Unpeened Surface Research Articles

Effect of process parameters involved in SWJP on surface integrity and biocompatibility characteristics of AZ31B Mg alloy

This study mainly investigates the influence of combined parametric conditions employed in submerged waterjet peening for biomedical applications. On measuring the microhardness and roughness on the peened surface, the finding corresponded directly with the strengthening of subsurface layers and different parametric combinations. The optimum parametric combination was determined by TOPSIS, considering microhardness and surface roughness as input. Under optimal parametric combinations, surface morphology and topography analysis, XRD analysis, corrosion behaviour examination, and cell morphology and viability analysis were studied and compared to unpeened surface. From the experimental results, the water bubbles create impulsive pressure that impacts the AZ31B Mg alloy's surface, promoting plastic deformation in the subsurface, enhancing the surface morphological and topographical characteristics, and making the alloy more suitable for bioimplant manufacture. Following that, these samples underwent an electrochemical corrosion study, which indicates the peened region's corrosion rate was reduced considerably compared to the unpeened region. In the course of biocompatibility analysis, the non-toxic nature of the peened region in the MG63 cell line was observed after 24 and 72 h.

Investigation of process parameters influence in waterjet peening of AZ31B-Mg alloy for bioimplant applications

The main objective of the study is to investigate the effects of parameters standoff-distance, waterjet pressure, traverse speed and number of passes used in the waterjet peening process on AZ31B-Mg alloy to assess surface grain refinement's potential and to improve the alloy's functionality in the biological environment, followed by the study of enhancement in hardening and roughening effect on AZ31B-Mg alloy surface. The optimal parametric condition required for investigating surface texture, X-ray diffraction analysis and electrochemical corrosion analysis with simulated body fluid was obtained through the multi-criteria optimisation, Technique for Order of Preference by Similarity to the Ideal Solution, which employs microhardness and surface roughness as response variables. The obtained results show significant improvement in mechanical and corrosion-inhibiting features of AZ31B-Mg alloy. Further, the biocompatibility of waterjet peened and unpeened surfaces was examined using 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-tetrazolium bromide assay and fluorescence analysis with MG63 cell lines.

Laser peening induced mitigation of severe pitting corrosion in titanium stabilized 321 steel

Titanium stabilized 321 stainless steel, as a structural component in oil and petrochemical industries, often susceptible to severe pitting corrosion under aggressive corrosion environments (sulfuric acid and chloride ions). This study explores the applicability of laser peening without protective coating (at 6 GW cm−2 intensity and 75 % pulse overlap rate) to enhance the electrochemical stability of 321 steel against such severe pitting corrosion. Laser peening altered surface-mechanical and electrochemical properties were investigated by AFM, SEM and potentiodynamic polarization/impedance spectroscopy techniques. On the account of laser peening induced compressive residual stress (-408 MPa) hardness enhancement, a stable/dense and uniform oxide film was formed on 321 steel, subsequently mitigating the pitting corrosion. By contrast, unpeened surface exhibited more number of wider and deeper merged pits (maximum diameter of ∼ 2197 μm). Laser peening promoted oxidation has been confirmed via elemental composition analysis. A twentyfold enhancement in charge transfer resistance and improved oxide film resistance, attributed from the laser peening increased defect density as demonstrated by the improved hardness, evidenced a highly electrochemical stability on 321 steel.

Surface and Tribological Properties of Powder Metallurgical Cp-Ti Titanium Alloy Modified by Shot Peening

The present study reveals for the first time the dry sliding wear behavior of a powder metallurgical pure titanium alloy (Cp-Ti) modified by shot peening. Cp-Ti samples were manufactured via powder metallurgy, and then their surface and subsurface features were modified using a custom-made, fully automated shot-peening system. The texture isotropy rate and the highest orientation angle of the shot-peened samples were 71.5% and 36°, respectively. The Abbott curves of the shot-peened surfaces revealed that the most common areal roughness value was 5.177 μm, with a frequency of 8.1%. Shot-peened surfaces exhibited an ~20% lower wear rate than unpeened surfaces under dry sliding wear, whereas the coefficient of friction was the same for both surfaces. Micro-ploughing, micro-cutting, oxidation, and three-body abrasion wear mechanisms were observed on the shot-peened and unpeened surfaces. High resolution 3D surface topographies of worn unpeened and shot-peened surfaces revealed micro-scratches and inhomogeneities along wear tracks, which are indicative of three-body abrasion mechanisms during contact. In addition, vertical and horizontal microcracks were visible just beneath the wear track, suggesting a clear indication of plastic deformation during contact. The cross-sectional hardness maps of shot-peened samples revealed the formation of a work-hardened surface layer with shot peening, which improved the wear resistance. These findings support that shot peening can be a useful tool to modify the surface and tribological properties of powder metallurgical Cp-Ti alloys.

