RESIDUAL STRESS AND DISTORTION ANALYSIS IN LASER BEAM WELDING PROCESSES

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1 Ind. J. Sci. Res. and Tech ():0-5/Kanthi ISSN: (Online) RESIDUAL STRESS AND DISTORTION ANALYSIS IN LASER BEAM WELDING PROCESSES Rajesh Goud Kanthi School of Mechanical Engineering,CEST, Fiji National University, Fiji *Author for Correspondence ABSTRACT An accurate and physically suitable heat flux simulation model is to be given as input for analysis of a laser beam welding. The outcome of the heat input is structural deformation and residual stress formation. In the literature Gaussian heat flux models are given and used by different authors. The load is vectorized so that it can be used in analytical FEM analysis or it can be used in FEA packages such as ANSYS. A representation of heat flux model is developed taking the area of heat source of fusion zone & heat affected zone which give rise to circular & elliptical areas. Comparative structural deformation and the related residual stresses are carried out in different heat flux applications. A realistic model to match the experimental work might require a combination of the present proposed heat flux models. This work can be used for selecting process parameters for reducing structural distortion and residual stresses simulation process. Key Words: Laser Beam Welding, Heat Flux, Residual Stress and Distortion INTRODUCTION Laser welding simulation was used in the recent past to understand evaluate and predict performance of weld for different input parameters. Mackwood and Crafter (005) summarized thermal modeling up to 00. Georgeta et al., (01) found that distortion and residual stress of laser welding increase with increase in laser beam power and reduces with increase speed of welding and also with increase in spot diameter. Distortion & residual stresses for a 1mm thin plate is simulated also validated with experiments by Deng and Murakawa (008). He found elastic plastic deformations for temperature dependent parameters. Moratis and Labeas (008) provided numerical simulation for aluminum lap joints for residual stress & distortions. Laser beam welding process includes heat radiation and convection losses. The laser beam gets embedded inside the molten metal and plume reflects within the dead end capillary key hole and loses energy which is transmitted mostly by heat conduction and also by absorption. The temperature dependent thermal - mechanical properties are used in the simulation. The phase changes causes release of latent heat from solid to molten state and heat of evaporation is also added in the case of LBW, again the transformation of molten state to solid state phase change occurs with absorption of latent heat of solidification. A 10000W/mm of laser power density, flux, causes a capillary key hole formation studied by Kaplan (1998). He obtained the front and rear wall of key hole from energy of heat flow equations. The reduction of residual stress & distortion was studied by Wencheng et al., (01), who studied reduction in residual stress in 316L material when welded by providing a heat sink. The simulation has shown a 0% decrease in longitudinal stress. Sanna (011), used ANSYS1 to model non linear transient three dimensional heat transfer model for the CO laser welding simulation for Al-6061-T6 alloy. In the present study stainless steel 304 material used in nuclear fusion reactors is analyzed with the simulation model adopted in ANSYS. The sequentially coupled thermal to structural model was used to estimate the transient temperature distribution. Subsequently, the residual stresses & distortions were calculated for a constant heat flux and next with the Gaussian flux from a Laser beam source. FORMATION & SIMULATION OF WELDING PROCESS The heat energy equations are referenced in many including Frewin and Scott (1999). T T T T T kx ky kz Q c[ V ] (1) x y z t x For a isotropic conductive material with equal coefficient of conductivity kx, ky, kz (W/mK) in all three chosen orthogonal co-ordinates. Equation (1) gives the heat energy in the weld area with temperature, T (K) obtained both in spatial, x, y, z (m) and temporal, t(s), terms. Q (W/m 3 or J/m 3 s) is the net heat from input and the losses in the form of convection & radiation. Density, ρ, kg/m 3, specific heat capacity, c, give the right hand terms of how much heat is retained with respect to time in the material and how much is taken away with the velocity of welding, v, m/s. 0

