Surface Compressive Residual Stress Research Articles

Misorientation and dislocation evolution in rapid residual stress relaxation by electropulsing

This study investigates the effect of high current density electropulsing on the material in a rapid stress relaxation process. An AISI 1020 steel was shot-peened to induce surface compressive residual stresses in a controlled manner and subsequently electropulsed to investigate the changes in microstructure and defect configuration. AISI 1020 steel was chosen as it has a simple microstructure (plain ferritic) and composition with low alloying conditions. It is an appropriate material to study the effect of transmitting electric pulses on the microstructural defect evolution. A combination of electron-backscattered diffraction and transmission electron microscopy proved to be an effective tool in characterizing the post-electro pulsing effects critically. By application of electropulsing, a reduction in the surface residual stress layer was noticed. Also, reductions in misorientation and dislocation density together with the disentanglement of dislocations within the cold-worked layer were observed after electropulsing. Additionally, the annihilation of shot-peening-induced deformation bands beyond the residual layer depth was observed. These effects have been rationalised by taking into account the various possibilities of athermal effects of electropulsing.

The effect of fine particle peening on aviation gear performance, Part I: Surface integrity

AbstractAs a fundamental component of aviation equipment, the gear has an essential effect on the service performance of the whole machine. Fine particle peening (FPP) technology is a good choice for the precision requirements of aviation gears. Therefore, a FPP model of aviation gears was constructed based on the discrete element‐finite element coupling method (DEM‐FEM) to simulate the evolution of surface roughness and residual compressive stress, and its results were verified by experiments. The effects of FPP intensity, coverage rate, and particle diameter on surface integrity were investigated. In addition, the importance degree of the intensity, coverage rate, and particle diameter of FPP on surface integrity was carried out by the random forest decision method. It demonstrates that the surface roughness Sa was mainly influenced by the FPP intensity and particle diameter. And the distribution state of the residual compressive stress was mainly affected by the particle diameter.

Composite effects of residual stress and hardness gradient on contact fatigue performance of rough tooth surfaces and optimization design of detailed characteristic parameters

Residual stress and hardness gradient significantly affect the contact fatigue failure process of rough tooth surfaces, and the meticulous design of their characteristic parameters can help improve the fatigue resistance of gears. In this paper, an efficient computational method for rough tooth surface contact is established based on the reconstruction modeling of asperities and mixed lubrication analysis. By comprehensively considering the effects of residual stress and hardness gradient, the Dangvan multiaxial fatigue criterion is adopted to predict and analyze the contact fatigue life of the gear surface. The correlation between the surface residual stress, maximum residual stress depth, surface hardness, and effective hardening layer depth, among other characteristic parameters, and the contact fatigue performance of tooth surfaces is established. The results show that: (1) Increasing the magnitude of surface residual compressive stress and surface hardness can significantly reduce the risk of fatigue failure in the near-surface layer of the gear. To achieve the same fatigue performance, there is a linear negative correlation between the design values of the two parameters. (2) Affected by the double stress peaks in the rough tooth surface stress field, the maximum residual stress depth significantly affects the depth at which the highest failure risk occurs. With changes in roughness, the optimal design value of the maximum residual compressive stress depth changes from about 0.5 times the Hertzian contact half-width depth in the subsurface layer (Sa < 0.2 μm) to about 15 μm in the near-surface layer (Sa > 0.4 μm). (3) Considering both process cost and gear surface fatigue resistance, the design threshold for the effective hardening layer depth should be greater than the Hertzian contact half-width. This paper establishes a composite correlation between the meticulous characteristic parameters of residual stress, hardness gradient, and the contact fatigue performance of tooth surfaces, providing theoretical support for optimal design of tooth surface anti-fatigue performance.

Surface morphology of API 5L X65 pipeline steel processed by ultrasonic impact peening: An integrated experimental and computational study

