Residual stress in the material is an unavoidable and important problem in machining, which not only increases the difficulty of the process but also affects the quality of the product.
Although aluminum alloy milling is a new processing technology, it has brought revolutionary changes to the traditional metal cutting theory and cutting processing technology.
At the same time, the aluminum alloy milling construction process has an ordinary milling speed of up to 150500 m/min and a high-speed milling speed of up to 30010000 m/min, which is far more than the speed of copper alloy, non-metallic composite materials, and other materials.
However, due to its high degree of milling process, the aluminum alloy will have a large number of residual internal stresses, seriously affecting its stability and making it prone to deformation.
Eliminating stress and ensuring product quality is a problem we must solve. Let us analyze and solve it from the following aspects.
Material structure
The test specimen is a high-strength deformation of aluminum alloy plate 7050-T7451. T7451 indicates the state of the material, meaning it has undergone hot-rolling after solid solution treatment.
Then, the material is subjected to 1.5% to 3% tensile plastic deformation along the rolling direction for a specified time, followed by aging treatment. The chemical composition of the material is shown in Table 1, and its mechanical properties are shown in Table 2.
From the specification of 915mm×265mm×45mm plate, 3 specimens with the size of 92mm×130mm×45mm were intercepted and used to measure the distribution of rolling direction and transverse residual stresses along the thickness of aluminum alloy plate, respectively.
Testing process
Ensure construction quality and complete the established tasks in the actual project construction. To achieve this, the construction process should be carried out and completed strictly with the project planning.
Internal stress detection is also necessary during aluminum alloy milling processing. The specific process is shown in Figure 1.
At the same time, in aluminum alloy milling processing, the detection of internal stress is an important step to ensure the quality and performance of parts. The points of attention for internal stress detection of aluminum alloy milling are as follows.
(1) Choose the appropriate detection method.
Nondestructive testing is required during the inspection. Commonly used nondestructive testing methods include ultrasonic testing, X-ray diffraction, magnetic particle inspection, and others. These methods do not destroy the parts.
These methods can detect the internal stress without destroying the parts. Through the stress table or related equipment for the measurement of residual stress, you can directly access the internal stress state of the parts.
(2) Focus on the detection environment.
When carrying out internal stress testing, ensure that the testing environment is stable in terms of temperature, humidity and other conditions, and avoid the influence of external factors on the test results.
(3) Data interpretation and analysis involves several key steps.
First, use appropriate equipment and methods to collect internal stress data. It is important to ensure the data’s accuracy and reliability.
Next, analyze and interpret the collected data. Compare the measured values with the standard values or historical data. Finally, judge whether the internal stress is within the acceptable range.
(4) Record and report.
Record and report the test results, including sample information, testing methods, data results and analysis conclusions for follow-up tracking and management.
By following the above precautions, internal stress can be detected effectively in aluminum alloy milling, ensuring that the quality and performance of aluminum alloy meet the requirements.
Uncertainty evaluation
When using the crack flexibility method for internal stress calculation, it is necessary to choose an interpolation function and its order.
The reasonableness of this choice is crucial. It determines whether the calculation results can best approximate the target value and maintain good endpoint stability.
For this reason, the uncertainty of the stress is evaluated. This includes the stress uncertainty caused by strain measurement errors, model errors, and the total uncertainty.
These evaluations are conducted using different interpolation functions and their orders. The goal is to find the reasonable interpolation function and the best convergence order.
The uncertainty in stress calculation has the following two main sources.
(1) Random errors from strain measurement data.
igure 2 shows the fit between strain distribution values and plate thickness. The values are obtained using the least squares method and compared with experimentally measured strain values.
In this case, the interpolation function is a Legendre polynomial of order 9. The deviation between the two results is one of the sources of uncertainty in the stress calculation.
(2) Modeling error.
When the selected level expansion can not be well fitted to the actual stress distribution in the plate and the error caused by the error is called model error.
The main source of this error is the choice of interpolation function order after the interpolation function is selected.
Strategy research
Internal stresses occur during milling of aluminum alloys, mainly due to cutting forces and thermal influences on the material during machining.
This internal stress affects the stability and performance of the part, so measures need to be taken to remove or reduce these internal stresses.
Heat treatment is an effective method of removing internal stresses generated during milling operations on aluminum alloys.
This process reorganizes the material’s microstructure through controlled heating and cooling to remove or reduce internal stresses. Below are commonly used heat treatment methods and strategies for removing internal stresses.
1. Annealing
Annealing is a common heat treatment used to reduce the hardness of aluminum alloys, increase toughness, and remove internal stresses generated during processing. The basic steps of annealing are as follows.
(1) Heating.
The aluminum alloy is heated above its recrystallization temperature, which is usually in the range of 300 to 400°C. This temperature is dependent on the specific composition of the alloy. This temperature depends on the specific composition of the alloy.
(2) Holding.
Holding at this temperature for a period of time allows the lattice structure within the material to rearrange itself and remove internal stresses. The holding time depends on the thickness of the material and the specific type of alloy and may vary from a few minutes to several hours.
(3) Slow Cooling.
The key to annealing is slow cooling, usually natural cooling to room temperature in the furnace. This step is critical because rapid cooling may reintroduce internal stresses.
