Effects of laser peening on the fatigue strength and defect tolerance of aluminum alloy

The effects of laser peening (LP) on the bending fatigue strength of the 7075‐T651 aluminum alloy were investigated. Accordingly, the defect tolerance of the aluminum alloy subjected to LP is discussed on the basis of fracture mechanics. The results indicate that a deeper compressive residual stress was induced by LP compared with the case of shot peening (SP). The fatigue strengths increased when both peening types were used. Semicircular slits with depths less than 0.4 and 0.1 mm were rendered harmless on the basis of the applications of LP and SP, respectively. The apparent threshold stress intensity factor range ΔK_th,ap increased by approximately five and two times owing to LP and SP, respectively. The increase of the ΔK_th,ap was caused by the compressive residual stress induced by the peening. The Kitagawa‐Takahashi diagram of the laser‐peened specimens shows that the defect tolerance of the aluminum alloy was improved by LP.     

Influence of thermal and mechanical surface modifications induced by laser shock processing on the initiation of corrosion pits in 316L stainless steel

Pure mechanical and thermo-mechanical laser shock processing treatments have been carried out on an AISI 316L stainless steel. Surface properties, mostly mechanical and metallurgical modifications, were analysed at different scales: a local scale using the nano-indentation technique and AFM analysis, and a more macroscopic scale, using microhardness, optical microscopy and residual stress determinations. After a pure mechanical laser-peening treatment, a significant improvement in the pitting corrosion resistance (+0.1 V) was observed in 0.05 M NaCl. This improvement was attributed to the combined effects of compressive residual stresses and work-hardening, and, in turn, to a mechano-electrochemical phenomenon by which a modification of cathodic reactions occurs during electrochemical tests. For the surface treated by thermo-mechanical laser peening (combination of a surface ablation and shock waves), a tendency for decreasing resistance against pitting corrosion was shown, and attributed to the processing-specific surface texture (ablation craters), which made the material susceptible for the creation of occluded cells.

---

Corrosion resistance of AA2219 aluminium alloy: electrochemical polarisation and impedance study

Abstract

The addition of copper increases the strength of aluminium, but decreases the localised corrosion resistance of the metal to seawater as the copper containing intermetallics exhibit different electrochemical properties from the matrix. The main requirement of corrosion resistance could be met through the surface modification by removing the copper rich phase. The present work involves investigations of the influence of prior copper removal treatment on the pitting and general corrosion resistance of AA2219 alloy containing a high copper content in different initial thermal tempers. Significant improvement in the pitting and general corrosion resistance after surface copper removal treatment was evident from the results of electrochemical polarisation and the impedance tests. Attempts were also was made to modify the surface further, with a conversion layer of cerium as an alternative to the chromate coating.     

Effects of electropulsing on inclusion size and corrosion-resistance of 304 stainless steel

Electropulsing has been imposed on molten AISI 304 stainless steel and its effects on the inclusion size and corrosion properties have been investigated. The average size of inclusions in electropulsing-treated samples was finer than that in untreated ones. A theoretical analysis shows that the mobility of solute atoms increases with electropulsing treatment. This promotes the nucleation of inclusions. Owing to a smaller inclusion size, the electrochemical experiments reveal that the pitting corrosion resistance of the electropulsing-treated stainless steel is significantly improved. The beneficial effects of electropulsing on the refinement of inclusions and the resistance to pitting corrosion increase with an increase in current density. Electropulsing treatment, therefore, provides a promising means to improve pitting corrosion resistance.     

