Evaluation of residual stresses in CO2 laser beam welding of SS316L weldments using FEA

Laser welding is used in critical component production when tight tolerances like minimal distortions and residual stresses are required. Laser beam welding offers a lower heat input, a smaller heat affected zone, lower residual stresses, minimum distortions, and greater mechanical joint characteristics than conventional welding does. In order to simulate the laser welding process used on SS316L plates, the Gaussian heat source model was used. The model is developed and simulated with volumetric heat source model with APDL coding using ANSYS. The thermal profiles at the joint cross sections via welded area, interface across joints is taken for the analysis. The maximum temperature was observed at the fusion zone and associated zones. The residual stresses are analysed in the same path and found the stresses are in safe limits of base material. Predicted and experimentally measured residual stresses are close agreement with 10%.


Introduction
Industries such as aircraft and chemistry use heat exchangers. Cladding is used in power plants and turbine generators to increase surface durability. Sandeep et al [1] investigated metallurgy for improving fatigue wear. The authors employed clad bead to increase shielded metal arc welding performance. However, implementing laser beam welding poses obstacles such as optimising process parameters, experimental setup and handling, and high expenses. Prior to the trials, simulation approaches employing Finite Element Analysis (FEA) are used to predict the heat input and optimise the weld process [2]. Similar materials connecting reduces component weight and cost, which is required in industries including nuclear, car, aerospace, power plants, and chemical plants. Welding is utilised for thick plates. Each heat pass takes the same amount of time to fill the material into the groove. During these operations, the substance is heated and cooled briefly. Even after the metal has cooled to normal temperature, the lattice size is strained. 3D models of the welding process were created to analyse temperatures, distortions, and residual stresses [3]. Balram et al [4][5][6] developed the numerical model for predicting thermal fields and residual stresses by employing Gaussian and double ellipsoidal models as heat source for TIG welding simulations. The peak temperatures at different location along the weld line was predicted and also, improper thermal gradients towards the base materials were observed in welding passes-1, 2. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Cooling curves up to 1000 s were shown clearly different profiles in dissimilar joints of Monel and 316 steel weldments.
Vasantha raja et al [7] investigated TIG and ATIG distortion and maximum stresses in SS316LN weldments. Double V-groove and square-butt connections are used to weld the 16 mm thick plate. The Vernier height gauge and ultrasonic method detect the distortion and residual stresses. Away from the weld zone, residual stresses were positive. It is 102 MPa on both sides. TIG joint double side welding 131 MPa & 187 MPa TIG welding has −0.972°distortion whereas A-TIG shows 1.03°distortion. Xu et al [8] used keyhole plasma arc welding for T-joints on Ti6Al4V samples. 5 mm plate is used to analyse residual stresses in metal joints. The first pass has significant tension while the second pass has mild stress. The longitudinal tensile stresses were 860 MPa. SYSWELD was used to simulate welding. Venkat et al [9] used SS304 to weld and measure heat, distortion, and stress fluctuations. 3 mm plates were GTAW joined. Welding speed was 2.11 mm s −1 and 3.06 mm s −1 . The K-thermocouple on the top surface was used to measure the temperatures. The weldment distortion was 1.16 mm. Their work also produced a 3D model with volumetric heat flux for welding process evaluation utilising a Gaussian distribution heat source. The weldment residual stress is 240 MPa. The simulation analysis is used to validate the ABAQUS stress model. In another work by Gulshan et al [10] had studied the residual stresses developed in the Zirc aloy-4 hallow tube of 8 mm thick using the XRD. In addition, the FEA simulations were used to obtain the residual stress and von misses stress in the tube.
Mohanty et al [11] performed laser weldment of SS 316 L butt joints and measured stress distributions. The CO 2 welding power is 3.5 kW with helium as a shielding zone and a frequency of 5000 Hz. X-ray diffraction measures residual stress on a weldment's top surface in the welding direction. The analysis is done to reduce residual stress in the weldment before and after utilizing vibratory stress relief therapy, which takes less time. The vibrational stress reduction procedure reduces residual stress by 100 MPa. Goldak et al [12] presented several heat fluxes for welding processes. Suitable heat flux with Gaussian surface flux was recommended for laser, electron and TIG welding processes for better weldability and deeper penetration. For realistic simulations, the power density hemispherical volume heat source is superior to the disc model. The Double Ellipsoidal Power Density Distribution was suggested to depict heat flow creation. Lee et al [13] created the dissimilar joint simulation model in ABAQUS 6.7. The steels utilized were 508 carbon and 316 stainless. GTAW multi-pass welding simulation with 8 passes. The thermal and structural analysis uses 8 node C3D8R components. The fusion zone temperature was 1900°C, while the HAZ was 1100°C. An axial residual stress distribution is carried out in the structure with 24 MPa to 203 MPa and −236 MPa to 80 MPa.
The purpose of this research was to create a model in ANSYS that would significantly cut down on the computational time required to predict the temperature field and thermal cycles which is close precise. The suggested method, which is based on the idea of the volumetric equivalent heat source, is shown by simulating high-power laser beam welding of an SS316L plate that is 5 mm thick. By providing the heat quantity to the FEA as volumetric creation of the internal energy, the tedious calibration process may be avoided. The transient thermal analysis is carried out to understand the temperatures distribution at various zones of the weldment and static structural analysis for residual stresses in the weldment. The process parameters used for simulation will be used for experimentation and be validated.

