 powder, with a particle size ranging from 15µm to 45μm, conforms to the compositions in Table 1. In this design, the build plate maintained a consistent temperature of 250 °C throughout fabrication. Six cuboid samples were produced, each measuring 15 × 10 × 5 mm. This research encompasses variations in multiple LPBF parameters: laser power, scanning strategy, and build plate temperature. Two distinct scanning strategies, namely ‘chess’ and ‘line,’ are shown in Fig. 2. The laser operated at a speed of 900 mm/s, with three laser power settings detailed in Table 2.

Table 1 IN718 LPBF samples’ raw powder’s chemical composition is expressed as a weight percentage
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Fig. 2
figure 2

Schematics of scanning strategies (A) chessboard scanning, (B) sample dimensions, (C) line scanning strategies

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Table 2 Detailed process parameters for samples
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The energy density is given by Eq 1 (Ref 47)

$$E =\frac{{\text{Laser}}\,\, {\text{Power}}}{({\text{speed}}*{\text{hatching}} \, {\text{distance}}*{\text{layer}} \, {\text{thickness}})}$$
(1)

For the fabrication of IN718, the literature suggests an energy density input range of 60–70 \({\text{J}}/{{\text{mm}}}^{3}\) (Ref 48, 49). Across all samples, a consistent layer thickness of 0.03 mm and a hatching distance of 0.1 mm were maintained. In the context of LPBF for IN718, elevated energy density values correlate with a reduction in RS. Conversely, utilizing a lower energy density in part production can lead to an increase in tensile RS. Alterations in process parameters to increase the energy density have been observed to significantly reduce the inherent tensile RS (Ref 50). The optimal range of the linear energy density for IN718 fabrication was 173–303 J/m (Ref 51). The speed decreased without increasing the power input for maximum linear energy density. This equation is expressed as follows:

$$E =\frac{{\text{Laser}} \, {\text{Power}}}{{\text{Speed}}}$$
(2)

2.2 Experimental Setup

RS on the surface was assessed using an x-ray diffractometer (XRD) equipped with small-angle x-ray scattering capabilities. The device, utilizing a Cu target, operated within a 20-60 KV tube voltage range, and for this particular research, it was specifically set to 40 KV and 44 mA with beam size 0.8 × 0.8 mm. The 2θ spectra obtained from the XRD were calculated by analyzing the scattering patterns of x-rays over a range of 2θ angles from 20 to 80°. The recorded intensity of the scattered x-rays has been determined as a function of the scattering angle within this specified range. The analysis included using the peak identified at a 2θ value of 75° to determine the corresponding d-space. The precision of the peak determination in the Jade software ensures the accuracy of the estimated d-space. Measurements spanned a variety of angles, with the sample angle increased in increments of 10 degrees, reaching a maximum of 40 degrees. The comprehensive collection of data, including both positive and negative ψ tilts, was not feasible due to the limits imposed by the experimental design and time constraints. A comprehensive analysis of the stress distribution was achieved using positive ψ tilts, effectively capturing the essential characteristics relevant to this research. The RS measured parallel to the direction of the laser movement during scanning was determined across all samples, with its magnitude influenced by various parameters (Ref 52). RS was determined for all samples by varying the parameters (Ref 47). Figure 3 illustrates the setup of the Rigaku XRD instrument.

Fig. 3
figure 3

X-ray diffraction setup with different ψ angles

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2.3 Heat Treatment Setup

The HT included a homogenization step before solution annealing, which was achieved by heating the samples at 1200 °C for 20 min then cooling the furnace at 980 °C for another 20 min before quenching with water (Ref 53). The heat treatments were then followed by standard aging heat treatment, according to AMS 5662 standard (Ref 54), at 720 °C for 8 h, then heated to 620 °C for 2 h, held at that temperature for another 8 h, followed by air cooled (Ref 55). Figure 4 shows the temperature–time profile of the heat treatment procedure.

