Liquid Film Migration in Aluminium Brazing Sheet?

Transcription

1 Materials Science Forum Vols (2006) pp Online available since 2006/Jul/15 at (2006) Trans Tech Publications, Switzerland doi: / Liquid Film Migration in Aluminium Brazing Sheet? A. Wittebrood a,c S.Desikan b R. Boom a,c,d L. Katgerman c,d a Corus Research Development & Technology, PO box 10000, 1970 CA, IJmuiden, the Netherlands b Corus Aluminium Rolled Products, Carl-Spaeter-Strasse 10, D-56070, Koblenz, Germany c Netherlands Institute for Metal Research, Mekelweg 2, 2628CD Delft, The Netherlands d Delft University of Technology, Rotterdamseweg 137, 2682AL Delft, The Netherlands Keywords: Aluminium brazing sheet, liquid film migration, recrystallization ABSTRACT From literature and own observations it is known that the clad and core alloys that make up aluminium brazing sheet can show severe interaction during the brazing cycle. This interaction leads to a complete re-distribution of elements, changing essential properties like strength and corrosion resistance. This interaction has been reported many times but up to present time no clear explanation is given why this interaction is actually occurring. There are a number of publications addressing the circumstances under which the interaction is more severe. Chemistry and low levels of strain applied before brazing have a significant influence on the severity of the interaction. As a yet possible mechanism behind the interaction Liquid Film Migration is mentioned. The observations done so far are in line with this described mechanism but no ultimate proof has been given so far. The question why the interaction takes place cannot be answered yet, clearly a change of free energy of the system is involved but the mechanism or mechanisms behind the change is unclear. 1 INTRODUCTION Aluminium brazing sheet is a sandwich material used for the mass production of automotive heat exchangers. Basically brazing sheet is a sandwich, consisting of an aluminium core alloy, typically an A3000 alloy (Mn containing) or an AA6000 alloy (Mg and Si containing) with a clad alloy of the AA4000 (Si containing) series. The core alloys are designed in such a way that after the brazing cycle, the condition is reached where the core has its optimum properties. Properties like strength and corrosion resistance are the main design parameters. The AA4000 alloy used for brazing sheet has a melting range between 570 and 610 C while the melting range of a typical AA3000 core alloy lies above 610 C. This difference in temperature between the two alloys is used to join complex shaped products in one shot". At brazing temperature, typically around 600 C, the AA4000 clad alloy is completely molten. Due to capillary forces and surface tension differences, the clad alloy will flow to connect adjacent pieces. In brazing, a liquid metal is used to join to metallic or non-metallic pieces. All aluminiumbrazing processes have in common that at a certain point in the brazing process molten aluminium is in contact with solid aluminium. During this stage, interaction takes place All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, (ID: /08/09,09:46:47)

2 1152 Aluminium Alloys 2006 between a liquid and solid phase. From literature it is known that this interaction can cause changes in the structure and properties of the core alloy involved. These changes are regarded as detrimental for the final product performance. In 1943 Miller [1] mentioned that the theoretically available clad alloy did not flow completely due to interaction of the molten clad with the solid core alloy. This article demonstrated clearly that the clad and core alloy showed interaction that could have a detrimental effect on the brazing process. Terril [2] claimed capillary action between the grains to be responsible for a reduced flow of clad alloy. However, no detailed micrographs were presented. Woods [3] described the interaction responsible for a reduced clad flow as core solution. Sharples [4] related the amount of cold work introduced by stretching to the severity of the interaction. Materials recrystallizing before brazing took place did show significantly less interaction compared to materials that did not recrystallize at brazing temperature. Depending on processing the critical amount of stretching lays between 3 and 20%. Schmatz [5] was the first to make a detailed chemical analysis of the different areas affected by the interaction. His conclusion was that the liquid originating at the clad-core interface progresses into the core alloy as a film, changing the element distribution on its way. The original grains of the core changed due to the liquid film passage to large grains. Okamoto [6] and Yamauchi [7] described the effect of dispersoid forming elements on the occurrence of the interaction and concluded that recrystallization inhibitors had a very detrimental effect. Nylén [8] tried to use the Thomson-Freundlich or Gibs-Thomson equation together with Fick's first law of diffusion to calculate the speed of the advancing film. However, the calculated velocity was only 2% of the actual observed velocity. In 1997 Woods [9] and Yang [10] published a detailed analysis of the observed interaction. Woods was the first to call the observation Liquid Film Migration. Liquid Film Migration has been observed in many other alloy systems [11,12,13]. The main observation made was the complete redistribution of elements when a liquid film has passed through. In the study of Yang [10] again the amount of cold work applied before brazing determined the severity of the interaction. In earlier work [14], Differential Scanning Calorimetry was used to determine the amount of the interaction between the clad and core alloy. The size of the joint (fillet) was correlated with the ratio of the solidification and melting enthalpies of the same sample. In the same publication [14] the effect of pre-straining before brazing with the resulting grain structure after brazing were given. A lower ratio was an indication of the severity of the interaction between liquid and solid. In this paper we attempt to systematically address the effect of homogenisation on the possible occurrence of the clad core alloy interaction. For this study we have chosen a core alloy chemistry and one clad alloy chemistry, see table 1, and processed a number of combinations to assure a different response to recrystallization during brazing. 2 MATERIALS AND EXPERIMENTAL PROCEDURES Table1: Core and clad chemistries Si Fe Mn Cu Core All other elements are below 0.01% Clad <0.01 <0.01 Before casting 1 gr/kg AlTiB5/1 was added. The core alloy was given different homogenisation cycles as given in table 2.

