1. The basic concept of residual stress

Residual stress refers to the internal stress that exists inside the part to maintain equilibrium even in the absence of external forces. According to the different stress properties, residual stress can be divided into residual compressive stress and residual tensile stress.

1. Residual compressive stress (-σ)

Residual compressive stress is the stress state in which the surface material of the part is compressed, and the stress pressure is usually conducive to improving the fatigue strength of the part.

2. Residual tensile stress (+σ)

Residual tensile stress is the stress state in which the surface material of the part is tensile, which usually adversely affects the performance of the part. The existence of residual stress will have an important impact on the machining dimensional stability, fatigue strength, stress corrosion resistance and other properties of mechanical parts, so special attention needs to be paid to the machining process of parts.

2. The main factors affecting residual stress

1、Tool geometry parametersThe impact of

1) Front angle impact

• The rake angle of the tool has a significant effect on the depth of existence of residual stress.
• Negative rake increases cutting forces and plastic deformation, resulting in a deeper stress-affected layer, so the depth of the residual stress layer can be doubled when using a negative rake tool.

2) Tool wear state

• As the amount of tool wear increases, the compressive stress value at a deeper point away from the surface of the part will increase significantly
• Severely worn tools can cause greater heat generation and mechanical deformation, which can significantly alter the distribution of residual stress.

2、Cutting parametersThe impact of

1) Feed volume

• When the feed volume increases, the tensile stress on the surface layer increases.
• As the feed rate increases, the position of the maximum compressive stress moves towards the inside of the workpiece.
• When processing large feeds, it is easy to form an unfavorable tensile stress state on the surface layer.

2) Cutting speed

• The heat generated during high-speed cutting is the most important influencing factor, and tensile stress is easy to generate at this time.
• Mechanical deformation is dominant when cutting at medium and low speeds, and compressive stress is generated at this time.
• There is a reasonable critical speed at the cutting speed, balancing the effects of heat and mechanical deformation to minimize residual stress.

3. The influence of processing methods

The residual stress distribution generated by different processing methods has different characteristics.

• Turning usually produces surface compressive stress and subsurface tensile stress.
• Grinding is prone to surface tensile stress and may also cause grinding burns.
• Milling stress states are the most complex and are closely related to tool geometry and cutting parameters.
• The stress concentration is obvious at the edge of the hole and the residual stress is large.
•  

3. Process measures to reduce residual stress

1. Tool selection and maintenance

1) Tool geometry optimization

• Choose the appropriate tool rake angle according to the material properties of the part.
• Keep the cutting edge of the tool sharp and be sure to avoid small jagged notches.
• The wear behind the control tool is about 0.2mm, and the tool is checked regularly.

2) Tool material selection

• Choose tool materials with good wear resistance and keep the state of the tool stable.
• The coating tool reduces friction during machining and reduces the effect of heat generation.

2. Optimization of process parameters

1) Cutting amount control

To reduce the impact of stress on part performance, it is necessary to select an appropriate combination of cutting speed, feed rate, and back-to-back cutter volume to find a suitable balance point that balances the requirements of machining efficiency and stress control.

2) Cooling and lubrication

Effective cooling methods are used to reduce the temperature during processing, such as high-pressure cooling, which can effectively reduce areas affected by heat.

3. Rigid guarantee of process system

1) Machine tools and fixtures

• Ensuring that the machine is as rigid as possible can reduce vibration during machining.
• Reasonable design of fixture tooling can reduce the deformation caused by clamping.

2) Drilling process improvement

• The use of guide sleeves can improve the drill bit stiffness.
• Chamfering the edges of the holes after drilling can reduce the concentration of stress.

4. Post-treatment process

1) Mechanical strengthening process

• The extrusion method is used to increase the residual compressive stress of the surface layer.
• The shot peening process can significantly improve the fatigue resistance of the parts.
• Fine extrusion gears can achieve higher tooth surface compressive stress than shaving teeth.

2) Heat treatment process

Destress annealingIt can effectively reduce residual stress, and for special parts materials, aging treatment can be adopted.

3) Composite process

Shot peening after EDM and electrolytic machining is particularly effective for superalloys and other materials.

Residual stresses are an inevitable condition in machining, but they can be effectively controlled by process optimization. By scientifically analyzing the formation principle of residual stress and taking effective control measures for the causes, the performance and service life of mechanical parts can be significantly improved.