Corrosion behavior of the friction-stir-welded joints of 2A14-T6 aluminum alloy

The corrosion behavior of friction-stir-welded 2A14-T6 aluminum alloy was investigated by immersion testing in immersion exfoliation corrosion（EXCO） solution. Electrochemical measurements（open circuit potential, potentiodynamic polarization curves, and electrochemical impedance spectroscopy）, scanning electron microscopy, and energy dispersive spectroscopy were employed for analyzing the corrosion mechanism. The results show that, compared to the base material, the corrosion resistance of the friction-stir welds is greatly improved, and the weld nugget has the highest corrosion resistance. The pitting susceptibility originates from the edge of Al-Cu-Fe-Mn-Si phase particles as the cathode compared to the matrix due to their high self-corrosion potential. No corrosion activity is observed around the θ phase（Al2Cu） after 2 h of immersion in EXCO solution.

Corrosion fatigue behavior of friction stir processed interstitial free steel

In this study, interstitial free （IF） steel plates were subjected to double-sided friction stir processing （FSP）. The fine-grained structure with an average grain size of about 12 μm was obtained in tbe processed zone （PZ）with a thickness of about 2.5mm. The yield strength （325 MPa） and ultimate tensile strength （451 MPa） of FSP IF steel were significantly higher than those of base material （BM） （192 and 314 MPa）, while the elongation （67.5%） almost remained unchanged compared with the BM （66.2%）. The average microhardness value of the PZ was about 130 HV, 1.3 times bigher than that of the BM. In addition, the FSP IF steel showed a more positive corrosion potential and lower corrosion current density than the BM, exhibiting lower corrosion tendency and corrosion rates in a 3.5 wt% NaCl solution. Furtbernlore, FSP IF steel exhibited higher fatigue life than the BM both in air and NaCl solution. Corrosion fatigue fracture surfaces of FSP IF steel mainly exhihited a typical transgranular fracture with fatigue striations, while the BM predominantly presented an intergranular fracture. Enhanced corrosion fatigue performance was mainly attributed to the increased resistance of nucleation and growth of fatigue cracks. The corrosion fatigue mechanism was primarily controlled by anodic dissolution under the combined effect of cyclic stress and corrosive solution.

An integrated hard- and soft-ware triboelectrochemical test rig for tribocorrosion experiments

Tribocorrosion is a degradation mechanism resulting from the interaction of mechanical(wear)and chemical(corrosion)phenomena.This interaction leads to surface damage that does not correspond to the simple sum of wear and corrosion taken separately.Testing tribocorrosion of material pairings thus requires the capability not only to control the mechanical and chemical test parameters but also to identify the relative contribution of corrosion and wear in the overall material degradation.This paper describes a novel laboratory tribocorrosion test set-up combining in one experiment the control of mechanical input(load,velocity,and frequency)and output(friction and wear)parameters as well as the control of corrosion phenomena through a series of dedicated electrochemical methods(open circuit,potentiostatic and potentiodynamic measurements).The possibilities offered by this set-up are illustrated by selected case studies.

Solid state crack repair by friction stir processing in 304L stainless steel

Friction stir processing （FSP） was investigated as a method of repairing cracks in 12 mm thick 304L stainless steel plate. Healing feasibility was demonstrated by processing a tapered crack using a PCBN/W- Re tool with a 25 mm diameter shoulder and a pin length of 6.4 mm. The experiment showed that it was possible to heal a crack that begins narrow and then progressively grows up to a width of 2 mm. Bead on plate experiments were used to find the best parameters for creating a consolidated stir zone with the least amount of hardness difference compared to the base metal. Grain refinement in some specimens resulted in much higher stir zone hardness, compared to base metal, A plot of grain size versus microhardness showed a very strong inverse correlation between grain size and hardness, as expected from the Hall- Perch relationship. Corrosion testing was carried out in order to evaluate the effect of FSP on potential sensitization of the stir zone. After 1000 h of intermittent immersion in 3.5% saline solution at room temperature it was found that no corrosion products formed on the base material controls or on any of the friction stir processed specimens.

Ultrasound enhanced friction stir welding of aluminum and steel： Process and properties of EN AW 6061/DC04-Joints

In this work, the joining of aluminum to steel was conducted by ultrasound enhanced friction stir weld- ing （USE-FSW）. The power ultrasound was introduced into one of the metal sheets by an ultrasonic roll seam module synchronously to the FSW-process. The effect of the ultrasound on the resulting welds, their microstructure and their corrosion properties was investigated by light and scanning electron microscopy and corrosion investigations. The USE-FSW-joints showed less and smaller steel particles in the nugget zone as well as a thinner continuous intermetallic phase of FeAl3 at the interface. The nondestructive testing method of computed laminography proved the observations made by optic microscopy due to non-porous joints for both techniques. Corrosion investigations showed only low corrosion current densities and no enhanced galvanic corrosion for the EN AW-6061/DC04-hybrid joints in sodium chloride solution.

Comparing the Fatigue and Corrosion Behavior of Nanograin and Coarse-Grain IF Steels

In the present work,a nanograin layer of about 150 μm thick was formed on the surface of an interstitial-free（IF） steel via friction stir processing.Then,the fatigue and corrosion behaviors of IF steel with nanograin layer were compared with that of coarse-structure counterpart.More than threefold increase in the hardness was observed due to the formation of nanograin layer.The size of nanograms in the stir zone was within 30-150 nm.This resulted in 50%increase in the fatigue strength of nanostructured specimen.Furthermore,the fracture surfaces were characterized using field emission scanning electron microscopy and scanning electron microscopy.As for the fatigue behavior of nanograin IF steel,the fracture surface was characterized by the formation of nanospacing striations and nanodimples.Besides,the nanograin structure pronounced the passivity and exhibited higher corrosion resistance.