Surface Strengthening Process Research Articles

Effect of surface high-density twinned structure on the fatigue crack initiation of Ti–6Al–4V

Fatigue crack initiation sites would transfer from surface to subsurface of metal specimens through shot peening (SP) or laser shock surface strengthening. This phenomenon is typically attributed to the introduction of compressive residual stress on the surface of the specimen during the surface strengthening process. In this study, Ti–6Al–4V specimens featuring a high-density twinned structure on their surfaces, which eliminated compressive residual stress, were fabricated. Fatigue test result revealed that fatigue crack initiation sites in specimens with twinned structures were predominantly located in the subsurface, near the junction area between the twinned structure and the matrix. This phenomenon was accompanied by an increase in fatigue life. The microstructural features in the fatigue crack source were analyzed using focused ion beam (FIB) coupled with electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). It was observed that primary crack formed in badly oriented grains that are difficult to initiate dislocation slip according to the Schmid factor (SF). Nonetheless, TEM observations revealed the activation of basal <a> dislocations in these grains, potentially leading to crack nucleation along basal slip bands and subsequent propagation along the basal plane. Our results suggest that a high-density twin structure on the surface can enhance the fatigue strength of Ti–6Al–4V by transferring the fatigue crack source to the subsurface.

Surface modification and its effect on fatigue performance of nickel-based superalloy treated by ultrasonic surface rolling process

In the process of material removal, the metallurgical and mechanical properties of the surface layer of GH4169 superalloy have changed significantly, which has an adverse effect on the fatigue life. Therefore, surface strengthening process is indispensable as the last process of part manufacturing. In the paper, once and four times of ultrasonic surface rolling process (USRP) are carried out on GH4169 superalloy. The results show that USRP treatment improves the degree of grain refinement, the proportion of low angle grain boundaries (LAGBs) and dislocation density of the surface layer. The USRP treatment with four times forms a preferred orientation (〈101〉//TD) on the surface layer, and, the grain refinement mechanism is revealed. Moreover, USRP treatment changes the crack propagation mode from transgranular to intergranular. The fatigue tests indicate that the fatigue life of samples treated with USRP with once and USRP with four times increase by 55.6% and 138.6%, respectively. It can be inferred that the surface topography, residual stress, microstructure and degree of working hardening are the main factors affecting the low cycle fatigue properties of USRP treated samples.

Near-in-situ investigation of surface crack healing and strengthening mechanism of Ti-43.5Al-4Nb-1Mo-0.1B alloy by novel electroshock treatment

The Ti-43.5Al-4Nb-1Mo-0.1B (TNM) alloy is prone to emerge surface cracks and other defects during processing and service, which will deteriorate the mechanical properties and fatigue life of the materials. In this work, a novel electroshock treatment (EST) for the surface crack repair and strengthening process has been proposed. Combined with near-in-situ experiments and theoretical analysis, the influence law and healing mechanism of different current densities and EST modes on the healing of surface cracks in the alloy were investigated. The scanning electron microscopy (SEM) near-in-situ observation reveals that the main factors influencing the healing of surface cracks relate to the time of energization, the current density, and the cracking angle. Meanwhile, the crack tip and locally bulging deflected positions are more likely to be healed in a short time under continuous EST mode and medium current density (75 A/mm2). The surface crack width at the calibration position decreases from 0.47 μm and 2.23 μm to 0 μm and 0.03 μm, respectively. The electron back scattered diffraction (EBSD) results show that the dislocation density increases around the surface crack healing area, and the dislocation density at the crack tip and away from the healing region is decreased and uniformly distributed after EST, which can further suppress the generation and propagation of the crack, resulting in a 20.1 % increase in tensile strength and a 40.3 % increase in fracture strain. Considering the coupling influence of the thermal and non-thermal effects of EST, a two-stage progressive atomic diffusion of thermal compressive stresses and crack bonding interfaces has been proposed to describe the evolution process of surface crack healing during EST.