Laser surface hardening of AISI 420 stainless steel treated by pulsed Nd:YAG laser

The AISI 420 martensitic stainless steel was surface-hardened by a pulsed Nd:YAG laser. The influences of process parameters (laser pulse energy, duration time and travel speed) on the depth and hardness of laser treated area and its corrosion behavior were Investigated. In the optimum process parameters, maximum hardness (490 VHN) in the laser surface treated area was achieved. The pitting corrosion behavior was studied by potentiodynamic polarization technique in 3.5% NaCl solution at 25 °C. Metallographical and electrochemical corrosion studies illustrated beneficial effects of laser surface hardening by refining the microstructure and enhancing the pitting corrosion resistance of the martensitic stainless steel. The pitting corrosion resistance of laser surface treated samples in 3.5% NaCl solution depends on the overlap ratio clearly. The pitting potential (E_pp) decreased significantly by increasing the ratio of pulse overlapping.     

Evaluating the improvement of corrosion residual strength by adding 1.0 wt.% yttrium into an AZ91D magnesium alloy

The influence of yttrium on the corrosion residual strength of an AZ91D magnesium alloy was investigated detailedly. Scanning electron microscope was employed to analyze the microstructure and the fractography of the studied alloys. The microstructure of AZ91D magnesium alloy is remarkably refined due to the addition of yttrium. The electrochemical potentiodynamic polarization curve of the studied alloy was performed with a CHI 660b electrochemical station in the three-electrode system. The result reveals that yttrium significantly promotes the overall corrosion resistance of AZ91D magnesium alloy by suppressing the cathodic reaction in corrosion process. However, the nucleation and propagation of corrosion pits on the surface of the 1.0 wt.% Y modified AZ91D magnesium alloy indicate that pitting corrosion still emerges after the addition of yttrium. Furthermore, stress concentration caused by corrosion pits should be responsible for the drop of corrosion residual strength although the addition of yttrium remarkably weakens the effect of stress concentration at the tip of corrosion pits in loading process.     

Electrochemical behaviour of Ti–Ni SMA and Co–Cr alloys in dynamic Tyrode’s simulated body fluid

The electrochemical behaviour of Ti–Ni shape memory alloy and Co–Cr alloys were investigated in dynamic Tyrode’s simulated body fluid on a Model CP6 Potentiostat/Galvanostat. The results indicated that, for all alloys, the anodic dissolution and the pitting sensitivity increased with the flow rate of the Tyrode’s solution increasing while the open-circuit potentials and pitting corrosion potentials decreased with the Tyrode’s solution increasing. Pitting corrosion of Ti–Ni alloy was easier than Co–Cr alloys. Since the solution’s flow enhanced oxygen transform and made it easy to reach the surface of electrodes, the plateau of oxygen diffusion control was diminished. All these indicated that the cathodic reduction and the corrosion reaction, which was controlled by the electrochemical mass transport process, were all accelerated in dynamic Tyrode’s simulated body fluid.     

Influence of surface roughness on the corrosion behaviour of magnesium alloy

In this study, the influence of surface roughness on the passivation and pitting corrosion behaviour of AZ91 magnesium alloy in chloride-containing environment was examined using electrochemical techniques. Potentiodynamic polarisation and electrochemical impedance spectroscopy tests suggested that the passivation behaviour of the alloy was affected by increasing the surface roughness. Consequently, the corrosion current and the pitting tendency of the alloy also increased with increase in the surface roughness. Scanning electron micrographs of 24 h immersion test samples clearly revealed pitting corrosion in the highest surface roughness (Sa 430) alloy, whereas in the lowest surface roughness (Sa 80) alloy no evidence of pitting corrosion was observed. Interestingly, when the passivity of the alloy was disturbed by galvanostatically holding the sample at anodic current for 1 h, the alloy underwent high pitting corrosion irrespective of their surface roughness. Thus the study suggests that the surface roughness plays a critical role in the passivation behaviour of the alloy and hence the pitting tendency.     