Ultrasonic impact peening (UIP) is a highly effective surface engineering technique to improve material durability through peening-induced surface plastic deformation. These improvements are determined by not only the generation of surface compressive residual stresses but also the alteration of surface morphological features. In literature, researchers have focused primarily on the average roughness (Sa) to understand the morphology changes induced by UIP. From the surface engineering perspective, in addition to Sa, other surface morphological characteristics play key roles in determining material durability, particularly surface skewness (Ssk) and surface kurtosis (Sku), which describe the asymmetrical distribution of surface profiles and the sharpness of peak features, respectively. In this study, surface morphological characteristics of API 5L X65 pipeline steel processed by UIP are investigated with a focus on the effects of processing parameters on Sa, Ssk, and Sku. The results indicate that the optimized UIP conditions lead to the preparation of surfaces with valley structures as the dominant feature accompanied by blunt peaks, resulting in the elimination of stress concentration sites towards enhanced material durability. In addition, surface hardness, residual stresses, and peening-induced deformation depth are measured. Furthermore, to gain a comprehensive understanding of the material response and the underlying mechanisms driving surface morphological alterations by UIP, a finite element method (FEM)-based model is developed. By investigating the effect of process parameters (peening amplitude, peening cycles, and pin size) on the surface deformation depth and stress/strain distribution, a correlation between these process variables with surface characteristics is established.

Effect of combined laser shock peening and nitrogen implantation on the microstructure and tribology of M50 bearing steel

The M50 bearing steel underwent a combined treatment of laser shock peening (LSP) and nitrogen ion implantation. The microstructure, surface mechanical properties and tribological behaviors were evaluated. Comparative analysis with the untreated sample revealed significant improvements in surface nano-hardness, increasing from 7.51 GPa to 11.35 GPa, 11.51 GPa, and 12.62 GPa for samples subjected to LSP, ion implantation, and combined strengthening, respectively. Correspondingly, surface compressive residual stress showed notable increases from −15.55 MPa to −1043.76 MPa, −451.34 MPa, and −1276.13 MPa, respectively. The improved surface mechanical properties observed in the combined strengthened sample are likely due to the synergistic effects of the combined strengthening. In friction tests, the combined strengthened sample exhibited the lowest friction coefficient, attributed to the presence of a metal nitride layer and a thicker amorphous layer. Wear mechanisms transitioned from fatigue spalling for the untreated specimen to slight wear and the formation of an oxidative adhesion layer after LSP and ion implantation. Remarkably, the surface of the combined strengthened specimen demonstrated minimal wear, with only an oxidative adhesion layer present.

Influence of transversal vibration on cutting performance and surface integrity during ultrasonic peening drilling of Al-Li alloys

Advanced hole-making process is of great importance to enhance the fatigue performance of Al-Li alloy part in aviation industry. Ultrasonic peening drilling (UPD), in which an ultrasonic transversal vibration is applied to the cutting tools, is a recently proposed hole-making method that integrates precision-machining and surface strengthening by single-shot operation. In the study, kinematics, material removal mechanism and strengthening mechanism for UPD of Al-Li alloy by helical fluted reamers are analyzed. The effect of transversal vibration on the cutting performance and surface integrity is studied through comparative experiments between UPD and conventional drilling (CD) of Al-Li alloy holes. The experimental results show that UPD exhibits superior cutting performance with a maximum reduction of 52.6% in thrust force and 52.3% in torque, respectively, compared to CD. Moreover, narrower dimensional tolerance is obtained in UPD due to the reduced transversal force and improved machining stability. Additionally, deeper plastic deformation, higher surface microhardness and residual compressive stress of machined holes are obtained by UPD. The electron back-scattered diffraction (EBSD) analysis confirms that deeper machined affect area and grain refinement are realized in UPD. Therefore, the results indicate that UPD is a feasible method for achieving high-precision and strengthened holes for Al-Li alloy.

Investigating the Dynamic Mechanical Properties and Strengthening Mechanisms of Ti-6Al-4V Alloy by Using the Ultrasonic Surface Rolling Process.

The demand for titanium alloy has been increasing in various industries, including aerospace, marine, and biomedical fields, as they fulfilled the need for lightweight, high-strength, and corrosion-resistant material for modern manufacturing. However, titanium alloy has relatively low hardness, poor wear performance, and fatigue properties, which limits its popularization and application. These disadvantages could be efficiently overcome by surface strengthening technology, such as the ultrasonic surface rolling process (USRP). In this study, the true thermo-mechanical deformation behavior of Ti-6Al-4V was obtained by dynamic mechanical experiment using a Hopkinson pressure bar. Moreover, USRP was applied on the Ti-6Al-4V workpiece with different parameters of static forces to investigate the evolution in surface morphology, surface roughness, microstructure, hardness, residual stress, and fatigue performance. The strain rate and temperature during the USRP of Ti-6Al-4V under the corresponding conditions were about 3000 s-1 and 200 °C, respectively, which were derived from the numerical simulation. The correlation between the true thermo-mechanical behavior of Ti-6Al-4V alloy and the USRP parameters of the Ti-6Al-4V workpiece was established, which could provide a theoretical contribution to the optimization of the USRP parameters. After USRP, the cross-sectional hardness distribution of the workpiece was shown to initially rise, followed by a subsequent decrease, ultimately to matrix hardness. The cross-sectional residual compressive stress distribution of the workpiece showed a tendency to initially reduce, then increase, and finally decrease to zero. The fatigue performance of the workpiece was greatly enhanced after USRP due to the effect of grain refinement, work hardening, and beneficial residual compressive stress, thereby inhibiting the propagation of the fatigue crack. However, it could be noted that the excessive static force parameter of USRP could induce the decline in surface finish and compressive residual stress of the workpiece, which eliminated the beneficial effect of the USRP treatment. This indicated that the choice of the optimal USRP parameters was highly crucial. This work would be conducive to achieving high-efficiency and low-damage USRP machining, which could be used to effectively guide the development of high-end equipment manufacturing.

