The Brazing Behavior of Aluminum-Steel Cladding Strip with Pre-Cladded Brazing Layer Used in Air Cooling System of Power Plants

The brazing behavior of 3003 fin assemble with 4343(filler layer)/4A60(transition layer)/08Al(steel layer) brazing sheet (the ratio of filler layer thickness to transition thickness were constant 1:2) were investigated. This study elucidates the brazing results vary with the layer thickness and brazing temperatures. Results showed that keeping the holding time at brazing temperature constant (the holding time of this study was 10 minutes in all brazing progress), under the condition of brazing temperature less than or equal to 580 C, thinner filler layer led to well formed joint, while thicker filler layer led to defects at brazing joints because of poor mobility. When brazing temperature was up to 600 C, a thinner transition layer trended to lead to the formation a thick intermetallic compounds layer on the Aluminum-Steel surface. It was inferred that Si diffused from molten filler metal to the interface of transition layer and steel layer and promoted the formation of intermetallic compounds. Furthermore, it was found that in the surface away from brazing joints, with lower Si content, the intermetallic compounds were mainly (Fe2Al5), while higher Si content region nearby brazing joints trended to form 5 (Al8Fe2Si) and 6(Al4.5FeSi). Meanwhile, a transition layer with thickness above 134 m could avoid the formation of intermetallic compound. The optimum brazing process was that temperature range was 590C to 595C with 10-minutes holding time and the transition layer thickness was suggested to be equal or more than 134 m.


Introduction
With the development of electric power technology, the high parameter steam turbine unit of the thermal power plants are increasing year by year.So there is a greater demand for water resources in the cooling system of the power plant.Air-cooling system for power plants was developed in 1970s in order to alleviate the contradiction between the shortage of water resource and the increasing water demand in power stations.Not only can the air-cooling system improve the efficiency of heat exchanger, but it also can save at least 75% of the water compared with the traditional ways [1].
One of the key technologies of producing air-cooling system for power plant is the manufacturing technology of the tube-fin heat exchanger.The air-cooling tube-fin exchanger is mainly manufactured by brazing the base tube and the fin.Base tube, made of aluminum-steel clad prepared by rolling-bonding process, has both good mechanical properties of the steel and good thermal conductivity of aluminum alloy [2][3].The fin, made of aluminum alloy which has good thermal conductivity, is brazed on the base tube.
Heating up to melt the brazing filler, with the aid of capillary force, the filler filled into the gap between fins and base tube so that brazed joint formed by metallurgical bonding after solidification [4].Aluminum-steel cladding strip with pre-cladded brazing layer has been developed in recent years.The filler layer is hot rolled with transition aluminum layer.The as-rolled sheet and steel are cold rolled by the rolling mill.Then, the monolayer layer fin and the three layer aluminum-steel clad are brazed.This can reduce the times of rolling and save cost.Besides, the filler coating on the surface of aluminum-steel clad is benefit to avoid erosion of the fin.
The brazing temperature and the thickness of transition layer are the key parameters of brazing process.During this process, it is prone to cause erosion defects of the transition layer.After the transition layer was penetrated by the molten filler, the Si element accumulated at the interface of aluminum-steel, which leads to the formation of brittle compounds at the interface of aluminum-steel.The brittle compounds can severely damage the bonding of the aluminum and steel, weaken the mechanical properties of material [5].
Thus it greatly reduced the service life of the material.In this paper, the monolayer 3003 fin and three layers of aluminum-steel clad material were brazed at different temperature and different thickness of the 4A60 transition layer by controlled atmosphere brazing(CAB) process.Silicon's effect on the erosion defects of the transition and formation of the intermetallic compound(IMC) at the interface was investigated.

Experimental Procedure
Three layers of aluminum-steel cladding strip 4343/4A60/08Al (filler layer/ transition layer/steel) with different thickness and 3003 fin were chosen as the experimental materials.
Elements and specifications of the materials are shown in Firstly, the specimens were scrubed with alcohol to remove grease and other impurities on the surface.And then the specimens were soaked in the NaOH solution with mass fraction of 10% for 2min, so that the oxide film on the the specimens was removed.On the filler surface of the aluminum-steel cladding strip, fluoaluminate flux suspension (NOCOLOK, AlF 3 -KF) with mass fraction of 10% was painted after rinsed.Then the specimens was dried at 80 o C in the oven.The fin was placed on the filler surface of the aluminum-steel cladding strip and ensure good contact between the two.Then the assembles were put into the brazing furnace to carry out brazing under the condition of Ar gas protection.
During the brazing process, to make the specimens heated uniformity, take the stage of heating and thermal insulation.The furnace was first evacuated to a vasumm less than 100Pa, purged with high-purify (99.999%)Ar, and then heated to   3a-b, there were parts of the position can't form metallurgical combination.But when the filler layer thickness 76ȝm, the filler cannot be fully melted.Owing to poor liquidity of the molten filler, the capillary force cannot completely overcome viscous resistance force.Therefore in figure 3f-g, only a small part of the filler flowed to the brazed seam, formed brazed joint with defects.Figure 3c-d and h-j show that when the brazing temperature range was 590 o C ~595 o C, the filler melted with good liquidity.The capillary force can overcome viscous resistance force.The filler flowed to the brazed joint, formed perfect brazed joint.When the thickness of the filler layer was 112ȝm, it was unable to form brazed joint.In figure 3e, a lot of intermetallic compound was generated at the aluminum-steel interface.Figure 3i shows that when the transition thickness 134ȝm.The brazed joint coud be formed and intermetallic compound was not found.The Si content in the filler (7.5wt.%) was much higher than that in the transition layer solid metal grain boundaries, namely the grain boundary penetration [6].The cause of penetration was that a large number of crystal defects existed in grain boundaries.The crystal lattice distortion seriously increased the energy of atoms in this region, leading to the increasing of the frequency of the atoms.Therefore, the grain boundary was conducive to the diffusion of elements [7], which was the high speed channel for the diffusion of Si.