3 Materials Science Forum Vols Table 2: Used homogenisation practices Heating T Hold t Hold Cooling T Cool rate rate Cycle Cycle 2 35 C/hour 600 C 168 hours 35 C/hour 100 C The 2, core and clad alloy combinations were processed identically to produce a 0.4 mm clad brazing sheet in O temper condition. Prior to brazing the samples were stretched for 0,1, 3, 5, 7 and 10% in plane strain mode to introduce dislocations. To simulate the behaviour during manufacturing a "standard" brazing cycle was applied. The brazing cycle applied was a typical one used for making evaporators with a maximum temperature of 595 C and a holding time of 3 minutes. Typical heating rate is 30 C/min; cooling rate is 60 C/min. To study the effect of heating rate, some of the samples were immersed in a salt bath at 595 C for 3 minutes and water quenched.after furnace and salt bath brazing, cross sections of the different samples for LIM and line scan investigations were made. The severity of the interaction can be seen in figure 1. After brazing only a part of the original core alloy is present. 3 RESULTS Figure 1: Before and after brazing In total 2 clad /core combinations stretched at 6 levels giving 12 specimens to study were produced. Cross sections of all specimens were examined for the precipitates and grain structures after brazing. Figure 2 shows the cross section etched for precipitates while figure 3 shows the grain structures. In both figures for every cross section, the amount of stretching prior to brazing is indicated. Cycle 1, non-homogenised Cycle 2, homogenised Figure 2: Cross sections etched for precipitates after furnace brazing Cycle 1, non-homogenised Cycle 2, homogenised Figure 3: Grain structures after furnace brazing

4 1154 Aluminium Alloys 2006 Line scans for Si, Fe, Mn and Cu were made for the two different alloys homogenised with the two cycles and stretch for 7% before salt bath and furnace brazing, see figure 4. cycle 1, 7% stretch Braze furnace cycle 1, 7% stretch Salt bath cycle 2, 7% stretch Braze furnace cycle 2, 7% stretch Salt bath Figure 4: Line scans of homogenised and non-homogenised core alloys, brazed in a furnace or salt bath 4 DISCUSSION 4.1 Recrystallization From the micrographs in figures 2 and 3, it is clear that the clad alloy interacts differently with the core alloy given different homogenisation cycles. The one without homogenisation shows the highest degree of interaction. The grain structures in figure 3 indicate that even at levels of 10% stretching, the non-homogenised core did not recrystallize while the homogenized core alloy showed recrystallization at a level of 3% stretching. Samples brazed in a salt bath did not show a difference in grain structure compared to the furnace-brazed samples, indicating that heating rate did not play an important role in the recrystallizing process. The key to the interaction seems to be the onset of recrystallization. This observation is completely in line with results published earlier [6,7]. Recrystallization of aluminium alloys during a thermal treatment follows a number of steps before reaching its final stage [15]. The first step before recrystallization is recovery and during this stage dislocations will organise into a structure. In proceeding to the next step

5 where reorganising of grain boundaries or recrystallization takes place, localised areas with high densities of dislocations play an important role in the onset of recrystallization. The levels of strain introduced in the experiments are relatively low, where the dominant recrystallization mechanism is Strain Induced grain Boundary Migration or SIBM. SIBM involves the bulging of a part of a pre-existing grain boundary leaving a dislocation free region behind the moving grain boundary. The driving force for SIBM is assumed to come from the difference in dislocation density on opposite sides of a grain boundary. SIBM results in a very coarse grain after recrystallization as can been seen in figure 3 for the alloy homogenised according to cycle 2. The movement of the grain boundary during recrystallization can be inhibited by small particles, which is called Zener pinning [15]. The size and number of particles responsible for the pinning action is strongly influenced by the processing of the alloy prior to brazing. According to Li [16] time and temperature have a serious impact on the size and number of precipitates formed during homogenisation. The non-homogenised alloy will form a large number of very small dispersoids during the brazing cycle, responsible for recrystallization inhibition or sub grain structure stabilization. This stabilized structure seems to be important in the interaction between clad and core alloy. In literature there are a few authors [17,18] discussing how a molten metal wets a dislocation core reducing the energy stored in the solid metal as a possible alternative to recrystallization. 4.2 Line scans Materials Science Forum Vols All non-homogenised samples show a constant level of elements over a certain distance; especially the salt bath brazed sample give a constant profile. Thermodynamic calculations show that the measured concentration of the elements is almost identical with the composition of the aluminium alloy to solidify at 595 C. The core alloy has been swept by a migrating interface leaving behind a new solid solution of equilibrium solidus composition at the experimental temperature [19]. Another observation is the possible detachment of pockets filled with particles from the moving liquid film. Figure 4 is a detailed micrograph from figure 1 of the non-homogenised alloy compared with the observations from McPhee [20]. Direction of film movement Figure 4: Detailed micrograph from figure 2 showing clusters of particles possibly detached from the advancing liquid film Yang [10] also reported a concentration of particles in the advancing film. The advancing liquid film becomes loaded with non-soluble particles that exert a drag force on the moving liquid film resulting in detachment of pockets filled with these particles. 5 CONCLUSIONS Brazing sheet exhibits under specific conditions a severe interaction between the liquid clad alloy and solid core alloy. The interactions depend strongly on the microstructure of the core alloy at the moment of liquefaction of the clad alloy. A microstructure with a dislocation structure is susceptible to the interaction. How the interaction between a dislocation structure

