Abstract:At this stage,titanium alloy materials have gradually become the main raw materials of large-scale precision castings.In order to effectively solve the practical production problems existing in the NC machining process,such as complicated process flow,difficult to control machining deformation,poor local rigidity of castings and difficult machining of local characteristics,it is necessary to start from residual detection The positioning mode and process equipment are studied,and the targeted optimization strategy is designed to improve the NC machining mechanism of titanium alloy castings.

Key words:application analysis；technical research；NC machining；titanium alloy

1 Introduction

  In the industrial field, numerical control technology is gradually applied to the process of mechanical mold manufacturing, which not only greatly improves the actual production efficiency of China 's industry, but also effectively reduces the industrial production cost and ensures the overall quality of mechanical products. The extensive application of numerical control processing technology in the industrial production process has significantly accelerated the development speed of industrial enterprises, and its available economic benefits have been improved linearly. In this context, the titanium alloy material itself has the characteristics of good mechanical properties, high basic strength and strong corrosion resistance. Therefore, it is widely used in the production process of modern industry. Titanium alloy castings can not only inherit many advantages of titanium alloy materials, but also easily and effectively form large thin-walled complex structures, so as to improve the utilization rate of raw materials, effectively reduce the workload of machining, optimize the production cycle to a certain extent, and reduce the cost of CNC machining.

2 Application Analysis of Titanium Alloy Precision Casting

2.1 Analysis of machining characteristics of titanium alloy precision castings

  Against the backdrop of continuous economic growth, China's comprehensive development capacity is constantly improving, with the degree of industrialization deepening. In the process of industrial modernization, modern industry, mainly the mechanical mold industry, is vigorously developing. The continuous optimization of industrial technologies such as CNC machining technology has raised higher requirements for the efficiency and quality of CNC machining in the social market. Due to the complex overall structure of titanium alloy precision castings, coupled with issues such as weak local rigidity, there are many and complex relationships that need to be coordinated during the production and processing of large titanium alloy precision castings, making it difficult to control the deformation of castings. Therefore, in terms of CNC machining, the control requirements for the machining process and the perfection of the process plan are very high, Specifically, it is reflected in the following three points: firstly, there are many processes that require positioning during CNC machining, resulting in repeated positioning problems, and the clamping work during CNC machining is difficult; Secondly, the material removal amount and local allowance of the casting are not uniform enough, such as machining vibration and cutting deformation problems that are prone to occur during CNC machining; Finally, the structure of large titanium alloy precision castings is relatively complex, so the CNC machining process is relatively long. At present, China's industry and the widespread use of precision casting processing technology have basically met the standards and needs of economic construction. However, due to the problems of excessive grinding volume by fitters, relatively cumbersome process flow, and numerous human intervention factors in the application of CNC machining technology, the processing efficiency and quality of titanium alloy precision castings have been seriously affected.

2.2 Characteristics of large titanium alloy precision castings

  The first aspect is the characteristics of large titanium alloy precision castings in terms of part structure. Large titanium alloy precision castings are generally parts of the overall frame type. As shown in Figure 1, the net height in the Z direction is usually above 650 mm, and the effective support area inside the component is small, and the local rigidity of the component is relatively poor. There are relatively many thin-walled structures on the surface of the casting, and most of the thickness of the ribs is 2-3mm, and the machinability is relatively poor ; titanium alloy precision castings generally also have four diameter steps of the focus hole and deep groove cavity lugs and so on numerical control processing is very difficult structure, at the same time, the groove width, coaxiality and aperture construction accuracy requirements are also very high.

Fig.1 Typical titanium alloy precision castings

  The second aspect is the characteristics of the casting blank. Although the blank size of titanium alloy precision castings has been basically fixed at present, it is difficult to control accuracy errors during the forging process of titanium alloy castings. Therefore, in the subsequent CNC machining process, the following two problems are also prone to occur: on the one hand, it is difficult to achieve effective coordination between non machined and machined surfaces, and it is easy to encounter machining step problems during the machining process, Significantly increased the actual workload of bench workers' polishing work; On the other hand, it will exacerbate the problem of uneven machining allowance of castings, leading to serious deformation of castings during CNC machining operations.