Interaction of electric current with burnishing parameters in surface integrity assessment of additively manufactured Inconel 718

One of main problem associated with surface sever plastic deformation (SSPD) techniques like low plasticity burnishing (LPB) of metallic materials is limitation of surface integrity improvement at further loads (i.e. static force or pass number). It is due to interlocking of dislocation motion that limits further plastic deformation. As a plausible solution, in the present work, electric current assisted burnishing (ECAB) is introduced to use the concept of electroplasticity in surface integrity (roughness, hardness and residual stress) enhancement of Inconel 718 by additive manufacturing. Nevertheless, there are some researches which use this method to enhance performance of SSPD, the interaction effect of electrical parameter like current density with other parameters haven’t been studied well. Thus, to make the process optimized in terms of surface integrity aspects, parametric study of ECAB process with focus on interaction of current density with other LPB parameters merits further investigations. Thus, in the present work, an experimental study was carried out to identify the interaction effect of current density (as main parameter of ECAB) with other burnishing parameters (spindle speed, feed rate, current density, pass number and static force) on surface integrity aspects. Analysis of variances was further carried out to identify the contribution of each parameter on performance measures. Then, the interaction plots based on polynomial regression were developed, analyzed and discussed based on the physic of plastic deformation and some microscopic analysis like surface topography, scanning electron microscopy and X-ray diffraction analysis. It was found from the results that electric current density has the greatest effect on surface integrity aspect with contribution of 60 % on surface roughness, 68.5 % on hardness and 30 % on surface compressive residual stress. On the other hand it was found that this parameter has high interaction with process factors while applying different current density changes the optimum burnishing setting for each performance measures. Application of LPB at optimum parameter setting causes improvement of surface roughness of as-built material about 54 %; further improvement of 70 % and 85 % in roughness values were also achieved when the electric current with densities of 5A/mm2 and 10A/mm2, respectively were applied to burnishing process at optimum setting. It was also found that application of optimum LPB as well as optimum ECAB with densities of 5A/mm2 and 10A/mm2, results in improving the surface hardness about 17.5 %, 42 % and 68 %, respectively. In addition, under foresaid post-processing sequence, the surface compressive residual stress enhanced from 121 MPa (tensile type) for as-built material to −208 MPa, −345 MPa and −488 MPa (compressive type). Finally, the most optimum solution results in minimum surface roughness as well as maximum hardness and residual stress were identified by desirability approach function as 300RPM spindle speed, 0.08 mm/rev feed rate, 10A/mm2 current density, 3 pass number and 300 N force. To show the application of this optimization in a real life problem, fatigue resistance of optimum samples for high cycle and low cycle testing were obtained and compared with as-built material. It was found that following this post-treatment, the life cycle of samples improves from 160 % to 270 %, respectively that affirms practical application and manufacturing efficiency of proposed approach.

Effect of ultrasonic surface rolling on the friction and wear properties of 8620H carburized steel at elevated temperatures

The surface strengthening treatment of 8620H carburized steel was carried out by ultrasonic surface rolling (USR) process, and the samples with and without the treatment were subjected to friction and wear experiments at 25°C, 100°C, and 150°C. The experimental results showed that the samples with USR treatment had higher surface hardness, lower surface roughness, deeper hardened layer depth, and higher surface residual compressive stress than the samples without USR treatment. Meanwhile, the wear degree of the samples deteriorated as the temperature increased, but the anti-wear performance of the samples with USR treatment increased by 52.9%, 29.8%, and 14.9% compared with the samples without USR treatment at 25°C, 100°C, and 150°C, respectively, indicating that the USR treatment still had a better strengthening effect at high temperatures.