Fig. 4
figure 4

Temperature–time profile of the heat treatment procedure for IN718 samples. AC: air cooling, FC: furnace cooling, WQ: water quenched

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To analyze RS, low-energy XRD was performed on the samples. The penetration depth of such x-rays is limited, approximately 10μm, enabling the resolution of strains near the subsurface. The XRD data were collected with a scanning rate of 2 and a step size of 0.400°. The Rietveld refinement technique was used to align with observed values. To reduce the surface roughness of the as-built samples and remove the oxide layer after the heat treatment, light sanding was uniformly applied in a similar way reported by other researchers (Ref 56) and consistently on all evaluated samples before and after heat treatment to ensure standardized treatment across all evaluated samples.

2.4 Surface Residual Stresses Calculation

Bragg’s equation was used to obtain the spacing value (d), while the angle ψ was determined using the conventional Sin2(ψ) method to represent the tilt angle of the sample surface. XRD provides an intensity graph versus a 2θ graph. Based on Bragg’s equation, we yield the ‘d’ spacing value, where ψ is the tilt angle of the sample surface defined by the conventional Sin2(ψ) method (Ref 57). Each sample had five different ‘d’ values obtained from five different ψ angles. These angles were 0, 10, 20, 30, and 40°. A graph was then plotted using d and Sin2(ψ). The graph provides the regression value using Eq 3, adapted from (Ref 58).

$$\upsigma =\frac{A*E}{2*v*A+\left(1+v\right)*B}$$
(3)

where σ is the RS A, and B are the slope and intercept of the linear regression line obtained by plotting sin2 ψ vs. Δ(2θ), E is Young’s modulus, and ν is Poisson’s ratio, which is used to calculate the RS in the material.

3 Results

3.1 RS in as-Built Samples

The technique used to determine the interplanar spacing of a connected 2θ scan of the sample obtained at various sample tilt angles is defined as the angle between the diffracting plane normal and sample surface normal. Therefore, RS can be derived from the slope of a linear plot between the fractional change in plane spacing and \({{\text{sin}}}^{2}\) ψ: (Ref 59, 60). Samples 1-6 were manufactured under various conditions. Sample 1, using the chess scanning strategy at 190 W, recorded d values between 0.12651 and 0.12696 nm and 2θ values from 74.707 to 75.016°. Table 3 presents the d-spacing and 2θ values for various ψ values. From the XRD data for IN718, additively manufactured RS samples were calculated based on the maximum spacing values d and the angle of the sample (Ref 61).

Table 3 As-built sample 1-6 data—d-spacing and 2θ values
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Sample 2 was manufactured using a chess scanning strategy. The sample was processed at a power setting of 230 W. Upon analysis, the interplanar spacing or d-values of the sample were determined to lie between 0.12647 and 0.12696 nm. These values provide insight into the atomic spacing and indicate the phase and structure of the material. The Bragg angle, denoted as 2θ, provides information regarding the diffraction pattern of the material. For sample 2, the 2θ values ranged from 74.706 to 75.044°, as detailed in Table 3. Sample 3, with chess as scanning strategy, 270 W as laser power, 900 mm/sec as speed, and the bed temperature at 250 °C reported the following measurements. The sample’s interplanar spacing, or d-values, fell within the 0.12672–0.12708 nm range. The 2θ values were between 74.625 and 74.874. Sample 4, with a line scanning strategy, 190 W as laser power, 900 mm/sec as speed, and the bed temperature of 250 °C, recorded measurements as shown in Table 3. The d value varied from 0.12668 to 0.12721 nm. The 2θ value varied from 74.531 to 74.90°. For sample 4, from Fig. 5, the linear regression coefficient R^2 value is 0.5501, and from the slope equation Y, the A value is 0.00088 and the B value is 0.12681. Sample 5, with a line scanning strategy, 230 W as laser power, 900 mm/sec as speed, and a bed temperature of 250 °C, records measurements as follows. The d value varied from 0.12646 to 0.12712 nm. The 2θ values varied from 74.595 to 75.051°. Table 3 presents the d-spacing and 2θ values for various ψ values. For sample 5, from Fig. 5, the linear regression coefficient \({R}^{2}\) value was 0.3869, and from the slope equation Y, the A value was 0.0009, and the B value was 0.1267. Sample 6, with the line as scanning strategy, 270 W as laser power, 900mm/sec as speed, and the bed temperature of 250 °C, recorded measurements, as shown in Table 3. The d value varied from 0.12687 to 0.12724 nm. The 2θ values varied from 74.479 to 74.767°. For sample 6, from Fig. 5, the linear regression coefficient \({R}^{2}\) value is 0.1224, and from the slope equation Y, the A value is − 0.0003, and the B value is 0.1270.

