From Process to Performance: an in-depth Analysis of the Surface Elements of Machined Metal Parts

I. Introduction

In the field of mechanical processing, the surface quality of metal parts is like a crucial screw. Although it seems insignificant, it plays a pivotal role in the performance of the entire mechanical system. Whether it's the components with stringent requirements for precision and reliability in the aerospace field, the various accessories that frequently endure friction and impact in automobile manufacturing, or the fine and tiny metal components within electronic equipment, the quality of the metal parts' surface is directly closely linked to the product's service life, stability, and functionality.

From the most intuitive perspective, a high-quality metal part surface means it is smooth and flawless. This not only enhances the aesthetic appearance of the product, meeting consumers' growing aesthetic demands, but also reduces frictional resistance and energy loss during actual use, enabling mechanical equipment to operate more smoothly and efficiently. Just imagine that in high-precision instruments and equipment, even the tiniest surface protrusions or depressions could cause excessive wear between components during long-term operation, which would then lead to a decline in equipment precision and even malfunctions.

Delving into the mechanical performance aspect, good surface quality of metal parts can effectively enhance the wear resistance of materials. Under working conditions where parts are subject to repeated friction and forces, they can still maintain stable dimensions and shapes, greatly extending their service life. Moreover, when facing complex and variable stress environments, a high-quality surface can evenly distribute stress and avoid the occurrence of stress concentration, thereby significantly improving the fatigue strength of parts and enabling the mechanical system to operate reliably under high work loads.

In today's highly competitive manufacturing market, controlling the surface quality of metal parts has become the foundation for enterprises to establish themselves. It is not only the key to meeting customers' demands for high-performance and high-quality products but also a core element for enterprises to reduce production costs, improve production efficiency, and enhance market competitiveness. Next, let's explore in depth what specific and strict requirements mechanical processing factories have for the surface of metal parts.

II. Surface Roughness — The Critical Influence of Micro-irregularities

(I) Definition and Measurement

Surface roughness refers to the micro-geometric shape errors composed of small intervals and tiny peaks and valleys on the machined surface. When we observe the surface of a metal part with the naked eye, it may seem smooth and flat, but under a microscope, we can clearly see the undulating peaks and valleys, just like mountains and valleys in the microscopic world. The intervals between these peaks and valleys are extremely small, usually less than 1mm, which are characteristics at the microscopic level.

In mechanical processing, the commonly used evaluation parameters for surface roughness include the arithmetic mean deviation of the profile Ra, the ten-point height of irregularities Rz, and the maximum height of the profile Ry, etc. Among them, Ra is the most commonly used. It is the arithmetic mean of the absolute values of the profile deviations within the sampling length and directly reflects the average degree of microscopic undulations on the surface. For example, in the processing of precision bearings, the Ra value may be required to be controlled within a very small range, such as below 0.2μm, to ensure the smoothness and stability of the bearings during high-speed operation.

There are various methods for measuring surface roughness, which can be divided into contact measurement and non-contact measurement. Contact measurement, such as the stylus method, uses an extremely fine diamond stylus to slowly slide across the measured surface at a constant speed. The sensor accurately records the up-and-down movement trajectory of the stylus and then obtains the value of surface roughness. This method has high precision and can directly obtain micro-profile information, but it requires careful operation to prevent the stylus from damaging the measured surface. Non-contact measurement includes the light-section method, laser interferometry, etc., which use optical principles to indirectly measure surface roughness through phenomena such as the reflection, refraction, or interference of light. It will not cause contact damage to the workpiece surface and is suitable for measuring some precision parts with extremely high requirements for surface integrity.

(II) Influence on Performance

The influence of surface roughness on the performance of metal parts is extensive and profound. It is like the butterfly effect, where slight surface differences can trigger a series of chain reactions that affect all aspects of the parts' performance in use.

In terms of wear resistance, surface roughness plays a crucial role. When two metal surfaces are in contact and have relative motion, it is actually the microscopic peaks on the surfaces that bear the pressure and friction. If the surface roughness is too large, the pressure at the peak points will increase sharply, and the material is extremely prone to plastic deformation and shear failure, thus exacerbating wear. Take the piston and the inner wall of the cylinder in an automobile engine as an example. A rough inner wall of the cylinder will cause the piston rings to wear out quickly during reciprocating motion, resulting in a decrease in engine power, an increase in fuel consumption, and even malfunctions. On the contrary, if the surface is too smooth, it will be difficult for lubricating oil to form a stable oil film on the surface. Under high-load working conditions, the probability of direct contact between metal surfaces increases, which will also lead to aggravated wear. Therefore, parts under different working conditions have their optimal values of surface roughness, which need to be determined through precise calculations and a large amount of practical experience.