6 1156 Aluminium Alloys 2006 and a liquid clad alloy takes place is yet unknown. Future research will focus on the possible mechanism(s) behind the interaction and the driving force. 6 ACKNOWLEDGEMENT This research was carried out under project number MC in the framework of the Strategic Research programme of the Netherlands Institute for Metals Research ( 7 REFERENCES [1] Mike A. Miller, Aluminum Brazing Sheet-Fundamentals of Metal Flow, Welding Journal 22 (12) 1943, Res. Suppl., pp. 596-s, 604-s. [2] J.R. Terrill, Diffusion of Silicon in Aluminum Brazing Sheet, Welding Journal 45 (5) 1966, Res. Suppl., pp. 202-s, 209.s. [3] R.A. Woods and I.B. Robinson, Flow of Aluminum Dip Brazing Filler Metals, Welding Journal 53 (10) 1974, Res. Suppl., pp.440-s, 445-s. [4] P. Sharples, Aluminum Brazing Problems Due to Grain Size, Welding Journal, 54 (1975), pp [5] D.J. Schmatz, Grain Boundary Penetration During Brazing of Aluminum, Welding Journal (10) 1983, Res. Suppl., pp.267s, 271s. [6] Ikuo Okamoto, Tadashi Takemoto and Kei Uchikawa, Transactions of JWRI, (1983), vol.12, no.1, pp [7] S. Yamauchi, k. Kato, Influence of Precipitate Dispersion on the Erosion of Lightly Deformed Brazing Sheets, Keinkizoku (1991), vol. 41, no.4, pp [8] Margaretha Nylén, Ulla Gustavsson, Bevis Hutchinson and Anna Örtnäs, Mechanistic Studies of Brazing in Clad Aluminium Alloys, Materials Science Forum Vols , (1996), pp [9] R.A. Woods, Liquid Film Migration During Aluminum Brazing, SAE paper [10] Henry S. Yang and Ralph A. Woods, Mechanisms of Liquid Film Migration (LFM) in Aluminum Brazing Sheet. SAE paper [11] D.N. Yoon, Chemically induced interface migration in solids, Annu. Rev. Mater. Sci., (1989).pp [12] D.Y. Yoon, Theories and observations of chemically induced interface migration, InternationalMaterials Reviews (1995) vol.40, No.4, pp [13] Y. Brechet and G.R. Purdy, Comments on the process of liquid film migration, Scripta Metallurgica, (1988), vol. 22, pp [14] A. Wittebrood, C.J. Kooij and K. Vierregge, Grain Boundary Melting or Liquid Film Migration in Brazing Sheet, Materials Science Forum Vols ,(2000), pp [15] F.J. Humphreys and M Hatherly, Recrystallization and Related Annealing Phenomena, Pergamon (1996), ISBN [16] Y.J. Li, L. Arnberg, Quantitative study on the precipitation behavior of dispersoids in DC cast AA3003 alloy during heating and homogenization, Acta Materialia 51, (2003), pp [17] E. Rabkin, V. Semenov, W. Gust, L.s. Shindlerman, Diffusion Induced Instabilities at Internal Interfaces in Solids, Defect and Diffusion Forum Vols , (1997), pp [18] E. Rabkin, I. Snapiro, Wetting of low angle grain boundaries, Acta Materialia. 48, (2000), pp [19] S.W. Barker, G.R. Purdy, On liquid film migration in aluminum-copper alloys, Acta Materialia 46, (1998), No 2, pp. 511, 524. [20] W.A.G. McPhee, G.B. Schaffer and J. Drennan, The effect of iron on liquid film migration and sintering of an Al-Cu-Mg alloy, Acta Materialia 51, (2003), pp

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