Fig. 5
figure 5

Linear regression plots for as-built samples: (A) Sample 1, (B) Sample 2, (C) Sample 3, (D) Sample 4, (E) Sample 5, and (F) Sample 6

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From Table 4, the RS on the surface of the AM IN718 samples was higher in samples manufactured at 190 and 230 W than in those manufactured at a power value of 270 W. In sample 6, using the line scanning strategy at 270 W power and 900 mm/s speed, compressive RS was observed at 258.03 MPa. Sample 3 was fabricated using the chess scanning strategy at 270 W and 900 mm/s. The XRD measurement showed a tensile RS observed at 188.91 MPa.

Table 4 Residual stresses in all specimens under the as-built condition
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3.2 XRD Analysis of Heat-Treated Samples

Following HT, sample 1 was processed using a chess scanning strategy, a laser power of 190 W, a scanning speed of 900 mm/sec, and a heated bed temperature of 250 °C and shows the following recorded measurements. Table 5 presents the d-spacing and 2θ values with changing ψ values obtained for samples 1 to 6.

Table 5 Post-Processed data for sample 1 to 6—d-spacing and 2θ values
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The d-spacing varied from 0.12646 to 0.12677 nm. The 2θ values varied from 74.837 to 75.127. For sample 1, as shown in Fig. 6, the linear regression coefficient R2 was 0.1804, and from the slope equation Y, the value of A was 0.0004, and the value of B was 0.1265. The d-spacing value was reduced for sample 1 under the as-built and post-processed conditions. Sample 2, with chess as scanning strategy, 230 W as laser power, 900 mm/sec as speed, and the bed temperature of 250 °C, recorded measurements as follows. The d value varied from 0.12637 nm to 0.12637 nm. The 2θ values varied from 74.809 to 75.115°. For sample 1, from Fig. 6, the linear regression coefficient \({R}^{2}\) value was 0.0073, and from the slope equation Y, the A value was 0.00009, and the B value was 0.1266. The d-spacing value was reduced from sample 1 without HT to sample 1 with HT.

Fig. 6
figure 6

Linear regression plots for post-processed samples: (A) Sample 1, (B) Sample 2, (C) Sample 3, (D) Sample 4, (E) Sample 5, and (F) Sample 6

