Brazing Sheet Research Articles

A closer look at the internal corrosion test for automotive heat exchangers: the OY water test

AbstractNowadays, automotive heat exchangers are mainly made from aluminum. Over the last few decades, the development of aluminum brazing sheet has been mainly focused on the improvement of the external corrosion resistance. Recently, the interest of the heat exchanger manufacturers has also moved toward the assessment of the internal corrosion resistance. Accelerated tests are used to simulate in‐service conditions. These tests are crucial as they provide information to predict the material and product performance. Various internal corrosion tests have been developed, the so‐called ‘OY water tests’. Among all companies, the testing parameters can be varied in terms of ion concentrations or pH. Although the OY water tests are constantly being improved, there is so far no common internal corrosion standard as existing for external corrosion assessment. The present work aims at providing better understanding corrosion mechanisms encountered in the OY water media. The role of various elements present in the solution, cupric, chloride, sulfate ions, has been determined. These parameters were assessed via immersion testing. In order to get better understanding of the corrosion mechanism, electrochemical analysis combined with SEM characterization was carried out. The electrochemical analysis enables the understanding of the thermodynamics and kinetics of the corrosion process while immersion tests are performed to quantify the level of corrosion and follow the surface evolution over time. Copyright © 2010 John Wiley & Sons, Ltd.

An effective warm forming process; numerical and experimental study

To investigate the effectiveness of the warm forming a coupled thermo-mechanical model is developed in LS-Dyna. The temperature dependant material properties of the material, a long-life 3000 series clad aluminum alloy brazing sheet are modelled using a user defined material subroutine accommodating the Barlat YLD2000 anisotropic plane-stress yield surface and the physically motivated Bergstrom hardening rule. The maximum thickness reduction is adopted as a simplified failure criteria to determine both the location and failure depth for the numerical models. The original progressive tooling is modified to incorporate the application of individual temperatures on the dies and the punch. The dies (die and blank holder) are heated with embedded cartridge heaters and the punch is cooled by circulation of chilled water. Select experimental cases are modelled. FEA model is validated against experimental results by comparing punch force versus displacement as well as failure location. The numerical results for either successful or failed simulations are compared with corresponding experimental samples.

Effect of Mn addition to core alloy on corrosion resistance of thin brazing sheet

To investigate effect of additional Mn in thin brazing sheet (core alloy) on corrosion resistance, SWAAT and electrochemical measurements have been carried out with brazed Al–Mn–1.6 mass%Si alloys. Intergranular corrosion clearly occured in the alloys which Mn content is 1 mass% or less. In the case of these alloys precipitates of Si particle were observed at grain boundary, but they were not observed in grain after brazing. As a result, the potential difference between neighborhood of the grain boundaries and grain matrix became bigger. Therefore, selective dissolution occured at neighborhood of the grain boundaries which was lower solid solution of Si. On the other hand, intergranular corrosion were not observed in the alloys which Mn content is 1.6 mass% or more. In the case of these alloys fine precipitates of AlMnSi were observed in grain after brazing. As a result, the formation of Si-SDZ (solute depleted zone) by Si precipitation at grain boundary seen in the former alloys were inhibited. In conclusion, addition of Mn in thin brazing sheet had improving effect of corrosion resistance after brazing.

Effects of Cu content in the core, Zn in the filler, and sheet thickness on the corrosion resistance of a brazing sheet

The effects of Cu content in the core and Zn in the filler of a brazing sheet of 0.2 mm thickness as well as those of 0.3 and 0.5 mm thicknesses has been investigated. The corrosion form changed from pitting to general corrosion by adding more than 0.2% Cu to the core in the sheets of 0.3 and 0.5 mm thickness, while pitting corrosion occurred in all the 0.2 mm thickness sheets even containing Cu after 20 days SWAAT. On the other hand, residual life after corrosion was greatly improved by suppression of localized corrosion even in the sheets of 0.2 mm thickness clad with filler containing more than 1% Zn, independently of Cu content in the core. The suppression of localized corrosion was caused both by higher potential difference between filler and core, and by the gradual potential slope formed in the core due to Zn diffusion during brazing.

