Brazing Material,Technology And Mechanism Of Thin-walled Stainless Steel Tubule

Condenser is the main component of refrigeration system,which is a kind of heat exchanger.It can transfer the heat in the pipe to the exterior of the pipe.It is often used in air conditioning,automobile,power plant,electronic devices and other industrial occasions.In order to improve the performance of the condenser,stainless steel tube was instead of copper tube for heat exchange.However,the connection mode between stainless steel and copper increases the difficulty of brazing.At present,Cu-P series filler metal used for brazing copper condenser is unable to brazing thin wall stainless steel tube and copper,so it is necessary to search for or develop a new filler metal.In order to complete the brazing of the new generation condenser,a series of related researches on brazing materials,technology and mechanism were carried out based on Cu-Sn solder.Four kinds of copper-based alloy powders were prepared by gas atomization equipment.Sn content ranged from 25% to 35% and Ni content ranged from 3% to10%.Their physical properties and brazing properties were tested.The prepared alloy powder has good quality,low oxidation discoloration,high sphericity and fine particle size.The 200 mesh powder accounts for more than 60%.DTA curve analysis of the prepared powders shows that the melting range of the four kinds of Cu-Ni-Sn alloy powders is between 780 and 850 ?.When Ni content in the alloy powders is low,the low melting point phase will precipitate at about 500 ?.But this phenomenon will not occur when Ni content is increased.The result of diffusivity test showed that the diffusivity of four kinds of Cu-Ni-Sn solders is good.When Sn content is too high,it is easy to melt with copper base metal.The effect of adding trace Zr content on diffusivity is little.The result of joint shear strength test showed that the shear strength of four kinds of Cu-Ni-Sn solders ranges from 53.7 MPa to68.5 MPa.Reducing Sn content and increasing Ni content can improve the shear strength.The addition of trace Zr element can play the role of homogeneous structure and fine grain strengthening,and the shear strength of joints increases obviously.When 0.2% Zr is added,the shear strength increases to about 20%.The brazing test of thin-walled stainless steel tubes was conducted in a protective atmosphere furnace.During the brazing process,the flow rate is 4L/min,and the brazing temperature rises to 500 ? for 10 min at the rate of 10 C/min.Then the brazing temperature rises to brazing temperature for 10 min and then cooles with the furnace.The brazing parts need to be pre-welded and post-welded.The brazing filler metal is pre-positioned on the side of the brazing gap and placed horizontally.Good thin-walled stainless steel tube brazing parts can be obtained by using the above brazing process.Increasing Ni content in the brazing filler metal can enhance the wettability on the surface of stainless steel and eliminate the weld imperfection caused by insufficient wettability.The above results showed that the brazing filler metal can meet the brazing requirements.Among all kinds of filler metals used in the experiment,CuNiSn-3,CuNiSnZr-1 and CuNiSnZr-2 filler metals could obtain better brazing joint effect.Using SEM,EDS to observe the interface of thin-walled stainless steel tube brazing seam.When low Ni content in the filler metal,too much Cu-Sn metallic compound was easy to form weld imperfection close to stainless steel tube.With increasing of Ni content,the wettability of thin walled stainless steel tube surface was improved,and no porosity,crack and other defects were found on the brazing joint interface.The addition of trace Zr element to the filler metal can promote nucleation and make the microstructure more uniform when the filler metal solidifies.When the filler metal contains a few P elements,P elements will accumulate on the surface of the stainless steel and form brittle metallic compound with Fe elements in the stainless steel.The brazing alloy is well combined with the based metal,forming a metallurgical bond with the copper base metal.The brazing joint area is mainly composed of three parts.First,copper based solid solution structure is continuously distributed around the interface junction.Second,the bright white structure is mainly composed of metal compounds,such as electronic phase Cu31Sn8,Cu3 Sn and Ni3 Sn.The third is the banded structure uniformly distributed in the center of filler metal,which is mainly composed of copper based solid solutions containing Ni and Sn.