THERMAL ANALYSIS OF WELDING IN T-JOINT PLATES USING FINITE ELEMENT ANALYSIS

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1 THERMAL ANALYSIS OF WELDING IN T-JOINT PLATES USING FINITE ELEMENT ANALYSIS Mr. K.KRISHNAMOORTHY 1, Mr.S.SHEIK SULAIMAN 2, Mr.R.KARTHIKEYAN. 3 Assistant Professor/Mechanical Engineering, Francis Xavier Engineering College, Tirunelveli, India 1 Assistant Professor/Mechanical Engineering, Francis Xavier Engineering College, Tirunelveli, India 2 Assistant Professor/Mechanical Engineering, Francis Xavier Engineering College, Tirunelveli, India 3 ABSTRACT: Welding is highly reliable and efficient metal joining process. The thermal response of materials to a welding heat source sometimes causes mechanical problems, e.g. residual stresses and distortion and changes in mechanical properties due to changes in the microstructure. The finite element method (FEM) is the most commonly used numerical technique, which provides accurate estimates of thermal parameters for this analysis. Finite element analysis (FEA) is a tool used especially in determining the thermal stresses and temperature distribution of the welded models, which are difficult to analyze by hand calculations. The objective of the current work is to evaluate transient thermal analysis in arc welded T- Joint 304L stainless steel plates. The object is modeled in 3D and analyzed using FEA with an element type of SOLID70. Energy is input into the thermal model using moving circular area heat source. The results obtained by thermal analysis are used to determine the temperature distribution and temperature histories. Keywords: FEA analysis, Heat sources, Temperature distribution, Temperature histories. I. INTRODUCTION To produce high strength welded structures, arc welding is an effective and economic joining method attracting world welding community. Due to non-uniform expansion and contraction of the weld metal and surrounding base metal by heating and cooling cycles during welding, thermal stresses occurs in the weld and adjacent areas. During the heating phase, the strains produced always induce plastic deformation of the metal. The stresses resulting from these strains combine and react to produce internal forces that cause a variety of welding distortions. Welded steel joints are sometimes considered the weakest part in the object owing to the possible reduced creep strength of the weld metal and surrounding heat affected zone (HAZ). Finite Element Analysis (FEA) as a reliable method for this analysis. During welding processes, heat can be transmitted by conduction, convection and radiation. For welding processes where an electric arc is used as the welding heat source, heat conduction through the metal body is the major mode of heat transfer and heat convection is less significant as for as the temperature field in the welded body is concerned. The heat flow in the welding process presents a very complex situation, which currently defies the detailed analysis by analytical calculations. However, this problem can be simplified by considering conduction only (on the basis of the limited effect of radiation) and treating the convection by making use of an arc efficiency parameter. 11

2 II. HEAT SOURCE The temperature vs. time relationship of welded components and structure can be theoretically obtained by carrying out a heattransfer analysis of a welding process. This involves many complicated heat-flow phenomena including heat radiation, convection, heat conduction as well as fluid flow of melting weld metal. This process would require solving many constitutive differential equations using finite element or finite different methods that are time consuming despite the fact that the computing power continuous to improve. Therefore, from a practical point of view, analytical solutions for the heat-transfer problem in welding are preferable despite their limitations. Their major advantage is that they are given in the closed form equation that could provide the temperature-time information for the welding thermal problems in a rabid and convenient way. In most welding processes performed on thick plates, the heat flow in three dimensional (3D). For cases of high power or fast moving heat sources, the heat flow along the travel direction of the heat source can be neglected, hence, the one-dimensional heat flow can be used to model this situation. The solution to the heat-conduction equation is based on the concept of an instantaneous heat source that is widely used in heat-conduction analysis. The concept of instantaneous heat sources assumes that the heat is released instantaneously at time t 0 in an infinite medium of initial temperature T 0 either across a plane for a uniaxial conduction, along a line for biaxial conduction is in a point for triaxial conduction. The general governing heat-conduction is as follows, T T T T x y z a t (1) Where a is thermal diffusivity of the body k c material ( a, where k- thermal conductivity, - density, c- specific heat of the material) III.GAUSSIAN - DISTRIBUTED AREA HEAT SOURCE The Gaussian-distributed heat source is a simplified model for a local concentration of a welding heat input when its density is assumed to follow the Gaussian distribution. Normally, the temperature field in the vicinity of the heat source is particularly dependent on the heat-flow density of the heat source. However, the temperature at the far field is less sensitive to the heat flow density; therefore, the distributed heat source gives a similar temperature distribution if the heat source is replaced by a concentrated point source in the centre of its area. Hence, the Gaussian-distributed heat source can be used to simulate the welding heat source to give a better prediction of the temperature field near the source center to overcome the weakness point and line heat source, which would predict an infinitely high temperature at the source location. The Gaussian heat source is used to simulate the welding arc, welding flame, or welding beam where the heat source density is represented by at an arbitrary point exp( kr 2 ) (2) 12

