Study of Control of Welding Defects by Using Externally-Applied Compressive Mechanism

• Main Authors: Terence Liu, 劉立仁
• Other Authors: K.H. Hou
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/65558431660740804284

碩士 === 長庚大學 === 機械工程研究所 === 92 === Arc welding has been proven to be a very economical and efficient manufacturing technology, and is widely used in various industries. However, there might still be some unexpected welding defects occurring during welding. In addition to human errors, the most noticeable defects are weld cracking, which are primarily caused by both microstructural changes and thermal stresses during welding. The formation of thermal stresses is due to the abrupt heating and cooling, and the non-uniform cooling of the weldments. The thermal stresses can be roughly divided into the thermal stress during cooling and the residual stress after the weldment being completely cooled. Weld cracking is directly related to these stresses, which are tensile in nature. It is probable to lower the cracking susceptibility such as fusion zone solidification cracking and heat-affected zone liquation cracking by reducing the thermal stresses during cooling. Moreover, the residual stress of the weldments may also be lowered and consequently result in better fatigue strength and resistance to stress corrosion cracking. The major goal of this study is to design an externally-applied compressive mechanism, which is based on the concept of shot peening, to produce a series of impacts or presses at various locations of the weld metal or HAZ. The impacts should be able to produce minor plastic deformation and also lower the tensile thermal stresses, which may even become compressive momentarily. Consequently, most tensile stress-related weld cracking may be prevented. In order to perform a systematic optimization study of each experimental variable, the following procedures will be conducted including FEM simulation. In addition, measurements of welding thermal cycles and residual stresses will be conducted. The welding defects to be evaluated in this study include HAZ liquation cracking, repair welding defects in extensively radiated materials, and ductility dip cracking.