Erosion Behavior of the Transition Layer
When the transition layer thickness was 112ȝm, filler penetrated transition layer along the grain boundaries of the transiton layer at 600 o C. Filler infiltrated to the aluminum-steel interface, transverse diffused along the aluminum-steel interface.At the same time, the increasing of filler at the aluminum-steel interface reduced melting point of the aluminum that was adjacent to steel, which led to the formation of intermetallic compounds.
According to the Kirkendall effect, in figure 3e some pores appeared in the transition layer [8] due to the the diffusion rate of Al atoms was faster than Fe atoms.The silicon's diffusion from brazing filler to transition layer during brazing heating process led to form Si deficiency in filler layer, resulting in decreasing fluidity of brazing filler and more chance to form defects, such as no-bonding and pores.At the brazed joint, filler almost completely lossed in figure 4a.Excessive diffusion of the filler caused pit and even perforation, which weakened the mechanical properties of the brazed joint.
When the transition layer thickness was 134ȝm, the transition layer was corroded by the molten filler but not be penetrated.The molten filler diffused to the surface of the transition layer, until reached saturation.Then, the surface of the transition layer began to dissolve into the molten filler.Under the condition that the molten filler was not flowing, the molten filler first solidificated along the surface of the transition layer.Due to the transition layer corroded to a certain depht by the molten filler, the heat-affected zone formed after the filler solidification.Figure 3h-j showed that coarse Į-Al distributed in the filler.However, interfacial compound was not found at the interface of the aluminum-steel.In figure 5, we can see that the higher the brazing temperature, the larger the heat-affected zone [9].The increasing of Si content in transition layer can promote the formation of intermetallic compound.Therefore, in order to prevent Si diffusing into the interface of aluminum-steel effectively, the transition layer thickness was at least 134ȝm.Fig. 5 The Curve For Range Of Heat Affected Zone-Brazing Temperature  The Si in filler diffused to the interface of aluminum-steel along the grain boundary of transition layer in the form of penetration.Then Si diffused along the interface of aluminum-steel in transverse direction.The increasing of Si at the interface will increase the diffusion rate of aluminum atom and promote the formation of intermetallic compound(Figure 7a b).

Intermetallic compound of aluminum-steel interface
The termination point of IMC is in position.IMC were embeded in the transition layer, which is a block like morphology(Figure 7b).The Si content of the point of IMC was not much increased.At the point of , Al-Fe interdiffusion phenomenon mainly occurs.The reaction process was the following three stages [11].
In the first stage, Al-Fe interdiffused at the interface, forming a certain range of diffusion layer.The diffusion coefficient of Fe atoms in aluminum is larger than the diffusion coefficient of Al atoms in iron.There will be a large number of Fe atoms across the interface with the increasing of temperature or time.On side of the aluminum layer, Fe reacted with aluminum generating FeAl 3 (ș phase) accroding to Fe-Al phase diagram [12].
Al atoms continued to diffuse into the iron through FeAl 3 , forming Fe-Al solid solution on the iron side.Fe atoms diffused into the transition layer through FeAl 3 , forming FeAl 3 by reacting with aluminum.As a result, the intermetallic compound grew to the transition layer.
The growth rate was controlled by the diffusion coefficient of Fe atoms.
In the second stage, with the increasing of the diffusion of Fe atoms, the FeAl The intermetallic layers can be identified as Ĳ 5 phase-Al 8 Fe 2 Si (adjacent to steel) and Ĳ 6 phase-Al 4.5 FeSi (adjacent to aluminum) by the EDS analysis.The initial Si content of the transition layer was 0.8 wt.%(A point in figure 6).According to the phase diagram of Al-Fe-Si, when the concentration of Si in the interface was more than A point in figure 7, which can decrease the melting point of aluminum alloy and acceleratethe IMC growth at the interface of aluminum-steel [13].Aluminum alloy at the interface of aluminum alloy melted.The Fe 2 Al 5 disappeared finally as the content of Si increasing at the interface.
Figure 7c shows that in the high Si concentrations region, Al-Fe-Si compounds formed at the aluminum-steel.The results of EDS show that the two intermetallic layers were Ĳ 5 phase(Al 8 Fe 2 Si) and Ĳ 6 phase(Al 4.5 FeSi), in figure 7c .
According to the report in Ref [14]Si atom was easy to fill the Fe 2 Al 5 vacancy because of the high vacancy density of Fe 2 Al 5 .For this reason, interfacial compound was two layers.
In the first stage, Si, contained in filler, diffused and penetrated into the interface of aluminum-steel.This lead to the first generation of phase Ĳ 5 (Al 8 Fe 2 Si).
In the second stage, diffusion of Fe through phase Ĳ 5 (Al 8 Fe 2 Si) in the transition layer and reacted with Ĳ 5 generating phase Ĳ 6 (Al 4.5 FeSi).From the steel layer to the transition layer was Ĳ 5 Ĳ 6 , respectively.