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Sample 3, with chess as scanning strategy, 270 W as laser power, 900 mm/sec as speed, and a bed temperature of 250 °C, records measurements as shown in Table 5. Sample 4, with the line as scanning strategy, 190W as laser power, 900 mm/sec as speed, and a bed temperature of 250 °C, recorded measurements, as shown in Fig. 6. The d value varied from 0.12663 to 0.12693 nm. The 2θ value varied from 74.726 to 74.932°. For sample 4, from Fig. 6, the linear regression coefficient \({R}^{2}\) value is 0.7562, and from the slope equation Y, the A value is 0.0006, and the B value is 0.1266. The d-spacing value was reduced from that of sample 4 without HT to that of sample 4 with HT. Sample 5, with the line as scanning strategy, 230 W as laser power, 900 mm/sec as speed, and the bed temperature of 250 °C, records measurements as shown in Table 5. The d value varied from 0.12689 to 0.12743 nm. The 2θ values varied from 74.383 to 74.757°. For sample 5, as shown in Fig. 6, the linear regression coefficient \({R}^{2}\) value was 0.2159, and from the slope equation Y, the A value was 0.0006, and the B value was 0.127. Sample 6, with line scanning strategy, 270W as laser power, 900 mm/sec as speed, and the bed temperature of 250 °C, records measurements as follows. The d value varied from 0.12633 to 0.12672 nm. The 2θ value varied from 74.872 to 75.14°. For sample 5, as shown in Fig. 6, the linear regression coefficient \({R}^{2}\) value was 0.0745, and from the slope equation Y, the A value was 0.00026, and the B value was 0.12657. All specific measurements of the d-spacing and 2θ values for these samples across varying ψ values are shown in Table 5. The results of the post-processing heat treatments on the RS of the IN718 sample produced by AM are listed in Table 6. Some specimens exhibited a transition from tensile to compressive RS because of the heat treatment. However, the efficacy of post-processing heat treatments may vary among samples, as evidenced by the high RS present in some samples.

Table 6 Residual stresses in all the specimens in post-processed condition
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3.3 Comparison of Residual Stresses in as-Built and Post-Processed

While the results indicated a significant 90% reduction in RS for specimen 2, specimens 1, 4, and 5 exhibited more modest reductions of 37, 32, and 33%, respectively. Interestingly, while sample 3 underwent a transition from tensile to compressive RS, sample 6 showed the opposite trend, shifting from compressive to tensile RS. Analyzing the RS values before and after post-processing offers insights into the efficacy of the applied HT. This can be attributed to the chessboard scanning strategy, which is more effective in minimizing residual stress due to its ability to effectively mitigate localized thermal gradients (Ref 62). Table 7 compares the RS values under the built and post-processed conditions.

Table 7 Comparison of residual stresses between as-built and post-processed conditions
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The influence of AM parameters and post-processing treatments on RS is illustrated in Fig. 7, which shows the RS in all six specimens under the as-built and post-processed conditions. The RS value in all samples varies from − 258 to 772 MPa under the built conditions. The RS value in all samples varied from − 25 to 515 MPa under post-processed conditions.

Fig. 7
figure 7

Evaluation of Residual Stresses in All Examined Samples

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4 Discussion

The laser power used during the LPBF process significantly affects the RS on the surface of the part. The results for the as-built conditions showed that several parameters were consistent across all six specimens. These parameters included a heated bed at 250 °C, the same part orientation, and a laser speed of 900 mm/s. However, the laser power and scanning strategies varied among specimens. As a result, the RS values across these samples exhibited a wide range, ranging from − 258 to 772 MPa. The power level used during the LPBF process has an important impact on the determination of the RS of a specimen. Using the chess scanning strategy, sample 1 at 190 W power demonstrated an RS value of 601 MPa, Sample 2 at 230 W power demonstrated an RS value of 772 MPa, and sample 3 at 270 W power demonstrated an RS value of 189 MPa. The manufacturer’s optimal laser parameters for IN718 were 230 W laser power and 900 mm/s scanning speed. Compared to this standard, the RS was reduced by 22% at 190 W and 75% at 270 W. This could be because lower power levels may result in a lower energy density, resulting in the incomplete fusion of the material or varying cooling rates, which can cause a higher RS. Lower laser power may not provide sufficient energy density to thoroughly dissolve the powder particles, resulting in poor layer fusion and influencing the mechanical properties of the part (Ref 63). Existing research has demonstrated various effects of scan vector length and rotation on the RS. The influence of the scanning strategy on the RS is complex. The decrease in the thermal gradient is a fundamental mechanism that may explain the lower RS observed with the chess scanning strategy. The chess scanning strategy divides the built area into smaller sections, allowing for more uniform heat distribution and reducing RS values (Ref 64). Sample 4 demonstrated an RS of 754 MPa fabricated using a 190 W laser speed and line scanning strategy. For sample 5, which was manufactured at 230 W power, the RS reached 772 MPa, while sample 6, fabricated using 270 W power, had an RS value of − 258 MPa. The RS was 2% lower at 190 W and 133% lower at 270 W. Therefore, this work suggests that the optimal parameters for the fabrication of IN718 with minimal RS are 270 W power and 900 mm/s speed in both line and chess scanning strategies.