Improvement of friction characteristics of cast aluminum-silicon alloy by laser shock peening

Piston friction is the primary cause of power loss in automotive engines and thus reducing the friction on piston surface is crucial for the improvement of engine performance. In this work, the effects of laser shock peening on friction characteristics of JIS-AC8A aluminum-silicon alloy, a piston material for automotive engines, were investigated experimentally. Laser shock peening was carried out using a Nd:YAG laser (wavelength = 532 nm, pulse width = 8 ns) at the conditions of laser intensity 4 GW/cm2, overlapping ratio 50% and spot diameter 2.06 mm with no protective coating. When laser shock peening was applied, surface hardness increased by 22% and significant enhancement of compressive residual stress was achieved. Friction coefficients of the laser peened surface decreased by 19%, 41% and 45% from those of unpeened surface at the 50, 100 and 150 N normal load conditions, respectively, demonstrating the effectiveness of laser shock peening in reducing surface friction. Also, the mass loss by wear decreased by 94%.

Corrosion Study on Laser Shock Peened 316L Stainless Steel in Simulated Body Fluid and Chloride Medium

In the present work, corrosion behavior of laser shock peened 316 L stainless steel was evaluated and compared with unpeened specimen in Hank solution (simulated body fluid) and chloride medium. Laser shock peening (LSP) was used for surface modification using Q-switched Nd-YAG laser. In the corrosion study, different electrochemical experiments such as open circuit potential (OCP)-time measurements, potentiodynamic anodic polarization, electrochemical impedance spectroscopy (EIS) measurements were performed in 0.5 M NaCl and Hank solution to examine the corrosion resistance of unpeened and laser peened 316 L SS specimens. The results of corrosion study demonstrated that LSP improved the pitting corrosion resistance despite marginally higher roughness (Ra = 1.01 μm) for laser peened surface than unpeened surface (Ra = 0.54 μm). Improvement in pitting corrosion resistance was indicated by ennoblement of OCP, more noble (anodic) value of pitting potential (Epit) and lower corrosion current density (Icorr). Conductivity and pH of NaCl and Hank solution increased after every polarization experiments which indicate the ionic dissolution during corrosion experiments. Qualitative analysis of Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) results also showed higher arc radius (higher polarization resistance) for laser peened specimen than unpeened, which is indicative of more protective passive film and hence improved corrosion resistance. Further, microstructural examination after polarization experiments in both NaCl and Hank solution showed less pitting sites in laser peened specimens than unpeened surfaces. These microstructural observations are in line with the results obtained in polarization experiments. The results of the study are important with respect to medical implants and their biocompatibility in the human body.

Influence of shot peening on surface quality of austenitic and duplex stainless steel

In the present investigation, an attempt has been made to enhance the surface quality of austenitic stainless steel 316L and duplex stainless steel 2205 through shot peening process. The study mainly focuses the surface morphology, microstructural changes, surface roughness and microhardness of the peened layers. Metallography analysis was carried out and compared with the unpeened surface characteristics. As result of peening process, surface recrystallization was achieved on the layers of the peened samples. It was found that shot peening plays significant role in enhancing the surface properties of 316L and 2205. Particularly it has greater influence on the work hardening of austenitic stainless steel than the duplex stainless steel due to its more ductility nature under the investigated shot peening parameters. The findings of the present study will be useful with regard to the enhancement of surface texture achieved through peening.

Laser shock peening without coating induced residual stress distribution, wettability characteristics and enhanced pitting corrosion resistance of austenitic stainless steel

Low energy laser shock peening without coating (LSPwC) was conducted on AISI 304 austenitic stainless steel specimens with varying pulse densities or overlapping. Highest magnitude of compressive residual stress (CRS) was achieved for an optimized pulse density of 2500 pulses/cm2 (75% overlapping). The 2-D and 3-D topographical analysis were indicative of the fact that controlled roughening of the surface was achieved after the LSPwC process. After the LSPwC process, the hydrophilic unpeened surface was converted into the hydrophobic surface, thus decreasing the wettability characteristics of the surface. The X-ray diffraction (XRD) results reveal that there is a beginning of the martensite transformation and the rise in the intensity value of the peaks after LSPwC indicates the presence of compressive residual stresses induced in the specimen. The optical microscope and high-resolution transmission electron microscope results provided evidence of grain refinement and deformation induced refinement features such as multidirectional mechanical twinning, dislocations lines, micro shear cells and stacking faults in the near and sub-surface areas. The average hardness value of the LSPwC specimens was found to be increased by 28% more than the untreated specimen. The potentiodynamic polarization revealed that there was a considerable amount of increase in the pitting corrosion resistance after the LSPwC process, thus, supporting to extend the fatigue life of the specimen. The electrochemical impedance spectroscopic (EIS) analysis depicts that the LSPwC process supports the formation of the strong passivation layer in 3.5% NaCl solution.

