Microstructural Changes in Brazing Sheet due to Solid-Liquid Interaction (2009)

Aluminium brazing sheet is the material of choice to produce automotive heat exchangers. Although in Dutch the official translation of aluminium brazing sheet is “aluminium hardsoldeerplaat” the English name is used in the industry. Aluminium brazing sheet is basically a sandwich material and consists of an aluminium core alloy, typically an AA3XXX alloy (containing Mn) or an AA6XXX alloy (containing Mg and Si) with a clad alloy of the AA4XXX (containing Si) series. The core alloy gives the final product the desired properties after brazing. 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 engineering parameters. Typically the core alloy is single side or both side clad and the thickness of the clad alloy ranges between 5 and 20% of the total thickness. The AA4XXX alloy used for aluminium brazing sheet has a melting range between 570°C and 610°C while the melting range of a typical AA3XXX 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 the brazing temperature, typically around 600°C, the AA4XXX clad alloy is completely molten. Due to capillary forces and surface tension differences, the molten clad alloy will flow to connect adjacent pieces. The process by which joining is taking place is called the brazing process. During this brazing process liquid metal from the clad alloy is in close contact with the solid core alloy. At this stage an interaction between the two phases can take place. Several types of interaction between the two phases can take place but the interaction that is referred to as Liquid Film Migration is the topic of study in this thesis. Liquid Film Migration is, however, not the only name given in literature to what seems to be the same interaction. Liquid Film Migration is causing a significant change in microstructure and element distribution of the core alloy. These changes are detrimental to the corrosion resistance of the final product. Although the name Liquid Film Migration was given to the process responsible for the observed changes no conclusive evidence has been presented to confirm the existence of a liquid film in aluminium brazing sheet. The literature available on Liquid Film Migration in aluminium brazing sheet has given some information on the conditions favourable for Liquid Film Migration to occur. These conditions are residual strain in the core alloy present at the peak brazing temperature and a small grain size of the core alloy. This thesis focussed on the occurrence of LFM in brazing sheet and the possible mechanism and driving forces behind it. Two different core alloys with the same clad alloy were processed and studied for their susceptibility to Liquid Film Migration. Conditions were created that according to literature should result in different degrees of Liquid Film Migration. Brazing took place in a standard brazing furnace or in a salt bath. The salt bath would enable a kinetic study since time and temperature are well controlled. The study of the samples after brazing indeed showed a different response to the applied processing. The main observation was that the onset of recrystallization of the core alloy plays a major role in the occurrence of Liquid Film Migration. A detailed study of the liquid film showed the accumulation of constituents and dispersoids which were originally present in the core alloy. The measured diffusion profiles of silicon in front of the liquid film coincide with theoretical diffusion profiles as if they originated from a moving boundary. From the diffusion profiles of silicon, the kinetics of the moving liquid film was extracted resulting in an inverse square root dependence of the velocity with time. An estimation of the energies available in the system showed that the coherency strain energy could not be the driving force for Liquid Film Migration in aluminium brazing sheet. Most likely the energy is supplied by the reduction of the (sub)grain size. This is supported by the fact that strained samples that do not recrystallize during brazing are the ones showing the highest degree of Liquid Film Migration. Based on these findings, the residual strain present in the form of sub-grains or dislocations is considered to provide the energy for the movement of the liquid film. A qualitative assessment of the residual strain present in samples after brazing supported the hypothesis that indeed this residual strain is the energy source needed for the movement of the liquid film. The mechanism behind liquid film migration has been found to be similar to the recrystallization process caused by Strain Induced Boundary Migration. It was presented that lowering the surface energy between grains by a liquid film would allow the boundary to move at lower dislocation densities compared to a non liquid infiltrated grain boundary. Recrystallization would reduce the dislocation density taking away the driving force to spport liquid film migration. Liquid film migration and strain induced boundary migration are both in competition for the same energy. Strain induced boundary migration can take place during the whole brazing cycle while liquid film migration only can occur in the presence of a liquid. This means that the clad alloy has to be at least partly molten to allow wetting of the grain boundaries. When recrystallization by strain induced boundary migration takes place no liquid film migration will occur. A theoretical approach to determine the velocity of the liquid film was in reasonable agreement with the observations. The main obstacle to completely quantify liquid film migration is the uncertainty of the development of the film thickness in time. As demonstrated, recrystallization plays a critical role in the onset of liquid film migration. A through Process Model was used to determine if such model could be used to predict recrystallization based on thermal mechanical process input parameters and alloy chemistry. Unfortunately the model lacks sensitivity to predict the onset of recrystallization in this study. However the model contains all necessary modules to ensure that after fine tuning for this application, more predictive power can be expected.