Influence of mechanical surface treatments on fatigue strength of commercial purity titanium

A comparative study of two mechanical surface treatments of shot peening (SP) and cold rolling (CR) on fatigue strength of commercial purity titanium has been conducted. The treated surface was characterized by using transmission electron microscopy (TEM), X-ray diffraction (XRD), and surface roughness measurements. Experimental result shows that SP and CR increased the fatigue strength of commercial purity titanium, and moreover, the following results were obtained: (1) The improvement of fatigue strength is related to the formation of deformation twins in strengthened layer. Before or after fatigue, the samples strengthened by SP or CR not only have twin shape and number change, but also have twin interactions in the SP and twin-grain boundary interactions in the CR. (2) XRD measurement demonstrated that SP leads to surface compressive residual stress are much higher than those after CR. Surface compressive residual stress has higher relaxation in the SP than in the CR condition during cyclic loading, then the surface compressive residual stress has the same values after fatigue deformation. (3) Surface roughness resulting from SP is ten times of CR.     

Effects of 5-(3-aminophenyl)-tetrazole on the inhibition of unalloyed iron corrosion in aerated 3.5% sodium chloride solutions as a corrosion inhibitor

The effects of 5-(3-aminophenyl)-tetrazole (APT) on the inhibition of unalloyed iron corrosion in aerated 3.5% NaCl solutions as a corrosion inhibitor have been studied using open circuit potential (OCP), cyclic potentiodynamic polarization (CPP), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) measurements. The inhibited iron surface was characterized by scanning electron spectroscopy (SEM) and energy dispersive X-ray (EDS) investigations. The OCP showed positive shifts of potential in the presence of APT and the increase of its concentration. CPP and CA measurements indicated that APT molecules decrease the pitting and uniform corrosions through decreasing the pitting and absolute currents, and corrosion rate as well as shifting the corrosion and pitting potentials of iron towards the noble values. EIS plots revealed that APT increases the surface and polarization resistances of iron. SEM/EDS investigations proved that the inhibition of iron corrosion in NaCl containing APT solutions is achieved by the adsorption of APT molecules onto iron to preclude the dissolution process by blocking the active sites on its surface.     

Shot peening of plasma nitrided steel for fretting fatigue strength enhancement

Abstract

The potential of fretting fatigue strength enhancement by a duplex surface engineering process involving shot peening of plasma nitrided steel, termed duplex SP/PN, is demonstrated. Specimens of 709M40 steel were individually plasma nitrided, shot peened, or duplex SP/PN treated. Fretting fatigue properties of the surface engineered specimens were evaluated. Surface roughness, residual stress, and hardening effect following the various surface treatments were examined and compared. It has been found that the duplex treatment can significantly improve the fretting fatigue strength of the investigated low alloy steel. Under the present testing conditions, the duplex SP/PN treatment increased the fretting fatigue strength (at 10⁷ cycles) of 709M40 steel by more than 70% relative to the nitrided, 120% to the shot peened, and 500% to the untreated steel. The improvement has been explained in terms of the significantly increased surface hardness and compressive residual stress in the near surface region following the duplex SP/PN treatment. By analysing the stress distributions in a shot peened surface, the influence of surface roughness on fretting fatigue strength is also discussed.     

Evolution of residual stress redistribution associated with localized surface microcracking in shot-peened medium-carbon steel during fatigue test

Shot peening (SP) is a conventional method used for improving material properties, especially fatigue strength, through work hardening and the induction of compressive residual stress (CRS) near the surface by plastic deformation. However, CRS is redistributed and relaxed by the occurrence of physical discontinuities such as microcracks. In this study, the effects of residual stress redistribution and relaxation during the fatigue life associated with microdamages on the properties of a material were considered. To this end, annealed medium-carbon steel was treated with SP at three levels of peening intensity to investigate the effects on fatigue life and residual stress distributions. Rotating–bending fatigue tests were carried out to clarify the fatigue life distributions, and X-ray diffraction and scanning electron microscopy were used for residual stress and full-width at half-maximum (FWHM) values, microscopic inspections, respectively. The results indicate that microcracks at the treated surface significantly influenced stress redistribution, depending on the initial residual stress distribution at the surface. Moreover, when the induced CRS was relaxed during mechanical loading, these microcracks caused fatigue life degradation regardless of peening treatment. The effects of surface microcracks on stress redistribution and relaxation were discussed and a valuable range of peening conditions of used material was proposed.     