Comparison of Chromia Growth Kinetics in a Ni-based Superalloy, with and without Shot-peening

The effect of shot-peening on the oxidation in air of the Ni-based superalloy RR1000 has been investigated over the temperature range 700–800°C. The surface oxide in both peened and un-peened conditions consisted of isolated grains of rutile on the outermost surface beneath which was a protective Ti-doped chromia scale. Internal oxidation of aluminium occurred within the alloy with the formation of alumina particles within a γ′ (nominally Ni3(Al,Ti)) denuded zone but the morphology of the sub-surface oxides differed between the two surface conditions examined. The kinetics of thickening of the chromia layer were sub-parabolic in most cases but closely approached parabolic behaviour for the un-peened surface condition at 800°C. An enhancement in the rate of chromia growth was found for both surface conditions compared with a Ti-free chromia layer. This enhancement has been attributed to increased Cr ion diffusion as a result of Ti-doping of the chromia layer but the effect is reduced over time because of Ti-depletion in the alloy. At 800°C, in the un-peened condition, the reduction in growth rate with exposure time is much less marked and this effect seems to be associated with the formation of a (Ti,Ta)O2 phase beneath the chromia layer.

Laser peening without coating on aluminum alloy Al-6061-T6 using low energy Nd:YAG laser

The present study investigates the effect of laser peening without coating on aluminum alloy Al-6061-T6 with a 300mJ infrared laser. The surface topography, microstructure, surface topography, surface residual stress and micro-hardness of peened and unpeened surfaces were studied. The study shows that laser peening without coating can significantly improve surface compressive stress and micro-hardness with trivial increase in surface roughness. Microstructure evaluation confirmed there was no near surface solidification after LPwC.

Effect of laser shot peening on precipitation hardened aluminum alloy 6061-T6 using low energy laser

Mechanical properties of engineering material can be improved by introducing compressive residual stress on the material surface and refinement of their microstructure. Variety of mechanical process such as shot peening, water jet peening, ultrasonic peening, laser shot peening were developed in the last decades on this contrast. Among these, lasers shot peening emerged as a novel industrial treatment to improve the crack resistance of turbine blades and the stress corrosion cracking (SCC) of austenic stainless steel in power plants. In this study we successfully performed laser shot peening on precipitation hardened aluminum alloy 6061-T6 with low energy (300 mJ, 1064 nm) Nd:YAG laser using different pulse densities of 22 pulses/mm 2 and 32 pulses/mm 2. Residual stress evaluation based on X-ray diffraction sin 2 ψ method indicates a maximum of 190% percentage increase on surface compressive stress. Depth profile of micro-hardness shows the impact of laser generated shock wave up to 1.2 mm from the surface. Apart from that, the crystalline size and micro-strain on the laser shot peened surfaces have been investigated and compared with the unpeened surface using X-ray diffraction in conjunction with line broadening analysis through the Williamson–Hall plot.

Investigation of Contact Fatigue of High Strength Steel Gears Subjected to Surface Treatment

In this paper the contact fatigue resistance of gearwheel teeth, subjected to shot-peening treatment, was investigated experimentally and analytically. The main objective was the evaluation and prediction of fatigue crack initiation, propagation, direction, and rate. A specially designed experimental rig was used to test a number of spur gears with the following characteristics: (a) unhardened, thermally untreated unpeened surfaces, (b) thermally treated unpeened surfaces, (c) unhardened peened surfaces, and (d) thermally treated peened surfaces. The theoretical model assumed initiation and propagation of surface cracks of gears operating in the elastohydrodynamic lubrication regime while loading was due to simultaneous rolling and sliding. Finite element modeling was used for the calculation of the stress field at the gear teeth. Comparison of the experimental and analytical results showed considerable improvement in the contact fatigue strength of thermally treated gear teeth and especially those that underwent shot peening, which increased surface durability. The residual stresses induced by shot peening are mainly effective in stopping microcrack propagation. When shot peening is applied on thermally untreated gear teeth surface, it increases the contact fatigue life of the material by 17% at 7 × 105 loading cycles. If shot peening is applied on carburized gear teeth surfaces, it increases the surface fatigue life by approximately 8% at 106 cycles. Contact fatigue and eventual pitting are treated as a normal consequence of the operation of machine elements. To study this failure process different types of testing machines have been designed. The purpose of this paper is the presentation and evaluation of a new design experimental rig for studying contact fatigue damage of gear teeth subjected to different load patterns.

