In the field of CNC precision machining, the surface quality and mechanical properties of parts directly determine their service reliability and lifespan. Shot peening, as a core surface strengthening and finishing technology, is widely used in high-end manufacturing fields such as aerospace, automotive manufacturing, and mold processing due to its advantages of high efficiency, environmental friendliness, and controllability.
Shot peening is not simply a surface cleaning process; rather, it uses scientific technology to precisely strengthen CNC-machined parts, fundamentally improving their overall performance. Compared to traditional surface treatment processes, it does not require high temperatures or chemical reagents, falling under the category of cold working. This avoids deformation caused by high temperatures and allows for precise performance control, making it a crucial bridge connecting CNC cutting and finished product applications.
The core principle of shot peening is to use high-speed spherical projectiles traveling at 50 to 100 meters per second to impact the surface of a workpiece, causing controlled plastic deformation of the surface metal and forming a residual compressive stress layer 0.1 to 0.5 millimeters deep. This compressive stress layer acts as a “protective layer,” effectively offsetting the tensile stress generated during the part’s service life and inhibiting the initiation and propagation of surface microcracks.
In practical applications, shot-peened parts can experience a 30% to 300% increase in fatigue life, with some critical components even seeing a 3 to 5-fold increase. Simultaneously, the impact of the high-speed projectiles efficiently removes tool marks, burrs, and surface oxide scale from CNC machining, creating a uniform matte texture on the part’s surface. This not only optimizes the appearance but also significantly improves the adhesion of subsequent processes such as spraying, electroplating, and bonding, laying a solid foundation for further processing.
Pre-treatment is a fundamental step in shot peening, directly affecting the uniformity of the subsequent strengthening effect. Its core task is to remove impurities, oil, and loose rust from the part surface. This typically involves degreasing, deburring, and ultrasonic cleaning to ensure full contact between the shot and the part surface, preventing impurities from isolating the shot and causing insufficient or uneven strengthening in certain areas.
Case Study: A precision mold manufacturer performed shot peening on CNC-machined injection mold cavities. Due to incomplete removal of cutting oil from the cavity surface, localized “pitting” appeared after shot peening, resulting in uneven strengthening. Subsequent optimization of the pre-treatment process, adding two steps of high-temperature degreasing and ultrasonic cleaning, thoroughly removed surface impurities, increasing the shot peening pass rate from 89% to 99.5%.
Shot peening is crucial for determining the strengthening effect of parts, and its core lies in the selection of the peening medium, parameter control, and equipment compatibility. The shot medium must be precisely selected based on the material and performance requirements of the parts. Common types include steel shot, glass beads, ceramic shot, and stainless steel shot. Steel shot is suitable for strengthening high-strength parts, glass beads are used for fine surface finishing, ceramic shot has excellent wear resistance, and stainless steel shot is suitable for parts with high rust prevention requirements.
Controlling the shot peening parameters is equally critical. The pressure needs to be controlled between 0.4 and 0.8 MPa, the shot diameter should be selected between 0.1 and 1.0 mm, the impact angle should be maintained between 30° and 90°, and the coverage rate should reach 100% to 200%. Utilizing the multi-axis linkage function of CNC shot peening machines, precise machining of complex curved surfaces, deep holes, and thin-walled CNC precision parts can be achieved, ensuring that every part receives a uniform strengthening effect.
The post-processing stage mainly includes dust removal, removal of residual shot, and performance testing. Dust removal prevents shot dust from adhering to the surface of parts and affecting subsequent processes; removing residual shot prevents wear or malfunctions caused by friction between residual shot and the internal parts during service; performance testing is the final check, using methods such as roughness testing, stress testing, and coverage testing to verify whether the shot peening effect meets design requirements and ensures that the part quality meets standards.
The core value of shot peening in CNC machining lies primarily in three aspects: improved mechanical properties, optimized surface quality, and environmental compatibility. In terms of mechanical properties, besides significantly enhancing the fatigue resistance of parts, it also strengthens wear resistance and stress corrosion resistance through the synergistic effect of surface hardening and residual compressive stress. Simultaneously, it eliminates tensile stress generated during CNC cutting and welding, reducing subsequent deformation and ensuring dimensional accuracy.
Regarding surface quality, shot peening can completely remove tool marks and flash left by CNC machining, controlling the surface roughness between Ra 0.8 and 6.3 μm, resulting in a uniform matte finish. Its environmentally friendly cold-working characteristics do not produce chemical pollution, meeting the green development needs of modern manufacturing and eliminating the need for additional investment in environmental protection equipment.
Shot peening is highly adaptable, and its performance can be improved for both common metal materials such as steel, stainless steel, aluminum alloy, and titanium alloy, as well as high-end special materials such as nickel-based alloys.
Engine blades, turbine disks, landing gear, and other components in the aerospace field must withstand high-frequency alternating loads and extreme environments, requiring extremely high fatigue life and resistance to stress corrosion. Statistics show that 78% of these critical components require shot peening.
Case Study: An aerospace component manufacturer produces titanium alloy engine blades for a certain type of passenger aircraft. After being machined by five-axis CNC, the blade surface has fine tool marks, which can easily become sources of fatigue cracks. Using 0.2mm diameter ceramic shot at a pressure of 0.5MPa, multi-axis simultaneous full-coverage shot peening (200% coverage) resulted in a 0.3mm deep residual compressive stress layer on the blade surface, increasing fatigue life by 4.2 times and fully meeting stringent aerospace standards.