Influence of laser surface alloying with chromium and nickel on corrosion resistance of type 304L stainless steel

Abstract

The influence of laser surface alloying (LSA) with Cr and Cr + Ni on the corrosion behaviour of type 304L stainless steel (SS) was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in chloride (0·5M NaCl) and acidic (1 N H₂SO₄) media. Surface alloying was carried out by laser cladding type 304L SS substrate with premixed powders of AISI type 316L SS and the desired alloying elements. The results indicated that Cr surface alloyed specimen exhibited a duplex (γ + α) microstructure with Cr content of ～24 wt-%, whereas Cr + Ni surface alloyed specimen was associated with austenitic microstructure with Cr and Ni contents of ～22 wt-% each. The potentiodynamic polarisation results in chloride solution indicated that LSA with Cr + Ni considerably enhanced the pitting corrosion resistance compared with LSA with Cr alone. In acidic media, such beneficial effects were not observed. Electrochemical impedance spectroscopy results showed an increase in semicircle arc for both chloride and acidic media for both Cr and Cr + Ni clad samples indicating improvement in the oxide film stability compared with untreated specimen. The polarisation resistance was higher and capacitance values of the laser clad specimen were lower than those in the untreated specimen. The microstructural changes and compositional variations produced by LSA are correlated to the corrosion behaviour.     

Microelectrochemical Studies of Pit Initiation on High Purity and Ultra High Purity Aluminum

The onset of pitting on high purity and ultra high purity aluminum has been studied in the micrometer range by using a microelectrochemical cell in order to evaluate influences that induce localized corrosion. Microelectrochemical tests on polished samples of Al 99.999 and Al 99.9999 in 1 M NaCl showed that pitting at large areas occurred at several hundreds mV more negative than at small areas, suggesting that even very pure Al contains weak points. On rough surfaces weak points are more “activated” than on smooth surfaces. Locally measured pitting potentials of polished samples were shifted to more positive values than those measured on ground samples. Larger areas contain always small scratches, even on polished samples. Hence, the surface roughness showed a minor influence on the onset of pitting on larger areas. Microelectrochemical measurements performed on pure Al 99.99 with small amounts of copper showed that a copper content below 30 ppm is too small to have a beneficial or detrimental effect, whereas a copper content above 300 ppm probably leads to the formation of small inclusions or precipitations that acted as preferential corrosion initiation sites. A copper content around 100 ppm showed a beneficial effect on very small areas but not on larger ones.     

Passivity and pitting of austenic stainless steel AISI 316 in the CH3OH-H2SO4 system

The passivity and pitting behaviour of AISI 316 stainless steel in methanol containing different concentrations (0.001 to 1 M) of H₂SO₄ has been investigated by an electrochemical method. Higher concentrations of sulphuric acid have been found to facilitate the passivation process and influence the passivity current and passivation range. The studies reveal the peculiarities of steel pitting and predicts the protective properties of the surface by a process of repassivation in this system. The kinetics of pitting, morphology of the pits and repassivation of the surface have been investigated for this steel. The beneficial effect of molybdenum as an alloying element of steel, on corrosion behaviour has been noticed.     

Effects of mechanical surface treatments on fatigue performance of extruded ZK60 alloy

The effects of shot peening (SP) and ball‐burnishing (BB) as mechanical surface treatments on the high cycle fatigue (HCF) performance of the extruded high‐strength magnesium alloy ZK60 were investigated. Various Almen intensities (0–0.33 mmA) and burnishing pressures (0–100 bar) were applied to reach optimum HCF performance of SP and BB conditions, respectively. SP and BB not only result in marked changes in the near surface microstructures but also in pronounced strengthening and in generation of residual compressive stresses in the near‐surface regions. Both SP and BB lead to significant improvements in the fatigue life of the electropolished reference (EP) provided that suitable process parameters were applied. Utilizing optimum process parameters, the 10⁷ cycles fatigue strength increased from 150 MPa (EP) over 175 MPa (SP) to 200 MPa (BB).     

