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Comparison of characteristics of advanced surface strengthening processes for metal materials

In high-tech and heavy industry fields such as aviation, aerospace, high-speed rail, and ocean, the reliability and durability of key components directly affect the safe operation and long-term benefits of the entire system. However, these components often serve in extremely complex working environments, such as high stress, high temperature differences, strong corrosion, etc., leading to fatigue failure becoming one of the main factors restricting equipment performance and service life. Fatigue failure not only causes huge economic losses, but may also lead to serious safety accidents, with profound impacts on society and the environment. Therefore, it is particularly important to explore effective anti fatigue technologies and improve the fatigue resistance of key components.     

Surface Severe Plastic Deformation (SSPD) surface strengthening technology, as an innovative anti fatigue strategy, has received widespread attention in recent years. This technology can effectively suppress the initiation and propagation of cracks by introducing beneficial residual compressive stresses, thereby significantly improving the fatigue life of materials. This article will provide a detailed introduction to several mainstream surface strengthening processes, including traditional shot peening, laser shock peening, ultrasonic shock peening, and ultrasonic rolling peening. The aim is to provide readers with a comprehensive overview of surface strengthening techniques and provide theoretical support and practical guidance for solving fatigue problems in practical engineering.

1、 Traditional shot peening strengthening

Basic principle:

The basic principle of traditional shot peening process is to use a projectile launching device to launch a large number of high-speed projectiles repeatedly hitting the surface of metal materials, causing them to produce subtle cold plastic deformation, thereby improving the strength and hardness of the part surface, and generating residual compressive stress on the surface, thereby improving the fatigue strength or stress corrosion resistance of mechanical components. Figure 1 is a schematic diagram of the shot peening process. Figure 1 Schematic diagram of shot peening process:

(a) The projectile impacts the surface of the workpiece; (b) Shot peening process

Advantages:

1. Low process cost, low energy consumption, simple equipment structure, easy operation method, high production efficiency, and significant strengthening effect.

2. The process is basically mature, and the process parameters and operating procedures have been highly standardized, suitable for various metal parts. For example, shot peening can be used to improve the fatigue strength of alloy steel, carbon steel, ultra-high strength steel, cast iron, aluminum alloy, magnesium alloy, titanium alloy, and even powder alloy parts.

3. Shot peening can achieve refinement of material surface grains, introduction of residual compressive stress, and changes in surface roughness through plastic deformation after projectile collision.

limitations:

1. Shot peening equipment occupies a large space and generates a large amount of metal dust and noise during the shot peening process, which has a significant impact on people's physical, mental, and health. It is necessary to enclose the operating space to prevent environmental pollution caused by particle splashing and dust floating, and to collect the particles.

During the shot blasting process, the projectile directly collides and squeezes the surface of the material, causing deformation, pits, and other occurrences; The smoother the surface before shot peening, the rougher the surface after shot peening, which is unfavorable for precision machined surfaces or parts that require a smooth surface. After high-intensity shot peening of the parts, deep craters not only increase the surface roughness value, but also form significant stress concentration.

The thickness of the plastic deformation layer on the surface of the material strengthened by shot peening is limited, generally ranging from 0.1 to 0.8 mm, depending on the selected process parameters. The higher the shot peening intensity, the greater the residual compressive stress obtained and the deeper the residual compressive stress layer, but the greater the macroscopic deformation caused. Due to the influence of the diameter of the projectile, it is easy to cause uneven reinforcement layers and cannot effectively impact the surface of precision parts.

2、 Laser shock enhancement

Basic principle:

The basic principle of laser shock peening technology is to use a strong laser beam with short pulses (tens of nanoseconds) and high peak power density (>109 W/cm2) to penetrate the confinement layer and irradiate the surface of the absorption layer of the workpiece. The absorption layer absorbs laser energy and vaporizes, producing a plasma with high temperature (>10000 K) and high pressure (>1 GPa). The plasma further absorbs the subsequent laser energy, but due to the constraint of the confinement layer, a high-strength shock wave is generated. When the peak pressure of the shock wave exceeds the dynamic yield strength of the workpiece, the surface of the workpiece undergoes strain strengthening, forming residual compressive stress. Under the action of plastic deformation, the microstructure undergoes significant changes, and the material's surface wear resistance and corrosion resistance are effectively improved, while the fatigue life is significantly increased. The schematic diagram of laser shock strengthening is shown in Figure 2 [8]. Figure 2 Schematic diagram of laser shock strengthening

Advantages:

1. Laser shock peening has the characteristics of high energy (GW level), high pressure (GPa level), short pulse (ns level), ultra-high strain rate (~107 s-1), non-contact, etc.