Automotive chassis and powertrain components such as crankshafts, connecting rods, gears, and suspension springs are subjected to long-term impact and alternating loads. Shot peening can effectively improve their wear resistance and fatigue resistance, reducing the probability of failure.
Case Study: A stainless steel steering knuckle from an automotive parts company, after CNC milling and turning, underwent stainless steel shot peening (0.6mm diameter, 0.6MPa pressure) to strengthen key stress-bearing areas and finish other parts. After treatment, the steering knuckle’s fatigue strength increased by 85%, and its wear resistance improved by 30%, successfully meeting the overseas customer’s dual requirements for appearance and performance, resulting in a 40% year-on-year increase in orders.
In mold making, shot peening improves the surface wear resistance and demolding performance of injection molds and die-casting molds. In medical devices, shot peening optimizes the surface roughness of implants and surgical instruments, improving biocompatibility and preventing tissue rejection.
Case Study: A medical device company’s titanium alloy orthopedic implants had a surface roughness Ra of 3.2μm after CNC machining, which did not meet implantation standards. Using glass bead shot peening (0.1mm diameter, 0.3MPa pressure), the roughness was optimized to Ra 0.8μm, creating a fine micro-texture on the surface. Biocompatibility improved by 30%, and the implant successfully passed medical certification.
In CNC machining, shot peening is often confused with sandblasting and shot blasting. Although all three are surface treatment processes, their core principles, process effects, and applicable scenarios differ significantly. Inappropriate selection can directly affect part quality. The specific differences are shown in the table below:
| Type | Power source | Media | Effect | Applications |
| Shot peening | Compressed air or centrifugal force | Spherical projectiles (steel shot, glass beads, etc.) | Residual compressive stress strengthening and precision finishing | Precision parts, complex curved surfaces, fatigue-critical components |
| Sandblasting | Compressed air | angular sand grains | Surface cleaning, roughening, and deoxidation | Pre-treatment before painting, rust removal of parts |
| Shot blasting | Centrifugal impeller | Steel shot or steel wire segments | High-efficiency cleaning, shallow reinforcement | Large structural components, batch castings |
Case Comparison: A certain agricultural machinery company initially used shot blasting to treat CNC precision gears. However, due to uneven shot blasting intensity, scratches appeared on the gear surface, and the fatigue life did not meet requirements. Subsequently, they switched to shot peening, using steel shot as the medium and precisely controlling the parameters. This resulted in a 2-fold increase in gear fatigue life, and the surface quality fully met standards, clearly demonstrating that precision parts are better suited for shot peening.
Shot peening is suitable for most metallic materials, including steel, stainless steel, aluminum alloys, titanium alloys, and nickel-based alloys. These materials have good plasticity and can withstand the plastic deformation caused by shot impact, forming a stable residual compressive stress layer.
However, it is crucial to note that brittle materials such as ceramics and glass have poor plasticity, and high-speed shot impact can easily cause parts to crack or break. Therefore, shot peening must be used with caution. If shot peening is necessary, extremely fine shot and extremely low pressure must be used, and small-batch tests should be conducted beforehand.
CNC shot peening machines can achieve a positioning accuracy of ±0.1mm, meeting the machining needs of CNC precision parts such as complex cavities, deep holes, and thin-walled parts. However, during the machining process, it is necessary to design the fixtures reasonably to avoid deformation of the parts during shot peening—especially for thin-walled parts, flexible fixtures should be used to disperse the impact force of the shot to prevent warping and deformation of the parts.
The hardness and size of the shot, as well as the pressure and velocity of the shot peening, all directly affect the strengthening effect. The harder and larger the shot, the deeper the compressive stress layer, but the higher the surface roughness of the part. Higher pressure and velocity result in a stronger strengthening effect, but over-peening must be avoided to prevent deformation or cracking of the part. In actual production, the optimal parameters need to be determined through experiments based on the part’s material, size, and performance requirements.
Post-shot peening inspection is crucial for ensuring quality. Typically, Almen test specimens are used to monitor shot peening intensity, and a coverage meter is used to ensure uniform surface hardening. This is combined with roughness testing and stress detection to comprehensively verify the shot peening effect. For parts in high-end fields such as aerospace and medical applications, fatigue life testing is also required to ensure full compliance with design standards.
In practical applications, shot peening processes are affected by factors such as parameters, equipment, and pretreatment, which may lead to various problems. Targeted countermeasures need to be taken to solve these problems and ensure process stability.
Shot peening, a key surface treatment technology in CNC precision machining, not only optimizes the surface quality of parts but also fundamentally improves their mechanical properties and extends their service life, providing crucial support for the high-quality development of high-end manufacturing.
With the continuous upgrading of CNC machining technology, shot peening is evolving towards greater precision, efficiency, and environmental friendliness. For example, the application of CNC shot peening robots has achieved automated, intelligent, and precise strengthening, while the development of new environmentally friendly shot has further reduced production pollution. In the future, shot peening will play a vital role in more precision manufacturing fields, helping companies create higher-quality core components and expand into broader market opportunities.