Studies on residual stress and surface topography after pneumatic shot peening using discrete element method–finite element method coupled model

After pneumatic shot peening, compressive residual stress is induced on the surface of the target, which will improve the fatigue life. During the process, surface roughness is also induced, which may reduce the fatigue life. To achieve the optimum compressive residual stress with the smallest surface roughness, the formation mechanism of the residual stress field and the surface topography are revealed. Then, pneumatic shot peening is simulated by using a discrete element method–finite element method coupled model. Based on the effects of four variables (the incident angle θ, the initial shot velocity v0, the shot radius R) and the mass flow rate rm on five parameters (the equivalent plastic strain, the equivalent stress, the surface compressive residual stress, the maximum compressive residual stress), the surface roughness is investigated. The results show that it induces a residual elastic–plastic strain after the impact of a single shot, which can form a crater and a hemispherical residual stress field. After the impact of many random shots, scattered craters connect with each other, therewith a rough and continuous surface topography is formed. Scattered residual stress fields couple with each other, and a constant residual stress layer with the compressive residual stress in the surface and the tensile residual stress in the subsurface are formed. All five parameters are determined by the residual elastic–plastic strain, which increases as the augment of the normal impact velocity, the shot mass, and the coverage rate. Along with the increase of θ and rm, these five parameters firstly increase and then decrease; with the increase of v0 and R, these five parameters increase. Therefore, reasonable values of θ, v0, R, and rm should be chosen to obtain the optimum compressive residual stress with as small surface roughness as possible.

In-process approach for editing the subsurface properties during single-lip deep hole drilling using a sensor-integrated tool

Single-lip deep-hole drilling (SLD) is characterized by high surface quality and compressive residual stress in the subsurface of the drill hole. These properties depend significantly on the thermomechanical conditions in the machining process. The desired subsurface properties can be adjusted in-process via process monitoring near the cutting zone with a sensor-integrated tool and closed loop control when the thermomechanical conditions are maintained in the optimum range. In this paper, a method is presented to control the thermomechanical conditions to adjust the properties in the subsurface. The process model integrated in the controller is implemented as a soft sensor and takes into account the residual stresses, the roughness, the hardness and the grain size in the surface as well as in the subsurface depending on the process control variables, such as the feed rate and cutting speed. The correlation between the process variables, the thermomechanical conditions of the cutting process and the subsurface properties are investigated both experimentally and by finite element (FE) simulations. Within a justified process parameter range, characteristic fields for the soft sensor were established for each property. In addition, the procedure of controller design and the employed hardware and interfaces are presented.

Effect of pulsed femtosecond laser shock peening surface modification on anti-wear failure properties of AISI 9310 gear steel

The wear resistance of gear components significantly influences operational reliability and lifespan of mechanical equipment. Compared with nanosecond time scale laser surface treatment, femtosecond laser shock peening (FLSP) has the advantage of no absorption layer and constraint layer in processing complex geometric structure parts. However, attempts at FLSP for gear materials are quite limited up to now. This paper introduces a methodology employing Pulsed FLSP to enhance the wear resistance of AISI 9310 gear steel. The research delves into the effects of FLSP on the surface morphology, mechanical characteristics, and wear performance of AISI 9310 steel under varying laser energy levels (50 μJ, 100 μJ, 150 μJ, 200 μJ) and pulse spacings (10 μm, 15 μm, 30 μm). The findings reveal that femtosecond laser surface treatment significantly augmented surface microhardness by 9.4 % to 17.4 % and induced a surge in surface residual compressive stress (RCS) by 30.5 % to 103.1 %. Moreover, it introduced a work hardening layer of 150–300 μm in depth direction, resulting in a wear rate reduction ranging from 10.1 % to 61.7 %. The sample treated with 15 μm pulse spacing and 150 μJ laser energy exhibited simultaneously the lowest wear rate and a minimum average coefficient of friction (COF) on 0.47. The advancement in wear resistance is attributed to the synergistic effects of markedly increased surface hardness, gradient hardness in depth direction, and reduced grain size induced by FLSP. A multiple linear regression (MLR) model indicates that deeper work hardening layers are most significant for improving wear resistance, which reduces risk of wear failure.