Diffusional solidification phenomena in clad aluminum automotive braze sheet

Differential scanning calorimetry (DSC) was used to investigate clad/core interactions and the evolving microstructure during simulated brazing of aluminum brazing sheet alloy No. 12, which consists of an AA3003 core with AA4343 cladding layers on either side. During the brazing operation the clad layers can alloy with the core as a result of Si diffusion before and after clad melting (i.e. in the solid-state and the solid/liquid state). This interaction affects the amount of liquid formed and its duration, which will impact on joint formation and mechanical performance. Metallographic data and DSC liquid fraction measurements show that solid-state interdiffusion prior to melting is responsible for some initial liquid suppression, but the amount of liquid present during the brazing cycle decreases rapidly as a result of isothermal, or diffusional, solidification. The liquid phase can be completely removed via diffusional solidification at the brazing temperature prior to cooling, leaving behind a significantly coarsened clad structure. Using this novel and practical DSC technique, experimental insight regarding Si diffusion into the core can be obtained as well as the evolving clad/core solidification structure.

Effect of thickness of filler on corrosion resistance of brazing sheet

The effects of Zn addition to the filler, Cu addition to the core and the filler thickness on the corrosion resistance of the brazing sheet were investigated with SWAAT. In case of the brazing sheets containing Zn in the filler or Cu in the core, the corrosion depth was decreased and the perforation period was increased with increasing the filler thickness. In case of those brazing sheets, the corrosion potential of the surface layer was less noble than that of the core, and the corrosion potential difference between the surface and the core was increased with increasing the filler thickness. According to the galvanic coupling immersion test, the corrosion potential difference between the surface and the core was required more than 10 mV in order to have the sacrificial anode effect of the surface. The thickness of the sacrificial anode layer, defined as the layer whose corrosion potential was less noble than that of the core for 10 mV or more, was increased with increasing the filler thickness. The perforation period in SWAAT was increased with increasing the corrosion potential difference and the thickness of the sacrificial anode layer.

Fatigue characteristics of aluminium brazing sheet after brazing

Fatigue characteristics of aluminium brazing sheet after brazing

Recent advance of brazing sheet and flux for aluminium brazing

Recent advance of brazing sheet and flux for aluminium brazing

Cross-section corrosion-potential profiles of aluminum-alloy brazing sheets observed by the flowing electrolyte scanning-droplet-cell technique

The scanning-droplet-cell (SDC), recently used for studying electrochemistry of electrodes in micro-areas, was modified to a flowing electrolyte-type one (f-SDC), in which fresh electrolyte was continuously supplied on the surface to be examined. The f-SDC with a coaxial double capillary structure could avoid the contamination of electrolyte by species dissolved from the measuring electrode surface. In the present study, the f-SDC technique was applied successfully to obtain profiles of the corrosion potential for the cross-sections of aluminum-alloy brazing sheets, used for the heat exchanger of the automobile. The brazing sheets consist of an Al–Zn alloy sacrificial anode layer, an Al–Mn–Cu core layer and an Al–Si brazing filler. The profiles of the corrosion potential, important to evaluate the corrosion protection performance of the sacrificial anode, revealed that the potential gradient arising from the sacrificial anode to the core layer is mainly controlled by zinc, which diffuses from the former layer to the latter during the cladding treatment and the post-heat treatment.

Development of Cu Brazing Sheet with Cu-P Composite Plating

A Cu brazing sheet has been developed using a Cu-P composite plating method. A Cu-P composite plating layer, which contains 7mass%P, was formed on a Cu plate with a copper sulfate solution including P particles. The melting start temperature of the Cu-P composite layer was determined to be approximately 765°C. Microstructure and joint strength of a brazed joint with the Cu-P composite layer were investigated and compared with those of the joint with a conventional Cu-7P filler foil. As the results of the study, it was clarified that the Cu-P composite layer developed is feasible to use as a brazing material for Cu and Cu alloys.

Macro- and micro-surface engineering to improve hot roll bonding of aluminum plate and sheet

Hot roll bonding of aluminum plate and sheet currently is the primary manufacturing method of fuselage skin sheet for aircraft and brazing sheet for automotive applications. Tremendous challenges and opportunities exist to improve quality and productivity of these products. This work explored the effects of macro- and micro-surface engineering on hot roll bonding process. It was found that micro-surface engineering yielded better bonding quality than macro-surface engineering did. The critical surface roughness of the core was about 0.58 μm below which bonding quality in terms of area contact and bonding energy can be significantly improved. However, no marked improvement was observed when the surface roughness was reduced further to 0.03 μm. Also, the oxide layer on the surface of the core and local deformation on the surface of liner played a very important role. Ultrasonic test and roller peel test were employed to assess the bonding strength and quality. The quantitative ultrasonic test results are in good agreement with the roller peel test results.

