TY - THES

T1 - Atomic Force Microscopy and Kelvin Probe Force Microscopy investigations of high-strength aluminium brazing sheets

AU - Huszar, Michael

N1 - no embargo

PY - 2019

Y1 - 2019

N2 - Aluminium and its alloys are excellent materials for lightweight constructions and transportation applications (due to their low density around of 2.7 g/cm³). In addition, the combination of high thermal conductivity (205 to 232 WK/m) with good corrosion resistance also make them especially suitable for heat exchangers in the automotive industry. For the development of these heat exchangers, multilayered brazing sheets with different aluminium alloys are used. Conventionally, these sheets consist of two layers where the core material (e.g. Al-Mn) provides the mechanical strength, and the brazing layer (e.g. Al-Si) with a lower melting point (e.g. 575 °C) is used to bond the final product together. To improve efficiency and service life, it is possible to change the core material to a high-strength Al alloy, although this may lead to an increased risk of corrosion. In this work, two different high-strength Al brazing sheets are studied via Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM) to explore the physical parameters which influence the corrosion processes. The core material in both sheets is AlZn4.5Mg1 and the brazing layer is an eutectic AlSi10 alloy. These two layers have to be separated by an intermediate layer which prevents Si diffusion and subsequent loss of mechanical strength. The first investigated sample had a pure Al (with 99.85 wt%) intermediate layer and the second one an AlMn1 layer. Metallographic cross-sections of the sheets were prepared for the examination after a simulated brazing process (12 min at 610 °C). The investigation included the measurement of the brazed state by means of AFM and KPFM as well as the topographic change after immersion in a solution (42 g/l NaCl, adjusted to pH 3 with acetic acid) for fixed time intervals of 1 h and 3 h only via AFM. With a modified experimental setup, KPFM measurements were also possible for the AlMn1 sample after the immersion testing. In both samples, the inclusions were found to have a more noble potential when compared with the surrounding Al-matrix. Al(Mn,Fe)Si-particles exhibited a potential difference of approximately 300 mV. Accurate values for the potential differences of smaller inclusions (< 2 µm) could not be determined, but a trend to higher potentials was identified. In the intermediate layers of the two samples, the inclusions were observed to be different. In the pure Al sample, a high number of very small inclusions (area around of 3 µm²) was observed. The size and number of these inclusions decreased upon increasing corrosion time. It was also possible to determine the alteration in the surface potential between the layers. Such change was more pronounced on the border of the intermediate layer to the core material (1.15 V/µm) and is a consequence of the diffusion processes during brazing. Based on the results obtained from the performed measurements, the possibility of estimating corrosion properties using the KPFM technique was demonstrated on the studied samples. Therefore, the methodology used in this work allowed the conclusion that AlMn1 as intermediate layer for high strength Al brazing sheets is the most appropriate candidate material to minimize the corrosion attack under the studied conditions. The outcomes of this research work shed light on the possibility of further investigating the effects of particle size on the corrosion resistance of these lightweight materials.

AB - Aluminium and its alloys are excellent materials for lightweight constructions and transportation applications (due to their low density around of 2.7 g/cm³). In addition, the combination of high thermal conductivity (205 to 232 WK/m) with good corrosion resistance also make them especially suitable for heat exchangers in the automotive industry. For the development of these heat exchangers, multilayered brazing sheets with different aluminium alloys are used. Conventionally, these sheets consist of two layers where the core material (e.g. Al-Mn) provides the mechanical strength, and the brazing layer (e.g. Al-Si) with a lower melting point (e.g. 575 °C) is used to bond the final product together. To improve efficiency and service life, it is possible to change the core material to a high-strength Al alloy, although this may lead to an increased risk of corrosion. In this work, two different high-strength Al brazing sheets are studied via Atomic Force Microscopy (AFM) and Kelvin Probe Force Microscopy (KPFM) to explore the physical parameters which influence the corrosion processes. The core material in both sheets is AlZn4.5Mg1 and the brazing layer is an eutectic AlSi10 alloy. These two layers have to be separated by an intermediate layer which prevents Si diffusion and subsequent loss of mechanical strength. The first investigated sample had a pure Al (with 99.85 wt%) intermediate layer and the second one an AlMn1 layer. Metallographic cross-sections of the sheets were prepared for the examination after a simulated brazing process (12 min at 610 °C). The investigation included the measurement of the brazed state by means of AFM and KPFM as well as the topographic change after immersion in a solution (42 g/l NaCl, adjusted to pH 3 with acetic acid) for fixed time intervals of 1 h and 3 h only via AFM. With a modified experimental setup, KPFM measurements were also possible for the AlMn1 sample after the immersion testing. In both samples, the inclusions were found to have a more noble potential when compared with the surrounding Al-matrix. Al(Mn,Fe)Si-particles exhibited a potential difference of approximately 300 mV. Accurate values for the potential differences of smaller inclusions (< 2 µm) could not be determined, but a trend to higher potentials was identified. In the intermediate layers of the two samples, the inclusions were observed to be different. In the pure Al sample, a high number of very small inclusions (area around of 3 µm²) was observed. The size and number of these inclusions decreased upon increasing corrosion time. It was also possible to determine the alteration in the surface potential between the layers. Such change was more pronounced on the border of the intermediate layer to the core material (1.15 V/µm) and is a consequence of the diffusion processes during brazing. Based on the results obtained from the performed measurements, the possibility of estimating corrosion properties using the KPFM technique was demonstrated on the studied samples. Therefore, the methodology used in this work allowed the conclusion that AlMn1 as intermediate layer for high strength Al brazing sheets is the most appropriate candidate material to minimize the corrosion attack under the studied conditions. The outcomes of this research work shed light on the possibility of further investigating the effects of particle size on the corrosion resistance of these lightweight materials.

KW - AFM

KW - KPFM

KW - Al brazing sheet

KW - heat exchanger

KW - AFM

KW - KPFM

KW - Al

KW - Lotblech

KW - Wärmetauscher

M3 - Master's Thesis

ER -