2. Laser shock peening can introduce a residual compressive stress influence layer with larger numerical values and deeper depths, which can reach a depth of 1-2mm, 5-10 times that of traditional mechanical shot peening [11-12]. After strengthening, gradient stress and microstructure can be formed on the surface of the material, resulting in better fatigue performance improvement. Moreover, due to the low cold work hardening rate, the stability of residual stress and fatigue resistance under thermal stress and mechanical load is better.

3. Laser shock peening propagates the shock wave into the interior of the material and undergoes plastic deformation in sequence with the material along the propagation path. The amount of plastic deformation is not concentrated on the surface of the material, and the depth of the impact micro pits left is only a few micrometers, with less impact on surface roughness.

During the laser shock peening process, the equipment and process do not need to be specifically designed for different parts, as they can accurately control the energy, pulse width, spot shape (circular or square), size, position, overlap rate, and laser incidence angle of each pulse laser. They can achieve "hit where you point" for complex surface components and special parts, and have good process design and implementation.

5. Laser shock enhancement usually uses flowing water as a constraint layer, and the waste liquid generated during the enhancement process is easy to collect and has low pollution.

6. Laser shock peening has a wide range of applicable materials, not only suitable for conventional metal materials, but also for surface modification treatment of hard and brittle non-metallic materials such as ceramics and silicon-based materials.

limitations:

Compared to the United States, the relevant strengthening equipment in China is relatively backward. Currently, lasers have problems such as low pulse energy, poor spot quality, and low repetition rate, which greatly limit the development of laser shock enhancement technology.

2. Laser shock peening equipment generally integrates multiple high-tech components such as high-power lasers, precision optical systems, complex control systems, and specialized fixtures. The research and development, manufacturing, and integration of these components require a significant amount of capital investment, which directly increases the purchase cost of the equipment. Usually used in high-end fields such as aerospace and marine vessels. For small and medium-sized enterprises, the cost of strengthening processing methods is too high, which limits the application and promotion of this technology.

3. The existing laser shock peening methods have cumbersome pre-treatment processes and generally low experimental efficiency. The optical system is complex and prone to light pollution, as well as having too many auxiliary devices, making it inconvenient to operate. In experiments, flowing water is often used as a constraint layer, but the uniformity of the thickness of the constraint layer is poor; Most absorption layers are made in the form of black tape, black paint, aluminum foil, etc., and the processing is relatively time-consuming.

3、 Ultrasonic shock enhancement

Basic principle:

The basic principle of ultrasonic shock enhancement technology is to convert the harmonic oscillation of the ultrasonic transducer into shock pulses, expand the ultrasonic amplitude through a amplitude rod, and install a spherical indenter on the end tool head to transmit high-density energy waves and shock vibrations to the workpiece. Before applying ultrasonic impact dynamic load, a static pressure is first applied to the workpiece through a spherical indenter. Under the action of ultrasonic impact dynamic load and static load, the cold plasticity characteristics of the metal are utilized to cause plastic deformation on the surface of the metal material, and a grain refinement effect is produced within a certain depth range on the surface of the material [18]. The schematic diagram of ultrasonic shock enhancement is shown in Figure 3. Figure 3 Schematic diagram of ultrasonic impact strengthening

Advantages:

During ultrasonic impact strengthening, energy conversion is more concentrated compared to other surface strengthening techniques, which can achieve lower surface roughness, higher hardness, and larger and uniform residual compressive stress on the surface of the workpiece material.

2. Ultrasonic impact equipment has low cost, high impact head strength, can be reused, and does not require frequent replacement. The equipment structure design is simple and compact, and the energy transfer process between various components is short and directly efficient, resulting in low energy consumption and high efficiency.

3. Ultrasonic impact is achieved by using a high-frequency impact head to enhance surface performance, so there will be no pollution such as smoke, dust, waste residue, and strong light during the processing, achieving dual protection of the environment and the health of operators.

4. There are various types of impact heads for ultrasonic impact equipment, which can be replaced according to usage requirements; The impact equipment can be manually operated or used in conjunction with automated industrial equipment to meet various processing requirements in complex environments, with strong processing adaptability.

5. Ultrasonic impact strengthening equipment is simple, lightweight, highly controllable, and has low noise. It has the advantages of high processing accuracy and wide processing range, and plays an important role in stress relief treatment of welded joints, surface strengthening of metal materials, surface pretreatment, and composite strengthening.

limitations:

1. Ultrasonic impact strengthening is mainly used for regular planes (cylindrical, flat, spherical), and the strengthening of complex components requires cooperation with robotic arms. In addition, for complex thin-walled curved parts, if the strengthening force is large, the part will deform and affect accuracy. If the strengthening force is small, the strengthening effect will not meet the standard.