Computational formulation and heat source model
3-Dimensional finite element model is developed for laser beam welding process using ANSYS software. Model is with a dimension of 110 mm × 80 mm × 5 mm plate, which is given figure 1. SOLID 70 [14] element is chosen for the simulation studies. It is an 8 node and has single degree freedom at each node, which supports both transient thermal and static structural analysis. During the coupling process, the element switches to SOLID 45 structural element. For the simulation models, we employ the two mesh types. As shown in figure 1, the finer mesh is put on the fusion zone with a mesh size of 1 mm, and the coarser mesh is put on the rest of the plate with a mesh size of 2 mm. The process parameters for the simulation process are chosen from the literature available for SS316L [15,16] grades and same properties is used. This is a procedure for transient finite element analysis using the Gaussian heat source model as given the figure 1(d). In the present study the simulation work is done using ANSYS. Welding power of 3.0 kW, welding speed of 1.0 m min −1 and a spot diameter of 2 mm were used for experimentation to validate the chosen parameter for confirm the quality of the joints which is given in table 1 and figure 1(e) shows the CO 2 laser beam welding used for experimentation. Finally, the simulated data is used for experimentation to validate the process parameter and to verify the quality of the joint with structural integrity. Whereas these parameters show the weldable for nickel-based alloys with full penetration [17]. The base materials SS316L thermal and mechanical properties are given in table 2. The iteration is continued for convergence of temperature or heat flux values.

Basic assumption and governing equation
The ambient temperature of 30°C was chosen for all the combinations. Temperature dependent thermal properties of the base material is taken [18] for transient thermal is shown in figure 1(c) and static structural analysis the mechanical properties of the base material are taken from table 2, on the upper surface of the Table1. Laser beam welding parameters for the simulation and experimentation process [16].  (1).  Welding of thick plates with high power densities may be accomplished by the use of a laser beam welding technique. In comparison to other procedures, the laser beam welding method produces connections that are free of faults and is hence one of the techniques that is employed. Because of the strict criteria placed on some industries, such as nuclear pressure vessels, chemical facilities, and aeronautical sectors, welding of similar materials has gained a lot of attention. Welding of austenitic steels, such as SS 316 L, has several fascinating uses in the chemical industry, the nuclear industry, and space applications.
The heat input that is delivered by LBW may be computed by making use of the formula that is provided further down [19].
here laser power is given in W in kW, and the travel speed is mentioned as V in m min −1 .

Residual stresses
In the joining process, the weld region is subjected to heating and cooling cycles. As a result, the weld metal and the parent material undergo differential thermal expansion and contraction, which results in welding Residual Stresses (RS). In this study, the RS is measured on top surfaces of the weldment in a transverse direction (perpendicular to the weld line) at a variety of distances from the weld line at different points. The ( ) Cos a measuring method with a spot size of 2 mm is adopted; the camera image is displayed in figure 4(b). Sample measuring parameters are the diffraction angle (=150.876°), inter-planar spacing (d = 1.077 Å) X-ray wavelength. X-ray diffraction was used to measure the residual stresses in the base metal, HAZ, and weld area in accordance with ASTM E2860 −12 standards [20]. For the test, weld samples were welded with a spot diameter of 0.5 mm, and the residual stress was measured at the weld zone. The residual stresses are measured at particular area and shown in figure 2.

Results and discussion
3.1. Thermal analysis Compared to arc welding, laser beam welding produces a smaller fusion zone and heat impacted zone. The fusion zone is the highest temperature in laser welding owing to the narrow beam spot local heating of the material. The analyst needs a heat source model that accurately forecasts the weldment temperature [21]. On the remaining top surface, convection and radiation boundary conditions are utilised.
In every location, the estimation of the residual stresses is given a thorough examination. Because of the variation in the temperature gradient, the model only accounts for the characteristics of the material when it is subjected to high temperatures. A transverse residual stress is a stress that acts in a direction that is perpendicular to the direction of the weld bead. The temperature and residual stresses were evaluated on the base plate surface, where the distributions are reported for fusion zone, heat-affected zone, and base plate of the weldments. The temperature distribution of the plates is given in figures 3(a)-(d). The plotted temperatures are in the transverse direction to the welding. The heat flux, geometry and boundary condition are symmetrically applied for all the combinations. Figure 4 shows the temperature distribution in the SS316L butt weldment obtained from the simulation studies. The highest temperature was seen at the fusion zone 1512 K and sharply decreased to a temperature of  1108 K at the heat-affected zone, and the base plate remains at the ambient temperature of 303 K. The temperature distributions are symmetrical. The isotherms are of the weldment represents all zone of the plate and also can be assess the distance from zone to other are given in figure 3(c). Figure 4 shows the temperature distribution, which replicates the Gaussian normal distribution (bell-shaped curves) at the butt junction of two welded plates. The weld simulation results estimate the heat flux distribution geometric parameters.