2. Solid solution treatment
Solution treatment is another commonly used heat treatment process, especially for heat-treatable strengthened aluminum alloys, and is performed as follows.
(1) Heating.
The aluminum alloy is heated to a higher temperature, usually 500 to 550°C, to allow some of the components of the alloy (e.g., magnesium and silicon) to enter the solid solution of the aluminum.
(2) Rapid cooling.
Rapidly lowering the temperature by water quenching or air cooling fixes the state of these elements in the solid solution. This step helps to harden the material and remove internal stresses due to uneven distribution of the alloying elements.
(3) Aging treatment.
After solid solution treatment, aging treatment (heating to a lower temperature, e.g., 150-200°C) is usually performed to enhance the material properties further and stabilize the microstructure.
3. Stress Relief Heat Treatment
Stress-relieving heat treatment is performed at lower temperatures, usually below the annealing temperature. This process reduces internal stresses introduced during processing without significantly altering the material’s mechanical properties.
Stress-relieving heat treatment typically includes the following processes.
(1) Heating.
Heat the material to 150~200℃.
(2) Holding.
Hold at this temperature for a certain period of time, usually 1 ~ 2h.
(3) Slow cooling.
Allow the material to cool naturally to room temperature in the furnace.
Among the stress relief strategies is the de-thermalization method, and the other is mechanical treatment, such as vibratory stress relief. Vibratory treatment of aluminum alloy parts after machining can reduce internal stresses and make them more stable.
Mechanical treatment methods can be used as a relatively simple and economical way to reduce internal stresses generated during milling and machining of aluminum alloys.
However, some applications that require high material properties may require a combination of other more elaborate treatments, such as heat treatment or residual stress measurement and analysis, to ensure that the part’s quality and performance meet the requirements.
4. Vibratory Stress Relief
Vibration stress relieving is a method for reducing internal stresses by applying mechanical vibration to the part after milling.
This method suits applications where material property requirements are not particularly stringent. It is often used for structural or appearance parts.
(1) Vibration equipment selection.
Selection of appropriate vibration equipment is very important to achieve effective stress relief.
Usually, a specially designed vibration table or system is used; its frequency, amplitude, and duration can be adjusted.
(2) Vibration parameter adjustment is essential for effective stress relief.
The frequency, amplitude, and duration of vibration must be adjusted based on the material type, part size and shape, and the degree of internal stress.
The optimal vibration parameters are usually determined through experiment and experience.
(3) Vibration post-treatment includes a vibration process to reduce internal stress.
Vibration process: The milled aluminum alloy part is placed on the vibration equipment, which is then activated to initiate the vibration process.
During vibration, the internal structure of the part moves and adjusts slightly. This adjustment helps to reduce internal stress.
(4) Vibration post-treatment involves additional steps after the vibration process.
Surface cleaning and inspection of the part may be required to ensure that the treatment does not adversely affect its size and shape.
Figure 3 shows the internal stress profile of an aluminum alloy after applying a vibratory stress relief strategy.
5. Ultrasonic treatment
Ultrasonic treatment is a mechanical method that uses high-frequency vibration on the material’s surface and internal structure. This process promotes the release and reduction of internal stress.
At the same time, ultrasonic impact treatment effectively removes local welding repair stress.
(1) Ultrasonic equipment selection.
The key is selecting the appropriate ultrasonic equipment. These devices usually produce high-frequency vibration that is transferred to the surface of the workpiece.
(2) Ultrasonic treatment parameter settings.
Ultrasonic treatment parameters such as frequency, power, and duration need to be set according to the type of material and size of the part.
Excessive power and frequency may damage the part, so they need to be carefully adjusted.
(3) The ultrasonic treatment begins with placing the milled aluminum alloy parts under the ultrasonic equipment.
The treatment is then initiated according to the preset parameters.
The vibratory action of the ultrasonic waves promotes the adjustment of the material’s internal structure at the microscopic level, which helps reduce internal stress.
(4) Ultrasonic post-treatment.
After treatment, the part may need to be cleaned and inspected to ensure that the treatment process does not adversely affect the part.
Using heat treatment, mechanical treatment and optimization of the cutting process, the internal stresses generated during the milling of aluminum alloys can be effectively removed or reduced, thus improving the quality and performance of the parts.
Conclusion
Aluminum alloy milling technology significantly improves product quality and reduces manufacturing costs.
However, high-speed milling is prone to residual stresses within the material. These stresses affect stability and lead to deformation.
Therefore, it is crucial to detect and eliminate internal stresses.
For detection, non-destructive testing methods (e.g., ultrasonic, X-ray diffraction, etc.) are commonly used. These methods help ensure a stable environment and maintain data accuracy.
When calculating stresses using the crack flexibility method, the interpolation function and order need to be optimized. This optimization improves accuracy.
For stress relief, the main strategies include:
Heat treatment: annealing, solution treatment and stress relief heat treatment to release stresses by heating and slow cooling.
Mechanical treatments: vibration stress relief and ultrasonic treatments to reduce stress through mechanical vibration or high frequency vibration.
The stability and performance of aluminum alloy parts can be effectively improved through scientific testing and elimination strategies.
