Thin-walled parts have a firm foothold in this modern s […]
Thin-walled parts have a firm foothold in this modern society by virtue of their own advantages such as light weight and compact structure, especially now in the aviation field, and they are bound to have a place in the future development. However, as far as the current processing technology and processing methods are concerned, due to the low rigidity of the thin-walled parts, the problem of deformation of the workpiece in the processing engineering still limits the large-scale development of thin-walled parts, especially some Traditional machining methods are now being eliminated, and CNC machining is an inevitable development trend of workpiece machining in the future.
1. Classification and characteristics of thin-walled parts
The common thin-walled parts in life are generally circular, shell-shaped and flat-shaped. The structural size of these parts is generally much larger than its thickness, and when the ratio of its structural size or radius of curvature to its own thickness is greater than 20 At that time, we called this kind of parts thin-walled parts. Thin-walled parts can be classified according to the most basic material properties. Generally, there are titanium alloys, plastics and some composite materials. Thin-walled parts processed from these different materials have different properties and can meet the needs of various markets. This article mainly focuses on The high-precision machining method of titanium alloy thin-walled parts is analyzed. Almost all thin-walled parts are composed of thin-shaped plates and reinforcing ribs. This is also the reason for the light weight of thin-walled parts. By manufacturing thin-walled parts of different specifications, they can be applied to various purposes. According to the structural purpose, thin-walled parts can be divided into beams, joints, siding and ribs. These different types of parts have their own unique shapes and complete their respective functions. However, the characteristics of this kind of thin-walled parts make the production process more difficult. Because the thin-walled has lower rigidity, it is prone to deformation during the processing process, resulting in the production of thin-walled parts that do not meet the standards. From this, it can be seen that the analysis of the processing method of thin-walled parts and the optimization of its processing technology are very necessary, and it is of great significance to promote the development of the thin-walled parts processing industry.
2. Factors affecting the machining accuracy of titanium alloy high-precision thin-walled parts
In the actual machining process of thin-walled parts, there are countless factors that affect the machining accuracy of titanium alloy high-precision thin-walled parts. Among them, the main influencing factors can be seen in Figure 1. Generally speaking, the properties of the workpiece, the tool, the machine tool, etc. Will affect the machining accuracy of thin-walled parts. For example, if the rigidity of the workpiece itself is insufficient, the deformation that may occur during the machining process will lead to a greater loss of accuracy. Therefore, during the machining process, minimize or avoid the deformation of the workpiece as much as possible. This is to improve the thin wall The most efficient method for parts machining accuracy.
2.1. the influence of clamping factors on part deformation
The clamping process is one of the core processes of the entire thin-walled parts processing technology. No matter which processing method is used, the quality of the clamping process will directly determine the processing quality of the thin-walled parts. For the processing of thin-walled parts, the clamping plan, clamping point position and clamping force determine the quality of the processed parts. If improper clamping point position or clamping force is used, parts may be deformed to varying degrees, which will also affect the machining accuracy of parts to a large extent. Especially when processing thin-walled parts on a machine tool, the importance of the clamping process is fully reflected. Among them, 30% to 50% of the machining errors are derived from the clamping process. In addition, during the processing of thin-walled parts, the fluctuation effect of the force between the ?A tightening force and the cutting force will produce a coupling effect, which will lead to the redistribution of the machining residual stress and the initial residual stress inside the part, which will also affect the machining quality of the part. To influence. Therefore, the clamping problem of thin-walled parts is still not to be ignored. Improving the clamping technology of thin-walled parts is of great significance for preventing part deformation during processing.
2.2. the influence of cutting force and cutting heat on deformation of parts
The processing parameters of thin-walled parts directly reflect the relationship between the thin-walled parts and the processing props. In the processing of thin-walled parts, due to the low elastic modulus of titanium alloy thin-walled parts, the surface of the finished parts will be affected. Produce larger springback, which will directly lead to an increase in the contact area between the machined surface and the flank surface of the tool, which will have a very large impact on the processing quality of thin-walled parts, and the processing accuracy of the parts will drop sharply. At the same time, It will also reduce tool durability. On the other hand, if the cutting force is too large and exceeds the elastic limit of the material, it will cause plastic deformation of the part. Moreover, the existence of cutting heat is also one of the key factors that affect the quality of parts processing. Cutting heat is generated by the friction between the chips and the rake face, the machined surface of the workpiece and the flank face. A large amount of cutting heat will cause the workpiece to be damaged. The uneven temperature will also aggravate the deformation of the parts, resulting in a decrease in the machining accuracy of the parts. At the same time, the surface quality of the parts cannot be well guaranteed.
2.3. the influence of residual stress on deformation
There are two main components of residual stress in thin-walled parts. One part is the initial residual stress generated during the initial forming process of thin-walled parts. There are many reasons for this part of the residual stress. Among them, the large-area splicing of thin-walled parts is affected by the residual stress during processing. more obvious. The second part is the residual stress of the machined surface. This part of the residual stress is mainly the result of the comprehensive influence of various factors such as the mechanical and thermal effects of the tool on the surface metal of the workpiece, and the elastic recovery of the inner metal. In the processing of thin-walled parts, it is extremely possible to break the balance of residual stress in the part blank, and at this time, the internal stress balance of the part is broken, resulting in a redistribution of stress, thereby causing deformation of the part.