Pitting behavior of SiCp/2024 Al metal matrix composites

The effects of the volume fraction of SiC particulate reinforcements and the concentration of chloride ions in solution on the localized corrosion characteristics of SiCp/2024 Al metal matrix composites (MMC) were investigated. A scanning micro reference electrode (SMRE) technique was employed to study the dynamic process of pitting initiation and development on the surface of the composites at open-circuit potential. Potentiodynamic polarizations were performed to characterize the electrochemical behavior of the MMCs. The morphology of the localized attack on the MMC sample after corrosion tests were examined by scanning electron microscopy (SEM). The results of electrochemical measurement showed that the composites were less resistant to pit initiation than the corresponding unreinforced metrix alloy. Increase in the volume fraction of SiCp reinforcement in the SiCp/2024 Al composites resulted in a significant decrease of pitting potential. In situ potential mapping of active centers on the surfaces of the composites revealed that local breakdown of passivity and initiation of micro pitting corrosion could take place even at an open-circuit potential which was more negative than the pitting potential, and the number of active centers on the surfaces of the composites increased as the volume fraction of SiC particulates in MMCs increased. Micro-structural analysis indicated that pitting attack on the composites mainly occurred at SiCp-Al interfaces or inclusions-Al interfaces.     

Effect of aging treatment on microstructure and corrosion behavior of Al-Zn-Mg aluminum alloy in aqueous solutions with different aggressive ions

The microstructure and corrosion behavior of Al-Zn-Mg alloy(namely 7×××)after natural aging treatment(NAT)and artificial aging treatment(AAT)in aqueous NaCl solutions containing different aggressive ions have been investigated in current work.Results of microstructure characterization demonstrate that the aging treatment has a great influence on the grain size and precipitates.The grain size is relatively sizeable and no evident precipitates are observed in alloy after NAT comparable with that after AAT.The electrochemical corrosion behavior of alloy was studied by polarization curve and electrochemical impedance spectroscopy(EIS).The corrosion potential(E_corr)of the aluminum alloy is more negative in 3.5 wt.%NaCl containing 0.052 wt.%NaHSO₃solution than that in 3.5 wt.%NaCl solutions with or without 0.907 wt.%NaHCO₃.Charge transfer resistance(Rct)results reveal that alloy after AAT has an enhancement of corrosion resistance compare with that after NAT.With the immersion time increasing,mostly pitting spreads over the surface of the alloy only in NaCl solution,whereas exfoliation corrosion mainly occurs in NaCl solutions containing NaHSO₃or NaHCO₃.     

Low-temperature carburised AISI 316L austenitic stainless steel: Wear and corrosion behaviour

Low temperature carburising (LTC) is a thermochemical treatment designed so as to achieve a good combination of wear and corrosion resistance in stainless and duplex steels. In this work, the influence of LTC on both corrosion and dry sliding behaviour of AISI 316L was investigated. LTC significantly enhanced surface hardness, due to the formation of the carbon-supersaturated S-phase. Consequently, the wear behaviour (evaluated against different countermaterials) improved, due to increased resistance to plastic deformation, as well as to decreased tendency towards adhesion. In order to evaluate the corrosion behaviour, electrochemical measurements were performed both in conventional environments and in reference conditions for the food industry. The results showed a significantly improved corrosion resistance in chloride environments, where the formation of a C-rich surface layer ennobles the treated steel, even though pitting corrosion was observed at very high anodic potentials. Conversely, the treated steel showed comparable (in acetic acid) or worse (in a sanitising solution) behaviour than the untreated one. In sulphuric acid the treated steel did not passivate, but it corroded at a limiting current density much lower that the critical current density for AISI 316L passivation.