The microstructure of the surface layer of the machined part and the nature of the surface metal material change after machining. The mechanically processed part surface is not an ideal smooth surface, but it has various degrees of surface roughness such as rough ripples, chills, and cracks. Although only a very thin layer (0.05 ~ 0.15mm), it has a great impact on the performance of machine parts; wear, corrosion and fatigue damage of parts start from the surface of the parts, especially the modern industrial production Machines are moving toward precision, high speed, and multi-functionality. Mechanical parts work under conditions of high temperature, high pressure, high speed, and high stress. Any defects in the surface layer will accelerate the failure of the parts. Therefore, we must pay attention to the surface quality of machining.
First, the meaning of the surface quality of machining
The machining quality of machine parts not only refers to the machining accuracy but also includes the quality of the machined surface. It is the characterization of the integrity of the surface layer after machining of the part. On the surface after machining, there is always a certain deviation of the micro geometry, and the physical and mechanical properties of the surface layer also change. Therefore, the machined surface quality includes two aspects of the geometric characteristics of the machined surface and the physical and mechanical properties of the surface layer.
(I) Geometrical features of the machined surfaceThe microscopic geometric features of the machined surface mainly include the surface roughness and surface waviness, as shown in Figure 5-1. The surface roughness is a surface micro-corrugation having a pitch L of less than 1 mm, and the surface waviness is a surface wave having a pitch L of 1 to 20 mm. Normally, when the L/H (Pitch/wave height) is less than 50, it is the surface roughness, and when L/H is 50 to 1000, it is the surface waviness.
1 . Surface Roughness Surface roughness is mainly caused by the shape of the tool and the plastic deformation and vibration during the cutting process. It refers to the micro geometry of the machined surface.
2 . The surface waviness is mainly caused by the periodic shape error (L 2/H 2 in Figure 5-1) caused by the low-frequency vibration of the process system in the machining process, between the shape error (L 1/H 1 > 1000) and the surface roughness. (L 3/H 3 < 50).
(b) Physical and mechanical properties of the machined surface layer
The physical-mechanical properties of the surface layer include work-hardening of the surface layer, residual stress, and metallographic structure changes of the surface layer. Due to the combined action of cutting force and heat during machining, the physical and mechanical properties of the surface layer metal change with respect to the physical and mechanical properties of the base metal. Figure 5-2a shows the variation of the surface layer of the part along the depth. The outermost layer is formed with an oxide film or other compound, and absorbs and infiltrates the gas particles, which is called an adsorption layer. Below the adsorption layer is the compression layer, which is the plastic deformation zone caused by the action of the cutting force, and the upper part is the fiber layer produced by the crushing friction of the cutter. The effect of cutting heat will also cause phase change and grain size change in the workpiece surface layer material.
1 . Surface layer work hardening
The work-hardening of the surface layer is generally assessed by the depth of the hardened layer and the degree of hardening N:
N= [(HH 0 ) / H 0]Ã— l00%
Where H - the microhardness of the surface layer after processing;
H. - Microhardness of raw materials.
2 . Surface layer microstructure changes
In the process (especially grinding) under the effect of high temperature, the surface temperature of the workpiece increases, when the temperature exceeds the critical point of the phase transition of the material, it will produce metallurgical structure changes, greatly reducing the part performance, this change Including grain size, shape, precipitates and recrystallization. The changes in metallographic structure are mainly determined by microscopic observations.
3 . Surface layer residual stress
During processing, residual stresses are generated in the surface layer due to plastic deformation, changes in metallurgical structure, and volume changes due to temperature. At present, the determination of residual stress is mostly qualitative, and its effect on the performance of the part depends on its orientation, size, and distribution.
Second, the surface integrity
With the development of science and technology, the requirements for the use of products are getting higher and higher. Some important parts need to work under conditions of high temperature, high pressure, and high speed. Any defects in the surface layer directly affect the working performance of the parts and are suitable for science and technology. In the field of research on surface quality, the concept of surface integrity has been proposed, mainly including:
(A) The surface topography mainly describes the geometric features of the processed parts, including surface roughness, surface waviness and texture.
(b) Surface defects
It refers to the appearance of macroscopic cracks, flaws, and corrosion on the surface of the machine and has a great influence on the use of parts.
(III) Metallurgical and chemical properties of microstructures and surface layers
Including micro-cracks, microstructure changes and intergranular corrosion.
(4) Physical and mechanical properties of the surface layer
Including the depth and degree of surface layer hardening, surface layer residual stress size, distribution.
(5) Other engineering features of the surface layerIncluding friction characteristics, light reflectivity, conductivity and permeability.