3. titanium alloy high-precision thin-walled parts processing technology and optimization
3.1.Considering from the structure of the workpiece
When designing the structure of the workpiece, not only the use performance of the workpiece should be considered, but also the adaptability of this structure to the processing process. The processing methods corresponding to different workpiece structures are different, in order to ensure the processing of thin-walled parts. The accuracy of the workpiece structure is particularly important. Generally speaking, the application of thin-walled parts processed from titanium alloy plates has higher accuracy requirements and usage requirements. The deformation of the parts will not only cause difficulties in the process of installation, but also may not be able to complete the design parts. What needs to be done. Therefore, in order to avoid deformation of the workpiece during processing, first, consider designing the workpiece into a symmetrical structure. This structure enables the release of internal forces of each part of the workpiece during processing to be synchronized to avoid internal force distribution. Unequal conditions. Second, in the design of the thin plate, try to ensure that the thickness of the entire thin plate is as consistent as possible, and at some corners of the workpiece, due to processing or heat treatment, stress concentration may occur. The transition can be made by designing the corner to a circular arc structure. , Thereby reducing the deformation of the workpiece.
3.2. Consider the clamping of the workpiece
The thin-walled parts themselves are thinner and only have lower rigidity, that is, the ability of the workpiece to resist elastic deformation is weak. Therefore, during the processing of the workpiece, the clamping will also affect the workpiece to a large extent. Deformed. Clamping is mainly used to fix the workpiece, using the clamping to locate the workpiece and ensure the stability of the workpiece during processing, as shown in Figure 3. Unreasonable clamping position and clamping force will cause the machining accuracy to decrease. Therefore, when selecting the clamping position, try to ensure that the clamping positions are in a symmetrical relationship, and the clamping force can be adjusted according to the rigidity of the workpiece. When the rigidity of the workpiece is high, a larger clamping force can be selected, but special attention should be paid to the fact that when the rigidity of the workpiece is low, an appropriate clamping force must be selected, otherwise it is easy to cause deformation of the workpiece during processing.
3.3.Considering from heat treatment
The general heat treatment of the workpiece is completed by quenching and artificial aging treatment, and the timing of the heat treatment of the workpiece is very important to reduce the deformation of the workpiece. Because when the workpiece is heat treated, the temperature stress and phase change stress will be generated inside the workpiece due to the change of the workpiece's own temperature, which is the main reason for the deformation of the workpiece. At the same time, heat treatment can't destroy the mechanical properties of the workpiece. Therefore, it is generally considered to arrange the timing of heat treatment before rough machining of the blank. Therefore, the timing of heat treatment should be rationalized as much as possible, so as to not only ensure the mechanical properties of the workpiece, but also reduce the deformation caused by the heat treatment of the workpiece.
3.4. considering the process method and cutting fluid
In the process arrangement of workpiece processing, firstly, according to the different composition and structure of different types of workpieces, the process arrangement should be carried out. Among them, special attention should be paid to the analysis of the easily deformed parts of the workpiece during processing and consider whether it can be Reduce the amount of deformation of the workpiece through adjustments in some processes. Secondly, when roughing the workpiece, it is necessary to reserve a large cutting margin at the beginning, and do a good job of positioning the reference surface. As the workpiece is processed, it is necessary to always pay attention to the correction of the reference surface, because the processing The reduction of the margin in the process will inevitably bring about a change in the reference level. The choice of cutting fluid is mainly based on the nature of processing and processing tools. The reasonable use of cutting fluid according to different process arrangements and tool usage will help improve the efficiency of workpiece processing.
3.5. Elimination of residual stress of thin-walled parts
The initial residual stress of thin-walled parts is generally determined by the heating factors of the blank material, and the processing residual stress is generally reflected after the processing of thin-walled parts, so the research on residual stress is worth paying attention to, how to predict The influence of residual stress and how to eliminate the influence of residual stress on the processing quality of parts.
Although the source of the residual stress in thin-walled parts is known, its influence on the deformation of thin-walled parts in processing can not be accurately determined, because the residual stress of thin-walled parts causes the deformation of thin-walled parts is generally caused by heat factors and mechanical forces. The result of the effect. At present, the control of residual stress generally uses the current more popular finite element analysis method to establish a finite element model of thin-walled parts, and uses numerical analysis to predict the impact of residual stress. In addition, this method can not only simulate the results of deformation correction of thin-walled parts, but also predict springback.
At present, the methods to eliminate the residual stress of the workpiece include pre-stretching, vibration aging, aging annealing and cryogenic treatment. Among these methods, the cryogenic treatment application is the most successful. Cryogenic treatment can effectively reduce the residual stress in thin-walled parts. At the same time, the treatment can also increase the hardness and strength of the parts, improve the wear resistance of the workpiece, and increase the service life of the parts. In addition, cryogenic treatment can ensure the dimensional accuracy of the parts and improve the internal stress distribution in the parts. In order to reduce the influence of machining residual stress on the deformation of parts, it is necessary to start from the aspect of reducing cutting heat.