The Effect Of Microstructure On The Fatigue Characteristics In Al-Mg-Si-(Cu) Alloys

The6xxx series Al-Mg-Si-(Cu) alloys stand out by a combination ofgood formability, good weldability, very good corrosion resistance, excellentsurface quality, a high ratio of strength to density as well as satisfactorystrengthening potential during paint bake cycles. Therefore, they are widely used inautomobiles and high-speed trains along with other energy-saving vehicles. Inthis work, the relations between microstructures and fatigue characteristics of theAl-Mg-Si-(Cu) alloys are investigated by means of hardness test, fatigue test,tensile test, optical microscopy, scanning electron microscopy (SEM) andtransmission electron microscopy (TEM). The first part (Chapter III) is tounderstand the mechanical behaviors under cyclic loading and the fractographs ofthe Al-Mg-Si-Cu alloy aged for different times. The second part (Chapter IV) is tounderstand the effects of microstructures on the fatigue properties of a dissimilarjoint made out of the Al-Mg-Si and the Al-Zn-Mg aluminum alloy and assembledtogether using pulsed metal inert gas (MIG) welding. The main contents andresults are summarized as follows:1) The effects of cyclic loading on the mechanical properties of anAl-Mg-Si-Cu alloy during fatigue process are studied. The yield strength ofunder-aged and peak-aged samples decline in the fatigue process, and thisphenomenon for the under-aged samples is more obvious. However, theelongation of the under-aged samples rises with the number of cycles. Thestrength of the over-aged samples improves in the fatigue process, but theelongation drops significantly in the late fatigue life.2) The fatigue fracture analysis of the Al-Mg-Si-Cu alloys reveals that thecracks are inclined to grow along the grain boundary after initiating at thespecimens surface and the crack-source-regions form there. Interestingly, grainboundary separation regions were observed to occur between the ray-like crackgrowth region and the over-loading region in the over-aged alloy.3) The hardness profile of the dissimilar joint shows that the hardnessincreases dramatically from the weld seam to Al-Zn-Mg base metal, while, thehardness value decreases significantly in the softened-zone of the Al-Mg-Si basemetal side. A large number of rod-like β ’ phase and U2phase were found in this softened-zone where fracture took place during tensile testing.4) The Scanning Electron Microscopy containing EBSD system isconducted to observe the crystallography of the dissimilar joint. The weld seamzone is comprised of columnar crystals. There is a fine grain zone along the fusionline near the weld seam side, and abnormal grain growth appears near the basemetal side. The line scanning analysis by Energy dispersive X-ray spectroscopy(EDS) shows quantitatively the width of the transition zone and a gradual change ofsolute concentration. Backscattered electron images and EDS analysis reveal a largenumber of dispersoids, containing Al, Si and Mg, distributed in the transition zone.SEM observations show fatigue cracks initiate at the transition zone of theAl-Mg-Si alloy base metal side. No nano-scale precipitates were observed by TEMin this zone.