Fatigue crack paths and properties in A356-T6 aluminum alloy microstructurally modified by friction stir processing under different conditions

A356-T6 cast aluminum alloy is a light weight structural material, but fatigue crack initiates and propagates from a casting defect leading to final fracture. Thus it is important to eliminate casting defects. In this study, friction stir processing (FSP) was applied to A356-T6, in which rotating tool with probe and shoulder was plunged into the material and travels along the longitudinal direction to induce severe plastic deformation,resulting in the modification of microstructure. Two different processing conditions with low and high toolrotational speeds were tried and subsequently fully reversed fatigue tests were performed to investigate theeffect of processing conditions on the crack initiation and propagation behavior. The fatigue strengths weresuccessfully improved by both conditions due to the elimination of casting defects. But the lower tool rotationalspeed could further improve fatigue strength than the higher speed. EBSD analyses revealed that the higher tool rotational speed resulted in the severer texture having detrimental effects on fatigue crack initiation andpropagation resistances.&nbsp

EBSD-assisted fractographic analysis of crack paths in magnesium alloy

 Magnesium (Mg) alloys are attractive as structural materials due to their light weight and high specific strength. It is well known that Mg alloy has hexagonal close-packed (HCP) structure and only basal slipor twinning can operate during plastic deformation because critical resolved shear stresses of the other slipsystems such as pyramidal or prismatic slips are much higher than the basal slip. Thus sometimes characteristicfracture surfaces are formed during stress corrosion cracking (SCC) or fatigue crack propagation (FCP) in Mgalloys, where many parallel lines are formed. These lines are different from so-called fatigue striations, becausethey are formed even under sustained load condition of SCC. Consequently, electron back scattered diffraction(EBSD) technique was applied on the fracture surface, and the formation mechanism of parallel lines wasinvestigated. EBSD-assisted fractography had revealed that the characteristic parallel lines were formed due tothe operation of basal slips, not twining. It is considered that hydrogen-enhanced localized plasticity (HELP)mechanism had been activated under corrosive environment

Influence of joint line remnant on crack paths under static and fatigue loadings in friction stir welded Al-Mg-Sc alloy

The influence of the joint line remnant (JLR) on tensile and fatigue fracture behaviour has been investigated in a friction stir welded Al-Mg-Sc alloy. JLR is one of the microstructural features formed in friction stir welds depending on welding conditions and alloy systems. It is attributed to initial oxide layer on butting surfaces to be welded. In this study, two different tool travel speeds were used. JLR was formed in both welds but its spatial distribution was different depending on the tool travel speeds. Under the tensile test, the weld with the higher heat input fractured partially along JLR, since strong microstructural inhomogeneity existed in the vicinity of JLR in this weld and JLR had weak bonding. Resultantly, the mechanical properties of this weld were deteriorated compared with the other weld. Fatigue crack initiation was not affected by the existence of JLR in all welds. But the crack propagated preferentially along JLR in the weld of the higher heat input, when it initiated on the retreating side. Consequently, such crack propagation behaviour along JLR could bring about shorter fatigue lives in larger components in which crack growth phase is dominant