Search

Precipitation hardening and corrosion behavior of a friction stir welding (FSW) based on the aluminum alloy A6005 reinforced with TiB2 nanoparticles have been studied. Mechanical alloying (MA) and hot extrusion techniques have been employed as processing route followed by FSW. Samples characterization has been performed by DSC and TEM, and precipitation strengthening of the bulk samples and the FSW joint has been evaluated by micro-hardness tests after T6 thermal treatment. TEM characterization revealed the presence of Mg–Si hardening phases, mainly of β′ phase, and dispersoids of α-Al(FeMnCr)Si into the aluminum matrix. The results revealed that samples subjected to MA had less susceptibility to T6 thermal treatment and that the presence of nano-TiB2 reinforcement accelerates aging time. In addition, electrochemical tests based on polarization tests have been performed in 3.5% NaCl solution to assess the effect of FSW process on corrosion behavior. The FSW joint had worse corrosion behavior since the passive Al2O3 film was not generated on the weld zone. SEM–EDS analysis revealed that pits nucleated mainly in sites with a higher presence of Fe contaminant which acts cathodically with respect to the aluminum matrix, producing galvanic corrosion.

High velocity oxy-fuel (HVOF) technique was used to deposit an Al2O3-30(Ni20Al) coating on AISI 304 stainless steel substrate to improve its corrosion and wear performance at high temperature. First, thermal cycling tests at 750 and 850 °C were conducted in air atmosphere on both coated and uncoated stainless steel substrates to study the effectiveness of coating protection system at different exposure times (2, 6, 10 and 15 days). Then, high temperature corrosion and wear mechanisms were studied by SEM and XRD techniques. These analyses revealed that uncoated specimens were severely corroded but no oxidation on the coated substrates was detected. Oxide growth on the uncoated substrate was mainly formed by spinel (MnCr2O4 and FeCr2O4) and corundum (Fe2O3 and Cr2O3) type oxides. Meanwhile, these oxides were not found on the coated substrate. Wear rate of the coated specimen was found to be reduced from 6.9·10−4 to 8.3·10−5 mm3/N·m compared to that of the uncoated specimen. The exposure to high temperatures improved coating wear resistance: wear rate was four times lower in the coated sample exposed to high temperatures compared to that of the non-exposed coated sample due to the formation of a harder phase of NiO.

International audience; Fireside corrosion of T24 steel and HVOF NiCr-coated counterparts is investigated. The samples were exposed to different deposits of NaCl, KCl, Na2SO4 and K2SO4 at 650 ºC for 360 h in dry air. The deposit with the highest chloride content (NaCl-KCl) was the most aggressive followed by the ones combining chlorides and sulfates. In contrast, the sulfate-based salts did not produce severe damage. The NiCr coating maintained the integrity of the substrate but the chlorides induced a great attack on the coating. The formation of K2CrO4 and the penetration of chlorides are identified as the main factors of this attack.

An A6005 aluminium alloy composite reinforced with different contents of titanium boride particles (2.5, 5 and 10 wt%) was produced by a mechanical alloying (MA) route. First, the aluminium alloy and titanium boride powders were blended by MA at different milling times, from 1 to 10 h. The samples obtained after MA processing were microstructurally and mechanically characterised by OM, SEM, TEM, LDS, XRD and micro-hardness tests. The optimum size distribution and suitable mechanical properties were obtained with 5 or 10 wt% of titanium boride particles after 8 h of MA, when the steady state is reached. Hot extrusion was employed as a consolidation process. An extruded sample of powders with a 5 wt% reinforcement content that was MA for 8 h was characterised by TEM, FE-SEM and nanoindentation tests. The results show a good distribution of the nano-reinforcement within the aluminium matrix. Nano-indentation test shows an increment in hardness values from 2.6 GPa for the reference extrusion of A6005 powders without MA and reinforcement to 3.3 GPa for the A6005 extrusion with 5 wt% TiB2 and MA for 8 h.

The high-temperature corrosion behavior of NiMoCrW and CoNiCrAlY base coatings produced by high velocity oxy-fuel technique (HVOF) has been analyzed in dry air atmosphere and in presence of NaCl-KCl salt mixture at 650 °C for 360 h. Their behavior was compared with that of NiCr coating used as reference. Their corrosion resistance in presence of NaCl-KCl was measured by dimensional metrology analysis and the corroded samples were characterized by XRD and SEM techniques. The coatings exhibited excellent oxidation resistance in dry air atmosphere due to the formation of Cr2O3 and Cr-rich spinels, as well as the formation of Al2O3 in CoNiCrAlY coating. Their corrosion resistance in presence of NaCl-KCl was significantly lower due to chlorine-induced active oxidation and due to the formation of chromates, identified by XRD as K2CrO4. However, both coatings presented higher corrosion resistance in presence of the alkali salts compared to that of NiCr coating. The presence of Mo in the NiMoCrW coating improved the high-temperature corrosion resistance in presence of the alkali salts and the formation of a stable Al2O3 oxide scale on CoNiCrAlY coating reduced the penetration of chlorides and the degradation of the protective oxide scale by chromates formation.

This investigation reports on the thermal diffusivity of NiCr HVOF coatings on three ferritic steels (T24, T92 and VM12) and one austenitic steel (AISI 304) determined by laser-flash technique from room temperature up to 800 °C. The NiCr HVOF coatings were compact, dense, and well-adhered to the substrates with an average thickness of 194 ± 51 μm. The thermal diffusivity of AISI 304 austenitic steel increases steadily with temperature. In contrast, the thermal diffusivity decreases from room temperature till the Curie's temperature and then increases up to 800 °C in the T24, T92 and VM12 ferritic steels. It is also shown that the substrate has a greater influence on the thermal diffusivity at lower temperatures while the coating increases thermal diffusivity of the overall sample (substrate + coating) at temperatures above 400 °C. Therefore, the intrinsic thermal diffusivity of NiCr, coating calculated with a 2-layer model, evolves similarly than the 304 steel with temperature.