Abstract:

Clad brazing sheets used in the production of automotive heat exchangers typically consist of a 4xxx (Al-Si) clad alloy and a 3xxx (Al-Mn) core alloy. During brazing of these sheets, the clad melts to provide the necessary filler metal for joining, while the core remains solid. Deformation during assembly of the heat exchangers can adversely impact the brazeability of these sheets, by increasing Silicon (Si) diffusion into the core, and potentially induce a phenomenon known as liquid-film migration (LFM). In an effort to mitigate the adverse effects of increased Si diffusion and strain-induced LFM, a novel braze sheet was designed incorporating a thermal barrier layer between the clad and core alloy. The effects of incorporating such a layer on the microstructure and mechanical properties is investigated in comparison with a typical industry standard grade control sheet with no barrier layer. The current work studies O tempered versions of these brazing sheets and observes the effects of varying pre-brazing deformation on the simulated braze performance of these sheets. Clad was made of AA4045, an Al-Si alloy while the core is made up of an Al-Mn-Cu-Mg combination. The barrier layer within the new brazing sheet was made of a 2.54 mm commercial grade O tempered AA1100. Ingots were prepared using direct chill casting. The clad alloy was an AA4045 Al-10Si while the core alloy was Al-1.5Mn-0.5Cu with 1 and 2 percent Mg for the Control and New Sheet, respectively. Two sets of brazing sheets were prepared: a New Sheet with barrier layer and a Control sheet without barrier layer. New Sheet was assembled with the clad, barrier and core being welded together. The assembly was reheated to 723.15 K (450^oC), hot roll bonded to 5 mm thickness and then annealed to reach the O tempered condition. The Control sheet is an industry grade single-side 1 mm clad commercial grade AA4045-X901 O temper long life brazing sheet. The clad fills 10 percent of the thickness and it is composed of Al and Si to ensure proper cladding. Tensile specimens with 12.7 mm gauge width and 19.1 mm gauge length were machined along the rolling direction. A pre-strain of 0%, 2%, 4%, 6%, 8% and 10% was applied to both the New Sheet and Control sets of samples. To simulate the heating cycle of furnace brazing, the specimens were flash annealed through a belt furnace. In addition, the same set of samples are exposed to a simulated brazing heat treatment using a Netzsch 404C Pegasus differential scanning calorimeter (DSC). The ratio of the solidification energy to the initial melting energy (ΔH_sol/ΔH_melt) allows the prediction of the brazing performance.

According to the DSC results, the amount of strain applied to the samples noticeably affects the ΔH_sol/ΔH_meltwith varying behavior. Strain is found to have a positive effect on the New Sheet up to 6% after which the ratio is relatively stable. Observing these samples under a scanning electron microscope (SEM) shows decreasing intensity of Si diffusion into the core of New Sheet samples due to the presence of the barrier layer. In contrast, ratio for the Control sheets decreased up to 4% strain which is caused to the presence of LFM reducing the amount of clad available during brazing. SEM scans showed occurrence of liquid film migration in 4% strain samples at the clad-core interface which is detrimental to brazing performance.

Keywords: brazing, cladding, aluminum, barrier layer, strain, DSC

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Dr. Haiou Jin