Fatigue strength is also an important performance indicator affected by surface roughness. Under the long-term action of alternating loads, stress concentration is very likely to occur at the microscopic valleys on the part surface, just like "fatigue bombs" buried on the part surface. In these stress concentration areas, with the increase in the number of load cycles, tiny cracks will gradually initiate, and the cracks will continue to expand until the part fractures and fails. For example, the blades of an aero-engine, which bear huge alternating stresses when rotating at high speed, have extremely high requirements for surface roughness. Even the slightest surface flaws could lead to catastrophic consequences during flight.

Corrosion resistance is also closely related to surface roughness. The microscopic valleys on a rough metal surface are like "traps" that accumulate dirt. Dust, moisture, corrosive media, etc. are easy to accumulate here and gradually penetrate into the metal interior, accelerating the corrosion process. For metal components used in the marine environment, such as the hulls of ships and the supporting structures of offshore platforms, if the surface roughness does not meet the standards, the corrosive components such as salts in seawater and sea wind will quickly erode the metal, greatly shortening the service life of the components.

In terms of fitting accuracy, the influence of surface roughness cannot be underestimated. For parts with clearance fits, such as the shaft and the bearing, if the surface roughness of the shaft is too large, due to the wear effect in the initial stage of operation, the fitting clearance will increase rapidly, resulting in a decline in equipment precision and the generation of vibration and noise. For parts with interference fits, such as the assembly of the hub and the shaft, a rough surface will reduce the actual interference amount during assembly, lower the connection strength, and affect the reliability of the entire structure.

III. Surface Hardness — A Solid Defense Against Wear

(I) Importance Manifestation

Surface hardness is a key performance indicator for metal parts to resist external pressure, scratching, and wear on their surfaces. It is like a "protective armor" on the metal surface, and its importance is self-evident.

In terms of wear resistance, hardness is closely related to wear resistance, just like the relationship between a foundation and a high-rise building. A metal surface with higher hardness can effectively resist the cutting and plowing effects of external particles and mating parts during the friction process, reducing material loss. For example, in the crusher hammers of mining equipment, when facing extremely hard ores, if the surface hardness of the hammers is insufficient, the impact and friction of the ores will quickly cause the surface materials of the hammers to peel off, resulting in changes in the shape of the hammers, a significant reduction in crushing efficiency, and a sharp shortening of the service life. However, when the surface of the hammers is specially treated to have high hardness, they can maintain good crushing performance during long-term and high-intensity operations, greatly improving production efficiency and reducing the frequency of equipment replacement, thus saving costs for enterprises.

Compressive resistance is also closely related to surface hardness. During the operation of the mechanical system, many metal parts need to bear huge pressures, such as the molds of large stamping machines and the guide rails of machine tools. If the surface hardness is insufficient, under the action of high pressure, the surface is very prone to plastic deformation, forming pits, collapses, and other defects, which not only affect the precision and function of the parts themselves but also may cause malfunctions of the entire mechanical system. Take the crankshaft of an automobile engine as an example. It bears periodic huge pressures from the piston connecting rods during work. If the surface hardness of the crankshaft is poor, after long-term operation, the journal part may have depressions, destroying the fitting accuracy with the bearing, resulting in increased engine vibration, a decrease in power, and even serious mechanical failures.

Surface hardness has a decisive influence on the service life of metal parts. High-quality surface hardness can continuously resist various wears, corrosions, and external impacts during the service period of the parts, ensuring stable dimensions and reliable performance of the parts. In the aerospace field, the key components of satellites need to operate in the extremely harsh space environment for many years. Facing challenges such as cosmic rays, micrometeorite impacts, and drastic temperature changes, metal parts with high-hardness surfaces can effectively prevent surface damage, maintain the integrity of the internal structure, ensure the long-term stable operation of the satellite system, avoid catastrophic consequences caused by component failures, and lay a solid foundation for the success of aerospace missions.

(II) Common Strengthening Methods

To improve the surface hardness of metal parts, the field of mechanical processing has developed a series of effective strengthening methods. These methods are like endowing metals with superpowers, enabling them to better cope with complex working conditions.

