Hot-dip Coating of Aluminum Alloy to Steel Substrates with Ultrasonic Vibration

Satoshi Tamura*, Naoki Ishihara**, Yoshiki Tsunekawa*** and Masahiro Okumiya*** In order to simplify the manufacturing process of complex structured slide-bearings used at the large ends of connecting rods in automotive engines, direct coating of bearing material mainly by thermal spraying is being examined. Newly substituted lead-free metals like aluminum圃basedalloys are appealing as slide-bearing material candidate. Hot-dip coatings practically employed in the manufacturing of zinc alloy coated steel sheets. However, itis difficult to coat aluminum alloy on bulky steel substrates without sufficient preheating, because a rapidly solidified layer containing gas babbles is formed on a substrate surface. A variety of ironaluminides are also formed at the interface of steel and aluminum hot-dip coatings, which is the main reason for the difficulty in joining steel with aluminum. Ultrasonic vibration was applied to a steel substrate during the hot-dip coating of aluminum alloy to suppress the rapidly solidified layer and且 brittlereaction layer. Hot dipping of columnar steel substrates into molten aluminum alloy (Al-2.7mass%Si-4.6mass%Sn) was carried out through the use of a Langevin oscillator with a resonant frequency of 19.5 kHz. The application of ultrasonic vibration is quite effective for removing rapidly solidified layers and surface oxide layers from the substrate surface by sonocapillary effects based on the cavitation phenomenon, so that intimate contact is achieved at the beginning of hot-dip coating. When the hot-dipped substrate surface is heated higher than liquid temperature, a brittle intermetallic reaction layer of Fe2A15 grows at the interface. However, the application of ultrasonic vibration to hot-dipping is effective for controlling reaction layers less than 5μm in thickness. Impact test showed good adhesive strength of hotdipped aluminum coatings with a thin reaction layer of approximately 5μm.