Numerical analysis of influence of expansion joints on welding residual stress of an EGR cooler

Li Jun,Wen Shuwen,Zhang Wenfeng,Zhang Shuyan

1.Centre of Excellence for Advanced Materials,Songshan Lake Laboratory of Materials Science,Dongguan 523808,China;

2.Zhejiang Yinlun Machinery Co.Ltd.,Tiantai 317200,China

Abstract The influence of expansion joints on the welding residual stress at the tube-plate junction of an exhaust gas recirculation (EGR)cooler was studied by numerical simulation method.The simulation results show that the expansion joints set on the housing of the EGR cooler mainly for the sake of protecting the tube-plate joints from bearing additional heating stress can also reduce the welding residual tensile stress.The expansion joints set on the EGR cooler can mitigate the tensile force acting on the edges of the main plates through its elastic extension,and thus reduce the magnitude of welding residual tensile stress at the tube-plate junction.

Key words exhaust gas recirculation,welding,stress,expansion joint,numerical simulation

0 Introduction

The rapid increase in the number of industrial and civil automobiles promotes economic development and improves people's life,but brings serious pollution to people's living environment[1].Exhaust gas recirculation (EGR)is an environmentally friendly technology developed to reduce NOx emission from automobile engines.

EGR cooler is a key functional component of the EGR system.Current research on EGR cooler mainly focuses on emission reduction performance or brazing technology[2-8],and few literatures pay attention to the welding residual stress field caused by fusion welding process,which in fact has great influence on the service reliability of EGR cooler.

Expansion joint,also called compensator,is a flexible structure set on the container housing or the pipe to compensate for additional stress caused by temperature difference or mechanical vibration.Expansion joint is also an essential design element in the manufacture of EGR cooler.However,out of the need of saving manufacturing cost,some heat exchanger manufacturers ignore this important structural design and do not set expansion joint on the housing when producing some types of EGR coolers,which will bring safety risks to the working reliability of EGR cooler products.

Taking a typical plate-fin EGR cooler as an example,this paper investigated the effect of expansion joints on the welding residual stress at the tube-plate junction which is prone to failure by using elastic-plastic finite element method.It is suggested that enough attention should be paid to optimizing the structure design of EGR cooler housing.

1 Structure and welding process

Fig.1 Structure diagram of the plate-fin EGR cooler (a)EGR cooler assembly (b)Core subassembly

Fig.1 is the structure diagram of the EGR cooler involved in this paper.The cooler is mainly composed of a core subassembly,a housing,two connecting flanges and two water chamber subassemblies.The water chamber subassemblies include respectively a connecting water pipe and a water pipe joint.The core subassembly is composed of two main plates,two auxiliary main plates,a plurality of channel fins and several tube subassemblies that include flat tubes and turbulent sheets.The material of the flat pipes and the channel fins is 316L stainless steel,and the material of the main plates and the auxiliary main plates is 304 stainless steel.The material of the housing is 1.5 mm thick 304 stainless steel sheet.The connecting flanges and the water pipes were made of 304 stainless steel.The housing is composed of two parts,which are connected with the core component,the connecting flanges and the water chamber components as a whole through the fusion welding method,so as to form the gas way and the water way that not interlinked with each other.The housing shown in Fig.1 is designed with three expansion joints of the same size.

The welding process of the EGR cooler is as follows:firstly,the core component was vacuum brazed,then the housing,the water chamber components,the inlet flange and the outlet flange were firmly spot-welded together with the core component,and finally sealed by TIG wire fusion welding.

Fig.2 Welding seam positions of the plate-fin EGR cooler

Fig.2 shows the TIG welding seam position of the EGR product.The detailed welding sequence of the TIG welding is as follows:firstly,the two parts of the housing were welded together at the places marked with 1 and 2,the next is the connection of the housing and the core component with welds located at the positions marked with 3 and 4,then,the water chamber components were welded to the housing at the positions marked with 5 and 6,and finally the gas-inlet flange and the gas-outlet flange were welded to the housing at the position marked with 7 and 8 to complete the whole assembly and manufacturing process of the EGR cooler.

2 Finite element modeling

The fabrication of plate-fin EGR cooler involves brazing and fusion welding.As the brazing process was carried out in a vacuum furnace,the core subassembly of the EGR cooler was slowly and uniformly heated and cooled,and the tube-plate joints had large degree of freedom during the brazing process,so the brazing process was not considered in the modeling analysis of this study,that is,all components of the model were assumed to be in a state of zero stress before the simulation of the welding process.

Fig.3 Finite element models of the plate-fin EGR cooler(a)With expansion joints (b)Without expansion joint

Fig.3 shows the finite element models of this study,whose dimensions are the same as that of the real product.In order to improve the modeling and calculation efficiency,some indifferent parts,such as the ends of the water-inlet pipe,the water-outlet pipe,the gas-inlet flange and the gasoutlet flange were removed without affecting the calculation accuracy.Some simplified treatments were also made for individual welds.The material property of each part of the model was uniformly set for 304 stainless steel,whose physical and mechanical parameters at room temperature are as follows:the heat conductivity coefficient is 14.6 W/(m·K),the specific heat is 502 J/(kg·℃),the thermal expansion coefficient is 1.60×10-5/℃,the yield strength is 335 MPa,the Young modulus is 193 GPa and the Poisson ratio is 0.3.The temperature dependence of physical and mechanical properties of materials is considered in the finite element modeling.In order to simulate the three-dimensional heat conduction accurately,the double ellipsoid heat source model was used to calculate all the welding paths.The displacement boundary conditions of the model were defined as shown in Fig.4.

