1. Field of the Invention
The present invention relates to a technique for joining of a steel pipe and a light metal pipe. In particular, the present invention relates to the joint structure of a vibration-absorbing hose used for the refrigerant circuit of a mobile air conditioning.
2. Description of the Related Art
Aluminum alloy pipes have recently been used in piping of refrigerant circuits of mobile air conditioning systems to reduce the weight of automotive bodies. Vibrations due to a compressor or the like may produce resonance of the piping so as to cause noise. Thus, composite hoses each formed of rubber and a resin have conventionally been incorporated in the middle of the piping in order to suppress resonance of the piping.
HFC 134a has been used in place of chlorofluorocarbons, which are ozone-destroying substances, as a refrigerant for mobile air conditioning systems. HFC 134a has an ozone depletion potential of zero but has high global warming potential to cause an increase in global warming. Thus, a CO2 refrigerant is recommended as an alternative to HFC 134a because CO2 has low global warming potential and is a naturally existing material.
However, in the use of the CO2 refrigerant, the refrigerant circuit piping needs to have a heat resistant temperature of 140° C. to 180° C. as compared with 120° C. to 140° C. in using the HFC 134a refrigerant. Furthermore, the refrigerant circuit piping using the CO2 refrigerant needs to have a discharge pressure of 13 to 15 MPa as compared with 1.7 to 1.8 MPa in using the HFC 134a refrigerant.
Known composite hoses each formed of rubber and a resin do not withstand such high-temperature, high-pressure conditions. Thus, Japanese Unexamined Patent Application Publication No. 2002-195474 (Patent Document 1) discloses a vibration-absorbing hose having a stainless-steel bellows in place of the known composite hoses. The stainless-steel vibration-absorbing hose has a metal hose wall and has thus significantly low gas permeability compared with known composite hoses each formed of rubber and a resin; hence, there is no leakage of the refrigerant from the stainless-steel hose. Thus, the stainless-steel vibration-absorbing hose is used for not only the CO2 refrigerant but also the current HFC 134a refrigerant and the like in order to bring the amount of leakage of the refrigerant to the exterior closer to zero.
However, when the vibration-absorbing hose is incorporate in the refrigerant circuit, there are the following problems: under the present circumstances, the bellows of the vibration-absorbing hose must be composed of stainless steel only in view of processability and strength. On the other hand, the refrigerant circuit piping needs to be composed of aluminum or an aluminum alloy in view of a reduction in the weight of an automotive body and costs; hence, it is difficult to change the material from aluminum to stainless steel. Thus, the stainless-steel vibration-absorbing hose needs to be connected with the aluminum piping. However, it is significantly difficult to achieve a reliable joint with high strength and high hermeticity by only mechanically fitting or screwing these metal pipes. Furthermore, joining of an aluminum component with a stainless-steel component by welding or brazing to form a joint easily forms a brittle intermetallic compound in the joint. Hence, also in this case, it is significantly difficult to achieve a reliable joint with high strength and high hermeticity.
Japanese Unexamined Patent Application Publication No. 7-310161 (Patent Document 2) discloses a method for joining a steel component to an aluminum component, the method including roughening a surface of the steel component to form irregularities; then, temporarily forming an aluminum layer; and thus forming a diffusion layer composed of a Fe—Al intermetallic compound by radio-frequency heating while pressing the aluminum layer from the surface side.
However, the method disclosed therein aims to improve abrasive resistance and smoothness of the surface of the steel component by forming the diffusion layer composed of the intermetallic compound. As long as the intermetallic compound is formed, a reliable joint with high strength and high hermeticity is not obtained.