Conventionally, various techniques have been proposed as joint structures for metallic pipes to be used in a heat exchanger or the like. For example, a joint portion between metallic pipes requires pressure resistance to withstand pressure of a refrigerant and thermal shock resistance to withstand thermal shock attributable to refrigerant temperature.
Patent Document 1 discloses a method of joining a copper pipe and an aluminum pipe with each other by flash-butt welding. In addition, Patent Document 2 discloses a method of joining a copper pipe and an aluminum pipe with each other by eutectic bonding. However, performances of these joining methods are not necessarily sufficient in terms of pressure resistance and thermal shock resistance due to small joint areas between pipes.
In addition, when joining different metals such as a copper pipe and an aluminum pipe with each other, an intermetallic compound layer of these metals is produced at an interface of aluminum and copper. Since this intermetallic compound is fragile, an increase in production of the intermetallic compound results in a decrease in strength of the portion.
Patent Document 3 discloses a method of joining a copper pipe and an aluminum pipe by brazing via a plated stainless-steel joint. Patent Document 3 describes that even if a certain amount of an intermetallic compound is produced at a joint interface between different metals, strength of stainless steel offsets the decrease in strength due to the intermetallic compound. As a result, a joint structure as a whole becomes less deformable and strength thereof increases.
However, with the joint structure described in Patent Document 3, the necessity of using a stainless-steel joint increases cost. In addition, plating applied to the joint further increases cost.
Patent Document 1: Japanese Patent Application Laid-open No. H9-182979
Patent Document 2: Japanese Patent Application Laid-open No. 2001-334371
Patent Document 3: Japanese Patent Application Laid-open No. H8-267228