3 where is the maximum value of the heat source density; r is the distance from to the centre of the heat source and k is the coefficient determining the concentration of the heat source, also known as distribution parameter, which represents the width of the Gaussian distribution the point or line heat source that give an unrealistic infinite temperature. Where, q = heat flux (W/m 2 ), k = distribution parameter (m -2 ), r = radius of the circular heat source (m), ρ = density of the material (kg/m 3 ). curve (higher value of corresponds to a more c = specific heat (kj/kg K), concentrated heat source).a schematic illustration of the Gaussian area heat source. T 0 = initial temperature of the plate ( 0 C). T= temperature distribution ( 0 C), v= welding speed (mm/sec), a= thermal diffusivity (m 2 /sec), t= time (sec), x= Distance along in x-direction (mm), y= Distance along in y-direction (mm). IV.FINITE ELEMENT ANALYSIS The finite element method (FEM) (its practical application often known as finite element analysis (FEA)) is a numerical technique for solving problems of Engineering and Mathematical Physics. Fig. (1) Gaussian distributed area heat source Let us assume that Q is the total output of the heat source. The heat equilibrium condition Q= (3) In this method, a body or a structure in which the analysis to be carried out is subdivided into smaller elements of finite dimensions called finite elements. Then the body is considered as an assemblage of these elements connected at a finite number of joints called Nodes or nodal points. The properties of each Subsequently, the maximum density of the heat source, depending on the heat output, Q, and distribution parameter k as = and equation becomes, type of finite element is obtained and assembled together and solved as whole to get solution. In this T-joint have two plates (Fig. 2) have dimensions, horizontal plate length=100mm, width=50mm, thickness=6mm and vertical square plate have same Qk exp( kr It is worth noting that when 2 ) (4) the heat density thickness and size=50mm. The finite element meshing is shown in (Fig. 3). It have 8476 nodes and 6175 brick node elements., this means that this Gaussian-distributed density heat source will predict a finite temperature at the heat source centre, which is more realistic than 13

4 Fig. (2) Finite Element model of T-joint plates Fig. (3) Finite Element meshing of T-joint plates V.MATERIAL PROPERTIES Temperature (ºc) Specific heat (J/Kg o C) Conductivity (W/m o C) Density (Kg/m 3 ) Yield stress (Mpa) Thermal expansion coefficient 10ˉ5/ºc Young s modulus (Mpa) Poisson s , ratio Table1: Temperature dependent thermal, physical and mechanical properties of 304L austenitic stainless steel. [1]. VI.THERMAL ANALYSIS A thermal analysis calculates the temperature distribution and related thermal quantities in a system or component. In this work transient thermal analysis was used To calculate the heat input for arc welding procedures, the following formula can be used (5) Where Q = heat input (W), V = voltage (V), I = current (A) =26 V, =210 A, η= 60% 14

5 In this thermal analysis of T-joint have totally 25 load steps were involved in the moving area heat source. Totally 10 seconds was consumed to complete the welding process. The nodal temperature solutions were obtained from the thermal analysis. In 3D analysis, during a time step, the welding arc is allowed to stay at the element with constant heat flux and then moved to next element. VII.RESULTS The moving continuous area heat source of the welded plates traveled along both sides of the vertical plate simultaneously and same direction. Under all these conditions, the thermal analysis carried out and the data obtained from analysis saved to a file. The thermal analysis results are shown in following figures from 4-13 Fig. (4) Temperature distribution at.3sec Fig. (6) Temperature distribution at 8 sec Fig. (5) Temperature distribution at 2 sec Fig. (7)Temperature distribution at 10sec 15

6 Fig. (8) Temperature distribution after cooling at 100sec Fig.(11)Temperature history at node 425 Fig. (9) Temperature history at node 386 Fig. (12) Temperature distribution after cooling at 400sec Fig.(10)Temperature history at node 410 Fig. (13) Temperature history after cooling at node

7 VII CONCLUSION The normal results of welded T-joint was obtained from thermal analysis, Show us the temperature distribution at the parts. It is so difficult to obtain this distribution from the experiments. Finite element method is an efficient technique in welding analysis. Differences in physical, mechanical and chemical properties of base metals cause non uniform temperature distribution and heat transfer. The obtained temperature distribution and temperature history is used to find out the residual stress produced in welding. ACKNOWLEDGMENT I am using this opportunity to express my gratitude to everyone who supported me throughout the project. I am thankful for their aspiring guidance, invaluably constructive criticism and friendy advice during the project work. I am sincerely grateful to them for sharing their truthful and illuminating views on a number of issues related to the project. I would also like to thank to all the people who provided me with the facilities being required and conductive conditions for my project. REFERENCES [1] S.Nadimi., R.J.Khoushehmehr., B.Rohini and A.Mostafapour, Investigation and Analysis of Weld Induced Residual Stresses in Two Dissimilar Pipes by Finite Element Modelling. Journal of Applied sciences 8 (6): , [2] ANSYS guide, ANSYS release 10.0 [3] Z.Barsoum, Residual Stress Prediction and Relaxation in Welded Tubular Joint. [4] Xiangyang Lu and Tasnim Hassan., Residual Stresses in Butt and Socket Welded Joints. Transactions, SMiRT 16, Washington DC, August [5] J.J.Dike., A.R.Ortega., C.H.Cadden., Finite Element Modeling and Validation of Residual Stresses in 304L Girth Welds. 5 th International Conference on Trends in Welding Research, June 1-5, 1998, Pine Mountain, GA. [6] Naeem Ullah Dar., Ejaz M.Qureshi., and M.M.I Hammouda., Analysis of Weld-induced Residual Stresses and Distortions in Thin-walled Cylinders. Journal of Mechanical Science and Technology 2 (2009) [7] N.T. Nguyen., Thermal Analysis of Welds. ETRS Pvt Ltd, WIT press, Australia, [8] John.A.Goldak and Mehdi Akhlaghi., Computational Welding Mechanics. Publication by Springer. 17