Functional Layers of Aluminium Alloy on Steel Made by Alternative Friction Processes, for Elements of Metal Structures

Abstract

Coating of steel with aluminium alloy is needed for the execution of a functional layer for corrosion protection. Some experiments have been performed on square-section tubes (50 mm x 50 mm) of S235 steel, according to EN 10 025, to be coated with 1 mm thick sheets of EN AW 5754 aluminium alloy that have been previously bended as U shaped profiles. A new experimental model of specialized equipment has been used for certain experiments to make these functional layers of aluminium alloy on steel. Firstly, friction drilling and threading by form tapping, followed by screws-mounting without nuts have been used to make such joints. Several holes have been executed by a Ø4.3 friction drilling tool, then an M5 form tap was used for threading. For friction drilling, tools with 90% tungsten carbide content and 1 micron grain size were applied. By threading, TiN coated form taps have been used. Secondly, overlap friction stir welding (FSW) has been applied, to make a functional layer of aluminium alloy on a 50 mm x 50 mm S235 steel tube. The wings of the U profiles were overlapped. A quenched FSW tool, own-made of C 45 grade steel, EN 10083, has been used for these joining tests. The joining parameters are mentioned for each process. The run of each joining process is described and the joint test samples are presented. The appearance of the screw-mounted functional layers is appropriate. The metallographic analysis has revealed adequate form of the burr formed below the hole. The burr height is 2.5 – 3.2 mm. The pattern of the M5 thread is appropriate. No defects have been detected on the holes and threads. The appearance of the FSW functional layers is adequate. Metallographic analysis shows that FSW joints of the overlapped aluminium alloy sheets are adequate, because there is no gap between these sheets. There is only a narrow gap between the aluminium alloy bottom sheet and the wall of the steel tube, which proves an appropriate positioning of the two metals. No defects were detected, except for a weld flaw, as a small and isolated cavity, with a section less than 0.1 mm2, considered within the acceptance limit, according to EN 25239-5. The U shaped sheets of aluminium alloy are firmly fixed on the square steel tube, for both coating types. The mentioned processes are proposed to increase productivity in industrial technologies for series production. The processes addressed in this paper are more rapid than conventional processes. Adequate preparation of the parts to be welded, mechanization and automation allow repeatability and quality. The target applications are coated structure elements for devices, appliances, tools, welded structures or automobiles. The involved industrial areas of the applications are: manufacturing, electro-technique, construction and automotive industries. The presented processes are ecological, because they do not need lubricants or other toxic substances and do not produce chips or harmfull substances.