3 Simulation results and discussion

Fig.5 compares the contour band of the welding residual stress in X direction on the front surface of the main plates for the EGR coolers with and without expansion joints.According to the color display of the welding residual stress value at both the gas-inlet end and the gas-outlet end of the EGR coolers,the welding residual tensile stress at the tube-plate junction near the round corners of the flat pipes is relatively concentrated.Compared with the EGR cooler with expansion joints,the welding residual stress at the tube-plate junction on the left-hand side of the main plate at the gas-inlet end of the EGR cooler without expansion joints is not significantly different while it appears slightly higher on the right side of that.At the gas-outlet end of the EGR cooler without expansion joints,the welding residual stresses at the tube-plate junction on both the lefthand side and right-hand side of the main plate appear higher than that at the corresponding parts of the EGR cooler with expansion joints.

Fig.4 Displacement boundary conditions of the model

Fig.5 Contour band comparison of the welding residual stress in x direction on the front surface of the main plates of the EGR coolers with and without expansion joints (a)Gas-inlet end (b)Gas-outlet end

In order to determine the influence of the expansion joints on the welding residual stress,several data points near the round corners of the flat tubes were selected as shown in Fig.6 and all of these were extracted the stress value in the length direction of the cross section of the flat pipes.Fig.7 compares the welding residual stresses at A-F points taken from the gas-inlet end and the gas-outlet end of the EGR coolers with and without expansion joints.The results show that except the welding residual stresses at A-C points at the inlet end of both EGR coolers are not significantly different,the welding residual stresses at other data points taken from the EGR cooler without expansion joints are all higher than those with expansion joints,especially for the residual stress values of point D and point E,which differ by the maximum difference of 67.8 MPa and 82.5 MPa respectively.The main purpose of setting expansion joints on the housing of EGR cooler is to avoid that the tube-plate joints bear additional heating stress due to the unequal thermal expansion and cold contraction between the cooling pipes and the housing in the working environment,however,the simulation results show that the expansion joint set on the EGR cooler housing could also reduce the welding residual tensile stress at the tube-plate junction.

Fig.6 Schematic diagram indicating the position of data points

The reason why the expansion joints help to reduce the welding residual tensile stress at the tube-plate junction is thought to have a lot to do with the two TIG welds used to connect the two parts of the housing as shown in Fig.2.Longitudinal shrinkage will occur during the cooling phase of welding process,and according to the formula (1)[9],the longer the weld,the greater the longitudinal shrinkage.

Fig.7 Comparison of welding residual stress at A-F points taken from the gas-inlet end and the gas-outlet end of EGR coolers with and without expansion joints (a)Gas-inlet end(b)Gas-outlet end

Where △L—Weld longitudinal shrinkage(cm)

L—Overall weld length(cm)

q—Weld heat input(J/cm)

U—Arc voltage(V)

I—Welding current(A)

η—Arc thermal efficiency

ν—Welding speed(cm/s)

As mentioned above,before the TIG wire filling welding,the two parts of the housing were firmly spot-welded together with the core subassembly.Therefore,when welding the first weld and the second weld,a tensile force would be exerted on the longer edges of the main plates because the weld and the metal in the heat affected zone were not free to contract.When the first weld and the second weld were completed,the tube-plate joints near the round corners of the flat pipes would bear tensile stress on the front surface and compressive stress on the back surface,as shown in Fig.8.This bending stress is destructive to the Ni-based brazed joints with gap sensitivity.The expansion joints of EGR cooler can reduce the tensile force acting on the longer edges of the main plates during the welding cooling process of the first weld and the second weld by its elastic extension,which can compensate for the longitudinal shrinkage of the two long welds,so as to reduce the welding residual tensile stress at the tube-plate junction near the round corners of flat pipes.The small difference between the EGR coolers with and without expansion joints in the welding residual stress value at A-C points at the gas-inlet end is due to the existence of the water-inlet chamber component located between the main plate at the gas-inlet end and the first weld.The water-inlet chamber subassembly weakens the force acting on the edge of the main plate from the longitudinal shrinkage of the first weld.This small difference stead fully proves that the expansion joints are very effective in reducing the welding residual tensile stress at the tube-plate junction of the EGR cooler.

Fig.8 Contour band of welding residual stress at the tubeplate junction of EGR cooler in x direction (a)Gas-inlet end(b)Gas-outlet end

Fig.9 Simulation results of the width variation of the middle expansion joint on the housing of EGR cooler with the increase of heat input of the second weld

In order to further explain the relationship between the expansion joints and the welding residual stress,different heat inputs were used for the second weld and the variation of the width of the middle expansion joint on the housing of EGR cooler with the increase of heat input was observed.Fig.9 indicates that the width of the expansion joint will increase after welding,and the magnitude of increase is in direct proportion to the heat input of the second weld seam.The simulation results strongly prove that the expansion joints set on the housing of the EGR cooler can mitigate the tensile force acting on the edges of the main plates by its elastic extension,thus playing a positive role in reducing the welding residual tensile stress in the EGR cooler.

4 Conclusions

(1)The welding residual tensile stress on the front surface of the main plate of plate-fin EGR cooler is mainly concentrated at the tube-plate junction near the round corners of flat tubes.

(2)The longitudinal contraction of the welds used for connecting the two parts of the housing of the EGR cooler is the main cause for the bending stress at the tube-plate junction near the round corners of flat pipes.

(3)The expansion joints set on the housing of the EGR cooler can decrease the tensile force acting on the edges of main plates during the welding cooling process through its elastic extension,so as to reduce the welding residual tensile stress at the tube-plate junction of the EGR cooler.

(4)More attention should be paid to optimize the structure design of EGR cooler housing.