Carburizing is a classic surface strengthening heat treatment process, just like injecting a "hard power" into the metal surface. It is mainly applicable to low-carbon steel or low-alloy steel. The specific operation is to place the workpiece in an active carburizing medium and heat it to the single-phase austenite region at 900 - 950 degrees Celsius and hold it for a sufficient time. During this process, the active carbon atoms decomposed from the carburizing medium are like well-trained little soldiers, infiltrating into the surface layer of the steel part one after another, greatly increasing the carbon content of the surface layer and thus obtaining high hardness. After carburizing, the workpiece also needs to undergo quenching and low-temperature tempering treatments to further optimize the organizational structure, improve surface hardness, wear resistance, and fatigue strength, while maintaining the toughness and plasticity of the low-carbon steel in the core. For example, in the gear processing of automobile transmissions, the carburizing process is widely used. After treatment, the hardness of the gear tooth surfaces is significantly improved, enabling them to withstand frequent meshing impacts and friction, ensuring efficient and stable power transmission and greatly extending the service life of the gears.

Nitriding, also known as nitriding treatment, is also an extremely important chemical heat treatment method, just like putting a hard "protective coat" on the metal surface. Nitriding usually adopts medium carbon steel containing Cr, Mo, Al, Ti, V, such as the commonly used 38CrMoAl steel. The workpiece is placed in a specific medium and heated to 500 - 570 °C to make the nitrogen atoms in the medium infiltrate into the surface layer of the workpiece. After nitriding, the surface hardness of the workpiece is extremely high, reaching 69 - 72HRC, and its wear resistance, fatigue strength, and corrosion resistance are all significantly enhanced. Moreover, since the nitriding temperature is relatively low, the deformation of the workpiece is small, which is particularly crucial for some precision parts. In the processing of the lead screws of precision machine tools, after nitriding treatment, the surface hardness of the lead screws is increased, enabling them to effectively resist wear during high-precision transmission, ensure positioning accuracy, and operate stably for a long time, providing a reliable guarantee for precision processing.

Quenching is a heat treatment process that improves hardness by changing the metal's organizational structure through rapid cooling, just like giving the metal a "quenching into steel" baptism. For materials such as medium carbon steel, the workpiece is heated above the critical temperature, held for a certain time, and then quickly immersed in water, oil, or other cooling media for cooling. During the rapid cooling process, a martensite structure is formed on the surface of the workpiece, and the hardness is greatly increased. For example, after quenching, the cutting edge hardness of cutting tools is significantly enhanced, enabling them to easily cut metal materials to meet the cutting requirements in mechanical processing. However, the quenching process requires precise control of the heating temperature, holding time, and cooling speed, otherwise, it is easy to cause defects such as workpiece cracking and deformation, affecting product quality.

In addition to the above heat treatment methods, surface coating technology also shines brightly in the field of strengthening the surface hardness of metal parts, providing diversified protection and strengthening means for metal parts. Electroplating is a common coating technology that deposits a metal coating on the surface of metal parts through electrolysis, such as chromium plating and zinc plating. The chromium plating layer has high hardness and good wear resistance and is often used for decoration and wear-resistant protection, such as in automobile wheels, bathroom hardware, and other products. The zinc plating layer is mainly used for the corrosion prevention of steel parts and is widely applied to outdoor steel structures, power transmission towers, and other facilities, extending the service life of metal parts. Electroless plating uses chemical reactions to reduce and deposit metal ions on the workpiece surface to form a coating without the application of an external current. For example, electroless nickel plating has a uniform and dense coating, which can effectively improve the surface hardness and corrosion resistance of parts and plays an important role in protecting precision parts in fields such as electronics and chemical industry. Thermal spraying technology is like putting a multifunctional "armor" on metal parts. It sprays materials in a molten or semi-molten state, such as metals, ceramics, alloys, etc., onto the workpiece surface after atomization by high-speed airflow to form a coating. This coating not only has high hardness but also has multiple functions such as wear resistance, corrosion resistance, and heat insulation. It performs excellently in protecting parts working in high-temperature, heavy-load, and corrosive environments, such as aero-engine blades and metallurgical rolls, greatly expanding the application range and service performance of metal parts.

IV. Surface Finish — The Dual Pursuit of Aesthetics and Function

(I) Symbol of Aesthetics and Quality

Surface finish is the direct manifestation of the appearance of metal parts, just like a mirror that reflects the quality and style of the product. In the consumer market, metal products with high surface finish can often attract consumers' attention at first sight and arouse their purchasing desire.