Structural analysis
In every location, the estimation of the residual stresses is given a thorough examination. Because of the variation in the temperature gradient, the model only accounts for the characteristics of the material when it is subjected to high temperatures. A transverse residual stress is a stress that acts in a direction that is perpendicular to the direction of the weld bead. It was helpful to get an assessment of the stress distribution with the provided heat thanks to the transient thermal analysis. In terms of the weld process parameters, this information was essential. Before beginning the manufacturing process, you may use this process simulation to gain a rough estimate of the degree of stress that will be experienced by the weld structure. The chosen input process parameters will be the right choice for with heat flux, for maintaining structural integrity. Residual stresses are obtained by giving thermal temperature distribution to plate and boundary conditions are fixed edges along the two boundaries along the lengths, in the Z direction given in figure 5. Because of boundary conditions the longitudinal central distortion is less and maximum at the edges. The stresses in transverse direction are strongly affected in the fusion zone. The residual stresses are varied from −0.114E 11 to −0.228E 10 is less than the yield stress of base plate. The behaviour of the stress is shown in the figure 5, the net force on the plate is in compressive forces.

Weld quality
Experimentation is carried out with the process parameter given in table1 and figure 6 shows the CO 2 laser beam weldment. The quality of the joints was assessed by visual inspection for surface defects and microstructure for internal defects and were observed the weldment is free from defects. The weldment's aspect ratio changed based on the values of the controlled variable. As shown in figure 7, bead geometry of each joint is provided at the lateral surfaces of the joints and the internal quality of weldments is checked using an optical microscope. Figures 7(A)-(C) shows the weld bead geometry of the laser beam weldment at top middle and centre of the weld bead with full penetration, and figure 7(D) shows the bead width with a length of 2.9 mm of the weldment, in mid span with a width of 1.02 mm and depth shows with full penetration. The solidification grain barrier (SGB) [22] may be seen in the middle of the fusion zone. The greater laser power density and travel speed are both thought to be caused by the SGB in the middle of the fusion zone. Figure 8 shows the Alpha angle of the stresses measured surface. When it comes to the distribution of residual stresses in the joints, thermal and mechanical characteristics of the base materials, such as thermal conductivity, the coefficient of thermal expansion, and yield strength, will play an extremely important role. This transient thermal stress leads to non-uniform strain which results in non-uniform residual stresses. From structural integrity point of view, it is important to keep the stress levels below the yield points point for SS316L is about 190 MPa. As all the residual stresses are below the yield stress with a factor of safety of more than 1.0 the structure is safe. Also, the rate of cooling is greater, which results in a quenching effect. The fact that this action leads to equiaxed grains in the weld zone may be related to the fact that the weld zone is −214 MPa subjected to induced compressive residual stresses [23]. In terms of fatigue life, tensile residual stress might potentially contribute to the onset of cracks and early failure, while compressive residual stress has the potential to increase fatigue life. Similar kind of observations were observed in research work related to joining of Ni-based alloy Monel 400 and AISI 316 steel where the weld zone is subjected to compressive nature residual stresses which could be attributed to improve the weld strength and reduces the growth of crack propagation [24,25]. The compressive stresses reported at the fusion zone for simulation and experimental analysis respectively. Lostado [26] had also reported in his work that any small differences between the welded joints and the welded joints based on FEM can be enlarged enormously in the presence of nonlinearities. This influence and results in the dropping of structural integrity.

Conclusions
A finite element model is constructed and effectively used to transient thermal analysis and static structural analysis on an SS316 stainless steel plate. The material properties used for the simulation studies is temperature dependant properties. The energy required was predicted and empirically validated in order to determine the constraints of a butt weld. Based on the thickness, a typical energy need is presented.  • Full penetration of 5 mm thick SS plate butt joint was obtained using a laser power which are much lower compared to those used in conventional fusion welding processes for the same material and geometry.
• The thermal distribution and the metallurgical investigation showed a typical epitaxial growth in the fusion zone with defect free and an extremely narrow heat-affected zone.
• The analysis of joints has maximum stress distributions within limits with the factor of safety of more than 1 assuring structural integrity. • The variation may attribute to the presence of factors such as convection and radiation effects. Further, it has been observed that the thermally induced stresses profiles were more or less maintained symmetry along the fusion zone.