2 Ind. J. Sci. Res. and Tech ():0-5/Kanthi ISSN: (Online) The boundary conditions given are To (x, y, z, 0) throughout the body at time zero or at the starting of the weld, this is an essential boundary condition. In addition the natural boundary conditions have to applied consisting of normal conduction kn T, heat flux q, convection h (T-T0), and the radiation term, 4 4 ( T T0 n ). Together, the boundary conditions are summed up as: 4 4 Kn - q + h(t-t0) + ( T T0 ) =0 () When symmetric boundary and insulation boundaries are considered as adiabatic, with no heat flowing through the surface it is obtained by making convection zero, and conduction zero from the surface. Where, kn is the thermal conductivity normal to the surface, W/mK, h is the convective heat transfer coefficient, W/m K, εis emissivity of surface radiating, ζis the Stefan Boltzmann s constant, , w/m K 4. When it is difficult to use radiation boundary condition, it is combined to convective heat flux by using a modified coefficient, h r, for hot rolled steel plates with an error of about 5% is, h r.410 T (3) Radiation inclusion will increase solution time by about three times and hence combined with convection. In laser welding pulsed heat can be given which is available in Ansys to be incorporated. For analytical solution Kronecker δ, is used which takes the value of 1 when pulse is on and a value of 0, when pulse is off. Finite Element Formulation The heat equations (1) can be represented in tensor form so the elemental transient heat equation is obtained and later summed to get the system equation which is analysed with time.. K T T C T T V Q T (4) Where K is a temperature dependent conductivity matrix. C is the temperature dependent capacitance matrix based on specific heat it s product with rate of temperature gives heat. V is the velocity vector and Q the heat load on the system. The above equation can be solved numerically, with standard FEM models with Crank Nicholson or Euler time integration models. An initial temperature T i is assumed K, C and Q are calculated at that temperature and the next temperature T at i+1 is obtained. Again K, C & Q are calculated and temperature at next temperature interval is calculated. The iteration is continued for convergence of temperature or heat flux values. Assumptions Thermal properties, i.e. conductivity, specific heat, density are temperature dependent. A combined convection and radiation boundary condition is used on the remaining of the top surface. Heat flux of constant used for fusion welding and Gaussian distribution for Laser welding is used on Heat Affected Zone (HAZ). FINITE ELEMENT MODEL The finite element model of dimensions 40 X150 X 5mm is used. The AISI 304 austenitic stainless steel material is considered for simulations to be carried out. The convection is applied on all the surface of the plate except on the heat applied area. The temperature dependent thermal properties for AISI 304 stainless steel material are given in Table 1. These properties were taken from the work carried out Amudala (01). In the fig.1 shows the meshing of the model with tetrahedral element of volume mesh of 0.0 for whole model. Table 1: Temperature dependent thermal properties for AISI 304 Austenitic stainless steels S. No. Temp (K) Thermal conductivity, W/m o K Density, Kg/m 3 Specific heat, J/Kg K

3 Ind. J. Sci. Res. and Tech ():0-5/Kanthi ISSN: (Online) Figure 1: Mesh model used for analysis Figure : Solid-70 3-D Thermal Solid Table : Properties of AISI 304 Steel Tensile Yield Density Melting point Thermal conductivity % Elongation strength strength 515 Mpa 05 Mpa 8000 kg/m o c 16. W/m o K at 100 o c 0-40 In the ANSYS the heat transfer analysis is conducted using element type SOLID-70. This element type has a threedimensional thermal conduction capability and eight nodes with single degree freedom (temperature) at each node. The element is applicable for three dimensional, steady-state or transient thermal analysis. The element can also compensate for mass transport heat flow from a constant velocity field. In this analysis, element SOLID-70 is replaced with by a three-dimensional (3-D) structural element SOLID45. The element is defined by eight nodes having three degrees of freedom at each node (translations in the nodal x, y and z directions). The element has plasticity, creep, swelling, stress stiffening, large deflection, and large strain capabilities ANSYS (User s manual, 001). In the present study AISI type 304 stainless steel is used having many advantages such as low thermal conductivity, high resistance of corrosion and high stability at elevated temperatures and also it is used in numerous industries, including electronics, medical instruments, home appliances, automotive and specialized tube industry. It has excellent forming and welding characteristics. The properties of a typical stainless steel sheet are given in table (Amudala, 01).