Taking high-end electronic products as an example, such as Apple's series of products, the metal parts of their casings are finely polished to present a mirror-like smooth texture. They not only feel smooth to the touch but also shine brightly under light, demonstrating a high-end and fashionable brand image. This excellent surface finish makes the products stand out among numerous competitors and becomes a symbol of quality and taste in consumers' minds, greatly enhancing the brand's added value and market competitiveness.

In the automotive field, the finish of metal panels, wheels, and other components on the vehicle body also attracts much attention. For a luxury car, the metal body under the paint is carefully polished to have extremely high flatness and minimal surface flaws. When light shines on it, it reflects a smooth and natural luster, just like a beautiful work of art. The polished wheels complement the body, showing the refinement and luxury of the vehicle, making the driver feel honored and also winning a good reputation for the brand.

For daily necessities, such as furniture hardware accessories and bathroom fixtures, good surface finish directly improves the user experience and the beauty of the home environment. Smooth door handles and faucets are not only easy to clean but also give people a clean and comfortable visual feeling, adding a touch of refinement and warmth to the home environment. It can be said that surface finish plays an irreplaceable and crucial role in the aesthetic presentation and quality shaping of products and is an important focus for enterprises to create high-quality products and establish brand images.

(II) Implementation Processes

To achieve the desired surface finish, the field of mechanical processing uses a series of exquisite processes, which are like the brushes in an artist's hands, carefully carving the metal surface.

Polishing is one of the most common and widely used processes. Its core principle is to gradually remove the tiny protrusions and flaws on the metal surface through mechanical, chemical, or electrochemical actions to make the surface smoother and flatter. Mechanical polishing is like a fine grinding feast. It uses tools such as polishing wheels and polishing pastes to contact and move relatively to the metal surface with a certain pressure and speed. The polishing wheel is usually made of soft and elastic materials such as wool and canvas. Different grit polishing pastes are used, progressing from rough polishing to fine polishing. In the rough polishing stage, a polishing paste with a larger grit is used to quickly remove obvious scratches and machining marks on the surface. As the polishing process progresses, a polishing paste with a finer grit is used for fine polishing. At this time, the friction between the polishing wheel and the metal surface is more delicate, which can effectively eliminate subtle surface defects and greatly reduce the surface roughness, finally presenting a mirror-like shiny effect. For example, in jewelry processing, the polishing treatment of gold and silver jewelry can make the jewelry surface shine brightly and show luxury.

The grinding process focuses on performing high-precision cutting processing on the metal surface through grinding tools such as grinding wheels to achieve the purpose of reducing surface roughness and improving surface finish. According to the requirements of processing precision, grinding can be divided into multiple stages such as rough grinding, semi-finish grinding, and finish grinding. During rough grinding, a grinding wheel with a larger grit and higher hardness is selected to quickly remove the allowance on the workpiece surface with a larger cutting depth, laying the foundation for subsequent processing. In the semi-finish grinding stage, the grit of the grinding wheel is reduced, and the cutting depth becomes shallower to further refine the surface. Finish grinding uses a grinding wheel with an extremely fine grit to perform micro-cutting on the surface with a very small cutting amount to ensure that the surface flatness and finish reach extremely high standards. In the processing of machine tool guide rails, the grinding process can make the surface of the guide rails obtain extremely high straightness and finish, ensuring the high precision and stability of the movement of the machine tool.

Electrolytic polishing is an advanced process based on electrochemical principles. It cleverly utilizes the selective dissolution characteristics of metals in the electrolyte to achieve surface finish. The metal workpiece is immersed as an anode in a specific electrolyte. After connecting to direct current, the microscopic protrusions on the metal surface dissolve first because of the relatively large current density, while the recessed parts dissolve at a slower rate. As the electrolytic process continues, the microscopic irregularities on the surface are gradually "smoothed out", the roughness is significantly reduced,

and at the same time, a dense oxide film is formed on the surface, further enhancing the surface brightness and corrosion resistance. This process is especially applicable to precision parts with complex shapes and extremely high requirements for surface quality, such as stainless steel surgical instruments in medical devices and some tiny components in the aerospace field. It can not only meet the requirements for high-precision surface finish but also avoid the stress concentration and deformation problems that may be brought about by traditional mechanical processing.