Durability Analysis of Shell TIG Weld and the Effects of Weld Penetration on the Weld Strength in Clamshell Catalytic Converter

摘要

A typical automotive catalytic converter consists mainly of a catalyst-coated cellular ceramic substrate, an intumescent mat, and a stainless steel shell. In the clamshell design, two shell halves are pressed together and TIG welded at the edge of the shell flanges. During the welding assembly process, the intumescent mat placed between the substrate and the shell is compressed to provide necessary pressure to retain the substrate. Such a manufacturing process creates a TIG weld joint with a pre-existing crack, which is subjected to a crack opening force by the mat pressure. In addition to the mat pressure force from the assembly process, during service, the catalytic converter is exposed to severe longitudinal excitation from the engine combustion and the rotational motion of various engine components. Preventing weld joint cracking has been a major issue in the converter design and manufacturing. The root cause of the converter shell weld cracking has been found to be excessive cyclic crack opening force associated with the circumferential vibration mode combined with the mean crack opening force from the mat pressure at assembly. Within the framework of linear fracture mechanics, the near weld tip singular fields of stress and strain are characterized by the stress intensity factor, which can be used to estimate the strength or durability of the weld joint. Modeling the edge weld joint as a two dimensional plane strain crack, the solution of the stress intensity factor is obtained, which contains a weld penetration factor. The numerical solution of the weld penetration factor is presented. Using this stress intensity factor solution and the "Paris Law," an equation for predicting fatigue life of the edge weld joint is derived. It is found that when the weld penetration decreases, the stress intensity factor increases quickly, which may cause the weld crack to grow. When the weld crack grows, lower weld penetration results in very short fatigue life. Therefore, to improve weld joint strength, larger weld penetration is needed. However, when the weld penetration reaches 1.5 times the shell thickness, no further improvement of weld strength results. It should be noted that the solution for the stress intensity factor and the equation for the fatigue life prediction obtained in this paper, apply to the edge weld joints in general, and are not limited to those in clamshell converters alone.