4 Ind. J. Sci. Res. and Tech ():0-5/Kanthi ISSN: (Online) RESULTS Thermal analysis The thermal analysis has been carried out for constant heat flux with welding speed =7.5mm/sec. After the welding phase is finished, the time step is progressively increased up to 1000 sec to allow the plate to cool down to room temperature. In the present work Finite Element Analysis of single-pass butt-welding has been carried out with constant heat flux. For this, a simple Butt-joint welding whose welding parameters are consistent to those of Friedman s model with heat input Q=100 W is considered and has been simulated using ANSYS. The temperature will gives understanding of weldability in the weldment and also results to effect of structures of due to temperatures in the weldment. In fig.3 shows the temperature distribution in the weldment. The temperature various from 308 o k to 374 o k, which results to understand the weldability of the material. Figure 3: Temperature distribution in the weldment Figure 4: Temperature distribution in the transverse direction of the weldment The perpendicular of the weld direction is called as the transverse direction. In fig.4 shows the temperature distribution in the transverse direction of the weldment, results shows the maximum temperature is at the fusion zone of the weldment. The temperatures in the model is various from 308 o K to 374 o K. In fig. 5 shows the temperature distribution in the weldment at three zones of the model, i.e. base material, heat effected zone and fusion zone. The fusion zone carries 3

5 Ind. J. Sci. Res. and Tech ():0-5/Kanthi ISSN: (Online) more amount of heat i.e. the temperature is around 800 o K, the heat affected zone is around 1700 o K. The simulation is carried for 1000sec the base material was not to cool down to ambient temperature. Structural results The estimate of residual stresses is analyzed in the weldment. Due to the variance in the temperature gradient, the thermal dependent material properties are given in the model. A stress acting normal to the direction of weld bead is known as a transverse residual stress. Figure 5: Temperatures distributions in the model Figure 6: Stress distribution in the model Figure 7: Distortion of the weldment 4

6 Ind. J. Sci. Res. and Tech ():0-5/Kanthi ISSN: (Online) The temperature near the weld bead and heat affected zone rapidly changes with distance from the heat source. Residual stress distribution over the plate area of heat input is shown in Fig.6 which shows more stress value in the weld bead area and gradually decreases from center line to the base plate end. Due to the effect of temperature distribution the model i.e. thermal changes which leads to effect in the structural changes; the changes are residual stresses and distortion. In the fig. 7 shows the distortion in the weldment is varied from mm to 0.34mm, we can see from the fig. 5 based the thermal distributions the residual stresses and the distribution are varied. The maximum distribution is at the welding joint. CONCLUSION A welding process has been simulated using a commercial finite element package, ANSYS. First for a constant heat flux and next with a Gaussian distribution more suitable for Laser weld. The temperature near the weld bead and the HAZ decreases rapidly with the distance from the centre of the heat source. The transverse residual is high near the weld and reduces as it moves further. The distortion of the weldment shows low even at the fusion zone. Based on simulation results, residual stress is of the weldment can be predicted. Thus, the experiment analysis, which might be costly, can be avoided. REFERENCES Amudala NSB (01). Finite Element Simulation of Hybrid Welding Process for Welding 304 Austenitic Stainless Steel Plate. International Journal of Research in Engineering and Technology, 1(3) Deng D & Murakawa H (008). Prediction of welding distortion and residual stress in a thin plate butt-welded joint. Computational Material Science, Frewin MR & Scott DA (1999). Finite element model of pulsed laser welding. Welding Journal, 15S-S. Georgeta A, Gheorghe S & Danut I (01). Input Parameters Influence on the Residual Stress and Distortions at Laser Welding Using Finite Element Analysis. UPB Scientific Bulletin, Series D, 74() 15. Kaplan A (1994). A model of deep penetration laser welding based on Calculation of the keyhole profile. Journal of Physics D: Applied Physics, Mackwood AP & Crafter RC (005). Thermal modeling of laser welding and related processes a literature review. Optics & Laser Technology, Moratis GA & Labeas GN (008). Residual stress and distortion calculation of laser beam welding for aluminum lap joints. Journal of Material Processing Technology, Sanaa NM (011). Analysis of Temperature and Residual stress Distribution in Co laser welded Aluminum 6061 Plates Using FEM. Al-Khwarizmi Engineering Journal, 7(3) Wenchung J, Yuchai Z & Wanchuck W (01). Using heat sink technology to reduce residual stress in 316L stainless steel welding Joint: Finite element simulation. International Journal of Pressure Vessels & Piping,