Strategy and mechanism to improve the fatigue properties of Ti6Al4V ELI alloy by microstructure modulation combined with surface strengthening process

The hot isostatic pressing (HIP) technique allows obtaining powder metallurgy Ti6Al4V parts with high densities, but its microstructure is not ideal, resulting in a lower fatigue life than expected. In this work, the ideal microstructure was obtained by heat treatment, but this process produces thermally induced porosity (TIP). After heat treatment, the fatigue strength of the bimodal Ti6Al4V alloy reached 680 MPa at 107 cycles, whereas the HIP Ti6Al4V alloy with a fully equiaxed microstructure was lower than 590 MPa. This is because the bimodal Ti6Al4V alloy has better resistance to fatigue crack initiation. This result confirmed that TIPs do not have a significant negative impact on fatigue properties. To further improve the fatigue performance, this study induced gradient nanostructured (GNS) in Ti6Al4V alloy by ultrasonic surface rolling process (USRP), and the fatigue strength of the USRP-treated specimen reached 750 MPa at 107 cycles. Combining microstructure observations and theoretical analysis, the surface hardening mechanism of the USRP-treated Ti6Al4V alloy was quantitatively described. The results showed that the plastic deformation layer is approximately 130 μm, but grain boundary strengthening maintained its effect to a depth of about 200 μm whilst dislocation strengthening maintained its effect to a depth of approximately 350 μm. Under the comprehensive effect of surface hardening and compressive residual stress field, the fatigue crack initiation period of the USRP specimen was significantly extended. This may be the primary reason behind the enhanced fatigue performance obtained through USRP treatment.

A simulation modeling methodology considering random multiple shots for shot peening process

Abstract Shot peening (SP) process is a typical surface strengthening process for metal and metal matrix composites, which can significantly improve the fatigue life and strength. The traditional SP simulation model falls short as it only takes into account one or a few shots, proving insufficient for accurately simulating the entire impact process involving hundreds of shots. In this study, a random multiple shots simulation modeling methodology with hundreds of random shots is proposed to simulate the impact process of SP. In order to reduce the simulation error, the random function Rand of MATLAB is used to generate the shot distributions many times, and the shot distribution closest to the average number is selected and the three-dimension parametric explicit dynamics numerical simulation model is built using ABAQUS software. Orthogonal experiments are carried out to investigate the influences of shot diameter, incident impact velocity, and angle on the residual stress distribution, roughness, and specimen deformation. Results showed that the average relative errors of maximum residual compressive stress, roughness, and deformation of specimen between simulation model and experimental value are 30.99, 16.14, and 16.73%, respectively. The primary factors affecting residual stress and deformation is shot diameter, and the main factor affecting roughness is impact velocity.

Multi-objective optimization of the subsurface residual stress field of TC4 alloy in machine hammer peening

Machine hammer peening (MHP) is a surface strengthening process by introducing residual compressive stress at the part's surface. Many indicators of residual stress, such as the surface residual compressive stress and depth of residual compressive stress, significantly impact the fatigue performance of parts and thus need to be optimized. The present study focuses on two main problems in multi-objective optimization of the subsurface residual stress field induced by MHP: the relationship between a specific residual stress field and a combination of impact velocity and indentation distribution and how to implement the MHP process that could produce such a combination. The latter problem was first addressed by a kinematic model of the MHP device, which can optimize the MHP device input parameters to achieve the target axial impact velocity. For the former problem, a finite element model was first established to simulate the residual stress field induced by MHP. Then, the response surface method was used to optimize the simulated residual stress field with multiple objectives. Based on the above two models, we can obtain the appropriate MHP parameters to induce a target subsurface residual stress field. A verification experiment was carried out, and the residual stress field obtained was relatively close to the theoretical one, with a deviation of 1.7 % for the depth of residual compressive stress and 13.8 % for the surface residual compressive stress.

State of the Art and Perspectives on Surface-Strengthening Process and Associated Mechanisms by Shot Peening

Shot peening is a surface-strengthening process that is widely used in various industries, such as aerospace, automotive, and biomedical engineering. The process involves the impact of small, spherical media, called shots, onto the surface of a material, resulting in residual compressive stress and improved surface properties. This review aims to provide an overview of the state of the art and perspectives on surface strengthening by shot peening. The review covers various aspects of shot peening, including process parameters, shot materials, and quality control techniques. The advantages and limitations of shot peening in comparison to other surface-strengthening techniques are also discussed. The findings of this review indicate that shot peening is a versatile and effective surface-strengthening technique with numerous applications, and further research is needed to fully realize its potential. In conclusion, this review provides insights into the current status and future perspectives on surface strengthening by shot peening, and it is expected to be useful for researchers, engineers, and practitioners in the field of material science and engineering.