  The third aspect is the analysis of the deformation characteristics of the parts. If the stress distribution of the titanium alloy large precision castings is not uniform, it is easy to cause serious deformation problems of the titanium alloy castings. Because most of the large titanium alloy precision castings are semi-closed frame structure, and the internal strong support is less, the rear end face of the precision casting will also show an open structure, the rigidity of the part structure is poor, and the process rib structure is not strengthened, so it is easy to appear in the process. Deformation problems such as opening end tension, height direction dislocation and surface bending, the main deformation factors include excessive removal of local materials and uneven release of internal stress. After the blank casting is formed, the distribution of the microstructure of the precision casting is not uniform enough, which leads to the problem of uneven release of thermal stress.

2.3 The development process of titanium alloy precision casting technology

In different periods, the forging technology and quality control process of titanium alloy precision castings have different characteristics, and are constantly improving and optimizing. Different process periods correspond to the following technologies respectively.：

The first is ordinary casting technology. In the early stage of the application of titanium alloy precision casting technology, this process was mainly used to produce medium-sized ordinary precision castings with medium size and simple shape. Most of the sizes of this type of precision castings are within 500 mm, and the shapes are mostly rod-shaped, disc-shaped, flat-shaped or ring-shaped, etc., and most of the wall thicknesses are within the range of 6-10 mm, such as prostheses implanted into the human body, engine blades and struts. Because there are no more complex process structures such as deep grooves, grooves and ribs, the structural stress during pouring is relatively small and the feeding is sufficient, and the forming process of this type of casting is relatively simple, so the forming quality can always be maintained in a relatively high quality state.

The second is a relatively large thin-walled complex casting, which is a type of casting that has been continuously developed to meet the practical application needs of the aerospace industry. With the gradual popularity of the aerospace field, the application of this type of casting forming technology has become the key research object of various large research institutions. The size of large thin-walled complex castings is mostly strictly controlled in the range of 500-1000 mm, and the wall thickness is 1-3 mm.

Finally, it is a super-large integrally formed casting. At present, in the more developed countries abroad, the forging technology of super-large titanium alloy integral forming precision castings has been fully mature. However, it is still in its infancy in China.Such castings cannot be compared with other castings in terms of quality and performance. The size of this type of castings is mostly above 1500mm, the minimum wall thickness is about 1mm, and the mass is mostly about 50-1000kg. This type of casting is often used to replace the structure formed by the assembly of a large number of fasteners and parts, thereby comprehensively improving the stability and accuracy of the casting structure, and significantly reducing costs and accelerating production speed. The research and technical application of super large integral forming castings is the research difficulty of forging technology of titanium alloy precision castings in China at present, and it is also the focus of future research.

3 Analysis of NC machining technology scheme for large titanium alloy precision castings

  On the basis of systematic analysis of the structural characteristics and actual machining difficulties of large titanium alloy precision castings, a targeted CNC machining technology plan is formulated, which includes several main links such as clamping, rapid positioning, deformation control, and dimensional accuracy control:

  The first is the application scheme of clamping technology. Since the upper part of the large titanium alloy precision casting structure is mostly in a free state, as shown in Figure 2, the openings at both ends of the casting and the large holes in the middle are relatively weak in rigidity. In general, vibration problems are prone to occur during the machining process, which has a negative impact on the quality of CNC machining. In the face of this problem, three sets of adjustable supporting tooling can be added to the CNC machining platform to effectively enhance the actual stiffness of the CNC machining part of the titanium alloy precision castings, so as to ensure the surface quality and performance basis of the precision castings during CNC machining.

Fig.2 The upper part structure of titanium alloy precision casting

  The second is the fast positioning method. The rapid positioning work is to set a fixed positioning pin device at the tooling part of the processing platform, and set a position hole structure at the corresponding position of the technical boss of the casting processing to ensure that the hole shaft gap can echo, so as to achieve rapid and accurate positioning in the numerical control machining process. At the same time, the positioning pin at the origin of the tooling needs to be designed as a cylinder, and the distal positioning pin needs to be designed as a hexagon, so as to leave extra allowance for the machining deformation of the casting on the basis of better fixing the precision casting, and lay the foundation for the subsequent clamping operation.