Influence of heat treatment on corrosion resistance of aluminum alloy brazing sheet

SWAAT for 20 days has been carried out on brazing sheets of 1 mm thickness to estimate outside corrosion resistance on the material for aluminum heat exchanger applied at higher temperature. Maximum corrosion depth after the test decreased in brazing sheets both of Al–1%Mn alloy core and of Al–1%Mn–0.5%Si alloy core clad with Al–Si alloy filler while it increased in that of Al–1%Mn–0.5%Si–0.5%Cu alloy core, with increase of heat treatment time at 200°C after brazing. Marked intergranular corrosion in the core was observed in the latter alloy. On the other hand, general corrosion was observed in brazing sheets clad with filler alloy containing Zn, and the attack stopped at near interface between filler and core, independent of core alloys and of heat treatment time. Corrosion characteristics and attack depth on the sheets were discussed in terms of electrochemical properties of filler and core alloys.

Liquid Film Migration in Aluminium Brazing Sheet?

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.

Effects of Si Content and Cold Rolling Condition on Brazeability of Al-Mn-Zn Alloy Core Brazing Sheet

Three layer clad brazing sheets composed of Al-7.5Si alloy (filler, thickness:10 ㎛ )/Al-1.2Mn-2Zn-(0.04-1.05)Si alloy (core, thickness:80 ㎛)/Al-7.5Si alloy (filler, thickness:10 ㎛), were produced by laboratory fabrication, through casting, hot rolling, cold rolling, intermediate annealing, and final cold rolling. The effects of Si content in the core(0.04-1.05wt.%) and reduction rate of the final cold rolling(10-45%) on microstructure and brazing behavior were investigated. The results revealed that the microstructure and brazeability of the brazing sheet are governed both by Si content in the core and by the reduction rate of the final cold rolling. The excellent brazeability was obtained when the core alloy has the Si content of/cold rolled to 0.04%/10-45%, 0.41%/20-45% and 0.64%/30-45%. In these cases, a coarse grain structure was developed in the core during the brazing process, which suppressed the penetration of melting filler into the core.

Diffusion-controlled melting and re-solidification of metal micro layers on a reactive substrate

The paper offers a theoretical approach to a prediction of residue formation inherent to melting and subsequent solidification of micro layers of molten aluminum alloys. The residue formation follows a reactive flow of a portion of the melt that is removed by a surface tension action. The residue portion solidifies in situ. The phenomenon studied is associated with materials’ processing during controlled atmosphere brazing of aluminum. The model assumes that diffusion of Silicon, present in an Al+Si clad of a brazing sheet, has a twofold role. First, a solid state Si diffusion prior to melting and across the clad–core interface of a composite brazing sheet takes place and modifies alloys’ composition on both sides of the interface. Subsequently, Si diffusion within clad controls the melting process. Both processes are essential for clad residue formation. The approach advocated in this paper leads to a prediction of the residue formation through a modeling of the non-equilibrium diffusion-controlled melting. A heuristic interpretation of physical mechanisms was discussed and a related mathematical model devised. The model was solved numerically in terms of Si concentration distributions for a moving boundary problem and corroborated with empirical data. Empirical data were gathered using an experimental controlled atmosphere brazing facility. The results of the modeling and their corroboration with the experimental data indicate a strong dependence of residue formations on the pre-melting state of the clad, in particular on the grain size within Al-clad matrix. A good agreement between numerically predicted residue mass and experimental findings is documented in detail.

Dendritic growth in Al–Si alloys during brazing. Part 2: Computational modeling

The computational heat and mass transfer modeling approach presented in this paper emphasizes the influence of undercoolings on dendrite structure formations of the alpha phase crystals inherent to advanced phases of an aluminum brazing netshape manufacturing sequence. In the first segment of this work, the empirical evidence involving the outcome of the solidification process and its kinetics was presented. In this paper, simulation of the alpha phase crystal pattern formation is corroborated with empirical findings obtained by utilizing an AA4343/AA3003 brazing sheet exposed to controlled atmosphere brazing (CAB) in ultra-high purity nitrogen.