Enhancement of fatigue properties of selective laser melting fabricated TC4 alloy by multiple shot peening treatments

This study investigated the impact of multiple shot peening (SP) treatments on the surface morphology, microstructure, residual stress and fatigue performance of TC4 alloy manufactured by selective laser melting (SLM). The multiple SP treatments were conducted sequentially using cast steel beads, ceramic beads and glass beads at different peening intensities. In comparison to the conventional single SP process, multiple SP treatments reduced surface roughness and stress concentration coefficient while further increasing the residual compressive stress on the alloy surface. SP significantly enhanced the fatigue life of the alloy by 6.4 to 9.6 times, with a more pronounced improvement observed with multiple SP. Fatigue fracture analysis revealed that the untreated alloy showed multiple fatigue sources which all located on the surface, with surface printing defects acting as potential sites for crack initiation. After SP treatments, all fatigue cracks were initiated beneath the surface. The improved fatigue performance of SLM-formed TC4 alloy could be mainly attributed to the SP-induced surface microstructure refinement and residual compressive stresses. Furthermore, results indicated that, for subsurface crack initiation, the initial cracks primarily stemmed from the fracture of α/α' martensite in the primary β columnar grains.

Microstructural characterization and wear performance of shot-peened TA15 titanium alloy fabricated by SLM

Ti-6Al-2Zr-1Mo-1 V (TA15) is a kind of near-α titanium alloy widely used in the aerospace industry, known for its excellent thermal strength and weldability. However, its low thermal conductivity and high deformation resistance make traditional machining methods challenging. Despite some researchers have manufactured TA15 alloy using SLM, the issue of tensile residual stresses on component surfaces remains prominent. This study aims to investigate the impact of shot peening on the surface residual stresses, microstructure, and wear performance of SLM-formed TA15 alloy. Experimental results demonstrate the successful introduction of surface compressive residual stresses through shot peening, with a maximum value of -897 MPa (at 0.45mmA intensity). Shot peening reduces the grain size within the deformation layer, resulting in a significant presence of dislocation entanglement and dislocation cells. When the intensity reaches 0.35mmA, the emergence of nanocrystals is observed, attributed to the influence of dislocation activity and dynamic recrystallization. Additionally, shot-peened samples exhibit notably improved wear resistance compared to their non-peened counterparts. However, excessive shot peening intensity may compromise surface quality. Therefore, under specific load conditions, higher intensity is not necessarily better. In conclusion, shot peening proves to be an effective technique for enhancing the surface stress state and wear performance of SLM-formed TA15 alloy.

Enhancement of the three-point bending behavior of a coarse-grained NiTi shape memory alloy with surface heterogeneous gradient microstructure and compressive residual stress

NiTi shape memory alloys offen subject to bending deformation as common functional materials. Laser shock peening (LSP) was utilized to fabricate a gradient layer to improve the bending mechanical behavior of a coarse-grained NiTi plate. A surface gradient layer with a thickness of approximately 100 µm was created in a 1.38 mm thick coarse-grained NiTi plate. The LSPed surface contained nanocrystals, dislocations, and amorphous, while the interior layer contained a substantial number of deformation twins, dislocations, and precipitates. The nanohardness of the surface treated with LSP can reach 10 GPa. The results of a three-point bending experiment indicate that LSP treatment increases the bending resistance of the NiTi plate. This study demonstrated that laser shock peening is an effective method for increasing the deformation resistance of a coarse-grained NiTi by means of surface gradient structures, residual stress, and precipitates.

Pre-mixed abrasive waterjet peening with post-low stress milling process and the effect of process parameters on the surface integrity of TC11 titanium alloy