  The third is deformation control processing. The first is the optimization of cutting tools and related parameters. In the cutting process of precision castings, cutting force is one of the factors that have the greatest influence on the processing quality of precision castings. The cutting force determines the cutting heat and machining deformation of titanium alloy castings to a large extent, and even affects the actual efficiency of cutting. Therefore, in the process of cutting work, sharp tools with sharp edges or large rake angles are usually selected, and the method of small cutting depth and layered processing is adopted, so as to reduce the probability of deformation problems in titanium alloy precision castings during CNC machining. Secondly, the realization of stress-free surface repair. Under the influence of various processing stresses, titanium alloy precision castings will have certain deformation problems. In this case, it is necessary to adopt stress-free clamping and stress-free surface repair methods to reduce the deformation of castings. In the free state of large titanium alloy precision castings, copper gaskets and other components are used to solidify the bottom surface of the positioning boss to ensure that the parts are clamped without stress during the pressing operation, and the positioning boss will not have deformation problems, so as to eliminate the clamping stress and casting deformation problems during the processing.

  The fourth is the control scheme of dimensional accuracy. Before the start of CNC machining, it is necessary to fully measure the machining surface and non-machining surface, so as to facilitate the subsequent inspection of deformation and machining allowance, and determine the machining allowance of CNC program through the inspection results. In the actual numerical control machining process, due to the obvious problem of tool removal in the thin-walled structure of large titanium alloy precision castings, the numerical control machining accuracy of precision castings is easily affected, which eventually leads to problems such as uneven local allowance and large size difference in precision castings.

  Finally, from the perspective of the overall process flow, the traditional CNC machining technology process generally includes 25 technical processes, among which there are many complex processes such as benchmark proofreading, allowance inspection, clamping operation and fitter grinding. The turnover times of parts are too many, but the processing efficiency is relatively low. Under the background of the continuous development of electronic information technology, CNC machining technology gradually combines advanced technical methods, process equipment and CNC equipment in the process of optimization and innovation, and optimizes the original technical process. The optimized CNC machining is only 12 processes. Therefore, from the perspective of dimensional accuracy control, deformation control processing, rapid positioning and clamping operation, a complete and high-quality CNC machining scheme can be established to ensure the CNC machining effect of large titanium alloy precision castings.

4 Strategies to optimize the application effect of NC machining technology for large titanium alloy precision castings

4.1 Comprehensive improvement of CNC machining technology to improve its applicability

  With the overall improvement of China 's industrial level, the market competition in the industrial market has become increasingly fierce. If industrial enterprises want to be in a favorable position in the fierce market competition, maintain their market position, and obtain higher social and economic benefits, they need to carry out comprehensive and in-depth research on the overall development trend of the industrial market while carrying out production optimization reform. The corresponding improvement of CNC machining technology ensures that the application of CNC machining technology can effectively promote the economic development of enterprises. In order to comprehensively improve the application of CNC machining technology, it is necessary for industrial enterprises to pay attention to the importance of improving CNC machining technology and increase investment in the research and development of CNC machining technology, so as to ensure that the improvement of CNC machining technology can achieve corresponding results. The enterprise has achieved the application improvement of CNC machining technology, which can improve the production quality of titanium alloy precision castings to the greatest extent, and can also speed up the production efficiency on the basis of cost saving. With the continuous optimization and improvement of CNC machining technology, it can also improve the application degree of CNC machining technology and solve the problems and difficulties in the application process of CNC machining technology, so as to further realize the production and processing of some larger and more complex titanium alloy precision castings.

