Tensile properties and drawability of thin bimetallic aluminum-scandium-zirconium / stainless steel foils and monometallic Al-Sc-Zr fabricated by magnetron sputtering

Abstract

Al-Sc-Zr alloys are interesting for the production of high strength micro components by micro deep drawing. These alloys show a good hardenability due to the formation of nanometer-scale spheroidal Al3(Sc, Zr) precipitates, which are highly coherent with the aluminum matrix. However, the formation of these precipitates in Al-Sc-Zr foils fabricated by conventional metallurgical methods dramatically reduces their ductility and drawability. In this work, magnetron sputtering was used to produce Al-Sc-Zr foils and Al-Sc-Zr / stainless steel bimetallic foils which are nearly free of these precipitates. Tensile tests were carried out to measure and compare the mechanical properties of monometallic Al-Sc-Zr foils and bimetallic Al-Sc-Zr / stainless steel foils deposited with varying plasma target powers and containing different volume fractions (layer thickness) of Al-Sc-Zr. Micro deep drawing was used to determine the drawability of selected monometallic and bimetallic foils. The results show that the density of monometallic Al-Sc-Zr foils can be improved significantly by increasing the DC target power and by using the high power impulse magnetron sputtering (HiPIMS) technology, resulting in foils with higher ductility. Bimetallic foils achieved higher strength and ductility than monometallic Al-Sc-Zr foils. Their mechanical properties vary with the target power and the volume fraction (thickness) of Al-Sc-Zr. The limit drawing ratio of HiPIMS deposited monometallic foil was 1.7 or 1.8 depending on the side of the foil facing the die, whereas a limit drawing ratio of 1.9 was observed for bimetallic foils.

Materials for Micro Forming

“…These properties also increased with the layer thickness (volume fraction). The yield and tensile strength as well as the apparent Young's modulus of the bimetallic foils are well predicted by rules of mixtures [Kov18]. Despite being submitted to higher strains than the elongation at fracture of monometallic Al-Sc-Zr foils, the Al-Sc-Zr layer in the bimetallic foils did not crack prematurely in the tensile tests.…”

“…On the top side (upper side or free surface), the columnar grains grow freely, resulting in high roughness and porosity. If the free surface of the foils faced towards the die, the LDR was only 1.7, whereas a LDR of 1.8 was reached if the bottom side faced toward the die [Kov18]. This is because strong tensile stress always develops on the outer shell of the cup.…”

“…6.23b. Al-Sc-Zr foils were produced using a PVD unit with a rotating substrate holder and with three DC Al-Sc targets (2 mass% Sc) and one DC Al-Zr target (2 mass-% Zr) or with two HiPIMS Al-Sc-Zr targets (1 mass-% Sc, 0.5 mass-% Zr) [Kov18]. The DC target plasma power was 2 or 4 kW.…”