Research on the edge effect of 7075 aluminium alloy damaged aircraft component strengthened by laser shock peening

Laser shock peening (LSP) is a promising surface strengthening process in the field of aircraft maintenance. However, unreasonable residual stress distribution often exists at the edge of the damaged aircraft component strengthened by LSP, which results in a significant decrease in strengthening effect of LSP. To explore the residual stress distribution after LSP in the edge region of components and reveal the influence of edge effects on deformation and inducing residual stress, the finite element (FE) models of LSP on 7075 aluminum alloy was established based on ABAQUS. The reliability of the FE model was verified by comparing the experiment and simulation results. The residual stress distribution in the edge and center region after LSP were analyzed. The results show that the edge effect enhances the flow of material to the edge and introduces tensile stress in this direction, thereby reducing the overall residual compressive stress level in the edge region. The influence range of edge effect on residual stress distribution is about 70% of the laser spot diameter. Besides, edge effects reinforce the phenomenon of residual stress holes, which results in a reduction of the maximum value by 109.2MPa. The influence of edge effect can be solved by multi surfaces LSP.

Understanding the protective role of a gradient titanium oxide ceramic layer on Ti6Al4V against corrosion via analyses of Mott-Schottky curve and electron work function (EWF)

Thermal oxidation (TO) process was employed to generate a gradient titanium oxide ceramic layer for improving corrosion performance and service safety of Ti6Al4V alloy. The semiconductor characteristic of the TO layer was evaluated in CO2-saturated simulated oilfield brine. The generated TO layer with a thickness of about 20 μm was dense and continuous without cracks or spalling characteristics. The TO layer mainly comprised of an oxide ceramic layer (rutile TiO2 ceramic phase, minor anatase one, and Al2O3) and an oxygen diffusion layer. The conducted electrochemical analysis suggested that the corrosion resistance of Ti6Al4V alloy was improved using TO surface strengthening process. It was demonstrated that the TO layer with semiconductor characteristics showed a transition from n-type (donor) to p-type (acceptor) with the increasing applied electric potential. The electron work function of the TO layer was higher than that of Ti6Al4V alloy with a naturally formed passive film. The improvement in corrosion properties was attributed to the excellent chemical stability and semiconductor properties of the metal oxide ceramic phases (TiO2, Al2O3) in the TO layer.

Influence of Ultrasonic Surface Rolling Process and Shot Peening on Fretting Fatigue Performance of Ti-6Al-4V

At present, there are many studies on the residual stress field and plastic strain field introduced by surface strengthening, which can well hinder the initiation of early fatigue cracks and delay the propagation of fatigue cracks. However, there are few studies on the effects of these key factors on fretting wear. In the paper, shot-peening (SP) and ultrasonic surface rolling process (USRP) were performed on Ti-6Al-4V plate specimens. The surface hardness and residual stresses of the material were tested by vickers indenter and X-ray diffraction residual stress analyzer. Microhardness were measured by HXD-1000MC/CD micro Vickers hardness tester. The effects of different surface strengthening on its fretting fatigue properties were verified by fretting fatigue experiments. The fretting fatigue fracture surface and wear morphology of the specimens were studied and analyzed by means of microscopic observation, and the mechanism of improving fretting fatigue life by surface strengthening process was further explained. After USRP treatment, the surface roughness of Ti-6Al-4V is significantly improved. In addition, the microhardness of the specimen after SP reaches the maximum at 80 μm from the surface, which is about 123% higher than that of the AsR specimen. After USRP, it reaches the maximum at 150 μm from the surface, which is about 128% higher than that of AsR specimen. It is also found that the residual compressive stress of the specimens treated by USRP and SP increases first and then decreases with the depth direction, and the residual stress reaches the maximum on the sub surface. The USRP specimen reaches the maximum value at 0.18 mm, about − 550 MPa, while the SP specimen reaches the maximum value at 0.1 mm, about − 380 MPa. The fretting fatigue life of Ti-6Al-4V effectively improved after USRP and SP. The surface integrity of specimens after USRP is the best, which has deeper residual compressive stress layer and more refined grain. In this paper, a fretting wear device is designed to carry out fretting fatigue experiments on specimens with different surface strengthening.

Effect of local equivalent stress on fatigue life prediction of carburized Cr-Ni alloy steel based on evaluation of maximum crack sizes

The fatigue assessment methods considering surface strengthening process were proposed through experimental investigation of fatigue properties of carburized Cr-Ni alloy under stress ratios of 0 and 0.3. Interior-failure is caused by continuous debondings of refined grains due to stress concentration around interior-inclusion, while surface-failure is caused by grain dislocation slip around surface-inclusion. In view of the effect of local equivalent stress (residual stress, mean stress and stress concentration around the inclusion), the PRx-S-N curves of carburized Cr-Ni alloy steel were constructed based on the prediction of crack characteristic sizes by Generalized Extreme Values (GEV) and Generalized Pareto (GP) distributions under different cumulative probabilities, and the comparison shows that GP distribution can predict the fatigue strength better than GEV distribution. The fatigue life prediction models can be established by GP distribution for surface and interior failures, and the predicted results are in good agreement with the experimental results.