Conclusions
To sum up, through the study of the brazing behavior of 3003 fin and 4343/4A60/08Al three layers aluminum-steel cladding strip used in power station air cooling system.The conclusion of this paper are listed as follows.
(1) The optimum brazing temperature range was 590 o C ~595 o C with a holding time of 10min, Meanwhile, the thickness of the transition layer must be equal to or greater than 134ȝm.
(2) At the condition of brazing temperature less than or euqal to 580 o C, the fluxility of the molten fillers was poor.Many defects were found at the brazing joint.
(3) At the condition of brazing temperature 600 o C, the transition layer was seriously corroded.Moreover, when the transition layer thickness was less than or equal to 112ȝm, the molten filler penetrated through the transition layer, resulted in the intermetallic compounds formation at the aluminum-steel interface.the generation of intermetallic compounds.At the edge of the intermetallic compounds away from the brazing joint, the Si concentration was lower, and the intermetallic compound were mainly Ș(Fe 2 Al 5 ).In the region nearby brazing joint, with higher concentration of Si, the intermetallic compounds had a two-layer structure.One layer closing to transition layer was mainly Ĳ 5 , while the other closing to steel layer was mainly Ĳ 6 .

Acknowledgement
Authors would like to thank the support from Jiangsu Key Laboratory for Clad

Fig. 1
Fig.1 Temperature rising curve of brazing process

Figure 2
Figure 2 and table 3 shows the different experimental results after brazing at different temperature.Brazing temperature can affect the liquidity of the filler effectively.It was able to form brazed joint when the thickness of the filler layer was 62ȝm, under the condition of 570 o C ~580 o C.However, in figure 3a-b, there were parts of the position can't form

Fig. 3
Fig.3 The Results Of Brazing With Different Thickness Of Filler Layer And Transition Layer (A-E) 62:112ȝm And (F-J) 76:134ȝm Under Different Brazing Temperature Conditions

( 0 .
81wt.%).Si diffused from filler (high concentration) to transition layer (low concentration).The diffusion behavior of Si had an important influence on the erosion of the transition layer.During brazing process, the molten filler gathered in brazed joint with the aid of capillary force.The silicon's diffusion led to silicon's segregation at grain boundaries of the transition layer.The increasing of Si content led to the decreasing of the melting point of grain boundaries.When the melting point of the grain boundaries was lower than the brazing temperature, the grain boundaries melting occured, which can produce grain boundary penetration.The phenomenon that liquid metal penetrated into the DOI

Fig. 6
Fig.6Al-Si phase diagram[10] 3 and Fe atoms reacted to form Fe 2 Al 5 , which is very brittle in nature.It's larger thickness is detrimental to forming properties of the material.The interface of the Fe 2 Al 5 phase shows highly irregular strip shape morphology (embeded in the transition layer).The thickness of intermetallic compound increased gradually.With the diffusion of Al atoms, the reaction among Al atoms FeAl 3 and Fe-Al solid solution occurred: Al+FeAl 3 +Fe(Al) Fe 2 Al 5 .The saturation atom of Fe 2 Al 5 is only 70%, and the vacancy density is high.There are a large number of vacancies in the lattice along the c axis.The Al atom can diffuse rapidly along the c axis in the Fe 2 Al 5 .Fe 2 Al 5 grew up to form a columnar crystal region(Figure7 ).In the third stage, the diffusion coefficient of Fe atoms in Fe 2 Al 5 (Ș phase)is faster than that of Al atoms.As a result, Fe 2 Al 5 grew to the transition layer side.The thickness of the intermetallic compound increased and thickness of the transition layer decreased.The diffusion of Fe and Al lead to the growth of Fe 2 Al 5 .Finally, the entire interfacial DOIFe 2 Al 5 columnar crystals.

( 4 )
During brazing progress, Si in the filler diffused along the grain boundaries of the transition layer and gathered along the interface of transition layer and steel layer, leaded to DOI Materials (No:BM2014006), International Science & Technology Cooperation Program of Wuxi (No:CZE02H1504), Program on Jiangsu Science and Technology Achievements Transformation (No:BA2015079).

Table 1 elements
table 1 and table 2. (wt.%) and properties of the fin and three layers of aluminum-steel cladding strip

Table 4
Intermetallic Compound (Imc) With Different Thickness Of Filler Layer And Transition Layer

Table 5
Energy Spectrum Analysis(Atom Percentage) Of Aluminum-Steel Brazing Interface