Estimating inherent deformation in thin-plate Al-alloy joint by means of inverse analysis with the help of cutting technique

Highlights

• A new method was developed to estimate inherent deformations in thin-plate Al-alloy joint.

• The proposed method is an effective tool to estimate inherent deformations for thin-plate joints with buckling distortion.

• The characteristics of inherent deformations in Al-alloy thin-plate joint were clarified by T-E-P FEM.

Abstract

The elastic finite element method based on inherent strain theory has been recognized as an effective tool to estimate the total welding deformation for large and complex welded structures. When this computational approach is employed to predict welding deformation in a weldment, one prerequisite is that the inherent deformations of each welded joint included in the welded structure should be known beforehand. The inverse analysis method based on the combination of measuring technology and finite element method can be used to obtain the inherent deformations for various welded joints. However, if buckling distortion occurs in a welded joint, it will be difficult for this method to accurately obtain the inherent deformations especially in thin-plate joints. To overcome this difficulty, an improved inverse analysis method with the help of cutting technique was developed in the current study. The effectiveness of the proposed method was demonstrated through obtaining the inherent deformations in an Al-alloy thin-plate joint with buckling distortion.

Introduction

Aluminum alloy, as lightweight structural material, has been widely used in the manufacturing of automobile, ship and passenger train. Welding technology is usually employed to assemble thin-plate elements because of its high productivity and design flexibility. Compared with the structural steels, the young's modulus of the aluminum alloy is only one third of them, and the thermal expansion coefficient is almost twice as large as that of the mild steel [1]. In addition, the thermal conductivity of aluminum alloy is significantly larger than that of steels. Therefore, it can be foreseen that welding deformation in an aluminum alloy welded structure is more serious than that generated in a steel-made weldment. Welding-induced distortion not only negatively affects the performance of product but also hinders the assembling process especially when automatic welding technology is used. Therefore, it is very vital to control and reduce welding distortion in practical engineering application.

To effectively control welding deformation, we must deeply understand the influences of various factors on the final deformation based on the experimental observations or through the predictions obtained by numerical method [2]. With the development of computer hardware and software, numerical simulation technology has been becoming a powerful tool to solve the complicated thermo-mechanical problems during welding process.

At present, there are two methods based on finite element method (FEM) which can be used to predict welding deformation. One is the thermal elastic plastic FEM [3], and the other is the elastic FEM based on the inherent strain theory [4]. When the former method is used, the temperature field, stress and strain fields and even the thermal-metallurgical-mechanical coupling behaviors can be calculated. However, there are some obvious disadvantages in this method. For example, a very long computing time is needed because welding process is coupled nonlinear problem. In addition, the temperature-dependent material properties even phase-dependent material properties should be measured by experiment. Also, heat source model and its parameters should be carefully selected. At present, a number of fast computational approaches such as ISM [5], [6] and ideal explicit method [7] based on thermal elastic plastic FEM have been developed, and these methods perhaps are promising ways to solve the thermo-mechanical problems in large-scale welded structures. However, it is inevitable to spend a relatively long time for preparing the huge mesh data and defining the required information related to welding procedure.

The elastic FEM based on inherent strain method has been recognized as an effective approach to estimate welding deformation in large scale welded structure because of the short computing time and the relatively simple preparation of input data [8]. When this method is used to predict welding distortion in a welded structure, one prerequisite is that the inherent deformations of each joint in the whole structure should be known beforehand.

The basic components of inherent deformation in a weld include longitudinal shrinkage, transverse shrinkage, longitudinal bending, and transverse bending (angular distortion) [9]. The first two components are in-plane deformation, and the last two belong to out-of-plane deformation.

At present, three methods can be employed to obtain inherent deformation for a welded joint. The first one is experimental method [9]. Using this method, each component of inherent deformation can be directly obtained by measuring technology. The measuring accuracy strongly depends on the measuring tools and the magnitude of deformation occurred in the joint. Generally, transverse shrinkage and angular distortion can be easily measured because of their relatively large magnitude. However, it is difficult to accurately measure longitudinal shrinkage and longitudinal bending because their values are often small. The inherent deformation in a weld can also be calculated by integration method based on thermal elastic plastic FEM [10]. However, the calculation accuracy strongly depends on the analyst's knowledge and experiences because too many influential factors of welding deformation should be carefully taken into account in thermal elastic plastic finite element model. The third method is the inverse analysis method [11]. This method was proposed by Liang and Murakawa [12] recently, and it has been used to successfully establish the database of inherent deformation in several typical thin-plate mild steel joints.

For mild steel or stainless steel joints, a relatively small size can be used to obtain the inherent deformation because the thermal conductivity is relatively small and the moving temperature field can reach a quasi-steady-state in a short time. For Al-alloy joints, the thermal conductivity is much larger than that of steel, so the moving weld pool needs a longer time to reach the quasi-steady-state. In such situation, a relatively large size of welded joint is required to obtain accurate inherent deformation. Because the stiffness of thin-plate Al-alloy joint is relatively small, it can be foreseen that buckling distortion will be apt to occur especially when a large heat input is applied or the thickness of plate is small. If buckling distortion occurs in a joint, it will be difficult to accurately obtain the inherent deformation by inverse analysis method. To avoid this problem, an improved inverse analysis method with the help of cutting technique was proposed to obtain inherent deformation of thin-plate Al-alloy joint in the current study.

In this study, the distribution shape and feature of each inherent deformation component along welding line in a thin-plate Al-alloy joint were clarified based on the results of residual plastic strain (inherent strain) distribution simulated by thermal elastic plastic FEM. Meanwhile, the welding deformation of the Al-alloy thin-plate joint predicted by thermal elastic plastic FEM was verified by experiment. Then, according to the distribution features of inherent deformation, an improved method based on inverse analysis with the help of cutting technique was proposed to estimate inherent deformations in the Al-alloy thin-plate joint. Finally, the welding deformation of Al-alloy thin-plate joint was computed using the inherent deformations obtained by the new method, and the computed result was verified by the corresponding thermal elastic plastic FE model.