Experimental investigation on effect of surface strengthening process and roughness on water droplet erosion behavior in turbomachinery

In this paper, the water droplet erosion (WDE) performance of a typical martensitic blade material 17-4 PH (0Cr17Ni4Cu4Nb), surface strengthened laser cladding stellite and brazed stellite alloy is studied based on high-speed water jet test system. The WDE resistances from high to low are the brazed stellite alloy, laser cladding stellite alloy and 17-4 PH. For the same material, the greater the initial roughness of the target surface, the more severe is the WDE damage. Improving the surface finish of the target is conducive to the diversion and expansion of the liquid flow, prolonging the incubation period, and thus enhancing the WDE resistance.

Influence of thermal effects in turn-rolling

The part life span significantly depends on microstructure and the residual stress state. The current state of research shows that the mechanical surface strengthening process deep-rolling is a suitable method for alteration of surface and subsurface properties to increase strength and life span of highly stressed components. A further increase in the life span can be reached by using the process temperature of the turning process to enhance the influences of deep rolling. For this purpose, a turn-rolling tool was developed. Especially concerning soft machining operations, the induced process temperatures can lead to higher compressive residual stresses, and finally, to higher part life spans. For this, it is necessary to consider the influence of heat on the component caused by the turning process. Therefore, a novel approach to the design of process parameters and tool geometries for turn-rolling based on 3D FEM simulations was developed in this work. Based on this approach, it was shown that depending on process parameters and tool microgeometry, the component temperature can be specifically influenced near the surface.

Formation Mechanism of Residual Stress Field Induced by Surface Strengthening Process

Abstract The formation of the residual stress field that accompanies severe plastic deformation is analyzed in view of the crystal structure evolution and mechanical response of the material. The conclusion drawn is that elastic release is the fundamental cause of the residual compressive stress field in the near-surface layer, while the residual tensile stress field in the deep region is attributed to load conservation. The elastic strain, which is embodied by the offset of a metal atom to the equilibrium position of the lattice, varies with the depth of the material owing to the differentiated effects of elastic release and load conservation.

Investigation on the effect of shot peening coverage on the surface integrity

Shot peening is a typical surface strengthening process that is widely applied in engineering practice for it can introduce finer grains in the near-surface layer and generate a compressive residual stress (CRS) layer. The finite element method, together with a Python subscript, is applied to create a multiple-shot model to estimate the evolution of the shot peening coverage. The influences of coverage on residual stress, plastic strain and surface topography are discussed. The predicted results reveal that as the shot peening coverage increases from 100% to 400% under the given processing condition, the maximum CRS and the plastic strain will increase gradually. But there is no evident alteration on the width of CRS layer and plastic strain layer. The variations of each roughness parameter with coverage are different.

Surface integrity and residual stress analysis of pulsed water jet peened stainless steel surfaces

Enhancement of the life of the engineering components is essential from the perspective of its performance, therefore, it is necessary to eliminate the negative effects that attenuates the life of these components. Numerous surface treatment methods are used at present out of which water jet peening process have been stated to overcome the shortcoming reported in other surface treatment methods like surface defects caused by the embedment of particles during shot peening, thermal effects caused by laser shock peening. It has been reported in studies that the technological modification of water jet technology, ultrasonically generated pulsed water jet (PWJ) can be used for the peening application but still the detailed study of parametric variations for the peening process is not clearly explained. The present work aims at reporting the effect of variation of parameters: jet pressure (40 MPa to 100 MPa), traverse speed of the nozzle (5 mm/s to 25 mm/s) and standoff distance (15 mm to 31 mm) during the ultrasonically generated PWJ peening process. The peened surface was quantified in terms of surface residual stress measurements, micro hardness and surface roughness measurements. The results indicate that the variations in the parameters shows significant changes in the surface residual stress enhancement and strengthening process. The lower pressure (40 MPa), lower traverse speed (5 mm/s) and higher standoff distance (31 mm) showed the maximum improvement in the surface residual stress (up to 540 MPa) and micro-hardness (up to 570 HV). The impact of the repeated pulses causes plastic deformation on the surface and sub-surface layers which results into the change in the microstructures of the affected area. The microstructural examination through field emission scanning electron microscopyshowed variations in these deformations under different parametric conditions. This study can give better understanding for the proper selection of the peening parameters for practical applications using ultrasonically generated pulsed water jet technology.