After the HT of all the samples, the RS values recorded across all samples ranged from − 25 to 515 MPa. This variation indicated a significant change in the RS levels owing to the HT process. Using a chess scanning strategy with a laser power of 190 W, sample 1 exhibited an RS of 379 MPa. When the power was increased to 230 W for sample 2, the RS decreased to 77 MPa and further declined to − 25 MPa for sample 3 at a power setting of 270 W. The RS observed was 390% higher at 190 W and 132% lower at 270 W than the manufacturer’s recommended process parameters.

In the case of the line scanning strategy, sample 4 displayed an RS of 515 MPa at 190 W laser power. The RS only slightly decreased to 514 MPa for sample 5 at 230 W laser power and further reduced to 223 MPa for sample 6 at 270 W laser power. The RS was 2% lower at 190 W laser power and decreased by 133% at 270 W laser power.

The post-processed chess scanning strategy specimens were observed to have less tensile/more compressive RS. Overall, it can be concluded that the chess scanning strategy is the best for manufacturing a sample of IN718, whereas HT and manufacturing with 270 W generated less RS on the surface of the IN718 samples.

5 Conclusion

This work investigated the measurement and characterization of RS in both as-built and post-processed parts. A summary of various RS measurement and characterization techniques is presented, emphasizing the significance of manufacturing process parameters. The primary objective of this study was to reduce the RS of the final AM IN718 product. Six IN718 samples were manufactured using two scanning strategies (chess and line) and three laser power levels (190, 230, and 270 W) on a heat-assisted bed at 250 °C. It is evident that employing a heat-assisted bed with a chess scanning strategy significantly mitigates RS. A laser power of 270 W and scanning speed of 900 mm/s produced a minimal surface RS. The optimal AM parameters for IN718 include the chess scanning strategy, laser power of 270 W, and scanning speed of 900 mm/s. Implementing these process parameters can significantly lower RS, thereby improving the performance of the fabricated IN718 parts. This study emphasizes controlled surface RS due to their critical role in mitigating risks of surface cracking and potential delamination. When these stresses are not optimally managed, they can initiate surface-level imperfections, which could compromise the material’s performance during operation.

Residual stresses are caused by nonuniform plastic deformation of thermal and/or mechanical sources. The main goal of the presented study is to highlight the effect of thermal gradients induced by the laser processing (laser power and scanning strategy) of IN718 and then the effect of subsequent heat treatment on the RS with the presence of an unavoidable oxidation layer that requires removal.

The study of RS of metallic materials requires surface preparation and previous treatments to the sample surfaces such as electropolishing (Ref 65,66,67,68), shot peening (SP), low plasticity burnishing (LPB), low stress grinding (LSG), and glass bead peened (GBP), all of which inevitably produce or alter RS to some extent on the samples. Efforts can be directed to use correction models that are available to estimate the true stress that existed per ASTM standards such as ASTM E2860 (Ref 67).

Future work branching from this study will focus on performing electropolishing to the heat-treated samples to eliminate any RS induced due to the effect of sample preparation that was necessary to remove the oxidation layer.

The RS values presented in this study are not absolute and are specific to the processing conditions of the IN718 samples. The results presented guide the expected RS in the LPBF fabrication procedure of IN718 and the expected RS values when a low-cost sample preparation procedure is followed after heat treatment without or before electropolishing.