Residual stress relaxation in shot peened high strength low alloy steel

The relaxation of residual stress in a shot peened high strength low alloy steel in response to applied loads has been investigated for both single shot and cyclic loading. The near surface residual stress was measured predominantly by X-ray diffraction (first 10 μm) but supplemented for some conditions by measurements using high resolution, fine increment hole drilling to yield depth profiling. The compressive residual stress showed a peak at ∼50 μm below the surface. Under single shot compressive loading, relaxation became significant when the combined surface and applied compressive stresses exceeded the proportional limit of the unpeened surface. The steel showed good resistance to residual stress relaxation under cyclic loading conditions, with only a 13% decrease despite cyclic load amplitudes at 50% of the 0˙2% proof stress in compression. Significant relaxation did occur at applied stress amplitudes of ∼80% of the 0˙2% proof stress in compression. While large single shot or cyclic stresses would represent rare occurrences, they could lead to a marked loss in beneficial compressive stress and should be accounted for in structural integrity assessment.

Fatigue cracking in high-cycle- and very-high-cycle-fatigue areas of peened and unpeened Al-based alloys because of fretting damages

In-service fretting damage influenced the fatigue crack origination in materials under the damaged surface for the very-high-cycle fatigue. Specimens of BS L65 Al–Cu alloy with peened and unpeened surfaces tested under cyclic tension and static compression had the crack initiation on the surface because of fretting damage. The static compression was used to reproduce the fretting damage on the specimen surface. The special methodology was applied to the crack growth analysis in the case of the biaxial stress-state with R = −1.0 for the investigated material. The fretting influence on the fatigue crack propagation was estimated on the basis of the functional correction, F(fr), for the stress intensity factor, K I. The fretting damage influence on the fatigue crack growth was briefly discussed regarding the kinetic curves reproduced from the fractographic analysis. The bimodal S–N curve was introduced to describe the influence of fretting damages on the behaviour of materials in high- and very-high-cycle-fatigue areas.

Shot peening effect on aluminide diffusion coating formation on alloy steels at low temperatures

This study reveals the enhancement effect of shot peening on the kinetics of aluminide diffusion coating growth on alloy steels at temperatures below 675 °C. This enhancement effect is larger at lower processing temperatures, but progressively diminishes as temperature increases. The activation energy for coating growth in the peened surface is lower than in the unpeened surface, indicating that the influence of diffusion through grain boundary and/or microstructure defects is larger in the peened surface than in the unpeened surface.

Influence of plastic deformation upon the half-width of engineering metallic materials in hard state

The half-width values of the X-ray diffraction profiles are frequently used to characterize the static strength of a strengthened surface, or the depth distribution of this mechanical parameter, in a strengthened surface layer, especially in a shot-peening affected layer. However, for the unpeened surface and the base material of the shot-peened specimen of an alloy steel treated in hard state, the experimental results shown in this article indicate that uniaxial tensile or compressive plastic deformation increases the yield strengths while it decreases the half-width values. The half-width values of the shot-peened surface and surface layer greatly decrease, whereas the yield strength of this surface remarkably increases. Accordingly, in the authors' opinion, the half-width values could not correctly describe the static strengths of hard metallic materials, and, contrary to the viewpoint put forward by a lot of researchers, the shot-peened surfaces of such materials are work hardened instead of work softened. A model demonstrating that plastic deformation reduces the half-width values by decreasing the second kind internal stresses is developed.

SHORT CRACK GROWTH BEHAVIOUR UNDER VARIABLE AMPLITUDE LOADING OF SHOT PEENED SURFACES

Abstract— The effect of variable amplitude loading on the initiation and propagation of fatigue cracks from shot peened and unpeened surfaces was investigated. The results confirm the well‐known effect of shot peening, i.e. the increase of fatigue life in comparative tests under constant amplitude loading. Shot peening increases the time to crack initiation and also decreases crack propagation rate, particularly in the earlier stages of growth.Variable amplitude tests were of various kinds: (i) half‐life at one stress level, followed by testing at the second stress level until failure, (ii) sequence loading at two stress levels, and (iii) a different number of overload cycles (1, 10, 100) following a given number of base amplitude cycles.The results of the two amplitude tests showed different behaviour depending on whether the first half‐life cycles were of higher or lower stress levels. These results are explained in terms of crack density and crack coalescence. The extent of damage accumulation during block loading depends on the stress levels employed. The benefits of shot peening are compromised by high stress levels.Results from the overload tests showed that the interpretation of results is not straight forward. A combination of the theories of crack retardation and microstructural‐fracture mechanics seems a necessary prerequisite in an explanation of the resultant behaviour.