A New Plasma Surface Alloying to Improve the Wear Resistance of the Metallic Card Clothing

A new surface strengthening process: Plasma surface chromizing was implemented on the metallic card clothing to improve its wear resistance based on double glow plasma surface metallurgy technology. A chromizing coating was prepared in the process, which consisted of a deposited layer and diffusion layer. The surface morphologies, microstructure, phase composition, and hardness were analyzed in detail. The friction behaviors of the metallic card clothing before and after plasma surface alloying were comparatively analyzed under various sliding speeds at room temperature. The results showed that: 1. The chromizing coating on the surface of metallic card clothing was dense and homogeneous without defects, and the metallic card clothing still maintained its integrity and sharpness. 2. The chromizing coating consist of [Fe,Cr], Cr, Cr23C6, and Cr7C3, which contribute to the high hardness. 3. The average microhardness of metallic card clothing increased from 365.4 HV0.05 to 564.9 HV0.05 after plasma surface chromizing. Nano hardness of the chromizing coating was approximately 1.87 times than the metallic card clothing. 4. At various sliding velocities of 2 m/min, 4 m/min, and 6 m/min, the specific wear rates of metallic card clothing were 16.38, 9.06 and 6.26 × 10−4·mm3·N−1·m−1, and the specific wear rates of metallic card clothing after plasma surface chromizing were 2.91, 3.30, and 2.95 × 10−4·mm3·N−1·m−1. Furthermore, the wear mechanism of the chromizing coating gradually changed from adhesive wear to abrasive wear as the sliding velocity increased. The results indicate that the wear resistance of metallic card clothing was improved obviously after plasma surface chromizing.

Experimental and Numerical Analysis of Water Jet Peening on 6061 Aluminum Alloy

Water jet peening (WJP) is a mechanical surface strengthening process, which can improve the residual stress (RS) of the peened surface and then improve the fatigue life of components. In this paper, erosion experiments are conducted to investigate the influence of peening parameters on erosion. On this basis, RSs induced by WJP are studied in relation to the peening parameters. In addition, the coupled Eulerian–Lagrangian (CEL) technique is used to model and simulate the dynamic impact process of WJP on Al6061-T6. The influence of peening parameters such as jet pressure p, jet traverse velocity vf, and the number of water jet pass n on the modification of residual stress field (RSF) is examined by simulation and experiment. The influence of incidence angle α and water jet diameter d on RSF is also investigated by simulation. Results show that compressive RS σcrs is a result of the action of water-hammer pressure alone. Furthermore, σcrs increases with an increase in p, n and α. The optimal peening parameters for Al6061-T6 are found to be p = 60 MPa, vf = 2000 mm/min, n = 4, α = 90 deg and d = 2.0 mm. Finally, the depth of compressive RS layer D0 increases greatly with an increase in water jet diameter d and can reach 984 μm.

Potential of Using Water Jet Peening as a Surface Treatment Process for Welded Joints

Water jet peening is a mechanical surface strengthening process which has gained momentum in the last two decades. It is an application of water jet abrasion process which improves the residual stresses of the treated surface. In comparison to other conventional methods like shot peening, laser shock peening water jet peening offers resistance to corrosion, improved fatigue resistance, simpler control over the process, full coverage of the treated area, flexibility in treating complex areas and eco-friendly environment. Since WJP uses water for improving the residual stresses it is a potential method for treating a variety of materials. The high pressure water jets has the tendency to cause the surface damage if the parameters are not maintained within optimum conditions therefore, optimum conditions must be determined so that surface treatment can be carried out without causing any detrimental effects. Hence, it is necessary to control the peening parameters which are: Jet pressure, Standoff distance, nozzle geometry, nozzle traverse rate and exposure time. The paper aims at reviewing the work contribution for using Water jet peening as a potential surface treatment process.

The Research about Parallel Mechanism Workspace for Quenching

To research 3-DOF parallel mechanism institutions applying to the process of plasma arc surface strengthening, the author establishes an inverse kinematics of institutions mathematical model, and then, basing on its structural constraints and using the boundary finding, draws the workspace. It has guiding significance for the establishment of robotic the plasma arc surface strengthening process.