4.2 Select the most appropriate machine to ensure the quality of CNC machining

  There are many types of machine tools that can be selected when constructing a CNC machining production line for large titanium alloy precision castings. Among them, EDM machine tools and milling machine tools are the most widely used types. In order to ensure the actual production efficiency and product quality of the production line and reduce the number of subsequent adjustments and maintenance, it is necessary to pay full attention to the selection of CNC machine tools. Because there are many types of large titanium alloy precision castings, and the structure of castings is often complex, it is difficult to process. Therefore, the corresponding production processes of different types of titanium alloy precision castings also have great differences. In order to accurately and high-quality produce titanium alloy precision castings in the actual production process, and to compress the production cost as much as possible while completing the actual production needs, it is necessary to comprehensively classify the types of large titanium alloy precision castings to be produced to ensure the actual production effect. Due to the classification of titanium alloy precision castings, it is also necessary to select the corresponding CNC machine tools when designing the production line. Only by selecting the most suitable CNC machine tools, can the CNC machining process be carried out smoothly and stably, so as to ensure the actual production efficiency and basic production quality of large titanium alloy castings.

4.3 Improve the machining accuracy of titanium alloy precision castings, so as to effectively improve the machining quality.

  The traditional precision casting processing method has gradually exposed its existing problems and shortcomings in actual production practice. The traditional precision casting processing method consumes a long processing time, but the quality of the obtained precision casting products is often very unstable, making it easy to produce a large number of low-quality products and waste a large amount of raw material resources. Moreover, due to the limitations of the manufacturing process, the resulting casting products still have the disadvantage of not being precise enough to meet the processing needs of precision castings. In the context of increasing demand for industrial production, traditional precision casting processing techniques are fundamentally unable to meet production needs, which has seriously affected the overall development of precision casting production enterprises and even led to their inability to effectively use raw materials such as titanium alloys, further weakening their market competitiveness. In the process of widely applying science and technology to various industries, the application of CNC machining technology in the processing of large titanium alloy precision castings has achieved good results. With the deepening of the application of CNC machining technology, traditional precision casting machining processes are gradually being phased out. While innovating in the industrial market, in order to ensure that CNC machining technology has sufficient market vitality, it is also necessary to optimize and reform CNC machining technology. Among them, improving the quality and accuracy of titanium alloy precision casting machining is the main challenge faced by CNC machining technology innovation. The precision and quality of large titanium alloy precision castings directly reflect the quality of production and processing technology. Only by paying attention to the quality and precision of the finished products and optimizing CNC processing technology in a targeted manner can we truly achieve overall progress in the industrial industry.

4.4 Deeply optimize the production line of precision castings and strengthen the application effect of CNC machining technology

  The deep optimization of CNC machining production line is a direct method to ensure the production and processing efficiency of CNC machining production line and strengthen the application effect of CNC machining technology. Because the application effect of numerical control technology is greatly affected by the programming level of numerical control processing technology, in order to deeply optimize the numerical control processing production line and improve the processing quality of large titanium alloy castings, programmers need to modify and adjust the numerical control programming program according to the actual needs. Programmers need to fully consider the three main factors of processing sequence, processing time and processing quality, so that the whole production line can complete the production and processing work in accordance with the relevant instructions, so as to effectively improve the application effect of CNC machining technology and improve the production and processing flow of large titanium alloy castings.

5 Conclusion

  On the whole, the application of numerical control technology has promoted the modernization of China 's industrial field, and the continuous improvement of the overall level of China 's industry has also put forward more stringent requirements for the further optimization of numerical control technology. China has always been a powerful country in the field of mechanical manufacturing. In the field of modern processing and manufacturing, in order to achieve efficient development on the basis of ensuring the quality of finished products, it is necessary to fully apply the numerical control processing technology. Relevant industrial enterprises should appropriately strengthen the research on NC machining technology, comprehensively improve the whole enterprise 's cognition of NC machining technology, and realize the overall improvement of technical ability. At the same time, industrial enterprises also need to conduct in-depth research on raw materials for production, especially pay attention to the research on the processing flow of titanium alloy materials, and fundamentally solve the deformation problems of titanium alloy large-scale precision castings in the production process as much as possible. It is difficult to control, the casting structure is complex, and the machining accuracy is high.

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