Pre-mixed abrasive waterjet peening (PAWJP) is progressively gaining traction in enhancing metal materials’ surface integrity and mechanical behavior. To avoid the disadvantage of PAWJP in the application, a hybrid manufacturing method of pre-mixed abrasive waterjet peening with post-low stress milling (PAWJP-LSM) is proposed and studied experimentally. The principle of PAWJP-LSM was first introduced. Subsequently, the effect of the PAWJP-LSM parameters (i.e., abrasive type, traverse speed, nozzle inlet pressure, and removal amount) on the surface integrity of TC11 titanium alloy and its variation trend were systematically investigated. Results show that the specimen treated with PAWJP using different process parameters formed a PAWJP treatment area with a strengthening depth of 6–275 μm. The minimum surface roughness obtained by PAWJP was 0.727 μm. The maximum surface microhardness significantly increased to approximately 516.3 HV from 402.8 HV, with a maximum micro-hardness rate of 28.2 % and a maximum work hardening depth of 225 μm. The surface compressive residual stress (CRS) induced by PAWJP ranged from −97 to −322 MPa, with a maximum CRS exceeding −890 MPa and a maximum CRS layer thickness of 305 μm. The surface integrity of PAWJP-LSM is further enhanced compared to PAWJP, resulting in a 98.9 % reduction in minimum surface roughness and a 73.7 % reduction compared to the as-received surface roughness. Additionally, PAWJP-LSM exhibited a 98.9 % increase in surface CRS and a 19.4 % increase in maximum CRS. Moreover, the fatigue life was significantly improved by 69.4 %, which was 294.5 % higher than that of the as-received specimen. Nanocrystals with an average size of 27.3 nm were formed in the topmost surface. Overall, the PAWJP-LSM method demonstrated significant potential for engineering applications and exhibited substantial value in the surface treatment of TC11 alloy. This study validated the effectiveness of the proposed PAWJP-LSM method and provided a valuable reference for selecting optimized process parameters.

Introducing transversal vibration in twist drilling: Material removal mechanisms and surface integrity

In twist drilling, severe extrusion is unavoidable at the chisel edge zone and results in high thrust force and poor surface integrity. Traditional axial vibration-assisted twist drilling only modifies the material removed from the continuous extrusion to the interrupted extrusion for the chisel edge centre, and must satisfy the chip-breaking conditions to ensure that the cutting edge is periodically separated from the workpiece. Hence, transversal vibration, has been introduced in twist drilling to overcome the severe extrusion behaviour of the chisel edges of the twist drills, referred to as ultrasonic transversal vibration-assisted drilling (UTVD). Theoretical models revealed that the material removal behaviour of the chisel edge centre changed from extrusion in CD to cutting in UTVD. Experiments were performed on UTVD of Ti-6Al-4V to confirm the feasibility with this novel method. The results indicated that transverse vibration significantly affected the thrust force and torque, and the maximum decline was 29.4% and 31.5%, respectively, compared with CD. Generated chips verified the material removal behaviour of the chisel edge centre zone changed from extrusion in CD to cutting in UTVD. Therefore, the hole quality and surface integrity were improved significantly by UTVD. Deeper plastic deformation (∼2.5 times), higher surface microhardness and compressive residual stress were realised using UTVD. This study enhanced the understanding of material removal and formation mechanisms of the surface and subsurface in UTVD, thereby providing a novel method and theoretical basis for the high-performance twist drilling of difficult-to-cut alloys.

Effect of laser shock peening on surface integrity and tensile fatigue behavior of TB8 bolts

Laser shock peening (LSP) treatment was introduced to improve tensile fatigue behavior of TB8 titanium alloy bolt. TB8 bolts surface integrity, tensile fatigue fracture and strengthening mechanism after LSP treatment were systematically investigated. Results showed that cracks and machining defects on the thread surface were eliminated, surface layer microhardness and compressive residual stress were increased after LSP treatment. Meanwhile, the grains sizes of the LSP treated bolts samples were reduced to 38.61 μm with the formation of high density dislocations. LSP treated bolts samples with 80 mJ laser energy exhibited the maximum average tensile fatigue life, which was increased from 66,452 cycles to 81,319 cycles. In addition, under the synergistic effect of compressive residual stress and grains refinement, cracks propagation path was changed, which were the primary factors for the improvement of TB8 bolt tensile fatigue life.

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.

Finite-element simulation of residual stresses induced by laser shock peening in TC4 samples structurally similar to a turbine blade

This study is devoted to the investigation of residual stresses distribution (RSD) in a TC4 sample treated with laser shock peening. The study placed special emphasis on analyzing the RSD at the part of the samples structurally similar to a turbine blade, which is more frequently subjected to damage during service according to the aircraft statistics. Results of simulation showed that low power density of 1.11 GWt/cm2 could not induce compressive residual stress on the surface of a treated object. Furthermore, increasing the overlapping of laser spots does not improve the situation and still fail to induce surface compressive residual stress at a laser intensity of 1.11 GWt/cm2. The compressive stresses occur only with the rise in power density. Reducing the spot size from 3 mm to 1 mm for the power density of 10 GWt/cm2 results in a 20% increase in the magnitude of compressive residual stress in the area of interest. Moreover, applying 35% overlapping further enhances this value. In addition to increasing the magnitude of residual stress, this approach also leads to a more homogeneous RSD of the treated material.