2. Ultrasonic impact may cause impact damage to the surface of components while introducing residual compressive stress, leading to the emergence of small crack sources. It is not suitable for welding tensile stress relief of large aerospace thin-walled structural components, polyhedra, or rotating thin-walled precision structural components.

4、 Ultrasonic rolling strengthening

Basic principle:

The basic principle of ultrasonic rolling strengthening technology is to perform reciprocating rolling processing on the surface of the workpiece by combining high-frequency ultrasonic vibration with static pressure. In the process of ultrasonic rolling strengthening, ultrasonic energy is transmitted to the surface of the workpiece through the ultrasonic machining system (the ultrasonic generator generates high-frequency electrical signals during the machining process, which are converted into high-frequency small vibrations by the transducer and amplified by the amplitude lever and applied to the ultrasonic rolling tool head. The periodic dynamic impact and rolling action induce greater elastic-plastic deformation of the material, further reducing the surface roughness of the material and achieving the effect of "peak shaving and valley filling". Due to the refinement of the microstructure and the improvement of the degree of surface work hardening, the hardness of the material is effectively promoted, and a deeper gradient nano hardening layer and residual compressive stress influence area are formed on the surface layer, thereby greatly improving the comprehensive performance of the material such as fatigue strength, wear resistance, and corrosion resistance. The schematic diagram of the ultrasonic rolling strengthening system is shown in Figure 4. Figure 4 Schematic diagram of ultrasonic rolling strengthening system.

Advantages:

1. Combining the characteristics of ultrasonic vibration and rolling strengthening techniques, ultrasonic rolling simultaneously applies impact force and extrusion force to the material surface, increases processing energy, reduces plastic processing threshold, and produces strong plastic deformation and work hardening effect on the processed surface [23]. Due to its ability to evenly distribute force on the surface of the workpiece, the degree of plastic deformation in the surface layer can be improved, achieving the goal of further refining the grain size and obtaining a surface layer nanocrystalline structure.

2. Ultrasonic rolling strengthening avoids process defects such as pits and cracks caused by traditional surface strengthening techniques by rolling impact balls on the material surface, achieving extremely low surface roughness [26]. Ultrasonic rolling strengthens and converts residual tensile stress into residual compressive stress, obtaining deeper nano gradient hardening layers and residual compressive stress affected areas, filling the gaps of low production efficiency and mismatch between working environment and high-performance material performance requirements in traditional mechanical processing methods.

3. Ultrasonic rolling strengthening has the advantages of stable performance and high processing efficiency, which can effectively improve the surface integrity of materials and further enhance their comprehensive properties such as wear resistance, fatigue resistance, erosion resistance, and corrosion resistance.

4. After ultrasonic rolling, no new substances will be produced, no delamination will occur, and the material will not peel off during service; Its processing path is controllable, and it can focus on strengthening local areas according to the actual wear of the parts; Ultrasonic rolling technology can perform integrated processing of materials through finishing and strengthening.

5. Ultrasonic rolling equipment is simple and cost-effective. No chips are generated during the processing, which can achieve low energy consumption, low pollution, and low emission production. It is a green and efficient surface engineering technology that helps achieve the "dual carbon" goal and is widely used in precision machining of mechanical processing.

limitations:

1. Ultrasonic rolling strengthening technology is generally only suitable for flat and rotating parts, and it is difficult to achieve strengthening processing for precision parts with complex structures.

When parameters such as static pressure and rolling times exceed the plastic deformation limit of the material, surface defects such as wrinkling, cracking, and crushing will appear on the material surface, and the surface after rolling treatment will also experience shear deformation and local fatigue damage, thereby reducing deformation resistance. Severe plastic deformation can cause the surface strengthening layer hardness to be too high, making it impossible for subsequent processes to continuously improve material properties.

3. High hardness alloys are difficult to machine deep strengthening layers on their material surfaces through ultrasonic rolling, thus failing to achieve the desired effect.

5、 Strengthen process comparison

Traditional shot peening, laser shock peening, ultrasonic shock peening, and ultrasonic rolling peening, as typical metal surface strengthening processes, each have their own advantages. In practical applications, users should comprehensively consider factors such as the material type, shape and size, performance requirements, and production cost of the parts to choose the most suitable strengthening process. Table 1 briefly compares the similarities and differences of four typical metal surface strengthening processes in terms of process parameters, surface roughness, residual stress, microhardness, grain size, and fatigue performance, using common titanium alloys as examples.