Dendritic growth in Al–Si alloys during brazing. Part 1: Experimental evidence and kinetics

This work provides empirical evidence needed for an in depth phenomenological study of dendrite growth phenomena during brazing of aluminum alloys in form of composite brazing sheets. The main objective of this study was (1) a collection of experimental evidence associated with heat and mass transfer modeling of the Al + Si solid solution dendrite macro morphology evolution inherent to joint formation during brazing, and (2) the dendrite growth kinetics analysis. The isothermal dwell and the quench that follow the clad molten aluminum binary alloy surface-tension-driven flow into the joint at the peak brazing temperature upon melting lead to the solidification of the metal micro layer and joint formation. Before, during and after the isothermal dwell a significant reduction of Si content in the melt is found. So, a subsequent dissolution may affect the interface zone between the molten clad and substrate. α-phase dendrite assemblies imbedded in an irregular eutectic in the joint zone are the main morphological features of the solidification microstructures. The major characteristic of the phenomenon is a sensitivity of the dendrite pattern selection and dendrite population on brazing process parameters, in particular on the temperature during the dwell.

Electrochemical Behaviors of Quad-Layer Aluminum Brazing Sheet Composite for Automotive Applications

The cross-sectional depth corrosion potential profiles for a quad-layer aluminum brazing sheet composite AA4045/ 7072/3003/4045 (UNS A94045/A97072/A93003/A94045) in both the pre- and post-brazed forms have been established using ASTM G69-97 standard test method. Galvanic corrosion behaviors between the AA7072 (UNS A97072) layer and the nonzinc-containing AA3003 core alloy and AA4045 clad alloy also have been investigated in ASTM G69-97 solution by galvanic current measurement method. The effect of zinc and other major alloying elements on the corrosion potential was analyzed. The electrochemical studies show that there are significant potential differences (≥100 mV) and galvanic currents (∼2 to 8 × 102 μA/cm2) between the AA7072 layer and the other nonzinc-containing core and clad layers both before and after brazing. A zinc-containing AA7072 layer appears to be able to provide a sacrificial protection to both core alloy AA3003 and high-silicon braze clad layers such as AA4045 in ASTM G69-97 solution. The zinc distribution across the brazing sheet material was derived from the relative corrosion potential measurements. The distribution of zinc within the material can be related to the diffusion of zinc during the brazing process.

アルカリ水溶液中におけるアルミニウム合金クラッド材の局部腐食

Aluminum alloy three-layered brazing sheets, clad with sacrificial anode alloy on one side of them, have widely been used for the tube of radiators for automotive in which cooling water circulates. Inner pitting corrosion resistance of the tube has been excellent both in acidified and in neutral corrosive aqueous solutions by the effect of sacrificial anode. On the other hand, it has been reported that the sacrificial anode does not work effectively in alkaline solution, and perforation by pitting corrosion occurs in earlier testing period of time. In this report, localized corrosion characteristics in alkaline solution were investigated by immersion corrosion tests and electrochemical measurements. Based on the experimental results, new corrosion mechanism was proposed in terms of solution environment formed on cathode surface. Pitting corrosion of the core alloy grew not with the attack by the alkaline bulk solution, but with accelerated dissolution by strong alkaline solution derived from the electrochemical cell formation between sacrificial anode and cathodic core alloy. Corrosion products formed in the pit were effective for maintaining strong alkaline environment on the core alloy surface.

Solid state Si diffusion and joint formation involving aluminum brazing sheet

The paper provides information about the use of in situ determined diffusion coefficients of silicon for modeling of a brazed joint formation, i.e. formation of the equilibrium surface of a molten Al+ xSi alloy at the onset of solidification in the joint. Diffusion coefficients of Si were determined (within both the joint and/or residue zone) to analyze its migration across the clad–core interface of an Al brazing sheet, including both the period prior to reaching brazing temperature range, and the peak brazing temperature range. Subsequently, diffusion coefficients were used to predict the joint formation during brazing. Migration of silicon is not uniform along the clad–core interface during brazing and depends, in addition to material characteristics and process parameters, on the vicinity of the joint zone. It is argued that, due to these alterations, the joint formation modeling must be performed by using in situ determined diffusion coefficients. The diffusion coefficients determined directly from electron probe microanalysis (EPMA) scans at different locations along the cladding sheet and within the joint zone differ between each other and when compared to the literature data. This variation influences the outcome of the residue formation modeling; hence the joint formation modeling may be affected. The relation between these phenomena is briefly discussed and quantitative data regarding diffusion coefficients, and in particular an approach to utilization of these data in modeling of joint formation, are provided.